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Monday, 29 July 2013

Big meltwater conduits beneath an ice stream

 This is a rather interesting paper which explains how speculation and theory can sometimes be confirmed by observation -- using new techniques which were not available in the bad old days......

Glaciologists and geomorphologists have speculated for a long time about the manner in which meltwater moves beneath an ice sheet or ice stream in a polar situation.  In some parts of Antarctica there are spectacular 'labyrynths' of meltwater channels, suggesting that catastrophic meltwater flows have occurred in the past, with erosion on a massive scale being the result.  when my colleague David Sugden and I found big meltwater systems on two of the ice-free peninsulas of the South Shetlands (Byers Peninsula on Livingstone Island and Fildes Peninsula on King George Island) we wondered where all the water had come from, and also wondered whether these channels had been cut during a rapid and catastrophic melting phase at the end of the last glacial episode.

 A famous photo of the Labyrynth, by George Denton.  These channels are naturally taken to have been formed by vast amounts of meltwater, probably flowing subglacially.

The Labyrynth, in the Taylor Dry Valleys system of East Antarctica, is the most famous example of a chaotic system of connected valleys -- suggested to have been formed during the Miocene by subglacial floods.  As long ago as 1965 CA Cotton was speculating as to the origin of the "scablands" in the Wright Valley.

There is also a considerable amount of speculation in the glaciology literature about how water moves beneath great thicknesses of ice -- and how meltwater actually assists in lubricating the glacier bed and thus encourages ice movement and glacial erosion.

But naturally enough, it is virtually impossible to OBSERVE meltwater activity in real ice sheet situations where ice may be kilometres thick and where ice surface conditions may be very hostile indeed for field parties undertaking research.  Drilling projects in many parts of Antartica have revealed the presence of meltwater lakes -- some of them very large indeed -- but little has been known about where the water comes from, how it is maintained, and where it goes to.

Now a new advance in radar technology has allowed researchers to identify two different types of meltwater flow beneath very thick ice, associated with rapid ice movement.  Beneath the Thwaites Glacier in West Antarctica, Schroeder and his colleagues have used radar signals to differentiate two types of meltwater flow -- one essentially like a swamp, with no deep channels but with sheets of water interconnected and flowing extensively across the land-ice interface; and the other involving concentrated meltwater flow in distinct channels:

The authors were most interested in the implications of this work for the possible instability of the West Antarctic Ice Sheet -- but there are a number of geomorphological implications which we should note.  First, it is quite possible for large quantities of meltwater to be flowing on the bed of a very thick ice sheet or ice stream when it is operating normally -- rather than just when there is catastrophic wastage going on.  Second, extensive or areal flow and concentrated linear flow can occur beneath the same glacier, with a transition zone between the two types of flow.  Third, it appears from the work that the water is flowing uphill, on the up-glacier flank of a rock ridge -- that confirms the assumption made many hears ago that subglacial drainage operates wherever there is sufficient hydraulic pressure, regardless of ground surface slope.  Fourth, when the meltwater is concentrated into channels beneath the ice, it seems to move in distinct tunnels on the glacier bed with semi-circular or arches cross profiles.  That is interesting, because we have not had any great evidence on this until now.  We still do not know from this work whether there are deep rock-cut channels beneath the ice tunnels -- but if water is flowing in great volumes under high pressure it is almost inevitable that substantial bedrock erosion will be taking place.

There are strong implications in all of this work for the processes that might have operated in North Pembrokeshire during the Anglian and Devensian glacial episodes, when the Irish Sea Glacier cvovered much of the landscape.  For example, what does this work tell us about the cutting of the Gwaun Valley meltwater channel system, or even about the cutting of the gorge at Rhosyfelin?  Watch this space.....

Scientists Image Vast Subglacial Water System Underpinning West Antarctica’s Thwaites Glacier 

Evidence for a water system transition beneath Thwaites Glacier, West Antarctica

Dustin M. Schroeder,
Donald D. Blankenship, and
Duncan A. Young

Thwaites Glacier is one of the largest, most rapidly changing glaciers on Earth, and its landward-sloping bed reaches the interior of the marine West Antarctic Ice Sheet, which impounds enough ice to yield meters of sea-level rise. Marine ice sheets with landward-sloping beds have a potentially unstable configuration in which acceleration can initiate or modulate grounding-line retreat and ice loss. Subglacial water has been observed and theorized to accelerate the flow of overlying ice dependent on whether it is hydrologically distributed or concentrated. However, the subglacial water systems of Thwaites Glacier and their control on ice flow have not been characterized by geophysical analysis. The only practical means of observing these water systems is airborne ice-penetrating radar, but existing radar analysis approaches cannot discriminate between their dynamically critical states. We use the angular distribution of energy in radar bed echoes to characterize both the extent and hydrologic state of subglacial water systems across Thwaites Glacier. We validate this approach with radar imaging, showing that substantial water volumes are ponding in a system of distributed canals upstream of a bedrock ridge that is breached and bordered by a system of concentrated channels. The transition between these systems occurs with increasing surface slope, melt-water flux, and basal shear stress. This indicates a feedback between the subglacial water system and overlying ice dynamics, which raises the possibility that subglacial water could trigger or facilitate a grounding-line retreat in Thwaites Glacier capable of spreading into the interior of the West Antarctic Ice Sheet.

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