Mercury concentrations, speciation, and isotopic composition in sediment from a cold seep in the northern Gulf of Mexico

Total-Hg, monomethylmercury (MMHg), and mercury isotopic composition was determined in sediment from a cold seep and background sites in the northern Gulf of Mexico (nGoM). Total-Hg averaged 50 ng/g (n = 28), ranged from 31 to 67 ng/g, and decreased with depth (0–15 cm). MMHg averaged 0.91 ng/g (n = 18), and ranged from 0.2 to 1.9 ng/g. There was no significant difference for total-Hg or MMHg between cold seep and background sites. δ202Hg ranged from −0.5 to −0.8‰ and becomes more negative with depth (r = 0.989). Mass independent fractionation (Δ199Hg) was small but consistently positive (0.04–0.12‰); there was no difference between cold seeps (Δ199Hg = +0.09 ± 0.03; n = 7, 1SD) and background sites (Δ199Hg = +0.07 ± 0.02; n = 5, 1SD). This suggests that releases of hydrocarbons at the cold seep do not significantly alter Hg levels, and that cold seeps are likely not major sources of MMHg to nGoM waters.