Understanding Firn Rheology Through Laboratory Compaction Experiments and Radar Data

南极洲和格陵兰岛的冰盖正在失去质量，这是全球海平面加速上升的原因之一。卫星测高提供了冰盖体积变化的精确测量，但计算冰盖质量变化--与估计冰盖的海平面贡献相关的量--需要知道冰盖的密度。冰盖表面附近的密度也影响到在冰芯中恢复的气泡的年龄估计，这是关于过去气候变化的关键信息来源。冰盖密度主要由降雪(冰盖上持续一年或更长时间的积雪)压实成冰的速度控制，但目前还没有广泛接受的理论来解释这种压实是如何发生的。因此，该项目的目标是促进对降雪如何致密的理解。该团队将进行实验室实验，并分析来自南极洲的穿冰雷达和冰芯数据。该项目的一个关键预期成果是一种新的降雪致密模型，可用于改进对当今冰盖变化的卫星测高测量，以及从冰芯重建过去的气候变化。该项目将结合实验室实验、数值模拟和地球物理技术来确定降雪在压实成冰时的流变性。该团队将使用两种方法来测量降雪压实度：(1)基于实验室的实验和(2)冰芯和雷达数据的分析。在这项以实验室为基础的工作中，该团队将在单轴应变、恒温和轴向应力下对合成降雪样品进行一系列压实实验。他们还将测量颗粒大小的演变。通过进行大量实验(25)，研究小组将约束决定降雪压实率如何取决于密度、温度、颗粒大小和轴向应力的关键参数。实验将在台式设备中进行，温度低至零下40摄氏度，轴向应力高达4兆帕。对于基于现场数据的组件，研究小组将分析冰芯和穿透冰的雷达数据，以产生第一组由雷达得出的降雪压实率、钻孔温度、降雪密度和降雪颗粒大小的重合集。实验室和现场数据的结果将使用数值降雪压实模型联系在一起。该模型采用质量守恒、动量守恒和能量守恒，并对雪花的流变学和粒度演化进行了明确的描述。雪松流变学的约束将被纳入到这个模型中，研究小组将利用它来研究关于气候变化如何影响雪松密度的基本问题。这是一个关键的未知因素，在过去和现在的气候变化估计中造成了很大的测量不确定性。这一奖项反映了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 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.