We evaluate the impact of exceptionally sparse plant cover (0–20%) and rainfall (2–114 mm/yr) on the stable carbon and oxygen composition of soil carbonate along elevation transects in what is among the driest places on the planet, the Atacama Desert in northern Chile. δ13C and δ18O values of carbonates from the Atacama are the highest of any desert in the world. δ13C (VPDB) values from soil carbonate range from -8.2% at the wettest sites to +7.9% at the driest. We measured plant composition and modeled respiration rates required to form these carbonate isotopic values using a modified version of the soil diffusion model of [Cerling (1984) Earth Planet. Sci. Lett.71, 229–240], in which we assumed an exponential form of the soil CO2 production function, and relatively shallow (20–30 cm) average production depths. Overall, we find that respiration rates are the main predictor of the δ13C value of soil carbonate in the Atacama, whereas the fraction C3 to C4 biomass at individual sites has a subordinate influence. The high average δ13C value (+4.1%) of carbonate from the driest study sites indicates it formed&mdahs;perhaps abiotically—in the presence of pure atmospheric CO2. δ18O (VPDB) values from soil carbonate range from -5.9% at the wettest sites to +7.3% at the driest and show much less regular variation with elevation change than δ13C values. δ18O values for soil carbonate predicted from local temperature and δ18O values of rainfall values suggest that extreme (>80% in some cases) soil dewatering by evaporation occurs at most sites prior to carbonate formation. The effects of evaporation compromise the use of δ18O values from ancient soil carbonate to reconstruct paleoelevation in such arid settings.