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doi:10.2204/iodp.proc.314315316.132.2009 BiostratigraphyDuring Expedition 316, calcareous nannofossils and radiolarians were systematically studied to assign preliminary ages to core catcher samples. Samples from within the cores were examined when a more refined age determination was necessary. Timescale and biohorizonsBiostratigraphic zones of calcareous nannofossils of sedimentary sequences recovered during Expedition 316 mainly follow the timescale used by biostratigraphic studies carried out during Expedition 315 (see “Biostratigraphy” in the “Expedition 315 methods” chapter). Ages for biostratigraphic data were compiled mainly from the recent review by Raffi et al. (2006). Biostratigraphic zonation of calcareous nannofossils is based on the studies of Martini (1971) and Okada and Bukry (1980) with zonal modifications by Young (1999). Radiolarian zonation used in this study follows that presented for middle to high latitudes in Kamikuri et al. (2004). Additional biohorizons, including some characteristics species used in tropical zonation (Nigrini and Sanfilippo, 2001), were employed and calibrated into the standard geomagnetic polarity timescale (GPTS). The lower boundary of the Botryostrobus acquilonaris Zone is defined by the last occurrence (LO) of Stylatractus universus. The first occurrence (FO) of Eucyrtidium matuyamai defines the upper boundary of the Eucyrtidium matuyamai Zone. Although characteristic species do not necessarily occur in the studied radiolarian assemblages, additional biohorizons, such as the occurrence/co-occurrence of Lamprocyrtis neoheteroporos, Lamprocyrtis heteroporos, and Anthocyrtidium angulare, places an assemblage into the lower part of the Stylatractus universus Zone to the Eucyrtidium matuyamai Zone. Diatom zonation used during ODP Leg 186 (Maruyama and Shiono, 2003) for the North Pacific was used as a reference for diatom zonation during this expedition. Because characteristic species were not found in this area, we used additional events (Barron, 1985) and co-occurrence of taxa to identify diatom biohorizons in the studied assemblages. Biostratigraphic zones of planktonic foraminifers used during Expedition 316 are based on the timescale used during Expedition 315 (see “Biostratigraphy” in the “Expedition 315 methods” chapter). This is derived from the planktonic foraminfer zonation presented in Blow (1969). Astrochronological age estimates for the Neogene rely on the geologic timescale developed by the International Commission on Stratigraphy (ICS) in 2004 (Lourens et al., 2004). The timescale and biostratigraphic zones of calcareous nannofossils and radiolarians are summarized in Figure F25 and Tables T8 and T9. Calcareous nannofossilsTaxonomic remarksSeveral species of the genera Gephyrocapsa are commonly used as Pleistocene biostratigraphic markers. Species of the genera show a great range of variation in sizes and other morphological features, causing problems in identification (Su, 1996). Size-defined morphological groups of this genera suggested by Young (1999) were used during shipboard study, including Gephyrocapsa spp. medium I (>3.5 to <4 µm), Gephyrocapsa spp. medium II (≥4.5 to <5.5 µm), and Gephyrocapsa spp. large (≥5.5 µm). Reticulofenestra pseudoumbilicus is identified by specimens having a maximum coccolith length >7 µm in its uppermost range (the lower Pliocene) and also following the suggestion of Young (1999). Identification of other calcareous nannofossils mainly follows the compilation of Perch-Nielsen (1985). MethodsStandard smear slides were made for all samples using photocuring adhesive as a mounting medium. In addition, a concentration method was applied to extract more nannofossils from fine sand or silt samples before making smear slides because sediments recovered from Expedition 316 are dominated by interbedded sand layers and volcanic ash layers in turbidite sequences, yielding few to rare nannofossils in most samples from core catchers and within cores. About 1–5 g of sediment was placed in a beaker with distilled water and stirred for several seconds to suspend the sample. The suspension was left to sit for a few minutes, so sand-sized particles could settle to the bottom first and small-sized nannofossils were floated and concentrated in the upper part of the suspension. Several drops from the upper part of the suspension were taken for making smear slides. Calcareous nannofossils were examined using standard light microscope techniques under crossed polarizers and transmitted light at 250× to 2500× magnification with a Zeiss Axio Imager.A1m. Abundance and preservation of nannofossils from the core catcher samples investigated were recorded in the J-CORES database. The degrees of calcareous nannofossil species preservation were based on the following:
Group abundance (at 250× magnification) and relative abundance of individual calcareous nannofossil species (at 1250× magnification) are estimated based on the following scale:
RadiolariansMethodsAll core catchers were prepared for radiolarian determination using 5–10 cm3 of sediment. Samples were disaggregated in a solution containing H2O2 and Calgon until completion of the reaction. The coalesced solution was then sieved through a 63 µm mesh sieve. In case of aggregated sediments, these steps were repeated until complete disaggregation. A subsample was then kept in solution for radiolarian examination, while the rest was filtered and dried for further examination. The radiolarian-bearing solution was directly mounted onto standard cover-slipped microscope slides. From all the core catcher samples prepared and kept in solution, few representative samples were selected and dried to pick radiolarian specimens and check their preservation. Highly silicified and spongeous shells were chosen for this analysis, as they occur consistently in poorly and well-preserved radiolarian-bearing samples. There has been no attempt to systematically quantify the relative abundance of radiolarians because of the large species diversity and variable preservation throughout the site. Total radiolarian assemblage composition (>63 µm) was assessed as follows:
Radiolarian species composition was assessed as follows:
Preservation of each sample was recorded using the following:
Other microfossilsIn the case of foraminifer-bearing sediments (core catcher and discrete samples), radiolarian preparations were dried and residues were inspected for foraminfer biostratigraphic markers. For diatom-bearing sediments, samples were set in a solution with water. The solution was then pipetted and directly mounted onto standard cover-slipped microscope slides. There has been no attempt to systematically quantify the relative abundance of diatoms. Diatom occurrence was assessed as follows:
Preservation of each sample was recorded using the following:
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