Decoding impacts of modern human development on terrestrial organic carbon sequestration seaward of the Ayeyarwady-Thanlwin rivers

Continental shelf mud deposits off large rivers are substantial repositories for organic carbon; however, many of these rivers and the deltas they build have been extensively modified to support the development of growing coastal populations. While such modification is dramatic for most large Asian rivers (e.g., the Chang Jiang, Huang He, and Ganges-Brahmaputra), the Ayeyarwady-Thanlwin rivers remain an exception because of the absence of dams on the mainstems. Despite this, increased deforestation and associated land use change over the last 50 years has begun to alter fluvial sediment loads for this system, likely impacting the flux of sediment-bound terrestrial organic carbon to the global ocean. Together the Ayeyarwady- Thanlwin rivers transport a globally significant ~ 485 Mt yr^-1 of sediment and as much as 7.7 Mt yr^-1 of particulate organic carbon to the Northern Andaman Sea, where extreme tides in the Gulf of Martaban cause extensive resuspension of material prior to accumulation as a muddy, mid-shelf clinoform. Based on bulk stable isotope analyses, frequent resuspension of the seabed in the Gulf of Martaban creates a low-pass filter for geochemical signatures, effectively limiting signals of land use change during the past century. Whereas previous bulk analyses have indicated that terrestrial organic carbon may be remineralized during across-shelf transport, ramped pyrolysis/oxidation and radiocarbon methods show consistent terrestrial organic carbon character and content across the shelf, indicating that refractory terrestrial organic carbon dominates shelf deposits.  Comparing these findings with organic signatures of the rivers’ sediments, we suggest that significant remineralization of labile terrestrial material may occur prior to reaching the open shelf. Our findings also suggest that bulk sediment analyses have generally underestimated offshore terrestrial organic carbon content, which substantially impacts the derivation of carbon budgets for the Ayeyarwady-Thanlwin and other systems. While terrestrial organic carbon content in the offshore delta is currently high, with little indication of modern human impacts in the sediment record, organic carbon accumulation has the potential to be drastically impacted by future planned mainstem dam installation as well as changing climate (e.g., monsoon patterns and cyclone frequency and strength). Anticipated mainstem damming will likely alter the nature and magnitude of terrestrial organic carbon on the shelf due to increased reservoir retention and reduced sediment load. The consequent reduction in sediment supply may also drive the erosion of the clinoform, re-exposing and redistributing previously sequestered material on the shelf. The frequency of cyclonic activity and monsoon strength are also likely important climatic controls on offshore carbon delivery and sequestration. While the offshore Ayeyarwady delta currently exhibits minimal impacts from human activity, we predict that future damming and changes in climate will substantially alter terrestrial organic carbon sequestration in the offshore Ayeyarwady Delta, impacting both regional and global carbon budgets.