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: 2142593
• 负责人: Jerry Mitrovica
• 金额: $ 24.07万
• 依托单位: Harvard 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 2025-01-31
• 项目状态: 未结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2142593&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海平面上升构成的迫在眉睫的威胁，迫切需要更好地了解过去、现在和未来的南极冰块变化以及由此产生的固体地球变形。后一个过程称为冰川均衡调整。决定变形速率的关键参数是变形材料的粘度。迄今为止，绝大多数全球冰川均衡调整模型都假设地球的粘度结构仅随深度而变化。然而，存在大量的地质和地球物理证据表明粘度存在显着的横向变化，并且南极洲下方的地幔中存在低粘度区域，这些区域在十年或更快的时间尺度上迅速变形。这种粘度的变化导致固体地幔区域发生不同的变形，因此需要三维粘度结构模型来更好地测量南极冰盖的重量变化，准确地模拟冰盖动力学，并更好地预测未来的海洋因南极冰融化而发生的水位变化。这里进行的研究将为南极洲下方的固体地球构建第一代参考三维粘度模型。该分析将使用全球导航卫星系统过去几十年在南极洲各地测量的水平和垂直变形、最先进的南极地幔地震模型、冰川均衡调整和冰盖稳定性的耦合模拟，以及一种新颖的、观测驱动的、数学上严格的方法来计算无法直接观测的冰川均衡调整参数。该项目支持两名早期职业研究人员和两名研究生。资金将用于支持美国研究生和教师参加冰川均衡调整培训学校，该培训学校将由南极研究科学委员会倡议“南极洲的不稳定性和阈值”的首席研究员和领导层组织。南极洲的现代冰量损失是限制未来海平面变化的一个关键因素。尽管卫星重力测量和冰面高程变化可用于估计冰质量变化，但这些观测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，对过去和未来的冰盖动力学和海平面变化进行建模需要精确的固体地球变形模型。因此，必须准确量化粘弹性地球对冰河时代和现代世界冰盖演化的持续响应的贡献，称为冰川均衡调整（GIA）。尽管来自 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和固体地球变形的适当流变模型的了解不完整，导致对当今冰块质量的估计存在很大的不确定性未来冰动态和海平面变化的变化和建模。幸运的是，在过去的几十年里，安装在南极洲基岩上的全球导航卫星系统（GNSS）站已经对固体地球变形进行了直接观测。这些观测结果已用于正演模型中以推断区域一维粘度结构，但尚未直接用于对大陆三维粘度结构进行成像。这将通过四个关键任务来解决：(1) 根据最新 ANT-20 地震层析成像模型确定的地震剪切波速度，使用基于矿物物理学和实验结果的逆校准方案，推断合理的稳态扩散蠕变粘度模型。一套地球物理约束； (2) 确定从末次盛冰期到耦合 GIA/冰盖模型呈现的冰历史，该模型探索了推断的三维粘度模型和控制冰动力学的合理参数的范围。这些冰历史将与冰块变化的现代估计相结合； （3）利用伴随方法探索和表征垂直和水平GNSS变形和相对海平面观测对任务1和2中产生的三维粘度结构和冰历史的时空敏感性； (4)利用伴随法反演GNSS地壳变形率和相对海平面观测结果，得到新的南极洲下方地幔粘度三维图。这些反演将使用任务 1 和 2 中的模型以及从任务 3 中获得的直觉来进一步细化三维粘度结构并探索观测结果是否包括瞬态或非线性变形信号。该奖项反映了 NSF 的法定使命，并已被通过使用基金会的智力优点和更广泛的影响审查标准进行评估，认为值得支持。