Kinetische Isotopentrennung bei der Verdampfung von Wasser

Abstract

Water evaporating into air not saturated with water vapour shows a stronger enrichment of D and O‐18 than could be ascribed to the simple equilibrium isotopic fractionation. Kinetic effects are responsible for the increase in isotopic fractionation between liquid and gaseous phase. The so‐called kinetic fractionation factor is calculated by means of a simple model. Comparison with the experimentally obtained fractionation factors proves the assumption that the kinetic fractionation is produced by the different diffusion velocities of H2O‐16, H2O‐18 and HDO‐16 in the surrounding turbulent air. The consequences of the kinetic separation for the evaporation of the oceans are briefly discussed. Those for the evaporation of a limited water reservoir in isotopic exchange with the water vapour of the air are more extensively investigated. In the latter case calculation and experiments show that during evaporation the D‐ and O‐18‐contents approach stationary values as the water mass decreases.

Ground ice recharge via brine transport in frozen soils of Victoria Valley, Antarctica: Insights from modeling δ18O and δD profiles

“…Isotope fractionation due to evaporation is a well known process (Dansgaard, 1964;Craig and Gordon, 1965;Ehhalt and Knott, 1965;Criss, 1993, 1998;Criss, 1999, see further explanations in Appendix A). If the relative humidity and isotopic composition of atmospheric vapor are known (and constant over time) the isotopic evolution of a well-mixed, closed water body during progressive evaporation can be modeled by an open system Rayleigh distillation:…”

Hydrogeologic evidence for a continuous basal shear zone within a deep-seated gravitational slope deformation (Eastern Alps, Tyrol, Austria)

“…The method makes use of the fact that during the transformation of atmospheric water vapour to precipitation, which then infiltrates and becomes groundwater, fractionation processes between the heavier 18 O and the lighter 16 O isotope within the water molecules occur (Dansgaard 1964;Ehalt and Knott 1965). During condensation, the heavier 18 O is enriched relative to the 16 O in precipitation whilst becoming depleted in the remaining vapour.…”

Water Stable Isotopes: Atmospheric Composition and Applications in Polar Ice Core Studies

“…Kinetic effects have to be taken into account when drops fall below cloud base in a subsaturated environment, and they become important for precipitation falling through a dry atmosphere. This process has been the subject of both experimental and theoretical studies (Dansgaard, 1964;Ehhalt and Knott, 1965;Ehhalt et al, 1963;Stewart, 1975). Evaporation results in an isotopic enrichment of the drop both for dD and d 18 O.…”

“…As far as oxygen-17 is concerned, one should rely on theoretical values (van Hook, 1968), as no experimental (Craig and Gordon, 1965;Ehhalt and Knott, 1965;Merlivat, 1978). Molecular diffusivity ratios D i (D HDO /D H2 16 O and D H2 18 O /D H2 16 O ) are practically independent of temperature.…”