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Basal Lubrication - Just Use Water

It sounds like something they might do to you at a health spa, doesn’t it? But to students of glaciers, basal lubrication is the key that unlocks a long list of puzzles.

Why do precise measurements of glacier motion often show stick-slip behaviour, that is, hours and hours of near motionlessness punctuated by half-hours of rapid movement? Why do some glaciers surge, that is, accelerate suddenly every few decades, flowing rapidly for a year or two before returning, sometimes suddenly but more often gradually, to normal? Why does the landscape of southern Ontario, which I can see from my window, undulate? Why, in the sediment of the northern Atlantic Ocean, are there occasional layers of sand, interrupting the blanket of ultra-fine-grained mud?

The layers of sand beneath the Atlantic are spaced irregularly, 10,000-15,000 years apart, according to the Principle of Superposition, at depths below the sea floor that correspond to the last ice age. They are thin on the European side, thicker towards the northwest, and thickest of all in the neighbourhood of Hudson Strait, which separates Quebec from Baffin Island. The simplest explanation of this pattern is that every so often the bed of the Laurentide Ice Sheet, that covered most of Canada, became much more slippery. Much of its interior was drained by the Hudson Strait Ice Stream, which accelerated occasionally and discharged icebergs in huge numbers. With the icebergs came the sand. All of the plausible accounts of this instability have variations in basal meltwater supply, or possibly just its behaviour, as a critical ingredient.

Around where I live, we are rather proud of our drumlin field. Somebody counted these egg-shaped hills and got up to about four thousand. But geomorphologists now reckon that the tunnel channels are even more interesting. Tunnel channels are drainage networks shaped by subglacial meltwater at the end of the last ice age, after the ice had shaped the drumlins and indeed not long before the ice disappeared altogether. For a long time I simply could not see these things, and I still suspect that the geomorphologists are asking for more meltwater than is probable, but recent evidence from beneath the modern ice sheets is vindicating their interpretations. Now I can see the ancient tunnel valleys in the light of modern ones, apparently hard at work, beneath the Antarctic Ice Sheet.

I don’t know why most glaciers do not surge but a few do. Nor does anyone else. Surging is a phenomenon that has eluded explanation over several decades of concentrated observation and analysis. But we are all positive that subglacial hydrology contains the answer if we can only put together the pieces of the puzzle. The most recent instance of a surging glacier, detected by the U.S. Geological Survey on 3 July 2009, happens also to be a famous glacier - Malaspina Glacier in Alaska.

Many glaciers go faster in summer, suggesting that meltwater supply has something to do with glacier speed. Where the ice is observed to move in short bursts, there is usually also a suggestion, from one line of evidence or another, that it spends most of the time frozen - that is, stuck - to its bed. Slip happens when that immobile state is disturbed, in other words when the bed is lubricated upon the arrival of meltwater. But where does the meltwater come from? And go to?

It might not go anywhere, if the stuff that is moving around is not water but heat. That is, stick-slip may be telling us not about patterns of meltwater flow but about patterns of thawing and freezing. In fact, there may not be any heat moving around either. The melting temperature depends, slightly but measurably, on the confining pressure. So the thaw-freeze patterns could actually be patterns of subtle fluctuations of pressure, not just squeezing the water from one place to another but determining which of the two states, solid or liquid, it is stable in.

It is all very complicated, at scales from sticky patches up to the width of the north Atlantic and beyond. Great fun for glaciologists, but not without consequences for society - for example, if the Antarctic or Greenland Ice Sheet should decide to do what, according to the lesson from the sand under the Atlantic, the Laurentide Ice Sheet did repeatedly.

Posted by Graham Cogley on July 27, 2009 3:00 PM | Permalink