5 NATURAL ABUNDANCE OF THE STABLE ISOTOPES ... - Falw.vu

Stable Isotopes of Hydrogen, Carbon and Oxygenocean waterarctic sea icemarine moisture(sub)tropical precipitationtemperate zone ,,polar ice ,,Alpine glaciersLake Chadwood cellulose temperatepeatzoneclay minerals200 150 100 50 0 + 502 VSMOW (‰)Fig.7.16General view of 2 H/ 1 H variations in natural compounds. The ranges are indicative for themajority of materials shown.7.4 STABLE HYDROGEN ISOTOPES7.4.1 THE NATURAL ABUNDANCEThe chemical element hydrogen consists of two stable isotopes, 1 H and 2 H (D or Deuterium),with an abundance of about 99.985 and 0.015% and an isotope ratio 2 H/ 1 H 0.00015 (Ureyet al., 1932). This isotope ratio has a natural variation of about 250‰, higher than the 13 and18 variations, because of the relatively larger mass differences between the isotopes(Fig.7.16).As with 18 O, high 2 H concentrations are observed in strongly evaporated surface waters, whilelow 2 H contents are found in polar ice. Variations of about 250‰ are present in the part of thehydrological cycle to be discussed here.7.4.2 HYDROGEN ISOTOPE FRACTIONATIONSThe most important hydrogen isotope fractionation is that between the liquid and the vapourphases of water. Under equilibrium conditions water vapour is isotopically lighter (containsless 2 H) than liquid water by amounts given in Table 7.6. Fig.7.3 shows some actual isotoperatios of equilibrium systems and the matching 2 values. The fractionation by diffusion ofH 2 O through air ( 2 d ) varies between 22.9 ± 1.7‰ (Merlivat, 1978) and 20.4 ± 1.4‰.(unpubl.), slightly more than the value calculated from Eq.3.34 (16.3‰) (cf. Sect. 7.3.2).117