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

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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.