Mercury Isotope Evidence for Contrasting Mercury Sources to Coastal Versus Offshore Marine Fish

An important question related to the oceanic biogeochemical cycle of Hg is the source of methyl-Hg (MeHg) to foodwebs of offshore migratory fish that spend little or no time in shallow coastal waters, where Hg methylation is known to occur at high rates in reducing sediments and where anthropogenic Hg inputs are greatly enhanced. Bio-advection of coastally-derived MeHg has been hypothesized as a source of MeHg to the open ocean. Alternatively, it has been suggested that methylation may occur in the open ocean at locations such as seafloor hydrothermal vents or the oxygen minimum zone. Here we use naturally occurring mercury stable isotope ratios in fish tissues to explore MeHg sources. Coastal fish in the Gulf of Mexico have negative values of δ²⁰²Hg(0 to -1.0‰) and Δ¹⁹⁹Hg values of ~0.5‰, whereas offshore migratory fish have positive values of δ²⁰²Hg (0.2 to 0.7‰) and Δ¹⁹⁹Hg values of ~1.5‰. Positive mass independent isotope anomalies (Δ¹⁹⁹Hg) suggest that MeHg in offshore foodwebs in the Gulf of Mexico has undergone a much greater extent of photochemical demethylation than MeHg in coastal foodwebs. Mass dependent isotope values (δ²⁰²Hg) of coastal fish are consistent with measurements of sediments in shallow coastal areas and Mesozoic sedimentary rocks, but the δ²⁰²Hg values of offshore fish are higher and require a source of Hg that has been elevated in δ²⁰²Hg either by loss of Hg with low δ²⁰²Hg due to biotic or dark abiotic reduction, or derivation from sources with δ²⁰²Hg similar to hydrothermal deposits. Our results support the possible decoupling between terrestrially-derived Hg inputs to coastal environments and Hg levels in offshore migratory fish such as tuna.