Abstract:
Although currently the third largest delta system in the world in terms of sediment supply, and second in terms of carbon delivery, the offshore Ayeyarwady Delta has remained, until recently, largely unstudied. We report here on an investigation of the shelf area off the Ayeyarwady, including data from a research cruise conducted in December 2017. Sediment dispersal and accumulation in the adjacent Andaman Sea and Bay of Bengal are controlled by: seasonal variations in sediment input; reversing monsoon winds and circulation patterns; strong forcing by tides and waves; and tectonic uplift/subsidence west/east of the N-S trending Sagaing fault that bisects the delta. Maximum sediment accumulation rates of ~10 cm/y are observed in the foreset region of a subaqueous delta that is prograding into the Martaban Depression east of the Sagaing fault. Accumulation rates in other areas of the shelf surrounding the delta are typically ~1 cm/y. The extensive shallow (topset) region of the eastern Gulf of Martaban is characterized by deep (up to a meter) physical mixing of the seabed and reduced net accumulation rates. Frequent and deep re-oxidation of pore waters by tide and wave sediment resuspension likely forms the distinctive reddish-brown coloration of the sediments, and has important implications for carbon sequestration. The shelf (ramp) off the western mouths of the Ayeyarwady Delta is characterized by a relatively narrow nearshore (<40 m water depth) mud belt with higher sand and silt content offshore. This area of the delta is subject to extreme tidal currents and wave resuspension (SW monsoon), which may be energetic enough to winnow most of the fine-grained sediments delivered by the river mouths, and transport this material eastward to the Gulf of Martaban. The western most area adjacent to the Bay of Bengal is characterized by a mud drape extending across the entire shelf to slope. As evidenced by its distinct provenance signatures and regional modeling simulations, this area could represent the distal portion of the Ganges-Brahmaputra sediment dispersal system, with some addition from the Ayeyarwady that is transported westward during the NE monsoon. It appears that off-shelf escape of sediment to the Bay of Bengal occurs, however, there is no evidence of escape through the Martaban submarine canyon to the deeper portions of the Andaman Sea, as has been previously reported. A centennial sediment budget for the area indicates efficient sediment trapping in the Andaman Sea region. This area represents a golden opportunity for future fundamental research in the region concerning carbon cycling, climate and oceanographic forcing, and the future effects of sea level rise on deltaic landscapes.