Divergent sediment magnetic records between alpine lakes during Anthropocene and implications for using magnetic...

Atmospheric deposition is an important pathway for importing exogenous substances into remote alpine regions and can create negative impacts on the local environment. The magnetic characteristics of sediment archives in alpine lakes can be used to reconstruct anthropogenically driven changes in atmospheric deposition, however, the potential influence of catchment natural processes on the magnetic records of these lakes remains poorly addressed and thus limit their use in assessing anthropogenic effects. Here, systematic magnetic analyses were carried out on dated sediment cores from two alpine lakes (Cuoqia: with small and less eroded catchment; Wodi Co: with large and severely eroded catchment) on the southeastern Tibetan Plateau to determine how and to what extent the catchment natural processes impacted magnetic records in alpine lakes. The results showed that the magnetic materials in sediments of both lakes were dominated by allochthonous detrital magnetite. In the Anthropocene (after 1950 AD), both magnetic concentration parameters (χ_lf, χ_ARM, and SIRM) and particle size parameters (χ_ARM/χ_lf and χ_ARM/SIRM) notably increased in the sediment of Lake Cuoqia relative to before, indicating an enrichment of magnetic minerals and becoming finer of magnetic grains in sediments of this period, and these changes might be caused by the input of anthropogenic magnetic materials through atmospheric deposition. However, the magnetic concentration and particle size parameters varied little in the sediments of Wodi Co during Anthropocene, indicating the absence of anthropogenic impacts. The contrasting results between the two lakes might be driven by the intensity of magnetic minerals input from natural catchment erosion, and a high input may be able to mask anthropogenic contributions through atmospheric deposition. After approximately 1980 AD, both χ_ARM/χ_lf and χ_ARM/SIRM substantially increased in the sediments of Lake Cuoqia, pointing to a decrease in magnetic grain size, and expansion of catchment land plants under warming climate in this period might be the driver because this process probably prohibited the delivery of coarse magnetic grains to the lake. Meanwhile, χ_ARM/χ_lf and χ_ARM/SIRM significantly decreased in the sediments of Lake Wodi Co, reflecting an increase in magnetic grain size, which might be related to the potential intensification of land surface runoff under warming climate during this period. These results all demonstrated the important influence of catchment natural processes on sediment magnetic records in alpine lakes, which provides crucial knowledge for the improved use of magnetic proxies in reconstructing anthropogenic environmental changes in alpine lake basins.