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

Chapter 7 3 : from 2 , 1 , and 1 ; 1.03086 is the exact figure quoted by Friedman and O'Neil (1977), inagreement with 2 = 1.04115 (Brenninkmeijer et al., 1983) 4 : from 3 and 1The conversion equations are in general:18 lower = i 18 upper + i (7.28)where i = , D , or , and "lower" and "upper" refer to the levels in the scheme.Henceforth, all 18 values of carbonates are reported relative to VPDB, 18 of gaseous(atmospheric) CO 2 relative to VPDBCO 2 , and all 18 values of water relative to VSMOW,unless stated otherwise.More details on the measurement and calculation procedures are given in Chapter 11.7.2.4 SURVEY OF NATURAL 18 O VARIATIONS18 variations throughout the hydrologic cycle will be discussed in detail in the other volumes.Here we present only a broad survey (Fig.7.12).7.2.4.1 SEAWATERThe oceans form the largest global water reservoir. The 18 O content in the surface layer israther uniform, varying between +0.5 and 0.5‰ (Epstein & Mayeda, 1953). Only in tropicaland polar regions larger deviations are observed. In tropical regions more positive values arecaused by strong evaporation: 18 of water in the Mediterranean amounts to +2‰ ( 2 =+10‰). In polar regions more negative values originate from water melting from isotopicallylight snow and ice.If ocean water were evaporating under equilibrium conditions, the vapour resulting would be8 to 10‰ depleted in 18 O, depending on temperature. However, oceanic vapour appears tohave an average 18 value of -12 to -13‰, which must partly be due to kinetic fractionation.The relative humidity of the air and the evaporation temperature influence the amount of nonequilibriumfractionation occurring (see Volume 2 on Precipitation).7.2.4.2 PRECIPITATIONThe transformation of atmospheric water vapour to precipitation depends on so manyclimatological and local factors, that the 18 variations in precipitation over the globe are verylarge. As a general rule 18 becomes more negative the further the rain is removed from the110