1. The study used stable carbon and oxygen isotopes from paleosol carbonates in loess deposits in eastern Washington state to reconstruct past atmospheric CO2 levels. 2. Two methods were used to calculate CO2 levels, either assuming a CO2 value to determine soil respiration rates or assuming soil respiration rates to determine CO2 levels, since soil respiration is difficult to constrain. 3. Results suggest soil carbonates preferentially form when soil respiration is low. Reconstructed CO2 levels using ice core data matched soil respiration rates expected under cold, dry glacial conditions better than using the full range of possible respiration rates.