Heavy isotopic water molecules are preferentially incorporated into the growing ice so that the solid is enriched in deuterium and oxygen 18 compared to the water. Equilibrium fractionation always occurs at the ice-water interface but the observed fractionation between ice and bulk water can be lower, depending on the isotopic concentration in the water at the interface. Souchez & Jouzel (1984) have demonstrated that, by partial freezing of an open or a closed system, samples of ice plot in a SD-S18O diagram on a so-called freezing slope different from the meteoric water line. The equation of the slope S is given by:
S _ «D K -1)(1000 +S,D) «18(«18 -1)(1000 + S,18O)
where aD and a18 are the equilibrium fractionation coefficients for deuterium and oxygen-18 respectively, and SiD and Si18O are the S-values for deuterium and oxygen-18 respectively of the initial water at the onset of freezing. The distribution of sample points along the freezing slope is dependent on several factors: percentage of freezing in a closing system with progressive disappearance of the liquid phase, variations of the isotopic composition of the liquid at the interface due to diffusion and to convection, freezing rate and trapping of unfractionated water pockets during ice
accretion that freeze completely afterwards. In such a freezing process, deuterium excess is inversely related to the S values. The significance of deuterium excess in such a non-meteoric ice is related to the distribution of sample points on the freezing slope, thus to the factors cited above. Variations in deuterium excess are in this case the result of the conditions prevailing during freezing. This is in strong contrast with the general acceptance of deuterium excess in precipitation. In Fig. 35.2 is shown the very good inverse linear relationship between d and SD values of samples of lake ice retrieved from a small lake frozen to the bottom, adjacent to Suess Glacier in Taylor Valley, South Victoria Land. In basal ice reached in a tunnel cut at the front of Suess Glacier, a freezing slope is also well displayed in a SD-S18O diagram, indicating the role of water freezing in its formation in a subglacial environment where the basal temperature is several °C below the pressure-melting point. The deuterium excess profile in this basal ice is a mirror image of the S18O profile (Sleewaegen et al., 2003). This inverse relationship between d and SD results from the fact that the freezing slope is lower than 8.
The study of deuterium excess in ice formed by water freezing can reveal specific processes involved as developed in the two cases described below.
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