Kinetic fractionation of the carbon isotopes ¹³C and ¹²C during transfer of CO₂ from air to seawater

Laboratory experiments have been performed to determine the kinetic fractionation between ¹³CO₂ and ¹²CO₂ as CO₂ invades seawater. Knowledge of this fractionation, which is regulated by physical and chemical processes in the ocean boundary layer, is important for ocean-atmosphere models which use ¹³CO₂ as a tracer. Until now, only theoretical calculations have been performed to determine this fractionation. The experimental results give a fractionation of –2.4 ± 2.0‰ while the calculated value for invasion is –2.1 ± 0.3‰ (i.e. ¹³CO₂ invading slower). The results show that the fractionation of CO₂ during invasion into a hydroxide solution (–14‰) is clearly too high as a value for invasion into seawater and should no longer be used as an upper limit in CO₂ models.