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Paleontology and biostratigraphy

Preliminary age assignments for Expedition 341 sediments were based on biostratigraphic analyses of diatoms, planktonic foraminifers, and radiolarians (Figs. F10, F11). Benthic foraminifers were used primarily for paleoenvironmental interpretation. Diatom datum depths were determined by examining core catcher samples, and, where appropriate, additional samples (sampling spacing of 1.5–0.25 m) were taken from the split core sections. Planktonic and benthic foraminifers and radiolarians were examined in core catcher samples only. The preservation, abundance, and zonal assignment for selected samples and for each microfossil group were entered via DESClogik into the LIMS database.


The diatom zonal scheme used here mainly follows biostratigraphic studies by Barron and Gladenkov (1995) (ODP Leg 145) and Yanagisawa and Akiba (1998). Datums are modified following the updated geologic timescale (Hilgen et al., 2012) (Figs. F12, F13). Two- or three-digit code numbers (D120–D20) are given to the primary and secondary biohorizons according to the Neogene North Pacific Diatom (NPD) zone code of Akiba (1986) and Yanagisawa and Akiba (1998).

Several taxonomic studies of diatoms preserved in northeast Pacific sediments are available. For the identification of diatoms to the lowest possible taxonomic level, we mainly followed Akiba and Yanagisawa (1986), Yanagisawa and Akiba (1990), and Barron and Gladenkov (1995). Concerning Pliocene and Pleistocene diatoms found in Expedition 341 sediments, a major taxonomic issue is the modification proposed for the taxonomy of Neodenticula seminae and Neodenticula koizumii, two key diatom biostratigraphic markers in northeast Pacific sediments. As discussed by Yanagisawa and Akiba (1990, 1998), valves of extant N. seminae closely resemble those of the extinct N. koizumii and the morphologically intermediate, extinct taxon Neodenticula sp. A (Yanagisawa and Akiba, 1998). The distinguishing feature among these taxa is the copula structure. Whereas the copula of N. seminae is closed, smooth, and rounded, that of N. koizumii and Neodenticula sp. A is open and pointed, if well preserved. When ambiguity occurred in identifying the species, counts of open versus closed copula were made to determine the first occurrence of N. seminae and the last occurrence of N. koizumii.

Method of study for diatoms

Smear slides of core catcher samples and of silty mud from split core sections that were expected to be diatomaceous (based on visual observation) were prepared by placing a drop of reverse osmosis water and a small amount of raw sediment (tip of a toothpick) onto a glass slide and then evaporating the water by heating on a hot plate. One to two drops of Norland optical adhesive was applied to the dry slide, which was then covered with a 22 mm × 22 mm glass coverslip. The adhesive was solidified by placing the slide under ultraviolet light for ~10 min. Smear slides were partially examined at 400× and/or 1000× for stratigraphic markers and other common taxa using a ZEISS Axioplan microscope.

The total abundance of diatoms and the species composition of the preserved diatom assemblage were determined for all slides. Total diatom abundance of the sediment was assessed according to Scherer and Koç (1996):

  • D = dominant (>60% valves).
  • A = abundant (~20%–60% valves).
  • C = common (~5%–20% valves).
  • F = few (2%–5% valves).
  • R = rare (<2% valves).
  • B = barren (no diatoms present).

The relative abundance of each diatom taxon, as reported in range charts, was estimated using the following qualitative scale:

  • A = abundant (>10 valves/field of view [FOV]).
  • C = common (1–10 valves/FOV).
  • F = few (≥1 valves/10 FOVs and <1 valve/FOV).
  • R = rare (≥3 valves/traverse of coverslip and <1 valve/10 FOVs).
  • X = present (<3 valves/traverse of coverslip, including fragments).

Diatom preservation categories, as reported in range charts, were described qualitatively according to Barron and Gladenkov (1995):

  • VG = very good (no breakage or dissolution).
  • G = good (majority of specimens complete, with minor dissolution and/or breakage, and no significant enlargement of the areolae or a dissolution of frustule rims detected).
  • M = moderate (minor but common areolae enlargement and dissolution of frustule rims, with a considerable amount of breakage of specimens).
  • P = poor (strong dissolution or breakage, some specimens unidentifiable, and strong dissolution of frustule rims and areolae enlargement).


The sites drilled during Expedition 341 in the northeast Pacific are located near ODP Site 887 (Leg 145), where several authors have addressed radiolarian biostratigraphy. Shilov (1995) proposed a new Pliocene to Miocene radiolarian zonation based on Sites 881–887 (Leg 145). Morley and Nigrini (1995) identified several radiolarian biostratigraphic events. Recently, Kamikuri et al. (2004, 2007) conducted detailed investigations of the Neogene radiolarian biostratigraphy at Site 887. During Expedition 341, we used the radiolarian zonation and datums established by Kamikuri et al. (2007). However, radiolarian events in Kamikuri et al. (2007) were tied to Cande and Kent (1995). Here, we recalculated each radiolarian datum and interpreted zones based on the updated geologic timescale (Hilgen et al., 2012).

The radiolarian zonal scheme used here follows Kamikuri et al. (2004, 2007) and uses 49 index fossils from early Miocene to present (Figs. F14, F15). The radiolarian zones from Kamikuri et al. (2007) are restricted to the middle Miocene to Pleistocene and thus cannot be used for older sediments. In addition to the biodatums, we also determined the relative abundances of environmentally sensitive species following Pisias et al. (1997).

Method of study for radiolarians

To prepare samples for light microscopy observation, ~5 cm3 of wet core catcher sediment was sieved and rinsed using a 63 µm mesh sieve. The >63 µm fraction was then processed with 10% v/v H2O2 and 15% v/v HCl to remove calcium carbonate and clay infillings. The sample was then resieved using a 63 µm mesh sieve, and smear slides were prepared using the same technique as for the diatom analyses. Opportunistic counts of radiolarian taxa from mudline samples were also made.

Total radiolarian abundances were determined based on light microscopic observations at 40× magnification using a ZEISS AXIOSKOP microscope as follows:

  • A = abundant (>300 specimens/slide).
  • C = common (200–300 specimens/slide).
  • F = few (100–200 specimens/slide).
  • R = rare (50–100 specimens/slide).
  • P = present (<50 specimens/slide).
  • B = barren (0 specimens/slide).

Radiolarian species abundances were determined by counting at least 300 specimens (where possible) and tabulated as follows:

  • A = abundant (>20 specimens/300 counts).
  • C = common (10–20 specimens/300 counts).
  • F = few (5–10 specimens/300 counts).
  • R = rare (1–5 specimens/300 counts).
  • P = present (1 specimen/300 counts).

Preservation was recorded as follows:

  • G = good (most of specimens complete, including fine structures).
  • M = moderate (several species with minor dissolution and/or breakage).
  • P = poor (most of specimens with common dissolution and/or breakage).


Biostratigraphic datums are infrequent for both planktonic and benthic foraminifers in the high-latitude Pacific Ocean. For planktonic foraminifers, we used the biostratigraphic framework established for the California margin, which focuses on the evolution of the genus Neogloboquadrina (Fig. F16). This framework provides six fossil zones (CM1–CM6) and eight biostratigraphic datum events from 3.6 Ma to present (Kennett et al., 2000; Kucera and Kennett, 2000). Latitudinal variation of several biotic events (e.g., last occurrence of Neogloboquadrina asanoi = 2.0–2.4 Ma; Kucera and Kennett, 2000) within the California margin suggests that some datum events are diachronous, and there may be an age uncertainty when applying these bioevents in the southern Gulf of Alaska margin. Benthic foraminifers are limited in their biostratigraphic use but provide information on water depth, productivity, and oxygenation (Bergen and O’Neil, 1979; Gooday, 2003; Murray, 2006; Jorissen et al., 2007).

Method of study for foraminifers

We analyzed ~40 cm3 of sediment from each core catcher sample for benthic and planktonic foraminifers. Samples were disaggregated in tap water and wet-sieved through a 63 µm mesh sieve. If the sample was semilithified and did not readily disaggregate, the sample was soaked in water with a detergent (Borax) or with a small amount of H2O2. After disaggregation, all washed samples were rinsed with reverse osmosis water. The coarse fraction was placed on filter paper or in a porcelain dish and dried in an oven at ~40°C. The sieve was placed in an ultrasonic bath and thoroughly checked for sample residues to minimize cross-contamination.

Faunal identifications were made using a ZEISS Discovery V8 binocular light microscope. Taxonomic composition, relative abundance, and preservation state were observed in the >63 µm size fraction for benthic foraminifers and in the >125 µm size fraction for planktonic foraminifers. The >63 µm size fraction was also checked for the presence of planktonic foraminifers, but none were identified. Taxonomic identifications of benthic foraminifers at the species level were made largely following Todd and Low (1967), Smith (1973), Bergen and O’Neil (1979), and the Ellis and Messina catalog (; generic identifications largely followed Loeblich and Tappan (1988). Taxonomic identifications of planktonic foraminifers followed Kennett and Srinivasan (1983) and Saito et al. (1981); detailed descriptions of the morphological variation in the genus Neogloboquadrina were taken from Kennett et al. (2000) and Kucera and Kennett (2000).

Semiquantitative estimates of the relative abundances of benthic foraminifers in relation to the total >63 µm size fraction and of planktonic foraminifers in relation to the total >125 µm size fraction were ranked as follows:

  • D = dominant (>30%).
  • A = abundant (>10%–30%).
  • F = few (>5%–10%).
  • R = rare (1%–5%).
  • P = present (<1%).
  • B = barren (no foraminifers present).

Relative abundances of taxa within the benthic foraminifers with respect to the total benthic foraminiferal assemblage (>63 µm) and relative abundances of taxa within planktonic foraminifers with respect to the total planktonic foraminiferal assemblage (>125 µm) were ranked as follows:

  • D = dominant (>30%).
  • A = abundant (>10%–30%).
  • F = few (>5%–10%).
  • R = rare (1%–5%).
  • P = present (<1%).

In addition to the relative abundance of planktonic foraminifer species, the relative abundance (%) of the dextral coiling Neogloboquadrina pachyderma was determined by counting specimens of N. pachyderma with respect to their coiling directions.

Preservation of foraminiferal tests was ranked as follows:

  • VG = very good (no breakage or dissolution).
  • G = good (minor dissolution or pitting on calcareous taxa and no recrystallization; <10% of specimens are broken).
  • M = moderate (frequent etching of calcareous taxa; 10%–30% of specimens are broken).
  • P = poor (frequent dissolution and recrystallization; >30% of specimens are broken).