Abstract

A set of 55 surface samples from Atacama Desert gypsisols was collected along four altitude transects and analyzed for triangle O-17(SO4), delta O-18(SO4), delta S-34(SO4), and Sr-87/Sr-86 in order to identify the main depositional mechanisms and quantify the proportional contribution of sulfate sources across the desert's main topographic units - the Coastal Cordillera, the Central Depression, and the Precordillera. Sulfate deposited at any location in the Atacama Desert may derive from up to five different source and process endmembers that can principally be identified by their isotopic composition. Three aerosol sources dominate sulfate deposition and accumulation in gypsisols throughout the Atacama. These are secondary atmospheric Ca- and Na-sulfate derived from global volcanic SO2 oxidized by ozone and its derivates, and a marine sulfate aerosol that is roughly a 9:1 mixture of sea salt (primary) sulfate from evaporated sea spray, and secondary non-sea salt sulfate, i.e., dimethyl sulfide (DMS) mainly oxidized by ozone. These marine sources dominate sulfate deposition in the Coastal Cordillera. Unlike in most places on Earth, the positive mass-independent triangle O-17(SO4) signature of secondary atmospheric sulfate is ubiquitous in Atacama Desert soils and is never completely erased by biologic cycling. Hence, biologic cycling of the sulfate inventory - i.e., the reduction of sulfate, subsequent oxygen isotope exchange with water, and re-oxidation - is apparently not quantitative in any of the Atacama Desert environments sampled here, including the most biologically active permanent salt lakes. The preservation of highest triangle O-17(SO4) values (similar to 1.0 parts per thousand) at altitudes of the Coastal Cordillera above the 1200 m altitude cut-off level of fog advection suggest that episodic rainfall events are insufficient for significant in situ biologic sulfate turnover. At lower altitudes and regular fog advection, moisture supply appears to be sufficient for limited in situ biologic sulfate turnover. Overall, biologically recycled sulfate constitutes a fourth major contributor to the desert's gypsisol sulfate inventory. This may comprise fluvially redeposited sulfate from older sediments, particularly on the debris fans protruding from the Precordillera into the Central Depression. In the Central Depression, Sr-isotopes may allow for a further distinction between redeposition-dominated and aerosol-dominated sedimentation environments. On the Precordillera, rainfall and run-off provide sufficient moisture for significant but still incomplete in situ biologic sulfate cycling. Hydrothermal non-evaporitic Na-sulfate - the fifth potential source - likely originates from larger outflow events or mobilization from rock veins over the course of the Atacama's geologic history may contribute locally to the sulfate inventory. The degree of bio-cycling and mobility of sulfate likely reflects a balance between overall water availability and the mixing proportion of very soluble (mobile) Na-sulfate and less soluble Ca-sulfate inherent in the various sulfate sources.