Historical variations in the isotopic composition of mercury in sediment cores from northern Adriatic coastal environments

The results presented in this study involve the analysis of mercury (Hg) isotopes in two sediment cores from the Northern Adriatic. One core was collected in the Gulf of Trieste, an area contaminated by Hg residues produced by the Idrija Hg mine (Slovenia) during 500 years of activity and transported by the Isonzo River to the sea. The second core was collected in the Marano and Grado Lagoon, where Hg from a chlor-alkali plant was discharged into the Aussa-Corno River system flowing into this coastal environment. In this study, we used the high precision stable Hg isotope analysis of environmental samples made possible by continuous flow cold vapour introduction system coupled with MC-ICP-MS. The main aims of this research were to determine the isotope signatures of Hg from the two major sources of contamination, to compare them with the same signatures of the regional sedimentary background and to determine if Hg isotope compositions can be used to distinguish between different products deriving from exploitation and processing of Hg-bearing ore. Analysis of the dated core from the Gulf of Trieste showed that Hg concentration increased sharply at the beginning of 1800s, peaked before the first World War (23.32 µg g-1 in 1913) and then declined to a constant average of ≈ 8 µg g-1 in recent times (1980s-1990s). δ-values of the 199Hg/198Hg, 200Hg/198Hg, 201Hg/198Hg and 202Hg/198Hg ratios increased upward from the core bottom. Background sediments, where Hg concentration is 0.13 µg g-1, have an average δ202Hg of -3.93‰ ± 0.92‰ (n=5) which is significantly lower compared to recent contaminated sediments. The isotope signature of Hg deriving from mining activity was not altered by mass-independent fractionation (MIF) related to natural processes. Conversely, it showed mass-dependent fractionation (MDF), due to the transformation processes of Hg-bearing ores (mostly cinnabar but also native Hg in carboniferous schists) to by-products by the retorting processes which caused preferential depletion in the lighter isotopes. The isotope signatures of Hg were quite variable in the contaminated section of sediment cores where δ202Hg values ranged between -2.95‰ and 0.88‰. The variability in MDF is due to multiple sources, such as unroasted Hg ores and Hg waste calcines which, with different mixture and isotopic fingerprinting, affected the total isotopic composition of sediments.