Effective Rheology Across the Fragmentation Transition for Sea Ice and Ice Shelves

Sea ice and ice shelves can be described by a viscoelastic rheology that is approximately linear elastic and brittle at high strain rates and viscously shear thinning at low strain rates. Brittle ice easily fractures under compressive shear and forms shear bands as the material undergoes a transition to a fragmented, granular state. This transition plays a central role in the mechanical behavior at large scales of sea ice in the Arctic Ocean or Antarctic ice shelves. Here we demonstrate that the fragmentation transition is characterized by an essentially discontinuous drop of three to five orders of magnitude in effective viscosity and stress relaxation time. Beyond the fragmentation transition, grinding in shear zones further reduces both effective viscosity and shear stiffness, but with an essentially constant relaxation time of ∼10 s. These results are relevant for ice rheology implementation in large‐scale climate‐related models of sea ice and thin ice shelves. Plain Language Summary Models of ice dynamics adopt various parameterizations of the material properties of ice. These parameterizations define a rheology for ice that may include viscous, plastic, elastic, and/or brittle behavior. Using a combination of theory and a discrete element model which by construction does not require a prescribed rheology, we find that an abrupt transition occurs as fracture density increases, with sudden drops in shear strength and effective viscosity separating low and high fracture states. The existence of this transition has important implications for, for example, understanding the stability of ice shelves and their ability to “buttress” the flow of inland ice, and for the development of continuum models for the dynamics of sea ice at the geophysical scale. Key Points A high‐resolution first‐principles ice fracture model is employed to determine an effective rheology for ice that undergoes fragmentation A sharp transition occurs between an elastic‐brittle solid and a viscous fluid, in contrast to many damage models of ice The fragmentation transition may be a key factor determining the stability of thinning ice shelves