Abstract
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 50ng/g (n=28), ranged from 31 to 67ng/g, and decreased with depth (0–15cm). MMHg averaged 0.91ng/g (n=18), and ranged from 0.2 to 1.9ng/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.
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