SOILS AT THE HYPERARID MARGIN: THE ISOTOPIC COMPOSITION OF SOIL CARBONATE FROM THE ATACAMA DESERT

We evaluate the impact of exceptionally sparse plant cover and rainfall on the stable carbon and oxygen composition of soil carbonate in what is probably the driest place on the planet, the Atacama Desert. Carbon isotope values from soil carbonate range from -8.2 ‰ at the wettest sites to +7.9 ‰ at the driest. C3 plants dominate the flora of most areas of the Atacama, simplifying the estimation of soil respiration rates using carbon isotope values from soil carbonate. We modeled respiration rates using a modified version of the soil diffusion model of Cerling (1984), and assumed an exponential form of the soil CO2 production function, and relatively shallow (20-30 cm) average production depths. Our estimates of respiration rates vary from 2-4 to <0.1 mmoles/m2/hr. Overall, we find that respiration rates are the strongest predictor of the carbon isotope value of soil carbonate; the fraction C3 to C4 biomass at individual sites has a subordinate influence. Oxygen isotope (VPDB) values from soil carbonate range from –8.4 ‰ at the wettest sites to +7.3 ‰ at the driest and show much less regular variation with elevation change than carbon isotope values. Oxygen isotope values for soil carbonate predicted from local temperature and oxygen isotope values of rainfall values suggest that extreme (>90% in some cases) soil dewatering by evaporation occurs at most sites prior to carbonate formation. Soil carbonate in the Atacama has the most positive carbon isotope and oxygen isotope values of any desert in the world.