Abstract

Variations in the composition and texture of detrital records preserved in the Himalayan foreland basin and the Indus and Bengal fans likely track stages in the evolution of the orogen, the Indian Summer Monsoon, and regional ecology. Yet, interpreting these records remains challenging, as sediment characteristics are shaped by shifting provenance, hydrological sorting, weathering intensity, incorporation of mineral and organic matter, and post-depositional alteration. To address some of these complexities, we examined the mineralogy and clumped isotope compositions of detrital carbonates from modern Ganga River sediments and from early Miocene to Holocene Bengal Fan turbidites. Our results show that variations in clumped and oxygen isotope values of detrital calcite in Himalayan rivers reflect mixing between lithic carbonates and authigenic calcite precipitated within the river system. In Bengal Fan turbidites, clumped isotope temperatures and calcite-to-dolomite ratios decline during the Late Miocene, marking an increase in the relative contribution of authigenic calcite. This trend cannot be explained by decreasing source temperatures, which would imply unrealistically high-temperature calcite formation, nor by intensified weathering, as K/Si* values (a weathering proxy) remain stable since the mid-Miocene. Instead, we argue that the shift primarily reflects a change in the relative abundances of calcite and dolomite in Himalayan-derived sediments, specifically a Late Miocene, Himalaya-wide provenance shift from calcite-rich (dolomite-poor) Tethyan and Greater Himalaya terrains to calcite-poor (dolomite-rich) Lesser Himalaya sources.