Rapid iceberg calving following removal of tightly packed pro-glacial mélange

Abstract

Iceberg calving is a major contributor to Greenland’s ice mass loss. Pro-glacial mélange (a mixture of sea ice, icebergs, and snow) may be tightly packed in the long, narrow fjords that front many marine-terminating glaciers and can reduce calving by buttressing. However, data limitations have hampered a quantitative understanding. We develop a new radar-based approach to estimate time-varying elevations near the mélange-glacier interface, generating a factor of three or more improvement in elevation precision. We apply the technique to Jakobshavn Isbræ, Greenland’s major outlet glacier. Over a one-month period in early summer 2016, the glacier experienced essentially no calving, and was buttressed by an unusually thick mélange wedge that increased in thickness towards the glacier front. The extent and thickness of the wedge gradually decreased, with large-scale calving starting once the mélange mass within 7 km of the glacier front had decreased by >40%.

Formation, flow and break-up of ephemeral ice mélange at LeConte Glacier and Bay, Alaska

“…8; note the lag between the yellow and purple dots). During these pulses the boundary of visually thick ice mélange propagated up fjord in discrete steps (Video S1), similar to the observations of Xie and others (2019). Other periods of rapid motion were caused or amplified by the energy released by calving icebergs.…”

“…Several observations suggest that ice mélange can be viewed as weak, granular ice shelves that transmit stresses to glacier termini and influence iceberg calving. First, iceberg calving rates are often well-correlated with the formation and dispersal of ice mélange (or strengthening and weakening, if the ice mélange persists year round) (Sohn and others, 1998; Reeh and others, 2001; Joughin and others, 2008; Amundson and others, 2010; Howat and others, 2010; Seale and others, 2011; Walter and others, 2012; Xie and others, 2019). Second, during periods of glacier terminus quiescence, ice mélange is pushed from behind by the glacier terminus at roughly the glacier flow speed (i.e., the ice mélange must also push back against the terminus) and motion is accommodated by shear bands along the fjord margins and deformation within the ice mélange (Joughin and others, 2008; Amundson and others, 2010; Sundal and others, 2013; Foga and others, 2014; Amundson and Burton, 2018).…”

Calving Seasonality Associated With Melt‐Undercutting and Lake Ice Cover

“…Furthermore, the seasonal advance and recession of the ice-margin coincides with the formation and disintegration of lake ice cover. In this regard, the dynamics of lacustrine termini appear similar to the observed relationships between tidewater glaciers and ice-melange, whereby tidewater termini advance and retreat in response to the presence or absence of buttressing by an ice-melange, respectively (Amundson et al, 2010;Cassotto et al, 2015;Vieli et al, 2002;Xie et al, 2019).…”

A simple model of mélange buttressing for calving glaciers

“…Obeservations in Store glacier and Jakobshavn glacier in Greenland have shown that in the winter, when sea ice is thick, ice mélange prevents calving (Walter et al, 2012;Xie et al, 2019). This has also been reproduced in modelling studies of grounded marine glaciers (Krug et al, 2015;Todd et al, 2018Todd et al, , 2019: Backstresses from the mélange reduce the stresses in the glacier terminus thereby limiting crevasse propagation and reducing calving rates or preventing calving completely.…”