Role of Rainfall on the Geochemical Evolution of Soils: The Atacama Desert of Chile

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ABSTRACTSoil formation, a process that profoundly impacts global atmospheric and aqueous chemistry, is defined by the balance of mass inputs to, and losses from, the soil system. Observations in humid regions indicate that as soils undergo biogeochemical alteration over geological time scales, weathering losses exceed atmospheric inputs, and soils experience pervasive chemical depletions of biologically essential elements and large volumetric collapse (Vitousek et al., 1997).Yet this pattern hinges on the availability of water for weathering and solute transport, and recent climate comparisons in Hawaii reveal that decreasing precipitation, as might be expected, results in decreased overall mass losses over geological time spans (Chadwick and Goldstein, 2004).However, there is a decided lack of quantitative mass balance research on the soils of arid andhyperarid regions, and at this stage it is difficult to compare the long-term geochemical behavior of these environments with the Earth.s more humid regions.Intellectual merit. This work, when combined with other research, will provide an integrated perspective of the changing roles of atmospheric solutes and both chemical and physical weathering as precipitation decreases. This proposal outlines a chronological study of soils at critical points along a precipitation gradient in the arid to hyper-arid Atacama Desert of Chile that, when combined with previously published data, will reveal how soil formation undergoes fundamental physical and chemical changes with decreasing precipitation. Presently, detailed chemical and isotopic (stable, cosmogenic radionuclides) studies have been initiated at three locations which reveal that, with decreasing precipitation, soils increasingly retain atmospheric solutes and undergo volumetric expansion. However, this work is restricted in both space (climate range) and time (to multi-million year old soils). Funds are requested here to expand this work and develop a rigorous perspective of a pedologically-unexplored region.The key objectives of this project are to: (1) develop four chronological sequences along a latitudinal (precipitation) gradient in northern Chile, (2) quantify chemical and physical properties of the soils and atmospheric deposition along the gradient, and (3) obtain numerical ages for the landforms on which the soils have developed and, to the extent possible, for the soils themselves.These data will allow us to (1) establish rates of chemical and physical changes in the soils with rainfall and (2) add constraints to the geomorphic evolution of fluvial features in this unique desert.Broader impacts. This work will impact science by demonstrating the pervasive importance of atmospheric deposition to the soils of the Earth, and will provide climatic trends on soil chemistry that can be integrated with other studies in humid areas. The project will impact institutions and international scientific exchange because it will be an integrated collaboration between US andChilean scientists, will provide for student exchange and training (Chilean student to US, and US students working with Chilean faculty and graduate students), and will provide research results that will be disseminated within Chile by both U.S. and Chilean colleagues. Finally, the key funded personnel in the project are graduate students, and these students will work closely with the diverse set of collaborators, and the new research techniques that the students (US and Chilean) will be exposed to will put them at the forefront of earth surface research techniques and concepts.