OXYGEN ISOTOPE RECORDS OF CALCAREOUS NANNOFOSSILS SINCE MIS 6 FROM THE MARGINAL AREA OF WEST PACIFIC

Three piston cores, WP7, Ph05-5 and Z14-6,recovered from the marginal area of the West Pacific were analyzed for their δ¹⁸O variations of calcareous nannofossils. The nannofossil δ¹⁸O values of all the three cores showed apparent glacial-interglacial cycles. The δ¹⁸O values were generally lower during interglacials than glacials since MIS 6,corresponding to lower sea surface temperatures (SST) during glacial stage,except for the penultimate glacial cycle in core Z14-6. The average δ¹⁸O values of MIS 6-1 show an increasing trend from core WP7 to Ph05-5 and Z14-6, that is explained by the gradually decreasing SST at these locations. At a single location, however,the average δ¹⁸O values of planktonic foraminifera Neogloboquadrina dutertrei were higher than those of nannofossils, reflecting different water temperatures at their living water depth. These differences show a decreasing trend from core WP7 to Ph05-5, and Z14-6,that can be explained by increased disturbance in upper water. The δ¹⁸O difference between nannofossils and N. dutertrei, as well as the downcore variations in the percentage of Florisphaera profunda in core WP7 and Ph05-5 implies that the thermocline of the coring regions of Western Pacific Warm Pool and the Western Philippine Sea was deeper during interglacials than that in glacials since MIS 6.