BIOSTRATIGRAPHY

During Leg 195, calcareous nannofossils were studied to assess biostratigraphic constraints on the sedimentary sections at Sites 1200, 1201, and 1202. Ages for calcareous nannofossil events in the Paleogene and Miocene were determined according to the geomagnetic polarity timescale of Cande and Kent (1995), with minor modification for the Pliocene and Pleistocene using the astrochronology of Lourens et al. (1996). Astronomically tuned datum levels were used whenever possible. Foraminifers in the nannofossil-bearing serpentine mud intervals of Site 1200 were also examined to assist in interpreting the depositional environment and the taphonomy of the microfossils.

We have referred primarily to the zonations of Martini and Müller (1986) for Cenozoic sediments. Numerical ages used are those compiled by Berggren et al. (1995b) (Fig. F9) to facilitate easy comparison with other studies. Calcareous nannofossil assemblages were described from smear slides prepared for each core catcher sample and for as many additional core samples as time permitted. Standard smear slides were made for all samples using Norland optical adhesive as a mounting medium. Examination was performed exclusively with a light microscope, using whatever optical configuration yielded useful results. Unless otherwise noted, we followed taxonomic concepts summarized in Perch-Nielsen (1985). In all cases, a magnification of 1000x was used to make semiquantitative estimates of abundances of individual species. Five levels of abundance were recorded, with the following approximate definitions:

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

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

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

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

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

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

B = barren (none).

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

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

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

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

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

The qualitative evaluation of the preservation of calcareous nannofossils was recorded as poor (P), moderate (M), good (G), or very good (VG), as prescribed in the Janus database system. These categories represent subjective impressions according to the following definitions:

P = poor. Severe dissolution, fragmentation, and/or overgrowth has occurred. Primary features may have been destroyed, and many specimens cannot be identified at the species level.

M = moderate. Dissolution and/or overgrowth is evident. A significant proportion (up to 10%) of the specimens cannot be identified to species level with absolute certainty.

G = good. There is little or no evidence of dissolution and/or overgrowth. Diagnostic characteristics are preserved, and nearly all specimens can be identified to species level.

VG = very good. No evidence of dissolution and or secondary overgrowth; diagnostic characteristics are perfectly preserved.

Core catcher samples of ~20 cm³ or more were washed in tap water over a 150-µm mesh sieve to retrieve free microfossil specimens. Washed residues were dried in an oven at ~50°C. The dried samples were examined under a binocular microscope, and the foraminifer faunal composition was recorded in qualitative terms, based on an assessment of species observed in a random sample from the >150-µm size fraction. Relative abundances were reported using the following categories:

A = abundant (>30%).

C = common (16%-30%).

F = few (4%-15%).

R = rare (2%-3%).

T = trace (<2%).

Preservation of planktonic foraminifer assemblages was recorded as follows:

G = good (little or no evidence of dissolution or recrystallization/replacement).

M = moderate (moderate signs of dissolution or recrystallization/replacement; nearly all specimens can be identified at the species level).

P = poor (severe signs of dissolution or recrystallization/replacement; primary features are largely destroyed).