Exploring Natural And Human Influences On Sediment Dispersal And Seabed Geochemistry In The Offshore Ayeyarwady Delta, Myanmar

Worldwide, large river deltas have been extensively modified to support thedevelopment of growing coastal populations. While this is especially true of Asian river deltas (e.g., the Changjiang, Huang He, and Ganges-Brahmaputra), the Ayeyarwady Delta in Myanmar has remained remarkably underdeveloped. However, over the last 50 years changes in land use and increased mining efforts in the Ayeyarwady and Thanlwin River catchments have resulted in the deforestation of coastal and terrestrial forests and begun to alter fluvial sediment fluxes. While no large dams are currently installed on either river mainstem, future construction in Myanmar is imminent. Together the Ayeyarwady and Thanlwin Rivers transport ~485 Mt/y of sediment and as much as 7.7 Mt/y of particulate organic carbon to the offshore delta on the Northern Andaman shelf where material is extensively mixed by tides in the Gulf of Martaban before accumulating in a mid-shelf clinoform. Despite intense seabed mixing, substantial exchange of marine and fresh waters, and the potential for organic carbon priming, the isotopic composition of terrestrial organic carbon is largely consistent from the Gulf of Martaban to the clinoform depocenter suggesting that terrestrial material accumulating on the shelf is largely refractory in nature. This can be explained by the nearshore remineralization of labile terrestrial components prior to across-shelf transport and entrainment in frequent resuspension and deposition cycles. In addition to the spatially consistent terrestrial organic carbon signatures, 210Pb profiles further indicate that sediment is transported from the inner shelf to the clinoform depocenter on seasonal and / or tidal timescales, promoting short shelf transport times and the effective sequestration of terrestrial material in the clinoform. As a result, terrestrial organic carbon content is high in offshore sediment deposits, making the offshore Ayeyarwady delta a greater sink for terrestrial material on the continental margin than previously expected. While temporal changes to sequestration may be anticipated due to the increased development of the river catchments, no observable change in terrestrial organic carbon character or content is identified in the marine sediment record over the last 100 years. This is likely attributed to the extensive mixing on the shelf, wherein geochemical signatures associated with human development are filtered through frequent seabed resuspension. Nonetheless, future damming will drastically reduce fluxes from the Ayeyarwady and Thanlwin Rivers, leading to delta shoreline erosion and coastal inundation as well as changes to offshore terrestrial organic carbon accumulation. Flux reductions may also induce the erosion of the subaqueous clinoform deposit which would re-introduce organic carbon and other sediment-bound particles, such as organic pollutants and trace metals, to the water column. The re-exposure of previously sequestered material will likely alter sediment and terrestrial organic carbon distribution trends and negatively impact aquatic ecosystem health. While development in Myanmar is currently minimal, we suggest that given baseline conditions, damming and continued development will substantially alter delta sustainability, shelf sediment dynamics, as well as regional and global biogeochemical budgets.