Freshwater input variability in a west coastal area of Korea and its links to the global monsoon and ITCZ shifts during the period 8500–7800 cal yr BP

Abstract

In East Asia, freshwater inputs in coastal areas are mainly controlled by rainfall occurring during the East Asian summer monsoon (EASM), which is linked to seasonal latitudinal shifts in the intertropical convergence zone (ITCZ). To investigate the centennial-time-scale relationship between those inputs and the seasonal shifts in the ITCZ, we examined the behaviors of terrestrial input proxies in coastal sediments of western Korea, including grain size, elemental ratios from XRF core scanning (e.g., Zr/Ti and Si/Al), and C/N ratios. Correlation tests linked these proxies to past freshwater input variability, suggesting that elemental ratios can serve as a proxy for past EASM changes. During the period 8500–7800 cal yr BP, the stronger EASMs described by high-resolution peaks of Zr/Ti ratios corresponded to periods of intensified Indian summer monsoons (ISMs) and weakened South American summer monsoon (SASMs), supporting the existence of synchronous global monsoon (GM) events induced by a northward ITCZ shift. This study also shows that the 8.2-ka cooling event recorded in Greenland ice cores synchronously influenced the GM through a southward shift in the ITCZ, resulting in a weakened EASM and ISM in the Northern Hemisphere and an intensified SASM in the Southern Hemisphere. Our study thus demonstrates that the elemental ratios in coastal sediments measured using an XRF core scanner can be utilized to trace high-resolution (at least centennial-time-scale) variability in the EASM in terms of GM-ITCZ coupling.