Investigating tidal flexure on an ice shelf using kinematic GPS

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Abstract
The positions of ice-stream grounding zones are uniquely sensitive to changes in the mass balance of the ice sheet. Present methods for locating groundingzone features are either imprecise or require considerable effort in interpretation and so are of little value for change studies. We present a new method which uses the kinematic GPS technique to locate the position of the limit of tidal flexure. The method involves the collection of at least two surface-elevation profiles along the same track through the grounding zone, at different times during the tidal cycle. The elevation profiles obtained coincide upstream of the: limit of flexure but diverge downstream of the limit of flexure. Subtracting the profiles produces a tidal-def1ection profile which shows directly the response of the ice shelf to the tidal forcing. We present two examples of the use of this method, both on Rutford Ice Stream. Antarctica. The first is across the grounding zone and shows that the method is capable of measuring grounding positions to around 200 m precision. The second, taken across an active shear margin, shows a tidal-deflection profile, with an absence of steps that would indicate the presence of fracture planes penetrating from the ice base to sea level.