BIOSTRATIGRAPHY

Only one microfossil group, calcareous nannofossils, was examined for biostratigraphic purposes during Leg 206. The primary biostratigraphic objective was to date sediments immediately overlying the basement. Our secondary objective was to estimate sedimentation rates.

The biostratigraphic zonation of calcareous nannofossils is based primarily upon the studies of Martini and Müller (1986) and Okada and Bukry (1980). The numerical ages for biostratigraphic datums were compiled mainly from Berggren et al. (1995a, 1995b) (Fig. F12) to facilitate easy comparison with other studies. Preliminary ages were assigned to core catcher samples, and these were used to estimate sedimentation rates. Samples from within the cores were examined when a more refined age determination was necessary.

Several Reticulofenestra species have been used as Neogene and Quaternary biostratigraphic markers. They are mainly distinguished by coccolith size and the relative magnitude of the central opening. Some species, however, show a great range of variation in these parameters, causing problems in identification (Backman, 1980; Gallagher, 1989; Young, 1990; Su, 1996). During Leg 206, only one Reticulofenestra species, R. pseudoumbilica, was used as a stratigraphic marker. This species is identified by specimens having a maximum coccolith length of >7 µm in its uppermost range (the lower Pliocene), which is in accord with the size of the holotype (Gartner, 1967). Identification of other calcareous nannofossils mainly follows the compilation of Perch-Nielsen (1985).

Preparation of smear slides followed standard techniques using Norland optical adhesive as a mounting medium. Slides were examined using a Zeiss Axiophot microscope under cross-polarized light, transmitted light, and phase-contrast light at 800x-1250x magnification. Relative abundance of each nannofossil species, overall preservation of the nannofossil assemblage, and the total abundance of nannofossils were recorded for each sample. In all cases, a magnification of 1000x was used to make semiquantitative estimates of abundance of individual species. Five levels of relative abundance of a species were recorded, with the following approximate definitions:

V = very abundant (>10 specimens per field of view).

A = abundant (1-10 specimens per field of view).

C = common (1 specimen per 2-10 fields of view).

F = few (1 specimen per 11-50 fields of view).

R = rare (1 specimen per 51 or more fields of view).

The total abundance of all calcareous nannofossils for each sample was estimated as follows:

V = very abundant (>20,000 specimens for 500 fields of view).

A = abundant (2,001-20,000 specimens for 500 fields of view).

C = common (51-2,000 specimens for 500 fields of view).

F = few (11-50 specimens for 500 fields of view).

R = rare (1-10 specimens for 500 fields of view [~3 traverses]).

B = barren (none).

The qualitative evaluation of the preservation of calcareous nannofossils was based on the following criteria:

VG = very good (no evidence of dissolution and/or overgrowth; diagnostic characteristics are perfectly preserved and all specimens can be identified at species level).

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

M = moderate (dissolution and/or overgrowth are evident; besides commonly broken nannofossils, the number of delicate forms is reduced; identification of a significant proportion [up to 10%] of the specimens is impaired and cannot be identified to species level with absolute certainty).

P = poor (severe dissolution, fragmentation and/or overgrowth have occurred; primary features may have been destroyed and many specimens can be identified only at genera level).