Sediment magnetic records of human activities in Lake Chaohu Basin over the past 166 years

Abstract

Variations of sediment magnetism characteristics in lake systems are generally attributed to human activities in the respective catchments; however, the relationships between sediment magnetism proxies and the types or intensities of human activities are generally poorly established, which restricts the use of magnetism proxies in paleoenvironmental reconstructions. Here, magnetic parameters were measured in a dated sediment core from Lake Chaohu, Anhui Province, China, to explore whether and to what extent environmental magnetism proxies indicate the complex human activities in the lake basin over the past 166 years. Results demonstrate that detrital magnetite minerals were the major magnetite fraction in the sediments. The lowest values of magnetic concentration proxies (χ_lf, χ_ARM, and SIRM) in sediments prior to approximately 1950 indicate minimal input of detrital magnetic minerals, implying limited disturbance of the catchment, which coincided with weak human activities in the lake basin in this period—mainly primitive agriculture. In sediments deposited during ca. 1950–1985, increased χ_lf and reduced grain size-dependent proxies (χ_fd%, SIRM/χ_lf, and χ_ARM/χ_lf) indicate increased input of detrital magnetic minerals and coarsening of the magnetic grains. This shift might have resulted from the rapid development of agricultural and industrial activities in the lake catchment since the 1950s and decreased hydrodynamic force within the lake due to dam construction in the 1960s. After ca. 1985, χ_lf, χ_ARM, and SIRM gradually decrease to the pre-1950 levels, indicating further decrease of the magnetic mineral input. This is likely due to decreased agricultural activities as a consequence of urbanization expansion and a shift from high pollution factories to high-tech industry. Overall, the trajectories of magnetic proxies in the sediment core reflect historical variations of the types and intensities of human activities in the lake basin. This study constitutes an important basis for the use of environmental magnetic proxies in paleoenvironmental reconstructions.