Cosmo & Ice Sheet Thinning
- Ice Sheet Thinning
How thick was ice sheet during past glacials?
Transects down mtns - help estimate ice thinning rates
Importance:
Need ice histories to inform glacial isostatic adjustment models
Trimline Uncertainty
- Upper limit of ice sheet?
↑ exposure + weathering during glacials
- Thermal boundary within ice sheet
englacial
cold based ice above trimline
= frozen to substrate ∴ doesn't erode
Trimline Appearance
Above
More weathered rock
Below
Evidence for glacial erosion e.g. RMs / striations /erosional landforms
Features characteristic of warm-based ice
Testing what trimline reps
- Upper limit ice sheet?
Relatively thin ice ∴ expect:
Characteristics of each scenario:
Young erratics below TL - from last glacial
Older erratics above TL - from older glacials
- Thermal boundary
Thicker ice w transition from cold (above) > warm (below) bases
Young & old erratics either side of TL
Why ?
Cold based ice still transports material
Just doesn't erode bedrock
Mountains
= 'dipsticks'
Dipsticks --> to assess ice sheet thickness
Often trimlines reps thermal boundary
Tho is obvs case-by-case matter
Ice sheet thinning
Mountains = 'dipsticks' to track altitudinal ∆ ice sheet thicknesses
How?
Sample erratics &/or erratic-bedrock pairs
Importance - better understand:
Ice melt & SLR
GIA models
correct gravimetry MB records
Ice sheet dynamics
what processes cause ∆ in ice sheet
Estimate IS ∆ w/mtn transect assumes
Assumes during glacials when IS thickens to upper limit
--> Glacier removes existing sed on mtn side
depositing it @ foot of mountain
Not always the case esp. in Antarctica
Often, material = recycled
∴ redeposited in different location
Depositional trimline
Change observed in depositional till
Above
Just a few scattered erratics on bare bedrock
Below
Area = draped in sediment
Assumption IS thinning studies
Assumption: Youngest exposure ages mark ice surface elevation @ time
Problem: Negative evidence
Perhaps just not finding younger samples @ higher altitude
AIS - more concerned w age overestimation
Doesn't confirm absence
inheritance/recycling
= most signif geological signals
erosion = negligible