J.K. Bohlke Brian J. Andraski Lynne S. Fahlquist Laura M. Bexfield Frank D. Eckardt John B. Gates Alfonso F. Davila Christopher P. McKay Balaji Rao Ritesh Sevanthi Srinath Rajagopalan Nubia Estrada Neil C. Sturchio Paul B. Hatzinger Todd A. Anderson Greta J. Orris Julio L. Betancourt David A. Stonestrom Claudio Latorre Yanhe Li Gregory J. Harvey W Andrew Jackson 2015 <p><span>Natural perchlorate (ClO</span>₄⁻<span>) is of increasing interest due to its wide-spread occurrence on Earth and Mars, yet little information exists on the relative abundance of ClO</span>₄⁻<span> compared to other major anions, its stability, or long-term variations in production that may impact the observed distributions. Our objectives were to evaluate the occurrence and fate of ClO</span>₄⁻<span> in groundwater and soils/caliche in arid and semi-arid environments (southwestern United States, southern Africa, United Arab Emirates, China, Antarctica, and Chile) and the relationship of ClO</span>₄⁻<span> to the more well-studied atmospherically deposited anions NO</span>₃⁻<span>and Cl</span>⁻<span> as a means to understand the prevalent processes that affect the accumulation of these species over various time scales. ClO</span>₄⁻<span> is globally distributed in soil and groundwater in arid and semi-arid regions on Earth at concentrations ranging from 10</span>⁻¹<span>to 10</span>⁶<span>&nbsp;μg/kg. Generally, the ClO</span>₄⁻<span> concentration in these regions increases with aridity index, but also depends on the duration of arid conditions. In many arid and semi-arid areas, NO</span>₃⁻<span> and ClO</span>₄⁻<span> co-occur at molar ratios (NO</span>₃⁻<span>/ClO</span>₄⁻<span>) that vary between ∼10</span>⁴<span>and 10</span>⁵<span>. We hypothesize that atmospheric deposition ratios are largely preserved in hyper-arid areas that support little or no biological activity (e.g. plants or bacteria), but can be altered in areas with more active biological processes including N</span>₂<span> fixation, N mineralization, nitrification, denitrification, and microbial ClO</span>₄⁻<span> reduction, as indicated in part by NO</span>₃⁻<span> isotope data. In contrast, much larger ranges of Cl</span>⁻<span>/ClO</span>₄⁻<span> and Cl</span>⁻<span>/NO</span>₃⁻<span>ratios indicate Cl</span>⁻<span> varies independently from both ClO</span>₄⁻<span> and NO</span>₃⁻<span>. The general lack of correlation between Cl</span>⁻<span> and ClO</span>₄⁻<span> or NO</span>₃⁻<span> implies that Cl</span>⁻<span> is not a good indicator of co-deposition and should be used with care when interpreting oxyanion cycling in arid systems. The Atacama Desert appears to be unique compared to all other terrestrial locations having a NO</span>₃⁻<span>/ClO</span>₄⁻<span> molar ratio ∼10</span>³<span>. The relative enrichment in ClO</span>₄⁻<span>compared to Cl</span>⁻<span> or NO</span>₃⁻<span> and unique isotopic composition of Atacama ClO</span>₄⁻<span> may reflect either additional </span><i>in-situ</i><span> production mechanism(s) or higher relative atmospheric production rates in that specific region or in the geological past. Elevated concentrations of ClO</span>₄⁻<span> reported on the surface of Mars, and its enrichment with respect to Cl</span>⁻<span> and NO</span>₃⁻<span>, could reveal important clues regarding the climatic, hydrologic, and potentially biologic evolution of that planet. Given the highly conserved ratio of NO</span>₃⁻<span>/ClO</span>₄⁻<span> in non-biologically active areas on Earth, it may be possible to use alterations of this ratio as a biomarker on Mars and for interpreting major anion cycles and processes on both Mars and Earth, particularly with respect to the less-conserved NO</span>₃⁻<span> pool terrestrially.</span></p> application/pdf 10.1016/j.gca.2015.05.016 en Elsevier Global patterns and environmental controls of perchlorate and nitrate co-occurrence in arid and semi-arid environments article