[M-IS06] [EE] Evolution and variability of Asian Monsoon and its linkage with Cenozoic global cooling

[MIS06-09] Variations in East Asian summer monsoon in the last 400 ky deduced from results of Mg/Ca-sea surface temperature and oxygen isotope of IODP Site U1429

*Yoshimi Kubota¹, Etsuko Wakisaka², Steven Clemens³, Ann Holbourn⁴, Kyung Eun Lee⁵, Martin Ziegler⁶, Keiji Horikawa² (1.National Museum of Nature and Science, 2.University of Toyama, 3.Brown University, 4.Christian-Albrechts University, 5.Korea Maritime and Ocean University, 6.Utrecht University)

Keywords:East Asian summer monsoon, IODP Exp.346, East China Sea

The East Asian summer monsoon (EASM) system involved in the hydrological cycle and in latent heat and energy transport, and thus plays a crucial role in the regional and global climate system. Modern summer sea surface salinity in the northern part of the East China Sea (ECS) is mainly controlled by the freshwater discharge of the Yangtze River, which reflects East Asian summer monsoon (EASM) precipitation in South China. Site U1429 was drilled by Integrated Ocean Drilling Program (IODP) Expedition 346 in the northern ECS to reconstruct the Yangtze River discharge in high temporal resolution (~100 year resolution). A ~200 m long sediment succession was recovered, which covers the last 400 ky based on a benthic foraminiferal oxygen isotope record. A record of oxygen isotope of seawater (δ¹⁸O_w), a proxy that is related to salinity change and Yangtze River discharge, was reconstructed, based on high-resolution Mg/Ca and oxygen isotope (δ¹⁸O_pf) of the planktic foraminifera Globigerinoides ruber. As the surface water in the northern ECS is formed by mixing salty Kuroshio water (high δ¹⁸O_w) and freshwater from the Yangrze River (low δ¹⁸O_w), the δ¹⁸O_w of U1429 is interpreted as reflecting mixing ratio between Kuroshio water and freshwater.
The results of U1429 indicate that both δ¹⁸O_pf and Mg/Ca are dominated by 100 ky (eccentlicty) cycle with 41 ky (obliquity) and 23 ky (precession) cycles. The δ¹⁸O_w shows variations that have been in concert with Chinese speleothem oxygen isotope records on millennial to orbital scales except for 100 ky cycles that is not found in the Chinese speleothem record. As the the δ¹⁸O_w of U1429 follows the global mean δ¹⁸O_w on 100 ky cycles, this cyclicity originates from global ice volume change maybe through the Kuroshio water. On the other hand, the prominent 23 ky cycles in U1429 δ¹⁸O_w may reflect the variations in the freshwater discharge, supporting that the EASM is reglated by summer insolation in the Northern Hemisphere.