Georespiration and soil respiration operate on organic carbon pools of vastly different sizes and mean residence times (MRT). Both processes occur in the shallow subsurface at the Dalmeny site in southern Saskatchewan, Canada. Steady and transient, heterogeneous simulations of vadose CO2, 13CO2, and 14CO2 show that at least 98% of all subsurface respiration occurs in the solum where the MRT of labile soil carbon is about 10 years. Root respiration dominates the total during the growing season. Remaining CO2 generation occurs near the capillary surface at 6.5--7.5 m depth, where &dgr;14C of respired CO2 indicates an MRT of about 22,000 years. This value is consistent with a respiration substrate dominated by Cretaceous-age kerogen in the till. The simulated oxidation/georespiration rate at this depth is also consistent with observed depletion of kerogen C from the vadose zone during the Holocene. Field relations in this setting indicate that georespiration is controlled hydrogeologically by the development of aerobic vadose zones; we speculate that this may be more generally true on a global basis. Where soil parent materials contain ancient carbon, georespiration should be considered as a possible factor complicating studies of soil carbon turnover. ¿ 1998 American Geophysical Union