Wilkes Subglacial Basin: Earth Structure Influence on Ice-Sheet Stability
In Antarctica, present-day ice loss is focused in regions grounded below sea level, where warm ocean water erodes the ice shelves from below, resulting in an acceleration of the ice streams draining these areas. If the bed slopes inward, this initial acceleration can lead to a run-away effect known as the "Marine Ice-Sheet Instability" (MISI, Weertman, 1974; Schoof, 2007). The magnitude and timing of the ice-sheet response strongly depends on the physical conditions at the ice-bed interface, and tectonic evolution exerts first-order control on these conditions. That is, local geologic conditions strongly affect the response to ongoing and future perturbations; however, in most cases, the relevant solid-Earth parameters are largely unknown. The Wilkes Subglacial Basin (WSB) in East Antarctica is one such region that may be prone to the MISI and which may significantly contribute to sea-level change in the future.
Figure 1. Impact of solid-Earth parameters on simulations of ice-sheet stability in the WSB. (A) Present day bedrock topography of Antarctica (Fretwell et al., 2013). (B) Simulation of increased ice retreat in the WSB during the Pliocene in response to lower bedrock elevations (relative to present day), predicted by mantle-flow estimates from Austermann et al. (2015). Decreased bedrock elevations will amplify the MISI. (C) Simulated sensitivity of the WSB region to mantle viscosity in response to climate warming (Gomez et al., 2015). A relatively low viscosity will tend to dampen the MISI during ice-sheet retreat by "lifting" the grounding line due to more rapid glacial isostatic adjustment and lead to less ice loss.
Uncertainties in key solid-Earth parameters make it challenging to assess the role that the WSB has played in past sea-level change or could play in the future (Fig. 1). In order to produce consensus in modeled ice-sheet projections for Antarctica, the Earth structure beneath the ice sheet must be constrained. The WSB is also of interest as its origin, possible relationship to the Transantarctic Mountains, and its role in the tectonic evolution of Antarctica are all matters of debate.
Uncertainties in key solid-Earth parameters make it challenging to assess the role that the WSB has played in past sea-level change or could play in the future (Fig. 1). In order to produce consensus in modeled ice-sheet projections for Antarctica, the Earth structure beneath the ice sheet must be constrained. The WSB is also of interest as its origin, possible relationship to the Transantarctic Mountains, and its role in the tectonic evolution of Antarctica are all matters of debate.
Figure 2. Map showing the location of the WSB, the proposed seismic network (red triangles), and previous seismic experiments (gray circles). Flight lines associated with previous airborne geophysical surveys are shown by gray lines.
To constrain the Earth structure beneath the WSB and resolve the impact that these parameters have on the stability of the East Antarctic Ice Sheet, we have proposed an integrated observational and modeling experiment that relies on the combined expertise from observational seismologists, computational geodynamicists, glaciologists, and ice-sheet modelers. Using data from a new seismic field campaign (Fig. 2), we will refine tectonic models for the WSB and will estimate the thermal, density, and viscosity structure of the upper mantle. These solid-Earth characteristics will be used to simulate mantle flow and paleotopography as well as to model past and future ice-sheet stability.
Figure 3. Significance of seismic resolution on dynamic topography estimates. Mantle flow computations are based on (A) the global tomography model S40RTS (Ritsema et al., 2011) and (B) S40RTS embedded with the continental-scale model from Danesi and Morelli (2001). Results are preliminary and meant to illustrate the effects of regional density anomalies only. Computations are based on the approach of Becker et al. (2015) but using global rather than regional mantle flow constraints. The computations allow for lateral viscosity variations based on the temperature dependence of viscosity.
The National Science Foundation has provided two years of "planning" funding for this project (OPP-1914698). The full (fieldwork-based) proposal will be reviewed following this time.