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Total Hg distributions and its speciation were determined in two sediment cores collected from the western continental marginal high of India. Total Hg content in the sediment was found to gradually increase (by approximately two times) towards the surface in both the cores. It was found that Hg was preferentially bound to sulfide under anoxic condition. However, redox-mediated reactions in the upper part of the core influenced the total Hg content in the sediment cores. This study suggests that probable increase in authigenic and allogenic Hg deposition attributed to the increasing Hg concentration in the surface sediment in the study area.

This study demonstrates that under abiotic dark conditions in aquatic system, humic substances are not only capable of converting Hg(II) to Hg(0) but also able to bind Hg(II) ion. The degree of Hg(II) reduction is significantly influenced by the ratio of -COOH/-OH groups and the sulfur content in the HS, revealing a strong competition between complexation and reduction of Hg(II). This study suggests that abiotic and dark Hg(II) reduction depends on the pH and salinity of aqueous medium. At lower pH (∼ 4.0) and lower salinity (≤ 5.0 PSU), the reduction of Hg(II) to elemental mercury (Hg(0)) was comparatively rapid. Higher -COOH/-OH ratios in HS, favors dark abiotic reduction of Hg(II) as did a lower sulfur (S) content of HS. This study provided a rigorously controlled experimental design that showed that dark abiotic Hg(II) reduction by HS can potentially be important in the aquatic environment and is independent of the photochemical reduction observed in both fresh water and sea water.