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This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institution
and sharing with colleagues.
Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third party
websites are prohibited.
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A late–Middle Pleistocene (Marine Isotope Stage 6) vegetated surfaceburied by Old Crow tephra at the Palisades, interior Alaska
Alberto V. Reyes a,*, Britta J.L. Jensen a, Grant D. Zazula b, Thomas A. Ager c, Svetlana Kuzmina a,Catherine La Farge d, Duane G. Froese a
a Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canadab Yukon Palaeontology Program, Government of Yukon, P.O. Box 2703, Whitehorse, Yukon Y1A 2C6, Canadac United States Geological Survey, MS 980, Box 25046, Federal Center, Denver, CO 80225, USAd Department of Biological Sciences, University of Alberta, Edmonton, Alberta T6G 2E9, Canada
a r t i c l e i n f o
Article history:Received 29 July 2009Received in revised form27 November 2009Accepted 2 December 2009
a b s t r a c t
A 40 cm thick primary bed of Old Crow tephra (131 � 11 ka), an important stratigraphic marker ineastern Beringia, directly overlies a vegetated surface at Palisades West, on the Yukon River in centralAlaska. Analyses of insect, bryophyte, and vascular plant macrofossils from the buried surface andunderlying organic-rich silt suggest the local presence of an aquatic environment and mesic shrub-tundra at the time of tephra deposition. Autochthonous plant and insect macrofossils from peat directlyoverlying Old Crow tephra suggest similar aquatic habitats and hydric to mesic tundra environments,though pollen counts indicate a substantial herbaceous component to the regional tundra vegetation.Trace amounts of arboreal pollen in sediments associated with the tephra probably reflect reworkingfrom older deposits, rather than the local presence of trees. The revised glass fission-track age for OldCrow tephra places its deposition closer to the time of the last interglaciation than earlier age deter-minations, but stratigraphy and paleoecology of sites with Old Crow tephra indicate a late Marine IsotopeStage 6 age. Regional permafrost degradation and associated thaw slumping are responsible for the closestratigraphic and paleoecological relations between Old Crow tephra and last interglacial deposits atsome sites in eastern Beringia.
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1. Introduction
The late Cenozoic sediments of unglaciated Yukon and Alaskahost numerous tephra beds, sourced from volcanoes in the Aleutianarc and Wrangell Mountains, that have been used as chro-nostratigraphic markers for over half a century (e.g. Taber, 1943;Pewe, 1975; Preece et al., 2000; Jensen et al., 2008). The develop-ment of a detailed tephrostratigraphic framework in the region, inconcert with advances in the geochemical characterization (e.g.Pearce et al., 2004) and geochronology of tephra (e.g. Berger, 2003;Westgate et al., 2007), has facilitated regional correlation ofterrestrial sedimentary and paleoenvironmental records well
beyond the limit of radiocarbon dating (e.g. Westgate et al., 1990;Hamilton and Brigham-Grette, 1991; Beget, 2001).
Old Crow tephra is the most widespread of the many known bedsfrom this region. Its distribution extends from the Seward Peninsulaof northwestern Alaska, across many localities in southern andinterior Alaska, to central and northern Yukon (Westgate et al.,1983). The regional importance of Old Crow tephra lies in itsconsistent stratigraphic position below woody, organic-rich sedi-ments that are thought to be associated with the last interglaciationsensu stricto (Marine Isotope Stage, MIS, 5e). The age of Old Crowtephra is commonly cited as 140� 10 ka, based on a weighted mean(1s uncertainty) of six glass fission-track ages (Westgate et al.,1990). However, its age has been refined recently. Pewe et al. (2009)obtained a weighted-mean age of 131 � 11 ka (1s uncertainty),based partially on inclusion of a new age determination on a coarse-grained sample of Old Crow tephra from Togiak Bay, Alaska(Kaufman et al., 2001). Using a new, more precise age estimate forthe Moldavite tektite glass, the age monitor in glass fission-trackdating (Westgate et al., 2007), gives an even younger mean age of124 � 10 ka for Old Crow tephra (J.A. Westgate, personal
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communication, Nov. 2009). These three age estimates overlap at1s, and are consistent with thermoluminescence (Berger, 2003) andinfrared-stimulated luminescence (Auclair et al., 2007) ages onbracketing loess in the Fairbanks region.
The new glass fission-track age of Old Crow tephra revivesquestions concerning the nature of the temporal relation of Old Crowtephra to the last interglaciation (e.g. McDowell and Edwards, 2001;Muhs et al., 2001). This problem is particularly acute at the typelocality for the Eva Interglaciation Forest Bed in the Fairbanks region,where thermokarst-related slope processes may have reworked thetephra into, and locally above, last interglacial organic-rich sedi-ments (Pewe et al., 1997; Muhs et al., 2001). The tephra and itsassociated sediments have been the subject of numerous paleoeco-logical studies across eastern Beringia (e.g. Schweger and Matthews,1985; Matthews et al., 1990; Hamilton and Brigham-Grette, 1991;Waythomas et al., 1993; Muhs et al., 2001), yet a coherent regionalinterpretation of the paleoenvironmental setting at the time of OldCrow tephra deposition remains elusive because of apparentcontradictory climate and ecological signals among sites (McDowelland Edwards, 2001). Moreover, despite its regional distribution, weare not aware of any published records of in situ vegetation directlyunderlying Old Crow tephra, which could provide paleoecologicaldata from the moment of tephra deposition and help refine itschronostratigraphic significance.
In this paper, we report the presence of a vegetated surfaceburied by Old Crow tephra at the Palisades, a series of bluffs on theYukon River in interior Alaska. We compare multiple paleoeco-logical indicators (bryophyte, pollen, vascular plant, and insectassemblages) from the vegetated surface and associated sediments.Our multiproxy data from the vegetated surface indicate that thisimportant chronostratigraphic marker dates to late-MIS 6.
2. Site description
The Palisades comprise over 8 km of north-facing river-cutexposures on the Yukon River in Alaska about 70 km downstream of
the village of Tanana (Fig. 1). Vegetation at the site is northern borealforest, characterized by white and black spruce (Picea glauca andPicea mariana, respectively), poplar and aspen (Populus balsamiferaand Populus tremuloides, respectively), willow (Salix spp.), birch(Betula spp.), alder (Alnus crispa), and heath (Ericaceae). Thick mosscover is common in the forest above the Palisades bluffs. Two mainexposures, termed Palisades East and Palisades West, are separatedby several kilometers of vegetated, low-angle bluffs. We accessed thesite by traveling by boat w280 km from the Trans-Alaska Pipelinecrossing over the Yukon River, though it is also possible to travelfrom Fairbanks via the Chena and Tanana rivers. The site is locallytermed ‘‘The Boneyard’’ because Pleistocene bones have been foundat the base of the bluffs, and has been known to scientists andexplorers for over 100 years (see Matheus et al., 2003). Beget et al.(1991) were the first to identify Old Crow tephra at the Palisades,prompting Matheus et al. (2003) to study the stratigraphy, teph-rochronology, and paleoecology at Palisades East.
Stratigraphic investigations at the Palisades are hindered by thesteep, commonly near-vertical nature of the exposures, as well aswidespread slumping and gully-fill associated with thawing of ice-rich sediments and fluvial dissection. We observed several suddenfalls and topples involving very large blocks of frozen sediment whileworking at the site. Nevertheless, Matheus et al. (2003) developeda general lithostratigraphic framework for sediments at PalisadesEast. They proposed that fluvial and eolian sediments in the lowerthird of the bluffs are late Pliocene in age (ca. 2 Ma; now earlyPleistocene; Gibbard et al., in press) based on the presence of PAtephra, which is also present in the Fairbanks region and has anisothermal plateau glass fission-track age of 2.02 � 0.14 Ma (West-gate et al., 1990; Preece et al., 1999). Matheus et al. (2003) alsoreported EC and MC tephra above a prominent ‘‘lower peat’’ unit atthe Palisades. These tephra are present in the Fairbanks region,where they are associated with the early Pleistocene Dawson CutForest Bed (Westgate et al., 2003; Pewe et al., 2009). However, ECand MC tephra have not yet been found in direct stratigraphicassociation with the fission-track-dated PA tephra at either Palisades
Fig. 1. Location of study sites at the Palisades and other locales mentioned in the text (inset map). BC – Birch Creek; CB – Ch’ijee’s Bluff; DC – Dawson City; FB – Fairbanks;HL – Holitna lowland; IL – Imuruk Lake; KY – Koyukuk River; PA – Palisades; TH – Thistle Creek.
or Fairbanks, so their age, and that of their surrounding sediments atPalisades, is not established firmly. Paleomagnetic analyses of silt atseveral Palisades exposures indicate that no magnetic reversals arepresent (Opalka et al., 2004; D.B. Stone and D.G. Froese, unpublisheddata). Thus, the early Pleistocene sediments described by Matheuset al. (2003) may be restricted to isolated exposures betweensections of younger sediment, though it is possible that normally-magnetized sediments near the base of the Palisades may date to theOlduvai sub-chron. Old Crow and Sheep Creek-F tephra (190� 20 ka;Berger et al., 1996; Westgate et al., 2008) are common in the middleand upper third of the Palisades exposures. Where they are found inthe same measured section, they are separated by w4–6 m ofmassive silt. The uppermost Palisades sediments are made up ofmassive to crudely stratified silt with interbedded thick peats, andhost numerous ice wedges.
In this paper we focus our attention on a section at PalisadesWest (Figs. 1 and 2a), which was not described by Beget et al. (1991)or Matheus et al. (2003). Yeend (1977) presented generalizedstratigraphy of the lowermost 30–40 m of sediments at PalisadesWest, and noted a 6–30 cm thick ‘‘light-colored ash (?) or diatom(?) bed’’, overlain by ‘‘muck’’ in the uppermost several metres of hiscomposite section. However, the only paleoecological data fromPalisades West are pollen counts from Miocene stratified sand andgravel near river level (Yeend, 1977). Extant thick vegetation andslump deposits cover most of the Palisades West bluff.
3. Methods
Exposures at the Palisades West were described and sampled inJune 2007 by Jensen and Reyes, and at Palisades East in June 2005
by Froese, Reyes, and Zazula. Relative elevations above river level ofkey stratigraphic horizons were measured with a handheld GPSequipped with a barometric altimeter in 2007, and with a Lasertech100XL laser rangefinder in 2005.
Sub-samples of wood and terrestrial plant macrofossils wereprepared for 14C dating by accelerator mass spectrometry (AMS).Pre-treatment at the University of Alberta followed standard acid-base-acid procedures, with solutions heated to 70 �C: 30 min in 1 MHCl, 60 min in 1 M NaOH with solution changed until clear, 30 minin 1 M HCl, and rinses with ultrapure water until neutral. Weanticipated that the Palisades samples would return non-finiteages, so we also pre-treated and submitted sub-samples of Queets-A, an internal standard for non-finite-age material at the Keck–Carbon Cycle AMS facility (University of California, Irvine).
Tephra samples were analyzed at the University of AlbertaElectron Microprobe Laboratory. Analytical conditions, standards,and sample preparation followed Jensen et al. (2008). Tephra bedswere identified on the basis of glass shard morphology, mineralogy,stratigraphic context, and glass major element composition. Pali-sades samples were analyzed with reference samples of knownbeds to confirm correlations.
Bulk peat and sediment samples were soaked in water andscreened through a 250 mm sieve; bryophyte, vascular plant, andinsect macrofossils were subsampled from the screened residue.G. Zazula identified vascular plant macrofossils; nomenclature andecological information follow Cody (2000) and the Flora of NorthAmerica (Flora of North America Editorial Committee, 1993). Bryo-phyte macrofossils were mounted on permanent slides, identifiedby C. La Farge, and deposited in the Cryptogamic Herbarium atUniversity of Alberta. Bryophyte classification follows Goffinet et al.(2009); paleoecological reconstruction utilizes Steere (1978), Jans-sens (1983), Hedenas (1989, 1992), La Farge-England (1989), and LaFarge-England et al. (1991).
Pollen and spores were concentrated from bulk sedimentsamples at the U.S. Geological Survey Cenozoic Palynology Labo-ratory using methods modified from Doher (1980). T. Ager identi-fied pollen and spores using modern reference material in the USGScollections in Denver, Colorado, and by reference to publishedpollen floras (e.g., Moriya, 1978). Interpretation of past vegetationtypes from pollen assemblages was guided by comparing fossilassemblages to modern pollen assemblages from known vegetationtypes in Alaska and Yukon (e.g. Ager, 1975; Birks, 1980; Short et al.,1986; Oswald et al., 2003).
4. Results
4.1. Stratigraphy and chronology
Site A at Palisades West is less than 30 m high and partiallycovered by willow thickets. From river level, a thick white tephra isvisible across the top of exposure (Fig. 2a). The lowermost 16 mconsist of massive and stratified sand and gravel, with one thin bedof laminated silt (Fig. 3). A sharp contact separates these sedimentsfrom w10 m of massive-to-faintly laminated tan/grey silt with thin,diffuse organic beds. These silts are typical of Quaternary loessdeposits found across interior Alaska and Yukon (Westgate et al.,1990; Beget, 2001).
A thin (<1 cm), phenocryst-rich tephra, characterized byabundant frothy pumice shards, is present 18 m above river level;its upper and lower contacts are sharp and locally undulatory withseveral cm of relief. The tephra bed hosts very thin whisps of tansilt. Major element geochemical data (Table 1; UA1297), glassmorphology, and stratigraphic relations with overlying sedimentsidentify the bed as the 190 � 20 ka Sheep Creek-F tephra (Bergeret al., 1996; Westgate et al., 2008).
Fig. 2. Overview photographs of study sites. (a) Site A from river level; note thick bedof Old Crow tephra at Site A1. The exposure is w26 m high. (b) Site B from river level.The tephra labeled UA1124 is w25 m above river level.
We examined the uppermost several metres of Site A sedimentsat three different measured sections (Fig. 3b). At 24 m above riverlevel, massive tan-to-grey silt grades into a 40-cm thick bed ofbrown, darkening-upward, organic silt that is capped by several cm
of plant material and wood fragments, including roots and stems ingrowth position (Fig. 4a-d). We interpret the sequence as a vege-tated surface built on a weakly developed paleosol, which isprobably an Entisol or Inceptisol based on the lack of apparent soil
Fig. 3. Lithostratigraphy, 14C ages, and tephrochronology at Palisades study sites (a), and details of Site A stratigraphy near Old Crow tephra (b). OC ¼ Old Crow tephra.
Table 1Major element composition of tephra beds at Palisades.a
a The JEOL superprobe was operated at 15 keV accelerating voltage, 10 mm beam diameter, and 6 nA beam current. Standardized to obsidian standard UA 5831 and recast to100% on a water-free basis; (#) ¼ standard deviation; FeOt ¼ total iron oxide as FeO; H2Od ¼ water by difference.
b UAxxxx ¼ University of Alberta Tephrochronology Lab accession number; n ¼ shards analyzed.c SC-F, Sheep Creek-F; OC, Old Crow tephra.d UA1471 experienced Na loss during analyses. Identification as Old Crow tephra was confirmed by comparison to a reference sample that was analyzed during the same
horizon development beyond an A horizon. The vegetated surfaceis in turn overlain by a tabular bed, up to 40 cm thick, of massive,grey/white tephra, composed primarily of bubble-wall shards, thatis laterally continuous across the exposure. The tephra is sharplyoverlain by 65 cm of compact peat; at two of our measured sections20–40 cm of massive silt divide the peat into a lower, more fibrouspeat and an upper peat with rare silt laminae (Fig. 3b). An up-turned bed of tephra, up to 6 cm thick, is present in the silt bed thatdivides the two peats at Site A3. The peat is overlain by a tephra bedup to 2.5 cm thick at all three measured sections. Massive grey siltoverlies this tephra at Site A1. All the tephra beds sampled from theuppermost several metres of Site A sediments were identified asOld Crow tephra on the basis of major element geochemistry, andglass morphology (Table 1).
We also examined Old Crow tephra at exposures in the down-stream half of Palisades East (Site B, Fig. 1), in a series of shallowgullies emanating from the edge of a retrogressive thaw slump(Fig. 2b; the ‘‘thermokarst amphitheatre’’ of Matheus et al., 2003).Active slumping in 2005 exposed frozen sediments near Old Crowtephra, but most of the upper exposure was covered in 2007. Thesection is dominated by massive to weakly laminated silt, with rarebeds of organic-rich silt and peat (Fig. 3a). Several tephra beds arepresent in the lower w15 m of the exposure, but here we focus onthe interval between 24 and 30 m above river level.
Old Crow tephra (UA1124; Table 1) at Site B is 3.5 m belowwoody, organic-rich silt and peat that host abundant and excep-tionally well-preserved large stumps and logs (Fig. 4e). The tephra isup to 10 cm thick and has sharp upper and lower contacts with the
Fig. 4. Photographs of tephra and paleoecology sampling locations. (a) Site A1; geologist for scale is 1.7 m tall; (b) Contact between Old Crow tephra and organic-rich silt (sample07PAL-4); note growth position stems extending into overlying tephra (circles); (c) Old Crow tephra at Site A2; (d) Vegetated surface underlying Old Crow tephra at Site A2;(e) Woody, organic-rich silt at Site B; note rooted stump in growth position (arrow); (f) Micro-faulted Old Crow tephra and frost crack (dashed line) at Site B.
surrounding grey silt, which is massive and contains rare rootlets.Unlike Site A, the tephra bed is not uniformly tabular; locally it isoffset by cm-scale faults (Fig. 4f), or appears as stretched pods orlenses. At least two frost cracks, including one w12 cm long,incorporate wisps of tephra and small angular blocks of silt (Fig. 4f).The tephra and the wood/peat horizon are separated by massive andfaintly laminated grey silt. Samples from two logs at two differentSite B gullies, in identical stratigraphic positions with respect to OldCrow tephra, yielded ages of 53,600 � 260 (UCIAMS-44407) and>53,700 (UCIAMS-49772) 14C yr BP (Table 2). We interpret theformer as a non-finite age based on the age of the non-finite Queets-A standard that was pre-treated and analyzed at the same time asthe Palisades sample (Table 2). Old Crow tephra and the 14C-datedlogs provide maximum and minimum ages, respectively, for thewoody, organic-rich silt and peat, which is interpreted as a remnantlast interglacial forest bed.
4.2. Paleoecology
We analyzed bulk sediment samples from Site A for insect(Table 3; Fig. 5), vascular plant (Table 4), and bryophyte (Table 5)macrofossils, and pollen (Table 6).
The 40 cm thick organic-rich silt underlying Old Crow tephra atSite A (sample 07PAL-4; Figs. 3b and 4b) contained a few identifiablevascular plant and insect remains, but no bryophyte macrofossilswere found. About 500 ml of organics were screened from a w2 Lbulk sample. Plant macrofossils included mare’s tail (Hippuris), pondweed (Potamogeton), water-crowfoot (Ranunculus cf. aquatilis), Salix,and sedge (Carex). Pollen recovery from this sample was poor; onlytraces of sedge (Cyperaceae), Salix, and stiff clubmoss (Lycopodiumannotinum) were identified. Trace amounts of pine (Pinus), Picea, andpoorly preserved indeterminate conifer pollen grains were alsopresent. The few identifiable insect remains included several generaof ground beetles (Carabus and Pterostichus (Cryobius) spp.), theweevils Isochnus flagellum and Lepidophorus lineaticollis, and onespecimen from the order Megaloptera of indeterminate genus; waterflea (Daphnia) macrofossils were also recovered.
Even fewer identifiable macrofossils were recovered from thew300 ml of organics that were screened from w1 L of samplecollected from the vegetated surface directly underlying Old Crowtephra (sample 07PAL-8; Figs. 3b and 4c–d). Unidentified woodfragments and Salix were common, but the only other identifiablevascular plant macrofossils were rare Ranunculus achenes anda probable Carex stem sheath. Almost no pollen grains wererecovered from this sample; only traces of Salix, Alnus and Cyper-aceae were present, as were rare grains of Picea and Pinus. Thebryophyte component of 07PAL-8 included three species: Pseudo-calliergon brevifolium, Campylium stellatum, and Calliergon
giganteum. Insect remains included the weevil Lepidophorus line-aticolis and several species of ground beetles (Pterostichus (Cryo-bius) spp.).
We sampled the peat directly overlying Old Crow tephra at SiteA1, where 20 cm of silt divide the peat into a lower (07PAL-6) andupper (07PAL-7) horizon (Figs. 3b and 4a). The lower horizon isa highly compressed bryophytic peat with almost no vascular plantor insect macrofossils. Only a few Carex achenes, some Daphniaindividuals, and a single fragment of the water beetle Colymbeteswere recovered. The pollen assemblage (>300 grains counted) isdominated by Cyperaceae, Salix, grass (Poaceae), and variousherbaceous types (Ranunculaceae, Thalictrum, Saxifragaceae, Arte-misia, Asteroideae, Brassicaceae, and Polemonium). Trace amountsof Betula, Alnus, Pinus, and Picea were also found. Two commonfreshwater colonial algae genera, Botryococcus and Pediastrum,were identified during pollen analysis. The bryophyte assemblage isdominated by Pseudocalliergon turgescens, with rare stems ofP. brevifolium.
The upper peat horizon (07PAL-7), which contained silt laminae,was less fibrous than the lower peat and yielded more plant andinsect macrofossils. Carex achenes and graminoid (Poaceae) vege-tative material are abundant, and Salix macrofossils are present.Identified insect remains include at least two species of groundbeetles (Pterostichus (Cryobius) spp.), the rove beetle Stenus, theminute brown scavenger beetle Corticaria fenestralis, and the waterbeetle Hydroporus morio; the water fleas Daphnia and Simocephalusare also present. Like the lower peat, the upper peat pollenassemblage (>300 grains counted) is dominated by Cyperaceae,with smaller percentages of Salix and Poaceae. Other taxa repre-sented by low percentages of pollen include Betula, Artemisia, astertype (Asteraceae, subfamily Asteroideae), pink family(Caryophyllaceae) and other herb types (Table 6). Trace amounts ofPicea, Pinus, western hemlock (Tsuga heterophylla), Alnus, Betula,and fern spores (Polypodiaceae type) were also present. No bryo-phyte material was recovered from the sample.
Table 2Radiocarbon dates from the Palisades.
Laboratory code Field code Description Age (14C yr BP) d13C (& VPDB) a
To our knowledge, the exposure at Palisades West represents thefirst locality where a vegetated surface has been identified, buriedby a primary deposit of Old Crow tephra. Indeed, we only know oftwo vegetated surfaces buried by Pleistocene tephra in unglaciatedYukon/Alaska, both dating to late-MIS 3 and MIS 2 (Hofle et al.,2000; Goetcheus and Birks, 2001; Froese et al., 2006; Zazula et al.,2006; Kuzmina et al., 2008). Unfortunately, there was low taxo-nomic diversity in plant and insect assemblages (10 taxa, plus trace
amounts of pollen) from the surface buried by Old Crow tephra atthe Palisades. However, we are able to infer the local presence ofcertain habitats based on the contemporary ecology for some of thepollen and macrofossils we identified from organic-rich depositsdirectly above and below the tephra and buried surface.
The presence of small frost cracks at Site B (Fig. 4f) suggests coldand locally dry conditions when Old Crow tephra was deposited.However, plant and insect macrofossils suggest that the buriedsurface at Site A likely represents a mesic shrub willow-tundraenvironment with wet meadows. Carex and Salix macrofossils werepresent in the buried surface (07PAL-8) and the 40-cm thickorganic silt (07PAL-4) that it directly overlies. Vascular plantaquatic indicators (Hippuris, Potamogeton, Ranunculus cf. aquatilis)were found in the organic silt. The moss assemblage in the buriedsurface, represented by P. brevifolium, C. stellatum, and C. giganteum,indicates wet meadow environments with aquatic to hydric habi-tats. These species are common circumpolar taxa, indicative of wetmeadow habitats in the arctic that form around shallow pools,along slow-flowing streams, or in saturated depressions during thegrowing season. P. brevifolium is an indicator species of rich fen(calcareous) habitats with high pH. Both P. brevifolium andC. giganteum commonly grow submerged in water, while
Fig. 5. Selected fossil insect macrofossils. (a) Pterostichus (Cryobius) tareumiut, pronotum, sample 07PAL-4; (b) P. (Cryobius) brevicornis, pronotum, sample 07PAL-4; (c) P. (Cryobius)brevicornis, right elytron, sample 07PAL 8; (d) P. (Cryobius) sp., right elytron, sample 07PAL-8; (e) Hydroporus morio, right elytron, sample 07PAL-7; (f) Stenus sp., pronotum, sample07PAL-7; (g) Corticaria fenestralis, left elytron, 07PAL-7; (h) Lepidophorus lineaticollis, pronotum, sample 07PAL-8; (i) Isochnus flagellum, right elytron, sample 07PAL-4.
Table 4Plant macrofossil data from the Palisades.a
C. stellatum is more typical of hydric to mesic meadows (Steere,1978). P. brevifolium has been reported in numerous Quaternaryassemblages from northern Yukon basins and had its extant andsubfossil distributions mapped by Janssens (1983). It is alsocommonly found in Holocene assemblages from arctic sites (e.g.,Banks Island; La Farge, unpublished data).
Insect assemblages in the organic silt and buried surfaceunderlying Old Crow tephra provide additional support for a mesicto moist tundra and the local presence of an aquatic environment.Several species of Pterostichus (Cryobius) are present. The groundbeetles of subgenus Cryobius (Lindroth, 1961–1969; Ball, 1963) aremost abundant in moist and mesic tundra. P. (C.) tareumiut andP. (C.) parasimilis are currently known as obligate tundra habitants,but are present in MIS 3 fossil assemblages from the upperPorcupine River, which contain tundra and forest indicators(Matthews and Telka, 1997). Other members of the subgenus suchas P. (C.) brevicornis, P. (C.) pinguedineus, and P. (C.) ventricosus arecommonly found in tundra environments, but also in the northernforest zone along river banks, and under mosses and leaf litter. Thesingle recovered specimen of Megaloptera indicates the presence offresh water. Fossil specimens of L. lineaticolis are commonly asso-ciated with late Pleistocene steppe-tundra (Matthews, 1983; Kuz-mina et al., 2008). This species prefers dry open tundra and forestenvironments, but they are also currently present in moist tundrain Yukon and Alaska (Anderson, 1997). The weevil I. flagellum iscommonly associated with Salix spp. across the circumpolar Arctic(Anderson, 1997). Too few pollen grains were recovered from theburied surface to make any definitive paleoecological inferences,though the trace amounts of Salix, Alnus, Lycopodium, and Cyper-aceae that were present are consistent with the plant and insectmacrofossil data.
The few plant and insect macrofossils from peat directly over-lying Old Crow tephra at Site A1 are similarly consistent with moistwillow-tundra and aquatic habitat. Carex spp. and Salix dominatethe plant macrofossil assemblage, particularly in the upper peatsample. Recovered mosses include P. brevifolium and P. turgescens,which together suggest a fen environment, with P. turgescenstypical of consistently submerged habitats. Both are common inmodern circumpolar environments. The water fleas Daphnia andSimocephalus indicate the presence of aquatic habitat, as do thebeetles Hydroporus, Colymbetes and Stenus, while the minute brownscavenger beetle C. fenestralis is known to inhabit damp, moldyenvironments, including leaf litter and rotten wood (Hinton, 1941;Bousquet, 1990).
Pollen assemblages from the two peat samples above Old Crowtephra, both of which yielded pollen sums in excess of 300 grains,represent a mesic sedge-herb-dominated tundra, in contrast withthe present Picea-dominated boreal forest at the site, suggestingthat climate at the time of tephra deposition was colder than today.Mesic conditions are suggested by the dominance of sedge pollenand the presence of peat. The aquatic habitat indicated by vascularplant and insect macrofossils is supported by the presence of thefreshwater colonial algae Botryococcus and Pediastrum. Immedi-ately after deposition of Old Crow tephra, when lowland tundragrew at the Palisades, the mean July air temperature was probablywell below 10 �C. Sedge-dominated lowland tundra vegetation insome areas of Alaska (e.g. the coastal plain on the North Slope nearBarrow) currently grows under conditions of low annual precipi-tation (<250 mm) and cool mean July temperatures (3.7–8.7 �C)(Haugen and Brown, 1980).
Rare grains of Picea, Pinus and Tsuga pollen were present in allSite A samples. It is possible that long-distance transport mayaccount for the trace amounts of conifer pollen, or, in the case ofPicea, that it was present in the area in low abundance around thetime of Old Crow tephra deposition. However, there is no plantmacrofossil evidence for any arboreal taxa, nor are there any fossilinsects that are ecologically associated with forest vegetation. Also,the abundance of indeterminate conifer pollen fragments andpresence of Tsuga and Pinus, which are thought to have been absentfrom interior Alaska since the middle Pliocene (White et al., 1997),suggest that these grains were probably reworked from the olderdeposits that underlie the Pleistocene sand and silt sequence atPalisades West (E. Leopold and E. Daniels, unpublished data, pre-sented in Yeend, 1977). Thus it seems that the regional vegetationwas treeless during the time that followed deposition of Old Crowtephra.
The depauperate plant and insect assemblages associated withOld Crow tephra at Palisades West contrast starkly with evidence forboreal forest conditions in organic-rich sediments, presumably oflast interglacial age, 3.5 m above Old Crow tephra at Site B in Pali-sades East (Fig. 4e). Matheus et al. (2003) studied plant macrofossilsin a similar stratigraphic setting, and noted the presence of borealforest species Picea, Vaccinium, Polygonum, Rumex, Betula cf. papy-rifera, and Sphagnum. No palynological work has been conducted onthe Last Interglacial sediments at Palisades, but sediments of thatage at bluffs on the Koyukuk River and in mining exposures nearFairbanks have Picea pollen percentages that exceed 20% and 60%,respectively (Schweger and Matthews, 1985; Muhs et al., 2001).
5.2. Regional comparison
Our paleoecological data from the vegetated surface buried byOld Crow tephra suggest that Picea was locally absent, or very rare,at the time of tephra deposition, unlike several other studies ofsediments directly associated with Old Crow tephra (Fig. 6).McDowell and Edwards (2001) describe Old Crow tephra deposited
a Percentages for pollen-producing plants, spore-producing plants, and algae arewith respect to pollen, pollen þ spore, and pollen þ spore þ algae sums, respec-tively. * indicates pollen types that are probably reworked from local Neogenesediments.
b Samples 07PAL-4 and 07PAL-8 did not contain enough pollen grains for quan-titative analyses.
on a paleosol at Birch Creek, in east-central Alaska; Picea macro-fossils were present in the paleosol, but they were not able torecover any fossil pollen. However, pollen recovered from organic-rich silt directly above Old Crow tephra at Birch Creek suggests thepresence of open Picea forest with minimal shrubby understory.Similarly, minor peaks in Picea pollen at and directly above the levelof Old Crow tephra are present at Seward Peninsula in Imuruk LakeCore V sediments and at the KY-11 section on the Koyukuk River ininterior Alaska (Shackleton, 1982; Schweger and Matthews, 1985).Pollen associated with Old Crow tephra at Ch’ijee’s Bluff, innorthern Yukon, is dominated by sedge and shrubs but alsocontains a minor Picea component (<10%; Matthews et al., 1990). Incontrast, Waythomas et al. (1993), citing unpublished pollen data ofS.K. Short, found no evidence for the presence of Picea in siltdirectly associated with Old Crow tephra at a site in the Holitnalowland, and showed that tephra deposition was coeval with per-iglacial eolian sandsheet aggradation. At all these sites, like atPalisades, about 3–6 m of loess, commonly with pollen assemblagestypical of cold climate conditions, separate Old Crow tephra froma prominent horizon of large wood macrofossils and organic-richsediments with high Picea pollen percentages (Fig. 6). This prom-inent organic horizon has been assigned to the last interglaciationbased on the Westgate et al. (1990) 140 � 10 ka weighted-meanglass fission-track age of Old Crow tephra, and because pollen,plant, and insect macrofossil assemblages contain taxa near or
beyond their present northern range limits, suggesting an intervalat least as warm or warmer than today (e.g. Matthews et al., 1990;Edwards and McDowell, 1991).
The stratigraphy and paleoecology of sediments associated withOld Crow tephra are more cryptic at other sites. A peat several cmbelow Old Crow tephra at Hogatza Mine, in the middle KoyukukRiver area, has a pollen assemblage that includes 38% Picea – thehighest Picea percentage associated with Old Crow tephra ineastern Beringia – and is indicative of local spruce forest (Hamiltonand Brigham-Grette, 1991). Many workers have studied theHalfway House locality near Fairbanks, but there is disagreement inthe literature on the number of paleosols exposed in the section,their stratigraphic relation to Old Crow tephra, and even themeasured thickness of the section (e.g. Muhs et al., 2003; Lagroixand Banerjee, 2004). Furthermore, the new 131 � 11 ka weighted-mean isothermal plateau fission-track age for Old Crow tephra(Pewe et al., 2009) raises the possibility that the temporal relationbetween the tephra and overlying interglacial deposits could becloser than previously thought (Muhs et al., 2001). The stratigraphyat some sites in eastern Beringia, particularly in valley bottomsettings, is characterized by re-transported loess and organic-richmaterial. At one such site, the type locality for the Eva Interglaci-ation Forest Bed near Fairbanks, Troy Pewe and colleagues (e.g.Pewe et al., 1997) described the Old Crow tephra as directlyunderneath, up to 4 m below, or truncated by the prominent Eva
Fig. 6. Summary of selected paleoecological proxy data associated with Old Crow tephra and the last interglaciation at five sites in eastern Beringia. Imuruk Lake Core V and KY-11data from Schweger and Matthews (1985), who presented the Imuruk Lake pollen data of Shackleton (1982); Ch’ijee’s Bluff data from Matthews et al. (1990); Palisades data fromthis paper and Matheus et al. (2003); Birch Creek data from Edwards and McDowell (1991) and McDowell and Edwards (2001). The shaded region denotes correlation of lastinterglacial deposits at the five sites. Black and grey bars/lines denote Picea and Cyperaceae pollen percentage, respectively (except for Birch Creek, where the grey line denotes theCyperaceae þ Poaceae percentage). Note changes in depth/elevation axis scale.
Forest Bed, suggesting that Old Crow tephra was deposited prior tothe last interglaciation. However, Muhs et al. (2001) identified OldCrow tephra within the wood-rich horizon and associated organic-rich sediments with high Picea pollen percentages.
Elsewhere, for example at Ch’ijee’s Bluff and Thistle Creek inthe south Klondike goldfields (Fig. 1), Old Crow tephra is truncatedby, and locally incorporated into, steeply dipping organic-rich siltsthat grade laterally into prominent poorly sorted accumulations oflarge wood macrofossils. Reyes et al. (2006) interpreted thesedeposits as evidence for permafrost degradation and associatedthaw slumping of boreal forest soils and vegetation during thewarmest part of the last interglaciation. The early Holocene thermalmaximum in west-central Yukon is marked by a prominent thawunconformity (Burn et al., 1986) that resulted from shallowpermafrost degradation and, locally, by concentrations of remobi-lized organic material in valley bottom settings (Fraser and Burn,1997; Kotler and Burn, 2000). The inconsistencies in Old Crowtephra stratigraphy at many sites are almost certainly the result ofsimilar processes taking place during the last interglaciation. Inthese cases, the stratigraphic context of the tephra could bedramatically different from site to site, or even at the same sitewhen described at different exposures or at different times, as is thecase at the Eva Forest Bed type locality (Pewe et al., 1997; Muhset al., 2001).
Sites with primary beds of Old Crow tephra, such as Birch Creek,some Palisades exposures, Kulukbuk Bluff in the Holitna lowland,and Imuruk Lake cores, provide better constraints on the temporal,stratigraphic, and paleoecological relations between MIS 6, OldCrow tephra, and the last interglaciation. At those sites where OldCrow tephra is associated with appreciable amounts of Picea pollenor macrofossils (e.g. Birch Creek, Imuruk Lake), a more pronouncedpeak in Picea is present several metres above the tephra (Fig. 6), andsediments between the two Picea peaks lack boreal forest indica-tors. Thus, our paleoecological data from Palisades West, along withstratigraphy and paleoecology of sediments associated with OldCrow tephra elsewhere in eastern Beringia, support a late-MIS 6 agefor tephra deposition.
6. Conclusions
The presence of Old Crow tephra (131 � 11 ka) and the under-lying Sheep Creek-F tephra (w190 ka) at Palisades West shows thatlate–Middle Pleistocene sediments are present at this exposure. OldCrow tephra is present across the exposure as a primary 40 cm thicktabular bed directly overlying a vegetated surface and organic-richsilt. Vascular plant, bryophyte, and insect macrofossils from theburied surface and associated sediments suggest the local presenceof mesic tundra, wet meadow, and aquatic habitat when Old Crowtephra was deposited. There is no evidence, either from pollen, orplant and insect macrofossils, for the presence of boreal forest atthe time of tephra deposition. Our paleoecological data from Pali-sades West indicate a late-MIS 6 age for Old Crow tephra.
Acknowledgements
Our research was funded by the Natural Sciences and Engi-neering Research Council of Canada (D.G.F.), an Alberta IngenuityNew Faculty Award (D.G.F.), and grants from the CanadianCircumpolar Institute and Northern Scientific Training Program(A.V.R., B.J.L.J.). Steve Kuehn assisted in the field, Nancy Bigelow andMatt Irinaga provided logistical support, and Sergei Matveevfacilitated electron microprobe analyses. A.V.R. thanks Paul Math-eus for encouraging us to visit the Palisades, and acknowledgesscholarship support from Alberta Ingenuity, the Killam Trusts, andan NSF Graduate Research Fellowship. The United States Fish and
Wildlife Service granted permission to work in Nowitna NationalWildlife Refuge. We thank Julie Brigham-Grette and John Westgatefor their reviews of the manuscript.
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