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doi:10.2204/iodp.proc.323.105.2011

Biostratigraphy

Core catcher samples from Site U1341 are dominated by highly diverse diatom microfossil assemblages. The samples also contain assemblages of radiolarians, calcareous nannofossils, foraminifers, and organic-walled microfossils with medium to high diversity. The preservation of different microfossil groups ranges from moderate to very good. Biostratigraphic datums were derived from diatom, radiolarian, calcareous nannofossil, dinoflagellate, ebridian, and silicoflagellate bioevents that indicate Site U1341 contains early Pliocene to Holocene sediments. However, the presence of early Pliocene species indicates that some reworking has occurred within the uppermost ~20 m. In all, 23 bioevents were identified (Fig. F12; Table T2).

Both Holes U1341A and U1341B exhibit broadly linear sedimentation rates. Siliceous microfossils occur consistently throughout the section and are mainly composed of high-latitude pelagic species that indicate changes in surface water productivity. Calcareous microfossils are mostly confined to the top of the section above ~250 m CCSF-A for nannofossils and 280 m CCSF-A for planktonic foraminifers. Below these levels, only sporadic occurrences of calcareous fossils and calcareous cemented agglutinated foraminifers were detected, which may be linked to changes in preservation. Dinoflagellates consistently occur throughout the section, indicating changes in the productivity and the sea ice cover of the surface waters.

Calcareous nannofossils

All core catcher samples from Holes U1341A–U1341C were sampled and examined for calcareous nannofossils to assess overall abundance, state of preservation, and abundance by taxa (Table T3). As at IODP Sites U1339 and U1340, calcareous nannofossils are a minor component of the recovered sediments, except in small intervals, and their abundance is generally low, with frequent barren intervals, particularly in the lower parts of the sequence (Fig. F13). Calcareous nannofossil preservation ranges from good to moderate, except for the following samples that contain poorly preserved specimens: 323-U1341B-14H-CC and 46H-CC and 323-U1341C-12H-CC and 16H-CC. Coccolithus pelagicus is present throughout the studied interval, although small and medium gephyrocapsids and small reticulofenestrids (small Dictyococcites spp.) dominate some of the assemblages present. Reworked specimens are not a major component of the sediments at this site, and only a few individuals of Pliocene age were observed.

The frequency and importance of barren intervals in the calcareous nannofossil record of Site U1341 prevented accurate age determinations in most cases, but some periods were identified. The occurrence of Emiliania huxleyi in Samples 323-U1341A-1H-CC and 3H-CC, 323-U1341B-1H-CC, and 323-U1341C-4H-CC allowed assignment of the intervals containing those samples to calcareous nannofossil Zone NN21 of Martini (1971), defined by the first occurrence (FO) datum of E. huxleyi at 0.29 Ma. This taxon is usually represented by the small (<4 µm) morphotype, but well-preserved larger specimens (>4 µm) also occur, mainly in Sample 323-U1341B-1H-CC.

Calcareous nannofossil Zone NN20 and the top of Zone NN19 can be well constrained only in Hole U1341C (Table T3), where no barren samples occur between the FO of E. huxleyi in Sample 323-U1341C-4H-CC and the last occurrence (LO) datum of Pseudoemiliania lacunosa in Sample 323-U1341C-6H-CC, establishing an age older than 0.44 Ma for the latter. However, the occurrence of a characteristic 25 cm thick nannofossil ooze layer in Core 323-U1341C-8H, which was also recovered in Cores 323-U1341A-8H and 323-U1341B-8H (see below), indicates that the top of Zone NN19 is located in Holes U1341A and U1341B, at least on the top sections of these cores.

In older sediments, the combination of barren intervals with the absence of marker taxa prevented the application of Martini (1971) zonal schemes and the assignment of ages to all samples. However, the changes observed in the composition of calcareous nannofossil assemblages between Samples 323-U1341B-36H-CC (dominated by C. pelagicus) and 52H-CC (predominantly Dictyococcites spp. and Reticulofenestra minuta) are similar to those recorded at nearby DSDP Site 883 by Sato et al. (2002), who indicated that the reversal in abundance between these taxa marks the onset of heavy glaciations in the Northern Hemisphere and dated this horizon (their Datum Plane A) to 2.75 Ma. According to this information, Samples 323-U1341B-52H-CC and below are older than 2.75 Ma. Independent dating by means of diatom, radiolarian, and silicoflagellate biostratigraphy as well as preliminary chronomagnetostratigraphic results for this site is consistent with this estimation (Fig. F12).

As stated above, the frequent occurrence of barren intervals, which sometimes last for tens of meters, is a major feature of the sequences obtained at Site U1341 and could probably be correlated to the lowermost 400 m recovered from Hole U1340A. In most cases, calcareous nannofossils and planktonic foraminifers (and, to a lesser extent, benthic foraminifers) are equally affected, indicating a preservational imprint on the carbonate signal with a probable association with authigenic dolomites. However, transient high-productivity intervals of carbonate-walled organisms should not be discounted as the cause of the occurrence of carbonate-rich horizons within the recovered sequence such as the thin (~25 cm) nannofossil ooze layer observed in all three holes drilled at Site U1341 (intervals 323-U1341A-8H-7, 10 cm; 323-U1341B-8H-3, 60–80 cm; and 323-U1341C-8H-5, 146 cm) (Table T3). This ooze is characterized by high numbers of coccoliths and an assemblage dominated by small gephyrocapsids, and it displays abundant heavily calcified specimens of G. caribbeanica and C. pelagicus. The occurrence of P. lacunosa above the ooze layer (Core 323-U1341C-6H) and the occurrence of the Brunhes/Matuyama Chron boundary below it (Cores 323-U1341A-10H, 323-U1341B-9H, and 323-U1341C-10H) indicate that sedimentation of the layer occurred between 0.44 and 0.78 Ma. The acme interval of G. caribbeanica is a well-known event that takes place in most oceans between marine isotope Stages (MIS) 14 and 8 (Flores et al., 2003) and may have started even before at mid-MIS 15 in the northeast Atlantic (Hine, 1990). Therefore, an age range of 0.44–0.6 Ma is suggested for this calcareous nannofossil ooze layer. The proliferation of G. caribbeanica in the oceans has been suggested as a possible cause of the mid-Brunhes dissolution interval in the oceans (Barker et al., 2006), centered on MIS 11. Therefore, shore-based research on the origin of this ooze will be of great interest.

Planktonic foraminifers

Planktonic foraminifers (>125 µm) were studied in all core catcher samples from Holes U1341A–U1341C (Table T4). Three mudline samples from the tops of Cores 323-U1341A-1H, 323-U1341B-1H, and 323-U1341C-1H were also analyzed. All residues are dominated by either diatoms or siliciclastics. Sponge spicules were also observed in the samples. The late Pleistocene fauna at Site U1341 is dominated by Neogloboquadrina pachyderma (sinistral), reflecting late Pleistocene cooling. This species is the most abundant taxon in the modern Bering Sea (Asahi and Takahashi, 2007). A small number of subpolar species, including Globigerina bulloides, Globigerina umbilicata, and Neogloboquadrina pachyderma (dextral), were also found in the late Pleistocene fauna. Below ~150 m CCSF-A, planktonic foraminifer faunas are occasionally dominated by the subpolar species G. bulloides, G. umbilicata, and N. pachyderma (dextral), which indicates warmer sea-surface temperatures because these subpolar species are strongly temperature dependent (e.g., Bé and Tolderlund, 1971). No planktonic foraminifers were found below ~300 m CCSF-A, except for Samples 323-U1341A-35H-CC and 38H-CC and 323-U1341B-36H-CC and 61X-CC from the late Pliocene (~2.5 Ma). Only subpolar species were found in these samples. Globigerina uvula appears in high numbers in Sample 323-U1341B-36H-CC. This species is relatively widespread in high-latitude oceans (Saito et al., 1981) and has been found on the shelf of Norway and the Barents Sea, where it tolerates slightly low salinities (34–35) (e.g., Husum and Hald, 2004). Preliminary studies in the Bering Sea indicate that G. uvula also tolerates slightly low salinities in this region (H. Asahi, pers. comm., 2009).

Benthic foraminifers

Around 60 species of benthic foraminifers were identified in 140 samples from Holes U1341A–U1341C (Tables T5, T6, T7). Assemblages from the tops of the sections down to around Sample 11H-CC in all holes are of relatively high diversity and abundance, showing affinities to assemblages within or near the OMZ in the Sea of Okhotsk (Bubenshchikova et al., 2008), and are also partially composed of more common deepwater Pacific Ocean species (Butt, 1980). In the remainder of the cores, benthic foraminifers are less abundant and the agglutinated species Eggerella bradyi and Martinottiella communis are more important components of the assemblages, indicating significant dissolution of calcareous material.

Assemblage I (Globobulimina pacifica–Uvigerina)

Assemblage I is characterized by high-diversity and high-abundance faunas between Samples 1H-CC and 11H-CC in all holes, with persistent occurrences of the species Globobulimina pacifica, Uvigerina auberiana, Gyroidinoides soldanii, and Islandiella norcrossi. Other common species include Bulimina aff. exilis, Melonis barleeanum, and Pullenia bulloides. 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 associated with changes to the OMZ.

Assemblage II (Eggerella bradyi)

Assemblage II consists of medium- to low-diversity faunas between Samples 12H-CC and 37H-CC in all holes, characterized by the relatively persistent occurrence of E. bradyi. Other common species include G. pacifica, G. soldanii, Epistominella pulchella, and P. bulloides. This interval may be affected by dissolution because other calcareous microfossil groups are less abundant.

Assemblage III (depauperate)

The major characteristic of Assemblage III is the high number of barren samples between Samples 38H-CC and 45H-CC in all holes. E. bradyi occasionally occurs.

Assemblage IV (Martinottiella communis)

This low-diversity and low-abundance assemblage contains relatively continuous occurrences of M. communis and, to a lesser extent, E. bradyi between Samples 46H-CC and 71H-CC in all holes. Other species occasionally present are G. soldanii, Lenticulina spp., M. barleeanum, and Uvigerina spp. Samples 61H-CC and 62H-CC in all holes have relatively high diversity and abundance. M. communis has been recorded in low-oxygen intermediate waters off the Pacific coast of Japan (Kaiho and Hasegawa, 1986), where oxygen levels are somewhat higher than those in the modern Bering Sea bottom water at this site.

Ostracodes

Ostracodes are extremely rare at Site U1341, and only four specimens were found in the interval between Samples 323-U1341A-9H-CC and 35H-CC (Table T5). Despite their very low numbers, ostracode valves are large and very well preserved, suggesting that dilution by high diatom abundance took place. Three species were identified: Henryhowella dassiderma, Bradleya normani, and Cytheropteron massoni. Species of Henryhowella are epifaunal and have worldwide distribution in water depths ranging from ~500 to 5000 m (Ayress et al., 2004; Yasuhara et al., 2008). The highest relative abundances of Henryhowella and Bradleya were recorded during interglacials at deep North Atlantic sites that are influenced today by North Atlantic Deep Water (NADW) (Cronin et al., 1999; Didié and Bauch, 2000, 2002; Alvarez Zarikian et al., 2009). The glacial–interglacial distribution of the species at these sites suggests that they prefer adequate food supply and well-oxygenated bottom waters. The genus Cytheropteron has worldwide distribution but the highest species diversity in cold waters (Ayress et al., 2004; Stepanova, 2006). In the North Atlantic, the highest occurrences of Cytheropteron are associated with deglacial transitions (Cronin et al., 1999; Alvarez Zarikian et al., 2009).

Diatoms

Diatom biostratigraphy is based on analysis of core catcher samples from Holes U1341A and U1341B. Depth positions and age estimates of biostratigraphic marker events are shown in Table T2. Diatoms are the dominant microfossil in all holes and show good preservation throughout (Tables T8, T9).

A biostratigraphic zonation was constructed back to the early Pliocene for both Holes U1341A and U1341B. Reworking was observed in the uppermost three cores (to 22.3 mbsf) of Hole U1341A and the uppermost two cores (to 18.5 mbsf) of Hole U1341B, which contain species from both the early Pleistocene and Pliocene.

The LOs of Proboscia curvirostris, Thalassiosira jouseae, and Proboscia barboi were identified in Sample 323-U1341A-5H-CC (36.39 mbsf), and only P. curvirostris and T. jouseae were identified in Sample 323-U1341B-4H-CC (32.83 mbsf), giving a composite estimated age of 0.3 Ma. Although the LO of P. curvirostris is set at 0.28 Ma in the Bering Sea (Barron and Gladenkov, 1995), in the subarctic Pacific and around Japan the datum is defined at 0.3 Ma (Barron and Gladenkov, 1995; Yanagisawa and Akiba, 1998) (see "Diatoms" in "Biostratigraphy" in the "Site U1339" chapter for further discussion). Cores above 323-U1341A-5H were assigned to the Neodenticula seminae Zone of 0.3 Ma and younger. This datum is closely matched in Hole U1341B in Sample 323-U1341B-4H-CC. The assemblage is dominated by N. seminae, Actinocyclus curvatulus, Thalassiosira spp. (i.e., Thalassiosira latimarginata s.l., Thalassiosira antarctica spores, and, to a lesser extent, Thalassiosira nordenskioeldii), Actinocyclus ochotensis, Odontella aurita, and Porosira glacialis.

The following age of 0.9 Ma in Hole U1341A is defined by the last common occurrence (LCO) of Actinocyclus oculatus in Sample 323-U1341A-15H-CC. The corresponding datum in Hole U1341B was established in Sample 323-U1341B-13H-CC; however, this species is not as abundant in this hole, making age determination more difficult. The assemblage for the A. oculatus Zone in Hole U1341B is dominated by N. seminae, A. curvatulus, P. curvirostris, Thalassiosira spp. (i.e., T. latimarginata s.l., T. antarctica spores, and T. oestrupii), Rhizosolenia barboi, and Coscinodiscus marginatus, along with the less common taxon Thalassiothrix longissima.

The A. oculatus Zone is constrained by the LCO of Neodenticula koizumii (2.1 ± 0.1 Ma). In Hole U1341A this datum is set in Sample 323-U1341A-23H-CC and co-occurs in Hole U1341B in Sample 323-U1341B-23H-CC. This zonation in Holes U1341A and U1341B is further supported by the LO of secondary order datum Stephanopyxis horridus and the LCO of Thalassiosira antiqua (1.7–2.0 Ma). In Hole U1341A the assemblage is composed mainly of N. koizumii, A. curvatulus, Thalassiosira spp. (i.e., T. latimarginata s.l. and T. oestrupii), and C. marginatus, along with the less common taxon T. longissima. A similar assemblage is observed in Hole U1341B, with the addition of S. horridus.

The LCO of Neodenticula kamtschatica in Sample 323-U1341B-41H-CC defines the next biostratigraphic zone at 2.7 Ma. This species is not observed in Hole U1341A, which indicates that the bottom of the hole is younger than 2.7 Ma. The assemblage composition in Hole U1341B is dominated by N. kamtschatica, N. koizumii, Actinocyclus spp. (A. curvatulus, A. ochotensis, and A. oculatus), Thalassiosira spp. (T. latimarginata s.l., T. oestrupii, and T. antiqua), S. horridus, Stephanopyxis turris, and, to a lesser extent, T. longissima and Thalassiosira eccentrica.

The datums and subsequent age model were further confirmed by comparison between the relative abundance of N. seminae and N. kamtschatica in both Holes U1340A and U1341B (Fig. F14). This correlation technique using tie points to match similar events in the two records was applied to confirm and support the age models at Sites U1340 and U1341, following the difficulties encountered with the identification of the FO and LO of N. seminae and N. koizumii. It is evident in Hole U1341B that N. seminae appears sporadically in older sediments but reveals a rapid increase (RI) that is mirrored in Hole U1340A. A similar correlation occurs for N. kamtschatica at both Sites U1340 and U1341, whereby an abrupt decrease in abundance (or rapid decrease [RD]) in Hole U1340A is matched in Hole U1341B.

The newly termed RI of N. seminae abundance co-occurs with the top of a paleomagnetic datum—the Olduvai section—giving a date of 1.778 Ma to Sample 323-U1341B-22H-CC. This event was matched in Hole U1340A, and the same datum was extrapolated (see "Biostratigraphy" in the "Site U1340" chapter for further discussion). The RD of N. kamtschatica was also matched to Hole U1340A, where the abrupt decrease coincides with the paleomagnetic Gauss Event, giving a subsequent datum of 2.581 Ma. (see "Biostratigraphy" in the "Site U1340" chapter for further details).

An age of 3.7–3.9 Ma was assigned to Sample 323-U1341B-51H-CC by the first common occurrence (FCO) of N. koizumii. The remainder of Hole U1341B from Samples 323-U1341B-51H-CC through 71X-CC was defined as the N. kamtschatica Zone. The absence of Rouxia californica and the secondary order datums Thalassiosira praeoestrupii, Cosmiodiscus insignis, and Thalassiosira temperei signifies that the bottom of Hole U1341B does not enter North Pacific Diatom (NPD) Zone 7Ba (6.6 Ma). The assemblage for this zone is typified by N. kamtschatica and by the heavily silicified species C. marginatus, Stephanopyxis spp. (S. horridus, S. turris, and Stephanopyxis zabelinae), and, to a lesser extent, T. latimarginata s.l., and T. longissima.

Overall, the datums show a consistent offset of 100–200 k.y. compared to the paleomagnetic datums (Fig. F12), suggesting that the marker species in this region are diachronous. Therefore, a major revision of the biostratigraphic zonation will be required for this region.

Silicoflagellates and ebridians

Silicoflagellate and ebridian counting was conducted in Holes U1341A and U1341B (Tables T10, T11). The general trend of silicoflagellate and ebridian abundances at this site is similar to that at Site U1340. Five certain datums and one uncertain datum were estimated at this site based on the silicoflagellate and ebridian zonation established by Ling (1973, 1992) (Table T2). The youngest datum event, the LO of silicoflagellate Distephanus octonarius, is uncertain because of the limited silicoflagellate abundance and the significant influence of reworked materials in the uppermost three or four cores in both holes. Suggested reworked silicoflagellates in these cores are as follows: Bachmannocena apiculata (LO: late Miocene) in Sample 323-U1341B-4H-CC, Corbisema cf. hastata cunicula (LO: Pliocene?) in Sample 323-U1341B-3H-CC, and Dictyocha subarctios (LO: 0.7 Ma) in Sample 323-U1341B-2H-CC. The LO of silicoflagellate D. subarctios was estimated in Sample 323-U1341A-7H-CC (50.95–60.41 mbsf) but is uncertain in Hole U1341B because of the species’ limited abundance and sporadic occurrence. The next oldest datum is the LO of ebridian Ammodochium rectangulare, which suggests an age of 1.9 Ma. This event was estimated in Samples 323-U1341A-27H-CC (233.21–241.4 mbsf) and 323-U1341B-26H-CC (228.52–238.11 mbsf). The LO of ebridian Ebriopsis antiqua antiqua was found in Samples 323-U1341A-37H-CC (328.09–332.81 mbsf) and 323-U1341B-37H-CC (325.83–335.28 mbsf). The LO datum of silicoflagellate Distephanus jimlingii was found in Samples 323-U1341A-38H-CC (332.71–342.26 mbsf) and 323-U1341B-38H-CC (335.18–345.78 mbsf). In both holes, these datum events fit the age-depth profiles established by other biostratigraphic results and paleogeomagnetic polarity events (Table T2; Fig. F12). Silicoflagellate and ebridian zones were assigned based on these datums, with the exception of the upper part of both holes, as listed in the abundance tables.

Silicoflagellate assemblages are composed mainly of Distephanus speculum, Distephanus medianoctisol, and D. octonarius in most core catcher samples from both holes. However, their dominance is not constant throughout, which may suggest a paleoenvironmental change in surface waters.

Radiolarians

Radiolarian biostratigraphy is based on analysis of core catcher samples from Holes U1341A–U1341C. Radiolarian stratigraphy at Site U1341 extends from the Botryostrobus aquilonaris Zone (upper Quaternary) to the Dictyophimus bullatus Zone (middle Pliocene) in the subarctic Pacific (Kamikuri et al., 2007). In Sample 323-U1341B-71X-CC the LO of D. bullatus (3.8–4.0 Ma) was identified by the occurrence of several specimens. However, these specimens are not the typical form described by Morley and Nigrini (1995) and Motoyama (1996), although they are similar to Dictyophimus sp. B described by Motoyama (1996). We assigned this datum because Motoyama (1996) described the simultaneous occurrence of the LOs of both D. bullatus and Dictyophimus sp. B. Hence, we suggest that the age of the core bottom is between 3.8 and 4.0 Ma. Clearly, further shore-based work will be required to confirm the datum. As at Site U1340, the Stylatractus universus Zone (0.4–0.9 Ma) is unclear because of very rare occurrences of S. universus. Nine radiolarian datums derived in the subarctic Pacific were identified at this site (Tables T2, T12). These datums are basically consistent with other biostratigraphic and paleomagnetic datums and indicate relatively high sedimentation rates (average = 15 cm/k.y.). Among the radiolarian datums, the FOs of Eucyrtidium matuyamai in Holes U1341A–U1341C significantly differ from paleomagnetic datums. Because the FO of this species in Hole U1340A (at another Bowers Ridge site) is consistent with paleomagnetic datums, the discrepancy at Site U1341 stems from missing the true FO for E. matuyamai due to its rare occurrence.

Radiolarian abundances and preservation are shown in Table T13. Radiolarian preservation is good to moderate in sediment samples from the uppermost 400 m at Site U1341 and moderate to poor in the section below ~400 mbsf. Below 400 mbsf, few radiolarian specimens occur, and some are broken or incomplete. Radiolarians are abundant to common in the uppermost 100 m, but their abundance decreases in the interval below ~100 mbsf in each hole.

Changes in the abundance of Cycladophora davisiana, a cold, well-ventilated intermediate water–dwelling and deepwater-dwelling species, show antiphase patterns with abundances of calcareous microfossils (calcareous nannofossils and planktonic foraminifers) in each hole at Site U1341. This abundance pattern suggests a relationship between intermediate water formation in the subarctic Pacific and carbonate preservation. Cycladophora sakaii is thought to be an ancestor species of C. davisiana (Motoyama, 1997). Occurrences of C. sakaii are very low at Site U1340 (water depth = ~1300 m). In contrast, C. sakaii is found constantly below ~100 mbsf at Site U1341 (water depth = ~2200 m), implying that C. sakaii dwelled mainly in deep water below ~1000 m.

Palynology: dinoflagellate cysts, pollen, and other palynomorphs

Forty-four core catcher samples from Holes U1341A and U1341B were analyzed for palynological content (Table T14). Five samples are barren with respect to dinoflagellate cysts, but they contain few to rare pollen and reworked palynomorphs. Palynomorphs are moderately to well preserved in the upper part of the sequence (the uppermost 300 m of Hole U1341A), whereas they are moderately to poorly preserved in the interval below 300 mbsf. Pollen grains and spores occur in most samples, with variable concentrations usually ranging between 0 and 400 grains/cm³. Samples are dominated by tree pollen Pinus and Picea. Shrub and herb pollen and spores of pteridophytes (fern and moss) occur in low numbers, except in Samples 323-U1341A-3H-CC, 5H-CC, and 11H-CC, where they are relatively abundant. This variability reflects changing vegetation in adjacent lands. Phycoma of prasinophytes (freshwater algae) are present in very low concentrations in only a few samples (Table T14). Reworked pre-Neogene palynomorphs, which indicate detrital transport, occur generally in few numbers, except for Samples 323-U1341A-5H-CC, 11H-CC, and 13H-CC and 323-U1341B-45H-CC, where they are common. Organic linings of benthic foraminifers, which can be used as an index of calcium carbonate dissolution (de Vernal and Mudie, 1992), occur throughout the sequence and peak in abundance between 340 and 440 mbsf and in Sample 323-U1341A-11H-CC. This peak suggests some calcium carbonate dissolution. However, shipboard palynological preparations and analyses were insufficient to accurately assess carbonate preservation.

With 21 taxa recorded, dinoflagellate cyst species diversity is slightly higher at Site U1341 than at Sites U1339 and U1340. Dinoflagellate cysts are common to very abundant in most samples from the uppermost 200 m, whereas their concentration is usually <400 cysts/cm³ below 200 mbsf. Note that the concentration of dinoflagellate cysts is extremely variable, ranging from <100 to >10,000 cysts/cm³. Such amplitude of variation greater than two orders of magnitude suggests varying dinoflagellate productivity and sea-surface conditions through geologic time. An extremely high abundance of dinoflagellate cysts at 98.41 mbsf (Sample 323-U1341A-11H-CC) may well be coeval to an interglacial or interstadial interval.

In summary, dinoflagellate cyst assemblages are dominated by protoperidinial taxa commonly related to heterotrophic productivity. However, autotrophic taxa such as Bitectatodinium tepekiense and Nematosphaeropsis lemniscata/labyrinthus occur in many samples, mainly from the uppermost 300 m, and dominate the assemblages in Samples 323-U1341A-7H-CC and 15H-CC. Their dominance may be related to the incursion of oligotrophic, more oceanic water. The polar and subpolar taxa Islandinium minutum, Operculodinium centrocarpum-Arctic morphotype, and Impagidinium pallidum, which are known to be abundant in regions where sea ice cover occurs up to 12 months per year and winter sea-surface temperature is <0°C, occur only in the upper part of the sequence starting at ~371 mbsf (Table T14). Relatively warm water species such as Impagidinium aculeatum and Impagidinium patulum occur only sporadically. Below 300 mbsf the diversity decreases, and the assemblages are dominated by the protoperidinial Brigantedinium spp. and Trinovantedinium variabile or by the extinct species Filisphaera filifera. Trinovantedinium variabile appears below 300 m CCSF-A and dominates the assemblage of Sample 323-U1341A-60X-CC. In modern sediments, this species has been recorded in the northeastern Pacific margins, with very low percentages at latitudes south of 40°N (Radi and de Vernal, 2004). It has not been recorded in the modern North Atlantic and Arctic oceans, but it was reported to occur in Miocene and Pliocene sediments of the southern North Sea (e.g., Louwye et al., 2007). In Hole U1341A, Lejeunecysta falax was recorded for the first time in Pleistocene sediments (Sample 323-U1341A-15H-CC). This species was previously known to occur only in Pliocene and Miocene formations with poorly constrained biostratigraphic occurrence ranges (Bujak, 1984; de Vernal and Mudie, 1992). Finally, F. filifera indicates an age of 1.4–1.7 Ma in Sample 323-U1341A-23H-CC according to its LO in the North Pacific and North Atlantic (Bujak, 1984; M. Smelror et al., unpubl. data).

Discussion

Quantitative and semiquantitative data were collected for each microfossil group at Site U1341 (Fig. F13). Diatoms are the most abundant microfossil group throughout the cores. Heavily silicified diatoms are dominant in the lower interval below ~400 m CCSF-A and decline thereafter, suggesting that nutrients became less available, perhaps because of enhanced stratification after the onset of Northern Hemisphere Glaciation. Below 300 mbsf, dinoflagellate cyst diversity decreases and assemblages are dominated by heterotrophic protoperidinial taxa associated with diatom production. Sea ice diatoms increase above 400 m and again above 100 m CCSF-A, indicating increasing influence of sea ice over time. Polar dinoflagellate cysts, which are known to occur in regions with seasonal sea ice coverage of up to 12 months per year, occur significantly only in the uppermost 200 m of the sequence, confirming the presence of seasonal sea ice coverage. Calcareous foraminifers and nannofossils have the best preservation in the uppermost part of the section above ~240 m CCSF-A, whereas in the lower part of the sequence their preservation has probably been affected by postdepositional processes such as diagenetic recrystallization. The best calcareous preservation broadly coincides with the highest abundances of sea ice diatoms, polar dinoflagellate cysts, and radiolarians living in cold, oxygen-rich intermediate water masses. Because the preservation of carbonate in deep-sea sediments is hindered by high productivity and associated low oxygen in the bottom waters, it appears that productivity may have been reduced by direct seasonal sea ice coverage and enhanced stratification above ~240 m CCSF-A. Sea ice diatoms, intermediate water–dwelling radiolarians, and calcite preservation increase markedly at ~110 m CCSF-A.

Benthic foraminifer assemblages vary upsection, with the gradual replacement of M. communis Assemblage IV with E. bradyi Assemblage II between ~430 and ~370 m CCSF-A (~3–2.5 Ma) (Fig. F13). The ecological significance of these species is not well known, but the change in assemblage could reflect a decrease in oxygen levels after this time via a change in surface water productivity and/or deepwater circulation regime.

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