Terrestrial organic carbon processing and dispersal on a high-energy, river-dominated margin

Abstract. The Ayeyarwady and Thanlwin Rivers deliver ~1.9 Mt y^-1 of particulate terrestrial organic carbon (TerrOC) to the Northern Andaman Sea making them one of the largest sources of TerrOC to the world ocean. Offshore, fluvial material is extensively mixed by tides and estuarine circulation in the Gulf of Martaban, concurrently incorporating marine organic carbon. While previous bulk stable isotope analyses have suggested that frequent resuspension enhances TerrOC degradation, limiting burial on high-energy margins, in this study we use ramped pyrolysis/oxidation (RPO) and radiocarbon analysis to identify high and uniform TerrOC content (by Wt. %) in continental shelf sediments offshore of the Ayeyarwady Delta. By analyzing sediment samples from the river mouths and across the shelf, RPO results demonstrate consistent radiocarbon and stable carbon isotope composition (δ¹³C) of TerrOC from the Gulf of Martaban to the mid-shelf clinoform depocenter, suggesting that extensive degradation does not occur during shelf transport. Instead, rapid remineralization and processing of labile material likely occurs near-shore, while refractory terrestrial components are efficiently transported to the mid-shelf depocenter. Given δ¹³C values of refractory TerrOC fractions that mimic marine sources, we suggest that offshore TerrOC content has been substantially underestimated by bulk δ¹³C-based mixing models. Providing a new conceptual framework for organic carbon preservation in this globally significant offshore delta, these findings indicate the necessity of re-evaluating our understanding of TerrOC processing on high-energy, river-dominated margins, and support recent hypotheses that these systems provide a larger sink for TerrOC than previously suggested.