Anomalous carbon isotope fractionation between atmospheric CO 2 and dissolved inorganic carbon induced by intense photosynthesis

Abstract

A stable isotope mass-balance of dissolved inorganic carbon during a blue-green algae bloom in a softwater lake demonstrates that at low partial pressure of carbon dioxide there must be a large net negative carbon isotope fractionation between atmospheric CO 2 and the CO 2 absorbed by lake water at pH = 9.5. The net fractionation of CO 2(g) with respect to HCO − 3 was about −13%. compared with about +8%. for water at equilibrium with atmospheric CO 2 at pH ≈ 7. Chemical enhancement of CO 2 invasion at high pH by the reaction CO 2 + OH −→ HCO − 3 at large apparent film thicknesses may result in carbon isotope fractionation approaching that for a hydroxide solution. This phenomenon, coupled with a decrease in the photosynthetic fractionation, forced the surface water of a softwater lake to achieve increasingly negative δ 13C values during an algal bloom, which is in the opposite sense to the trend that results from photosynthesis under less extreme conditions. This and other similar systems must operate under non-equilibrium (kinetic) conditions, causing a large kinetic fractionation during CO 2 invasion at pH > 8 and relatively large film thicknesses ( i.e., low wind stress).