Processes controlling the oxygen isotope ratio of soil CO{sub 2}: Analytic and numerical modeling
- Univ. of California, Berkeley, CA (United States)
The {sup 18}O/{sup 16}O ratio of soil CO{sub 2} is important for both global and ecosystem scale budgets of the {sup 18}O/{sup 16}O ratio of atmospheric CO{sub 2}, as well as for using this ratio as a proxy for the isotopic ratio of soil water. The {sup 18}O/{sup 16}O ratio of soil CO{sub 2} reflects that of soil water due to the isotope exchange which occurs during the CO{sub 2} hydration reaction. The rate of this isotope exchange reaction strongly influences the oxygen isotope ratio of soil CO{sub 2}, and may be about two orders of magnitude slower than the isotope exchange reaction between CO{sub 2}(aq) and water (effective first order rate constant, k{sub eff}, of 10{sup {minus}4} versus 10{sup {minus}2} s{sup {minus}1} at 25 C). This difference in rate reflects transport across the soil water/soil air interface, which limits the overall rate of reaction. A diffusion-production-reaction model (Hesterberg and Siegenthaler, 1991) suggests that the {sup 18}O/{sup 16}O ratio of soil CO{sub 2} at depth deviates from the equilibrium value by {approximately}1--2{per_thousand} if k{sub eff} = 10{sup {minus}4} s{sup {minus}1}, negating the assumption that soil CO{sub 2} is in isotopic equilibrium with soil water. A sensitivity analysis of this model indicates the following factors affect {sup 18}O/{sup 16}O ratio of soil CO{sub 2} (in order of decreasing importance): (1) the isotope ratio of soil water; (2) the rate constant of the isotope exchange; (3) soil air-filled pore space and tortuosity. Notably, soil respiration rate has only a minor effect. Advection of soil gas due to wind pumping has a negligible effect on the {delta}{sup 18}O value of soil CO{sub 2} except in most extreme cases. Likewise, transient effects on the oxygen isotope ratio of soil CO{sub 2} due to changes in soil respiration rate are small.
- OSTI ID:
- 355672
- Journal Information:
- Geochimica et Cosmochimica Acta, Vol. 63, Issue 6; Other Information: PBD: Mar 1999
- Country of Publication:
- United States
- Language:
- English
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