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. delta C-13 and delta O-18 values of carbonates from the Atacama are the highest of any desert in the world. delta C-13 (VPDB) values from soil carbonate range from -8.2%o 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 delta C-13 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 delta C-13 value (+4.1 parts per thousand) of carbonate from the driest study sites indicates it formed-perhaps abiotically-in the presence of pure atmospheric CO2. delta O-18 (VPDB) values from soil carbonate range from -5.9 parts per thousand at the wettest sites to +7.3 parts per thousand at the driest and show much less regular variation with elevation change than delta C-13 values. delta O-18 values for soil carbonate predicted from local temperature and 8180 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 6180 values from ancient soil carbonate to reconstruct paleoelevation in such and settings.