Sedimentary geochemical evidence for recent eutrophication of Lake Chenghai, Yunnan, China

Geochemical anomalies and stable isotope ratios (δ18O, δ13C) in authigenic carbonates and organic matter (δ13C) from a 660-year sediment core from Lake Chenghai, southern China, provide a continuous history of recent lake eutrophication. The multi-proxy geochemical and isotopic record can be divided into a three-part history of contrasting limnological development, including: (1) a clear-water, oligotrophic open lake system (1340 and 1690 AD); (2) an environmentally unstable, hydrologically closed, oligotrophic lake system (1690-1940 AD); and (3) an increasingly eutrophic, closed lake system marked by higher organic matter, nitrogen, CaCO3, and pigment concentrations, and lower δ18O and δ13C values in authigenic calcite (1940-1999 AD). The unanticipated lowering of δ18O and δ13C of authigenic calcite during eutrophication is thought to be the result of disequilibrium water-carbonate fractionation of oxygen and carbon isotopes during periods of elevated primary production, pH, and [CO 32-] activities in the water column. The recent eutrophication of Lake Chenghai indicated by these geochemical proxies is essentially simultaneous with large-scale human migration and the application of agricultural fertilizers in the catchment area during the 20th century.