Basal Lubrication - Just Use Water
by Graham Cogley, environmentalresearchweb.org, July 27, 2009
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.
Thanks Hank! That's great stuff!
Comments on stoat:
Curious -- I read this:
" ...The research team found that the Recovery stream accelerates significantly as it passes over the lakes.
"Upstream of the lakes, it flows at two to three metres per year; after passing them, at about 50 metres per year.
"Whether there is a link to climate change is another question. The lakes lie in the eastern portion of Antarctica, where evidence suggests the icecap may be gaining mass rather than losing it.
"As this research team puts it: 'The Recovery sub-glacial lakes and the associated Recovery ice stream tributaries have the potential greatly to affect the drainage of the East Antarctic ice sheet, and its influence on sea level rise in the near future.'"
and read the abstract for this:
"Rapid Sediment Erosion and Drumlin Formation Observed Beneath a Fast-Flowing Antarctic Ice Stream - AM Smith, T Murray, KW Nicholls, K Makinson, G ... - American Geophysical Union, Fall Meeting 2005
Couple questions: at the bottom of the icecap (everywhere, I think) there's enough ice thickness that it's grounded. How close are any areas of the ice to neutral? I realise the water pressure at the bottom of the ice is ---- whatever it is, at a mile or two below sea level.
[There are bits of W Antarctica that are fairly close to neutral - part of its possible instability -W]
Does water under pressure carry more silt than water at 1 atmosphere pressure?
I ask because the rapid drumlin article says, yes, they looked through the ice and saw one form, really fast -- these had been thought to be slow creatures.
But -- given that liquid water is flowing along the interface between ice and ground, whatever that ground is (presumably rock) --- how much of what kind of rock flour can that stream carry?
I know it's possible to "fill up" a moving stream's capacity to carry a load -- any time the flow becomes turbulent it drops some and then when it gets laminar it can and will pick up more again. It's one of the conundrums of restoration: if I take a nasty eroding stretch of stream and methodically make check dams and secure eroding banks and plant willow, and otherwise do everything I can to make that stretch of streambed turbulate the flow and be dropping rather than carrying all the sediment it can.
Anytime you turbulate a flow, whatever's flowing drops some of what it's carrying.
If there's a dead air spot on the interface, anyplace a vortex or ripple consistently leaves undisturbed, whatever silt (for a stream), leaves and dust and seeds (for a breeze), or household lint (for a fan).
So --- we're at the bottom of a glacial ice cap. There's a lot of melting way above but we're two miles down and it's been dark and quiet for a while. But every now and then the ice does flow far enough to cause the contact plane to shift downstream a bit.
There will be some flow, where there's excess heat or friction or impurities in the water if anything can change its melting point in those conditions.
We get flows of water; some of them are carrying silt.
That passes through a space where there's a bit of a void, the stream spreads out and slows down and drops what it's carrying.
So, finally, a question -- isn't a drumlin seen happening so fast, likely to be built up by silt filling a void that's melted a bit, on the bottom of the ice, and so going to get silted up as fast as the flowing water going by can provide the silt?
How else could they be happening, under the ice and so fast? And doesn't this lead to some ideas about streamflow rate?
And, has anyone had a look at the Channeled Scablands recently? They were an icecap letting go --- are we sure the water was on top of or behind that ice, or could it have been building up underneath the ice like this?
Because there's one other thing a very silty fast strong flow will do going downhill --- cut away what's in front of it and just rearrange it if it's so full of silt it can't keep any more suspended. A topside melt lake will be mostly water; an under-ice-cap flow must be quite a bit of silt.
Done handwaving; I'll go catch up on the drumlin stories. Turns out they're seen on Mars, resembling those in the Scablands. Hmmmm.
[Ah, I know nothing of drumlins - sorry -W]