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One of glaciology’s bigger surprises

For an object that is supposed to be in the frozen state, your typical glacier has a surprising amount of liquid water in it. Even in the coldest parts of the Antarctic Ice Sheet, you can find water in the spaces between the ice crystals. It is liquid partly because it is under pressure but mainly because it is very salty, and in truth the amounts are tiny.

At the glacier bed, though, there is often a great deal of water. This makes sense when you think about the energy balance down there. The ice may, as in Antarctica, begin its sojourn on the Earth’s surface at a temperature tens of degrees below the freezing point, but as more of it accumulates the ice at the bottom can only get warmer. Some of the energy comes from friction as the ice moves over the bed. More comes from geothermal heat, the slow leakage of energy from the Earth’s interior. The ice has to warm up to its melting point eventually.

The basal meltwater has to go somewhere. A lot of it ends up in subglacial lakes, such as Subglacial Lake Vostok in East Antarctica. Subglacial Lake Vostok is about the same size, and coincidentally about the same shape, as Lake Ontario, but the resemblance is superficial. This is hardly an appropriate use of “superficial”: Subglacial Lake Vostok, apart from being hundreds of metres deep and millions of years old, is buried under 3.5 kilometres of ice. By comparison Lake Ontario is an ephemeral puddle.

Vostok is the biggest one we know of, but a couple of hundred other subglacial lakes are also known and more are being found all the time. One was newly identified last year only 15 km from Amundsen-Scott, the American base at the South Pole. Now, in a paper in Journal of Glaciology, Helen Fricker and Ted Scambos have described in more detail than before a complex system of connected subglacial lakes in West Antarctica.

Two things fascinate me about this report. First, although you can find subglacial lakes by hauling a radio echo-sounder across the surface of the ice sheet, these were found and their behaviour tracked by repeated observation from 600-700 km away. MODIS is an imaging satellite, and ICESat is a satellite travelling in a precisely repeating orbit from which it points a laser altimeter at the Earth’s surface. Subglacial lakes show up in ICESat track records as flat patches. The flatness means that the ice must be afloat. When Fricker and Scambos compared earlier with later observations, they saw these flat patches rising and falling through a few tenths of a metre, sometimes more. The only reasonable explanation is filling and draining of the subglacial meltwater. Filling pushes the surface up, draining allows it to subside.

The second fascinating thing is the connection of the subglacial lakes one with another. When one drains, one or more others fill a short time later. This kind of thing has been reported before (with an interesting very recent update by Sasha Carter and others), but the accumulation of evidence is showing that Antarctic subglacial hydrology must be a lively and a complicated business.

Fricker and Scambos show a block diagram of the subglacial topography, in which the lakes are arranged in the sequence you would expect. The uphill ones, when they are full (whatever that means), feed water to downhill ones. But “downhill” doesn’t mean what you would expect. This topography is a fiction, consisting of the real topography of the bed of the ice sheet plus the effect of the varying thickness of ice. The water is pressurized by this overburden, so to understand the flow of the water you have to redefine downhill so that it means “direction of the force of gravity plus the force due to the ice-thickness gradient”.

There is no suggestion that this work is revealing environmental change. Antarctic meltwater has presumably been behaving like this for ages. But there are two outstanding questions, both with intriguing implications. How, as it surely must, does subglacial hydrology affect the behaviour of the ice sheet? And how does the meltwater get out?


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