GOCE+Antarctica

GOCE+Antarctica- Dynamic Antarctic Lithosphere

Start date
29 January, 2016
End date
30 November, 2017

GOCE+Antarctica- Dynamic Antarctic Lithosphere -is an international project supported by the European Space Agency (ESA) that is using GOCE satellite gravity gradient data, GPS data and innovative 3D modelling to study the Antarctic lithosphere and its influence on the overlying ice sheets, including the process of glacio-isostatic adjustment (GIA).

Highlights

Enhanced satellite gravity gradient images

Publishing in the Nature journal Scientific Reports –Ebbing et al (2018) present global satellite gravity curvature maps that enable new interpretations of the lithospheric setting of Antarctica.

New curvature maps for Antarctica from GOCE satellite gravity.

The contrasts between the cratonic lithosphere of East Antarctica and the younger lithosphere of West Antarctica are apparent from these enhanced satellite gravity images. They also shed new light into the extent of the West Antarctic Rift System hidden beneath the West Antarctic Ice Sheet and offer new geophysical perspectives into the Marie Byrd Land hotspot.

Shape Index for Antarctica from GOCE satellite gravity.

Three distinct lithospheric domains can now be recognised from their satellite gravity signatures in interior East Antarctica. One corresponds to the composite Mawson Craton; another overlies the Tonian Oceanic Arc Superterrane in interior Dronning Maud Land; a currently less-well understood third province likely marks a cratonic margin region between the Weddell Sea and South Pole. This frontier is also being investigated in the ESA PolarGAP project, which also helps fill the data void in GOCE south of 83°S.

Plate tectonic animation from 200 Ma to present and Shape Index from GOCE

The “shape index” derived from satellite gravity for Antarctica and other continents were placed into a plate reconstruction. This provides new views of the similarities and the contrasts in lithospheric architecture and tectonic processes between Antarctica and formerly adjacent continents (Australia, Zelandia, South America, Africa and India) within Gondwana.

 

Rapid bedrock uplift beneath the West Antarctic Ice Sheet in the Amundsen Sea Embayment

Publishing in Science –Barletta et al (2018) show that the viscosity of the mantle under the Amundsen Sea Embayment – the mostly rapidly changing sector of the West Antarctic Ice Sheet- is particularly low and this explains the remarkably rapid bedrock uplift rates observed.

Observed bedrock motions (a) and West Antarctic ice sheet changes (b).

The marine portion of the West Antarctic Ice Sheet in the Amundsen Sea Embayment is a major contributor to global sea-level rise and is vulnerable to catastrophic collapse. But what is the role of the Solid Earth and in particular bedrock motion in modulating ice sheet retreat?

New GPS data reveal extremely rapid, up to 41 millimeters per year, bedrock uplift in the region, one of the fastest GIA-related uplift rates ever recorded. This could potentially slow down future grounding line retreat of Thwaites and Pine Island Glacier.

Schematic showing Solid Earth deformation caused by ice sheet changes.

Over 800 Earth models with about 10,000 variations of these models were combined with different scenarios of ice sheet history. By comparing the residual with predictions from viscoelastic modelling, a viscosity range for the shallow upper mantle between 1018 and 6.3 × 1018 Pa s was found to best fit the remarkably high uplift rates observed.

Grounding line migration and bedrock uplift.

Such low viscosity is likely linked to the hot upper mantle beneath parts of the West Antarctic Rift System, as suggested by seismological models and an elevated geothermal heat flux. The low viscosity in deeper layers may be related to a mantle plume, such as hypothesized in Marie Byrd Land. However, a mantle chemically altered by protracted Paleozoic-Cretaceous subduction along the former West Antarctic active margin also helps explain the low viscosity.

 

Current 3D modelling

PhD student Pappa (Univ. of Kiel) and his co-workers are finalising their efforts to combine modelling of GOCE satellite gravity data with independent seismological constraints and petrological and thermal modelling to derive a new 3D model of the Antarctic lithosphere.

The results provide new insights on crustal thickness variations beneath East and West Antarctica and the depth of the Lithosphere-Asthenosphere Boundary (LAB). Important new constraints on the thermal structure of the lithosphere, with implications for our understanding of the controls on upper mantle viscosity and geothermal heat flux variations are emerging. This is particularly important in the quest to better comprehend Solid Earth influences on the dynamics of the Antarctic ice sheets, including processes such as GIA.

Map showing the best-fit Moho (crust-mantle) depth estimate for Antarctica from inversion of satellite gravity data and the differences with previous seismically-derived estimates.

 

Map showing the best-fit depth estimate for the Lithosphere-Asthenosphere Boundary (LAB). Note the location of the profile in next image.

 

Profile view extracted from the new 3D model showing the contrast in crustal and lithosphere thickness between West and East Antarctica and variations in both density structure and architecture.

 

3D view of the new thermal model for the Antarctic lithosphere.

 

GIA modelling

Viscosity can be computed using a stress-strain rate relationship. For this a creep law for olivine is selected, which is assumed to be the material that governs deformation in the upper mantle. Using the temprature in the figure above, viscotiy maps, shown below, were calculated for dry olivine with 4 mm grain size. High viscosity in East Antarctica result in a thick effective viscosity there.

Viscosity map at different depths for the temperature model (credit to Wouter van der Wal)

Uplift rates are predicted with a 3D GIA model for the viscosity model for different creep parameters and ice loading model of Whitehouse et al (2012). Both large and small viscosity can result in a lower uplift rates. The figure below shows the effect of a weaker rheology which leads to subdued uplift rates.

Uplift rates for the temperature model in figure 5 and creep parameters for dry and 4 mm grain size olivine (left) and wet olivine of 10 mm grain size (right). (credit to Wouter van der Wal)

PhD student Blank (Delft University of Technology) developed a new GIA model which has higher resolution in Antarctica (see figure below). This allows studying smaller scale signals in Antarctica related to recent ice melting, in combination with low viscosity. The model has been benchmarked with tests shown in Martinec et al. (GJI, 2018)

Finite-element mesh for the new model (left) vs a model with the mesh characteristics of the old model (right). (credit to Wouter van der Wal)

 

The GOCE+Antarctica project will enable

1) studies of the structure of the lithospheric cradle on which Antarctic ice sheets flow;
2) new assessments of how lithospheric processes influence bedrock topography;
3) increased knowledge of the interactions between the Solid Earth and the overlying Antarctic ice sheets, in particular GIA processes.

This project promises to shed fundamental new light into deep structures beneath Antarctica. By addressing how the deeper Solid Earth influences ice sheet behavior it will also impact on our ability to comprehend and predict how these affect both present and future sea-level rise.

Project Work and Institutions

The GOCE+Antarctica project involves a research team from several European Universities and research institutions and can be subdivided into six main components:

1) Review of Antarctic lithosphere studies (including seismological, airborne geophysical, satellite gravity and xenolith studies);
2) Geophysical data assembly;
3) Bedrock topography studies;
4) Crust and lithosphere modelling;
5) GIA modelling and analysis;
6) Presentations, publications and outreach activities.

Partner responsibilities

  • The Christian-Albrechts-University in Kiel, Germany (Jorg Ebbing) is responsible for project management, satellite gravity gradient data analyses and lithosphere modelling. Prof. Ebbing will also supervise a PhD student (Folker Pappa) working on the project.
  • The British Antarctic Survey in Cambridge, UK (Fausto Ferraccioli) will investigate bedrock topography, contribute to crustal modelling, support geological and tectonic interpretation, and lead outreach/web page activities.
  • The DTU Space- Technical University of Denmark (Prof Rene Forsberg) will be responsible for PolarGAP airborne gravity data analysis and ice sheet modelling activities (Valentina Barletta).
  • The Delft University of Technology in the Netherlands (Wouter van der Wal & Bart Root) will lead work to incorporate the new Antarctic lithosphere models into GIA models.

Fausto Ferraccioli

Science Leader

BAS Science strategy team, Geology and Geophysics team

 

International partners include:

CAU

Jorg

Jorg Ebbing 

Folker_Pappa

Folker Pappa PhD Student

DTU UK B1 RGB

rf1

Rene Forsberg Professor, Head of Geodynamics

valentina

Valentina Barletta Postdoc

Delft

FOTO GUUS SCHOONEWILLE -

Wouter van der Wal Assistant Professor – Planetary Exploration, Astrodynamics and Space Missions group

Bas Blank PhD Student