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doi:10.2204/iodp.proc.323.109.2011

Biostratigraphy

Diatoms dominate the microfossil assemblage; however, diatom abundance and preservation are relatively variable, with high to moderate abundance and good to poor preservation. Other siliceous microfossils, radiolarians, and silicoflagellates are also present throughout, with low abundance and poor preservation. These fossils are mainly composed of high-latitude pelagic species, indicating changes to surface water productivity. Core catcher samples from Site U1345 contain well-preserved assemblages of calcareous microfossils throughout the majority of the section (0–168 m core composite depth below seafloor [CCSF-A]). The diversity of planktonic foraminifers and calcareous nannofossils is relatively low, as expected in high-latitude regions, and is consistent with all other sites. Benthic foraminifer diversity is high and consists of calcareous species associated with oxygen-limited environments. Organic-walled microfossils are present in high abundances and have very good preservation. Dinoflagellate assemblages indicate changes between low and high primary productivity, together with generally low sea-surface temperatures (SST) and short seasonal sea ice coverage. Six biostratigraphic datum events based on radiolarians, diatoms, and silicoflagellates reveal a relatively high average sedimentation rate of ~30 cm/k.y. and an age of ~0.5 ± 0.1 Ma for the base of the section (Fig. F14; Table T2).

Calcareous nannofossils

The abundance and state of preservation of calcareous nannofossils at Site U1345 were assessed by examining all core catcher samples from Holes U1345A–U1345D and by performing additional observations of laminated intervals on smear slides from the sedimentology collection (Table T3). The abundances of specific age-marker taxa were also evaluated. Calcareous nannofossils are generally rare or absent in the samples, but they are common to abundant at some horizons (Fig. F15), particularly in samples corresponding to laminated intervals, which are interpreted as belonging to interglacial periods (see "Lithostratigraphy"). Their preservation ranges from good to moderate, with only Sample 323-U1345D-12H-CC containing poorly preserved calcareous nannofossils. Both Coccolithus pelagicus and small gephyrocapsids are present throughout the sequence, with the former dominating the assemblage in the topmost samples (Samples 323-U1345A-1H-CC and 323-U1345C-1H-CC, plus additional samples from Cores 323-U1345A-1H and 2H) and the latter being more common in the lower levels that contain abundant calcareous nannofossils. Coccolithus braarudii, Calcidiscus leptoporus (medium and small forms), medium-sized gephyrocapsids, and Emiliania huxleyi are also present. Emiliania huxleyi is present in both the large (>4 µm) and small (<4 µm) varieties and is dominant in Sample 323-U1345A-1H-CC.

The presence of E. huxleyi in the topmost intervals of Holes U1345A–U1345C (Samples 323-U1345A-1H-CC and 2H-CC, 323-U1345B-1H-CC, and 323-U1345C-1H-CC) allows the assignment of those samples to calcareous nannofossil Zone NN21 (Martini, 1971), which ranges from the first occurrence (FO) datum of this species at 0.29 Ma to the present (Lourens et al., 2004). Barren samples and low calcareous nannofossil abundances prevent the assignment of this FO datum to a specific depth; however, the proposed age model based on other datums (Fig. F14) indicates that the distribution of E. huxleyi at Site U1345 seems to reflect its acme interval rather than its occurrence record.

No other biostratigraphic marker species were observed at Site U1345. The absence of Pseudoemiliania lacunosa in the bottom samples may indicate an age younger than 0.44 Ma for the oldest sediments at this site. However, other evidence (see "Lithostratigraphy" and "Paleomagnetism") indicates that the recovered sequence surpasses this age, which shows that the distribution of this species in this area might be hampered by environmental factors. More detailed shore-based study is needed to assess this datum.

Planktonic foraminifers

The distribution of planktonic foraminifers (>125 µm) was studied in all core catcher samples from Holes U1345A–U1345D (Table T4). The mudline sample from the top of Core 323-U1345D-1H was also analyzed. The late Quaternary fauna at Site U1345 is dominated by Neogloboquadrina pachyderma (sinistral), and, in general, there is a high abundance of well-preserved but commonly yellow-stained planktonic foraminifers at this site. Neogloboquadrina pachyderma (sinistral) is a polar species that also dominates modern-day faunas in the Bering Sea (Asahi and Takahashi, 2007). Additional species found in the late Quaternary include the subpolar species Globigerina bulloides, Globigerina umbilicata, and Neogloboquadrina pachyderma (dextral), which appear in low numbers. Samples 323-U1345A-1H-CC, 5H-CC, and 13H-CC contain faunas with an elevated content of subpolar species, with G. bulloides being the most frequent. This also applies to Samples 323-U1345C-1H-CC, 4H-CC, and 9H-CC and 323-U1345D-1H-CC. All samples contain siliciclastic grains in varying amounts (few to dominant). Pyrite and mica were also observed in the samples.

Benthic foraminifers

More than 40 species of benthic foraminifers were recovered in 49 samples from Holes U1345A–U1345D (Tables T5, T6, T7). The faunal composition shows no long-term change or trend over the cored interval and is therefore designated as one assemblage characterized by generally medium diversity (typically 8–12 species per sample) and high abundance (typically dominant to abundant), with persistently high occurrences of the species Bulimina aff. exilis, Uvigerina cf. peregrina, and Globobulimina pacifica. Other common and persistent species include Cassidulinoides tenuis, Elphidium cf. batialis, Brizalina earlandi, Epistominella pulchella, Islandiella norcrossi, Nonionella labradorica, and Valvulineria sp. The dominance of deep and shallow infaunal species fluctuates, and this is most likely related to changes in the extent of bottom water oxygen concentrations in association with changes to surface water productivity and/or deepwater ventilation.

The species composition is similar to that found in the modern OMZ of the Sea of Okhotsk (Bubenshchikova et al., 2008). Moreover, this site is located within the modern OMZ, and mudline samples contain the majority of species found downcore, supporting our low oxygen interpretation (Table T7). As at previous sites in the Bering Sea, faunal composition exhibits large changes in species dominance within the assemblage (Fig. F14). These changes are interpreted to be responses to changes in local oxygen concentration associated with productivity and/or deepwater ventilation shifts.

Ostracodes

All core catcher samples were examined for the presence of ostracodes, but only one specimen of the genus Cytheropteron was found in Sample 323-U1345D-11H-CC (Table T7).

Diatoms

Diatom biostratigraphy is based on the analysis of core catcher samples from each core from Holes U1345A, U1345C, U1345D, and U1345E. The depth positions and age estimates of biostratigraphic marker events are shown in Figure F14 and Tables T8, T9, T10, and T11. Diatom preservation is poor to good in all holes, and abundance is common to very abundant throughout this late Pleistocene record.

In Holes U1345A, U1345C, and U1345D, the last occurrence (LO) of Proboscia curvirostris and the LO of Thalassiosira jouseae were observed in Samples 323-U1345A-8H-CC (66.25 mbsf), 323-U1345C-8H-CC (67.7 mbsf), and 323-U1345D-8H-CC (69.8 mbsf), giving an age of 0.3 Ma (Barron and Gladenkov, 1995; Yanagisawa and Akiba, 1998). This depth is consistent with nearby gateway Site U1343. In Hole U1345E, T. jouseae was observed in abundance in Sample 323-U1345E-9H-CC (79.74 mbsf); however, P. curvirostris is absent until Sample 323-U1345E-10H-CC (89.1 mbsf), which may be related to preservation issues.

In general, diversity is lower at 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, Paralia sulcata, Thalassiothrix longissima, Thalassionema nitzschioides, Thalassiosira latimarginata s.l., and, to a lesser extent, Neodenticula seminae, Bacteriosira fragilis, and Actinocyclus curvatulus.

The cored interval above the LO of P. curvirostris was assigned to N. seminae Zone NPD12. This zone is dominated by T. antarctica spores, T. latimarginata  s.l., P. sulcata, Thalassiosira hyalina, and B. fragilis, with N. seminae and A. curvatulus having minor presence. In general, this site has a higher proportion of coastal neritics and freshwater species than the other gateway Sites U1343 and U1344.

Silicoflagellates and ebridians

Silicoflagellate and ebridian counting was conducted in Hole U1345A (Table T12). The youngest datum, LO of Distephanus octonarius (0.2–0.3 Ma), is estimated to occur in Core 323-U1345A-9H (71.11–80.64 mbsf). Distephanus speculum is dominant in all counted samples except Sample 323-U1345A-16H-CC (147.33–147.43 mbsf), which contains many Distephanus medianoctisol specimens. Ebridians were not found in any sample slides.

Radiolarians

Radiolarian biostratigraphy is based on the analysis of core catcher samples from Holes U1345A–U1345E. Radiolarian zones at Site U1345 could not be established because Stylatractus universus is absent. Four radiolarian datums derived from the subarctic Pacific were identified at this site (Table T13). The LOs of Lychnocanoma nipponica sakaii (50 ka) and Spongodiscus sp. (280–320 ka) are well determined by their abundant occurrences. In contrast, the LOs of Amphimelissa setosa (70–90 ka) and Axoprunum aquilonium (250–410 ka) are supported only by seldom occurrences, making precise stratigraphic datums difficult. Estimated sedimentation rates between the LOs of L. nipponica sakaii and Spongodiscus sp. are ~25 cm/k.y. in each hole.

Radiolarian abundances and preservation are shown in Table T14. Total radiolarian abundances are few to common throughout the cores. Radiolarian preservation is generally moderate. The radiolarian assemblage at Site U1345 is composed mainly of typical subarctic Pacific species such as Ceratospyris borealis, Cycladophora davisiana, L. nipponica sakaii, Spongodiscus sp., Spongotrochus glacialis, Stylodictya validispina, and Stylochlamydium venustum. Among these species, the relative abundance of C. davisiana fluctuates greatly, possibly in relation to ventilation changes with glacial–interglacial cycles. Sphaeropyle langii-robusta group, which is commonly found at Sites U1343 and U1344, has very low abundances. Because the abundances of S. langii-robusta group at shallower sites (U1339, U1340, and U1342) are also very low, their dwelling depth might be in deep water below ~1000 m.

Palynology: dinoflagellate cysts, pollen, and other palynomorphs

Core catcher samples from Holes U1345A and U1345C and the mudline sample from Hole U1345B were analyzed for their palynological content (Table T15). Moderately to well-preserved palynomorphs were encountered in all samples. Terrestrial palynomorphs, including pollen grains, spores of pteridophytes, and phycoma of prasinophytes, occur throughout the cores in variable numbers. Picea and Sphagnum are the most dominant palynomorphs among pollen and spores. Their concentrations are usually >500 grains/cm³, with variability throughout the sequence.

The concentration of dinoflagellate cysts is highly variable but is usually >1000 cysts/cm³ (Fig. F16). Dinocyst assemblages are composed mainly of heterotrophic-related dinocysts. Eighteen taxa were recorded, but only Brigantedinium spp. and Islandinium minutum occur in significant numbers, and they dominate the assemblages throughout. The composition of dinoflagellate cyst assemblages is modern in character, suggesting a Pleistocene age for all analyzed samples.

Discussion

High-frequency variation can be seen in the abundance and composition of all microfossil groups. Figures F15, F16, and F17 are marked with shaded boxes at the approximate locations of what appear to be distinct interglacials (~5, 40, 130, and 145 m CCSF-A). This locations are based on the reduction in sea ice diatoms, the increase in dinoflagellates, planktonic foraminifers, and calcareous nannofossil abundances, the increase in the open ocean diatoms Neodenticula and Actinocyclus spp., and the increase in the high-productivity dinoflagellate I. minutum and the associated low-oxygen benthic foraminifer B. aff. exilis. These intervals also coincide with lower GRA bulk density and are consistent with the age model.

Overall, calcareous nannofossil abundance at Site U1345 (Fig. F15) seems to follow glacial–interglacial cyclicity (with higher numbers during interglacials) and generally appears to reflect environmental factors such as temperature and nutrients rather than an overprinted diagenetic signal. Calcareous nannofossils do not proliferate in areas of sea ice.

Elevated contents of subpolar planktonic foraminifer species, including G. bulloides, appear at ~5, 40, 90, 130, and 145 m CCSF-A, largely coinciding with the interglacials defined (Fig. F15). This reveals increased SST during these intervals because 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 SST probably reflect interglacial conditions.

As at previous sites in the Bering Sea, the benthic foraminifer faunal composition shows large changes in species dominance within the assemblage (Tables T5, T6, T7). These changes are interpreted as responses to shifts in local oxygen concentration associated with productivity and/or deepwater ventilation. Bulimina aff. exilis is generally regarded as a low-oxygen/deep infaunal species (e.g., Kaiho, 1994; Bubenshchikova et al., 2008) and occurs in samples associated with high productivity and low sea ice. This suggests that higher productivity during some interglacials caused an expanded, more intense OMZ. Deepwater ventilation may also affect these faunas.

Among all radiolarian species, C. davisiana has the highest fluctuations in abundance, possibly related to ventilation changes with glacial–interglacial cycles (Ohkushi et al., 2003).

Low proportions of sea ice diatoms and high proportions of open water diatoms correspond well to the interglacial horizons (Fig. F17). In general, this site has a higher proportion of coastal neritic and freshwater species than the other gateway Sites U1343 and U1344. These coastal benthic and freshwater species are regarded as endemic to sea ice and are not a factor of continental run-off and subsequent lateral transport. Several of the species found at Site U1345 have been documented in sea ice from the Chukchi Sea (Von Quillfeldt et al., 2003). Horner (1985) found that these species formed an inverted ecosystem, living in pockets within the ice or attaching themselves to the ice strata.

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 such as the northeastern Pacific margins (e.g., Radi and de Vernal, 2004). However, it is also abundant in polar and subpolar regions of the North Atlantic and Arctic oceans, where seasonal sea ice coverage occurs (Rochon et al., 1999). Islandinium minutum is one of the principal, if not dominant, components of assemblages in the modern Arctic Ocean (Rochon et al., 1999; Head et al., 2001). It has been found to be very abundant in polar upwelling zones such as the North Water Polynia (Hamel et al., 2002). The overall abundance of dinocysts, and particularly the above-mentioned species, suggests high productivity and upwelling during strong interglacials. Dinocyst abundance also increases from 80 m CCSF-A to the top of the sequence. The extremely high abundance of dinocysts, especially in Samples 323-U1345B-Mudline and 323-U1345A-5H-CC (44.4 m CCSF-A) and Core 323-U1345A-13H (130.6 m CCSF-A), suggests interglacial periods. High dinocyst abundance coincides with relatively low pollen and spore concentrations. Such low terrestrial contribution during these intervals could be due to several mechanisms, including changes in sea level that drastically affected the proximity of land, changes in vegetation in source areas, and changes in the strength and pattern of atmospheric/oceanic circulation trajectories.

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