Si Isotope Fractionation During Rock Weathering and Pedogenesis

这一建议的智力价值在于对土壤发育过程中导致30 Si和28 Si分馏的过程有了更全面的了解。在此，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，表现出可解释的变化模式。在风化初期，土壤固体的d30 Si值相对于母质降低，而土壤溶液的d30 Si值相对于母质升高，最终次生矿物开始风化释放出d30 Si值低于母质的Si，从而降低了土壤溶液和淋溶水的d30 Si值。在全球范围内，结果与基于火成岩、大型河流和海水的d30 Si值的预测不一致，但它们提出了关于Si同位素作为矿物风化、次生矿物形成和溶解Si浸出的命运的基本问题。风化反应物和风化产物的d30 Si特征应该是最有效的Si示踪剂，但需要更多地了解Si同位素在这个复杂体系中不同方面的行为，该研究将对风化和土壤形成过程中产生Si同位素分馏的过程有更全面和定量的了解。它将结合系统的现场和实验室为基础的实验和测量在固体和水相的Si同位素组成的变化，以隔离的反应，是最强烈负责驱动d30 Si值的变化。实验针对Si参与的以下过程：1）原生和次生矿物的溶解，2）次生矿物的沉淀，3）几乎同时发生的溶解和沉淀反应的综合影响，以及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实验台实验;第四个将利用美国地质调查局正在进行的两个实验，一个利用沥滤柱，另一个是监测良好的土壤时序。前3个实验简化土壤过程，以确保最大的可解释性，而柱和现场实验测量复杂土壤形成过程的综合结果。年代序列是在长石母质，因此将增加一个有用的维度，现有的数据库的硅同位素变化与夏威夷玄武岩年代序列的渐进风化。化学风化解锁元素从岩石圈，使他们能够参与在陆地和最终海洋系统的dynamicaeochemical反应。硅是地壳中第二丰富的元素，也是排干大陆的河流中的主要溶质，至少80%的溶解硅进入海洋。这一建议的更广泛的影响在于捕获硅同位素作为硅酸盐风化的示踪剂的重要性，以及硅在地球表面和生态系统过程中的后续参与。硅酸盐风化对于缓冲酸雨、长期调节大气中的CO2至关重要，并且是自然沃茨中溶解硅的最终来源。硅的丰度、化学反应性以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，即使它的损失支持下游生态系统，并在沉积物中积累。拟议中的研究将增加对全球硅循环的理解，特别是受天气控制的硅从大陆到海洋的转移。

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.