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Palsa Formation :Mathematical Modelling and Field Information AN Weidong PALSA FORMATION :MATHEMATICAL MODELLING AND FIELD INFORMATION Thèse présentée à la Faculté des Études des Études Supérieures de L'Université Laval pour I'ob tention du grade de Philosophiai Doctor (Ph-D.) FAcULTÉ DES ÉTUDES SUPÉRIEURES UNIVERSITÉ LAVAL QUÉBEC AVRIL 1997 O An Weidong, 1997 National Library Bibliothèque nationale 14 of,,,, du Canada Acquisitions and Acquisitions et Bibliographie Services services bibliographiques 395 Wellington Street 395. tue Wellington ûttawa ON K1A ON4 Ottawa ON K1A ON4 Canada Canada The author has granted a non- L'auteur a accordé une licence non exclusive licence allowing the exclusive permettant à la National Library of Canada to Bibliothèque nationale du Canada de reproduce, loan, distribute or seil reproduire, prêter, distribuer ou copies of this thesis in microfom, vendre des copies de cette thèse sous paper or electronic formats. la forme de micro fi ch el^ de reproduction sur papier ou sur format électronique. The author retains ownership of the L'auteur conserve la propriété du copyright in this thesis. Neither the droit d'auteur q~ protège cette thèse. thesis nor substantial extracts fiom it Ni la thèse ni des chtssubstantiels may be printed or otherwise de celle-ci ne doivent être imprimés reproduced without the author's ou autrement reproduits sans son permission. autorisation. ABSTRACT Palsas are defrned as "Peaty permafrost mounds possessing a core of altemathg laye1 of segregated ice and pt or mineral soi1 material". Although such mounds can be compose exclusively of frozen peat and ice, the overwhelming field evidence shows that the peat i pdsas usuaiiy overlies sonme minera! sediments. When a frost susceptibIe soi1 is subjected : freezing, heave occurs as the result of the growth of ice lenses fed by water migration from tf adjacent unfiozen soil. The ice segregation responsible for the topographie heave of thes landforms takes place dominantly in the freezing sediments, the peat cover pIaying the singl role of an insulating layer tbat does not heave itself. A theory for the ongin of palsas was published as early as 1910 by Fries an Bergstrom. Since then, for a century, several hypotheses have been proposed to explain t€ inception, growth and degradation of palsas. RecentIy, it has been found that palsas, like oh periglaciai landforms, are signif~cantly affected by climatic changes: aggradation an degradation of the paisas are contemparaneous processes. Studies of the palsas and oth~ permafrost indicators do not have only an interest for geornophology but also for climatic an environmentai changes. The formation of palsas and cryogenic mounds and their evolution is principal1 controlled by the interaction of thermodynamics processes, ice segregation, frost heave thaw consolidation during decades and centuries. So far, our knowledge was mainly obtaine from field observations and investigations; very few field experiments were reported. Using the I -Dmodel presented, the long terni aggradation of permafrost, the buiidup ( segregated ice and frost heave that takes place during paisa formation have been numericd simulated with reference to field conditions met near the village of Kangiqsualujjuaq, Northem Quebec. The model suggests th: a) discrete ice Iens aggradation near permafro base is the dominant process of palsa heave, and b) palsas cm form under the present climat conditions under snowfree sites. These results and conclusions generdy agree with fie observations, and the modelling approach wili help increase the understanding of still unsolvt aspects of the processes of palsa formation such as the effect of surface geomophologic changes and climatic variations on pemiafrost. Also, using the 1-D model, a numericd prediction of the influence of dimate cooling 01 the palsas is presented. The predicted resuIts show that the cooling spells lead to formation O small ground ice layers in the palsas or cryogenic mounds. Despite a lack of exact data fron deep drill-hoIes and long-term observation of formation of thick layer of ground ice, thc existing cryostatigraphic information in the literature agrees with our model. The 2-D modelling sirn~Aatesthe inception of palsas in an open syseem impacted by a the simultaneous effects of the snow cover and snowfree ground conditions, side by side, b the seasonai variations of water content in the peat cover. The computed results are in agreement with observations from both experimental pals; inception and new natural ones in the field. The modelling results show that, because of th4 insulating effects of seasonal snow cover, the permafrost core fmt foms beneath the snowfrei peat surface and then develops graduaily and yearly in the cross-section. After 6 years O freeze-thaw cycles, the frost heave finally builds up a small incipient palsa 53 cm high. Thi conclusions are that: a) the seasonal snowpack is a major insulating factor directly impacting 01 the soi1 thermal regime for the origin of palsas; b) palsas are not only the product of pas climates, but also forrn in the current climate conditions in discontinuous permafrost regions, i the extemal and internai thermodynamics, mass supply and other relevant conditions are me1 The basic understanding of the origin of paIsas is very useful for studies of cold region climatic and environmentai changes. The conclusions pertaining to palsas should also b applicable to a large array of landforms, such as cryogenic mounds, and to permafrost in fin soils of the discontinuous zone. SHORT ABSTRACT This thesis presents two mathematical models that reproduce the formation and the aggradation of permafrost in palsas. Three major problems are adressed: 1) A one-dimensional mathematicai model of palsa formation; 2) The influence of climate cooling on palsas; and 3) A two-dimensionai model of the origin of palsas. The numerical modeiiing, assurning a saturated inhomogeneous soi1 medium, provides a basis for the advancement of theorical knowledge relative to palsas. Ground thermal regime, discrete ice lensing and frost heave problems had to be numerically soIved in the process. Cornparison of the numerical results with field data shows good agreement. 1 would iike to express my great gratitude to rny advisor, Professor Michel Aiiard of the Centre d'études nordiques for his wise guidance. encouragements and continuous suppori £rom the inception to the present form of this thesis. He has given me an invaluable help. Comrnents of the two other members of my supernision conmittee, Dr. Jean-Claude Dionne and Jean-Marie Konrad are aiso acknowledged. I wouId iike to acknowledge the help 1 received from many friends and the staff of the Centre d'études nordiques and of the département de Géographie, Université Lavai, particuiarly Dr. Janusz Frydecki, in charge of automatic data acquisition systerns, Martin Poitras, computer specialist and Serge Caron, research assistant. The contributions to field work by the following persons was also appreciated: Christian Bouchard, Richard Fortier and Éric Ménard. 1 owe a dept of gratitude to the National Science and Engineeering Research Council of Canada and "Fonds pour la formation de chercheurs et l'aide à la recherche" of Quehc for financial support for this project through grants to my supervisor and to the Centre d'études nordiques. I also want to express rny thanks to professor Wu Ziwang, director of the State Key Laboratory for Frozen Ground of the Lanzhou institute of Geocryology and Glaciology, my mentor in China, who encouraged and supported me to pursue rny doctoral degree. Finally, my parents and my whole farnily desserve rny full reconnaissance for their Life long support of my education. 4.8 Summary...................................................................................... 52 Chapter V A ONE-DIMENSIONAL MATHEMATICAL MODEL OF PALSA FORMATION AND GROWTH ................................ 54 5.1 Background ............................................................................... 54 5.2 Study Site ................................................................................. 56 5.2.1 General characteristics of the study site ........................................... 56 5.2.2 Peat properties ..........................~.......~....................................58 5.3 Mathematical Modelling .............................................................. 60 5.4 Numerical Computation .............................................................. 65 5.5 Numerical Results ...................................................................... 66 5 .5. 1 Themai regime .................... .. ............................................. 66 5.5.2 Ice segregation conditions .......................................................... 68 5.6 Discussion ................................................................................ 71 5.7 Summary .................................................................................. 73 ChapterVI INFLUENCE OF CLIMATE COOLING ON PALSAS ............ 75 6.1 Introduction .............................................................................. 75 6.1.1 Background .......................................................................... 76 6.1.2 Regional clirnate cooling ............................................................ 77 6.2 Synopsis of Modelling .......................................................... 77 6.2.1 Initial condition ...................................................................... 77 6.2.2 Boundary condition ................................................................
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