CGRG Bibliography of Canadian Geomorphology
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Author : Froese, D.G.; Westgate, J.A.; Preece, S.J.; Sandhu, A.S.; Schweger, C.E.; and White, J.M.
Date : 2003.
Title : Developing a chronology for the record of late Cenozoic climate change in NW North America: Integrated paleomagnetic, biostratigraphic, and tephrostratigraphic approaches.
Publication : New Frontiers in the Fourth Dimension: Generation, Calibration and Application of Geological Timescales. Mt. Tremblant, Québec, Canada March 15-18, 2003.
Issue :
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Abstract
The unglaciated region of Yukon and Alaska, collectively eastern Beringia, contains a record of the evolution of Arctic climate over the late Cenozoic, largely preserved in fluvial terraces and their overlying loess mantles. These records, although discontinuous, can be correlated between sites through their paleomagnetism and interbedded tephra, and linked to global changes through independent radiometric ages. The most complete records, to date, come from the extens ive placer mining exposures of the Klondike goldfields, Yukon Territory and the Fairbanks District, Alaska. Integratred paleomagnetic, tephrochronologic, and paleoecology provide a robust terrestrial record on the conditions preceding the onset of extensive northern hemisphere glaciation in the Pliocene and changes in Arctic climate through the Pleistocene. Early Pliocene deposits lack evidence of permafrost and pollen from these units is characterized by abundant Pinus (up to 40%) and Picea (up to 30%) and is part of a pre-glacial boreal forest that extended to 80º N. Evidence for regional mid-Pliocene (ca. 3.2-3 Ma) cooling is present in both east-central Alaska and west-central Yukon based on fossil ice-wedges associated with tephra beds. This provides the first evidence of North American permafrost. Pollen from mid-Pliocene units shows a decrease in Pinus abundance (~10%) and likely represents the latest age of a taxonomically richer boreal forest that existed in northern North America prior to the onset of glaciation. The first evidence of glaciation in the central Yukon is recorded by the Klondike gravel, an extensive glaciofluvial outwash derived from the first Cordilleran ice sheet advance in the region, prior to 2.6 Ma. The early Pleistocene record is generally scarce in central Yukon and Alaska, but scattered remnants suggest that interglacial forests by 2.3 Ma were essentially similar to known middle and late Pleistocene boreal forest. No major changes in the nature of glaciations or biostratigraphy are known around the official Plio-Pleistocene boundary at ca. 1.8 Ma. Studies of Pleistocene records allows characterization of at least 10 interglacials over the last 2.4 Ma in addition to the Holocene. Establishing regional tephras associated with interglacials allows timeslices (at decadal resolution) of regional vegetation and latitudinal climate gradients to be tested. And further, since many of these sites are associated with vertebrate records interchanges of fauna between the old and new worlds can be calibrated to these independent ages. In sum, integrated studies of the late Cenozoic of eastern Beringia promises one of the most detailed and chronologically-controlled records of climate/environmental change in North America.
Bibliography of Canadian Geomorphology