Mass-dependent and mass-independent variations in the isotope composition of mercury in a sediment core from Lake Ontario as related to pollution history and biogeochemical processes

Abstract

Historical variations in the isotope composition of mercury (Hg) in Lake Ontario were examined for the purpose of characterising the isotope signatures of the sources of Hg contamination and investigating their fate in the environment. The lake has been polluted with Hg, organochlorine compounds, and other contaminants from industrial and urban sources, chlor-alkali plants being, historically, the principal sources of Hg. The pollutants were discharged into the Niagara River, transported to the lake by fluvial action, and deposited in fine-grained sediments. Analysis of a dated sediment core from the lake revealed that the Hg concentration increased sharply in the 1890s, peaked in the 1930s–1940s and 1960s, and then declined, levelling off in the late 1900s. The δ-values of the 198Hg/202Hg, 199Hg/202Hg, 200Hg/202Hg, and 201Hg/202Hg ratios of the Hg that formed the peaks decreased with increasing total Hg concentration. Thus, the isotope signature of the anthropogenic Hg was formed by mass-dependent fractionation (MDF) which caused preferential depletion in the lighter isotopes, suggesting evaporation from Hg(0) electrodes in the chlor-alkali plants. The background Hg above and below the peaks also showed inverse correlations with total Hg but formed separate regression lines, reflecting the fact that its origins and pre-depositional history were different. The isotope signatures were altered appreciably by both MDF and mass-independent fractionation (MIF) attributable to natural processes in the lake. MDF caused secondary enrichment in lighter isotopes, and MIF caused intermittent 201Hg enrichment associated with episodes of Fe and Mn oxyhydroxide precipitation. FeOOH abundance showed a marked dip coinciding with maximum concentrations of organochlorine compounds in the 1960s, but 201Hg enrichment by MIF increased significantly with the concentrations of certain chlorobenzene compounds. These findings suggest that MIF was mediated by bacteria, including oxyhydroxide-precipitating species, and that organochlorines inhibited FeOOH-producing bacteria, whereas the chlorobenzenes stimulated bacterial enzyme activity involved in MIF.