Fate of Ayeyarwady and Thanlwin Rivers Sediments in the Andaman Sea and Bay of Bengal

Collectively, the modern Ayeyarwady (Irrawaddy) and Thanlwin (Salween) rivers deliver> 600 Mt/yr of sediment to the sea. To understand the fate of Ayeyarwady and Thanlwin river-derived sediments to the sea, we conducted a 14-day geophysical and geological survey in the northern Andaman Sea and eastern Bay of Bengal in December 2017. Overall, similar to 1500-km of high-resolution Chirp-sonar profiles and 30 sediment cores from the shelf were acquired. This paper presents the results of the processed high-resolution profiles together with sediment analyses. Our findings indicate: 1) There is little modern sediment accumulating on the shelf immediately off the Ayeyarwady River mouths. In contrast, a major mud wedge with a distal depocenter, up to 60 m in thickness, has been deposited seaward in the Gulf of Martaban, extending to similar to 130 m water depth into the Martaban Depression. Further, 2) There is no evidence showing that modern sediment has accumulated or is transported into the Martaban Canyon; 3) There is a mud drape/blanket wrapping around the narrow western Myanmar Shelf in the eastern Bay of Bengal. The thickness of the mud deposit is up to 20 m nearshore and gradually thins to the slope at -300 m water depth, and likely escapes into the deep Andaman Trench; 4) The estimated total amount of Holocene sediments deposited offshore is similar to 1290 x 10(9) tons. If we assume this has mainly accumulated since the middle Holocene highstand (similar to 6000 yr BP) like other major deltas, the historical annual mean depositional flux on the shelf would be 215 Mt/yr, which is equivalent to similar to 35% of the modern Ayeyarwady-Thanlwin rivers derived sediments; 5) Unlike other large river systems in Asia, such as the Yangtze and Mekong, this study indicates a bi-directional transport and depositional pattern controlled by the local currents that are influenced by tides, and seasonally varying monsoons winds and waves. Organic carbon bio-markers and isotope compositions show a gradual changing pattern with the along-shelf transport from the river to the Gulf of Martaban in the east and to the Bay of Bengal in the west.