The deepest geoid low on Earth and its possible relation to the instability of the West Antarctic Ice Sheet

Abstract

Although the geoid is usually displayed with respect to the reference ellipsoid, the difference between geoid and the Earth's hydrostatic equilibrium figure is geodynamically more meaningful, and has its deepest low in the Ross Sea area. Nearby in West Antarctica, there is also a residual topography high. This region is characterized by thin lithosphere, and a mantle plume has been suggested beneath. Hence upper mantle viscosity could be regionally reduced, allowing for faster rebound than elsewhere upon melting of the West Antarctic Ice Sheet (WAIS) which is one of the tipping elements of the global climate system. To study the possible causes of the geoid low / topography high combination, we compute the effects of density anomalies with the shape of a cylindrical disk of a given radius and depth range. With a density anomaly of -1% we find that a geoid low of the right size and magnitude can be explained with a disk radius of about 10° of arc and the base of the disk in the lower transition zone or even lower mantle; with a shallower base the amplitude is under-predicted. On the other hand, if in this case the top of the disk is shallower than ~150 km, dynamic topography amplitude is over-predicted. The fact that the residual topography high (more sensitive to density anomalies at shallower depth) is laterally displaced relative to the geoid low (more sensitive to greater depths) could indicate a plume or upwelling that is tilted due to large-scale flow. Alternatively, there may be two separate disks somewhat laterally displaced, one just below the lithosphere and mainly causing a dynamic topography high and one below the transition zone causing the geoid low.In order to test the feasibility of such density models, we perform computations of a plume that enters at the base of a box corresponding to a 3300 km x 3300 km region in the upper mantle, as well as some whole-mantle plume models, with the Aspect mantle convection code. However, these plume models have typically a narrow conduit (much narrower than ~10° of arc) and the plume tends to only become wider as it spreads beneath the lithosphere, i.e.\ at depths typically shallower than about 300 km, hence it would tend to rather under-predict the amplitude of the geoid compared to dynamic topography. We discuss how to possibly overcome the discrepancy between what is required to explain geoid and dynamic topography, and the outcome of numerical forward models.