Evidence for Mass-Independent Fractionation of Mercury Isotopes by Microbial Activities Linked to Geographically and Temporally Varying Climatic Conditions in Arctic and Subarctic Lakes

Abstract

Cores from Arctic and subarctic Canadian lakes were subjected to isotopic, chemical, micropaleontological, and geochronological analyses for the purpose of investigating mass-independent fractionation (MIF) of mercury isotopes. The cores preserved records of early twentieth century climatic warming (∼1915–1940), subsequent cooling (∼1940–1970), and renewed warming (∼1970–2004) [phases W1, C1, and W2, respectively]. Per mil deviations of 199Hg/202Hg and 201Hg/202Hg ratios due to MIF (Δ199Hg and Δ201Hg values) correlated with biological and biogeochemical factors linked to geographical and temporal climatic variations but varied, in large part, independently of each other. Δ201Hg tended to increase from east to west. Among subarctic lakes this trend paralleled westward decreases in annual precipitation, diatom concentration, and the post-1990 organic carbon/pre-1900 organic carbon ratio, and Δ201Hg increased in the order C1 ≤ W1 < W2. Δ201Hg varied inversely with diatom concentration, but Δ199Hg increased with increasing abundance of cyanobacteria. Arctic lakes, however, showed a south-to-north decrease in Δ199Hg/Δ201Hg ratios, paralleling a decrease in annual precipitation and an increase in Chlorophyta and cyanobacteria. Δ-values of individual lakes depended on the abundances of specific phylogenetic groups of phytoplankton, pyrolysis products of organic matter, and manganese, and on the manganese/iron ratios of oxyhydroxides, displaying clear separation of data representing different climatic trends. These results suggest that MIF was caused by microorganisms, such as bacteria which decomposed dead phytoplankton and mediated oxidation-reduction reactions of manganese and iron, and that the nature and isotope-fractionating activities of the microflora varied with climate-related environmental and biotic factors, including the community structure of the phytoplankton.