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doi:10.2204/iodp.proc.311.102.2006

Biostratigraphy

During Expedition 311, diatoms were studied to assign preliminary ages to core catcher samples. Samples from within the cores were examined when a more refined age determination was necessary. The timescale of Berggren et al. (1995) was used. The biostratigraphic zones of diatoms are summarized in Figure F9.

We used the Neogene North Pacific diatom zones of Akiba (1986) and Yanagisawa and Akiba (1998). Ages for bioevents that define the zones or characterize certain horizons are based primarily on Yanagisawa and Akiba (1998). The biohorizon ages of Barron (1992), Barron and Gladenkov (1995), Koizumi (1992), and Koizumi and Tanimura (1985) were also used.

Unprocessed strewn slides (Akiba, 1986) were prepared for each sample using a 22 mm x 22 mm coverslip and Pleurax mounting medium. The slides were examined on a normal light microscope at a magnification of 400x. One hundred diatom valves were counted for each sample. After counting, the slides were scanned to record the presence of other diatom species missed in the original tally. More than 200 and usually ~1000 valves were observed for samples containing sufficient diatom remains. For those samples that contained <100 diatom valves per slide, all the diatom valves on each slide were counted. Resting spores of Chaetoceros were counted separately during the routine count of diatom valves. Approximate numbers of diatom valves per slide were calculated based on the length of scanning lines to get the 100 count. In the site report tables, the "+" symbol indicates the occurrence of <1% diatom valves and/or valve fragments.

Diatom abundance is indicated as

• B = barren,
• VR = very rare (<100 valves per slide),
• R = rare (100–500 valves per slide),
• C = common (501–1000 valves per slide),
• A = abundant (1001–5000 valves per slide), and
• VA = very abundant (>5000 valves per slide).

Based on the degree of breakage and dissolution of diatom valves, the state of diatom preservation is recorded as

• VP = very poor,
• P = poor,
• M = moderate, and
• G = good.

Most of the diatom species recognized were tabulated, but several species that occur only sporadically and are not important for biostratigraphic or paleoceanographic studies were included as "miscellaneous" in the tables for each site chapter.

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