Production and Post Depositional Transformation of (Per)chlorate on Earth and Mars
Perchlorate (ClO4-) and chlorate (ClO3-) are ubiquitous throughout the terrestrial surface environment but only accumulate in terrestrial hyper-arid areas. ClO3- and ClO4- are also present in meteorites, lunar regolith and on the surface of Mars, where in the latter case concentrations equal are in excess of the maximum concentrations observed terrestrially. Terrestrial ClO4- isotopic composition supports a stratospheric source including variable involvement of O3-mediated oxidation reactions and perhaps other non-O3 photochemical pathways. Terrestrial ClO3- isotopic composition suggests only a portion of ClO3- is atmospheric in origin. Heterogeneous photochemical reactions may also occur at the Earth’s surface, although their overall significance is unclear but may play a larger role on the surface of Mars. ClO3-/ClO4- molar ratios terrestrially and extra-terrestrially are typically >1:1, except in cases where ClO3- has been preferentially lost due to biotic or abiotic post depositional transformation. Terrestrially in arid areas and on Mars, the occurrence of (per)chlorate is significantly correlated to the occurrence of NO3- albeit at vastly lower NO3-/ClO4- ratios on Mars compared to Earth. This correlation is likely related to common atmospheric production mechanisms and/or similar biogeochemical stability in areas devoid of plants. (Per)chlorate isotopic composition can be further used to constrain the potential for post depositional transformation and in some cases the time of accumulation based on either mass or reductions of 36Cl/Cl ratios in accumulated ClO4-. (Per)chlorate occurrence and variation in post depositional stability can be used to evaluate biogeochemical cycling of Cl and other oxy-anions (e.g. NO3-) on Earth and may aid in interpreting (per)chlorate and NO3- occurrence and activity on Mars. Of particular relevance are data from Mars Earth analogues. Data obtained on (per)chlorate from the McMurdo Dry Valley soils suggest that ClO3- but not ClO4- may be abiotically transformed in reactions that could be relevant to ancient Mars while biotic transformation in MDV lakes may be useful in predicting the potential for life in other extreme environments.