Modeling the Cordilleran Ice Sheet
Barry L. Robert
ABSTRACT
A time-dependent ice flow model is used to provide detailed reconstructions of ice growth and retreat for the southern portion of the Late Wisconsinan Cordilleran Ice Sheet. The two-dimensional, time-dependent model provides ice surface elevations and flow directions at a grid spacing of 15 km. Input to the model includes subglacial topography, a net mass balance function, and two ice flow parameters. The net mass balance function uses a polynomial equation to estimate equilibrium line altitude (ELA) across the study area. A quadratic equation is then used to provide net mass balance values as a function of elevation relative to the ELA. Late Wisconsinan glacial conditions are simulated by systematically lowering the ELA. The general timing of the model ice advance and retreat is tested against radiocarbon dated localities which place limits on the ice sheet's areal extent for different times during the Late Wisconsinan glaciation. In addition, glacial-geologic evidence directly attributable to the latest Cordilleran Ice Sheet is used in assessing the model reconstructions. Results from these experiments show that an ice growth and retreat chronology consistent with the limiting radiocarbon dates can be generated using the model, and provide information on flow directions and ice growth and retreat patterns.
RÉSUMÉ
Modélisation de l’Inlandsis de la Cordillère
On a élaboré un modèle d'écoulement glaciaire spatio-temporel qui permette la reconstitution précise de la croissance et du retrait de la partie centrale de l'Inlandsis de la Cordillère, au Wisconsinien supérieur. Le modèle bi-dimensionnel et temporel illustre les différentes altitudes de la surface glaciaire et les directions d'écoulement dans une grille dont les carreaux ont 15 km de côté. Les données de base comprennent la topographie sous-glaciaire, la fonction du bilan de masse net et deux paramètres de l'écoulement glaciaire. Une équation polynomiale sert a estimer l'altitude de la ligne d'équilibre dans la région à l'étude à partir de la fonction du bilan de masse net. Une équation quadratique permet ensuite d'obtenir les valeurs du bilan de masse net en tant que fonction de l'altitude relative de la ligne d'équilibre. Les conditions glaciaires au Wisconsinien supérieur sont simulées en abaissant de façon systématique l'altitude de la ligne d'équilibre. La chronologie générale de la croissance et du retrait établie dans le modèle est vérifiée par les sites datés au radiocarbone qui donnent des repères quant à l'étendue de l'inlandsis à différentes périodes de la glaciation du Wisconsinien supérieur. Les données de géologie glaciaire directement attribuables au dernier inlandsis ont également servi à vérifier le modèle. Les résultats tirés de ces expériences indiquent qu'une chronologie de la glaciation conforme aux datations au radiocarbone peut être établie en utilisant le modèle, qui livre aussi des renseignements sur les directions d'écoulement, ainsi que sur les modes de croissance et de retrait.
ZUSAMMENFASSUNG
Modellierung der Kordilleren-Eisdecke.
Um Eisausdehnung und Eisrùckzug fur den sudlichen Teil der Kordilleren-Eisdecke im Spàt-Wisconsin detailliert zu rekonstruieren, hat man ein zeitabhàngiges EisfluBmodell benutzt. Das zweidimensionale, zeitabhângige Modell gibt Auskunft ûber die verschiedenen Hôhen der Eisoberflàche und die Richtungen des FlieBens in einem Raster von Quadraten mit 15 km Seitenlànge. Zu den Eingaben in das Modell gehôren die subglaziale Topographie, die Funktion der Netto-Masse-Bilanz und zwei EisfluB-Parameter. Die Funktion der Netto-Masse-Bilanz benutzt eine polynomische Gleichung, um die Hôhe der Gleichgewichtslinie quer durch das untersuchte Gebiet abzuschàtzen. Eine quadratische Gleichung wird dann benutzt, um die Werte der Netto-Masse-Bilanz als eine Funktion der Hôhe in Bezug auf die Gleichgewichtslinie zu berechnen. Man simuliert die glazialen Bedingungen im spâten Wisconsin, indem man die Gleichgewichtslinie systematisch senkt. Die allgemeine Chronologie des Eisvor-und RùckstoBes im Modell wird ùberprùft mittels Vergleich mit Plâtzen, zu denen es Radiokarbondaten gibt und die der Ausdehnung der Eisflàche zu verschiedenen Zeiten wàhrend der spàtwisconsinischen Vereisung Grenzen setzen. Zusâtzlich werden bei der Festsetzung der Modell-Rekonstruktionen glazialgeologische Zeugnisse genutzt, die der spàtesten Kordilleren-Eisdecke direkt zuzuschreiben sind. Ergebnisse dieser Expérimente zeigen, daf3 mittels des Modells eine Eisausdehnungs- und Eisrûckzugschronologie geschaffen werden kann, die mit den Grenzwerten der Radiokarbondaten ùbereinstimmt und Informationen ùber FlieBrichtungen und Eisausdehnungs und Eisrùckzugsmuster liefert.
Bibliographie
Alley, N. F., 1979. Middle Wisconsin stratigraphy and climatic reconstruction, southern Vancouver Island, British Columbia. Quaternary Research, 11 : 213-237. DOI:10.1016/0033-5894(79)90005-X
Andrews, J. T. and Mahaffy M. A. W., 1976. Growth rate of the Laurentide Ice Sheet and sea level lowering (with emphasis on the 115,000 B.P. sea level low). Quaternary Research, 6: 167-183. DOI:10.1016/0033-5894(76)90048-X
Booth, D. B., 1986. Mass balance and sliding velocity of the Puget Lobe of the Cordilleran Ice Sheet during the last glaciation. Quaternary Research, 25: 269-280. DOI:10.1016/0033-5894(86)90001-3
Booth, D. B., 1987. Timing and processes of déglaciation along the southern margin of the Cordilleran Ice Sheet, p. 71-90. In W. F. Ruddiman and H. E. Wright Jr., eds., North America and Adjacent Oceans During the Last Déglaciation. The Geology of North America, K-3. Geological Society of America, Boulder, Colorado. 509 p.
Budd, W. F. and Smith, I. N., 1981. The growth and retreat of ice sheets in response to orbital radiation changes, p. 369-409. In I. Allison, éd., Sea Level Ice and Climatic Change. International Association of Hydrological Sciences: 131.
Budd, W. F. and Smith, I. N., 1987. Conditions for growth and retreat of the Laurentide Ice Sheet. Géographie Physique et Quaternaire, 41: 279-290.
COHMAP Members, 1988. Climatic change of the last 18,000 years: observations and model simulations. Science, 241: 1043-1052. DOI:10.1126/science.241.4869.1043
Clague, J. J., 1975. Glacier-flow patterns and the origin of Late Wisconsinan till in the southern Rocky Mountain Trench, British Columbia. Geological Society of America Bulletin, 86: 721-731. DOI:10.1130/0016-7606(1975)86<721:GPATOO>2.0.CO;2
Clague, J. J., 1980. Late Quaternary geology and geochronology of British Columbia, part 1 : radiocarbon dates. Geological Survey of Canada, Paper 80-12,28 p.
Clague, J. J., 1981. Late Quaternary geology and geochronology of British Columbia, part 2: summary and discussion of radiocarbon-dated Quaternary history. Geological Survey of Canada, Paper 80-35, 41 p.
Clague, J. J., 1983. Glacio-isostatic effects of the Cordilleran Ice Sheet, British Columbia, p. 321-343. In D. E. Smith and A. G. Dawson, eds., Shorelines and lsostasy. Academic Press, London, 387 p.
Clague, J. J., 1989. Cordilleran Ice Sheet, p. 40-42. In R. J. Fulton, éd., Chapter 1 of Quaternary Geology of Canada and Greenland. The Geology of North America, K-1, Geological Society of America, Boulder, Colorado, 839 p.
Clague, J. J., Saunders, I. R. and Roberts, M. C., 1988. Ice-free conditions in southwestern British Columbia at 16,000 years B.P. Canadian Journal of Earth Sciences, 25: 938-941. DOI:10.1139/e88-093
Denton, G. H. and Hughes, T. J., 1981. The Last Great Ice Sheets. Wiley Interscience, New York, 484 p.
Easterbrook, D. J., 1986. Stratigraphy and chronology of Quaternary deposits of the Puget Lowland and Olympic Mountains of Washington and the Cascade Mountains of Washington and Oregon, p. 145-159. In V. Sibrava, D. Q. Bowen and G. M. Richmond, eds., Quaternary Glaciations in the Northern Hemisphere. Pergamon Press, New York, 514 p.
Fulton, R. J., 1967. Déglaciation studies in Kamloops region, an area of moderate relief, British Columbia. Geological Survey of Canada, Bulletin 154, 36 p.
Fulton, R. J., 1984. Quaternary glaciation, Canadian Cordillera, p. 39-48. In R.J. Fulton, éd., Quaternary Stratigraphy of Canada - A Canadian Contribution to IGCP Project 24. Geological Survey of Canada, Paper 84-10, 210 p.
Haeberli, W., 1985. Fluctuations of glaciers 1975-1980 (vol. IV). International Commission on Snow and Ice of the International Association of Hydrologie Sciences and United Nations Education, Science and Culture Organization, Paris, 256 p.
Haeberli, W. and Muller, P., 1988. Fluctuations of glaciers 1980-1985 (vol. V). International Association of Hydrologie Sciences/United Nations Environmental Program/United Nations Education Science and Culture Organization, World Glacier Monitoring Service, Zurich, 289 p.
Heusser, C J., 1973. Environmental sequence following the Fraser advance of the Juan de Fuca Lobe, Washington. Quaternary Research, 3: 284-306. DOI:10.1016/0033-5894(73)90047-1
Heusser, C J., Heusser, L. E. and Streeter, S. S., 1980. Quaternary temperatures and precipitation for the north-west coast of North America. Nature, 286: 702-704. DOI:10.1038/286702a0
Hughes, T. J., 1985. The great Cenozoic Ice Sheet. Palaeogeography, Palaeoclimatology, Palaeoecology, 50: 9-43. DOI:10.1016/S0031-0182(85)80004-3
Hughes, T., 1987. Ice dynamics and déglaciation models when ice sheets collapsed, p. 183-220. In W. F. Ruddiman and H. E. Wright Jr., eds., North America and Adjacent Oceans During the Last Déglaciation. The Geology of North America, K-3, Geological Society of America, Boulder, Colorado, 509 p.
Locke, W. W. and Semerad, E., 1988. A testable hypothesis regarding Late-Pleistocene ice-cap glaciation of the northern Rocky Mountains of Montana. Geological Society of America Abstracts with Programs, 20: 428.
Mahaffy, M. A. W., 1974. A three-dimensional numerical method for computing the load distribution of ice sheets with time. M.S. thesis. University of Colorado, 74 p.
Mahaffy, M. A. W., 1976. A three-dimensional numerical model of ice sheets: tests on the Barnes Ice Cap, Northwest Territories. Journal of Geophysical Research, 81: 1059-1066. DOI:10.1029/JC081i006p01059
Mehringer, P. J. Jr., Sheppard, J. C and Foit, F. F. Jr., 1984. The age of Glacier Peak tephra in west-central Montana. Quaternary Research, 21: 36-41. DOI:10.1016/0033-5894(84)90087-5
Meier, M. F., Tangborn, W. V., Mayo, L. R. and Post, A., 1971. Combined ice and water balances of Gulkana and Wolverine Glaciers, Washington, 1965 and 1966 hydrologie years. U.S. Geological Survey Professional Paper 715-A, 23 p.
Mullineaux, D. R., Waldron, H. H. and Rubin, M., 1965. Stratigraphy and chronology of late interglacial and early Vashon glacial time in the Seattle area, Washington. U.S. Geological Survey Bulletin 1194-0, 10 p.
Oerlemans, J., 1981. Modeling of Pleistocene European ice sheets: some experiments with simple mass-balance parameterizations. Quaternary Research, 15: 77-85. DOI:10.1016/0033-5894(81)90115-0
Porter, S. C, 1977. Present and past glaciation threshold in the Cascade Range, Washington, U.S.A.: topographic and climatic controls, and paleoclimatic implications. Journal of Glaciology, 18: 101-116.
Porter, S. C, 1978. Glacier Peak tephra in the North Cascade Range, Washington: stratigraphy, distribution, and relationship to late-glacial events. Quaternary Research, 10: 30-41. DOI:10.1016/0033-5894(78)90011-X
Porter, S. C, Pierce, K. L. and Hamilton, T. D., 1983. Late Wisconsin mountain glaciation in the western United States, p. 71-111. In H. E. Wright Jr., éd., Late-Quaternary Environments of the United States, vol. 1. University of Minnesota Press, Minneapolis.
Prest, V. K., Grant, D. R. and Rampton, V. N., 1968. Glacial map of Canada. Geological Survey of Canada, Map 1253A.
Radok, U., Barry, R. G., Jenssen, R. A., Kiladis, G. N. and Mclnnes, B., 1982. Climatic and physical characteristics of the Greenland Ice Sheet, parts I and II. Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, 193 p.
| Auteur : | Barry L. Robert |
|---|---|
| Titre : | Modeling the Cordilleran Ice Sheet |
| Revue : | Géographie physique et Quaternaire, Volume 45, numéro 3, 1991, p. 287-299 |
| URI : | http://id.erudit.org/iderudit/032876ar |
| DOI : | 10.7202/032876ar |
Tous droits réservés © Les Presses de l'Université de Montréal, 1991