Understanding controls on soil formation: tracing and dating weathering transformations using U series disequilibria

土壤是生物圈、岩石圈、水圈和大气界面上极其重要的自然资源。土壤的形成对地表水、海洋和大气的化学性质有着深远的影响，然而土壤形成的速率和过程还没有得到很好的理解。U系列同位素不平衡对了解土壤形成及其相关的地球化学通量具有重要的潜力，包括土壤形成对二氧化碳的吸收、向生态系统提供岩石来源的营养物质以及向海洋输送溶质和沉积物。这个项目研究了土壤形成速率如何随时间和气候的变化而变化。该项目将提供U系列不平衡方法的首次全面应用，通过检查已知母质年龄的土壤剖面，这些土壤剖面经历了最小的侵蚀。该项目将通过研究不同母质年龄、不同景观位置和相同母质的降水梯度土壤的变化来阐明对土壤形成速率和过程的控制。这项拟议中的研究将利用夏威夷先前建立的土壤研究点。夏威夷是进行这项研究的理想的自然实验室，因为它是一个独特的环境，可以研究梯度，只有一个主要的土壤形成因素变化，而其他因素保持不变。利用夏威夷遗址已有的详细的物理和化学特征，结合基于U系列不平衡的风化时间尺度和强度的测定，我将检验土壤形成速率与降水和侵蚀速率正相关的假设，以及土壤形成速率在土壤的生命周期中按数量级下降的假设。牛津大学的主持人吉迪恩·亨德森博士拥有使用U系列同位素作为示踪剂的丰富经验，并在了解土壤形成的基本控制方面有共同的兴趣。在这项研究中收集的数据和发展的想法将提供年龄和气候在控制土壤形成和风化速率方面的作用的全面检查。更好地了解对土壤形成的控制将改善全球碳循环模型，提高我们预测全球变化影响的能力。

Project summary Understanding controls on soil formation: tracing and dating weathering transformations using U-series disequilibria Soils are critically important natural resources at the interface of the biosphere, lithosphere, hydrosphere, and atmosphere. Soil formation has a profound impact on the chemistry of surface waters, the oceans, and the atmosphere, yet the rates and processes involved in soil formation are not well understood. U series isotope disequilibria has important potential for understanding soil formation and its associated geochemical fluxes, including the uptake of CO2 by soil formation, the supply of rock-derived nutrients to ecosystems, and the delivery of solutes and sediment to the oceans. This project investigates how soil formation rates change over time and as a function of climate. This project will provide the first thorough application of the U series disequilibria approach by examining previously well-characterized soil profiles of known parent material age that have experienced minimal erosion. This project will elucidate controls on soil formation rates and processes by examining variations across soils of different parent material ages, different landscape positions, and across precipitation gradients with identical parent material. The proposed research will take advantage of previously established soil research sites in Hawaii. Hawaii is the ideal natural laboratory to do this research because it is an environment where it is uniquely possible to study gradients where only one of the major soil forming factors varies while the others are held constant. Using detailed physical and chemical characterization already available for the Hawaii sites in combination with determinations of the weathering timescale and intensity based on U series disequilibria, I will test the hypotheses that soil formation rates are positively correlated with precipitation and erosion rate, and that soil formation rates decline by orders of magnitude over the lifetime of a soil. The Oxford host, Dr. Gideon Henderson, has extensive experience employing U series isotopes as tracers, and shares a mutual interest in understanding the fundamental controls on soil formation. Data collected and ideas developed in this study will provide a comprehensive examination of the role of age and climate in controlling soil formation and weathering rates. Better understanding of the controls on soil formation will improve global carbon cycle models and enhance our ability to predict effects of global change.