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Abstract The land surface beneath the Greenland and Antarctic Ice Sheets is isostatically suppressed by the mass of the overlying ice. Accurate computation of the land elevation in the absence of ice is important when considering, for example, regional geodynamics, geomorphology, and ice sheet behaviour. Here, we use contemporary compilations of ice thickness and lithospheric effective elastic thickness to calculate the fully re-equilibrated isostatic response of the solid Earth to the complete removal of the Greenland and Antarctic Ice Sheets. We use an elastic plate flexure model to compute the isostatic response to the unloading of the modern ice sheet loads, and a self-gravitating viscoelastic Earth model to make an adjustment for the remaining isostatic disequilibrium driven by ice mass loss since the Last Glacial Maximum. Feedbacks arising from water loading in areas situated below sea level after ice sheet removal are also taken into account. In addition, we quantify the uncertainties in the total isostatic response associated with a range of elastic and viscoelastic Earth properties. We find that the maximum change in bed elevation following full re-equilibration occurs over the centre of the landmasses and is +783 m in Greenland and +936 m in Antarctica. By contrast, areas around the ice margins experience up to 123 m of lowering due to a combination of sea level rise, peripheral bulge collapse, and water loading. The computed isostatic response fields are openly accessible and have a number of applications for studying regional geodynamics, landscape evolution, cryosphere dynamics, and relative sea level change. 

Abstract The landscape hidden beneath the Greenland Ice Sheet remains one of the most sparsely mapped regions on Earth, but offers a unique record of environmental conditions prior to and during widespread glaciation, and of the ice sheet's response to changing climates. In particular, subglacial valleys observed across Greenland may preserve geomorphological information pertaining to landscape and ice sheet evolution. Here we analyze the morphology of a subglacial valley network in northern Greenland using bed elevation measurements acquired during multi‐year airborne radio‐echo sounding surveys. Channel profile morphologies are consistent with a primarily fluvial origin of the network, with evidence for localized modification by ice and/or meltwater. Gravity and magnetic anomalies suggest that the spatial organisation of the valley network is influenced by regional‐scale geological structure, implying a long‐lived and well‐established hydrological system. We also document two knickzones in the valley longitudinal profile and terraces above the channel floor in the lower course of the network. These observations, combined with stream power modeling, indicate that northern Greenland experienced two episodes of relative base level fall during the Neogene (∼150 m at ca. 12–3.7 Ma and ∼380 m at ca. 8.2–2.8 Ma) that resulted in channel profile adjustment via incision and knickzone retreat. The timing of the inferred base level fall correlates with other onshore and offshore records of uplift, denudation, and/or relative sea level change, and we suggest that tectonic and/or mantle‐driven uplift played an important role in the genesis of the modern landscape of northern Greenland. 

Abstract Contemporary crustal uplift and relative sea level (RSL) change in Greenland is caused by the response of the solid Earth to ongoing and historical ice mass change. Glacial isostatic adjustment (GIA) models, which seek to match patterns of land surface displacement and RSL change, typically employ a linear Maxwell viscoelastic model for the Earth's mantle. In Greenland, however, upper mantle viscosities inferred from ice load changes and other geophysical phenomena occurring over a range of timescales vary by up to two orders of magnitude. Here, we use full‐spectrum rheological models to examine the influence of transient deformation within the Greenland upper mantle, which may account for these differing viscosity estimates. We use observations of shear wave velocity combined with constitutive rheological models to self‐consistently calculate mechanical properties including the apparent upper mantle viscosity and lithosphere thickness across a broad spectrum of frequencies. We find that the contribution of transient behavior is most significant over loading timescales of 10²–10³ years, which corresponds to the timeframe of ice mass loss over recent centuries. Predicted apparent lithosphere thicknesses are also in good agreement with inferences made across seismic, GIA, and flexural timescales. Our results indicate that full‐spectrum constitutive models that more fully capture broadband mantle relaxation provide a means of reconciling seemingly contradictory estimates of Greenland's upper mantle viscosity and lithosphere thickness made from observations spanning a range of timescales.