Distribution and sources of organic matter in surface sediments of the eastern continental margin of India

Abstract

The sources and distribution of Organic Matter (OM) in surface sediments of the eastern continental margin of India, including the region influenced by river discharge, were investigated using content, molar C:N ratios and stable isotopes of carbon and nitrogen. Despite relatively high water column integrated chlorophyll-a concentrations were found in the continental shelf than the slope; however, the lower Sediment Organic Carbon (SOC) was found in the former than the latter region suggesting that in situ production did not play significant role on preservation of SOC in the coastal Bay of Bengal. The broad range of δ¹³C of SOC (−23.2 to −16.7‰) suggests that OM is a broad mixture of terrestrial and marine OM. Relative contributions from terrestrial C₃ and C₄ plants and marine sources are quantified as 34%, 23% and 43%, respectively, indicating that dominant source of allochthonous OM (∼57%) in the coastal Bay of Bengal. Relatively higher contribution of OM from C₄ plants was found in the sediments at off river Krishna indicating that this region received detritus of agricultural crops such as jowar, bajra and sugar cane, which are dominant in its drainage basin, during SW monsoon. This study revealed that relatively high OM preserved in the slope than shelf region along the coastal Bay of Bengal and the composition of OM is primarily controlled by the type of agricultural crops and vegetation in the drainage basin of the river.