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Water beneath Ice sheets (Water flow beneath IS (2. Distributed cavity…
Water beneath Ice sheets
Sources
Basal melting due to ice sheet flow
frictional heating during sliding
Geothermal heating
due to heatflux from Earth's core
Surface melt reaches bed
via crevasses/moulin etc
Subglacial
water impact
Small amounts melt - limited RSL contribution
Influence on ice flow - can contribute signif to RSL rise
Ice velocities increase due to subglacial water
Flows into ablation zone /ocean ∴ mass loss increased
Acceleration sea level rise driver
Ice flow acceleration at IS margins
= possible driver
Antarctica Mass Loss Mechanism
Little surface melting
Mostly fast flow velocities
& discharge into ocean via ice streaming
Subglacial water Antarctica
Vast lakes
Streams (& sometimes channels)
Lake
Vostok
= largest freshwater lake on Earth
Beneath ~ 4km ice in EAIS
Cored 2.2
miles Z
did not penetrate bottom
microbes isolated from atmos for Myr
could be useful / dangerous
Discovered from sonar soundings in 50s
Observing water beneath ice sheets
Surface roughness
Ice surface - rough texture normally
expression of uneven topog ice flows over
Smooth & flat
surface above lake
WHen ice flows over lake,
ice surface = smooth & flat
since lake surface ∴ ice bed = flat
Ice penetrating radar
Airborne surveys - radar penetrates ice surface
Radargram
exhibits:
Long flat reflector
@ 4 km Z
= surface of Lake Vostok
Hyperbola (curve) either side
= freeze on of ice
accreted basal ice on Lake Vostok
Lake Vostok Water
Source: Basal melting
subglacial melting causing water to [ool / collect
Removal: Accretion of ice onto base of ice flow
Water carried to other parts of ice shet
Residence time ~ 13 kyr
Freeze-on Hyperbola
Ice flows into lake & accretion focussed there at bottom
Freeze-on occurs rapidly
since base of ice very cold
∴ will freeze liquid as soon as exposed to it
flowing over cold bedrock for ~kyr
Accretion doesn't occur elsewhere
Freezing releases heat (latent)
Heat warms base a little
Hence accretion doesn't occur elsewhere
Hyperbola found either side Lake Vostok on radargram
Thwaites
Glacier
Amongst fastest & largest glaciers in world
Important - could dictate sealevel contribution of Antarctica in next 100 years
Water flow beneath IS
Water that is not ponded beneath ice sheets
1. Concentrated channels
= huge channels with large water throughput
2. Distributed cavity networks
= small, distributed network of cavities
streams travel parallel to eachother
Drainage network - influences flow velocity
influences sensitivity ice flow to water input