Effects of bottom water oxygen concentrations on mercury distribution and speciation in sediments below the oxygen minimum zone of the Arabian Sea

This study investigated the influences of varying dissolved oxygen (DO) levels in overlying bottom water, sediment grain size, total sedimentary organic carbon (Corg), and total sedimentary sulfide concentrations on the distribution and speciation of mercury (Hg) and methylmercury (MeHg) in coastal sediments from a transect across the oxygen minimum zone (OMZ), in the Arabian Sea, off the west coast of India. The Corg concentration increased with the increasing finer particulate in the sediments and decreasing DO level of the overlying bottom water. Increasing concentrations of Corg (under suboxic overlying water) increased accumulation of Hg in the sediments. However, depleted DO level of the overlying bottom water did not enhance the concentration of MeHg in the underlying sediments. The concentration of sedimentary MeHg decreased with the increasing Corg and total Hg concentrations in the sediments. The nature of the Corg played a key role in controlling the distribution and speciation of Hg in the sediments. This study suggests that increased concentrations of inert Corg (with C/N>11) increased Hg-Corg complexation and decreased the net methylation rate of Hg in the sediments from the study area. Overall, the nature of Corg controlled Hg-methylation in the surface sediments of the OMZ in the Arabian Sea.