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

考虑到全球大部分地区海平面上升所带来的迫在眉睫的威胁，我们迫切需要更好地了解过去、现在和未来南极冰质量的变化以及由此导致的固体地球变形。后一过程称为冰川均衡调整。决定这种变形速率的一个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黏度结构仅随深度而变化。然而，有大量的地质和地球物理证据表明，黏度存在显著的横向变化，并且在南极洲以下的地幔中存在低黏度区域，这些区域在年代际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的变形不同，因此需要一个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，并更好地预测南极冰盖融化对未来海平面变化的响应。在这里进行的研究将为南极洲下面的固体地球建立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在过去几十年中在南极洲各地点测量的水平和垂直变形、南极地幔的最先进地震模型、冰川均衡调整和冰盖稳定性的耦合模拟，以及一种新的、观测驱动的、数学上严格的方法来计算冰川均衡调整参数，这些参数不能直接观测到。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教师参加冰川均衡调整培训学校，该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组织。量化南极现代冰块损失的规模是限制未来海平面变化的一个关键因素。虽然卫星重力测量和冰面高程的变化被用来估计冰的质量变化，但这些观测不能提供直接的估计，因为它们也记录了固体地球的变化。同样，对过去和未来冰盖动态和海平面变化的建模需要一个精确的固体地球变形模型。因此，粘弹性地球对冰原演变的持续响应（称为冰川均衡调整（GIA））的贡献必须精确量化。虽然来自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的不完全了解和固体地球变形的适当流变模型导致在估计当今冰质量变化和模拟未来冰动力学和海平面变化方面存在很大的不确定性。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拟来推断区域一维粘度结构，但它们尚未直接用于大陆三维粘度结构的成像。这将通过四个关键任务来解决：(1)使用基于矿物物理学和一套地球物理约束的实验结果的逆校准方案，从最新ANT-20地震层析成像模型确定的地震横波速度推断出合理的稳态扩散蠕变粘度模型；(2)通过一个耦合的GIA/冰盖模型确定从末次极大期到现在的冰史，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。这些冰川史将与现代估计的冰质量变化相结合；(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和任务2产生的三维粘度结构和冰史的时空敏感性；(4)利用伴随方法反演GNSS地壳变形率和相对海平面观测数据，得到新的南极地幔粘度三维图。这些反演将使用任务1和任务2中的模型以及从任务3中获得的直觉来进一步完善三维粘度结构，并探索观测是否包含瞬态或非线性变形的信号。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.