ISOTOPIC EVIDENCE FOR THE SOURCE OF CA AND S IN SOIL GYPSUM/ANHYDRITE AND CALCITE IN THE ATACAMA DESERT, CHILE
In areas where the Coastal Cordillera is >1200 m in elevation, however, coastal fog cannot penetrate inland and the contribution of marine aerosols to soils is greatly reduced. Most pedogenic salts from inland soils have d34S values between +5.0 to +8.0 and 87Sr/86Sr ratios between 0.7070 and 0.7076. These values are similar to average d34S and 87Sr/86Sr values of salts from local streams, lakes, and salars (+ 5.4 ± 2 d34S and 0.70749 ± 0.00045 87Sr/86Sr) in the Andes and Atacama, suggesting extensive eolian reworking of salar salts onto the surrounding landscape. Ultimately, salar salts are precipitated from evaporated ground water, which has acquired its dissolved solutes from water-rock interactions (both high and low-temperature) along flowpaths from recharge areas in the Andes. Therefore, the main source for Ca and S in gypsum/anhydrite in non-coastal soils is indirect and involves bedrock alteration, not surficially on the hyperarid landscape, but in the subsurface by ground water, followed by eolian redistribution of ground-water derived salar salts to soils. The spatial distribution of high-grade nitrate deposits appears to correspond with areas that receive the lowest fluxes of local marine and salar salt, supporting arguments for tropospheric nitrogen as the main source for soil nitrate.