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

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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.