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doi:10.2204/iodp.proc.323.102.2011 BiostratigraphyPreliminary ages were assigned based on the biostratigraphic assessment of calcareous nannofossil, planktonic foraminifer, diatom, silicoflagellate, ebridian, dinoflagellate cyst, and radiolarian events. Paleoenvironmental interpretations are based on all of these groups, including benthic foraminifers and ostracodes. Biostratigraphy was correlated to the geomagnetic polarity timescale (GPTS) used for Expedition 323, which is based upon a composite of several timescales (Lourens et al., 2004). Microfossil datums were established by analyzing core catcher samples. When a more refined age determination was necessary, samples were taken from the core sections at appropriate intervals. The shipboard methods used to assess each microfossil group are detailed below. Calcareous nannofossilsThe standard Tertiary and Quaternary calcareous nannoplankton zonation proposed by Martini (1971) was selected as the main temporal framework for Expedition 323. This scheme contains well-established additional events such as the first occurrence (FO) and last occurrence (LO) datums of Reticulofenestra asanoi (Sato and Takayama, 1992). However, the application of this zonation to materials from the Bering Sea is problematic because the known biogeographical distribution of marker taxa (such as discoasterids) does not cover high latitudes in the Pacific Ocean (Sato et al., 2002). Therefore, the standard calcareous nannofossil zonation and the bioevents list were adapted to the specific composition of the assemblages present in the studied samples, and additional high-latitude calcareous nannofossil events, such as abundance reversals noted by Sato and Kameo (1996) and Sato et al. (2002), were added to Table T1. Age estimates for the proposed datums were adopted from Lourens et al. (2004), with the exception of the reversal in dominance between Dictyococcites spp. (small) and Coccolithus pelagicus, which was dated to 2.75 Ma by Sato et al. (2002). MethodsSamples were prepared following a standard smear slide preparation method. A small amount of sediment was placed onto a glass slide with a few drops of deionized water. The sediment was then smeared with a wooden toothpick and allowed to dry on a hot plate at low temperature. A drop of Norland optical adhesive was added as a mounting medium to the sample, which was covered with a coverslip and set to harden under an ultraviolet lamp for 15 min. The slide was then examined with a Zeiss light microscope at 1000× magnification using cross-polarized and transmitted light. As a general rule, the taxonomic concepts summarized in Young (1998) and Hine and Weaver (1998) were followed. However, terms were occasionally adjusted to adapt classification to the special characteristics of the assemblages present in the Bering Sea. Onboard taxonomic classification and data files were also updated for specific taxa such as Coccolithus pelagicus (which was divided into C. pelagicus and Coccolithus braarudii, in accordance with Sáez et al., 2003). Calcareous nannofossil preservation was estimated using the criteria established by Raffi and Flores (1995), who proposed four categories depending on the "average" state of preservation of the specimens examined in the smear slides:
The total abundance of calcareous nannofossils in each sample was assessed using semiquantitative abundance codes:
The abundances of individual nannofossil taxa were assessed as follows:
ForaminifersHigh-latitude Bering Sea planktonic foraminiferal assemblages have low diversity, and thus biostratigraphic datums are infrequent. However, preliminary ages were assigned using Neogene planktonic biostratigraphic zonations from the North Atlantic (Weaver and Clement, 1987; Spezzaferri, 1998) for Pliocene and Pleistocene samples from Expedition 323 (Table T2). Planktonic foraminiferal taxonomy largely follows that of Kennett and Srinivasan (1983). The Neogene timescale correlated to magnetostratigraphy, as defined by Lourens et al. (2004), was applied in this study. Benthic foraminifers have limited biostratigraphic use, but they provide indispensable information about water masses, including productivity and oxygen levels (e.g., Jorissen et al., 1995; Van der Zwaan et al., 1999). Benthic foraminifers are defined as faunal assemblage zones following Salvador (1994). The assemblage zones are named after the most frequent benthic foraminifers. Benthic taxonomy largely follows the definitions by Loeblich and Tappan (1988). MethodsMore than 40 cm3 of sediment from each core catcher sample was analyzed for planktonic and benthic foraminifers. In addition, mudline samples from most holes were also analyzed. Mudline samples were collected by emptying the sediment/water material from the top core liner of each hole into a bucket. Tests using Rose Bengal staining confirmed the presence of some living foraminifers in the mudline samples. All samples were disaggregated by being soaked and washed with warm deionized water through a 63 µm mesh sieve. Weakly lithified sediments were disaggregated with detergent (borax). Strongly lithified sediments were disaggregated before wet sieving and dried and soaked for 6–12 h in kerosene. The kerosene was decanted, and boiling water was added to aid disaggregation. After wet sieving, sample fractions were dried in an oven (maximum temperature = 50°C). To minimize contamination of foraminifers between samples, the sieve was placed into a sonicator for several minutes and thoroughly inspected. Faunal analysis was carried out using a Zeiss Stemi SV11 or Zeiss DR binocular microscope. The >63 µm (benthic) and >125 µm (planktonic) size fractions were analyzed for foraminifer preservation, abundance, and faunal composition. Preservation of foraminiferal tests includes any effects of diagenesis, abrasion, encrustation, and/or dissolution. Benthic and planktonic foraminifer preservation is categorized as follows:
Planktonic foraminifer abundance in the >125 µm fraction in relation to the total residue is defined as follows:
Benthic foraminifer abundance in the >63 µm fraction is recorded as follows:
Benthic foraminifer abundance, as plotted in site chapter figures, is calculated as the sum of total relative species abundance, with the following values assigned to each category:
OstracodesOstracodes (calcareous bivalve microcrustaceans) are the only metazoan organisms commonly preserved as microfossils in deep-sea sediments. Like benthic foraminifers, ostracodes have limited biostratigraphic use, but they can provide important environmental information and are valuable proxies in paleoceanographic studies (Dingle and Lord, 1990; Corrège 1993; Cronin et al., 2002; Ayress et al., 1997; Whatley et al., 1998; Alvarez Zarikian et al., 2009). Assemblage zones are named after the most frequent ostracode taxa. Taxonomic identification follows the taxonomy provided by Joy and Clark (1977), Whatley and Coles (1987), Didié and Bauch (2001), Stepanova (2006), Stepanova et al. (2004), Alvarez Zarikian (2009), and Yasuhara et al. (2009). Ostracodes were picked, identified, and counted from core catcher samples following the sample methodology used for benthic foraminifers (see above). Carapace or valve preservation is classified according to three categories ranging from transparent (best) to white and chalky (poor). DiatomsZonationDiatom zonation used for the Neogene closely follows the zonation by Yanagisawa and Akiba (1998). Datums were modified in line with the updated geologic timescale (Lourens et al., 2004). A major alteration was made in the technique used to identify Neodenticula species N. seminae and N. koizumii. As discussed by Yanagisawa and Akiba (1990, 1998), the extant species N. seminae is closely related to the extinct species N. koizumii and the intermediary extinct species Neodenticula sp. A in the light microscope. The distinguishing feature is the copula formation. The copula of N. seminae is closed, smooth, and rounded, whereas the copula of N. koizumii and Neodenticula sp. A (which is classified as an intermediary form of N. koizumii [Yanagisawa and Akiba, 1998]) is open and pointed when well preserved. Thus, when ambiguity was found in distinguishing the species, counts of open versus closed copula were made to determine the first common occurrence (FCO) of N. seminae and the last common occurrence (LCO) of N. koizumii. MethodsStrewn slides were prepared following the methods of Barron and Gladenkov (1995). A small amount of material was placed in a snap-cap vial. Deionized water was added, and the vial was shaken. The upper suspension was then removed with a pipette. Strewn slides were examined in their entirety at 630× magnification for stratigraphic markers and paleoenvironmentally sensitive taxa. Identifications were also checked routinely at 1000× magnification. Whenever possible, all diatom taxa were tabulated until 100 specimens were counted (with the exception of Chaetoceros resting and vegetative spores). In addition, because several age indicators (e.g., Proboscia spp.) are relatively large and are usually rare in total diatom abundances, additional counting was performed with slides prepared for silicoflagellate studies, whereby abundant small diatoms were removed by the 20 µm grain-size fraction (see "Silicoflagellates and ebridians" below). Total diatom abundance was assessed in relation to the remaining residue according to Scherer and Koç (1996):
The relative abundance of each diatom taxon, as reported in range charts, was estimated using a qualitative scale. Values indicate counts of 100 diatom valves; a blank indicates that specimens are absent. The relative abundances of species are defined as follows:
Diatom preservation categories, as reported in range charts, are described qualitatively according to Barron and Gladenkov (1995):
Silicoflagellates and ebridiansDuring Expedition 323, the biostratigraphic framework used for silicoflagellates and ebridians was Ling (1977), which is the compiled version of Bukry (1973) and Ling (1973) from the Bering Sea and other previous studies in the North Pacific. The methods of Ling (1973, 1977), as well as Kobayashi (1988; Hole 438A) and Ling (1992; Sites 798 and 799), were applied for the age estimation of datum levels. All estimated ages were adjusted to the astronomically tuned Neogene timescale (ATNTS2004: Lourens et al., 2004). Silicoflagellate and ebridian microslides for Sites U1340–U1345 were prepared as follows. A small amount (~0.5 cm3) of core catcher sample was sieved with a 20 µm mesh and hot water. The coarser fraction (>20 µm) was mounted on a glass slide with Norland optical adhesive (number 61). Microscopic observation for silicoflagellates and ebridians was conducted using the >20 µm fraction slides. In addition, microslides prepared for diatom observation (see "Diatoms") were also examined to identify the small-sized ebridian Ammodochium rectangulare. For Site U1339, only microslides prepared for diatom microscopic observations were used for species identification and counting of both silicoflagellates and ebridians because sediment samples from Site U1339 contain a sufficient number of skeletons for counting. Overall abundances of silicoflagellates and ebridians were determined by counting specimens over eight traverses (perpendicular to the length of the slide) at 400× using the following conventions:
Preservation of silicoflagellates and ebridians is recorded as follows:
RadiolariansAlthough no detailed radiolarian biostratigraphic study in the Bering Sea was conducted before Expedition 323, a biostratigraphic zonation was developed for the subarctic Pacific. Hays (1970) first proposed upper Pliocene to Quaternary radiolarian zones from piston core samples in the subarctic Pacific. Shilov (1995) proposed new Pliocene to Miocene radiolarian zonation based on Sites 881–887 of ODP Leg 145. Morley and Nigrini (1995) identified several radiolarian events during the Pleistocene to Miocene. Motoyama (1996) and Kamikuri et al. (2004, 2007) conducted detailed investigations of Neogene radiolarian biostratigraphy in the subarctic Pacific employing Deep Sea Drilling Project (DSDP) Leg 19, ODP Leg 145, and ODP Leg 186 samples. They established synthetic radiolarian zones and identified detailed radiolarian events. Essentially, the radiolarian zonation given by Kamikuri et al. (2007) was used; however, the timing of radiolarian events was recalculated according to the standard geologic timescale (Lourens et al., 2004). Sample preparation for light-microscopic observation was performed as follows. A ~3 cm3 sample of wet core catcher sediment was sieved through a 63 µm mesh sieve. A microslide was prepared by pipetting the coarse fraction onto a slide glass. After the sample on the slide was dry, a glass coverslip was mounted with Norland optical adhesive. When unclear radiolarian specimens were encountered because clay and organic matter were attached to the skeletons, Calgon or 10% H2O2 was added after sieving and another microslide was prepared. Eucyrtidium matuyamai, an important radiolarian stratigraphic marker occurring between 1 and 2 Ma, is a rare species (Hays, 1970). To determine the horizon of this datum, the >106 µm fractions were used in addition to the >63 µm fractions because E. matuyamai is generally >200 µm. Total radiolarian abundances were determined by light-microscopic observations at 100×: magnification (10×: objective and 10×: eyepiece lenses) as follows:
Radiolarian species abundances were determined by light-microscopic observations at 100×: as follows (the slide traverse employed was perpendicular to the longer distance of a microslide):
Preservation is recorded as follows:
Palynology: dinoflagellate cysts, pollen,
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