Sediment magnetism records of anthropogenic impact on a typical alpine lake in southeastern Tibetan Plateau

Alpine lakes provide a valuable opportunity for studying anthropogenic airborne contamination due to the absence of direct human activities in their catchments. However, to date, few in-depth investigations have employed magnetic proxies to assess anthropogenic impacts on these lakes. In this study, we analyzed the magnetic characteristics of a dated sediment core from Lake Jiren, a typical alpine lake in the southeastern margin of the Tibetan Plateau, to elucidate when, how, and to what extent anthropogenic contamination has impacted the lake basin. The results indicate that the magnetic minerals in the core are mainly composed of magnetite, with hematite accounting for a small percentage. In sediments deposited prior to ca. 1933 AD, low values of magnetic concentration-dependent proxies (χ_lf, χ_ARM, and SIRM) and relatively abundant detrital hematite reflect a dominant input of locally derived magnetic minerals to the lake, implying a relatively pristine environment with minimal anthropogenic impact. The relatively low χ_ARM/χ_lf and χ_ARM/SIRM ratios indicate coarser magnetic particles, likely resulting from the weathering of the bedrock in the basin. For sediments deposited after ca. 1947 AD, the values of χ_ARM/χ_lf and χ_ARM/SIRM ratios increased, pointing to a finer size of the magnetic particles. This change, along with the marked increase in SIRM and S_-300, suggests an important shift in the source of magnetic particles, indicating a notable anthropogenic contribution—most likely from airborne deposition of anthropogenic magnetic materials over the lake basin, followed by surface runoff to the lake. This transition temporally coincides with large-scale industrial development, such as mining, in southwestern China. A causal relationship is plausible, considering that the magnetic minerals generated by industrial activities have a high potential to be transported to adjacent alpine lakes through atmospheric processes. This study demonstrates that magnetic proxies, which can be acquired rapidly, non-destructively, and cost-effectively, are highly sensitive indicators of anthropogenic airborne contamination in alpine lakes.