The isotopic response of Lake Chenghai, SW China, to hydrologic modification from human activity

The human modification of lake hydrology is widespread in the industrial era; however, anthropogenic impacts have occurred for thousands of years in regions of the world with long histories of human occupation. Here, we use geochemical analyses of lake sediment to document the isotopic and geochemical response of Cheng Lake (Chenghai) in southwestern China to catchment modification, including the construction of a downstream dam. The Chenghai record indicates that land-use changes affected the lake/catchment by at least AD 1150 as evidenced by an increased flux of terrestrial organic matter, sediment with high concentrations of sorbed metals, and variations in stable isotopes of oxygen in precipitated aragonite. Decreases in magnetic susceptibility and metal concentrations after AD 1360 indicate a change in sediment basin dynamics. The construction of a downstream dam between AD 1573 and 1620 caused positive shifts in the isotopic composition of aragonite, indicative of increased evaporative loss and a longer water residence time. A lake-level drop by AD 1779 is accompanied by a decrease in organic carbon and a lighter nitrogen isotopic composition that suggests the oxidation of organic matter and/or reduced primary productivity. These changes closely coincide with shifts in climate variability (such as the termination of the ‘Little Ice Age’) inferred from other, regional paleoclimate records, highlighting the challenges in distinguishing between natural and anthropogenically driven environmental variations. This study demonstrates the importance of historical and cultural context in the interpretation of lake sediment records with substantial human settlement proximal to the lake system.