Natural perchlorate has a unique oxygen isotope signature

Perchlorate is known to be a minor component of the hyperarid Atacama Desert salts, and its origin has long been a subject of speculation. Here we report the first measurement of the triple-oxygen isotope ratios (O-18/O-16 and O-17/O-16) for both man-made perchlorate from commercial sources and natural perchlorate extracted from Atacama soils. We found that the 6180 values (i.e., normalized O-18/ 160 ratios) of man-made perchlorate were at -18.4 1 1.2%., whereas natural perchlorate has a variable delta(18)O value, ranging from -4.5%, to -24.8%o. The delta(18)O and delta(17)O values followed the bulk Earth's oxygen isotope fractionation line for man-made perchlorate, but all Atacama perchlorates deviated from this line, with a distinctly large and positive O-17 anomaly ranging from +4.2%o to +9.6%o. These findings provide a tool for the identification and forensics of perchlorate contamination in the environment. Additionally, they confirm an early speculation that the oxidation of volatile chlorine by O-3 and the formation of HClO4 can be a sink (albeit a minor one) for atmospheric chlorine.