The Belgian Antarctic Programme (OSTC) is gratefully acknowledged for supporting the work.

negative) owing to a successive rainout process that continuously transfers the heavier isotopomers into the liquid phase during condensation. The amount of vapour lost as precipitation is controlled by condensation temperatures, which is why the water isotopes eventually depend on temperatures.

Even in the early years of isotope geochemistry (Craig, 1961a; Dansgaard, 1964), however, it was noted that when calculating the scaled differences between the two isotopomers the result should be independent of the described first-order processes, that is, the dominating temperature control, and second-order processes should be detectable. The slope of the global meteoric water line, i.e. the relation between SD and S18O in global precipitation, is close to 8. Therefore the deuterium excess was defined originally (in Craig, 1961a) as d = SD - 8S18O. Several non-equilibrium processes influence the deuterium excess d.

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