EAR - Climate; Investigating Effects of 3-Dimensional and Non-Newtonian Mantle Viscosity on Relative Sea-Level Changes and Deglaciation History Since the Last Glacial Maximum

大约26000年前，北美和欧洲的大部分地区被几公里厚的冰盖覆盖。这些冰盖开始融化，大约8000年前它们大部分消失了，但它们的融水导致全球海平面上升了约130米。随着冰盖的融化，曾经支撑冰盖的陆地相应地逐渐上升，而广阔的海底则在融水增加的重量下下沉。这种地球表面缓慢的上升和下降是对冰川消融的反应，被称为冰川均衡调整（GIA）。GIA告诉我们海平面变化和冰盖融化的历史，以及地球地幔的流动，甚至地球的自转，但这是一个巨大的主题，需要许多科学家的努力和高度复杂的计算模型。为了使这项工作更有效，钟博士将更新他的软件（CitcomSVE），并与其他研究人员免费分享。钟博士自己的研究小组将研究过去26000年冰川消融历史的不确定性，并寻求解决以往研究中相互矛盾的建模结果。如果他成功了，他的软件可以充满信心地用于预测不久的将来海平面的变化。这将有助于科学家、工程师和其他利益相关者了解与海平面上升相关的风险，并采取措施提高风险最高的沿海地区的抵御能力。GIA研究确定了地幔粘度，并构建了自末次极大期（LGM）以来最近~ 26000年的脱冰历史。然而，两种广泛使用的ICE6G [Peltier et al.， 2015]和ANU [Lambeck et al.， 2017]冰模型存在显著差异，其附带的地幔粘度模型也存在显著差异。此外，尽管包括实验室实验和地球动力学建模在内的各种岩石变形研究表明，地幔粘度依赖于温度和应力（即非牛顿粘度）[例如，Karato, 2008]，但非牛顿粘度在GIA过程中的影响的观测证据仍然难以获得。该项目有三个目标：1)构建一个改进的冰模型和地幔粘度模型，以解释Peltier和Lambeck团队使用的联合相对海平面（RSL）数据集；2)从靠近原冰盖边缘的位置寻找准l形RSL曲线中非牛顿地幔在GIA过程中的观测证据；3)开发一个公开可用的有限元软件包CitcomSVE，用于模拟GIA和潮汐变形问题。为实现这些目标，本项目包括以下三项任务。任务1是进一步开发新升级的开源包CitcomSVE代码，包括更有效的计算方法来求解地球引力场和物理上更真实的特征，如地幔压缩性。任务2是进一步分析Peltier和Lambeck小组考虑的RSL数据，寻求更好地解释近场和远场RSL数据以及GRACE数据的冰模型和1-D地幔粘度模型，并测试地震模型和板块边界粘度导出的3-D地幔粘度对GIA观测值的影响。任务3是检查非牛顿粘度对GIA过程模拟的影响，表征近场站点的准l形RSL曲线，从消冰历史和非牛顿粘度方面了解准l形RSL曲线的原因，并验证非牛顿粘度在RSL观测中影响明显的假设。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。

Some 26,000 years ago, much of North America and Europe was covered by ice sheets several kilometers thick. These ice sheets started to melt and by about 8,000 years ago they were mostly gone, but their meltwater caused the sea level to rise globally by about 130 meters. As the ice sheets melted, the land that used to support them gradually rose in response, while the vast ocean floors subsided under the load of added weight from the meltwater. This slow rising and falling of the Earth’s surface in response to deglaciation is called glacial isostatic adjustment (GIA). GIA tells us about sea-level change and ice sheet melting history, as well as flow in the Earth's mantle and even rotation of the Earth, but it is a huge topic requiring the effort of many scientists and highly sophisticated computational models. To make this effort more efficient, Dr. Zhong will be updating his software (CitcomSVE) and sharing it freely with other researchers. Dr. Zhong's own research group will address uncertainties in deglaciation history for the last 26,000 years, and seek to resolve conflicting modeling results from previous studies. If he is successful, his software can be used with confidence to project sea level change in the near future. This will help scientists, engineers and other stakeholders understand risks associated with sea level rise and take steps to improve resilience in the highest-risk coastal regions. GIA studies determine mantle viscosity and construct deglaciation history for the last ~26,000 years since the last glacial maximum (LGM). However, the two widely used ICE6G [Peltier et al., 2015] and ANU [Lambeck et al., 2017] ice models differ significantly, and so do their accompanying mantle viscosity models. Furthermore, while various studies of rock deformation including laboratory experiments and geodynamic modeling indicate that mantle viscosity is temperature- and stress-dependent (i.e., non-Newtonian) [e.g., Karato, 2008], observational evidence for the influence of non-Newtonian viscosity in GIA process remains elusive. This project has three objectives: 1) to construct an improved ice model and mantle viscosity model that explains the combined relative sea level (RSL) datasets used by the Peltier and Lambeck groups, 2) to seek observational evidence in GIA process for non-Newtonian mantle in quasi-L shaped RSL curves from locations near former ice sheet edges, and 3) to develop a publicly available finite element package CitcomSVE for modeling GIA and tidal deformation problems. This project consists of the following three tasks to accomplish these objectives. Task 1 is to further develop the newly upgraded, open-source package, CitcomSVE code, by including more efficient computational methods for solving Earth’s gravitational field and physically more realistic features such as mantle compressibility. Task 2 is to further analyze the RSL data considered by Peltier and Lambeck groups, to seek ice models and 1-D mantle viscosity models that better explain these RSL data at near-field and far-field sites and GRACE data, and to test the effects of 3-D mantle viscosity derived from seismic models and plate boundary viscosity on GIA observables. Task 3 is to examine the effects of non-Newtonian viscosity on simulations of the GIA process, to characterize the quasi-L shaped RSL curves at near-field sites, to understand causes of the quasi-L-shaped RSL curves in terms of deglaciation history and non-Newtonian viscosity, and to test the hypothesis that the effect of non-Newtonian viscosity is evident in the RSL observations.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.