87Sr/86Sr in recent accumulations of calcium sulfate on landscapes of hyperarid settings: A bimodal altitudinal dependence for northern Chile (19.5°S–21.5°S)

An elevation-dependent relationship of the 87Sr/86Sr ratio of Holocene surface accumulations of sulfate salts is demonstrated for a continental margin hyperarid setting. In the Atacama Desert of north- ern Chile, gypsum and anhydrite of multiple origins exist widely on superficial materials that originated dur- ing the last 10,000 years. An important source of calcium sulfate is from offshore-generated stratocumulus clouds that are advected onto the continent, where they generate fog that transfers water droplets to the ground surface which, upon evaporation, leaves calcium sulfate crystals. Meteorological measurements of the cloud base and top altitudes average 400 m and 1100 m above sea level (masl), respectively. The seawater ratio of 87Sr/86Sr (0.70917) is distinctively higher than that reported for weathered mean Andean rock (less than 0.70750). Samples of 28 modern surface salt accumulations for locations between 200 and 2950 masl and between 198300 and 218300S verify that 87Sr/86Sr varies as a function of site altitude. Sites below 1075 masl and above 225 masl display calcium sulfate 87Sr/86Sr of mean value 0.70807 6 0.00004, while the ratio outside this altitudinal domain is 0.70746 6 0.00010. Thus, the 87Sr/86Sr ratio of Holocene salt accumulations differentiates two altitudinal domains.