Ice Sheet Reconstruction Flashcards
Calculations needed for isostatic inversion ice sheet reconstruction
1) current uplift rates
2) location and age of raised beaches (geomorphic evidence)
3) density of crust, ice, mantle
4) viscosity of mantle
5) rigidity of mantle
To get total original crustal depression
Pros and cons of geomorphic ice reconstruction
- Lots of different types of geomorphic evidence
- Info on extent, thickness, volume
- Geike 1867 was just a snapshot in time, did not have dates
What are the 4 methods of ice sheet reconstruction?
1) Geomorphological evidence
2) Isostatic inversion (raised beaches and uplift)
3) Relics (coral reefs in far away places)
4) Ice sheet modelling
Example of ice sheet model
British- chrono. Clark et al., 2022
Went and dated all of the moraines in the UK (more on land than in the sea). Used this data to constrain the model. Could get a rate of retreat. Could start to get ice sheet dynamics as well like ice streams.
How is geomorphic evidence used in ice sheet reconstruction?
Moraines.
Ridges of sediment deposited by a glacier.
Terminal used for extent of ice sheet.
Lateral used for lateral boundaries and ice surface elevation,
Tells us about extent and thickness of ice sheet.
Erratics.
Boulders, often slightly rounded which have been transported and are not from the underlying geology.
Tells us about source geology and dispersal trains.
Drumlins and glacial lineations.
Show direction of ice flow. Dynamics of ice sheet
More elongated lineations means faster ice flow. Reveals possible ice streams.
Why is ice sheet reconstruction useful?
- understanding ice sheet dynamics and rates of retreat
- how long will our current ice sheets last
- how do ice sheets affect other components of the climate systems
- global sediment transport useful for mineral exploration and mining.
Early reconstructions
Pro: Southern boundary known due to moraines
Con: Just art/shading for parts, with little geomorphic evidence
Pro: Then some geomorphological evidence
Eskers, striated bedforms.
Multiple domes and main ice flow patterns
Con: But no concept of thickness or time
No ice over Hudson Bay
Shilts 1980
Used erratics to find source and deposit areas
Reconstructed ice flow dynamics
1985
Ice flow model
Tweaked physics so could get multiple ice domes
But thickness was too small
Clark 1990
Pro: Used cross cutting, didn’t ignore and cherry pick data
Evidence is not synchronous
Evidence is not time-transgressive
Landsat data is now available
Con:
Doesn’t tell about ice thickness
Ice sheet models
Ice sheets are still too thick compared to isostatic inversion unless assume:
-soft, low-viscosity ice
- slippery base (deformable subglacial sediments)
Models don’t show ice sheet dynamics like ice streams
Models don’t account for all of sea level lost from ocean globally(hidden/lost ice sheets)
What are the 4 phases of Lauren tide reconstruction
Phase 1: early reconstructions
Phase 2: single dome
Phase 3: multi dome
Phase 4: multi temporal concept/evolving ice sheet
Phase 2
Single dome reconstruction:
First model (Sugden 1977)
Looked at Margins, geomorphic evidence.
Calculated back where ice surface elevation must have been
Used modern analogies (antártica and Greenland)
Ice is too thick tho and says single rather than multdome- conflict with isostatic inversion and geomorphic evidence
Phase 3
Early 1980s
Shilts- multi dome. Based on mapping of drumlins.
Models better, simulate multi dome
But still too thick.
Assumed drumlins were all created at the same time
Phase 4
Mid 1980s
Evolving ice sheet
Boulton 1985 thought ice forms created as ice retreats
Drumlins formed at margins as deglacation occurs
Ice flow and divides/domes change
Clark 1990
Cross cutting
