Mercury Stable Isotopes in Seabird Eggs Reflect a Gradient from Terrestrial Geogenic to Oceanic Mercury Reservoirs

Elevated mercury concentrations ([Hg]) were found in Alaskan murre (<i>Uria</i> spp.) eggs from the coastal embayment of Norton Sound relative to insular colonies in the northern Bering Sea-Bering Strait region. Stable isotopes of Hg, carbon, and nitrogen were measured in the eggs to investigate the source of this enrichment. Lower δ¹³C values in Norton Sound eggs (−23.3‰ to −20.0‰) relative to eggs from more oceanic colonies (−20.9‰ to −18.7‰) indicated that a significant terrestrial carbon source was associated with the elevated [Hg] in Norton Sound, implicating the Yukon River and smaller Seward Peninsula watersheds as the likely Hg source. The increasing [Hg] gradient extending inshore was accompanied by strong decreasing gradients of δ²⁰²Hg and Δ¹⁹⁹Hg in eggs, indicating lower degrees of mass-dependent (MDF) and mass-independent Hg fractionation (MIF) (respectively) in the Norton Sound food web. Negative or zero MDF and MIF signatures are typical of geological Hg sources, which suggests murres in Norton Sound integrated Hg from a more recent geological origin that has experienced a relatively limited extent of aquatic fractionation relative to more oceanic colonies. The association of low δ²⁰²Hg and Δ¹⁹⁹Hg with elevated [Hg] and terrestrial δ¹³C values suggested that Hg stable isotopes in murre eggs effectively differentiated terrestrial/geogenic Hg sources from oceanic reservoirs