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

Chapter 7for water18 VSMOW (‰)for carbonate18 VPDB (‰)60 40 20 0 +20ocean waterarctic sea icemarine moisture(sub)tropical precipitationDead Sea/Lake Chadtemperate zone precipitationAlpine glaciersGreenland glaciersAntarctic iceQuatern. marine carbonatesfresh-water carbonatesocean watermarine carbonatesigneous rocksmarine atmospheric CO 2atmospheric oxygenorganic matter20 0 +20 +40 +6018 VSMOW (‰)Fig.7.9General view of 18 O/ 16 O variations in natural compounds. The ranges are indicative forthe majority of materials shown. The relation between the VPDB and VSMOW scales isgiven in Sect.7.2.3 and Fig.7.11.Kinetic effects are observed during the evaporation of ocean water, as oceanic vapour isisotopically lighter than would result from equilibrium fractionation alone. The naturalisotope effect for oxygen ( -12‰) is smaller than could be brought about by fractionation bydiffusion. Laboratory measurements resulted in 18 d = -27.3 ± 0.7‰ (Merlivat, 1978) and 18 d= -27.2 ± 0.5‰ (unpubl.). These experimental values are again smaller than results fromEq.(3.35) (-31.3‰), which may be explained by the water molecules forming clusters of104