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

土壤形成是一个深刻影响全球大气和水化学的过程，由土壤系统的质量输入和损失的平衡来定义。湿润地区的观察表明，随着土壤在地质时间尺度上经历生物地球化学变化，风化损失超过了大气输入，土壤经历了生物必需元素的普遍化学耗竭和大体积崩塌(Vitousek等人，1997)。但这种模式取决于风化和溶质运输所需的水的可用性，最近夏威夷的气候比较显示，正如可能预期的那样，降水减少导致总质量损失在地质时间跨度内减少(Chadwick和Goldstein，2004)。然而，对于干旱和极度干旱地区的土壤，确实缺乏定量的质量平衡研究，在这个阶段，很难将这些环境的长期地球化学行为与地球上更潮湿的地区进行比较。这项工作与其他研究相结合，将为大气溶质作用的变化以及随着降雨量减少而发生的化学和物理风化提供一个综合的视角。这项建议概述了智利干旱到极度干旱的阿塔卡马沙漠沿降水梯度临界点的土壤年代学研究，当与以前公布的数据相结合时，将揭示土壤形成如何随着降雨量的减少经历基本的物理和化学变化。目前，已经在三个地点开始了详细的化学和同位素(稳定的、宇宙成因的放射性核素)研究，这些研究表明，随着降雨量的减少，土壤越来越多地保留大气中的溶质并经历体积膨胀。然而，这项工作受到空间(气候范围)和时间(数百万年前的土壤)的限制。这个项目的主要目标是：(1)沿着智利北部的纬度(降水)梯度开发四个时间序列，(2)量化土壤的化学和物理性质以及沿梯度的大气沉积，(3)获得土壤发育的地貌的数字年龄，并尽可能地，对于土壤本身，这些数据将允许我们(1)建立土壤中随降雨而发生的化学和物理变化的速率，以及(2)增加对这一独特沙漠中河流特征的地貌演变的约束。这项工作将通过展示大气沉积对地球土壤的普遍重要性来影响科学，并将提供土壤化学的气候趋势，可与潮湿地区的其他研究相结合。该项目将影响机构和国际科学交流，因为它将是美国和智利科学家之间的综合合作，将提供学生交换和培训(智利学生对美国，以及美国学生与智利教职员工和研究生合作)，并将提供将由美国和智利同事在智利境内传播的研究成果。最后，该项目的关键资助人员是研究生，这些学生将与不同的合作者密切合作，学生(美国和智利)将接触到的新研究技术将使他们处于地球表面研究技术和概念的前沿。

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.