Variability of chemical weathering from 7000 years ago to the present in a basin influenced by mining activity in central Mexico

In this paper, we evaluate chemical weathering and its climatic significance in different periods (pre- and post-industrialization) in a basin impacted by 500 years of mining activities. We consider the geochemical and mineralogical composition of stream sediments and overbank deposits from the Guanajuato River Basin (GRB) in central Mexico. This research provides information about the weathering modifications caused by anthropogenic impacts before and after mining activities. The elemental concentrations of major elements were determined using X-ray fluorescence spectrometry, and the clay mineralogy was determined using X-ray diffraction (XRD). The chemical weathering intensity was estimated using the chemical index of alteration (CIA), the plagioclase index of alteration (PIA), and the weathering index of Parker (WIP) indices. The clay minerals in overbank sediments are kaolinite > smectite > chlorite > and vermiculite influenced by climatically driven weathering. In-stream sediments, the clay minerals are chlorite > vermiculite > kaolinite > smectite, influenced by the origin of the sediments and showing that physical weathering has been dominant in recent times because of changes in the vegetation cover and accelerated erosion in the stream sediments of the GRB. The weathering indices show that chemical weathering was greater in the preindustrial epoch (overbank sediments) compared with the present day (stream sediments) in the GRB. According to the CIA, clay content, and chemical composition, we identified four paleoclimatic conditions from 2450 ± 40 yr BP to 9410 ± 40 BP. There is a wet period at the base of the column (9410 ± 40 BP), followed by an early cold period and a wet period surrounding 2450 ± 40 yr BP.

The combination of weathering indices and the clay content proved to be a useful tool; it was used to determine the weathering evolution of the basin and the anthropogenic impacts in a basin where there are historical references to mining activity over the last 500 years.