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doi:10.2204/iodp.proc.341.103.2014

Paleontology and biostratigraphy

Samples from Site U1417 reveal rich assemblages of siliceous microfossils, whereas calcareous microfossils are less abundant in most intervals. Diatoms and radiolarians are abundant to rare and well to poorly preserved in several intervals. These diatom and radiolarian assemblages permit the establishment of biostratigraphic schemes, which are complemented by planktonic foraminifers (Fig. F17; Table T5). Changes in the preservational quality and abundance of siliceous microfossils and in the diagenetic alteration of calcareous microfossils appear to correlate with changes in pore water chemistry (silica concentrations and magnesium, calcium, and alkalinity concentrations, respectively; see “Geochemistry”). According to our biostratigraphic datums, summarized in Table T5, the deepest recovered sediments from Site U1417 are ~10 Ma. Beyond the biostratigraphic schemes, the micropaleontologic assemblages provide insight into paleoclimatologic and paleoceanographic conditions of the subarctic northeast Pacific Ocean. In particular, the relative abundances of certain diatoms, radiolarians, and planktonic foraminifers suggest climatic variations between warm and cool intervals (Figs. F18, F19). Elements of the benthic foraminiferal fauna and diatom flora also suggest the transport of biogenic material from shallower water environments (Fig. F19).

Diatoms

We investigated core catcher samples and sediment from selected split core sections from Holes U1417A–U1417E to define the sediment age and paleoenvironmental conditions (Tables T5, T6). For a detailed description of diatom zonal scheme and taxonomy, see “Paleontology and biostratigraphy” in the “Methods” chapter (Jaeger et al., 2014).

Diatom biostratigraphy

A total of 464 slides, which include samples from the core catcher and split core sections, were prepared for diatom analyses from Holes U1417A–U1417E (Table T6). Eight diatom datums were observed at Site U1417 (Table T5). Diatoms associated with North Pacific Diatom Zones 6B–12 were present (late Miocene–Pleistocene).

Diatom paleoenvironmental considerations

Valve abundance and preservation varies strongly throughout the sediment column (Figs. F17). High to moderate abundances are mostly recorded in the upper ~200 m CCSF-B and between ~300 and ~420 m CCSF-B (Fig. F19). Except for a few samples, the intervals from 245 to 300 and 440 to 800 m CCSF-B are barren of diatoms. In general, valve preservation closely matches the total diatom abundance and tends to be good when abundance is high. Variations in preservation and abundance and shifts in the assemblage possibly reflect past paleoceanographic changes in the pelagic and the coastal regions of the Gulf of Alaska.

The Site U1417 diatom community mostly consists of Pliocene and Pleistocene species, typical of cold waters of the high-latitude northeastern and subarctic Pacific Ocean. The well-preserved, cold-to-temperate water species Neodenticula seminae (Simonsen et Kanaya) Akiba et Yanagisawa is common in the uppermost ~200–225 m CCSF-B (Table T6; Fig. F18). In present-day observations, periods of high relative abundances of N. seminae correspond to periods of high opal and organic carbon fluxes in the northeast Pacific (Takahashi et al., 1990). Based on her survey of the diatom community preserved in surface (Holocene) sediments of the Bering and Okhotsk Seas, Sancetta (1982) concluded that N. seminae is a good tracer of the main path of the Alaska Current and a good indicator of the variability of paleohydrographic and paleoclimatic conditions in the Gulf of Alaska. However, because not all diatom valves are equally affected by dissolution, its downcore record should be cautiously interpreted.

The composition of the cold-water species diatom assemblage differs between the uppermost ~200 m and the interval from ~300 to 435 m CCSF-B. A major downhole shift toward a relative increase of Coscinodiscus marginatus, a decrease of Actynocyclus curvatulus, and the disappearance of N. seminae occurs in the cold-water assemblage around ~225 m CCSF-B (Table T6; Fig. F18). This shift may be associated with the intensification of Northern Hemisphere glaciation after 2.6–3 Ma and the development of the modern halocline system in the subarctic Pacific (Swann et al., 2006). Because C. marginatus is apparently well adapted to present-day high nutrient supply during short periods of low-incident radiation (Takahashi et al., 1990), its higher relative contribution between ~300 and 470 m CCSF-B at Site U1417 can be interpreted as reflecting enhanced productivity during periods of reduced solar radiation. According to the diatom flux study of Takahashi et al. (1990), the well-silicified centric diatom A. curvatulus peaks during modern spring flux maxima in the northeast Pacific. During the late Quaternary, it had its highest relative contribution during glacials in the Bering Sea (Katsuki and Takahashi, 2005).

Other cold-water species present at Site U1417 are Actinocyclus ochotensis Jousé, Asteromphalus robustus Castracane, Bacteriosira fragilis (Gran) Gran, Rhizosolenia hebetata f. hiemalis J.W. Bailey, Shionodiscus trifultus, Thalassiosira gravida Cleve, and Thalassiosira hyalina (Grunow in Cleve et Grunow) Gran (Sancetta, 1982; Medlin and Priddle, 1990; Koizumi 2008). Their occurrences are sporadic, and they have low abundances at Site U1417 (Table T6). Warm-water species Rhizosolenia hebetata f. semispina (Hensen) Gran, Shionodiscus oestrupii (G. Fryxell) A.J. Alverson et al., and Thalassiosira leptopus (Grunow in Van Heurck) Hasle et G. Fryxell (Koizumi, 2008) are rare components of the Site U1417 assemblage. Temperate-water species Thalassiothrix spp. and Stephanopyxis spp. (Hasle and Syvertsen, 1996) are sporadically recorded.

Rare occurrences of Achnanthes spp., Cocconeis spp., Grammatophora spp., Rhabdonema spp., and Paralia sulcata (Ehrenberg) Cleve at Site U1417 also point to a certain degree of downslope transport from the Alaskan coast into the deeper pelagic waters overlying Site U1417. In addition, the sporadic occurrences of several freshwater/neritic marine species (e.g., Achnanthes lanceolata [Brébisson ex Kützing] Grünow and Cyclotella spp.) (Table T6) may reflect periods of enhanced terrigenous input into the pelagic realm of the Gulf of Alaska. Similarly, the recurrent presence of resting spores of Chaetoceros in the uppermost ~250 m CCSF-B (Fig. F19) is indicative of downslope transport into deeper waters of the Gulf of Alaska.

A striking feature of the sediments from Site U1417 is the occurrence of interbedded diatom-rich sediments suggestive of diatom ooze (See “Lithostratigraphy”). These diatom-rich sediments between ~20 and 180 m CCSF-B and between ~360 and 430 m CCSF-B are primarily composed of N. seminae, Thalassiothrix spp., and Coscinodiscus spp. and alternate with mixed sediments (Fig. F16).

Radiolarians

A total of 196 samples in Holes U1417A–U1417E were prepared for radiolarian analyses. Radiolarians are abundant in the upper part of Site U1417 (Fig. F17). However, for depths below ~210 m CCSF-B, the occurrence of radiolarians is sporadic and abundances are low. The preservation of radiolarians is good to poor in the upper ~200 m CSF-A and decreases to poor at greater depths. Fluctuations in species abundance are presented in Table T7.

Radiolarian biostratigraphy

Nine radiolarian datums are present at Site U1417; they indicate sediment ages from late Miocene to present (Table T5; see Fig. F10 in the “Methods” chapter [Jaeger et al., 2014]).

Radiolarian paleoenvironmental considerations

The paleoenvironmental conditions at Site U1417 are inferred by using key radiolarian species grouped according to their common ecological responses to environmental control factors (sea-surface temperature and salinity). These groups are based on multivariate cluster and factor analyses of radiolarians preserved in surface sediment from the North Pacific (Kamikuri et al., 2008) and the Central Indian Ocean (Gupta, 1996), in the sediment traps of the Indian Ocean (Gupta et al., 2002), in the modern radiolarian mapping of the world oceans (Boltovskoy et al., 2010), and from late Miocene sediments (Gupta and Srinivasan, 1992). Cycladophora davisiana and Spongopyle osculosa are grouped as cold deepwater species (>200 m), whereas Ceratospyris borealis, Actinomma boreale, Stylodictya validispina, and Larcopyle buestchlii are cold-water species living in the upper ~200 m of the water column. The subarctic-temperate group is composed of Lithelius minor and Larcopyle weddelium. Cold-water radiolarians (both deepwater and surface species) have higher relative abundances in the upper ~225 m CCSF-B of the sediment record, whereas between ~300 and 400 m CCSF-B, temperate species increase in relative abundance (Fig. F18).

Foraminifers

Core catcher samples from Holes U1417A–U1417D were examined for planktonic foraminifers from the >125 µm size fraction in 85 samples (Table T8) and for benthic foraminifers from the >63 µm size fraction in 105 samples (Table T9). Samples from Hole U1417E would not disaggregate and were not investigated. Most core catcher samples were dominated by siliciclastic material in the >63 µm size fraction, although a few samples were dominated by diatoms or had very little sand-sized material.

Planktonic foraminifers

Sixteen planktonic foraminifer species were encountered in Holes U1417A–U1417D (Table T8). Assemblages are composed of subarctic to temperate water species, dominated by Neogloboquadrina pachyderma. Preservation varies from poor to good, generally decreasing in quality with depth (Fig. F17). Furthermore, the high number of barren samples (40 of 89 samples) suggests diagenetic dissolution of foraminifer shells in the sediments.

The lack of a continuous planktonic foraminiferal record because of poor preservation at Site U1417 makes it difficult to determine precise biozones. At Site U1417, the last occurrence (LO) of Neogloboquadrina inglei (0.7 ± 0.1 Ma) (Kucera and Kennett, 2000) is recognized (Table T5). Neogloboquadrina kagaensis (LO 1.9 ± 0.1 Ma) (Kennett et al., 2000) was also found but appears in only two samples (Table T8).

The most striking change in the planktonic foraminiferal assemblage is a change in the coiling direction of neogloboquadrinids (Figs. F18, F19). An abrupt decrease in dextral coiling neogloboquadrinids occurs between 255.29 and 275.74 m CCSF-B). The timing of this shift roughly corresponds to the interval from the latest part of the Cycladophora sakaii Zone (1.9–3.3 Ma) to the earliest part of the Eucyrtidium matuyamai Zone (1.3–1.9 Ma) and roughly corresponds to the datum event defined by a change in the coiling direction of N. pachyderma on the California margin (~2.0 Ma) (Lagoe and Thompson, 1988). However, numerous barren intervals make it difficult to constrain the timing of this event in the Gulf of Alaska margin. The relative abundance of dextral coiling specimens of neogloboquadrinids has long been used as an indicator of sea-surface temperature during the Quaternary because of their preference for warm-water environments (Kucera, 2007). Taking this environmental preference, as well as the timing of evolutionary changes in the Neogloboquadrina plexus (Kucera and Kennett, 2000), into account, the change in coiling direction is probably due to a major cooling of surface waters beginning around 2.0–2.2 Ma in the northeastern Pacific.

Brown-colored foraminifer shells were occasionally observed in the core catcher samples. Brown-colored shells are dominant in Samples 341-U1417C-14H-CC through 16H-7W, 49–59 cm (121.90–140.58 m CCSF-B), and 341-U1417D-28H-CC (222.42 m CCSF-B). Brown-colored foraminifers were also reported in the Bering Sea (Cook et al., 2011) and in the northwestern Pacific (Ohkushi et al., 2005). Further studies suggested that this color change in foraminifer shells is attributed to the postdepositional diagenesis during authigenic carbonate precipitation associated with anaerobic methane oxidation (Cook et al., 2011; Uchida et al., 2004). At Site U1417, the pore water profile shows an abrupt drop in alkalinity and a magnesium minimum in the pore waters between ~200 and 300 m CCSF-B. These pore water features are coincident with the occurrence of brownish foraminifer shells, suggesting the precipitation of authigenic carbonates within that interval (see “Geochemistry”).

Benthic foraminifers

Of the 105 core catcher samples that were examined from Holes U1417A–U1417D, 52 contained benthic foraminifers, but abundances were generally low (Figs. F17). Low abundances and numerous barren intervals may reflect dissolution and/or dilution by siliciclastic particles and diatoms. Sixty-nine species or species groups are identified (Table T9), although identifications should be taken as tentative. Benthic foraminiferal assemblages at this site are dominated by individuals in the 63–150 µm size fraction in most samples. However, assemblages from Samples 341-U1417D-62X-CC (435.36 m CCSF-B) and 64X-CC (444.49 m CCSF-B) are dominated by individuals >150 µm. Assemblages are diverse, with ~3–25 species or species groups per sample for those samples that contain at least 10 individuals (median = 10 species/sample). Preservation of foraminifers ranges from very good to poor, with poor preservation observed in ~40% of the core catchers (Table T9; Fig. F17).

The taxonomic composition among samples is highly variable with respect to the rare taxa (Table T9). However, the samples can be grouped into three main assemblage types united by their most abundant taxa. The majority of samples contain assemblages with dominant to abundant Gyroidina spp. and major contributions from Melonis pompilioides, Pullenia salisburyi, and Eilohedra vitrea. This composition is consistent with middle to lower bathyal environments from the modern Gulf of Alaska (Bergen and O’Neil, 1979) and with the present water depth of Site U1417.

Cassidulina teretis dominates some samples, including Samples 341-U1417D-62X-CC and 64X-CC, where coal and plant material is also preserved (see “Lithostratigraphy”). In the Gulf of Alaska, C. teretis and other Cassidulina species are most abundant in middle to outer neritic environments (Bergen and O’Neil, 1979), suggesting transport from shallower waters. Species considered indicative of shelf environments from Oregon to Alaska, including Triferina angulosa, Quinqueloculina sp., Lagena laevis, Eggerella advena, and Karrierella baccata (see Culver and Buzas, 1985), are also occasionally encountered at Site U1417, further suggesting transport of foraminifers from the shelf to the deep ocean.

Nonionella labradorica and Buccella inusitata characterize a third assemblage that occurs in Samples 341-U1417D-3H-CC to 6H-CC (23.06–47.80 m CCSF-B). N. labradorica is reported to reach abundances >10% at ~200 m water depth in the Gulf of Alaska, with a second increase below ~1500–2000 m water depth (Bergen and O’Neil, 1979). These species are also characteristic of Pliocene to Pleistocene assemblages from the Japan Trench area interpreted to be from 1500–2000 m water depth (Keller, 1980). Thus, this assemblage also suggests transport from environments shallower than the present water depth at Site U1417.

Other microfossils

Ostracods

Ostracods were present in 10 samples examined for benthic foraminifers but were not taxonomically described (comments in Table T9).

Calcareous nannofossils

Calcareous nannofossils were found in isolated intervals in Holes U1417B–U1417D. Overall, the number of species observed was very low (~2–5 species/sample), which is likely the result of either dissolution or environmental influence. The most common species found are Coccolithus pelagicus, Dictyococcites productus, and small reticulofenestrids (<3 µm). The small reticulofenestrids may be a combination of specimens of Reticulofenestra and Gephyrocapsa, but no central area structures were observed that allowed identification of Gephyrocapsa. The best-preserved nannofossils are found within diatom-rich intervals in the upper ~130 m CCSF-B.

Summary

Age models derived from diatoms, radiolarians, and planktonic foraminifers generally agree (Table T5; Fig. F20). In some intervals, ages were inverted stratigraphically, suggesting reworking or transport of older material to the site. Reworking and transport is also indicated by shallower water benthic foraminifers, diatom resting spores, and coastal diatom specimens (Fig. F19).

The preserved microfossils at Site U1417 provide an excellent opportunity to document the biotic response to climatic change in both the surface and deeper ocean. Marked variations the relative abundances of taxa potentially reflect major changes in global climate variations linked to fluctuations in ice volume and reorganization of North Pacific circulation during the Pliocene and the Pleistocene (Figs. F18, F19). Numerous barren intervals make it difficult to interpret the environmental conditions during the Miocene. However, in intervals where radiolarians are present, the species composition indicates temperate surface water conditions during the Miocene. Both planktonic foraminifers and radiolarians from this site record cooler conditions in the upper ~225 m CCSF-B than in older parts of the record. Coincidently, the diatom flora shifts to a more diverse assemblage above ~215 m CCSF-B, with only a few monospecific C. marginatus intervals. Low preservation of siliceous microfossils prevents the observation of this transition around this specific depth. Observed barren intervals at the site may be due to either lower diatom and radiolarian productivity or to increased dissolution suggested by the retention of dissolution resistent taxa in nearly barren intervals.

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