Geochemical comparisons of weathering, provenance and tectonics in the fluvial sediments from Yarlung Zangbo to Brahmaputra River

Globally, large river sediments record crucial signatures of geology and climate change from source to sink. The Yarlung Zangbo River (YZR) is upstream of the Brahmaputra River (BJR). It originates in the active Himalaya Tibetan Plateau (HTP) and finally enters the Indian Ocean. Therefore, YZR-BJR sediments are best for studying Earth surface processes and climate variations in Asia. However, few comparisons of their sediment chemistry have been conducted with respect to the weathering, climate, provenance and tectonic setting. Here, we examined 80 fluvial sediments measuring major and trace elements with chemical indicators. The YZR-BJR sediments are derived from a felsic source on an active continental margin with a continental island arc. Both the Lhasa terrane (LT) and Himalayan orogen are possibly the main sources of upstream (YZR) sediments that are chemically close to the upper crustal continent (UCC). In contrast, downstream BJR sediments, such as the postArchean Australian Shale (PAAS), are predominantly sourced from the eastern Himalayan orogen, followed by LT. Due to the uplift of HTP and global cooling, the YZR sediments underwent low to moderate silicate weathering, while the downstream BJR sediments experienced weaker weathering processes with lower chemical index of alteration (CIA) values. This means dominant physical weathering and erosion in the source area and thus provides substantial sediments. Although the CIA values of finer overbank sediments are slightly higher than those in coarser riverbank sediments, they can still reveal weathering combined with other chemical indicators. The results suggest that there is no obvious contrast between YZR and BJR in the HTP due to river mixing and strong physical weathering.

Automated summary

The sediments of the Yarlung Zangbo River and the Brahmaputra River are important resources for studying surface processes and climate change in Asia. This study compared the sediment chemistry of the two rivers and found differences in their sources, weathering processes, and sediment types. Due to river mixing and strong physical weathering, there is no obvious contrast between the two rivers in the Himalaya Tibetan Plateau.