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doi:10.2204/iodp.proc.323.104.2011

Biostratigraphy

Core catcher samples from Site U1340 are dominated by diatom microfossil assemblages with high diversity. Samples also contain assemblages of radiolarian, calcareous nannofossil, foraminiferal, and organic-walled microfossils with medium to high diversity. The preservation of different microfossil groups ranges from very good to moderate. Biostratigraphic datums are derived from diatom, radiolarian, dinoflagellate, ebridian, and silicoflagellate bioevents and show that Site U1340 contains Pliocene to recent sediments. In total, 17 bioevents were identified in Hole U1340A (Fig. F19; Table T2). Age estimation below 2 Ma (~300 mbsf) is difficult because few well-confirmed biostratigraphic datums were identified. Our strategy to establish a preliminary age framework was based on percent Neodenticula species patterns (diatoms) compared with those at Site U1341 and tied with paleomagnetic datums (Fig. F20; see "Diatoms"). Samples 323-U1340A-1H-CC through 30H-CC (~1.8 Ma) exhibit a broadly linear sedimentation rate of ~15 cm/k.y. and rapidly increase to ~28 cm/k.y. between Samples 323-U1340A-30H-CC and 63H-CC. The sedimentation rate below Sample 323-U1340-63H-CC is apparently very low (~4 cm/k.y.) according to this preliminary age model. Siliceous microfossils occur consistently throughout the section and are mainly composed of high-latitude pelagic species that indicate changes in surface water productivity. Calcareous microfossils are mostly confined to the top of the section (approximately Samples 323-U1340A-23H-CC and above for nannofossils and 31H-CC and above for foraminifers). Below this level, only sporadic occurrences of calcareous cemented agglutinated foraminifers were detected. Benthic foraminifers are largely characteristic of those found within or near the oxygen minimum zone (OMZ) in high-latitude regions. Dinoflagellates consistently occur throughout the section, indicating changes to the productivity and sea ice cover of the surface waters.

Calcareous nannofossils

The existence, abundance, and preservation of calcareous nannofossils in the sedimentary sequences recovered at Site U1340 were determined by examining all core catcher samples from Holes U1340A–U1340D. Additional analyses of the composition of calcareous nannofossil assemblages and the abundance of individual taxa were performed simultaneously. The recorded data are shown in Table T3 (see "Biostratigraphy" in the "Methods" chapter for a definition of the codes used).

Calcareous nannofossils are present in the upper third of the sediment record of Site U1340 (Samples 323-U1340A-1H-CC through 23H-CC and Holes U1340B–U1340D), but their presence is not continuous and some samples are barren. The remaining samples recovered from Hole U1340A are devoid of calcareous nannofossils, with the exception of Samples 323-U1340A-42H-CC and 43X-CC, which yielded few specimens. Preservation of the observed specimens ranges from good to moderate in Hole U1340A and from good to moderate-good in Holes U1340B–U1340D. In all holes, assemblages are mainly dominated by Coccolithus pelagicus, although small gephyrocapsids are more abundant in Samples 323-U1340B-3H-CC and 6H-CC, Hole U1340C, and Sample 323-U1340D-3H-CC. In Holes U1340C and U1340D, the total abundance of small gephyrocapsids exceeds 50 specimens per field of view (FOV). Reworked specimens, mostly of Miocene and Paleogene ages, were found in some samples.

A chronology based on calcareous nannofossil datums was not achieved at Site U1340 because of discontinuous records that prevented the full allocation of datums to specific samples. Emiliania huxleyi was identified in Samples 323-U1340B-1H-CC and 4H-CC, 323-U1340C-2H-CC, and 323-U1340D-1H-CC and 3H-CC. In these three holes, the first occurrence (FO) datum of E. huxleyi (0.29 Ma), which marks the base of nannofossil Zone NN21 (Martini, 1971), could not be reasonably constrained because the holes were terminated before the end of this zone. All samples from Holes U1340B–U1340D were assumed to occur within Zone NN21. Specimens of Pseudoemiliania lacunosa were observed in Samples 323-U1340A-7H-CC, 15H-CC, 17H-CC, 19H-CC, and 23H-CC, but only Sample 323-U1340A-7H-CC was considered to occur in Zone NN19. This zone is defined at its top by the last occurrence (LO) datum of P. lacunosa at 0.44 Ma and at its bottom by the LO of Discoaster brouweri at 2.06 Ma. Because no specimens of D. brouweri were found in Hole U1340A, the bottom of Zone NN19 could not be constrained and the location of Samples 323-U1340A-8H-CC through 23H-CC could not be determined. All extant taxa present at Site U1340 are characteristic of the subarctic and transitional coccolithophore zones in the Pacific (Okada and Honjo, 1973).

Planktonic foraminifers

Eighty-three core catcher samples from the >125 µm fraction from Holes U1340A–U1340D were analyzed for planktonic foraminifers (Table T4). In addition, the >63 µm fraction of the mudline sample from the top of Core 323-U1340A-1H was analyzed. Most core catcher samples and the mudline sample are dominated by diatoms and siliciclastics. Sponge spicules are also present in the samples. However, core catcher samples from Cores 323-U1340A-1H, 9H, and 10H; Cores 323-U1340B-2H and 6H; and Core 323-U1340C-3H are dominated by planktonic foraminifers with low abundances of diatoms and siliciclastics. Planktonic foraminifers disappear downcore from Core 323-U1340A-24H only to reappear in Cores 323-U1340A-31H, 42H, and 43H. The fauna does not change significantly throughout the Pleistocene (above 200 mbsf) and is dominated by Neogloboquadrina pachyderma (sinistral), a subpolar–polar species (Bé and Tolderlund, 1971). This species is also the dominating species in the water column of the Bering Sea today (Asahi and Takahashi, 2007). The faunal assemblages also include the subpolar species Globigerina bulloides, Globigerina umbilicata, and Neogloboquadrina pachyderma (dextral). For the duration of the scattered reappearances of the planktonic fauna in the early Pleistocene–late Pliocene (below 200 mbsf), only these subpolar species are present. The distribution of N. pachyderma (dextral) is controlled by temperature (Bé and Tolderlund, 1971). Globigerina bulloides and G. umbilicata are controlled both by temperature and food availability (Reynolds and Thunell, 1985), but studies in the Bering Sea indicate that these species are also mostly controlled by temperature in this region (Asahi and Takahashi, 2007).

Benthic foraminifers

More than 40 species of benthic foraminifers were recovered in 83 samples (>63 µm fraction) from Holes U1340A–U1340D (Tables T5, T6, T7, T8). The majority of core catcher samples down to Sample 31H-CC (Assemblage I) in all holes contain abundant calcareous foraminifers. Calcareous foraminifers from this assemblage resemble species from OMZs on Umnak Plateau and in the Sea of Okhotsk (Bubenshchikova et al., 2008), as well as more common deepwater Pacific Ocean species (Butt, 1980). Abundance and diversity fall markedly below Sample 323-U1340A-32H-CC (282.17 mbsf), below which samples are either barren or consist of a monospecific agglutinated assemblage.

Assemblage I (Islandiella norcrossi–Uvigerina aff. peregrina)

Assemblage I is varyingly dominated by Bulimina aff. exilis, Brizalina earlandi, Epistominella pulchella, Globobulimina pacifica, Uvigerina aff. peregrina, and Islandiella norcrossi in Samples 1H-CC to 31H-CC in all holes. Many other species occur only occasionally, with diversity generally ranging from 5 to 15 species per sample. Variation in species dominance is likely controlled by oxygen and nutrient flux to the seafloor and may be linked to glacial–interglacial cycles, although further high-resolution work is required.

Assemblage II (depauperate)

The major characteristic of Assemblage II is the high number of barren samples between Samples 32H-CC and 55H-CC in all holes. Occasional occurrences of Eggerella bradyi, Globocassidulina sp., and Gyroidinoides soldanii are present. This interval may have been affected by dissolution because other calcareous microfossil groups are largely absent.

Assemblage III (Martinottiella communis)

This low-abundance assemblage is characterized by the more or less continuous and singular occurrence of Martinottiella communis in Samples 56H-CC through 71H-CC in all holes. This species is cosmopolitan in the modern ocean and dominant in the low-oxygen intermediate water (300–700 m) along the Pacific coast of Japan (Kaiho and Hasegawa, 1986). This interval may have suffered from some dissolution, but M. communis has calcareous cement, and preservation of this species is fair to good.

Ostracodes

Core catcher samples were examined for the presence of ostracodes, but no specimens were found.

Diatoms

Diatom biostratigraphy is based on analysis of core catcher samples from Hole U1340A. Depth positions and age estimates of biostratigraphic marker events are shown in Table T9. Diatoms are the dominant microfossil in all holes and show good preservation throughout. At lower depths, gaps exist in the record because of poor core retrieval during XCB coring.

Biostratigraphic zonation was constructed as far back as the early Pliocene for Hole U1340A. The LOs of Proboscia curvirostris, Thalassiosira jouseae, and Proboscia barboi were identified in Sample 323-U1340A-5H-CC (42.35 mbsf), giving a composite estimated age of 0.3 Ma. The LOs of P. curvirostris and T. jouseae were set at 0.26 and 0.31 Ma, respectively, based on a piston core recovered from the northernmost Emperor seamount (K. Katsuki and K. Takahashi, unpubl. data). However, their datums are defined as 0.3 Ma in this report because both datums co-occur in Core 323-U1340A-5H and have also been recorded at other IODP Expedition 323 sites. This age estimation is the same as previous results from the northern subarctic Pacific and around Japan (Barron and Gladenkov, 1995; Yanagisawa and Akiba, 1998). Hence, estimated diatom datums are essentially valid for age determination at this site, although minor datum revisions at the 0.01 m.y.-scale resolution are needed in the near future. Cores above Sample 323-U1340A-5H-CC were assigned to the Neodenticula seminae Zone of 0.3 Ma and younger.

An age of 0.9 Ma was assigned to Sample 323-U1340A-18H-CC (165.89 mbsf) by the LO of Actinocyclus oculatus, which is followed by the first common occurrence (FCO) of P. curvirostris at 1.7–2.0 Ma in Sample 323-U1340A-23H-CC. The last common occurrence (LCO) of Neodenticula koizumii was found in Sample 323-U1340A-25H-CC (215.65 mbsf), giving an age of 2.1 ± 0.1 Ma. The FCO of N. koizumii was found in Sample 323-U1340A-59X-CC (515.1 mbsf), and the LCO of Neodenticula kamtschatica was found in Sample 323-U1340A-57X-CC (2.7 Ma; Barron and Gladenkov, 1995; Yanagisawa and Akiba, 1998). The interval between Samples 323-U1340A-57X-CC and 59X-CC possibly corresponds to North Pacific Diatom (NPD) Zone 8 of ~2.7–3.9 Ma. Although N. kamtschatica occurs earlier, beginning in Sample 323-U1340A-46X-CC, specimens are sporadic and become consistent only in Sample 56X-CC. Indeed, the LCO of N. kamtschatica was used to define this zone.

The FCO of N. seminae was initially determined in Sample 323-U1340A-38H-CC, giving an age of 2.7 ± 0.1 Ma (Barron and Gladenkov, 1995). The distinction between N. seminae and N. koizumii in the light microscope is problematic and is discussed in greater depth by Yanagisawa and Akiba (1998). The distinguishing feature between N. seminae and the intermediary species Neodenticula sp. A (which is now defined as N. koizumii: Yanagisawa and Akiba, 1998) under a light microscope is the presence of a closed versus an open copula. Therefore, samples were revised to verify the occurrence of N. koizumii and N. seminae by counting the presence of closed and open copulas (see "Biostratigraphy" in the "Methods" chapter for further discussion). The datums and subsequent age model were further confirmed by comparison of the relative abundance of N. seminae and N. kamtschatica in Holes U1340A and U1341B (Fig. F20). This correlation technique using tie points to match similar events in the two records was applied to confirm and support the age models at Sites U1340 and U1341 following the difficulties encountered with the identification of the FO and LO of N. seminae and N. kamtschatica. It is evident in Hole U1341B that N. seminae appears sporadically in older sediments but reveals a rapid increase (RI) that is mirrored in Hole U1340A. A similar correlation occurs for N. kamtschatica at both Sites U1340 and U1341, whereby an abrupt decrease in abundance (or rapid decrease [RD]) in Hole U1340A is matched in Hole U1341B.

The newly termed RI of N. seminae abundance co-occurs with the top of a paleomagnetic datum—the Olduvai section—giving a date of 1.778 Ma to Sample 323-U1341B-22H-CC. This event was matched in Hole U1340A, and the same datum was extrapolated for Hole U1341B (see "Biostratigraphy" in the "Site U1341" chapter for further discussion). The RD of N. kamtschatica was also matched to Hole U1340A, where the abrupt decrease coincides with the paleomagnetic Gauss Event, giving a subsequent datum of 2.581 Ma (see "Biostratigraphy" in the "Site U1341" chapter for further details).

The FCO of N. koizumii and the dominance of N. kamtschatica above Sample 323-U1340A-56X-CC define this zone as Subzone NPD7Bb with an age of 3.9 Ma. The absence of Rouxia californica and second-order species Cosmiodiscus insignis, Thalassiosira praeoestrupii, and Thalassiosira temperei signify that the bottom of Hole U1340A does not enter Subzone NPD7Ba (5.5 ± 0.1 Ma). The continued presence of Thalassiosira latimarginata s.l. until Sample 323-U1340A-71X-CC also eliminates the possibility that these species are absent because of low abundances or as a consequence of limited sample volume for slide preparation.

Diatom assemblages are mainly composed of pelagic species N. seminae, Actinocyclus curvatulus group, and Thalassiosira spp. (Thalassiosira antarctica spores, T. latimarginata group, and T. oestrupii) throughout the Pleistocene and upper Pliocene. The assemblage shifts during the late Pliocene (around Sample 323-U1340A-39H-CC) to an assemblage dominated by Stephanopyxis horridus and N. koizumii. Several significant abundance peaks of Thalassiothrix longissima, a high-productivity indicator, occur throughout the upper and lower Pliocene. In general, few freshwater diatoms or coastal water diatoms, including Chaetoceros spores, were observed below the upper Pleistocene, which may be attributed to this site's distal location to continental influence. At this site there is a clear shift from a lower diversity, heavily silicified assemblage (e.g., Coscinodiscus marginatus, Stephanopyxis spp., and T. longissima) to a more diverse species composition at ~260 mbsf, before the LCO of N. koizumii.

Silicoflagellates and ebridians

In general, silicoflagellate and ebridian abundances at this site are lower than those at Site U1339 (Table T10). This is most likely because (1) their main habitats are in coastal waters rather than pelagic waters and (2) they are diluted by mass diatom occurrences. Therefore, the slide preparation procedure used for Site U1339 was changed for Site U1340, as described in "Biostratigraphy" in the "Methods" chapter.

Silicoflagellate and ebridian counting at this site was conducted on samples from Hole U1340A. Based on the silicoflagellate and ebridian zonation established by Ling (1973, 1992), the following datum events were located: the LO of silicoflagellate Dictyocha subarctios and the LO of ebridian Ebriopsis antiqua antiqua. The youngest datum, LO of silicoflagellate Distephanus octonarius, was not defined because its LO in Sample 323-U1340A-2H-CC is too young. The LO of Dictyocha subarctios was estimated to occur between Samples 323-U1340A-12H-CC and 13H-CC (108.78–118.63 mbsf). The Brunhes/Matuyama Chron boundary in this hole was found in Section 323-U1340A-16H-3 (134.9–136.4 mbsf). The LO of Dc. subarctios was estimated at 0.74 Ma based on a previous study in the Sea of Japan (Ling, 1992). In this hole, the LO of Dc. subarctios may be revised to 0.60–0.65 Ma if the linear sedimentation rate is applied from the top to the depth of the Brunhes/Matuyama boundary. The LO of E. antiqua antiqua (2.47–2.48 Ma) was found between Samples 323-U1340A-34H-CC and 35H-CC (300.36–310.59 mbsf). Although this taxon was observed in Sample 323-U1340A-12H-CC, it is considered reworked. The bottom of this hole reaches ~4.0 Ma based on micropaleontological and paleomagnetic results. However, characteristic species and their datum events were not defined for sediments older than 2.5 Ma.

Because of limited datum points, silicoflagellate and ebridian zonation is undefined for most of the cored interval in this hole. Sample 323-U1340A-12H-CC and the above interval are assigned to the Distephanus octangulatus Zone or the Ds. octonarius Zone. Zonation from Samples 323-U1340A-13H-CC to 34H-CC is undefined, but this interval may be correlated to the Dc. subarctios Zone and the Ammodochium rectangulare Zone. The interval from Sample 323-U1340A-35H-CC to the bottom of the hole is also undefined because the characteristic species Distephanus jimlingii needed for the zone definition is absent except for one specimen in Sample 323-U1340A-36H-CC. This undefined zone may be correlated to the E. antiqua antiqua Zone and the Ds. jimlingii Zone based on the age-depth profile of this hole.

Radiolarians

Radiolarian biostratigraphy is based on analysis of core catcher samples from Hole U1340A. Radiolarian stratigraphy at Site U1340 includes the Botryostrobus aquilonaris Zone (upper Quaternary) to the Dictyophimus bullatus Zone (middle Pliocene) in the subarctic Pacific (Kamikuri et al., 2007). However, the Stylatractus universus Zone (between 0.4 and 0.9 Ma) is missing because of the absence of S. universus, which may be attributed to very low species occurrence or a sedimentary hiatus. In addition, review of the Cycladophora sakaii Zone in the Bering Sea may be necessary because the occurrence of typical C. sakaii in Hole U1340A is very rare. Eleven radiolarian datums derived in the subarctic Pacific were identified at this site (Table T11). These radiolarian datums indicate unusual deposition, with extremely high sedimentation rates below ~500 mbsf that are not consistent with other datums. The LO of D. bullatus (3.8–4.0 Ma) was identified between Samples 323-U1340A-58H-CC and 59H-CC. However, its form is not typical and is similar to the Dictyophimus sp. B of Motoyama (1996). Thecosphaera akitaensis (FO: 3.8–4.0 Ma), Ceratospyris borealis (FO: 4.7–5.2 Ma), and Spongurus pylomaticus (FO: 5.2–5.5 Ma) were found in Sample 323-U1340A-71X-CC. Although the datum of T. akitaensis is not widely confirmed in the subarctic Pacific, we can safely say that the core bottom is younger than 5 Ma. Further investigations for the datums of the FO of T. akitaensis and LO of D. bullatus are necessary. Radiolarian abundances and preservation are consistent (Table T12), with abundant to common abundances and good to moderate preservation in the upper interval (above ~200 mbsf) and few abundances with moderate to poor preservation in the lower interval (below ~200 mbsf).

Palynology: dinoflagellate cysts, pollen,
and other palynomorphs

Palynological assemblages were examined in core catcher samples from Hole U1340A (Table T13). Heavy liquid separation was systematically used because of abundant diatoms and siliciclastic debris. Although this method resulted in relatively clean residues for palynological analysis, it may have biased abundance estimates.

All samples contain poorly to well-preserved palynomorphs. The concentration of terrestrial palynomorphs is low to moderate in most samples (Table T13). Pollen grains and spores are present throughout the sequence and are composed mainly of Picea, Pinus, and Sphagnum. Their concentration is usually <200 grains/cm³, suggesting minor input through atmospheric or ocean circulation. However, reworked pre-Neogene palynomorphs, including pollen, spores, and dinocysts, are relatively abundant in most samples (Table T13), with concentrations reaching >1000 grains/cm³ at 100–156 mbsf and at 400 mbsf. They are generally accompanied by high numbers of wood microfragments. The abundance of reworked palynomorphs indicates detrital inputs originating from older sediments. Freshwater palynomorphs (Pediastrum, Botryococcus, and tintinnids) occur only in the upper part of the sequence from 42 to 186 mbsf. Organic linings of benthic foraminifers are common in most samples, suggesting low to moderate carbonate dissolution.

Dinoflagellate cysts occur in all samples, with concentrations ranging between 55 and 22,000 cysts/cm³. They are very abundant only in the uppermost 200 m of the sequence (Table T13). The assemblages are mainly dominated by the protoperidinial heterotrophic Brigantedinium spp., which can be associated with high productivity and upwelling. The gonyaulacale Operculodinium centrocarpum co-dominates the assemblage with Brigantedinium spp. in Sample 323-U1340A-1H-CC. Operculodinium centrocarpum is one of the most ubiquitous species in the Northern Hemisphere, but its distribution in surface sediments of the Bering Sea is closely related to seasonal sea ice cover (Radi et al., 2001). All accompanying taxa except the extinct species Filisphaera filifera are known to be abundant in polar and circumpolar regions.

The presence of F. filifera in Sample 323-U1340A-25H-CC indicates an age of 1.7 Ma (LO) at 215.6 mbsf. The stratigraphic range of this species (Miocene to lower Pleistocene; LO at ~1.7 Ma) was established by Matsuoka (1983) in the Niigata Basin (Japan) and by Bujak (1984) in the Bering Sea (DSDP Site 185). However, in the eastern North Atlantic the LO of F. filifera was established at ~1.41 Ma at DSDP Site 610 (De Schepper, 2006) and 1.4 Ma in the Norwegian-Greenland Sea (M. Smelror et al., unpubl. data).

Discussion

Quantitative and semiquantitative data were collected for each microfossil group at Site U1340 (Fig. F21). Diatoms are by far the most common microfossils present and show that older sediments below ~500 m CCSF-A (~2.75 Ma) contain no sea ice species. Heavily silicified valves below ~250 m CCSF-A suggest that nutrient availability may have been higher in the past, allowing valves to become more heavily silicified. The open water species Neodenticula spp. is also more prevalent at this time. Although its ecological preference is not well constrained, the presence of benthic foraminifer species M. communis above ~500 m CCSF-A (~2.75 Ma) suggests that a different bottom water nutrient regime was in place because this species is not found in younger Bering Sea sediments. Today, M. communis is found dominating the low-oxygen zone along the Pacific coast of Japan (Kaiho and Hasegawa, 1986). Calcareous microfossil preservation is poor below ~250 m CCSF-A, suggesting dissolution as the primary cause.

Above ~250 m CCSF-A, sea ice diatoms increase, and the absence of heavily silicified diatoms indicates that nutrients may have become less available after this time, perhaps via enhanced stratification from sea ice. Temperature does not appear to be the limiting factor because the heavily silicified C. marginatus valves persist in Pleistocene sediments at the gateway sites (IODP Sites U1343, U1344, and U1345). The general co-occurrence of intermediate water–dwelling radiolarian species and sea ice diatoms provides evidence for elevated intermediate water production at this time. Benthic foraminifers above ~250 m CCSF-A are indicative of low-oxygen environments, suggesting that bottom waters were affected by high organic carbon export production and/or low bottom water ventilation. The absence of warmer water planktonic foraminifers above ~200 m CCSF-A indicates that surface water cooling occurred above this interval.

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