Rapid Ice Unloading in the Southern Antarctic Peninsula and its Effect on Bedrock Uplift Rates

The Wordie Ice Shelf, lying off the Antarctic Peninsula's (AP) west coast, has undergone a long-term disintegration since the 1960s with a substantial calving event occurring around 1989, followed by a steady retreat and its almost-complete disappearance. The dynamic response of the upstream glaciers to the ice shelf collapse, and the response of the solid Earth to the associated mass loss, are not fully understood. To quantify the mass loss from the system, we generated a digital elevation model (DEM) using airborne vertical and oblique imagery from 1966 and compared it to a DEM from 2008 SPOT data. This analysis reveals lowering over that time of approximately 60 m at the front of Fleming Glacier. Using IceBridge and ICESat-2/GLAS data during 2002-2014 we show a higher mean elevation change rate post-2008 than pre-2008. We use these load change data as a basis for simulation of viscoelastic solid Earth deformation. We subtract modelled elastic deformation rates, and a suite of modelled viscous rates, from GPS-derived three-dimensional bedrock velocities at sites to the south of Fleming Glacier. Assuming the pre-breakup bedrock uplift was positive due to post-Last Glacial Maximum (LGM) ice retreat, our viscoelastic-corrected GPS uplift rates suggest upper mantle viscosities are >2×1019 Pa s in this region, about an order of magnitude greater than previously found for the northern AP. The horizontal velocities at the site nearest to the Fleming Glacier, after elastic and plate tectonic corrections, point away from Marguerite Bay rather than the present glacier front, suggesting that the horizontal motion is dominated by signal from the earlier retreat of the glacier system following the LGM, compatible with a relatively strong mantle in this region. These findings are in accord with those of an earlier study of southern Palmer Land that inferred present-day uplift associated with late Holocene loading changes south of this region, and highlights the need for improved ice loading changes through the late Holocene.