Collaborative Research: The Impact of Impurities and Stress State on Polycrystalline Ice Deformation

极地冰盖的冰是地球上最纯净的物质之一，但少量的杂质-如酸-对冰如何流动以及从冰芯中可以了解到过去的气候非常重要。该项目的目标是通过比较纯的和掺杂硫酸的样品的变形行为来了解这种酸对多晶冰变形的作用。选择硫酸是因为它对解释过去的气候很重要，也因为它在低温下可以导致冰中的水纹。这项工作将集中在多晶冰中酸的位置，运动和影响，这些多晶冰比单晶冰更复杂。通过使样品变形和进行微观结构表征，酸对变形速率、晶粒演化和酸本身的运动的作用将被评估。这项工作将导致博士教育。达特茅斯学院的学生，向本科生介绍华盛顿大学和达特茅斯学院的研究。尽管普遍使用的冰的本构关系通常被称为“格伦流动定律”，显着的不确定性存在，特别是关于杂质的作用和定向织物的发展。该项目的目的是通过对纯冰和掺硫酸冰进行变形试验和微观结构表征来改善冰的本构关系。该项目将重点关注硫酸对冰粘度，结构演变和扩散率的影响。硫酸对多晶冰的力学性能有直接和间接的影响。 直接效应改变位错速度和/或密度，间接效应改变晶粒尺寸和组构。这些影响的复杂性和相互作用意味着不可能通过简单地检查冰芯标本来了解硫酸的影响。在这个项目中，该团队将使四种类型的冰变形：实验室生长的掺杂有类似于天然浓度的硫酸的冰样品，实验室生长的高纯度冰，分层掺杂和纯冰，以及来自南极冰芯的天然冰。变形将在单轴压缩和简单剪切下进行。简单的剪切试验的增加是关键的实验室观察到的变形行为的行为的极地冰盖的剪切应变占主导地位的冰运动在基础冰。在变形至应变从5%到25%之后，将使用包括各种扫描电子显微镜技术、拉曼显微镜、基于同步加速器的纳米X射线荧光和离子色谱法的方法来评估微观结构的发展。这些分析技术将允许确定1）杂质的分离和移动，2）晶界迁移的速率，3）再结晶晶粒的数量;以及4）冰晶的完全取向。结果将使微观结构模拟的影响，硫酸和数值模拟的扩散冰芯。最终的结果将是更好地了解冰的变形，提高冰芯解释和冰盖建模。这个奖项反映了NSF的法定使命，并已被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。

The ice of the polar ice sheets is among the purest substances on Earth, yet the small amount of impurities --such as acids-- are important to how the ice flows and what can be learned from ice cores about past climate. The goal of this project is to understand the role of such acids on the deformation of polycrystalline ice by comparing the deformation behavior of pure and sulfuric acid-doped samples. Sulfuric acid was chosen both because of its importance for interpreting past climate and because it can lead to water veins in ice at low temperatures. This work will focus on the location, movement, and impact of acids in polycrystalline ice that are more complex than in single crystals of ice. By deforming samples and performing microstructural characterization, the role of acids on deformation rate, grain evolution, and the movement of the acids themselves, will be assessed. The work will lead to the education of a Ph.D. student at Dartmouth College, introduce undergraduate students to research at both the University of Washington and Dartmouth College. Despite the ubiquitous use of the constitutive relation for ice commonly referred to as "Glen's Flow Law", significant uncertainty exists particularly with regard to the role of impurities and the development of oriented fabrics. The aim of this project is to improve the constitutive relationship for ice by performing deformation tests and microstructural characterization of pure and sulfuric acid-doped ice. The project will focus on sulfuric acid's impact on ice viscosity, fabric evolution, and diffusivity. Sulfuric acid can have both direct and indirect effects on the mechanical properties of polycrystalline ice. The direct effects change the dislocation velocity and/or density, and the indirect effects change the grain size and fabric. The complexity and interaction of these effects means that it is not possible to understand the effects of sulfuric acid by simply examining ice core specimens. In this project, the team will deform four types of ice: lab-grown ice samples doped with similar-to-natural concentrations of sulfuric acid, lab-grown high-purity ice, layered doped and pure ice, and natural ice from Antarctic ice cores. Deformation will be performed in both uniaxial compression and simple shear. The addition of simple shear tests is critical for relating the laboratory-observed deformation behavior to the behavior of polar ice sheets where the shear strain dominates ice motion in basal ice. After deformation to strains from 5 percent up to 25 percent, the microstructural development will be assessed with methods including a variety of scanning electron microscope techniques, Raman microscopy, synchrotron-based Nano-X-ray fluorescence, and ion chromatography. These analysis techniques will allow the determination of 1) the segregation and movement of impurities, 2) the rate of grain-boundary migration, 3) the number of recrystallized grains; and 4) the full orientation of the ice crystals. The results will enable both microstructural modeling of the effects of sulfuric acid and numerical modeling of diffusion in ice cores. The net result will be a better understanding of ice deformation that improves ice-core interpretation and ice-sheet modeling.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.