Tracing life in the Atacama Desert by multi-isotope analyses of sulfates

Calcium sulfates are the dominating salts in soils of the Atacama Desert (Chile) but respective sulfate sources are debated. Various sulfate sources comprise distinct isotopic compositions. To map the spatial distribution of sulfate sources and to identify biological sulfate recycling, we determined Δ17OSO4, δ18OSO4, δ34SSO4, and 87Sr/86Sr of surface samples from Atacama Desert soils taken along four W-E transects from the Pacific coast to the Pre-Andean Cordillera.

Distinct mass-independent 17O anomalies in sulfates originate from atmospheric oxidation of reduced sulfur species derived from volcanic emissions or dimethyl sulfide (DMS) a biogenic sulfur gas that emits from the ocean. Marine aerosols represent a mixture of marine sulfate and atmospherically oxidized DMS and thus comprise elevated Δ17OSO4. Biological sulfate recycling leads to oxygen isotope exchange with ambient water, resulting in Δ17OSO4 values that converge towards 0‰ and generally increasing δ18OSO4 values because water in the Atacama Desert is highly evaporated. Volcanic and marine sulfate aerosols are effectively distinguished by δ34SSO4. The combination of δ34SSO4 and 87Sr/86Sr allows to identify dissolution and re-precipitation of sulfate.

Our dataset reveals the distribution of sulfate sources for Atacama Desert soils. Post-depositional bio-alteration processes lead to changes in the isotopic composition allowing to draw conclusions on bio activity and water availability in such hyper-arid environments like the Atacama Desert.