Week 4: Marine glaciers, ice shelves and calving

Question 1

Importance of marine terminating glaciers

Answer 1

Main ice sheet drainage

Ocean contact

Calving

Potential for rapid changes

• retreat
• flow velocity

SL contribution
- e.g. Larsen ice shelf 2002

Question 2

Calving in Antarctica and Greenland

Answer 2

Effective ablation mechanism

90% A 50% G

Question 3

Ice shelf =

Answer 3

Floating ice body, sits at glacier front

Question 4

Ice shelf characteristics

Answer 4

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

Question 5

Tidewater margin glacier =

Answer 5

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

Question 6

Tidewater margin glacier characteristics

Answer 6

T > -5’C

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

Surface melt (e.g. Greenland/Alaska)

DIAGRAM

Question 7

Flow of marine ice masses

Answer 7

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

Bed below SL = trough (fjord)

Flow generally increases towards terminus

Question 8

Why does flow increase towards terminus?

Answer 8

Thins towards terminus, as approach floatation:

• low basal resistance
• enhanced sliding
• reduced friction

ALSO beds below SL = soft (marine) basal sediment

Question 9

Calving models

Answer 9

WATER-DEPTH CALVING

FLOTATION CALVING

CREVASSE-DEPTH CALVING

Question 10

Water-depth calving model

Answer 10

Only works with grounded terminus i.e. TIDEWATER glaciers

Brown 1982 eqn relating calving rate to water depth

Question 11

Implications of water-depth calving model

Answer 11

1. Unstable retreat for reverse bed slope

2. Non-linear response

Question 12

Reverse bed slope

Answer 12

Deepens inland

Question 13

Issues with water-depth calving model

Answer 13

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

Question 14

Floatation calving model

Answer 14

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

Question 15

Flotation calving model; model simulation results

Answer 15

Basal topography is crucial
- rapid retreat through overdeepenings

Trigger = thinning due to climatic change

Glacier speeds up during retreat

Question 16

Problem with flotation calving model AND water-depth model

Answer 16

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

Question 17

Crevasse-depth calving

Answer 17

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

Question 18

Key control on ice stability…

Answer 18

TOPOGRAPHY

Question 19

Topography’s effect on ice stability

Answer 19

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

Question 20

Stable topography

Answer 20

DIAGRAM

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

Question 21

Unstable topography

Answer 21

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

Question 22

MISI

Answer 22

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

Question 23

Theoretical (!mathematical) analysis of MISI

Answer 23

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

Question 24

IMB =

Answer 24

Integrated Mass Balance

Tells us how much mass has come in

Question 25

GL flux =

Answer 25

Grounding Line flux

Tells us how much mass can leave

Will not stop moving until reaches another stable point

Question 26

MISI: GL > IMB

Answer 26

Retreat occurs until equalise

Question 27

MISI GL < IMB

Answer 27

Advance occurs until equalise

Question 28

MISI GL = IMB

Answer 28

Stable glacier

Question 29

Ice shelves and floating tongues, provide:

Answer 29

Lateral resistance

Buttressing

Question 30

Buttressing =

Answer 30

Backstress onto grounded ice via longitudinal stresses

Question 31

Ice shelves/floating tongues in action example

Answer 31

Jakobshavn Greenland

Retreat of floating ice tongue/shelf

• increases flow velocity
• propagates inland

Question 32

Case study

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

Answer 32

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

Question 33

Stress intensity =

Answer 33

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

Question 34

Case study

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

Answer 34

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)

Question 35

Case study

Changes in AIS: Pine Island Glacier

Answer 35

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

Question 36

Case study

Changes in AIS: Pine Island Glacier

Basal melting beneath ice shelves

Answer 36

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