Carbon isotopic fractionation during the CO 2 exchange process between air and sea water under equilibrium and kinetic conditions

Abstract

Carbon isotopic fractionation during the air/sea exchange process is not fully understood at present. Information on the equilibrium and kinetic fractionation factors is an essential requirement, together with the value of the CO 2 partial pressure, for understanding the carbon cycle in the atmosphere and marine environments. Using a specially designed countercurrent equilibrator system, the fractionation factors between gaseous CO 2 and dissolved inorganic carbon in sea water were determined under both kinetic and equilibrium conditions. The following results were obtained: kinetic fractionation factor for air to sea ( α as ) is 0.998 at 288.2 K; kinetic fractionation factor for sea to air ( α sa ) is 0.990; equilibrium fractionation factor ( α eq ) is 0.991 at pH = 8.3 and 288.2 K. From these results, the carbon isotopic ratio of CO 2 passed through the air/ sea interface is estimated to be about −10 %. for air to sea and −8 %. for sea to air when CO 2 exchange takes place between air ( δ 13 C = −8 %.) and surface sea water ( δ 13 C = 2 %.) at 288.2 K.