is mechanical heterogeneity controlling the stability of the larsen c ice shelf? bernd kulessa 1,...
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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, [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)
+ 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)
1000
1050
1100
1150
1200
1250
1300
y (k
m)
Ice thickness based on combined ICESat and Bedmap
-140
-120
-100
-80
-60
-40
-20
0 20 40 60 80
‘Combined’minus
‘Bedmap only’
How does this compare with Griggs and Bamber, GRL, in press?
500 600 700 800 900
Density (kg/m³)
-600
-550
-500
-450
-400
-350
-300
-250
-200
-150
-100
-50
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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1300
0
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m/a
x (km)
y (k
m)
Velocity inversion for strain/stress
not good enough for fracture
criterion
Updated velocity map (RAMP + feature tracking)
0
100
200
300
400
500
600
700Modelled vs.measured velocities
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x (km)
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1300
y (k
m)
-200
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-50
0
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-50
0
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0
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700
m a-1
m a-1
~ 5% difference to GPSderived velocities (2008/09)
-200 -150 -100 -50 0 50 100 150 200
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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1050
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1150
1200
1250
1300
y (k
m)
0
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kPa/m0.5
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x (km)
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y (k
m)
0
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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
450
550
650
750
850
950
0 10 20 30 40 50 60 70 80 90 100
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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800
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1000
0 10 20 30 40 50 60 70 80 90 100
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
200
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400
500
600
700
800
900
1000
0 10 20 30 40 50 60 70 80 90 100
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