In order to better calibrate pedogenic carbonate as a proxy for past environments, we compared the stable isotopic composition of soil CO2, soil water, and pedogenic carbonate in young soils from central New Mexico, USA. Seasonal changes in the δ13C value of soil CO2, the δ18O value of soil water, and the soil temperature were monitored to establish the timing of isotopic equilibrium with the carbonate. Calcite solubility was calculated from measured temperatures and CO2 concentrations in the soil. This approach allowed us to determine the conditions associated with pedogenic carbonate formation. Carbon isotope equilibrium, oxygen isotope equilibrium, and minimum calcite solubility all occurred simultaneously during warm, dry conditions in May 2008 when soil CO2 concentrations were low. It is therefore concluded that pedogenic carbonate forms during warm, dry periods and does not record mean growing season conditions as typically assumed. The seasonal bias in pedogenic carbonate formation may explain the occurrence of pedogenic carbonate in monsoon climates and its absence in regions where annual precipitation is more uniformly distributed. The implications of the seasonal bias for stable isotope–based paleoenvironmental reconstructions are that paleoelevations may have been previously over- or underestimated, paleoatmospheric CO2 concentrations likely have been significantly overestimated, and pedogenic carbonate provides a C4-biased record of paleovegetation, especially in dry soils.