Increasing lake evaporation over the Holocene revealed by oxygen stable isotope in Indian-monsoon dominated southwestern China

Oxygen isotope from lacustrine carbonate has been widely used to infer changes in monsoon rainfall isotope and thus monsoon variability in Asian monsoon region. Similar to speleothem, existing lacustrine records show a decreasing monsoon intensity during the Holocene. Since lacustrine carbonate directly incorporates oxygen isotope from lake water, potential evaporative enrichment of lake water isotope compositions could further enrich lacustrine carbonate isotopes. To date, however, this potential evaporative effect has not been well evaluated. In this study, we present isotope compositions of 30 lake water samples collected from 6 lakes, which indeed show evaporative isotope enrichment in modern lakes in Indian monsoon-influenced southwestern China. We then report an oxygen isotope record of authigenic carbonate for the past 7800 years inferred from Lake Chenghai. The record, together with other two published lacustrine records (Lake Xingyun and Lake Qilu) in the region, show a larger-magnitude of Holocene oxygen isotope enrichment than that of speleothem data. We calculate parallel Holocene rainfall and lake water isotope changes for each lake, based on a simplified isotope-mass and water-mass balance model, which enables us to tentatively evaluate the changes in evaporation effect on lake water during the Holocene. We indeed find that the oxygen isotope from Lake Chenghai and other two lakes has been influenced by an overall increasing evaporative enrichment effect throughout the Holocene, which approximately equivalents to that the evaporative lake water losses have increased from <20% at the early Holocene to similar to 20%-40% at the late Holocene. Our new finding highlights the significance of evaporation effect on isotope of lake water in the humid Indian monsoon region, which has implications for a better understanding of long-term changes in broad-scale terrestrial water balance.