Temporal and spatial variations in sedimentary characteristics of Lake Lugu during the last hundred years and the influence factors analysis

Abstract

Twelve short sediment cores were collected from Lake Lugu to analyze the particle size composition, magnetic susceptibility (χlf), metal content and chemical index of alteration (CIA) in this study. Referencing to the 210Pb and 137Cs dating results, natural factor such as precipitation on the sedimentary characteristics, and human activity records such as agricultural expansion and deforestation, the temporal and spatial variations in sedimentary characteristics of Lake Lugu in recent 150 years and were discussed. The results indicated that the sediments of Lake Lugu were mainly (> 80%) composed of clay and fine silt. The fine fractions (such as clay) showed typical positive correlations with χlf and CIA. The sedimentary proxies showed similar vertical changes in the cores from different lake areas. Before 1920s, χlf and CIA remained low values and the grain size compositions were stable, suggesting weak human disturbance and stable particle input from the catchment. The clay content, χlf and CIA increased gradually since 1920s, which were more remarkable change since ca.1970, indicating enhanced erosion of well-weathered and fine-grained topsoil probably relating to intensive agricultural development and deforestation in Lake Lugu Catchment. The χlf and CIA tended to decreasing and median grain size increased since around 2002, which suggested decreasing topsoil erosion attributing to the implement of environmental protection measures such as vegetation recovery in the catchment. Study between the sedimentary proxies and instrumental data showed that climate factors such as precipitation had a weaker effect on the sedimentary evolution of Lake Lugu when compared to human perturbation. Spatially, the sedimentary proxies showed obvious heterogeneity. There was higher clay content in the surface sediments in north and south lake areas where they were near the pluvial-alluvial plains. The mean mass accumulation rates were 0.020-0.043 g/(cm2·a) since 1964, which showed higher values in the south Lake Lugu than in the north Lake Lugu. The high accumulation rate in the south lake should be ascribed to a fact that are influenced by inflow and outflow rivers and the strong soil erosion from the south catchment caused by intensive human activities.