Variations in the isotope composition of mercury in a freshwater sediment sequence and food web

Abstract

Analysis of a sediment core and food-web animals from the Niagara Basin of Lake Ontario yielded the first recorded evidence for systematic variations in the stable-isotope composition of mercury (Hg) in natural environments on Earth. The sediments comprised younger strata enriched in Hg by recent pollution overlying older strata containing background Hg only. Several Hg isotope ratios, including 199Hg/201Hg, 201Hg/204Hg, and 202Hg/204Hg, varied significantly with the Mn/Fe ratio of the extractable readily reduced oxyhydroxide fraction of the sediment and with the concentrations of extractable Mn or Fe fractions, or both, in the sediment, and the zone of recent Hg pollution gave strikingly different results than the zone of background Hg. Food-web animals displayed a progressive increase in the 202Hg/204Hg ratio from lower to higher trophic levels in the order crustaceans < forage fish < trout; but superimposed on this trend were secondary trends owing to systematic decreases in the ratio from planktonic to benthic crustaceans and, correspondingly, from plankton-eating to benthos-eating forage fish. The results of the research suggest fractionation of Hg isotopes by natural processes, including Hg methylation, with effects linked to temporal variations in the oxidation-reduction potential of the sediments. These findings show that Hg isotopes could provide valuable but as yet untapped information about the sources and biogeochemical cycling of natural and anthropogenic Hg.