Deciphering the physical controls on the fate of terrestrially-derived organic carbon in a high-yield tectonically-active margin

Sediment carried by large rivers accumulates on the adjacent continental shelves forming major deposits of mineral and associated organic matter. Globally, these deposits represent the major burial sites for organic matter in the ocean and help to regulate atmospheric CO2. The amount of carbon preserved in a paticular shelf deposit depends on how fast the sediment is buried.

When sediment is buried faster, more organic carbon is removed. In very energetic shelf settings, however, waves and tides can cause sediment resuspension. This resuspension leads to oxidation and breakdown of organic matter that releases CO2 to the atmosphere.

The Ayeyarwady-Thanlwin river system in the northern Andaman Sea is the third largest globally but has not previously been assessed for its carbon sequestration potential. It is the only remaining large river system in Asia that has not been severely affected by human activity. Humans have altered the delivery of river sediment to the ocean in many systems with unknown consequence on the global carbon budget.

This study uses an existing set of core samples and other data collectd during a 2017 research cruise. The goal is to examine the transformation of organic matter from the rivers to the ocean. The study also explores the physical processes that determine the burial efficiency for orgainc carbon.

This will be determined through modeling of ocean currents, waves, tides and storms. The results of this study will provide a baseline for future changes, including planned major dam construction on these rivers. With increasing human modifications to this system and increased storm frequency driven by climate change, this research will help better understand the future of this heavily populated and environmentally sensitive system.

This project will support a Ph.D. student and a post-doctoral scientist. In addition, the models developed for this project will be available for community modeling efforts.

Continental margins are the primary depocenters of mud delivered by rivers, record a rich history of terrestrial and oceanographic conditions, and account for most of the organic carbon (OC) burial in the ocean. The Ayeyarwady and Thanlwin rivers enter an active margin in the northern Andaman Sea and eastern Bay of Bengal. Relatively understudied, their combined inputs rank in the top three of the world’s river systems in terms of sediment and OC supply.

A 2017 research cruise conducted by the PIs recovered a unique and unprecedented set of observations and sediment samples, and this study leverages these existing samples and modifies a numerical model to address exciting new questions regarding the dispersal and fate of sediment and OC in this globally important system. Specifically: 1) what is the residence time of sediment in the Gulf and mechanism(s) for transfer of this material to the basin depocenter?, 2) how do the associated physical processes affect the transformation and sequestration of organic carbon as it transits the shallow shelf to the basin?, and 3) what are the relative roles of seasonal (monsoonal) oceanographic conditions and episodic events (cyclones) in affecting sediment dispersal to the depocenter and northwestern shelf.

At the time of the 2017 cruise, the Ayeyarwady and Thanlwin were the last free-flowing mega-rivers in tropical and subtropical Asia. Therefore, this study provides a baseline for certain future change. Numerical models developed for this project will be available for application to other river-influenced margins via community modeling efforts.

This research supports the education of a promising Ph.D. candidate who will focus on the organic geochemistry proposed herein, and a post-doctoral scientist who will focus on the potentially critical role of fluid mud transport in this system. The PIs forged strong relationships with the Myanmar research community during the previous project, and propose to continue this engagement through joint participation at an international meeting.

With rapidly increasing human modifications to the A-T system and predictions of future increased cyclone frequency driven by climate change, this research will be instrumental in understanding the future trajectory of this heavily populated and environmentally sensitive system.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.