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doi:10.2204/iodp.proc.341.104.2014

Paleontology and biostratigraphy

The abundances of microfossils at Site U1418 can be divided into two distinct depth intervals. The upper 200 m CCSF-B is rich in both siliceous and calcareous microfossils. Within this sediment interval, preservation is often moderate to good for planktonic and benthic foraminifers, diatoms, and radiolarians, with abundance fluctuations from barren to abundant. Deeper than 200 m CCSF-B, numerous intervals are barren of siliceous microfossils but calcareous microfossils are more consistently observed (Fig. F15). The biostratigraphic age model of Site U1418 was established by combining diatom, planktonic foraminifer, and radiolarian datums (Table T5). The oldest datum encountered is the last occurrence (LO) of the planktonic foraminifer Neogloboquadrina inglei, which suggests that the sediments around 600 m CCSF-B are older than 0.7 Ma. Beyond the biostratigraphic schemes, the micropaleontologic assemblages may provide a record of climatic shifts. We recognize downcore variation in the relative abundances of environmentally sensitive planktonic foraminifers and radiolarians that suggest periods of warming and cooling (Fig. F16). In addition, variation in the diatom assemblages may indicate changes in paleoprimary productivity, sediment transport from coastal areas, and the influence of sea ice (Figs. F17, F18). Changes in the benthic foraminiferal faunas may suggest variation in benthic oxygenation (Fig. F18).

Diatoms

In order to define the sediment age and paleoenvironmental conditions, core catcher samples and samples from selected split core sections from Holes U1418A–U1418F were analyzed (Table T6). For a detailed description of diatom zonal scheme and taxonomy, see the “Methods” chapter (Jaeger et al., 2014a).

The sampling resolution for each of the studied holes is ~3 samples per core; however, 431 of the 573 samples investigated are barren of diatoms (Fig. F15; Table T6). Diatom occurrence and valve preservation vary strongly throughout the sediment column of Site U1418. Diatoms occur more frequently at depths shallower than 190 m CCSF-B and only sporadically at greater depths.

Diatom biostratigraphy

We recognize the bioevent LO of Proboscia curvirostris (Jousé) Jordan et Priddle (D120; 0.3 ± 0.1 Ma) between Samples 341-U1418F-11R-2W, 100 cm, and 4R-2W, 100 cm (Table T5), marking the base of Zone NPD 12 (present–[0.3 ± 0.1] Ma). The LO datum of Actinocyclus oculatus Jousé (D110; 1.1 ± 0.1 Ma) is not observed; therefore, sediments deeper than the LO of P. curvirostris are within Zone NPD 11 ([0.3 ± 0.1]–[1.1 ± 0.1] Ma).

Diatom paleoenvironmental considerations

Valve preservation ranges from good to poor and tends to be better whenever abundance is higher (Fig. F15). Strong variations in abundance and preservation and shifts in the species composition of the diatom assemblage can be used to reconstruct paleoceanographic changes in the pelagic and coastal regions of the Gulf of Alaska.

Independent of the downcore depth, the diatom community of Site U1418 is diverse and mainly consists of Pleistocene to Holocene species typical of cold–temperate, high-latitude ocean waters.

Cold-water species, including Neodenticula seminae (Simonsen et Kanaya) Akiba et Yanagisawa, Actynocyclus curvatulus Janisch in Schmidt, and Rhizosolenia hebetata f. hiemalis Gran (Sancetta, 1982; Medlin and Priddle, 1990; Koizumi 2008), tend to dominate whenever total diatom abundance is greater than “few” (Table T6; Fig. F16). Warm- and temperate-water species are always less abundant than cold-water species (Figs. F16, F18). Main contributors to the warm and temperate group include Rhizosolenia hebetata f. semispina (Hensen) Gran, Stephanopyxis spp., and Bacterosira spp. (Hasle and Syvertsen, 1996; Koizumi, 2008).

The influence of coastal waters on Site U1418 is suggested by the recurrent presence of coastal and benthic diatoms; the resting spores of Chaetoceros, Thalassionema nitzschioides var. nitzschioides (Grunow) Mereschkowsky; and the tycoplanktonic diatom Paralia spp. (Hasle and Syvertsen, 1996). The common occurrence of Chaetoceros resting spores at sediment depths shallower than 200 m CCSF-B is indicative of transport of highly productive coastal waters into the pelagic realm of the Gulf of Alaska. The relative abundance of Chaetoceros resting spores is greater whenever the total diatom abundance is greater than “few.” The sporadic presence of Paralia spp. and some other tycoplanktonic/benthic diatoms (mainly Achnanthes spp., Cocconeis spp., and Odontella aurita [Lyngbye] Agardh) at Site U1418 (shown as coastal and neritic diatoms in Fig. F17) likely reflects input of coastal and neritic sediment into the pelagic realm of the Gulf of Alaska. Rare occurrences of the sea ice–related species Bacterosira bathyomphala (P.T. Cleve) Syvertsen et Hasle, Fragilariopsis cilyndrus (Grunow) Frenguelli, and Thalassiosira gravida Cleve (Armand et al., 2005; Koizumi, 2008) are mainly observed between 50 and 80 m CCSF-B and between 100 and 195 m CCSF-B (Fig. F18).

Radiolarians

A total of 179 samples in Holes U1418A–U1418F were prepared for radiolarian analyses (Table T7). Radiolarian preservation is generally good, and abundances range from few to abundant in Holes U1418A–U1418E, with few barren intervals (Fig. F15). However, in Hole U1418F, we observe long intervals (from 500 to 950 m CCSF-B) where radiolarians do not occur or are rare in abundance with poor preservation.

Radiolarian biostratigraphy

We encountered three radiolarian datums at Site U1418. We encountered only one radiolarian datum in Holes U1418A–U1418E (Fig. F15). We observe the LO of Lychnocanoma sakaii Morley and Nigrini between Samples 341-U1418D-9H-CC (75.21 m CCSF-B) and 341-U1418A-7H-CC (62.15 m CCSF-B), giving an estimated age of 0.03 ± 0.03 Ma (Fig. F19; Table T5). The LO of Stylacontharium acquilonium (Hays) (0.4 ± 0.1 Ma) is not observed in Holes U1418A–U1418E, suggesting that the collected sediment from those holes is younger than 0.4 ± 0.1 Ma and is located within the radiolarian Botryostrobus acquilonaris Zone (0–0.5 Ma). The LO of S. acquilonium is recorded between Samples 314-U1418F-17R-CC (414.65 m CCSF-B) and 11R-CC (354.82 m CCSF-B) (Fig. F19; Table T5). The LO of Stylatractus universus Hays (0.5 ± 0.1 Ma), which defines the top of the radiolarian S. universus Zone (0.5–1.3 Ma), is also encountered in Hole U1418F between Samples 341-U1418F-27R-CC (509.83 m CCSF-B) and 18R-CC (424.13 m CCSF-B) (Fig. F19; Table T5).

Radiolarian paleoenvironmental considerations

Radiolarians are grouped according to their environmental preferences following Kamikuri et al. (2008). Cold-water species, such as Stylodictya validispina and Stylochlamydium venustum, dominate the assemblages (Fig. F16; Table T7). Cycladophora davisiana, an intermediate-water dweller, frequently occurs at depths shallower than 250 m CCSF-B (Fig. F14). We also record the presence of subtropical fauna, including several species of the polyniids group (Gupta and Fernandes, 1997; Gupta, 2002) and Spongaster tetras irregularis. The abundances of each group oscillate downcore. Cold-water species are abundant from 0 to 180 m CCSF-B (Fig. F16). Warm-temperate species occur infrequently; however, when present, their abundances are high from 0 to 180 m CCSF-B (Fig. F16). Variations of cold- and warm-water species probably suggest glacial–interglacial climate changes. An abundance peak of L. sakaii near 120 m CCSF-B suggests that the sediment age is ~60 ka at this depth (Matul et al., 2002). Deeper than 200 m CCSF-B, radiolarians are only occasionally observed (Fig. F15), but when present, several sporadic abundance peaks of cold-water radiolarian species are recorded (Fig. F16). Deeper than 500 m CCSF-B, all of the studied intervals are barren of radiolarians, which suggests high dissolution of siliceous microfossils.

Foraminifers

Core catcher samples from Holes U1418A–U1418D and U1418F were examined for planktonic foraminifers from the >125 µm size fraction in 72 samples (Table T8) and for benthic foraminifers from the >63 µm size fraction in 84 samples (Table T9). Some samples, especially those taken deeper than ~700 m CCSF-B, were difficult to disaggregate and could not be investigated.

Planktonic foraminifers

Within 72 samples, we observed 12 planktonic foraminifer species at Site U1418. Planktonic foraminifers are present in all examined core catcher samples, with abundances ranging from present to dominant. Their preservation ranges from moderate to very good (Fig. F15; Table T8). Neogloboquadrina pachyderma (sinistral) dominates in the planktonic foraminiferal assemblage, followed by Globigerina umbilicata and Globigerina bulloides.

The planktonic foraminiferal faunal assemblage at Site U1418 shows cyclic changes, as seen in the relative abundances of G. bulloides and G. umbilicata (Fig. F16). Because these species have a preference for relatively warm water compared to the preferences of N. pachyderma in the modern North Pacific (e.g., Asahi and Takahashi, 2007; Kuroyanagi et al., 2008; Sautter and Thunnell, 1989), the cyclic changes in their relative abundances likely indicate temperature changes associated with glacial–interglacial intervals.

N. inglei was observed from Sample 341-U1418F-64R-CC (863.39 m CCSF-B) to its LO (0.7 ± 0.1 Ma; Kucera and Kennett, 2000) in Sample 40R-CC (638.32 m CCSF-B) (Tables T5, T8; Fig. F17). The presence of N. inglei in Sample 341-U1418F-21R-CC (454.01 m CCSF-B) is interpreted as being reworked. This reworking hypothesis is also supported by the presence of Neoglolobigerina kagaensis (LO 2.0 ± 0.1 Ma; Kennet et al., 2000) (Fig. F17) and is consistent with the presence of neritic and coastal diatoms and Chaetoceros resting spores, which indicate occasional sedimentary inputs from the shelf margin to Site U1418.

Benthic foraminifers

Of the 84 samples that were examined at Site U1418, 79 contained benthic foraminifers with abundances ranging from present to abundant (Table T9). Preservation ranged from poor to very good with ~38% of samples judged as poorly preserved (Fig. F15). Fifty species or species groups were identified. Median assemblage diversity was 6 genera per sample with a maximum of 21 genera per sample (not sample-size standardized). The number of genera per sample has a generally recurring pattern through time, suggestive of glacial–interglacial intervals, which may partially reflect variations in preservation quality and abundance (Fig. F16).

The majority of assemblages are characterized by dominant to abundant Cassidulina cushmani and/or Nonionella labradorica. Elphidium spp. is also frequently encountered and is abundant to dominant in some samples. C. cushmani is most abundant at middle to lower bathyal water depths (~500–2000 m), and N. labradorica increases in abundance deeper than 1500 m water depth in the Gulf of Alaska (Bergen and O’Neil, 1979). A majority of Elphidium spp. specimens are similar to Elphidium sp. B in Keller (1980), which is described as an upper middle bathyal species (500–1500 m). Occasionally, Stainforthia spp. is dominant and, along with increased abundances in other elongate buliminids (Fig. F18), may indicate periodic low-oxygen conditions at the seafloor. Melonis pompilioides, Pullenia bulloides, Globobulimina auriculata, Cibicides spp., and a small low-trochospiral species tentatively identified as Alabaminella weddellensis are also characteristic of assemblages at Site U1418.

Some species more typical of shelf settings, including Cassidulina teretis, Karreriella baccata, and Elphidium excavatum, are occasionally observed and may have been transported to the site. However, the tests of E. excavatum are very irregular and appear to have developmental abnormalities, which may suggest they were living in suboptimal conditions (i.e., deep water).

Other microfossils

Ostracods

Seven samples examined for benthic foraminifers contained ostracod valves (see comments in Table T9).

Calcareous nannofossils

Calcareous nannofossils are present in isolated intervals in the upper ~600 m CCSF-B of Site U1418. Samples deeper than 600 m CCSF-B were not analyzed. When present, preservation varies from poor to good and abundance varies from very rare to few. The species present include Coccolithus pelagicus, Cruciplacolithus neohelis, Dictyococcites productus, Emiliania huxleyi, Gephyrocapsa muellerae, and other reticulofenestrids.

Summary

Based on the downcore occurrence of microfossils at Site U1418, two distinct intervals were identified. A rich siliceous and calcareous community is observed at depths shallower than 200 m CCSF-B. Deeper than 200 m CCSF-B, calcareous microfossils are consistently preserved, but numerous intervals are barren of siliceous microfossils (Fig. F15). The rare occurrence of diatoms and radiolarians deeper than 200 m CCSF-B impedes identification of reliable biostratigraphic datums. The oldest datum encountered is the LO of the planktonic foraminifer N. inglei, which suggests that sediments around 600 m CCSF-B are older than 0.7 Ma. This age-depth tie point agrees with the Matuyama/Brunhes polarity reversal observed at ~660 m CCSF-B (Fig. F20).

Strong variations in environmentally sensitive planktonic foraminifers and radiolarians record the alternation of warming and cooling intervals (Fig. F16). In addition, variations in bottom water oxygenation are suggested by species changes in the benthic foraminiferal fauna (Fig. F17). Variations in diatom abundance and changes in species composition indicate changes in paleoproductivity, transport from shallow coastal waters, and a period of sea ice influence over Site U1418 (Fig. F17).

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