ALBERT

Description: The interpretation of trace element/calcium ratios of speleothems as indicators of local hydroclimatic variability in the vicinity of caves has led to controversy in reconstructing the evolution of moisture conditions in arid central Asia (ACA) during the Holocene. Here we present records of Mg/Ca, Sr/Ca, Ba/Ca, and U/Ca from precisely dated stalagmites from Baluk cave in Xinjiang (northwest China), spanning the past 9370 years. The co-variations of the trace element ratios, together with the slopes of the regression lines of the corresponding logarithmically transformed data, suggest that they are dominated by prior calcite precipitation (PCP) and thus can be used as reliable proxies of changes in moisture/precipitation. The trace element ratios are relatively high during ~9 to 5 ka and lower from 5 ka to the present, indicating a trend of increasing mid–late Holocene moisture in ACA. The long-term trend of variation of the trace element ratios is correlative with two other records of speleothem trace element ratios from caves in ACA: Kesang cave (western Xinjiang) and Ton cave (Uzbekistan). This spatial coherency of the trend of inferred moisture conditions from three caves that are separated by hundreds of kilometers demonstrates that speleothem trace element ratios are indicative of large spatial scale rather than local hydroclimatic variability in ACA during the Holocene. However, the long-term trend of variation of the trace element ratios is the inverse of the corresponding oxygen isotope (δ18O) records from the same cave sites, which implies that Holocene speleothem δ18O records do not represent changes in the precipitation amount in ACA; rather, they most likely reflect moisture sources and related water vapor transport controlled by Northern Hemisphere summer insolation (NHSI). Our findings provide new evidence for a ‘westerlies-dominated climatic regime’, which influenced hydroclimatic changes in ACA during the Holocene.

Description: 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.