The Atacama Desert, in the rain shadow of the Andes of Chile, is one of the driest and most lifeless environments on Earth and is considered a test environment for astrobiology. It also contains an abundance of nitrate minerals, enough that its soils have been mined for fertilizer for nearly two centuries. The origin of these nitrates has been debated since Charles Darwin visited here on his Beagle voyage, with suggestions ranging from bacterial fixation, to atmospheric deposition of sea salt, to volcanic inputs, guano, and more. Michalski et al.examined the three oxygen isotopes (16O, 17O, and 18O) of soil nitrates and sulfates to characterize their origin. Biological processes and most inorganic reactions produce fractionate isotopes depending on their mass difference. In contrast, photochemistry in Earth's atmosphere yields a distinctive relation among the isotopes that is independent of their mass. Most of the nitrate and other salts in the Atacama appear to originate from conversion of gaseous nitrogen and other compounds to particles in the atmosphere followed by deposition. In wetter places, these salts would be processed further by biological activity. Deposition over the past 2 million years is probably sufficient to have produced the large deposits observed. — BH
Geochim. Cosmochim. Acta 68, 4023 (2004).