Modeling the elastic transmission of tidal stresses to great distances inland in channelized ice streams

摘要

Geodetic surveys suggest that ocean tides can modulate the motion ofAntarctic ice streams, even at stations many tens of kilometers inland fromthe grounding line. These surveys suggest that ocean tidal stresses canperturb ice stream motion at distances about an order of magnitude fartherinland than tidal flexure of the ice stream alone. Recent models exploringthe role of tidal perturbations in basal shear stress are primarily one- ortwo-dimensional, with the impact of the ice stream margins either ignored orparameterized. Here, we use two- and three-dimensional finite-elementmodeling to investigate transmission of tidal stresses in ice streams andthe impact of considering more realistic, three-dimensional ice streamgeometries. Using Rutford Ice Stream as a real-world comparison, wedemonstrate that the assumption that elastic tidal stresses in ice streamspropagate large distances inland fails for channelized glaciers due to anintrinsic, exponential decay in the stress caused by resistance at the icestream margins. This behavior is independent of basal conditions beneath theice stream and cannot be fit to observations using either elastic ornonlinear viscoelastic rheologies without nearly complete decoupling of theice stream from its lateral margins. Our results suggest that a mechanismexternal to the ice stream is necessary to explain the tidal modulation ofstresses far upstream of the grounding line for narrow ice streams. Wepropose a hydrologic model based on time-dependent variability in tillstrength to explain transmission of tidal stresses inland of the groundingline. This conceptual model can reproduce observations from Rutford Ice Stream.