Si Isotope Fractionation During Rock Weathering and Pedogenesis

这一建议的学术价值在于对土壤发育过程中导致30Si和28Si分馏的过程有了更全面的理解。在这里，测量结果表明，来自玄武岩和花岗岩的未风化矿物在风化过程中发生变化，这是可以解释的变化模式。在风化初期，土壤固溶体的d30Si值相对于母质降低，而土壤溶液的d30Si值相对母质增大，最终次生矿物开始风化释放比母质低的硅，土壤溶液和淋洗水的d30Si特征降低。在全球范围内，结果与基于火成岩、大河和海水的d30Si值的预测不一致，但它们提出了关于硅同位素命运的根本问题，即矿物天气、次生矿物的形成和溶解的硅被淋滤。反应物和风化产物的d30Si特征应该是硅的最有效示踪剂，但需要更多地了解硅同位素在这个复杂系统中不同方面的行为。拟议的研究将对风化和土壤形成过程中产生硅同位素分馏的过程进行更全面和定量的理解。它将结合系统的现场和实验室实验和固体和水相中硅同位素组成变化的测量，以便分离出对驱动d30Si值变化最强烈的反应。实验针对硅参与的以下过程：1)原生矿物和次生矿物的溶解，2)次生矿物的沉淀，3)几乎同时溶解和沉淀反应的综合影响，4)风化、淋滤和粘土合成的综合方面。这些实验中的前三个将是实验室内的台式实验；第四个将利用美国地质调查局正在进行的两个实验，一个使用淋滤柱，另一个使用监测良好的土壤年代序列。前3个实验简化土壤过程以确保最大的可解释性，而柱子和田间实验测量复杂土壤形成过程的综合结果。年代序列是在硬岩母物质中的，因此将为现有的与夏威夷玄武岩年代序列中逐渐风化有关的硅同位素变化数据库增加一个有用的维度。化学风化解锁了岩石圈中的元素，使它们能够参与陆地系统和最终海洋系统中的动态生物地球化学反应。硅是地壳中第二种最富含的元素，也是排出大陆的河流中的主要溶质，提供了进入海洋的至少80%的溶解硅。这一提议的更广泛影响在于捕捉硅同位素作为硅酸盐风化示踪剂的能力的重要性，以及硅随后参与地球表面和生态系统过程的重要性。硅酸盐风化对缓冲酸雨和大气二氧化碳的长期调节至关重要，也是天然水中溶解硅的最终来源。硅的丰度、化学反应活性及其宿主矿物结构稳定性的变化确保了它在土壤中持续存在，即使它的损失支撑着下游生态系统，当它在沉积沉积物中积累时也是如此。拟议的研究将增进对全球硅循环的了解，特别是硅从大陆到海洋的风化控制转移。

The intellectual merit of this proposal lies in the development of a more complete understanding of theprocesses that cause fractionation of 30Si and 28Si during soil development. Herein, measurements arepresented that demonstrate interpretable patterns of change as unweathered minerals derived from basaltand granite are transformed during weathering. During the early stages of weathering d30Si values of soilsolids decrease whereas the d30Si values of soil solution increase relative to the parent material.Eventually, secondary minerals begin to weather releasing Si that has lower d30Si that parent material andtherefore the d30Si signatures of soil solution and leaching water decreases. Globally, the results are inagreement with predictions based on the d30Si values of igneous rocks, large rivers and ocean water, butthey raise fundamental questions regarding the fate of Si isotopes as minerals weather, secondaryminerals are formed, and dissolved Si is leached. The d30Si signatures of the reactants and products ofweathering should be the most effective tracer of Si, but more needs to be understood about the behaviorof Si isotopes in different aspects of this complex system.The proposed research will develop a more comprehensive and quantitative understanding of theprocesses that produce Si isotopic fractionations during weathering and soil formation. It will combinesystematic field- and laboratory-based experiments and measurement of the changes in Si-isotopecomposition in solid and aqueous phases in order to isolate the reactions that are most stronglyresponsible for driving changes in d30Si values. The experiments target the following processes in whichSi participates: 1) dissolution of primary and secondary minerals, 2) precipitation of secondary minerals,3) the combined impact of near-simultaneous dissolution and precipitation reactions, and 4) the integratedaspects of weathering, leaching and clay synthesis. The first 3 of these experiments will be in-lab benchtopexperiments; and the fourth will utilize two ongoing experiments at the US Geological Survey, oneutilizing leaching columns, the other a well-monitored soil chronosequence. The first 3 experimentssimplify soil processes to insure maximum interpretability whereas the column and field experimentsmeasure the integrated results of complex soil forming processes. The chronosequence is in arkosic parentmaterial, and thus will add a useful dimension to an existing database on silicon isotopic changesassociated with progressive weathering in the Hawaiian basaltic chronosequence.Chemical weathering unlocks elements from the lithosphere allowing them to participate in dynamicbiogeochemical reactions in terrestrial and ultimately marine systems. Silicon is the second mostabundant element in the Earth's crust and the dominant solute in rivers that drain continents supplying atleast 80% of the dissolved Si entering oceans. The broader impact of this proposal lies in the importanceof capturing the power of Si isotopes as a tracer for silicate weathering, and the subsequent participationof Si in Earth surface and ecosystem processes. Silicate weathering is critical for buffering acid rain, forlong-term regulation of atmospheric CO2, and is the ultimate source of dissolved Si in natural waters.Silicon's abundance, chemical reactivity, and the variation in the structural stability of its host mineralsensure that it persists in soils even as its loss supports downstream ecosystems and as it accumulates insedimentary deposits. The proposed research will augment understanding of the global Si cycle, andspecifically the weathering-controlled transfer of Si from continents to oceans.