Fates of the Ayeyarwady and Thanlwin River-Derived Sediments to the Andaman Sea and Gulf of Bengal

Collectively the modern Ayeyarwady and Thanlwin rivers deliver more than 500 Mt/yr of sediment to the sea. However, the distribution, thickness, accumulation rate, transport and depositional process, and stratigraphic sequences of these two rivers' sediments on the adjacent shelves are not well known. To understand the fate of the river-derived sediments to the sea, we conducted a 14-day geophysical and geological survey off the Ayeyarwady and Thanlwin deltas in December 2017. Overall we have acquired 1500-km high-resolution CHIRP sonar profiles and 30 sediment cores in the northern Andaman Sea and eastern Gulf of Bengal. Here we present the results of the processed high-resolution seismic profiles together some sediment core analysis. We find: 1) There is little Holocene sediment accumulating on the shelf immediately off the Ayeyarwady River mouth, in contrast, a major mud depocenter, up to 60 m in thickness, has been deposited southeastward and extends -150 m water depth into the Gulf of Martaban. 2) Further, there is no evidence that shows modern sediment has accumulated or has been transported into the Martaban Canyon; 3) There is also a mud belt wrapping around the narrow shelf along the western side of the Ayeyarwady River delta in the eastern Bay of Bengal. The thickness of the mud clinoform along the northwestern coast is up to 20 m nearshore and, although it thins slightly seaward, it extends beyond the shelf break at -200 to -300 m water depth, where sediment escape to the slope and basin is likely; 4) Accumulation rates are typically 0.5-1 cm/y in nearshore areas, however these rates increase to a maximum of 10 cm/y in the mud depocenter off the Gulf of Martaban. Unlike other large river systems, such as the Yangtze and Mekong, this study highlights a unique bidirectional transport pattern controlled by the alternating monsoonal seasons.