Disturbance impacts on mercury dynamics in northern Gulf of Mexico sediments

We evaluated the impacts of physical disturbance on sediment Hg biogeochemistry in the northern Gulf of Mexico by exploring changes in Hg abundance and speciation in cores collected between July 2005 and July 2006, a time period that included the passages of Hurricanes Katrina and Rita in this region. Comparisons across space and time reveal large changes in sediment characteristics, both with respect to Hg biogeochemistry and other measures of sediment composition, that indicate substantial disturbance. While the degree of disturbance varied between stations, at sites that clearly received new surface deposits considerable increases in OC and total Hg were observed, along with shifts in major element composition. At the station closest to Hurricane Katrina's track ( station A'2), Pb-210 levels are consistent with the episodic deposition of > 10 cm of sediments. These surface sediments ( 0-10 cm) at A'2 had the highest %MeHg of all stations and all dates, suggesting that the disturbance resulted initially in increased net methylation. While the observed disturbances elsewhere could not in all cases be definitively linked to hurricane activities, the substantial thickness of deposits (> 10 cm) at multiple sites is consistent with a major event, and the similarity in the deposits' chemical fingerprint across all impacted sites suggests similar sources or processes. We estimate that the two hurricanes redistributed approximately 5 times the annual Hg input from the Mississippi-Atchafalaya River system and atmospheric deposition. These observations highlight the need to consider the effects of major disturbances on the biogeochemical cycling of Hg in coastal systems.