Isotopic composition of natural and synthetic chlorate (δ18O, Δ17O, δ37Cl, 36Cl/Cl): Methods and initial results

Abstract Natural chlorate (ClO3−) is widely distributed in terrestrial and extraterrestrial environments. To improve understanding of the origins and distribution of ClO3−, we developed and tested methods to determine the multi-dimensional isotopic compositions (δ18O, Δ17O, δ37Cl, 36Cl/Cl) of ClO3− and then applied the methods to samples of natural nitrate-rich caliche-type salt deposits in the Atacama Desert, Chile, and Death Valley, USA. Tests with reagents and artificial mixed samples indicate stable-isotope ratios were minimally affected by the purification processes. Chlorate extracted from Atacama samples had δ18O = +7.0 to +11.1‰, Δ17O = +5.7 to +6.4‰, δ37Cl = −1.4 to +1.3‰, and 36Cl/Cl = 48 × 10−15 to 104 × 10−15. Chlorate from Death Valley samples had δ18O = −6.9 to +1.6‰, Δ17O = +0.4 to +2.6‰, δ37Cl = +0.8 to +1.0‰, and 36Cl/Cl = 14 × 10−15 to 44 × 10−15. Positive Δ17O values of natural ClO3− indicate that its production involved reaction with O3, while its Cl isotopic composition is consistent with a tropospheric or near-surface source of Cl. The Δ17O and δ18O values of natural ClO3− are positively correlated, as are those of ClO4− and NO3− from the same localities, possibly indicating variation in the relative contributions of O3 as a source of O in the formation of the oxyanions. Additional isotopic analyses of ClO3− could provide stronger constraints on its production mechanisms and/or post-formational alterations, with applications for environmental forensics, global biogeochemical cycling of Cl, and the origins of oxyanions detected on Mars.