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Elevation Feedback & Binge-Purge (Laurentide IS Binge-Purge (1.…
Elevation Feedback & Binge-Purge
Ice Sheet Termination
What makes ice sheets disappear rapidly ?
Jagged sawtooth pattern
= sharp transition from glacial to interglacial
Feedbacks
at play
Albedo loss from ice melt --> accelerates warming rate
Altitude-melt feedback
Temp - Altitude Dependence
Ice: Small lowering of ice surface due to instability
--> Runaway melt
= Possible cause rapid ice sheet decay ?
Mountains: with increasing altitude-temps decrease ∴ more snow
Altitude-Melt Feedback
Gregoire 2012
Modelled rapid collapse Laurentide IS
--> using altitude-melt feedback
Elevation dependence of temp
--> means ice sheet surface warms as it thins
∴ Runaway effect:
Thinned ice sheet --> greater surface melt --> faster thinning
Possible Causes 100 kyr Cyclicity
Clarke & Pollard Regolith Mechanism
2. Elevation feedback & binge-purge
Rapid Collapse Laurentide IS
Gregoire 2012
Slight depression developed just E of Rockies
∴ lowered ice surface elevation
Altitude-melt feedback ∴ runaway melt
∴ Portion of Laurentide decayed
from 2 km thick to nothing in ~500 years
Cause of Meltwater Pulse 1a?
Meltwater
Pulse 1a
16-25 m sea level rise in ~ 500 years
When ? ~ 14 kyr
Cause MWP1a
Debated whether caused by melt from Laurentide or Antarctica
If from Antarctica, could it happen again?
Laurentide IS Binge-Purge
1. Thickening Laurentide IS insulates bed
causing relative warming allowing melt
2. Meltwater lubricates bed
∴ ice flows into ocean @ Hudson Bay
3. Ice thins to maintain mass continuity
following mass loss to ocean
--> Insulation stops ∴ bed refreezes --> Ice sheet builds up again
Result: Fast drawdown of ice sheet
Limited Evidence IS Drawdown
Only evidence binge purge = Ice rafted debris
Lithic fragments of continental origin on Atlantic seafloor
Ice rafting
Bergs melt out at sea in Atlantic
∴ Releasing debris they entrained on IS
Ice bergs calve from Hudson Bay
= only plausible mechanism for these deposits to reach seafloor
Heinrich
Events
Two peaks ice rafted debris
1. Initial IRD output (first ice bergs released)
= armada of debris laden ice bergs
Mid purge IRD hiatus
when initial debris melted off
2.Late purge IRD output
(last icebergs)
Freeze-on of basal debris
--> indicates return to frozen bed ?
Heinrich
Event Cause
Possibly binge purge oscillations ?
Explain Heinrich events (2 pulses IRD on Atlantic seafloor)
May have cooled climate?
freshwater flux into N.Atantic interrupted overturning circulation?