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doi:10.2204/iodp.proc.323.107.2011

Biostratigraphy

All core catcher samples from Holes U1343A–U1343E and three additional paleontological samples from Hole U1343A (Samples 323-U1343A-8H-4, 55 cm; 10H-6, 52 cm; and 12H-7, 35 cm) were examined for biostratigraphic purposes. Biostratigraphic datums are derived from radiolarian, diatom, dinoflagellate, silicoflagellate, ebridian, calcareous nannofossil, and planktonic foraminiferal bioevents and are summarized in Table T2. The composite age model derived from all five holes shows that the sediments recovered at Site U1343 span the last ~2.2 m.y. (Fig. F13), indicating a broadly linear trend in sedimentation rates, with values of ~26 cm/k.y. in the uppermost 400 m core composite depth below seafloor (CCSF-A) and increasing to ~56 cm/k.y. in the lowermost 350 m CCSF-A. Although this age model shows that Site U1343 contains Holocene to late Pliocene sediments, the presence of Miocene and early Pliocene species indicates that some reworking has occurred at certain levels.

Samples from Site U1343 are dominated by diatom assemblages and also contain radiolarians, silicoflagellates, ebridians, organic-walled microfossils (dinoflagellates, pollen, and spores), calcareous nannofossils, and planktonic and benthic foraminiferal assemblages. Ostracodes are also present in some samples. The preservation of different microfossil groups ranges from very good to poor, with dissolution processes and barren intervals affecting the calcareous fossil groups in the lowermost 350 m, probably related to dissolution and diagenetic recrystallization. In the uppermost 250–300 m, siliceous, calcareous, and organic fossils have relatively high abundances (Fig. F14), and all groups exhibit distinct large, frequent oscillations likely associated with fluctuations in sea ice cover (Fig. F15) and productivity in the upper water column and perhaps the deepwater environment as well.

Calcareous nannofossils

The abundance and preservation of calcareous nannofossils was determined in all core catcher samples from Holes U1343A and U1343C and in 61 core catcher samples from Hole U1343E. Samples 323-U1343A-8H-4, 55 cm; 10H-6, 52 cm; and 12H-7, 35 cm, were also examined, as were several toothpick samples from each hole (Table T3). The abundances of specific taxa were assessed in all of the samples examined; Coccolithus pelagicus is the most common taxon, followed by small and medium gephyrocapsids. Coccolithus leptoporus (both medium and small morphotypes) was also observed at some levels. Reworked specimens of mostly Miocene and Pliocene age were found in some samples from Holes U1343A and U1343E. The preservation of calcareous nannofossils ranges from good to poor, with most samples having moderate to good preservation. Etching was the most common phenomenon observed, and it is especially prominent in Samples 323-U1343A-22H-CC, 323-U1343C-15H-CC, and 323-U1343E-73X-CC. Barren levels are common throughout the upper part of the record and are the major feature below 240 m CCSF-A (Fig. F14), except in brief intervals represented by Samples 323-U1343E-35H-CC, 41H-CC through 45X-CC, and 73X-CC.

Biostratigraphic marker species (Emiliania huxleyi and Pseudoemiliania lacunosa) were only observed in Hole U1343A (Table T3). The occurrence of E. huxleyi in Sample 323-U1343A-3H-CC indicates that this sample and Samples 323-U1343A-1H-CC and 2H-CC can be assigned to calcareous nannofossil Zone NN21 (Martini, 1971), which is defined by the first occurrence (FO) datum of E. huxleyi at 0.29 Ma. However, the occurrence of a barren interval below this level hampers the precise positioning of the FO of E. huxleyi and the boundary with Zone NN20. In contrast, the last occurrence (LO) datum of P. lacunosa at 0.44 Ma (Lourens et al., 2004) in Sample 323-U1343A-12H-7, 46 cm, which defines the upper limit of calcareous nannofossil Zone NN19, is well constrained because it is located within an interval rich in calcareous nannofossils. The lower limit of Zone NN19 at 1.93 Ma (Lourens et al., 2004) is not contained in the record recovered from Hole U1343A.

Planktonic foraminifers

All core catcher samples from Holes U1343A–U1343E were examined for planktonic foraminifers using the >125 µm fraction (Table T4). In addition, mudline samples from the top of Core 1H in all holes were analyzed using the same size fraction. The samples display fluctuating amounts of siliciclastic grains, pyrite, and mica downcore. Yellow to brownish staining of the foraminifer tests was also observed throughout the holes (Tables T4). Mudline samples contain very low abundances of planktonic foraminifers, except for the mudline sample from Core 323-U1343D-1H, where planktonic foraminifers are abundant. Core catcher samples contain generally high abundances of planktonic foraminifers in the uppermost 250 m CCSF-A. In the lower part of the record, planktonic foraminifers are absent or low in abundance (Fig. F14). The fauna in the uppermost 250 m CCSF-A is mainly dominated by Neogloboquadrina pachyderma (sinistral). This species dominates modern subpolar–polar environments and is controlled by sea-surface temperature (SST) (Bé and Tolderlund, 1971). SST is also the controlling factor of the distribution of N. pachyderma (sinistral) in the Bering Sea today (Asahi and Takahashi, 2007). Other species present are Globigerina bulloides, Globigerina umbilicata, and Neogloboquadrina pachyderma (dextral). These species are also found today in the Bering Sea, reflecting subpolar–polar temperatures (Asahi and Takahashi, 2007). The faunal change observed at ~250 m CCSF-A is dated at ~1 Ma (Fig. F13), which coincides with the mid-Pleistocene Transition. Planktonic foraminifers are absent or very low in abundance before this time, and mainly subpolar species are present. In Samples 323-U1343E-44H-CC, 46H-CC, and 47H-CC, Neogloboquadrina atlantica (sinistral) is present in very low numbers (1–2 specimens). The LO of this species is between 2.4 and 2.5 Ma according to high-latitude stratigraphy from the North Atlantic (Weaver and Clement, 1987; Spezzaferri, 1998). Neogloboquadrina atlantica was first identified in the North Pacific at Deep Sea Drilling Project (DSDP) Site 883 in the middle Pliocene (Dowsett and Ishman, 1995). However, these ages appear to be too old when compared to other datums derived at this site (Fig. F13). It is difficult to identify N. atlantica because of its preservation and the transition forms between N. pachyderma (sinistral) and N. atlantica (sinistral) (Dowsett and Poore, 1990; Dowsett and Ishman, 1995), and further taxonomic work is required. We note that the occurrence of N. atlantica at this site coincides with a unique lithology: clays without diatoms, which were drilled using the XCB (see "Lithostratigraphy").

Benthic foraminifers

Around 50 species of benthic foraminifers were identified in 135 samples from Holes U1343A–U1343E (Tables T5, T6, T7). Assemblages are of relatively low diversity (typically 4–8 species per sample) and variable abundance (abundant to dominant), with a marked decline in both diversity and abundance from Sample 323-U1343E-25H-CC downhole. Both assemblages have a similar species composition, with close similarities to assemblages from Site U1339 and within or near the OMZ in the Sea of Okhotsk (Bubenshchikova et al., 2008). Variations in species dominance are most likely linked to changes in bottom water oxygenation, with the most important mechanisms likely being surface water productivity and/or intermediate water ventilation variability. High-frequency variations in oxygenation are apparent throughout the section, but initial results show that shallow infaunal high-oxygen indicators (Elphidium cf. batialis and Islandiella norcrossi) are generally dominant from Sample 323-U1343E-24H-CC downhole.

Assemblage I (Globobulimina–Nonionella)

Assemblage I is characterized by largely medium-diversity and high-abundance faunas between the top of the section and Sample 323-U1343E-25H-CC, with persistent occurrences of the species Globobulimina pacifica, Nonionella labradorica, Bulimina aff. exilis, Cassidulinoides tenuis, E. cf. batialis, and I. norcrossi. Other common species include Uvigerina auberiana, Uvigerina cf. peregrina, and Valvulineria sp. Fluctuations in the dominance of deep and shallow infaunal species occur and are most likely related to changes in bottom water oxygen concentrations in association with changes to surface water productivity and/or intermediate water ventilation.

Assemblage II (Elphidium–Islandiella)

Assemblage II consists of low-diversity and medium-abundance faunas from Sample 323-U1343E-26H-CC downhole, characterized by the relatively persistent occurrences of E. cf. batialis and I. norcrossi. Other common species include B. aff. exilis and C. tenuis. The most dominant species are regarded as shallow infaunal species in the Sea of Okhotsk (Bubenshchikova et al., 2008) and likely have higher oxygen tolerances than those of Assemblage I, although they are still within proximity to the OMZ.

Ostracodes

Only three ostracode taxa were found in 2 out of 135 samples studied at Site U1343: Samples 323-U1343A-8H-CC and 323-U1343E-9H-CC. The specimens are articulated (full carapace) and well preserved, suggesting that they were in situ. Their low numbers can be explained by sediment dilution. The taxa observed include Krithe sp., Pseudocythere cf. Pseudocythere caudata, and Munseyella sp. The species P. caudata is found worldwide in the deep ocean and is common in the Arctic Ocean (Joy and Clark, 1977) and in sediments deposited during glacial times in the subarctic North Atlantic (Didié and Bauch, 2000; Alvarez Zarikian, 2009; Alvarez Zarikian et al., 2009). Krithe is a dominant genus in deep-sea sediments worldwide. Species of Krithe have depth distributions ranging from 500 to >5000 m, but previous studies have shown that the genus is mainly associated with cold water masses and high-productivity areas (Coles et al., 1994; Rodriguez-Lázaro and Cronin, 1999). Further examination is needed to identify the third species found in the samples.

Diatoms

Diatom biostratigraphy is based on analysis of core catcher samples from each core in Holes U1343A, U1343C, and U1343E. Depth positions and age estimates of biostratigraphic marker events are shown in Figure F13 and Tables T8, T9, T10, and T11. Diatom preservation is moderate to good in all holes, and diatom abundance is common to very abundant throughout this record.

The LO datums of Proboscia curvirostris and Thalassiosira jouseae were observed in Holes U1343A, U1343C, and U1343E (Table T2), giving an age of 0.3 Ma (Barron and Gladenkov, 1995; Yanagisawa and Akiba, 1998). This age is consistent with results from IODP Site U1339 at Umnak Plateau. The drilled interval above the LO of P. curvirostris is assigned to Neodenticula seminae North Pacific Diatom (NPD) Zone 12.

In general, diversity is high throughout Zone NPD12 in each hole. This zone is dominated by N. seminae, Thalassiosira spp. (Thalassiosira antarctica spores and Thalassiosira latimarginata s.l.), Thalassionema nitzschioides, Fragilariopsis spp., Paralia sulcata, and, to a lesser extent, Actinocyclus curvatulus and Thalassiosira oestrupii.

The last common occurrence (LCO) datum of Actinocyclus oculatus was observed in Sample 323-U1343E-35H-CC (301.4 mbsf). At this time the interval between this datum level and the LO datum of P. curvirostris in Hole U1343E is assigned to P. curvirostris Zone NPD11. The interval below this datum and above the FO of Neodenticula koizumii is A. oculatus Zone NPD10. No specimens were observed in Hole U1343A, defining the bottom of the hole as Zone NPD11. In Hole U1343C, only one valve was found in Sample 323-U1343C-13H-CC; therefore, no clear datum was defined.

The FO of P. curvirostris was defined in Sample 323-U1343E-56X-CC and assigned the age of 1.85 ± 0.1 Ma in the A. oculatus Zone. 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 sieve concentrated larger diatom valves, and counts were made to obtain the datum (Table T11). In addition, the LO of Stephanopyxis horridus (1.9–2.0 Ma) was estimated in Core 323-U1343E-61X. The A. oculatus Zone (NPD10) is defined by N. seminae, Porosira glacialis, Stephanopyxis spp. (S. horridus, Stephanopyxis turris, and Stephanopyxis zabelinae), Paralia sol, P. sulcata, Thalassiosira spp. (T. antarctica spores and T. jouseae), and, to a lesser extent, Delphineis cf. angustata and Coscinodiscus marginatus.

In the deeper Hole U1343E, the LCO of N. koizumii was observed in Sample 323-U1343E-77X-CC, assigning it an age of 2.1 Ma (Yanagisawa and Akiba, 1998). In accordance with Yanagisawa and Akiba (1998), N. koizumii was distinguished by the open copula of the valves, which differs from the closed copula of N. seminae. In general, the assemblage is composed of T. latimarginata s.l., T. antarctica spores, T. oestrupii, and, to a lesser extent, Rhizosolenia spp. The absence of biostratigraphic marker species Neodenticula kamtschatica (Zone NPD8) means that the interval between this datum and the bottom of Hole U1343E is assigned to N. koizumii Zone NPD9.

Silicoflagellates and ebridians

Silicoflagellate and ebridian counting was conducted in Holes U1343C and U1343E (Table T12). However, not all core catcher samples could be examined for species counts because of limited time. Therefore, only selected intervals containing datum events were processed. The abundance of silicoflagellates and ebridians at Site U1343 is typically lower than at Sites U1340 and U1341 because of the high abundance of coastal and marginal sea ice diatoms. The number of datums in Holes U1343C and U1343E are one and three, respectively. The youngest datum, LO of Distephanus octonarius (0.2–0.3 Ma), was estimated in Core 323-U1343E-8H (64.00–74.89 mbsf). The LO of Dictyocha subarctios (0.6–0.8 Ma) was estimated in Cores 323-U1343C-23H (196.16–205.35 mbsf) and 323-U1343E-23H (183.86–193.41 mbsf). The LO of Ammodochium rectangulare is probably located in Core 323-U1343E-64X (550.64–561.15 mbsf). Because of the trace abundance of ebridians in the lower part of Hole U1343E, the LO of A. rectangulare may be revised by more detailed shore-based work. The bottom age of Hole U1343C is younger than 1.9 Ma because of the absence of A. rectangulare throughout the record. The bottom age of Hole U1343E is younger than 2.5 Ma because of the absence of Ebriopsis antiqua antiqua.

Dictyocha spp., rather than D. subarctios, exhibits a relative abundance of >30% of total silicoflagellates at the base of Core 323-U1343E-27H. This common occurrence is the first record found in this offshore work. Dictyocha spp. is mainly observed in modern temperate–subtropical waters (Poelchau, 1976) and, according to sediment trap studies, is rare in the western subarctic Pacific (Station 50N: Onodera and Takahashi, 2005) and the southern Bering Sea (Station AB: Onodera and Takahashi, unpubl. data), whereas it is abundant in the eastern subarctic Pacific (Takahashi, 1985, 1989). Therefore, the characteristic occurrence of Dictyocha spp. may suggest significant temporal input of warmer, less eutrophic waters into the Bering Sea from the eastern subarctic Pacific. The high abundance of N. seminae also supports this contention. Because the number of examined samples was significantly limited, further analysis is warranted for postexpedition work.

Radiolarians

Radiolarian biostratigraphy is based on the analysis of core catcher samples from Holes U1343A–U1343E. Radiolarian stratigraphy at Site U1343 (Table T13) 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 common in the subarctic Pacific were identified at this site (Table T13). These datums indicate high sedimentation rates (~20 cm/k.y.) in the uppermost 200 m of each hole. The LO and FO datums of E. matuyamai (0.9–1.5 Ma and 1.7–1.9 Ma, respectively) were identified in samples from Hole U1343E, providing constraints for age estimation of the lower intervals and an average sedimentation rate in Hole U1343E of 30–40 cm/k.y. This suggests sedimentation rates of >50 cm/k.y. in the intervals below 200 m.

Radiolarian abundance and preservation are shown in Table T14 and Figure F14. In general, both radiolarian abundance and preservation at Site U1343 decrease downhole. Radiolarian preservation is good to moderate in all samples from the uppermost 200 m. On the other hand, preservation in samples below 200 m is moderate to poor. Radiolarians are also abundant to common in the uppermost 200 m interval in each hole, whereas they are few in the interval below 200 m in Hole U1343E. The low abundance and poor preservation of radiolarian skeletons in the lower intervals prevents us from estimating the bottom age of Hole U1343E. Because it is not possible to find the next oldest radiolarian datum (LO of Thecosphaera akitaensis: 2.4–2.7 Ma), the bottom age of Hole U1343E cannot be assigned until more detailed shore-based work is accomplished.

Radiolarian assemblages at Site U1343 are mainly composed of typical subarctic Pacific species such as Ceratospyris borealis, Cycladophora davisiana, Spongopyle osculosa, Spongotrochus glacialis, and Stylodictya validispina.

Palynology: dinoflagellate cysts, pollen, and other palynomorphs

Palynological assemblages were examined in 55 core catcher samples from Holes U1343A and U1343E. Additional samples (323-U1343A-8H-4, 55–57 cm; 10H-6, 52–54 cm; and 12H-7, 35–37 cm) were also analyzed (Table T15). The preservation of all palynomorphs is generally good to moderate except in a few samples (323-U1343A-19H-CC and 323-U1343E-52X-CC, 66X-CC, and 84X-CC). Abundant terrestrial palynomorphs occur throughout the sequence, with concentrations as high as 2500 grains/cm³. Palynomorphs are mainly dominated by Picea, Sphagnum spores, and the freshwater algae Botryococcus. Their absolute abundances are variable but are usually >500 grains/cm³ and can be as high 2500 grains/cm³. These terrestrial palynomorph abundances can be related to significant input through atmospheric and/or ocean circulation.

Dinoflagellate cysts are common to abundant (10³–10⁴ cysts/cm³) in most samples (Fig. F14). Assemblages show relatively high species diversity, with 25 taxa recorded. However, only Brigantedinium spp., Islandinium minutum, and Filisphaera filifera occur in significant numbers (Table T15). The occurrence of F. filifera in Sample 323-U1342E-44X-CC (376.6 mbsf) suggests an age of 1.41–1.7 Ma according to its LO datum in the North Pacific and the North Atlantic (Bujak, 1984; Smelror et al., unpubl. data).

In the uppermost 250 m, dinoflagellate cysts undergo high-amplitude changes in both species composition and abundance (Fig. F14). In general, typical high-productivity and upwelling assemblages dominate when abundance is high, whereas the polar species I. minutum dominates the assemblages when abundance is relatively low. This change in dinoflagellate assemblages indicates surface water condition changes from high productivity and upwelling to pronounced sea ice cover (Fig. F15). High variability above 250 mbsf is also observed in pollen and spore abundance, suggesting changing vegetation in adjacent land masses.

Below 250 mbsf, dinoflagellate cyst assemblages have low species diversity and low variability. However, two events marked by both high dinoflagellate cyst abundance (>5000 cysts/cm³) and the dominance of F. filifera occur at 695 and 398 mbsf (Fig. F14). The ecological affinity of F. filifera is still poorly documented; however, it is considered a cold-tolerant species because it has been recorded in Arctic Pliocene–Pleistocene sediments (Mudie, 1985), although its first appearance in the northwest Atlantic coincides with the beginning of a warming episode in the late Miocene (Head et al., 1989; Aksu and Hillaire-Marcel, 1989).

Discussion

The fossil record contained in the sequence recovered at Site U1343 displays distinct changes in the general trend of abundance and/or assemblage composition in all major fossil groups (Figs. F14, F15). These changes seem to occur gradually or abruptly between 200 and 300 m CCSF-A, which, according to the age model proposed for this site (Fig. F13), falls between 0.8 and 1.1 Ma. Thus, the trend variations recorded in all fossil groups coincide with the mid-Pleistocene Transition, when global glacial–interglacial cycles gradually shifted from a predominantly mid-amplitude 41 k.y. cycle to a high-amplitude ~100 k.y. cycle. This agrees with the increased range of variability in fossil records observed in the uppermost 250 m at Site U1343, which coincides with increased sea ice cover, upper water column productivity, and variability in the deepwater environment (Figs. F14, F15).

The diatom assemblage reveals high variability related to shifts in the overlying surface water masses at this site. Sanchetta (1982) described the pelagic species N. seminae as a tracer for the relatively warm Alaskan Stream waters, whose presence decreases in surface sediments north of the Aleutian Islands. Here, however, N. seminae is clearly in antiphase with sea ice species, suggesting it is a good indicator of open, ice-free waters. This species dominates the record, showing strong cyclicity with a major downturn after ~1 Ma, in line with subpolar planktonic foraminifers and other temperate/open water dinoflagellates and silicoflagellates. After ~1 Ma, sea ice diatoms and dinoflagellate species dominate their respective assemblages. The occurrence of intermediate-water radiolarian species C. davisiana also increases at this time. A comparison of diatom sea ice species and C. davisiana reveals a similar overall trend. However, at higher frequency the two do not always co-vary, revealing phases of opposing trends. This is also seen in a comparison of dinoflagellate sea ice species and C. davisiana and suggests that the overall increase in glacial–interglacial intensity after ~1 Ma invoked a colder prevailing climate at this site. This high-frequency relationship is not apparent at the Bowers Ridge sites, and therefore these interpretations require further investigation.

Benthic faunas indicate high-frequency changes in bottom water oxygen content over the entire sequence, likely related primarily to documented large changes in surface water productivity and possibly also to bottom water ventilation changes and/or methane seeps. The general increase in abundance and diversity above ~240 m CCSF-A (Assemblage I) coincides with an increase in bottom water oxygen variability. Abundance and diversity may be related to a change in sedimentation rate (Fig. F13), but the assemblage change marks the onset of more variable and occasionally lower oxygen conditions in the uppermost 240 m, which may be linked to regional cooling after the mid-Pleistocene Transition.

The imprint of diagenetic processes in the records of calcareous fossil groups is strong in the sequence recovered at Site U1343. Intervals barren of calcareous nannofossils are synchronous with the occurrence of microscopic to macroscopic authigenic carbonate minerals, either in the form of dolomite or authigenic carbonate (see "Lithostratigraphy"). Yellow-brownish stained foraminiferal tests were encountered throughout Holes U1343A–U1343E. This phenomenon was previously reported in the northwest Pacific, where it was shown to result from postdepositional authigenic carbonate formation in the foraminiferal tests (e.g., Ohkushi et al., 2005). Stable isotope measurements were affected by this carbonate formation and had slightly higher values (Ohkushi et al., 2005). The formation of authigenic carbonate is connected to the release of methane from the sediments, which may have been the cause of its formation at this site. Yellow-brown foraminifers also co-occur with layers of authigenic carbonate in the sediments (see "Lithostratigraphy").

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