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

由于其复杂的热结构，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。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，以用于流变学和去冰川冰盖变化。模型梯度将使用伴随方法有效地计算。这个框架允许我们超越有限的一组正演模拟，并使我们第一次能够有效地将海平面数据同化到3D GIA模型中。我们将使用一个新汇编的数据集，其中包含超过13,000个去冰川海平面的数据点，以产生第一张地球内部粘性结构的断层图像。我们将进一步使用这种方法来生成与3D地球模型一致的冰盖重建。除了探索地球和冰结构之间的权衡之外，我们还将开发和实施二阶伴随方程来评估不确定性传播。我们的模型成果将使我们能够解决两个有针对性的研究问题，(1)全新世期间不同冰盖的融化或重新推进的数量和(2)全球海平面变化的当今贡献及其对沿海主要城市海平面变化的不确定性。

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 isostaticadjustment (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 observationsfor rheology and deglacial ice sheet changes using gradient-based optimization. Model gradients will be efficiently calculated using the adjoint method. This framework allows us to move beyond a limited set of forward simulations and enables us for the first time to efficiently assimilate sea level data into a 3D GIA model. We 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. We 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, we will develop and implement second-order adjoint equations to assess uncertainty propagation. Our model output will allow us to address two targeted research questions, (1) the amount of melt or readvance of different ice sheets during the Holocene and (2) the present-day contribution of GIAand its uncertainty to sea level change in major coastal cities.