Geochemistry and mineralogy of the late Neogene alkaline megalake sediments in the Qaidam Basin (China): Implications for provenance, tectonics, paleoclimate, paleoenvironment and organic matter accumulation

Abstract

The late Neogene is widely regarded as a suitable analogy for future climate scenarios, but the paleoclimatic conditions of this period on the Qinghai-Tibet Plateau remain poorly understood. This study investigates the mineralogical composition and elemental geochemistry of 24 shale samples from two wells in the Yiliping Depression of the Qaidam Basin to reconstruct the paleodepositional conditions of the sediments and the late Neogene paleoclimate conditions. The results indicate that the chemical composition of the late Neogene alkaline lacustrine sediments was little affected by sediment recycling, sorting, diagenesis and metasomatism. Provenance analysis indicates a predominantly felsic source with minor contributions from intermediate volcanic rocks, formed under a tectonic setting similar to continental island arc tectonic settings in geochemistry. Paleoclimate reconstructions reveal consistently arid and cold conditions on the Qinghai-Tibet Plateau, characterized by weak chemical weathering. These climatic conditions were colder than those achieved in most contemporaneous regions globally, with a distinct cooling trend from the late Neogene to the Pleistocene. The deposition occurred predominantly in hypersaline environments, with localized transitions to brackish conditions. The redox conditions of the water column varied spatially, i.e., oxic waters dominated in the central depression (the H 1 well area), while the margins of the depression (the Y 3 well area) experienced fluctuations between oxic to anoxic conditions. The primary paleoproductivity in the Y 3 well exhibited similarities to that in the Holocene lakes of Peru and Brazil, whereas the H 1 well showed resemblances to most marine sediments. Notably, the primary paleoproductivity of the studied samples was lower than that of typical upwelling systems. Organic matter preservation was principally controlled by depositional conditions, with little effects from primary paleoproductivity and sedimentation rate/detrital input. This work provides valuable insights into the late Neogene climate dynamics on the Qinghai-Tibet Plateau and enhances the understanding of the formation of microbial gas source rocks.