Rit (Vísindafélag Íslendinga) - 01.06.1976, Qupperneq 38

4. ISOTOPIC CONTENT OF THE PRECIPITATION
4. 1 General.
The saturated vapour pressure of H20 is slightly higher than
that of HDO and H2018. Therefore, when water vapour is in equili-
brium with liquid water the vapour phase will contain somewhat
less HDO and H2018 than the liquid phase.
The isotope ratio of the vapour phase relative to that of the water
phase is given by the fractionation constant, a, where a is simply
given by the ratio of the vapour pressures of the respective compo-
nents:
_ Ph2o
Phdo
and
a0
18
Ph2o16
Ph2o18
The fractionation constants aD and a018 are thus hasic for frac-
tionation of deuterium and O18 in evaporation and condensation
processes. The fractionation constants are independent of the total
pressure under which the system exists, hut each varies with the
temperature. Numerical values for aD and a018 for the tempera-
tures at which evaporation and condensation processes occur in
nature are given by Dansgaard (1964).
When water evaporates from the ocean imder equilibrium condi-
tions, for example at 20 °C, the vapour should be depleted hy about
80%o in deuterium and 9%0 in O18 relative to the ocean water.
This is however not quite so simple, since the evaporation has also
been found to be govemed by kinetic effects as regards the oxygen
isotopes (Craig 1961 a, Friedman et al. 1964 and Dansgaard 1964).
Therefore the isotopic content of the water vapour which has eva-
porated from the ocean surface can differ in its isotope ratio from
what would be expected from the fractionation constants alone.