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

Abstract

The scarcity of modern process studies of the relationship between climatic variables and lake-water oxygen isotope (δ18OLW) composition has restricted our understanding of the climatic significance of oxygen isotope (δ18O) records from lake sediments. We measured changes in δ18OLW 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 δ18OLW at Lake Nanyihu, located in the lower reach of the Yangtze River, are controlled by precipitation oxygen isotope (δ18OP) composition, temperature and evaporation during the non-monsoon season. However, the variations in δ18OLW at the lakes close to the northern limit of the EASM (Gonghai, Mayinghai and Xihaizi) are affected not only by δ18OP, 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 δ18OLW 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 δ18OLW, the variations in δ18OLW are more influenced by the supply of abundant precipitation with lower δ18OP to the lakes, which caused a substantial decrease in δ18OLW across the EASM region in this season. Therefore, the variations in δ18OLW mainly reflect the δ18OP of the EASM during the monsoon season. However, the timing of the peak in monsoon precipitation does not correspond well to minima in δ18OLW, and correlation analysis reveals that there is only a weak relationship between δ18OLW and precipitation/humidity at all four lakes during the monsoon season. These findings demonstrate that although δ18OLW can represent the δ18OP of the EASM, it does not reflect precipitation/humidity during the monsoon season. Overall, our findings indicate that the seasonal variations of δ18OLW across the EASM region are controlled by different climatic variables, which implies that the interpretation of lacustrine δ18O records across the EASM region as an indicator of regional effective precipitation/humidity needs to be reassessed.

Dipolar mode of precipitation changes between north China and the Yangtze River Valley existed over the entire Holocene: Evidence from the sediment record of Nanyi Lake

“…However, without the influence of multiple factors, the geochemical proxies from the sediments of hydrologically‐open lakes are potentially much simpler to interpret and thus are much more suitable for reconstructing paleoprecipitation intensity, compared to hydrologically‐closed lakes. Nanyi Lake is hydrologically‐open and the precipitation in the surrounding region is high; thus large volumes of inflowing and outflowing water lead to vigorous water exchange (Zhang et al ., 2019). Soluble elements such as Sr 2+ , Na + , and Ca 2+ would be flushed from the lake basin via the outflow and would not be retained within the lake sediments.…”

“…There are many hydrologically‐open lakes in Anhui Province in the YRV and they are all located within the EASM region. Given that precipitation in the region is high, large volumes of inflowing and outflowing water lead to vigorous water exchange (Zhang et al ., 2019). Here, we present a record of paleoprecipitation intensity based on geochemical analysis of a radiocarbon‐dated lacustrine sediment record from Nandi Lake in the YRV.…”