L9: Arctic Coasts Flashcards
What proportion of Earth’s coastlines are periglacial?
1/3rd (Ballantyne, 2018)
30-34% permafrost alone (Lanuit et al., 2013).
What affect does the ice on coastlines have?
Prevents erosion in intertidal zone in winter and spring, and offshore ice reduces waves and fetch.
But…storms and open water conditions allow thermal erosion of permafrost cliffs.
What are the series of controls on polar coastal processes?
Short term marine factors (freezing, sea and beach ice; energy of wind/ wave/ tide/ current.
Short term terrestrial factors (geology, topography, permafrost rivers, terrestrial debris supply).
Long term factors (plate tectonics, climate change, glacio eustacy and isostacy)
Distinctive polar coastal factors
- Sea level change
Macro forms are dominated by glacio-eustatic submergence.
Recent glacio-isostatic rebound is giving widespread emergence with raised beaches.
Climate change will reinforce submergence.
Distinctive polar coastal factors
- Available coastal sediment
Beaches fed by mass movement (thermokarst flows?), talus slopes and rivers.
Very highs sediment availability may explain high deposition (and erosion) rates.
This may be a kind of coastal paraglaciation.
Distinctive polar coastal factors
- Coastal permafrost
Permafrost is widespread in cliffs, beaches
Also in the offshore zone if sea level is rising or the coast eroding
Adds thermal erosion to normal mechanical processes (thermoabrasion)
May help to explain annual recession rates of 10m-30m a year.
How do tides and currents impact coasts?
Most polar coast is micro tidal (<2m range).
This means that although there is not a lot of tidal energy, what there is is concentrated over a restricted zone.
Tides essential in growth of beach ice.
Tides break the bond between sea ice and coast during thaw.
How does wave action impact coasts?
Highly seasonal with coasts ice bound (no wave action) for most of year.
Wave action only possible after break up of sea ice.
Even in high summer, floating ice reduces fetch and thus limits wave generation.
Polar coasts in general are thus seen as low energy, despite possible storm exposure.
How do thermal processes impact coasts?
Thermal abrasion at the cliff foot (in association with mechanical wave processes) produces an erosional niche.
Backwearing and down wearing thermokarst on the cliff face create a mix of degredational features.
The inshore seabed may subside irregularly due to melt (submarine thermokarst) thus increasing wave energy transmission.
How does beach ice deposition impact coasts? 1
Ice deposition on the beach in autumn and winter creates characteristic beach forms- the ice foot.
Ice freezes on beach in each falling tide- accumulates several metres.
Storms add debris layers in this sequence periodically.
Ice foot growth ends once the sea ice freezes permanently.
How does beach ice deposition impact coasts? 2
The main process active in Winter- but much more important in Spring and Summer.
Erodes, deforms sediments and deposits (after melt).
Active at less scale in lakes.
Significant applied threat.
How does beach ice deposition impact coasts? 3
The sea ice itself is highly reflective, and resists thaw.
Tidal hinging ultimately breaks the coast/ ice link.
Water also runs off coastal snowmelt to help produce a shore lead.
Shallow-water melt from underneath.
