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.

Keywords:

• bacteria
• climate change
• lakes
• mercury isotopes
• sediment cores
• phytoplankton

Acknowledgements

The cores were collected by Environment Canada personnel and university collaborators, as explained more fully by Kirk et al. (2011). The total Hg and Hg isotope analyses were performed by Activation Laboratories Ltd. (Ancaster, Ontario) under the direction of Y. Kapusta, whom I thank for valuable technical information and discussions as well as his expert technical services. The NH₂OH·HCl/HNO₃ and 1M HCl extractions of Mn and Fe oxyhdyroxides were carried out by N. Nguyen and the late V. Cheam, respectively, and the Mn and Fe analyses of the extracts together with the total Mn, Fe, org. C, and org. N analyses were performed under the direction of G. Sardella at the National Laboratory for Environmental Testing (CCIW). The Rock-Eval 6 analyses were performed by Baseline Resolution Inc. (Shenandoah, Texas, USA), the radiometric dating was done by F. Yang (Environment Canada), and the micropaleontological analyses were carried out by H. Kling (Algal Taxonomy and Ecology, Winnipeg, Manitoba). I thank D.C.G. Muir (Section Head, Aquatic Contaminants Research Division, CCIW) for his support of the research and for helpful comments and suggestions. Other relevant acknowledgments have been presented elsewhere (Jackson et al. 2004, 2008; Kirk et al. 2011; Muir et al. 2009).

Funding

The research was financed by the Northern Ecosystem Initiative of Environment Canada (Department of the Environment, Government of Canada), ArcticNet, the Northern Contaminants Programme of Indian and Northern Affairs Canada, and the Toxic Substances Research Initiative (Health Canada and Environment Canada, 1999–2002).