Origin of the isotopic composition of natural perchlorate: Experimental results for the impact of reaction pathway and initial ClO_x reactant

Natural perchlorate (ClO₄⁻) exists in many places on Earth, in lunar regolith, meteorites, and on the surface of Mars. Terrestrial natural ClO₄⁻ has widely variable Cl and O stable isotopic compositions (δ³⁷Cl, δ¹⁸O, Δ¹⁷O). The δ¹⁸O and Δ¹⁷O values of ClO₄⁻ from the most hyper-arid locations co-vary. ClO₄⁻ from less arid areas has relatively little ¹⁷O excess and poor Δ¹⁷O-δ¹⁸O correlation. ClO₄⁻ from the Atacama Desert has unusually low δ³⁷Cl (<−10‰) and exhibits a positive correlation between δ³⁷Cl and δ¹⁸O, while the δ³⁷Cl of ClO₄⁻ from all other locations varies between −5 and +7‰ with no δ³⁷Cl-δ¹⁸O covariation. To evaluate the impact of different precursors (ClO_x) and reaction pathways on the isotopic composition of ClO₄⁻, we measured the isotopic composition of ClO₄⁻ produced in the laboratory by UV or O₃ mediated aqueous oxidation of Cl⁻, OCl⁻, ClO₂⁻, and ClO₂° as well as O₃ mediated oxidation of dry NaCl. ClO_x oxidation in aqueous or dry systems enriched in O₃ produced ClO₄⁻ with Δ¹⁷O values that generally increased with the number of O atoms required and included evidence that the site-specific ¹⁷O anomaly in O₃ was preferentially transferred to ClO₄⁻. Based on the inferred number of O atoms sourced from O₃, and known Cl and O reaction pathways, it appears that ClO₂° and ClO₃* were required intermediates in the production of ClO₄⁻ in the O₃ experiments. ClO_x aqueous oxidation by UV irradiation produced ClO₄⁻ with a large range of δ¹⁸O values and little or no ¹⁷O anomaly. ClO₃⁻ was produced to a much greater extent than ClO₄⁻ in all experiments except dry oxidation of NaCl by O₃. The isotopic composition of ClO₃⁻ was distinct from that of ClO₄⁻ produced from the same initial reactants. Combined results of O₃ and UV mediated reactions largely bracketed the range of natural ClO₄⁻ δ¹⁸O and Δ¹⁷O values as well as δ³⁷Cl values of non-Atacama natural samples, but no conditions produced the low δ³⁷Cl values of Atacama ClO₄⁻. Our results indicate that variation in production mechanisms, possibly combined with isotopically variable precursors, could be responsible for much of the observed isotopic variation in natural ClO₄⁻ and ClO₃⁻.