Studies on the pore water sulfate, chloride and sedimentary methane to understand the sulfate reduction process in the eastern Arabian Sea

Abstract

Sediment cores (~5 m length) from ten stations collected in the water depths of 2665-3210 m in the eastern Arabian Sea were studied for pore water sulfate (SO42-), chloride (Cl-) and lighter-hydrocarbons (methane: C1, ethane:C2 and propane: C3) along with other parameters such as calcium carbonate (CaCO3), total organic carbon (TOC) and porosity. Also dissolved oxygen (DO) content in the near bottom waters has been measured. The CaCO3 content of the sediments ranged from 33.6 to 69.9% where as TOC varied from 0.21 to 0.86%. Porosity of the sediments ranged from 54 to 57% while DO of the near bottom waters is in the range of 81 to 150 mM. The sulfate content of the sediments in all the cores varies from 39.8 mM to 12.7 mM indicating SO42- reduction. Similarly chloride concentration varies from 561 to 407 mM. The SO42- / Cl- ratio is in the range of 0.025 to 0.072 with an average ratio of ~ 0.05 indicating that there is no significant loss of sulfate in the pore water. The down-core methane concentration in the sediment ranges from 1.5 to 12.5 nM. In addition, ethane and propane concentrations ranged from 0.04 to 1.2 nM and 0.09 to 0.46 nM respectively. The slow sulfate reduction process probably is due to a low content of Corg, moderate sedimentation rates as well as low porosity, in addition to the absence of strong anoxic environment in the sediments.