Controls on methylmercury accumulation in northern Gulf of Mexico sediments

• Sediment THg and MeHg lacked well-defined spatial trends in nGOM.
• Potential methylation and demethylation rate constants (km, kdm) were high.
• The two major controls for sediment MeHg were organic carbon (OC) and km.

We examined Hg biogeochemistry in northern Gulf of Mexico (nGOM) sediments along a ∼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 (km and kdm), from enriched isotope spikes (201Hg(II) and Me199Hg) 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 km decreased with sediment depth and were significantly correlated, but neither was correlated with organic carbon (OC). Together, OC and km 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.