What has driven a late Miocene drop in clumped isotope temperatures of detrital calcite in Himalayan river systems?

Variations in the composition and texture of detrital records preserved in the Himalayan foreland basin and the Indus and Bengal fans, potentially records stages in evolution of the orogen, the Indian Summer Monsoon climate, and regional ecology. However, interpreting these records is challenging, as sediment properties are influenced by changing provenance, hydrological sorting, weathering intensity, incorporation of mineral and organic matter, and post-depositional alteration. To address some of these complexities, we analyzed the mineralogy and clumped isotope compositions of detrital carbonates from modern Ganga River sediments and from early Miocene to Holocene Bengal Fan turbidite deposits. We show that variations in clumped and oxygen isotope values of detrital calcite in Himalayan river sediments reflect mixing between Himalayan-derived lithic carbonates and authigenic calcite precipitated within the river system. Clumped isotope temperature values and calcite-to-dolomite ratios in Bengal-Fan turbidite deposits drop in the late Miocene, indicating an increase in the relative contribution of authigenic calcite. This shift cannot be explained by a decrease in source temperatures, as it would imply unrealistically high-temperature lithic or authigenic calcite formation during the Miocene. Nor can it be attributed to intensified weathering, given that turbidite K/Si* values (proxy for weathering intensity) remain stable since the mid-Miocene. We therefore suggest that these trends are primarily driven by a change in the abundancies of both calcite and dolomite in Himalayan derived sediments. Specifically, we propose that this observation reflects a Late Miocene Himalaya-wide shift in provenance of detrital carbonate, from calcite‐rich (dolomite‐poor) Tethyan‐ and Greater‐Himalaya to calcite‐poor (dolomite‐rich) Lesser Himalaya terrains.