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

摘要 大陆上硅酸盐矿物的风化是硅 (Si) 的主要来源，硅 (Si) 是形成土壤中的次生硅酸盐矿物、植物中的蛋白石植硅体以及最终形成海洋中的硅藻的硅 (Si)。尽管硅是地球生物地球化学中的重要元素，但它没有成熟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或水或有机体时所遵循的路径。锗 (Ge) 和硅的化学行为类似，并被视为硅在地表环境中行为的伪同位素示踪剂。我们进行的初步研究表明，Ge/Si 比率会因土壤和植物过程而发生清晰且符合逻辑的变化。在这里，我们提出了对硅中Ge分馏过程的系统分析：1）不均匀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体的形成。我们提出了一个基于现场的项目，量化岩石矿物、土壤矿物、土壤水和在喷出物形成的土壤中植物植硅体的Ge/Si比率。潮湿热带风化环境中的火山岩和侵入深成岩。 Ge/Si 比率将根据来自夏威夷（玄武岩基质）和波多黎各卢基约（花岗岩基质）一系列经过充分研究的热带土壤地点的新鲜母质和土壤来确定。孔隙水和矿物分离物（如适用）与分析土壤样品位于同一层位。我们还将分析主要植被中的植物植硅体。我们将使用这些数据来测试有关控制风化环境中 Ge/Si 分馏过程的具体假设。拟议的研究将比较和对比隐晶质玄武岩环境中的 Ge/Si 系统与深成花岗岩环境中的行为，我们预计由于化学成分、矿物学和结晶度的差异，分馏机制会有所不同。它还将专门研究植物对硅的吸收和土壤系统中生物硅循环的作用。这项研究的结果将提供开发 Ge/Si 比率所需的基本数据，作为风化、植物循环和地表水中二氧化硅的定量示踪剂。

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.