Distribution and Source of Organic Matter in Cored Sediments From the Andaman Sea off Myanmar

Knowing the distribution, and source of sedimentary organic matter (OM) in the Andaman Sea off Myanmar, the second highest sink of organic carbon in the global ocean, is significant for understanding its biogeochemical cycle. In order to better establish these parameters, we determined the total organic carbon (TOC), total nitrogen (TN), total sulfur (TS), δ¹³C_org and δ¹⁵N_tot, and n-alkanes contents of multi-core surface sediments. TOC and TN concentrations varied from 0.61 to 1.18 wt.% (average 0.87 wt.%) and 0.07 to 0.17 wt.% (average 0.13 wt.%), respectively, with high TOC in the clay-rich sediments indicating that hydrodynamic sorting readily controlled TOC abundances. Bulk TOC/TN ratio (5.51 to 11.92, average 7.89) is attributed to the high inputs from planktonic sources of OM and appreciable amounts of terrestrial OM. The δ¹³C_org and δ¹⁵N_tot values for the sediment cores ranged from -8.34 to -20.3‰ and +3.65 to +6.53 ‰, respectively suggesting that OM in the sediments is largely derived from planktonic sources, with significant contributions from a mixture of terrestrial and aquatic sources and sea grasses. n-Alkane distributions are characterized mainly by C₁₆ to C₃₅, with odd-over-even predominance after C₂₅, suggesting a diverse origin of land plant OMs. The relatively high content of odd long-chain n-alkanes from C₂₇ to C₃₃, with high peaks at C₂₉ and C₃₁, were detected in most investigated samples, reflecting a terrigenous origin of OM (e.g. shrubs, woody or herbaceous plants). Average chain length (ACL), carbon preference index (CPI), P_aq and P_wax ratio values indicate that high input of OM from non-emergent aquatic macrophytes followed by grasses and herbs.