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RELICT MIDDLE PLEISTOCENE PERMAFROST IN CENTRAL YUKON TERRITORY

FROESE, DUANE G  University of Alberta.
Westgate, John A  University of Toronto.
Preece, Shari J  University of Toronto.
Mayer, Bernhard  University of Calgary.
Zazula, Grant D  Simon Fraser University.
Reyes, Alberto V  University of Alberta.

Models of projected surface air temperatures changes suggest significant increases in active layer thickness and consequent loss of permafrost from large areas of the discontinuous permafrost zone (Stendel and Christensen, 2002). These projections have some support from paleo-studies in the Fairbanks area of central Alaska, which suggest that permafrost completely melted out during the last interglaciation (Pw, 1975; Pw et al., 1997). Indirect evidence from speleothem growth in northern Yukon, however, suggests that permafrost may have persisted throughout much of the Brunhes Chron (last 780 ka)(Lauriol et al., 1997). Here we report the occurrence of relict permafrost dating to the middle Pleistocene in central Yukon Territory. Our discovery of ancient permafrost, which appears to be widespread in central Yukon, implies that large areas of relict permafrost should exist throughout northwestern North America.

At the Dominion Creek site in the southern Klondike goldfields, at least three generations of ice wedges are recognized in association with two independently-dated tephra beds (Fig.1). The first generation wedges, locally exceeding 10 m in depth, cross-cut underlying in-situ forest paleosols with rooted Picea stumps. Sheep Creek tephra (190 20 ka) is present in the paleo-active layer above the first generation ice wedges, but is cross-cut and folded along the margins by the second generation ice wedges. At least one first generation ice wedge includes a bed of Sheep Creek tephra parallel to the ice foliation, indicating ice wedge cracking was contemporaneous with tephra deposition. Dominion Creek tephra (170 20 ka) is present within a weakly developed graminoid-rich paleosol above the second generation ice wedges, but is cross-cut and folded by a third generation of ice wedges. The association of the ice wedges with the tephra beds dates the relict ice to the early part of marine isotope stage (MIS) 6, while the ice-rich silt associated with the underlying forest bed dates to MIS 7. The isotopic composition of the ice wedges supports this interpretation. Ice sampled from the wedges is strongly depleted in δD with values ranging from -230 to -233 , while ice from the underlying forest beds has δD values of -175 to -189 , comparable to Holocene values in the region (Kotler and Burn, 2000). These findings indicate that ground ice has persisted, at least locally, in interior Yukon Territory from MIS 7 through the present. This exceptional record of permafrost 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.

REFERENCES
Kotler, E., and Burn, C. R. (2000). Cryostratigraphy of the Klondike "muck" deposits, west-central Yukon Territory. Canadian Journal of Earth Sciences 37, 849-861.



Lauriol, B., Ford, D. C., Cinq Mars, J., and Morris, W. A. (1997). The chronology of speleothem deposition in northern Yukon and its relationships to permafrost. Canadian Journal of Earth Sciences 34, 902-911.



Pw, T. L. (1975). Quaternary Geology of Alaska, USGS Professional Paper 835. 143 p.



Pw, T. L., Berger, G. W., Westgate, J. A., Brown, P. M., and Leavitt, S. W. (1997). Eva Interglaciation Forest Bed, unglaciated east-central Alaska: global warming 125,000 years ago. Geological Society of America, Special Paper 319.



Stendel, M., and Christensen, J. H. (2002). Impact of global warming on permafrost conditions in a coupled GCM. Geophysical Research Letters 29, 10.1029/2001GL01345.



Figure 1. A. Syngenetic ice wedge (IW) folding Sheep Creek tephra (SCt) along ice margin. Vertical arrows indicate locations of SCt. B. SCt recovered from frost crack within ice wedge to a depth of 4 m. C. At least three thaw unconformities (ice contacts) are recognized within the cryostratigraphy (1) First generation wedges up to 5 m wide, (2) Second generation wedges up to 1.5 m wide, (3) Third generation wedges less than 30 cm wide. SCt is present within the paleo-active layer above the first generation ice wedges. D Large ice wedge cross-cutting, and compression folding the MIS 7 forest bed (FB).


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