Abstract
AbstractThe long‐wavelength negative gravity anomaly over Hudson Bay coincides with the area depressed by the Laurentide Ice Sheet during the Last Glacial Maximum, suggesting that it is, at least partly, caused by glacial isostatic adjustment (GIA). Additional contributions to the static gravity field stem from surface dynamic topography, core‐mantle boundary (CMB) topography, and density anomalies in the subsurface. Previous estimates of the contribution of GIA to the gravity anomaly range from 25% to more than 80%. However, these estimates did not include uncertainties in all components that contribute to the gravity field. In this study, we develop a forward model for the gravity anomaly based on density models and dynamic models, investigating uncertainty in all components. We derive lithospheric densities from equilibrium constraints but extend the concept of lithospheric isostasy to a force balance that includes the dynamic models. The largest uncertainty in the predicted gravity anomaly is due to the lower mantle viscosity, uncertainties in the ice history, the crustal model, the lithosphere‐asthenosphere boundary, and the conversion from seismic velocities to density have a smaller effect. A preference for lower mantle viscosities >1022 Pa s is found, in which case at least 60% of the observed long‐wavelength gravity anomaly can be attributed to GIA. This lower bound on the lower mantle viscosity has implications for inferences based on models for mantle convection and GIA.
Highlights
The global gravity model XGM2016 exhibits a negative anomaly of about 50 mGal near Hudson Bay for wavelengths larger than 600 km (Figure 1) (Pail et al, 2018)
The anomaly is thought to be caused by the incomplete rebound following the deglaciation of the Laurentide Ice Sheet (Kaula, 1972; Walcott, 1973), a process known as glacial isostatic adjustment (GIA)
We fit the viscosity to observations, and from our preferred viscosity, we obtain the GIA and mantle convection contributions to the gravity field
Summary
The global gravity model XGM2016 exhibits a negative anomaly of about 50 mGal near Hudson Bay for wavelengths larger than 600 km (Figure 1) (Pail et al, 2018). The shape of this anomaly resembles the contours of the deflection due to the former Laurentide Ice Sheet. The anomaly is thought to be caused by the incomplete rebound following the deglaciation of the Laurentide Ice Sheet (Kaula, 1972; Walcott, 1973), a process known as glacial isostatic adjustment (GIA). Before using the gravity anomaly to constrain GIA, these contributions need to be quantified
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