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Glaciers and Glaciation > Week 2: Water in glaciers > Flashcards

Week 2: Water in glaciers Flashcards

1
Q

Importance of glacial meltwater

A

RELIABLE WATER RESOURCE
- Sorg et al 2012; Central Asia/Andes

HEP
- 98% Norway’s electricity

HAZARDS (OUTBURST FLOODS)

INFLUENCES GLACIER FLOW

SEDIMENT EROSION/TRANSPORT/DEPOSITION

LARGE VOLS DISRUPTION OCEAN CIRCULATION
- Younger Dryas cold reversal caused by large glacial lake flowing into N Atlantic = slowed ocean circulation = - 6-8’C

EXTREME ENVIRONMENTS FOR MICROBIAL SYSTEMS

  • Christner et al 2014
  • e.g. Antarctica isolated from sunlight
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2
Q

Meltwater sources

A

Melting ice/snow (MOST)

Adjacent land/groundwater runoff

Rainfall/dew

Stored water (lakes) release

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3
Q

How does ice/snow melt?

A

1) Surface melt; surface energy balances varies daily/seasonally/annually
2) Subglacial friction/P melting
3) Geothermal e.g. Iceland ice sheets

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4
Q

Primary permeability

A

Tiny interconnected air spaces/thin lenses/veins between ice crystals

Greatest in snow/firn b/c air spaces but with increased pressure gradients = in ice too

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5
Q

Secondary permeability

A

Large channels/tunnels (mm to m diameter)

Bulk of meltwater drainage

Supra/en/sub-glacial channels

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6
Q

Channel scalloping

A

Represents high water pressure

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7
Q

Temperate glaciers discharge patterns

A

Diurnal:
- high in day low in night

Seasonal
- base discharge increases (due to efficiency) gradually throughout melt season

= diurnal amplitudes increase
(N.B. Smaller changes e.g. snowstorm inhibits melt briefly)

= peak daily discharge arrives earlier

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8
Q

Supraglacial meltwater drainage

A

If melting > refreezing = water accumulates in ponds/channels

= streams in ablation zone

Extensive networks e.g. Greenland IS

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9
Q

Ablation zone =

A 
Impermeable ice (with respect to acc zone where snowfall occurs)
Higher melt rates
Smooth channels mm-m depths
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10
Q

Englacial meltwater drainage

A

Enters through moulins
Water P fluctuates rapidly = v DYNAMIC

e.g. Snow = fill = plug = refreeze = abandon (Holmlund 1988)

Likened to karst systems where internal weakness exploited (Gulley and Benn 2007)

Investigate with manual descents/ice penetrating radar/dye tracing

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11
Q

Moulin =

A

Hole which forms due to structural weakness e.g. crevasse/extension

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12
Q

Movement within englacial conduits

A

HYDRAULIC POTENTIAL (gdt where water moves from one place to another

= (P/shape/size) + (potential due to elevation i.e. water weight and elevation) + Pw

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13
Q

Hydraulic potential in supraglacial

A

Dictated by elevation and water flows down-slope

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14
Q

Water pressure in englacial conduits

A

Might be at atmospheric P or might be influenced by weight of overlying ice (CRYOSTATIC P)

Open to air = atmospheric

Closed = depends on N (effective pressure)

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15
Q

N (effective pressure) =

A

Pi - Pw

Influence and englacial and subglacial drainage and glacier motion (Benn and Evans 1998)

Pw = 0, N = Pi (Pi>Pw)
- max N = conduit narrows

Pw = Pi, N = 0
- ice supported

(Pi never steady state

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16
Q

Englacial conduit size depends on:

A

1) Ice deformation due to N

2) Water flow
- frictional heat melts = enlarges

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17
Q

Subglacial meltwater drainage

A

Through moulins to bed, similar to englacial

18
Q

Sources of subglacial meltwater

A

PREDOMINANTLY surface meltwater/rain

Basal melting from geothermal (Paterson 1994(

Frictional heat from bed sliding

19
Q

Subglacial meltwater systems depend on:

A

Water discharge

T distribution at ice-bed interface

Bed permeability

Bed topography

Bed rigidity

  • switch from one to another over time
20
Q

The subglacial systems:

A

Benn and Evans 1998:

1) Bulk movement with deforming till
2) Darcian porewater flow
3) Pipe flow
4) Dendritic channel network
5) Linked cavity systems
6) Braided canal system
7) Thin film at ice-water interface

Distributed = 1,2,5,6,7
Discrete = ?3,4
21
Q

Distributed systems

A

Numerous, extensive smaller films/conduits

Inefficient drainage
- water trapped longer = ice can flow

Higher Pw

Slippery bed

22
Q

Discrete systems

A

Few large channels/conduits
- larger and flat bottoms (O Cofaigh 1996)

Efficient drainage

Low Pw

Sticky bed

Channels cut and incise into:
1) Ice = R channels

2) Bed = N channels

23
Q

R channels

A

Similar to englacial conduits i.e. kept open by tunnel wall melting/frictional heat

Low P = capture nearby water = branching tributaries (follow hydraulic potential)

?! Subglacial water flowing uphill?!

24
Q

R channel resulting formation

A

Esker = sand/gravel formation from sedimentation in R channels then subsequent deglaciation

25
Q

N channels

A

Singly/braided networks

Imply prolonged meltwater erosion

26
Q

7) Water films

A

Difficult to measure

Originally thought = primary drainage system BUT Walder 1982 =

27
Q

Regelation

A

High pressure point on stops size of bumps = melting

Refreezes in low P on lee side = regelation

–> smooths irregularities in bed

CONTRIBUTES TO GLACIAL SLIDING

28
Q

5) Linked cavity system =

A

Passageways with fluctuating diameters (Paterson 1994)

Fluctuating Pw = unstable

Shape due to bed topography

Low transit velocities

Intermittently connected to main drainage network

  • melt season = connect = slippery bed = flow
  • small amount water required!
29
Q

Linked cavity system to dendritic network

A

Glacier surging

30
Q

Glacial lakes =

A

Store water on/in/under/adjacent to glaciers

31
Q

Supraglacial lake example

A

Greenland Ice Sheet

- debris covered glaciers are conducive

32
Q

Supra and englacial lakes

A

Small and temporary

Englacial = rare/ephemeral

1) Temporate glaciers, supra form early in abl season and drain
2) Crevasses/conduits close off

Cold polar glaciers = supra persist

33
Q

Zwally effect

A

Drainage/access of lake to bed through moulin = increases flow velocity

CONTENTIOUS

  • long/short term?
  • more water encourages more efficient drainage system = slows down????
34
Q

Hydrofracturing =

A

Drainage of supra glacial lakes to englacial/subglacial positions fractures apart ice shelves

35
Q

Example of hydrofracturing

A

Larsen B, Antarctic Peninsula

> 2750 lakes drained in a few days (Banwell et al 2013)

= speed up of tributary glaciers and increase in SL contribution

36
Q

Subglacial lakes

A

Vary mm-1000s km2

Accumulate in areas of low hydraulic potential

Form from basal melting from high geothermal heat flux
- e.g. Vatnajokull, Iceland

Extreme environment
- analogous to Jupiter’s ice covered moon, Europa

37
Q

Antarctica subglacial lakes

A

More than 140

May influence ice velocity (Bel et al 2007/Stearns et al 2008)

Satellite altimetry = short term uplift/lowering of ice surface with filling/drainage
= increase in velocity (Fricker et al 2007)

38
Q

Ice dammed lake =

A

Marginal lakes

Common adjacent to cold glaciers

Variable size due to en/subglacial drainage

39
Q

Proglacial lakes =

A

In front of glacier margins

Blocked by topography not ice

Commonly form inside old moraines

Dam breached = drain catastrophically

40
Q

Jokulhlaups =

A

Sudden ice-dammed lake drainage below/through dam

Rapid fluvial dam incision

Growth/collapse subglacial reservoirs

POSITIVE FEEDBACK

  • channel produced
  • melts ice
  • increases channel
  • …

Transports large amounts debris/ice

41
Q

Jokulhlaups hydrograph

A

Rise steeply to peak discharge then ends abruptly as lake empties

42
Q

Patterns of jokulhlaups

A

Predictable (ish)
- Grimsvotn, Iceland ~6 years

During ablation season

Volcanic activity

  • Grimsvotn 4-5km3 at 50,000m3/s
  • Missoula 2184km3 up to 3 million m3/s (Clarke et al 1984)

Glaciers and Glaciation (29 decks)

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