HR: 0800h
AN: B11A-1031    [Abstracts]
TI: Variability of Deuterium Fractionation Associated With Soil Uptake of Atmospheric Molecular Hydrogen
AU: * Rahn, T
EM: trahn@lanl.gov
AF: Los Alamos National Lab, Earth and Environmental Sciences MS D-462, Los Alamos, NM 87545 United States
AU: Randerson, J T
EM: jranders@uci.edu
AF: University of California Irvine, Department of Earth System Science 3212 Croul Hall Mail Code: 3100 , Irvine, CA 92697 United States
AU: Eiler, J
EM: eiler@gps.caltech.edu
AF: California Institute of Technology, Geological and Planetary Sciences Mail Stop 100-23, Pasadena, CA 91125 United States
AB: Molecular hydrogen (H₂) is the second most abundant reduced gas in the atmosphere (after methane) with a globally averaged mixing ratio of ~530 nmol/mol. Its largest sources are photochemical oxidation of methane and non-methane hydrocarbons with other recognized sources that include biomass burning, fossil fuel burning, nitrogen fixation, and ocean degassing. These sources are balanced by reaction of H₂ with hydroxyl radicals (~25%) in the atmosphere and by deposition at the terrestrial soil surface (~75%). As with other atmospheric trace gases, the stable isotopic content of H₂ has the potential to help quantify the various aspects of its production and destruction. The average deuterium content of H₂ is dD_H2 = ~130 ‰ relative to Standard Mean Ocean Water. While recent studies have begun to elucidate the deuterium content of the individual sources of H₂ and the fractionation associated with hydroxyl oxidation has been well established in the laboratory, there are still few data documenting the fractionation associated with soil uptake. We measured the fractionation associated with soil uptake in May, June and August of 2002 in three upland ecosystems that were part of an Alaskan fire chronosequence. Fire occurred at these sites in 1999, 1987, and ~1920. Grasses and herbaceous vegetation establish initially after fire and are gradually replaced by deciduous trees and finally by evergreen trees and moss. All three sites were in interior Alaska near the town of Delta Junction (63° 54'N, 145° 40'W). Fluxes were measured with a Plexiglas flux chamber (8 liter volume) with a manifold of four ~400 ml double-valved glass flasks in parallel and a diaphragm pump for circulation (5 SLPM). Flasks were continuously flushed by the circulating system and isolated sequentially; they were then returned to the laboratory at Caltech for subsequent analysis. In the field, the chamber was seated on Plexiglas collars that were installed prior to initiating the study and left in place for the duration of the study. Permanent collars insured that placement of the chamber/collar system did not cause any localized disturbance and unintentionally perturb the H₂ flux and that sampling locations could be reoccupied to investigate seasonal differences. Prior to field deployment the entire system was tested on a plain sheet of Plexiglas to ensure that no H₂ was produced or consumed by the diaphragm pump or any of the system parts. While earlier studies have measured HD/HH deposition velocities with a ratio of ~0.96, results of our Alaskan study show deposition velocity ratios of 0.89 ± 0.03 (n=17). Furthermore, sorting of data by season reveals no significant variability in this ratio but sorting by location over all seasons reveals the greatest fractionation at the most recent burn, 0.87 ± 0.02 (n=8) and the least fractionation at the mature spruce forest, 0.93 ± 0.01 (n=3) with fractionation at the intermediate burn site being 0.90 ± 0.02 (n=6). While not statistically conclusive, our results suggest that soil/ecosystem variability may affect fractionation associated with soil uptake of H₂, adding further complication to modeling efforts that until now have assumed this parameter to be a constant value.
DE: 0330 Geochemical cycles (1030)
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0426 Biosphere/atmosphere interactions (0315)
DE: 0454 Isotopic composition and chemistry (1041, 4870)
SC: Biogeosciences [B]
MN: Fall Meeting 2005