A total of 183 sediment samples were collected at 30-cm intervals from Cores 186-1150A-1H through 3H and 186-1151C-1H through 3H of the two Japan Trench sites Holes 1150A and 1151C (Fig. F1). Sediment samples were prepared as smear slides using standard techniques (Bown and Young, 1998), and calcareous nannofossils were examined at 1000x magnification under a Nikon E600 polarizing light microscope.
To understand a general trend in the biostratigraphy, samples were examined at intervals of approximately two samples per section in the first stage of investigation. Following this observation, the remaining samples were examined to refine the biostratigraphy.
For the Quaternary, relative abundances of placolith-type species are useful as datum events (e.g., Hine and Weaver, 1998). Florisphaera profunda is usually the only lower photic zone dweller that is preserved well in marine sediment, and it often dominates the fossil assemblage. Lower photic zone dwellers respond to environmental changes in a different manner than upper photic zone dwellers (Molfino and McIntyre, 1990a, 1990b; Hagino et al., 2000). For the purpose of biostratigraphy and paleoceanography, more than 300 upper photic zone specimens were identified and counted and the number of F. profunda coccoliths present in the same view fields were counted separately.