Abstract
Abstract. The δ18O of seawater (δ18Ow), an indirect indicator 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 mainly controlled by the Changjiang (Yangtze River) discharge, which reflects summer rainfall in its drainage basin. Thus, changes in the summer SSS in the northern ECS are interpreted as reflecting variations in the East Asian summer monsoon (EASM) precipitation in the Changjiang Basin. This interpretation is confirmed by a strong relationship between the SSS in the northern ECS and the Changjiang discharge during the wet season (May–October) based on instrumental salinity records from 1951 to 2000. However, it is difficult to estimate absolute salinity values in the past with high accuracy, because the past salinity–δ18Ow regression slope, end member salinity, and δ18Ow values are not well understood. Here, we conduct δ18Ow mass-balance calculation to estimate the freshwater contribution to the surface water of the northern ECS during the last 7 kyr by assuming a simple mixing between two end members – the seawater and the Changjiang freshwater. The result indicates that there has been no gradual decreasing secular trend in the Changjiang freshwater flux from the middle Holocene to the present day, suggesting that summer insolation in the Northern Hemisphere does not regulate the EASM precipitation in the Changjiang Basin. Instead, internal feedback appears to have been more important during the Holocene. The absence of a decreasing trend in regional summer precipitation over the Changjiang Basin since the middle Holocene is contradictory to Chinese speleothems' δ18O records, suggesting that it is not possible to explain orbital changes in Chinese speleothems' δ18O during the Holocene by changes in summer precipitation, but that such changes are related to other factors such as changes in the moisture source.
Highlights
The East Asian summer monsoon (EASM) is generated by land–ocean thermal contrast between Asia and northwestern Pacific
Precipitation/evaporation ratio deduced from several lake-level studies within the Changjiang (Yangtze River) Basin showed no decreasing trend or even slight increase from early-middle to late Holocene (An et al, 2000), which is inconsistent with the classical interpretation of the speleothem δ18O during the Holocene that the speleothem δ18O reflects the decrease in EASM precipitation
We estimated the relative contribution of the Changjiang freshwater in the northern East China Sea (ECS) during the Holocene by applying the two-end member mixing model and by considering temporal changes in end member δ18O of seawater (δ18Ow) of KTW and CFW
Summary
The East Asian summer monsoon (EASM) is generated by land–ocean thermal contrast between Asia and northwestern Pacific. J. et al, 2001; Yuan et al, 2004; Dykoski et al, 2005; Wang Y. et al, 2005; Cheng et al, 2009), indicating that orbital-scale variations in the speleothem δ18O follow the summer insolation curve of the Northern Hemisphere without any obvious time lag (0.77 ± 0.45 kyr; Wang et al, 2008) Based on these speleothem records, the concept that intensity of the EASM is mainly controlled by the local summer insolation on orbital timescale, as suggested by modeling studies (Kutzbach, 1981; Kutzbach et al, 2008), has been widely accepted (e.g., Cheng et al, 2012). Precipitation/evaporation ratio deduced from several lake-level studies within the Changjiang (Yangtze River) Basin showed no decreasing trend or even slight increase from early-middle to late Holocene (An et al, 2000), which is inconsistent with the classical interpretation of the speleothem δ18O during the Holocene that the speleothem δ18O reflects the decrease in EASM precipitation
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
