HR: 0800h
AN: V51E-0835    [Abstracts]
TI: A Distinct Magnetic Isotope Effect Measured in Atmospheric Mercury in Epiphytes
AU: * Ghosh, S
EM: sulataghosh@gly.fsu.edu
AF: Department of Geology, NHMFL-Geochemistry, Florida State University, 1800 E Paul Dirac Dr., Tallahassee, FL 32310, United States
AU: Odom, A L
EM: odom@magnet.fsu.edu
AF: Department of Geology, NHMFL-Geochemistry, Florida State University, 1800 E Paul Dirac Dr., Tallahassee, FL 32310, United States
AB: Due to the importance of Mercury as an environmental contaminant, mercury cycling in the atmosphere has been extensively studied. However, there still remain uncertainties in the relative amounts of natural and anthropogenic emissions, atmospheric deposition rates as well as the spatial variation of atmospheric mercury. Part of a study to determine the isotopic composition of mercury deposited from the atmosphere has involved the use of epiphytes as monitors. The greatest advantage of such natural monitors is that a widespread, high-density network is possible at low cost. One of the disadvantages at present is that these monitors likely contain different mercury species (for example both gaseous, elemental mercury trapped by adsorption and Hg (II) by wet deposition). The project began with the understanding that biochemical reactions involving metallothioneins within the epiphytes might have produced an isotopic effect. One such regional network was composed of samples of Tillandsia usenoides (common name: Spanish moss) collected along the eastern Coastal Plain of the U.S. from northern Florida to North Carolina.
The isotopic composition of a sample is expressed as permil deviations from a standard. The deviations are defined as δ^AHg = \left(\frac{R_sample}{R_std}-1 \right)1000 ‰ , where A represents the atomic mass number. R=\frac{^AHg}{²⁰²Hg} were measured for the isotopes ¹⁹⁸Hg, ¹⁹⁹Hg, ²⁰⁰Hg, ²⁰¹Hg, ²⁰²Hg and ²⁰⁴Hg relative to the mercury standard SRM NIST 3133, by a standard-sample bracketing technique. For all samples, the delta values of the even-N plotted against atomic mass numbers define a linear curve. For the odd-N isotopes, δ¹⁹⁹Hg and δ²⁰¹Hg deviate from this mass-dependent fractionation (MDF) relationship and indicate a mass-independent fractionation (MIF) effect and a negative anomaly, i.e. a depletion in ¹⁹⁹Hg and ²⁰¹Hg relative to the even-N isotopes. These deviations are expressed as Δ¹⁹⁹Hg = δ¹⁹⁹Hg_total - δ¹⁹⁹Hg_MDF.
A Δ²⁰¹Hg/Δ¹⁹⁹Hg ratio of 1.11 is predicted by isotope fractionation due to the Magnetic Isotope Effect (MIE), because 1.11 is the ratio of the magnetic moments of the two odd-N isotopes. A plot of Δ¹⁹⁹Hg versus Δ²⁰¹Hg values obtained reveals a striking pattern. All samples plot well within analytical uncertainly along a straight line passing through zero and having a slope of 1.11. Based on thermodynamic principles, some have argued that nuclear spin effects are quite insignificant in producing isotopic fractionation. However the MIE is a kinetic one in which those isotopes with non-zero magnetic moments effect the rates of recombination of free radical pairs by nuclear-electron hyperfine interaction and can become enriched or depleted in either reactants or products. In the samples studied here, the nuclear spin is far more important than either nuclear mass or nuclear volume in effecting isotopic fractionation of Mercury.

DE: 1041 Stable isotope geochemistry (0454, 4870)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting