Recycling control on chemical weathering indices (Yarlung River, South Tibet)

ABSTRACT Geochemical proxies are widely considered to effectively reflect weathering intensity across drainage basins, without duly considering the possibility that weathering features can be inherited from previous sedimentary cycles. To elucidate the impact of the recycling effect on weathering assessments, this study investigates sandbar and suspended sediments in the Yarlung River, representing the Tibetan headwaters of the Brahmaputra River, by comparing weathering proxies in largely first‐cycle sediments derived from igneous rocks of the Lhasa Block versus overwhelmingly recycled sediments from sedimentary rocks of the Himalayan belt. First‐cycle sediments record modern weathering intensity, whereas recycled sediments reflect an integrated weathering history incorporating also chemical effects acquired during previous sedimentary cycles. Detritus derived from the Himalayan belt thus misleadingly appears to be more weathered than detritus shed from the Lhasa Block in the same modern semiarid climate. Weathering signatures in the Yarlung catchment are thus primarily governed by grain‐size controlled variability in sediment contribution from the Lhasa Block versus Himalayan belt. Because of the inherited weathering signals, recycled sediments systematically have higher CIA and lower WIP than first‐cycle sediments, a discrepancy particularly manifest under lower temperature and precipitation conditions. Because all major rivers flow across sedimentary basins, recycled detritus is abundant in deltaic to turbiditic sediments and sedimentary rocks worldwide. This implies that the weathering signal they carry is invariably the sum of weathering acquired in the catchment plus paleo‐weathering. Detangling coeval versus inherited weathering effects poses a fundamental challenge in sedimentary research if reliable climatic and paleoclimatic reconstructions must be obtained.