Seasonal variations in the lake-water oxygen isotope composition of four lakes in the East Asian summer monsoon region: Implications for the interpretation of paleo-isotope records

摘要

The scarcity of modern process studies of the relationship between climatic variables and lake-water oxygen isotope (delta O-18(LW)) composition has restricted our understanding of the climatic significance of oxygen isotope (delta O-18) records from lake sediments. We measured changes in delta O-18(LW) at four lakes (Nanyihu, Gonghai, Mayinghai and Xihaizi) at various locations across the East Asian summer monsoon (EASM) region, from December 2016 to January 2018. The variations in delta O-18(LW) at Lake Nanyihu, located in the lower reach of the Yangtze River, are controlled by precipitation oxygen isotope (delta O-18(P)) composition, temperature and evaporation during the non-monsoon season. However, the variations in delta O-18(LW) at the lakes close to the northern limit of the EASM (Gonghai, Mayinghai and Xihaizi) are affected not only by delta O-18(P), temperature and evaporation, but also by the replenishment of groundwater during the non-monsoon season. In general, during the non-monsoon season, the variation of delta O-18(LW) exhibits a variety of patterns and the controlling factors across the EASM region are complex. During the monsoon season, although the effect of evaporation-induced isotopic enrichment still affects delta O-18(LW), the variations in delta O-18(LW) are more influenced by the supply of abundant precipitation with lower delta O-18(P) to the lakes, which caused a substantial decrease in delta O-18(LW) across the EASM region in this season. Therefore, the variations in delta O-18(LW) mainly reflect the delta O-18(P) of the EASM during the monsoon season. However, the timing of the peak in monsoon precipitation does not correspond well to minima in delta O-18(LW), and correlation analysis reveals that there is only a weak relationship between delta O-18(LW) and precipitation/humidity at all four lakes during the monsoon season. These findings demonstrate that although delta O-18(LW) can represent the delta O-18(P) of the EASM, it does not reflect precipitation/humidity during the monsoon season. Overall, our findings indicate that the seasonal variations of delta O-18(LW) across the EASM region are controlled by different climatic variables, which implies that the interpretation of lacustrine delta O-18 records across the EASM region as an indicator of regional effective precipitation/humidity needs to be reassessed.