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

摘要大陆硅酸盐矿物的风化作用是硅（Si）在土壤中形成次生硅酸盐矿物、在植物中形成蛋白石植物岩以及最终在海洋中形成硅藻的主要来源。尽管硅是地球生物地球化学中的重要元素，但它还没有一种发达的示踪剂来清楚地了解有机和无机过程如何影响硅的行为，以及硅从岩石转移到粘土、水或生物体时所遵循的途径。锗（Ge）和硅的化学行为相似，并已被视为表面环境中硅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比在响应土壤和植物过程中明显和合乎逻辑的变化。在此，我们提出了一个系统的分析Ge从Si分离的过程：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，以及3)植物吸收和蛋白石生物合成导致植物岩的形成。我们提出了一个基于野外的项目，我们量化了在潮湿的热带风化环境下，在挤压火山岩和侵入深成岩上形成的土壤中岩石矿物、土壤矿物、土壤水和植物岩的Ge/Si比值。Ge/Si比值将在夏威夷（玄武岩基质）和波多黎各Luquillo（花岗岩基质）等热带土壤遗址的新鲜母质和土壤上测定。孔隙水和矿物分离物（如适用）来自与土壤样品相同的层位，将被分析。我们还将分析优势植被的植物岩。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。本研究将比较隐晶玄武岩环境中Ge/Si体系与深凹花岗岩环境中Ge/Si体系的分选行为，我们预计由于化学成分、矿物学和结晶度的差异，分选机制将有所不同。它还将专门研究植物对硅的吸收和土壤系统中生物源二氧化硅循环的作用。这项研究的结果将为开发Ge/Si比率作为风化、植物循环和地表水中二氧化硅的定量示踪剂提供所需的基本数据。

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.