Controls on methylmercury accumulation in northern Gulf of Mexico sediments

We examined Hg biogeochemistry in northern Gulf of Mexico (nGOM) sediments along a similar to 400 km. east-west transect off the Louisiana coast in order to characterize primary controls on net methylmercury (MeHg) production and accumulation in sediments and evaluate the potential influence of water column hypoxia. Total Hg (THg) and MeHg concentrations ranged from 52 to 340 pmol g(-1) and 0.08 to 1.4 pmol g(-1), respectively, and exhibited no clear east-west spatial trends, or trends related to water column hypoxia. Potential methylation and demethylation rate constants (k(m) and k(dm)), from enriched isotope spikes (Hg-201(II) and (MeHg)-Hg-199) to intact sediment cores, were substantially higher (0.02-0.19 d(-1) and 39-63 d(-1), respectively) than in other coastal sediment systems. The percentage of Hg present as MeHg in sediment cores (%MeHg = 0.04-1.1%) was comparable to other systems. Both %MeHg and k(m) decreased with sediment depth and were significantly correlated, but neither was correlated with organic carbon (OC). Together, OC and k(m) explained 56% of the variation in [MeHg]. These results suggest that OC primarily acts as a MeHg-binding ligand in nGOM sediments, unlike some other coastal systems Where OC has been shown to directly influence Hg methylation. (C) 2015 Elsevier Ltd. All rights reserved.