Measuring long-term, mineral-specific weathering rates in diverse climatic settings

化学风化和物理侵蚀是相互依存的过程，它们塑造了景观，调节了土壤的组成，并向河流输送了溶质和沉积物。它们在地球的长期气候演变中也起着关键作用。硅酸盐风化是大气中二氧化碳的长期汇。因此，在一定程度上，自然环境下硅酸盐的风化速率随着温度的升高而增加（正如理论考虑和实验室实验表明的那样），风化反馈将在数百万年的时间里缓冲地球气候对大温度变化的影响。从某种程度上说，硅酸盐的风化速率取决于物理侵蚀带来的矿物供应速率，侵蚀的构造强迫可能影响地球的长期气候演变。然而，量化这些机制仍然很困难，因为很少在野外条件下测量单个矿物相的风化速率。最近发展的方法现在允许从积极侵蚀土壤的化学和同位素组成来测量物理侵蚀和化学风化的长期速率。宇宙成因核素在活跃侵蚀土壤中的浓度可以用来推断它们的长期剥蚀速率。我们最近的研究表明，这些剥蚀率估计可以与质量平衡计算（基于土壤及其母基岩的总体化学成分）相结合，以产生长期化学风化率的测量结果。我们建议进一步扩展这些最新的进展，通过扩展我们的宇宙成因方法来测量单个矿物相的风化速率。这些方法应广泛适用，因为它们不需要特殊的现场情况；相比之下，传统的土壤质量平衡方法通常需要已知年龄和已知初始成分的非侵蚀土壤。因此，这些方法将为野外测量矿物风化速率提供一种重要的新工具。在以前的工作中，我们已经使用宇宙生成核素在跨越不同气候制度的广泛网络中测量剥蚀率和散装化学风化率。在这里，我们建议测量其中四个地点的土壤和母质的矿物相组成，从而量化每个主要矿物相的风化速率。因为这个项目利用了我们以前对宇宙核素和整体地球化学的测量，所以它可以非常经济有效地进行。此外，我们提出的XRD，薄片和微探针分析将使用我们已经收集的样品，从而消除了现场费用。在拟建的地点，年平均气温在2到25摄氏度之间，平均降水量在67到420厘米/年之间。该项目旨在确定和量化气候对长期矿物风化率的影响。由于在以前的工作中已经测量了长期剥蚀率，并且由于基岩矿物组成将作为拟议工作的一部分进行量化，因此应该有可能明确地解释基岩侵蚀造成的矿物供应率在不同地点之间的差异所产生的潜在混淆效应。因此，这些结果将有助于建立更好的养分循环和长期气候演变模型，并有助于更定量地了解矿物风化和土壤发育过程。更广泛的影响本研究将为一名研究助理提供博士后培训。本科生将通过参与项目的许多阶段获得实践研究经验。这项工作的结果将在包括地貌学、地球化学和土壤学在内的广泛领域得到应用。

ABSTRACTChemical weathering and physical erosion are interdependent processes that sculpt landscapes, regulate the composition of soils, and deliver solutes and sediment to streams. They also play critical roles in Earth's long-term climatic evolution. Silicate weathering is the long-term sink for atmospheric CO2. Hence, to the extent that silicate-weathering rates increase with temperature in natural settings (as theoretical considerations and laboratory experiments indicate that they should), weathering feedbacks will, over millions of years, buffer Earth's climate against large temperature shifts. To the extent that silicate-weathering rates depend on rates of mineral supply from physical erosion, tectonic forcing of erosion may affect Earth's long-term climatic evolution. Quantifying these mechanisms has remained difficult, however, because weathering rates of individual mineral phases have rarely been measured under field conditions.Recently developed methods now allow long-term rates of physical erosion and chemical weathering to be measured from the chemical and isotopic composition of actively eroding soils.Concentrations of cosmogenic nuclides in actively eroding soils can be used to infer their long-term denudation rates. We have recently shown that these denudation rate estimates can be combined with mass balance calculations (based on the bulk chemistry of soils and their parent bedrock), to yield measurements of long-term chemical weathering rates.Intellectual meritHere we propose to further extend these recent advances, by extending our cosmogenic methods to measure weathering rates of individual mineral phases. These methods should be widely applicable, because they do not require unusual field situations; by contrast, conventional soil mass-balance methods typically require non-eroding soils of known age and known initial composition. Thus these methods should provide an important new tool for measuring mineral weathering rates in the field.In previous work we have used cosmogenic nuclides to measure denudation rates and bulk chemical weathering rates in a widespread network of sites spanning diverse climatic regimes. Here we propose to measure the mineral phase composition of soils and parent materials at four of these sites, and thus quantify the weathering rates of each of the major mineral phases. Because this project leverages our previous measurements of cosmogenic nuclides and bulk geochemistry, it can be pursued very cost-effectively. Moreover, our proposed XRD, thin section, and microprobe analyses will use samples that we have already collected, thus eliminating field expenses.Across the proposed sites, mean annual temperatures span a range of 2 to 25C, and average precipitation spans a range of 67 to 420 cm/yr. This project aims to identify and quantify the effects of climate on long-term mineral weathering rates. Because long-term denudation rates have already been measured in previous work, and because bedrock mineral composition will be quantified as part of the proposed work, it should be possible to explicitly account for potentially confounding effects arising from site-to-site differences in rates of mineral supply from erosion of bedrock. Hence, these results should contribute to better models of nutrient cycles and long-term climatic evolution, and to a more quantitative understanding of mineral weathering and soil development processes.Broader impactsThis study will provide postdoctoral training for one research associate. Undergraduates will gain hands-on research experience through involvement in many phases of the project. Results from this work should find application in a wide range of fields, including geomorphology, geochemistry, and pedology.