Fate of Ayeyarwady (Irrawaddy) and Thanlwin (Salween) Rivers Sediments in the Andaman Sea and Bay of Bengal

Paul Liu, North Carolina State University, Department of Marine, Earth, and Atmospheric Sciences, Raleigh, United States, Steven A Kuehl, College of William & Mary, Williamsburg, United States, Austin Chandler Chandler Pierce, North Carolina State University, Department of Marine, Earth, and Atmospheric Sciences, Raleigh, NC, United States, Joshua R Williams, Texas A&M University, Department of Marine Sciences, Galveston, TX, United States, Neal Edward Blair, Northwestern University, Evanston, United States, Dr. Courtney Kay Harris, Ph.D., Virginia Institute of Marine Science, William & Mary, Gloucester Point, VA, United States and Day Wa Aung, University of Yangon, Department of Geology, Yangon, Myanmar
Abstract:
Collectively, the modern Ayeyarwady (Irrawaddy) and Thanlwin (Salween) rivers deliver more than 600 Mt/yr of sediment to the sea. To understand the fate of the 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, ~1500-km of high-resolution Chirp-sonar profiles and 30 sediment cores from the shelf were acquired. Our findings based on the results of the processed high-resolution profiles together with sediment analyses 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 and extends -130 m water depth into the Martaban Depression. Further, there is no evidence showing modern sediment has accumulated or transported into the Martaban Canyon; 2) There is a mud drape wrapping around the narrow shelf along the 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 sea trench in the Bay of Bengal; 3) The 100-yr-scale sediment accumulation rates in the Gulf of Martaban range between 1 and 10 cm/yr, while rates in the Western Myanmar Shelf are <1.0 cm/yr; 4) The estimated total amount of Holocene sediments deposited offshore is ~1290×109tons. If we assume this has been accumulated since the middle Holocene highstand (~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 ~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 under the alternating seasonal monsoons, tides and waves. Organic carbon biomarkers 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.
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