CARBON-ISOTOPE FRACTIONATION DURING GAS-WATER EXCHANGE AND DISSOLUTION OF CO2

The kinetic and equilibrium fractionation effects for C-13 during CO2 gas transfer (epsilon(k) and epsilon(aq-g)) have been measured in acidified distilled water. The equilibrium fractionation effects between bicarbonate and carbonate and gaseous CO2 (epsilon(HCO3-g) and epsilon(CO3-g) have been measured in NaHCO3 and NaHCO3 + Na2CO3 solutions, respectively, from 5-degrees to 25-degrees-C. The measured fractionations, except epsilon(CO3-g), agreed with earlier work to within 0.2 parts per thousand. epsilon(CO3-g) was about 2 parts per thousand smaller than most values previously reported. The temperature dependence of the fractionation for C-13 between bicarbonate and carbonate and gaseous CO2 was found to be epsilon(HCO3-g) = -(0.141 +/- 0.003)T(degrees-C) + (10.78 +/- 0.05) parts per thousand and epsilon(CO3-g) = -(0.052 +/- 0.03)T(degrees-C) + (7.22 +/- 0.46) parts per thousand, respectively. The fractionation during gas dissolution was epsilon(aq-g) = -(0.0049 +/- 0.003)T(degrees-C) - (1.31 +/- 0.06) parts per thousand and the kinetic effect during gas transfer, epsilon(k), was -0.81 +/- 0.16 parts per thousand at 21-degrees-C and -0.95 +/- 0.20 parts per thousand at 5-degrees-C. The equilibrium fractionation between total DIC in seawater and CO2 in air (epsilon(DIC-g)) was measured and compared with that calculated from the concentration of aqueous CO2, HCO3-, and CO3= and individual fractionations between the three C species and CO2 gas. The measured and calculated results showed a significant difference of up to 0.2 parts per thousand. We hypothesize that carbonate ion complexes likely complicate the calculation of epsilon(DIC-g) from individula C species. We obtain the following empirical function of epsilon(DIC-g) in seawater vs. temperature and the carbonate fraction (fCO3), epsilon(DIC-g) = (0.014 +/- 0.01)Tf(CO-3) - (0.105 +/- 0.002)T + (10.353 +/- 0.05) parts per thousands, when 0.05 < f(CO3) < 0.20 and 5-degrees-C < T < 25-degrees-C.