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doi:10.2204/iodp.proc.323.101.2011

Summary of expedition results

Overview of ages and sedimentation rates

Expedition 323 focused on analyzing long-term ocean and climate trends and the evolution of higher frequency glacial–interglacial to millennial-scale oscillations through the Pliocene and Pleistocene. As such, our primary drilling objective was to obtain sediments whose components could be used to elucidate a detailed evolutionary history of climate, surface-ocean, and intermediate-water conditions since the earliest Pliocene in the Bering Sea, where amplified high-resolution changes of climatic signals are recorded. In addition, we sought to explore subseafloor microbial activity in the Bering Sea because of its extremely high surface productivity.

Our objectives, in terms of acquiring sediment core samples as well as conducting shipboard research, have been adequately met because our targeted drill sites had extremely high sedimentation rates (Fig. F5) and contained abundant microfossils and other paleoceanographic proxies.

Among the three drill sites explored in the Bowers Ridge region (Fig. F6), both of the deepest holes drilled, Holes U1340A (605 meters below seafloor [mbsf]) and U1341B (600 mbsf), represent time spans between the Holocene and ~5 Ma in the Pliocene (Fig. F5). The expedition's initial goal of penetrating to ~5 Ma was adequately accomplished at both sites, despite the failure of the extended core barrel (XCB) cutting shoe spacing sub in Hole U1340A, which had a target depth of penetration of 700 m. At the gateway region sites (at the Bering slope), two deep holes were drilled: Holes U1343E (744 mbsf) and U1344A (745 mbsf) (Fig. F7). Hole U1343E reached ~2.1 Ma, whereas Hole U1344A reached ~1.9 Ma (Fig. F5). At other drill sites, the bottom ages of the sedimentary sequences based on biomagnetostratigraphy are as follows: Site U1339 (Umnak Plateau; 0.74 Ma) (Fig. F8), Site U1342 (Bowers Ridge; 1.2 Ma [with the exception of middle Miocene sediments found just above basement]) (Fig. F6), and Site U1345 (gateway; 0.5 Ma) (Fig. F7).

The sediments recovered from Bowers Ridge display high sedimentation rates (average = ~14.5 cm/k.y. at Sites U1340 and U1341) (Figs. F9, F10; Tables T3, T4) without apparent hiatuses and are generally appropriate for high-resolution Pliocene–Pleistocene paleoceanography, with adequate calcareous benthic foraminiferal preservation in the Pleistocene but much lower preservation in the Pliocene. The sediments at these sites are generally barren of planktonic foraminifers and calcareous nannofossils, except for some intervals younger than ~2.5 and ~3 Ma. The abundance of all siliceous microfossils is generally high, enabling good biostratigraphy and paleoceanographic reconstruction. Furthermore, the upper part of Site U1340 (~20–150 m core composite depth below seafloor [CCSF-A]) had obvious soft-sediment deformation due to mass movement possibly caused by local seismic activity. Although such deformation hinders the continuous reconstruction of late Pleistocene high-resolution paleoceanography at this site, information from other drill sites can fill the gap. At Bowers Ridge Site U1342 (Fig. F6), sedimentation rates are relatively low, averaging ~4.5 cm/k.y. (Fig. F11; Table T5).

At Site U1339, to the south on the Bering slope, sedimentation rates are comparably high relative to other Bering slope sites, averaging ~29 cm/k.y. (Fig. F12; Table T6). In the region of the Arctic gateway sites proximal to the Bering slope, the observed sedimentation rates are overwhelmingly high: Hole U1343E has a sedimentation rate of 21–58 cm/k.y. (Fig. F13; Table T7) and Hole U1344A has a rate of 29–50 cm/k.y. (Fig. F14; Table T8). Sedimentation rates are so high, in fact, that drilling reached ages of only 2.1 and 1.9 Ma, respectively, despite penetration to 745 mbsf at each site (Fig. F5). At Site U1345, to the north on the Bering slope, sedimentation rates are also relatively high, averaging ~28 cm/k.y. (Fig. F15; Table T9). Such high sedimentation rates stem from the deposition of silt and clays transported by the Yukon and other rivers as well as the terrigenous sediments once deposited on the shelf. In spite of the high percentage of terrigenous materials, pertinent biotic proxies, including benthic foraminifers and siliceous microfossils, are adequately preserved. Therefore, the overall coverage of excellent cores to ~5 Ma in the Bowers Ridge region and ~2 Ma in the gateway region allows for detailed, continuous high-resolution paleoceanographic studies relevant to global climate change.

Lithostratigraphic summary

The seven sites drilled during Expedition 323 provide a continuous high-resolution record of the evolution of marine sedimentation in the marginal Bering Sea (Fig. F16). Onboard lithologic and sedimentologic analyses of the core sediments were carried out with a combination of visual core description, smear slide analysis, and, for selected samples, X-ray diffractometry. Overall, the sediments recovered in the Bering Sea are a mixture of three components: biogenic (mainly diatom frustules with varying proportions of calcareous nannofossils, foraminifers, silicoflagellates, and radiolarians), siliciclastic (mainly silt and, secondarily, sand and isolated pebble- to cobble-sized IRD clasts), and volcaniclastic (mainly fine ash). Other accessory lithologies identified include authigenic carbonates (dolomite and aragonite) and sulfides. The most prominent sedimentary features observed are decimeter- to meter-scale bedded alternations of sediment color and texture reflecting alternations in lithology. The sediments are generally highly bioturbated. However, fine-scale lamination preserving alternations between millimeter-scale laminae of biogenic and terrigenous material are also present at several of the drilled sites (Fig. F16).

The distributions of the sedimentary components and sedimentary structures as well as their variability both within and between the Expedition 323 sites account for the changes in biogenic, glaciomarine, terrigenous, and volcanogenic sediment sources and the environmental conditions present during sediment deposition. The scales of these lithologic variations indicate that sedimentation in the Bering Sea has recorded long-term trends that include the critical period of reorganization of Earth's climate from the warm early Pliocene and the transition into the ice ages. Milankovitch-scale (bedding) to high-frequency, potentially annual (lamination) variability is also recorded. The physiographic setting of the different sites, their depths, and their locations relative to sediment source areas (continents, ice sheets, volcanoes, and upwelling centers) account for the marked regional differences in sediment composition, especially between the Pleistocene sections of the Bowers Ridge and Bering slope sites.

The preliminary results from Expedition 323 suggest that the history of sedimentation in the Bering Sea is broadly characterized by three main sedimentary phases that occurred between ~5 and ~2.7 Ma, ~2.7 and ~1.74 Ma, and ~1.74 Ma to recent (Fig. F16).

Sediments of middle and early Pliocene age (between ~5 and ~2.7 Ma) were recovered only at Bowers Ridge Sites U1340 and U1341 (Figs. F17, F18). Sedimentation is relatively high during this period (Fig. F5) and is characterized by biogenic diatom ooze with minor amounts of diatom silt, sponge spicules, and vitric ash. Although the Pliocene sediments are commonly bioturbated, distinct intervals characterized by extensive lamination also occur. The oldest laminated intervals (<3.8 Ma) were observed at Site U1341. Stratigraphic intervals where laminations are concentrated are possibly indicative of the depth of the OMZ. Isolated IRD pebbles were observed in sediments older than 3.8 Ma only at Site U1340. Limited dropstone occurrence prior to 2.7 Ma was also reported at two sites drilled in the northern Pacific during Leg 145 (Sites 881 and 883) and in the Yakataga Formation in Alaska (Lagoe et al., 1993), which suggests the development of ice sheets prior to the onset of NHG (Krissek, 1995).

The middle sections of Sites U1340 and U1341 (between ~2.7 and 1.74 Ma) are characterized by beds of diatom ooze with minor amounts of calcareous nannofossils and foraminifer ooze, alternating with diatom silt beds. The latter are composed of subequal proportions of siliciclastic (silt-sized quartz, feldspar, and rock fragments and/or clay) and biogenic (mainly diatom and, secondarily, calcareous nannofossils, foraminifers, silicoflagellates, and sponge spicules) components and minor volcaniclastic components. Dropstone occurrence is common—indicating a peak in siliciclastic deposition that was also observed at Leg 145 sites—and coincides with the beginning of NHG. However, the dramatic drop in opal content recorded at Site 882 (Haug et al., 1999) is not present at the Bowers Ridge sites, where, conversely, the biogenic component and inferred paleoproductivity is high throughout the late Pliocene and Pleistocene.

All sites drilled during Expedition 323 preserve a record of sedimentation ranging from the late Pliocene through the Holocene (between 1.74 Ma and recent). Lithologies and sedimentation rates vary between the different sites, as indicated by a basin-wide comparison of the evolution of sedimentation in the Bering Sea during this period (Fig. F16). The lowest sedimentation rates (only 4.5 cm/k.y.) were observed at Site U1342, where laminated foraminifer-rich diatom ooze beds alternate with silty clay beds at scales ranging in the Milankovitch band (Fig. F19). The same temporal interval corresponds to a much thicker section at Sites U1340 and U1341 (Figs. F17, F18), where the bedding alternations are less distinct and the abundance of IRD is higher. Although lamination is common at Sites U1342 and U1340, laminae are very rare at the deeper Site U1341, suggesting a shallow depth of the OMZ during most of the Pleistocene. At the Bering Sea slope site, sedimentation rates are about three times higher than at the Bowers Ridge sites. At Sites U1339 (Fig. F20), U1343 (Fig. F21), U1344 (Fig. F22), and U1345 (Fig. F23), siliciclastic-rich beds (mostly diatom-rich silty clay) and mixed siliciclastic-biogenic beds (clay, silt, and diatom ooze with varying abundances of foraminifers, nannofossils, and sponge spicules) alternate rhythmically. The sections are pervasively bioturbated, and laminated intervals are rare. Overall, sedimentation on the Bering slope is characterized by a higher influence of both siliciclastic material delivered by ice sheets and terrigenous sedimentation derived from the continental shelf and slope, which are indented by some of the largest submarine canyons in the world. However, because of their proximity to the continents, it is not clear whether the sediments characterized by high siliciclastic content are recording periods of ice sheet expansion (stadials) or increased runoff (interstadials). Dropstones are common at all sites during this time period and increase significantly at ~1 Ma, as was also observed in coeval sediments from the North Pacific based on the results of Leg 145 (Krissek, 1995).

Cyclical siliciclastic deposition and its relationship to climate change

Expedition 323 is the first expedition to recover deep, continuous sections of sediment from the Bering Sea, providing an opportunity to study sediment distributions in time and space in this marginal sea. The plethora of climatic and sedimentary structure information in the cores will allow us to develop an understanding of the influence of climate changes on sedimentation. Clearly, siliciclastics are a dominant component of all Bering slope sites and are less dominant at the Bowers Ridge sites. Because the sediments at each site are primarily composed of mixtures of siliciclastics and diatom frustules, the physical magnetic properties and natural gamma ray (NGR) measurements made on the tracks and with the logging tools generally provide information about the relative proportions of clays/silts versus diatoms (although ash is a significant component of the sediment at Site U1339). NGR records (Fig. F16) indicate clear cyclicity; amplitude and wavelength appear to change markedly with depth at the Bering slope sites but not at the Bowers Ridge sites. Specifically, in sediment younger than ~1 Ma at the Bering slope sites (U1339, U1343, U1344, and U1345), the NGR record appears to vary with lower amplitudes and across a wider range of frequencies >1/40 k.y. compared to NGR records in sediment older than 1 Ma (Fig. F16; see "Natural gamma radiation" in "Physical properties" in the "Site U1343" chapter and "Natural gamma radiation" in "Physical properties" in the "Site U1344" chapter). Because Sites U1339 and U1345 are short records younger than 1 Ma, long-term trends in the character of the variability can only be evaluated in the longer NGR records of Sites U1343 and U1344; however, the relatively high frequency variability of NGR data from the younger sediments is evident at all four slope sites.

This change in the character of variability is also apparent in the logging data from Sites U1343 and U1344 (Fig. F24). The downhole logging data provide our only opportunity to derive continuous records of lithologic variability in the deeper sections where only one hole was drilled and where continuous composites of data generated on the cores cannot be spliced together as they are in sections with multiple drill holes. Potassium concentrations (K%), as measured by the logging tool, are generally related to terrigenous clay concentrations, although variations in the mineralogy of the terrigenous fraction could have a secondary influence. In this sedimentary environment, it appears that NGR represents the amount of terrigenous material relative to biogenic material in the sediment. Although much more research is needed, shipboard data indicate that high siliciclastic relative to biogenic deposition probably occurred during colder periods or possibly during deglaciations.

Applying the shipboard age model to the K% record at Site U1343 and performing spectral analyses on the record (not shown) indicate that variability in K% dominantly occurs with 40 k.y. periodicity in periods older than ~1 Ma. In the portion of the record younger than 1 Ma, the variance is dispersed across many frequencies, and 40 k.y. periodicity does not dominate the spectrum. Benthic δ18O records (Lisiecki and Raymo, 2005) indicate that the variability of ice volume, and therefore of eustatic sea level, occurs primarily with 40 k.y. periodicity prior to ~1 Ma (referred to as the "40 K world"). As such, it appears that sea level may control the timing of pulses of sedimentation at the Bering slope sites. Sea level highstands in warm periods may have been times when sediment deposition on shelves occurred at the expense of deposition on the slope. As ice volume increased, sea level fell, the coastline moved seaward, and the flux of terrigenous material to the slope may have increased. This simple explanation may indicate how global climate change indirectly controlled the timing of sediment deposition at the slope sites during the 40 K world. After 1 Ma, the variability of the sediment composition changed, as reflected by logging K%. NGR core measurements do not appear to be simply related to sea level change, which occurred dominantly with ~100 k.y. variability; rather, variability in sedimentation appears to occur with much higher frequency. It is unclear why this happened, but the increase in sea ice diatom and sea ice dinoflagellate forms (discussed in the next section) at ~1 Ma may indicate that high-frequency variability in siliciclastic deposition related to sea ice processes is important—perhaps more important than global ice volume and concomitant sea level changes, which occur with lower frequency.

A close look at magnetic (Fig. F25) and density (not shown) data provides additional details about processes that may dictate terrigenous deposition. At first glance, the record of relative magnetic paleointensity of the sediment at Sites U1343 and U1344 agrees with the K% records and indicates a change in depositional style and cyclicity at ~1 Ma (Fig. F25), with lower frequency and higher amplitude changes prior to 1 Ma at the Bering slope sites. The ratio of natural remanent magnetization (NRM) to magnetic susceptibility is an index that attempts to normalize intensity by the concentration of magnetic grains. However, changes in grain-size distributions may also impact this ratio; low NRM/magnetic susceptibility values indicate that magnetic grains are coarser than those in sediment with higher NRM/magnetic susceptibility values. Grain-size differences can be related to a variety of climatically controlled factors as well as to diagenetic dissolution of the smallest clay-sized grains. The fact that NRM and magnetic susceptibility both show distinct variability on the same scale representing 40 k.y. cycles suggests that the primary factor controlling magnetic susceptibility and paleointensity records is related to the relative amount of terrigenous material. This also indicates that diagenetic overprints may alter but not obliterate the primary signal. The fact that the amplitude of changes in the paleointensity record (Fig. F25) downcore is even more extreme than that in the NGR and K% data suggests that, if anything, diagenetic overprints may amplify the contrast between sediments with varying amounts of siliciclastics grains.

After 1 Ma, during the time period of large 100 k.y. ice volume cycles, gamma ray attenuation (GRA) bulk density records vary across a spectrum of frequencies. However, these records show long-period variance characteristic of the 100 k.y. ice age cycles more clearly than other physical parameter properties do. For example, after 1 Ma, paleointensity (Fig. F25) and NGR (Fig. F16) variability have power distributed across low (orbital) and suborbital frequencies. This indicates that terrigenous deposition at the Bering slope sites in the late Pleistocene was likely a combination of many processes, such as bottom water current deposition of fine-grained material (drift), IRD from sea ice and icebergs, and mass sediment transfer from the shelf to the lower slope and abyss. Superimposed on this are possibly independent or nonlinearly related changes in biogenic fluxes and postdepositional processes such as diagenesis and precipitation of authigenic minerals. The periodic and stochastic forces that drive each of these processes could be different, resulting in rich records of lithologic variability in the latest Pleistocene.

On Bowers Ridge at Sites U1340, U1341, and U1342, the trends and variability in siliciclastics appear to be notably different than those at the Bering slope sites, as indicated by NGR (Fig. F16), K% (Fig. F24), and NRM/magnetic susceptibility (Fig. F25) records. In contrast to the Bering slope sites, there are some marked long-term trends in the Bowers Ridge records. A notable change in Site U1341 logging K% data occurs at ~425 m depth (Fig. F24), along with an increase in observed silt content and dropstones in the sediment (Fig. F16). This depth in the section is equivalent in time to ~3 Ma and may signify that the Bering Sea regional expression of NHG includes an increase in clays and silts delivered via sea ice or icebergs to Bowers Ridge. The fact that K% decreases above this point in the section even while the ice ages persisted is difficult to explain. One possibility is that the warmer climate of the Pliocene facilitated the formation of K-rich clays on land, which were then shed into the Bering Sea once the ice ages began. K-rich clay formation could have then decreased when the climate cooled. NGR, magnetic susceptibility, and sedimentological smear slide and descriptive data indicate increasing amounts of terrigenous material upsection, with a pronounced increase at ~1 Ma, probably due to a greater supply of ice-rafted siliciclastics to Bowers Ridge once large 100 k.y. glaciations occurred.

As at the Bering slope sites, the variability of physical properties in sediments younger than 1 Ma at Bowers Ridge appears to include higher frequency variability than the 100 k.y. ice volume cycles. Again, higher frequency climate variability, including changes in the distribution of sea ice and icebergs delivering sediment to Bowers Ridge, may dominate the mode of sediment deposition in the Bering Sea after 1 Ma. However, in contrast to the Bering slope sites, the variability of all studied lithologic parameters (NGR, magnetic susceptibility, and GRA bulk density) in sediment older than 1 Ma at Bowers Ridge is not dominantly paced by 40 k.y. ice volume cycles. One explanation is that, even in the 40 K world, sediment delivery to Bowers Ridge was paced at least in part by processes other than sea level change. Spectral analyses of K% data from Site U1341 at Bowers Ridge indicate that variability occurs across a spectrum of frequencies, with notable concentrations of variance at 40 k.y. and 23 k.y. periodicities. During the 40 K world, cycles and episodes of IRD deposition at Bowers Ridge were possibly impacted by regional climate changes responding to both obliquity (40 k.y.) and precession (23 k.y.) solar radiation changes. In contrast, at the Bering slope, sea level variations that occur dominantly with 40 k.y. periodicity were the main factor controlling sediment transport from the continent/shelf to the slope. The observation that high-latitude climate responded to precession forcing during the 40 K world has important implications, not only for theories that explain ice volume changes, but ultimately for our understanding of ice sheet dynamics and our ability to understand ice sheet response to radiative forcing.

History of sea ice development

The investigation of the evolutionary history of climate and surface-ocean conditions is primarily approached by studying microfossils, sedimentology, and the physical properties of sediments. This is particularly true for the influence of sea ice formation and the flow of the relatively warm oligotrophic Pacific surface waters into the Bering Sea.

One of the most striking findings of Expedition 323 is the general sea ice distribution history of the Bering Sea for the past 5 m.y. As described in "Overview of ages and sedimentation rates," the first sign of ice is the presence of a few pebbles, which are thought to have been transported as IRD starting at >3.8 Ma at Site U1340 (Fig. F16), indicating the formation of sea ice or iceberg transportation to the Bowers Ridge region. However, the bulk of the evolution of sea ice distribution has been interpreted from shipboard analyses of sea ice diatoms, sea ice–related dinoflagellate taxa, and, to a lesser extent, other diatom taxa and intermediate water–dwelling radiolarians (see below for details).

The details of sea ice evolution are derived from changes in the relative abundance of sea ice diatom taxa, which are represented mainly by Thalassiosira antarctica spores (Abelmann, 1992a) and sea ice dinoflagellates (Fig. F26). The first signs of sea ice diatoms and dinoflagellates are subtle increases in their abundance at Bowers Ridge at Site U1340, starting at ~3.4 Ma for dinoflagellates and ~2.7 Ma for diatoms, coincident with NHG (Maslin et al., 1996). After ~2 Ma and into the present, sea ice assemblage signals become progressively stronger, reaching ~10%–20%. In contrast to the Bowers Ridge sites, sea ice cover at the Bering slope sites is markedly severe, indicated by sea ice assemblage percentages that are significantly higher than those observed at Bowers Ridge. Sea ice diatom values range from ~10% to 50% during the latest Pliocene and increase from ~30% to 70% during the Pleistocene. The abundance of sea ice dinoflagellates also increases but not to the same extent as sea ice diatoms. Notably, sea ice diatom and sea ice dinoflagellate assemblages clearly show a significant increase in both abundance and amplitude of variability around the mid-Pleistocene Transition (MPT) (Fig. F26). Analogous to sea ice–associated diatom and dinoflagellate taxa, a clear increasing trend in abundance of intermediate water–dwelling radiolarian taxa at the MPT was also observed at Sites U1343 and U1344. This is consistent with the interpretation that the surface water became gradually more affected by the formation of sea ice as climate progressively cooled; in the presence of sea ice, surface-dwelling radiolarians disappeared, and, as a consequence, the relative percentages of intermediate water dwellers such as C. davisiana were higher (Abelmann, 1992b; Okazaki et al., 2003).

At Sites U1343 and U1344, which are closer than the Bowers Ridge sites to the southern boundary of today's seasonal sea ice maximum extent, a dramatic change in the dominance of dinoflagellate cyst assemblages from autotrophic to heterotrophic taxa is evident at ~1.2–1.5 Ma. This suggests that sea ice formation occurred well before the time when the abundance of sea ice taxa significantly increased at ~1 Ma. Heterotrophic dinoflagellates feed mainly on diatoms (Jacobson and Anderson, 1986), and their abundance is most likely the result of food availability; hence, they can persist in ice-laden environments. On the other hand, the low production of their autotrophic counterparts, which dwell mainly in open waters without sea ice, could be related to these taxa being out-competed in the presence of blooming diatom populations in regions of seasonal sea ice cover. Furthermore, heterotrophic dinoflagellate cysts dominate the assemblages in polar areas where sea ice occurs as long as 12 months per year (e.g., Rochon et al., 1999). The sea ice–associated dinoflagellate species Islandinium minutum, known to be very abundant or dominant in the Arctic Ocean and in subarctic regions with pronounced sea ice cover (Head et al., 2001; Hamel et al., 2002), appears to increase at ~1 Ma at Sites U1343 and U1344 (Fig. F26). Along with significant increases in both sea ice dinoflagellates and sea ice diatoms (e.g., Thalassiosira antarctica spores) at ~1 Ma at both sites, a rather significant drop in the abundance of N. seminae, a typical pelagic diatom, also occurs (Fig. F27). All of these biotic events are within the time interval of the MPT, which spans ~1.2–0.8 Ma and marks the transition from 41 k.y. obliquity ice volume cycles to larger ice age cycles that vary at ~100 k.y.

As noted above, the Bowers Ridge and Bering slope regions show distinct differences in the extent of sea ice cover throughout the last ~2.1 m.y., the time period covered by the sedimentary records of both regions. The extent of sea ice cover of the latter is substantially greater than that of the former because of the distal locations from the relatively warm Alaskan Stream subarctic Pacific water entry of the three Bering slope sites, which are most prone to perennial sea ice cover in the Bering Sea. The spatial differences in sea ice cover today are mainly attributed to the surface water circulation pattern (Fig. F2); this spatial difference appears to have persisted since at least 2.1 Ma, implying that surface water circulation patterns have also persisted (see below for details with N. seminae and other taxa). This is further supported by the fact that at Site U1339 on Umnak Plateau, the site closest to an entry point of Alaskan Stream subarctic Pacific water, sea ice taxa suggest lower levels of sea ice cover compared to the Bering slope sites farther north, which are not as heavily influenced by Alaskan Stream water. Furthermore, Katsuki and Takahashi (2005) illustrated that during the low sea level stand of the LGM, the relatively warm subarctic Alaskan Stream water entered the Bering Sea through the western Aleutian passes rather than through the eastern passes, whose shallow water depth would have inhibited flow during times of low sea level. After entry, the Alaskan Stream water turns eastward, flowing past Bowers Ridge and reaching the southeast corner of the Bering Sea close to Site U1339 on Umnak Plateau before turning northwest on its counter-clockwise path. Therefore, the spatial pattern of sea ice and surface water circulation observed in the last glacial period, including the stronger influence of warm Alaskan Stream water at Umnak Plateau, compared to its influence at the northern Bering slope, persists over long timescales.

Changes in biological productivity and the influence of the subarctic Pacific surface watermass flowing into the Bering Sea

Based on the spatial distributions of long-term temporal changes of three diatom taxa (Coscinodiscus marginatus, Neodenticula spp., and Actinocyclus curvatulus), it is clear that the influence of subarctic Pacific (Alaskan Stream) waters flowing into the Bering Sea, which are relatively warm and less eutrophic than Bering Sea waters, has typically been strongest at the Bowers Ridge sites, followed by the Umnak Plateau site. The weakest influence of this relatively warm watermass occurred at the Bering slope sites (Fig. F27). This is the same pattern found by Katsuki and Takahashi's (2005) study of past watermass circulation patterns, in which they inferred sea ice distributions during the last glacial period. The longer records from Expedition 323 indicate that, as climate cooled through the Pleistocene, pelagic water influence at all sites progressively weakened. Furthermore, the sites closest to straits through which pelagic water flows into the Bering Sea have consistently higher abundances of subarctic pelagic diatom species than those downstream in the counter-clockwise circulation pattern of the surface watermasses.

From the bottom of the holes upward at the Bowers Ridge sites, a marked drop in C. marginatus was seen at ~3 Ma at Site U1341 and at ~2.6 Ma at Site U1340 (Fig. F27). This can be interpreted as resulting from a sharp reduction in nutrient supply due to the development of upper layer stratification. It is apparent that the diatom taxon C. marginatus requires a relatively high nutrient supply and tolerates low light intensity. This is substantiated by the fact that today this diatom taxon (1) dwells in the lower euphotic zone off of Spain (Nogueira et al., 2000; Nogueira and Figueiras, 2005) and (2) occurs during early winter (~November–January) in the subarctic Pacific and in the Bering Sea based on time-series sediment trapping (Takahashi, 1986; Takahashi et al., 1989; Onodera and Takahashi, 2009). Thus, a high nutrient supply demand and low light intensity demand is apparent for this taxon. In addition, heavily silicified species persist in glacial-like Pleistocene sediments at the Bering slope sites, supporting the contention that temperature is not the principal limiting factor. This timing of 3–2.6 Ma coincides approximately with the end of the so-called "opal dump" observed in the subarctic Pacific at ~2.7 Ma, which is coincident with the onset of NHG (Maslin et al., 1996). Although the reduction in C. marginatus persisted around the time of NHG, an overwhelmingly continuous presence of diatom ooze and interbedded diatom ooze and silt sediments accumulated throughout the Pliocene–Pleistocene in the Bering Sea. This clearly suggests that a high amount of opal sedimentation continued after the onset of NHG well into the Pleistocene.

The 5 m.y. long-term trend of Neodenticula (Neodenticula kamtschatica, Neodenticula koizumii, N. seminae, and Neodenticula sp.) in the Bowers Ridge region shows the following two patterns. Slightly higher percentages were observed from the bottom of the hole until ~2.9–2.7 Ma compared to the sections above, particularly at Site U1340, but generally Neodenticula is an important component of the assemblage throughout. On the other hand, the abundance of Actinocyclus spp. (Fig. F27) clearly increases through the Pliocene, suggesting changes in stratification/watermass circulation as the colder climate developed. This is generally true for Sites U1340 and U1341 at Bowers Ridge and for Site U1343 off of the Bering slope. Data from Site U1344, however, provide a more complicated picture than this and do not show a clear trend, probably because of the overwhelmingly high abundance of sea ice–associated diatoms such as T. antarctica spores (Abelmann, 1992a) (~25% during 4–0.9 Ma; ~40% during 0.9–0 Ma) (Fig. F27). Both at Bowers Ridge and at the Bering slope, marked declines were seen for both taxa (Actinocyclus spp. and N. seminae) around the MPT (Fig. F27), although the exact timing of the decline is site and taxa specific. Nevertheless, the upheaval of both taxa at the MPT is notable. During intervals of high eutrophic levels, suggested by high Actinocyclus spp., cooler, low trophic subarctic Pacific waters, indicated by N. seminae, were introduced into the Bering Sea. This is based on the geographic and seasonal distributions of A. curvatulus, which appears to be ecologically similar to Thalassiosira latimarginata s.l. and, to a lesser extent, N. seminae (Sancetta, 1982; Takahashi, 1986; Takahashi et al., 1989). As surface waters became increasingly stratified, especially after ~0.9 Ma with Milankovitch-scale 100 k.y. climatic cyclic regimes, N. seminae declined with the emerging sea ice diatoms.

Changes in bottom water and intermediate-water conditions

Much of the Pacific water entering the Bering Sea is matched by outflow through the Aleutian Islands. The most significant outflow is through the Kamchatka Strait, which has a maximum water depth of 4420 m. If some component of North Pacific intermediate or deep water formed in the Bering Sea in the past, particularly when sea level was lower, it would have flowed out through the Kamchatka Strait or a secondary outlet near the Commander–Near Strait at 2000 m (Fig. F1). One of the scientific objectives of Expedition 323 was to elucidate the history, temporal variability, and intensity of NPIW and deepwater formation in the Bering Sea and its links to surface water processes. This objective was partially fulfilled during the expedition but would have been more adequately addressed had we obtained permission to drill two important sites located in Russian territorial waters, Shirshov Ridge Site SHR-3B at 2232 m water depth and Site KST-1B inside the Kamchatka Strait at 3435 m water depth (Takahashi et al., 2009). The main objective at these sites was to monitor deepwater masses flowing out of the Bering Sea to the Pacific Ocean. Nevertheless, seven sites were successfully cored, and complete records of environmental variability of intermediate-water and deepwater sites spanning the Pleistocene to the Pliocene were recovered during Expedition 323. The sites range from 818 to 3174 m and allow for the characterization of past vertical watermass distribution and for the reconstruction of the history and distribution of the OMZ in the region (Fig. F4).

Previous observations made in glacial records from the Bering Sea and just outside the Bering Sea on the Detroit Seamount in the North Pacific suggest a source of ventilated intermediate water coming from the Bering Sea and/or the Detroit Seamount region (Gorbarenko, 1996). Furthermore, other paleoceanographic reconstructions of the Bering Sea, made possible by the Hakuhou-Maru piston coring survey cruise in 1999, show past NPIW formation during four different time slices, reconstructed based on the high-oxygen indicator and intermediate water–dwelling radiolarian species C. davisiana. According to these studies, the role of the Bering Sea in NPIW formation is apparently visible during the cold intervals (Tanaka and Takahashi, 2005).

Shipboard analyses of sediment samples recovered during Expedition 323 show a continuous recovery of Pliocene to Holocene deep-sea benthic foraminifers and midwater radiolarians at all sites (Figs. F28, F29), although calcareous benthic foraminifers appear to be rare in the middle and early Pliocene. The benthic foraminifer faunal composition displays large assemblage changes and shifts in species dominance. These changes are likely related to variability in local oxygen concentration in the bottom waters associated with surface water productivity and/or deepwater ventilation, possibly on Milankovitch and other timescales. For example, Bulimina aff. exilis, a common species in the Bering Sea samples, is generally regarded as a low oxygen/deep infaunal species and has been found in samples associated with high productivity and low sea ice (e.g., Bubenshchikova et al., 2008; Kaiho, 1994).

Bering Sea benthic foraminifers show affinities to assemblages found within or near the OMZ in the Sea of Okhotsk and also to more common deep Pacific Ocean assemblages. The faunal similarity between the two marginal seas and the deep Pacific Ocean will allow for reconstruction of the physico-chemical characteristics of deep and intermediate watermasses as a result of changes in surface productivity, deepwater ventilation, sea ice coverage, and continental glaciation.

Previous piston core studies showed a significant intensification of the OMZ during the last deglacial at Umnak Plateau (Okazaki et al., 2005), suggesting a relationship between productivity and terrestrial nutrient supply from melting. However, there is no information regarding the longer timescale relationship through the Pleistocene. Analysis of faunas from the newly drilled Bering Sea sites will be particularly important in extending this record through the entire Pliocene (at Bowers Ridge) and Pleistocene (at Bowers Ridge and the Bering slope) and will allow us to decipher the onset and evolution of the OMZ and provide further insight into NPIW production at this marginal sea. Furthermore, Site U1344 (~3200 m) is presently located below the OMZ and also has the potential to monitor past deepwater changes.

A striking finding of the expedition was the relatively low oxygen content of intermediate watermass conditions at most sites during the last 5 m.y., as indicated by the presence of episodic laminated sediment intervals throughout the entire sections, particularly in the late Pliocene and Pleistocene. The benthic foraminifer faunas recovered from deep cored Sites U1340 and U1341 generally support this, indicating that Bering Sea intermediate and deep waters were better oxygenated in the early Pliocene, as suggested by the presence of the agglutinated foraminifer Martinottiella communis (Fig. F29), and less oxygenated during the last ~2 m.y., as suggested by the increase in deep infaunal benthic foraminifers (Fig. F28). This assemblage is composed of abundant calcareous benthic genera that are typically indicative of reduced-oxygen conditions (Bubenshchikova et al., 2008).

High sedimentation-rate Sites U1339 and U1345, located within the OMZ today (Fig. F4), reveal high-amplitude variability in the relative abundance of the deep infaunal assemblage since 0.8 Ma. This appears to be associated with interglacial–deglacial cyclicity, showing a higher abundance of deep infaunal species (reflecting the lowest bottom water oxygen conditions) during interglacials, in particular strong interglacial marine isotope Stages (MIS) 1, 5, and 11. Higher bottom water oxygen conditions appear to correlate with glacials. Furthermore, Sites U1340, U1343, and U1344 contain well-preserved foraminifers over the last 2 m.y., with increasing absolute abundances of benthic and planktonic taxa across the MPT (~0.8–1.1 Ma), in association with an increase in abundance of the polar planktonic foraminifer Neogloboquadrina pachyderma (sinistral). This cooling trend was also observed as an increase in the abundance of sea ice dinoflagellates and diatoms and intermediate water–dwelling radiolarians (e.g., C. davisiana). Cooling of the surface waters would have enhanced ventilation of the intermediate waters during glacials and increased density stratification during interglacials, contributing to the drop in dissolved-oxygen content in the intermediate and bottom waters at these times, which is suggested by the increase in deep infaunal benthic foraminifer taxa.

Microbiology in high–surface-productivity environments

The microbiological objectives of Expedition 323 were to constrain global models of subseafloor biomass and microbial respiration by quantifying subseafloor cell abundance and interstitial water chemistry in an extremely high productivity region of the ocean. We also sought to determine how subseafloor community composition is influenced by high productivity in the overlying water column.

To meet these objectives, high-resolution sampling for microbiological analyses and interstitial water chemistry took place at five sites in the Bering Sea. Each site was selected based upon its distance from land and its marine productivity, as determined by annual chlorophyll-a concentrations in the water column.

Four cores were drilled using the APC system to ~40 mbsf at each microbiology-dedicated hole. Contamination tests using perfluorocarbon tracer (PFT) and fluorescent beads (Smith et al., 2000) were performed in each hole, showing that contamination from drilling fluid was insignificant.

Sediment samples were taken for determining the cell abundance (a proxy for microbial biomass) and diversity of the microbial community. In general, these samples were taken every 25 cm for Sections 1H-1 through 2H-3 (or 2H-6) and every 75 cm for Core 3H. Below Core 3H, the sampling resolution was once per core. Interstitial water whole-rounds adjacent to microbiology whole-rounds were taken at the same resolution. In addition, 10 cm whole-rounds were taken for interstitial water and microbiological analyses once per core until APC refusal. Interstitial water sampling continued at this resolution in XCB cores, and microbiology samples for prokaryotic cell abundance, diversity, and community structure were collected at ~400 and ~700 mbsf. PFT analyses showed no drilling fluid contamination at these depths.

Interstitial water samples were extracted on board, and aliquots were fixed for shore-based and shipboard analyses. Shipboard aliquots were used to determine concentrations of dissolved inorganic carbon (DIC), alkalinity, sulfate, sulfide, ammonium, phosphate, major ions (e.g., Ca, Na, and K), and minor ions (e.g., Fe and Mn). These data, along with formation factor, allow for a quantitative determination of microbial respiration rates (D'Hondt et al., 2002).

Geochemical data obtained during the expedition show that microbial activity along the slope sites (i.e., Sites U1339, U1343, U1344, and U1345) is substantially higher than at Site U1342 on Bowers Ridge. At the slope sites, the concentrations of microbial respiration products such as DIC, ammonium, and phosphate are approximately an order of magnitude higher than at Bowers Ridge (Fig. F30). A shallow sulfate–methane transition zone (SMTZ) (~6–11 mbsf) is present (Fig. F30) at these sites, highlighting the importance of methanogenesis and sulfate reduction coupled to the anaerobic oxidation of methane (AOM) for sediment geochemistry. Interstitial water data suggest the presence of microbially mediated Fe- and Mn-oxide reduction. Interestingly, the geochemical profiles suggest significant microbial activity as deep as 700 mbsf. In contrast, at Bowers Ridge, sulfate concentrations decrease subtly and penetrate to the basement at Site U1342, suggesting very low rates of microbially mediated sulfate reduction and the absence of AOM at present. The difference in microbial activity at these sites might be caused by different rates of water column productivity and sedimentation.

We expect that the differences in geochemistry between the slope and ridge sites will be reflected in microbial cell abundance and diversity. Specifically, we expect elevated cell abundance and a consortium of sulfate-reducing bacteria and methane-oxidizing archaea in the SMTZ.