Role of oxygen and salinity on biogeochemical processes controlling mercury and monomethylmercury flux from estuarine sediments

UNCW Author/Contributor (non-UNCW co-authors, if there are any, appear on document)
Joshua S. Vinson (Creator)
The University of North Carolina Wilmington (UNCW )
Web Site:
Stephen Skrabal

Abstract: Estuarine sediments are complex biogeochemical microenvironments that play an important role in the transformation and redistribution of mercury and methylmercury (MeHg) to overlying waters. The presence of sulfate-reducing bacteria is widely accepted to be a critical factor in mercury methylation. As freshwater contains typically low concentrations of sulfate and seawater is sulfate-rich, salt water intrusion to freshwater sediments by sea level rise or anthropogenic causes may significantly increase MeHg production and release by sediments. This study used high-resolution (millimeter-scale) pore water profiling of redox-active analytes, combined with simultaneous measurement of benthic fluxes, to discern how biogeochemical processes affect sediment-water exchange of dissolved mercury and MeHg in predominantly tidal freshwater and estuarine wetlands. Sediment incubations and manipulations of oxygen and salinity conditions were performed using sediment cores from two contrasting sites in the Cape Fear and White Oak estuaries. The dominance of manganese reduction in Cape Fear sediments in February and June apparently inhibited the expected thermodynamic progression toward sulfate reduction and subsequent MeHg fluxes. Maximum MeHg fluxes of 29-36 pmol m-2 d-1 were observed in early fall (September) under anoxic, low salinity conditions and were coincident with of Fe2+ and Mn2+ pore water accumulation at the sediment-water interface (SWI) and low concentrations of H2S (5-40 µM) below the SWI. Presence or absence of Mn2+ and Fe2+ at the SWI was consistently associated with MeHg fluxes, suggesting the reduction of metal oxyhydroxides in conjunction with sulfate reduction are factors controlling sediment-water exchange of MeHg. Variable fluxes of MeHg (0, -6, 6 pmol m-2 d-1) during summer in White Oak sediments was observed in the presence of sulfate reduction under low salinity, suggesting the presence of other methylationinhibiting mechanisms. These data suggest increasing salinities of freshwater sediments due to sea level rise or human activities promote sulfate reduction and associated MeHg production resulting in elevated concentrations of MeHg in the Cape Fear River sediments. In addition, Fe and Mn oxyhydroxides, which are effective barriers against the diffusion of MeHg to overlying water, are reduced during periods of anoxia and release MeHg to the overlying waters.

Additional Information

A Thesis Submitted to the University of North Carolina Wilmington in Partial Fulfillment Of the Requirements for the Degree of Master of Science
Language: English
Date: 2008

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