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

Stable Isotopes of Hydrogen, Carbon and Oxygen2 VSMOW (‰)+ 40meteoric water line: 2 = 8 18 + 10 ‰0ocean 40condensationrainwater 80marine atmosphericvapour at equatorwater vapourin equilibriumat 20 0 C120160remainingvapour201612840+418 VSMOW (‰)Fig.7.17Relation between natural variations of 18 and 2 ocean water, atmospheric vapour andprecipitation. The black round represents the hypothetical value of water vapour inisotopic equilibrium with ocean water, black square the observed isotopic composition ofequatorial marine vapour, originated from the more realistic non-equilibriumfractionation. Marine vapour gradually condenses into precipitation (hatched arrow) witha positive fractionation, leaving the vapour progressively depleted in 18 O and 2 H (greyarrow) (cf. Fig.7.13).is a non-equilibrium (partly kinetic) process. However, from the observed vapour compositiononward the vapour and precipitation remain in isotopic equilibrium, because the formation ofprecipitation is likely to occur from saturated vapour (i.e. vapour in physical equilibrium withwater). Consequently the 18 and 2 values both move along the meteoric water line (Eq.7.33).121