Collaborative Research: Imaging the 3D Viscosity Structure of the Antarctic Mantle with Existing Observations from GPS and Relative Sea Level

合作研究：利用 GPS 和相对海平面的现有观测结果对南极地幔的 3D 粘度结构进行成像

• 批准号: 2142592
• 负责人: Andrew Lloyd
• 金额: $ 43.78万
• 依托单位: Columbia University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2022
• 资助国家: 美国
• 起止时间: 2022-02-15 至 2026-01-31
• 项目状态: 未结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2142592&HistoricalAwards=false
• 关键词: Collaborative; Research; Imaging; 3D; Viscosity

Given the imminent threat posed by rising sea levels across much of the globe, there is a critical need to better understand past, present, and future Antarctic ice mass change and the resulting solid Earth deformation. The latter process is referred to as glacial isostatic adjustment. A key parameter that determines the rate of this deformation is the viscosity of the deforming material. To date, the vast majority of global glacial isostatic adjustment models assume that Earth's viscosity structure varies with depth alone. However, there exists extensive geological and geophysical evidence for significant lateral variations in viscosity and for the existence of low viscosity regions in the Earths mantle below Antarctica that deform rapidly on decadal or faster time scales. This variability in viscosity causes regions of the solid mantle to deform differently and thus a model of three-dimensional viscosity structure is needed to better measure the changing weight of the Antarctic ice sheet, to accurately model ice sheet dynamics, and to better project future sea level changes in response to Antarctic ice melt. The research conducted here will construct a first-generation reference three-dimensional viscosity model for the solid Earth underlying Antarctica. The analysis will use horizontal and vertical deformations measured by the Global Navigation Satellite System over the last few decades at sites across Antarctica, a state-of-the-art seismic model of the Antarctic mantle, coupled simulations of glacial isostatic adjustment and ice sheet stability, and a novel, observationally driven and mathematically rigorous approach to calculating the glacial isostatic adjustment parameters that cannot be directly observed. This project supports two early-career researchers and two graduate students. Funding will be used to support the participation of U.S. graduate students and instructors in a glacial isostatic adjustment training school, which will be organized by the principal investigator and leadership of the Scientific Committee on Antarctic Research initiative Instabilities and Thresholds in Antarctica.Quantifying the magnitude of modern ice mass loss from Antarctica is a key element in efforts to constrain future sea level change. Although satellite gravimetry and changes in ice surface elevation are used to estimate ice mass change, these observations cannot provide a direct estimate because they also record changes in the solid Earth. Similarly, modeling of past and future ice sheet dynamics and sea level change require an accurate model of solid earth deformation. Thus, the contribution from the ongoing response of the viscoelastic Earth to ice sheet evolution across the ice age and into the modern world, termed glacial isostatic adjustment (GIA), must be accurately quantified. Although the signal from GIA is widely recognized as being a significant component of modern Antarctic deformation, our incomplete knowledge of earths three-dimensional viscosity structure and the appropriate rheological model for the solid Earth deformation leads to large uncertainties in estimates of present-day ice mass change and modeling of future ice dynamics and sea level change. Fortunately, direct observations of solid Earth deformation have been made over the last few decades by Global Navigation Satellite System (GNSS) stations installed on bedrock across Antarctica. These observations have been used in forward modeling to infer regional one-dimensional viscosity structure, but they have not been directly used to image the continents three-dimensional viscosity structure. This will be addressed through four key tasks: (1) Inferring plausible steady-state diffusion creep viscosity models from the seismic shear wave speeds determined with the latest ANT-20 seismic tomography model using an inverse calibration scheme based on experimental results from mineral physics and a suite of geophysical constraints; (2) Determining ice histories that span from the Last Glacial Maximum to present from a coupled GIA/ice sheet model, which explores the range of inferred three-dimensional viscosity models and plausible parameters governing ice dynamics. These ice histories will be merged with modern estimates of ice mass change; (3) Exploring and characterizing the spatiotemporal sensitivities of vertical and horizontal GNSS deformation and relative sea level observations to the three-dimensional viscosity structure and ice history produced in tasks 1 and 2 using the adjoint method; and (4) Inverting observations of GNSS crustal deformation rates and relative sea level using the adjoint method to derive a new three-dimensional map of mantle viscosity below Antarctica. These inversions will use the models from task 1 and 2 and intuition gained from task 3 to further refine the three-dimensional viscosity structure and to explore whether observations include signals of transient or non-linear deformation.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.

鉴于全球大部分地区的海平面上升所带来的迫在眉睫的威胁，要更好地理解过去，现在和未来的南极冰质量变化以及由此产生的固体地球变形。后一个过程称为冰川等静力调节。确定这种变形速率的关键参数是变形材料的粘度。迄今为止，绝大多数全球冰川等静力调节模型假设地球的粘度结构仅随深度而变化。然而，存在广泛的地质和地球物理证据，证明粘度的显着横向变化以及在南极以下地球地面中的低粘度区域存在，这些区域在南极以下的地下室中迅速变形在衰老或更快的时间尺度上。粘度的这种变异性会导致固体地幔的区域以不同的形式变形，因此需要三维粘度结构的模型，以更好地衡量南极冰盖的变化，以准确地对冰盖动态进行建模，并更好地投射未来的海平面对南极冰层的响应。这里进行的研究将为南极固体地球构建第一代参考三维粘度模型。 The analysis will use horizo​​ntal and vertical deformations measured by the Global Navigation Satellite System over the last few decades at sites across Antarctica, a state-of-the-art seismic model of the Antarctic mantle, coupled simulations of glacial isostatic adjustment and ice sheet stability, and a novel, observationally driven and mathematically rigorous approach to calculating the glacial isostatic adjustment parameters that cannot be directly观察到。该项目支持两名早期研究人员和两名研究生。资金将用于支持美国研究生和讲师参与冰川等静止调整培训学校，该学校将由南极的南极研究倡议稳定性和科学委员会的首席研究员和领导层组织。尽管使用卫星重量表和冰面升高的变化用于估计冰块的变化，但这些观察结果无法提供直接估计，因为它们还记录了固体地球的变化。同样，对过去和将来的冰盖动力学和海平面变化的建模需要精确的固体变形模型。因此，必须准确地量化粘弹性土对冰期进化以及对现代世界的进化的反应，称为冰川等静态调节（GIA）。尽管GIA的信号被广泛认为是现代南极变形的重要组成部分，但我们对地球三维粘度结构的不完整知识以及固体地球变形的适当的流变学模型，导致估计当今的冰质量变化和对未来冰的动力学和海平面变化的估计。幸运的是，在过去的几十年中，通过安装在南极洲基岩上的全球导航卫星系统（GNSS）站进行了直接观察到固体变形。这些观察结果已用于推断区域的一维粘度结构中，但尚未直接用于成像大陆的三维粘度结构。这将通过四个关键任务来解决：（1）通过基于矿物质物理学和地球物理约束套件的实验结果，使用最新的ANT-20地震层析成像模型来推断出从最新的ANT-20地震层析成像模型确定的地震剪切波速度的合理稳态扩散粘度模型； （2）确定从最后一个冰川最大值到耦合的GIA/冰盖模型的冰历史，该模型探讨了有关冰冰动力学的推断的三维粘度模型和合理参数的范围。这些冰历史将与现代冰块变化的现代估计合并。 （3）探索和表征垂直和水平GNSS变形的时空灵敏度以及使用邻接方法在任务1和2中产生的三维粘度结构和冰历史的相对海平面观察结果； （4）使用伴随方法对GNSS地壳变形率和相对海平面的反相观察，以在南极洲低于南极洲的地幔粘度的新三维图。 These inversions will use the models from task 1 and 2 and intuition gained from task 3 to further refine the three-dimensional viscosity structure and to explore whether observations include signals of transient or non-linear deformation.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.