Approximately 1500 smear slides were prepared for calcareous nannofossil biostratigraphy at the three Blake Ridge sites (Sites 994, 995, and 997). The slides were examined with a Zeiss Axioplan microscope in cross-polarized light. The occurrence of biostratigraphically important species was examined in phase-contrast. Sampling intervals vary between two samples per section for the upper Quaternary to one sample per section for the Neogene sequence. Nannofossil zones employed here are those of Bukry (1973, 1975) and Okada and Bukry (1980). Supplementary nannofossil datum events summarized in Young et al. (1994) were also determined where possible. Chronology of the datum events used for this study is mostly based on Berggren et al., (1995a, 1995b). A summary of the chronological data employed is illustrated in Figure 2.
To study the floral assemblages in Hole 994C, 236 samples were selected representing approximately 30-k.y. time intervals. The smear slides of the samples were examined under a light microscope for the occurrence of all nannofossil taxa.
Florisphaera profunda is usually the only lower photic-zone species preserved in marine sediment, and it responds to environmental changes in a different manner from all other taxa that are basically upper photic-zone dwellers (Molfino and McIntyre, 1990a, 1990b). For the convenience of discussion, therefore, all taxa excluding F. profunda will be called as upper photic-zone species.
Because an increase in relative abundance of one taxon automatically results in a decline of the other taxa, relative abundance data has inherent limitation for reconstructing the biosphere's response to changing environment. Selective dissolution also distorts composition of fossilized assemblage. To minimize the total sum problem, the abundance change of F. profunda was expressed as its abundance ratio against the all other coccoliths that are regarded as upper photic-zone species.
Although the dissolution effect cannot be eliminated, the mass accumulation rate of each taxon is a better data to use to analyze the paleo-environment. For this study, however, severe disturbance of recovered cores caused by the expansion of interstitial gas prevented reliable calculation of the mass accumulation rate.
For each smear slide, three separate observation were made.