Massive atmospheric nitrate accumulation in a continental interior desert, northwestern China

Abstract

Large-scale nitrate deposits are rare on Earth’s surface due to the high solubility of nitrate minerals. Exceptions are found in extremely old (to 14 m.y.) and hyperarid deserts such as the Atacama Desert in northern Chile, or the McMurdo Dry Valleys in Antarctica. The nitrate in both of these regions has been determined to originate from atmospheric oxidation of NOx. Here we report a new type of massive atmospheric nitrate deposit, with resources equivalent to the Atacama deposits, in the Turpan-Hami area, northwestern China. This deposit is characterized by (1) a location in the center of a large continent; (2) young age (Pleistocene); (3) general enrichment near the surface rather than at depth; and (4) high spatial variability in the nitrogen and triple oxygen isotope composition within this arid region, the δ15N ranging from 0.7‰ to 27.6‰, δ18O from 30.2‰ to 46.7‰, and Δ17O from 5.9‰ to 20.7‰. The Turpan-Hami nitrate deposit nitrogen and triple oxygen isotope composition is closely between those of the Mojave Desert (southwestern United States) and the Atacama Desert, suggesting that (1) Earth’s low-middle latitudes (i.e., non-polar sites) have been receiving atmospheric nitrate deposition of similar nitrogen and triple oxygen isotope composition; and (2) local or regional isotope differences in nitrate deposits can be attributed to postdepositional processes, namely the difference in aridity and associated microbial activities.