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

摘要

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 (approximate to 1915-1940), subsequent cooling (approximate to 1940-1970), and renewed warming (approximate to 1970-2004) [phases W1, C1, and W2, respectively]. Per mil deviations of Hg-199/Hg-202 and Hg-201/Hg-202 ratios due to MIF (Hg-199 and Hg-201 values) correlated with biological and biogeochemical factors linked to geographical and temporal climatic variations but varied, in large part, independently of each other. Hg-201 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 Hg-201 increased in the order C1 W1 < W2. Hg-201 varied inversely with diatom concentration, but Hg-199 increased with increasing abundance of cyanobacteria. Arctic lakes, however, showed a south-to-north decrease in Hg-199/Hg-201 ratios, paralleling a decrease in annual precipitation and an increase in Chlorophyta and cyanobacteria. -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.