Phosphate oxygen isotope fingerprints of past biological activity in the Atacama Desert.

• Low δ 18O HCl-P values were constant with parent material at the driest sites. • δ 18O HCl-P values increase at less arid sites. • δ 18O HCl-P values trace past biological activity in the Atacama Desert. The Atacama Desert (Chile) is one of driest places on Earth, with a hyper-arid climate and less than 2 mm yr−1 precipitation; nevertheless, it has experienced rare periods of sporadic rainfall. These periods shortly enhanced vegetation growth and microbial activity, which must have utilized major nutrients such as phosphorus (P). However, any biological cycling of P involves an oxygen exchange with water, which should now reside in the hyperarid soils as tracer of life. In order to identify such evidences, we performed sequential P fractionation and analyzed the oxygen isotope composition of HCl-extractable phosphate (δ 18O HCl–P) in the surface soil (0–15 cm) of a climatic gradient along the rising alluvial fans of the Central Depression to the Precordillera, Chile. At the driest sites, the δ 18O HCl-P values were constant with depth and deviated from biologically-driven isotopic equilibrium. In contrast, we observed a considerable increase of δ 18O HCl-P values below the soil surface at less arid sites, where some isotope values were even within the range of full isotopic equilibrium with biologically cycled phosphate. For the latter sites, this points to most efficient biological P cycling right below the uppermost surface of the desert. Critically, the absolute concentrations of this biologically cycled P exceeded those of P potentially stored in living microbial cells by at least two orders of magnitude. Therefore, our data provides evidence that δ 18O HCl-P values trace not recent but past biological activity, making it a powerful tool for assessing the existence, pathways and evolution of life in such arid ecosystems on Earth and, thus, potentially on other planets such as Mars. [ABSTRACT FROM AUTHOR]