Physical, chemical and biological characteristics* of CO₂ gas flux across the air‐water interface

Carbon dioxide gas flux across the air‐water interface is most often treated as a ‘simple’ physical process, primarily responding to wind speed and water temperature. Available experimental data yield an exponential regression equation relating wind speed to the thickness of a stagnant boundary film through which gas diffuses to or from the water. Flux of CO 2 is influenced by CO 2 hydration reactions in the stagnant boundary layer. High pH and a thick stagnant boundary layer favour chemical enhancement of the CO 2 gas flux. The rate of CO 2 flux reflects the sum of net organic metabolism plus CaCO 3 reactions. Some interesting gas‐flux constraints on the rate of net organic carbon production and on global geochemical cycling of CaCO 3 emerge. At high pH (circa 10), the maximum net organic carbon production which can be supported by CO 2 flux across the air‐water interface is about 0.06 mol C m &2 d &1 . On a global scale, organic C, not atmospheric C, appears to account for the ‘CO 2 ’ term in the classical CaCO 3 dissolution‐precipitation reaction.