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Mantle melt generation in response to glacial unloading has been linked to enhanced magmatic volatile release in Iceland and global eruptive records. It is unclear whether this process is important in systems lacking evidence of enhanced eruptions. The deglaciation of the Yellowstone ice cap did not observably enhance volcanism, yet Yellowstone emits large volumes of CO₂due to melt crystallization at depth. Here we model mantle melting and CO₂release during the deglaciation of Yellowstone (using Iceland as a benchmark). We find mantle melting is enhanced 19-fold during deglaciation, generating an additional 250–620 km³. These melts segregate an additional 18–79 Gt of CO₂from the mantle, representing a ~3–15% increase in the global volcanic CO₂flux (if degassed immediately). We suggest deglaciation-enhanced mantle melting is important in continental settings with partially molten mantle – including Greenland and West Antarctica – potentially implying positive feedbacks between deglaciation and climate warming.

 

Abstract Accurate representation of the viscous flow of ice is fundamental to understanding glacier dynamics and projecting sea-level rise. Ice viscosity is often described by a simple but largely untested and uncalibrated constitutive relation, Glen’s Flow Law, wherein the rate of deformation is proportional to stress raised to the power n . The value n  = 3 is commonly prescribed in ice-flow models, though observations and experiments support a range of values across stresses and temperatures found on Earth. Here, we leverage recent remotely-sensed observations of Antarctic ice shelves to show that Glen’s Flow Law approximates the viscous flow of ice with n  = 4.1 ± 0.4 in fast-flowing areas. The viscosity and flow rate of ice are therefore more sensitive to changes in stress than most ice-flow models allow. By calibrating the governing equation of ice deformation, our result is a pathway towards improving projections of future glacier change. 

The accelerated calving of ice shelves buttressing the Antarctic Ice Sheet may form unstable ice cliffs. The marine ice cliff instability hypothesis posits that cliffs taller than a critical height (~90 m) will undergo structural collapse, initiating runaway retreat in ice‐sheet models. This critical height is based on inferences from preexisting, static ice cliffs. Here we show how the critical height increases with the timescale of ice‐shelf collapse. We model failure mechanisms within an ice cliff deforming after removal of ice‐shelf buttressing stresses. If removal occurs rapidly, the cliff deforms primarily elastically and fails through tensile‐brittle fracture, even at relatively small cliff heights. As the ice‐shelf removal timescale increases, viscous relaxation dominates, and the critical height increases to ~540 m for timescales greater than days. A 90‐m critical height implies ice‐shelf removal in under an hour. Incorporation of ice‐shelf collapse timescales in prognostic ice‐sheet models will mitigate the marine ice cliff instability, implying less ice mass loss.