| 研究実績の概要 |
Twenty-four Porites samples were collected from Kikaijima, Japan and Northwest Luzon, Philippines. Sr/Ca-based reconstructed mean, summer, and winter SSTs from 4960 yrBP coral show higher values relative to the modern SST in Kikaijima. 5712 and 3235 yr BP were relatively cooler than the present-day. Reconstructed mean SST from 4200 yr BP coral from Northwest Luzon was comparable to the present-day but the duration of warmer months was shorter. The tropical Philippines was warmer than subtropical Southern Japan during mid- to late Holocene transition as supported by geochemical and coral growth evidence.
The ocean condition in Northwestern Pacific is modulated by the seasonal shift of monsoon circulation with warm (cool) sea surface temperature (SST), high (low) rainfall, low (high) salinity and negative (positive) δ18Oseawater during EASM (EAWM). However, the timing of these seasonal signals varies in different time-windows across the tropical and subtropical region. Together with δ18Ocoral records of previous studies from Southern Japan, we demonstrate the weakening of EASM rainfall from mid- to late Holocene. Negative δ18Oseawater and δ13Ccoral define the duration of the EASM in Northern Philippines from June to November, which was relatively longer than the present-day. In contrast to the sustained warm (28-30°C) modern-day SST, our 4200 BP coral suggests that sudden drop of tropical SST in August has reduced the meridional (15-30°N) SST gradient and moisture supply in extratropical East Asia.
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