Historical variations in the stable isotope composition of mercury in Arctic lake sediments

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 Hg-198/Hg-202, Hg-199/Hg-202, Hg-200/Hg-202, Hg-201/Hg-202, and Hg-211/Hg-202 expressed as delta-values (per mil deviations relative to a standard) reveal enrichment in Hg-198, Hg-199, Hg-200, and Hg-201, with depletion in Hg-204, the degree of enrichment varying inversely with atomic mass. A plot of delta(198)Hg, delta(199)Hg, delta(200)Hg, and delta(200)Hg against depth gave parallel profiles characterized by large, regular undulations superimposed on an overall trend toward increase with depth (i.e. age), and the delta(204)Hg profile is a mirror image of them. The delta(198)Hg, delta(199)Hg, delta(200)Hg, and delta(201)Hg values of the oldest (1899-1929) strata vary inversely with (NH2OHHCI)-H-./HNO3-extractable manganese concentration, but those of the youngest (1963-1997) strata give a positive correlation; intermediate (1936-1956) strata show no correlation and negligible variation in delta-values, possibly signifying a transition phase in which the two opposite trends offset each other. The delta-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 delta-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.