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Monday, September 21, 2009

Fiords and unstable glacier behaviour (Marinelli Glacier)

Fiords and unstable glacier behaviour

When Slartibartfast was given the job of shaping the Earth’s surface, the part he enjoyed most was the fiords. I am sure he could explain how to create a fiord much better than I can, but I will have a go. The explanation is informative but disturbing.

To make a fiord, you need fast-flowing ice, which implies that the glacier bed must be a site of intense dissipation of energy. Most of the motion is by basal sliding, which implies that the bed cannot be frozen and also suggests that much of the energy will be used up in entraining and removing bed material. This agrees with the visible fact that fiords are much deeper than the mountain and plateau terrains through which they are threaded. It also agrees with an explanation of what is going on that appears in a recent study.

Michèle Koppes and colleagues measured the volume of sediment beneath the waters of Marinelli Fiord in Tierra del Fuego. These waters have taken the place of the tongue of Marinelli Glacier, which has retreated 13 km since about 1945. Before then, its terminus was stable at the mouth of the fiord. The sediment must have been delivered by the glacier since its retreat began. The implied rate of erosion is an astonishing 39 mm/yr, give or take 40%. That is, the sediment implies that the glacier has stripped 39 mm off the land surface during each of the past 50-60 years. It seems certain that most of the erosion must be happening beneath the fast-flowing trunk of the glacier. If so, its bed is being overdeepened very rapidly indeed.

Why was Marinelli Glacier stable before 1945 and unstable thereafter? The answer has something to do with climatic change, but more immediately with the glacier’s own behaviour. When it was stable, its terminus rested on the pile of sediment it had itself deposited at the mouth of its fiord. Beyond that there was deeper water, into which the glacier did not advance because the ice arriving at the terminus simply broke off as icebergs. The deeper the water, the faster the iceberg discharge rate. When the climate changed, the glacier was no longer able to deliver the amount of ice needed to keep the terminus where it was. So it retreated.

The trouble is that the retreat moved the terminus down a slope. Not towards the deeper water beyond the glacier’s terminal moraine, but towards a deeper part of its own bed, carved by its own erosive handiwork. Just as it was unable to advance into deeper water, so Marinelli Glacier has been unable to stop retreating into deeper water. Its overdeepened bed has aggravated its inability to deliver the ice to keep up the iceberg discharge rate. It has done its best, by thinning and accelerating, with the side effect of increasing its erosive performance dramatically and thus setting itself up for renewed unstable behaviour after the next period of cool climate. But it is not going to settle down this time until it finds a part of its bed that slopes upwards inland, bringing the delivery of ice and the discharge of icebergs back into balance.

Many of the fiords we know about have shallow lips at their mouths, suggesting that they are deglaciated analogues of Marinelli. Most of the spectacular glacier retreats that have been documented in recent years, such as those of Jakobshavn Glacier in Greenland, and Columbia Glacier and the glaciers of Icy Bay in Alaska, have, like that of Marinelli, been from shoals produced at the heads of fiords by the glaciers themselves during less benign climatic times. But the scariest fiords of all are the ones we can’t see because they are still full of ice, and the scariest retreats are the ones that haven’t begun yet.

Marinelli Glacier is a shrimp (area about 160 km2) beside the leviathans of West Antarctica. Pine Island Glacier, draining about 185,000 km2 of the Antarctic Ice Sheet, resembles Marinelli in having a deep channel and a bed that slopes inland. As yet its terminus position hasn’t changed by much, but in a study just published Duncan Wingham and colleagues report that the ice just upstream from the terminus of Pine Island Glacier is now thinning at about 80 m/yr, more than 10 times the rate of just 10 years ago.


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