solis

Project researchers

  • Dr Bernd Kulessa, Department of Geography, University of Wales, Swansea
  • Dr Adrian Luckman, Department of Geography, University of Wales, Swansea
  • Professor Peter Sammonds, Department of Earth Sciences, University College London
  • Dr Edward King, Physical Sciences Division, British Antarctic Survey 
  • Dr Daniela Jansen, Department of Geography, University of Wales, Swansea

Introduction

Many ice shelves on the Antarctic Peninsula have retreated in the light of ongoing climatic warming. The dramatic break-ups of the Larsen A and B ice shelves in 1995 and 2002 have seen particularly pronounced scientific and public interest. Many outlet glaciers accelerated as the buttressing ice-shelf force was removed, increasing the volumes of ice discharged from the ice sheet interior into the ocean. This ‘domino effect’ highlights the importance of ice shelves in controlling sea level rise. As climatic warming is progressing southwards on the Antarctic Peninsula, scientific focus has been shifting to the southern neighbour, the Larsen C ice shelf. This ice shelf is one of the largest in Antarctica and is buttressing a considerable number of outlet glaciers that evacuate large quantities of ice from the Antarctic interior. Identifying the present and future stability of the Larsen C ice shelf is therefore a global research priority.

A multi-disciplinary, multi-national research program is currently underway on the Larsen C ice shelf to elucidate the physical properties and processes that govern its stability, and to predict future stability as forced by climatic warming. The fundamental hypothesis of the SOLIS project is that mechanically softer ‘flow stripes’ of ice originate at mountains fronts in the vicinity of the ice-shelf grounding line. These flow stripes are sandwiched between mechanically stiffer units of glacier ice that originates in the ice-sheet interior. Satellite and structural glaciological observations suggest that the softer flow stripes critically control rates of rift propagation and thus represent a governing control on ice shelf stability.

SOLIS aims to integrate the field geophysical and glaciological findings with satellite data to constrain a multi-dimensional numerical model of ice-shelf fracture mechanics to infer the present and predict the future stability of the Larsen C ice shelf.

Field Campaigns

First field season: Dec/Jan 2008/9

The field site was located in the South-East of the Larsen C Ice Shelf, close to the tips of the large scale rifts originating downstream of Kenyon Peninsula. Three key common off-set radar profiles (all ~ 19 kilometres long) were acquired using the Sensors & Software PE Pro System with 50 MHz antennas. Two profiles were directed along the ice flow direction, one acquired on the 5 km wide flow band formed downstream of Joerg Peninsula, and one on glacier-derived ice to the South of the flow band. The third profile was directed perpendicular to the flow, crossing the other profiles at right angles. Two orthogonal seismic profiles (respectively aligned south-north and west-east) were acquired at each of the two cross-over points.

Second field season: Nov/Dec 2009

In the second field season we focussed on the same flow band further upstream, about ? km east of Joerg Peninsula. A 20 km grid of GPR-profiles with a spacing of 4 km between the profiles was acquired to sample ice thickness and structure across and along the flow band and the adjacent glacier-derived ice units. A 2 km interval was employed close to the centre of the grid to focus the key flow unit originating from Joerg Peninsula.

In addition to the 20km grid, two forays were made further afield. The first was to local features to the North-East of the grid thought to be the surface expression of basal crevasses. The second was a radar line to the North of the grid designed to sample transitions between a number of different flow units.

Maps and additional information are to be made available.

Preliminary Results

The flow bands visible on remote sensing images are connected with irregular reflectors within the ice column, partly disrupted by normal depth basal reflectors.

The depth of the irregular reflector is increasing with distance to the grounding line, due to accumulation of snow while moving downstream.

The flow stripe features are also evident in the vertical component of the GPS data as depressions. The vertical displacement relative to the adjacent flow units levels out further downstream, which is an indication for further basal ice accretion due to freezing of sea water in the gaps.

A linear radar profile to the north perpendicular the ice flow direction shows well defined basal reflectors in the flow units originating from glaciers feeding the ice shelf and diffuse, shallow reflectors within the suture zones in between. The findings derived from the higher resolution survey downstream of Joerg Peninsula might therefore be upscaled to other suture zones within the Larsen C ice shelf.

Reporting

For maps & detailed technical information see the complete field report.

Modelling

We modelled the flow of the Larsen C and northernmost Larsen D ice shelves using an adapted continuum-mechanical model, and applied a fracture criterion to the simulated velocities to investigate its present-day stability. Constraints come from satellite data and geophysical measurements in the 2008-09 and 2009-2010 austral summer. We obtained excellent agreements between modelled and measured ice-flow velocities, and inferred and observed distributions of rifts and crevasses. Ice-shelf thickness was derived from BEDMAP and ICESat data and depth-density inferred from our seismic data. Notable exceptions occur in regions of modelled basal accretion down flow of promontories, thus placing the first quantitative constraints on their mechanical effects. Anomalously soft marine ice, advected into the ice shelf in flow-parallel bands, controls rates of rift propagation downstream.

Our model simulations confirm that the Larsen C ice shelf is stable in its current dynamic regime. Ice-mechanical heterogeneities in ice-stream suture zones, sustained by marine-ice production down flow of promontories, have significant stabilising effects on the ice shelf. Reduction in rates of marine-ice production could therefore lead to weakening of suture zones and possibly development of Larsen B-style dynamic conditions prior to its disintegration. First model studies with an extended continuum-mechanical flow model and fracture criterion allowing for ice-shelf mechanical heterogeneities show, that weakening of prominent marine ice-rich flow bands, inferred to be dominant in the north and south of Larsen C ice shelf would, promote Larsen B-style mechanical evolution. This emphasizes the importance of further research into the mechanics of suture zones and their dependency on marine ice provenance, together with thorough quantification of their modification of the ice-shelf stress regime and thus its stability.