Estuarine-deltaic controls on coastal carbon burial in the western Ganges-Brahmaputra delta over the last 5,000 years

Abstract

The Ganges–Brahmaputra fluvial system drains the Himalayas and is one of the largest sources of terrestrial biosphere carbon to the ocean. It represents a major continental reservoir of CO2 associated with c. 1–2 billion tons of sediment transported each year. Shallow coastal environments receive substantial inputs of terrestrial carbon (900 Tg C yr−1), with allochthonous carbon capture on connected floodplains. Vegetated coastal ecosystems play a dominant role in the sequestration of carbon and operate as highly efficient carbon sinks. Mangrove sediments are subject to intense carbon-fixing processes that have a potentially high impact on the global carbon budget. The Sundarbans is the largest tidal mangrove forest in the world (10,200 km2 in area) and is located on the marine-terrestrial boundary of the Ganges-Brahmaputra delta and the Bay of Bengal, in West Bengal (India) and Bangladesh. Estimates of sedimentation on the tidal delta plain of the Ganges-Brahmaputra delta reveal mean rates of ∼1.1 cm yr−1 with accretion understood to approximately equal the regional rate of sea-level rise of ∼1.0 cm yr−1. In this study, the properties of sediments from the western Ganges-Brahmaputra delta are used to investigate controls on coastal carbon burial over the past 5,000 years. Our main findings are: (1) Beta regression of aluminium and silica ratio data is a robust method of estimating total organic carbon in sediment from the Indian Sundarbans; (2) the estimated rate of sediment deposition over last 5,000 years is between 1.0 and 2.5 mm yr−1, with uncertainty surrounding the reworked origins of sediment; and (3) temporal variation of total organic carbon accumulation through the last 5,000 years is generated by varying sedimentary depositional processes. The delivery and burial of total organic carbon is predicated on the continual supply of sediment to the Sundarbans, which future management strategies may need to consider given changing rates of deposition.