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doi:10.2204/iodp.proc.323.106.2011

Biostratigraphy

Core catcher samples above 36 m CCSF-A at Site U1342 are dominated by diatom microfossils exhibiting high diversity and variably good to poor preservation. These samples also contain well-preserved, low-diversity assemblages of calcareous nannofossils and planktonic foraminifers typical of high latitudes, very high to low abundances of calcareous benthic foraminifers, and variable abundances of other siliceous microfossils. Palynological residues contain high to low abundances of dinoflagellate cysts, pollen, and other palynomorphs. Below 36 m CCSF-A, the majority of microfossils disappear, and samples contain only rare diatoms and palynomorphs and are barren of calcareous microfossils. Abundant sponge spicules and fragments occur. Biostratigraphy below 36 m CCSF-A is hampered by poor microfossil preservation. Eight biostratigraphic datum events were interpreted based on radiolarians, diatoms, silicoflagellates, and calcareous nannofossils. These, together with paleomagnetic datum events, indicate that the base of the microfossil-rich section (uppermost 36 m CCSF-A) is 1.0 Ma (Fig. F15; Table T2), giving a low sedimentation rate of 3.6 cm/k.y. compared to other Expedition 323 sites. Siliceous microfossils are mainly composed of high-latitude pelagic species similar to those found at nearby Sites U1340 and U1341. Dinoflagellate assemblages vary between low and high primary productivity indicators and indicate generally low sea-surface temperatures (SST) and low seasonal sea ice coverage. Calcareous planktonic microfossils at this site reflect a high-latitude environment, indicating variation in SST. Benthic foraminifers are dominated by species typical of environments within the OMZ. The appearance of Pliocene diatom specimens in shallow samples and Pleistocene specimens close to the basement rock suggest reworking at this site. Middle Miocene diatoms were observed below ~45 mbsf, and further investigation will be needed to determine the age of the basement.

Calcareous nannofossils

All core catcher samples from Holes U1342A–U1342D were sampled and examined to assess the abundance and state of preservation of calcareous nannofossils. Evaluations of the abundance of taxa within the assemblages were also made. The results (Table T3) show that Coccolithus pelagicus is the dominant taxon and is present in all samples containing calcareous nannofossils; small and medium gephyrocapsids are also frequently present. Other taxa include Emiliania huxleyi, Pseudoemiliania lacunosa, Reticulofenestra minuta, and Calcidiscus leptoporus. Reworked specimens occur in most samples from Hole U1342D, and some of these were identified as typical middle Miocene to Pliocene taxa. The sequences recovered at Site U1342 are also characterized by frequent barren intervals, particularly in all samples below ~36 m CCSF-A (Samples 323-U1342A-6H-CC and 7H-CC, 323-U1342B-2H-CC through 5H-CC, 323-U1342C-4H-CC through 6H-CC, and 323-U1342D-5H-CC and 9H-CC) and apparently at random in Holes U1342B and U1342C, hampering the continuity of the calcareous nannofossil record.

Emiliania huxleyi, which characterizes calcareous nannoplankton Zone NN21 (Martini, 1971), is present in Samples 323-U1342A-1H-CC and 323-U1342D-1H-CC and 2H-CC. Therefore, these samples can be assigned to Zone NN21, which ranges from 0.29 Ma to the present. Sample 323-U1342A-3H-CC contains P. lacunosa and is therefore assigned to calcareous nannofossil Zone NN19 (Martini, 1971), which ends at 0.44 Ma.

It was not possible to assign an age based on calcareous nannofossils to the sandy silt bottom sediments of Site U1342. The existence of Pliocene and middle Miocene reworked taxa in samples above this level may provide an age estimate for those sediments; however, long-distance transportation of reworked taxa cannot be discounted because warm-water Discoaster spp. was found among the reworked population.

Planktonic foraminifers

The >125 µm fraction of 25 core catcher samples from Holes U1342A–U1342D and an extra sample from Hole U1342C were analyzed for planktonic foraminifers (Table T4). Additionally, mudline samples from the top of Core 1H in all holes were analyzed using the same size fraction. High proportions of sand, sponge spicules, and diatoms were observed in the majority of samples, and Sample 323-U1342D-9X-CC contains only rock fragments. The uppermost samples of Site U1342 cores (uppermost ~36 m CCSF-A) contain dominant to few planktonic foraminifers, and samples are mostly barren of planktonic foraminifers below. The fauna is largely dominated by Neogloboquadrina pachyderma (sinistral). The frequency of subpolar species Globigerina bulloides, Globigerina umbilicata, and Neogloboquadrina pachyderma (dextral) varies between holes and cores. In Hole U1342A, both G. bulloides and N. pachyderma (dextral) are variably abundant, and G. bulloides is abundant in Sample 323-U1342D-4H-CC.

The dominating species at Site U1342, N. pachyderma (sinistral), is also the dominating species in the water column of the Bering Sea today (Asahi and Takahashi, 2007). The temporal variability of this species at this site shows how subpolar–polar sea-surface conditions prevailed over the last 1 m.y. Globigerina bulloides and G. umbilicata are controlled both by temperature and food availability (Reynolds and Thunell, 1985); however, studies in the Bering Sea show that in this region these species are mostly influenced by temperature (Asahi and Takahashi, 2007).

Benthic foraminifers

More than 20 species of benthic foraminifers were recovered in 29 samples from Holes U1342A–U1342D (Table T5). The majority of core catcher samples down to around Sample 4H-CC in all holes contain varyingly diverse calcareous assemblages ranging from high to low abundance. Occasionally dominant species are Cassidulina sp. and Uvigerina peregrina. Persistently occurring species include Brizalina pygmaea, Brizalina spathula, Bulimina aff. exilis, Globobulimina pacifica, and Valvulineria spp. This assemblage shows similarities to the assemblages found in the uppermost ~100 m of Sites U1339, U1340, and U1341 and also to those within or near the OMZ in the Sea of Okhotsk (Bubenshchikova et al., 2008). This fauna is likely strongly affected by the local OMZ, and variation in species abundance is probably linked to changes in oxygen concentrations and, in turn, surface water productivity and/or intermediate water ventilation. This assemblage does not appear to reflect shallow-water (shelf) deposition.

The assemblage largely disappears from Sample 5H-CC and below in all holes and is replaced with glauconite-rich sands likely derived from a more shallow (shelf) setting.

Ostracodes

No ostracodes were found in core catcher samples at Site U1342.

Diatoms

Diatom biostratigraphy is based on the analysis of core catcher samples from all cores from Holes U1342A–U1342D. Depth positions and age estimates of biostratigraphic marker events are shown in Figure F15 and Table T6. Diatom preservation is poor to moderate in all holes, and abundance is common to rare throughout this Pleistocene record.

Only 5–6 cores were retrieved at Site U1342 before basement was reached. Continued drilling into the rock strata provided some interludes of softer material that were analyzed for diatoms (Samples 323-U1342A-7H-CC and 323-U1342B-7H-CC). However, little information could be derived from these older intervals, and clear reworking was evident from the presence of the early Pleistocene–Pliocene species Neodenticula koizumii and the Miocene species Actinocyclus ingens. The presence of this middle Miocene species suggests a minimum age of 14 Ma (Baldauf and Barron, 1980) for the basement at this site. This datum is, however, tentative and requires further investigation.

In Holes U1342A and U1342C, the last occurrence (LO) of Proboscia curvirostris was observed in Samples 323-U1342A-3H-CC and 323-U1342C-3H-CC. This datum, however, was observed at a shallower depth in Samples 323-U1342B-2H-CC and 323-U1342D-2H-CC and concurs with radiolarian datum Spongodiscus sp. (Table T2). Therefore, the stratigraphic marker observed at the shallower depths was used to assign an age of 0.3 Ma (Barron and Gladenkov, 1995; Yanagisawa and Akiba, 1998).

In general, diversity is higher in the P. curvirostris Zone in every hole, including the species Neodenticula seminae, Actinocyclus curvatulus, Thalassiosira spp. (Thalassiosira antarctica spores, Thalassiosira latimarginata s.l., and Thalassiosira oestrupii), Thalassiothrix longissima, and Porosira glacialis. In Hole U1342B, stratigraphic biomarker Thalassiosira jouseae co-occurs with P. curvirostris, and its absence in the other holes further suggests poor preservation at this site.

The last common occurrence (LCO) datum of Actinocyclus oculatus marks the following stratigraphic zonation and was not observed in any of the holes, although it was observed sporadically throughout the record. Therefore, no clear datum was defined beyond P. curvirostris Zone 11. The LO of N. koizumii was observed in Samples 323-U1342A-5H-CC, 323-U1342B-5H-CC, and 323-U1342D-5H-CC. This datum of 2.1 Ma could not be assigned because this zone is established by the LCO of N. koizumii according to Yanagisawa and Akiba (1998). Because of poor diatom preservation below Core 5H in all holes, biostratigraphic zonation was constrained by one species, P. curvirostris, which places N. seminae North Pacific Diatom (NPD) Zone 12 in Samples 323-U1342A-1H-CC through 2H-CC and 323-U1342C-1H-CC through 2H-CC (1.65–11.41 and 7.09–17.04 mbsf, respectively). In Holes U1342B and U1342D, Zone NPD12 is shallower and covers only Samples 323-U1342B-1H-CC and 323-U1342D-1H-CC (0–5.33 and 0–6.11 mbsf, respectively). In general, this short interval is poorer in diatom preservation than Zone 11 and is composed of T. latimarginata s.l., T. antarctica spores, T. oestrupii, and, to a lesser extent, Rhizosolenia spp.

Silicoflagellates and ebridians

Silicoflagellate and ebridian counting was conducted in Holes U1342A and U1342D (Table T7). Silicoflagellate and ebridian preservation at Site U1342 is essentially poor to moderate and is worse than the preservation at previous sites. Zonation datum events could not be defined at this site because of low abundances, poor skeleton preservation, and limited sample numbers. Based on the observed species, the age of the uppermost four cores from Holes U1342A and U1342D is probably Pleistocene. The age of Core 323-U1342A-5H and below may be older than 2.5 Ma based on the occurrence of Ebriopsis antiqua antiqua, assuming it is not reworked. The biostratigraphic results at this site are still unclear, but they will be revised with the increased sample numbers available for shore-based study.

Radiolarians

Radiolarian biostratigraphy is based on the analysis of core catcher samples from Holes U1342A–U1342D. The radiolarian zone defined during this expedition (see "Biostratigraphy" in the "Methods" chapter) could not be determined because there was no occurrence of Stylatractus universus. However, the stratigraphy at Site U1342 (Table T8) extends from the Botryostrobus aquilonaris Zone (upper Quaternary) to the Eucyrtidium matuyamai Zone (middle Quaternary) in the subarctic Pacific (Kamikuri et al., 2007). Six radiolarian datums derived in the subarctic Pacific were identified at this site (Table T8). These datums indicate lower sedimentation rates (~5 cm/k.y.) in the uppermost 20 m intervals of each hole than those at other Bowers Ridge sites (U1340 and U1341). The Amphimelissa setosa LO datum (0.08–0.10 Ma) was found only in Hole U1342C, probably because its small skeleton size passed through the 63 µm mesh. Note that the occurrence of S. universus in Sample 323-U1342A-7H-CC is not used as a datum because Samples 323-U1342A-6H-CC and 7H-CC possibly constitute flow-in materials. Although radiolarian datums in the lower intervals are scarce, the LO of E. matuyamai (0.9–1.5 Ma) was found in Sample 323-U1342B-4H-CC. This datum provides a constraint for age estimation for the lower intervals and an average sedimentation rate in Hole U1342B of 2–4 cm/k.y. Below 36 m CCSF-A, radiolarian abundances are very low and only a few Pleistocene species were found. No Miocene radiolarian species were found in the interval.

Radiolarian abundances and preservation are shown in Table T9. Radiolarian preservation is generally moderate to poor in all samples. The preservation conditions at this site are worse than those at other Bowers Ridge sites (U1340 and U1341). In particular, radiolarian skeletons in Sample 323-U1342A-3H-CC suffered significant dissolution and appear thin and frail, similar to specimens from deep-sea sediments found below 4000 m water depth. The dissolution is possibly related to sediment winnowing by bottom currents (see "Lithostratigraphy"). Radiolarian abundances are common to few in all holes at Site U1342 and very few in the lower intervals. Radiolarian assemblages are similar to those found at other Bowers Ridge sites (U1340 and U1341). However, the abundance of Stylochlamydium venustum, with its fragile skeletal parts, is apparently low, suggesting poor preservation. Interestingly, Phorticium pylonium Heackel, which is abundant in tropical to subtropical Pacific waters, was found in Sample 323-U1342D-1H-CC. With its characteristic squarelike cortical shell, this specimen differs from those found at low latitudes.

Palynology: dinoflagellate cysts, pollen, and other palynomorphs

Palynological assemblages were examined in core catcher samples from Holes U1342A and U1342B (Table T10). In spite of relatively poor diatom preservation compared to Sites U1340 and U1341, samples were difficult to process because of abundant detrital and biogenic silica. Pollen grains (mostly dominated by Picea) and pteridophyte spores occur throughout the sequence, with concentrations ranging between 10 and 900 grains/cm³. The highest abundance occurs in Samples 323-U1342A-2H-CC and 3H-CC at ~11.86 and 22.02 m CCSF-A, respectively. Variable pollen concentrations indicate vegetation changes in the source area and/or changes in the strength and pattern of atmospheric and/or oceanic circulation trajectories. Reworked palynomorphs are common only in Samples 323-U1342B-3H-CC and 5H-CC and likely reflect detrital input. Organic linings of benthic foraminifers are common throughout the sequence. Their abundance could be related to moderate calcium carbonate dissolution or high benthic foraminifer production.

Dinoflagellate cysts are common to abundant in most samples, with concentrations ranging between 100 and >1000 cysts/cm³. However, dinoflagellate cysts are very few to rare in the sandy layer (Samples 323-U1342A-6H-CC and 7H-CC). Poor preservation of organic-walled dinoflagellate cysts in the lower part of the sequence, particularly in the sandy layer (Samples 323-U1342A-6H-CC and 7H-CC and 323-U1342B-5H-CC), suggests significantly high oxygen concentrations in the bottom water, probably caused by sediment remobilization. The species composition of the assemblages is modern (Table T10), suggesting a Pleistocene age for all analyzed samples.

Discussion

Diatomaceous sediments above ~36 m CCSF-A span the last ~1.2 m.y. (Fig. F15). These sediments and those below contain occasional reworked diatoms and silicoflagellates. The age of the underlying rock strata could not be clearly defined; however, middle Miocene diatoms observed in the deepest sample (323-U1342A-7H-CC) suggest a tentative age older than 14 Ma. Biostratigraphic investigations will continue postexpedition and be improved with shore-based studies.

Planktonic foraminifer abundances are high throughout most of the record. Calcareous nannofossil abundances are low but follow the same general trend as planktonic foraminifers. At the top of the sandy silt basal unit (~36 m CCSF-A), both calcareous nannofossils and planktonic foraminifers are absent (Fig. F16). Barren intervals usually coincide with coarser sediments, which probably indicates the existence of winnowing processes that washed away the finer fraction during these intervals. The barren levels also match levels with lower percentages of CaCO₃. In fact, the large similarities between carbonate abundance and calcareous nannofossil records might indicate that most of the carbonate at this site accumulated within the clay fraction, as at Site U1341. The percentages of heavily silicified diatom valves are low throughout the section except at ~42 m CCSF-A (Fig. F16), where there is a single peak.

Both dinoflagellate cyst assemblages and sea ice diatoms suggest low seasonal sea ice coverage, and planktonic foraminifers indicate polar to subpolar conditions (Fig. F17). In general, dinoflagellate cysts suggest high primary productivity, low SST, and seasonal sea ice coverage (Fig. F17). However, the co-dominance of the autotrophic-related productivity species Operculodinium centrocarpum and the heterotrophic Brigantedinium spp. in Sample 323-U1342A-3H-CC could be associated with relatively low primary productivity and/or an incursion of oceanic/oligotrophic waters. Ecological interpretation of diatom assemblages at this site is complicated by the low abundance of diatoms and sample intervals and may only be resolved by higher resolution work.

Benthic foraminifers generally have high abundances, and variation in species dominance is probably related to changes in oxygen levels caused by productivity changes and/or intermediate water ventilation, although it may also have been affected by current winnowing. Figure F18 shows the variability in the proportions of two common genera, Bulimina spp. and Uvigerina spp., against gamma ray attenuation (GRA) bulk density data. Uvigerina is regarded as a shallow infaunal (intermediate oxygen) genus, and Bulimina is regarded as deep infaunal (low oxygen) (e.g., Bubenshchikova et al., 2008; Kaiho, 1994). Low-oxygen species abundance generally correlates with variations in GRA bulk density in the uppermost ~9 m because low GRA is associated with high biogenic production rates. Low oxygen at these times suggests that productivity was a factor affecting the OMZ extent; however, the OMZ may also have been affected by intermediate water ventilation during times of low production and possibly extensive sea ice coverage. Further analysis is needed to fully understand this relationship.

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