Molecular and Isotopic Signs of Life and Climate in the Hyperarid Atacama Desert

Abstract:

The hyperarid region of the Atacama Desert is considered one of the driest places on earth. Geomorphological studies suggest that this area has maintained a near-continuous hyperarid climate for at least the past 2 million years. Water-limited biological and geochemical processes have created a unique landscape with many similarities to Mars. While precipitation is rare in the Atacama Desert, coastal fog occurs regularly and microbial communities capable of utilizing fog and dew water are able to persist. Within this region, we found soils with lichen-dominated biological soil crusts living in association with physical sulfate crusts on the soil surface. Due to their high tolerance of desiccation and ability to utilize fog water, biologic soil crusts are able to exist in this hyperarid environment. We chose two sites ~30 km apart along a fog frequency transect which showed visible differences in the degree of biological coverage to study how shifts in fog water availability affect the biogeochemical processes occurring. Our previous radiocarbon dating and δ¹³C analysis of soil carbonates here indicates that soil in both locations has been accreting for over 15,000 years and confirmed that biological activity and rates of C cycling are greater at the higher fog frequency site. This study expands on that work to characterize the isotopic imprint of extreme aridity and evaporative processes in this environment, examining both organic and inorganic materials. A standard fatty acid extraction method was used and we were able to detect fatty acids in all soils analyzed, even those over 15,000 years old. Compound specific isotope analysis of these fatty acids clearly showed an enrichment of ²H at the drier site, with ~ 25 per mil difference between the surface samples. Similarly, analysis of δ¹⁸O of soil carbonates show ~10 per mil enrichment of ¹⁸O at the drier site. We attribute these differences to a prolonged and consistently greater evaporative stress at the eastern (drier) site, and note that this is one of the only studies to show an isotopic signature of evaporation in both organic and inorganic materials.