FRACTIONATION OF SOIL GASES BY DIFFUSION OF WATER-VAPOR, GRAVITATIONAL SETTLING, AND THERMAL-DIFFUSION

Air sampled from the moist unsaturated zone in a sand dune exhibits de pletion in the heavy isotopes of N-2 and O-2. We propose that the depl etion is caused by a diffusive flux of water vapor out of the dune, wh ich sweeps out the other gases, forcing them to diffuse back into the dune. The heavy isotopes of N-2 and O-2 diffuse back more slowly, resu lting in a steady-state depletion of the heavy isotopes in the dune in terior. We predict the effect's magnitude with molecular diffusion the ory and reproduce it in a laboratory simulation, finding good agreemen t between field, theory, and lab. The magnitude of the effect is gover ned by the ratio of the binary diffusivities against water vapor of a pair of gases, and increases similar to linearly with the difference b etween the water vapor mole fraction of the site and the advectively m ixed reservoir with which it is in diffusive contact (in most cases th e atmosphere). The steady-state effect is given by delta(i) = [i/j/i(0 )/j(0) - 1] 10(3) parts per thousand congruent to [(1 - x(H2O)/1 - x(H 2O0))((Dj-H2O/Di-H2O)-1) -1] 10(3) parts per thousand, where delta(i) is the fractional deviation in permil of the gas i/gas j ratio from th e advectively mixed reservoir, x(H2O) and x(H2O0) are respectively the mole fractions of water vapor at the site and in the advectively mixe d reservoir, and D-i-H2O is the binary diffusion coefficient of gas i with water vapor. The effect is independent of scale at steady state, but approaches steady state with the time constant of diffusion set by the length scale. Exploiting the mechanism, we make an experimental e stimate of the relative diffusivities of O-2 and N-2 against water vap or, finding that O-2 diffuses 3.6 +/- 0.3% faster than N-2 despite its greater mass. We also confirm in the study dune the presence of two a dditional known processes: gravitational fractionation, heretofore see n only in the unconsolidated firn of polar ice sheets, and thermal dif fusion, well described in laboratory studies but not seen previously i n nature. We predict that soil gases in general will exhibit the three effects described here, the water vapor flux fractionation effect, gr avitational fractionation, and thermal diffusion. However, our analysi s neglects Knudsen diffusion and thus may be inapplicable to fine-grai ned soils.