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The oxygen isotopic composition of modern soil carbonate is well correlated with the isotopic composition of local meteoric water. The carbon isotopic cycle for CO2 in soils can be described in terms of the proportion of biomass using the C4 photosynthetic pathway and the CO2 respiration rate of the soil; at low soil respiration rates significant atmospheric CO2 mixing can occur. In general, the carbon isotopic composition of soil carbonate is related to the proportion of C4 biomass present in soil, but soils that freeze to the depth of carbonate formation often have a significant atmospheric component. This suggests that freezing of the soil solution should be considered as another important mechanism for soil carbonate formation. Because of these relationships, the isotopic composition of soil carbonate may be a paleoclimatic and paleoecologic indicator in cases in which diagenetic alteration has not occurred.