doi:10.1016/S0016-7037(02)01175-4 Isotopic evidence for the source of Ca and S in soil gypsum, anhydrite and calcite in the Atacama Desert, Chile JASON A. RECH,* ,‡ JAY QUADE, and WILLIAM S. HART Department of Geosciences and Desert Laboratory, University of Arizona, Tucson, AZ 85721 USA (Received November 20, 2001; accepted in revised form August 26, 2002) Abstract—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  34 S and 87 Sr/ 86 Sr values of soil gypsum/anhydrite and 87 Sr/ 86 Sr 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 ( 34 S values  14‰ and 87 Sr/ 86 Sr 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  34 S values between +5.0 to +8.0‰ and 87 Sr/ 86 Sr ratios between 0.7070 and 0.7076. These values are similar to average  34 S and 87 Sr/ 86 Sr values of salts from local streams, lakes, and salars (+5.4 2‰  34 S and 0.70749  0.00045 87 Sr/ 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 © 2003 Elsevier Science Ltd 1. INTRODUCTION The Atacama Desert is likely the oldest and driest desert on earth, optimal for the accumulation of highly soluble soil salts. Moreover, landscape surfaces in the Atacama are exceptionally stable and thought to be largely mid-Miocene in age (Alpers and Brimhall, 1988), virtually unaltered by chemical weather- ing and erosion. Atacama soil salts, composed mainly of sul- fate, nitrate, and chloride salts, are unique on earth (Ericksen, 1981), and their origin has been debated for over a century (see Ericksen, 1981, for citations between 1861 and 1980; van Moort, 1985; Chong, 1988; Alpers and Whittemore, 1990; Searl and Rankin, 1993; Ericksen, 1994; Berger and Cooke, 1997; Bo ¨hlke et al., 1997). Some of the origins suggested by previous researchers include: 1) marine aerosols associated with upwelling in the Pacific Ocean immediately to the west of the Atacama (nitrates, sulfates, chlorides, and iodates) (Erick- sen, 1981; van Moort, 1985; Chong, 1988); 2) dry and wet deposition of extra-local atmospheric aerosols (nitrate, iodate, perchlorate) (Claridge and Campbell, 1968; Ericksen, 1981; Bo ¨hlke et al., 1997); 3) salts derived from the weathering of Andean volcanics (sulfates, chlorides) (Ericksen, 1961; Erick- sen, 1981; Searl and Rankin, 1993, Berger and Cooke, 1997); 4) direct deposition of salts associated with volcanic emissions (sulfates) (Searl and Rankin, 1993; Berger and Cooke, 1997); 5) the decay of bird guano and remains of organic lake deposits (nitrate) (Ericksen, 1983). The importance of desert dust goes well beyond its influence on soil formation in the Atacama Desert. Atmospheric dust originates mainly in deserts and is transported to soils world- wide (Uematsu et al., 1983, 1985; Nakai et al., 1993; Rea, 1994, Tegen and Fung, 1994; Simonson, 1995; Kohfeld and Harrison, 2001). For example, dust from the deserts of Central Asia is transported to Hawaii (Dymond et al., 1974; Parrington et al., 1983) and Greenland (Biscaye et al., 1997), and dust from the Sahara Desert reaches Israel (Dan, 1991), South America (Prospero et al., 1981; Swap et al., 1992), and the Bahamas (Muhs et al., 1987). This fine-grained dust is com- posed of silicate minerals and soluble salts and is thought to play an essential role in sustaining many tropical ecosystems, where plant nutrients are rapidly weathered and removed from soil systems (Chadwick et al., 1999). There is also a component of atmospheric dust (e.g., nitrate), however, that forms directly in the atmosphere by various chemical reactions (Crutzen, 1974; Simonaitis and Heicklen, 1975; Noxon, 1976, 1978). Little is known about the deposition rates of these compounds that are commonly utilized by plants on the land surface or quickly removed by weathering processes. Local dust and aerosol sources, especially in arid regions, can also influence * Author to whom correspondence should be addressed (rechja@muohio.edu). ‡ Present address: Department of Geology, Miami University, Oxford, OH 45056 USA Pergamon Geochimica et Cosmochimica Acta, Vol. 67, No. 4, pp. 575–586, 2003 Copyright © 2003 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/03 $22.00 + .00 575