HR: 08:00h
AN: H41J-01    [Abstracts]
TI: Mass Dependent and Mass Independent Fractionation of Hg Isotopes and Estimation of Photochemical Loss of Hg in Aquatic Systems
AU: * Bergquist, B A
EM: bbergqui@umich.edu
AF: University of Michigan, Department of Geological Sciences 1100 N. University Ave 2534 C.C. Little, Ann Arbor, MI 48109, United States
AU: Blum, J D
EM: jdblum@umich.edu
AF: University of Michigan, Department of Geological Sciences 1100 N. University Ave 2534 C.C. Little, Ann Arbor, MI 48109, United States
AB: Mercury is a globally distributed and highly toxic pollutant, the mobility and bioaccumulation of which is dependent on its redox cycling. Hg isotope analysis is an important new tool for identifying Hg sources and tracking Hg transformations in the environment. Most natural samples analyzed for Hg isotopes display mass-dependent isotope fractionation (MDF), but a small body of data suggests that some natural samples also display mass- independent isotope fractionation (MIF) of the odd Hg isotopes. Here we document MIF of Hg isotopes during an important natural process, constrain the potential mechanism of isotope fractionation, and apply the MIF observed in natural samples to quantify the photochemical reduction of Hg species in the environment. Reduction of Hg species to Hg⁰ vapor is an important pathway for removal of Hg from aqueous systems into the atmosphere and occurs by abiotic and biotic mechanisms. In laboratory experiments, we find that photochemical reduction Hg species by natural sunlight leads to large MIF of the odd isotopes. Also, the relationship between MIF for the two odd isotopes of Hg is significantly different for different photo-reduction pathways. In contrast, both biological reduction (Kritee et al., 2006) and dark abiotic organically-mediated reduction follow MDF. Natural samples from aquatic ecosystems preserve both MDF and MIF. In fish, MDF increases with the size and Hg concentration of fish suggesting MDF may be useful in understanding Hg bioaccumulation. Fish also display a large range in MIF (4‰), and the relationship between the MIF of the two odd isotopes in fish has a similar slope to the slope found for photo-reduction of CH₃Hg⁺. Since fish bioaccumulate CH₃Hg⁺, fish may be recording the extent to which CH₃Hg⁺ is lost via photochemical reduction in an aquatic ecosystem. Fish populations from different locations have different MIF values, but mostly display similar MIF within a given locale. This suggests that MIF is preserved in the food web and could be used to quantify photo-reduction of CH₃Hg⁺ in ecosystems. Both MDF and MIF of Hg isotopes will be useful for quantifying and understanding Hg biogeochemical cycling in the environment.
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 0746 Lakes (9345)
DE: 0792 Contaminants (0432)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1065 Major and trace element geochemistry
SC: Hydrology [H]
MN: 2007 Fall Meeting