Chemical weathering processes of Himalaya-river systems since Miocene recorded by bulk and clay mineralogy of deep-sea sediments from IODP Expedition 354

The Ganga-Brahmaputra river system transports up to 600 million tons of sediment annually from the Himalayan range to the Bengal Fan. The catchment of the Ganga-Brahmaputra river system is characterized by highly contrasting lithologies and exhumation rates which strongly influence erosion and chemical weathering processes. Recent studies have emphasized the importance and role of floodplains for the chemical weathering of sediments eroded from Himalayan mountains. Changes in sediment chemical weathering are controlled by climatic changes, such as variations in Indian summer monsoon precipitation and glacial interglacial cycles. However, further exploration is needed to understand the impact of anthropic changes and long-term climatic and tectonic forcings on the chemical weathering regime of the Ganga-Brahmaputra system. We present the bulk and clay mineralogy, obtained with XRD, of turbiditic sediments collected from the Bengal Fan in the Indian Ocean during the IODP Expedition 354. The clay mineralogical assemblages of IODP Expedition 354 present a dominance of illite and chlorite throughout the record with a relative increase of smectite and kaolinite content during the Miocene. Such clay mineralogy is consistent with the clay mineralogy of sediments from Leg 22 site 218 from DSDP (which was reoccupied for  IODP Expedition 354). Miocene bulk sediments are relatively enriched in smectite, kaolinite, goethite, and terrigenous carbonates (calcite and dolomite). Therefore, our mineralogy results are showing a change in chemical weathering regime affecting the Himalaya system between the Miocene and Quaternary.  The Quaternary is characterized by a lower content of smectite, kaolinite and carbonates, the presence of amphiboles and an enrichment in micaceous minerals (muscovite/illite, biotite, chlorite) and plagioclases as also inferred from Raman spectroscopy (Limonta et al., 2023). This indicates that during the Miocene the chemical weathering of ferro-magnesian minerals and calco-sodic feldspars was more efficient as shown by geochemical data (Tachambalath, 2023). The decrease in chemical weathering intensity from Late Miocene is consistent with the concurrent Late Cenozoic global cooling and drying of Himalayan front associated with the decrease in Indian monsoon seasonality and/or precipitation after 10-8 Ma (Clift and Webb, 2019). Here, we show that the change in the Indian monsoon system from 10-8 Ma is marked in the Bengal Fan turbiditic sediments mineralogy.