Sea‐Level–Modulated Sediment Routing and Redox‐State Control of Land–Ocean Carbon Coupling in the Tropical Northern Indian Ocean Over the Past 37 kyr

Abstract

Continental margins regulate land‐ocean carbon transfer, but the mechanisms linking monsoon variability, sediment routing, and organic carbon (OC) burial remain incompletely resolved. We present a high‐resolution ∼37 kyr multi‐proxy reconstruction from sediment core E87‐32B in the middle Bengal Fan, integrating pollen assemblages, bulk geochemistry, stable isotopes, mass accumulation rates, redox‐sensitive proxies (Th/U), and Monte Carlo source apportionment to resolve the relative roles of source, transport, and preservation controls. Marine OC constitutes ∼50%–70% of sedimentary carbon, while terrestrial inputs account for ∼30%–50%. Within the terrestrial fraction, C ₄ ‐derived carbon comprises 55%–70% during MIS 3–2 and exceeds C ₃ ‐derived carbon despite pollen evidence for expanded C ₃ vegetation, indicating preferential export of C ₄ ‐rich grassland and floodplain carbon. Bulk δ ¹³ C values (−20.5‰ to −18.0‰) further demonstrate a persistent decoupling between vegetation composition and exported OC, while burial flux does not correlate with vegetation composition. Terrestrial OC mass accumulation was 150%–230% higher during glacial lowstands than during the Holocene, coincident with higher sedimentation rates and more reducing depositional conditions inferred from Th/U. During transgression, total OC accumulation declined by ∼53% despite a ∼60% increase in marine organic matter, while higher Th/U values indicate enhanced oxygen exposure associated with shelf trapping and sediment reworking. These results demonstrate that sea‐level‐controlled sediment connectivity and oxygen exposure time, rather than monsoon‐driven productivity or vegetation shifts alone, govern carbon burial efficiency. Preservation efficiency therefore emerges as a dominant control on tropical margin carbon sequestration, and provides a framework for understanding natural carbon‐cycle dynamics across glacial‐interglacial sea‐level changes.