Ayeyarwady-Thanlwin Rivers Shelf Sediment Dispersal: A High-Discharge Tidally-Dominated Monsoon-Influenced Tectonically-Active Setting

The Ayeyarwady-Thanlwin rank among the world’s top three river systems in terms of sediment load, and discharge into the energetic shelf environment of the Northern Andaman Sea.  The fate of this material has been investigated through a range of interdisciplinary studies since a 2017 field campaign to the present, and here we synthesize the resulting findings based on a combination of sedimentological, geochemical and oceanographic insights.  With no dams along the mainstems, this system has remained in a relatively steady-state condition during the past century, despite increasing human pressure, primarily from land-use changes and river sand mining. Tectonic setting plays a first-order control on the fate of the rivers’ sediment, with the formation of a mid-shelf pull-apart basin, the Martaban Depression, that serves as the major depocenter for this system.  Oceanographic conditions conspire to feed the rivers’ sediment into the Depression through the action of tides, waves and monsoon-driven circulation.  Extreme tides up to 7 m in amplitude keep sediment in suspension in an extensive shallow embayment, the Gulf of Martaban, before this material is released to the offshore Depression, likely because of some combination of spring-neap excursions, near-bed turbidity flows, or rapid offshore transport during cyclones.  Monsoon winds drive circulation toward the east during the SW Monsoon, coincident with the period of highest river discharge, further focusing sediment discharged from the numerous western Ayeyarwady distributaries into the Gulf.  Modeling results suggest surface and bottom net transport toward the Gulf may occur throughout the year.

Seabed geochemistry contributes much toward our understanding of shelf circulation and sediment dispersal patterns, and the preservation of the immense sediment-associated terrestrial organic carbon discharged by the rivers.  Core-scanning XRF elemental ratios show pronounced east-west trends that are attributed to three distinct sources: the Ayeyarwady, Thanlwin, and small rivers draining the Indo-Burman Range (IBR).  Distinct geochemical signatures on the shelf fronting the IBR suggest that sediment dispersal from the Ayeyarwady-Thanlwin is largely constrained to the Andaman Sea shelf. Downcore profiles of stable carbon isotopes from the Northern Andaman Sea show remarkably uniform values during the past century, suggesting that land-use changes evident in the catchment, especially during the past 50 years, are not preserved in the offshore record.  We suggest that extensive tidal reworking in the Gulf efficiently mutes such signals in the downcore record. Organic carbon studies further suggest that very little remineralization of terrestrial organic matter occurs during transport from the Gulf to the Depression, despite reworking and consequent oxidation in the Gulf.  Based on geochemical budgets of particle-reactive radionuclides scavenged from seawater, we estimate that significant onshore flow of open ocean water must occur, along with the substantial input of marine organic matter.  The absence of an observed “sediment priming” effect reflets the recalcitrant nature of this carbon pool.  The Ayeyarwady-Thanlwin system represents an end member within the family of such systems, and underscores the role of tectonic and oceanographic conditions in determining sediment dispersal and accumulation patterns in the marine environment.