For more than 40 yr the stable water isotopomers, H218O and HD16O, have provided the principal means of deriving past temperatures from ice-core drillings in polar regions. Their concentration relative to the main isotopomer H216O (H218O/H216O and HD16O/H216O) is expressed as a S value (S18O, SD), that is, as a deviation from the Standard Mean Ocean Water—SMOW (Craig, 1961b), usually given in parts per mil. In many studies the isotopic composition of the ice was linearly related to past site temperatures using a calibration based on the modern spatial S-value-temperature relationship (Johnsen et al., 1989; Jouzel & Koster, 1997; Jouzel et al., 2003a). This empirical approach is supported theoretically by Rayleigh distillation models (Dansgaard, 1964; Ciais & Jouzel, 1994; Jouzel & Merlivat, 1984). On their way from low to high latitudes water vapour masses become isotopi-cally more and more depleted (that is the S value becomes more
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