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doi:10.2204/iodp.proc.319.202.2013

Materials and methods

In this study, calcareous nannofossils in 32 cuttings samples from 715.7 to 1603.7 m mud depth below seafloor (MSF) in Hole C0009A were investigated. Smear slides prepared on board (see the “Site C0009” chapter [Expedition 319 Scientists, 2010b]) were used and the counting method was applied in order to characterize quantitatively the overall fossil assemblage changes throughout the section. This study aims to provide quantitative data of fossil assemblages and further refine the shipboard nannofossil biostratigraphic results. Calcareous nannofossils were observed at 1500× magnification under an Olympus BX51 polarized light microscope. We collected quantitative assemblage data by counting ~200 specimens in each sample and also searched for stratigraphically important but rare species beyond the counting effort. This counting method has not been applied during offshore examination and can accurately show relative abundance of each nannofossil taxon.

Calcareous nannofossil datums summarized by Lourens et al. (2004) and Raffi et al. (2006) are applied in order to determine the numerical age of sediment. Downhole contamination of specimens was commonly observed, and, to avoid misinterpretation of ages, we basically utilized last occurrences of index species.

This study mostly followed the taxonomy of nannofossils by Young (1998) as well as Raffi’s subdivision of Gephyrocapsa (Raffi, 2002), an important age-diagnostic genus in the Quaternary. Coccolith sizes of both Gephyrocapsa and Reticulofenestra varied throughout the Cenozoic (e.g., Young, 1990), and sudden changes of maximum size of their specimens can be used as important datums (e.g., Raffi et al., 2006). To provide biostratigraphic markers based on size variations, maximum sizes of both Gephyrocapsa and Reticulofenestra specimens were measured using an eyepiece graticule with an intermediate magnification changer 0.5 µm in diameter. Gephyrocapsa specimens are divided into four taxa, large Gephyrocapsa (>5.5 µm in diameter), medium Gephyrocapsa (≥4 µm, Gephyrocapsa omega morphotype), other medium Gephyrocapsa (4–5.5 µm), and small Gephyrocapsa (<4 µm). On the other hand, Reticulofenestra specimens are divided into Reticulofenestra pseudoumbilicus (elliptical coccolith ≥7 µm), Reticulofenestra asanoi (circular and/or subcircular coccolith ≥6 µm), and other Reticulofenestra specimens. Stratigraphic distributions of other Reticulofenestra specimens are shown in every one-micron (Fig. F2).

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