GeoActa Special Publication 1
2008
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GeoActa
an international Journal of Earth Sciences
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Giampiero Venturelli, Paola Iacumin
Dipartimento di Scienze della Terra, Universita` di Parma, Parco Area delle Scienze 157/A, 43100 Parma, Italy. E-mail: gvt@unipr.it
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Oxygen and hydrogen isotope fractionation in precipitation. A simple approach for teaching
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PDF (492 KB)
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Abstract
Equations which describe oxygen and hydrogen stable isotopes fractionation during water condensation/precipitation are summarised and presented in deductive simplified form which may be useful for teaching. They are mostly based on Rayleigh-type fractional distillation and elementary thermodynamics. The following hypothetical conditions are considered. (a) During condensation/precipitation the wet air mass only loses pure liquid water and does not acquire any gaseous or liquid water from the environment. (b) Condensation/precipitation occurs under two different extreme conditions: (i) the liquid water is immediately removed (no clouds present) during condensation; (ii) the condensed water is in part kept within the system (clouds present), in part removed as precipitation; in both cases condensation/precipitation may occur at either constant or variable pressure. (c) During condensation/precipitation equilibrium is maintained between the gas mixture and the liquid water of the reservoir. The equations show that temperature at the beginning of precipitation, initial isotope features of the reservoir and liquid water/total water ratio (Wl/Wr) in the reservoir are critical parameters in determining the isotopic variations at decreasing temperature. Although the equations reported represent limit conditions in nature, they have the following advantages: they are easily manageable, constitute the background for more complex fractionation models, and allow the identification of significant deviation of the natural processes from the Rayleigh-type models.
Keywords: Oxygen and hydrogen isotopes; Precipitation; Equation of isotope distribution
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