Evidence for mass-independent and mass-dependent fractionation of the stable isotopes of mercury by natural processes in aquatic ecosystems Togwell A. Jackson a, * , D. Michael Whittle b , Marlene S. Evans c , Derek C.G. Muir a a Aquatic Ecosystem Protection Research Division, Water Science and Technology Directorate, Environment Canada, 867 Lakeshore Road, P.O. Box 5050, Burlington, Ontario Canada, L7R 4A6 b Great Lakes Laboratory for Fisheries and Aquatic Sciences, Fisheries and Oceans Canada, 867 Lakeshore Road, P.O. Box 5050, Burlington, Ontario Canada, L7R 4A6 c Aquatic Ecosystem Protection Research Division, Water Science and Technology Directorate, Environment Canada, 11 Innovation Blvd., Saskatoon, Saskatchewan Canada, S7N 3H5 Available online 1 January 2008 Abstract Isotopic and chemical analyses were performed on crustaceans, forage fish, top predator fish, and sediment cores from Lake Ontario and two boreal forest lakes to investigate fractionation of the stable isotopes of Hg in aquatic ecosystems. Multicol- lector inductively coupled mass spectrometry was used to determine Hg isotope abundances. The Hg isotope data for all three lakes showed mass-independent variation in the organisms but only mass-dependent variation in the sediments. The mass- independent isotope effect was characterised by (1) selective enrichment in isotopes of odd mass number ( 199 Hg and 201 Hg), (2) enrichment in 201 Hg relative to 199 Hg, (3) an inverse relationship between isotopes of odd and even mass number in fish, and (4) a positive correlation with methylHg (CH 3 Hg + ) concentration, and hence with trophic level (although lake whitefish were consistently anomalous, possibly owing to biochemical demethylation). Isotope signatures of species at the same trophic level varied with habitat and diet, differentiating between planktonic and benthic crustaceans and their predators, and between fish that frequent deep, cold water and fish of similar diet that prefer warmer, shallower water, because of corresponding dif- ferences in CH 3 Hg + and inorganic Hg content. Isotopic analysis of CH 3 Hg + and inorganic Hg extracted from lake trout proved that the mass-independent isotope effect was due to anomalously high abundances of 199 Hg and 201 Hg in CH 3 Hg + , as implied by the data for whole organisms, suggesting mass-independent fractionation during microbial methylation of Hg. The purely mass-dependent variation in the sediments is attributable to the fact that Hg in sediments is mostly inorganic. The mass-independent fractionation of Hg isotopes can be explained by effects of nuclear spin or nuclear field shift, or both, and penetration of the inner electron shells of Hg by valence electrons of Hg-binding ligands. The results of the research dem- onstrate that isotopic analysis of Hg could yield valuable information about the biogeochemical cycling of Hg. Crown Copyright Ó 2007 Published by Elsevier Ltd. All rights reserved. 1. Introduction Mercury has seven stable isotopes, whose mass numbers and conventional terrestrial abundances 0883-2927/$ - see front matter Crown Copyright Ó 2007 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2007.12.013 * Corresponding author. Fax: +1 905 336 6430. E-mail address: t.a.jackson@ec.gc.ca (T.A. Jackson). Available online at www.sciencedirect.com Applied Geochemistry 23 (2008) 547–571 www.elsevier.com/locate/apgeochem Applied Geochemistry