Changes in East Asian summer monsoon precipitation during the Holocene deduced from a freshwater flux reconstruction of the Changjiang (Yangtze River) based on the oxygen isotope mass balance in the northern East China Sea

摘要

The δ18O of seawater (δ18O_w), an indirectindicator of sea surface salinity (SSS), in the northern East China Sea(ECS) is reconstructed for the Holocene using paired analyses of Mg / Ca ratio and δ18O of planktic foraminiferal tests. According to modern observation,interannual variations in SSS during summer in the northern ECS are mainlycontrolled by the Changjiang (Yangtze River) discharge, which reflectssummer rainfall in its drainage basin. Thus, changes in the summer SSS inthe northern ECS are interpreted as reflecting variations in the East Asiansummer monsoon (EASM) precipitation in the Changjiang Basin. Thisinterpretation is confirmed by a strong relationship between the SSS in thenorthern ECS and the Changjiang discharge during the wet season (May–October)based on instrumental salinity records from 1951 to 2000. However, it isdifficult to estimate absolute salinity values in the past with highaccuracy, because the past salinity–δ18O_w regressionslope, end member salinity, and δ18O_w values are not wellunderstood. Here, we conduct δ18O_w mass-balance calculation to estimate thefreshwater contribution to the surface water of the northern ECS during the last 7 kyr by assuming asimple mixing between two end members – the seawater and the Changjiang freshwater. The result indicatesthat there has been no gradual decreasing secular trend in the Changjiangfreshwater flux from the middle Holocene to the present day, suggesting thatsummer insolation in the Northern Hemisphere does not regulate the EASMprecipitation in the Changjiang Basin. Instead, internal feedback appears tohave been more important during the Holocene. The absence of a decreasingtrend in regional summer precipitation over the Changjiang Basin since themiddle Holocene is contradictory to Chinese speleothems' δ18Orecords, suggesting that it is not possible to explain orbital changes inChinese speleothems' δ18O during the Holocene by changes insummer precipitation, but that such changes are related to other factorssuch as changes in the moisture source.