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doi:10.2204/iodp.pr.323.2010

Principal site results

Site U1339

Background and objectives

The primary objective of drilling at Site U1339 (prospectus Site UMK-4D) was to study high-resolution paleoceanography in the easternmost part of the Bering Sea, a marginal sea expected to exhibit large variations during times of global climate change. Umnak Plateau (Fig. F14) is located off Bristol Bay and is well situated for the study of past changes in surface water conditions, sea ice extent, and associated biological productivity. Today, parts of the relatively warm Alaskan Stream surface water flow into the Bering Sea through Unimak and Amukta passes and hence sea ice is not formed in this region. However, substantial sea ice coverage has been noted during the LGM, when sea level was ~100 m lower than today, indicating that the influence of relatively warm water from the distal end of the Alaskan Stream was reduced. This may have been because water entering the Bering Sea from the Pacific Ocean through Unimak and Amukta passes was at least partially restricted when sea level dropped; the warm Pacific water could have more easily passed through the deeper passes located in the central and western Aleutians, such as Amchitka Strait (Katsuki and Takahashi, 2005). Thus, monitoring past environmental conditions at Umnak Plateau can provide an understanding of the impact of changes in water exchange between the Pacific and Bering Sea waters. Since these eastern passes are fairly shallow (~50 m and 430 m), little intermediate or deep water flows out to the Pacific in this region. As such, records from Umnak Plateau should provide us with different information than the western sites, which are closer to deep passes where surface water flow may not have been strongly inhibited by sea level changes and where dense water exchange with the Pacific Ocean is more likely to occur (Tanaka and Takahashi, 2005). To make this west–east comparison, records from the drill site on Umnak Plateau (Site U1339) can be compared to those of Site U1341 (water depth = 2177 m) at Bowers Ridge.

The drill site at Umnak Plateau (Site U1339) is situated on the northwest flank of a section of the plateau that is separated from the main shelf by a canyon (Fig. F15). This location was selected with the assumption that much of the siliciclastic material from the shelf would have been transported into the canyon during sea level lowstands. As such, this site location would have received more pelagic biogenics relative to terrigenous sediments than a site located on the main shelf. There is clear evidence of continuous horizontal reflectors on the seismic profiles (Fig. F16).

The drill site at Umnak Plateau can also be used to study the impact of subseafloor microbes on biogeochemical fluxes in the highest surface ocean productivity areas of the drill sites in the Bering Sea. Organic-fueled subseafloor respiration and its impact on biogeochemistry in such a highly productive region has not previously been quantified. To do this, drilled sediments from Umnak Plateau were used to determine subseafloor cell abundance and to investigate the link between the mass and characteristics of subseafloor microbes and the extent of export productivity from the surface ocean.

Scholl and Creager (1973) found Pleistocene diatomaceous sediments with ash layers in the uppermost 120 m at DSDP Sites 184 and 185, both drilled at Umnak Plateau, followed by Pliocene diatomaceous sediments below. They recorded sedimentation rates of ~67 m/m.y. and indicated that the diatomaceous sediments have neritic components, which were likely influenced from the Bristol Bay region. A piston core study from the same general region provided a sedimentation rate of 262 m/m.y. (Takahashi, 2005). Thus, prior to drilling the predicted age for the bottom of Site U1339 at ~200 meters below seafloor (mbsf) ranged from the mid-Pleistocene to the Pliocene.

Lithostratigraphy

The sediments recovered at Site U1339 are a mixture of three components: biogenic, volcaniclastic, and siliciclastic. Other accessory lithologies identified include authigenic dolomite and pyrite. In general, the color of the sediments reflects their lithologic characteristics. Sediment composed of mixed lithologies of diatom silt or diatom ashy silt is dark greenish gray, whereas diatom ooze is olive-gray to olive. Most of the volcaniclastic ash layers are black. Only one lithologic unit spanning the Pleistocene was defined and it was divided into four subunits (Fig. F17). The largest-scale sedimentary features are decimeter- to meter-scale bedded alternations of sediment color and texture reflecting alternations in lithology. Well-preserved lamination caused by alternating millimeter-scale dark and light laminae was mainly observed in five distinct intervals, each ranging from 10 to 40 cm in thickness. Distinct volcaniclastic layers ranging from a few millimeters to 10 cm in thickness occur throughout the unit. Some black volcaniclastic layers are normally graded, indicating either time-transgressive fining of grains or redeposition by gravity flows.

The major sediment component is biogenic and predominantly composed of diatoms with generally good preservation. Diatom frustules hosting pyrite framboids are also observed. Both benthic and planktonic foraminifers are observed, although their abundance levels were low. Thin laminae dominated by foraminifer tests are observed with dominance of benthic foraminifer Bulimina sp., which is characteristic of low oxygen content. Calcareous nannofossils, radiolarians, and sponge spicules are rare. Terrigenous grains, which are common sediment components, are composed of silt-sized quartz and feldspar, clay, mica, and rock fragments. Gravel- to pebble-sized, rounded to angular clasts are interpreted as dropstones delivered by melting sea ice or icebergs. Some pebbles are composed of pumice or obsidian, suggesting a volcanic source. Authigenic dolomite was found in five distinctive intervals, mostly in the upper and lower part of the sedimentary record. It occurs either as dolomite rhombohedra scattered in the sediment or as (semi-) lithified layers 5–10 cm thick. Because of the presence of authigenic carbonates at this site, carbonate-stable isotope measurements are potentially contaminated by the isotopic signature of the overgrowths. However, the presence of authigenic carbonates can be detected and screened out of the isotopic records. Depth variations of the color reflectance (CR) parameters a* and b*, GRA porosity, and magnetic susceptibility were compared to the lithologic variations. Overall, these four parameters show distinct short-term variability and longer term trends that can be correlated to lithologic variations at both short- and long-term scales. The depth variations of CR parameter b*, which reflects the yellowness of the sediment, show a remarkably negative correlation with the GRA data, with lower b* values corresponding to denser, more siliciclastic-rich sediments. CR value a* is tentatively correlated to the concentration of diatom ooze. Changes of magnetic susceptibility with depth mainly reflect volcaniclastic content, and a very good correlation exists between the thickest volcanic ash layers and the highest magnetic susceptibility excursions.

Micropaleontology

In contrast to their calcareous counterparts, siliceous microfossil groups had constant presence and relatively high abundance throughout all sections at this site. All age determinations were achieved with the siliceous microfossil groups (Table T3; Figs. F17, F18). Diatoms provided two important last occurrence (LO) datums of Thalassiosira jouseae (0.31 Ma) and Proboscia curvirostris (0.28 Ma) in the middle section of the four holes studied. The most important datums were derived by studying silicoflagellates, despite the fact that their abundances were generally "trace" to "few" with rare instances of "common." These are the first occurrence (FO) of Distephanus octangulatus (0.741 Ma) and the LO of Dictyocha subarctios (0.736 Ma), respectively. These two datums are considered to occur above the Brunhes/Matuyama boundary (0.781 Ma). However, without definite paleomagnetic datums of the Brunhes/Matuyama boundary (see "Paleomagnetism" section for detail), the silicoflagellate datums are the only reliable datums at this time. The LO of Distephanus octonarius (0.244 Ma), another silicoflagellate datum, provided harmonious age information in concordance with diatoms and radiolarians. Despite their relatively low abundances due to massive occurrences of diatoms in the bulk of samples, radiolarians provided five pertinent datums that span from near the top through the middle part of the drilled sections. These include Lychnocanoma nipponica sakaii (50 ka), Amphimelissa setosa (80–100 ka), Spongodiscus sp. in Ling (1973) (0.28–0.32 Ma), Axoprunum acquilonium (0.25–0.43 Ma), and Stylatractus universus (0.41–51 Ma).

Because of this site's proximity to the Bering shelf, particularly during the glacial lowstands, coastal water diatoms including Chaetoceros spores and freshwater diatoms were observed. Furthermore, the relative abundances of the ubiquitous Neodenticula seminae, an indicator species of the Alaskan Stream, fluctuated throughout the sections, indicating change in the Pacific water entry into the Bering Sea. The extent of sea ice–associated diatoms also fluctuated throughout the sections.

Calcareous nannofossils were either barren or sparse throughout the sections. Reworked specimens of calcareous nannofossils were found in the upper section, indicating transport by IRD or other processes. Planktonic foraminifers were present in all but a few samples. Both planktonic and benthic foraminifers were abundant enough to proceed with more detailed studies such as oxygen and carbon isotopic analyses. Abundant benthic calcareous foraminifers with generally low diversity showed close affinities to those recorded in recent sediments within or near the OMZ in the Sea of Okhotsk. The exceptionally high productivity in surface waters greatly expanded the OMZ.

Dinoflagellate assemblages were moderate to abundant with relatively low species diversity. They showed heterotrophic protoperidinial dinoflagellates feeding on diatoms. Such assemblages are related to extremely high diatom production. In the upper section, a taxon indicative of the North Water Polynia condition was noted, implying significant sea ice formation.

Paleomagnetism

Paleomagnetic analyses indicate that all sediments are likely from the Brunhes Chron (0–780,000 y before present [BP]). However, anomalous NRM intensities along with low or negative inclinations in the same intervals may be due to authigenic growth of greigite related to sulfate reduction or methanogenesis. As such, it is possible that the Brunhes/Matuyama boundary was reached at ~180 mbsf but that the reversal in polar intensity was obscured by the presence of greigite. At this time, it is difficult to say which hypothesis is more likely.

Stratigraphic Correlation

Three holes dedicated to paleoceanographic studies were drilled at this site. Cores were drilled at offset depths in order to obtain a continuous sedimentary section. By comparing fast track GRA density and magnetic susceptibility variations, the cores from different holes can be placed into a continuous stratigraphic framework. An affine table indicating offsets for each core in each hole was constructed (Table T4). A continuous record mainly using Holes U1339C and U1339D was constructed by splicing together the GRA and magnetic susceptibility records; this splice (Table T5) will guide the postcruise sampling strategy designed to generate continuous paleoceanographic records. The splice at Site U1339 is continuous from the mudline to ~200 m CSF (Fig. F17).

Geochemistry

Interstitial water samples taken from Hole U1339A at low resolution from 0 to 200 m CSF and at high resolution from Hole U1339B were analyzed for chlorinity, salinity, alkalinity, DIC, pH, sulfate, hydrogen sulfate, ammonium, phosphate, silica, Ca, Mg, Na, K, Fe, Mn, B, and Sr. In addition, ethane and methane were analyzed. Methane is detectable throughout, and at ~10 m CSF, where sulfate is depleted, methane concentrations increase dramatically (Fig. F18). The analyses show that the SMTZ at present is relatively shallow at 8–10 m CSF. Notably, alkalinity and DIC increase from 3 to 30 mM in the uppermost 10 m, reaching a maximum at ~120 m CSF (Fig. F18). Solid-phase analyses show that calcium carbonate content ranges from 0 to 13.3 wt% (average = 2.1 wt%) and total organic carbon is 0.47 to 1.83 wt% (average = 0.98 wt%).

Physical properties

Whole-round measurements of magnetic susceptibility and GRA bulk density (using the fast track) and magnetic susceptibility, GRA bulk density, and P-wave velocity (PWV) (using the whole-round multisensor logger [WRMSL] slow track) were made on each core recovered at Site U1339. One thermal conductivity (Tcon) measurement per core was also analyzed. Discrete samples of cores from Holes U1339A and U1339B were taken to analyze moisture, density, porosity, water content, and grain density. Magnetic susceptibility varied downcore, with the highest peaks likely related to volcanic ash layers and cyclical variations likely due to varying amounts of siliciclastics versus diatoms. Such variations in lithology likely contributed to the variations in GRA bulk density and discrete bulk density measurements, which indicate cyclical variations from 1.6 to 1.3 g/cm³. Porosity, water content, and grain density measurements exhibit an oscillating cyclicity similar to the wet bulk density record and are likely tracking variations in the concentration of biogenic debris—in particular, highly porous diatom frustules—with respect to lower porosity terrigenous sediment with higher grain density. Discrete bulk density measurements increase gradually with depth at a rate of ~0.1 g/cm³/100 m, most likely because of sediment compaction. Compaction likely explains the decrease in porosity downhole from a near-surface value of ~85% to ~65% at a depth near 200 m CSF.

NGR counts/s varied rhythmically from highs of 25 to 40 counts/s to a low of ~10 counts/s, evidently tracking clay mineral–bearing sediment in contrast to sediments with more biogenics. Because the PWV measurements were seriously degraded by expansion of hydrate-released methane gas, the P-wave logger was turned off for all core sections deeper than ~33 m CSF. Tcon values ranged widely from a low of ~0.5 W/(m^.K) to >1 W/(m^.K), with lower values presumably reflecting water-rich diatomaceous sections and higher values reflecting abundant terrigenous debris.

Electrical conductivity to determine formation factor was measured every 10 cm in the working half of the first core of Hole U1339A, then every 20 cm in Cores 323-U1339A-2H and 3H. Sediment conductivity ranges from 1.37 to 3.70 μS/cm. The highest values were recorded in ash layers and formation factor generally increased downhole.

Downhole measurements

By employing the deployed triple combination (triple combo) and the Formation MicroScanner (FMS)-sonic tool string combinations, data were recorded below the bottom of the pipe depth. They indicate good hole conditions and only minor excursions from the nominal size of the drill bit. The comparison of the gamma ray and density logs with the NGR and GRA track measurements on cores recovered and with moisture and density (MAD) measurements shows good agreement, which should allow for reliable core-log integration. All logs were referenced to the seafloor depth of 1875 m wireline log depth below rig floor (WRF) identified during the last pass of the FMS-sonic tool string. Comparison of the gamma ray logs measured during the main pass of the two runs shows an excellent repeatability between the two runs. The resistivity values measured by the electrode spherically focused resistivity (SFLU) measurement were lower than those recorded by induction measurements, probably because of current loss at the electrodes. The higher induction resistivities are closer to values typically measured in deep-sea sediments.

The display of the high coherence in sonic waveforms used to derive the compressional velocity suggests that despite the closeness of the formation velocity to the sound velocity in the borehole fluid (~1500 m/h), the Dipole Sonic Imager (DSI) was able to capture compressional wave arrivals and measure a reliable V_P profile over the entire open interval logged. Additional postcruise processing will, however, be necessary to derive V_S logs from the recorded dipole waveforms.

The downhole log measurements of bulk density, porosity, and electrical resistivity correlate very well. Variations in sediment composition result in variations of porosity that affect bulk density and resistivity in a similar manner. The measurements also correlate significantly with the gamma ray logs as a result of the overall uniform mineral matrix of the sediments in the interval logged. In particular, the parallel decrease with depth in gamma ray, density, and resistivity from 86 to 102 m wireline log matched depth (below seafloor) (WMSF) is typical of a retrograding fining-upward sequence.

The downhole variations of gamma ray radioactivity are controlled by the sediment content of naturally occurring radioactive elements (K, U, and Th). The computed gamma ray, or gamma ray without uranium, is a more accurate measure of the clay content than is the total gamma ray, which can be influenced by such factors as organic matter or detrital minerals. The most significant feature in the gamma ray logs is the increase at ~142 m WMSF, which is associated with an increase in the three radioactive components and a peak in uranium. This increase coincides with the transition from lithologic Subunits ID to IE (diatom ooze and diatom fine ash to diatom ooze). The peak in uranium can be related to the occurrence of dolostones observed in the cores in this interval.

The downhole third-generation advanced piston corer temperature tool (APCT-3) measurements show a range of 3.65°C at 23.0 m drilling depth below seafloor (DSF) to 12.83°C at 158.0 m DSF. These closely fit a linear geothermal gradient of 68.0°C/km. The temperature at the seafloor was 2.1°C based on the average of the measurements at the mudline during all the APCT-3 deployments. The obtained heat flow—the product of the geothermal gradient by the average thermal conductivity (0.80 W/[m^.K])—gives a value of 54.4 mW/m², which is within the range of previous measurement in the area.

Sedimentation rates

The age model for Site U1339 was constructed using biostratigraphic datums (Fig. F19). The datums that were selected to constrain the sedimentation rates were the ones judged to be best resolved (Table T6). The calculations indicate some variability in rates, which range from 22 to 50 cm/k.y. (Fig. F19).

Site U1340

Background and objectives

The primary objective of drilling at Site U1340 (prospectus Site BOW-12B) was to study high-resolution Pliocene and Pleistocene paleoceanography in the southernmost part of the Bering Sea at a topographic high on Bowers Ridge (Fig. F20), where relatively good calcium carbonate preservation is expected. Bowers Ridge is well situated to allow study of the past extent of water mass exchange with the Pacific Ocean through the adjacent Aleutian passes such as the Amukta, Amchitka, and Buldir passes. In particular, its location allows examination of the influence of the warm Alaskan Stream water mass entry into this region, which influenced the distribution of past sea ice coverage (Katsuki and Takahashi, 2005). Although productivity in the Bering Sea in general is very high with respect to other parts of the global oceans, the expected productivity at this site was lower than at Site U1339, which has substantially greater influence from the nearby Bering shelf and which was exposed during glacial lowstands. We selected this site at relatively shallow water depth to study physical and chemical changes in upper water mass conditions such as the low dissolved oxygen concentration conditions that caused the formation of laminated sediments at a site on the Bering slope at similar water depths (Cook et al., 2005). The vertical structure of past water masses can be determined by comparing results at this site with those at the other drilling sites on Bowers Ridge (Site U1341, water depth = 2177 m; Site U1342, water depth = 837 m) (Fig. F20).

Site U1340 at Bowers Ridge can also be used to study the impact of subseafloor microbes on biogeochemical fluxes in the highest surface ocean productivity areas of the drill sites in the Bering Sea. Samples used to study organic-fueled subseafloor respiration and its impact on biogeochemistry at the highly productive region of the previous Site U1339 were compared to analyses at the Bowers Ridge sites, including Site U1340, although the high-resolution sampling that occurred at Site U1339 was not performed at Site U1340. The Bowers Ridge sediments were used to determine subseafloor cell abundance and to investigate the link between the mass and characteristics of subseafloor microbes and the extent of export productivity from the surface ocean.

Site U1340 is located on the eastern flank of the southern part of Bowers Ridge (Fig. F20) in a basin ~10 km east of the ridge crest (Fig. F21). Close-ups of the seismic images in the basin (Figs. F22, F23, F24) indicate that strata dip to the east (Fig. F23). Some shallow sections do not have continuous parallel strata, but most of the rest of the section appears to have continuous features in the seismic images (Figs. F23, F24). The basement age and the age of the deeper sediments are unknown, but at DSDP Site 188 on the western flank of Bowers Ridge (Scholl and Creager, 1973) sediments as old as upper Miocene were found. Specifically, Scholl and Creager (1973) found recent to upper Miocene diatom ooze interbedded with silt-rich diatom ooze and diatomaceous silt. They also found calcareous nannofossils in the upper 120 m and planktonic foraminifers in the upper 300–400 m. In the interval between 580 and 638 mbsf they encountered mudstone. They reported sedimentation rates of ~100 m/m.y. A piston core study from the same general region provided sedimentation rates of 80 m/m.y. (Takahashi, 2005). Thus, prior to drilling sections ranging from the entire Pleistocene to the Pliocene and possibly Miocene were expected.

Lithostratigraphy

The sediments recovered at Site U1340 are a mix of biogenic, volcaniclastic, and siliciclastic sediments. Authigenic dolomite also occurred as an accessory sediment type. In general, the color of the sediments reflects their lithologic characteristics. Sediment composed of siliciclastic sediment or mixed lithologies tends to be very dark greenish gray to dark gray, whereas diatom ooze tends to be olive-gray to olive or dark gray. Volcaniclastic ash layers are dark gray to black or shades of light gray to white and, rarely, weak red. Intervals showing soft-sediment deformation were observed between Core 323-U1340A-3H and Section 8H-2.

Three lithologic units were defined at Site U1340 (Fig. F25). Unit I is composed of alternating beds of diatom ooze and diatom silt with minor amounts of mixed and volcaniclastic sediment from the Pleistocene. The most prominent sedimentary structure in Unit I is soft-sediment deformation of laminated and bedded diatom ooze and diatom silt. Intervals with soft-sediment deformation were observed from Core 323-U1340A-3H to Section 8H-2 and in Cores 11H, 12H, 14H, and 15H; from Cores 323-U1340B-3H to 6H; in Cores 323-U1340C-2H and 3H; and in all cores from Hole U1340D. Folded and tilted bed boundaries are clearly visible in these cores, suggesting the occurrence of synsedimentary slumps as a potential mechanism for the deformation. Distinct volcaniclastic layers ranging in thickness between a few millimeters and 3 cm occur throughout the unit. Volcanic ash is a common secondary or trace lithologic component. A major component of the sediment in Unit I is biogenic and predominantly composed of diatoms with generally good preservation. Diatom frustules hosting pyrite framboids were observed. Terrigenous particles are also a major component in Unit I. The most abundant terrigenous grain types are silt-sized feldspar, quartz, clay, mica, and rock fragments. Some pebbles are composed of basalt and pumice or scoria, indicating a volcanic source. Bioturbation varies from slight to strong throughout all holes and is typically characterized by a mottling defined by color changes. The main lithologies are olive to dark gray diatom ooze and dark gray to dark greenish gray diatom silt with variable amounts of dispersed vitric ash, isolated pebbles, and distinct ash layers as well as bioturbated ash/diatom ooze layers and laminated intervals. The light-colored sediments (olive) tend to contain predominantly biogenic components, whereas the dark-colored sediments (gray) tend to contain subequal proportions of siliciclastic and biogenic components. The volcanic ash layers are typically black, light gray, and, rarely, weak red. Unit II comprises diatom ooze with minor amounts of diatom silt and mixed and volcaniclastic sediment from the Pliocene. Unit II differs from Unit I in having significantly more diatom ooze, less interbedded diatom silt, and fewer laminated intervals. A layer of gravel was recovered at 380 m CSF from Hole U1340A. This is directly below an interval of frequent deposition of gravel-sized clasts at ~220–360 m CSF. This gravel-rich interval overlaps the base of Unit I and the top of Unit II. Unit III, which is further divided into Subunits IIIA and IIIB, comprises diatom-bearing, sponge-spicule-rich ashy sand; sponge-spicule-rich diatom ooze; diatom ooze; and minor amounts of volcaniclastic and siliciclastic sediment from the Pliocene in age. Core disturbance was observed in all sections collected in Unit III. The sediment was described as a slurry with water-rich areas at the core surface and along the liners.

Mixed and siliciclastic lithologies in Unit I correlate well with magnetic susceptibility changes. The increase occurred at ~260 m CSF, dated to around 2.6 Ma, and may reflect an increase in sea ice- or glacial ice-rafted debris delivered to the site after the onset of large glacial–interglacial cycles. The rounded shape of many of the gravel-sized grains suggests that their source was a coastal environment and therefore favors the hypothesis that they were deposited as a consequence of sea ice rafting. Relatively large clasts interpreted as dropstones were observed most frequently between 360 and 220 m CSF (~3.6–1.8 Ma), and their occurrence overlaps with the onset of increased siliciclastic components, coeval with a subtle increase in GRA. This may reflect the onset of NHG after 3.6 Ma (Mudelsee and Raymo, 2005). The occurrence of extensive intervals with soft-sediment deformation related to slumping is somewhat unexpected because the slope of Bowers Ridge is only slightly inclined at the drill sites. An alternative explanation for the triggering of sediment mass movements could be the seismogenic activity of Bowers Ridge, which is representing a buried subduction zone and is relatively close to the volcanic Aleutian arc. Another possibility could be water loss during mineral phase changes in deeper sediments, which may result in less cohesive sediment packages prone to deformation. The primarily biogenic components of Unit II and Subunit IIIB may reflect warmer, highly productive Pliocene conditions or a higher sea level.

Micropaleontology

Core catcher (CC) samples from Site U1340 are dominated by diatom microfossil assemblages with high diversity. Seventeen datums have been identified in Hole U1340A (Table T7; Fig. F26). Sections 323-U1340A-1H-CC to approximately 34H-CC exhibit a broadly linear sedimentation rate. Thereafter, the sedimentation rate appears to increase drastically, with the last three datums all giving the same age of 3.8–4 Ma. Siliceous microfossils show consistent occurrences throughout the section and are mainly composed of high-latitude pelagic species, indicating changes to surface water productivity. Calcareous microfossils are mostly confined to the top of the section above Section 323-U1340A-23H-CC for nannofossils and 31H-CC for foraminifers. Reworked calcareous nannofossil specimens, mostly of Miocene and Paleogene age, were found in some samples. The planktonic foraminifer fauna does not change radically throughout the Pleistocene (above 200 m CSF), and it is dominated by the subpolar–polar species Neogloboquadrina pachyderma (sinistral) together with the subpolar species Globigerina bulloides, Globigerina umbilicata, and Neogloboquadrina pachyderma (dextral). Benthic foraminifers are largely characteristic of those found within or near the OMZ in high-latitude regions. Dinoflagellates consistently occur throughout the section, indicating changes to the productivity and ice cover of the surface waters.

Over 40 species of benthic foraminifers recovered in 83 samples resemble species from oxygen-depleted zones on Umnak Plateau and elsewhere. Abundance and diversity fell markedly below 282.17 m CSF in Hole U1340A, with samples either barren or consisting of a monospecific agglutinated assemblage. Core catcher samples were checked for ostracodes, but no specimens were found.

Diatoms are the dominant microfossil in all holes and they show good preservation throughout. The LOs of Proboscia curvirostris, Thalassiosira jouseae, and Proboscia barboi were identified at 37.60 m CSF for all three species, giving a composite estimated age of 0.3 Ma based on the result from a piston core from Site ES on the northern Emperor Seamount. The age of 0.9 Ma was assigned at 161.24 m CSF by the last common occurrence (LCO) of Actinocyclus oculatus, which is followed by the first common occurrence (FCO) of Proboscia curvirostris at 1.8 Ma. The LCO of Neodenticula koizumii was determined at Section 323-U1340A-25H-CC, giving an age of 2.1 Ma. Neodenticula koizumii and Neodenticula kamtschatica occur jointly between Sections 323-U1340A-46X-CC and 57X-CC, corresponding to North Pacific Diatom (NPD) Zone 8 (2.7–3.9 Ma). The FO of Neodenticula koizumii and the dominance of Neodenticula kamtschatica above Section 323-U1240A-56X-CC defines this zone as NPD Zone 7Bb (3.9 Ma). Diatom assemblages are mainly composed of pelagic species throughout the Pleistocene and upper Pliocene. Several significant abundance peaks of a high-productivity indicator occur throughout the upper and lower Pliocene. In general, few coastal water diatoms, including Chaetoceros spores, or freshwater diatoms were observed below the upper Pleistocene, which may be explained by the distant location of this site to continental influence.

Despite the low abundances of silicoflagellates and ebridians, two LOs were obtained. The LO of Dc. subarctios (0.74 Ma) fits conformably with those of other siliceous microfossils as well as that of paleomagnetism. The LO of Ebriopsis antiqua antiqua (2.47–2.48 Ma) is placed at 305.47 m CSF.

Consistently abundant to common radiolarians with good to moderate preservation occurred in the upper interval (above ~200 m), whereas radiolarians with few abundances and moderate to poor preservation occurred in the lower interval (below ~200 m). The radiolarian stratigraphy spans from the Botryostrobus aquilonaris Zone (upper Quaternary) to the Dictyophimus bullatus Zone (middle Pliocene). A missing Stylatractus universus Zone (between 0.4 and 0.9 Ma) is due to the absence of S. universus. Eleven radiolarian datums derived in the subarctic Pacific were identified at this site.

All samples contain poorly to well-preserved palynomorphs. The concentration of terrestrial palynomorphs is low to moderate in most samples. Freshwater palynomorphs (Pediastrum, Botryococcus, and tintinides) occur only in the upper part of the sequence from 42 to 186 m CSF. Reworked palynomorphs are generally accompanied by a high number of wood microfragments. Very abundant dinoflagellate cysts occur only in the uppermost 200 m CSF and lower abundance occurs below. The LO of Filisphaera filifera (1.7 Ma) was determined at 216 m CSF. The Gonyaulacale Operculodinium centrocarpum co-dominates the assemblage together with Brigantedinium spp. in the core catcher samples from the uppermost cores. O. centrocarpum is closely related to seasonal sea ice cover. All accompanying taxa except extinct species (Filisphaera filifera and Batiacasphaera minuta) are known to be abundant in polar and circumpolar regions.

Paleomagnetism

The inclinations measured in Site U1340 sediments average almost 70° over the entire depth range of the cores. The site axial dipole inclination is ~72°. Several distinct intervals of reversed inclinations are interpreted to be polarity epochs. The declinations, after correction with the FlexIt tool to orient the declination data with North, suggest that there are multiple polarity intervals in the uppermost 17 cores from Hole U1340A; however, the FlexIt corrections are too poor to be of much detailed use in assigning polarity boundaries. The inclinations provide an initial guide to polarity zonation in Hole U1340A; we could discern the Brunhes, Jaramillo, Olduvai, and Gauss normal polarity chrons (Fig. F25). Polarity boundaries and paleontological age estimates are generally in agreement (Fig. F26).

The NRM intensities largely remain at the same level throughout most of Hole U1340A. The NRM and Chi intensities vary over more than an order of magnitude on a meter scale. We interpret this to stem mostly from variable flux of detrital sediment versus biogenic sediment flux (mostly diatoms at this site). The large changes in NRM intensity also appear to be associated with notable detrital (and presumably magnetic) grain size changes. Both of these variations make the relative paleointensity estimates, determined by normalizing the cleaned NRM (20 mT) by magnetic susceptibility, questionable in interpretation. The relative paleointensity variability is quite large, but most of it is strongly correlated with NRM and magnetic susceptibility variability and is probably not due mostly to geomagnetic field variability. As at our last site, we see no notable evidence for the presence of magnetic field excursions in any of the cores.

Stratigraphic correlation

The composite depth scale and splice at Site U1340 is complete from 0.0 to 47.7 m CCSF-A (Tables T8, T9). This splice is tentative because of some evidence for soft-sediment deformation in some of the cores based on observed sedimentary structures, data features that do not appear to correlate between holes, and in some cases sections that appear repeat within a single hole. An interval that contains a distinctive pink ash appears to be present three times in Hole U1340C: twice in Core 323-U1340C-2H and once in Core 3H. This appearance of the same sedimentary interval at different subseafloor depths as well as the replication of a section within a single hole is consistent with visual evidence for tilted beds and nonconformable surfaces. We suggest that sediment deformation has produced thickened and noncorrelative intervals in the depth interval from ~20 to ~45 m CCSF-A in Holes U1340B and U1340C and perhaps also in Holes U1340A and U1340D. We are not confident that the full sedimentary sequence is represented in the composite splice. We tentatively splice Core 323-U1340B-4H as the bottom interval of the splice, which includes the least disturbed intervals through the interval of inferred slumping. Affine growth factors in the spliced interval have values of 1.14 in Hole U1340A and 1.18 in Hole U1340B, which are within the normal range typical of many drill sites. The remaining cores at Site U1340 are not tied to the splice but are appended to the bottom of the splice with a constant affine value of 7.31.

Geochemistry

In Hole U1340B, nine interstitial water samples ranging from 2.9 to 53.3 m CSF were retrieved by the whole-round squeezing technique. Interstitial water chloride concentrations varied between 528 and 570 mM, but downhole salinity remained constant at 36. Alkalinity increased from 2.9 to 18 m CSF, whereas only a subtle increase was seen below. DIC showed a similar trend as alkalinity with a maximum concentration at 42.3 m CSF. pH remained unchanged throughout, averaging 7.7. The dissolved sulfate concentrations slightly decreased. Hydrogen sulfide was detected at low concentrations, averaging 4.4 µM. Ammonium concentrations increased as depth increased. Phosphate concentrations gradually increased throughout the uppermost ~12 m, followed by a gradual decrease to 53 m CSF.

Methane was the only hydrocarbon gas detected in Holes U1340A and U1340B. Concentrations of methane in Holes U1340A and U1340B ranged from 0 to 3.1 ppmv and from 2.2 to 6.6 ppmv, respectively. Ethane and other volatile hydrocarbons were not detected. Fourteen core catchers from Hole U1340A were used for the preliminary analysis of solid-phase total inorganic carbon (TIC), TOC, total nitrogen, and total sulfur. Calcium carbonate contents in Hole U1340A ranged from 0 to 3.6 wt%. A part of increased CaCO₃ contents between 300 and 400 m CSF corresponds to the intervals where authigenic dolomite and calcite were observed in sediments. Because of the presence of authigenic carbonates at this site, carbonate-stable isotope measurements are potentially contaminated by the isotopic signature of the overgrowths. However, the presence of authigenic carbonates can be detected and screened out of the isotopic records. TOC and total nitrogen contents range from 0.25 to 1.19 wt% and from 0.02 to 0.09 wt%, respectively. TOC decreased below 400 m CSF. Total sulfur contents ranged from 0.09 to 0.42 wt%. Undetectable methane, deep SO₄^2– penetration, and low values of DIC, alkalinity, NH₄⁺, and PO₄^3– suggest extremely low microbial activity compared to Site U1339 despite similar TOC content.

Microbiology

Samples for abundance of prokaryotes were collected adjacent to interstitial water whole-rounds at the same resolution. Samples were fixed for further shore-based analyses.

Physical properties

Cores from all holes were placed on the fast or Special Task Multisensor Logger (STMSL) track and scanned for magnetic susceptibility and GRA bulk density and on the WRMSL for GRA, magnetic susceptibility, and P-wave scanning. Because of noisy data, noncontact resistivity values were not recorded. P-wave velocity and sediment shear strength measurements were not determined on working section halves. Magnetic susceptibility values spike irregularly, with values ranging from 500 to 1300 SI units in the uppermost 250 m CSF, but values were relatively low and invariant below ~250 m CSF until 525 m CSF, when they increased sharply to above 1000 SI units, registering a thick tephra unit. Bulk density determinations reveal high excursions and an apparent rhythmic pattern of higher values alternating with lower ones. The average reading in the tephra-bearing diatom silt of Unit I (surface to ~360 m CSF) is ~1.4 g/cm³. Within Unit I, bulk density decreases with depth to ~1.38 g/cm³ at ~360 m CSF. Below ~384 m CSF and a shift to XCB coring, values lowered additionally to ~1.32 g/cm³. In Hole U1340A, NGR counts/s, which principally reflect clay mineral abundance, decrease with depth from near-surface readings averaging ~20 counts/s to ~10 counts/s at ~380 m CSF and below. Presumably the downhole decrease in NGR counts reflects increasing relative abundance of siliceous biogenic tests and debris of lithologic Unit II.

The P-wave velocity profile for Hole U1340A is variable but generally increases from a near-surface velocity of ~1.52 to ~1.55 km/s at ~280 m CSF. This section corresponds to lithologic Unit I. In the underlying Unit II, and in particular below the transition from APC to XCB coring at ~384 m CSF, average P-wave readings exhibit only a subtle continuation of a downsection increase in velocity of ~10 m/s (from ~1.55 to ~1.56 km/s) at the bottom of Hole U1340A (~604 m CSF), possibly because the diatomaceous ooze section of Unit II that included tephra beds is mechanically stronger and less yielding to compaction than the overlying diatomaceous silt beds of lithologic Unit I.

The depth distribution of MAD wet bulk density is similar to that traced by the WRMSL GRA sensor. The MAD profile documents a slight downhole trend of decreasing density from ~1.42 g/cm³ near the seafloor to ~1.40 g/cm³ at ~360 m CSF. Below this depth, which marks a switch to XCB drilling and a ~25 m thick section of poor core recovery (~360 to 384 m CSF), a low average density of ~1.32 g/cm³ is recorded. However, bulk density increases farther downhole to ~1.4 g/cm³ at the bottom of Hole U1340A at ~604 m CSF. Perhaps a water-rich and partially load-bearing section signaled by the recovery of gravelly and sandy beds in the upper part of lithologic Unit II separates the upper decreasing and lower increasing trends in bulk density.

Porosity and water content profiles in Hole U1340A are similar and exhibit three contrasting trends. For porosity, the upper trend from seafloor to ~350 m CSF displays an average value near 74% that remains virtually constant with depth. The middle trend, which begins below the transition from APC to XCB drilling at ~384 m CSF and the zone of poor recovery from ~360 to 384 m CSF, documents a shift to a higher average porosity value near 80%, below which the average decreases progressively to ~66% at ~550 m CSF. Porosity measurements decrease to ~75% at the base of Hole U1340A at ~604 m CSF (Fig. F22). Hydraulically, the middle porosity sections appear to be separated from the upper ones by a permeability barrier in the zone of poor core recovery between ~360 and 384 m CSF. This inferred barrier occurs at the level of a gravel-bearing sequence within the diatom ooze section of lithologic Unit II. Isolation of the middle trend from the basal one is coincident with lithologic Unit III, a coarse ashy layer overlying the diatom ooze of Unit IV.

The average grain density decreases downsection from near-surface values of ~2.61 g/cm³ to ~2.5 g/cm³ at the bottom of Hole U1340A at ~604 m CSF. Wide excursions, some of which are so low (<1.5 g/cm³) or high (>2.9 g/cm³) that measuring error is suspected, occur about the mean, which is ~2.45 g/cm³. The downsection decreasing values appear to reflect an increase in biogenic silica (chiefly diatom frustules) with respect to terrigenous mineral debris and tephra.

Downhole measurements

The measured temperatures ranged from 5.67°C at 70.4 m DSF to 9.79°C at 165.4 m DSF, and they closely fit a linear geothermal gradient of 43.4°C/km. The temperature at the seafloor was 2.8°C based on the average of the measurements at the mudline during all the APCT-3 deployments. A simple estimate of the heat flow can be obtained from the product of the geothermal gradient by the average thermal conductivity (0.851 W/[m^.K]), which gives a value of 36.9 mW/m², within the range of previous measurement in the area. Alternatively, if the thermal regime is purely conductive, then the resulting linear fit of the temperature gives a slightly higher heat flow value of 37.8 mW/m².

Sedimentation rates

Sedimentation rates in the upper Pleistocene section appear to be 13–17 cm/k.y. (Fig. F26; Table T10). However, below ~30 m CSF there is evidence of soft-sediment deformation, which could have caused postdepositional thickening of the section. From ~150 m CSF to ~330 m CSF, sedimentary structures indicative of deformation were almost completely absent and the sedimentation rate was 15–24 cm/k.y. Below 330 m CSF to ~500 m CSF the sedimentation rate was as high as 32 cm/k.y. From ~500 m CSF to the bottom of the hole the sedimentation rates appeared to be quite low (~4 cm/k.y.)

Site U1341

Background and objectives

The primary objective of drilling at Site U1341 (prospectus Site BOW-14B) was to study high-resolution Pliocene–Pleistocene paleoceanography in the southern part of the Bering Sea at a western flank location of Bowers Ridge. Previous DSDP coring (Site 188) and other piston core studies in the region documented relatively high sedimentation rates of 100–138 m/m.y., respectively, and the presence of appropriate microfossils for paleoceanographic studies. Bowers Ridge is well situated to allow study of the past extent of water mass exchange with the Pacific Ocean through the adjacent Aleutian passes such as Amukta, Amchitka, and Buldir passes. In particular, its location allows for examination of the influence of the warm Alaskan Stream water mass entry into this region and presumable impact the distribution of past sea ice coverage. Although the productivity in the Bering Sea in general is very high with respect to other parts of the global oceans (Takahashi et al., 2002), the expected productivity at this site is lower than at Site U1339, which experienced substantially greater influence from the adjacent Bering shelf, which was subaerially exposed during the glacial low sea level stands. Drilling at this site—located at a relatively deep water depth of 2177 m—provides us with past intermediate water conditions, including chemical compositions. For example, this site is located just below the modern dissolved OMZ, which causes the formation of laminated sediments. Slight fluctuations in the intensity or depth of the OMZ should be captured by proxy records of past oxygenation measured at this site and compared to other, shallower sites. This site and the shallower drill sites at Bowers Ridge (Site U1340, water depth = 1295 m; Site U1342, water depth = 819 m) will be used to compare the vertical extent of water mass conditions.

This drill site at Bowers Ridge also allowed for study of the impact of subseafloor microbes on biogeochemical fluxes in the highest surface ocean productivity areas of the drill sites in the Bering Sea. Organic-fueled subseafloor respiration and its impact on biogeochemistry in such a highly productive region has not previously been quantified. To do this, sediments drilled at Bowers Ridge were used to determine subseafloor cell abundances and then subjected to intensive geochemical analysis to investigate the link between the mass and characteristics of subseafloor microbes and the extent of export productivity from the surface ocean.

Site U1341 is located on the western flank of the central part of Bowers Ridge (Fig. F20) in a depression (Fig. F27) ~40 km west of the ridge crest. Close-ups of the seismic images in the basin (Figs. F27, F28, F29, F30) indicate that strata dip gently to the west (Figs. F27, F29). Some shallow sections do not have continuous parallel strata, but most of the rest of the section appears to have continuous features in the seismic images (Figs. F29, F30). The basement age and the age of the deeper sediments are unknown, but at DSDP Site 188 on the western flank of Bowers Ridge (Scholl and Creager, 1973) sediments as old as upper Miocene were found. Specifically, Scholl and Creager (1973) found recent to upper Miocene diatomaceous silt as well as diatom ooze interbedded with silt-rich diatom ooze. They also found calcareous nannofossils in the uppermost 120 m and planktonic foraminifers in the uppermost 300–400 m. In the interval between 580 and 638 m they encountered mudstone. They reported sedimentation rates of ~100 m/m.y. A piston core study from the same general region provided a sedimentation rate of 138 m/m.y. (Takahashi, 2005), permitting the study of high-resolution paleoceanography. Thus, prior to drilling, recovery of Pleistocene to Pliocene sections was expected.

Lithostratigraphy

Three holes were drilled at Site U1341; the deepest, Hole U1341B, reached 604.5 m CSF. The sediments recovered are a mix of biogenic and siliciclastic sediments, whereas volcaniclastic material was of minor importance, perhaps reflecting the more distal location of Site U1341 to the Aleutian arc. The most abundant terrigenous grain types are silt-sized feldspar, quartz, clay, mica, and rock fragments. Some pebbles are composed of basalt, pumice, or scoria, indicating a volcanic source. Dolostones and micrometer-scale crystals of dolomite occur in high concentrations at various depths but particularly at the bottom of Hole U1341B. Authigenic carbonate patches, nodules, and layers are light olive-gray and olive-gray and often characterized by a granular texture and a stronger induration than the surrounding sediments. Because of the presence of authigenic carbonates, carbonate-stable isotope measurements in sediments from this and other sites where authigenic carbonates occur are potentially contaminated by the isotopic signature of the overgrowths. However, the presence of authigenic carbonates can be detected and screened out of the isotopic records. Sediment composed of siliciclastic sediment or mixed lithologies tends to be very dark greenish gray to dark gray, whereas diatom ooze tends to be dark gray to olive-gray to light olive. Boundaries between different sediment colors and/or lithologies are dominantly gradational and bioturbated, but occasional sharp contacts occur as well. Sharp contacts are occasionally coeval with distinct changes in magnetic susceptibility, with higher values corresponding to mixed biogenic-siliciclastic sediments and lower values corresponding to diatom ooze. Intervals with thin, distinct parallel laminations were relatively rare and confined to the upper part of the record (0–20 m CSF). At lower depths, thickly laminated to thinly bedded material with wavy boundaries occurred more frequently. Site U1341 sediments are indicative of low oxygen conditions, as implied by the decrease in the benthic foraminifer diversity index below 200 m and by the occurrence of laminated intervals throughout the cores. Also, today an OMZ impinges on parts of the western Bowers Ridge.

Two lithologic units were defined at Site U1341 by a change from alternating diatom silt, diatom clay, and diatom ooze to solely diatom ooze (Fig. F31). The boundary between the two units, dated at 1.6 Ma, is paralleled by a significant change in the intensity of the magnetic susceptibility record (208 m CSF in Hole U1341B), in the abundances of calcareous tests, and in interstitial water pH and Ca values. The two units are further divided into Subunits IA to ID and Subunits IIA to IIE, respectively.

Whole-round density (GRA) (Fig. F31) correlates well with the abundance of diatoms recorded in smear slide data, with a high percentage of diatoms correlating with low GRA values and low siliciclastic components. Diatom ooze may reflect interglacial conditions, whereas mixed diatom-siliciclastic lithologies may reflect glacial conditions. This is consistent with the pattern of biogenic opal MARs observed in piston cores from the Bering Sea. Interestingly, sediment intervals rich in nannofossils were observed at this site.

Biostratigraphy

Core catcher samples from Site U1341 are dominated by highly diverse diatoms together with radiolarian, calcareous nannofossil, foraminiferal, and organic-walled microfossils with medium to high diversity and preservation ranging from moderate to very good. Biostratigraphic datums are derived from diatom, radiolarian, dinoflagellate, ebridian, silicoflagellate, and calcareous nannofossil bioevents and show that Site U1341 contains early Pliocene to Pleistocene sediments (Table T11; Fig. F31). However, the presence of early Pliocene species indicates that some reworking has occurred within the uppermost ~20 m.

Cores from Holes U1341A and U1341B exhibit a broadly linear sedimentation rate. Siliceous microfossils show consistent occurrences throughout the section and are mainly composed of high-latitude pelagic species, indicating changes to surface water productivity. Calcareous microfossils are mostly confined to the top of sections from cores at ~250 m CCSF-A for nannofossils and 280 m CCSF-A for planktonic foraminifers. Thereafter, only sporadic occurrences of calcareous fossils and calcareous cemented agglutinated foraminifers are detected, which may be linked to changes in preservation. Benthic foraminifers are largely characteristic of those found within or near the OMZ in high-latitude regions. Dinoflagellates consistently occur throughout the section, indicating changes to the productivity and ice cover of the surface waters.

Calcareous foraminifers and nannofossils show greatest preservation in the upper part of the section from ~240 m CCSF-A. This broadly coincides with the greatest abundances of sea ice diatoms and radiolarians living in cold and oxygen-rich intermediate water masses. As the preservation of carbonate in deep-sea sediments is hindered by high productivity and associated low oxygen in the bottom waters, productivity may have been reduced by a direct seasonal sea ice coverage and an enhanced stratification. Sea ice diatoms, intermediate water–dwelling radiolarians, and calcite preservation all increase markedly again at ~110 m CCSF-A.

The first occurrence datum of Emiliania huxleyi at 0.29 Ma provided the zonal assignment of calcareous nannofossil Zone NN21 in the upper section of this site. Calcareous nannofossil Zone NN20 and the top of calcareous nannofossil Zone NN19 can only be well constrained in Hole U1341C, establishing an age older than 0.44 Ma.

The late Pleistocene fauna at Site U1341 is dominated by Neogloboquadrina pachyderma (sinistral) and reflects late Pleistocene cooling. Additional fauna are the subpolar species Globigerina bulloides, Globigerina umbilicata, and Neogloboquadrina pachyderma (dextral), which appear in low numbers. Around 2.5 Ma only the subpolar species are present. Around 60 species of benthic foraminifers were recovered in 140 samples from three holes at this site. Assemblages in the top of the section from Sample 323-U1341B-11H-CC are of relatively high diversity and abundance and show affinities to assemblages within or near the OMZ in the Sea of Okhotsk and also more common deepwater Pacific Ocean species. In the remainder of the holes foraminifers become less abundant and the agglutinated species Eggerella bradyi and Martinottiella communis become more important components of the assemblages.

Cores above 41.2 m CSF in Hole U1341A are assigned to the Neodenticula seminae Zone (0.3 Ma and younger). This datum was closely matched in Hole U1341B at 37.7 m CSF. The following age of 0.9 Ma in Hole U1341A is defined by the LCO of Actinocyclus oculatus at 79.6 m CSF. The top of the A. oculatus Zone is constrained by the LCO of Neodenticula koizumii (1.7 ± 0.1 Ma). The LCO of Neodenticula kamtschatica at 366.2 m CSF in Hole U1341B defines the base of the next biostratigraphic zone at 2.7 ± 0.1 Ma. The age of 3.9 Ma is assigned between 458.3 and 458.8 m CSF by the FO of N. koizumii.

The LO of silicoflagellate Dictyocha subarctios was found between 50.95 and 60.41 m CSF in Hole U1341A. The LO of ebridian Ammodochium rectangulare, with the age of 1.9 Ma, was assigned to 233.21–241.4 m CSF in Hole U1341A and 228.52–238.11 m CSF in Hole U1341B. The LO of ebridian Ebriopsis antiqua antiqua was found between 328.09 and 332.81 m CSF in Hole U1341A and between 325.83 and 335.28 m CSF in Hole U1341B. The LO of silicoflagellate Distephanus jimlingii lies between 332.71 and 342.26 m CSF in Hole U1341A and between 335.18 and 345.78 m CSF in Hole U1341B. Silicoflagellate assemblages were mainly composed of Distephanus speculum, Distephanus medianoctisol, and Distephanus octonarius in most core catcher samples in both holes.

The radiolarian stratigraphy at Site U1341 spans from the Botryostrobus aquilonaris Zone (upper Quaternary) to the Dictyophimus bullatus Zone (middle Pliocene) in the subarctic Pacific. At the bottom of Hole U1341B, the LO of Dictyophimus bullatus (3.8–4.0 Ma) is identified by the occurrence of several specimens of the species. Changes in abundances of Cycladophora davisiana intermediate water–dwelling species showed antiphase patterns with abundances of calcareous microfossils (calcareous nannoplanktons and planktonic foraminifers) in each hole of Site U1341. This suggests the relationship between intermediate water formation in the subarctic Pacific and carbonate preservation. Cycladophora sakaii is thought to be an ancestor species of Cycladophora davisiana. Occurrences of Cycladophora sakaii were very low at Site U1340 (water depth = ~1300 m). On the other hand, Cycladophora sakaii was consistently found below the 1000 m interval at Site U1341 (water depth = ~2200 m), implying Cycladophora sakaii dwelled mainly in deep water below 1000 m water depth.

The polar and subpolar dinoflagellate taxa Islandinium minutum, Operculodinium centrocarpum (arctic morphotype), and Impagidinium pallidum—which are known to be abundant in regions where sea ice cover occurs up to 12 months per year and winter sea-surface temperature is <0°C—occur only in the upper part of the sequence starting at ~371 m CSF. Below 300 m CSF the diversity decreases and the assemblages are dominated by the Protoperidinial Brigantedinium spp. and Trinovantedinium variabile or by the extinct species Filisphaera filifera. T. variabile appears below 300 m CCSF-A and dominates the assemblage of Sample 323-U1341A-60X-CC.

Paleomagnetism

Archive halves of all APC cores recovered at Site U1341 were measured on the three-axis cryogenic magnetometer at 2.5 cm intervals. NRM was measured before (NRM step) and/or after stepwise alternating-field (AF) demagnetization (demagnetization step) in peak fields of up to 20 mT. Cores 323-U1341A-1H through 12H and Sections 323-U1341C-5H-3 through 11H-5 were measured at NRM step and 20 mT demagnetization step; other cores from Site U1341 were measured only at 20 mT demagnetization step to maintain core flow. The obtained inclinations average nearly 70° over the entire depth range of the cores, whereas the site axial dipole inclination is ~72°. The inclinations show several distinct intervals of reversed inclinations that we interpret to be polarity epochs (Table T11; Fig. F31). The declinations, after correction with the FlexIt tool to orient the declination data with North, suggest that there are multiple polarity intervals in the uppermost 17 cores in Holes U1341A and U1341B. The FlexIt tool appears to show the declination change of ~180° at the Brunhes/Matuyama boundary in Hole U1341A but it does not seem to identify older polarity changes (Jaramillo onset or termination) or the Brunhes/Matuyama boundary in Hole U1341B. Within the Matuyama reversed polarity interval, approximately a dozen previously identified excursions are discernible. Relative paleointensity variations (Chi or INT/Ms) show good correlation with benthic foraminifer diversity related to dissolved oxygen contents of the bottom waters. This indicates that extensive reducing conditions near the sediment/water interface, especially in the lower half of the hole, were responsible for the degradation of paleointensity.

Geochemistry

Undetectable methane, deep SO₄^2– penetration, and low values of DIC, alkalinity, NH₄⁺, and PO₄^3– suggest low present-day microbial activity compared to Site U1339 despite similar TOC contents (Fig. F32). Preliminary model estimates based on the measured DIC profiles suggest that microbial respiration at Site U1341 in the uppermost 30 m is ~20% of the activity estimated for Site U1339. This difference may partly be attributed to differences in sedimentation rates. The major metabolic pathway in the sections studied is organoclastic sulfate reduction, and according to the DIC, alkalinity, and sulfate profiles this process is mainly confined to the upper 30 m.

The most striking feature of Site U1341 is the unusual shape of the DIC, alkalinity, and phosphate profiles as well the pattern of the SO₄^2– profile (Fig. F32). DIC, phosphate, and alkalinity profiles indicate either nonsteady state caused by recent changes in microbial activity or unusually high net consumption of DIC and PO₄^3– in strata below 50 m. The SO₄^2– profile suggests a curiously high net consumption in the 300–400 m depth interval despite the lack of CH₄ and the presence of a presumable, rather refractory, organic carbon pool. It is possible that the interstitial water chemistry still to a large extent shows the overprint by past events such as sulfate removal by extreme high rates of organic matter mineralization during high productivity periods. The present-state interstitial water chemistry thus reflects the transition toward a new steady state.

Microbiology

Seventy-three microbiological samples were collected adjacent to interstitial water whole-rounds for postcruise analyses for abundance of prokaryotes.

Physical properties

The uppermost ~210 m CSF of the sediment section at Site U1341 exhibits rapid single-point excursions to readings >200–400 SI units of whole-core magnetic susceptibility measurements. Below this depth magnetic susceptibility readings are subdued and rapid deflections to values >100 SI units are uncommon except at ~575 m CSF, where a broad band of high susceptibility occurs between 565 and 575 m CSF. The upper section of rapidly varying and high values is coincident with lithologic Unit I. Although ash layers occur in this unit, they are equally abundant in the underlying Unit II, which displays only background variations in readings. The contrast in the profiles of magnetic susceptibility readings between Units I and II is thought to reflect the occurrence in the diatom silt of Unit I of presently unidentified alteration products having magnetic susceptibility properties.

The GRA sensor records a trend of slightly decreasing average values of wet bulk density from a near-surface reading of ~1.35 g/cm³ to ~1.32 g/cm³ at the base of Hole U1341B at ~605 m CSF (Fig. F31). At ~220 m CSF, which is below the transition from Unit I to Unit II, a discernible but small shift to lower density near ~125 g/cm³ is evident. The entire vertical profile of bulk density undulates broadly from average values of ~1.30 g/cm³ to ~1.45 g/cm³. The wavelength of fluctuations narrows downhole. Measurement of P-wave velocity by the slow track WRMSL documents a downhole trend of increasing velocity. The gradient of average P-wave velocity values ranges from ~1.51 km/s for near-surface sediment to ~1.56 km/s at the base of the hole at 605 m CSF. A slight shift to lower readings (0.02 km/s) appears to occur across the transition from lithologic Unit I to Unit II. The low overall gradient in downhole velocity, which is estimated at only 0.08 km/s/km, demonstrates the ability of diatomaceous sediment to resist compaction.

Downhole NGR readings show spiking to high values >40 counts/s above an undulatory and generally decreasing trend of values from near-surface averages of ~15 counts/s to less than ~5 counts/s at the base of Hole U1341B (Fig. F31). A shift to slightly lower values is just perceptible near the boundary between Units I and II. The implication of the overall decreasing trend and broad superimposed oscillations is that the clay mineral content decreases irregularly downhole, an interpretation that is consistent with the decreasing terrigenous content of the drilled section from the diatom silt of Unit I to the dominantly siliceous ooze of Unit II.

The MAD downhole trend clearly reveals the contrasting density characteristics of the diatom silt of lithologic Unit I and the siliceous microfossil ooze of Unit II. The sediment in Unit I shows a higher fluctuation in values ranging from 1.62 to 1.2 g/cm³ superimposed on a perceptible but slight increase with depth to the unit's boundary with underlying Unit II. The lower overall bulk density of Unit II appears to reflect a higher concentration of low-density siliceous microfossils than that found in the diatom silt of overlying Unit I. Just beneath the surface, porosity values average ~80% and the corresponding water content is ~60%. At the bottom of Hole U1341B water content decreases only slightly to ~58% and, correspondingly, porosity decreases to ~75%. Similar to the depth profiles of most other physical properties, the downhole distribution of sediment porosity exhibits undulations or excursions to higher and lower values. The downhole variation in grain density is prominently offset, from an average density of ~2.39 g/cm³ to 2.23 g/cm³ at the boundary between Units I and II. The average density also decreases with depth from a near-surface value of ~2.50 g/cm³ to as low as 2.10 g/cm³ at the base of Hole U1341B. These trends are interpreted as tracking the downhole increase in relative abundance of low-density diatoms. The depth distribution of thermal conductivity decreases overall from a near-surface value of 0.85 W/(m^.K) to 0.80 W/(m^.K) at the base of Hole U1341B. This profile thus parallels the downhole decreasing values of most other physical properties measured on cores recovered in Hole U1341B.

Stratigraphic correlation

The composite depth scale and splice at Site U1341 is constructed from 0.0 to 374.40 m CCSF-A (Tables T12, T13). The splice consists of one continuous splice from the mudline to 141.30 m CCSF-A and two appended "floating" splices, the first from 141.3 to 326.43 m CCSF-A and the second from 326.44 to 374.40 m CCSF-A. The continuous splice ranges from the top of Core 323-U1341A-1H to Section 323-U1341B-14H-7, 88 cm. The first floating splice ranges from Section 323-U1341B-15H-1, 0 cm, to Section 323-U1341A-34H-7, 64 cm. The second floating splice ranges from Section 323-U1341A-35H-1, 0 cm, to the base of Section 323-U1341B-39H-7. These appended intervals are supported by wireline logging data compared to core logging of natural gamma radiation. Additional cores below the splice are included in the composite depth framework by appending them with a constant affine value specific to each hole.

The cumulative offset between CSF and CCSF-D depth scales is approximately linear. The affine growth factor (a measure of the fractional stretching of the composite section relative to the drilled interval) at Site U1341 is 1.06 between 0 and 374.4 m CSF. Cores deeper than the spliced interval, that is, Cores 323-U1341A-41H and 323-U1341B-40H through 71X, are not tied to the splice but are appended with a constant affine value of 19.95. MARs in this interval should not be divided by the affine growth factor because their depths are a linear transformation of drilling depths.

Downhole measurements

Two downhole logging tool strings were deployed in Hole U1341B to the total depth of 600 m DSF (2750 m drilling depth below rig floor [DRF]): the triple combo and the FMS-sonic combination. Overall, the caliper of the density sonde on the triple combo tool string indicated an enlarged and irregular borehole with many intervals with hole diameter >20 inches. During the runs, the tool encountered significant drag in many places above 260 m wireline log depth below seafloor (WSF), producing a stick-slip motion that was detrimental to the quality of the data. As a result, the logging speed was increased from 900 ft/h to 1200 ft/h, and the tools were reconfigured to prevent any impact on the vertical resolution of the data.

The readings of the two orthogonal FMS calipers suggest that the borehole section was far from circular, probably elliptical. One caliper read <10 inches over most of the lower half of the interval logged, while the other one kept close to ~14 inches, near the limit of its range. The fact that the curves display variability over most of the hole suggests that both sets of arms were making some kind of contact with the formation, possibly with one pad only in some places. The larger Hostile Environment Litho-Density Sonde (HLDS) caliper readings show that this single-arm caliper was likely following the longest axis of the hole and that the stronger and skinnier arm was actually pushing inside the formation.

The large hole size had an effect mostly on measurements that require good contact with the formation, namely density and porosity. The very high neutron porosity values above ~275 m WMSF indicate that porosity readings are erroneous above this depth. Similarly, the anomalously low density values between 180 and 210 m WMSF also indicate poor tool contact and are also erroneous. Even if the FMS arms seem to have been in contact with the formation over most of the interval logged, this contact was likely only partial in places, resulting in blurry or featureless images in many intervals. It is still possible to identify many fine layers, mostly in the deeper part of the hole.

Logging Unit 1 (80–220 m WMSF) is characterized mainly by decreasing trends with depth in gamma radiation and resistivity, accompanied with several high peaks in these measurements (Fig. F33). It coincides mostly with lithologic Unit I, which consists of diatom ooze and diatom silt. Most of the peaks in gamma radiation are related to high uranium content. The coincidence of these higher uranium values with higher resistivity, and to some extent with higher density, is an indication that they are due to authigenic carbonate, which was observed at many of these depths. Logging Unit 2 (220–350 m WMSF) is defined by increasing trends with depth in gamma radiation and density measurements, whereas resistivity mostly decreases. Several peaks in gamma radiation can be observed in this unit as well, again generally due to higher uranium content and often associated with authigenic carbonate observed in the core. The top of logging Unit 3 (350–425 m WMSF) is defined by a sharp drop in density at ~350 m WMSF and by similar changes in gamma radiation and density. Because velocity does not display any significant change at this depth, the change in density is likely responsible for the strong reflector that can be observed in seismic Line Stk5-1 at 3340 ms two-way traveltime. The top of logging Unit 4 (425–600 m WMSF) is defined by a drop in resistivity, which decreases with depth over the entire unit. It coincides also with an inflection in the overall increase with depth of shear velocity and, to a lesser extent, of compressional velocity.

Formation temperature measurements were successfully made with the APCT-3 tool at three depths in Hole U1341A. The measured temperatures ranged from 4.68°C at 41.0 m DSF to 11.12°C at 136.0 m DSF and closely fit a linear geothermal gradient of 67.8°C/km. The temperature at the seafloor was 1.95°C based on the average of the measurements at the mudline during all the APCT-3 deployments. A simple estimate of the heat flow can be obtained from the product of the geothermal gradient by the average thermal conductivity (0.825 W/[m^.K]), which gives a value of 55.9 mW/m², within the range of previous measurement in the area.

Age model and sedimentation rates

Only two relatively deep holes in Expedition 323 reached the bulk of the Pliocene age: Holes U1340A (cored to 604.6 m CSF) and U1341B (cored to 600.0 m CSF). Stratigraphic efforts using diatoms, silicoflagellates, ebridians, radiolarians, and paleomagnetism were employed to determine a correct age model; however, this method proved difficult. Among the many parameters examined on board the R/V JOIDES Resolution, the percentage counts of three species of abundant diatoms—Neodenticula kamtschatica, Neodenticula koizumii, and Neodenticula seminae—were very useful in constructing tie points between the two holes in question.

The total percentage of each of these species exceeds 50% during the Pliocene (Fig. F34). Because of their high abundances, higher reliability on the tie points was given to their rapid increase (RI) or rapid decrease (RD) than was given to the LO or FO of taxa with only a single or few specimen counts (Fig. F35). These taxa were interpreted to have occurred to a similar extent at about the same times because Holes U1340A and U1341B are not too far apart in the Bowers Ridge region. Detailed compilation of all the pertinent biostratigraphic and paleomagnetic datums clearly suggests that the tie points are more or less in agreement, with the exception of Ebriopsis antiqua antiqua, whose abundance is very rare (Fig. F35). Especially note that the agreed paleomagnetic datums are in line with the tie points illustrated. Six reliable tie points were assigned from the Pliocene and Pleistocene section: the FO of N. koizumii, the top of the Gauss, the RI of N. seminae, the bottom of the Jaramillo, the top of the Jaramillo, and the Brunhes/Matuyama boundary (Fig. F35).

Sedimentation rates in the upper Pleistocene section appear to be 10–11 cm/k.y. (Fig. F36; Table T14). During the Jaramillo the sedimentation rate is very high, 46 cm/k.y., but more work is needed to refine the depth of the polarity reversals at the top and bottom of the Jaramillo. Below ~150 m CCSF-A sedimentation rates are at 15 cm/k.y. until the top of the Gauss at 368 m CCSF-A. Below that depth the sedimentation rate is 9 cm/k.y. These age model and sedimentation rate estimates may be altered with postcruise research.

Site U1342

Background and objectives

The primary objective of drilling at Site U1342 (prospectus Site BOW-15A) was to study high-resolution Pliocene–Pleistocene paleoceanography at a relatively shallow water depth on Bowers Ridge, where relatively low sedimentation rates were observed in an earlier site survey piston core study (Takahashi, 2005). Bowers Ridge is well situated for study of the past extent of water mass exchange with the Pacific Ocean through adjacent Aleutian passes such as Amukta, Amchitka, and Buldir passes (Figs. F37, F38, F39, F40). As with the other Bowers Ridge sites, the record of changes in the flow of the warm Alaskan Stream water mass into the Bering Sea and its impact on the distribution of past sea ice coverage is of particular interest.

A previous site survey piston core study found more open water conditions during the LGM at Site BOW-8A, which is practically the same location as Site U1342, than at Site BOW-12A near the ridge crest at the location of Site U1340. Although productivity in the Bering Sea in general is very high with respect to other parts of the global oceans, the expected productivity at this site, along with the other Bowers Ridge sites, is lower than at Site U1339 on Umnak Plateau, which experiences substantially greater influence from the adjacent Bering Sea shelf. Site U1342, with its relatively shallow water depth of 818 m, serves as the shallow end site in comparison to the other Bowers Ridge sites (Site U1340, water depth = 1295 m; Site U1341, water depth = 2140 m); therefore, Site U1342 provides an important constraint on the intensity and depth of the water column OMZ. A previous site survey piston core study reported sedimentation rates of ~32 m/m.y., and Pliocene-age sediments at the bottom of the sedimentary section are expected.

This drill site at Bowers Ridge can also be used to study the impact of subseafloor microbes on biogeochemical fluxes. Organic-fueled subseafloor respiration and its impact on biogeochemistry in such a highly productive region has not previously been quantified. To do this, sediments drilled at Bowers Ridge will be used to determine subseafloor cell abundances and to investigate the link between the mass and characteristics of subseafloor microbes and the extent of export productivity from the surface ocean (Takahashi et al., 2000, 2002). Compared to the other Expedition 323 drill sites where detailed microbiological studies have occurred, Site U1342 is expected to have lower (but still high) surface ocean productivity. As such, because of its more open ocean location farthest away from the high-productivity zone of the shelf, Site U1342 serves as the low-productivity end-member of the expedition's microbiological study.

Lithostratigraphy

The sediments recovered at Site U1342 are a mix of biogenic, volcaniclastic, and siliciclastic sediments. In general, the color of the sediments reflects their lithologic characteristics: sediment composed of siliciclastic sediment or mixed lithologies tends to be very dark greenish gray to dark gray, whereas biogenic sediment is olive-gray to olive. Volcaniclastic ash layers are dark gray to black or shades of light gray to white. The sedimentary sequence can be divided into three lithologic units: Unit I, consisting of silt and laminated ooze; Unit II, consisting of sponge spicule–bearing, diatom-bearing sand; and Unit III, consisting of volcanic rock and volcaniclastic sedimentary rock.

The occurrence of well-preserved laminations in Unit I indicates the absence of bioturbating fauna and thus suggests low oxygen conditions in the bottom waters and the sediment pore water. Burrows or mottles at the gradational tops of laminated sediment intervals indicate an increase in oxygenation of bottom waters after the deposition of the laminated sediments. In contrast, the sharp bottom boundaries suggest either a sudden increase in sedimentation rate at the onset of laminated intervals or a hiatus between the laminated sediments and underlying siliciclastic sediments. The winnowing of sediment by bottom currents may have caused the apparent low average sedimentation rate (3 cm/k.y.), and the sedimentary record might inherit strongly variable sedimentation rates or even phases of nondeposition or erosion. The total number of well-correlated laminated intervals is roughly equal to the total number of interglacial cycles that occurred during the last 1.1 m.y. The occurrence of laminated ooze reflects the interglacial times, which is consistent with the preliminary paleomagnetic age model.

Moderately to well-rounded and mixed volcaniclastic and metamorphic mineral or rock fragments in the sand encountered in Unit II indicate a terrestrial source. Although the base of Unit I is dated to only 1.2 Ma, several Miocene diatom species were observed in Unit II. This suggests the following possible processes: extremely low sedimentation, a hiatus, or reworking by sediment gravity flows.

The basement volcanic rock (basalt) was most likely deposited as lava, as indicated by the flow alignment of the plagioclase laths and the vesicular texture. The volcaniclastic rocks beneath have a mafic volcanic source and contain both lava fragments and tephra fragments (scoria).

Biostratigraphy

Eighteen biostratigraphic datum events indicating an upper to middle Quaternary sequence were identified based on radiolarians, diatoms, silicoflagellates, and calcareous nannofossils. The sedimentation rate at Site U1342 is relatively low (3 cm/k.y.), as implied by the estimated age of 1.2 Ma for the bottom of Unit I based on biostratigraphic markers. Calcareous microfossils reflect a high-latitude environment, indicating variations of sea-surface temperatures. The benthic foraminifers generally reflect environments within or near the OMZ in high-latitude regions. The preserved siliceous microfossils are mainly composed of high-latitude pelagic species, which indicates changes to surface water productivity. Dinoflagellates consistently occur throughout the record with poor preservation in the lower part. The dinoflagellate assemblages show changes between low and high primary productivity together with general low sea-surface temperatures and low seasonal sea ice coverage.

Both dinocyst assemblages and sea ice diatoms suggest low seasonal sea ice coverage. In general, dinocysts suggest high primary productivity, low sea-surface temperature, and seasonal sea ice coverage. However, the co-dominance of the autotrophic Operculodinium centrocarpum and the heterotrophic Brigantedinium spp. at 20.96 m CSF in Hole U1342A could be associated with relatively low primary productivity and/or an incursion of oceanic/oligotrophic waters. Planktonic foraminifers are present throughout the section from ~36 m CSF, showing polar–subpolar conditions. Benthic foraminifers generally exhibit high abundances, and variation in species dominance is probably related to changes in oxygen levels and/or organic fluxes.

The uppermost one or two cores, depending on the hole, are assigned to calcareous nannoplankton Zone NN21 (0.29 Ma to the present) (Martini, 1971; Lourens et al., 2004), characterized by Emiliania huxleyi. Sample 323-U1342A-3H-CC contains Pseudoemiliana lacunosa and is therefore assigned to calcareous nannofossil Zone NN19 (>0.44 Ma). Planktonic foraminifers are present only in the uppermost four cores of each hole except Hole U1342D, where they also occur in Core 323-U1342D-5H. The fauna is dominated by Neogloboquadrina pachyderma (sinistral) in all samples. More than 20 species of benthic foraminifers were recovered from this site. Occasionally dominant species are Cassidulina sp. and Uvigerina peregrina. Persistently occurring species include Brizalina pygmaea, Brizalina spathulata, Bulimina aff. exilis, Globobulimina pacifica, and Valvulineria spp. This assemblage shows similarities to those in the uppermost ~100 m CSF of Sites U1339, U1340, and U1341 and also to those within or near the OMZ in the Sea of Okhotsk (Bubenshchikova et al., 2008). This assemblage does not appear to reflect shallow-water (shelfal) deposition.

The LO of Proboscia curvirostris (0.3 Ma) is observed at the base of Cores 323-U1342A-3H and 323-U1342C-3H, which is consistent with Sites U1340 and U1341, also located on Bowers Ridge. Because of poor diatom preservation below Core 5H in each hole, the biostratigraphic zonation was constrained by one species, P. curvirostris, which places the N. seminae Zone 12 at 0–11.41 m CSF in Hole U1342A and 0–17.04 m CSF in Hole U1342C, respectively. Based on the observed silicoflagellate species the age of the uppermost four cores in Holes U1342A and U1342D is probably Pleistocene. Cores 323-U1342A-5H and below may possibly be older than 2.5 Ma given the occurrence of Ebriopsis antiqua antiqua, assuming it is not reworked.

Six radiolarian datums derived in the subarctic Pacific were identified at this site. These datums showed relatively low sedimentation rates (~5 cm/k.y.) in the uppermost 20 m of each hole compared to the other Bowers Ridge sites (Sites U1340 and U1341). Although radiolarian datums are scarce in the lower intervals, the LO of E. matuyamai (0.9–1.5 Ma) was found at the base of Core 323-U1342B-4H. This datum provides a constraint for the age estimation for the lower intervals and an average sedimentation rate in Hole U1342B of 2–4 cm/k.y.

Poor preservation of organic-walled dinoflagellate cysts in the lower part of the sequence and particularly in the sandy layer suggests significant oxygen concentrations in the bottom water. The dinoflagellate assemblage composition is modern, suggesting Pleistocene age for all analyzed samples.

Paleomagnetism

The average inclination value is nearly 70° over all normal polarity intervals, which is close to the site axial dipole inclination of ~72° (Table T15). This indicates that we can effectively remove overprint magnetization caused by drill pipes and/or core barrels from the NRM records. We defined a polarity zonation from the inclination record and correlated the zonation to the polarity timescale based on micropaleontology datums (Fig. F41). The Brunhes/Matuyama boundary and the base of the Jaramillo Subchron are clearly identified through Holes U1342A, U1342C, and U1342D. The top of the Jaramillo Subchron and both the top and base of the Cobb Mountain Subchron are identified in Holes U1342C and U1342D but not in Hole U1342A. We also tentatively note the existence of three excursions visible in all three holes: the Kamikatsura, Santa Rosa, and Punaruu excursions.

It is possible to see a correlatable pattern of relative paleointensity at the present site and at Site U1341, as indicated by the relative numbering scheme within the Brunhes Chron (the last 780,000 y). Note, however, that the relative paleointensity estimates are both significantly influenced by lithologic variability and should not be considered high-resolution estimates of true geomagnetic field intensity variations. We estimate that fine-grained hemipelagic marine sediments were deposited over the last 1 m.y. with essentially a constant sediment accumulation rate. Note the discrete intervals where both magnetic parameters undergo more than order-of-magnitude decreases. These indicate the intervals of significant magnetic mineral dissolution. They appear to be closely related to the laminated sediment intervals, and both are probably related to enhanced rates of reduction diagenesis during those selected time intervals. Our chronostratigraphic estimates suggest that these dissolved intervals (and the associated laminations) occur in the interglacial sediments. Note that a few narrow intervals have significantly stronger remanence because of authigenic greigite. They are interpreted to always occur within the glacial-stage sediments with lower porosity.

Stratigraphic correlation

The composite depth scale and splice at Site U1342 is complete and continuous from 0.0 to 46.71 m CCSF-A (Tables T16, T17). Sediment cores below the splice are included in the composite depth framework by appending them with a constant affine value of 2.783 m. Color reflectance data were examined: the b* parameter is considered useful for correlation, but L* and a* do not appear to correlate between holes. Within the splice the composite CCSF-A depth scale is defined as the CCSF-D depth scale. Note that CCSF-D rigorously applies only to the spliced interval. Intervals outside the splice, although available with CCSF-A composite depth assignments, should not be expected to correlate precisely with fine-scale details within the splice or with other holes because of normal variation in the relative spacing of features in different holes. Such apparent depth differences may reflect coring artifacts or fine-scale variations in sediment accumulation and preservation at and below the seafloor. The cumulative offset between CSF and CCSF-D depth scales is approximately linear. The affine growth factor at Site U1342 is 1.06 between 0 and 44.0 m CSF. Calculation of MARs based on the CCSF-A or CCSF-D scales should account for this affine growth factor by dividing apparent depth intervals by the appropriate growth factor. After dividing by the growth factor this scaled depth scale should be referred to as CCSF-B.

Geochemistry

Interstitial water sulfate, DIC, PO₄^3–, and NH₄⁺ profiles indicate that sediments at Site U1342 are characterized by low rates of anaerobic carbon mineralization predominantly driven by organoclastic sulfate reduction (Fig. F42). Additionally, small increases in Mn concentration might indicate microbial Mn reduction as a further mineralization pathway. It is more likely, however, that dissolved Mn is released during reaction of hydrogen sulfide with Fe/Mn (oxyhydr)oxides. Comparison with Site U1341, also located on Bowers Ridge, reveals that the present site has one order of magnitude lower NH₄⁺ concentrations and ~50% lower phosphate concentrations. This highlights the extremely low mineralization rates at Site U1342 despite its shallower depth and the similar TOC at both sites. The low extent of anaerobic carbon mineralization at the present site can be attributed to the extremely low sedimentation rates. Very low sedimentation rates probably prolong the time that organic matter is degraded via oxic respiration and nitrate reduction in the oxic/suboxic sediment zone. This leaves rather refractory organic material, which is only inefficiently degraded during anaerobic carbon mineralization (Hulthe et al., 1998).

Solid-phase data, however, suggest that the present-day geochemistry might only give a limited picture of past conditions. High TOC concentrations that correlate strongly with high total sulfur concentrations in several laminated intervals discovered throughout the sediment column indicate events of high organic carbon input that probably resulted in high sulfate reduction rates during these periods and hence elevated hydrogen sulfide production, which is reflected in high pyrite (total sulfur) concentrations. However, the contents of CaCO₃ are not always high in the laminated layers.

Another interesting feature of Site U1342 is the inverse relationship of the (almost linear) calcium and magnesium profiles, which indicates the influence of signals linked to the alteration of the underlying basalt on the interstitial water calcium and magnesium concentrations. Most likely, both profiles are the result of diffusion between the seawater and the relatively shallow basaltic basement. Low-temperature interactions of seawater with the basaltic basement (e.g., the dissolution of basaltic glass, calcic plagioclase, and olivine) result in the liberation of calcium, whereas the precipitation of smectite leads to the consumption of magnesium (see, for example, Gieskes, 1981; Staudigel and Hart, 1983; Thompson, 1983; Lyons et al., 2000).

Microbiology

Samples for abundance of prokaryotes were collected adjacent to interstitial water whole-rounds in sections drilled using the APC. High-resolution sampling took place in the microbiology-dedicated cores and in additional samples taken once per core to APC refusal in Hole U1342A. These additional samples were taken to evaluate cell abundance and community structure in the deepest portion of Site U1342. PFT analyses performed on these cores showed no contamination from the drill fluid. Samples from all cores were fixed. These analyses assist the understanding of global distribution and abundance of subseafloor life in a highly productive oceanic regime. Special focus will be directed toward the function of Archaea in the sulfate reduction zone, the SMTZ, and the methanogenesis zone.

Physical properties

The downhole-decreasing trend of magnetic susceptibility ranges from a near-surface value of ~100 SI units to about half this value at ~35 m CSF, which is the base of lithologic Unit I. Below this depth magnetic susceptibility readings in Unit II increase in steps to as high as 400 SI units at 40 m CSF. The higher values in Unit II likely track the downward-increasing abundance of sandy material derived from the underlying volcanic basement, which was wave-based leveled sometime in the early late Miocene. Magnetic susceptibility values are much higher in Hole U1342D at ~73–74 m CSF (29–30 m below the sediment/basement contact), averaging ~1500 SI units, and decrease deeper in the section to ~750 SI units at ~116–117 m CSF (~72–73 m into basement).

A prominent downhole profile of increasing average wet bulk density is noted. The higher gradient of the lower trend is within the sandy sediment of Unit II, which is presumably granular material derived during the wave-beveling final stage of destruction of a stratavolcano that formerly rose above Ulm Plateau. P-wave velocity increases downhole from near-surface readings of just over 1.45 km/s to ~1.55 km/s in Unit II near the contact with basement rock at ~45 m CSF (Fig. F39). In the sandy sediment of Unit II in Hole U1342D, P-wave velocity increased to ~1.65 km/s and somewhat higher.

NGR readings increase irregularly downhole from a near-surface average of ~18 counts/s to ~24 counts/s at the base of Unit I. The gradient of increasing counts/s steepens within the sandy beds of Unit II to reach ~33 counts/s just above basement contact at ~44 m CSF. Presumably, the elevated NGR readings record a downhole increase in clay minerals and, at least within Unit II, clay minerals derived from the underlying bedrock of arc lava and volcaniclastic deposits. Rock clasts recovered in core sections exhibit evidence of oxidation. Clay minerals that account for the higher readings in the basal sediment of Unit II presumably reflect subaerial weathering of the stratavolcano that formerly towered above Ulm Plateau prior to its early late Miocene destruction.

In general, MAD values of discrete samples track those of gamma ray attenuation but with a much greater spacing of measurements. The contrast between Unit I and denser Unit II sediments is nonetheless well shown by MAD data. The near-surface porosity is ~80% but decreases to ~55% in the sandy sediment of Unit II and just above basement at 44 m CSF. The downhole trend of overall decreasing porosity and water content presumably reflects compaction of the section, with notable excursions to higher values in the basal 10–12 m of Unit I that may identify a higher relative content of siliceous microfossils. The relatively elevated grain density in Unit I in comparison to the upper beds of Site U1341 on the deeper flank of Bowers Ridge can be attributed to the availability of nearby bedrock sources for Site U1342 deposits.

Downhole measurements

The temperatures measured with the APCT-3 tool in Hole U1342C were 4.46°C at 26.2 m DSF and 5.32°C at 35.0 m DSF. From these two measurements the geothermal gradient should be 97.7°C/km. The estimated heat flow is 96.9 m W/m², which is significantly higher than existing measurements in the area. Alternately, considering the variations in thermal conductivity with depth, a more accurate measure of the heat flow in a conductive regime can be given by a Bullard plot, which gives a heat flow value of 80.9 m W/m², which is closer to other measurements in the Bowers Ridge area.

Sedimentation rates

The apparent sedimentation rate in the uppermost section between the mudline and 3.6 m CCSF-A (marked by the LO of Amphimelissa setosa) was 5 cm/k.y. with relatively high water contents and hence may not be significantly different from those below it (Fig. F43; Table T18). Sedimentation rates stayed at fairly constant values of 2–3 cm/k.y. between 3.6 m CCSF-A and the magnetic Kamikatsura Excursion at ~29 m CCSF-A. This is followed by an increasing trend up to 9 cm/k.y. down to the bottom of magnetic Cobb Mountain at ~43 m CCSF-A. The presence of Miocene diatoms in lithologic Unit II warrants further refinement of the age model.

Site U1343

Background and objectives

The primary objective of drilling at Site U1343 (prospectus Site GAT-4C) was to study high-resolution Pliocene–Pleistocene paleoceanography at a location proximal to the gateway to the Arctic Ocean. Additionally, this site is closer to the current seasonal sea ice limit and because of its 2 km water depth it provides information regarding the history of mid-depth water in the Aleutian Basin. This site is located at a topographic high clearly separated from the Bering shelf (Figs. F44, F45, F46, F47, F48). Hence, it was anticipated to have received fewer reworked terrigenous sediments from the shelf during the interglacials or the subaerially exposed land during the glacials than a location directly downslope of the Bering shelf.

Site U1343 is in the area of high biological productivity called the Green Belt. The Green Belt is formed by the Bering Slope Current (BSC), which originates from the Alaskan Stream water that flows into the Bering Sea through the western Aleutian Islands. The water that enters the Bering Sea moves eastward along the Aleutian Islands and consequently encounters the Bering shelf. The base of the BSC is at ~300 m water depth and its flow is forced to turn to the northwest once it meets the slope and shelf; eddies and instabilities in its flow cause upwelling along the shelf break. Moreover, tidal mixing causes further vertical mixing of the water masses along the BSC, enhancing biological productivity within the Green Belt, which is adjacent to the northwest-trending shelf break where high primary productivity in the surface waters and high organic carbon accumulation at the seafloor take place. Because of the expected high organic carbon supply to the seafloor, especially during the interglacial sea level highstands, it is possible the OMZ previously expanded down to the depth of this site. In order to compare the vertical extent of water mass conditions across the basin and relate the OMZ to paleoproductivity, the records from the shallower drill sites on Bowers Ridge (Site U1340, water depth = 1295 m; Site U1342, water depth = 818 m) as well as the other slope sites will be used. This site is also located close to the maximum extent of the present-day seasonal sea ice cover. Thus, this site was expected to have been covered by seasonal or perennial sea ice during the glacial sea level lowstands.

This drill site in the gateway region to the Arctic Ocean can also be used to study the impact of subseafloor microbes on biogeochemical fluxes in the highest surface ocean productivity areas of the Bering Sea drill sites. Organic-fueled subseafloor respiration and its impact on biogeochemistry in such a highly productive region has not previously been quantified. To do so, the drilled sediments in the gateway region were used to determine subseafloor cell abundance and to investigate the link between the mass and characteristics of subseafloor microbes and the extent of export productivity from the surface ocean.

Sedimentation rates at this site were not previously known because of a lack of piston cores. However, rates of ~180 m/m.y. were observed at Site U1344 (proposed Site GAT-3A) in an earlier site survey piston core study (Takahashi, 2005). Thus, prior to drilling, recovery of Pleistocene to Pliocene sections was expected at this site.

Lithostratigraphy

Sediments at Site U1343 are primarily composed of silt with varying amounts of clay and diatoms and minor amounts of sand, ash, foraminifers, calcareous nannofossils, and sponge spicules. The sediment is predominantly dark/very dark greenish gray to dark/very dark gray. One lithologic unit spanning the early Pleistocene to the Holocene is defined at this site. Six laminated intervals are observed and can be correlated between the holes based on lithologic, reflectance, and magnetic susceptibility data. Unit I encompasses a time period comparable to Unit I defined at Bowers Ridge Sites U1340 and U1341. However, Site U1343 is distinct in having a higher proportion of siliciclastic grains and a higher occurrence of sand-sized grains. This is probably related to the location of this site on the continental slope and the relative proximity to the source of terrigenous sediments. During glacial sea level lowstands, and in particular during early stages of deglaciations, significant amounts of coarser grained detrital material might have been mobilized from the exposed Bering Sea shelf and redeposited farther down the continental slope. Unlike Site U1339, drilled on the slope at Umnak Plateau, volcaniclastic material is only a minor component of sediment at Site U1343 because it is more distant from the Aleutian arc. There is a shallow SMTZ and abundant methane in the sediment column at Site U1343, like at the other slope sites. Benthic and planktonic foraminifers encrusted with yellow minerals are common, and they often coincide with samples in which authigenic carbonates are found. Association of authigenic carbonates and apparent overgrowths implies that authigenic carbonates nucleated on the foraminifer tests. In this case the stable isotope records from this site are potentially contaminated by the isotopic signature of the overgrowths; however, pristine foraminifer shells were also observed, and the generation of uncontaminated isotope stratigraphies should be possible after careful screening for authigenic carbonates.

Biostratigraphy

The composite age model derived from all five holes shows that the sediment record recovered at Site U1343 spans the last 2.0–2.4 m.y., yielding a broadly linear trend in sedimentation rates, with values around 26 cm/k.y. in the uppermost 400 m CCSF-A that increase to ~56 cm/k.y. in the bottom 350 m CCSF-A. Siliceous, calcareous, and organic fossils show the greatest abundances in the uppermost 300–250 m CCSF-A relative to the section below. In the upper section all microfossil groups show distinct large and frequent oscillations, indicating fluctuations in the sea ice cover and productivity in the upper water column and in the deepwater environment. All major microfossil groups show distinct changes, occurring gradually or abruptly between 200 and 300 m CCSF-A (0.8 to 1.1 Ma), in their general trend of abundances and/or assemblage composition. These changes are thus linked to the MPT. Benthic foraminifers indicate high-frequency changes in bottom water oxygen content over the entire sequence, probably related primarily to surface water productivity, but possibly also to bottom water ventilation changes and methane seeps. The cooling trend observed after the MPT is reflected in the increase of abundances of sea ice dinoflagellate and diatom taxa and in the decrease of the predominance of pelagic/open water conditions evidenced in the decline of subpolar planktonic foraminifers and the diatom species Neodenticula seminae.

The occurrence of calcareous nannofossil Emiliania huxleyi at the base of Core 323-U1343A-3H assigns an age for this core to calcareous nannofossil Zone NN21, which is defined by the FO datum of E. huxleyi (0.29 Ma) to the present. However, the occurrence of a barren interval below this level made it impossible to constrain the FO of E. huxleyi and the boundary with Zone NN20. The LO datum of Pseudoemiliania lacunosa (0.44 Ma) in Sample 323-U1343A-12H-7, 46 cm, which defines the upper limit of calcareous nannofossil Zone NN19, is well constrained because it takes place within an interval rich in calcareous nannofossils. Although the trace amount of planktonic foraminifer Neogloboquadrina atlantica (sinistral) with the LO (2.4–2.5 Ma) is present, the age derived by this taxon appears too old when compared to other datums. Benthic foraminiferal assemblages have a similar species composition to the assemblages found at Site U1339 and are within or near the OMZ in the Sea of Okhotsk. High-frequency variation in oxygenation is apparent throughout the section, but initial results show generally higher oxygen indicators such as Elphidium cf. batialis and Islandiella norcrossi are dominant in and below Core 323-U1343E-24H.

The LO datum of diatom Proboscia curvirostris (0.3 Ma) and the LO datum of Thalassiosira jouseae (0.3 Ma) were observed at ~70–77 m CSF in three holes at Site U1343. This is consistent with results from Site U1339 at Umnak Plateau. The cored interval above the LO of P. curvirostris (0.3 Ma) is assigned to Neodenticula seminae Zone NPD12. The interval between the LCO datum of Actinocyclus oculatus observed at 296.4 m CSF and the LO datum of P. curvirostris at 69.4 m CSF in Hole U1343E is assigned to P. curvirostris Zone NPD11. The interval below this datum above the FO of Neodenticula koizumii is A. oculatus Zone NPD10. The FO of P. curvirostris (1.85 ± 0.1 Ma) was defined in Sample 323-U1343E-50X-CC and assigned to the age in the A. oculatus Zone. The LO of Pyxidicula horridus (1.9–2.0 Ma) was estimated in Core 323-U1343E-61X. The LCO of N. koizumii (2.1 Ma) was observed in Sample 323-U1342E-77X-CC. The interval between this datum and the bottom of Hole U1343E is assigned to N. koizumii Zone NPD9. The LO of silicoflagellate Distephanus octonarius (0.2–0.3 Ma) is estimated to be between 64.0 and 74.9 m CSF. The LO of Dictyocha subarctios (0.6–0.8 Ma) is estimated to be between 196.2 and 205.35 m CSF (Hole U1343C) and between 196.1 and 205.45 m CSF (Hole U1343E). The bottom age in Hole U1343E is younger than 2.5 Ma because of the absence of ebridian Ebriopsis antiqua antiqua. In Sample 323-U1343E-27H-CC relatively warm water taxa belonging to Dictyocha spp. are observed in high numbers (30%), indicating a possible increase of subarctic Pacific water entry into the Bering Sea. Six radiolarian datums common in the subarctic Pacific were identified at this site. The LO and FO datums of E. matuyamai (0.9–1.5 Ma and 1.7–1.9 Ma, respectively) were identified in samples from Hole U1343E. In the uppermost 250 m CSF changes in dinoflagellate assemblages are observed, indicating surface water conditions that vary from high productivity and upwelling to conditions with pronounced sea ice cover. Such a high variability above 250 m CSF is also observed in pollen and spore abundances, suggesting changing vegetation in adjacent land masses as well.

Paleomagnetism

The Brunhes/Matuyama boundary is clearly identified in Holes U1343A, U1343C, and U1343E between 180 and 185 m CSF, and both the termination and onset of the Jaramillo Subchron were identified below this (Table T19). Deeper in the section, whereas inclination tends to cluster around normal polarity values making it difficult to identify polarity zonation, the top boundary of the normal polarity zone at ~292 m CSF has been tentatively identified as the termination of the Cobb Mountain Subchron (Fig. F49). The paleointensity variation has quite a large amplitude and obviously shows a coherent change with the magnetic susceptibility, suggesting that NRM intensity has been largely influenced by environmental changes.

Geochemistry

Interstitial water SO₄^2–, DIC, PO₄^3–, and NH₄⁺ concentration profiles indicate that the sediments at Site U1343 are characterized by high rates of carbon turnover compared to sites at Bowers Ridge (Fig. F50). Values are, in general, at least one order of magnitude higher than at Site U1342 on Bowers Ridge. Profiles of CH₄ and SO₄^2– suggest that sulfate reduction is largely driven by the diffusion of CH₄ into the sulfate zone. The SO₄^2– profile is nearly linear in the uppermost 8 m CSF, indicating no significant consumption there. The CH₄ flux into the SO₄^2– zone, as calculated from the concentration gradient between 8 and 11 m CSF, is ~50%–60% of the SO₄^2– flux into the SMTZ. Hydrogen sulfide is also at a maximum in the SMTZ. The ratio between autotrophic and organoclastic sulfate reduction is higher at Site U1343 than at the Bowers Ridge or Umnak Plateau sites. A relatively high flux of Ca²⁺ into the SMTZ further stresses the importance of anaerobic methane oxidation (AOM), which commonly leads to the formation of CaCO₃. The Ca²⁺ flux into the SMTZ is ~35% of the methane flux, indicating that an equivalent fraction of the DIC produced through AOM is deposited as CaCO₃.

The curvature of the NH₄⁺ profile suggests NH₄⁺ production from organic matter degradation throughout the sediment column. This is confirmed through preliminary modeling exercises and suggests organic matter degradation and hence microbial activity even at depths below 400 m CSF. Organic matter degradation also leads to the accumulation of DIC and PO₄^3– in the interstitial water. The accumulation of these species, however, is much lower than predicted by the NH₄⁺ profile assuming steady state and a constant ratio between C, N, and P of remineralized organic matter. This suggests both production and consumption of DIC and PO₄^3– in the sediment. Consumption of these species is most likely due to formation of apatite and calcium carbonates (e.g., dolomite). The pore water profiles suggest that rates of net consumption of PO₄^3– are highest between 180 and 200 m CSF and net DIC consumption is highest between 300 and 350 m CSF.

The decrease in salinity and pore water chloride concentrations indicate freshening of the pore fluids with depth. A possible explanation for this trend is the dissociation of gas hydrates during core recovery, which releases fresh water and causes depletions in dissolved ion concentrations. Alternatively, decreases in pore water salinity and chloride concentrations can result from meteoric water input, clay membrane ion filtration, and clay mineral dehydration.

Microbiology

Samples for abundance of prokaryotes were collected adjacent to interstitial water whole-rounds in sections cored using the APC. High-resolution sampling took place in the microbiology-dedicated cores from Hole U1343B as well as in samples taken in each core to APC refusal in Hole U1343A. Additional samples were taken from XCB Cores 323-U1343E-78X to 80X to evaluate cell abundance and community structure in the deepest portion of Hole U1343E. PFT analyses performed on all cores from Hole U1343B and in the deeper cores from Hole U1343E show no contamination from the drill fluid. Samples from all cores were fixed. These analyses will assist with understanding the global distribution and abundance of subseafloor life in a highly productive oceanic regime. A special focus will be directed toward the function of Archaea in the sulfate reduction zone, the SMTZ, and the methanogenesis zone.

Physical properties

Downhole from the uppermost ~10 m CSF, wet bulk density increases slightly from an average of ~1.60 g/cm³ to ~1.65 g/cm³ at ~100 m CSF. The average value below this depth, although oscillatory, does not seem to change until ~360 m CSF, the calculated depth (~360 m) of the bottom-simulating reflector (BSR), where a shift to a lower average of ~1.60 g/cm³ occurs. This shift coincides with the change from APC to XCB coring and the consequent recovery of drilling-disturbed core sections. Magnetic susceptibility exhibits cyclicity from lower values averaging ~20–25 SI units to higher readings of ~250 SI units. Peak readings, which are roughly separated by 30–50 m, are prominent to ~360 m CSF. Below this depth the wavelength increases and average values decrease.

Sonic velocities, V_P and V_S, recorded by the downhole FMS-sonic logging tool increase with depth from a near-surface value of ~1550 m/s to ~1840 m/s at the bottom of Hole U1343E at ~744 m CSF. Three gradients of increasing V_P can be recognized: the first gradient extends to ~360 m CSF, increasing at ~110 m/s/km; the second extends from 360 to 520 m CSF, increasing at ~550 m/s/km; and the third gradient extends from 530 to ~744 m CSF, increasing at ~890 m/s/km. NGR counts generally track abundance of clay minerals and their absorbed radioactive nuclei. Evidently, in Hole U1343E higher bulk density sediment is also richer in clay and other siliciclastic minerals. Siliceous microfossils that resist compaction and sediment consolidation are not the dominant component constructing the stratigraphic section. Perhaps because of this circumstance, NGR readings appear to track compaction-driven densification of clay-rich beds, an observation consistent with the progressive downhole increase in logging-tool-measured V_P.

In general, cores collected above the BSR at ~360 m CSF exhibit higher variability of thermal conductivity values, and these are also the most gas-disrupted sections measured. In cores collected below the transition at ~520 m CSF to higher carbonate bearing, V_P and bulk density display the highest range of thermal conductivity values. Average values of discrete sample density increase downhole in Hole U1343A from a near-surface measurement of ~1.50 g/cm³ to near 1.70 g/cm³ at ~100 m CSF. Below this subsurface level the average MAD bulk density changes little. Similar profiles of water (moisture) content and sediment porosity are recorded in Holes U1343A and U1343E. Near-surface porosity is ~70%, noticeably lower than that measured at Sites U1339 (~80%), U1340 (~75%), U1341 (~78%), and U1342 (~80%). This difference is ascribed to the lower overall content of siliceous microfossils constructing the sedimentary section. Porosity and water content decrease sharply downhole to ~60% at ~80 m CSF, below which porosity only gradually decreases to ~56% at ~744 m CCSF-D. Average grain density seems to show three density-fluctuating groupings: an upper group from the seafloor to ~100 m CSF with an average density of ~2.68 g/cm³; a middle group between ~100 and 540 m CSF with an average density ~2.65 g/cm³; and a basal group that shifts to a lower density of ~2.55 g/cm³ at ~540 m CSF but that increases to 2.70 g/cm³ at 744 m CSF.

Stratigraphic correlation

The composite depth scale and splice at Site U1343 is constructed from 0 to 779.18 m CCSF-A (Tables T20, T21). The splice consists of one complete and continuous interval from the mudline to 270.47 m CCSF-A. The continuous splice ranges from the top of Core 323-U1343C-1H to Section 323-U1343E-29H-7, 79 cm, and below this are appended cores ranging from Sections 323-U1343E-29H-1, 0 cm, to 83X-7, 31 cm, (779.18 m CCSF-A), with a constant affine value of 35.62 m. Within the splice the CCSF-A depth scale is defined as the CCSF-D depth scale. CCSF-D rigorously applies only to the spliced interval. The cumulative offset between CSF and CCSF-D depth scales is nonlinear. The affine growth factor at Site U1343 between 0 and 36.4 m CSF is 1.03. At greater depths all cores have an affine growth factor of 1.15. Calculation of MARs based on the CCSF-A or CCSF-D scales should account for the affine growth factor by dividing apparent depth intervals by the appropriate growth factor for the depth interval. After dividing by the growth factor (accounting for the different depth intervals) this scaled depth scale should be referred to as CCSF-B. MARs calculated for the interval of appended cores deeper than the spliced interval should not be divided by the affine growth factor because their depths are a linear transformation of drilling depths.

Downhole measurements

Two tool strings were deployed in Hole U1343E: the triple combo and the FMS-sonic combination. Although both runs indicated an irregular hole, particularly above 430 m WSF, all of the calipers showed that the tools were making contact with the formation over most of the interval logged, suggesting that the overall quality of the data is good. Although the HLDS caliper suggested that the tools made good contact between 300 and 360 m WSF and that the hole was even smaller than the nominal bit size in part of this interval, the density and neutron porosity data in this interval seem questionable. The anomalously low density readings between 307 and 322 m WSF and the very high neutron porosity values between 300 and 360 m WSF suggest that the tool was not properly measuring formation properties. Comparison with the density measurements made with the GRA track sensor on cores recovered from Hole U1343E and with the MAD measurements made on samples from Site U1343 shows generally good agreement except in this interval, where logging data are significantly lower than core measurements.

Logging Unit 1 (100–330 m WMSF) is characterized mainly by a steady increase with depth in velocity, whereas the other log data remain mostly uniform despite some variability, such as in gamma radiation. The velocity increase at the bottom of logging Unit 1 is likely responsible for the strong reflector observed at 2860 ms two-way traveltime in the seismic data. A synthetic seismogram was produced using a wavelet extracted from the seafloor reflection in traces adjacent to Shotpoint 350 and Site U1343 in Line Stk-1. Although we speculated this reflector might be a BSR, indicating the existence of gas hydrate overlying free gas, no conclusive indication from the logs supported the occurrence of gas hydrate. However, slightly higher velocity and resistivity trends and lower dipole waveform amplitudes above the reflector, as well as low chlorinity values measured on several interstitial water samples, suggest that some amount of gas hydrate might be present. Logging Unit 2 (330–510 m WMSF) is defined by slightly decreasing trends with depth in resistivity and by gamma ray values slightly higher than the shallower and deeper units. The top of logging Unit 3 (510–745 m WMSF) is defined by a drop in gamma radiation, an increase in V_P, and a change in the trends of all the logs. The gamma ray, potassium, thorium, density, resistivity, V_P, and V_S logs all display higher amplitude and lower frequency variability than in the upper units, suggesting a significant change in the deposition history and rates. A dolostone recovered in this unit can be recognized in the FMS images.

The APCT-3 tool was successfully deployed three times in Hole U1343A. The measured temperatures ranged from 4.34°C at 43.5 m DSF to 8.53°C at 129.0 m DSF and closely fit a linear geothermal gradient of 49.0°C/km. A simple estimate of the heat flow can be obtained from the product of the geothermal gradient and the average thermal conductivity (0.985 W/[m^.K]), which gives a value of 48.2 mW/m², within the range of previous measurement in the area.

Sedimentation rates

Very high sedimentation rates were observed throughout the drilled depths at Site U1343 (Fig. F51; Table T22). From the seafloor to the top of the Jaramillo Subchron (0.998 Ma; 267.6 m CCSF-A) sedimentation rates were relatively constant within the range of 25–29 cm/k.y. An increase in the rate to 46 cm/k.y. was observed between the top and the bottom of the Jaramillo Subchron (1.072 Ma; 302.0 m CCSF-A). Farther down at 407.7 m CCSF-A, or 1.55 Ma as indicated by the LO of dinoflagellate F. filifera, sedimentation rates are ~21–25 cm/k.y. Sedimentation rates increase to 54–58 cm/k.y. between 408 and 716 m CCSF-A. The age at this depth is estimated at 2.1 Ma by the LCO of diatom Neodenticula koizumii.

Site U1344

Background and objectives

The primary objective of drilling at Site U1344 (prospectus Site GAT-3C) was to study high-resolution Pliocene–Pleistocene paleoceanography at a proximal gateway location to the Arctic Ocean at the deepest water depth of Expedition 323. The site is located ~3200 m along the small summit of a canyon interfluve ~10–15 km southeast of Pervenets Canyon, a large submarine canyon that deeply and widely incises the Beringian continental slope (Figs. F1, F52, F53, F54, F55) (Normark and Carlson, 2003). Pervenets Canyon, along with companion Zhemchug Canyon adjacent to Site U1343, was discovered in the early 1960s by the Soviet fishing industry and named after one of the discovering trawlers. At times of glacially lowered sea level the head of Pervenets Canyon is commonly presumed to have been one of the outfall locations for the Anadyr River, which presently drains the Russian northeast and enters the Bering Sea at the Gulf of Anadyr. It is anticipated to have received a supply of terrigenous sediments from the shelf during both the interglacials and the glacials.

This is also the area of high biological productivity called the Green Belt. The Green Belt is formed by the BSC, which originates from the Alaskan Stream water that flows into the Bering Sea through the western Aleutian Islands. The water that enters the Bering Sea moves eastward along the Aleutian Islands and consequently encounters the shallow blocking Bering shelf. The bottom depth of the BSC is at ~300 m, and its flow is forced to turn to the northwest once it meets the slope and shelf; eddies and instabilities in its flow cause upwelling along the shelf break. Moreover, tidal mixing causes further vertical mixing of the water masses along the BSC, enhancing biological productivity within the Green Belt (Taniguchi, 1984; Springer et al., 1996), a zone adjacent to the northwest-trending shelf break where high primary productivity in the surface waters and high organic carbon accumulation at the seafloor take place (Springer et al., 1996). However, we anticipated finding a lower organic carbon supply to the seafloor than at the other gateway sites or at Site U1339 because of the deeper water depth of Site U1344. Thus, expectation of the impingement by the dissolved OMZ in the past is relatively small at this site. Nevertheless, it is important to compare the vertical extent of water mass conditions on a basin-wide scale that includes this site. Hence, the records from the shallower drill sites on Bowers Ridge as well as the other gateway sites can be fully employed for the comparison.

This site is also located close to the maximum extent of the present-day seasonal sea ice cover. Thus, this site was expected to have been covered by seasonal or perennial sea ice during the glacial low sea level stands. Since it is adjacent to the Bering shelf, a high amount of terrigenous sediment supply was expected, especially during the glacial lowstands.

This relatively deep drill site in the gateway region to the Arctic Ocean can also be used to study the impact of subseafloor microbes on biogeochemical fluxes in the highest surface ocean productivity areas of the drill sites in the Bering Sea. Organic-fueled subseafloor respiration and its impact on biogeochemistry in such a highly productive region has not previously been quantified. To do this, the drilled sediments in the gateway region were used to determine subseafloor cell abundances and to investigate the link between the mass and characteristics of subseafloor microbes and the extent of export productivity from the surface ocean.

Sedimentation rates at this site have been estimated at 170–180 m/m.y. based on earlier site survey piston core studies (Takahashi, 2005; T. Sakamoto et al., unpubl. data). Neither of the piston cores taken in these studies recovered the Holocene section, possibly indicating erosion during the recent past. Prior to drilling, recovery of sections from the Pleistocene to Pliocene were expected.

Lithostratigraphy

One lithologic unit spanning the early Pleistocene to the Holocene was defined at this site. Unit I at Site U1344 encompasses a time period comparable to Unit I defined at Bowers Ridge Sites U1340 and U1341, and it is in general very similar to Unit I at the other Bering Sea margin Sites U1339 and U1343. However, Site U1344 is distinct in that it has an even higher proportion of siliciclastic components and a higher occurrence of sand-sized grains than Site U1343. This is probably related to the location of this site on the continental slope and its relative proximity to sources of terrigenous sediments from the continental margin. Sandy lithologies are concentrated in three relatively distinct intervals at Site U1344. These can be correlated not only between the holes at Site U1344 but also to Site U1343, where three distinctly sandy intervals occur. Whereas lithologies dominated by diatoms are associated with changes in color reflectance and are analogous to those at other sites, the lithology changes here are more subtle because of overall higher abundances of siliciclastic detritus. Higher abundances of diatoms may reflect high diatom flux during interglacials, as previously observed in the Bering Sea (Okazaki et al., 2005).

Only one diatom-rich laminated interval was observed. All other laminations are defined by faint color changes with gradational boundaries with the surrounding lithologies. It appears that well-oxygenated bottom water conditions probably prevailed throughout most of the Pleistocene, preventing preservation of laminations. Almost all dropstones are well rounded, indicating a period of reworking prior to incorporation in the ice. The rounding therefore favors a coastal provenance and sea ice rafting rather than icebergs (Lisitzin, 2003). Unlike most other sites (though similar to Site U1343) volcaniclastic material is only a minor component of the sediment at Site U1344 because it is more distant from the Aleutian arc. Authigenic carbonate occurs throughout the sediment and is not constrained to deeper parts of the sequence as it was at Site U1343. The shallowest appearance in Hole U1344A is at 63 m CSF. The presence of gas in the sediments caused several types of coring disturbance—mostly cracks as wide as several centimeters. In some cases, this affected the stratigraphic integrity of the sediment sequence, similar to Sites U1339 and U1343.

Biostratigraphy

The water depth of Site U1344 is ~3200 m and has a potential to reconstruct past deepwater changes because it is presently located below the OMZ. Benthic foraminiferal fauna indicate high-frequency changes in the bottom water oxygen content over the entire section, probably related primarily to surface water productivity but possibly to bottom water ventilation changes and methane seeps as well. There is a general increase in abundance and bottom water oxygen variability from ~300 m CSF to the top of the section. The low-oxygen indicator Bulimina aff. exilis is more abundant at both Sites U1343 and U1344 after ~0.8 Ma, as is benthic foraminifer abundance maxima. Both high-abundance and low-oxygen benthic fauna were found to be common during the last deglacial at Bowers Ridge (Okazaki et al., 2005), and the increase in such characteristics from 0.8 Ma may mark the onset of more intense deglacials, greater nutrient availability, and higher surface water productivity. Similarly to Site U1343, the increases of planktonic foraminifers also coincide with the highest numbers of sea ice diatoms and sea ice dinoflagellate cysts after 1 Ma. The change from low to high abundances of planktonic foraminifers coincides with the increased abundances of dinoflagellate cysts, calcareous nannofossils, benthic foraminifers, and the number of low-oxygen benthic foraminifers, analogous to the data shown by Okazaki et al. (2005) for the CaCO₃ preservation peaks during the last deglaciation in the Bering Sea.

This site is characterized by very low abundance of calcareous nannofossils. Only samples from the uppermost cores to the base of Core 323-U1344D-4H can be assigned to calcareous nannofossil Zone NN21 with an estimated age of <0.29 Ma. The planktonic foraminifer faunal assemblage found during the late Pleistocene is dominated by Neogloboquadrina pachyderma (sinistral) throughout. Below 260 m CCSF-A, N. pachyderma (sinistral) is reduced or absent from the assemblage and the fauna is replaced by subpolar assemblage dominated by G. bulloides. The occurrence of this species is mainly ruled by sea-surface temperature (Asahi and Takahashi, 2007), indicating that the late Pliocene–early Pleistocene was warmer than the late Pleistocene at this site.

The drilled interval above the LO of diatom P. curvirostris (ranging from 107.2 to 122.3 m CSF, depending on the hole) is assigned to Neodenticula seminae Zone NPD12. Because of the absence of Actinocyclus oculatus in Holes U1344D and U1344E, the bottom of each hole is assigned to Zone NPD 11. The FO datum of P. curvirostris was defined in Sample 323-U1344A-56X-CC and assigned the age of 1.85 ± 0.1 Ma in the A. oculatus Zone. The LO of Pyxidicula horridus (1.9–2.0 Ma) was estimated at the base of Core 323-U1344A-63X. The LO of silicoflagellate Dictyocha subarctios was assigned to Cores 323-U1344A-30H (270.25–280.31 m CSF) and 323-U1344D-26H (224.41–234.09 m CSF). The LO of ebridian Ammodochium rectangulare appears to be located in Core 323-U1344A-78X (733.13–739.75 m CSF). The radiolarian ages at Site U1344 span from the Botryostrobus aquilonaris Zone (upper Quaternary) to the Eucyrtidium matuyamai Zone (middle Quaternary) in the subarctic Pacific. Five radiolarian datums derived from the subarctic Pacific were identified at this site. Estimated sedimentation rates in the uppermost 150 m in Holes U1344A, U1344D, and U1344E are >30 cm/k.y., which is slightly higher than at neighboring Site U1343 (~20 cm/k.y.). The LO of E. matuyamai (0.9–1.5 Ma) was identified in samples from Hole U1344A. The occurrence of dinoflagellate Filisphaera filifera at the base of Core 323-U1244A-50X (473.4 m CSF) suggests an age of 1.41–1.7 Ma, according to its LO datum in the North Pacific and North Atlantic. This species dominates the assemblages in a few samples above this depth, quite similarly to Site U1343. The occasional occurrence of the autotrophic species Operculudinium centrocarpum may be related to oceanic conditions with relatively low productivity.

Paleomagnetism

The Brunhes/Matuyama boundary is clearly identified at ~280 m CSF. The Jaramillo, Cobb Mountain, and Olduvai Subchrons might be correlatable with the extracted normal polarity zones placed at ~380, 420, and 680 m CSF, respectively (Fig. F56; Table T23). The paleointensity variation has large amplitude and obviously shows a coherent change with magnetic susceptibility, suggesting that NRM intensity has been largely influenced by environmental changes. The relative paleointensity pattern seen at this site is consistent with those observed at Sites U1340, U1341, U1342, and U1343. Based on the correlations, MIS 1–19 have been assigned to ~280 m CSF. The dramatic changes in NRM indicate notable effects of early sediment diagenesis. Significant magnetic mineral dissolution starts within 10 m CSF due to AOM–sulfate reduction processes, which are also evident at Sites U1343 and U1339.

Geochemistry

The rate of carbon turnover in the sediment at Site U1344 is similar to or slightly higher than at Site U1343, as evidenced by similar SO₄^2–, DIC, PO₄^3–, and NH₄⁺ concentration profiles (Fig. F57). Similar to Site U1343, profiles of CH₄ and SO₄^2– at Site U1344 suggest that sulfate reduction is largely driven by CH₄ diffusion into the sulfate zone. The CH₄ flux into the SO₄^2– zone, as calculated from the concentration gradient between 8 and 13 m CSF, is ~70%–80% of the SO₄^2– flux into the SMTZ. The importance of AOM for overall carbon turnover is also stressed by the curvature in the DIC profile. The steepest concentration gradient in the uppermost 10 m CSF is observed directly above the SMTZ, suggesting that the highest DIC flux occurs from this zone. Preliminary modeling of the DIC profile suggests that net DIC production in the SMTZ accounts for 80% of the DIC production in the uppermost 30 m CSF of sediment. Hydrogen sulfide is also at a maximum in the SMTZ, most likely because sulfate reduction rates are the highest and the content of oxidized iron is the lowest in this zone. Magnetic susceptibility data obtained during fast scan of the cores confirm a low content of oxidized iron in the SMTZ.

AOM is well known to favor the deposition of carbonates in the SMTZ. At Site U1344 a relatively high flux of Ca²⁺ into the SMTZ is observed, which indicates the formation of calcium carbonate. There were also indications for Mg²⁺ flux into the SMTZ, which may suggest dolomite formation. The curvature of the NH₄⁺ profile suggests production from organic matter degradation throughout the sediment column. Microbial-mediated degradation is either conducted via a respiratory or fermentative pathway. According to the classical reduction scheme in sediments, only fermentation and hydrogenotrophic methanogenesis occur below the SMTZ; however, at this site the Fe profile suggests that Fe reduction occurs below the SMTZ. Organic matter degradation also leads to the accumulation of DIC and PO₄^3– in the interstitial water. The accumulation of these species, however, is much lower than that predicted by the NH₄⁺ profile, assuming steady state and a constant ratio between C, N, and P of remineralized organic matter. This suggests both production and consumption of DIC and PO₄^3– in the sediment. Consumption of these species is most likely due to formation of apatite and calcium carbonates (e.g., dolomite). The interstitial water profiles suggest that rates of net consumption of PO₄^3– and DIC are the highest between 300 and 350 m CSF. Ca²⁺ and Mg²⁺ concentration profiles likewise indicate net consumption of these species between 300 and 350 m CSF.

Microbiology

Samples for abundance of prokaryotes were collected adjacent to interstitial water whole-rounds in sections drilled using the APC. High-resolution sampling took place in the microbiology-dedicated cores as well as in additional samples taken once per core to APC refusal in Hole U1344A. Additional samples were taken from XCB Cores 323-U1344A-78X to 80X to evaluate cell abundance and community structure in the deepest portion of Hole U1344A. PFT analyses performed on these cores show no contamination from the drill fluid. Samples from all cores were fixed.

It is of interest to examine the relationship between microbial productivity and diversity in the uppermost 25 m of the sediment column. Special attention will be directed toward the function of Archaea in the sulfate reduction zone, the SMTZ, and the methanogenesis zone. The sulfate-methane transition is a hot spot for microbial activity and abundance within deep-sea sediments (D'Hondt, Jørgensen, Miller, et al., 2003), and we will expect an increase in the abundance and activity of microbial life, while the remainder of the core should see a significant decrease with depth in both active and benign microbial life.

Physical properties

The downhole profile of density for Hole U1344A is remarkably similar to that of Hole U1343E. The overall downhole increase in bulk density is interpreted to record compactive dewatering in a generally lithologically uniform sequence of fine-grained sediment. Magnetic susceptibility, as measured by the WRMSL, exhibits little change in average value and character with depth. Based on what was learned at previous sites, the rhythmic oscillations are presumed to be a function of lithologic composition and patterns of in situ sediment alteration.

Except for the uppermost three cores in Hole U1344A, P-wave velocity readings for the sedimentary section penetrated at this hole were only collected by the FMS-sonic downhole logging tool. Sonic P-wave velocity data reveal a profile similar to that recorded in Hole U1343E in that the average velocity increases downsection in steplike sectors. Except for the uppermost ~80–100 m CSF, across which NGR readings increase from a near-surface measurement of ~25 count/s to ~34 counts/s, NGR values at deeper depths oscillate around this average to the base of the Hole U1344A at 745 m CSF. Presumably, variations in counts per second reflect downhole changes in content of clay and siliciclastic minerals.

Thermal conductivity measurements can be grouped into an upper and lower sequence. The upper vertical sequence displays an estimated average reading of ~0.905 W/(m^.K) and extends downhole from the near surface to ~260 m CSF, below which APC refusal caused a change to XCB coring and V_P shifts abruptly to higher readings. In Hole U1344A porosity decreases most rapidly in the upper part of the drilled section, falling to an average value of ~60% at 80–100 m CSF. Below this depth, other than oscillating readings, there are no notable shifts in average value or changes in trend. The overall downhole decrease in porosity tracked by MAD and logging data is presumably a manifestation of compaction dewatering.

The upper group of dry grain density, extending from the surface to ~160 m CSF, is ~2.70 g/cm³. The middle sequence, from ~160 to 620 m CSF, exhibits an average density of 2.65 g/cm³, and the underlying basal group has a lower density of ~2.62 g/cm³. It appears that an overall uphole increase in deposition of denser siliciclastic mineral debris is recorded.

Stratigraphic correlation

The complete and continuous composite depth scale and splice at Site U1344 is constructed from 0.0 to 332.02 m CCSF-A (Tables T24, T25). The continuous splice ranges from the top of Core 323-U1344A-1H to Section 323-U1344A-31X-5, 50 cm. The appended cores range from Cores 323-U1344A-32X to 79X (790.37 m CCSF-A), with a constant affine value of 43.78 m. All splice points in the interval of 0–50 m CCSF-A are clear and convincing based on multiple data types. The splice tie point between Sections 323-U1344A-5H-7, 4.44 cm, and 323-U1344D-6H-3, 79.88 cm (51.99 m CCSF-A), is uncertain and could be moved ~2.4 m shallower in Core 323-U1344D-6H with equal uncertainty. This is a point to be resolved with postcruise data. The splice tie points between Sections 323-U1344E-6H-6, 112.47 cm, and 323-U1344A-6H-4, 122.06 cm (60.40 m CCSF-A), between Sections 323-U1344E-14H-1, 10.92 cm, and 323-U1344D-14H-2, 136.02 cm (140.43 m CCSF-A), and Sections 323-U1334A-16H-7, 1.04 cm, and 323-U1334D-17H-4, 1.34 cm (175.52 m CCSF-A), are uncertain because of low signal amplitude in magnetic susceptibility.

Downhole measurements

Two tool strings were deployed in Hole U1344A: the triple combo and the FMS-sonic combination. Overall, the caliper of the density sonde shows an irregular borehole with a particularly large-diameter interval between 170 and 260 m WSF but with very good conditions in the lower section. Deeper in the borehole, small enlargements regularly spaced every ~9.5 m indicate where the bit was sitting whenever a core was recovered. However, all the calipers show that the tools made at least partial contact with the formation over most of the interval logged, suggesting that the overall quality of the data is good. Irregular hole size has an effect on measurements that require good contact with the formation, namely density and porosity. The anomalously low density values between 230 and 250 m WSF within the 100 m interval with the largest hole size are probably erroneous, as are most neutron porosity measurements in this entire interval. The quality of the logs can also be assessed by comparison with the NGR and GRA track data and with the MAD measurements made on cores recovered from Hole U1344A. Except for two short intervals with lower density logging data (230–250 and 420–430 m WSF), all density data sets are in good agreement, confirming overall good data quality despite the enlarged hole. Comparison of the gamma ray logs measured during the main pass of the two runs shows excellent repeatability.

Logging Unit 1 (100–330 m WMSF) is characterized mainly by a steady increase with depth in V_P and V_S, whereas the other log data remain mostly uniform despite some variability such as in gamma radiation. The bottom of this unit is defined by a noticeable drop in V_P, V_S, gamma radiation, density, and resistivity immediately above a sharp peak in these measurements, particularly in V_S and resistivity, indicating a fine, stiff layer. This sequence corresponds to a core with poor recovery. Logging Unit 2 (330–460 m WMSF) is almost uniquely defined by the V_P and V_S logs, both of which increase steadily through the unit. Gamma radiation and density also increase with depth in this unit in a more subdued manner. The top of logging Unit 3 (460–620 m WMSF) is defined by an inflection in the velocity profiles, which, combined with a decreasing trend in density, generates the strong reflector at 4.83 s two-way traveltime. The variability with depth in gamma radiation and in most logs displays a cyclicity more clearly defined than in the upper units. Finally, the top of logging Unit 4 (620–745 m WMSF) is defined by a sharp increase in V_P, V_S, gamma radiation, and density, as well as by a significant change in the trends of all the logs. As in the deepest unit of Site U1343, the gamma ray, potassium, thorium, density, resistivity, V_P, and V_S logs all display a variability with depth of wider amplitude and lower frequency than in the upper units, suggesting a significant change in deposition history and rates.

The APCT-3 tool was successfully deployed three times in Hole U1344A. The measured temperatures ranged from 4.51°C at 47.1 m DSF to 9.57°C at 142.1 m DSF and closely fit a linear geothermal gradient of 53.3°C/km. The temperature at the seafloor was 1.65°C based on the average of the measurements at the mudline during all the APCT-3 deployments. A simple estimate of the heat flow can be obtained from the product of the geothermal gradient and the average thermal conductivity (0.911 W/[m^.K]), which gives a value of 48.5 mW/m², within the range of previous measurement in the area

Sedimentation rates

Sedimentation rates observed at Site U1344 are mostly similar to values within a narrow range of 29–50 cm/k.y. throughout Holes U1344A, U1344D, and U1344E (Table T26). One exception is the interval between the top and the bottom of the Cobb Mountain Subchron (459.0–469.6 m CCSF-A), which resulted in 89 cm/k.y. Based on sedimentation rates, the bottom age of Hole U1344A was determined to be ~1.9 Ma (Fig. F58).

Site U1345

Background and objectives

The primary objective of drilling at Site U1345 (prospectus Site NAV-1B) was to study high-resolution Holocene–late Pleistocene paleoceanography at a proximal gateway location to the Arctic Ocean at a water depth of ~1008 m. The drill site is located on an interfluve ridge near the large, broad head of Navarin submarine channel off the Bering Sea shelf (Figs. F1, F59, F60, F61, F62) (Normark and Carlson, 2003). We anticipated that this site received an ample supply of terrigenous sediments from the shelf during the glacials. This is also in the area of high biological productivity called the Green Belt. The Green Belt is formed by the BSC, which originates in the incoming Alaskan Stream water that flows through the western Aleutians into the Bering Sea (Taniguchi, 1984; Springer et al., 1996). Farther northwest, higher primary productivity and organic carbon is observed at the seafloor (Springer et al., 1996). Thus, past impingement by the dissolved OMZ was highly expected at this site. Because the expected sedimentation rates were high with intermittently laminated sediment of millimeter to submillimeter thickness (see below), we expected to be able to reconstruct detailed climate change of submillennial timescales. Therefore, we anticipated comparing data from this site with those of other pertinent high-resolution records from such places as the Santa Barbara Basin, the Cariaco Basin, and GISP2. It is also important to compare the vertical extent of water mass conditions on a basin-wide scale that includes the Bowers Ridge, gateway, and Umnak sites.

Furthermore, this site is located close to the maximum extent of the present-day seasonal sea ice cover. Thus, we expected this site to have been extensively covered by seasonal or perennial sea ice during the glacial low sea level stands. Due to its proximity to the location of sea ice formation, where cold and dense brine is expelled when sea ice is formed, this site, as well as Site U1344, provides crucial information regarding the formation of the North Pacific Intermediate Water.

This relatively shallow drill site in the gateway region to the Arctic Ocean can also be used to study the impact of subseafloor microbes on biogeochemical fluxes in the highest surface ocean productivity areas of the drill sites in the Bering Sea. Organic-fueled subseafloor respiration and its impact on biogeochemistry in such a highly productive region has not previously been quantified. To do this, the drilled sediments in the gateway region were used to determine subseafloor cell abundance and to investigate the link between the mass and characteristics of subseafloor microbes and the extent of export productivity from the surface ocean (Takahashi et al., 2000).

Reports of sedimentation rates at this site location vary significantly, ranging from 14 cm/k.y. during the Holocene and 91 cm/k.y. during the LGM to as much as 242 cm/k.y. during the deglaciation (Cook et al., 2005). Prior to drilling, recovery of sections from the Holocene to late Pleistocene were expected at this site.

Lithostratigraphy

Only one lithologic unit was recognized at Site U1345. Unit I comprises the same time period as Unit I at the other sites: the middle Pleistocene to the Holocene. Site U1345 is distinct among the near-shelf sites due to the abundance and generally coarser texture of the siliciclastic component in the sediments as well as the higher frequency of laminated intervals. Intervals characterized by >25% sand and by thin sandy layers occur at all depths in all the holes drilled at this site. The laminations and thin-bedded sediments are numerous and well correlated between holes.

Site U1345 is located in the central portion of the modern OMZ. The sediments deposited at this site can provide important information concerning Pleistocene to Holocene variability of bottom water oxygen concentrations. The preservation of laminated and thinly bedded sediments (beds <10 cm thick) could be interpreted as the result of a reduction of the activity of benthic macrofauna due to low oxygen concentrations in the bottom waters and surface sediments. Laminations do not have a clear signature in the physical property or reflectance data as observed at Site U1342.

The laminated intervals can be divided into two categories based on the abundance of biogenic grains: (1) couplets or triplets of diatom oozes, mixed siliciclastic/biogenic sediments, and siliciclastic sediments, or (2) couplets of siliciclastic sediments of alternating textures that may include minor (<40%) amounts of diatoms. Laminated sediments of the first category are similar to laminated intervals at other sites, which are typically biogenic rich, olive-green, dark olive-gray, and very dark greenish gray. This category of lamination seems to occur mainly during interglacials. This relationship supports previous observations of higher flux of diatoms during interglacial compared to glacial periods (Okazaki et al., 2005). The second category of laminated sediments is mainly siliciclastic and unique to Site U1345. This type of lamination occurs in sediments that are tentatively identified as deposited during glacial conditions. Since these sediments are not biogenic rich, changes in intermediate water ventilation may have been the controlling parameter for bottom water oxygen concentrations during these periods.

We observed intermittent finely disseminated authigenic carbonates deeper than ~30 m CSF in all holes at this site. The SMTZ is at ~6.5 m CSF, the shallowest observed during Expedition 323. Calcium and magnesium concentrations in the pore water decrease toward the SMTZ, suggesting active authigenic carbonate precipitation at and below this depth today.

Few ash layers were observed at Site U1345. The ashes that do appear are light colored, suggesting that their source is explosive rhyolitic volcanism. This site is distant from the nearest likely source of volcaniclastic grains, the Aleutian arc, so the transport mechanism must have been one capable of widespread dissemination.

The lithologies at Site U1345 are sandier than at any other site from Expedition 323. Lithologies with >25% sand and thin sandy layers occur throughout all holes. The presence of this coarse material is probably related to the position of Site U1345 at the crest of an interfluve at the mouth of Navarin Canyon. The siliciclastic grain sizes at Site U1345 contrast even more strongly with Site U1339, located on a submarine plateau isolated from the continental shelf. At the latter site virtually no sand-sized grains were recorded. This may be due either to decreased transport of terrigenous material to the site or to a high biogenic flux to Site U1339 sediments.

Biostratigraphy

High-frequency variations can be seen in the abundance and composition of all microfossil groups. The decrease in sea ice diatoms, the increase in dinoflagellates, planktonic foraminifers, and calcareous nannofossils, and the percent of open ocean diatoms Neodenticula and Actinocyclus and the high-productivity dinoflagellate Islandinium minutum, which are associated with increases in the low-oxygen benthic foraminifer Bulimina aff. exilis, indicate the approximate depth intervals of distinct interglacials (at ~5, 40, 130, and 145 m CCSF-A). These intervals also coincide with low GRA bulk density and are consistent with the age model.

Overall, the distribution of calcareous nannofossils at Site U1345 seems to follow glacial–interglacial cyclicity with higher numbers during interglacials. Changes in their abundances generally reflect changes in environmental factors such as temperature and nutrients. Calcareous nannofossils do not become dominant components of the biota in areas of sea ice coverage. Elevated content of subpolar planktonic foraminifer species with G. bulloides appear at ~5, 40, 90, 130, and 145 m CCSF-A, largely coinciding with the inferred interglacials. This shows increased sea-surface temperatures during these intervals. G. bulloides is controlled by temperature rather than food availability in the Bering Sea (Reynolds and Thunell, 1985; Asahi and Takahashi, 2007). These periods of elevated sea-surface temperatures probably reflect interglacial conditions.

As at the previous Bering Sea sites, the benthic foraminifer assemblage faunal composition shows large changes in species dominance. These changes are interpreted as shifts in local oxygen concentrations associated with surface productivity and/or deepwater ventilation on Milankovitch timescales. Bulimina aff. exilis is generally regarded as a low-oxygen/deep infaunal species (Bubenshchikova et al., 2008; Kaiho, 1994) and occurs in samples associated with high productivity and low sea ice. This suggests that higher productivity during some interglacials may have caused an expansion and intensification of the OMZ.

The LO of Proboscia curvirostris and the LO of Thalassiosira jouseae were observed from 71.1 to 73.3 m CSF, depending on the hole, giving an age of 0.3 Ma. In general, diversity is lower for this site than at the other gateway sites. The diatom assemblage for this zone (NPD11) is dominated by Thalassiosira antarctica spores, Fragilariopsis spp., Paralia sol, P. sulcata, Thalassiothrix longissima, Thalassionema nitzschioides, T. latimarginta s.l., and to a lesser extent Neodenticula seminae, Bacteriosira fragilis, and Actinocyclus curvatulus. The core interval above the LO of P. curvirostris to the recent is assigned to Neodenticula seminae Zone NPD12. This zone is dominated by T. antarctica spores, T. latimarginta s.l., P. sulcata, T. hyalina, B. fragilis, and the minor presence of N. seminae and A. curvatulus. In general, this site reveals a higher proportion of coastal neritic diatoms, together with freshwater species, than the other gateway sites (Sites U1343 and U1344). Low proportions of sea ice diatoms and high proportions of open water diatoms correspond well with the interglacial horizons. The last occurrence of silicoflagellate Distephanus octonarius (0.2–0.3 Ma) was observed in Core 323-U1345A-9H (71.01–80.64 m CSF).

Radiolarian zones at Site U1345 could not be established due to the absence of Stylatractus universus. Four radiolarian datums derived in the subarctic Pacific were identified at this site. The LO of Lychnocanoma nipponica sakaii (50 ka) and Spongodiscus sp. (280–320 ka) were determined. The LOs of Amphimelissa setosa (70–90 ka) and Axoprunum aquilonium (250–410 ka) were supported only by seldom occurrences, indicating uncertain top positions of the stratigraphic age. Estimated sedimentation rates between the LO of L. nipponica sakaii and Spongodiscus sp. are ~25 cm/k.y. in each hole. Among all radiolarian species, C. davisiana shows high fluctuations in abundance, possibly relating to ventilation changes with glacial–interglacial cycles. The Sphaeropyle langii/robusta group, which are commonly found at Sites U1343 and U1344, show very low abundances. Because abundances of Sphaeropyle langii/robusta group at the shallower Sites U1339, U1340, and U1342 were also very low, their dwelling depth might be in deep water below 1000 m.

The dinoflagellate species Brigantedinium spp. is one of the most ubiquitous taxa among protoperidinials, and its distribution in modern sediments is closely related to primary productivity in temperate regions and also to polar and subpolar regions of the North Atlantic and Arctic oceans with seasonal sea ice coverage (Rochon et al., 1999). Islandinium minutum is one of the principal, if not the dominant, component of assemblages in the modern Arctic Ocean (Rochon et al., 1999; Head et al., 2001). The overall abundance of dinocysts and particularly the above-mentioned species suggests high productivity and upwelling during prominent interglacials. Extremely high abundances of dinocysts, especially at the mudline in Hole U1345B and Sections 323-U1345A-5H-CC (44.4 m CCSF-A) and 13A-CC (130.6 m CCSF-A) suggest interglacial periods. This coincides with relatively low pollen and spore concentrations.

Paleomagnetism

No polarity reversal boundary was observed in the cores at Site U1345; therefore, the whole sequence is assigned to the Brunhes normal polarity zone (Fig. F63; Table T27). The relative paleointensity pattern is consistent with that seen at all other sites. Based on the correlations, MIS 1–12 were assigned. The significant changes in NRM indicate notable effects of early sediment diagenesis, as this has also been seen at the previous sites. Significant magnetic mineral dissolution starts within 5 m CSF due to processes related to AOM and sulfate reduction. This is also evident at Sites U1344, U1343, and U1339. The active zone of dissolution appears to be limited to the top 10 m so that magnetization does not change significantly at deeper depths.

Geochemistry

Of all the sites investigated, Site U1345 shows the shallowest SMTZ at ~6.25 m CSF (Fig. F64). Likewise, this site is characterized by the steepest flux of methane into this zone and the highest interstitial water hydrogen sulfide concentrations. Similar to the other shelf sites (Sites U1343 and U1344), the almost linear sulfate and methane profiles suggest that AOM coupled with sulfate reduction accounts for most of the sulfate consumption in the sediment. Preliminary modeling of the DIC profile suggests that net DIC production in the SMTZ accounts for 70% of the DIC production in the top sediment layers. The organic matter degradation products phosphate and ammonium show accumulation in the pore water, with the distinct minimum in phosphate concentration between 22.25 and 27.25 m CSF. However, this also indicates that the consumption of this species is most likely due to the formation of phosphate-bearing minerals such as apatite.

The occurrence of high concentrations of interstitial water hydrogen sulfide in the SMTZ can be attributed to very high sulfate reduction rates at this depth and probably also to a lack of a sufficient pool of reactive Fe mineral phases (e.g., Fe [oxhydr]oxides) that can react with hydrogen sulfide on short timescales. Distinct peaks in dissolved Fe and Mn concentrations immediately below the SMTZ are the result of microbial dissimilatory Fe reduction. Calcium and magnesium profiles show depletion at the depth of the present SMTZ, suggesting the formation of authigenic Mg-rich carbonate (e.g., dolomite) driven by the production of DIC during AOM and an increase in pH leading to oversaturation of the pore water with respect to carbonate. Interestingly, the dissolved calcium profile shows a further decrease with depth and a minimum concentration at ~40 m CSF. This depth corresponds to a dolostone layer found at 40.27 m CSF. Sites U1343, U1344, and U1345 furthermore show high concentrations of dissolved Ba in the pore water and indicate a sink of this ion just above the SMTZ. The distribution of Ba at these sites can be attributed to diagenetic remobilization of Ba deposited as biogenic barite into the sulfate-depleted pore water (von Breymann et al., 1992). The upper end of the SMTZ where the sulfate and dissolved Ba profiles overlap marks the present front of authigenic barite formation.

Microbiology

Samples for abundance of prokaryotes were collected adjacent to interstitial water whole-rounds. High-resolution sampling took place in the microbiology-dedicated cores as well as in additional samples taken once per core to APC refusal. It is of interest to examine the relationship between microbial productivity and diversity in the uppermost 25 m of sediment dedicated for microbial ecology. A special focus will be directed toward the function of Archaea in the sulfate reduction zone, the SMTZ, and the methanogenesis zone. The sulfate–methane transition is a hot spot for microbial activity and abundance within deep-sea sediments (D'Hondt, Jørgensen, Miller, et al., 2003), and we will thus expect an increase in the abundance and activity of microbial life; the remainder of the core should see a significant decrease with depth in both active and benign microbial life. To obtain an estimate of active subseafloor life, samples were also taken in low resolution for catalyzed reporter deposition-fluorescence in situ hybridization (CARD-FISH) at all aforementioned zones and at depth.

We will rely on estimates generated by shipboard participants (cell counts and geochemical profiles) and shore-based participants (amino acid and amino sugar composition) as indicators of productivity. We will examine overall bacterial and archaeal diversity by a combination of conventional 16S ribosomal ribonucleic acid (rRNA) clone libraries and quantitative polymerase chain reaction (qPCR) and/or a new quantitative community fingerprinting method involving automated ribosomal intergenic spacer analysis (ARISA) (Ramette, 2009).

Physical properties

Wet bulk densities in Hole U1345A appear to be higher by 0.1–0.2 g/cm³ than those measured in Hole U1344A in the uppermost 150 m of the sedimentary section; the higher densities of sediment in Hole U1345A probably reflect their higher sand content.

Similar to stratigraphic sections drilled at Beringian margin sites (Sites U1343 and U1344), GRA density values in Hole U1345A also document rhythmic fluctuations. Magnetic susceptibility measurements seem to have realistically recorded the downhole contour of changing values that are functions of many factors. An explanation for the higher counts for the Hole U1345A section is its coarser and higher content of siliciclastic mineral debris. In the downhole profile the contour of NGR readings is broadly similar to that of GRA bulk density. The downhole distribution of thermal conductivity readings displays an overall trend of increasing conductivity. Downsection profiles of MAD-measured porosity and water content record a progressive decrease in average values. The downhole distribution of water content and porosity is rhythmic. Little change is seen in average grain density with depth. The higher average grain density (2.75 g/cm³) of Unit I in Hole U1345A is interpreted to be a consequence of its greater abundance of coarse siliciclastic grains.

Stratigraphic correlation

The composite depth scale and splice at Site U1345 is complete and continuous from 0.0 to 167.6 m CCSF-A (Tables T28, T29). The splice ranges from the top of Core 323-U1345A-1H to Section 323-U1345D-16H-7, 146.6 cm. There are no appended intervals. Most of the splice points are clear and convincing based on the multiple copies of the section recovered in five holes. The splice tie point between Sections 323-U1345A-10H-4, 50.0 cm, and 323-U1345C-10H-1, 100.18 cm (93.02 m CCSF-A), is uncertain because Core 323-U1345A-10H contains disturbed flow-in starting approximately in the middle of Section 10H-4 and extending through the bottom of Section 10H-7. The disturbed section is not included in the splice. The splice tie point between Sections 323-U1345D-10H-7, 58.4 cm, and 323-U1345A-11H-3, 115.4 cm (103.284 m CCSF-A), and between Sections 323-U1345D-13H-6, 138.4 cm, and 323-U1345A-14H3, 94.7 cm (133.637 m CCSF-A), are tentative because of low signal amplitudes in MS477. The cumulative offset between CSF and CCSF-D depth scales is roughly linear. The affine growth factor at Site U1345 is 1.11.

Downhole measurements

The only downhole measurements made at Site U1345 were three deployments of the APCT-3 tool in Hole U1345A. The measured temperatures ranged from 4.92°C at 42.4 m DSF to 8.15°C at 108.9 m DSF, indicating a local geothermal gradient of 48.5°C/km. A simple estimate of the heat flow can be obtained from the product of the geothermal gradient and the average thermal conductivity, which gives a value of 51.6 mW/m², in agreement with existing measurements in the area.

Sedimentation rates

Based on the four holes studied, the following two mean radiolarian biostratigraphic datums were employed for the determination of sedimentation rates: the LO of Lychnocanoma nipponica sakaii and the LO of Spongodiscus sp. (Ling, 1973). Only one sedimentation rate of 28 cm/k.y. appears to be applicable to this site (Fig. F65; Table T30). This sedimentation rate is lower than that of the adjacent, deeper Site U1344 (~39 cm/k.y.).

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