Thermokarst Processes Flashcards
What happens when permafrost thaws on slopes?
Thermokarst slumps- river or sea cliffs, unconsolidated, naturally exposed to sun/air/water and thus potential to thaw.
Back-wearing: slumps occur as ground ice thaws and sediment flows- occurs along riverbanks and sea cliffs: open faces exposed to sun, air, rain. c
What is “liquid limit”?
Amount of water needed to liquefy the surface sediment.
If excess ice > liquid limit, thermokarst will generate sediment flow- sediment lost alone.
What do thermokarst lakes require?
Unconsolidated sediments to have accumulated. Rare in glaciated landscapes.
How does thermokarst occur?
Excess ice = water locked in large ice bodies
Thawing leads to overall loss of volume (ice to water)
Super saturated (more water than available pore space) leads to collapse and/ or flow-age
What are syngenetic ice wedges?
Grow as substrate accumulates over time (e.g loess).
Wind blown sediment.
What is an ice complex?
With holey cheese, imagine the holes are filled with silt and the cheese is ice and the ice gets eroded away.
Climate and disturbance-generated thermokarst
What does a mean warming of the environment change?
Everything under the ground changes its temperature regime if above the grounds temperature changes.
Cold -> hot = get an increase in depth of thaw (active layer thaws deeper).
Generate melting of ice.
What are some effects of disturbance on thermokarst?
Removal of insulating vegetation (including forest moss layer) or thinning/ loss of peaty soil can lead to more surface heating and enhance thermokarst.
Fire severely affects resilience of landscape to thermokarst.
Warming plus more/ severe fire may lead to a tipping point, where permafrost is permanently degraded.
What are some human induced thermokarst disturbances?
Emplacement of infrastructure, forest clearance, roads and tracks can all disturb the surface heat budget and affect the active layer.
How is methane generated in thermokarst lakes?
Deep aeolian sediments containing carbon.
Inside thaw bulb is >0oC and anaerobic decomposition f carbon leads to methane production.
Summary: when does thermokarst occur?
When ice rich ground is subject to a change in conditions that encourage thaw (disturbance, climate change).
Summary: what does excess ice lead to?
Thaws and leads to loss of mass, karsting and slumping.
Summary: what happens if liquid limit exceeds sediment flows?
Rapid erosion (sea, cliffs, river banks)
Summary: what is it that gives rise to lakes of different depths?
Shallow epigenetic wedges and deep syngenetic wedges may thaw in level ground.
Thermokarst: sequences of landscape transformation
Disruption of land surface Usually increase in wetness May form ponds -> lakes Changes in shape and depth can be rapid Enhanced emission of methane to atmosphere
