NSFGEO-NERC: Adjoint tomography of mantle viscosity using deglacial sea level observations

未来海平面因应对持续变暖而发生的变化是社会关注的问题。适应海平面上升是公共卫生、沿海基础设施和经济稳定面临的紧迫挑战。今天和过去的海平面变化是由于融水增加到海洋盆地，但也是由于海岸线的抬升或下沉。地球表面翘曲的一个原因是，在上一次冰川盛期，主要的冰盖向下挤压地球表面，并在融化时释放出来。研究人员将使用计算模型来模拟海平面和固体地球如何随着过去25,000年来冰盖的变化而变化，当时地球正在经历冰川高峰期的过渡。为了校准他们的模型，研究小组将他们的预测与地质记录中的13,000多次海平面观测进行了比较。这使他们能够改进模型，特别是更好地理解地球的内部粘度，这是这些海平面模型中的一个关键参数。这项工作将使研究小组能够回答关于地球内部变形的速度或速度的基本问题，以及固体地球变形对今天海岸线上的海平面变化的贡献有多大。这与美国东海岸和西海岸的城市尤其相关，在这些城市，与海洋变暖和冰盖融化相比，固体地球变形对海平面上升的贡献类似。该项目将在国际化和高度跨学科的背景下培训两名博士后研究科学家和一名研究生，以了解地球的内部结构、气候历史以及全球变化的速度和幅度。由于地幔复杂的热结构，地幔的粘度在径向和横向上都有变化。虽然从地震学、地球动力学、矿物物理学和海平面观测中都知道存在这些变化，但它们很难加以限制。通过冰川均衡调整(GIA)--地球对冰盖消长的粘弹性反应--这种不完整的知识传播到过去的冰盖和海平面变化的问题上。这一提议是为了更好地理解地球流变学及其对冰冻圈演化的影响。这里提出的新的方面是使用基于梯度的优化来反演海平面、大地测量和重力观测的流变学和去冰盖变化。模型梯度将使用伴随方法有效地计算。这一框架允许调查人员超越有限的一组正演模拟，并首次使他们能够有效地将海平面数据和其他GIA观测数据同化到3D GIA模型中。国际团队将使用一个新汇编的数据集，该数据集包含13,000多个退冰期海平面数据点，以产生第一张地球内部粘性结构的断层图像。他们还将使用这种方法来制作与3D地球模型一致的冰盖重建图。除了探索地球和冰结构之间的权衡之外，该团队还将开发和实施二阶伴随方程来评估不确定性传播。模型输出将使研究人员能够解决两个有针对性的研究问题，(1)全新世不同冰盖的融化或重新推进的数量和(2)GIA的当今贡献及其对主要沿海城市海平面变化的不确定性。这是一个由国家科学基金会地球科学理事会(NSF/GEO)和英国国家环境研究委员会(UKRI/NERC)通过NSF/GEO-NERC牵头机构协议共同资助的项目。该协定允许美国和英国提交一个单一的联合提案，并由该机构进行同行审查，该机构的调查员在预算中所占比例最大。在成功地共同确定奖项后，每个机构将为预算的比例以及与其自己的调查人员和工作部分相关的调查人员提供资金。该奖项反映了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。

Future sea level change in response to ongoing warming is of societal concern. Adapting to rising seas is a pressing challenge for public health, coastal infrastructure, and economic stability. Sea level today and in the past changes due to the addition of meltwater to the ocean basins, but also due to uplift or sinking of coastlines. One reason for the warping of Earth’s surface is that major ice sheets during the last glacial maximum have pressed down on Earth’s surface and released it as they melt. The investigators will use computational modeling to simulate how sea level and the solid Earth change in response to changing ice sheets over the past 25,000 years when Earth transitioned out of a glacial maximum. To calibrate their model, the team compares their predictions to more than 13,000 sea level observations from the geologic record. That allows them to improve the model and in particular better understand Earth’s internal viscosity, which is a key parameter in these sea level models. This work will allow the team to answer fundamental questions about how fast or slowly Earth’s interior deforms, but also how much solid Earth deformation contributes to sea level change along coastlines today. This is particularly relevant for cities along the U.S. East and West coast in which solid Earth deformation contributes a similar amount of sea level rise compared to warming oceans and melting ice sheets. This project will train two postdoctoral research scientists and one graduate student in an international and highly interdisciplinary setting to understand Earth's internal structure, its climatic history and the pace and magnitude of global change.The viscosity of Earth's mantle varies radially and laterally as a result of its complex thermal structure. While it is known from seismology, geodynamics, mineral physics, and sea level observations that these variations exist, they are difficult to constrain. Through glacial isostatic adjustment (GIA) -- the viscoelastic response of the Earth to waxing and waning ice sheets -- this incomplete knowledge propagates into questions of past ice sheets and sea level change. This proposal constitutes an effort to better understand Earth rheology and its implications for cryosphere evolution. The novel aspect proposed here is to invert sea level, geodetic, and gravitational observations for rheology and deglacial ice sheet changes using gradient-based optimization. Model gradients will be efficiently calculated using the adjoint method. This framework allows the investigators to move beyond a limited set of forward simulations and enables them for the first time to efficiently assimilate sea level data and other GIA observations into a 3D GIA model. The international team will use a newly compiled dataset with over 13,000 datapoints of deglacial sea level to produce the first tomographic image of Earth's internal viscosity structure. They will furthermore use this approach to produce ice sheet reconstructions that are consistent with 3D Earth models. In addition to exploring trade-offs between the Earth and ice structure, the team will develop and implement second-order adjoint equations to assess uncertainty propagation. The model output will allow the investigators to address two targeted research questions, (1) the amount of melt or re-advance of different ice sheets during the Holocene and (2) the present-day contribution of GIA and its uncertainty to sea level change in major coastal cities.This is a project that is jointly funded by the National Science Foundation’s Directorate of Geosciences (NSF/GEO) and the National Environment Research Council (UKRI/NERC) of the United Kingdom (UK) via the NSF/GEO-NERC Lead Agency Agreement. This Agreement allows a single joint US/UK proposal to be submitted and peer-reviewed by the Agency whose investigator has the largest proportion of the budget. Upon successful joint determination of an award, each Agency funds the proportion of the budget and the investigators associated with its own investigators and component of the work.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.