Abstract
ABSTRACTGrounding zones are vital to ice-sheet mass balance and its coupling to the global ocean circulation. Processes here determine the mass discharge from the grounded ice sheet, to the floating ice shelves. The response of this transition zone to tidal forcing has been described by both elastic and viscoelastic models. Here we examine the validity of these models for grounding zone flexure over tidal timescales using field data from the Southern McMurdo Ice Shelf (78° 15′S, 167° 7′E). Observations of tidal movement were carried out by simultaneous tiltmeter and GPS measurements along a profile across the grounding zone. Finite-element simulations covering a 64 d period reveal that the viscoelastic model fits best the observations using a Young's modulus of 1.6 GPa and a viscosity of 1013.7 Pa s (≈ 50.1 TPa s). We conclude that the elastic model is only well-constrained for tidal displacements >35% of the spring-tidal amplitude using a Young's modulus of 1.62 ± 0.69 GPa, but that a viscoelastic model is necessary to adequately capture tidal bending at amplitudes below this threshold. In grounding zones where bending stresses are greater than at the Southern McMurdo Ice Shelf or ice viscosity is lower, the threshold would be even higher.
Highlights
The vast majority (74%) of Antarctica’s outer margins are confined by freely-floating ice shelves and ice tongues (Bindschadler and others, 2011)
We present the effects of changing the ice rheology on the modelled flexure curves by systematically varying Young’s modulus and viscosity
An elastic and a viscoelastic model have been compared with field observations for a range of plausible values of the Young’s modulus and ice viscosity
Summary
The vast majority (74%) of Antarctica’s outer margins are confined by freely-floating ice shelves and ice tongues (Bindschadler and others, 2011). These areas are separated from the ice sheets by grounding zones where the ice detaches from the bedrock or sediment at its base. Thinning ice shelves (Pritchard and others, 2009, 2012) reduce the amount of buttressing (Dupont and Alley, 2005) and may lead to an enhanced ice discharge to the ocean on a retro-grade bed (Gudmundsson, 2013). Recent decrease in ice-shelf volume (Paolo and others, 2015) is attributed to both atmospheric variability (Scambos and others, 2013) and modified oceanic conditions (Dutrieux and others, 2014). The mass input from tributary ice streams to ice shelves, combined with knowledge of accumulation on the ice sheets, provides a basis for modelling the mass balance behaviour of Antarctica (Rignot and Thomas, 2002)
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