Impact of Late Neogene Dynamic Topography on Antarctic and Greenland Ice-Sheet Stability

Ice-sheet models that underpin current projections of future sea-level change often calibrate sensitivity to changes in climate using palaeo-ice volume estimates for the Mid-Pliocene Warm Period (MPWP; ∼3 Ma), the most recent interval with climatic conditions approximating those expected in the near future. The ice-sheet model runs used in these calibrations generally assume MPWP bedrock topography equal to that of the present day. Bedrock topography is a major control on ice-sheet volumes predicted by these models, since marine-based regions are highly susceptible to runaway destabilisation. However, dynamic topography (DT; i.e., topography supported by convectively generated stresses) is likely to have evolved substantially over the past ∼3 Ma, invalidating this assumption. To our knowledge, no study has yet assessed this impact on Greenland’s MPWP equilibrium ice volume, while only one study has done so for the Antarctic Ice Sheet (AIS) using relatively low-resolution seismic tomography to predict mantle flow patterns.This study aims to more accurately quantify DT impacts on Pliocene ice-sheet stability at both poles using high-resolution seismic tomographic and geodynamic models. Existing DT predictions and observations of present-day DT are compared, finding generally good agreement, though a +0.8 km offset in observed values is seen across the North Atlantic region. This feature can be explained by invoking isostatic elevation of melt-depleted oceanic mantle lithosphere resulting from longstanding interaction between the Iceland plume and North Atlantic mid-ocean ridge spreading centres. Improved correlations between shear-wave velocity (VS) and residual depth anomalies motivate incorporation of a higher-resolution Antarctic seismic tomographic mantle model into DT predictions by merging it in temperature space with a lower resolution global model. The resulting mantle convection simulations and reconstructions of post-MPWP DT change enable more accurate prediction of Pliocene bedrock topography. Ice sheet models are run on both DT-corrected MPWP topography and present-day topography, showing differences in steady-state ice volume of ∼1.8 m sea-level equivalent (SLE), with complete loss of the Ross Ice Shelf occurring in the former. This substantial DT-related component of observed MPWP sea-level excess suggests existing estimates of climatically controlled AIS contributions need to be lowered, reducing inferred ice-sheet sensitivity. Recalibrating existing sea-level projections accordingly reduces predicted end-of-century Antarctic contributions to future sea level change by 45%. Ice sheet models are also run under a representative Pliocene climate to verify whether this bedrock-topography driven equilibrium ice volume difference is independent of climatic forcing, as suggested by recent palaeo-ice-sheet modelling work.