Sediment And Terrestrial Organic Carbon Budgets For The Offshore Ayeyarwady Delta: Establishing A Baseline For Future Change

Large river deltas serve as globally important archives of terrestrial and shallow marine biogeochemical signatures because of rapid sediment burial, and have the potential to impact global biogeochemical cycling on modern and geologic timescales. This study investigates modern sediment and terrestrial organic carbon (TerrOC) accumulation within the offshore Ayeyarwady Delta in order to determine sediment and TerrOC budgets for this largely understudied mega-delta. The Ayeyarwady is the world’s third largest delta in terms of sediment supply, and remains one of the last long free-flowing rivers in Asia. However, recent increases in regional anthropogenic impacts risk severe alterations to sediment and TerrOC supplies and transport within this major system, proving the motivation for constructing baseline sediment and TerrOC budgets. Based upon 210Pb geochronology of 30 sediment cores collected offshore of the Ayeyarwady Delta, we estimate that 392 +77/-15 Mt of sediment, or ~ 70 - 80% of the Ayeyarwady-Thanlwin sediment load, accumulates on the shelf annually with the highest accumulation rates being found in the foreset beds of the subaqueous delta. Sediment not retained on the shelf is likely partitioned between the Ayeyarwady floodplain, shoreline accretion, and minor deep sea export. δ13C mixing models were coupled with modern sediment accumulation rates to identify TerrOC burial fluxes which indicate that 1.96 +1.07/-0.22 Mt/yr of TerrOC is accumulating on the shelf, with TerrOC burial fluxes also being highest in the foreset beds. Comparing annual shelf-wide accumulation to Ayeyarwady-Thanlwin TerrOC input we estimate that an apparent ~ 100% of TerrOC input is preserved offshore. However, an across shelf trend of increasing TerrOC degradation with increased distance offshore is also found, indicating that while the shelf has a high apparent TerrOC sequestration potential, oxidation is also occurring prior to deposition within the subaqueous delta. Based on these conflicting outcomes, we suggest that input of TerrOC from additional sources other than the Ayeyarwady-Thanlwin River, including ungauged small and medium rivers, and the Ayeyarwady delta plain, maintains high TerrOC burial fluxes. As anthropogenic development within the Ayeyarwady and Thanlwin watersheds continues to increase, these sediment and TerrOC budgets provide a baseline from which future human and environmentally induced changes within the offshore Ayeyarwady Delta can be monitored