BIOSTRATIGRAPHY

Only one microfossil group, calcareous nannofossils, was examined for biostratigraphic purposes during Leg 190. The goal of the nannofossil analysis was to provide a high-resolution biostratigraphy of sediments in the Nankai Trough. To obtain more detailed records, biostratigraphic zonal assignments during Leg 190 were made on a number of samples within each core (one sample per section).

Preparation of smear slides for light microscope examination followed standard procedures. Taxon identification was made under plane-, cross-polarized light, and phase contrast using a Zeiss Axioscope microscope at 1000× magnification. Abundance, preservation, and zonal data for each sample investigated were recorded in the Janus paleontology database.

The following scale was used to estimate the relative abundances of individual species. Estimates of percentage of calcareous nannofossils present in each sample were as follows:

B = barren, none.

R = rare, <0.1%.

F = few, from 0.1% to <1%.

C = common, from 1% to <10%.

A = abundant, from 10% to 50%.

D = dominant, >50%.

The assessments of preservation of calcareous nannofossils were based on the following criteria:

P = poor, severe dissolution, fragmentation and/or overgrowth has occurred; primary features may have been destroyed and many specimens can not be identified at the species level.

M = moderate, dissolution and/or overgrowth are evident; besides frequently broken nannofossils, the number of delicate forms is reduced.

G = good, there is little or no evidence of dissolution and/or overgrowth; diagnostic characteristics are preserved and nearly all species (~95%) can be identified.

VG = very good, there is no evidence of dissolution and/or overgrowth; diagnostic characteristics are preserved and all specimens can be identified.

The calcareous nannofossil zonation used here (Fig. F8) is that of Martini (1971). Zonal modifications adopted are those proposed by Young (1998). For the Pleistocene, the zonation scheme of Gartner (1977) is employed to provide a higher resolution. Biostratigraphic event zonal markers for the Cenozoic are shown in Figure F8, with the events defining the zonal boundaries. In Table T4, well-dated nannofossil datums used during Leg 190 are listed.