CGRG Bibliography of Canadian Geomorphology
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Author : Froese, D.G.; Westgate, J.A.; Preece, S.J.; Mayer, B.M.; Zazula, G.D.; and Reyes1, A.V.
Date : 2004
Title : Relict middle Pleistocene permafrost in Yukon Territory.
Publication : 49th Annual Meeting of the Geological Association and the Mineralogical Association of Canada. May 12-14, 2004. Brock University, St. Catharines, Ontario.
Issue :
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Abstract
How long permafrost has persisted in Arctic regions is poorly known. This reflects the few truly old ice complex exposures known, and more likely, the difficulty in establishing reliable chronologies beyond the limit of the radiocarbon method ca. 40,000 yrs). However, understanding the future of permafrost in modern Arctic regions can be informed through documenting the responses of permafrost regions to the warming of past interglaciations. In northern Siberia, published estimates for the age of relict permafrost vary from 3 million years, based largely on the seeming survival of ancient bacteria in the ice, to as little as 200,000 years from U/Th dating of permafrost peats. The record in North America is even more poorly known and direct records of permafrost do not extend beyond the limit of the radiocarbon dating method (ca. 40,000 yrs). Here we report the occurrence of relict permafrost dating to the middle Pleistocene from the Discontinuous Permafrost Zone of central Yukon Territory. In the southern Klondike goldfields, Yukon Territory, large ice wedges, locally exceeding 10 m in depth, cross-cut underlying in-situ forest beds. The isotopic composition of the ice wedges is depleted in dD with values ranging from -230 to -233 ‰, while the underlying forest beds have dD values of -175 to -189 ‰, comparable to Holocene values in the region. Two tephra beds occur in association with the ice wedges. Sheep Creek tephra (190 ± 20 ka) is cross-cut by the ice wedges, but also occurs within the paleo-active layer above, and veins of the tephra occur along foliation cracks within an ice wedge indicating ice wedge formation was contemporaneous with tephra deposition. Dominion Creek tephra occurs on the surface of a paleosol 1 m above Sheep Creek tephra, and has an age of 170 ± 20 ka. Permafrost at the site is relict from the early part of marine isotope stage 6, while the underlying forest bed and associated aggradational ice is likely associated with marine isotope stage 7. These findings indicate that ground ice has persisted, at least in preferred locations in interior Yukon Territory through the last interglaciation. This finding provides the opportunity for paleoenvironmental reconstruction using permafrost pore-ice and permafrost-preserved fossils through at least the last two glacial cycles, and presumably much longer in large areas of northwestern North America.
Bibliography of Canadian Geomorphology