Spatial pattern and methylation process of mercury enrichment in lake sediments during glacial periods in cold and arid regions

Underlying surface in lake watersheds and mercury concentrations in lake inflows are key factors affecting the accumulation of mercury in lake sediments. Lake characteristics play a crucial role in the process of mercury methylation in sediments. Lakes in cold regions have unique environmental features, including a long ice-cover period, during which mercury undergoes complex physicochemical processes. However, the extent of mercury accumulation and methylation in cold region lake sediments remains unclear. We studied the concentrations, pollution levels, and ecological risks of mercury and methylmercury in surface sediments from six lakes in China's cold regions, and analyzed the mechanisms by which lake characteristics influence mercury methylation. The results indicate significant mercury enrichment in surface sediments of typical lakes in Inner Mongolia, with some regions exceeding the average mercury levels found in lakes across China. Mercury concentrations in surface sediments of lakes from different land use types within their watersheds show considerable spatial variability, with the following pattern: agricultural irrigation areas > agro-pastoral transition areas > grassland and sand areas. Agricultural activity intensity in lake watersheds has the most pronounced impact on the spatial heterogeneity of surface sediment mercury concentrations and their associated ecological risks. Lake water input and geographical location can indirectly control the spatial distribution of mercury concentrations and ecological risks in Inner Mongolia lakes by affecting external mercury inputs. The methylation process in lake surface sediments during the ice-cover period is significant. Based on a correlation analysis model, water depth was found to be a key factor controlling methylmercury content and mercury methylation rates in lake sediments during the ice-cover period. Deep water lakes promote the conversion of mercury into methylmercury in sediments. Water depth influences the redox conditions of sediments and the amount of light radiation received by the sediments, thereby affecting the methylation and demethylation processes of mercury, ultimately controlling the levels of methylmercury in sediments.