Mélange, landfast sea ice, ice velocities: What controls seasonal calving rates in North West Greenland?

Calving glaciers in Greenland frequently exhibit seasonal cycles of terminus advance and retreat and changes in ice velocity and calving rate associated with changing air temperatures, surface melt rates and local sea ice conditions. Landfast sea ice that forms each winter in Greenland and glacier mélange ( sikussaq ) are part of a continuum of ice forms in front of marine-terminating outlet glaciers, posited to offer a buttressing effect on marine calving glaciers equivalent to floating ice shelves and potentially thereby helping to reduce the risk of marine ice sheet instability feedbacks. However, the role of mélange in controlling calving rates and front position is controversial, with previous studies showing mixed results and limited effects on terminus ice velocities. Here, we use a comprehensive range of different types of data including a novel in situ dataset of high time resolution GNSS position information, combined with satellite datasets of ice velocity and calving front position for three representative glaciers in north west Greenland. Our study at the Tracy, Melville, and Farquhar glaciers took place during the period from late winter (March) to peak melt season (July) in 2022 and 2023. Seasonal variations in outlet glacier velocity, calving activity and terminus position vary in-step with the seasonal cycle of air temperature and landfast sea ice formation and break-up. Our observations are consistent with previous granular material theoretical frameworks where fast ice acts to delay the removal of mélange. We also observe large calving events at the peak of the fast ice season suggesting that neither landfast ice nor mélange fully suppress calving activity. We find no evidence of tidally driven movements within the mélange zone during the fast ice season. Our observations show that the unbonded mélange at these outlet glaciers has very little influence on glacier velocity or calving rates and it is only when the separate ice blocks in the mélange zone are frozen into a matrix of landfast sea ice, losing the granular material properties that are characteristic of mélange but more associated with rigid multiyear sea ice, that they appear to exert any kind of (limited) influence on the calving rates. Our observations form a comprehensive and useful dataset for evaluating models of mélange interactions and developing insights into the material properties of fast ice and mélange. We conclude that at these representative small and medium-sized Greenland outlet glaciers, factors including surface melt and glacier velocity as well as the presence of landfast sea ice bonding mélange modulate seasonal calving behaviour.