George VI Ice Shelf

Holt, T.O., Glasser, N.F., Quincey, D. and Siegfried, M.R., 2013. Speedup and fracturing of George VI Ice Shelf, Antarctic Peninsula. The Cryosphere, 7: 797-816.

George VI Ice Shelf

George VI Ice Shelf, Alexander Island

George VI Ice Shelf, Alexander Island, showing ice flowing onto the ice shelf from both the Antarctic Peninsula and Alexander Island

Alexander Island and George VI Ice Shelf is an area I’m particularly interested in (see our project details), and the ice shelf is worth investigating for several reasons. For a start, it’s unusual, being trapped between the mainland and Alexander Island, and secondly, because it’s right on the -9°C mean annual air temperature isotherm (like a contour, but of mean annual air temperatures).  Some people have argued that this mean annual air temperature is the critical threshold above which ice shelves may dramatically collapse, which has implications for accelerated flow of glaciers and ice-sheet thinning. Ice shelves are also susceptible to warming from below by warm currents penetrating onto the continental shelf. So, research into this important part of the peninsula is always welcome. Holt and colleagues have just completed a study (open access) that investigates the response of George VI Ice Shelf to environmental change (i.e., oceanic and atmospheric temperature variations), and offer an assessment as to its future stability (Holt et al., 2013).

Alexander Island and George VI Ice Shelf, from the Landsat Image Mosaic of Antarctica

Alexander Island and George VI Ice Shelf, from the Landsat Image Mosaic of Antarctica

Ice shelf collapse

Simplified schematic figure of an ordinary ice shelf (such as the Larsen Ice Shelf) and George VI Ice Shelf, which abutts Alexander Island, forming pressure ridges and ice-shelf moraines.

Simplified schematic figure of an ordinary ice shelf (such as the Larsen Ice Shelf) and George VI Ice Shelf, which abutts Alexander Island, forming pressure ridges and ice-shelf moraines.

Holt et al. comment that ice-shelf collapse is usually preceded by sustained ice-front retreat, with a frontal geometry that bows inland (ice shelves are more stable where they are ‘pinned’ against the land, at their lateral margins) (Doake et al., 1998); continued thinning from below and above by atmospheric and oceanic warming (Fricker and Padman, 2012); increasing flow speeds; and finally, structural weakening along suture zones (where ice shelves or glacier ice meets) (Glasser and Scambos, 2008). Meltwater ponding on the ice surface is also critical, because hydrofracture may drive rapid ice-shelf break up.

George VI Ice Shelf is approximately 24,000 km2 and is the second largest ice-shelf on the Antarctic Peninsula. The northern ice front calves into Marguerite Bay, and the southern ice front calves into the Ronne Entrance, where there are several ice rises (Holt et al., 2013), which are bedrock highs on which the ice front is grounded and pinned. Along its centreline, this means that the ice shelf is around 450 km long, and is around 20 km wide and 100 m thick in the north, 600 m thick in the centre, and 75 km wide in the south. Ice flows from both Alexander Island and the Antarctic Peninsula into the ice shelf. The ice shelf primarily loses mass (ablates) as a result of calving at its termini and basal melting.

 

Glacier-ice shelf interactions: In a stable glacier-ice shelf system, the glacier's downhill movement is offset by the buoyant force of the water on the front of the shelf. Warmer temperatures destabilize this system by lubricating the glacier's base and creating melt ponds that eventually carve through the shelf. Once the ice shelf retreats to the grounding line, the buoyant force that used to offset glacier flow becomes negligible, and the glacier picks up speed on its way to the sea. Original Image by Ted Scambos and Michon Scott, National Snow and Ice Data Center.

Glacier-ice shelf interactions: In a stable glacier-ice shelf system, the glacier’s downhill movement is offset by the buoyant force of the water on the front of the shelf. Warmer temperatures destabilize this system by lubricating the glacier’s base and creating melt ponds that eventually carve through the shelf. Once the ice shelf retreats to the grounding line, the buoyant force that used to offset glacier flow becomes negligible, and the glacier picks up speed on its way to the sea. Original Image by Ted Scambos and Michon Scott, National Snow and Ice Data Center.

Changes in George VI Ice Shelf

Holt and others (2013) conducted an extensive analysis of ice shelf velocity, structural glaciology, recession, thinning and grounding-zone retreat. They found that the ice shelf is retreating at an annual rate of around 1.0 km (1936-1974) to 1.1 km per year (1974-2010). However, recession of this particular ice shelf generally occurs during a limited number of periodic, large-scale calving events, with a two-phase recession recorded from 1974-2010 (Holt et al., 2013). Calving events followed the gradual development of large, deep rifts, with a large calving event in January 2010. Much of the ice-shelf recession is therefore controlled by long-standing rifts in the ice, which develop many years before the calving event. Rifts are structurally controlled, with areas of rifted ice occurring between different flow units of the in-flowing glaciers. In the northern part of the ice shelf, the ice flows under an extensional regime and does not undergo longitudinal compression following rift formation. Fractures that therefore originate where glacier ice flows into the ice shelf therefore continue without impediment towards the northern end of the ice shelf, where they form large rifts, making the northern end susceptible to large, periodic, calving events.

The photographs below show the northern part of George VI Ice Shelf from above.

The southern margin of George VI Ice Shelf is steadily receding (Holt et al., 2013), with recession concentrated in the centre of the ice front, where the ice is thinnest. The islands on which the ice shelf is pinned cause regular fracturing and rifting as the ice flows over and around them, as the ice shifts from compressional stresses to tensile stresses downstream. The ice rises also cause ice flow to slow around them, so they effectively act as an impediment to smooth flow.

Parts of the ice shelf are now flowing faster. Recession of the most northerly margin has reduced the backstress and buttressing effect on Riley Glacier, on the Antarctic Peninsula, which is now flowing faster. In the south, the removal of large areas of ice has led to increased longitudinal extension, causing more fracturing and increased ice velocities (Holt et al., 2013).

Future stability

The northern end of George VI Ice Shelf is currently free from large crevasses, which take several decades to form. No large-scale rifting is therefore immediately expected from this area. However, it has changed considerably over the last 40 years, and response to continued environmental change is expected (Holt et al., 2013). However, surface melt is prevalent on the ice shelf, and continued longer, warmer summers and more meltwater ponding may result in ice-shelf recession by hydrofracture, where meltwater ponds melt downwards through the ice, weakening it structurally and contributing to increased iceberg calving.

The southern ice front of the ice shelf has changed rapidly following climatic and oceanic changes, with sustained recession, mostly in the thinner central portion. This part of the ice shelf is structurally weak and prone to iceberg calving. The ice shelf is thinning following increased basal melting from the incursion of warm waters beneath the ice shelf. This part of the ice shelf is therefore very vulnerable.

Further reading

References

Doake, C.S.M., Corr, H.F.J., Rott, H., Skvarca, P. and Young, N.W., 1998. Breakup and conditions for stability of the northern Larsen Ice Shelf, Antarctica. Nature, 391 (6669): 778-780.

Fricker, H.A. and Padman, L., 2012. Thirty years of elevation change on Antarctic Peninsula ice shelves from multimission satellite radar altimetry. Journal of Geophysical Research: Oceans, 117 (C2): C02026.

Glasser, N.F. and Scambos, T.A., 2008. A structural glaciological analysis of the 2002 Larsen B ice shelf collapse. Journal of Glaciology, 54 (184): 3-16.

Holt, T.O., Glasser, N.F., Quincey, D. and Siegfried, M.R., 2013. Speedup and fracturing of George VI Ice Shelf, Antarctic Peninsula. The Cryosphere, 7: 797-816.

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