Glacial Isostatic Adjustment in Antarctica : a rheological study

The Antarctic Ice Sheet (AIS) is the largest ice sheet on Earth that has known important mass changes during the last 20 kyrs. These changes deform the Earth and modify its gravity field, a process known as Glacial Isostatic Adjustment (GIA). GIA is directly influenced by the mechanical properties and internal structure of the Earth, and is monitored using Global Navigation Satellite System positioning or gravity measurements. However, GIA in Antarctica remains poorly constrained due to the cumulative effect of past and present ice-mass changes, the unknown history of the past ice-mass change, and the uncertainties of the mechanical properties of the Earth. The viscous deformation due to GIA is usually modeled using a Maxwell rheology. However, other geophysical processes employ Andrade (tidal deformation) or Burgers (post-seismic deformation) laws that could result in a more rapid response of the Earth. We investigate the effect of using these different rheology laws to model GIA-induced deformation in Antarctica. Employing the ALMA and TABOO softwares, we use the Love number and Green functions formalism to compute the surface motion and the gravity changes induced by the past and present ice-mass redistributions. We use the elastic properties and the radial structure of the preliminary reference Earth model (PREM) and the viscosity profile given by Hanyk (1999). The deformation is computed for the three rheological laws mentioned above using ICE-6G and elevation changes from ENVISAT (2002-2010) to represent the past and present changesof the AIS, respectively. We obtain that the three rheological laws lead to significant Earth response within a 20 kyrs time interval since the beginning of the ice-mass change. The differences are the largest between Maxwell and Burgers rheologies during the 500 years following the beginning of the surface-mass change. Regarding the response to present changes in Antarctica, the largest discrepancies are obtained in regions with the greatest current melting rates, namely Thwaites and Pine Island Glacier in West Antarctica. Uplift rates computed twelve years after the end ofthe present melting using Burgers and Andrade rheologies are five and two times larger than those obtained using Maxwell, respectively.