Local bed-controlled grounding-line retreat at Cook Ice Shelf, East Antarctica, revealed by differential InSAR

Differential synthetic aperture radar interferometry (DInSAR) of COSMO-SkyMed (CSK) SAR 1-day tandem pairs was used to map the 2020-2021 grounding line (GL) of the Cook Ice Shelf, East Antarctica, and to assess the observed retreat relative to the 1996 GL and 2018 grounding zone (GZ). To interpret the dynamics of the retreat, we combined annual ice velocities, ice-thickness change rates, and bed topography. Agreement between tidal predictions and DInSAR-derived vertical displacements within the hydrostatic-equilibrium zone indicates that the GL mapping is robust. The CSK-DInSAR GL shows inland retreat of ~ 5.8 km relative to the 2018 upstream GZ boundary in the western Cook East Ice Shelf and ~ 8.8 km relative to the 1996 GL across Cook West Ice Shelf, while other sectors remain stationary. In the retreating sector of western Cook East Ice Shelf, ice flow slowed in 2017-2018 and then increased by ~ 60 m/yr in 2018-2019, returning to prior speeds with no further acceleration. The retreat occurred where the 2018 upstream GZ already lay on a retrograde bed. The 2020-2021 GL occupies either a local prograde bed or a retrograde segment immediately seaward of a prograde transition, suggesting threshold-like retreat controlled by bed geometry.