Week 2: Water in glaciers Flashcards
Importance of glacial meltwater
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
Meltwater sources
Melting ice/snow (MOST)
Adjacent land/groundwater runoff
Rainfall/dew
Stored water (lakes) release
How does ice/snow melt?
1) Surface melt; surface energy balances varies daily/seasonally/annually
2) Subglacial friction/P melting
3) Geothermal e.g. Iceland ice sheets
Primary permeability
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
Secondary permeability
Large channels/tunnels (mm to m diameter)
Bulk of meltwater drainage
Supra/en/sub-glacial channels
Channel scalloping
Represents high water pressure
Temperate glaciers discharge patterns
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
Supraglacial meltwater drainage
If melting > refreezing = water accumulates in ponds/channels
= streams in ablation zone
Extensive networks e.g. Greenland IS
Ablation zone =
Impermeable ice (with respect to acc zone where snowfall occurs) Higher melt rates Smooth channels mm-m depths
Englacial meltwater drainage
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
Moulin =
Hole which forms due to structural weakness e.g. crevasse/extension
Movement within englacial conduits
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
Hydraulic potential in supraglacial
Dictated by elevation and water flows down-slope
Water pressure in englacial conduits
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)
N (effective pressure) =
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
Englacial conduit size depends on:
1) Ice deformation due to N
2) Water flow
- frictional heat melts = enlarges
Subglacial meltwater drainage
Through moulins to bed, similar to englacial
Sources of subglacial meltwater
PREDOMINANTLY surface meltwater/rain
Basal melting from geothermal (Paterson 1994(
Frictional heat from bed sliding
Subglacial meltwater systems depend on:
Water discharge
T distribution at ice-bed interface
Bed permeability
Bed topography
Bed rigidity
- switch from one to another over time
The subglacial systems:
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
Distributed systems
Numerous, extensive smaller films/conduits
Inefficient drainage
- water trapped longer = ice can flow
Higher Pw
Slippery bed
Discrete systems
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
R channels
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?!
R channel resulting formation
Esker = sand/gravel formation from sedimentation in R channels then subsequent deglaciation
N channels
Singly/braided networks
Imply prolonged meltwater erosion
7) Water films
Difficult to measure
Originally thought = primary drainage system BUT Walder 1982 =
Regelation
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
5) Linked cavity system =
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!
Linked cavity system to dendritic network
Glacier surging
Glacial lakes =
Store water on/in/under/adjacent to glaciers
Supraglacial lake example
Greenland Ice Sheet
- debris covered glaciers are conducive
Supra and englacial lakes
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
Zwally effect
Drainage/access of lake to bed through moulin = increases flow velocity
CONTENTIOUS
- long/short term?
- more water encourages more efficient drainage system = slows down????
Hydrofracturing =
Drainage of supra glacial lakes to englacial/subglacial positions fractures apart ice shelves
Example of hydrofracturing
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
Subglacial lakes
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
Antarctica subglacial lakes
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)
Ice dammed lake =
Marginal lakes
Common adjacent to cold glaciers
Variable size due to en/subglacial drainage
Proglacial lakes =
In front of glacier margins
Blocked by topography not ice
Commonly form inside old moraines
Dam breached = drain catastrophically
Jokulhlaups =
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
Jokulhlaups hydrograph
Rise steeply to peak discharge then ends abruptly as lake empties
Patterns of jokulhlaups
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)
