Organic matter and sedimentary accumulation rates in a transect of cores in the Bay of Bengal offshore Bangladesh and Andaman Sea offshore Myanmar

Abstract

Eight sediment multi-cores samples collected from the Bay of Bengal and the Andaman Sea were examined to evaluate organic matter (OM) sources to the marine setting for a large sediment and carbon sink. Total organic carbon (TOC) for sediments from the northern Bay of Bengal (NBB), eastern Bay of Bengal (EBB) and the northern Andaman Sea (NAS) range from 0.34 to 0.79 wt%, 0.84 to 1.06 wt%, and 0.20 to 1.18 wt%, respectively, with higher concentrations in fine silts than in the sandy fractions suggesting hydrodynamic sorting and surface area control TOC. The TOC to total nitrogen (TN) ratio (C/N) (NBB: 5.31–8.52; EBB: 5.51–6.97; NAS: 6.44–11.92) indicates OM is a mixture of dominant contributions from marine plankton and lesser contributions from land plants. The organic carbon (δ13Corg) isotopic values display a similar range of values in each of the regions of the NBB (−12.0 to −19.9‰), EBB (−10.1 to −20.2‰) and NAS (−8.3 to −20.3‰). δ13Corg values may derive from marine phytoplankton, terrestrial C4 plants and/or aquatic macrophytes. The total nitrogen (δ15N) isotopic compositions (NBB: 3.7–4.1; EBB: 4.1–5.9; NAS: 4.2–6.5‰) diagnose input from planktonic sources or aquatic macrophytes. Hydrocarbons from the sediments have a bimodal distribution with high relative abundance of mid-chain n-alkanes (C23 and C25 peaks) and long-chain n-alkanes (C29 and C31). Average chain length (ACL25-35), carbon preference index (CPI25-35), and grassland (Pgrass) ratios suggest contributions from terrestrial vegetation including grasses. High TOC to total sulphur (TS) ratios (C/S) observed suggest well-oxygenated interval associated with vertical mixing of seawater about 1000 years ago. The average sedimentation rates of Bay of Bengal and Andaman Sea are about 0.4 mm yr−1 and 0.07 mm yr−1, respectively. The organic carbon accumulation rate of the Bay of Bengal is found to be several times higher than that of Andaman Sea. Rapid deposition of both marine and terrestrial OM in the Bay of Bengal could be a favourable sedimentary system for methane fermentation to form methane hydrate in this regime.