The Middle Wisconsinan History of the Laurentide Ice Sheet

Evidence for Middle Wisconsinan ice limits and climates comes from sites scattered around the periphery of the Laurentide Ice domain and from the Hudson Bay Lowlands. Interpretations are based on dated wood, peat, shell and sediment; biological climate indicators (chiefly cool-climate indicators); and stratigraphie sequences of both glacial and nonglacial units. The best evidence comes from the prairie provinces and the Great Lakes areas, both of which indicate substantial ice retreat from earlier southern glacial limits, and cool, boreal-tundra climates. The western arctic may have experienced an early warm period but both the western arctic — northwestern plains and eastern maritime areas may later have become ice accumulation areas. Three maps portray various possible ice limits. The first shows substantial ice cover in the arctic, but reduced ice cover in the prairies and Great Lakes, and expanded maritime ice caps (rather than Laurentide Ice) in the southeast and on Baffin Island. This ice mass distribution may reflect Middle Wisconsinan shifts in air masses and ocean currents. Ice volumes generated by this model are in accord with the marine oxygen isotope record and perceived global sea level changes. A modification to this model, which resolves some of the controversy in the Hudson Bay Lowlands, shows a calving bay penetrating into the heart of the ice sheet, induced by dynamic instability of the marine-based ice mass in Hudson Bay during relatively high glacial isostatic and eustatic seas. A third reconstruction portrays extensive climatically induced déglaciation and retains Laurentide ice only in parts of the northwest and Labrador-Ungava, with local ice in the Appalachian-Atlantic region. This model is based on alternative genetic interpretations of lithologic units and reassessment of age assignments.

RÉSUMÉ

Évolution de la calotte glaciaire laurentidienne au Wisconsinien moyen

Les indices sur les limites glaciaires et les climats du Wisconsinien moyen proviennent d'endroits situés en bordure de la calotte glaciaire laurentidienne et des basses terres de la baie d'Hudson. Les interprétations se fondent sur la datation du bois, de la tourbe, des coquilles et des sédiments, sur les indicateurs biologiques de climat et sur les séquences stratigraphiques des unités glaciaires et non glaciaires. Les meilleurs indices proviennent des Prairies et des Grands Lacs; ils révèlent un recul marqué de la glace à partir des limites méridionales antérieures et des climats frais de type boréal ou de toundra. L'ouest de l'Arctique a peut-être initialement connu une période chaude, mais cette région, les plaines du nord-ouest et les Maritimes seraient par la suite devenues des zones d'accumulation de glace. Trois cartes montrent les diverses limites glaciaires proposées. Dans la première, la couverture glaciaire est très étendue dans l'Arctique et moins étendue dans les Prairies et les Grands Lacs; on y voit de grandes calottes glaciaires maritimes dans le sud-est et dans l'île de Baffin. Cette répartition de la glace pourrait résulter du déplacement des masses d'air et des courants océaniques au cours du Wisconsinien moyen. Un autre modèle, légèrement différent, résout certains problèmes associés aux basses terres de la baie d'Hudson; la baie de vêlage, qui s'avance jusqu'au centre de l'inlandsis, est le produit de l'instabilité dynamique de la masse glaciaire marine dans la baie d'Hudson. Un troisième modèle fait voir une déglaciation étendue provoquée par des changements climatiques; la glace laurentidienne persiste uniquement à certains endroits dans le nord-ouest, dans la région du Labrador et de l'Ungava et dans la zone appalachienne atlantique.

ZUSAMMENFASSUNG

Geschichte der laurentischen Eisdecke im mittleren glazialen Wisconsin

Anhaltspunkte fur Eisgrenzen und Klima im mittleren glazialen Wisconsin ergeben sich aus Plätzen, die um die Peripherie des von der laurentischen Eisdecke bedeckten Gebiets verstreut sind, sowie aus dem Tiefland der Hudson Bay. Die lnterpretationen grunden sich auf datiertes HoIz, Torf, Muscheln und Sedimente, biologische Klimalndikatoren und die stratigraphischen Abfolgen sowohl glazialer als auch nichtglazialer Einheiten. Die besten Zeugnisse stammen aus den PrärieProvinzen und dem Gebiet der Gro(3en Seen. Aus beiden Regionen zeigt sich ein bedeutender EisRückgang von den früheren sud-lichen Eisgrenzen und kühles nördliches Tundra-Klima. In der westlichen Arktik mag wohl eine frühe warme Période aufgetreten sein, aber sowohl die westarktischen Ebenen als auch die ôstlichen Meeres-Gebiete sind wohl später Gebiete starker Vereisung geworden. Drei Karten illustrieren verschiedene mögliche Eisdecken. Die erste zeigt eine beträchtliche Eisdecke in der Arktis, jedoch in den Prärien und dem Groften Seen-Gebiet eine geringere Eisdecke und im Südosten und auf der Baffin-lnsel ausgedehnte Meeres-Eiskappen. Diëse Verteilung der Eismasse mag die Folge von Verschiebungen von Luftbewegungen und ozeanischen Strömungen sein. Eine Abweichung von diesem Modell, welche einige der Kontroversen im Hudson-Bay Tiefland klärt, zeigt eine Kalbungs-Bucht, welche in den Kern der Eisdecke vordringt. Diëse Bucht wurde durch die dynamische Instabilität der marinen Eismasse in der Hudson-Bay während relativ hoher glazialisostatischer und eustatischer Gezeiten hervorgerufen. Ein drittes Modell beschreibt ausgedehnte klimatisch bedingte Enteisung und zeigt laurentisches Eis nur in Teilen des Nordwestens und von Labrador-Ungava mit örtlich begrenztem Eis im Gebiet der Atlantik-Appalachen.

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Autores :	Lynda A. Dredge y L. Harvey Thorleifson
Título :	The Middle Wisconsinan History of the Laurentide Ice Sheet
Revista :	Géographie physique et Quaternaire, Volumen 41, número 2, 1987, p. 215-235
URI :	http://id.erudit.org/iderudit/032680ar
DOI :	10.7202/032680ar