Late Pliocene to late Pleistocene environments preserved at the Palisades Site, central Yukon River, Alaska

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Quaternary Research 60 (2003) 33– 43

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Late Pliocene to late Pleistocene environments preserved at the Palisades Site, central Yukon River, Alaska Paul Matheus,a,* James Bege´t,b Owen Mason,c and Carol Gelvin-Reymillerd b

a Alaska Quaternary Center and the Institute of Arctic Biology, University of Alaska Fairbanks, Fairbanks, AK 99775-5540, USA Department of Geology & Geophysics and the Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK 99775-5780, USA c Department of Anthropology, University of Alaska Anchorage, 3211 Providence Drive, Anchorage, AK 99508, USA d 1191 Dolphin Way, Fairbanks, AK 99709, USA

Received 30 December 2002

Abstract The Palisades Site is an extensive silt-loam bluff complex on the central Yukon River preserving a nearly continuous record of the last 2 myr. Volcanic ash deposits present include the Old Crow (OCt; 140,000 yr), Sheep Creek (SCt; 190,000 yr), PA (2.02 myr), EC (ca. 2 myr), and Mining Camp (ca. 2 myr) tephras. Two new tephras, PAL and PAU, are geochemically similar to the PA and EC tephras and appear to be comagmatic. The PA tephra occurs in ice-wedge casts and solifluction deposits, marking the oldest occurrence of permafrost in central Alaska. Three buried forest horizons are present in association with dated tephras. The uppermost forest bed occurs immediately above the OCt; the middle forest horizon occurs below the SCt. The lowest forest bed occurs between the EC and the PA tephras, and correlates with the Dawson Cut Forest Bed. Plant taxa in all three peats are common elements of moist taiga forest found in lowlands of central Alaska today. Large mammal fossils are all from common late Pleistocene taxa. Those recovered in situ came from a single horizon radiocarbon dated to ca. 27,000 14C yr B.P. The incongruous small mammal assemblage in that horizon reflects a diverse landscape with both wet and mesic environments. © 2003 University of Washington. Published by Elsevier Inc. All rights reserved. Keywords: Alaska; Yukon River; Beringia; Interglacial; Peat; Dawson Cut Forest Bed; Tephra; Permafrost; Mammals

Introduction The Palisades Site is a tall bluff complex comprised of unconsolidated sediments stretching nearly 11 km along the southern banks of the Yukon River in central Alaska, USA (Figs. 1 and 2). The bluffs are referred to locally as “The Boneyard” because Pleistocene-age bones regularly erode from their faces. The sheer walls, erosional gullies, massive ice bodies, and bones of Pleistocene mammals exposed at the Palisades caught the attention of early surveyors exploring central Alaska (Russell, 1890; Spurr, 1898; Maddren, 1905; Gilmore, 1908). Later, Eardley (1938) reported on the general lithology in the context of sedimentary processes of the * Corresponding author. Alaska Quaternary Center, CSEM, 900 Yukon Drive, University of Alaska Fairbanks, Fairbanks, AK 99775-5940. Fax: ⫹1-907-474-6293. E-mail address: [email protected] (P. Matheus).

Yukon River, and Yeend (1977) described the Tertiary deposits that underlie the bluffs. The discovery of the Old Crow tephra (OCt) (140,000 ⫾ 20,000 yr) in the upper portions of the bluffs by Bege´t et al. (1991) suggested there was considerable time depth recorded in lower sections of the Palisades. Young and Walker (1982) noted the occurrence of plant macrofossils, including large logs, and bones of large mammals, but the Palisades’ paleontological resources have never been studied in the context of modern paleoecological and paleoclimatic paradigms in Beringia. Here, we report preliminary data and conclusions regarding the age, major stratigraphic units, and paleoenvironmental record of the Palisades. General site description The Palisades Site (65° 6⬘ N, 153° 15⬘ W) is located 55 km downstream from the village of Tanana, in the Nowitna

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Fig. 1. Location of the Palisades Site, Yukon River, Central Alaska. PW, Palisades West; PE, Palisades East.

Wildlife Refuge (Fig. 1). The regional valley fill is mostly unconsolidated loess and alluvium, forming a broad tableland into which the river is moderately incised (Eardley, 1938). The bluffs of the Palisades, comprised mostly of perennially frozen silts, are an escarpment formed by lateral migration and erosion of the Yukon River into a portion of the tableland that has been uplifted by movement along the Kaltag Fault (Patton and Hoare, 1968). The escarpment rises from east to west, the tallest sections being approximately 75 m (Fig. 3). On the eastern limit, the silt bluffs reside unconformably on unconsolidated to loosely consolidated late Miocene sands and sandstones interbedded with lignite seams. On the western limit, a bedded, late Miocene conglomerate underlies the silts unconformably (Yeend, 1977). The bluff faces are steep due to active undercutting, and runoff from melting permafrost bodies causes extensive gullying, making it possible to study sections in three dimensions. However, these processes also generate large slump blocks and colluvium that present interpretive difficulties. The Palisades actually consists of two escarpments: “Palisades East” and “Palisades West,” 8 and 3 km long, respectively, and separated by a 3-km zone of stable knolls (Fig. 1). This paper only reports on Palisades East and does not discuss the Tertiary sequences.

Lithostratigraphy and major chronostratigraphic horizons We divided the silt sequences at Palisades East into five lithostratigraphic units (Fig. 3), and identified 10 major chronostratigraphic horizons—seven tephras and three peat beds (Figs. 3 and 4). Most horizons were mapped discontinuously in Figure 3 because of colluvium or inaccessibility, and some horizons were visible only where there was recent slumping. Lithostratigraphic units (Fig. 3) Unit 1 is a thin, finely bedded silt and clay unit at the base of the bluffs. It extends less than 1 m above spring water level and was not studied in detail. Unit 2 designates alternating laminated and massive silt-loams in the lower third of the bluffs. Laminated sediments were probably deposited in low-energy fluvial environments in close association with the paleo-Yukon River. Occasional forest beds and other terrestrial facies are present low in the unit, and it is likely that the massive components are eolian, indicating that Unit 2 preserves both fluvial and subaerially exposed facies. Numerous small erosional unconformities may be present, and one or more major unconformities probably

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Fig. 2. Typical bluff section from the central portion of Palisades East. Horizontal distance spanned in photo is ⬃0.3 km; the length of the entire Palisades East complex is ⬃7 km. Vertical height of bluffs in this section is ⬃70 m. Note ice bodies, ice-thaw gullies, thermokarst amphitheater, lower peat, and upper peat (middle peat covered by colluvium).

separate Units 2 and 3. We never located a clear contact between Units 2 and 3. Tephras near the base of Unit 2 place its age near the Plio-Pleistocene boundary. Unit 3 is composed of primary and secondary loess manifested as massive to weakly laminated greyish-tan silt and fine sand, with orange ferric oxidation staining and thin organic horizons. Elsewhere in eastern Beringia, similar loess deposits reflect a pattern of temporarily stable surfaces alternating with periods of higher rates of airfall loess (Pe´we´, 1975a; Bege´t, 1988, 2001). Unit 3 was deposited during marine oxygen isotope stage (MIS) 6 as indicated by a suite of chronostratigraphic markers discussed below. The contact between Unit 3 and Unit 4 is abrupt, marked by a thick buried forest horizon (attributable to MIS 5). In 1998, we observed an ice body in Unit 3, ovate to wedge-shaped, ⬃4 m wide, and ⬃3 m tall. This is a significant find because it indicates that not all permafrost melted in central Alaska during MIS 5, contrary to arguments of Pe´we´ (Pe´we´, 1975a; Pe´we´ et al., 1997), and it means that the Palisades may preserve MIS-6 ice for future studies. Unit 4 is an ice-rich muck-loam unit of variable thickness. It is rich in large ice bodies, probably both syngenetic and epigenetic. Individual ice bodies extend downward from Holocene deposits above (Unit 5) into the MIS-5 interglacial forest bed below. Ice-rich muck-loams of this

sort are common valley fill in central Alaska, and typically form from reworked loess (Pe´we´, 1975a; Hamilton et al., 1988; Bege´t, 2001). Unit 5 demarcates Holocene peat and silt-loam. The basal portion is loess-like and contains buried forest material (correlative with the Ready Bullion Formation of Pe´we´, 1975b). There are modern epigenetic ice wedges in this unit and polygonal ground on the surface. Unit 5 is capped by paludified, black spruce forest with frequent muskegs and a thick layer of forest duff and mosses. Major chronostratigraphic markers and horizons (Figs. 3 and 4) We traced 10 major horizons and chronostratigraphic markers across the lateral surface of Palisades East (described below). From bottom to top, they are: (1) PA tephra, (2) PAL tephra, (3) a compressed lower peat/forest bed, (4) EC tephra, (5) PAU tephra, (6) Mining Camp tephra, (7) a sporadic middle peat/forest bed, (8) Sheep Creek tephra, (9) Old Crow tephra, and (10) a thick upper peat/forest bed. We also observed numerous organic-rich subhorizons which may record early to mid Pleistocene warm periods or paleosols, but they are small and discontinuous, making correlation difficult.

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Fig. 3. Generalized longitudinal profile of Palisades East showing major lithostratigraphic units and horizons. Note: the co-magmatic PA and PAL tephras both occur as pink tephras immediately below the lowest peat.

Tephras and associated features Tephras are numerous at the site, especially in lower sections. Several correlate with dated tephras found elsewhere in Alaska and the Yukon. Previously, only the OCt had been identified from the Palisades (Bege´t et al., 1991). Tephras in Alaska and Yukon are classified as either Type I or Type II based on their source area, mineralogy and geochemistry. Type I originate from vents in the Aleutian Arc-Alaska Peninsula region; Type II originate in the Wrangell volcanic field (Preece et al., 2000; Westgate et al., 2001). PA tephra. A pink Type I tephra occurs at elevations of 2–7 m in the eastern half of Palisades East (Fig. 3). It occurs in both massive and bedded silts, and typically is stratigraphically just below the lowest peat/forest bed. Tephra beds vary from thin interdigitated lenses a few millimeters thick to pods up to 15 cm in thickness. The field occurrence and stratigraphic position of this tephra are similar to the PAL tephra, and their relative ages and identities are still unresolved (see below). Samples of this tephra analyzed at the Alaska Tephrochronology Center (ACT 2421 and ACT 2423) have majorelement glass geochemistries that are statistically indistinguishable from the PA tephra at Gold Hill and Cripple Sump (Ester Island) near Fairbanks (Table 1) (Preece et al., 1999) The PA tephra has been dated by isothermal plateau

fission-track (IPFT) techniques to 2.02 ⫾ 0.14 myr (Westgate et al., 2000). PAL tephra. On a separate occasion, samples were collected from tephra beds fitting the description for the PA tephra above, except these beds were traced over a greater lateral extent, and were found at elevations only 1–2 m above water level near the east and west limits, but ⬃14 m above water level in the central bluff faces (Fig. 3). Like the PA tephra, this tephra also is found just below the lower peat/ forest bed. Two samples (UT1280 and UT1281) were submitted to the University of Toronto for geochemical analyses. John Westgate (written communication, 2002) identified both as belonging to a single tephra having a geochemistry and glass composition intermediate between the PA and EC tephras described by Preece et al. (1999) near Fairbanks. Westgate et al. (2000, 2003) suggest that the PA and EC tephras are comagmatic, with the EC eruption postdating the PA eruption by a few millennia. Thus, samples UT1280 and UT1281 may represent a third, intermediate eruption that we informally name the PAL tephra (also see Westgate et al., 2003). This description presumes that the PAL tephra predates the PA tephra, but we could not determine their stratigraphic relation because we were unaware during sampling that they represent two different tephras.

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and CaO concentrations, and higher SiO2 and K2O concentrations, and can be distinguished from the PA tephra by its slightly lower SiO2, and K2O concentrations, and higher Al2O3 concentrations. Analysis of grains of PAU tephra revealed some with compositions very similar to the underlying EC tephra, while no EC chemistries were identified within the PA tephra. Although the EC tephra was found only at one site, where it was preserved as pods of ash within a presumed paleosol, the PAU tephra could be traced laterally for many meters across the exposures at the Palisades, and everywhere clearly lies above the lower peat/ forest bed. Mining Camp tephra. A Type II tephra (ACT 2417) occurs in section directly above the EC and PAL tephras. This tephra is more silicic and has lower FeO and Al2O3 compositions than the PA, PAL, PAU, and EC ashes. Westgate et al. (2003) have identified a tephra of very similar composition in the Fairbanks area that they named the Mining Camp tephra. Based on the chemical similarities and stratigraphic position, tephra ACT 2417 is correlated with the Mining Camp tephra.

Fig. 4. Composite section profile of typical bluff exposure at Palisades East. Lithostratigraphic unit descriptions same as in Figure 3.

EC tephra. Twenty five centimeters above the lowermost peat/forest bed, are pods (1 cm thick) of a gray, Type I ash in a 20-cm-thick gleyed zone of massive silt. Electron microprobe analyses (ACT 2418) indicate that this tephra closely resembles the EC tephra that Westgate et al. (2000, 2003) described from Engineer Creek, near Fairbanks (Table 1). It is geochemically distinguishable from the PA tephra by its lower SiO2 concentrations, less evolved glass composition, and stratigraphic position. PAU tephra. A fourth Type I tephra (ACT 2424 and ACT 2418),) occurs 10 cm above the EC tephra, where it is preserved as 1–2 mauve ash beds several centimeters thick. No tephras of correlative age are known from anywhere in Beringia, and we propose informally naming this ash bed the PAU tephra. The PAU tephra is broadly similar to the underlying EC tephra, and also the PA and PAL tephras, and apparently records another comagmatic eruption close in age to those events (Table 1). However, the PAU tephra can be distinguished from the EC by its lower Al2O3, FeO,

Sheep Creek tephra (SCt). A white Type II tephra (ACT 2412, ACT 2414, ACT 2416, and ACT 2420) is preserved in very thin beds 0.5–2 cm thick that are easily observed at elevations of 25–30 m at both Palisades East and West. It occurs about 4 – 8 m below the OCt and the uppermost forest bed and about 5 m above the middle forest bed (Figs. 3 and 4). We identified this unit as the Sheep Creek tephra using electron microprobe analyses (Table 1). Thermoluminescence (TL) dating of sediments surrounding the SCt elsewhere in Alaska and Yukon indicate its age to be about 190,000 yr (Berger et al., 1996). Old Crow tephra (OCt). The Old Crow tephra at the Palisades is preserved as a prominent white ash horizon occurring 2–5 m below the upper forest bed (Figs. 3 and 4). The average IPFT date for the OCt is 140,000 ⫾ 10,000 yr (Westgate, 1988, 1989), correlating the upper peat/forest bed with MIS 5 and the Eva Creek forest bed in the Fairbanks area (Bege´t et al., 1991; Pe´we´ et al., 1997; Muhs et al., 2001). Age of the Dawson Cut Forest Bed. In the Fairbanks area, the EC tephra occurs immediately above the Dawson Cut Forest Bed (DCFB) (Pe´we´ 1975b). The PA tephra, which presumably predates the EC, has not been found at sites containing the DCFB (Preece et al., 1999; Westgate et al., 2003). At Palisades East, the EC tephra occurs immediately above the lowest buried forest bed. The comagmatic PAL and PA tephras, while never found in direct association with thick forest deposits, both occur slightly below and at times interdigitated with organic stringers that can be traced laterally to the forest bed. Consequently, we correlate the lower forest bed with the DCFB. Furthermore, the relation

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Table 1 Major element chemistry of glass shards from Tephras at the Palisades site, central Alaska Sample names OLD CROW TEPHRA ACT 2415 ACT 2422 SHEEP CREEK TEPHRA ACT 2412 ACT 2414 ACT 2416 ACT 2420 MINING CAMP TEPHRA ACT 2417 PAU TEPHRA ACT 2421 EC TEPHRA ACT 2418 PA TEPHRA ACT 2423 ACT 2424

Category

Na2O

MgO

Al2O3

SiO2

CI

K2O

CaO

TiO2

Fe2O3

H2O

ave (n ⫽ 39) std dev ave (n ⫽ 44) std dev

3.48 0.13 3.51 0.16

0.29 0.03 0.30 0.05

13.10 0.12 13.02 0.15

75.54 0.31 75.47 0.32

0.27 0.03 0.27 0.03

3.75 0.15 3.81 0.14

1.41 0.08 1.44 0.07

0.26 0.22 0.26 0.16

1.89 0.13 1.92 0.17

3.36 0.97 3.50 3.27

ave (n ⫽ std dev ave (n ⫽ std dev ave (n ⫽ std dev ave (n ⫽ std dev

4.54 0.20 4.53 0.47 4.23 0.21 4.03 0.65

0.69 0.08 0.71 0.11 0.75 0.07 0.72 0.11

15.71 0.30 15.73 0.53 15.81 0.34 15.64 0.38

72.13 0.76 72.04 0.81 72.22 1.03 72.55 0.29

0.05 0.02 0.05 0.03 0.03 0.03 0.07 0.06

2.03 0.11 1.96 0.17 2.06 0.18 2.06 0.14

2.49 0.18 2.61 0.24 2.50 0.30 2.53 0.16

0.33 0.19 0.27 0.25 0.29 0.21 0.21 0.23

2.05 0.20 2.11 0.27 2.11 0.23 2.19 0.32

3.56 2.53 4.42 2.86 3.88 2.91 4.41 2.90

ave (n ⫽ 40) std dev

3.49 0.23

0.26 0.03

13.07 0.15

77.40 0.36

0.25 0.03

2.72 0.13

1.24 0.05

0.28 0.28

1.28 0.10

5.23 2.23

ave (n ⫽ 28) std dev

4.06 0.22

0.51 0.16

13.88 0.63

71.47 1.76

0.51 0.06

4.21 0.30

1.94 0.44

0.56 0.31

2.87 0.56

3.31 0.00

ave (n ⫽ 17) std dev

3.95 0.33

0.62 0.10

14.35 0.54

70.49 0.80

0.48 0.06

3.87 0.15

2.77 0.32

0.54 0.18

2.80 0.40

3.36 2.07

ave (n ⫽ 17) std dev ave (n ⫽ 12) std dev

3.73 0.34 3.68 0.13

0.29 0.04 0.32 0.04

13.02 0.24 13.27 0.16

74.17 0.46 73.86 0.46

0.53 0.13 0.58 0.05

4.44 0.21 4.39 0.12

1.34 0.12 1.44 0.08

0.29 0.22 0.25 0.18

2.20 0.19 2.21 0.23

5.28 0.87 4.94 0.70

22) 14) 6) 8)

Notes: All glass totals normalized to 100%; total iron shown as Fe2O3. H2O determined by difference, based on analytical totals. Number of analyses averaged for each tephra shown by (n ⫽ x). Table is arranged in stratigraphic order, from youngest (top) to oldest tephra (bottom). Analytical conditions, standards, and microprobe equipment discussed in Bege´t et al. (1991).

of the PA tephra and DCBF at Palisades East indicates the age of DCFB to be ⬃2.02 ⫾ 0.14 myr. These stratigraphic relations also corroborate Westgate et al.’s (2000, 2003) assertion that the EC tephra is slightly younger than the PA tephra. Ancient solifluction, permafrost, and plant root-casts preserved by the PA tephra. In some exposures at Palisades East, beds of PA tephra are highly convoluted and even overturned, suggesting downslope movement by solifluction and other forms of cryoturbation. About 1500 m west of the bluffs’ eastern limit we found the PA tephra slumped into a wedge-shaped collapse feature roughly 1.5 m wide at the top and 2 m deep, which we interpreted to be an ice-wedge cast. This evidence indicates that permafrost existed in central Alaska shortly before 2.02 myr ago, and just before the development of the DCFB. Based on the distribution of active ice wedges today in Alaska, it has been argued that ice wedges form where mean annual air temperatures (MAT) are ⫺7° C or colder (Pe´we´ 1975a). But it since has been shown that ice wedges can form where MAT is as high as ⫺3° to ⫺4°C (Hamilton et

al., 1983), depending on factors such as winter snow depth and rapidity of cooling. Today, MAT in central Alaska near the Palisades is ⫺2° to ⫺4°C and there is active ice wedge formation at colder microsites. The presence of ice-wedge casts immediately below the PA tephra indicates that the MAT in central Alaska at the end of the Pliocene was at most ⫺3°C, but probably closer to ⫺7°C The discovery of ice-wedge casts with inclusion of PA tephra at the Palisades constitutes the oldest evidence of permafrost yet found in Alaska. Westgate and Froese (2001) reported ice-wedge casts at Quartz Creek near Dawson, Yukon, dating to 2.6 –3.3 myr. That stands as the oldest minimum date for permafrost in all of eastern Beringia. About 1000 m west of the eastern limit at Palisades East, the PA tephra infills thin, vertically oriented root casts typically 2– 8 cm long, 1–3 mm wide, in densities of ⬃1 cast per 5–30 mm (Figs. 5b and c). They clearly are not casts of tree roots, and they do not resemble tundra roots, which are thicker, less uniformly spaced, less straight, and have chaotic, subhorizontal orientations. They best indicate an homogeneous vegetation such as a grass or sedge meadow. In one location, the tephra and root casts are

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Fig. 5. Lower peat complex and PA tephra. (a) Alternating subunits of fluvial beds, massive silts, poorly manifested peat layer, and the underlying PA tephra. (b) Presumed root casts infilled by the PA tephra. (c) Similar root casts, PA tephra, and the lower peat incorporated into a solifluction lobe.

completely overturned in the leading edge of a rolled solifluction lobe (Fig. 5c). Peats, forest beds and plant macrofossils Lower peat/forest bed (Fig. 5). This wood and organic-rich peat complex occurs low in the bluffs of Palisades East, within lithostratigraphic Unit 2. It is most pronounced in eastern sections, where it is near water level, and central sections, where it is as high as 14 m (Fig. 5a). The presence of occasional large tree trunks, mostly isolated and horizontal, but occasionally rooted, makes the lower peat stand out as a stratigraphic unit. However, the stratigraphy and composition of the peat complex is varied and organics are sparse. Most often, it is a 5- to 20-cm-thick compressed subpeat layer with poor organic preservation. In other exposures, it occurs as brown (organic-rich) massive silt beds 3–10 cm thick containing isolated wood fragments. In its poorest expression, the lower peat is represented by alternating laminae (⬃1–3 cm) of brown organic-rich and gray organic-poor silts and fine sands, where the organic-rich laminae contain small (⬍1 cm diameter) wood fragments and/or grade into subpeaty pods.

In most cases, the lower peat complex seems to be preserving overbank types of facies, where the paleo Yukon River or a slough periodically flooded low-lying terrestrial environments. Because there are fluvial components, some of the large tree trunks may have been deposited by floods. We recovered tree sections up to 40 cm in diameter and 3 m long, but they have not been identified to taxon. Their spirally cracked wood structure suggests they are Picea (spruce). Rarely, the lower peat occurs locally as a thick, compact organic mat layer 1 m thick. Macrofossil analyses of one of these mats yielded few intact plant parts, but we did identify carbonized, friable needles and small branches of Picea; fragments of graminoids and Equisetum; and pholidia of Sphagnum; plus statoblasts and adult structures of the bryozoan Cristatella mucedo. The presence of adult Cristatella indicates that standing freshwater was in the immediate vicinity. The cumulative presence of Picea, Equisetum, and Sphagnum indicates a moist, lowland spruce forest characteristic of low-lying areas of Interior Alaska today. The lower peat/forest bed is correlated with the DCFB of Pe´we´ (1975b) (see discussion above).

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Middle peat/forest bed. This horizon is weakly expressed, discontinuous, and difficult to access, but stands out as a recurring feature about halfway up bluffs of Palisades East. Like the lower peat/forest bed, the middle peat/forest bed is conspicuous because it contains occasional large horizontal logs. Generally, organic preservation is poor, but it often contains brown silts 30 –50 cm thick, probably representing an interglacial paleosol. We were able to sample the middle peat only at two locations. At one of these, the peat was unusually thick (50 – 60 cm), wood-rich, and contained logs with roots. Logs were not identified to taxon, but had spiral cracks suggestive of Picea. The organic matrix is compressed and fragmented and composed of nearly 50% Sphagnum. There also are abundant fragments of leaves that appear to be Salix. Seeds of Cyperaceae are present; those that could be identified to genus are all Carex. We recovered small leaves and woody stems that are tentatively identified as an ericad, and anthers from male catkins of Betula or possibly Alnus. No Betula fruits or bracts are present, however, nor is there any macrofossil evidence of Picea or any other arboreal taxon, other than the occasional log. Unlike the lower peat, it contains no aquatic components. The combined presence of large logs along with significant Sphagnum indicates that the lower peat records a moist environment, probably within a mature spruce forest. The middle peat/forest bed occurs a few meters below the SCt, dating it to at least MIS 7. However, it is near the contact of units 2 and 3, a suspected unconformity. The only other MIS 7 site reported in Beringia is at Ash Bend in Yukon (Westgate et al., 2001). There, the SCt occurs immediately above MIS-7 organic material. Therefore, it is possible that the middle peat/forest bed dates to an even older interglaciation. Upper peat/forest bed. The most prominent horizon at Palisades East is a thick, laterally extensive peat/buried forest deposit in the upper third of the bluffs (height of 40 – 60 m) containing abundant remains of large trees (Picea and Betula). The stratigraphy of the upper peat is not uniform. In a few sections, mostly in the east and west margins of the bluffs, it is only ⬃20 cm thick, with few large wood fragments, while in other areas, mostly in the central bluffs, it is a 2- to 3-m-thick wood-laden buried forest deposit. In about half of the exposures, it occurs as two wood-rich zones (up to 3 m) separated by ⬃1–2 m of weakly laminated to massive silts. The upper peat is assigned to MIS 5 because it is 2–5 m above the OCt and is capped by a typical late Wisconsinan ice-wedge/alluvial loess sequence followed by clearly recognizable Holocene deposits (Bege´t et al., 1991). Bege´t et al. (1991) also reported that this peat occurred in two distinct layers, but provided no explanation for it. In 2000, we discovered beaver-chewed wood (up to 15 cm in diameter) in great abundance in portions of the lower woodrich zone, suggesting that sometime early in MIS 5, a beaver dam flooded a portion of the boreal forest. We suggest that

the lower wood-rich zone preserves the predam boreal forest, the middle zone of bedded silts represents either the pond sediments or the diverted stream course, and the upper wood-rich zone preserves the successional return to boreal forest. Preservation of wood and other plant material in the upper peat is excellent. Logs of spruce and paper birch up to 30 cm in diameter and a few meters long are present, some with attached roots. Most are lying horizontally, but some are in rooted position. Many birch trees still retain their delicate paper-like white bark, and near-pristine spruce cones are preserved. Spruce logs from the upper peat can still be burned, and retain fine, intact internal structure. Macrofossil analyses of the upper wood-rich zone revealed two types of organic matrices, reflecting very different habitats. One is light brown, pungent, and rich in Sphagnum but contains no arboreal taxa. Over 90% of this matrix type is composed of tightly matted (compressed) Sphagnum moss, but includes minor amounts of ericale leaves (most closely resembling Oxycocccus microcarpus— bog cranberry), grass sheaths, wingless fruits of Betula, and seeds of Vaccinium. The community represented by this assemblage can still be found in modern boreal forest ecosystems in wet acidic bogs and near the edge of shallow ponds or where there are seeps and/or high groundwater. The second matrix type is dark brown, fibrous, and contains a greater variety of macrofossils, mostly borealassociated taxa, but little moss of any type (small amounts of Sphagnum were found). Approximately one-third of all macrofossils in the second matrix type are fruits and female catkins of Betula, most likely Betula papyrifera, based on the abundance of its wood. Seeds (perigenia) from at least two species of Carex are numerous, as are three other types of sedges (identification to genus was not possible). Polygonum and Rumex are present, along with needles and cones from Picea. There are a few taxa which grow in watersaturated or aquatic environments, including Calla palustris (water arum) and a seed that we have tentatively identified as Menyanthes trifoliata (buck bean). When collecting samples, we were not aware that the upper peat preserves two distinct habitats, thus are unable to provide relational information about them. Presumably, they reflect microhabitats and seral successional stages that occur in paludified, lowland boreal forest, and do not necessarily reflect any climatically driven change in ecosystem structure over time.

Mammal remains Large mammals Early investigators at the Palisades collected bones of mammoth (Mammuthus primigenius), bison (Bison priscus), and muskox (Ovibos moschatus) (Spurr 1898; Maddren, 1905; Gillmore, 1908, Quakenbush, 1909; Eardley,

P. Matheus et al. / Quaternary Research 60 (2003) 33– 43

1938). Apparently, all were detrital finds on the river bank and debris slides. The U.S. Fish and Wildlife Service in Galena, Alaska, maintains an informal collection of bones picked up at the Palisades; we examined the collection and found it contains mostly bison, mammoth, and caribou (Rangifer tarandus), with very little horse (Equus lambei) or muskox. Overall, these finds and the morphology of the specimens suggest a late Pleistocene age. However, Eardley (1938) reported finding a mammoth tusk in situ from a basal layer that he claimed was early Pleistocene. The pre-Wisconsinan mammal record in eastern Beringian, especially Alaska, is poor. Consequently, we excavated and test-screened sediments surrounding older chronostratigraphic markers, but recovered no bones. We found 12 bones of large mammals lying loose on colluvium and at the base of bluffs. In addition, we recovered 24 bones of large mammals and shells from the fresh water snail Succinea strigata in situ in a slump block, which had slid from high in the bluffs to near river level. The bones and snails, along with numerous small sticks, were concentrated in a ⬃20-cm bed, most likely a fluvial trash layer or an alluvial wash deposit. Radiocarbon dates on three bones in the slump block confirm that it slid from high in the section: 26,010 ⫾ 290 14 C yr B.P. (Rangifer tarandus, Beta-110937), 26,300 ⫾ 300 14C yr B.P. (Bison priscus, Beta-110938), and 27,890 ⫾ 300 14C yr B.P. (Mammuthus primigenius, Beta-110939). Two pieces of wood from the same bed returned infinite radiocarbon dates (⬎50,320 14C yr B.P. (Beta-110940); ⬎51,270 14C yr B.P. (Beta-110941)), indicating that old wood (probably from the upper peat) has been reworked into sections of the upper bluffs. The large mammal taxa we recovered (both in situ and detritally) were bison, mammoth, and caribou, common elements of late Pleistocene mammal assemblages in eastern Beringia (Guthrie, 1968, 1990). We did not recover any horse, and they are rare in other Palisades reports and the USFWS collection. Horses were common during the lateWisconsinan glaciation elsewhere in Alaska and Yukon. Recent studies by Elias et al. (1996), Goetcheus and Birks (2001), and Guthrie (2001) suggest that late Pleistocene ecosystems in far western Alaska, including much of the Bering Land Bridge, may have supported more mesic vegetative communities with more tundra elements than elsewhere in Beringia. Such vegetation would have been less productive for grazing, arid-adapted horses. The lack of horses at the Palisades suggests that the boundary between this Bering Land Bridge mesic belt and more arid environments of central Alaska and Yukon may have extended farther east than previously thought. Small mammals We wet-screened (1.6 mm mesh) ⬃1 m3 of sediment from the ca. 27,000-year-old bone-rich bed in the slump block, and found it to be rich in teeth of microtine rodents.

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Taxa recovered were singing vole (Microtus miurus), brown lemming (Lemmus trimucronatus), tundra vole (Microtus oeconomus), and collared lemming (Dicrostonyx torquatus). Their respective abundance ratios in the assemblage are approximately 3:2:1:1. All still occur in Alaska today; all but Dicrostonyx (which occupies coastal tundra) occur in central Alaska. The assemblage includes four of the six species normally occurring in eastern Beringia during the late Pleistocene (Guthrie, 1968; Morlan, 1984, 1989). Missing are Clethrionomys rutilus (red-backed vole), and Microtus xanthognathus (yellow-cheeked vole), but this is not surprising as neither probably appeared in eastern Beringia until the last few millennia of the Pleistocene (Guthrie, 1968; Morlan, 1989). A modern microtine assemblage from the area’s wet taiga forest and surrounding hillsides today would include Lemmus trimucronatus and Microtus oeconomus, along with Microtus xanthognathus, Microtus pennsylvanicus (meadow vole), and Synaptomys borealis (northern bog lemming) (Lehmkuhl, 2000). The four microtine species recovered occupy a range of habitats today. In general, Lemmus and Microtus oeconomus utilize wet habitats while Dicrostonyx and Microtus miurus are found in drier habitats. Lemmus and Microtus oeconomus can co-occur in generally flat, wet tundra areas where they feed on the bases of grasses and sedges. However, in many areas of northern Alaska where Lemmus is abundant, Microtus oeconomus is mostly absent (Bee and Hall, 1956). Ground cover appears to be more important to Lemmus and it occupies a variety of landforms from polygonal ground to alluvial fans, whereas Microtus oeconomus prefers extensive, continuous wet grass-sedge communities (Bee and Hall, 1956; Youngman, 1975; Guthrie, 1968). Dicrostonyx inhabits cold dry tundra habitats but only occurs today in western and northern coastal Alaska where its range seems to be tightly associated with upland Eriophorum (cotton grass) meadows. Lemmus and Dicrostonyx, while often co-occurring on tundra in northern and western Alaska, rarely overlap in specific habitats: Lemmus mainly inhabits wet lowland sites, whereas Dicrostonyx occupies drier ground (Bee and Hall, 1956). Microtus miurus avoids wet areas and is generally found in vegetation dominated by dwarf willow (Salix). Its preference for dwarf willow is twofold: willow leaves and leaves of other forbs occurring with dwarf willow (e.g., Lupinus, Astragalus, Petasites, Oxytropis, Hedysarum, Draba, and Equisetum) are a principal component of its diet, and it seems to have a strict requirement for the overhead cover (anti-predation) afforded by willows. Overhead cover is important because Microtus miurus is predominantly diurnal (Bee and Hall, 1956; Guthrie, 1968). The fact that nearly half the specimens recovered in the ca. 27,000-yr-old bone bed were Microtus miurus indicates that the region was covered by a drier and better drained herbaceous plant community than today, probably with dwarf willow and a variety of mesophylous to xerophylous forbs, but there must have been significant habitat diversity

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with some moister habitats available as evidenced by the presence of Lemmus sibiricus and Microtus oeconomus. Summary and conclusion The Palisades preserve the most continuous record of Plio-Pleistocene terrestrial environments yet reported for Alaska. Its abundant tephras allow absolute and relative dating of horizons and stratigraphic correlation to sites elsewhere in Alaska and Yukon. We report the farthest west occurrences of the PA, EC, and Mining Camp tephras, and the first occurrence of the PAL and PAU tephras. The PA and EC tephras bracket a lower buried forest unit, which we therefore correlate to the Dawson Cut Forest Bed. This provides the first clear evidence that the PA tephra underlies and predates the DCFB, and helps refine the age of DCFB to just under 2.02 myr. Similarly, ice-wedge casts infilled by PA tephra record the oldest known occurrence of permafrost in Alaska (2.02 myr). Further study into the relative identities and stratigraphies of the PA and PAL tephras is warranted. The limited number and type of plant macrofossils recovered from three buried forest units at the Palisades suggest that moist boreal forests floristically similar to the modern lowland boreal forests occurred in west-central Alaska during interglacial periods of the past 2 myr. Even though all mammal fossils recovered at the Palisades so far are late Pleistocene age, the Palisades still stands as perhaps the most promising site in Alaska to prospect for pre-late Pleistocene mammals, due to the presence of abundant dated horizons and sedimentary processes that concentrate bone. Acknowledgments John Westgate was integral to interpretations of the regional tephrochronology and identification of the PAL tephra. This work was funded in part by the Nowitna National Wildlife Refuge, U.S. Fish and Wildlife Service. We thank Pete Dematteo, Fred Huntington, Heather Johnson, and Debra Corbett for their assistance at the USFWS offices in Galena and Anchorage. Additional funding was provided for tephra analyses by the Alaska Volcano Observatory, U.S. Dept. of the Interior. Glenn Berger provided valuable assistance and discussion in the field. We thank Nora Foster for identifying snails and bivalves. This paper was improved by comments on earlier drafts from John Westgate, Duane Froese, John Storer, Nancy Bigelow, and Alan Gilllespie. References Bee, J.W., Hall, E.R., 1956. Mammals of Northern Alaska. Allen Press, Lawrence, KS.

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