14:30 〜 14:45
[MIS16-03] Hydroclimate seasonality during mid- to late Holocene transition: Evidence of East Asian Monsoon dynamics from fossil corals
キーワード：East Asian monsoon, corals, hydroclimate, Holocene, seasonal variation
Hydroclimate (SST, salinity and rainfall) associated with the East Asian summer and winter monsoon (EASM and EAWM) is reconstructed for the mid-to late Holocene transition using fossil corals in Kikai Island, Southern Japan. Here we present monthly-resolved Sr/Ca, δ18Ocoral and estimated δ18Oseawater from modern (1989-2015AD), 3235, 4960 and 5712 years BP corals. The ocean condition in Kikai Island 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). Modern coral and in-situ SST, salinity, δ18Oseawater data were calibrated to demonstrate the sensitivity of the coral geochemical records to the monsoon-driven oceanographic response. Summer δ18Ocoral values at 5712 and 4960 years BP are lower than the coral data from 3232 years BP. The double negative δ18Oseawater peaks in June and September at 5712 and 4960 years BP compared to the single negative peak at 3235 years BP, are indications of increased salinity and/or reduced rainfall from mid-to-late Holocene. The warm (cool) SST, low (high) salinity and negative (positive) δ18Oseawater during the winter season are indicators of a weaker (stronger) EAWM during 4960 and 3235 (5712) years BP. The negative δ18Oseawater from average winter values at 4960 and 3235 years BP are opposite to the value recorded at 5712 years BP. Weaker EAWM in recent decades is marked by negative winter δ18Oseawater in the 26-year modern coral, which signals the increased winter rainfall.
Together with published δ18Ocoral records of previous studies from Southern Japan, we demonstrate the weakening of EASM rainfall from mid-to late Holocene. The coral-derived mean SST anomalies from northwestern Pacific and southwestern Indo-Pacific show that the expansion (contraction) of the northern margin of West Pacific Warm Pool (WPWP) may have influenced the northerly (southerly) position of the intertropical convergence zone (ITCZ) and the intensified (weakened) summer monsoon rainfall. Weak EAWM at 4960 and 3235 years BP and strong EAWM at 5712 years BP are consistent with the established winter monsoon records in East Asia. The decline of Sr/Ca and δ18Ocoral seasonalities from mid-to-late Holocene was caused by reduced seasonal ocean heating and monsoon convection, which led to the weakened state of EASM and EAWM. A wealth of paleoclimate records agrees that the complex interaction of the extent of WPWP, ITCZ position, tropical SST forcing, and Northern Hemisphere insolation changes have dominated the mechanism of highly varied EASM and EAWM intensities from mid-to late Holocene.
Together with published δ18Ocoral records of previous studies from Southern Japan, we demonstrate the weakening of EASM rainfall from mid-to late Holocene. The coral-derived mean SST anomalies from northwestern Pacific and southwestern Indo-Pacific show that the expansion (contraction) of the northern margin of West Pacific Warm Pool (WPWP) may have influenced the northerly (southerly) position of the intertropical convergence zone (ITCZ) and the intensified (weakened) summer monsoon rainfall. Weak EAWM at 4960 and 3235 years BP and strong EAWM at 5712 years BP are consistent with the established winter monsoon records in East Asia. The decline of Sr/Ca and δ18Ocoral seasonalities from mid-to-late Holocene was caused by reduced seasonal ocean heating and monsoon convection, which led to the weakened state of EASM and EAWM. A wealth of paleoclimate records agrees that the complex interaction of the extent of WPWP, ITCZ position, tropical SST forcing, and Northern Hemisphere insolation changes have dominated the mechanism of highly varied EASM and EAWM intensities from mid-to late Holocene.