Variations in the Yangze River outflow in the last 7 ky deduced from δ18O and Mg/Ca in the northern East China Sea

Abstract:

The East Asian summer monsoon (EASM) system is involved in hydrological cycle, and latent heat and energy transport, thus plays a significant role in global climate system. Observation records of the precipitation (~50-year long) are not long enough to reveal the frequency and extremity of the EASM precipitation. Therefore, quantitative reconstruction of the past precipitation changes using geological records is necessary.

 The δ¹⁸O of seawater (δ¹⁸O_w), an indirect indicator of sea surface salinity, in the northern East China Sea (ECS) is reconstructed for the last 7 kyr using paired Mg/Ca ratio and δ¹⁸O of planktic foraminiferal tests. According to modern observation, inter-annual variations in sea surface salinity during summer in the northern part of the ECS are mainly controlled by the discharge from the Yangtze River, which reflects summer rainfall in the drainage area of the Yangtze River. Thus, changes in the summer sea surface salinity in the northern ECS are interpreted as reflecting variations in the East Asian summer monsoon (EASM) precipitation in South China. This interpretation is confirmed by the relationship between salinity in the northern ECS and the discharge from the Yangtze River during wet season (May-October) based on the observational salinity data from 1951 to 1995.

 We calculate a fraction of the Yangtze River freshwater by using two endmembers, freshwater δ¹⁸O and seawater δ¹⁸O. During the last 7 ky, the freshwater fraction varies between 5% and 0% on multi-centennial to millennial–scale. However, there is no long-term trend from the middle Holocene that is commonly known in Chinese speleothem δ¹⁸O. Our record suggests that changes in summer insolation in the Northern Hemisphere do not seem to impact on changes in summer precipitation in the Yangtze Basin. Instead, other factors such as inter-seasonal migration speed of the monsoon front likely control on the precipitation in the Yangtze Basin even in the long time scale.