Abstract
Abstract. We examine tidal flexure in the grounding zone of the McMurdo Ice Shelf, Antarctica, using a combination of TerraSAR-X repeat-pass radar interferometry, a precise digital elevation model, and GPS ground validation data. Satellite and field data were acquired in tandem between October and December 2014. Our GPS data show a horizontal modulation of up to 60 % of the vertical displacement amplitude at tidal periods within a few kilometres of the grounding line. We ascribe the observed oscillatory horizontal motion to varying bending stresses and account for it using a simple elastic beam model. The horizontal surface strain is removed from nine differential interferograms to obtain precise bending curves. They reveal a fixed (as opposed to tidally migrating) grounding-line position and eliminate the possibility of significant upstream bending at this location. The consequence of apparent vertical motion due to uncorrected horizontal strain in interferometric data is a systematic mislocation of the interferometric grounding line by up to the order of one ice thickness, or several hundred metres. While our field site was selected due to its simple boundary conditions and low background velocity, our findings are relevant to other grounding zones studied by satellite interferometry, particularly studies looking at tidally induced velocity changes or interpreting satellite-based flexure profiles.
Highlights
Ice shelves are the floating seaward extensions of the Antarctic Ice Sheet
While our field site was selected due to its simple boundary conditions and low background velocity, our findings are relevant to other grounding zones studied by satellite interferometry, studies looking at tidally induced velocity changes or interpreting satellite-based flexure profiles
We examined tidal motion of ice in an ice shelf grounding zone using a combination of satellite interferometry, GPS, and modelling
Summary
Ice shelves are the floating seaward extensions of the Antarctic Ice Sheet. They move vertically with ocean tides and are continuously bent in the grounding zone which forms their landward margin. The assumption that horizontal velocity remains constant between satellite passes may be incorrect as flexural bending effects may alter the flow field (Doake et al, 2002), and variations in subglacial water pressure can modify the ice flow up to tens of kilometres upstream of the ground line over tidal frequencies (Anandakrishnan et al, 2003; Gudmundsson, 2007; Marsh et al, 2013) These changes may not be linearly related to the tides (King et al, 2011; Rosier et al, 2015), further complicating the interpretation of interferograms. We discuss the results in the final section along with implications for the significantly improved observation of tidal flexure using SAR interferometry
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