Sediment biomarkers record hydrological and anthropogenic-driven environmental changes since 1800 AD in the Ili-Balkhash Basin, arid Central Asia

Abstract

Human activity and hydroclimate change greatly influence the environment in a lake and its catchment, particularly in areas with fragile ecosystems, such as arid Central Asia. In this study, lipid biomarkers (n-alkanes and n-fatty acids) were measured in a 210Pb- and 137Cs-dated sediment core from Lake Balkhash to determine their environmental significance and infer the history of environmental change over the last ~200 years. The terrestrial origin of long-chain n-alkanes and the aquatic origin of both n-fatty acids and mid-/short-chain n-alkanes were inferred from molecular distributions and diagnostic ratios. Three major environmental phases were identified over the past two centuries based on stratigraphic shifts in biomarker indicators. During 1800-1860 AD, the lake exhibited a high-water level with abundant submerged/floating macrophytes and limited phytoplankton, as suggested by multiple indicators, e.g., high proportions of aquatic macrophytes (Paq) and long-chain n-fatty acids (L-FAs). Overall, minor terrestrial inputs were revealed by low concentrations of long-chain n-alkanes (L-ALKs), suggesting dense vegetation cover in the catchment. The lake environment experienced a pronounced change in the subsequent phase from 1860–1930 AD, during which the cover of submerged/floating macrophytes gradually diminished, as revealed by the decreasing trend of aquatic proxies, e.g., L-FAs and Paq. In contrast, the number of emergent plants and terrestrial inputs increased, as suggested by the decreased Paq values. A great variation in water levels likely resulted in the shrinkage of the submerged/floating macrophyte cover. During 1930-2017 AD, anthropogenic impacts began to appear on the sediment profile. The highest terrestrial inputs, as revealed by the maximum L-ALK abundance on record, indicated intensive exploitation of the catchment during 1935-1959 AD. The lowest L-FA and Paq values suggested that submerged/floating macrophytes were at the lowest levels during this phase, possibly in response to the decreased water levels and increasing salinity. Increased human-induced nutrient loading coupled with elevated regional temperature prompted the lake to become an increasingly productive lake system, especially in more recent decades, as indicated by the highest levels of short-chain lipids. These results highlight the important role of hydrological variation and human activity in the environmental evolution of the Ili-Balkhash Basin.