Geochemical records of Qionghai Lake sediments in southwestern China linked to late Quaternary climate changes

Abstract

Knowledge of the chemical weathering history under the subtropical monsoonal climate will help to resolve the discrepancies regarding how climate conditions constrain chemical weathering. Major element and rare earth element (REE) compositions of the Qionghai Lake sediments in Sichuan Province, southwestern China were analyzed, aiming to provide information on sediment source discrimination and history of chemical weathering intensities and its responses to variations of Indian summer monsoon (ISM) and paleoclimate conditions. As revealed by the A-CN-K diagram and other provenance proxies including TiO2/Zr ratio, Th/Sc vs. Zr/Sc and La/Th vs. Hf diagrams, as well as the REE characteristics, the source of Qionghai Lake sediments was felsic rocks. Additionally, these sediments were subjected to small changes in provenance, sedimentary sorting and recycling, and weak diagenesis after deposition and/or metasomatism, indicating original signals were maintained on the intensity of chemical weathering of source area. The chemical weathering intensities inferred from CIA (chemical index of alteration) values have significantly fluctuated over the past 28 kyr, reflecting three stages of paleoclimate evolution, which is consistent with grain size characteristics, carbonate contents, low-frequency magnetic susceptibilities, TOC and biomarker proxy (Paq). The CIA values changed significantly during the typical warm and cold climate events including the Last Glacial Maximum, Heinrich 1 cold event, Younger Dryas and Holocene Climatic Optimum, and generally exhibited an increasing trend in amplitude correspondingly, in concert with the climate shifting from cold and dry to warm and humid conditions. Generally warm and humid climate conditions suggest enhanced chemical weathering intensity, whereas weak chemical weathering denotes cold and dry climate in the ISM region. In addition, sequential variations of the CIA values during the past 28 kyr may reflect changing chemical weathering processes, in responses to the varying intensities of Indian summer monsoon and the climate conditions forced by the Northern Hemisphere solar insolation.