METHODS

Preparation for light microscopy and measurement of absolute abundance followed the procedure described by Beaufort (1991). Approximately 0.02 g of dried sediments was placed in a flask, and 100 mL of water was added. This flask was placed in an ultrasonic cleaner at a moderate vibration setting to homogenize the suspension. After 5 s, the flask was removed from the ultrasonic cleaner and shaken. The suspensioned particles were poured into the 100-mL beaker. The cover glass was heightened above the bottom of this beaker. The beaker was placed in the oven and left at room temperature for a day. The temperature of the oven was raised to ~30°-40°C so that water could evaporate after the grainy particles descended quietly to the cover glass. The cover glass was then mounted to the slide glass with the mounting medium "Photocuring Adhesive."

The coccolith absolute abundance was estimated under a polarized microscope at 1500x magnification. The absolute abundance was calculated from the number of coccoliths observed per a field of view, the volume of suspension poured in the beaker, and the volume of dry sediment.

Three hundred coccoliths were identified to species level in each slide prepared for the light microscopy under a magnification of 1500×. The relative abundance of each nannofossil taxon is defined as follows:

D = dominant (>50% of the total assemblages);

A = abundant (32%-50% of the total assemblages);

C = common (8%-32% of the total assemblages);

F = few (2%-8% of the total assemblages);

R = rare (1%-2% of the total assemblages); and

VR = very rare (<1% of the total assemblages).

The preservation of the nannofossils was recorded using the criteria of Steinmetz (1979) where

G = good (fossils lack evidence of dissolution or overgrowth); and

M = moderately good (the majority of the specimens slightly etched [fine structures are missing, but no diagnostic changes of form are evident in light microscopy; all taxa may be easily identified]).