HR: 0830h
AN: B31E-0363    [PDF]
TI: Changes in Carbon Isotope Composition of Methyl Halides Resulting from Biological and Chemical Degradation
AU: * Baesman, S M
EM: sbaesman@usgs.gov
AF: U.S. Geological Survey, Mail Stop 480 345 Middlefield Road, Menlo Park, CA 94025 United States
AU: Miller, L G
EM: lgmiller@usgs.gov
AF: U.S. Geological Survey, Mail Stop 480 345 Middlefield Road, Menlo Park, CA 94025 United States
AU: Oremland, R S
EM: roremlan@usgs.gov
AF: U.S. Geological Survey, Mail Stop 480 345 Middlefield Road, Menlo Park, CA 94025 United States
AB: Methyl bromide (MeBr), methyl chloride (MeCl) and methyl iodide (MeI) are reactive trace gases that are produced and released to the atmosphere at the Earths surface. These methyl halides have the potential to influence ozone levels in the stratosphere. Current estimates of the relative contributions of natural and anthropogenic sources of these methyl halides are the subject of considerable debate. In addition, there is uncertainty in the magnitude of some of the largest sinks for these compounds. Hence, the atmospheric budgets of MeBr, MeCl and MeI, while uncertain at present, may be better constrained using stable isotope ratio ($^{13}$C/$^{12}$C) mass balances of sources and sinks. Our work has focused on characterizing the effects upon $\delta^{13}$C values of methyl halides released after reactions which discriminate in favor of $^{12}$C during removal processes. Previously, we determined very large fractionations of carbon isotopes by pure cultures of soil bacteria. Further, we have documented large fractionations (kinetic isotope effects or KIEs) of methyl halides in live soils. In the case of MeBr and MeI, substantial fractionation also occurred in heat-killed soil, suggesting that chemical degradation resulted in a shift in the stable isotopic composition. At elevated concentrations, for instance during agricultural soil fumigations, the $\delta^{13}$C value of MeBr or MeI released from soil can be determined by flux measurements or soil profiles. However, more information is needed regarding the processes responsible for isotope fractionation to be able to extrapolate to areas where the concentration is low or direct measurement is not otherwise possible. We report here on measurements of the fractionation of carbon isotopes in methyl halides during degradation by chemical processes that are likely to occur in soil or seawater. These processes include aqueous hydrolysis and halide exchange and the methylation of organic matter using humic acid as the model methyl acceptor. Results are compared with fractionation achieved during the uptake of methyl halides by live and heat-killed soils.
DE: 0315 Biosphere/atmosphere interactions
DE: 0330 Geochemical cycles
DE: 1615 Biogeochemical processes (4805)
DE: 4840 Microbiology
DE: 4870 Stable isotopes
SC: Biogeosciences [B]
MN: 2003 Fall Meeting