Mercury isotope signatures in sediments and marine organisms as tracers of historical industrial pollution

Abstract

Isotopic composition of mercury (Hg) in marine organisms and sediment cores was used to identify sources and reconstruct historical trends of contamination in the coastal-marine area of Rosignano Solvay (Italy), affected by Hg pollution from a chlor-alkali plant on the near land. Sediments show a wide range of Hg concentration and Hg isotope signatures. Particularly, coupled Hg concentration and δ²⁰²Hg values trace inputs from different sources. The two depth-profiles clearly indicate three distinct periods: "pre-industrial" (before 1941), "industrial" (between 1941 and 2007) and "post-industrial" (after 2007) ages. This is also corroborated by sediment chronology, using ²¹⁰Pb dating method, validated through ¹³⁷Cs. Marine organisms are characterized by Hg isotope signatures comparable to "post-industrial" surface sediments. Notably, specimens of Mullus spp. evidence isotope composition comparable to the "industrial" sediments, thus suggesting a still active role of those sediments as source of Hg for the benthic fish compartment. The small amount of MIF and the Δ¹⁹⁹Hg/Δ²⁰¹Hg ratio recorded in organisms are reasonably consistent with limited processes of MMHg demethylation in the water column.

Keywords: Dating; Historical trends; Marine pollution; Mercury isotope ratios; Sources.