The cosmogenic iodine and stable chromium isotopic signal of Atacama's giant nitrate deposits (Invited)

Massive nitrate accumulations on Earth's surface are scarce and its existence is restricted largely to hyperarid environments like the Atacama Desert in northern Chile, with minor amounts of nitrate present in other desert environments. The nitrate deposits of Atacama are unique because of their extension but also due to the presence of iodates, chromates and perchlorates, in a complex soil/mineral layer composed of nitrates, sulfates and chlorides. Although several hypotheses have been proposed, the formation of these deposits still remains highly controversial. Oxygen and nitrogen isotope data indicate that there is a significant atmospheric component in nitrate formation, while sulfur and oxygen data in sulfates indicate an existence of atmospheric, terrestrial and marine sources. Perchlorate has a distinct atmospheric signature, and has raised attention as it is closest in abundance to perchlorate measured at NASA's Phoenix Lander site in Mars. In this report we focus on the iodine and chromium components of Atacama's nitrates. Iodine occurrence is so exceptional that these deposits are currently the world's prime source for iodine (100-1000's ppm abundance). Chromium is widespread in the Atacama nitrates in the 10's of ppm range but can also be sufficiently enriched to form chromate-bearing minerals. We present the first cosmogenic iodine and stable chromium isotope data of nitrates from the hyperarid core of the Atacama Desert. The isotopic ratios of iodine in the nitrates are low (¹²⁹I/I~150-600×10^-15) and deviate significantly from atmospheric and sea spray sources (¹²⁹I/I~1500×10^-15), sharing similarities with shales that form part of the Jurassic marine basin at the eastern border of the Atacama Desert. The positive and highly fractionated δ^53/52Cr_SRM979 values of nitrates (+0.050 to +3.088‰) deviate from Andean rock reservoirs (-0.192 to -0.078‰) and are comparable to groundwater δ^53/52Cr values from the Mojave Desert, pointing to chromium redox cycling due to groundwater transport. These results strongly suggest that groundwater played a major role in leaching and transporting iodine and oxidized chromium from sedimentary rocks and volcanic sequences in the high Andes to the Central Basin. The precipitation of iodates and chromates was probably favored downstream by the 'impermeable' barrier effect of the Coastal Range that allowed saline and reduced groundwater to efficiently rise, evaporate and oxidize in the presence of a continuous flux of atmospheric nitrate/perchlorate and excursions of sea spray sulfate/chloride from the Pacific Ocean, under increasing desiccation and tectonic uplift conditions. These results not only shed new lights on Atacama's nitrate formation but also provide new insights on the potential large-scale effects of groundwater flow in Earth and Mars.