NSFGEO-NERC: Adjoint Tomography of Mantle Viscosity using Deglacial Sea Level Observations

Lead PI: Jacqueline Austermann

Unit Affiliation: Seismology, Geology and Tectonophysics, Lamont-Doherty Earth Observatory (LDEO)

June 2020 - May 2022
Project Type: Research

DESCRIPTION: 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.