Historical Variations in the Stable Isotope Composition of Mercury in Arctic Lake Sediments

• Published in: Environmental science & technology 2004-05, Vol.38 (10), p.2813-2821
• Main Authors: Jackson, Togwell A, Muir, Derek C. G, Vincent, Warwick F
• Format: Article
• Language: English
• Subjects: Applied sciences; Arctic Regions; Canada; Earth sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Environmental science; Exact sciences and technology; Freshwater; Geologic Sediments - analysis; Geologic Sediments - chemistry; History, 19th Century; History, 20th Century; Isotopes; Lakes; Mass Spectrometry - methods; Mercury; Mercury Isotopes - analysis; Mercury Isotopes - history; Oxidation; Pollution; Pollution sources. Measurement results; Pollution, environment geology; Sediments; Soil and sediments pollution; Water - analysis; Water Pollutants, Chemical - analysis; Water Pollutants, Chemical - history
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/es0306009

The stable isotope composition of mercury (Hg) in a dated core from the anoxic zone of a saline, meromictic Arctic lake was found to vary as a complex function of the age and chemical composition of the sediment. Throughout the stratigraphic sequence, which spans the years 1899−1997, the ratios 198Hg/202Hg, 199Hg/202Hg, 200Hg/202Hg, 201Hg/202Hg, and 204Hg/202Hg expressed as δ-values (per mil deviations relative to a standard) reveal enrichment in 198Hg, 199Hg, 200Hg, and 201Hg, with depletion in 204Hg, the degree of enrichment varying inversely with atomic mass. A plot of δ198Hg, δ199Hg, δ200Hg, and δ201Hg against depth gave parallel profiles characterized by large, regular undulations superimposed on an overall trend toward increase with depth (i.e. age), and the δ204Hg profile is a mirror image of them. The δ198Hg, δ199Hg, δ200Hg, and δ201Hg values of the oldest (1899−1929) strata vary inversely with NH2OH·HCl/HNO3-extractable manganese concentra tion, but those of the youngest (1963−1997) strata give a positive correlation; intermediate (1936−1956) strata show no correlation and negligible variation in δ-values, possibly signifying a transition phase in which the two opposite trends offset each other. The δ-values show similar but weaker relationships with organic carbon. The results strongly suggest fractionation of Hg isotopes by microbial activities linked to oxidation−reduction reactions in the lake, although effects of isotopic signatures indicative of the sources of the Hg have not been ruled out. The radical change in the nature of the relationship between δ-values and sediment chemistry over time may reflect environmental and biotic changes that altered the isotope-fractionating processes. These findings imply that variations in the isotopic makeup of Hg, together with related physical, chemical, and biological data, could yield important new information about the biogeochemical cycle of Hg.