Current models of Philippine Sea (PHS) Plate motion assume a general and large northward shift since the Eocene. In order to constrain better the age and amount of this northward shift, we have conducted a paleomagnetic study on drill and gravity cores, respectively, taken from the seafloor of the northern part of the PHS Plate. The core samples consist of sedimentary rocks or semi-consolidated sediments, and their ages, as estimated from microfossils and strontium isotope ratios, range from the Eocene to late Miocene. The results of stepwise alternating-field and thermal demagnetization experiments revealed that 19 sections at 17 sites out of 58 sections at the 29 sites examined yielded mean paleomagnetic directions with a 95% confidence limit (α 95) of <25 • , and 14 sections at 13 sites have α 95 < 15 •. An estimation of the amount of the northward shift at each site was obtained from the difference between the paleolatitude and the present latitude. This estimation revealed that the northern part of the PHS Plate was located near the equator at 50 Ma and that the majority of the northward shift took place between about 50 and 25 Ma. Very little northward movement occurred after 15 Ma. Based on our data, together with the available paleomagnetic information suggesting clockwise rotation of about 90 • since the Eocene and the requirements from geometry with the surrounding plates, we present a model in which the PHS Plate rotated 90 • clockwise between 50 and 15 Ma on the Euler pole near 23 • N, 162 • E, although it is impossible to specifically determine the Euler pole position.

We studied high-resolution Quaternary calcareous nannofossil biostratigraphy to clarify the relationship between nannofossil events and oxygen isotope stratigraphy using the continuous sediment sequence from Integrated Ocean Drilling Program Site U1308 in the North Atlantic Ocean. Results indicate that some nannofossil events found in the section, such as the last occurrences of Reticulofenestra asanoi and Gephyrocapsa spp. (large), are located in a different stratigraphic position compared to previous studies. We also clarify the critical stratigraphic positions of both the first occurrence of Emiliania huxleyi and the last occurrence of Pseudoemiliania lacunosa, which occur just below the highest peaks of each marine isotope Stage 8 and 12.

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The late Quaternary calcareous nannofossil assemblages from Ocean Drilling Program Holes 807A and 846B, located in the western and eastern equatorial Pacific Ocean, respectively, were analyzed to clarify changes in surface-water conditions during the last 500 000 years. The uppermost Quaternary sediments in both holes contain abundant nannofossils, and their assemblages are characterized by high species diversity. The absolute abundances of coccoliths (specimens/g) and relative numbers of small reticulofenestrids decreased drastically in both holes between 0.3 and 0.2 Ma, whereas the relative abundance of warm-water species and Florisphaera profunda increased suddenly at this time. These data indicate that upwelling around the western and eastern equatorial Pacific regions weakened after 0.2 Ma, which was caused by a decrease in trade-wind strength.

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