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30Si和28Si分异的过程有了更全面的了解。在此，测量结果显示了可解释的变化模式，因为来自玄武岩和花岗岩的未风化矿物在风化过程中发生了转变。在风化初期，固相物的d30Si值相对于母质降低，而土壤溶液的d30Si值相对于母质增加。最终，次生矿物开始释放d30Si低于母质的Si，因此土壤溶液和浸出水的d30Si特征降低。在全球范围内，这些结果与基于火成岩、大河和海水d30Si值的预测不一致，但它们提出了关于Si同位素在矿物天气、次生矿物形成和溶解的Si被浸出过程中的命运的基本问题。反应物和风化产物的d30Si特征应该是最有效的Si示踪剂，但Si同位素在这个复杂体系中不同方面的行为还需要进一步了解。拟议的研究将对风化和土壤形成过程中产生硅同位素分异的过程进行更全面和定量的了解。它将结合系统的现场和实验室实验，以及固体和水相中si同位素组成变化的测量，以分离出对d30Si值变化最强烈负责的反应。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；4)风化、淋溶和粘土合成的综合影响。前3个实验将在实验室进行；第四个将利用美国地质调查局正在进行的两个实验，一个利用淋滤柱，另一个利用监测良好的土壤时序。前3个试验简化了土壤过程以确保最大的可解释性，而柱试验和现场试验测量复杂土壤形成过程的综合结果。该年代序列属于黑质母物质，因此将为夏威夷玄武岩年代序列中与递进风化有关的硅同位素变化的现有数据库增加一个有用的维度。化学风化作用将岩石圈中的元素释放出来，使它们参与陆地和最终海洋系统的动态生物地球化学反应。硅是地壳中含量第二丰富的元素，也是流经大陆的河流中的主要溶质，至少80%的溶解硅流入海洋。这一建议的更广泛的影响在于捕捉硅同位素作为硅酸盐风化示踪剂的力量的重要性，以及随后Si在地球表面和生态系统过程中的参与。硅酸盐风化对缓冲酸雨、长期调节大气二氧化碳至关重要，是天然水体中溶解硅的最终来源。硅的丰度、化学反应性和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的生态系统，并在沉积物中积累。拟议的研究将增加对全球硅循环的理解，特别是气候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.