A total of 39 samples from Holes 1251C and 1251B and 41 samples from Hole 1252A were analyzed (Tables T1, T2). About 0.1 g of sample was soaked in 10 mL of distilled water for 1 hr and then stirred. Strewn slides were prepared by sampling the solution with a pipette, spreading the sample on an 18 mm x 18 mm coverslip, drying it on a hot plate, and then mounting the sample with mounting medium.
More than 100 diatom valves were counted for each sample at 600x magnification. Resting spores of Chaetoceros and its allied forms were counted separately during this routine count of diatom valves. At least half of the slide was scanned after the routine counting to find rare but important diatoms. Identifications of diatoms were checked at 1000x magnification.
Only one diatom biohorizon was recognized during shipboard analysis of the middle Pleistocene to Holocene sediments from both sites. In order to improve the resolution of the age determination for this interval, it was necessary to correlate the fluctuation of diatom assemblages from both sites to the standard marine oxygen isotope stages (MIS). The abrupt changes of the frequencies of a few taxa, such as Fragilariopsis doliolus, Thalassionema nitzschioides, and resting spores of Chaetoceros spp., for the past 30 k.y. were dated or correlated to the standard oxygen isotope curve by Sancetta et al. (1992) off Oregon and California. The high-resolution study by Barron et al. (2003) at Ocean Drilling Program Site 1019 off northern California revealed detailed changes in the diatom assemblage for the past 16 k.y. with precise age control by radiometric ages. The fluctuation of diatom assemblages from Sites 1251 and 1252 was correlated with the standard MIS by comparing the results of the present study with the results of Barron et al. (2003) and Sancetta et al. (1992).