The Eva Interglaciation Forest Bed represents a frozen, buried, ancient boreal forest in the Yukon-Tanana Upland of east-central Alaska. It consists of excellently preserved peat lenses, sticks, roots, and logs as well as rooted and unrooted stumps of trees, mainly spruce and birch. Consistent with the modern boreal forest, the largest and most common tree in the fossil forest is spruce, mainly white spruce (Picea glauca). Remains of birch trees are common, mostly Betula papyrifera. The forest remains were buried by loess that became frozen and so are well preserved. None of the wood is mineralized. Many of the fragments are black from buring, suggesting forest fires were widespread in the Yukon-Tanana Upland during the interglaciation. Also, evidence is presented for the first time of the existence of spruce bark beetles (Scolytidae) during the last interglaciation in Alaska. Efforts to determine the age of the Eva Forest Bed in this study have covered the past 50 years. Methods applied have varied from the use of stratigraphic interpretation of sedimentological events and preserved evidence of climatic changes to the use of modern geochronometry. Several methods of dating have come to fruition in the 1990s. New radiocarbon dating by liquid scintillation (LS) detectors indicates the forest wood to be older than 70,000 years. Perhaps the greatest breakthrough is the development of the isothermal-plateau fission-track method of dating geologically young volcanic glass shards. The Old Crow tephra closely underlying the Eva Forest Bed has been dated at 140 ± 10 ka and strongly supports the original interpretation of the forest bed as of last interglaciation. In the early 1990s, highly improved thermoluminescence (TL) sediment dating techniques were utilized for dating loess above and below the forest bed indicating the age of the Eva Forest Bed is probably 125,000 years with a duration of the Eva Interglaciation of probably only a few thousand years (Sangamon, Oxygen Isotope Substage 5e). Stratigraphically, the Eva Forest Bed lies at the prominent unconformity between the underlying massive, green Gold Hill Loess (pre-Sangamon) and the overlying blackish, ice-wedge-rich retransported loess of the Goldstream Formation (Wisconsin). Studies of the frozen Gold Hill Loess indicate that the warm interglacial interval was characterized by deep and rapid thawing of permafrost and erosion of loess accompanied by gullying and block slumping of frozen loess. After extensive slumping, the topography became smooth and the forest became extensive. Tilting of enclosed tephra layers outline the slump blocks. Evidence for deep permafrost thawing is supported by the absence today of ice wedges, buried pingos, and mammal carcasses in the presently refrozen loess of pre-Wisconsin age. Deep thawing is also indicated by reduction of iron on loess grains from ferric to ferrous turning the traditional tan color of loess to greenish in the buried Gold Hill Loess. It is the unique sequence of refreezing in Wisconsin time that has preserved the remarkable evidence for deep thawing in earlier Sangamontime - the green color. The forest bed formed after much of the thawing, erosion, and slumping activity had ceased, and it overlies the angular unconformity. More than halfa-dozen distinct tephra layers have been identified, characterized, and correlated in the upper part of the Gold Hill Loess, aiding in the reconstruction of the sequence of events leading to the erosion, thawing, and emplacement of the Eva Forest Bed. Dendrochronology studies of trees and 13C/12C isotopic ratios of wood from the Eva Forest Bed, and comparisons with wood from the modern boreal forest, strongly suggest environmental conditions at least similar to those of today. Some plant remains and ground beetle taxa of Eva Forest time in Canada represent species that extended farther north than they do today. Also, buried spruce macrofossils suggest that the boreal forest may have extended north of the Brooks Range in Alaska. These botanical and physical data indicate an environmentt warmer than the present interglaciation with the mean annual air temperature warmer than 0 °C, perhaps +1 or +2 °C or warmer to permit the ice to melt and permafrost to thaw from the surface downward. Supporting this concept are astronomical inferences that during the last interglacial (Oxygen Isotope Substage 5e) the July insolation anomaly at 65°N. latitude reached values of almost 50% higher than 10,000 years ago, the beginning of the Holocene Interglaciation.
|Original language||English (US)|
|Number of pages||54|
|Journal||Special Paper of the Geological Society of America|
|State||Published - 1997|
ASJC Scopus subject areas