Cryogenic Soil Structure and Cryopedogenesis

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Cryogenic Soil Structure and Cryopedogenesis

• Chien-Lu Ping
• Agricultural and Forestry Experiment Station
• University of Alaska Fairbanks, Palmer, AK
• Yuri L. Shur
• Dept. of Civil Engineering and Environmental
  Sciences
• University of Alaska Fairbanks, Fairbanks, AK

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Cryogenic Structures
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Cryogenic Structure

• Structures formed by cryogenic processes
  cryogenesis.
• Cryogenesis processes in soils associated with
  soil freezing and thawing or deformation due to
  ice formation and or freeze-thaw cycles.
• Cryopedogenesis Soil formation affected by
  cryogenesis.
• Occurrence in areas affected by permafrost and
  seasonal frost.

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Why Study Cryogenic Structure

• The need to study soils in frozen state because
  soils are frozen gt75 of the time.
• To understand the relationship between soil
  formation and cryogenesis.
• To use cryogenic structure for horizonation.
• To estimate the depth of active layer.
• To determine paleopermafrost features.

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Crumb structure and cryptogamic crust on the
surface of frost boil
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Crumb Structure and Needle Ice

• The primary mechanism of crumb structure
  formation - needle ice formation.
• Needle ice often develops on the surface of the
  ground during night frost in autumn and spring.
• Elongation of the ice crystal is perpendicular to
  the cooling surface (ground).
• Soil particles on the ground surface are lifted
  by needle ice, then fall to the ground surface
  when thaw commences.
• Different from granular.

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Needle ice formation and crumb structure
0
10
20
30
0
5
0
5
A. Needle ice formation.
B. Crumb structure accumulated on surface after
needle ice thaw.
(Scale in cm)
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Granular structure formed in earth hummocks under
boreal forest, Yukon Territory
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Cryogenic Granular Structure

• Granular structures form on the surface horizons
  under tundra vegetation cover, especially on
  hummocks.
• The granular structures have a nearly rounded
  shape, 2-4 mm in diameter with a firm moist
  consistence.
• It started with platy structures that broken into
  pieces due to horizontal stress during
  refreezing.
• Then the broken plates are turned around by the
  root mat action and gradually become spheroids
  with repeated freeze-thaw cycles.

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Granular Structure formed under organic horizons
in a moist acidic tundra soils, Alaska
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Platy structure formed at the lower active layer
of a tundra soil, Alaska
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Cryogenic Platy Structure

• Platy structures in frost-affected soils form due
  to thin ice layers lens formation.
• Ice lens forms perpendicular to the freezing
  front, thus its orientation is generally parallel
  to the ground surface.
• When ground surface start to freeze a freezing
  front is created due to thermal gradient and
  phase change in water.
• Water, either in liquid or vapor forms, moves
  toward the freezing front and form a thin layer
  of ice.
• More layers of ice lens form as the frost
  penetrates deeper.
• Soil particles become somewhat orientated as a
  result of repeated freeze-thaw cycles.

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Massive structure as fractured by power hammer in
a boreal forest soil, Alaska
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Massive Structure

• In Arctic, during freezing, water moves to 2
  freezing fronts one from surface and the other
  from permafrost, a process called freeze back.
• A desiccation zone is formed in the middle of
  active layer with a massive structure.
• Coarse blocky or platy structures may form and
  the frost cracks are filled with sublimation ice,
  an indication of vapor movement.
• Ice content lt15 in frozen state

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Water contents of 2 Gelisols in
late March
0
0
Oe
Oi/Oa
A
A
20
20
Bw
Haplorthel (boreal forest)
Bw
Depth (cm)
40
40
2Bw2f
Molliturbel (arctic tundra)
60
60
BC
2BCf
80
80
Oaf
0
40
200
80
120
160
Water Content ( by wt.)
100
Cryogenic Structure
Reticular w/ continuous ice lenses
Structureless
Cf1
120
Platy
Ataxitic (suspended) ice-rich layer
Cf2
Coarse platy or massive
140
0
40
80
120
160
200
Micro- lenticular
Lenticular
Water Content ( by wt.)
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Lenticular structure formed in lower active
layer, Lower Kolyma, NE Rissia
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Lenticular Structure

• Deformed platy structure due to horizontal stress
  during freezing.
• Platy structures being fractured into
  discontinuous and curved lenses.
• Size ranges 2 8 mm thick and 7 15 mm long.
• Formed in loamy soils, friable to slightly firm
  when moist and very firm when frozen.
• Contains 30 to 50 ice when frozen.

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Reticulate Structure (77 85 cm)
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Reticulate Structure

• Horizontal orientation provided by coarse
  lenticular structure caused by ice lens, and
  vertical orientation by ice net (ice vein)
  formation due to freeze back.
• Ice lens and ice net become wider due to water
  accumulation from repeated freeze cycle ice
  content gt40
• Usually formed at the contact of the active layer
  and permafrost.
• Angular blocky structure after thaw.

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Ice net (ice vein) formed by vertical ice veins
at the bottom of the active layer, (top view)
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Ice net formation

• Formed by desiccation cracks due to freeze back
  in early winter.
• The cracks filled with sublimation or segregation
  ice in winter an filled with water in the fall.
• Contribution to the formation of reticulate
  structure.

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Ataxitic structure (right above ice wedge) in a
tundra soil, NE Russia
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Ataxitic Structure

• A term used by Russian geocryologists describing
  an ice-rich horizon in which soil blocks
  suspended in an ice matrix.
• Late stage of reticulate structure development,
  subject to decadal freeze thaw cycle and slow
  decrease of the active layer depth.
• Usually occur at upper permafrost (some buried
  horizon also have ataxitic structure).
• Angular blocky structure when thawed.
• Ice content gt60.

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Micro-lenticular structure
4 cm
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Micro-lenticular Structure

• Characterized by alternate very thin ice lens and
  soil, usually lt0.5 mm and ice content gt60.
• Extremely firm consistence
• Syngenic formation in loess or sediment deposit
  (permafrost table gradually rise due to
  thickening of the overburden deposits).
• Usually below decadal thaw cycle, thus considered
  as true permafrost.
• Typical for Late Pleistocene syngenetic
  permafrost

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Cryopedogenic Processes

• Cryogenic structures after thawing reflects in
  post-cryogenic structure of soil, which forms
  soil structure and provide passages for air and
  water after thaw.
• Provides surfaces for biogeochemical weathering.
• Induce cryoturbation through differential frost
  heave, frost churning of SOM into lower active
  layer and upper permafrost.

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Common occurrence of cryogenic structures in soil
horizons
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Acknowledgement

• This research is supported by the following
  projects
• USDA Hatch project
• NSF ARCSS Flux and ATLAS projects
• USDA-NRCS Global Change project
• International Permafrost Association
• University of Alaska Foundation
• University of Alaska EPSCoR pogram