Sources of mercury in deep-sea sediments of the Mediterranean Sea as revealed by mercury stable isotopes

Mercury (Hg) and its stable isotope composition were used to determine the sources of Hg in deep-sea sediments of the Mediterranean Sea. Surface and down-core sediment δ 202 Hg values varied widely between −2.30 and +0.78‰, showed consistently positive values for mass independent fractionation of odd Hg isotopes (with average values of Δ 199 Hg = +0.10 ± 0.04‰ and Δ 201 Hg = +0.04 ± 0.02‰) and near-zero Δ 200 Hg values, indicating either multiple Hg sources or a combination of different Hg isotope fractionating processes before and after sediment deposition. Both mass-dependent and mass-independent fractionation processes influence the isotopic composition of Hg in the Mediterranean Sea. Positive Δ 199 Hg values are likely the result of enhanced Hg 2+ photoreduction in the Mediterranean water column before incorporation of Hg into sediments, while mass-dependent fractionation decreases δ 202 Hg values due to kinetic isotope fractionation during deposition and mobilization. An isotope mixing model based on mass-dependent and mass-independent fractionation (δ 202 Hg and Δ 199 Hg) suggests at least three primary Hg sources of atmospheric deposition in the surface sediments: urban, industrial and global precipitation-derived. Industry is the main source of Hg in Algerian and Western Basin surface sediments and at two sites in the Adriatic Sea, while the urban contribution is most prominent at the Strait of Otranto (MS3) and in Adriatic surface sediments. The contribution from precipitation ranged from 10% in Algerian to 37% in W Basin sediments. Overall, results suggest that atmospheric Hg deposition to Mediterranean surface sediments is dominated by gaseous elemental mercury (58 ± 11%) rather than wet deposition.