Decoupled local climate and chemical weathering intensity of fine-grained siliciclastic sediments from a paleo-megalake: An example from the Qaidam basin, northern Tibetan Plateau

Abstract

Clay mineralogy and elemental geochemistry of lacustrine sedimentary records have been widely used in paleo-weathering and paleo-climate studies. However, different paleo-climatic interpretations may be produced via analyses on siliciclastic sediments and other sedimentary components (e.g., authigenic minerals and organic materials), especially for megalake systems. To test this hypothesis, we focus on late Cenozoic lacustrine fine-grained deposits from the northwestern Qaidam basin and combine clay mineralogical and geochemical data (including neodymium isotope) to interpret sediment provenance, reconstruct paleo-weathering history and characterize sediment-climate feedback processes. Provenance analysis results show that the lacustrine siliciclastic sediments were derived from lithologically similar sources that provided dominant felsic parent-rocks. The kaolinite/illite ratio, illite chemistry index, Chemical Index of Alteration (CIA) values are low and demonstrate overwhelmingly mild-moderate chemical weathering intensity, seemingly matching well with the middle–late Miocene regional arid climate. However, these weathering intensity index values are also highly fluctuant and display increasing trends during ~13.4–12 Ma and ~8.8–8.4 Ma. The reconstructed paleo-weathering history is discrepant with local intensified aridity conditions documented by previously published sedimentary carbonate oxygen isotope, evaporite mineral, biomarker and sporopollen data. Given the occurrence of a middle–late Miocene megalake in the Qaidam basin, the intense exhumation at ~13–12 Ma and the ~9 to ~7 Ma intensified East Asian summer monsoon precipitation in northeastern Tibet regions, we suggest that the lacustrine fine-grained siliciclastic compositions and corresponding weathering records were likely influenced by tectonic and climatic changes in far regions to the east. The observed clay mineralogical and geochemical variations reflect weathering conditions involving sediment sources and transport pathways, not merely the depositional areas. Our findings verify the possible differences in paleo-climate reconstruction results from different lacustrine sediment indicators. This study emphasizes the importance of a sediment source-to-sink perspective for paleo-climate study based on siliciclastic materials in megalake systems.