The Onset of Fast Flow of Ice Streams (Onset Experiment)
Current StatusThe first year of the experiment successfully collected over 600 km of short pulse radar data, 300 km of 50 MHz radar data, and over 200 motion stakes were surveyed by GPS.Scientific MotivationThe West Antarctic Ice Sheet (WAIS) is a marine ice sheet, grounded below sea level and potentially unstable in reponse to sea-level rise and/or climate warming. The flowof this ice sheet is variable on timescales ranging from centuries to millenia. The future of this ice sheet is a topic of considerable debate. The ice streams of the Ross Sea Embayment (A--F) drain the interior West Antarctic Ice Sheet by rapidly moving vast quantities of ice to the calving front of the Ross Ice Shelf. These ice streams are key factors in any discussion of WAIS stability because they are strongly out of balance.Understanding the role of the ice streams as buffers between theinterior ice and the floating ice shelves; the reasons for their fast flow; the factors controlling their current grounding-line-, margin-, and head-positions are crucial to any attempt at modeling the WAIS system and predicting the future of the ice sheet. For these reasons a fundamental objective of the WAIS project is to: Determine the physical controls on the ice motion, particularly the ice streams, how these processes are coupled to the atmosphere, lithosphere and ocean, and what aspects of this linked system drive the system toward or away from stability; For the Antarctic ice streams of the Siple Coast, we define the transition from no-sliding (or all internal deformation) to motion dominated by sliding as the ``onset-region''. To fully understand (and adequately model) the WAIS, this onset region must be better understood. The lateral margins of the ice streams are also a transition that need to be explained --- conceivably subject to hypotheses similar to those for the onset. The definition of the onset region is necessarily ambiguous because the formation of the ice streams is so poorly understood. The physical manifestion of the onset of streaming may be a change in the velocity (a peak in acceleration), a change in driving-stress, or both. The relationship between glacial parameters (bed roughness, bed wetness, fabric)and the onset is unknown and the new-found relation between the subglacialgeology and the onset is poorly understood. Hypotheses on controls of the location of the onset region range from the ``purely-glaciologic'' to the ``purely-geologic''; the answer is likely to be some combination of the two. Under one purely-glaciologic hypothesis, the basal water layer thickness increases sufficiently to drown controlling obstacles (under a Weertman-type theory) and ice streaming occurs. For this hypothesis, the boundaries of the catchment of the ice stream, the basal hydrologic potential and the driving stress are the parameters that control both the onset and the margins of the ice stream. Thus to model the ice sheet effectively, one would need a detailed surface and bed topography map, an accumulation map and geothermal heat flux; all of which, except for heat flux, can be estimated from radar work. Another purely glaciologic hypothesis for ice stream onset would involve the initiation of fast creep (e.g., Jakobshavn Isbr\ae). Ice stream A on the Siple coast and the ice streams that flow into the Amundsen Sea (Pine Island and Thwaites) are likely to have a large component of fast-creep. Ice streams C and D, however, are thought to be characterized by a fast-slidingcondition although it is possible that some percentage of thestrain of the ice streams in the onset region is due to fast-creep on orientedbasal planes near the sole. Carefully coupled seismic and radar experimentscan be designed to test this. The other extreme, a purely geologic hypothesis, would argue that the ice streams'onset-position and margins are entirely controlled by the subglacial sedimentary structure and properties. The sedimentary basins would determine where there are erosional source regions to produce till, which when mobilized and of sufficient thickness, would cause the ice stream to form. Thus, to model the ice sheet we need a good subglacial geologic map showing the distribution, thickness, and properties of the sedimentary basins, which can be estimated from seismic and other geophysical work. |
Engineering Details
LinksInfo for IO-205 Project Participants such as how to fill out the RPSC forms and how and when to ship boxes South. Some academic and manufacturer links of interest. Some of my own links (Linux, glaciology, seismology, news, etc.) Night/Day TerminatorToday's terminator at Byrd Surface Camp at noon. Click for a larger image. Image courtesy of the xearth program
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