HR: 10:50h
AN: B12B-03 INVITED     [Abstracts]
TI: Application of compound-specific hydrogen isotope analyses to study anaerobic processes
AU: * Valentine, D L
EM: valentine@geol.ucsb.edu
AF: University of California, Department of Geological Sciences, Santa Barbara, CA 93106 United States
AU: Sessions, A L
EM: als@gps.caltech.edu
AF: California Institute of Technology, Division of Geological and Planetary Sciences, Pasadena, CA 99999 United States
AU: Chidthaisong, A
EM: amnat_c@jgsee.kmutt.ac.th
AF: King Mongut's Technical University, Joint Graduate Program for Energy and Environment, Bangkok, 99999 Thailand
AU: Tyler, S
EM: styler@uci.edu
AF: University of California, Department of Earth System Science, Irvine, CA 92697 United States
AB: Compound specific isotope analysis provides a potentially-powerful tool to study Earth system processes. In order to realize this goal we must first understand the many factors that impact isotopic distributions in nature. In pursuit of this goal we have initiated a series of investigations to assess the applicability of compound specific hydrogen isotope analyses for studying microbial processes in nature. Laboratory investigations thus far have focused on anaerobic microbial processes involving H$_{2}$, including methanogenesis and acetogenesis. Our experimental approach has been to grow pure and enrichment cultures of autotrophic, anaerobic bacteria and archaea under isotopically-defined conditions ($\delta$D of water and H$_{2}$ are known), and to relate these conditions to the $\delta$D of major metabolic products (such as methane and lipids). Investigations of methanogenic archaea indicate variable fractionation patterns between H$_{2}$, H$_{2}$O and CH$_{4}$. Fractionation is dependent on the microbial community structure and on the growth stage of the organism. Alternate biochemical routes of methanogenesis yield methane with $\delta$D variations as large as 150 per mil. These results help to explain much of the hydrogen isotope variability in methane from deep subsurface versus surficial environments. Studies with homoacetogenic bacteria show fractionations between H$_{2}$O and cellular lipids which are larger than has been observed in other culture studies. Results may be due to greater fractionation in the biochemical pathway, or may be due to the indirect incorporation of hydrogen atoms from H$_{2}$ into the lipids. Either way, these results indicate that the hydrogen isotopic composition of lipid biomarkers in nature may provide an indication of anoxia and/or H$_{2}$ cycling. All studies performed to date also indicate rapid isotopic equilibration of H$_{2}$ and H$_{2}$O, and indicate the likelihood of achieving isotopic equilibrium in nature. Ongoing investigations focus on the variability of hydrogen isotope fractionation in other microbial processes and on the hydrogen isotopic composition of organic metabolites in sediments and soils.
DE: 1055 Organic geochemistry
DE: 1615 Biogeochemical processes (4805)
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting