Radiocarbon and 230Th- 234U dates of calcic horizons from calciorthid soil profiles in the Mojave Desert were used to calculate the rate of deposition of pedogenic CaCO 3. A major period of CaCO 3 deposition appears to have occurred about 20000 yBP forming calcic horizons below 100-cm depth during a climatic regime with greater effective rainfall than in the present. The overall rate of deposition has been 1.0 to 3.5 g CaCO 3/m 2/yr during soil formation. This rate is consistent with present-day rates, assuming that the atmospheric deposition of Ca limits the process. Stable isotope ratios in calcic horizons indicate that CaCO 3 precipitated from a soil environment with CO 2 of - 15.5%. 13C /12C ( vs. PDB) and H 2O of + 2.0%. 18O /16O ( vs. SMOW). These values suggest that CaCO 3 precipitates when seasonal drought simultaneously lowers soil pore pCO2 and enriches soil water 18O by evaporation. The role of soil calcic horizons in the global geochemical cycle of carbon is discussed.