The Rates and Mechanisms of Calcium Isotope Transport in Aquifers

含水层中钙同位素迁移的速率和机制

• 批准号: 0617585
• 负责人: Andrew Jacobson
• 金额: $ 9.1万
• 依托单位: Northwestern University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2006
• 资助国家: 美国
• 起止时间: 2006-08-01 至 2010-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0617585&HistoricalAwards=false
• 关键词: Rates; Mechanisms; Calcium; Isotope; Transport

EAR-0617585JACOBSONCalcium (Ca) isotope geochemistry holds much promise as a tool for examining the Ca cycle and its major controls (e.g., climate, tectonics, and biology). A considerable amount of recent work has focused on understanding Ca isotope variations in the marine realm with the ultimate aim of elucidating the nature of Earth's climate system. However, few studies have examined the behavior of Ca isotopes during chemical weathering and transport in continental waters. Arguably, interpretations of seawater Ca isotope values require a firm understanding of Ca isotope cycling on the continents. To address this problem, this proposal seeks to examine the Ca isotope geochemistry of two aquifers: the organic-rich Wyodak-Anderson Coal Bed (WACB) aquifer, WY and the carbonate Madison aquifer, SD. The primary intellectual merit objective is to develop, for the first time, mass-transport models that describe the downgradient evolution of dissolved Ca2+ isotopes in groundwater systems. By examining the Ca isotope values of ground waters, rock digests, and rock leachates, the project will specifically explore how microbial activity, chemical weathering, carbonate mineral equilibria, and ion-exchange influence Ca isotope cycling during fluid transport. The PI will also undertake laboratory-scale batch experiments designed to elucidate the magnitude of Ca isotope fractionation during Ca-for- Na ion-exchange. The PI will accomplish the research in collaboration with Dr. Chris Holmden at the University of Saskatchewan, who will conduct the Ca isotope measurements by using a43Ca-42Ca double tracer and a Triton Multi Collector Thermal Ionization Mass Spectrometer (MC-TIMS). The study will build on the PI's previous work in both aquifers, where major ion and Sr isotope analyses were used to determine the pathways and kinetics of solute transport. These studies resulted in extensive model frameworks that will be used to interpret the proposed Ca isotope results. The PI expects that the study will proceed rapidly because the samples are in hand, and the major controls on dissolved Ca2+ concentrations are known. Preliminary Ca isotope values for groundwater and coal samples from the WACB aquifer are presented and discussed in the text. Dissolved Ca isotope values increase downgradient, while the coal itself is isotopically light. The Ca isotope composition of the coal is consistent with biological Ca isotope fractionation during ancient plant growth. The dissolved Ca isotope values may reflect fractionation during fluid transport. Ultimately, this study will yield highly detailed information about the rates and mechanisms of Ca isotope cycling in two distinct groundwater systems. More generally, the project will provide much needed information about the Ca isotope composition of Earth materials, and it will lead to an improved understanding of the global Ca cycle and the rapidly developing marine Ca isotope record. Broader impact: The project will include the teaching and training of an undergraduate student enrolled in the Integrated Science Program (ISP) at Northwestern University (NU). The ISP program is a highly selective mathematics and science curriculum that emphasizes interdisciplinary learning at an accelerated pace. The student will undertake Ca isotope measurements, and he/she will use the data to write a senior honors thesis. Results from the study will be broadly disseminated. The project will also include a minority student from Evanston Township High School who is participating in the EXCITE program at NU. EXCITE provides educational and social support to minority students and their families. The goal of the program is to increase the number of underrepresented minorities who choose careers in mathematics and science. Lastly, the WACB aquifer contains a commercially viable reservoir of natural gas, and the Madison aquifer is a major water resource for the northern plains states. Thus, the proposed research will benefit society as a whole by providing valuable information about the hydrogeochemistry of two aquifers relevant to energy resources and economic development.

钙（Ca）同位素地球化学作为检查Ca循环及其主要控制因素（例如，气候、构造和生物学）。最近的大量工作集中在理解海洋领域的钙同位素变化，最终目的是阐明地球气候系统的性质。然而，很少有研究探讨钙同位素在大陆沃茨中的化学风化和迁移过程中的行为。可以说，海水钙同位素值的解释需要一个坚定的理解大陆上的钙同位素循环。为了解决这个问题，本建议旨在研究两个含水层的钙同位素地球化学：富含有机质的Wyodak-Anderson煤层（WACB）含水层，WY和碳酸盐麦迪逊含水层，SD。主要的智力价值的目标是开发，第一次，质量传输模型，描述了地下水系统中溶解的Ca 2+同位素的向下梯度演变。通过检查地下沃茨水、岩石消化物和岩石沥滤物的Ca同位素值，该项目将专门探索微生物活动、化学风化、碳酸盐矿物平衡和离子交换如何影响流体输送期间的Ca同位素循环。PI还将进行实验室规模的批量实验，旨在阐明钙对钠离子交换过程中钙同位素分馏的大小。PI将与萨斯喀彻温大学的Chris Holmden博士合作完成这项研究，他将使用43 Ca-42 Ca双示踪剂和Triton多收集器热电离质谱仪（MC-TIMS）进行Ca同位素测量。这项研究将建立在PI以前在两个含水层，其中主要离子和锶同位素分析，以确定溶质运移的途径和动力学的工作。这些研究产生了广泛的模型框架，将用于解释拟议的钙同位素结果。PI预计研究将快速进行，因为样品在手，并且溶解Ca 2+浓度的主要控制已知。文中介绍并讨论了WACB含水层地下水和煤样的初步Ca同位素值。溶解钙同位素值增加向下梯度，而煤本身是同位素轻。煤的钙同位素组成与古植物生长过程中的生物钙同位素分馏相一致。溶解Ca同位素值可能反映了流体运移过程中的分馏作用。最终，这项研究将产生高度详细的信息，在两个不同的地下水系统的钙同位素循环的速率和机制。更广泛地说，该项目将提供有关地球物质钙同位素组成的急需信息，并将导致更好地了解全球钙循环和快速发展的海洋钙同位素记录。更广泛的影响：该项目将包括在西北大学（NU）的综合科学计划（ISP）注册的本科生的教学和培训。ISP计划是一个高度选择性的数学和科学课程，强调跨学科学习的速度加快。学生将进行钙同位素测量，他/她将使用的数据写一个高级荣誉论文。研究结果将广泛传播。该项目还将包括一名来自埃文斯顿镇高中的少数民族学生，他正在参加NU的EXCITE项目。EXCITE为少数民族学生及其家庭提供教育和社会支持。该计划的目标是增加选择数学和科学职业的代表性不足的少数民族的数量。最后，WACB含水层含有商业上可行的天然气储层，而麦迪逊含水层是北方平原各州的主要水资源。因此，拟议的研究将有利于整个社会，提供有关两个含水层的水文地球化学的宝贵信息，有关能源和经济发展。