Investigation of magnesium isotope fractionation during basalt weathering

该奖项是根据2009年《美国复苏和再投资法案》(公法111-5)提供资金的。我们建议研究夏威夷地表环境中风化过程和生物地球化学循环所产生的镁同位素变化。最近的研究表明，在硅质集水区的新鲜岩石、土壤、溪流、泉水和地下水中，以及天然碳酸盐和水之间，镁同位素存在差异。最近的其他实验工作证明了在高等植物吸收过程中镁同位素的分馏作用。然而，地表环境中镁同位素分馏的大小、机制和甚至方向仍然没有得到很好的约束。我们假设，与风化有关的分馏作用是产生的次生矿物类型的函数。我们将利用夏威夷群岛两个地点的一系列气候梯度地点，这些地点的次生矿物组合因气候变化而发生重大变化。虽然所有地点都有相似的玄武岩基岩岩性，但主要次生矿物从干燥科哈拉地点的镁碳酸盐和埃洛石，到较潮湿地点的尿磷矿。在东南部库劳山脉，随着降雨量的增加，蒙皂石让位于埃洛石和高岭石。我们将分析这些地点的岩石、土壤和土壤提取物、雨水、溪水和地下水以及叶片组织，以表征玄武岩蚀变和生物地球化学循环期间26 mg的变化。我们还将合成粘土矿物，并测量反应物和产物相的26 mg，以更好地约束对镁同位素分馏的矿物学控制。该项目将与法国南希CRPG的Nathalie Vigier博士密切合作，她已经制定了在这些类型的样品中进行26 mg分析的协议。科学价值：该项目将为理解表层环境中镁同位素分馏的机制提供关键数据，从而发展这一新工具的使用，以追踪风化和生物地球化学过程中镁的来源和途径。镁的循环与碳和硅的循环在多个时间尺度上密切相关，具有一阶重要性。该项目将具有变革性，因为它将使26 mg能够作为一种新的和强大的定量工具用于研究生物地球化学循环。广泛的影响：由于夏威夷作为一个自然实验室和模式系统的价值，我们以前在夏威夷地点对示踪系统的研究在科学界产生了重大影响，我们预计这将是类似的。其次，我们将与一位有前途的年轻法国研究人员和她的学生发展强有力的国际合作，他们是独立资助的，与我们合作。我们还将让我们的学生接触到美国没有的最先进的MCICP-MS仪器。美国研究生将参观法国的实验室，法国学生将参观我们的实验室和夏威夷的现场。第三，该项目将与康奈尔大学S在夏威夷的本科生实地考察项目密切结合。在夏威夷期间，学生将与项目研究人员互动，并就该项目的各个方面开展工作。我们将赞助至少两个与该项目相关的高级论文项目。

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). We propose a study of magnesium isotope variations produced by weathering processes and biogeochemical cycling in the surface environment of Hawaii. Recent work has demonstrated Mg isotopic variations among fresh rocks, soils, stream, spring and ground waters in silicatehosted watersheds, and between natural carbonates and water. Other recent experimental work has demonstrated Mg isotope fractionation during uptake by higher plants. However, the magnitudes, mechanisms, and even direction of Mg isotopic fractionation in the surface environment remain poorly constrained. We hypothesize that weathering related fractionation is a function of the type of secondary minerals produced. We will utilize a series of climate gradient sites at two locations in the Hawaiian Islands where significant changes in secondary mineral assemblages arise because of climatic variation. While all sites have similar basalt substrate lithology, dominant secondary minerals range from Mg-carbonates and halloysite at the dry Kohala sites, to allophane at wetter sites. In the SE Koolau Range, smectite gives way to halloysite and kaolinite with increasing rainfall. We will analyze rocks, soils and soil extracts, rain, stream and ground waters, and leaf tissues from these sites, to characterize 26Mg variations during basalt alteration and biogeochemical cycling. We will also synthesize clay minerals and measure 26Mg on reactant and product phases, to better constrain mineralogical controls on Mg isotope fractionation. The project will be carried out in close collaboration with Dr. Nathalie Vigier of the CRPG, Nancy, France, who has developed protocols for 26Mg analysis in these kinds of samples. Scientific merit: This project will provide critical data for understanding the mechanisms of Mg isotope fractionation in the surface environment, and so develop the use of this new tool to trace sources and pathways of Mg in weathering and biogeochemical processes. The cycle of Mg is closely linked to the cycles of carbon and silicon at multiple times scales, and is of first order importance. The project will be transformative in that it will enable the use of 26Mg as a new and powerful quantitative tool in studying biogeochemical cycles.Broader impacts: Previous studies of tracer systems at our sites in Hawaii have had substantial impact in the scientific community because of the value of Hawaii as a natural laboratory and model system, and we expect this to be similar. Secondly, we will develop a strong international collaboration with a promising young French researcher and her students, who are funded independently to work with us. We also will gain access for our students to state of the art MCICP- MS instrumentation not available in the US. US graduate students will visit French laboratories, and French students will visit our labs and Hawaiian field sites. Third, this project will interface closely with Cornell?s undergraduate field program in Hawaii. Students will interact with project researchers and work on aspects of this project while in Hawaii. We will sponsor at least two senior thesis projects associated with the project.