Rates and geochemical processes of soil and salt crust formation in Salars of the Atacama Desert, Chile Kari Finstad a, ⁎, Marco Pfeiffer a,b , Gavin McNicol a,c , Jaime Barnes d , Cecilia Demergasso e , Guillermo Chong f , Ronald Amundson a a Department of Environmental Science, Policy and Management, University of California, Berkeley, 137 Mulford Hall, Berkeley, CA 94720, USA b Departamento de Ingeniería y Suelos, Facultad de Ciencias Agronómicas, Universidad de Chile, Santa Rosa 11315, La Pintana, Chile c Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA d Department of Geological Sciences, University of Texas, Austin, TX 78712, USA e Centro de Biotecnología, Universidad Católica del Norte, Antofagasta, Chile f Departamento de Ciencias Geológicas, Universidad Católica del Norte, Antofagasta, Chile abstract article info Article history: Received 3 May 2016 Received in revised form 25 August 2016 Accepted 28 August 2016 Available online xxxx The hyperarid Atacama Desert contains numerous local basins with surficial salt crusts, known as salars, where evaporation of shallow groundwater drives the major soil processes. We examine chemical and isotopic profiles in two soils of differing ages from the Salar Llamara to determine the geochemical processes involved in their for- mation. Evaporation, which provides salts to the soils through mineral precipitation, decreases with increasing salt crust thickness, and average ~ 0.03 mm m -2 d -1 over geological time frames. Salt distribution varies predict- ably with depth and soil age, with the most soluble compounds concentrated nearest to the land surface, indicat- ing the direction of fluid flow. δ 34 S values of mineral sulfate tend to decrease with decreasing soil depth, following a pattern indicative of Rayleigh-like fractionation as solute-rich waters migrate toward the land surface. δ 13 C and δ 18 O values of carbonate suggest that the uppermost halite layers, which contain very small amounts of carbon- ate, have a strong biological signature. In contrast, carbonate-rich layers deeper in the profiles consist of largely unmodified lacustrine carbonate that formed in highly evaporitic lake conditions. The continuous upward evap- oration of water and dissolved solutes creates a rugged and physically dynamic halite crust composed of rounded salt nodules. The crust undergoes deliquescence as atmospheric relative humidity rises from marine air intru- sions, and we found that the halite nodules on the surface of the Salar Llamara are nearly always at or above del- iquescence relative humidity. The interiors of these nodules are therefore able to buffer the large diurnal changes in atmospheric relative humidity allowing for the survival of halophilic microbial communities in an otherwise very dry environment. Radiocarbon measurements of occluded organic C in the surface crusts indicate that C cy- cling occurs at differing rates depending on local micrometeorological conditions, and that a given salt crust fea- ture may persist for thousands of years once formed. © 2016 Published by Elsevier B.V. Keywords: Groundwater Salts Evaporation Isotope Salt crust 1. Introduction Geochemical processes in the Atacama Desert are limited by water. Most of the region consists of uplands and alluvial fans dependent on sparse rainfall (b 2 mm y -1 ) to drive soil and geochemical processes. However, the region also contains closed basins that receive both sur- face runoff and subsurface flow from the adjacent High Andes. Once a lake or wetland is desiccated, the evaporation of shallow groundwater by capillary flow promotes an upward movement of solutes, a direction that is the reverse of the normal water trajectory of most desert soils (Finstad et al., 2014). Studies of soil formation in these geological set- tings are very limited, and none have been conducted in northern Chile. In Chile, salt covered evaporitic basins are called salars and are distinguishable by the hard salt crusts commonly found on their sur- faces (Chong, 1984; Ericksen and Salas, 1990). Approximately half of the salars in northern Chile contain halite (NaCl) crusts, an area of N 4000 km 2 (Stoertz and Ericksen, 1974). Most of the salars are locat- ed in the Andes, with only a handful in the Central Depression near to the Pacific coast. Salars in this region of the world are of growing sig- nificance. First, they contribute to an understanding of climate and hydrologic change, providing insight into both regional environmen- tal changes and patterns of occupation by early hunter-gatherers and agriculturalists (Latorre et al., 2013). Second, fluid migration and chemical fractionation have locally deposited economically viable concentrations of iodine, boron, and other salts (Boschetti et al., 2007; Perez-Fodich et al., 2014; Chong et al., 2000). Finally, and somewhat paradoxically, salt crusts on the surface of salars can Geoderma 284 (2016) 57–72 ⁎ Corresponding author. E-mail address: karifinstad@berkeley.edu (K. Finstad). http://dx.doi.org/10.1016/j.geoderma.2016.08.020 0016-7061/© 2016 Published by Elsevier B.V. Contents lists available at ScienceDirect Geoderma journal homepage: www.elsevier.com/locate/geoderma