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

Stable Isotopes of Hydrogen, Carbon and Oxygenor in terms of values:1313T13a a / b b c c / b 13 a a / b c c / b DIC b(7.8)CCwhere the brackets indicate the respective concentrations, which are also denoted by a (acid),b (bicarbonate), and c (carbonate ions), so that a + b + c = C T . The values are given in Table7.2. The chemical fractions are quantitatively treated in Chapter 9.Originally 13 k values for the uptake of CO 2 by seawater of about 15‰ were assumed, basedon experimental results of CO 2 absorption by an alkaline solution (Baertschi, 1952).However, Siegenthaler & Münnich (1981) have reasoned that this effect does not apply to thedissolving action of seawater. Calculations by these authors as well as by Inoue & Sugimura(1985) and Wanninkhof (1985) have shown the kinetic fractionation during CO 2 uptake to be13 k (air sea) = 13 k (atm.CO 2 to CO 2 taken up) = 2.0 ± 0.2‰13T13This value was confirmed by experiments of the last author (2.4 ± 0.8‰).(We have toremind the reader that these 13 k values as well as those below are for kinetic fractionationsand do not refer to Table 7.2).The kinetic fractionation during CO 2 release by the ocean reported by Siegenthaler &Münnich (1981) needs a correction (Mook, 1986). By application of the most recentequilibrium fractionations of Table 7.2 the equilibrium fractionation ( 13 a/DIC ) for dissolvedCO 2 with respect to total dissolved carbon at 20 o C is 0.99055. This fractionation factor is alsodetermined by the chemical composition of seawater, which adds to the overall temperaturedependence.At 20 o C the respective relative concentrations in seawater at pH = 8.20 are: CO 2 (aq)/C T =0.006, HCO 3 /C T = 0.893 and CO 3 2 /C T = 0.102 where C T is the total inorganic carbonconcentration. By incorporation of the CO 2 + OH reaction (0.9995), the resulting 13 value is10.1 to 10.6‰, depending on whether the hydration of CO 2 to H 2 O is to be included.According to Inoue & Sugimura (1985) the value is about -10‰, so that we can conclude to afractionation of released CO 2 with respect to DIC of:13 k (sea air) = 13 k (DIC rel. to CO 2 released) = 10.3 ± 0.3‰Figure 7.4 represents a review of equilibrium and kinetic fractionations relative to gaseousCO 2 (left-hand scale) and the actual 13 values based on 13 (atm.CO 2 ) = 7.0‰ (right-handscale). It is obvious that the isotopic compositions of CO 2 released and taken up by the oceanare equal, as is required by the condition of stationary state of isotopic equilibrium betweenocean and atmosphere.95