ABSTRACT: Appearance of early permafrost in the Pliocene of Yukon and Alaska.

CGRG Bibliography of Canadian Geomorphology
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Abstract

The appearance of permafrost in northern stratigraphic records is intimately related to changes occurring in the Pliocene leading toward the onset of extensive northern hemisphere glaciation. In central Yukon, ice-wedge casts are present at several sites in the Upper White Channel gravel of the Klondike goldfields. The addition of paleomagnetic chronology pushes the age of these features to at least 3.2 Ma in the mid-Gauss Chron. Recent investigation of the Lost Chicken mine in east-central Alaska, shows the presence of frost-cracks and a large ice-wedge cast developed in fluvial gravel underlying the Lost Chicken tephra. The weighted mean age of the Lost Chicken tephra, based on 3 separate age determinations, is 2.92 ą 0.15 Ma. In northern Yukon Territory, small ice-wedge casts are present in Unit 1 at Ch'ijee's Bluff near the village of Old Crow. While these periglacial structures in the Old Crow Basin are not directly associated with independently-dated tephra beds, pollen and plant macrofossils suggest they date to the early Pliocene and may be considerably older than the central Yukon and Alaska records. The early and mid-Pliocene was an interval of warmer than modern climate while these periglacial structures suggest temperatures (at least winter temperatures) were as cold or colder than modern at this time. The modern occurrence and southerly extension of permafrost in this region largely reflects the pronounced continentality associated with the rainshadow of the St. Elias and Alaska Range of southern Yukon and Alaska. Similarly, the appearance of permafrost in these stratigraphic records suggests that this continentality was established at least by the mid-Pliocene and perhaps earlier.

Bibliography of Canadian Geomorphology