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

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.

Relative roles of land- and ocean-atmosphere interactions in Asian-Pacific thermal contrast variability at the precessional band

“…The precipitation anomaly pattern is similar to that associated with the recent global warming45. Moreover, our results also highlight that the paleo-APO modulates the orbital-scale EASM variabilities10 (including southerly winds91112 and precipitation212223242526 over East Asia).…”

Millennial-scale variability of East Asian summer monsoon inferred from sea surface salinity in the northern East China Sea (ECS) and its impact on the Japan Sea during Marine Isotope Stage (MIS) 3

“…Inoue et al (2012) calculated the relative contribution of the Changjiang freshwater in this area as approximately 2-3% in July and October, based on mass balance calculation of the SSS and 228 Ra concentration of surface water samples collected between 2008 and 2010. Instrumental SSS data around the core site shows a negative correlation with the Changjiang River discharge during the wet season for the past 50 years (Kubota et al 2015), confirming that the studied site is an appropriate location to reconstruct SSS.…”

New northwest Pacific radiolarian data as a tool to estimate past sea surface and intermediate water temperatures

“…The reconstruction of intermediate water hydrography also contributes relevant information, as such water masses play an important role in ventilation and carbon sequestration through glacial/interglacial climate cycles [e.g., Broecker et al, 2004;Okazaki et al, 2012]. In this context, studies targeting the reconstruction of changes in the intermediate water circulation across the last Termination were conducted in this area [Kubota et al, 2015a[Kubota et al, , 2015bMatsuzaki et al, 2014b;Okazaki et al, 2010;Sagawa and Ikehara, 2008].…”

“…Most studies reconstructing past changes in sea water temperature are based on geochemical analysis of planktic and benthic foraminifera [e.g., Kubota et al, 2010Kubota et al, , 2015aKubota et al, , 2015bOkazaki et al, 2012;Sagawa and Ikehara, 2008]. However, because foraminifera (planktic and benthic) produce calcareous tests, their fossil remains are strongly affected when deposited at depths close to the regional carbonate compensation depth (CCD).…”