Soils at the hyperarid margin: The isotopic composition of soil carbonate from the Atacama Desert, Northern Chile

Abstract

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. δ 13C and δ 18O values of carbonates from the Atacama are the highest of any desert in the world. δ 13C (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 CO 2 production function, and relatively shallow (20–30 cm) average production depths. Overall, we find that respiration rates are the main predictor of the δ 13C value of soil carbonate in the Atacama, whereas the fraction C 3 to C 4 biomass at individual sites has a subordinate influence. The high average δ 13C value (+4.1‰) of carbonate from the driest study sites indicates it formed—perhaps abiotically—in the presence of pure atmospheric CO 2. δ 18O (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 δ 13C values. δ 18O values for soil carbonate predicted from local temperature and δ 18O 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 δ 18O values from ancient soil carbonate to reconstruct paleoelevation in such arid settings.