Rates and geochemical processes of soil and salt crust formation in Salars of the Atacama Desert, Chile

Abstract

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 formation. Evaporation, which provides salts to the soils through mineral precipitation, decreases with increasing salt crust thickness, and average~0.03mmm−2d−1 over geological time frames. Salt distribution varies predictably with depth and soil age, with the most soluble compounds concentrated nearest to the land surface, indicating the direction of fluid flow. δ34S 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. δ13C and δ18O values of carbonate suggest that the uppermost halite layers, which contain very small amounts of carbonate, 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 evaporation 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 intrusions, and we found that the halite nodules on the surface of the Salar Llamara are nearly always at or above deliquescence 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 cycling occurs at differing rates depending on local micrometeorological conditions, and that a given salt crust feature may persist for thousands of years once formed.