The Rates and Mechanisms of Calcium Isotope Transport in Aquifers

钙（Ca）同位素地球化学作为研究钙循环及其主要控制因素（如气候、构造和生物）的工具大有希望。最近相当多的工作集中在了解海洋领域的Ca同位素变化，最终目的是阐明地球气候系统的性质。然而，很少有研究对大陆水域钙同位素在化学风化和运输过程中的行为进行了研究。可以说，对海水钙同位素值的解释需要对大陆钙同位素循环有明确的了解。为了解决这个问题，本提案试图检查两个含水层的Ca同位素地球化学：富含有机物的Wyodak-Anderson煤层（WACB）含水层，WY和碳酸盐麦迪逊含水层，SD。主要的智力价值目标是开发，第一次，质量传输模型，描述地下水系统中溶解Ca2+同位素的下降梯度演化。通过检查地下水、岩石消化物和岩石渗滤物的Ca同位素值，该项目将具体探索微生物活动、化学风化、碳酸盐矿物平衡和离子交换如何影响流体运输过程中的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Ca- Na离子交换过程中Ca同位素分馏的幅度。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a43Ca-42Ca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。该研究将建立在PI之前在两个含水层的工作基础上，其中主要离子和Sr同位素分析用于确定溶质运输的途径和动力学。这些研究产生了广泛的模型框架，将用于解释提出的Ca同位素结果。PI希望这项研究能够迅速进行，因为样品已经到手，而且溶解Ca2+浓度的主要控制因素是已知的。本文介绍并讨论了WACB含水层地下水和煤样的初步钙同位素值。溶解钙同位素值沿梯度下降增大，而煤本身同位素较轻。煤的Ca同位素组成与古植物生长过程中的生物Ca同位素分馏一致。溶解的Ca同位素值可以反映流体输运过程中的分馏。最终，这项研究将获得关于两个不同地下水系统中Ca同位素循环速率和机制的详细信息。更广泛地说，该项目将提供地球物质中Ca同位素组成的急需信息，并将有助于提高对全球Ca循环和快速发展的海洋Ca同位素记录的理解。更广泛的影响：该项目将包括西北大学（NU）综合科学计划（ISP）的一名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，强调跨学科的学习在加速的步伐。学生将进行Ca同位素测量，并将使用这些数据撰写高级荣誉论文。这项研究的结果将广泛传播。该项目还将包括一名来自埃文斯顿镇高中的少数族裔学生，他正在参加纽大的EXCITE项目。EXCITE为少数族裔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少数族裔人数。最后，WACB含水层包含一个商业上可行的天然气储层，麦迪逊含水层是北部平原各州的主要水资源。因此，拟议的研究将通过提供与能源资源和经济发展有关的两个含水层的水文地球化学的宝贵信息，使整个社会受益。

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.