Closing the budget of 20th century true polar wander

Abstract

We revisit the budget of 20th century true polar wander (~1°/Myr in the direction of 70°W) using a state-of-the-art adjoint-based reconstruction of mantle convective flow and predictions of ongoing glacial isostatic adjustment that adopt two independent models of Pleistocene ice history. Both calculations are based on a mantle viscosity profile that simultaneously fits a suite of data sets related to glacial isostatic adjustment (Fennoscandian Relaxation Spectrum, post-glacial decay times) and a set of present-day observations associated with mantle convection (long-wavelength gravity-anomalies, plate motions, excess ellipticity of the core-mantle boundary). Our predictions reconcile both the magnitude and direction of the observed true polar wander rate, with convection and glacial isostatic adjustment contributing signals that are 25-30 per cent and ~75 per cent of the observed rate, respectively. The former assumes that large-scale seismic velocity heterogeneities are purely thermal in origin, and we argue that our estimate of the convection signal likely represents an upper bound due to the neglect of hypothesized compositional variations within the large low-shear velocity provinces in the deep mantle.