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. delta Hg-202 ranged from -0.5 to -0.8 parts per thousand and becomes more negative with depth (r = 0.989). Mass independent fractionation (Delta Hg-199) was small but consistently positive (0.04-0.12 parts per thousand); there was no difference between cold seeps (Delta Hg-199 = +0.09 +/- 0.03; n = 7, 1SD) and background sites (Delta Hg-199 = +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. (C) 2013 Elsevier Ltd. All rights reserved.