ISOTOPIC FRACTIONATION OF ATMOSPHERICALLY DERIVED OXYANIONS IN SOILS ALONG A RAINFALL GRADIENT IN THE ATACAMA DESERT, CHILE

Soils of the Atacama Desert have been mined for their oxyanions. Recent work has established that the large inventories of nitrates and sulfates in these soils must be derived from atmospheric photochemistry, because they possess a characteristic "mass independent" oxygen isotope signal (Δ¹⁷O). We here present results of atmospheric and soil nitrate and sulfate analysis, showing that the rate and isotopic composition of solute deposition is relatively constant along a broad latitudinal (rainfall) gradient in northern Chile, from the arid margin to the hyperarid core of the present Atacama Desert. Despite constant oxyanion inputs along the rainfall gradient, nitrate concentrations in the soils themselves vary greatly: from 0.05 kg nitrate m^-2 and 60 kg sulfate m^-2 at 21 mm rain y^-1, to 50 kg nitrate m^-2 and 1300 kg sulfate m^-2 at ≤2 mm rain y^-1. Nitrate deposition at these sites has a mean Δ¹⁷O value of 23.0‰, in agreement with the observed range for airborne nitrate elsewhere (20-30‰). Profile average nitrate Δ¹⁷O values range from 9‰ at 21 mm y^-1, to 18‰ at ≤2 mm y^-1, the latter value representing about 80% pure atmospheric nitrate. The nitrate Δ¹⁷O values indicate that soil N processes in the core of the Atacama Desert have been dominantly abiotic since the late Pliocene. Sulfate Δ¹⁷O values are nearly constant (0.4-0.7‰) in all soils, indicating that a consistent and significant fraction of soil sulfate is the direct result of photochemically driven oxidation and does not vary with average rainfall. At the same time, enormous depth dependent variations in δ³⁴S and δ¹⁸O of sulfate in the driest soil span the entire range of values found anywhere in the desert. We show that these varations with depth are due to mass dependent fractionation with dissolution and reprecipitation during downward transport. These depth variations reveal that δ³⁴S and δ¹⁸O of sulfate in these areas are not conservative tracers of source (only Δ¹⁷O has that attribute), but instead provide important information about the direction and magnitude of water-driven oxyanion transport during hyperarid soil development.