Tracing mercury seawater vs. atmospheric inputs in a pristine SE USA salt marsh system: Mercury isotope evidence

Abstract

Salt marshes can be a significant source of MeHg in coastal marine organisms, however the sources and cycling of Hg in salt marsh sediments, and in coastal environments in general, remain unclear. We analyzed surface and cored sediments from a salt marsh and adjacent upland sand dune in Cabretta Island, Georgia, USA, for total mercury (HgT), Hg isotopes and trace metal concentrations to better constrain the sources of Hg in this coastal environment. HgT concentrations are lower in the upland sands (1–6ng/g) than in the marsh sediments (6–16ng/g). HgT shows a positive correlation with total organic content (TOC) and increasing proportion of fines in the sediments. Trace metal concentrations also show a positive correlation with HgT in the marsh sediments regardless of chemical affinity (i.e. chalcophile, siderophile or lithophile). All these data are consistent with surface adsorption (either directly on to sediments or on to organic matter) as a dominant mechanism of Hg and metals accumulation in the marsh sediments. The surface sediments show significant mass independent fractionation (MIF) of odd Hg isotopes: the upland dune sands have zero to slightly negative Δ199Hg (−0.07‰ to −0.14‰) and the marsh surface sediments have positive Δ199Hg (0.48‰ to 0.79‰). As in the surface samples, well-sorted sand layers in the cored sediments have low Hg concentrations and zero to slightly negative Δ199Hg, whereas fine particle-rich layers have higher Hg concentrations and positive Δ199Hg values. In the absence of any evidence for MIF of Hg isotopes by bacteria mediated methylation–demethylation reactions, the observed Hg isotope differences between marsh and upland sands can be explained by different sources of Hg. We suggest that local atmospheric Hg deposition dominates the Hg budget in the ombrotrophic upland sand, while the positive MIF in the marsh sediments are consistent with Hg dominantly of seawater origin. While in situ processes, like photoreduction and evasion can overprint the Hg depositional isotope signature, their effect on the sediments is currently unconstrained. Nevertheless, our data show that Hg isotopes can delineate different sources of Hg in a pristine coastal environment and at small spatial scales. Thus natural background isotope variability should be assessed as a baseline when Hg isotopes are used as tracers of Hg in anthropogenically-influenced sites.