Collaborative Research: Ge/Si as a Tracer of Terrestrial Silica Cycling

摘要硅酸盐矿物的风化作用是土壤中次生硅酸盐矿物、植物中蛋白石植硅体以及海洋中硅藻形成的主要硅源。尽管硅是地球地球化学中的重要元素，但它没有一个很好的示踪剂来提供有机和无机过程如何影响硅行为的清晰理解，以及硅从岩石移动到粘土或水或生物体的路径。锗（Ge）和硅的化学行为相似，并已被视为硅在表面活性剂环境中的行为的伪同位素示踪剂。我们已经进行了初步的研究，证明了Ge/Si比在土壤和植物过程中的明显和合理的变化。在这里，我们提出了一个系统的分析Ge分馏从Si期间：1）不一致风化和次生铝硅酸盐粘土矿物的形成，2）土壤中氧化铁的形成，3）植物吸收和蛋白石生物合成导致植硅石的形成。我们提出了一个基于实地的项目，我们量化了岩石矿物，土壤矿物，在潮湿的热带风化环境中，在喷出火山岩和侵入深成岩上形成的土壤中的土壤水和植物植硅体。Ge/Si比值将在新鲜母质和土壤上进行测定，这些母质和土壤来自夏威夷（玄武岩基质）和波多黎各的卢基略（花岗岩基质）一系列经过充分研究的热带土壤地点。将分析与土壤样品相同层位的孔隙沃茨和矿物分离物（如适用）。我们还将分析植物的植硅体从占主导地位的植被。我们将使用这些数据来测试有关风化环境中控制Ge/Si分馏过程的特定假设。拟议的研究将比较和对比的Ge/Si系统在隐晶质玄武岩环境与深成花岗岩类环境中的行为，我们预计，分馏的机制将不同，因为在化学成分，矿物学和结晶度的差异。它还将专门研究植物吸收硅和土壤系统中的生物硅循环的作用。这项研究的结果将提供所需的基本数据，开发Ge/Si比作为定量示踪剂的二氧化硅在风化，植物周期，并在地表沃茨。

ABSTRACTWeathering of silicate minerals on continents is a major source of silicon (Si) for formation of secondary silicate minerals in soil, opal phytoliths in plants and ultimately for diatoms in the oceans. Even though Si is a significant element in the biogeochemistry of Earth, it does not have a well developed tracer to provide a clear understanding of how organic and inorganic processes affect Si behavior and the pathways that Si follows as it moves from rock to clay or water or organism. Germanium (Ge) and Si behave chemically in a similar fashion and have been treated as a pseudo-isotopic tracer of the behavior of Si in the surficial environment. We have conducted preliminary research that demonstrates clear and logical changes in the Ge/Si ratio in response to soil and plant processes. Here we propose a systematic analysis of Ge fractionation from Si during: 1) incongruent weathering and the formation of secondary alumino-silicate clay minerals, 2) formation of iron oxides in soil, and 3) plant uptake and opal biosynthesis leading to formation of phytoliths.We propose a field-based project where we quantify the Ge/Si ratios of rock minerals, soil minerals, soil water and plant phytoliths in soils forming on extrusive volcanic rock and on intrusive plutonic rocks in a humid tropical weathering environment. Ge/Si ratios will be determined on fresh parent materials and soils from a series of well-studied tropical soil sites in Hawaii (basaltic substrate) and Luquillo, Puerto Rico (granitoid substrate). Pore waters and mineral separates (where applicable) from the same horizons as the soil samples will be analyzed. We will also analyze plant phytoliths from dominant vegetation. We will use the data to test specific hypotheses about processes that control Ge/Si fractionation in the weathering environment. The proposed research will compare and contrast the behavior of the Ge/Si system in aphanitic basaltic environments with plutonic granitoid environments, where we expect that the mechanisms of fractionation will differ because of the differences in chemical composition, mineralogy, and crystallinity. It will also specifically investigate the role of plant uptake of Si and the biogenic silica cycle in soil systems. The results of this study will provide the basic data needed to develop Ge/Si ratios as a quantitative tracer of silica during weathering, plant cycling, and in surface waters.