Permafrost is ground that is permanently frozen below
Permafrost is ground that is permanently frozen below the surface. In areas of permafrost a permanent zone of frozen soil exists below the surface even if the surface soil thaws in summer. Permafrost covers most of the higher northern land areas of earth, such as Siberia, Canada, and Alaska. Permafrost
Periglaciation is a set of processes that happen in areas near to ice sheets e.g. Alaska or in very cold mountain areas e.g. Himalayas. These processes produce a unique set of periglacial landforms. Many areas in Europe and North America show evidence of relic periglacial processes from the last Ice Age. A key characteristic of periglacial areas is permafrost or permanently frozen ground. Today, permafrost still covers 25% of the Earth’s surface although this figure is declining due to global warming. The depth and continuity of the permafrost vary with latitude, altitude and ocean currents. Map source:http://atlas.nrcan.gc.ca/site/english/index.html Diagram source: http://gsc.nrcan.gc.ca/permafrost/images/wheredoes2.jpg
In Northern Canada continuous permafrost extends down to depths of around 700 metres. Here mean annual air temperatures is below -5°C and winter temperatures may fall to -50°C. In summer only the top of the ground will thaw. This unfrozen surface layer is known as the active layer. The active layer varies from a few centimetres to about 5 metres and even on gentle slopes this saturated layer can become very mobile. Below the active layer, ground temperatures fluctuate but remain below freezing point. The unfrozen layer beneath the active layer is known as talik. The lower limit of the talik (permafrost base) is determined by geothermal heat rising up from the mantle. Diagram source: http://gsc.nrcan.gc.ca/permafrost/whatis_e.php
The Geothermal Gradient Figure 1.9 Diagrammatic cross-section through the upper 200-300 km of the Earth showing geothermal gradients reflecting more efficient adiabatic (constant heat content) convection of heat in the mobile asthenosphere (steeper gradient in blue) ) and less efficient conductive heat transfer through the more rigid lithosphere (shallower gradient in red). The boundary layer is a zone across which the transition in rheology and heat transfer mechanism occurs (in green). The thickness of the boundary layer is exaggerated here for clarity: it is probably less than half the thickness of the lithosphere.
The Geothermal Gradient Figure 1.11 Estimates of oceanic (blue curves) and continental shield (red curves) geotherms to a depth of 300 km. The thickness of mature (> 100Ma) oceanic lithosphere is hatched and that of continental shield lithosphere is yellow. Data from Green and Falloon ((1998), Green & Ringwood (1963), Jaupart and Mareschal (1999), McKenzie et al. (2005 and personal communication), Ringwood (1966), Rudnick and Nyblade (1999), Turcotte and Schubert (2002).
The Geothermal Gradient Fig 1.13. Pattern of global heat flux variations compiled from observations at over 20,000 sites and modeled on a spherical harmonic expansion to degree 12. From Pollack, Hurter and Johnson. (1993) Rev. Geophys. 31, 267-280. Cross-section of the mantle based on a seismic tomography model. Arrows represent plate motions and large-scale mantle flow and subduction zones represented by dipping line segments. EPR =- East pacific Rise, MAR = Mid-Atlantic Ridge, CBR = Carlsberg Ridge. Plates: EA = Eurasian, IN = Indian, PA = Pacific, NA = North American, SA = South American, AF = African, CO = Cocos. From Li and Romanowicz (1996). J. Geophys. Research, 101, 22,245-72.
The Geothermal Gradient Thermal structure in a 3D spherical mantle convection model (red is hot, blue is cold). J. H. Davies and H.-Peter Bunge http://www.ocean.cf.ac.uk/people/huw/AGU99/mantlecirc.html
Plate tectonics Cooling mechanisms for a hot planet If the viscosity is low enough, plumes (in blue) will descend from the cooled upper layer: a form of convection. Figure 12-18. Cold plumes descending from a cooled upper boundary layer in a tank of silicone oil. Photo courtesy Claude Jaupart. But the upper mantle is too viscous for this
Simple Model of the Surface Energy Budget Rn = Total Radiation H = Surface Sensible Heat Flux LE = Latent Energy Heat Flux G = Ground (Soil) Heat Flux Role of Soil in Each Term: H: Heat from soil warms (-)/cools air (+) LE: Heat used to evaporate water/freeze water G: Heat stored in soil (remember C1 and CV terms from thermodynamic equation)
Evaporation Rates and Model Initialization Nonlinear evaporation rate Limit = hydraulic diffusivity/moisture threshold (remember soil structure!) How will model initialization runs vary as a result? Warner, 2004
Linked In: Evapotranspiration Etot=Edir+Et+Ec Etot = Total Evaportranspiration from Soil and Vegetation Edir = Direct Evaporation from Soil Et = Transpiration from Plant Canopy Ec = Evaporation from Canopy Intercepted Rainfall Represents a moisture flux that can be approximated by comparing resistances to potential flux (Ohm’s Law: Flux=P/R) Resistances include: Available Soil Moisture Canopy (Stomatal) Resistance (Vegetation type, ‘Greeness’) Atmospheric Winds, Stability Bottom Line: Many Interacting Factors in Soil Moisture/Energy Budget !!!
One bucket hydrologic model P/WBM (no temperature profile) Vörösmarty et al., 1998 M. Rawlins, R. Lammers, S. Frolking, B. Fekete & C. J. Vörösmarty, 2003
A simple permafrost-hydrology model M. Rawlins, D. Nickolsky, V. Romanovsky et al., 1D heat equation with phase change Two bucket hydrological model
Hydro-Thermo Dynamic Model - HTDM-1.0 S. Marchenko, D. Wisser, V. Romanovsky, Frolking, S., and Vörösmarty, C. We couple a macroscale hydrologic model WBMplus and one of the versions of the GIPL thermo dynamic (permafrost) model Several key parameters: Field capacity Wilting point Infiltration rate Soil porosity Soil Thermal Properties Unfrozen Water Content Freezing-point depression WBM plus GIPL equal HTDM-1.0
Vertical water exchange between the land surface and the atmosphere Horizontal water transport along a prescribed river network Soil temperature dynamics Depth of seasonal freezing and thawing by solving 1D non-linear heat equation with phase change numerically Time of freeze up HTDM-1.0 is a fully coupled soil water balance and heat transfer model that simulates:
Modeled distribution of peat depth The peat depth is computed from the the carbon content [kg/m2] of the FAO soil map (Webb et al., 2000,
Modeled distribution of soil thermal conductivity within the upper layer
Mean annual air temperature (ECHAM5) and soil temperature at 0.5 m depth reconstructed for 2001 and predicted for 2050 and 2100
Mean annual soil temperature at 2 m and 5 m depth reconstructed for 2001 and predicted for 2050 and 2100
In general, the periglacial processes remain poorly understood and there is debate as to whether the processes were more active in the past than now. An increasing amount of research has been done as humans have tried to exploit the resources, particularly oil, in these difficult environments. An important process in periglacial areas is frost heave. This results from ice crystals or ice lenses forming in fine-grained soils. As the ice expands, the ground above is domed up and stones get pushed to the surface. Frozen ground cracks and forms patterned ground in which loose stones fall into the cracks to highlight the outlines. These so-called stone polygons vary from 1-5 metres in diameter. On slopes above 2°, the stone polygons become stretched and may form stone stripes. Photo source: http://www.mun.ca/botgarden/limestonebarrens/Geology.htm Photo source: http://sis.agr.gc.ca/cansis/taxa/landscape/ground/sorted_nets_nt.jpg
Photos source: http://gsc.nrcan.gc.ca/beaufort/images/ground_ice3.jpg In winter, the active layer freezes and the soil layer contracts and forms natural polygonal patterns. The following summer, the cracks may close up or sometimes become filled with meltwater and debris. In the subsequent winter, any water filled cracks expand as ice forms and the cracks widen to form ice wedges. Repeated freezing and thawing leads to wedges a metre wide and up to three metres deep. In the UK, fossil ice wedges can be found in many areas. The cracks are filled with sand and silt deposited by meltwater. Ice-wedge polygons can be distinguished from the stone polygons by their greater size (up to 30 metres across) and the depression, rather than a dome at the centre. Photo source: http://www.ucalgary.ca/~dgsmit/duane.jpg
In a periglacial landscape, the most dramatic landform is the pingo. These are dome-shaped hills which may rise to around 50 metres above the generally flat tundra landscape. Some of the pingos have depressions in their surfaces and some have ice cores at their centre. Photo source: http://www.geo.uu.nl/fg/berendsen/pictures/photography/alaska/Pingo.jpg It is believed that the origins of these landforms may vary and two types are now recognized – the open system (hydraulic) pingos and the closed-system (hydrostatic) pingos. Diagram source: http://www.erudit.org/revue/gpq/1998/v52/n3/004847ar.html
Open-system (hydraulic) pingos occur in valley bottoms and in areas of thin or discontinuous permafrost. They are common in East Greenland. When the surface layer freezes, water is trapped in the talik (unfrozen layer). This water comes under pressure and may move towards the surface where, surrounded by permafrost, it freezes into an ice core. This causes the surface to dome upwards. As water under pressure finds its way to the surface, the ice dome and thus the pingo, continues to grow. Photo source: http://sis.agr.gc.ca/cansis/taxa/landscape/ground/ice_wedge_nt2.jpg
Closed-system (hydrostatic) pingos are generally found in lowland areas where permafrost is more continuous. They often form on the sites of small lakes. As these lakes fill with sediments from meltwater, the surrounding permafrost advances and squeezes the unfrozen sediments below the lake. When the lake itself is frozen, the water in the underlying sediments causes the surface to dome upwards creating the pingo. If the dome cracks, the ice core may melt leading to a collapse of the pingo and a pond forming in the central crater. Diagram source: http://www.nsidc.net/fgdc/glossary/illustrations.pdf
Mass movement processes are very active in periglacial areas and help to shape the landscape in areas where there is gentle relief. Frost creep is common in the top 50 centimetres of soils in summer on steep slopes. Ice crystals in the soil push up stones and soil particles which collapse back when the ice melts. In this way the stones and soil particles zigzag their way down the slope moving perhaps 20-30 centimetres in a year. Solifluction (sometimes known as gelifluxion) is a more obvious process as it operates on a larger scale. In the short summer season, the saturated active layer sits on top of an impermeable frozen layer. The active layer is unstable on even quite gentle slopes of 1-2 degrees. As a result of this, the active layer becomes mobile, particularly in spring, and can flow large distances. The thin vegetation layer sometimes restricts movement but often breaks. Diagram source: http://www.school-portal.co.uk/GroupDownloadFile.asp?GroupId=5313&ResourceId=18976
Unpleasant surprises? 1980-90s – increasing evidence for dramatic and abrupt shifts in climate Abrupt climate change “when the climate system is forced to cross some threshold, triggering a transition to a new state at a rate determined by the climate system itself and faster than the cause” (Alley et al., 2003). An unpleasant surprise in store? Broecker (1987) IPCC models not able to model rapid changes well Focus shifts to sediments
Heinrich events Abrupt collapse of NH ice sheets - cooling Last hundreds of years, onset in years 6 identified in late Quaternary – more earlier? Bond et al., 1992 Lithics in N Atlantic ocean cores – ice bergs Other evidence 2.3x106 km3 freshwater 7 kyr cyclicity Cause – internal or external? Focused on N. Atlantic but do they propagate to other locations?
Dansgaard-Oeschger events Identified strongly in glacials, weakly in Holocene Identified from Greenland (5-8 °C warming) ice core records NH – rapid warming (decades or less), slow cooling SH – slow warming, less intense – precede? So called AIM events. Causes – solar (Bond et al. 2001)/ internal (THC/ice sheets/wind field/harmonic) Greenland Ice Core Data Rapid climate changes of period 1.47±0.5 ka
H and D-O events – recent picture Pervasive – D-O events seen in pre-Late Quaternary records (loess, Antarctic ice – 650-800 ka) Near global extent – Asia, Africa, Americas, Antarctica Many rapid proxy changes are seen as regional manifestations Porter and An, 1995, Nature Typified by the loess record in China (e.g. Porter and An, 1995 and more recently Guan et al., in press). Some recent caveats…
Sublimation is the freeze drying process in which solvents, such as water, pass directly from the solid to the vapor state. The rate of sublimation is dependent on temperature and pressure. Definition
Soils may be non-structured (e.g., single grain or massive) or consist of naturally formed units known as peds or aggregates. The initial stage in the formation of soil structure is the process of flocculation. Individual colloids typically exhibit a net negative charge which results in an electrostatic repulsion.
Reduction of the forces of electrostatic repulsion allows the particles to come closer together. Flocculation This process allows other forces of attraction to become more dominant. The formation of these “flocs” in suspension represents the early stages of aggregation.
Platy and spherical soil structure is common to the surface soil horizons, blocky and columnar/prismatic are associated with the deeper subsurface soil horizons
38 soil particle The larger the particles are, the more SiO2 the soil has, the more barren it is. http://netc.nwsuaf.edu.cn/jingpin/2003/turangxue/ppt/2.1.ppt
Surface Area In comparing clay with sand and silt, it is important to be aware of the relative amount of surface area of these particle size groups, bc it is on the surface that many chemical and physical processes take place. Smaller = more surface area (clay is tiny!)
Soil Horizons: O (organics = humus), A, B, C. Boundaries usually transitional rather than sharp.
Soil Structure Soils Composed of: Organic Matter (>80% organic soil, <10% mineral soil) Minerals (From parent geology, ~55% in mineral soil) Air Water Type, abundance, arrangement of particles govern heat flow, water flow, nutrient availability
5 General Soil Structure Profiles Place matters!!!
Factors Controlling Soil Formation ORGANIC ACTIVITY: Required to develop humus. TOPOGRAPHY: Soil development is difficult when topography is steep. TIME: Typical development = 2.5 cm/100 yrs. to 2.5 cm/1,000 yrs.
WEATHERING: Process that transforms high temperature minerals to low-temperature ones stable at the Earth’s surface. Dissolves ALL minerals to some extent. EROSION: Transport away from weathering site. This process exposes intrusive igneous rocks to weathering & the process continues. Transport by Wind, Ice, Gravity, Water (solution/ suspension).
Weathering & Erosion: how the hydrologic cycle interacts with geology. Weathering: group of destructive processes that change physical/chemical character of rock at/near surface. Weathering: in situ breakdown of rocks in contact with water, air, or organisms. Forms sediment and soil.
Physical Weathering… …Is the actual disintegration of rocks due to SCOURING by wind, water, and/or ice In simple terms… time Water and Wind in Death Valley Melt/Freeze, Wet/Dry = Expansion/Contraction (cracks in sidewalk) Plants help too!!!!
Chemical Weathering Climate important: Kinetic rates increase with temp. Rocks dissolve due to reactions between rock minerals and water, acid, or other chemicals Hydrolysis Mg2SiO4 + 4H+ + 4OH- ⇌ 2Mg2+ + 4OH- + H4SiO4 Dissolution CO2 + H2O -> H2CO3 then H2CO3 + CaCO3 -> Ca(HCO3)2 Oxidation 4Fe + 3O2 → 2 Fe2O3
Mechanical Weathering Frost-Wedging: water expands by 9% upon freezing – most significant where freeze-thaw cycle occurs often. Frost Heaving: cooler under rocks, freezes first, expands and lifts.
Frost Wedging, very effective at breaking up rocks. Only in climates with frequent freeze/thaw cycles.
Freeze-thaw action Moisture migration and redistribution Soil after freezing, Closed System Cold Warm Consolidated Soil Prior to Freezing Soil after freezing, Open System Heave
Frost heave: is the vertical expansion of soil caused by freezing the water in the voids. Water expands when it freezes Frost heave is not uniform in a horizontal direction and this causes serious damage of pavements and foundations supported by soil: Cracking of pavements, building walls, and floors When the temperature increases above the freezing point, frozen soil thaws and settlement of structures occurs Alternate lifting and settling of pavements and structures due to frost heave can cause serious structural damage
Frost Susceptible Soils
Need for Soil Improvement Frost Heave
Mechanical Weathering Precipitation of Crystals: salts precipitating from water in rock crevices/cracks. Forces the opening wider. Root Systems: dominant in cold/dry climates.
Mechanical Weathering Temperature Changes: differential expansion (deserts, mountains, & forest fires).
Fig. 05.15
Soil Development
Fig. 05.18 Water and cations loosely held by clay and available to plants
Soil Texture = The Sand, Silt & Clay in a soil. Soil texture is the single most important physical property of the soil. Knowing the soil texture alone will provide information about: 1) water flow potential, 2) water holding capacity, 3) fertility potential, 4) suitability for many urban uses.
Glacial Deposits Glacial deposits formed as massive ice sheets, moving across North America, approximately one million years ago. As the glaciers expanded, they "bulldozed" rocks, minerals and soil in front of them. As the ice sheets melted, the exposed parent material began to weather and soil was formed. http://www.soils.umn.edu/academics/classes/soil2125/img/1usglac.jpg
Glacial Soils Boulders and Rocks
12-lekcii_po_obschey_geokriologii_dobavleniya.ppt
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