Constraining Mercury Sources Along an Estuarine-to-Marine Sediment Gradient using Mercury and Carbon Stable Isotopes

The coastal ocean receives more mercury from riverine input (land-derived Hg) than from direct atmospheric deposition. Land-derived Hg is primarily particle-bound and buried in coastal sediment, where it may undergo methylation. However, its contribution to coastal sediment Hg loads remains poorly constrained. Here, we investigate Hg sources along an estuarine-to-marine gradient in the Bay of Biscay (Atlantic Ocean) combining Hg and carbon (C) stable isotopic composition in sediments. We find a significant (p<0.05) difference in δ202Hg and δ13C between estuarine (δ202Hg -0.83 ± 0.15‰, δ13C -27.3 ± 0.50‰) and marine (δ202Hg -0.54 ± 0.16‰, δ13C -25.2 ± 0.80‰) sediments, and a significant correlation (p<0.05) between δ13C and δ202Hg. While in-situ processes had a negligible impact on Hg isotopic composition, the observed trends could be explained by the mixing of land-derived Hg (e.g. vegetational uptake, anthropogenic) and direct atmospheric deposition to the marine environment. Applying a binary-mixing model with regional endmembers to δ13C and δ202 signatures reveals a near 1:1 relationship between land-derived Hg and terrestrial C along our estuarine-to-marine gradient. This relationship, if confirmed for other regions, suggests that joint observations of C stable isotopes and Hg concentrations could directly constrain the transfer of land-derived Hg into coastal sediments.