Abstract
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 δ202Hg 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 Δ199Hg = +0.10 ± 0.04‰ and Δ201Hg = +0.04 ± 0.02‰) and near-zero Δ200Hg 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 Δ199Hg values are likely the result of enhanced Hg2+ photoreduction in the Mediterranean water column before incorporation of Hg into sediments, while mass-dependent fractionation decreases δ202Hg values due to kinetic isotope fractionation during deposition and mobilization. An isotope mixing model based on mass-dependent and mass-independent fractionation (δ202Hg and Δ199Hg) 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.
Highlights
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
This similarity may suggest that the marine sediments reflect the Hg isotopic composition of ambient seawater, or may indicate that the isotopic composition of Hg in the ocean reflects that of Hg in the upper crust
Recent measurements of volcanic and sedimentary deposits in California showed Hg isotopic compositions close to values reported for Pleistocene Mediterranean seawater[14]
Summary
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. In order to understand Hg cycling in water and sediments of the Mediterranean Sea, as well as the relative contributions of Hg from natural and anthropogenic sources, detailed measurements and modelling studies were conducted[1,2]. Stable isotope ratios of Hg were utilized to better understand Hg sources and processing pathways in marine deep-sea sediments including depth profiles. We suggest that the determination of δ202Hg, and Δ199Hg values in sediments can be used to develop a triple mixing model that provides estimates of the relative amounts of Hg derived from urban, industry and global atmospheric deposition
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