Week 4: Marine glaciers, ice shelves and calving Flashcards

1
Q

Importance of marine terminating glaciers

A

Main ice sheet drainage

Ocean contact

Calving

Potential for rapid changes

  • retreat
  • flow velocity

SL contribution
- e.g. Larsen ice shelf 2002

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2
Q

Calving in Antarctica and Greenland

A

Effective ablation mechanism

90% A 50% G

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3
Q

Ice shelf =

A

Floating ice body, sits at glacier front

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4
Q

Ice shelf characteristics

A

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

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5
Q

Tidewater margin glacier =

A

Not an ice shelf, a straight cliff face at front of glacier

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6
Q

Tidewater margin glacier characteristics

A

T > -5’C

Grounded ice terminus/tidewater glaciers i.e. no ice shelf

Surface melt (e.g. Greenland/Alaska)

DIAGRAM

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7
Q

Flow of marine ice masses

A

High flux due to narrow outlets of large drainage basins = channelised

Bed below SL = trough (fjord)

Flow generally increases towards terminus

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8
Q

Why does flow increase towards terminus?

A

Thins towards terminus, as approach floatation:

  • low basal resistance
  • enhanced sliding
  • reduced friction

ALSO beds below SL = soft (marine) basal sediment

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9
Q

Calving models

A

WATER-DEPTH CALVING

FLOTATION CALVING

CREVASSE-DEPTH CALVING

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10
Q

Water-depth calving model

A

Only works with grounded terminus i.e. TIDEWATER glaciers

Brown 1982 eqn relating calving rate to water depth

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11
Q

Implications of water-depth calving model

A
  1. Unstable retreat for reverse bed slope

2. Non-linear response

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12
Q

Reverse bed slope

A

Deepens inland

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13
Q

Issues with water-depth calving model

A

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

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14
Q

Floatation calving model

A

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

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15
Q

Flotation calving model; model simulation results

A

Basal topography is crucial
- rapid retreat through overdeepenings

Trigger = thinning due to climatic change

Glacier speeds up during retreat

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16
Q

Problem with flotation calving model AND water-depth model

A

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

17
Q

Crevasse-depth calving

A

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
18
Q

Key control on ice stability…

A

TOPOGRAPHY

19
Q

Topography’s effect on ice stability

A
  1. Weertman 1973: ice discharge increases with water depth
    - greater space for water to flow through
    - calving/flotation rate higher
  2. Slope of bed
20
Q

Stable topography

A

DIAGRAM

Bed deepens towards open water
Increase/decrease in volume compensated vertically rather than laterally = doesn’t retreat very much with warming

21
Q

Unstable topography

A

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

22
Q

MISI

A

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

23
Q

Theoretical (!mathematical) analysis of MISI

A

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

24
Q

IMB =

A

Integrated Mass Balance

Tells us how much mass has come in

25
Q

GL flux =

A

Grounding Line flux

Tells us how much mass can leave

Will not stop moving until reaches another stable point

26
Q

MISI: GL > IMB

A

Retreat occurs until equalise

27
Q

MISI GL < IMB

A

Advance occurs until equalise

28
Q

MISI GL = IMB

A

Stable glacier

29
Q

Ice shelves and floating tongues, provide:

A

Lateral resistance

Buttressing

30
Q

Buttressing =

A

Backstress onto grounded ice via longitudinal stresses

31
Q

Ice shelves/floating tongues in action example

A

Jakobshavn Greenland

Retreat of floating ice tongue/shelf

  • increases flow velocity
  • propagates inland
32
Q

Case study

Ice self buttressing: collapse of Larsen B ice shelf, Antarctica 2002

A

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

33
Q

Stress intensity =

A

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

34
Q

Case study

Ocean water t (and buttressing): rapid outlet glacier changes in Greenland

A

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)

35
Q

Case study

Changes in AIS: Pine Island Glacier

A

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

36
Q

Case study

Changes in AIS: Pine Island Glacier

Basal melting beneath ice shelves

A

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