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Is mechanical heterogeneity controlling
the 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, [email protected]
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)
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
+ Constraints on the future evolution of these parameters
Ice thickness based on combined ICESat and Bedmap
Bedmap & ICESat (m)
1300
1250
‘Combined’
minus
‘Bedmap only’
y (km)
1200
1150
1100
1050
80
60
40
-2000
20
-2050
0
-2100
-20
-2150
-40
-2200
-60
-2250
-80
-2300
-100
-2350
-120
-140
1000
Difference to Bedmap:
mainly thinner ice front
x (km)
How does this compare with Griggs and Bamber, GRL, in press?
Firn / ice densities based on seismic data (from 2008/09 season)
-50
-100
Ice thickness (m)
-150
-200
-250
-300
-350
-400
-450
-500
In-situ density
-550
Mean density of
overlying ice column
-600
500
600
700
800
Transition from firn to consolidated ice
(915 kg/m³) at ~ 80 m depth
900
Mean density of upper layer: 770 kg /m³
Density (kg/m³)
Firn density correction here + in Griggs and Bamber, GRL, in press?
Updated velocity map (RAMP + feature tracking)
1300
800
700
600
1250
500
400
300
1200
200
• Preliminary velocity
map partly noisy
• More filtering could
smooth out real velocity
gradients
y (km)
100
0
1150
m/a
• No predictive capability
1100
Velocity inversion for
strain/stress not
good enough for
fracture criterion
1050
1000
-2350
-2300
-2250
-2200
-2150
x (km)
-2100
-2050
-2000
700
Modelled vs.
measured velocities
700
600
600
500
500
400
400
300
1300
m a-1
300
200
200
100
1250
0
100
1200
y (km)
0
m a-1
1150
200
1100
150
1050
200
150
1000
100
50
100
-2350
-2300
-2250
-2200
-2150
-2100
-2050
-2000
x (km)
~ 5% difference to GPS
derived velocities (2008/09)
50
0
-50
0
-50
-100
-100
-150 -150
-200
-200
Deviations in regions
with major rifts
-200 -150 -100 -50
0
50 100 150 200
Fracture mechanics: regions of potential crevasse opening
Stress intensity factor
(Fracturing > ~ 50)
1300
50
1250
00
1200
y (km)
00
50
kPa/m0.5
80
80
70
70
60
60
50
50
40
40
1150
00
1100
30
30
50
1050
20
20
10
10
0
0
00
1000
-2350 -2300
-2350 -2250
-2300 -2200
-2250 -2150
-2200 -2100
-2150 -2050
-2100 -2000
-2050 -2000
x (km) x (km)
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
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.
Solberg Inlet
Trail Inlet
Ice
Flow
~ 505 m a-1
10 km
50 MHz Common-Offset GPR
(0.8 ns SI, 8 stacks)
1 trace ~ every 3 m
incl. GPS position +/- 5m
Ice
Flow
~ 320 m
View
Holland, P. R. et al. (2009),
Marine ice in Larsen Ice Shelf
Geophys. Res. Lett., 36, L11604
doi:10.1029/2009GL038162.
Comparison with modelling reveals
characteristic two-lobe structure
N
Better defined englacial reflectors parallel than orthogonal to flow
Englacial debris ‘stringers’ by analogy with
Filchner-Ronne ice shelf?
View
N
CMP2-South W-E Vertical Geophones
2
3
4
1: Multiply reflected diving waves  firn density profiles
2: 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 Shot offset (0 – 1110
6: Multiple of P-wave reflection from seabed
5
6
Shot offset (0 – 1110 m)
1: Multiply reflected diving waves  firn density profiles
Two-way travel time (0 – 1500 ms)
6
2: P-wave reflection from ice-shelf base
CMP2-Sout
3
7
5
8
CMP2-South W-E Horizontal Geophones
3: P-S conversion at ice shelf base
34: Multiple of P-wave reflection from ice-shelf base
75: P-wave reflection from seabed
6: Multiple of P-wave reflection from seabed
5
8
Two-way travel time (0 – 1500 ms)
Two-way travel time (0 – 1500 ms)
1
Two-
Shot offset (0 – 1110 m)
High-quality seismic
and GPR CMP data
to estimate
mechanical properties
of firn, meteoric and
marine ice
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
Synthesis and modelling of future scenarios
• 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.)
Footnote 1: significant temporal changes in firn density?
950
King&Jarvis 1989
Density (kg m^-3)
850
750
2008/09 Seismic-N W-E
650
2008/09 Seismic-N S-N
2008/09 Seismic-S W-E
2008/09 Seismic-S S-N
550
1989 - 0km
1989 - 15km
1989 - 24km
450
0
10
20
30
40
50
60
Depth (m)
70
80
90
100
CMP1-North
1000
900
Density (kg m^-3)
Footnote 2: significant
differences in seismic vs.
GPR derived densities
800
700
600
500
CMP1-North Seismic W-E
400
CMP1-North Seismic S-N
CMP1-North GPR S-N
300
CMP1-North GPR W-E
200
1000
CMP2-South
900
Density (kg m^-3)
0
10
20
700
600
500
CMP2-South Seismic W-E
400
CMP2-South Seismic S-N
CMP2-South GPR S-N
CMP2-South GPR W-E
300
200
10
20
30
40
50
60
Depth (m)
800
0
30
40
50
60
Depth (m)
70
80
90
100
70
80
90
100
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