Deriving Sediment and Terrestrial Organic Carbon Budgets for the Offshore Ayeyarwady River Delta, Myanmar

Abstract:

Large river deltas serve as globally important archives of terrestrial and shallow marine biogeochemical signatures and, because of rapid burial, have the potential to impact global biogeochemical cycling on both 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, second in TerrOC delivery, 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 for this major system, proving the motivation for constructing baseline sediment and TerrOC budgets. Based upon ²¹⁰Pb geochronology of 30 sediment cores collected across the subaqueous delta, sediment accumulation rates (SARs) are found to reach near 10 cm/yr in the subaqueous delta foreset, while other regions are closer to 1cm/yr. Areal integration of SARs suggests that modern offshore accumulation accounts for a large fraction of the > 600 Mt/yr sediment load. This is in contrast to the Holocene budget that accounts for only ~1/3 of the sediment load, indicating a shift in the distribution of regional sedimentation from primarily subaerial to subaqueous progradation during the late Holocene. Despite the large volume of TerrOC delivered to the delta, oceanographic factors and sedimentological data qualitatively indicate that, landward of the subaqueous delta foreset, TerrOC is rapidly oxidized, preventing efficient TerrOC burial offshore. TerrOC sequestration potential is being further quantitatively assessed through the use of stable isotopes; however, preliminary elemental analyses support qualitative findings. This data in combination with spatially integrated SARs provides both sediment and TerrOC budgets as baseline values from which future human and environmental impacts can be monitored within the Ayeyarwady Delta. These budgets also indicate changes in regional sedimentation over the late Holocene and this mega-delta’s role in global carbon cycling.