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

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

Paul Matheus,a,* James Beget,b Owen Mason,c and Carol Gelvin-Reymillerd

a Alaska Quaternary Center and the Institute of Arctic Biology, University of Alaska Fairbanks, Fairbanks, AK 99775-5540, USAb 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, USAd 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 MiningCamp (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 buriedforest horizons are present in association with dated tephras. The uppermost forest bed occurs immediately above the OCt; the middle foresthorizon 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 allfrom common late Pleistocene taxa. Those recovered in situ came from a single horizon radiocarbon dated to ca. 27,000 14C yr B.P. Theincongruous 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 ofunconsolidated sediments stretching nearly 11 km along thesouthern banks of the Yukon River in central Alaska, USA(Figs. 1 and 2). The bluffs are referred to locally as “TheBoneyard” because Pleistocene-age bones regularly erodefrom their faces.

The sheer walls, erosional gullies, massive ice bodies,and bones of Pleistocene mammals exposed at the Palisadescaught the attention of early surveyors exploring centralAlaska (Russell, 1890; Spurr, 1898; Maddren, 1905; Gil-more, 1908). Later, Eardley (1938) reported on the generallithology in the context of sedimentary processes of the

Yukon River, and Yeend (1977) described the Tertiarydeposits that underlie the bluffs. The discovery of the OldCrow tephra (OCt) (140,000 � 20,000 yr) in the upperportions of the bluffs by Beget et al. (1991) suggested therewas considerable time depth recorded in lower sections ofthe Palisades. Young and Walker (1982) noted the occur-rence of plant macrofossils, including large logs, and bonesof large mammals, but the Palisades’ paleontological re-sources have never been studied in the context of modernpaleoecological and paleoclimatic paradigms in Beringia.Here, we report preliminary data and conclusions regardingthe age, major stratigraphic units, and paleoenvironmentalrecord of the Palisades.

General site description

The Palisades Site (65° 6� N, 153° 15� W) is located 55km downstream from the village of Tanana, in the Nowitna

* Corresponding author. Alaska Quaternary Center, CSEM, 900 YukonDrive, University of Alaska Fairbanks, Fairbanks, AK 99775-5940. Fax:�1-907-474-6293.

E-mail address: [email protected] (P. Matheus).

R

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0033-5894/03/$ – see front matter © 2003 University of Washington. Published by Elsevier Inc. All rights reserved.doi:10.1016/S0033-5894(03)00091-7

Wildlife Refuge (Fig. 1). The regional valley fill is mostlyunconsolidated loess and alluvium, forming a broad table-land into which the river is moderately incised (Eardley,1938). The bluffs of the Palisades, comprised mostly ofperennially frozen silts, are an escarpment formed by lateralmigration and erosion of the Yukon River into a portion ofthe tableland that has been uplifted by movement along theKaltag Fault (Patton and Hoare, 1968). The escarpmentrises from east to west, the tallest sections being approxi-mately 75 m (Fig. 3). On the eastern limit, the silt bluffsreside unconformably on unconsolidated to loosely consol-idated late Miocene sands and sandstones interbedded withlignite seams. On the western limit, a bedded, late Mioceneconglomerate underlies the silts unconformably (Yeend,1977).

The bluff faces are steep due to active undercutting, andrunoff from melting permafrost bodies causes extensivegullying, making it possible to study sections in three di-mensions. However, these processes also generate largeslump blocks and colluvium that present interpretive diffi-culties.

The Palisades actually consists of two escarpments: “Pal-isades East” and “Palisades West,” 8 and 3 km long, re-spectively, and separated by a 3-km zone of stable knolls(Fig. 1). This paper only reports on Palisades East and doesnot discuss the Tertiary sequences.

Lithostratigraphy and major chronostratigraphichorizons

We divided the silt sequences at Palisades East into fivelithostratigraphic units (Fig. 3), and identified 10 majorchronostratigraphic horizons—seven tephras and three peatbeds (Figs. 3 and 4). Most horizons were mapped discon-tinuously in Figure 3 because of colluvium or inaccessibil-ity, and some horizons were visible only where there wasrecent slumping.

Lithostratigraphic units (Fig. 3)

Unit 1 is a thin, finely bedded silt and clay unit at thebase of the bluffs. It extends less than 1 m above springwater level and was not studied in detail.

Unit 2 designates alternating laminated and massivesilt-loams in the lower third of the bluffs. Laminatedsediments were probably deposited in low-energy fluvialenvironments in close association with the paleo-YukonRiver. Occasional forest beds and other terrestrial faciesare present low in the unit, and it is likely that themassive components are eolian, indicating that Unit 2preserves both fluvial and subaerially exposed facies.Numerous small erosional unconformities may bepresent, and one or more major unconformities probably

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

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

separate Units 2 and 3. We never located a clear contactbetween Units 2 and 3. Tephras near the base of Unit 2place its age near the Plio-Pleistocene boundary.

Unit 3 is composed of primary and secondary loessmanifested as massive to weakly laminated greyish-tan siltand fine sand, with orange ferric oxidation staining and thinorganic horizons. Elsewhere in eastern Beringia, similarloess deposits reflect a pattern of temporarily stable surfacesalternating with periods of higher rates of airfall loess(Pewe, 1975a; Beget, 1988, 2001).

Unit 3 was deposited during marine oxygen isotope stage(MIS) 6 as indicated by a suite of chronostratigraphic mark-ers discussed below. The contact between Unit 3 and Unit 4is abrupt, marked by a thick buried forest horizon (attribut-able 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 isa significant find because it indicates that not all permafrostmelted in central Alaska during MIS 5, contrary to argu-ments of Pewe (Pewe, 1975a; Pewe et al., 1997), and itmeans that the Palisades may preserve MIS-6 ice for futurestudies.

Unit 4 is an ice-rich muck-loam unit of variable thick-ness. It is rich in large ice bodies, probably both syngeneticand epigenetic. Individual ice bodies extend downwardfrom Holocene deposits above (Unit 5) into the MIS-5interglacial forest bed below. Ice-rich muck-loams of this

sort are common valley fill in central Alaska, and typicallyform from reworked loess (Pewe, 1975a; Hamilton et al.,1988; Beget, 2001).

Unit 5 demarcates Holocene peat and silt-loam. Thebasal portion is loess-like and contains buried forest mate-rial (correlative with the Ready Bullion Formation of Pewe,1975b). There are modern epigenetic ice wedges in this unitand polygonal ground on the surface. Unit 5 is capped bypaludified, black spruce forest with frequent muskegs and athick layer of forest duff and mosses.

Major chronostratigraphic markers and horizons(Figs. 3 and 4)

We traced 10 major horizons and chronostratigraphicmarkers across the lateral surface of Palisades East (de-scribed 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) asporadic middle peat/forest bed, (8) Sheep Creek tephra, (9)Old Crow tephra, and (10) a thick upper peat/forest bed. Wealso observed numerous organic-rich subhorizons whichmay record early to mid Pleistocene warm periods or pa-leosols, but they are small and discontinuous, making cor-relation difficult.

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 PalisadesEast 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 upperpeat (middle peat covered by colluvium).

35P. Matheus et al. / Quaternary Research 60 (2003) 33–43

Tephras and associated featuresTephras are numerous at the site, especially in lower

sections. Several correlate with dated tephras found else-where in Alaska and the Yukon. Previously, only the OCthad been identified from the Palisades (Beget et al., 1991).

Tephras in Alaska and Yukon are classified as eitherType I or Type II based on their source area, mineralogy andgeochemistry. Type I originate from vents in the AleutianArc-Alaska Peninsula region; Type II originate in theWrangell volcanic field (Preece et al., 2000; Westgate et al.,2001).

PA tephra. A pink Type I tephra occurs at elevations of 2–7m in the eastern half of Palisades East (Fig. 3). It occurs inboth massive and bedded silts, and typically is stratigraph-ically just below the lowest peat/forest bed. Tephra bedsvary from thin interdigitated lenses a few millimeters thickto pods up to 15 cm in thickness. The field occurrence andstratigraphic position of this tephra are similar to the PALtephra, and their relative ages and identities are still unre-solved (see below).

Samples of this tephra analyzed at the Alaska Tephro-chronology Center (ACT 2421 and ACT 2423) have major-element glass geochemistries that are statistically indistin-guishable from the PA tephra at Gold Hill and CrippleSump (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 (West-gate et al., 2000).

PAL tephra. On a separate occasion, samples were collectedfrom tephra beds fitting the description for the PA tephraabove, except these beds were traced over a greater lateralextent, and were found at elevations only 1–2 m abovewater level near the east and west limits, but �14 m abovewater level in the central bluff faces (Fig. 3). Like the PAtephra, this tephra also is found just below the lower peat/forest bed.

Two samples (UT1280 and UT1281) were submitted tothe University of Toronto for geochemical analyses. JohnWestgate (written communication, 2002) identified both asbelonging to a single tephra having a geochemistry andglass composition intermediate between the PA and ECtephras described by Preece et al. (1999) near Fairbanks.Westgate et al. (2000, 2003) suggest that the PA and ECtephras are comagmatic, with the EC eruption postdatingthe PA eruption by a few millennia. Thus, samples UT1280and UT1281 may represent a third, intermediate eruptionthat we informally name the PAL tephra (also see Westgateet al., 2003). This description presumes that the PAL tephrapredates the PA tephra, but we could not determine theirstratigraphic relation because we were unaware during sam-pling that they represent two different tephras.

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


EC tephra. Twenty five centimeters above the lowermostpeat/forest bed, are pods (1 cm thick) of a gray, Type I ashin a 20-cm-thick gleyed zone of massive silt. Electron mi-croprobe analyses (ACT 2418) indicate that this tephraclosely resembles the EC tephra that Westgate et al. (2000,2003) described from Engineer Creek, near Fairbanks (Ta-ble 1). It is geochemically distinguishable from the PAtephra by its lower SiO2 concentrations, less evolved glasscomposition, and stratigraphic position.

PAU tephra. A fourth Type I tephra (ACT 2424 and ACT2418),) occurs 10 cm above the EC tephra, where it ispreserved as 1–2 mauve ash beds several centimeters thick.No tephras of correlative age are known from anywhere inBeringia, and we propose informally naming this ash bedthe PAU tephra. The PAU tephra is broadly similar to theunderlying EC tephra, and also the PA and PAL tephras,and apparently records another comagmatic eruption closein age to those events (Table 1). However, the PAU tephracan be distinguished from the EC by its lower Al2O3, FeO,

and CaO concentrations, and higher SiO2 and K2O concen-trations, and can be distinguished from the PA tephra by itsslightly lower SiO2, and K2O concentrations, and higherAl2O3 concentrations. Analysis of grains of PAU tephrarevealed some with compositions very similar to the under-lying EC tephra, while no EC chemistries were identifiedwithin the PA tephra. Although the EC tephra was foundonly at one site, where it was preserved as pods of ashwithin a presumed paleosol, the PAU tephra could be tracedlaterally for many meters across the exposures at the Pali-sades, and everywhere clearly lies above the lower peat/forest bed.

Mining Camp tephra. A Type II tephra (ACT 2417) occursin section directly above the EC and PAL tephras. Thistephra is more silicic and has lower FeO and Al2O3 com-positions than the PA, PAL, PAU, and EC ashes. Westgateet al. (2003) have identified a tephra of very similar com-position in the Fairbanks area that they named the MiningCamp tephra. Based on the chemical similarities and strati-graphic position, tephra ACT 2417 is correlated with theMining Camp tephra.

Sheep Creek tephra (SCt). A white Type II tephra (ACT2412, ACT 2414, ACT 2416, and ACT 2420) is preservedin very thin beds 0.5–2 cm thick that are easily observed atelevations of 25–30 m at both Palisades East and West. Itoccurs about 4–8 m below the OCt and the uppermostforest bed and about 5 m above the middle forest bed (Figs.3 and 4). We identified this unit as the Sheep Creek tephrausing electron microprobe analyses (Table 1). Thermolumi-nescence (TL) dating of sediments surrounding the SCtelsewhere in Alaska and Yukon indicate its age to be about190,000 yr (Berger et al., 1996).

Old Crow tephra (OCt). The Old Crow tephra at the Pali-sades is preserved as a prominent white ash horizon occur-ring 2–5 m below the upper forest bed (Figs. 3 and 4). Theaverage IPFT date for the OCt is 140,000 � 10,000 yr(Westgate, 1988, 1989), correlating the upper peat/forestbed with MIS 5 and the Eva Creek forest bed in the Fair-banks area (Beget et al., 1991; Pewe et al., 1997; Muhs etal., 2001).

Age of the Dawson Cut Forest Bed. In the Fairbanks area,the EC tephra occurs immediately above the Dawson CutForest Bed (DCFB) (Pewe 1975b). The PA tephra, whichpresumably predates the EC, has not been found at sitescontaining the DCFB (Preece et al., 1999; Westgate et al.,2003). At Palisades East, the EC tephra occurs immediatelyabove the lowest buried forest bed. The comagmatic PALand PA tephras, while never found in direct association withthick forest deposits, both occur slightly below and at timesinterdigitated with organic stringers that can be traced lat-erally to the forest bed. Consequently, we correlate thelower forest bed with the DCFB. Furthermore, the relation

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


of the PA tephra and DCBF at Palisades East indicates theage of DCFB to be �2.02 � 0.14 myr. These stratigraphicrelations also corroborate Westgate et al.’s (2000, 2003)assertion that the EC tephra is slightly younger than the PAtephra.

Ancient solifluction, permafrost, and plant root-casts pre-served by the PA tephra. In some exposures at PalisadesEast, beds of PA tephra are highly convoluted and evenoverturned, suggesting downslope movement by solifluc-tion and other forms of cryoturbation. About 1500 m west ofthe bluffs’ eastern limit we found the PA tephra slumpedinto a wedge-shaped collapse feature roughly 1.5 m wide atthe top and 2 m deep, which we interpreted to be anice-wedge cast. This evidence indicates that permafrost ex-isted in central Alaska shortly before 2.02 myr ago, and justbefore the development of the DCFB.

Based on the distribution of active ice wedges today inAlaska, it has been argued that ice wedges form where meanannual air temperatures (MAT) are �7° C or colder (Pewe1975a). But it since has been shown that ice wedges canform where MAT is as high as �3° to �4°C (Hamilton et

al., 1983), depending on factors such as winter snow depthand rapidity of cooling. Today, MAT in central Alaska nearthe Palisades is �2° to �4°C and there is active ice wedgeformation at colder microsites. The presence of ice-wedgecasts immediately below the PA tephra indicates that theMAT in central Alaska at the end of the Pliocene was atmost �3°C, but probably closer to �7°C

The discovery of ice-wedge casts with inclusion of PAtephra at the Palisades constitutes the oldest evidence ofpermafrost 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 oldestminimum 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 caststypically 2–8 cm long, 1–3 mm wide, in densities of �1cast per 5–30 mm (Figs. 5b and c). They clearly are notcasts of tree roots, and they do not resemble tundra roots,which are thicker, less uniformly spaced, less straight, andhave chaotic, subhorizontal orientations. They best indicatean homogeneous vegetation such as a grass or sedgemeadow. In one location, the tephra and root casts are

Table 1Major element chemistry of glass shards from Tephras at the Palisades site, central Alaska

Sample names Category Na2O MgO Al2O3 SiO2 CI K2O CaO TiO2 Fe2O3 H2O

OLD CROW TEPHRAACT 2415 ave (n � 39) 3.48 0.29 13.10 75.54 0.27 3.75 1.41 0.26 1.89 3.36

std dev 0.13 0.03 0.12 0.31 0.03 0.15 0.08 0.22 0.13 0.97ACT 2422 ave (n � 44) 3.51 0.30 13.02 75.47 0.27 3.81 1.44 0.26 1.92 3.50

std dev 0.16 0.05 0.15 0.32 0.03 0.14 0.07 0.16 0.17 3.27SHEEP CREEK TEPHRAACT 2412 ave (n � 22) 4.54 0.69 15.71 72.13 0.05 2.03 2.49 0.33 2.05 3.56

std dev 0.20 0.08 0.30 0.76 0.02 0.11 0.18 0.19 0.20 2.53ACT 2414 ave (n � 14) 4.53 0.71 15.73 72.04 0.05 1.96 2.61 0.27 2.11 4.42

std dev 0.47 0.11 0.53 0.81 0.03 0.17 0.24 0.25 0.27 2.86ACT 2416 ave (n � 6) 4.23 0.75 15.81 72.22 0.03 2.06 2.50 0.29 2.11 3.88

std dev 0.21 0.07 0.34 1.03 0.03 0.18 0.30 0.21 0.23 2.91ACT 2420 ave (n � 8) 4.03 0.72 15.64 72.55 0.07 2.06 2.53 0.21 2.19 4.41

std dev 0.65 0.11 0.38 0.29 0.06 0.14 0.16 0.23 0.32 2.90MINING CAMP TEPHRAACT 2417 ave (n � 40) 3.49 0.26 13.07 77.40 0.25 2.72 1.24 0.28 1.28 5.23

std dev 0.23 0.03 0.15 0.36 0.03 0.13 0.05 0.28 0.10 2.23PAU TEPHRAACT 2421 ave (n � 28) 4.06 0.51 13.88 71.47 0.51 4.21 1.94 0.56 2.87 3.31

std dev 0.22 0.16 0.63 1.76 0.06 0.30 0.44 0.31 0.56 0.00EC TEPHRAACT 2418 ave (n � 17) 3.95 0.62 14.35 70.49 0.48 3.87 2.77 0.54 2.80 3.36

std dev 0.33 0.10 0.54 0.80 0.06 0.15 0.32 0.18 0.40 2.07PA TEPHRAACT 2423 ave (n � 17) 3.73 0.29 13.02 74.17 0.53 4.44 1.34 0.29 2.20 5.28

std dev 0.34 0.04 0.24 0.46 0.13 0.21 0.12 0.22 0.19 0.87ACT 2424 ave (n � 12) 3.68 0.32 13.27 73.86 0.58 4.39 1.44 0.25 2.21 4.94

std dev 0.13 0.04 0.16 0.46 0.05 0.12 0.08 0.18 0.23 0.70

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 Beget et al. (1991).


completely overturned in the leading edge of a rolled solif-luction lobe (Fig. 5c).

Peats, forest beds and plant macrofossilsLower peat/forest bed (Fig. 5). This wood and organic-richpeat complex occurs low in the bluffs of Palisades East,within lithostratigraphic Unit 2. It is most pronounced ineastern sections, where it is near water level, and centralsections, where it is as high as 14 m (Fig. 5a).

The presence of occasional large tree trunks, mostlyisolated and horizontal, but occasionally rooted, makesthe lower peat stand out as a stratigraphic unit. However,the stratigraphy and composition of the peat complex isvaried and organics are sparse. Most often, it is a 5- to20-cm-thick compressed subpeat layer with poor organicpreservation. In other exposures, it occurs as brown (or-ganic-rich) massive silt beds 3–10 cm thick containingisolated wood fragments. In its poorest expression, thelower peat is represented by alternating laminae (�1–3cm) of brown organic-rich and gray organic-poor siltsand fine sands, where the organic-rich laminae containsmall (�1 cm diameter) wood fragments and/or gradeinto subpeaty pods.

In most cases, the lower peat complex seems to bepreserving overbank types of facies, where the paleo YukonRiver or a slough periodically flooded low-lying terrestrialenvironments. Because there are fluvial components, someof the large tree trunks may have been deposited by floods.We recovered tree sections up to 40 cm in diameter and 3 mlong, but they have not been identified to taxon. Theirspirally cracked wood structure suggests they are Picea(spruce).

Rarely, the lower peat occurs locally as a thick, com-pact organic mat layer 1 m thick. Macrofossil analyses ofone of these mats yielded few intact plant parts, but wedid identify carbonized, friable needles and smallbranches of Picea; fragments of graminoids and Equise-tum; and pholidia of Sphagnum; plus statoblasts and adultstructures of the bryozoan Cristatella mucedo. The pres-ence of adult Cristatella indicates that standing freshwa-ter was in the immediate vicinity. The cumulative pres-ence of Picea, Equisetum, and Sphagnum indicates amoist, lowland spruce forest characteristic of low-lyingareas of Interior Alaska today. The lower peat/forest bedis correlated with the DCFB of Pewe (1975b) (see dis-cussion above).

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.


Middle peat/forest bed. This horizon is weakly expressed,discontinuous, and difficult to access, but stands out as arecurring feature about halfway up bluffs of Palisades East.Like the lower peat/forest bed, the middle peat/forest bed isconspicuous because it contains occasional large horizontallogs. Generally, organic preservation is poor, but it oftencontains brown silts 30–50 cm thick, probably representingan interglacial paleosol.

We were able to sample the middle peat only at twolocations. 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 crackssuggestive of Picea. The organic matrix is compressed andfragmented and composed of nearly 50% Sphagnum. Therealso are abundant fragments of leaves that appear to beSalix. Seeds of Cyperaceae are present; those that could beidentified to genus are all Carex. We recovered small leavesand 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 thereany macrofossil evidence of Picea or any other arborealtaxon, other than the occasional log. Unlike the lower peat,it contains no aquatic components. The combined presenceof large logs along with significant Sphagnum indicates thatthe lower peat records a moist environment, probably withina mature spruce forest.

The middle peat/forest bed occurs a few meters belowthe SCt, dating it to at least MIS 7. However, it is near thecontact of units 2 and 3, a suspected unconformity. The onlyother MIS 7 site reported in Beringia is at Ash Bend inYukon (Westgate et al., 2001). There, the SCt occurs im-mediately above MIS-7 organic material. Therefore, it ispossible that the middle peat/forest bed dates to an evenolder interglaciation.

Upper peat/forest bed. The most prominent horizon at Pal-isades East is a thick, laterally extensive peat/buried forestdeposit in the upper third of the bluffs (height of 40–60 m)containing abundant remains of large trees (Picea and Bet-ula). The stratigraphy of the upper peat is not uniform. In afew sections, mostly in the east and west margins of thebluffs, it is only �20 cm thick, with few large wood frag-ments, while in other areas, mostly in the central bluffs, it isa 2- to 3-m-thick wood-laden buried forest deposit. In abouthalf of the exposures, it occurs as two wood-rich zones (upto 3 m) separated by �1–2 m of weakly laminated tomassive silts. The upper peat is assigned to MIS 5 becauseit is 2–5 m above the OCt and is capped by a typical lateWisconsinan ice-wedge/alluvial loess sequence followed byclearly recognizable Holocene deposits (Beget et al., 1991).

Beget et al. (1991) also reported that this peat occurredin two distinct layers, but provided no explanation for it. In2000, we discovered beaver-chewed wood (up to 15 cm indiameter) in great abundance in portions of the lower wood-rich zone, suggesting that sometime early in MIS 5, a beaverdam flooded a portion of the boreal forest. We suggest that

the lower wood-rich zone preserves the predam boreal for-est, the middle zone of bedded silts represents either thepond sediments or the diverted stream course, and the upperwood-rich zone preserves the successional return to borealforest.

Preservation of wood and other plant material in theupper peat is excellent. Logs of spruce and paper birch up to30 cm in diameter and a few meters long are present, somewith attached roots. Most are lying horizontally, but someare in rooted position. Many birch trees still retain theirdelicate paper-like white bark, and near-pristine sprucecones are preserved. Spruce logs from the upper peat canstill be burned, and retain fine, intact internal structure.

Macrofossil analyses of the upper wood-rich zone re-vealed two types of organic matrices, reflecting very differ-ent habitats. One is light brown, pungent, and rich in Sph-agnum but contains no arboreal taxa. Over 90% of thismatrix type is composed of tightly matted (compressed)Sphagnum moss, but includes minor amounts of ericaleleaves (most closely resembling Oxycocccus microcarpus—bog cranberry), grass sheaths, wingless fruits of Betula, andseeds of Vaccinium. The community represented by thisassemblage can still be found in modern boreal forest eco-systems in wet acidic bogs and near the edge of shallowponds or where there are seeps and/or high groundwater.

The second matrix type is dark brown, fibrous, andcontains a greater variety of macrofossils, mostly boreal-associated taxa, but little moss of any type (small amountsof Sphagnum were found). Approximately one-third of allmacrofossils in the second matrix type are fruits and femalecatkins of Betula, most likely Betula papyrifera, based onthe abundance of its wood. Seeds (perigenia) from at leasttwo species of Carex are numerous, as are three other typesof sedges (identification to genus was not possible). Poly-gonum and Rumex are present, along with needles and conesfrom Picea. There are a few taxa which grow in water-saturated or aquatic environments, including Calla palustris(water arum) and a seed that we have tentatively identifiedas Menyanthes trifoliata (buck bean).

When collecting samples, we were not aware that theupper peat preserves two distinct habitats, thus are unable toprovide relational information about them. Presumably,they reflect microhabitats and seral successional stages thatoccur in paludified, lowland boreal forest, and do not nec-essarily reflect any climatically driven change in ecosystemstructure over time.

Mammal remains

Large mammals

Early investigators at the Palisades collected bones ofmammoth (Mammuthus primigenius), bison (Bison pris-cus), and muskox (Ovibos moschatus) (Spurr 1898; Mad-dren, 1905; Gillmore, 1908, Quakenbush, 1909; Eardley,


1938). Apparently, all were detrital finds on the river bankand debris slides. The U.S. Fish and Wildlife Service inGalena, Alaska, maintains an informal collection of bonespicked up at the Palisades; we examined the collection andfound it contains mostly bison, mammoth, and caribou(Rangifer tarandus), with very little horse (Equus lambei)or muskox. Overall, these finds and the morphology of thespecimens suggest a late Pleistocene age. However, Eardley(1938) reported finding a mammoth tusk in situ from a basallayer that he claimed was early Pleistocene.

The pre-Wisconsinan mammal record in eastern Bering-ian, especially Alaska, is poor. Consequently, we excavatedand test-screened sediments surrounding older chronostrati-graphic markers, but recovered no bones. We found 12bones of large mammals lying loose on colluvium and at thebase of bluffs. In addition, we recovered 24 bones of largemammals and shells from the fresh water snail Succineastrigata in situ in a slump block, which had slid from highin the bluffs to near river level. The bones and snails, alongwith numerous small sticks, were concentrated in a �20-cmbed, most likely a fluvial trash layer or an alluvial washdeposit.

Radiocarbon dates on three bones in the slump blockconfirm that it slid from high in the section: 26,010 � 29014C 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 infiniteradiocarbon dates (�50,320 14C yr B.P. (Beta-110940);�51,270 14C yr B.P. (Beta-110941)), indicating that oldwood (probably from the upper peat) has been reworkedinto sections of the upper bluffs.

The large mammal taxa we recovered (both in situ anddetritally) were bison, mammoth, and caribou, commonelements of late Pleistocene mammal assemblages in east-ern Beringia (Guthrie, 1968, 1990). We did not recover anyhorse, and they are rare in other Palisades reports and theUSFWS collection. Horses were common during the late-Wisconsinan glaciation elsewhere in Alaska and Yukon.Recent studies by Elias et al. (1996), Goetcheus and Birks(2001), and Guthrie (2001) suggest that late Pleistoceneecosystems in far western Alaska, including much of theBering Land Bridge, may have supported more mesic veg-etative communities with more tundra elements than else-where in Beringia. Such vegetation would have been lessproductive for grazing, arid-adapted horses. The lack ofhorses at the Palisades suggests that the boundary betweenthis Bering Land Bridge mesic belt and more arid environ-ments of central Alaska and Yukon may have extendedfarther east than previously thought.

Small mammals

We wet-screened (1.6 mm mesh) �1 m3 of sedimentfrom the ca. 27,000-year-old bone-rich bed in the slumpblock, and found it to be rich in teeth of microtine rodents.

Taxa recovered were singing vole (Microtus miurus), brownlemming (Lemmus trimucronatus), tundra vole (Microtusoeconomus), and collared lemming (Dicrostonyx torqua-tus). Their respective abundance ratios in the assemblageare approximately 3:2:1:1. All still occur in Alaska today;all but Dicrostonyx (which occupies coastal tundra) occur incentral Alaska. The assemblage includes four of the sixspecies normally occurring in eastern Beringia during thelate Pleistocene (Guthrie, 1968; Morlan, 1984, 1989). Miss-ing are Clethrionomys rutilus (red-backed vole), and Micro-tus xanthognathus (yellow-cheeked vole), but this is notsurprising as neither probably appeared in eastern Beringiauntil the last few millennia of the Pleistocene (Guthrie,1968; Morlan, 1989). A modern microtine assemblage fromthe area’s wet taiga forest and surrounding hillsides todaywould include Lemmus trimucronatus and Microtusoeconomus, along with Microtus xanthognathus, Microtuspennsylvanicus (meadow vole), and Synaptomys borealis(northern bog lemming) (Lehmkuhl, 2000).

The four microtine species recovered occupy a range ofhabitats today. In general, Lemmus and Microtus oecono-mus utilize wet habitats while Dicrostonyx and Microtusmiurus are found in drier habitats. Lemmus and Microtusoeconomus can co-occur in generally flat, wet tundra areaswhere they feed on the bases of grasses and sedges. How-ever, in many areas of northern Alaska where Lemmus isabundant, Microtus oeconomus is mostly absent (Bee andHall, 1956). Ground cover appears to be more important toLemmus and it occupies a variety of landforms from polyg-onal ground to alluvial fans, whereas Microtus oeconomusprefers extensive, continuous wet grass-sedge communities(Bee and Hall, 1956; Youngman, 1975; Guthrie, 1968).Dicrostonyx inhabits cold dry tundra habitats but only oc-curs today in western and northern coastal Alaska where itsrange seems to be tightly associated with upland Eriopho-rum (cotton grass) meadows. Lemmus and Dicrostonyx,while often co-occurring on tundra in northern and westernAlaska, rarely overlap in specific habitats: Lemmus mainlyinhabits wet lowland sites, whereas Dicrostonyx occupiesdrier ground (Bee and Hall, 1956). Microtus miurus avoidswet areas and is generally found in vegetation dominated bydwarf willow (Salix). Its preference for dwarf willow istwofold: willow leaves and leaves of other forbs occurringwith dwarf willow (e.g., Lupinus, Astragalus, Petasites,Oxytropis, Hedysarum, Draba, and Equisetum) are a prin-cipal component of its diet, and it seems to have a strictrequirement for the overhead cover (anti-predation) af-forded by willows. Overhead cover is important becauseMicrotus miurus is predominantly diurnal (Bee and Hall,1956; Guthrie, 1968).

The fact that nearly half the specimens recovered in theca. 27,000-yr-old bone bed were Microtus miurus indicatesthat the region was covered by a drier and better drainedherbaceous plant community than today, probably withdwarf willow and a variety of mesophylous to xerophylousforbs, but there must have been significant habitat diversity


with some moister habitats available as evidenced by thepresence of Lemmus sibiricus and Microtus oeconomus.

Summary and conclusion

The Palisades preserve the most continuous record ofPlio-Pleistocene terrestrial environments yet reported forAlaska. Its abundant tephras allow absolute and relativedating of horizons and stratigraphic correlation to sites else-where in Alaska and Yukon. We report the farthest westoccurrences of the PA, EC, and Mining Camp tephras, andthe first occurrence of the PAL and PAU tephras. The PAand EC tephras bracket a lower buried forest unit, which wetherefore correlate to the Dawson Cut Forest Bed. Thisprovides the first clear evidence that the PA tephra underliesand predates the DCFB, and helps refine the age of DCFBto just under 2.02 myr. Similarly, ice-wedge casts infilled byPA tephra record the oldest known occurrence of permafrostin Alaska (2.02 myr). Further study into the relative iden-tities and stratigraphies of the PA and PAL tephras is war-ranted.

The limited number and type of plant macrofossils re-covered from three buried forest units at the Palisades sug-gest that moist boreal forests floristically similar to themodern lowland boreal forests occurred in west-centralAlaska during interglacial periods of the past 2 myr. Eventhough all mammal fossils recovered at the Palisades so farare late Pleistocene age, the Palisades still stands as perhapsthe most promising site in Alaska to prospect for pre-latePleistocene mammals, due to the presence of abundantdated horizons and sedimentary processes that concentratebone.

Acknowledgments

John Westgate was integral to interpretations of the re-gional tephrochronology and identification of the PAL te-phra. This work was funded in part by the Nowitna NationalWildlife Refuge, U.S. Fish and Wildlife Service. We thankPete Dematteo, Fred Huntington, Heather Johnson, and De-bra Corbett for their assistance at the USFWS offices inGalena and Anchorage. Additional funding was providedfor tephra analyses by the Alaska Volcano Observatory,U.S. Dept. of the Interior. Glenn Berger provided valuableassistance and discussion in the field. We thank Nora Fosterfor identifying snails and bivalves. This paper was im-proved by comments on earlier drafts from John Westgate,Duane Froese, John Storer, Nancy Bigelow, and AlanGilllespie.

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