Week 4: Marine glaciers, ice shelves and calving Flashcards
Importance of marine terminating glaciers
Main ice sheet drainage
Ocean contact
Calving
Potential for rapid changes
- retreat
- flow velocity
SL contribution
- e.g. Larsen ice shelf 2002
Calving in Antarctica and Greenland
Effective ablation mechanism
90% A 50% G
Ice shelf =
Floating ice body, sits at glacier front
Ice shelf characteristics
T < -5’C
Grounding line = grounded to floating transition
Hardly any surface melt (e.g. Antarctic/N Greenland)
Bigger = more contact force for melting/refreezing
DIAGRAM
Tidewater margin glacier =
Not an ice shelf, a straight cliff face at front of glacier
Tidewater margin glacier characteristics
T > -5’C
Grounded ice terminus/tidewater glaciers i.e. no ice shelf
Surface melt (e.g. Greenland/Alaska)
DIAGRAM
Flow of marine ice masses
High flux due to narrow outlets of large drainage basins = channelised
Bed below SL = trough (fjord)
Flow generally increases towards terminus
Why does flow increase towards terminus?
Thins towards terminus, as approach floatation:
- low basal resistance
- enhanced sliding
- reduced friction
ALSO beds below SL = soft (marine) basal sediment
Calving models
WATER-DEPTH CALVING
FLOTATION CALVING
CREVASSE-DEPTH CALVING
Water-depth calving model
Only works with grounded terminus i.e. TIDEWATER glaciers
Brown 1982 eqn relating calving rate to water depth
Implications of water-depth calving model
- Unstable retreat for reverse bed slope
2. Non-linear response
Reverse bed slope
Deepens inland
Issues with water-depth calving model
Data only from 12 ALASKAN glaciers
Small data set
All stable i.e. slowing retreating/advancing so causation not clear
Model doesn’t incorporate feedbacks with other components (e.g. adjustment to reduced buttressing) of glacier dynamics
e.g. Columbia Glacier results suggest another process contributing to ice flow acceleration after retreat through basal depression
Floatation calving model
Only works with grounded terminus i.e. TIDEWATER GLACIERS
Terminus position = where surface reaches critical height before flotation
Everything bigger = calved off
SO calving rate due to dynamics - surface elevation change (flow/surface mass balance)
Thinner glacier = flotation height reached further back = retreat and enhanced calving
NO FLOATING TONGUE POSSIBLE
Flotation calving model; model simulation results
Basal topography is crucial
- rapid retreat through overdeepenings
Trigger = thinning due to climatic change
Glacier speeds up during retreat
Problem with flotation calving model AND water-depth model
Represent calving as a function of an independent variable
BUT glaciers have inherently unstable behaviour e.g. readvance after retreat which these models do not allow for
Crevasse-depth calving
Benn et al 2007
Linked to ice stretching rate
When crevasse depth = waterline, calving occurs
When crevasses are :
- wider/deeper = ice blocks more unstable
- closer to terminus = increase instability
- filled with water = forced open
Key control on ice stability…
TOPOGRAPHY
Topography’s effect on ice stability
- Weertman 1973: ice discharge increases with water depth
- greater space for water to flow through
- calving/flotation rate higher - Slope of bed
Stable topography
DIAGRAM
Bed deepens towards open water
Increase/decrease in volume compensated vertically rather than laterally = doesn’t retreat very much with warming
Unstable topography
Bed deepens behind grounding line “reverse sloping bed”
Thinner = easier to float
Grounding line retreats slightly = more ice discharge can get through b/c deeper bed
= RUNAWAY FEEDBACK
MARINE ICE SHEET INSTABILITY HYPOTHESIS (MISI)
DIAGRAM
MISI
MARINE ICE SHEET INSTABILITY HYPOTHESIS
(Thomas 1979 and others)
If bed deepens inland, catastrophic ‘unstable’ retreat can potentially occur
Assumes acc cannot increase to compensate for increased discharge
Theoretical (!mathematical) analysis of MISI
Ignores potential influence of calving
IMB = (all snow upstream of given location ) - (any melted from surface)
GL flux = how much ice req to achieve stable rounding line in relation to topography (water depth) at given location “GATE”
Controlled by topography diagram
IMB =
Integrated Mass Balance
Tells us how much mass has come in
GL flux =
Grounding Line flux
Tells us how much mass can leave
Will not stop moving until reaches another stable point
MISI: GL > IMB
Retreat occurs until equalise
MISI GL < IMB
Advance occurs until equalise
MISI GL = IMB
Stable glacier
Ice shelves and floating tongues, provide:
Lateral resistance
Buttressing
Buttressing =
Backstress onto grounded ice via longitudinal stresses
Ice shelves/floating tongues in action example
Jakobshavn Greenland
Retreat of floating ice tongue/shelf
- increases flow velocity
- propagates inland
Case study
Ice self buttressing: collapse of Larsen B ice shelf, Antarctica 2002
Isotherm map = -5’C isotherm migrating south over t
Ice shelves disintegrated as moves
- Crucial T?
Sheperd et al 2003: 30cm/yr thinning previous decade
Enhanced surface melt = crevasses
Rignot et al 2004, Hulbe et al 2008: loss of back stress = acceleration/tributaries thinning
Indirect consequence on SL b/c already floating = water already displaced BUT grounded ice displacing into seawater quickly = affects
Stress intensity =
Stress limit reached in relation to crevasses
Depends on:
Tensile stress
Lithostatic stress
Water pressure
N.B. Banwell et al 2013: one supra glacial (crevasse) lake drains = chain reaction
Case study
Ocean water t (and buttressing): rapid outlet glacier changes in Greenland
Retreat, thinning, acceleration of outlet glaciers at terminus and rapidly propagating upstream
Jakobshavn Isbrai shows retreat starts BEFORE surface melting, when sea ice opens (Joughin et al 2008):
- ocean source?
- basal melt beneath floating tongue?
Arrival of WARM OCEAN WATER coincides with beginning of acceleration (Holland et al 2008)
= makes contact with ice
= key = fjord circulation (Straneo 2010)
Case study
Changes in AIS: Pine Island Glacier
Large ice shelves = stable
Small = rapid inland thinning/acceleration/grounding line retreat of marine ice streams
Synchronous thinning of ice shelves
- enhanced basalt melt beneath?
- ocean warming?
Highest thinning rates where troughs allow warm water to access deep ice shelf bases
SHELF = COUPLING ELEMENT B/W OCEAN AND ICE SHEET INTERIOR/INLAND
Case study
Changes in AIS: Pine Island Glacier
Basal melting beneath ice shelves
Confirmed by satellite altimetry (Pritchard et al 2012)
Hydrostatic P at depth = lowers MP
Saline-dense, ~warm water reaches grounding line = melts ice = freshwater (less dense)
= rising freshwater plume
Towards end/top of shelf P drops = increases MP = supercooled water freezes on
DIAGRAM
