Mélange or landfast ice: What controls seasonal calving at Greenland outlet glaciers?

Abstract. Landfast sea ice and glacier mélange are part of a continuum of ice forms in front of marine-terminating outlet glaciers in Greenland. Mélange (sikussaq) has been posited to offer a buttressing effect on marine margins equivalent to floating ice shelves, potentially thereby helping to reduce the risk of marine ice sheet instability feedbacks. However, the role of mélange in buttressing marine termini is controversial with previous studies showing mixed results and only limited effects on terminus ice velocities. Here, we use a comprehensive and 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. However, 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 therefore suggest modelling landfast ice and glacier mélange as part of a glacier continuum that can modulate the response of glaciers to climate forcing on a seasonal cycle where landfast ice is seasonally present. The postulated buttressing or backstress effect from the mélange appears mainly when it is bound by landfast sea ice, however we note that our observations show movements of the mélange away from the glacier fronts at a similar velocity, rendering the assumption that landfast ice or mélange exert a significant back stress on termini unlikely. The break-up of landfast ice and onset of surface glacier melt occur concurrently in the summer melt season and both are probably therefore important in driving the seasonality of glacier front positions. We find no evidence of tidally driven movements within the mélange zone during the fast ice season, and no effects from surface winds that may explain calving events. Our observations also 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 Greenland outlet glacier, landfast sea ice and not the presence of mélange controls the seasonal calving front behaviour.