Mercury Isotopic Composition of Marine Sediments in the Baltic Sea

            Mass dependent (d²⁰²Hg) and mass independent (Δ¹⁹⁹Hg) variation in the isotopic composition of Hg has been observed in a wide range of natural materials. In this study we measured the Hg isotopic composition of marine sediments from the Baltic Sea that predate and post-date the Industrial Revolution to see whether anthropogenic inputs of Hg had altered the isotopic composition of the sediments.  We analyzed surface (0-4 cm) and pre-Industrial (24-34 cm) sediments from spatially distributed, hydrographically separated locations in the Baltic Sea.  Sediments from the Baltic Proper and Arkona basins demonstrate a three-fold enrichment in Hg concentration between pre-Industrial and post-Industrial sediments, with Hg concentrations increasing from an average of 19 ng/g to 51 ng/g in the Baltic Proper basin and from 51 ng/g to 155 ng/g in the Arkona basin.  All sediments analyzed have a narrow range of d²⁰²Hg and Δ¹⁹⁹Hg values.  Deeper sediments from the two basins have d²⁰²Hg values ranging from -1.2‰ to -0.9‰ and Δ¹⁹⁹Hg values of +0.14‰ to +0.15‰.  Near-surface sediments from each site have d²⁰²Hg values ranging between -0.9‰ and -0.8‰ and Δ¹⁹⁹Hg values of +0.09‰.  The sediments analyzed have indistinguishable Hg isotopic compositions within analytical uncertainty (d²⁰²Hg = ±0.1‰, Δ¹⁹⁹Hg = ±0.05‰).  The isotopic composition of Baltic Sea sediments is nearly identical to mid-Pleistocene Mediterranean Sea marine sediments.  The similar Hg isotopic composition of these marine sediments deposited at different times and locations suggests a characteristic marine Hg isotopic signature preserved in sediments of  d²⁰²Hg » -0.9‰ and Δ¹⁹⁹Hg » +0.1‰.  Implications of this isotopic signature for global Hg cycling will be discussed.