Is mechanical heterogeneity controlling the stability of the Larsen C ice shelf? Bernd Kulessa 1, Daniela Jansen 1, Edward King 2, Adrian Luckman 1, Peter Sammonds 3 1School of the Environment and Society, Swansea University, UK, 2British Antarctic Survey, High Cross, Cambridge, UK 4Department of Earth Sciences, University College London, UK

What we want to do (SOLIS Project) Assess present + model the future stability of the Larsen C ice shelf Identify regions of crevasse opening using 2-D fracture criterion (Rist et al., 1999; updated for ice shelf mechanical heterogeneities) + Constraints on the future evolution of these parameters Stress fieldContinuum-mechanical flow model calibrated by present surface velocities (updated RAMP) (Sandhäger et al., 2000, 2005; Jezek et al., 2008) 3-D ice thickness / structure GPR, seismic reflection, BEDMAP, satellite altimeter data / modelling (Holland et al., 2009; Griggs et al., in press) Various ice mechanical properties Seismic reflection, GPR, model calibration by present patterns of fracturing

Difference to Bedmap: mainly thinner ice front Ice thickness based on combined ICESat and Bedmap ‘Combined’ minus ‘Bedmap only’ How does this compare with Griggs and Bamber, GRL, in press?

In-situ density Mean density of overlying ice column Transition from firn to consolidated ice (915 kg/m³) at ~ 80 m depth Mean density of upper layer: 770 kg /m³ Firn / ice densities based on seismic data (from 2008/09 season) Firn density correction here + in Griggs and Bamber, GRL, in press?

Preliminary velocity map partly noisy More filtering could smooth out real velocity gradients No predictive capability m/a x (km) y (km) Velocity inversion for strain/stress not good enough for fracture criterion Updated velocity map (RAMP + feature tracking)

Modelled vs. measured velocities m a -1 ~ 5% difference to GPS derived velocities (2008/09)

Deviations in regions with major rifts

Stress intensity factor (Fracturing > ~ 50) Fracture mechanics: regions of potential crevasse opening kPa/m 0.5 D. Jansen, B. Kulessa et al., Fracturing of Larsen C and implications for ice-shelf stability, J. Glaciol., shortly in review

Next: model improvements - structural / mechanical heterogeneities 10 km Ice Flow ~ 505 m a -1 Solberg Inlet Trail Inlet Glasser, N., B. Kulessa, A. Luckman, E. C. King, P. R. Sammonds, T. Scambos, K. Jeczek The structural glaciology and inferred ice mechanical properties of the Larsen C ice shelf. Journal of Glaciology, 55(191),

~ 320 m 50 MHz Common-Offset GPR (0.8 ns SI, 8 stacks)  1 trace ~ every 3 m incl. GPS position +/- 5m Ice Flow View

Comparison with modelling reveals characteristic two-lobe structure Holland, P. R. et al. (2009), Marine ice in Larsen Ice Shelf Geophys. Res. Lett., 36, L11604 doi: /2009GL N ~ 5 km

N View Better defined englacial reflectors parallel than orthogonal to flow Englacial debris ‘stringers’ by analogy with Filchner-Ronne ice shelf?

High-quality seismic and GPR CMP data to estimate mechanical properties of firn, meteoric and marine ice

Can do a pretty job reproducing current observations, know what the problems / weaknesses are (eliminate them) Estimate and implement ice structural / mechanical heterogeneities (if / as they matter) Thinner future ice shelf (due to basal or surface melting) Increasing local / regional stresses due to surface ponding Altered density / temperature profiles (surface melting, melt water percolation and refreezing) Different temperature profiles for the flow lines, e.g. marine ice, warmer (?) Different environmental conditions (waves, wind, etc.) Synthesis and modelling of future scenarios

King&Jarvis 1989 Footnote 1: significant temporal changes in firn density?

Footnote 2: significant differences in seismic vs. GPR derived densities CMP1-North CMP2-South