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doi:10.2204/iodp.proc.323.103.2011

Biostratigraphy

Core catcher samples from Site U1339 are dominated by high-diversity siliceous microfossil assemblages. The samples also contain assemblages of calcareous and organic-walled microfossils with high diversity. In general, the preservation of the different microfossil groups ranges from very good to moderate. Biostratigraphic datum events derived from radiolarians, diatoms, and silicoflagellates show that Site U1339 covers a Pleistocene sequence (Fig. F14; Table T2). Age control of the bottom of the sequence is difficult because of the very rare occurrences of biostratigraphic markers. A silicoflagellate datum event indicates that the bottom of the hole is slightly above the Brunhes/Matuyama Chron boundary (0.781 Ma), but this requires further investigation. The microfossils recovered comprise polar to subpolar floral and faunal assemblages that reflect the geographic setting of Site U1339 and glacial–interglacial changes. The dominance of Neodenticula seminae in the diatom assemblage clearly reflects fluctuations of Alaskan Stream influence. In addition, benthic foraminifer faunal changes likely reflect changes in dissolved oxygen concentrations and/or nutrient contents of the bottom water mass.

Calcareous nannofossils

All core catcher samples from Holes U1339A–U1339D were examined to assess the presence, abundance, state of preservation, and taxonomic composition of calcareous nannofossils (Table T3). These data were recorded semiqualitatively following the codes defined in "Biostratigraphy" in the "Methods" chapter. At Site U1339, more than half of the samples are barren of calcareous nannofossils, whereas only eight samples contain abundant to common nannofossil assemblages. In general, preservation of the observed specimens varies from good to moderate throughout the samples. Coccolithus pelagicus and small gephyrocapsids are generally the dominant taxa. Reworked nannofossils were found in some core catcher samples and are particularly important in Sample 323-U1339B-5H-CC, which contains abundant specimens of Coccolithus miopelagicus (Miocene).

No calcareous nannofossil datum was proposed for this site because barren intervals prevented full interpretation. Emiliania huxleyi was identified in Samples 323-U1339B-1H-CC through 4H-CC, 323-U1339C-1H-CC, and 323-U1339D-1H-CC and 4H-CC. However, its first occurrence (FO) datum (0.29 Ma), which marks the base of nannofossil Zone NN21 (Martini, 1971), could not be reasonably constrained because of the barren intervals below in Holes U1339B–U1339D. A single specimen of Pseudoemiliania lacunosa was observed in Sample 323-U1339-16H-CC, but no specimens were found in the upper and lower sections of this hole or in other holes, and thus the last occurrence (LO) datum of this species (0.44 Ma) was not considered for age model construction of this site. The following samples occur within Zone NN21: Samples 323-U1339B-1H-CC through 4H-CC, 323-U1339C-1H-CC, and 323-U1339D-1H-CC through 4H-CC, whereas Samples 323-U1339B-16H-CC through 22H-CC are within Zone NN19 (defined at its top by the LO of P. lacunosa). The limits of these two zones and the extension of Zone NN20 (which spans the LO of P. lacunosa at 0.44 Ma and the FO of E. huxleyi at 0.29 Ma) are thus not well constrained at this site.

Because of low specimen numbers, relative abundance counts were not performed for samples from Site U1339. However, all taxa observed are characteristic of the subarctic and transitional coccolithophore zones in the Pacific (Okada and Honjo, 1973) and have been observed frequently in cold-water environments and glacial sediments.

Planktonic foraminifers

All core catcher samples from Holes U1339A–U1339D were examined for planktonic foraminifers from the >125 µm size fraction (Table T4). The 125 µm fraction of mudline samples from each hole was also studied. All core catcher samples and mudline samples are dominated by diatoms and, to some degree, coarse-grained clasts. Planktonic foraminifers are present in almost all samples except Samples 323-U1339B-9H-CC, 16H-CC, and 22H-CC; 323-U1339C-12H-CC; and 323-U1339D-9H-CC, 11H-CC, and 20H-CC. The fauna is mainly dominated by Neogloboquadrina pachyderma (sinistral). This species dominates modern subpolar–polar environments and is largely temperature dependent (Bé and Tolderlund, 1971). It is also the dominating species in sediment traps in the Bering Sea (Asahi and Takahashi, 2007). Additional species include Globigerina bulloides, Globigerina umbilicata, and Neogloboquadrina pachyderma (dextral), which are also found in sediment traps in the Bering Sea, reflecting subpolar conditions (Asahi and Takahashi, 2007). This fauna does not change significantly throughout the Pleistocene. Exceptions from the main distribution are Samples 323-U1339B-18H-CC and 323-U1339C-20H-CC, where N. pachyderma (dextral) is found in higher numbers than N. pachyderma (sinistral), and Sample 323-U1339C-20H-CC, where G. bulloides is as abundant as N. pachyderma (sinistral). Both N. pachyderma (dextral) and G. bulloides are characteristic of warmer sea-surface temperatures (SST) than N. pachyderma (sinistral) (e.g., Bé and Tolderlund, 1971).

Benthic foraminifers

Twenty-nine species of benthic foraminifers were recovered in 79 samples from Holes U1339A–U1339D (Tables T5, T6, T7, T8). The majority of samples contain abundant calcareous foraminifers with generally low diversity ranging from ~5 to 10 species per sample. These species show close affinities to those recorded in recent sediments within or near the oxygen minimum zone (OMZ) in the Sea of Okhotsk between 600 and 1750 m water depth (Bubenshchikova et al., 2008), and many have been recorded in Bering Sea deepwater sediments over the last glacial period (Okazaki et al., 2005; Khusid et al., 2006). Assemblages are somewhat different from those of the North Pacific Emperor seamounts (Butt, 1980). Although this site is beneath relatively deep waters, high productivity in surface waters greatly expanded the OMZ. There appears to be no long-term shift in fauna throughout the section; rather, samples exhibit changes in species dominance over timescales shorter than the sampling resolution is able to resolve. These assemblages are described below.

Assemblage I (Islandiella norcrossi–Elphidium cf. batialis)

Assemblage I is varyingly dominated by Islandiella norcrossi, Elphidium cf. batialis, Stainforthia spp., Uvigerina cf. peregrina, and Uvigerina auberiana in the samples listed in Tables T5, T6, T7, and T8. These species are predominantly found in the shallow infauna (0–1.7 cm) in the Sea of Okhotsk and are therefore generally indicative of higher dissolved-oxygen levels in the bottom water within the OMZ.

Assemblage II (Nonionella labradorica–Globobulimina pacifica)

Assemblage II is dominated by the intermediate and deep infaunal species Nonionella labradorica, Globobulimina pacifica, Valvulineria sp., and Nonionella turgida digitata in Samples 323-U1339A-3H-CC, 7H-CC, 12H-CC, and 20H-CC; 323-U1339B-1H-CC, 3H-CC, and 6H-CC; and 323-U1339D-6H-CC and 8H-CC. These species are found predominantly in the intermediate to deep infauna (1.8–5.2 cm) in the Sea of Okhotsk and are generally considered more tolerant of lower dissolved-oxygen levels.

The variation in abundance of these two assemblages is likely linked to glacial–interglacial changes in primary productivity and deepwater ventilation, although further high-resolution work is needed to ascertain its relationship with global climate.

Ostracodes

Core catcher samples were examined for the presence of ostracodes, but only one specimen belonging to the genus Argilloecia was found in Sample 323-U1339B-10H-CC.

Diatoms

Diatom biostratigraphy is based on the analysis of core catcher samples from each core from all holes at Site U1339. Wherever possible, datums were refined by analyzing additional toothpick samples taken at regular intervals from the core in question. Depth positions and age estimates of biostratigraphic marker events are shown in Tables T9, T10, T11, and T12. Diatoms are the dominant microfossils at Site U1339 and show good preservation throughout this Pleistocene record.

Only four cores were retrieved from Hole U1339A because of mechanical problems with the coring system. As a result of the high sedimentation rate in this hole, no datums were observed.

In Hole U1339B, the LO of Proboscia curvirostris was identified between Samples 323-U1339B-9H-CC and 8H-CC, giving a preliminary age of 0.28 Ma based on the result from the piston core at Site ES on the northernmost Emperor seamount (T. Katsuki, unpubl. data). A second age of 0.31 Ma was initially assigned to the interval between 323-U1339B-9H-3, 45 cm, and 9H-4, 45 cm, by the species Thalassiosira jouseae. However, in Hole U1339C, both datums were estimated in the same interval between Samples 323-U1339C-10H-CC and 10H-1, 25 cm. Furthermore, the results in Hole U1339D (Samples 323-U1339D-7H-CC and 8H-CC, for P. curvirostris, and Cores 323-U1339D-6H and 7H, for T. jouseae) contradict the LO relationship in Hole U1339B. Previous studies located the LO of T. jouseae just below the LO of P. curvirostris (Yanagisawa and Akiba, 1998; T. Katsuki, unpubl. data, Site KH99-03 ES-PC). The different LO patterns for both species at this site may be the result of the low abundance of P. curvirostris or the limited sample volume used for slide preparation. However, the extra sampling conducted for Core 323-U1339C-10H revealed that the LOs of P. curvirostris and T. jouseae co-occur in Sample 323-U1339C-10H-1, 25 cm, confining the two datums to an interval of 25 cm. This suggests that the LOs of both species are closely tied, and a finalized LO age of 0.3 Ma was assigned at this site. This age estimate is further supported by the LO datum of radiolarian Spongodiscus sp., which suggests an age of 0.3 Ma at similar depths. In the subarctic Pacific and around Japan, the datum of P. curvirostris is defined as 0.3 Ma (Barron and Gladenkov, 1995; Yanagisawa and Akiba, 1998). Thus, estimated diatom datums are used for general age determination at this site, although minor datum revisions at the 0.01 m.y.-scale resolution will be needed in the near future. The preceding older datum at 0.9 Ma was not observed in all holes. Using the biostratigraphic results of other microfossils and magnetic stratigraphy, Site U1339 cores younger than 0.3 Ma were assigned to the Neodenticula seminae Zone, and cores from 0.3 Ma to the bottom of Holes U1339B–U1339D were assigned to the P. curvirostris Zone (Yanagisawa and Akiba, 1998). All Hole U1339A cores were assigned to the N. seminae Zone.

Diatom assemblages at Site U1339 are composed mainly of N. seminae, Actinocyclus spp., and Thalassiosira spp. (T. antarctica spores, T. latimarginata s.l., and T. oestrupii) throughout the obtained cores. In addition, relative abundances of coastal water diatoms, including Chaetoceros spores, and freshwater diatoms are common in most samples at this site. The occurrence of these coastal/freshwater diatoms can be explained by the proximal location of this site to the Bering Sea shelf. However, such species have been documented to reside in sea ice (von Quillfeldt et al., 2003) and therefore may be produced in situ and may not be a result of lateral transport. In the northeastern subarctic Pacific, the relative abundance of N. seminae in relation to the total number of diatoms has been used as a proxy for the Alaskan Stream (Sancetta, 1982). Indeed, at this site, a clear negative relationship was observed between the Alaskan Stream indicator and pelagic cold-water species for all cores, suggesting that Site U1339 is located at the salinity margin.

Silicoflagellates

Because of diatom abundances, silicoflagellate skeletons are a minor component of the siliceous microfossils at this site. Silicoflagellate abundances are trace to few and, rarely, common (Table T13). Based on the silicoflagellate zonation established by Ling (1973b, 1992), one datum event was detected in Holes U1339B and U1339C and three were detected in Hole U1339D. The bottom of Hole U1339A did not reach the first (youngest) datum event at 0.24 Ma.

The first datum, LO of Distephanus octonarius (0.244 Ma), was estimated between Samples 323-U1339B-5H-CC and 6H-CC, between Samples 323-U1339C-7H-CC and 8H-CC, and between Samples 323-U1339D-5H-CC and 6H-CC. The age estimation for this datum is based on the results at Site 185 of DSDP Leg 19 (Ling, 1973b), and the calibrated age is 0.24 ± 0.05 Ma. The interval above this datum event was assigned to the Distephanus octangulatus Zone in Ling (1973b).

The other two events, FO of D. octangulatus (0.741 Ma) and LO of Dictyocha subarctios (0.736 Ma), were estimated between Samples 323-U1339D-20H-CC and 21H-CC. In Hole U1339B, D. octangulatus was observed in the bottommost sample (323-U1339B-22H-CC), and thus the datum event may be located lower than the bottom of the hole. These datums were found slightly above the Brunhes/Matuyama Chron boundary during Leg 19 in the North Pacific (Ling, 1973b) and Ocean Drilling Program (ODP) Leg 128 in the Sea of Japan (Ling, 1992). However, geomagnetic analysis hints that the Brunhes/Matuyama boundary may possibly be located at ~189 mbsf in Hole U1339B. If this geomagnetic information becomes conclusive, the datum information of the FO of D. octangulatus and the LO of D. subarctios must be reconsidered at this or other sites in the Bering Sea.

Radiolarians

Radiolarian biostratigraphy is based on the analysis of core catcher samples from Holes U1339A–U1339D. The preservation of radiolarians in samples from all cores is generally good, but abundance is common or few and diversity is low (Table T14). Radiolarian stratigraphy at Site U1339 extends from the Botryostrobus aquilonaris Zone to the Stylatractus universus Zone (Kamikuri et al., 2007). The radiolarian assemblage is composed mainly of Cycladophora davisiana, Ceratospyris borealis, and Stylodictya validispina, which occur in most of the sections. Five radiolarian LO datums derived from the subarctic Pacific were identified at this site (Table T15):

• Lychnocanoma nipponica sakaii (50 ka)

• Amphimelissa setosa (80–100 ka)

• Spongodiscus sp. in Ling (1973a) (280–320 ka)

• Axoprunum acquilonium (250–430 ka)

• Stylatractus universus (410–510 ka)

The datums L. nipponica sakaii, A. setosa, and Spongodiscus sp. are basically consistent with each other in a comparison of all holes, indicating consistent occurrences and reliable age controls at this site. However, the datums A. acquilonium and S. universus are supported only by seldom occurrences in Holes U1339B–U1339D, indicating slightly uncertain top positions of stratigraphic age. Plausible datum events of both species occur in Hole U1339C (LO of A. acquilonium, between Cores 323-U1339C-9H and 10H, and LO of S. universus, between Cores 323-U1339C-11H and 12H). No radiolarian datum was identified below the LO of S. universus. Further age control for the lower sections is difficult because of very rare occurrences of the lower radiolarian stratigraphic marker (LO of Eucyrtidium matuyamai [0.9–1.5 Ma]).

Palynology: dinoflagellate cysts, pollen, and other palynomorphs

Palynological assemblages were examined in core catcher samples from Holes U1339A, U1339B, and U1339D (Table T16). These samples were extremely difficult to process because of abundant detrital silica and diatoms. Heavy liquid (sodium polytungstate) was systematically used to separate organic compounds from the silica. This preparation may have resulted in an underestimation of palynomorph abundances because some organic particles may have been trapped and entrained within the silica. Nevertheless, well-preserved and abundant palynomorphs, including mainly dinoflagellate cysts accompanied by variable numbers of pollen and spores (mostly bisaccate pollen and sphagnum), freshwater green algae (Pediastrum and Botryococcus), and organic linings of benthic foraminifers, were recovered in most samples. Preservation is mostly good, and very few reworked palynomorphs were recorded (Table T16).

All investigated samples yielded moderate to abundant dinoflagellate cysts (Table T16). However, the assemblages have relatively low species diversity (16 recorded taxa), probably because of insufficient onboard sample preparation. The dinoflagellate cyst assemblages are highly dominated by two taxa, Brigantedinium spp. and Islandinium minutum, which are produced by heterotrophic protoperidinial dinoflagellates feeding on diatoms (Jacobson and Anderson, 1986). Such assemblages could be related to extremely high diatom production and high upwelling intensity (Radi and de Vernal, 2004). The assemblages in Samples 323-U1339B-1H-CC and 3H-CC are marked by the dominance of I. minutum. In the modern ocean, this species shows its highest abundance in the Arctic and in circum-Arctic environments with very low SST and seasonal sea ice cover (Rochon et al., 1999; Head et al., 2001). In these samples (323-U1339B-1H-CC and 3H-CC), the percentage of I. minutum reaches up to 67%, remarkably analogous to occurrences in the North Water Polynia (Hamel et al., 2002). The assemblages also include taxa that characterize high-latitude environments, such as Spiniferites frigidus and the Arctic morphotype of Polykrikos. On the whole, the assemblages reflect a high-productivity environment with low SST and probably seasonal sea ice cover. Two main assemblage zones were distinguished: an assemblage dominated by I. minutum in Samples 323-U1339B-1H-CC and 3H-CC and an assemblage dominated by Brigantedinium spp. throughout the sequences.

Terrestrial palynomorphs are dominated by Picea and Pinus pollen grains, which occur in moderate to low numbers throughout the sequences. Their occurrence indicates atmospheric and/or oceanic long-distance transport and likely reflects the vegetation of adjacent land (Alaska and Siberia). Freshwater algae occur only in Samples 323-U1339A-1H-CC through 4H-CC, 323-U1339B-1H-CC through 7H-CC, and 323-U1339D-2H-CC.

Sample 323-U1339B-22H-CC contains a single specimen of Hystrichospaeropsi obscura, with an LO at 0.7 Ma in the North Atlantic (Mudie, 1987). However, no specimens of this species were seen in other slides from the same sample or after verification in the bottommost cores of Hole U1339D (Samples 323-U1339D-19H-5, 75 cm; 20H-3, 73 cm; and 21H-2, 77 cm). Thus, this biostratigraphic marker cannot be applied with confidence.

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