Different sulfate sources and their post-depositional migration in Atacama soils

Sulfate is one of the major salts accumulated in the hyperarid Atacama Desert, Chile. Its origin has been attributed to local weathering, volcanic source, sea salt, and atmosphere deposition. Previous studies concluded that atmospheric sulfate is present in the desert, but this term was poorly defined in terms of chemical origin, and its contribution to the total sulfate budget in soil is debated. The contribution of each potential sulfate source is expected to vary geographically due to different source locations. Efforts to quantify the contribution from each source have been hampered by the lack of well-defined isotopic compositions for its end-members. In this study, we discard the vague term of "atmospheric sulfate" by recognizing two major sulfate sources on the basis of their formation pathways: primary and secondary sulfate. Primary sulfate, including those from rock weathering, sea salt, and volcanic ash leachates, do not have an O-17 anomaly. Secondary sulfate, derived from the oxidation of reduced sulfur gases (biological or volcanic emissions) by atmospheric ozone or hydrogen peroxide, however, has an O-17 anomaly. We developed a method to extract water-soluble anions sequentially or entirely from bulk soil samples, and analyzed the sulfate concentrations and O-17 anomalies for four geographically representative soil profiles in the central Atacama Desert (23degreesS to 25.5degreesS). We found that sequential sulfate extractions on the same soil sample produce sulfate with decreasing Delta(17)O values. Except for a positive bulge at 20 to 50 cm in depth, there is a general increase in the Delta(17)O value for total sulfate with soil depth for all the four soil profiles that we examined. These findings are consistent with the hypothesis that secondary sulfates (O-17-anomalous sulfates) are more soluble and thus preferentially leached in liquid water than primary sulfate. The finding also warns that incomplete sulfate extraction from sulfate-rich soils yields unpredictable results. We infer a Delta(17)O value of similar to4.6parts per thousand for secondary sulfate based on data from partial sulfate extraction experiments. Total contribution of secondary sulfate in the central Atacama Desert is estimated to range from 9% to 24%, with the highest percentage in the Central Depression. (C) 2004 Elsevier B.V. All rights reserved.