A Late Holocene Record of Variations in the Chemical Weathering Intensity and Pedogenesis in a Lake Catchment from Southern India

Abstract

We investigated the detrital influx, chemical weathering intensity, provenance and pedogenesis over the past 2,500 years in the catchment of Pookot Lake, southern India. The down-core variations of metal/Al ratios (Na/Al, K/Al, Mg/Al, Ca/Al, Fe/Al, Mn/Al, Zn/Al, Ba/Al) of the Pookot sediments indicate changes in the rainfall-induced terrigenous inflow to the lake. In contrast, fluctuations in the chemical index of alteration (CIA) and Rb/Sr values denote the variability in the strength of chemical weathering in the watershed of the lake. The results show that the detrital influx, and hence rainfall, remained steady except during 1500–600 cal. years B.P. (high) and 600–300 cal. year B.P. (low) in the Pookot lake catchment. However, the periods of high/low chemical weathering intensity in the catchment do not correspond to periods of high/low detrital influx to the lake basin. The similar shale-normalized rare earth elemental curves point to a uniform provenance. The past pedogenic activity is indicated by pedogenic χ_lf and pedogenic χ_fd derived from citrate-bicarbonate-dithionite (CBD) extraction. The data indicate that the fine-grained magnetite/maghemite formed during the pedogenesis mainly contributes to the magnetic signal of sediments. The degree of pedogenesis was strong during 2500–2000 cal. years B.P. and moderate throughout 1500–600 cal. years B.P. The pedogenic intensity became stronger again during ~ 600 cal. years B.P., which weakened between 600 and 300 cal. years B.P. and remained steady thereafter. The present study indicates that detrital influx proxies like metal/Al ratios are more suitable for reconstructing past climate in tropical climate rather than chemical weathering indices.