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• Clay-sized geochemistry reveals the Qaidam Basin (QB) Paleogene weathering history. • QB Paleogene silicate weathering intensity presents a continuously decreasing trend. • Global cooling was the main regulator of Paleogene chemical weathering processes. • Clay-sized geochemical indices aptly reveal weathering histories in coarse sediment. An integrated multiproxy dataset involving both mineralogical and geochemical investigations of the northern Tibetan Plateau helps in assessing regional weathering processes linked to topographic evolution and environmental change. However, changes in landscape and drainage reorganizations caused by active tectonics and remarkable climate change during the Cenozoic characterize the basins in and around the Tibetan Plateau by frequent facies changes and coarse lithologies, which largely limit the reliability of bulk geochemical indicators for revealing the regional weathering history. Here, we present detailed geochemical investigations of clay-sized sediments collected from the Hongliugou section (54–26 Ma) in the northern Qaidam Basin to reconstruct high-resolution chemical weathering records of the northern Tibetan Plateau. To prevent chlorite bias when calculating the chemical index of alteration (CIA) in the clay-sized sediments, we propose a modified CIA formula (CIA Mg+), in which Mg is added as a soluble cation that can be removed during weathering, similar to Na, K and Ca; thus, CIA Mg+ = Al 2 O 3 /(Al 2 O 3 + Na 2 O + K 2 O + CaO* + MgO) × 100. The CIA Mg+ and illite chemistry indices are generally the highest values in the early Eocene Climate Optimum (∼52–49.5 Ma) and a long-term decreasing trend is present from ∼ 49.5–26 Ma; the Mg/Al ratios of the clay-sized sediments and hydrochloric acid leachates are the lowest during the EECO and present a long-term increasing trend in the following 49.5–26 Ma, thus collectively indicating a continuous decrease in the regional chemical weathering intensity. The weakening weathering process during the Paleogene is mainly attributed to the decreasing global temperature and decline in precipitation caused by global cooling. Our study suggests that the proposed geochemical index based on clay-sized sediments can serve as an efficient proxy for regional chemical weathering reconstruction, even in thick sedimentary sequences with frequently changing facies and lithology. [ABSTRACT FROM AUTHOR]

The grain-size composition of fluvial-lacustrine deposits in terrestrial basins is commonly used to reconstruct temporal variations in sedimentary processes, tectonic activities and paleoclimatic conditions. However, previous studies have generally ignored possible grain-size alteration induced by post-depositional weathering and pedogenesis, which undermines the use of grain-size analysis for reconstructing depositional environments and paleoclimatic changes. In the present study, the grain-size distributions of late Cenozoic fluvial-lacustrine deposits in the Xining Basin (XNB) in the NE Tibetan Plateau (TP) were analyzed. The results demonstrate that the contents of the ultrafine (<1 μm), clay (<4 μm) and fine silt (<10 μm) fractions of bulk siltstone/mudstone samples show an upward-increasing trend, with the depositional environment changing from a shallow lake to a fluvial and finally an alluvial fan environment. Combined with the results of detailed geochemical and rock magnetic analyses of size-fractionated samples, a positive relationship was found between the content of fine-grained sediments and the weathering/pedogenic intensity in the fluvial and lacustrine sedimentary sequences. These results indicate that, under suitable conditions, chemical weathering can significantly affect the grain-size composition of fluvial-lacustrine deposits in terrestrial basins. The results are therefore very important for understanding the processes of formation of fine-grained sediments in complex natural environments. • Intensified pedogenesis caused the increase in the fine particle content of late Miocene fluvial-lacustrine sediments • Chemical weathering affect the grain-size compositions of fluvial-lacustrine deposits • Applying grain-size parameters to reconstruct palaeoenvironment should consider the effects of post-depositional alteration. [ABSTRACT FROM AUTHOR]