Understanding Firn Rheology Through Laboratory Compaction Experiments and Radar Data

通过实验室压实实验和雷达数据了解冷杉流变学

• 批准号: 1935438
• 负责人: Christine McCarthy
• 金额: $ 73.75万
• 依托单位: Columbia University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2020
• 资助国家: 美国
• 起止时间: 2020-04-01 至 2025-03-31
• 项目状态: 未结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1935438&HistoricalAwards=false
• 关键词: Understanding; Firn; Rheology; Through; Laboratory

The ice sheets of Antarctica and Greenland are losing mass and contributing to accelerating global sea-level rise. Satellite altimetry provides precise measurement of ice-sheet volume change, but computing ice-sheet mass change—the quantity relevant for estimating the ice sheet’s sea-level contribution—requires knowing the density of the ice sheet. The density near the ice-sheet surface also affects age estimates of air bubbles recovered in ice cores, which are a key source of information on past climate changes. Ice-sheet density is primarily controlled by the rate at which firn (snow that has persisted for a year or more on ice sheets) compacts into ice, but there is currently no widely accepted theory of how this compaction occurs. The goal of this project is thus to advance understanding of how firn densifies. The team will conduct laboratory experiments and analyze ice-penetrating radar and ice-core data from Antarctica. A key desired outcome of the project is a new model of firn densification that can be used to improve satellite-based altimetry measurements of present-day ice-sheet change and reconstructions of past climate changes from ice cores. This project will combine laboratory experiments, numerical modeling, and geophysical techniques to determine the rheology of firn as it compacts to form ice. The team will use two methods to measure firn compaction: (1) lab-based experiments and (2) analysis of ice-core and radar data. For the lab-based work, the team will conduct a suite of compaction experiments on synthetic firn samples under uni-axial strain and constant temperature and axial stress. They will also measure the grain-size evolution. By running a large number of experiments ( 25), the team will constrain key parameters that determine how firn compaction rate depends on density, temperature, grain size, and axial stress. The experiments will be conducted in a table-top apparatus at temperatures as low as -40 degrees C and axial stresses up to 4 MPa. For the field-data-based component, the team will analyze ice-core and ice-penetrating radar data to produce the first coincident set of radar-derived firn compaction rates, borehole temperatures, firn densities, and firn grain sizes. Results from lab and field data will be tied together using a numerical firn compaction model. This model is formulated using conservation of mass, momentum, and energy, along with an explicit description of firn rheology and grain-size evolution. Constraints on firn rheology will be incorporated into this model and the team will use it to examine fundamental questions about how changes in the climate affect firn density. This is a crucial unknown that contributes significant measurement uncertainty in estimates of past and present climate change.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.

南极和格陵兰的冰盖正在失去质量，并有助于加速全球海平面上升。卫星高度测定提供了冰片体积变化的精确度量，但是计算冰片质量变化（与估计冰盖海平面贡献相关的数量）需要了解冰盖的密度。冰盖表面附近的密度还会影响冰芯中回收气泡的年龄估计，这是过去气候变化的关键信息来源。冰片密度主要由FIR（在冰盖上持续了一年或更长时间的雪）紧凑到冰中的速度（降雪）的速度控制，但是目前尚无关于这种压实如何发生的广泛接受的理论。因此，该项目的目的是提高人们对首次致密方式的理解。该团队将进行实验室实验，并分析来自南极洲的冰根雷达和冰核数据。该项目的一个关键期望结果是一种新的FIR致密化模型，可用于改善基于卫星的高度测量测量值，对当今的冰层变化以及对冰核过去气候变化的重建。该项目将结合实验室实验，数值建模和地球物理技术，以确定FIR的流变形成冰。该团队将使用两种方法来测量FIR压实：（1）基于实验室的实验和（2）冰核和雷达数据的分析。对于基于实验室的工作，该团队将在单轴应变以及恒定温度和轴向应力下对合成FIR样品进行一系列压实实验。他们还将测量晶粒大小的演变。通过运行大量实验（25），团队将限制关键参数，以确定FIR压实率如何取决于密度，温度，晶粒尺寸和轴向应力。该实验将在低至-40摄氏度的温度下进行台式设备进行，轴向应力高达4 MPa。对于基于野外DATA的组件，该团队将分析冰核和冰冰雷达数据，以产生首个雷达衍生的FIR压实率，井眼温度，FIR密度和第一粒度的一致集合。实验室和现场数据的结果将使用数值FIR压实模型将其绑定在一起。该模型是使用质量，动量和能量的保护，以及对firheology和晶粒大小进化的明确描述来制定的。对Firheology的限制将纳入该模型，团队将使用它来研究有关气候变化如何影响FIR密度的基本问题。这是一个至关重要的未知，在过去和当前气候变化的估计中造成了重大的测量不确定性。该奖项反映了NSF的法定任务，并通过使用基金会的知识分子优点和更广泛的影响评估标准来评估值得支持。

项目成果

Grain-size evolution controls the accumulation dependence of modelled firn thickness

晶粒尺寸演化控制模拟云层厚度的累积依赖性

• DOI: 10.5194/tc-16-3413-2022
• 发表时间: 2022
• 期刊: The Cryosphere
• 作者: Kingslake, Jonathan;Skarbek, Robert;Case, Elizabeth;McCarthy, Christine
• 通讯作者: McCarthy, Christine