Global patterns and environmental controls of perchlorate and nitrate co-occurrence in arid and semi-arid environments

Geochimica et Cosmochimica Acta
By: , and 

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Abstract

Natural perchlorate (ClO4) is of increasing interest due to its wide-spread occurrence on Earth and Mars, yet little information exists on the relative abundance of ClO4 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 ClO4 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 ClO4 to the more well-studied atmospherically deposited anions NO3and Cl as a means to understand the prevalent processes that affect the accumulation of these species over various time scales. ClO4 is globally distributed in soil and groundwater in arid and semi-arid regions on Earth at concentrations ranging from 10−1to 106 μg/kg. Generally, the ClO4 concentration in these regions increases with aridity index, but also depends on the duration of arid conditions. In many arid and semi-arid areas, NO3 and ClO4 co-occur at molar ratios (NO3/ClO4) that vary between ∼104and 105. 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 N2 fixation, N mineralization, nitrification, denitrification, and microbial ClO4 reduction, as indicated in part by NO3 isotope data. In contrast, much larger ranges of Cl/ClO4 and Cl/NO3ratios indicate Cl varies independently from both ClO4 and NO3. The general lack of correlation between Cl and ClO4 or NO3 implies that Cl 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 NO3/ClO4 molar ratio ∼103. The relative enrichment in ClO4compared to Cl or NO3 and unique isotopic composition of Atacama ClO4 may reflect either additional in-situ production mechanism(s) or higher relative atmospheric production rates in that specific region or in the geological past. Elevated concentrations of ClO4 reported on the surface of Mars, and its enrichment with respect to Cl and NO3, could reveal important clues regarding the climatic, hydrologic, and potentially biologic evolution of that planet. Given the highly conserved ratio of NO3/ClO4 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 NO3 pool terrestrially.

Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Global patterns and environmental controls of perchlorate and nitrate co-occurrence in arid and semi-arid environments
Series title Geochimica et Cosmochimica Acta
DOI 10.1016/j.gca.2015.05.016
Volume 164
Year Published 2015
Language English
Publisher Elsevier
Contributing office(s) National Research Program - Eastern Branch, Nevada Water Science Center, Toxic Substances Hydrology Program
Description 21 p.
First page 502
Last page 522