The impact of weathering, iron oxides, and aluminous clays on organic carbon storage and stability in an extensive tropical river system

• Clay minerals and pedogenic iron oxides control OC stability in the Ganges–Brahmaputra system. • Variations in δ 13 C indicate partial replacement of OC by C 3 vegetation during downstream transport. • Illite significantly enhances OC retention, while smectite plays a negative role. • Chemical weathering destabilizes organo-mineral association, reducing OC storage by ∼ 25 %. • Pedogenic iron oxides play a crucial role in preserving OC during sediment transport and weathering. Understanding the factors controlling organic carbon (OC) dynamics in tropical river systems is essential for evaluating their role in global carbon cycling. This study explores how mineralogical composition, particularly aluminous clays and pedogenic iron oxides (Fe d ), influences OC storage and stability in the Ganges–Brahmaputra (G–B) river system within the Bengal Basin. The G-B river system transports substantial terrestrial organic matter (OM) into the Bay of Bengal, making it a key component of the global carbon cycle. The Brahmaputra River sediments are enriched with C 3 plant-derived OM, as indicated by lower δ 13 C values (−24.0 ‰) compared to those from the Ganges (−23.8 ‰ to − 22.4 ‰). A significant drop in OC content in the Indo-Gangetic Floodplain (0.27 % to 0.03 %), followed by an increase in the Bengal Basin (0.15 %), suggests partial oxidation of Himalayan OC, which is subsequently replaced by OC from floodplain sources during downstream transport. Clay content (3.7 ± 2.0 %) and Fe d concentration (0.13 ± 0.07 %) in Ganges River sediments are significantly lower than those of the Brahmaputra (7 ± 2.3 % and 0.30 ± 0.07 %, respectively). Illite, a dominant clay mineral, exhibits a strong positive correlation with OC content, while smectite shows a negative correlation, likely due to its secondary formation, short interaction time with OM, and limited OC retention capacity. Fe d significantly contributes to OC preservation through interacting with minerals, particularly in chemical-weathered regions. Regression analysis identifies Fe d and illite as the most influential factors in OC storage, contributing 52 % and 48 %, respectively, while grain size shows a minor negative impact. The heavy fraction of OM, comprising over 90 % of total OC, underscores the importance of mineral-associated OC (OC MA ) in long-term storage. Enhanced chemical weathering in the Ganges floodplains destabilizes pre-formed OC MA , reducing OC storage (∼25 %) and increasing CO 2 emissions. These findings highlight that floodplain weathering significantly influences OC fate during sediment transport, and position tropical river systems as significant CO 2 sources to the atmosphere.