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The deuterium and oxygen 18 concentrations of water evaporating into air of nonzero humidity do not follow the simple batch distillation equation but increase asymptotically to a stationary isotopic state as the mass of water decreases to zero. This effect is due to a rapid molecular exchange between liquid and vapor, which predominates over the simple separation effect of removal of vapor. In diy air the batch distillation law is obeyed, and the isotopic enrichment continually increases; however, the separation factor is larger than the equilibrium isotopic vapor pressure ratio when the evaporation rate is comparable to natural rates. The HDO and H2O18 enrichments in our experiments show a linear correlation with a slope of about 5.5, as found in the evaporation of natural water bodies. The mechanism of the molecular exchange will be described in following papers.

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Atmospheric gases trapped in polar ice at the firn to ice transition layer are enriched in heavy isotopes (nitrogen-15 and oxygen-18) and in heavy gases (O(2)/N(2) and Ar/N(2) ratios) relative to the free atmosphere. The maximum enrichments observed follow patterns predicted for gravitational equilibrium at the base of the firn layer, as calculated from the depth to the transition layer and the temperature in the firn. Gas ratios exhibit both positive and negative enrichments relative to air: the negative enrichments of heavy gases are consistent with observed artifacts of vacuum stripping of gases from fractured ice and with the relative values of molecular diameters that govern capillary transport. These two models for isotopic and elemental fractionation provide a basis for understanding the initial enrichments of carbon-13 and oxygen-18 in trapped CO(2), CH(4), and O(2) in ice cores, which must be known in order to decipher ancient atmospheric isotopic ratios.

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