Presence of Frozen Fringe Impacts Soft‐Bedded Slip Relationship.

Glaciers and ice streams flowing over sediment beds commonly have a layer of ice‐rich debris adhered to their base, known as a "frozen fringe," but its impact on basal friction is unknown. We simulated basal slip over granular beds with a cryogenic ring shear device while ice infiltrated the bed to grow a fringe, and measured the frictional response under different effective stresses and slip speeds. Frictional resistance increased with increasing slip speed until it plateaued at the frictional strength of the till, closely resembling the regularized Coulomb slip law associated with clean ice over deformable beds. We hypothesize that this arises from deformation in a previously unidentified zone of weakly frozen sediments at the fringe's base, which is highly sensitive to temperature and stress gradients. We show how a rheologic model for ice‐rich debris coupled with the thermomechanics of fringe growth can account for the regularized Coulomb behavior. Plain Language Summary: Many glaciers move by sliding over sediment beds. As the glacier flows downslope, ice can infiltrate the underlying sediments, forming a layer of ice‐rich debris attached at the glacier's base. We investigated how this frozen fringe impacts glacier motion by simulating glacier slip in a cold‐room facility with a specialized ring shear device. We recreated glacier conditions, sliding ice over granular beds to form the fringe, and then assessed how frictional resistance at the slip interface varied under different stresses and ice speeds. We found that frozen fringe influences the relationship between ice speed and frictional resistance, known as the "slip law." As ice speed increased, basal friction increased to a threshold that matches the strength of the ice‐free sediment bed—mirroring the "regularized Coulomb slip law" inferred for clean ice over soft beds. We attribute this behavior to deformation in a weakly frozen zone at the base of the frozen fringe and show how this behavior can be incorporated into existing parameterizations of glacier slip. Key Points: Ring shear experiments show frozen fringe alters basal slip dynamics for soft‐bedded glaciersDeformation in a zone of weakly frozen sediments within the fringe leads to a regularized Coulomb slip response [ABSTRACT FROM AUTHOR]