Exploring the impact of Cordilleran Ice Sheet size on marine-terminating ice stream grounding line dynamics

The growth and retreat of the Cordilleran Ice Sheet prior to the Last Glacial Maximum, during Marine Isotope Stage 3 (MIS 3; 57 ka to 29 ka), is elusive. Yet ice sheet size impacts underlying topography through glacial isostatic adjustment, which can modulate ice stream grounding zone dynamics. In this study, we explore bounds on MIS 3 Cordilleran Ice Sheet volume, and predict how topographic change due to glacial isostatic adjustment would shift the zones where grounding lines can persist for key Cordilleran ice streams. We identified three key ice streams (Yakutat Sea Valley, Skeena Valley, Juan de Fuca Strait), two of which are located near sites with sediment cores recording active ice dynamics across this time interval. We used the reconstructed bedrock topography for these ice streams to assess how glacial isostatic adjustment would change the potential zones of grounding line persistence, based on plausible end-member Cordilleran Ice Sheet histories. We found that glacial isostatic adjustment shifts the locations of persistent grounding line zones differently, based on the location of the Cordilleran ice stream with respect to the predicted spatial pattern of topographic change. Depending on the size of the Cordilleran Ice Sheet in the period leading into the Last Glacial Maximum, glacial isostatic adjustment could have acted to either stabilize or destabilize marine-terminating grounding lines across the Pacific shelf. Future work refining the Cordilleran Ice Sheet history across the glacial build-up phase will disentangle the role of solid Earth feedbacks on grounding line dynamics for individual ice streams, and provide insight into the mechanisms causing abrupt climate events, ranging from rapid ice discharge to megafloods, documented in the North Pacific across the last ice age. • Size of MIS 3 Cordilleran Ice Sheet modulates topography beneath ice streams. • Glacial isostatic adjustment shifts Cordilleran ice stream grounding zones. • Solid Earth impacts ice stream stability and response to climate forcing.