Records of Chemical Weathering and Plateau Saline Lake Sediments Linked to Paleoclimate Fluctuations During the Eocene

Abstract

Interactions between mountain building, weathering and climate draw significant attention within the geoscience community. In this study, the mixed rocks from the Eocene Lower Ganchaigou Formation (LGCG) of the Qaidam Basin are employed for weathering intensity evaluation by petrology, mineralogy, and geochemistry. Lithological characteristics indicate high-frequency sedimentary cycles (<10 m) of the LGCG mixed rocks, which are identified by a single cycle with four lithofacies from bottom to top: massive sandstones, laminated silty shales, laminated silty limestones, and bedded dolomites. Provenance analysis suggests a predominant felsic volcanic source within a continental island arc background for these mixed sediments. Chemical weathering intensity indices (CIA, chemical index of weathering, and plagioclase index of alteration) show a high coherence with sedimentary cycles, initially increasing and gradually decreasing in a single cycle. Chemical index of alteration values ranged from 51.1% to 67.2%, indicating weak to moderate weathering intensity. Additionally, correlation analysis reveals that the cyclical variations of chemical weathering intensity correlate strongly with regular paleoclimate evolution and terrigenous input changes, which were controlled by the shift of the westerlies. During the westerly strengthening periods, stronger precipitation results in a warm and humid paleoclimate conditions, leading to higher terrigenous input dominated by fine-grained clay minerals with moderate chemical weathering intensity. During the westerly weakening periods, less rainfall leads to a cold and dry paleoclimate conditions, resulting in lower terrigenous input dominated by coarser clastic sediments and weak weathering intensity. Our study provides new evidence and insights into the westerly shift and paleoclimate evolution in the Eocene from the perspective of high-resolution chemical weathering records.