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Late Pliocene Dawson Cut Forest Bed and new tephrochronological findings in the Gold Hill Loess, east-central Alaska

T.L. Péwé^1,*, J.A. Westgate^2,, S.J. Preece², P.M. Brown³ and S.W. Leavitt⁴

¹ Department of Geology, Arizona State University, Tempe, Arizona 85287, USA
² Department of Geology, University of Toronto, Toronto, Ontario M5S 3B1, Canada
³ Rocky Mountain Tree-Ring Research, Inc., 2901 Moore Lane, Fort Collins, Colorado 80526, USA
⁴ Laboratory of Tree-Ring Research, University of Arizona, Tucson, Arizona 85721, USA

Correspondence: E-mail: westgate{at}geology.utoronto.ca.

Thick loess deposits in the Fairbanks region of interior Alaska are a rich source of information on past climates and environments during the late Cenozoic, and the numerous distal, silicic tephra beds preserved within them offer the potential for good chronological control. The Dawson Cut Forest Bed lies in the lower part of this loess cover. Plant macrofossils consist of Picea glauca, Picea mariana, Betula papyrifera, and Populus balsamifera but no Pinus. These fossils, together with the abundance and size of spruce remains, pollen, tree-ring characteristics, and ¹³C values of spruce wood, demonstrate that the boreal forest represented by the Dawson Cut Forest Bed was similar to the modern boreal forest of central Alaska. Warming conditions during the early part of the Dawson Cut Interglaciation initiated thawing of permafrost and melting of ground ice, as evidenced in the presence of ice-wedge casts and major erosion of the lower Gold Hill Loess. Tephrochronological, magnetostratigraphic, and glass fission-track dating studies in the Fairbanks area and at the Palisades site on the Yukon River in central Alaska suggest an age for the Dawson Cut Forest Bed of ca. 2 Ma. Hence, the northern boreal forest of northwestern North America, as we know it today, has a long history that probably extends back to at least 2 Ma.

Key Words: buried forest beds • plant macro-fossils • pollen analysis • tephrochronology • glass fission-track dating • paleoenvironmental reconstruction • tree-rings • carbon isotopes • late Pliocene