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）资助的。我们提出了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的研究。最近的工作已经证明了镁同位素在含硅流域的新鲜岩石、土壤、溪流、泉水和地下水中以及在天然碳酸盐和水之间的变化。最近的其他实验工作已经证明了高等植物在吸收过程中Mg同位素的分馏。然而，在地表环境中，Mg同位素分馏的幅度、机制甚至方向仍然知之甚少。我们假设与风化有关的分选作用是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站点，在那里，由于气候变化，次生矿物组合发生了重大变化。虽然所有地点都具有相似的玄武岩基底岩性，但主要的次生矿物范围从干燥Kohala地点的mg -碳酸盐和高岭土到潮湿地点的allophane。在库劳山脉东南部，随着降雨量的增加，蒙脱石让位给高岭石和高岭石。我们将分析这些地点的岩石、土壤和土壤提取物、雨水、溪流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过程中26Mg的变化。我们还将合成粘土矿物，并在反应物和生成物相上测量26Mg，以更好地约束矿物对Mg同位素分馏的控制。该项目将与法国南希CRPG的Nathalie Vigier博士密切合作，她已经制定了对这类样品进行26Mg分析的方案。科学价值：该项目将为了解地表环境中Mg同位素分馏的机制提供关键数据，从而开发使用这种新工具来追踪风化和生物地球化学过程中Mg的来源和途径。镁的循环在多个时间尺度上与碳和硅的循环密切相关，具有一级重要性。该项目将具有变革性，因为它将使26Mg成为研究生物地球化学循环的一种新的强大的定量工具。更广泛的影响：由于夏威夷作为一个自然实验室和模型系统的价值，我们在夏威夷的站点对示踪系统的先前研究对科学界产生了重大影响，我们希望这是相似的。其次，我们将与一位有前途的年轻法国研究人员及其学生开展强有力的国际合作，他们将获得独立资助，与我们一起工作。我们还将为我们的学生获得在美国无法获得的最先进的MCICP- MS仪器。美国研究生将参观法国实验室，法国学生将参观我们的实验室和夏威夷的实地考察地点。第三，这个项目将与康奈尔大学紧密合作。他在夏威夷的本科实习项目。在夏威夷期间，学生将与项目研究人员进行互动，并就该项目的各个方面进行工作。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.