Isotopic evidence for the source of Ca and S in soil gypsum, anhydrite and calcite in the Atacama Desert, Chile

The origin of pedogenic salts in the Atacama Desert has long been debated. Possible salt sources include in situ weathering at the soil site, local sources such as aerosols from the adjacent Pacific Ocean or salt-encrusted playas (salars), and extra-local atmospheric dust. To identify the origin of Ca and S in Atacama soil salts, we determined delta(34)S and Sr-87/Sr-86 values of soil gypsum/anhydrite and Sr-87/Sr-86 values of soil calcite along three east-west trending transects. Our results demonstrate the strong influence of marine aerosols on soil gypsum/anhydrite development in areas where marine fog penetrates inland. Results from an east-west transect located along a breach in the Coastal Cordillera show that most soils within 90 km of the coast, and below 1300 m in elevation, are influenced by marine aerosols and that soils within 50 km, and below 800 m in elevation, receive >50% of Ca and S from marine aerosols (delta(34)S values > 14parts per thousand and Sr-87/Sr-86 values >0.7083). 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 delta(34)S values between + 5.0 to + 8.0parts per thousand and Sr-87/Sr-86 ratios between 0.7070 and 0.7076. These values are similar to average delta S-34 and Sr-87/Sr-86 values of salts from local streams, lakes, and salars (+5.4 +/-2% delta(34)S and 0.70749 +/- 0.00045 Sr-87/(86) Sr) 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. Copyright (C) 2003 Elsevier Science Ltd.