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doi:10.2204/iodp.proc.323.108.2011

Biostratigraphy

All core catcher samples from Holes U1344A–U1344E were examined for biostratigraphic purposes. Biostratigraphic datums were derived from radiolarian, diatom, dinoflagellate, silicoflagellate, and ebridian bioevents and are summarized in Table T2. The composite age model derived from all five holes shows that the sedimentary record recovered at Site U1344 spans the last 2 m.y. (Fig. F16), yielding a broadly linear trend in sedimentation rates with average values of ~40 cm/k.y. Samples from Site U1344 are dominated by diatom assemblages and also contain radiolarians, silicoflagellates, ebridians, organic-walled microfossils (dinoflagellate cysts, pollen, and spores), calcareous nannofossils, and planktonic and benthic foraminiferal assemblages.

The preservation of microfossil groups ranges from very good to poor, with dissolution processes and barren intervals affecting calcareous fossil groups in the lowermost 250 m CCSF-A, probably related to dissolution and diagenetic recrystallization. Siliceous, calcareous, and organic fossils have relatively high abundances and show distinct, large-scale oscillations in the uppermost 250 m CCSF-A (Fig. F17). These microfossil groups possibly reflect fluctuations in sea ice and primary productivity in the upper water column that affect the deepwater environment as well.

Calcareous nannofossils

All core catcher samples from Holes U1344A–U1344E were examined to assess the general abundance and state of preservation of calcareous nannofossils as well as specific marker taxa (Table T3). Site U1344 is characterized by very low calcareous nannofossil abundances; sample abundances range from few to barren, with only three samples (323-U1344A-6H-CC and 323-U1344E-6H-CC and 20H-CC) having abundant nannofossils. Barren intervals are common throughout the record and are prevalent in the lower 450 m of the sequence in Hole U1344A. Coccolithus pelagicus is the most abundant taxon in calcareous nannofossil–bearing samples, followed by the group of small gephyrocapsids; other taxa such as medium gephyrocapsids and Coccolithus leptoporus (normal and small varieties) appear only in very low numbers and at specific intervals. The preservation of all taxa ranges from good to poor, with better preservation in the uppermost ~250 m. Reworked specimens are not abundant at this site and occur irregularly throughout the sequence.

Age-marker species are rare in the calcareous nannofossil record of Site U1344, and no ages were obtained from this group of fossils (Table T2; Fig. F16). Emiliania huxleyi was found only in the uppermost samples of Holes U1344A–U1344D and is always rare (Table T3). Therefore, only Samples 323-U1344A-1H-CC, 323-U1344B-1H-CC, 323-U1344C-1H-CC, and 323-U1344D-1H-CC through 4H-CC could be assigned to calcareous nannofossil Zone NN21 (Martini, 1971) with an estimated age of younger than 0.29 Ma (Lourens et al., 2004). Because barren samples exist beneath samples bearing E. huxleyi, the bottom of Zone NN21 could not be established. Moreover, it is likely that those samples are located within the acme interval of E. huxleyi, which commonly ranges from 0.85 Ma to the present (Thierstein et al., 1977).

Planktonic foraminifers

All core catcher samples from Holes U1344A–U1344E were analyzed for planktonic foraminifers from the >125 µm fraction (Table T4). In addition, mudline samples were analyzed using the same size fraction. All samples contain abundant siliciclastic grains, pyrite, and mica. The samples display relatively high planktonic foraminifer abundance in the uppermost ~260 m. Only a few yellow foraminifer tests were encountered at this site (Table T4). The lower portions in each hole contain fewer planktonic foraminifers, and some samples are barren. Only one short continuous barren interval (712–750 m CCSF-A, at the late Pliocene–early Pleistocene transition) was encountered (Fig. F17). The faunal assemblage found in the uppermost ~260 m is dominated by Neogloboquadrina pachyderma (sinistral) throughout. This polar species also dominates faunal assemblages in the Bering Sea today (Asahi and Takahashi, 2007). Additional species found include the subpolar species Globigerina bulloides, Globigerina umbilicata, and Neogloboquadrina pachyderma (dextral), which also occur in the Bering Sea today (Asahi and Takahashi, 2007). However, these species are less abundant in Site U1344 core samples than they are today. This indicates that conditions were colder during the late Pleistocene, because these species are controlled by sea-surface temperatures (e.g., Bé and Tolderlund, 1971; Asahi and Takahashi, 2007). Below 260 m CCSF-A, N. pachyderma (sinistral) is less abundant or absent from the faunal assemblages. The assemblages at this depth consist of subpolar species dominated by G. bulloides. This species is characteristic of transitional–polar environments, and its abundance is influenced by food availability and sea-surface temperature (e.g., Reynolds and Thunell, 1985); however, recent studies in the Bering Sea indicate that G. bulloides mainly responds to sea-surface temperature (Asahi and Takahashi, 2007). This shows that the late Pliocene–early Pleistocene was warmer than the late Pleistocene at this site.

Benthic foraminifers

Around 40 species of benthic foraminifers composing two assemblages were recovered in 138 samples from Holes U1344A–U1344E (Tables T5, T6, T7, T8). These assemblages have relatively low diversity (typically 4–8 species per sample) and variable abundance (frequent to dominant), with a marked decline in both from around Sample 323-U1344A-32X-CC downhole. Both assemblages are dominated by low-oxygen species and are similar to the assemblages found at Site U1343 (Fig. F18), with the exception of the lack of abundant Islandiella norcrossi and the persistent presence of Melonis pompilioides from Sample 323-U1344A-34X-CC downhole (Fig. F19). Both assemblages have some similarities to assemblages found at Site U1339, within or near the OMZ in the Sea of Okhotsk (Bubenshchikova et al., 2008), and at the Emperor seamounts (Butt, 1980). The variation in species dominance is most likely linked to changes in bottom water oxygenation, with the most important mechanisms probably being surface water productivity and/or deepwater ventilation variability. High-frequency variation in oxygenation is apparent throughout the section, but initial results show that the generally higher oxygen indicator Elphidium cf. batialis and possibly M. pompilioides are dominant from Sample 323-U1344A-34X-CC downhole. Melonis pompilioides is more abundant at this site than at any other site and may therefore be a higher oxygen indicator, as all other sites are shallower and more affected by the OMZ today.

Assemblage I (Elphidium–Bulimina)

Assemblage I is characterized by generally medium-diversity, high-abundance faunas in samples from the top of the section to Sample 323-U1344A-32X-CC, with persistent occurrences of the species Bulimina aff. exilis, Globobulimina pacifica, Cassidulinoides tenuis, and E. cf. batialis (Fig. F18). Other common species include Nonionella labradorica, Nonionella turgida, Uvigerina auberiana, and Valvulineria sp. Fluctuations in the dominance of deep and shallow infaunal species occur and are 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.

Assemblage II (Elphidium–Melonis)

Assemblage II consists of low-diversity, medium-abundance faunas from Sample 323-U1344A-33X-CC downhole and is characterized by the relatively persistent occurrence of E. cf. batialis and M. pompilioides (Fig. F18). Other common species include G. pacifica, Valvulineria sp., Cibicidoides mundulus, and C. tenuis. The most dominant species, E. batialis, is regarded as a shallow infaunal species in the Sea of Okhotsk (Bubenshchikova et al., 2008). Melonis pompilioides may also be a higher oxygen indicator because it does not persistently occur in any of the shallow sites.

Ostracodes

Site U1344 produced five core catcher samples with ostracodes, more than any other site cored during Expedition 323. Samples 323-U1344A-3H-CC, 5H-CC, and 22H-CC; 323-U1344C-3H-CC and 4H-CC; and 323-U1344D-13H-CC and 14H-CC contain Krithe, Argilloecia, Munseyella, and one unidentified specimen. Ostracode abundances range from present (1 specimen) to few (5–10 specimens). Preservation varies from very good (transparent) to moderate (white and chalky). Despite their very low numbers, most ostracodes are articulated (full carapace) and well preserved, suggesting that they were in situ and their low numbers are caused by dilution in the sediments. Postcruise examination will allow full taxonomic identification of the unidentified species.

Diatoms

Diatom biostratigraphy is based on the analysis of core catcher samples from each core from Holes U1344A, U1344D, and U1344E. Depth positions and age estimates of biostratigraphic marker events are shown in Figure F16 and Tables T2, T9, T10, T11, T12, and T13. Diatom preservation is moderate to good in all holes and is common to very abundant throughout the Pleistocene record.

The last occurrence (LO) datums of Proboscia curvirostris and Thalassiosira jouseae were observed in Samples 323-U1344A-12H-6, 70 cm (118.7 mbsf), 323-U1344D-11H-CC (93.2 mbsf), and 323-U1344E-14H-CC (117.7 mbsf), giving an age of 0.3 Ma (Barron and Gladenkov, 1995; Yanagisawa and Akiba, 1998). This is consistent with nearby Site U1343 and with Site U1339 on Umnak Plateau. The drilled interval above the LO of P. curvirostris is assigned to Neodenticula seminae Zone NPD12. In general, diversity is high for this zone in each hole. The zone is dominated by N. seminae, Actinocyclus curvatulus, Thalassiosira spp. (Thalassiosira antarctica spores and Thalassiosira latimarginata s.l.), Bacteriosira fragilis, Fragilariopsis spp., and, to a lesser extent, Paralia sol and Paralia sulcata.

The last common occurrence (LCO) datum of Actinocyclus oculatus was observed in Sample 323-U1344A-44X-CC (409.3 mbsf). Species abundance is low at this site, and therefore this datum may be refined further. Because of the absence of A. oculatus, the bottom ages of Holes U1344D and U1344E were assigned to Zone NPD11.

The first common occurrence (FCO) datum of P. curvirostris was defined in Sample 323-U1344A-56X-CC and assigned an age of 1.7–2.0 Ma in the A. oculatus Zone NPD10. This datum was not established through conventional counts because few specimens were observed on the standard smear slides. However, many valves were found on slides prepared for silicoflagellate counts. The >20 µm sieved material concentrated larger diatom valves, and counts were made to obtain the datum (Tables T12, T13). In addition, the LO datum of Stephanopyxis horridus (1.9–2.0 Ma) was defined in Sample 323-U1344A-63X-CC. This zone is defined by species N. seminae, Porosira glacialis, Stephanopyxis spp. (S. horridus, Stephanopyxis turris, and Stephanopyxis resting spores), P. sol, P. sulcata, Thalassiosira spp. (T. antarctica spores and T. jouseae), and, to a lesser extent, Delphineis cf. angustata and Coscinodiscus marginatus.

Silicoflagellates and ebridians

Silicoflagellate and ebridian counting at this site was conducted in Holes U1344A and U1344D (Table T14). However, not all core catcher samples could be counted because of limited time. Therefore, only several intervals with anticipated datum events were examined. The youngest datum, LO of Distephanus octonarius (0.2–0.3 Ma), was estimated in Core 323-U1344D-12H (97.86–107.27 mbsf). The LO datum of Dictyocha subarctios (0.6–0.8 Ma) was estimated in Cores 323-U1344A-30H (270.35–280.31 mbsf) and 323-U1344D-26H (224.51–234.09 mbsf). The LO datum of Ammodochium rectangulare (1.9 Ma) is most likely located in Core 323-U1344A-78X (733.13–739.75 mbsf). Because of the trace abundances of ebridians in the lower part of Hole U1344E, the LO of A. rectangulare may be revised by more detailed shore-based work. Older datum events, such as the LO of Ebriopsis antiqua antiqua (2.47–2.48 Ma), were not observed at this site.

Radiolarians

Radiolarian biostratigraphy is based on the analysis of core catcher samples from Holes U1344A–U1344E. Radiolarian stratigraphy at Site U1344 (Table T15) extends from the Botryostrobus aquilonaris Zone (upper Quaternary) to the Eucyrtidium matuyamai Zone (middle Quaternary) in the subarctic Pacific (Kamikuri et al., 2007). Five radiolarian datums derived from the subarctic Pacific were identified at this site (Table T15). Estimated sedimentation rates in the uppermost 150 m of Holes U1344A, U1344D, and U1344E are >30 cm/k.y., which is slightly higher than rates 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, but its first occurrence (FO) datum (1.7–1.9 Ma) was not determined because of very low abundances in the lower interval.

Radiolarian abundances and preservation are shown in Figure F17 and Table T16. In general, both radiolarian abundance and preservation at Site U1344 are lower than at Site U1343. Radiolarian preservation is good to moderate in all samples from the uppermost 300 m. Conversely, preservation in samples below 300 m CCSF-A is moderate to poor. Radiolarians are mostly common in the uppermost 200 m in each hole, whereas they are few in the intervals below 200 m CCSF-A in Hole U1344A. Radiolarian assemblages at Site U1344 are mainly composed of typical subarctic Pacific species such as Ceratospyris borealis, Cycladophora davisiana, Sphaeropyle langii-robusta group, Spongotrochus glacialis, and Stylodictya validispina. Radiolarian preservation, abundance, and assemblages are similar to those at neighboring Site U1343. Considering the water-depth differences between Sites U1343 (~2000 m) and U1344 (~3200 m), future work may be able to compare onshore radiolarian data and discuss water mass structural changes.

Palynology: dinoflagellate cysts, pollen, and other palynomorphs

Palynological assemblages were examined in 70 core catcher samples from Holes U1344A and U1344E (Table T17). The preservation of all palynomorphs is generally good in the uppermost 200 m and moderate to poor in the lower part. The composition of palynomorphs is very similar to that at Site U1343. Variable numbers of pollen and spores were found throughout the sequence. They are mostly dominated by Picea grains and Sphagnum spores, with concentrations as high as 1600 and 1400 grains/cm³, respectively. To a lesser extent, freshwater algae Botryococcus and Pediastrum and reworked pre-Neogene palynomorphs also occur throughout the sequence, with concentrations usually of <400 grains/cm³. Analogous to Site U1343, these abundances and the variability of terrestrial compounds likely reflect significant contributions of terrigenous material by atmospheric and/or oceanic circulation.

Dinoflagellate cysts are common to abundant in most samples, with concentrations ranging between 10² and 10⁴ cysts/cm³ (Fig. F17). They are, however, very few to rare between 607.6 and 683.6 mbsf. In the uppermost 420 m, the assemblages are dominated by the heterotrophic protoperidinial Brigantedinium spp. or by the polar species Islandinium minutum. Variability in their relative abundances is likely related to changes in surface water conditions between high-productivity and upwelling conditions and pronounced sea ice coverage (see the "Site U1343" chapter). The occasional occurrence of the autotrophic species Operculodinium centrocarpum (Table T17) may be related to oceanic conditions with relatively low productivity. The occurrence of Filisphaera filifera in Sample 323-U1344A-50X-CC (473.4 mbsf) suggests an age of 1.41–1.7 Ma, according to its LO datum in the North Pacific and North Atlantic (Bujak, 1984; M. Smelror et al., unpubl. data). This species dominates the assemblages in a few samples above this depth, much like at Site U1343.

Discussion

As at previous sites, Site U1344 is dominated by siliceous microfossils. Again, the most dominant of these species are diatoms, which have high abundances and are generally well preserved. In contrast, silicoflagellates and ebridians were found in significantly lower abundances, although this did not appear to be due to preservation issues because skeletons were intact and had good preservation. Radiolarians, on the other hand, show moderate to good preservation in the uppermost ~350 m in all holes and a gradual reduction in both abundance and preservation thereafter. This shift is reflected in the diatom assemblage, whereby open water or ice-free water indicator N. seminae dominates at this site until ~350–400 mbsf, below which it shows a significant decrease that continues to the present day (Fig. F17). The switch in the diatom record to a sea ice species–dominated assemblage suggests a major shift in the climate regime that could be described as intensification to glacial-like conditions. This event is registered at ~1 Ma and coincides with the mid-Pleistocene Transition (MPT). The increased presence of radiolarians in the sediments after ~1 Ma is also mirrored at Sites U1343 and U1341 and suggests a regional response to these changes in Bering Sea hydrography.

The water depth of Site U1344 is ~3200 m, and thus Site U1344 is presently located below the OMZ and can potentially be used to monitor past deepwater changes. Benthic foraminiferal faunas indicate high-frequency changes in the bottom water oxygen content over the entire section, possibly related to surface water productivity, bottom water ventilation changes, and/or methane seeps (Fig. F18). Abundance generally increases from ~300 mbsf to the top of the section (Assemblage I), as does bottom water oxygen variability. The low-oxygen indicator B. aff. exilis is abundant at both Sites U1343 and U1344 after ~0.8 Ma, along with benthic foraminifer abundance maxima (Fig. F19). Both high-abundance and low-oxygen benthic faunas are common during the last deglacial at Bowers Ridge (Okazaki et al., 2005), and the increase in such characteristics after 0.8 Ma may mark the onset of more intense deglacials, greater nutrient availability, and higher surface water productivity.

The poor preservation below 200 mbsf of organic microfossils, including dinoflagellate cysts, pollen, and spores, is probably related to the depth of this site, which is below the OMZ. Dinoflagellate cysts vary greatly in both abundance and species composition throughout the sequence (Fig. F17). In a few samples below 500 mbsf, the dinoflagellate cysts show extremely high production associated with the dominance of the extinct species F. filifera.

Calcium carbonate dissolution events are recorded in sediments at Site U1344. The calcareous nannofossil record exhibits the highest abundances in the uppermost ~200 m, similar to Site U1343. Large variations in calcareous nannofossil content are displayed in the uppermost 250 m, whereas the lower part is generally characterized by barren samples (Fig. F17). The highest abundances of planktonic foraminifers were encountered in the upper part of the holes corresponding to the late Pleistocene, which supports Zahn et al.'s (1991) conclusion that CaCO₃ is preserved under glacial conditions. Because authigenic carbonates are ubiquitously present in the record (see "Lithostratigraphy"), diagenetic dissolution of biogenic carbonate may also be one of the mechanisms controlling the presence of calcareous microfossils. The water depth at this site (~3200 m) should also be considered as a possible cause of the abundance differences between this site and Site U1343 because carbonate ion concentration drops with water depth. As at Site U1343, the increase of calcareous microfossils also coincides with the highest numbers of sea ice diatoms and polar dinoflagellate cysts after 1 Ma. However, planktonic foraminifers are also preserved in the earlier part of the record, and these preserved intervals consist of subpolar species. Changes in calcareous microfossils (Figs. F17, F18, F19) may be explained by the "CaCO₃ compensation hypothesis" (Broecker and Peng, 1987), as Okazaki et al. (2005) suggested that CaCO₃ preservation peaked during the last deglaciation in the Bering Sea.

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