Carbon isotope fractionation during gas-water exchange and dissolution of CO2

• Published in: Geochimica et cosmochimica acta 1995-01, Vol.59 (1), p.107-114
• Main Authors: Zhang, J., Quay, P.D., Wilbur, D.O.
• Format: Article
• Language: English
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/0016-7037(95)91550-D

The kinetic and equilibrium fractionation effects for 13C during CO2 gas transfer (εk and εag−g) have been measured in acidified distilled water. The equilibrium fractionation effects between bicarbonate and carbonate and gaseous C02 (εHCO3−g and εCO3−g) have been measured in NaHC03 and NaHC03 + Na2C03 solutions, respectively, from 5° to 25°C. The measured fractionations, except εCO3−g, agreed with earlier work to within 0.2‰. εCO3−g was about 2‰ smaller than most values previously reported. The temperature dependence of the fractionation for 13C between bicarbonate and carbonate and gaseous CO2 was found to be εHCO3−g = −(0.141 ± 0.003)T(°C) + 0.05)‰ and εCO3−g = −(0.052 ± 0.03) T(°C)E+ (7.22 ± 0.46)‰ respectively. The fractionation during gas dissolution was εCO3−g = −0.03)T(°C) + (1.31 ± 0.06%. and the kinetic effect during gas transfer, εk, was −0.81 ± 0.16‰ at 21°C and −0.95 ± 0.20‰ at 5°C. The equilibrium fractionation between total DIC in seawater and CO2 in air (εDIC−g) was measured and compared with that calculated from the concentration of aqueous CO2, HC03− 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‰. We hypothesize that carbonate ion complexes likely complicate the calculation of εDIC−g from individual C species. We obtain the following empirical function of εDIC−g in seawater vs. temperature and the carbonate fraction (fCO3), εDIC−g = (0.014 ± 0.01) TfCO3 − (0.105 ± 0.002) T + (10.53 ± 0.05)%., when 0.05 < fCO3 < 0.20and5°C< T < 25°C