87Sr/86Sr of Ca-sulfates and water in Atacama Desert as tracer for Ca sources and hydrological distribution processes

Abstract

The Atacama Desert, known as the driest region on Earth, has accumulated substantial salt deposits owing to hyperarid conditions prevailing since the Miocene. These salt deposits mainly include Ca-sulfates and halite, but are also known for their large nitrate deposits. This study focuses on the sources of Ca-sulfates, which occurs in a great mineralogical and compositional diversity. Previous research argued that main sources contributing sulfate to the Atacama salt budget include secondary atmospheric sulfate and biologically recycled sulfate. Secondary atmospheric sulfate originates mostly from marine and atmospheric sources. Additionally, sources delivering Ca into the Atacama Desert include weathered material from the Andes, which is transported by rivers and groundwater flow into the Central depression and to a somewhat lesser extent also marine aerosols. However, there are still open issues, in particular with respect to the elemental mass balances of Ca and sulfur that are different because of contrasting relative abundances in seawater derived components and weathering products. This study uses ⁸⁷Sr/⁸⁶Sr isotope compositions of both lacustrine as well as pedogenic Ca sulfates and water samples to further investigate sources and transport pathways of Ca within the Atacama Desert. Two case studies from the driest portion of the Atacama Desert, namely the Tiliviche basin (−19.5°S) and the Quillagua-Llamara-Basin (−21.5°S), are investigated in detail. The sulfate samples and their detrital impurities were dissolved utilizing a newly developed method for selective digestion of Ca-sulfates, using anion exchange resin. This method allows for rapid dissolution of Ca-sulfate samples without affecting inherent siliciclastic material. The water samples analyzed exhibit a moderately radiogenic range in ⁸⁷Sr/⁸⁶Sr of ∼0.7063 to ∼0.7075. Fossil salar Ca-sulfates display less variation, ranging from ∼0.7065 to ∼0.7072, whereas pedogenic samples diverge toward higher values, spanning a range from ∼0.7067 to ∼0.7081. Siliciclastic particles, incorporated within the Ca-sulfates, span a wider ⁸⁷Sr/⁸⁶Sr range from ∼0.7058 to ∼0.7095.

The relatively narrow ⁸⁷Sr/⁸⁶Sr range of the water samples contrasts with the variable Sr isotope composition of the Andean basement. This observation is attributed to mixing and efficient pooling, arising from larger scale water-rock interaction with diverse geological units. The even narrower range observed for lacustrine Ca-sulfate samples likely results from additional homogenization processes, both laterally and vertically, driven by erosion and dissolution-reprecipitation of Ca-sulfates and other salts. In marked contrast, pedogenic Ca-sulfate samples diverge to more radiogenic Sr isotope values. This reflects more local patterns and is indicative for the presence of a high-⁸⁷Sr/⁸⁶Sr endmember other than sea spray. In the two case studies investigated, Jurassic limestones in the Coastal Cordillera appear as plausible high-⁸⁷Sr/⁸⁶Sr sources. Moreover, the ⁸⁷Sr/⁸⁶Sr composition of water sources and fossil salar Ca-sulfates in the Tiliviche basin allows identifying past hydrologic networks with different catchments. For example, ⁸⁷Sr/⁸⁶Sr ratios of fossil lacustrine Ca-sulfates and recent water courses in the Tiliviche basin indicate a former hydrologic connection between two different drainage systems, the Tana/Tiliche drainage and the Aroma drainage.