VEGETATION AND PALEOCLIMATE OF THE LAST INTERGLACIAL PERIOD, CENTRAL ALASKA

INTRODUCTION

The last interglacial period is of considerable interest to both paleoclimatologists and climate modellers because it is thought to be the last time global climates were significantly warmer than present, and is a possible analog for future warm climates. During the peak of the last interglacial period around 125 ka, sea level was about 6 m higher than present, based on dating of emergent coral reefs on tectonically stable coastlines distant from plate boundaries (see examples given in Muhs et al., 1994). These data indicate that global ice volumes were significantly lower than present, by an amount equivalent to the present volume of the Greenland or West Antarctic ice sheets. Such inferences about lower ice volume are supported by oxygen isotope records in foraminifera found in deep-sea cores, which show strongly negative values during stage 5e, generally interpreted to be the peak of the last interglacial (Martinson et al., 1987). Data from the Vostok ice core of Antarctica indicate that atmospheric carbon dioxide concentrations were higher than the highest preanthropogenic Holocene levels (Lorius et al., 1985; 1990; Barnola et al., 1991). Pollen data from many northern hemisphere continental records including North America (King and Saunders, 1986; Zhu and Baker, 1995), Europe (Woillard, 1978; Zagwijn, 1996), and Asia (Lozhkin and Anderson, 1995) show significant warming and migration of biomes (relative to present) during the last interglacial. Of particular interest to paleoclimate modeling is the response of high-latitude regions to global warming during the last interglacial. Such regions are particularly sensitive to climate warming, may respond earliest to external climate forcing, and are important in feedback mechanisms in global climate systems (LIGA Members, 1991).

Figure 1

Recent atmospheric general circulation model (AGCM) and vegetation reconstructions of the northern hemisphere for the last interglacial (Kutzbach et al., 1991; Harrison et al., 1991; de Noblet et al., 1996; Montoya et al., 1998) show that summer temperatures in coastal Alaska could have been as much as 2°C higher than present and as much as 3-4°C warmer than present in interior Alaska. Harrison et al. (1995) presented a vegetation simulation for the last interglacial that showed an expanded boreal forest, relative to the present (see Fig. 1 for present distribution of boreal forest), in Alaska and northwestern Canada, but this same experiment also resulted in the emergence of a considerable area of cool-season steppe in interior Alaska and Yukon.

PRESENT CLIMATE AND VEGETATION OF INTERIOR ALASKA

Figure 2

The present climate of central Alaska is strongly continental, and temperature regimes in interior Alaska reflect this character. Winters are long and cold, and summers are warm but relatively short. Summer (June, July, and August) is the period when the majority of precipitation occurs. Because of the presence of the Alaska Range to the south, air masses from the north Pacific Ocean lose much of their moisture before penetrating very far inland. As a consequence, much of interior Alaska has annual precipitation totals of only about 250-400 mm; the 1961-1990 mean at Fairbanks is 276 mm (Fig. 2). The presence of the Brooks Range to the north of interior Alaska has little effect on the precipitation regime because Arctic air masses from the north are relatively dry.

The dominant vegetation at low elevations in interior Alaska is northern boreal forest (Fig. 1). The factors responsible for the boreal forest boundaries in North America are complex, and include length of growing season, growing season temperatures, desiccating winds and low humidity, nutrient-poor, unstable soils, amount of snow cover (as it affects survival of seedlings), and effects of fire. Spruce-dominated boreal forest is rare in areas that have mean July air temperatures lower than about 12°C (Fig. 1). In Alaska, the northern climatic boundary to spruce forest is determined altitudinally by the presence of the Brooks Range.

RECORD OF THE LAST INTERGLACIAL IN CENTRAL ALASKA

Figure 3

In central Alaska, stratigraphic and geochronologic studies by Péwé et al. (1997) and Berger et al. (1996) indicate that Eva Forest Bed, a prominent feature in the loess record of this region, probably dates from the last interglacial period. The bed contains abundant Picea and Betula (probably B. papyrifera) macrofossils, which indicates a boreal forest much like the present. This interpretation differs from those of earlier studies by Guthrie (1968) and Matthews (1970). These workers suggested, on the basis of faunal data, that tundra was the dominant vegetation at the time of formation of this unit. In this study we present new stratigraphic, geochronologic, pollen, and soil data from the type locality of the Eva Forest Bed, found at Eva Creek, west of Fairbanks (Fig. 3). Although Péwé et al. (1997) presented a comprehensive study of this important deposit, they did not conduct pollen or soils studies. We also present a new compilation of last-interglacial vegetation records in Alaska and Yukon, updating the summaries of Hamilton and Brigham-Grette (1991) and LIGA Members (1991), and discuss how these data compare to AGCM results of Harrison et al. (1995).

Interpretation based on pollen data

RECORD OF THE LAST INTERGLACIAL IN CENTRAL ALASKA

Figure 4

Figure 5

The very high percentages of Picea pollen and Polypodiaceae spores in most of the forest bed samples may in part be a result of differential preservation. The thick walls of Picea pollen and Polypodiaceae spores make these types less vulnerable to oxidation and fungal attack than the thinner-walled pollen types present such as Betula, Alnus, and Salix. Many of these thinner-walled pollen grains showed evidence of damage by decomposition. Nevertheless, the samples contain sufficient amounts of the thinner-walled taxa to permit reconstruction of the main elements of the Eva Forest Bed flora. The pollen flora is strikingly similar to that of the present-day northern boreal forest of interior Alaska, and is consistent with the macrofossil specimens of Picea and Betula collected in the present study, as well as macrofossil specimens of these same taxa reported by Péwé et al. (1997).

Figure 6

Two major shifts in biota and inferred climate are represented by pollen from an organic-rich paleosol and overlying loess that are thought to be stratigraphically above the Eva Forest Bed (Fig. 4). Three samples within the paleosol yielded sufficient pollen grains for counting, as did four samples in the overlying grey loess (or reworked loess). There is a striking difference in the composition of pollen in the paleosol compared to the overlying loess (Fig. 6). In the paleosol, there is a sedge-dominated tundra assemblage with willows. Willow is also present as a macrofossil in the lower part of the paleosol, and gave an AMS radiocarbon age of >47.8 ka (Fig. 4). This vegetation assemblage represents a substantially cooler climate than existed during the interval represented by the Eva Forest Bed. It implies elimination of forest from the Fairbanks area some time after formation of the Eva Forest Bed.

Above the paleosol, the four samples in loess and reworked loess show progressive increases in pollen percentages of Picea, Betula, and Alnus. Such a trend can be interpreted as a reinvasion of boreal forest into the Fairbanks area following the cold interval represented by the sedge-grass-herb tundra samples in the paleosol. However, the abundance of Picea pollen is substantially lower than the samples found in the Eva Forest Bed. This suggests that the vegetation in the Eva Creek area during this interval of reinvading trees may have been open forest-tundra, or perhaps upland tundra with some lowland gallery forests. A corollary intepretation is that the climate was not as warm as the time when the Eva Forest Bed was deposited.

Interpretation based on soils data

Buried soils can be identified in section by the presence of key soil horizons, by the relative abundance of organic matter, and by chemical properties that reflect dominant soil-forming processes. Soils usually have high organic matter values in their upper horizons and abundances diminish rapidly with depth, and modern boreal forest soils in the Fairbanks area show this characteristic depth function very clearly.

Soil morphological and chemical properties are strongly linked to the soil-forming factors of climate and vegetation, and therefore these properties in paleosols can be useful in inferring past climate and vegetation. Under forest vegetation in many environments, soils develop leached E horizons below the A or O horizon, and over time usually develop subsurface horizons charactertized by distinctive structure or color (Bw horizons), clay accumulation (Bt horizons), or accumulation of Fe-oxyhydroxides (Bs horizons). In soils developing under environments characterized by leaching, such as boreal forest, primary minerals, particularly aluminosilicates, experience chemical weathering and lose SiO₂ in surface (O and A) horizons. The development of Bw or Bs horizons is marked by the accumulation of Fe-oxyhydroxides, resulting in an increase in Fe₂O₃ content in subsurface horizons.

Figure 7

Soil morphological and chemical data support the vegetation and climate inferences for the last interglacial complex based on pollen data. The soil under the Eva Forest Bed, although clearly disturbed by cryoturbation, thawing, and slumping, has easily distinguished O and Bw horizons that resemble those found under boreal forest in Alaska today (Fig. 7). It may also have an E horizon, a leached zone that usually does not form under tundra vegetation, but is common under boreal forest in Alaska. Organic matter shows highest values in the O horizon, and diminishes rapidly with depth. In addition, SiO₂/TiO₂ values are relatively low in the O horizons and Fe₂O₃/TiO₂ values are very high in all horizons, but particularly the Bw horizon. Overall, the soil under the Eva Forest Bed shows more evidence of chemical weathering than is found in modern Fairbanks area Inceptisols, and is similar to the degree of chemical weathering in Kenai area Spodosols. These observations indicate the Eva Forest Bed paleosol may have developed under higher-than-present precipitation during the last interglacial, which is consistent with the higher-than-modern abundance of monolete spores observed in the Eva Forest Bed itself.

Correlation of last-interglacial records at Eva Creek

Figure 8

The pollen and soil record at Eva Creek reflect the changing climatic conditions of the last interglacial complex. Based on the previously reported combined fission-track and TL age estimates discussed above and the stratigraphic evidence presented here, we correlate the spruce-bearing portions of the Eva Forest Bed to the peak of the last interglacial period around 126 ka, when summer insolation near Fairbanks was at its highest values of the past 200 kyr (Fig. 8). Pollen in what is inferred to be a younger loess and organic-rich paleosol containing the Dome tephra (Fig. 4) indicates that the peak of the last interglacial period was followed by a cold, dry period where tundra vegetation dominated. This cold period could correlate with either of the two summer insolation lows at ~114 ka or ~93 ka (Fig. 8). Tundra vegetation under a cold, dry climate was then replaced by warmer and somewhat moister conditions (relative to the previous climate with its tundra vegetation). The pollen assemblage suggests a less-extensive boreal forest (relative to present) or forest-tundra, but in any case without the abundance of spruce or ferns recorded at the peak of the last interglacial. This later period of boreal forest growth could correlate to either of the insolation maxima at ~103 ka or ~82 ka (Fig. 8).

PALEOCLIMATE OF THE LAST INTERGLACIAL PERIOD IN ALASKA AND YUKON

Other last interglacial boreal forest evidence in Alaska and Yukon

Enough pollen and macrofossil localities have been reported for Alaska and Yukon that it is now possible to sketch a regional picture of the probable extent of boreal forest during the last interglacial period. In compiling a mapped summary of these localities, we consider "probable" last interglacial localities those where the hypothesized last-interglacial deposits have infinite radiocarbon ages and are in close association with either the Old Crow tephra or aminostratigraphically correlated Pelukian (=last interglacial) shoreline deposits. We consider "possible" last interglacial deposits to be those with infinite radiocarbon ages and in a stratigraphic position that permits a last-interglacial correlation (i.e., overlain by dated deposits of last-glacial age).

Figure 9

Results of this compilation show a last-interglacial boreal forest that was more extensive than the modern one (Fig. 9). Well-studied, spruce-pollen-bearing or spruce-macrofossil-bearing localities associated with the Old Crow tephra are numerous in the interior of Alaska and Yukon Territory. The overall picture that emerges for Alaska and Yukon during the peak warmth of the last interglacial is a region with warmer-than-present summers, an absence of permafrost in the interior, and probably greater precipitation in the interior. The most surprising implication of our study, based primarily on spore data and secondarily on soil data, is the notion of higher precipitation in the interior of Alaska during the last interglacial. This unexpected finding has important implications for climate models of the last interglacial.

Four independent AGCM reconstructions indicate that summer warming of about 3-4°C could have occurred in interior Alaska during the last interglacial period, driven by the higher summer insolation at ~126 ka (Kutzbach et al., 1991; Harrison et al., 1991, 1995; de Noblet et al., 1996; Montoya et al., 1998). The presence of boreal forest in interior Alaska and Yukon during the last interglacial does not provide a definitive test of this hypothesis, although its presence permits such a reconstruction. However, the distribution of last-interglacial boreal forest beyond its present limits on the Seward Peninsula indicates summer warming of at least 1-2°C. The pollen evidence for boreal forest on St. Lawrence Island, if correctly assigned to the last interglaciation, requires a warming of about 3-5°C during summer (Fig. 1). The AGCM results of Harrison et al. (1995) also indicate that this summer warming indirectly resulted in warmer winters in Alaska, even though winter insolation would have been lower at this time. This unexpected result derives from a simulation of less extensive Arctic Ocean sea ice, in turn derived from delayed sea ice formation in the fall, and earlier sea ice melting in the spring. Montoya et al. (1998) also simulated a decrease in Arctic sea ice during the last interglacial, although not as much as that produced by the model of Harrison et al. (1995). Faunal data from Pelukian shorelines studied by Brigham-Grette and Hopkins (1995) are in good agreement with a simulation of reduced sea ice, as discussed above.

A major difference between Harrison et al.’s (1995) AGCM simulation with its linked biome simulation and our compilation of last-interglacial localities is the extent of boreal forest in the interior of Alaska and Yukon. The AGCM results, despite the diminished sea ice extent, do not indicate increased precipitation in Alaska during the last interglacial. In fact, the model indicates a net moisture deficit, and the linked biome model produces a cool steppe in much of interior Alaska and Yukon (Fig. 10). The compilation of boreal forest localities presented here disagrees with the simulation of a cool steppe, and in particular the abundance of Polypodiaceae spores at Eva Creek indicates that precipitation must have been higher than the present during the last interglacial. It is interesting to note that Lozhkin and Anderson (1995) report an expanded boreal forest in northeast Siberia during the last interglacial, based on paleobotanical data, with both increased summer temperatures and slightly increased precipitation. Harrison et al.’s (1995) model also shows an expanded boreal forest in northeast Siberia during the last interglacial, in agreement with the data of Lozhkin and Anderson (1995), but also produced a moisture decrease compared to present. The comparison of model results for both Alaska and Siberia suggests that Harrison et al.’s (1995) AGCM is better at simulating temperature than precipitation.

CONCLUSIONS

(1) Stratigraphic studies at Eva Creek near Fairbanks indicate a complex last-interglacial record wherein periods of loess deposition alternated with periods of soil formation, when loess deposition rates were lower. The Eva Forest Bed appears to have formed after the deposition of loess containing the Sheep Creek tephra (~190 ka), perhaps partly overlapping in time with deposition of the Old Crow tephra ( ~160 ka to ~120 ka), and before deposition of the Dome tephra. Spruce wood fragments from this bed also have radiocarbon ages beyond the range of the method; we therefore correlate the forest bed with the peak of the last interglacial period, in agreement with Péwé et al. (1997).

(2) Pollen, macrofossil, and paleosol data from the Eva Forest Bed indicate that boreal forest was the dominant vegetation near Fairbanks during the peak of the last interglacial period, also in agreement with Péwé et al. (1997). New fossil spore and paleosol data indicate, however, that precipitation may have been greater than present at this time. The period of boreal forest growth was followed by cold, dry conditions with tundra vegetation, in turn followed by a second period of boreal forest or forest-tundra growth. These dramatic changes in vegetation probably reflect the widely varying summer insolation conditions at high latitudes during the last interglacial complex, from about 126 ka to 82 ka.

(3) A new compilation of last-interglacial localities indicates that boreal forest was extensive over interior Alaska and Yukon Territory. Boreal forest also extended beyond its present range onto the Seward and Baldwin Peninsulas, may have reached St. Lawrence Island, and probably extended to higher elevations now occupied by tundra in the interior. Under such implied warm conditions, it is possible that boreal forest spread to areas north of the Brooks Range as well.

(4) Comparison of last-interglacial pollen and macrofossil data with atmospheric general circulation model (AGCM) results shows both agreement and disagreement. Model results of warmer-than-present summers and less extensive sea ice are in agreement with fossil data. However, numerous and widespread localities with boreal forest records are in conflict with model reconstructions of an extensive cool steppe in interior Alaska and much of Yukon Territory.

REFERENCES
(*references in bold are project products)

Ager, T.A. (1975). Late Quaternary environmental history of the Tanana Valley, Alaska. Institute for Polar Studies Report 54, Ohio State University, 117 pp.

Ager, T.A. (1982). Vegetational history of western Alaska during the Wisconsin glacial interval and the Holocene. In Hopkins, D.M., Matthews, J.V., Jr., Schweger, C.E., and Young, S.B. (eds), Paleoecology of Beringia, pp. 75-93. New York, Academic Press.

Ager, T.A. (1983). Holocene vegetational history of Alaska. In Wright, H.E., Jr. (ed), Late Quaternary Environments of the United States, Volume 2: The Holocene, pp. 128-141. Minneapolis, University of Minnesota Press. Ager, T.A. (1989). History of late Pleistocene and Holocene vegetation in the Copper River Basin, south-central Alaska. U.S. Geological Survey Circular 1026, pp. 89-92.

Ager, T.A., and Brubaker, L. (1985). Quaternary palynology and vegetational history of Alaska. In Bryant, V.M., Jr., and Holloway, R.G. (eds), Pollen Records of Late- Quaternary North American Sediments, pp. 353-383. Dallas, Texas, American Association of Stratigraphic Palynologists Foundation.

Anderson, P.M., Lozhkin, A.V., Eisner, W.R., Kozhevnikova, M.V., Hopkins, D.M., Brubaker, L.B., and Colinvaux, P.A. (1994). Two late Quaternary pollen records from south-central Alaska. Géographie physique et Quaternaire, 48, 131-143.

Barnola, J.M., Pimienta, P., Raynaud, D., and Korotkevich, Y.S. (1991). CO2-climate relationship as deduced from the Vostok ice core: A re-examination based on new measurements and on a re-evaluation of the air dating. Tellus, 43B, 83-90.

Begét, J.E. (1996). Tephrochronology and paleoclimatology of the last interglacial-glacial cycle recorded in Alaskan loess deposits. Quaternary International, 34-36, 121-126.

Begét, J.E., Edwards, M.E., Hopkins, D., Keskinen, M., and Kukla, G. (1991). Old Crow tephra found at the Palisades of the Yukon, Alaska. Quaternary Research, 35, 291-297.

Berger, A., and Loutre, M.F. (1991). Insolation values for the climate of the last 10 million years. Quaternary Science Reviews, 10, 297-317.

Berger, G.W., Péwé, T.L., Westgate, J.A., and Preece, S.J. (1996). Age of Sheep Creek tephra (Pleistocene) in central Alaska from thermoluminescence dating of bracketing loess. Quaternary Research, 45, 263-270.

Berger, G.W., Pillans, B.J., and Palmer, A.S. (1994). Test of thermoluminescence dating of loess from New Zealand and Alaska. Quaternary Science Reviews, 13, 309-333.

Brigham-Grette, J., and Hopkins, D.M. (1995). Emergent marine record and paleoclimate of the last interglaciation along the northwest Alaskan coast. Quaternary Research, 43, 159-173.

Carter, L.D., and Ager, T.A. (1989). Late Pleistocene spruce (Picea) in northern interior basins of Alaska and the Yukon: Evidence from marine deposits in northern Alaska. U.S. Geological Survey Circular 1026, pp. 11-14.

Colinvaux, P.A. (1964). The environment of the Bering Land Bridge. Ecological Monographs, 34, 297-329.

Colinvaux, P.A. (1967). A long pollen record from St. Lawrence Island, Bering Sea (Alaska). Palaeogeography, Palaeoclimatology, Palaeoecology, 3, 29-48.

de Noblet, N., Braconnot, P., Joussaume, S., and Masson, V. (1996). Sensitivity of simulated Asian and African summer monsoons to orbitally induced variations in insolation 126, 115 and 6 kBP. Climate Dynamics, 12, 589-603.

Edwards, M.E., and McDowell, P.F. (1991). Interglacial deposits at Birch Creek, northeast interior Alaska. Quaternary Research, 35, 41-52.

Elias, S.A., and Short, S.K. (1992). Paleoecology of an interglacial peat deposit, Nuyakuk, southwestern Alaska, U.S.A. Géographie Physique et Quaternaire, 46, 85-96.

Elias, S.A., Short, S.K., and Waythomas, C.F. (1996). Late Quaternary environments, Denali National Park and Preserve, Alaska. Arctic, 49, 292-305.

Fernald, A.T. (1965). Glaciation in the Nabesna River area, upper Tanana River valley, Alaska. U.S. Geological Survey Professional Paper 525-C, pp. C120-C123.

Guthrie, R.D. (1968). Paleoecology of a late Pleistocene small mammal community from interior Alaska. Arctic, 21, 223-244.

Hamilton, T.D., and Brigham-Grette, J. (1991). The last interglaciation in Alaska: Stratigraphy and paleoecology of potential sites. Quaternary International, 10-12, 49-71.

Harrison, S.P., Kutzbach, J.E., and Behling, P. (1991). General circulation models, palaeoclimatic data and last interglacial climates. Quaternary International, 10-12, 231-242.

Harrison, S.P., Kutzbach, J.E., Prentice, I.C., Behling, P.J., and Sykes, M.T. (1995). The response of northern hemisphere extratropical climate and vegetation to orbitally induced changes in insolation during the last interglaciation. Quaternary Research, 43, 174-184.

Heusser, C.J. (1960). Late Pleistocene environments of North Pacific North America. American Geographical Society Special Publication No. 35, 308 pp.

Heusser, C.J. (1983). Holocene vegetation history of the Prince William Sound region, south-central Alaska. Quaternary Research, 19, 337-355.

Jouzel, J., Barkov, N.I., Barnola, J.M., Bender, M., Chappellaz, J., Genthon, C., Kotlyakov, V.M., Lipenkov, V., Lorius, C., Petit, J.R., Raynaud, D., Raisbeck, G., Ritz, C., Sowers, T., Stievenard, M., Yiou, F., and Yiou, P. (1993). Extending the Vostok ice-core record of paleoclimate to the penultimate glacial period. Nature, 364, 407-412.

King, J.E., and Saunders, J.J. (1986). Geochelone in Illinois and the Illinoian-Sangamonian vegetation of the type region. Quaternary Research, 25, 89-99.

Kutzbach, J.E., Gallimore, R.G., and Guetter, P.J. (1991). Sensitivity experiments on the effect of orbitally-caused insolation changes on the interglacial climate of high northern latitudes. Quaternary International, 10-12, 223-229.

LIGA Members. (1991). Report of the 1st discussion group: The last interglacial in high latitudes of the northern hemisphere: terrestrial and marine evidence. Quaternary International, 10-12, 9-28. Lorius, C., Jouzel, J., Ritz, C., Merlivat, L., Barkov, N.I., Korotkevich, Y.S., and Kotlyakov, V.M. (1985). A 150,000-year climatic record from Antarctic ice. Nature, 316, 591-596.

Lorius, C., Jouzel, J., Raynaud, D., Hansen, J., and Le Treut, H. (1990). The ice-core record: climate sensitivity and future greenhouse warming. Nature, 347, 139-145.

Lozhkin, A.V., and Anderson, P.M. (1995). The last interglaciation in northeast Siberia. Quaternary Research, 43, 147-158.

Maeda, Y. (1985). Pollen analysis of the drilling cores in St. Lawrence Island. In: Nakao, K. (ed), Hydrological Regime and Climatic Changes in the Arctic Circle, pp. 34-36. Laboratory of Hydrology, Department of Geophysics, Hokkaido University, Japan.

Mann, D.H. (1986). Wisconsin and Holocene glaciation of southeast Alaska. In Hamilton, T.D., Reed, K.M., and Thorson, R.M. (eds), Glaciation in Alaska-The Geologic Record, pp. 237-265. Alaska Geological Society. Martinson, D.G., Pisias, N.G., Hays, J.D., Imbrie, J., Moore, T.C., Jr., and Shackleton, N.J. (1987). Age dating and the orbital theory of the ice ages: Development of a high-resolution 0 to 300,000-year chronostratigraphy. Quaternary Research, 27, 1-29.

Matthews, J.V., Jr. (1970). Quaternary environmental history of interior Alaska: Pollen samples from organic colluvium and peats. Arctic and Alpine Research, 2, 241-251.

Matthews, J.V., Jr., Schweger, C.E., and Janssens, J.A. (1990). The last (Koy-Yukon) interglaciation in the northern Yukon: Evidence from Unit 4 at Ch’ijee’s Bluff, Bluefish Basin. Géographie Physique et Quaternaire, 44, 341-362.

McCulloch, D.S., Taylor, D.W., and Rubin, M. (1965). Stratigraphy, non-marine mollusks, and radiometric dates from Quaternary deposits in the Kotzebue Sound area, western Alaska. Journal of Geology, 73, 442-453.

Montoya, M., Crowley, T.J., and von Storch, H. (1998). Temperatures at the last interglacial simulated by a coupled ocean-atmosphere climate model. Paleoceanography, 13, 170-177.

Muhs, D.R., Kennedy, G.L., and Rockwell, T.K. (1994). Uranium-series ages of marine terrace corals from the Pacific coast of North America and implications for last-interglacial sea level history. Quaternary Research, 42, 72-87.

Muhs, D.R., Ager, T.A., and Begét, J.B. (1998). Vegetation and paleoclimate of the last interglacial period, central Alaska: Quaternary Science Reviews, in press.

Muhs, D.R., Ager, T., Stafford, T.W., Jr., Pavich, M., Begét, J.E, and McGeehin, J.P. (1997). The last interglacial-glacial cycle in late Quaternary loess, central interior Alaska. In Elias, S.A., and Brigham-Grette, J. (eds.), Program and Abstracts, Beringian Paleoenvironments Workshop, Florissant, Colorado, pp. 109-112.

Péwé, T.L. (1975a). Quaternary geology of Alaska. U.S. Geological Survey Professional Paper 835, 145 pp.

Péwé, T.L. (1975b). Quaternary stratigraphic nomenclature in unglaciated central Alaska. U.S. Geological Survey Professional Paper 862, 32 pp. Péwé, T.L., Wahrhaftig, C., and Weber, F.R. (1966). Geologic map of the Fairbanks quadrangle, Alaska. U.S. Geological Survey Miscellaneous Investigations Map I-455, scale 1:250,000.

Péwé, 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, 54 pp.

Schweger, C.E., and Matthews, J.V., Jr. (1991). The last (Koy-Yukon) interglaciation in the Yukon: Comparisons with Holocene and interstadial pollen records. Quaternary International, 10-12, 85-94.

Shackleton, J. (1982). Environmental histories from Whitefish and Imuruk Lakes, Seward Peninsula, Alaska. Ohio State University Institute of Polar Studies Report 76, 50 pp.

Waythomas, C.F., Lea, P.D., and Walter, R.C. (1993). Stratigraphic context of Old Crow tephra, Holitna Lowland, interior southwest Alaska. Quaternary Research, 40, 20-29.

Weber, F.R., and Ager, T.A. (1984). Glacial-lake deposits in the Mount Harper area, Yukon-Tanana Upland. U.S. Geological Survey Circular, 868, 68-70.

Westgate, J. (1988). Isothermal plateau fission-track age of the late Pleistocene Old Crow tephra, Alaska. Geophysical Research Letters, 15, 376-379.

Westgate, J. (1989). Isothermal plateau fission-track ages of hydrated glass shards from silicic tephra beds. Earth and Planetary Science Letters, 95, 226-234.

Westgate, J.A., Hamilton, T.D., and Gorton, M.P. (1983). Old Crow tephra: A new late Pleistocene stratigraphic marker across north-central Alaska and western Yukon Territory. Quaternary Research, 19, 38-54.

Woillard, G.M. (1978). Grand Pile Peat Bog: a continuous pollen record for the last 140,000 years. Quaternary Research, 9, 1-21.

Zagwijn, W.H. (1996). An analysis of Eemian climate in western and central Europe. Quaternary Science Reviews, 15, 451-469.

Zhu, H., and Baker, R.G. (1995). Vegetation and climate of the last glacial-interglacial cycle in southern Illinois, USA. Journal of Paleolimnology, 14, 337-354.