The Atacama Desert, Chile, one of the oldest and driest deserts on Earth, is unique because it contains the largest known nitrate deposits in the world. The origin of these nitrate deposits has been a mystery since their discovery in the 1800s. There are two possible sources of natural nitrate: microbiological processes and photochemical reactions. The majority of material on Earth follows mass-dependent fractionation between stable oxygen isotopes with the abundance of 17Ο (denoted by δ) as half that of 18O. This relationship is quantified by Δ17O = δ17O – ½ δ18O, where Δ17O=0 for most terrestrial material, including microbial nitrate. Photochemically-produced atmospheric nitrate, however, has a large mass-independent 17O anomaly with Δ17O values of ~23‰. Therefore, a novel stable oxygen isotope analysis of nitrate was performed on soils collected from two Atacama sites to delineate between the two main possible sources of nitrate. The observed Δ17O values of 17.1-20.4‰ at both sites indicate the Atacama nitrate is mainly from the atmosphere, suggesting that microorganisms are severely limited by hyperaridity. However, small nitrate Δ17O variations with depth suggest the relative importance of nitrification may have varied in the past, which is probably related to climate controlled water availability. Both isotopic and geochemical data suggest that the hydrological history differed at the two sampling sites, resulting in different depth profiles of soluble ions and isotopic signals. Overall, atmospheric inputs and water activities play pivotal roles in the Atacama nitrate deposit formation, providing an important basic insight into the nitrogen cycle in hyperarid regions.

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