I. Sasgen et al. MAGMA Seminar, May 25, 2005, Prague Geodetic signatures of glacial changes in Antarctica Ingo Sasgen Supervision: Detlef Wolf, Zdeněk Martinec GeoForschungsZentrum Potsdam Email: [email protected]
Dec 25, 2015
I. Sasgen et al.
MAGMA Seminar, May 25, 2005, Prague
Geodetic signaturesof glacial changes in Antarctica
Ingo Sasgen
Supervision: Detlef Wolf, Zdeněk MartinecGeoForschungsZentrum Potsdam
Email: [email protected]
Geodetic signatures of Antarctica I. Sasgen
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Larsen B ice-shelf collapse
National Snow and Ice DataCenter, http://nsidc.gov (2005)
Jan. 31 – Mar. 05, 2002
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Causes and consequences of collapse
mean temperature trend of +1.2°C in 100 a for all Antarctic stations
regional warming of +2.5°C in 50 a along the Antarctic Peninsula
warming likely cause for collapse of the Larsen B ice shelf
glacier acceleration observed after desintegration of the Larsen B ice shelf, i.e. ice-velocity increase by factor of 5 to 8
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Scientific importance
Antarctic ice sheet (AIS) largest ice mass on earth 10 times larger than the Greenland ice sheet glacial variations closely linked to global climate
and sea-level changes knowledge on present-day state and near-future
developement improves climate models, which predict global temperature and sea-level changes
for the centuries to come
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Antarctic ice sheet
13.6 x 106 km2 grounded portion,i.e. 95 % of the continent
volume of 61 m equivalent sea level (ESL)
~ 4 km maximum ice thickness drained by ice streams which feed ice
shelves mountain glaciers along
Antarcic Peninsula
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Ice sheet volume changes
accumulation: ~ + 5 mm ESL/a accumulation – discharge: ~ - 0.1 mm ESL/a discharge since last glacial maximum (LGM),
21 ka BP: ~ -12 m ESL
present sea-level rise: ~ 1.8 mm/a ESL sea-level rise since LGM: ~ 110 m ESL
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Antarctic continent
area of rock outcrops < 0.4 % East Antarctica is a
Precambrian shield West Antarctica comprises
younger, tectonically more active terranes
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Transantarctic Mountains
Transantarctic Mountains
Transantarctic Mountains mark tectonic suture zone lateral variations of the lithosphere thickness and the
viscosity expected
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Modelling components
Elastic
Last glaciation~ 1000 a
Seasonal~ 1 a
Secluar~ 10 – 100 a
Viscoelastic
Geoid-height change
Radial displacement
Gravity Recovery andClimate Experiment (GRACE)
Global PositioningSystem (GPS)
Load model
Earth model
Earth response
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Seasonal ice-mass changes (VAUG)
Vaughan et al. (1999) Cazenave et al. (2000)
accumulation of ~ 5 mm ESL/a based on ice cores
temporal variation from global mean sea-level changes inferred from satellite altimetry
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Seasonal earth response
Geoid-height change Radial displacementElastic,
cut-off degree 256
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East Antarctica roughly in balance most prominent changes of up to 1
m/a for glaciers draining into the Amundsen Sea (PIG, THW, SMI, KOH)
East Antarctica in balance Byrd (BYR) likely in balance (former
0.05 mm ESL/a) ice-thickness changes along Antarctic
Peninsula and West Antarctic coast several m/a
Update 2004:
Secular ice-mass balance (RT02)
Region Mass change Volume change
Gt/a mm ESL/a
West Antarctica (75 %) - 45 - 0.12
East Antarctica (55 %) 19 0.05
Antarctica (58 %) - 26 - 0.07
Rignot & Thomas (2002)
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Earth response to secular changes
Geoid-height change Radial displacementElastic,
cut-off degree 256
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Viscoelastic earth model
present-day post-glacial rebound (PGR) due tolast glaciation is calculated with a
lateral homogenous viscoelastic earth model based on the
spectral-finite element code developed by Martinec (2000)
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Earth model parameters
Model Lith.
thickness Viscosity Elasticity
hL (km) UM (Pa s) LM (Pa s)
LVM 100 5.2 1020 5.9 1021 PREM
MP 100 1.0 1021 4.0 1021 PREM
HVM 100 2.0 1021 1.0 1022 PREM
WestAntarctica
East Antarctica
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15 ka BP7 ka BP4 ka BP
Last glaciation and its retreat (HUY)
thermomechanical model allows regional retreat history, e. g. late
retreat from the Ronne ice shelf ice volume of – 12 m ESL at the LGM
compared to present day
Huybrechts (2002)
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Earth response to last glaciation
Geoid-height change Radial displacementViscoelastic,
cut-off degree 256
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International GPS Service stations
7 stations along the Antarcic coast
continuous time series > 6 a
nominal accuracy ~ 1 mm/a
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Land-uplift rates at GPS stations
mm/a mm/a
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Possible GPS transects
GPS measurements along A-A‘, B-B1 and B1-B2 can constrain the glacial history
Measurements along B2-B questionable: tectonic displacements large and influenced by rheological transition
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Summary of GPS comparison
Interpretation of IGS data difficult, because stations located
at rheological transition (e.g. Mawson, Davis) lateral heterogenous earth model
at ice margin where rebound is complex accurate last glaciation models
in tectonically active regions (e.g. Mc Murdo) ignore particular station
not in the former load center
Large solution differences between regional networks, e.g. Amery ice shelf region
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GRACE satellite mission
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GRACE satellite mission
Primary mission objective of the GRACE:monthly high-accuracy determination of the earth‘s gravity field,i.e. temporal variations of the geoid height
Possible application:mass balance of ice sheetsocean-current changespost-glacial rebound
Mission status:operational since October 200218 monthly solutions existcurrent spatial resolution ~ 1000 km with anestimated accuracy ~ mm/a
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Schematic principle of GRACE
Satellite A Satellite B
∆m
Satellite-satellite distance ∆l tracked by microwave
link
Satellite B
∆l = f (∆m)
GPS GPS GPS GPS
Precise orbitdetermination by GPS
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May. 2003 – Apr. 2003May 2003 – Apr. 2002Nov. 2002 – Aug. 2002
Comparison of spectral geoid change
Nov. 2003 – Nov. 2002
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May 2003 – Apr. 2003
Spatial geoid change comparison
Aug. 2003 – Aug. 2002 Nov. 2002 – Aug. 2002
Prediction,
Observation,
cut-off degree 13
cut-off degree 13
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Discussion of geoid-height interpretation
Predicted and GRACE measured geoid changes do not correspond yet:
Strong anomalies over the ocean dominate the signal artificial ocean phenomenon: tides not successfully removed? real ocean phenonmenon: circumpolar current?
Seasonal changes not visible (not even the sign) temporal variation not realistic?
Secular changes not detectable at the current resolution expected 8 a of measurments sufficient for a linear-tend estimate?
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Outlook: seasonal ice-mass changes
Include metereological parameters accumulation from moisture flux onto
the Antarctic continent discharge, i.e. mainly calving, from
surface-air temperature Patagonia as proxy for the Antarctic
Peninsula?
accumulation discharge
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Outlook: GRACE data
Quantify errors introduced by ocean model
Remove ocean signal (e.g. by filtering)
Focus on (regional) total ice-mass changes, not spatial distribution
Allow error dependent weighing of degree power to include maximum information
May 2003 – May 2002 minusAug. 2003 – Aug. 2002
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Glacial changes of the AIS induce geoid changes and land uplift : with measurable magnitudes and specific signatures
Observations by GRACE and GPS
do not correspond to the predictions yet
need to be refined and extended according to the expected signature
Summary
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Questions
???X?!!?!
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Glacial changes of the AIS induce geoid and surface displacement changes: with measurable magnitudes and specific signatures
Observations of the geoid (GRACE) and the surface displacement changes (GPS)
do not correspond to the predictions yet
need to be refined and extended according to the expected signature
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Present ice-mass balance (updated)
Rignot & Thomas (2002), Thomas et al. (2004), Rignot et al. (2004)
Sasgen et al. (2005)
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Secular
Spectral geoid change
Last glaciation
Secular ice-mass balance induces geoid change well above GRACE accuracy
High power even at high degrees However, seasonal changes ~ one
order of magnitude larger Interannual variation can introduce
„pseudo“-secular trend
Last glaciation induces high power at degree low degrees well above the GRACE accuracy
Up to degree 9 the employed earth model is not importance