Is mechanical heterogeneity controllingthe stability of the Larsen C ice shelf?

Bernd Kulessa1, Daniela Jansen1, Edward King2,Adrian Luckman1, Peter Sammonds3

1School of the Environment and Society, Swansea University, UK, b.kulessa@swansea.ac.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 field Continuum-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-2350 -2300 -2250 -2200 -2150 -2100 -2050 -2000

x (km)

Bedmap & ICESat (m)

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y (k

m)

Ice thickness based on combined ICESat and Bedmap

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‘Combined’minus

‘Bedmap only’

How does this compare with Griggs and Bamber, GRL, in press?

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Density (kg/m³)

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Ice

thic

knes

s (m

)

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

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m/a

x (km)

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m)

Velocity inversion for strain/stress

not good enough for fracture

criterion

Updated velocity map (RAMP + feature tracking)

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700Modelled vs.measured velocities

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m a-1

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~ 5% difference to GPSderived velocities (2008/09)

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Deviations in regions with major

rifts

Stress intensity factor(Fracturing > ~ 50)

Fracture mechanics: regions of potential crevasse opening

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x (km)

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y (k

m)

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kPa/m0.5

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D. Jansen, B. Kulessa et al., Fracturing of Larsen C andimplications for ice-shelf stability, J. Glaciol., shortly in review

Next: model improvements - structural / mechanical heterogeneities

10 km

IceFlow

~ 505 m a-1

Solb

erg

In

let

Tra

il I

nle

t

Glasser, N., B. Kulessa, A. Luckman, E. C. King, P. R. Sammonds, T. Scambos, K. Jeczek. 2009.The structural glaciology and inferred ice mechanical properties of the Larsen C ice shelf.

Journal of Glaciology, 55(191), 400-410.

~ 320 m

50 MHz Common-Offset GPR

(0.8 ns SI, 8 stacks)1 trace ~ every 3 m

incl. GPS position +/- 5m

IceFlow

View

Comparison with modelling revealscharacteristic two-lobe structure

Holland, P. R. et al. (2009),Marine ice in Larsen Ice ShelfGeophys. Res. Lett., 36, L11604doi:10.1029/2009GL038162.

N

~ 5 km

NView

Better defined englacial reflectors parallel than orthogonal to flow

Englacial debris ‘stringers’ by analogy withFilchner-Ronne ice shelf?

CMP2-South W-E Vertical Geophones

2

1

34

5

1: Multiply reflected diving waves firn density profiles2: P-wave reflection from ice-shelf base

3: P-S conversion at ice shelf base

4: Multiple of P-wave reflection from ice-shelf base

5: P-wave reflection from seabed

6: Multiple of P-wave reflection from seabed

6

Shot offset (0 – 1110 m)Tw

o-w

ay t

ravel ti

me (

0 –

15

00

ms)

CMP2-South W-E Vertical Geophones

2

1

34

5

1: Multiply reflected diving waves firn density profiles2: P-wave reflection from ice-shelf base

3: P-S conversion at ice shelf base

4: Multiple of P-wave reflection from ice-shelf base

5: P-wave reflection from seabed

6: Multiple of P-wave reflection from seabed

6

Shot offset (0 – 1110 m)

Tw

o-w

ay t

ravel ti

me (

0 –

15

00

ms)

CMP2-South W-E Horizontal Geophones

37

3: P-S conversion at ice shelf base

5: P-wave reflection off seabed

7: S-wave reflection from ice-shelf base

8: P-S conversion of seabed reflection at ice shelf base

58

Shot offset (0 – 1110 m)

Tw

o-w

ay t

ravel ti

me (

0 –

15

00

ms)

CMP2-South W-E Horizontal Geophones

37

3: P-S conversion at ice shelf base

5: P-wave reflection off seabed

7: S-wave reflection from ice-shelf base

8: P-S conversion of seabed reflection at ice shelf base

58

Shot offset (0 – 1110 m)

Tw

o-w

ay t

ravel ti

me (

0 –

15

00

ms)

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

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Depth (m)

De

nsi

ty (

kg

m^

-3)

2008/ 09 Seismic-N W-E

2008/ 09 Seismic-N S-N

2008/ 09 Seismic-S W-E

2008/ 09 Seismic-S S-N

1989 - 0km

1989 - 15km

1989 - 24km

King&Jarvis 1989

Footnote 1: significant temporal changes in firn density?

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Depth (m)

De

nsi

ty (

kg

m^

-3)

CMP2-South Seismic W-E

CMP2-South Seismic S-N

CMP2-South GPR S-N

CMP2-South GPR W-E

Footnote 2: significant

differences in seismic vs.

GPR derived densities

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Depth (m)

De

nsi

ty (

kg

m^

-3)

CMP1-North Seismic W-E

CMP1-North Seismic S-N

CMP1-North GPR S-N

CMP1-North GPR W-E

CMP1-North

CMP2-South