Sources, sinks and long-term cycling of iodine in the hyperarid Atacama continental margin

Abstract

The Atacama region in northern Chile hosts the driest desert on Earth and is the world’s premier iodine production province. The origin of iodine enrichment in Atacama is controversial and fundamentally different processes have been invoked over the years that involve marine, eolian and more recently deep sedimentary fluid and groundwater sources. As a result of the very limited geochemical iodine data in Atacama and the western South American margin, the origin of iodine enrichment in this region still remains elusive. In this study, we present a comprehensive survey of iodine concentrations and isotopic ratios (129I/I) of different reservoirs in the Atacama Desert of northern Chile, including nitrate soils, supergene copper deposits, marine sedimentary rocks, geothermal fluids, groundwater and meteoric water. Nitrate soils along the eastern slope of the Coastal Cordillera are found to have mean iodine concentrations of at least three orders of magnitude higher than the mean crustal abundances of 0.12 ppm, with a mean concentration of 700 ppm. Soils above giant copper deposits in the Central Depression are also highly enriched in iodine (100’s of ppm range), and Cu-iodide and iodate minerals occur in the supergene enrichment zones of some of these deposits. Further east in the Precordillera, Jurassic sedimentary shales and limestones show above-background iodine concentrations, the latter averaging 50 ppm in the southern portion of the study area. The highest iodine concentrations in fluids were measured in groundwater below nitrate soils in the Coastal Range ( 3.5–10 ppm) and in geothermal waters (1–3 ppm) along the volcanic arc. Although highly variable, the iodine isotopic ratios (129I/I) of Jurassic marine sedimentary rocks ( 300–600 10 15), nitrate soils ( 150–1500 10 15) and waters ( 215 10 15) are consistently low (<1500 10 15), indicating that recent anthropogenic additions are almost negligible in most surficial and deeper reservoirs. Geochemical mixing models reveal that the measured 129I/I ratios in Atacama are in agreement with multiple sources of iodine that include variable contributions from old organic iodine sources (i.e., marine sedimentary rocks) and younger fluids such as pore waters, geothermal fluids and meteoric waters. Our results show that the large variation observed in the iodine isotopic ratios of different reservoirs (129I/I from 150 to 1580 10 15) is indicative of significant mixing and circulation of fluids of meteoric, sedimentary and volcanic origin along the Chilean continental margin in the last 30 million years. We conclude that this protracted and large-scale fluid flow was driven by tectonic uplift and highly influenced by the climatic history of the Atacama Desert. The combination of such factors has played an unforeseen role in transporting and accumulating iodine and other soluble components in the Atacama region, and is evidence that elemental remobilization is a key process in the overall crustal cycle of iodine over scales of millions of years.