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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.

	FOOTNOTES

 
*Deceased.

GSA Data Repository Item 2008165, one photograph from Gold Hill Cut, two photographs of stem disks of wood from Dawson Cut Forest Bed, and one detailed table documenting the basic data for localities and wood samples of the Dawson Cut Forest Bed, is available at www.geosociety.org/pubs/ft2008.htm. Requests may also be sent to editing{at}geosociety.org.

	ABSTRACT

TOP ABSTRACT INTRODUCTION GEOLOGIC SETTING, MODERN... GOLD HILL LOESS AND... TEPHROCHRONOLOGICAL ADVANCES STRATIGRAPHIC SETTING,... AGE OF THE DAWSON... ENVIRONMENTAL CONDITIONS IN... CONCLUSIONS REFERENCES CITED

 
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

	INTRODUCTION

TOP ABSTRACT INTRODUCTION GEOLOGIC SETTING, MODERN... GOLD HILL LOESS AND... TEPHROCHRONOLOGICAL ADVANCES STRATIGRAPHIC SETTING,... AGE OF THE DAWSON... ENVIRONMENTAL CONDITIONS IN... CONCLUSIONS REFERENCES CITED

 
There is much concern today about the extent to which humans are influencing Earth's climate. This is especially true of the sensitive arctic and subarctic regions, where small increases in temperature can lead to major environmental changes, such as melting glaciers and sea ice, thawing of permafrost, widespread development of thermokarst terrain, and enhanced erosion, as a result of positive feedbacks (CAPE Project Members, 2006). Studies of past climate and vegetation are important because they provide a baseline reference for making judgments on the importance of this anthropological factor. This report deals with a period of global warming during the late Pliocene, when a boreal forest developed in interior Alaska. It details the character, stratigraphic setting, distribution, age, and paleoenvironmental significance of this interglacial forest bed, called the Dawson Cut Forest Bed, which is now represented mainly by rooted stumps and prostrate logs in the lower part of thick, perennially frozen loess in the Fairbanks area of interior Alaska, a region that remained unglaciated throughout late Cenozoic time (Figs. 1 and 2). Although the emphasis is on the Dawson Cut Forest Bed and the new tephrochronological findings, we also broadly consider the history and environment in the Fairbanks area from ca. 3 Ma to 200 ka and so complement an earlier report that considered the deposits and events from ca. 200 ka to the present day (Péwé et al., 1997).

View larger version (60K): [in this window] [in a new window]   Figure 1. Extent of late Cenozoic glaciations (shaded) in Alaska and northwestern Canada and location of Fairbanks and Yukon-Tanana Upland. HH—site of Halfway House exposure, B—Birch Creek site, P—Palisades site, LC—Lost Chicken Mine. Figure was modified from Péwé (1975a) and Tarnocai and Schweger (1991).

View larger version (34K): [in this window] [in a new window]   Figure 2. Index map of the Fairbanks area, south-central Yukon-Tanana Upland, Alaska, showing Dawson Cut Forest Bed sites (*) and locations of new tephra studies. Heavy lines in inset map (B) show position and orientation of geological sections shown in the indicated figures. CA—Chatanika site; DC—Dawson Cut; WD—West Dawson; EN—Engineer Creek; GI—Geophysical Institute; U—University of Alaska campus; CH—Church site; BH—Birch Hill.

	GEOLOGIC SETTING, MODERN ENVIRONMENT, AND SUMMARY OF LATE CENOZOIC STRATIGRAPHY

TOP ABSTRACT INTRODUCTION GEOLOGIC SETTING, MODERN... GOLD HILL LOESS AND... TEPHROCHRONOLOGICAL ADVANCES STRATIGRAPHIC SETTING,... AGE OF THE DAWSON... ENVIRONMENTAL CONDITIONS IN... CONCLUSIONS REFERENCES CITED

The Yukon-Tanana Upland is an east-trending upland between the Yukon and Tanana Rivers (Fig. 1); it consists of a dissected area of accordant rounded ridges 700–1000 m in elevation. Scattered, discontinuous groups of higher mountains project above the upland ridges to altitudes of 2000 m. Rounded upland ridges near Fairbanks have summits at 500–600 m above sea level. Bedrock in the southern part of the upland is chiefly schist, slate, and gneiss but also includes local masses of basalt, quartz, diorite, and granite (Péwé et al., 1966; Robinson et al., 1990).

Sediment-laden glacial rivers deposited hundreds of meters of sand and gravel in the Tanana and Yukon Valleys during glacial advances. Aggradation of these trunk valleys raised base level and caused tributaries from the unglaciated Yukon-Tanana Upland to aggrade their lower valleys. Winds blowing across outwash plains and vegetation-free bars of glacier-derived braided streams in trunk valleys picked up great quantities of silt and redeposited it as loess (Péwé, 1951), especially during times of glacial expansion. Loess was deposited on the Yukon-Tanana Upland and other nearby areas, blanketing ridges with thicknesses of a few centimeters to more than 60 m near rivers. Much of the loess was retransported to valley bottoms, where thicknesses of 100 m have been recorded (Péwé, 1958).

Modern Climate and Vegetation
To understand the environmental conditions at the time of the Dawson Cut Interglaciation in east-central Alaska more fully, it would be well to review briefly the climate, permafrost characteristics, and vegetation of the present Holocene Interglaciation.

The Fairbanks region of interior Alaska has a distinctly continental climate and a large variation of temperature from winter to summer. Winters are normally long, dark, cold, and dry, and the temperature frequently drops to –45 °C. Summers are short, sunny, and warm with temperatures exceeding 30 °C. The mean annual air temperature at the Fairbanks airport for the period 1949–2005 is –2.7 °C. The mean annual air temperature in low areas and on the north side of hills is colder. The mean number of days in Fairbanks with freezing temperature is 233, and freezing temperatures have been reported every month except July. The wind regime in central Alaska is generally composed of long, relatively calm conditions from September to May, and a short, slightly windy summer from June to August. The prevailing surface-wind direction in winter is north to northeast, and in summer, it is south to southwest. Mean annual precipitation for the period 1949–2006 is 268 mm. Precipitation normally reaches a minimum in spring and a maximum in August, when rainfall is common. Most of the annual precipitation is from May through September, with ~30% falling as snow, which normally covers the ground from mid-October through mid-April. Mean annual snowfall for 1949–2005 was 172 cm.

The Fairbanks area is in the discontinuous permafrost zone, and perennially frozen ground is widespread, except beneath hilltops and moderate to steep south-facing slopes. Although some permafrost in the area is probably relict from colder Wisconsin-age conditions, perennially frozen ground forms today under favorable conditions. Floodplain sediments are perennially frozen to 80 m (Péwé, 1954). Permafrost is up to 100 m thick in the valley bottoms and lower slopes of the Yukon-Tanana Upland. Here, large masses of ice in the form of horizontal to vertical sheets, wedges, saucers, and irregular shapes occur in the retransported loess of Wisconsin age. Ice wedges range from less than 30 cm to more than 15 m in length. All ice wedges in central Alaska are now inactive, except in local areas of particularly cold microclimates (Péwé, 1962; Hamilton et al., 1983). Permafrost temperature at the level of zero amplitude (depth of 8–15 m) is between –0.5 °C and 0 °C (Péwé and Paige, 1963). Thawing of ice-rich retransported loess in valley bottoms results in considerable differential subsidence of the ground surface and produces thermokarst topography. Many landforms indicative of permafrost with large ice masses are present in the vicinity of Fairbanks, including open-system pingos, ice-wedge polygons, thermokarst pits, beaded drainage, and thaw ponds (Péwé, 1982).

The Yukon-Tanana Upland is in the taiga, or northern boreal forest, of central Alaska (Fig. 3). The northern boreal forest consists of primarily open, slow-growing spruce interspersed with occasional dense, well-developed forest stands and treeless bogs (Viereck, 1973). Vierick and Little (1972, p. 9) stated that the extensive boreal forest of interior Alaska is composed of only three coniferous tree species, white spruce (Picea glauca), black spruce (Picea mariana), and tamarack (Larix laricina); three large deciduous tree species, balsam poplar (Populus balsaminifera), quaking aspen (Populus tremuloides), and paper birch (Betula papyrifera); and several species of willow (Salix) and two alder (Alnus).

View larger version (63K): [in this window] [in a new window]   Figure 3. Extent of present-day boreal (taiga) forest in Alaska (after Viereck and Little, 1972). T—Tofty, P—Palisades site, K—Koyukuk River.

Treeline on south-facing slopes is ~800 m in elevation and is slightly lower on north-facing slopes. White and black spruce varieties typically form an open woodland just below tree-line. Above 1000 m elevation in the southern Yukon-Tanana Upland, hillslopes and summits bear a discontinuous carpet of low-growing herbs, grasses, shrubs, cushion plants, and lichens. This cover is interrupted by rocky rubble and contains low, dense, pruned thickets of willows and resin birch in hollows where snow collects each winter.

Summary of Late Cenozoic Stratigraphy
We present a brief survey of the late Cenozoic deposits in the Fairbanks area of east-central Alaska in order to illustrate the stratigraphic context of the Dawson Cut Forest Bed (Fig. 4). The oldest unconsolidated deposit is the Cripple Gravel (Péwé, 1975a, 1989). It is a gold-bearing, brownish, coarse sandy gravel with angular clasts and is preserved on buried bedrock benches. The Cripple Gravel is of late Tertiary age (Westgate et al., 1990; Péwé et al., 1997) with no reported fossils. It is interpreted as being composed of solifluction debris that was produced in a periglacial climate and partly reworked by streams. The gravel formed when drainage directions were different from those of today (Péwé, 1965).

View larger version (21K): [in this window] [in a new window]   Figure 4. Schematic composite cross section of a creek valley near Fairbanks, Alaska, illustrating stratigraphic relations of late Cenozoic deposits. The unconformity between the Gold Hill Loess and overlying beds is emphasized by a heavier line.

Gold Hill Loess is a thick, massive, frozen, tan to grayish, mainly air-fall loess with no ice wedges (Péwé, 1975b). On lower slopes, it locally underlies the prominent, frozen, well-preserved Eva Forest Bed of last interglacial age with a marked unconformity (Péwé et al., 1997) or the Goldstream Formation (Fig. 4). On upper slopes, where it could not be differentiated from loess of Wisconsin or Holocene age, it has been lumped under the general term of Fairbanks Loess (Péwé, 1958, 1975b). With the identification and correlation of numerous tephra beds in the Gold Hill Loess, it is now possible to recognize Gold Hill Loess on upper slopes where it appears near or at the surface (Fig. 4). Gold Hill Loess embraces air-fall and retransported loess with ages from ca. 3 Ma to ca. 130 ka and represents several periods of loess deposition and erosion.

The Dawson Cut Forest Bed of the Dawson Cut Interglaciation (Péwé, 1952, 1975b) is a gray to black silt unit, 1–3 m thick, which contains peat lenses, logs, and forest beds and lies in the lower part of the Gold Hill Loess. White and black spruce, birch, alder, and other species have been identified in this deposit. Some white spruce logs are as much as 30 cm in diameter.

Goldstream Formation overlies the Eva Forest Bed. It is a widespread deposit of perennially frozen, poorly bedded, organic-rich, gray to black retransported loess that is 10–35 m thick. This silt fills the valley bottoms and forms low-angle silt fans extending from lower slopes into the valley bottoms (Fig. 4). Remains of mammoth, horse, and bison are common, but frozen carcasses are rare (Péwé, 1957; Guthrie, 1990). Palynological analyses and studies of mammal fossils indicate that trees were absent on the landscape during deposition of the Goldstream Formation (Matthews, 1968; Guthrie, 1968a, 1968b). All these characteristics, plus the presence of large ice wedges, are interpreted as indicating a harsh, periglacial climate. The Goldstream Formation is of Wisconsin age (Péwé, 1975b).

Engineer Loess unconformably overlies the Goldstream Formation and is located on the hillslopes, whereas Ready Bullion Formation occupies the valley bottoms (Fig. 4). The latter unit is a poorly to well-stratified, perennially frozen, organic-rich, retransported loess, and it contains the Giddings Forest Bed, the remains of a boreal forest less than 10 ka (Péwé et al., 1997). Both the Engineer Loess and Ready Bullion Formation are of Holocene age and have basal radiocarbon ages of ca. 10 ka.

	GOLD HILL LOESS AND ITS SUBDIVISION

TOP ABSTRACT INTRODUCTION GEOLOGIC SETTING, MODERN... GOLD HILL LOESS AND... TEPHROCHRONOLOGICAL ADVANCES STRATIGRAPHIC SETTING,... AGE OF THE DAWSON... ENVIRONMENTAL CONDITIONS IN... CONCLUSIONS REFERENCES CITED

 
The rolling hills of central Alaska are blanketed with layers of loess that thicken in valleys and thin on upper hills and ridge tops. For the past 100 yr, placer gold miners have sunk shafts and adits in the frozen silt cover, and since 1926, miners have sliced open the frozen loess cover for aboveground gold dredging, making 1–2-km-long, steep-sided excavations in valley bottoms and lower slopes and exposing the underlying gold-bearing, coarse creek gravels.

In the late 1940s and 1950s, there were many active mining exposures of frozen sediments in the lower creek valleys of the Yukon-Tanana Upland (Fig. 1) that changed daily in the summer and provided three-dimensional views of the late Cenozoic stratigraphy. These sections gave a type of stratigraphic fence diagram (e.g., Péwé et al., 1997), in contrast to a single thawed face of loess that must be trenched today to permit even a limited view. Furthermore, hundreds of mining boreholes to bedrock permitted determination of loess thickness and delineation of areas of no permafrost.

Prior to the creation of the 2.5-km-long, 68-m-deep excavation in the loess at Gold Hill (Fig. 2), starting in 1951, Péwé determined from the borehole data that the loess there was on a low rounded bedrock hill, situated only 1–2 km from the floodplain of the Tanana River, the dominant source of the windblown silt (Muhs and Budahn, 2006). The elevation, shape, and location of this buried bedrock hill account for the great loess thickness and long loess record at Gold Hill. Greatest loess deposition occurs close to the source and at an elevation near to that of the source, rather than hundreds of meters higher on upper slopes and hilltops. Higher and steeper terrain contains thinner loess because of its greater susceptibility to erosion. River bedrock bluffs near the Tanana River floodplain receive much windblown silt, but because of steep faces and higher altitudes, most of this loess is easily removed. For example, loess deposits on Birch Hill (Fig. 2) adjacent to the floodplain, and at the Halfway House exposure on a hilltop overlooking the Tanana River, 35 km west of Fairbanks (Fig. 1), contain only short records of Pleistocene loess (Muhs et al., 2003). On the other hand, the low rounded buried bedrock hill at Gold Hill, near the silt source, permitted major accumulations with less erosion, resulting in the preservation of a thick late Cenozoic loess record (Fig. 5).

View larger version (73K): [in this window] [in a new window]   Figure 5. Panoramic view of the late Cenozoic loess record at the eastern end of the Gold Hill Cut mining exposure, now known as the "Troy L. Péwé Climatic Change Permafrost Reserve." Maximum height of loess cliffs is 50 m. Inset map is plan view of Gold Hill Cut showing location of trenches 1, 2, 3, and 4. Location is shown in Figure 2, and detailed stratigraphy is shown in Figure 6.

View larger version (40K): [in this window] [in a new window]   Figure 6. This sketch of late Cenozoic loess stratigraphy at the east end of Gold Hill Cut illustrates the most complete and detailed record of the major climatic warmings and coolings in central Alaska over approximately the last 3 m.y. Here is the only known record of all three members of the Gold Hill Loess and five major unconformities in the loess, each indicating major topographical, climatic, and permafrost changes in the environment. The ages and names of these unconformities are as follows, in order of increasing age: Holocene-Wisconsin unconformity, 10 ka; the Eva Forest Bed Interglaciation unconformity, 125 ka; the Ester Interval unconformity, younger than 780–older than 610 ka; the Dawson Cut Forest Bed unconformity near the PA tephra, ca. 2 Ma; the unconformity between the lower Gold Hill Loess and Cripple Gravel, ca. 3 Ma. Generalized stratigraphy, loess polarity, and tephra beds are indicated in four vertical trenches in Gold Hill Loess: 1, 2, 3, 4 (see Fig. 5). Black bar—normal polarity; white bar—reversed polarity; diagonal lines—covered or no polarity recorded. Ages of polarity units are from standard geomagnetic time scales (Shackleton et al., 1990; Valet and Meynadier, 1993). xxxxxx—tephra bed, —ice-wedge cast, —concretions, tree stump. Details of stratigraphy, tephra, ice wedges, and forest beds have been omitted in Wisconsin and Holocene formations for simplicity. Basal date of ca. 3 Ma is from paleomagnetic studies (Westgate et al., 1990). Thermoluminescence (TL) dates on loess are by G.W. Berger (Berger and Péwé, 2001; Berger, 2003). Age of tephra beds is from glass fission-track methods (Preece et al., 1999). Old Crow tephra and Dome Ash Bed (DAB) were not recognized at this site but were identified at stratigraphic levels indicated on the figure during gold-mining operations when bluff was farther south. Tephra bed names as originally defined, except for: DAB—Dome Ash Bed, MG—Mosquito Gulch tephra, HH—Halfway House tephra, and VT—Variegated tephra.

The Dawson Cut Forest Bed unconformity is so named because its age is close to that of the Dawson Cut Forest Bed. This cannot be demonstrated at Gold Hill, but at Ester Island (Fig. 2), PA tephra (ca. 2 Ma) is stratigraphically near black silt of the Dawson Cut Forest Bed (Fig. 7), and at Engineer Creek (Fig. 2), EC tephra occurs in the upper part of the Dawson Cut Forest Bed (Fig. 8) and is only 75 cm above PA tephra at the Palisades site on the Yukon River (Fig. 1) (Matheus et al., 2003). This unconformity represents a time when huge amounts of loess were removed from the valley bottoms and lower slopes of the Fairbanks area. For example, Mosquito Gulch (MG) tephra (1.45 Ma) sits directly on the Cripple Gravel just west of trench 1 at Gold Hill (Fig. 6); the lower Gold Hill Loess has been entirely removed. The same is true at Ester Island, where PA tephra and WP tephra (1.03 Ma) occur in the basal part of the loess sequence (Fig. 7). Likewise, there is no pre–Dawson Cut Forest Bed loess at Engineer Creek (Fig. 8). Loess with a normal remanent magnetic polarity in the lower Gold Hill Loess of trench 3 at Gold Hill is interpreted as belonging to the Gauss chron, given the age of the overlying PA tephra (Preece et al., 1999). Therefore, the 2.5 m of loess between this tephra bed and the top of the underlying normal-polarity sediments must represent over 500,000 yr of time, again suggesting extensive removal of sediments in this part of the stratigraphic column.

View larger version (26K): [in this window] [in a new window]   Figure 7. Geologic cross section at the Ester Island mining exposure 2 km west of the confluence of Ester and Cripple Creeks, and 16 km west of Fairbanks, Alaska, showing distribution of sediments prior to removal of loess and dredging of the gravels by gold-mining operations during the period 1940–1965. The type section of the Ester Ash Bed is in the middle Gold Hill Loess, 1.5 m above the Cripple Gravel. Sheep Creek F tephra age by thermoluminescence (TL) dating on bracketing loess is from Berger et al. (1996). Other tephra ages are by glass fission-track methods (Preece et al., 1999). Holocene Engineer Loess and the Ready Bullion Formation, as well as ice wedges and stratigraphic details in the Goldstream Formation, are omitted for simplicity. MGHL—middle Gold Hill Loess. Surface topography, top of gravel, bedrock, and the fault are based on survey and drill-hole data from U.S. Smelting, Refining and Mining Company, Fairbanks, Alaska. This figure is modified after Péwé (1952). See Figure 2 for location of section.

View larger version (35K): [in this window] [in a new window]   Figure 8. (A) Dawson and Engineer placer mining cuts created in the 1940s and 1950s near the mouth of Engineer Creek, 13 km north of Fairbanks (Fig. 2). Cuts expose perennially frozen Pliocene-Pleistocene loess and auriferous stream gravel in creek valley bottoms and on adjacent rock terraces. Dawson Cut Forest Bed is preserved only in creek valley bottoms on younger Fox Gravel. (B) Composite sketch of stratigraphy at north wall of Engineer Creek. Exposure is type locality of EC tephra, Mining Camp tephra, and Dawson Cut Forest Bed. See supplementary information Table S1 (see text footnote 1) for data on wood of Dawson Cut Forest Bed.

The absence of the Dawson Cut Forest Bed at Gold Hill is probably because the old loess here was unfrozen during several interglacial periods, allowing groundwater to move through the loess and help weather away the remains of the ancient boreal forest. In the Mississippi Valley, for example, the widespread presence of woodland terrestrial mollusk shells in the loess of Wisconsin age indicates the existence of forests during loess accumulation, even though the old forest vegetation has since weathered away (Cameron, 1940; Roberts et al., 2003).

The middle Gold Hill Loess is defined as the loess that is bracketed by the Dawson Cut Forest Bed unconformity and the younger Ester Interval unconformity (Fig. 6), the latter of which is well exposed at Ester Island and Gold Hill (Fig. 2). A major erosional unconformity indicating pronounced cutting into the Gold Hill Loess can be seen just above the Ester Ash Bed at Ester Island (Fig. 9), and its crosscutting relationship to that tephra bed shows that it must be younger than 0.81 ± 0.07 Ma. A waterlaid conglomerate, 30–40 cm thick and composed of angular silt pebbles and blocks together with rare schist fragments, lies on this unconformity. The upper limiting date for the Ester Interval unconformity is not available from this locality, but it is old because this erosional surface is covered by more than 50 m of upper Gold Hill Loess, Goldstream Formation, and the Engineer Loess.

View larger version (27K): [in this window] [in a new window]   Figure 9. Stratigraphic sketch of the lower part of the 60 m loess cliff at Ester Island, 16 km west of Fairbanks, Alaska, illustrating relationship of the Ester Interval unconformity and overlying conglomerate bed to the underlying Ester Ash Bed in middle Gold Hill Loess. Conglomerate is composed of pebbles and blocks of silt and rare schist fragments. Paleomagnetic ages on extreme right of figure: M—Matuyama reversed chron; J—Jaramillo normal subchron. Note difference in scale.

The age of the unconformity that marks the top of the middle Gold Hill Loess at Gold Hill can be assessed from several observations. The presence of SP tephra in loess blocks in the basal gully sediments indicates an age younger than 0.87 ± 0.06 Ma (Westgate et al., 1990), and, because the gully fill has a normal magnetic polarity, this maximum age estimate can be refined to younger than 0.78 Ma (Fig. 6). HP tephra (0.61 ± 0.05 Ma; Preece et al., 1999) occurs ~4 m above this unconformity, so that the latter's age must be younger than 0.78 Ma but older than 0.61 Ma. Given these time constraints, the Ester Interval unconformity at Ester Island is thought to represent the same environmental break as the prominent unconformity that marks the top of the middle Gold Hill Loess at Gold Hill—a time of climatic warming and erosion involving removal of considerable loess, but that still left a remnant of much older loess.

The upper Gold Hill Loess is stratigraphically above the Ester Interval unconformity and below the Eva Forest Bed unconformity (Fig. 6) (Péwé et al., 1997). During mining operations in the 1950s, the Eva Forest Bed unconformity was very conspicuous, separating the frozen, dark retransported loess of the Goldstream Formation from the underlying massive, light-colored Gold Hill Loess (Fig. 10). However, this stratigraphic break is not prominent in the thawed loess at Gold Hill today. It is defined by remnant occurrences of Eva Forest Bed, thermoluminescence (TL) ages, and the occurrence of tephra beds, such as Old Crow tephra (131 ± 11 ka, see following), just below the unconformity (Fig. 6). This paleoenvironmental break, involving thermokarst development, has been studied in detail in the Yukon-Tanana Upland (Péwé et al., 1997), and it indicates a major period of loess erosion, when the climate was at least as warm as that of today in central Alaska and may well have been warmer (Brigham-Grette and Hopkins, 1995). Most, if not all, permafrost disappeared. The Gold Hill Loess has refrozen during the last 100,000 yr.

View larger version (158K): [in this window] [in a new window]   Figure 10. A 20-m-thick section of Pliocene-Pleistocene perennially frozen Gold Hill Loess at Gold Hill mining excavation overlain by late Pleistocene Goldstream Formation (loess). Upper part of the late Pleistocene and Holocene loess has been removed from the top of the section by placer gold-mining operations. The buried Eva Forest Bed represents the Sangamon Interglaciation. Lower part of section of Gold Hill Loess was temporarily covered by frozen and tumbled blocks of Gold Hill Loess, which were removed by jets of cold water under high pressure. North wall of Gold Mining Cut, 10 km west of Fairbanks, Alaska. Photograph PK 2159 by T.L. Péwé, 3 June 1952.

	TEPHROCHRONOLOGICAL ADVANCES

TOP ABSTRACT INTRODUCTION GEOLOGIC SETTING, MODERN... GOLD HILL LOESS AND... TEPHROCHRONOLOGICAL ADVANCES STRATIGRAPHIC SETTING,... AGE OF THE DAWSON... ENVIRONMENTAL CONDITIONS IN... CONCLUSIONS REFERENCES CITED

 
Here, we continue to develop the late Cenozoic tephrochronological record of the Fairbanks area, building on our earlier studies (Preece et al., 1999; Westgate et al., 1990, 2003). In this report, we include the results of new tephra characterization studies on samples that were collected by us during earlier field seasons, including the remainder of Péwé's tephra collection. Tables 1, 2, and 3 list the tephra beds in the upper Gold Hill Loess, middle Gold Hill Loess, and in sediments younger than the Gold Hill Loess, respectively. No tephra beds have been discovered in the lower Gold Hill Loess. These tables provide information on the major-element composition of the glass shards, the number of distinct chemical populations present, an assignment to a type I or type II category, with those samples that do not fit into these categories being noted as "other" (Preece et al., 1999), and a classification based on the normalized glass composition using the International Union of Geological Sciences total alkali-silica diagram (Le Bas et al., 1986). Mineralogical and chemical differences between type I and type II tephra beds are detailed in Preece et al. (1999). It will suffice here to say that bubble-wall glass shards are abundant in type I tephra beds, which contain mostly anhydrous minerals (feldspar, orthopyroxene, and clinopyroxene, with minor amounts of amphibole, titanomagnetite, ilmenite, apatite, zircon, and biotite), whereas highly inflated pumice and amphiboles are much more abundant in type II tephra beds, where a typical mineral assemblage consists of feldspar, amphibole, and orthopyroxene, with minor amounts of magnetite, ilmenite, apatite, and zircon. A chart summarizing the major attributes of tephra beds in the Fairbanks area is given in Table 4.

The current status of the tephra record at Gold Hill is shown in Figure 6. We note only the new additions; information on the other tephra beds is given in Preece et al. (1999). Mosquito Gulch (MG) tephra has been found immediately to the west of trench 1 in the lowermost part of the Gold Hill Loess just above the Cripple Gravel. It has a weighted mean age of 1.45 ± 0.14 Ma based on four glass fission-track ages (Westgate et al., 2001), so the lower member of the Gold Hill Loess does not extend this far west at Gold Hill (Fig. 6). Another occurrence of this rhyolitic tephra is located 4 m above the PA tephra at trench 4. Blebs of an orange, rhyolitic tephra bed, OT tephra, are positioned just below the Ester Interval unconformity at trench 2, and, near the top of trench 3, MF and PH tephra beds have been identified above LW tephra, providing, for the first time, the relative stratigraphic position of these three units.

The reference site for the Dome Ash Bed (DAB, UT745) is along the west side of Eva Creek (Figs. 2 and 11), where it occurs above Old Crow tephra and Sheep Creek F tephra (Westgate et al., 2008). Preece et al. (1999) placed DAB in the uppermost Gold Hill Loess. Additional DAB samples, recognized when the remainder of Péwé's tephra samples were analyzed (Fig. 12), occur at Gold Hill (UT415), the north end of Eva Creek (UA374), and West Dawson (Fig. 13). Péwé's field notes show that all these DAB correlatives are in the uppermost part of Gold Hill Loess; we can now say that this stratigraphic placement is incorrect because our recent identification of DAB in horizontal, finely laminated lacustrine silts of unit F-2 at Birch Creek (Fig. 1) (Edwards and McDowell, 1991; tephra samples and stratigraphy provided by Tom Hamilton, 2001, written commun.) indicates that it was deposited when a closed boreal forest covered the Birch Creek site—that is, at a time of interglacial conditions when temperatures were at least as warm as those of today (McDowell and Edwards, 2001). This new information supports the view of Muhs et al. (2001), who stated that DAB is just above the main development of the Eva Forest Bed at Eva Creek.

View larger version (43K): [in this window] [in a new window]   Figure 11. (A) Longitudinal geological section of late Cenozoic stratigraphy of perennially frozen sediments and forest beds exposed in the east wall of lower Eva Creek Cut mining exposure, 16 km west of Fairbanks, Alaska. Rectangle indicates area covered by Figure 15. (B) Geological cross section in upper part of Eva mining cut and the adjacent Eva Bench Cut. (C) Composite stratigraphic sketch of perennially frozen late Cenozoic loess, tephra, gravel, and forest beds on west wall of upper Eva Creek Cut mining exposure. Old Crow tephra age (131 ± 11 ka) is from this study; thermoluminescence (TL) age (190 ± 20 ka) of Sheep Creek F tephra on bracketing loess is from Berger et al. (1996). Location of sections is shown in Figure 2, and data on the pollen sample are given in Table 6. DAB—Dome Ash Bed.

View larger version (18K): [in this window] [in a new window]   Figure 12. Oxide bivariate plots showing correlatives of the Dome Ash Bed in the Fair-banks region (crosses) and the Birch Creek site (UT1835). UT745 is Dome Ash Bed at the reference site at Eva Creek (Figs. 2 and 11C).

View larger version (24K): [in this window] [in a new window]   Figure 13. Stratigraphic setting and identity of tephra beds in Gold Hill Loess in West Dawson area (see Fig. 2). (A) Three-dimensional sketch made by Péwé during placer gold-mining operations in 1956. Dome Ash Bed (DAB) is shown to be close to the unconformity separating the "Wisconsin Muck" from the "Illinoisan Silt" (now called Gold Hill Loess), but its precise stratigraphic position cannot be assessed on this diagram. (B–D) Other vertical exposures with tephra beds in upper part of Gold Hill Loess.

A coarse-grained sample (UT1434) of Old Crow tephra was sent to us by Darrell Kaufman (Northern Arizona University) for an age determination using our glass fission-track methods. It was collected from a 40-cm-thick bed at Togiak Bay, Alaska (58°47.38'N, 161°10.75'W) and must be <500 km from its source vent in the Alaska Peninsula (Westgate et al., 2000). An age of 129 ± 14 ka was obtained using the isothermal plateau technique (Westgate, 1989), and the result on the internal standard shows that an accurate estimate of the neutron dose was obtained because its glass fission-track age is well within 1 of the ⁴⁰Ar/³⁹Ar age (Table 5). Other glass–isothermal plateau fission track ages for Old Crow tephra are listed in the footnotes of Table 5. They were taken from Preece et al. (1999), but their associated errors have been recalculated using the formula recommended by Bigazzi and Galbraith (1999). These errors are much larger than in UT1434 because fewer fission tracks were counted due to the much finer grain size and resultant smaller glass surface areas. For example, whereas 159 spontaneous tracks were counted in UT1434, only 36 were counted in UT613, and 19 were counted in UT501. The weighted mean age and error (Bevington, 1969) on four glass ITPFT determinations of Old Crow tephra are 131 ± 11 ka (1). This age estimate agrees with TL ages 15 cm above and below Old Crow tephra at Birch Hill (Fig. 2) of 128 ± 22 ka and 144 ± 22 ka, respectively (Berger et al., 1996), as well as with the infrared stimulated luminescence (ISL) ages for bracketing loess at the Halfway House site (Fig. 1) of 131 +21/–13 ka and 135 +21/–13 ka (Auclair et al., 2007). Given these age data and the varying lithologic and paleoecological contexts of Old Crow tephra (Hamilton, 1993; Elias et al., 1999; McDowell and Edwards, 2001; Muhs et al., 2001, 2003), it seems likely that Old Crow tephra was deposited during the stage 6 to stage 5 transition, prior to growth of the last interglacial boreal forest, when conditions were similar to or warmer than those of today.

Other newly characterized tephra beds in the Fairbanks area include the Weigh Scale tephra, which was collected in 1949 by Péwé near the base of the Gold Hill Loess overlying the Dawson Cut Forest Bed at the lower end of the Eva Creek exposure (Fig. 11). This tephra is a rhyolitic, type I bed with abundant bubble-wall glass shards (Table 2). Unfortunately, insufficient sample was available for fission-track dating. College Hill tephra is from the excavation for the utility corridor on the University of Alaska campus (Fig. 2). Its pumice has a dacitic composition (Table 1). Feldspar, green amphibole, orthopyroxene, and opaques were identified in the sample. A TL age of 207 ± 40 ka was obtained on silt just above this tephra bed (Berger and Péwé, 2001; Péwé et al., 1997). EI tephra occurs as a 2-cm-thick bed on the north side of Ester Island in the upper Gold Hill Loess below the Eva Forest Bed (Fig. 7). Its pumiceous glass has a rhyolitic composition, and the mineral assemblage indicates a type II identity (Table 1). EB tephra lies near the base of the Holocene Engineer Loess, which overlies the Goldstream Formation at the Eva Bench Cut (Fig. 11B). It also is a type II bed with rhyolitic glass (Table 3). The Chatanika Ash Bed is ~1 cm thick and occurs ~4 m below the top of the Goldstream Formation in exposures along the Chatanika River, 40 km north of Fairbanks (Fig. 2). Its composition is very similar to that of EB tephra (Table 3), and its age is ca. 14,000 ¹⁴C yr B.P. (Table 4; Péwé, 1975b). A new occurrence of GI tephra (Preece et al., 1999) is located at the Church site at the western end of Birch Hill (Fig. 2; Table 1).

	STRATIGRAPHIC SETTING, DESCRIPTION, AND DISTRIBUTION OF THE DAWSON CUT FOREST BED

TOP ABSTRACT INTRODUCTION GEOLOGIC SETTING, MODERN... GOLD HILL LOESS AND... TEPHROCHRONOLOGICAL ADVANCES STRATIGRAPHIC SETTING,... AGE OF THE DAWSON... ENVIRONMENTAL CONDITIONS IN... CONCLUSIONS REFERENCES CITED

 
Dawson Cut Forest Bed as a Stratigraphic Unit
The Dawson Cut Forest Bed represents an ancient, buried, boreal forest and has been identified at several localities in the Fairbanks area (Fig. 2). It is a 0.5–3-m-thick bed of peat lenses, sticks, logs, and rooted and unrooted stumps of trees in perennially frozen gray to black retransported loess that is rich in small carbonized plant fragments. The bed crops out as a black zone at the base of placer gold-mining exposures in valley bottoms at Eva Creek (Fig. 11), Engineer Creek (Fig. 8), Cripple Sump (Fig. 7), and Sheep Creek Cut (Fig. 14).

View larger version (49K): [in this window] [in a new window]   Figure 14. (A) Stratigraphic sketch of transverse section across the upper end of Sheep Creek Cut mining exposure at bend in mining cut, 15 km NW of Fairbanks, Alaska. Dawson Cut Forest Bed of iron-stained and cracked spruce logs and overturned stumps occurs at top of and in upper meter of Fox Gravel and underlies green middle Gold Hill Loess. Holocene silt overlying Goldstream Formation of Wisconsin age is generalized for simplicity. Unconformity beneath Goldstream Formation is indicated by heavy line. (B) Stratigraphy of west wall of Sheep Creek Cut. Dawson Cut Forest Bed consists of smashed, flattened, iron-stained spruce stumps, logs, and sticks in 1–3 m of organic-rich silt and in upper meter of Fox Gravel. All underlie green Gold Hill Loess. Goldstream Formation and Holocene loess are generalized for simplicity.

The Dawson Cut Forest Bed is overlain at most localities by green middle Gold Hill Loess. However, on the east wall of lower Eva Creek, the loess has been removed locally so that the Goldstream Formation lies directly on the Dawson Cut Forest Bed (Fig. 11). The forest bed overlies Fox Gravel at upper and lower Eva Creek, middle and upper Sheep Creek Cut, lower Engineer Creek, and Cripple Sump (Fig. 2). Detailed examinations were made at the contact of the Gold Hill Loess and underlying Cripple Gravel at other mining cuts, specifically Ester Island, Eva Bench Cut, Gold Hill, and Dawson Cut, but no forest bed was present. Elsewhere, the Dawson Cut Forest Bed was removed by erosion during the last interglacial or earlier (Péwé et al., 1997), resulting in the Goldstream Formation lying directly on Fox Gravel. Some examples are Ready Bullion Creek (Péwé, 1965, his Figs. 1–10), lower Ester Creek, central Cripple Creek, Fairbanks Creek (Péwé, 1952, his Fig. 34), and upper Goldstream Creek (Fig. 2).

The appearance of the forest bed varies from exposure to exposure. Locally, it is present as an extensive buried forest bed, 100 m long or more, or in local patches. The bed may exist as isolated logs and stumps (Fig. 15), as simple wood fragments, or as woody silt or gravel. For the most part, the organic silt of the Dawson Cut Forest Bed, which is associated with the logs and stumps, is indistinguishable from the dark retransported loess of the Goldstream Formation. The contact between the silt-rich forest bed and the overlying Gold Hill Loess is gradational; the black silt grades upward to brown or green loess. The contact with the underlying creek gravel is also gradational. Lenses of dark alluvial silt up to 1 m thick occur in the uppermost 2 or 3 m of the Fox Gravel. In situ and prostrate stumps (Fig. 16) up to 30 cm in diameter occur, but smashed and flattened logs up to 2 m long are more common and occur in both the silt lenses and in the heavily iron-cemented gravel (supplementary information, Table S1 [see ^{footnote 1}]).

View larger version (180K): [in this window] [in a new window]   Figure 15. White spruce stump of Dawson Cut Forest Bed rooted in silt and gravel of the Fox Gravel at the base of the Gold Hill Loess at Eva Creek (see Fig. 11A). (Photograph 471 by Troy L. Péwé, 18 September 1949; previously published in Péwé, 1975).

View larger version (142K): [in this window] [in a new window]   Figure 16. Formerly prostrate fossil black spruce stump (Picea mariana) preserved near top of silty, coarse Fox Gravel with angular clasts at elevation of 250 m in Eva Creek valley bottom and excavated from level of Troy Péwé's feet. This stump is part of Dawson Cut Forest Bed (see sample no. 191, supplementary information, Table S1 [see text footnote 1]). Photograph PK29313 by T.L. Péwé, 14 June 1989.

An exceptionally good, 100-m-long exposure of the Dawson Cut Forest Bed was noted at the base of the 35-m-high loess cliff in the west wall of Sheep Creek Cut, 15 km northwest of Fair-banks (Fig. 14B). Spruce logs, commonly stained with a bright yellow iron-oxyhydroxide, occur near the top of the Fox Gravel, and smashed logs and stumps can be seen at the base of the overlying 3-m-thick black silt, which contains gravel stringers, clear ice segregations, and logs. The two wedge-shaped structures in this dark silt, with their vertically oriented spruce fragments, are probably ice-wedge casts (Fig. 14B).

Trees of the Dawson Cut Forest Bed
Spruce dominates the tree remains at all the exposures of the Dawson Cut Forest Bed (supplementary information, Table S1 [see ^{footnote 1}]). The largest spruce samples are up to 30 cm in diameter and 3 m long. The abundance and size of spruce remains appear to be consistent with the modern boreal forest, where spruce is the most common and largest tree present (Viereck, 1975; Lutz, 1956). Most of the spruce fossils are white spruce (Picea glauca) rather than black spruce (Picea mariana) as indicated by the presence of white spruce cones and needles, the larger size of the stem remains, and, possibly, evidence of spruce bark beetles. Black spruce (Picea mariana), birch (Betula papyrifera), and cottonwood (Populus balsamifera) are much less common and less well preserved. The size of the stem and the thin rings of a black spruce specimen from the Dawson Cut Forest Bed at Sheep Creek are characteristic of modern black spruce (supplementary information, Fig. S2 [see ^{footnote 1}]). Tree-ring counts on this specimen and another spruce fossil from the same bed at Eva Creek (supplementary information, Fig. S3 [see ^{footnote 1}]) indicate that some trees were more than 200 yr old when they died. A cut and polished stem disk from the latter log (no. 191, supplementary information, Fig. S3, Table S1 [see ^{footnote 1}]) illustrates rings that are very similar to those of a stem disk from modern white spruce (no. M2; Péwé et al., 1997, his Fig. 17) near Fairbanks from about the same elevation.

View larger version (32K): [in this window] [in a new window]   Figure 17. Sketch of stratigraphy of perennially frozen late Cenozoic sediments of the basal part of Cripple Sump mining exposure (Fig. 7) exhibiting tephra and black silt of Dawson Cut Forest Bed; 16 km west of Fairbanks, Alaska. Tephra samples were collected by Péwé in 1949.

Regarding the method of preservation, logs, stumps, roots, cones, and a host of wood fragments on the floor of the Dawson Cut forest were probably buried by the last of the alluvial silt and gravel of the shifting creeks. This material, in turn, was soon buried by the initial deposits of the middle Gold Hill Loess. Some wood on slopes was probably buried rapidly as layers of silt from the Gold Hill Loess were washed downhill. At other sites, wood remains were gradually buried by middle Gold Hill air-fall loess. Some weathering and decay of the buried specimens probably continued until the silty organic bed became perennially frozen.

Spruce wood is the best preserved of the tree species. Even though logs and stumps occur, most of the wood is fragmentary, smashed, flattened, iron-stained, and shows evidence of burning. Only small bits of bark are preserved. The wood is not mineralized, although most specimens are moderately to heavily stained to a brownish, reddish, or yellow color by iron-oxyhydroxides deposited by seeping ground-water when the ground and the forest bed were thawed. As the wood remains are exposed by mining and thaw, they become soft, punky, and fetid. Upon drying, they mostly split into many fragments. Close examination indicates that much of the wood cell structure is deformed.

The poorly preserved state of wood in the Dawson Cut Forest Bed is in great contrast to wood from the perennially frozen Eva Forest Bed, which is moderately to excellently preserved, not compressed, and retains good cell structure. Likewise, wood from the Giddings Forest Bed of Holocene age is excellently preserved and differs from modern wood only in color (Giddings, 1938).

The occurrence of extensive and repeated fires in the boreal forest of interior Alaska and Russia in prehistoric, historic, and modern times is well substantiated (Lutz, 1956; Shostakovitch, 1925; Viereck, 1973; Péwé et al., 1997). It is of no surprise, therefore, that abundant evidence exists for the presence of widespread forest fires in the Fairbanks area during the Dawson Cut Interglaciation. Although only a few burnt wood specimens are listed in Table S1 of the supplementary information (see ^{footnote 1}), almost every one of the many exposures of the forest bed in the Fairbanks area yielded evidence of former fires. Burnt rooted stumps, fallen logs, and sticks, mainly of spruce, attest to the presence of forest fires, as does fire charring on bark and the debarked surface of trees and the ubiquitous accumulation of small carbon particles and charcoal fragments in the silty soil. Locally, the burnt debris forms thin layers in the soil of the Dawson Cut Forest Bed.

Pollen and Faunal Remains
Pollen preserved in the organic silt of the forest bed can give us a more complete record of the composition of the forest than can macrospecimens of trees alone. Unfortunately, our experience is that pollen grains are poorly preserved in ancient loess, most probably as the result of destruction by freezing and thawing, and oxidation during their long burial history.

Black organic sediment at the base of Dawson Cut Forest Bed above Fox Gravel at Eva Creek was sampled by Péwé in 1987 and proved to be rich in pollen (Table 6). Charles Schweger, University of Alberta, reported that the slightly lower Picea and high Betula and Alnus frequencies suggest a slightly more open boreal forest when compared to surface samples of modern boreal forest (Schweger, 1997, written commun.). The work of Anderson and Brubaker (1986) is in agreement with this interpretation; they suggest >20% Picea pollen indicates a closed canopy boreal forest. These comparisons suggest that boreal forest conditions in the Fair-banks area during the Dawson Cut Interglaciation were more typical of what one would now encounter between the Yukon River and Koyukuk River (Fig. 3).

Spruce bark beetles (Scolytidae) lived in the forests of the Fairbanks area during the Dawson Cut Interglaciation. Small, meandering beetle galleries, ~3–4 mm wide, on the debarked surface of white spruce logs were first observed by Péwé and H.J. Lutz, Professor of Forestry, Yale University, in the Fairbanks area in 1957 (supplementary information, Table S1 [see ^{footnote 1}]). Subsequently, Péwé has examined numerous beetle-scored spruce logs from a number of exposures of the Dawson Cut Forest Bed.

	AGE OF THE DAWSON CUT FOREST BED

TOP ABSTRACT INTRODUCTION GEOLOGIC SETTING, MODERN... GOLD HILL LOESS AND... TEPHROCHRONOLOGICAL ADVANCES STRATIGRAPHIC SETTING,... AGE OF THE DAWSON... ENVIRONMENTAL CONDITIONS IN... CONCLUSIONS REFERENCES CITED

 
Constraints on the age of the Dawson Cut Forest Bed have followed from the application of tephrochronologic and fission-track dating, and magnetostratigraphic methods (Preece et al., 1999; Westgate et al., 1990, 2003). Tephra beds in the Fairbanks area that are stratigraphically close to Dawson Cut Forest Bed, and hence critical to the resolution of its age, are the Weigh Scale tephra (Fig. 11), Mining Camp tephra (Figs. 7 and 8), EC tephra (Fig. 8), and PA tephra (Fig. 6; Table 4). EC tephra is older than the Mining Camp tephra (Fig. 8), but the Fairbanks exposures do not reveal the stratigraphic relationship of the PA and Weigh Scale tephra beds within this group. EC tephra occurs in the uppermost part of the Dawson Cut Forest Bed, Mining Camp, and Weigh Scale tephra beds in the lowermost part of the middle Gold Hill Loess, just above Dawson Cut Forest Bed, but PA tephra does not occur in direct association with the forest bed, although a close stratigraphic relationship is suggested at the Cripple Sump excavation (Figs. 7 and 17). Only PA is datable by glass fission-track methods; the other tephra beds are either too pumiceous or of insufficient sample size. Two glass–isothermal plateau fission track ages (Westgate, 1989) were determined and gave a weighted mean age and error of 2.02 ± 0.14 Ma (1), in agreement with the reversed magnetic polarity of the enclosing loess (Fig. 6) (Westgate et al., 1990).

Several of these tephra beds are closely associated with the lowermost peat/forest bed at the Palisades site (Fig. 1). The EC and Mining Camp tephra beds are 25 and 50 cm above the forest bed, respectively (Fig. 18). PA tephra occurs immediately below this forest bed (Matheus et al., 2003), which, therefore, must correlate with Dawson Cut Forest Bed at Fairbanks and have an age of ca. 2 Ma. Another possible correlative occurrence of the Dawson Cut Forest Bed is at the Tofty placer district near Manley Hot Springs in western Yukon-Tanana Upland, east-central Alaska (Péwé et al., 1997, their Figure 12, p. 24). All three sites of the Dawson Cut Forest Bed are well within the present-day distribution of boreal forest in Alaska (Fig. 3).

View larger version (29K): [in this window] [in a new window]   Figure 18. Stratigraphy of tephra beds close to the lower peat–forest bed at the Palisades site, Yukon River, Alaska (Fig. 1). Information was taken from Matheus et al. (2003). These tephra beds demonstrate that this lower peat–forest bed correlates with the Dawson Cut Forest Bed in the Fairbanks region.

	ENVIRONMENTAL CONDITIONS IN FAIRBANKS REGION DURING DAWSON CUT INTERGLACIATION

TOP ABSTRACT INTRODUCTION GEOLOGIC SETTING, MODERN... GOLD HILL LOESS AND... TEPHROCHRONOLOGICAL ADVANCES STRATIGRAPHIC SETTING,... AGE OF THE DAWSON... ENVIRONMENTAL CONDITIONS IN... CONCLUSIONS REFERENCES CITED

Botanical Evidence
The Dawson Cut Forest Bed has long been considered to represent the typical taiga, or northern boreal forest, of central Alaska (Péwé, 1952, 1965, 1975a). Plant macrofossils indicate a spruce-birch forest with at least the following taxa: white spruce, black spruce, paper birch, poplar, and Sphagnum moss. The size and frequency of spruce and birch macrofossils are similar to representatives in the modern taiga. Forest fires during the Dawson Cut Interglaciation were similar to those of the modern boreal forest, and the ubiquitous spruce bark beetle was present then as now in the boreal forest of Alaska. Such a forest exists today from the south flank of the Brooks Range to the Anchorage area (Figs. 1 and 3), an area with a range of mean annual air temperatures from about –7 °C in the north (1970–1977 records) to +2 °C in the south (1952–2007 records).

Pollen from organic silt at the base of the Dawson Cut Forest Bed supports a spruce-birch boreal forest interpretation, but the high birch and alder frequencies suggest a more open boreal forest, perhaps comparable to that which now exists in the region between the Yukon and Koyukuk Rivers.

Tree Rings
Tree rings from white spruce in central Alaska have been used to reconstruct past climate variability (Blasing and Fritts, 1975; Cropper, 1982; D’Arrigo et al., 1992). However, there has been little dendroclimatic research using black spruce. Studies have demonstrated that white spruce growth is strongly influenced by temperature regimes (Larsen, 1980; Blasing and Fritts, 1975; Cropper, 1982; D’Arrigo et al., 1992), although precipitation may also limit growth in more interior stands, especially on south-facing slopes. Chronologies of ring width and ring density, particularly maximum latewood density, have been developed from modern Alaskan white spruce trees for purposes of climatic reconstruction (Blasing and Fritts, 1975; Cropper, 1982; D’Arrigo et al., 1992). To assess qualitatively the annual variability of climate during the time when the Dawson Cut Forest Bed trees grew, descriptive statistics from annual ring widths from Dawson Cut Forest Bed fossil wood were compared to similar statistics from modern white spruce from central Alaska for which climate relationships are known. Dawson Cut Forest Bed wood was also compared to wood from the Eva Forest Bed (Péwé et al., 1997).

White and black spruce wood samples from the Dawson Cut Forest Bed are summarized in Table 7. Species identifications were made by D. Marguerie and Y. Bégin, Université Laval (Marguerie et al., 2001). In order to examine the tree-ring structure, all samples were first surfaced with 400 grit sandpaper using a belt sander and hand sanding. Ring numbers were counted on each sample, and notes were made when unusual ring structures, such as reaction wood or scars, were observed within the ring series. Reaction wood is an area of expanded cell structure generally formed in response to a tree leaning off center, and it is characterized in conifer species by eccentric growth on the downhill portion of the ring circumference (Fritts, 1976). Ring widths were measured using a rotary-encoder measuring system connected to a microcomputer (Robinson and Evans, 1980).

Modern Alaskan white spruce tree-ring data used to compare to the Dawson Cut Forest Bed wood were compiled from raw measurements from X-ray density scans from six white spruce sites from central Alaska archived in the International Tree-Ring Data Bank (ITRDB; see site descriptions in Péwé et al., 1997). These six sites were collected and measured by Fritz Schweingruber of the Swiss Federal Institute of Forest Research. Statistics for Eva Forest Bed wood samples are from Péwé et al. (1997).

Descriptive statistics of ring width series measured from white and black spruce samples from the Dawson Cut Forest Bed are presented in Table 7. Ring series from the Dawson Cut Forest Bed material were generally very short owing to fragmentation of the wood. The Dawson Cut Forest Bed wood is more fragile and less well preserved than wood from the Eva Forest Bed (Péwé et al., 1997). The longest-lived tree examined, no. 185A, was heavily flattened, and its rings were distorted. Other sections also appeared to have been flattened in circumference. In contrast to the Eva Forest Bed samples, areas of reaction wood are rare in the Dawson Cut Forest Bed cross sections, suggesting that these trees were growing on more stable soils.

We compare means and standard errors for ring width measurements from the Dawson Cut Forest Bed white spruce wood to similar statistics from modern and Eva Forest Bed series in Table 8. The only significant difference in ring statistics between the Dawson Cut Forest Bed material and either the modern or Eva Forest Bed trees is in the first-order autocorrelation, which is a measure of the degree of similarity in ring width between years.

In general, and allowing for the very small sample size for the statistical parameters for wood from the Dawson Cut Forest Bed (five series), it is possible that environmental conditions responsible for formation of the tree-ring series during the climatic optimal period when the Dawson Cut Forest Bed trees grew were comparable to conditions in operation today. Little difference is seen between modern Alaskan tree rings and the Dawson Cut Forest Bed ring series in any of the descriptive characteristics examined. The only significant difference is that there was apparently less annual persistence between years in the Dawson Cut Forest Bed wood, as indicated by the lower autocorrelation value. Autocorrelation in trees is the result of either carry-over of stored reserves or leaf area between years, or it can be the result of autocorrelation in climate, especially temperature.

Carbon Isotopes
The ¹³C/¹²C ratios (expressed as ¹³C = [{¹³C/¹²C_sample/¹³C/¹²C_standard} – 1] x 1000 in units) of ancient plant matter, including wood, have been used in efforts to reconstruct past environmental changes (e.g., Krishnamurthy and Epstein, 1990; Leavitt and Danzer, 1991, 1992; Leavitt, 1993; Van de Water et al., 1994). This is possible because current plant carbon isotope fractionation models (Farquhar et al., 1982; Francey and Farquhar, 1982) indicate that plant ¹³C is a function of atmospheric CO₂ concentration and ¹³C, and any factors that influence rates of carbon isotope fixation and stomatal conductance of CO₂. Such factors include relative humidity, moisture availability, light levels, and nutrient status. If one of these values shifts through time (with others remaining constant), the consequence may be a shift in ¹³C. The isotopic composition of the Dawson Cut Forest Bed trees can be compared to that of other trees from Alaska representing the last 125,000 yr (Péwé et al., 1997) to infer the occurrence of such environmental shifts.

Ten trees from the Dawson Cut Forest Bed were subsampled by slicing complete, thin cross sections (~3–5 mm thick) from each. Full cross sections reduce any confounding effects from intratree isotopic variability (Leavitt and Long, 1986). These subsamples were ground to 20 mesh, and the holocellulose component of the wood was isolated using a modified sodium chlorite extraction method (Leavitt and Danzer, 1993). Cellulose is relatively resistant to decomposition and alteration, and it is commonly used in isotopic research with wood. Unlike modern cellulose, which is usually very white, the cellulose isolated from these trees ranged from light tan to reddish brown. We believe this to represent iron-oxide staining, which is resistant to the chemical processing. This is consistent with earlier discussion on the presence of iron-oxy-hydroxides, but because the latter contains no carbon, it does not affect the isotopic analyses. Although we did not calculate percentage cellulose yield, visual observation indicated substantial recovery of cellulose in spite of the great age of the wood, suggesting excellent preservation. Cellulose was combusted in a vacuum recirculating system at 800 °C in the presence of CuO and free O₂, followed by collection and purification of the CO₂ combustion product. The CO₂ was analyzed mass spectrometrically, and the results are reported as ¹³C with respect to the international Peedee belemnite (PDB) standard. Repeated combustion and analysis of a cellulose standard gave a precision of ~±0.2 (1 standard deviation).

The ¹³C results for the Dawson Cut Forest Bed trees are presented in Figure 19, together with results from the Eva Forest Bed and Holocene/modern trees from central Alaska, previously analyzed in the same laboratory as these specimens (see Tables 1, 2, and 3 in Péwé et al., 1997). The range in the Dawson Cut Forest Bed ¹³C spruce values is nearly identical to that seen in the Eva Forest Bed spruce trees. Hardwood (angiosperm) trees are more negative (¹³C depleted) relative to the spruce trees, but this difference in hardwood and conifers is well documented (Leavitt and Newberry, 1992; Stuiver and Braziunas, 1987). The ¹³C means (±1 standard deviation) for Dawson Cut Forest Bed, Eva, Holocene, and modern spruce trees are –22.32 ± 0.96, –22.14 ± 0.95, –23.45 ± 0.76, and –23.08 ± 1.03, respectively. Thus, spruce trees from Eva Forest Bed and Dawson Cut Forest Bed are indistinguishable isotopically, although both are less negative (more ¹³C enriched) than Holocene and modern spruce. In the case of modern spruce, its ~1 ¹³C depletion compared to spruce from the Eva Forest Bed and Dawson Cut Forest Bed is consistent with the Industrial Revolution inputs of ¹³C-depleted CO₂ to the atmosphere from fossil-fuel combustion. A ¹³C decline in tree rings of ~1–1.5 is observed in growth rings of modern trees over the past 200 yr (Leavitt and Long, 1988; Leavitt and Lara, 1994).

View larger version (11K): [in this window] [in a new window]   Figure 19. 13C measured in cellulose component of modern and fossil wood samples from the Fairbanks area, Alaska. Holocene, Eva Forest Bed, and Dawson Cut Forest Bed samples are from perennially frozen loess and retransported loess. Numbered samples refer to specimens from Dawson Cut Forest Bed (see supplementary information, Table S1 [see text footnote 1]); unnumbered data points refer to samples in Figure 27 of Péwé et al. (1997).

Longevity of Northern Boreal Forest in Northwestern North America
Current knowledge supports the view that the northern boreal forest (taiga) as we know it today has a long history that extends back to at least 2 Ma. Conditions favoring the development of this type of boreal forest came into being following the onset of global cooling ca. 2.7 Ma (Prueher and Rea, 1998), because prior to this time, a very different boreal forest existed in northwestern North America. A coniferous forest richer in genera and species covered much of Alaska and northern Canada in early late Pliocene times. It was dominated by pines, spruce, fir, and larch, and it also included several now-extinct species (Matthews et al., 2003). Climate was wetter and less continental than now, with little or no permafrost. This diverse boreal forest survived to at least 2.92 ± 0.15 Ma, the glass fission-track age of Lost Chicken tephra, which sits directly on peat (with its diverse, diagnostic flora at the Lost Chicken Mine in east-central Alaska) (Figs. 1 and 20). It is possible, therefore, that earlier examples of the modern-type northern boreal forest (taiga) will be discovered in the time slot 2.7–2.0 Ma.

View larger version (161K): [in this window] [in a new window]   Figure 20. Lost Chicken tephra above late Pliocene, preglacial peat at Lost Chicken Mine, east-central Alaska. This tephra bed has a glass fission-track age of 2.92 ± 0.15 Ma, the weighted mean and standard error of three age determinations. Scale is 10 cm long.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION GEOLOGIC SETTING, MODERN... GOLD HILL LOESS AND... TEPHROCHRONOLOGICAL ADVANCES STRATIGRAPHIC SETTING,... AGE OF THE DAWSON... ENVIRONMENTAL CONDITIONS IN... CONCLUSIONS REFERENCES CITED

1. Gold Hill Loess makes up the greater part of the thick loess cover in the Fairbanks region and is a rich archive of late Cenozoic climate change, and good chronological control is provided by numerous distal tephra beds, derived from vents in the Wrangell volcanic field and the Aleutian-Alaska Peninsula arc.
   
2. Major unconformities in the Gold Hill Loess serve to divide the formation into three informal members: lower, middle, and upper Gold Hill Loess. The upper and middle members can be recognized by their suite of tephra beds, but no tephra beds have been found in the lower Gold Hill Loess.
   
3. Dome Ash Bed and Old Crow tephra serve as important markers for the last interglaciation in interior Alaska. The former was deposited during the last interglaciation when boreal forest was well established at Birch Creek, interior Alaska, whereas lithologic and paleoecological contexts of Old Crow tephra, in conjunction with new age estimates, suggest deposition during the stage 6 to stage 5 transition, just prior to growth of boreal forest in interior Alaska during the last interglaciation.
   
4. Dawson Cut Forest Bed overlies the Fox Gravel and lies near the base of the middle Gold Hill Loess. Plant macrofossils consist of Picea glauca, Picea mariana, Betula papyrifera, and Populus balsamifera, but no Pinus. A single pollen analysis shows slightly lower Picea and high Betula and Alnus frequencies, suggesting a more open boreal forest when compared to surface samples of the modern boreal forest.
   
5. The Dawson Cut Forest Bed in the Fair-banks area correlates with the lower peat–forest bed at the Palisades site based on associated tephra beds at both sites. Another probable correlative occurs at the Tofty placer mining district near Manley Hot Springs. All three sites are well within the present-day distribution of boreal forest in Alaska.
   
6. Recognition of PA tephra immediately below the lower peat–forest bed at the Palisades indicates that the Dawson Cut Forest Bed has an age of ca. 2 Ma.
   
7. Plant macrofossils, abundance and size of spruce remains, pollen, tree-ring characteristics, and ¹³C values of spruce wood all demonstrate that the boreal forest represented by the Dawson Cut Forest Bed was similar to the modern boreal forest of central Alaska.
   
8. The northern boreal forest (taiga) of northwestern North America as we know it today likely has a long history that extends back to at least 2 Ma.
   

	ACKNOWLEDGMENTS

 
Péwé has drawn heavily on the experience of colleagues in the Branch of Alaskan Geology, U.S. Geological Survey, and the Departments of Geology at the University of Alaska–Fairbanks and at Arizona State University during the course of his studies in the Fairbanks area. In the 1940s and 1950s. Otto William Geist, University of Alaska, showed him many exposures produced by placer gold-mining activities in the Yukon-Tanana Upland. Many miners generously provided access to their properties. Special thanks go to Roy Earling, John Metcalfe, and James Crawford, administrators of the U.S. Smelting, Refining, and Mining Company; Pete Eagan, Manager, Fairbanks Department, Alaska Gold Company; and Walter and Mike Wigger, owners of properties in the Eva Creek area.

Field work by Péwé in the 1940s and 1950s was with the Alaska Terrain and Permafrost Section of the U.S. Geological Survey, supported in part by the Engineer Intelligence Division, United States Army. Stratigraphic investigations by Péwé in 1977 were supported by the State of Alaska Division of Geological and Geophysical Surveys. Financial support from the U.S. National Academy of Sciences permitted field checking by Péwé in 1981, 1982, and 1983.

Péwé was aided in the field by many assistants of the U.S. Geological Survey and University of Alaska, notably E.S. King Jr., A.M. Gooding, D.R. Loftus, G. Herman, E.W. Marshall, D.D. Smith, R.A. Paige, L.R. Mayo, R.D. Reger, P.V. Sellman, N.W. Rutter, J.M. Blackwell, N.R. Rivard, J.W. Bell, and E.J. Bell. Valuable field assistance to Westgate was provided by B. Stemper, K. Kemp, Qiang Hu, A. Westgate, and G. Westgate. E.H. Beistline, former Dean of the School of Mines at the University of Alaska, provided information on mining activities and personnel. G.E. Weller and G.D. Guthrie, University of Alaska, kindly provided climatic and vertebrate paleontological information, respectively. F.R. Weber and J.D. Townshend, U.S. Geological Survey, provided logistic aid in the field over several summers, and James and Sally Murphy of Fairbanks provided logistic support to Péwé for many summers. We thank Susan Selkirk, graphic artist, Arizona State University, for providing us with digital copies of figures she had prepared for Péwé, a number of which were later updated. Thanks also go to Brian Gootee, Tempe, Arizona, who helped us retrieve a number of Péwé's research files. Tom Hamilton, U.S. Geological Survey, kindly provided the tephra samples from Birch Creek, Alaska.

R.D. Reger, formerly of the Alaska State Division of Geological and Geophysical Surveys, Fairbanks, was associated with various aspects of the project from 1954 to 1994. He aided in field work and argued scientific ideas in the field and office. We are grateful for his stimulating discussions over the years.

From 1985 to 1995, Péwé and Westgate received grants from national agencies, including National Science Foundation grant EAR-8520143 (1985–1989) and EAR-9022590 (1991–1995), and National Geographic grants 4100-89 and 5196-94 to Péwé for trenching and radiocarbon dating, respectively. Support for Westgate's tephra studies was provided by the Natural Sciences and Engineering Research Council of Canada and the American Chemical Society. Brown thanks James Burns, Laboratory of Tree-Ring Research, University of Arizona, Tucson, for X-ray densitometry of the Dawson Cut Forest Bed samples, and Bruce Bauer, International Tree-Ring Data Bank, National Geophysical Data Center, Boulder, Colorado, for providing the raw density data from Fritz Schweingruber's white spruce sites in Alaska. Leavitt thanks S. Danzer for helping prepare samples and T. Newberry and B. McCaleb for helping to analyze the samples. CO₂ samples prepared from combustion of the tree-ring cellulose were analyzed in the Laboratory of Isotope Geochemistry, Department of Geosciences, University of Arizona (Austin Long, Director). Reviews by D. Muhs, U.S. Geological Survey, and J. Begét, University of Alaska–Fairbanks, resulted in a significantly improved paper.

Finally and importantly, Westgate wishes to acknowledge the help and encouragement given by Mary Jean Péwé. Her knowledge of her husband's academic files, samples, and field notebooks greatly facilitated access to important documents that allowed this project to be completed.

Point of Information. In 1989, Péwé submitted a proposal to the Alaska State Legislature that 25.5 acres of the northeast end of the Gold Hill Cut be purchased by the state of Alaska and set aside for scientific study as a Past Climate Permafrost Preserve. This proposal was approved by the Alaska State Legislature in 1990, and, in 1996, the area was secured and preserved for future studies. In 1998, the Geophysical Institute of the University of Alaska Fairbanks–took over this research site and named it the Troy L. Péwé Climatic Change Permafrost Reserve. Troy Péwé visited this site just prior to his death in 1999.

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TOP ABSTRACT INTRODUCTION GEOLOGIC SETTING, MODERN... GOLD HILL LOESS AND... TEPHROCHRONOLOGICAL ADVANCES STRATIGRAPHIC SETTING,... AGE OF THE DAWSON... ENVIRONMENTAL CONDITIONS IN... CONCLUSIONS REFERENCES CITED

 

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RECEIVED FOR PUBLICATION September 5, 2007

REVISED MANUSCRIPT RECEIVED February 9, 2008

MANUSCRIPT ACCEPTED April 22, 2008

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