L11: Cryogenic Mounds

Question 1

What do palsas contain?

Answer 1

A segregated ice core

Question 2

What can palsas be classified on the basis of?

Answer 2

Structure

Question 3

What size are palsas?

Answer 3

Oval or circular in plan and from 0.2m to 8m high and 2-100m long axis.

Question 4

What is a pure peat palsa made of?

Answer 4

Peat entirely

Question 5

What is a mature peat palsa made of?

Answer 5

Peat and thin layer of silt underneath

Question 6

What is mineral (cored) palsa made of?

Answer 6

Thin layer of peat on top and mainly silt

Question 7

Why does the freezing front penetrate deeper into certain parts of the palsa than others?

Answer 7

Cracks

Question 8

What happens when the freezing front penetrate deeper into certain parts of the palsa than others?

What happens when it gets cold/ wet?

Answer 8

It forms segregated ice and ice lenses and pushes up the surface material.

This is drier to the area around and lighter vegetation on top affects albedo.

This insulates the core dry heat (stays frozen).

When it gets cold/ wet cold penetrates faster as the thermal conductivity and heat capacity is higher when wet.

Cracks start to form, penetration increases and ice begins to melt out.

Question 9

What does dry and wet peat have as a key different?

Answer 9

The thermal conductivity and heat capacity is higher when wet.

Question 10

Palsas snow removal hypothesis

Answer 10

Snow is a good insulator.

Snow removal leads to deep frost penetration.

Snow on the flanks of the palsa retard permafrost in the mire.

Question 11

Palsas vegetation change hypothesis

Answer 11

An increase in albedo reduces summer heat influx preserving the frozen core.

Question 12

Palsas buoyancy hypothesis

Answer 12

Idea is that if you’ve got frozen peat it will have a lower density than the material around it and it will float up.

Once this has happened, peat dries out and insulates the core beneath.

Question 13

Case study: Palsas

(Melanie and Payette, 2014)

Where?

Answer 13

Northern Quebec around the Boniface River

Circumpolar discontinuous zone

Mean annual air temperature -7oC

Question 14

Case study: Palsas

(Melanie and Payette, 2014)

What?

Answer 14

Palsa- organic or mineral soil mounds with a permafrost core.

Diameter 20-70m…height 2-9m

Vegetated with lichens and dwarf shrubs, mosses some bare patches.

Uneven topography and variable depth of organic layer, snow depth and tree height.

Question 15

Case study: Palsas

(Melanie and Payette, 2014)

How?

Answer 15

Methods
• Evaluate impact of forest cover on ground thermal regime of wooded palsas by comparing active layer thickness in different soils and vegetation types
• Aerial photos and field validation of 4 palsa types
• Data on vegetation, snow depth, organic layer thickness, location and size statistically analysed

How in terms of process might be appropriate – how do palsas form?

Question 16

Case study: Palsas

(Melanie and Payette, 2014)

Why?

Answer 16

• Why are the interrelationships identified so (results)
• Annual variations in thaw rate and thickness of active layer (why?)
• Organic layer critical – bare soil thawed much quicker and was deeper (why)
• Vegetation shows fastest thawing in forest opening and shrubs and lowest under moss and lichen spruce stands (why?)

Question 17

Case study: Palsas

(Melanie and Payette, 2014)

What if?

So what?

Answer 17

• From your “why”,s
• Snow cover had a lesser impact than might be expected because of? (time of snow melt, other more dominating factors such as vegetation)

• Give your opinion or predictions eg.
• Future growth or degradation of palsasin this area – degradation more likely because of
changing climate of internal factors such as tree growth?
• What is causing this differential degradation (70% in non-wooded area ….)

Question 18

Case study: Palsas

(Melanie and Payette, 2014)

What next?

Answer 18

What next?
• Recommendations for further research or analysis
• What else do we need to know?
• Repeat study in other palsa areas such as Finland to compare environmental controls and use
growth /degradation data from there to contextualise this study
• “Interactions between environmental factors such as oranic layer, snow depth, topography,
vegetation types and thermokarst, and the development of the active layer at a local scale
still require further work.”(page 95)

Question 19

What are pingos?

Answer 19

Important features of permafrost terrain
• Indicative of permafrost
• From the Inuit word for ‘conical hill’
• Contains a massive ice core (ground ice)
• Oval or circular in plan
• From 2 to 55 m high and 30 to 100 m long axis

Question 20

Pingo Classification
Can be classified on the basis of origin
(genesis) into one of 3 types:

Answer 20

• Hydrostatic (formerly ‘closed system’
or ‘Mackenzie Delta’
• Hydraulic (formerly ‘open system’ or
‘East Greenland’)
• Polygenetic (controversial, perhaps)

Question 21

Hydrostatic Pingos

Answer 21

• Lake drains and the moisture in the unfrozen area
(talik) freezes from base and sides.
• Water is trapped and then freezes, first as
segregated ice, then massive ice develops.
• This pushes up the sediment above to form a hill.

….Water movement is by hydrostatic pressure

Question 22

Ground ice types

Answer 22

Main one is intrusive ice (injection ice) where water at the top of the sub-pingo water lens occurs = pure ice core

But also:
Segregation ice
Pore ice
Dilation crack ice
Ice-wedge ice

Question 23

Case study: Pingos

(Jones et al., 2012)

Overview

Answer 23

96% of these pingos originated in drained
lake basins
Formed by the injections and freezing of
pressurised water forming an ice core with
heave or updoming of the overlying
sediments and the associated 9% volume
increase and continued injection
Rates of growth = 6m vertical growth in
first 20 years, then only 1m in the next 12
years…….

Question 24

Palsas and Pingos represent the main what?

Answer 24

Non-seasonal cryogenic mounds

Question 25

Palsas occur because of what?

Answer 25

the unique behaviour of peat when it dries and

gets wet, and the associated thermal characteristics.

Question 26

Pingos form through what?

Answer 26

the freezing of pressurized ground water, where

the pressure can be hydrostatic or hydraulic

Question 27

Both palsas and pingos undergo what?

Answer 27

a cycle of growth and degradation, irrespective of climate change