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

大陆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来源。尽管硅是地球生物地球化学中的一种重要元素，但它没有一个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，以及硅从岩石移动到粘土、水或有机体时所遵循的路径。锗(Ge)和硅的化学行为相似，被认为是硅在地表环境中行为的伪同位素示踪剂。我们已经进行了初步研究，证明了Ge/Si比值随着土壤和植物过程的变化而发生明显和合理的变化。本文系统地分析了硅中Ge的分馏过程：1)不一致风化和次生铝硅酸盐粘土矿物的形成；2)土壤中铁氧化物的形成；3)植物吸收和蛋白石生物合成导致植硅体的形成。我们提出了一个野外项目，我们量化了在潮湿的热带风化环境下形成于喷出火山岩和侵入深成岩上的土壤中岩石矿物、土壤矿物、土壤水和植物植硅体中的Ge/Si比值。将在夏威夷(玄武岩基质)和波多黎各卢基洛(花岗岩基质)的一系列研究良好的热带土壤遗址的新鲜母质和土壤上测定Ge/Si比率。将分析与土壤样本相同层位的孔隙水和矿物质(如适用)。我们还将分析优势植被中的植物植硅体。我们将使用这些数据来检验有关风化环境中控制Ge/Si分馏过程的具体假设。拟议的研究将比较和对比Ge/Si系统在隐晶质玄武岩环境和深成花岗岩环境中的行为，我们预计，由于化学成分、矿物学和结晶度的不同，分馏机制将不同。它还将专门研究植物对硅的吸收和土壤系统中生物成因二氧化硅循环的作用。这项研究的结果将提供开发Ge/Si比率作为风化、植物循环和地表水中二氧化硅定量示踪剂所需的基本数据。

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.