Investigations into the Fate and Occurrence of Chlorate in the Environment: Implications for Oxy-Chlorine Species on Mars and Earth

ClO3- occurrence, production, and post depositional transformation has significant implications to our understanding of chlorine (Cl) cycling and potential biogeochemical reactions on Earth and Mars. However, little information is known on the natural isotopic composition of ClO3- and the post-depositional processes that can reduce ClO3- in the environment. The objective of this study was to develop a method to measure the stable isotope composition (δ18O, δ17O and δ37Cl) of ClO3- and to determine the isotopic composition of ClO3- in natural desert salt accumulations that have been studied previously for NO3- and ClO4-. We also determine the potential abiotic transformation of ClO3- by Fe (II)-bearing minerals, similar to known reactions between NO3- and Fe (II) minerals. Additionally, ClO3-, nitrate (NO3-), and ClO4- were evaluated for use as electron acceptors in comparison to oxygen (O2) by comparing oil transformation and mineralization in mesocosms consisting of oiled salt marsh sediment from an area impacted by the BP Horizon oil spill. The isotopic composition of oxyanions can be used to evaluate their production mechanisms and post-depositional alteration. The process of ClO3- purification and analysis of δ18O, δ 17O and δ37Cl is problematic, but has recently been resolved by adapting previously published methods for ClO4-. Competitive anions (e.g. NO3-, Cl-, ClO4-, and SO4-2) are removed through a series of processes including biological reduction, solid phase extraction, and anion or cation exchange and for the first time we report the natural isotopic composition of ClO3- from Death Valley and the Atacama Desert. As the presence of iron-derived minerals has been established in Antarctica, Martian soils, and chondrite meteorites, batch experiments were conducted by reacting four Fe (II)-bearing minerals (wustite, siderite, magnetite, and green rust) with ClO3- at various pH (4.5, 6.5, 8.9). Chlorate reduction was rapid and generally ClO3- was quantitatively converted to Cl-, establishing a previously unknown abiotic reaction that could reduce ClO3-. In order to determine the potential biotic reduction of oxy-anions during oil transformation, mineralization rates were determined by measuring CO2 production and δ13C of the produced CO2 and compared to transformation evaluated by measuring the alkane/hopane ratios over a 4 month period. Oil mineralization was greatest for the aerated treatments and least for the perchlorate amended. Results of this study will increase our understanding of production and surface reactions that produce and transform oxy-chlorine compounds.