Depth distribution of phosphate 18O isotope values along the Paposo transect

The extreme conditions of the Chilean Atacama Desert preserve life's fingerprints. The subsoil, in particular, offers protection for microbial communities. Our study sampled at Paposo four pits at 10 cm intervals, reaching a depth of 2 m, along a transect with increasing distances from the coast and thus aridity. The oxygen isotope composition of HCl-extractable phosphate (δ¹⁸O_HCl-Pi) analysis was conducted to explore the biological P cycling. None of the measured values of δ¹⁸O_HCl-Pi fell within the equilibrium range, indicating that soil phosphate had only undergone partially biological cycling. However, variations were observed at different depths. In the fog-nourished zone near the coast (3.5 km), the highest δ¹⁸O_HCl-Pi value of 16.6 ‰ was found at the surface, gradually decreasing with depth. However, a relatively higher value of 15.8 ‰ was detected at a depth of 80‒90 cm, pointing to wetter period in the past. At a location 13.4 km from the coast with rare rainfall, both profiles had lower average δ¹⁸O_HCl-Pi values of 8.9 and 7.7 ‰ at the surface, in an active and abandoned section respectively. Interestingly, in the abandoned section, the value increased to 13.1‰ in the second layer (10‒20 cm), while it remained stable in the active section. This suggests that topography plays a role in detecting past signs of life. Our investigation sheds new light on the historical presence and variability of (microbial) life in relation to changes in soil wetness due to climate fluctuations.