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--对于冰的流动方式以及从冰芯中可以了解到过去的气候是很重要的。该项目的目标是通过比较纯样品和掺硫酸样品的变形行为，了解这些酸在多晶冰变形中的作用。之所以选择硫酸，既是因为它对解释过去的气候很重要，也是因为它在低温下会在冰中形成水纹。这项工作将集中在多晶冰中酸的位置、移动和影响，这些酸比单晶冰中的酸更复杂。通过对样品进行变形和微观结构表征，将评估酸对变形速率、颗粒演化和酸本身运动的作用。这项工作将为达特茅斯学院的博士生提供教育，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”，但在杂质的作用和取向织物的发展方面仍然存在重大的不确定性。该项目的目的是通过对纯冰和掺硫酸冰进行变形测试和微观结构表征来改善冰的本构关系。该项目将重点研究硫酸对冰粘度、结构演变和扩散性的影响。硫酸可以对多晶冰的机械性能产生直接和间接的影响。直接效应改变位错速度和/或密度，间接效应改变晶粒度和组构。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样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：掺入接近自然浓度的硫酸的实验室生长的冰样品，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两种情况下进行。为了将实验室观察到的变形行为与极地冰盖的行为联系起来，添加简单的剪切试验是至关重要的，在极地冰盖中，剪切应变主导着基冰中的冰运动。在变形到5%到25%的应变后，将使用各种方法评估微观结构的发展，包括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。这些分析技术将允许确定1)杂质的偏析和移动，2)晶界迁移速度，3)再结晶颗粒的数量，以及4)冰晶的全取向。这些结果将使硫酸影响的微观结构模拟和冰芯扩散的数值模拟成为可能。最终结果将是更好地了解冰的变形，从而改进冰芯解释和冰盖建模。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.