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

Paul Liu, North Carolina State University Raleigh, Raleigh, NC, 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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