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1.
Glacial landforms and outwash terraces in the Nenana River valley, Reindeer Hills and Monahan Flat in the central Alaska Range were dated with 60 10Be exposure ages to determine the timing of Late Pleistocene glaciation. In the Nenana River valley, glaciation occurred at 104–180 ka (Lignite Creek glaciation), ca. 55 ka (Healy glaciation), and ca. 16 ka (Carlo Creek phase); glaciers retreated in the Reindeer Hills and Monahan Flat by ca. 14 ka and ca. 13 ka, respectively. The Carlo Creek moraine is similar in age to at least six other moraines in the Alaska Range, Ahklun Mountains and Brooks Range. The new data suggest that post‐depositional geological processes limit the usefulness of 10Be methods to the latter part (≤60 ka) of the late Quaternary in central Alaska. Ages on Healy and younger landforms cluster well, with the exception of Riley Creek moraines and Monahan Flat‐west sites, where boulders were likely affected by post‐depositional processes. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

2.
Four glacial drifts that are interstratified with lava flows and tephra layers on the upper slopes of Mauna Kea demonstrate that an ice cap formed repeatedly at the summit of the volcano during the middle and late Pleistocene. The oldest drift (Pohakuloa Formation) probably was deposited shortly after eruption of a lava flow having a KAr age of 278,500 ± 68,500 yr. Drift of the Waihu Formation, marked by a belt of subdued end moraines, is correlated with hyaloclastite cones and associated lava flows that were erupted beneath an ice cap about 170,000–175,000 yr ago. One of four younger subglacially erupted lavas at the crest of the volcano has a KAr age of 41,300 ± 8300 yr. Tephra layers that antedate the last glaciation are about 29,700 to 37,200 14C yr old and underlie dune sand that is believed to correlate with drift of the Makanaka Formation deposited during the last ice advance. The late Makanaka ice cap, which covered an area of about 70 km2 and was as much as 100 m thick, is reconstructed from end moraines and limits of erratic stones that encircle the summit region. The ice cap disappeared from the summit before about 9080 yr ago. Postglacial lavas and tephra overlie the youngest drift on the upper south flank of the mountain and buried a widespread post-Makanaka soil on the lower south rift zone about 4500 14C yr ago. The island of Hawaii is subsiding isostatically due to crustal loading by Quaternary volcanic rocks, with subsidence near the midpoint of Mauna Kea estimated as about 2.5 ± 0.5 mm/yr. A curve depicting an inferred long-term subsidence rate has been used to adjust equilibrium-line altitudes (ELAs) of former ice caps that are calculated on the basis of reconstructed glacier topography and an assumed accumulation-area ratio of 0.6 ± 0.05. The results indicate that ELA depression was greatest during Waihu glaciation, least during Pohakuloa glaciation, and that the ELA during late Makanaka glaciation was somewhat lower than that of the early Makanaka advance. Available radiometric dates show that late Makanaka glaciation correlates with stage 2 of the marine oxygen-isotope record, and suggest that early Makanaka, Waihu, and Pohakuloa glaciations correlate, respectively, with isotope stages 4, 6, and 8. Because ice caps could have formed on Mauna Kea only after the snowline was lowered many hundreds of meters below its inferred present level, episodes of Hawaiian glaciation probably were restricted to times of maximum ice volume on the continents. The asymmetry of the late Makanaka ice cap and the southeast-descending gradient of its equilibrium line are consistent with a southeast (tradewinds) source of precipitation during the last glaciation. Although departures of glacial-age temperature and precipitation from present values are difficult to assess quantitatively, growth of former ice caps on Mauna Kea most likely was due to enhanced winter snowfall and to reduced ablation rates brought about by lower air temperature and increased cloudiness.  相似文献   

3.
Two widespread tephra deposits constrain the age of the Delta Glaciation in central Alaska. The Old Crow tephra (ca. 140,000 ± 10,000 yr), identified by electron microprobe and ion microprobe analyses of individual glass shards, overlies an outwash terrace coeval with the Delta glaciation. The Sheep Creek tephra (ca. 190,000 yr) is reworked in alluvium of Delta age. The upper and lower limiting tephra dates indicate that the Delta glaciation occurred during marine oxygen isotope stage 6. We hypothesize that glaciers in the Delta River Valley reached their maximum Pleistocene extent during this cold interval because of significant mid-Pleistocene tectonic uplift of the east-central Alaska Range.  相似文献   

4.
In 1980 a large proboscidean femur, probably Mammuthus sp., was found in situ in a bluff exposure at the mouth of the Tyone River in the northwestern part of the Copper River Basin, Alaska. The regional setting, stratigraphy, radiocarbon chronology, flora, and implications of the fossil locality, which represents the first documented occurrence of Pleistocene terrestrial mammalian fauna in southern Alaska, are described. Radiocarbon dates and stratigraphic relations at the site indicate that the sediments containing the fossil accumulated during the transition from interstadial to glacial conditions during terminal middle Wisconsin time. During this interval the immediate vicinity was unforested and large areas of south-central Alaska may have been available for faunal and possibly human habitation. This documented find, dated at 29,450 ± 610 14C yr B.P., extends the known range for Pleistocene mammals and possibly steppe-tundra conditions south-ward at least 150 km, and suggests that mountain passes through the Alaska Range to the north were ice free during the last part of the middle Wisconsin interstadial.  相似文献   

5.
Cover sediments of the York Terrace exposed near the California River, western Seward Peninsula, Alaska, yield mollusks, ostracodes, and foraminifera that lived during the Anvilian transgression of early Pleistocene age. The fossiliferous sediments lie at the inner edge of the York Terrace, a deformed wave-cut platform that extends eastward from Bering Strait along much of the southern coast of Seward Peninsula. The seaward margin is truncated by the little-deformed Lost River Terrace, carved during the Pelukian (Sangamonian) transgression. The early Pleistocene sediments seem to have been deposited between the first and second of four glaciations for which evidence can be found in the California River area.The California River fauna includes several extinct species and several species now confined to areas as remote as the northwestern Pacific and north Atlantic. The fauna probably lived in water temperatures much like those of the present time but deeper water on the Bering Shelf is suggested.The presence of an early Pleistocene fauna at the inner edge of the York Terrace at California River shows that the terrace was largely carved before and during early Pleistocene time. However, a marine fauna apparently of middle Pleistocene age is found on the York Terrace near Cassiterite Peak, and this seems to indicate that the terrace remained low until middle Pleistocene time. Uplift of the York Terrace probably was accompanied by uplift of Bering Strait. The strait may have been deeper, and there may have been no land bridge between the Seward Peninsula of Alaksa and the Chukotka Peninsula of Siberia during most of early and middle Pleistocene time.  相似文献   

6.
Pleistocene fluvial landforms and riparian ecosystems in central California responded to climate changes in the Sierra Nevada, yet the glacial history of the western Sierra remains largely unknown. Three glacial stages in the northwestern Sierra Nevada are documented by field mapping and cosmogenic radionuclide surface-exposure (CRSE) ages. Two CRSE ages of erratic boulders on an isolated till above Bear Valley provide a limiting minimum age of 76,400±3800 10Be yr. Another boulder age provides a limiting minimum age of 48,800±3200 10Be yr for a broad-crested moraine ridge within Bear Valley. Three CRSE ages producing an average age of 18,600±1180 yr were drawn from two boulders near a sharp-crested bouldery lateral moraine that represents an extensive Tioga glaciation in Bear Valley. Nine CRSE ages from striated bedrock along a steep valley transect average 14,100±1500 yr and suggest rapid late-glacial ice retreat from lower Fordyce Canyon with no subsequent extensive glaciations. These ages are generally consistent with glacial and pluvial records in east-central California and Nevada.  相似文献   

7.
The Dry Creek archeologic site contains a stratified record of late Pleistocene human occupation in central Alaska. Four archeologic components occur within a sequence of multiple loess and sand layers which together form a 2-m cap above weathered glacial outwash. The two oldest components appear to be of late Pleistocene age and occur with the bones of extinct game animals. Geologic mapping, stratigraphic correlations, radiocarbon dating, and sediment analyses indicate that the basal loess units formed part of a widespread blanket that was associated with an arctic steppe environment and with stream aggradation during waning phases of the last major glaciation of the Alaska Range. These basal loess beds contain artifacts for which radiocarbon dates and typologic correlations suggest a time range of perhaps 12,000–9000 yr ago. A long subsequent episode of cultural sterility was associated with waning loess deposition and development of a cryoturbated tundra soil above shallow permafrost. Sand deposition from local source areas predominated during the middle and late Holocene, and buried Subarctic Brown Soils indicate that a forest fringe developed on bluff-edge sand sheets along Dry Creek. The youngest archeologic component, which is associated with the deepest forest soil, indicates intermittent human occupation of the site between about 4700 and 3400 14C yr BP.  相似文献   

8.
The Neogene Yamadağ volcanics occupy a vast area between Sivas and Malatya in eastern Anatolia, Turkey. These volcanic rocks are characterized by pyroclastics comprising agglomerates, tuffs and some small outcrops of basaltic–andesitic–dacitic rocks, overlain upward by basaltic and dacitic rocks, and finally by basaltic lava flows in the Arapkir area, northern Malatya Province. The basaltic lava flows in the Arapkir area yield a 40Ar/39Ar age of 15.8 ± 0.2 Ma, whereas the dacitic lava flows give 40Ar/39Ar ages ranging from 17.6 through 14.7 ± 0.1 to 12.2 ± 0.2 Ma, corresponding to the Middle Miocene. These volcanic rocks have subalkaline basaltic, basaltic andesitic; alkaline basaltic trachyandesitic and dacitic chemical compositions. Some special textures, such as spongy-cellular, sieve and embayed textures; oscillatory zoning and glass inclusions in plagioclase phenocrysts; ghost amphiboles and fresh biotite flakes are attributable to disequilibrium crystallization related to magma mixing between coeval magmas. The main solidification processes consist of fractional crystallization and magma mixing which were operative during the soldification of these volcanic rocks. The dacitic rocks are enriched in LILE, LREE and Th, U type HFSE relative to the basaltic rocks. The basaltic rocks also show some marked differences in terms of trace-element and REE geochemistry; namely, the alkaline basaltic trachyandesites have pronounced higher HFSE, MREE and HREE contents relative to the subalkaline basalts. Trace and REE geochemical data reveal the existence of three distinct magma sources – one subalkaline basaltic trachyandesitic, one alkaline basaltic and one dacitic – in the genesis of the Yamadağ volcanics in the Arapkir region. The subalkaline basaltic and alkaline basaltic trachyandesitic magmas were derived from an E-MORB type enriched mantle source with a relatively high- and low-degree partial melting, respectively. The magmatic melt of dacitic rocks seem to be derived from an OIB-type enriched lithospheric mantle with a low proportion of partial melting. The enriched lithospheric mantle source reflect the metasomatism induced by earlier subduction-derived fluids. All these coeval magmas were generated in a post-collisional extensional geodynamic setting in Eastern Anatolia, Turkey.  相似文献   

9.
Dortch, J. M., Owen, L. A., Caffee, M. W. & Brease, P. 2009: Late Quaternary glaciation and equilibrium line altitude variations of the McKinley River region, central Alaska Range. Boreas, 10.1111/j.1502‐3885.2009.00121.x. ISSN 0300‐9483 Glacial deposits and landforms produced by the Muldrow and Peters glaciers in the McKinley River region of Alaska were examined using geomorphic and 10Be terrestrial cosmogenic nuclide (TCN) surface exposure dating (SED) methods to assess the timing and nature of late Quaternary glaciation and moraine stabilization. In addition to the oldest glacial deposits (McLeod Creek Drift), a group of four late Pleistocene moraines (MP‐I, II, III and IV) and three late Holocene till deposits (‘X’, ‘Y’ and ‘Z’ drifts) are present in the region, representing at least eight glacial advances. The 10Be TCN ages for the MP‐I moraine ranged from 2.5 kyr to 146 kyr, which highlights the problems of defining the ages of late Quaternary moraines using SED methods in central Alaska. The Muldrow ‘X’ drift has a 10Be TCN age of ～0.54 kyr, which is ～1.3 kyr younger than the independent minimum lichen age of ～1.8 kyr. This age difference probably represents the minimum time between formation and early stabilization of the moraine. Contemporary and former equilibrium line altitudes (ELAs) were determined. The ELA depressions for the Muldrow glacial system were 560, 400, 350 and 190 m and for the Peters glacial system 560, 360, 150 and 10 m, based on MP‐I through MP‐IV moraines, respectively. The difference between ELA depressions for the Muldrow and Peters glaciers likely reflects differences in supraglacial debris‐cover, glacier hypsometry and topographic controls on glacier mass balance.  相似文献   

10.
Recurring glacial outburst floods from the Yukon-Tanana Upland are inferred from sediments exposed along the Yukon River near the mouth of Charley River in east-central Alaska. Deposits range from imbricate gravel and granules indicating flow locally extending up the Yukon valley, to more distal sediments consisting of at least 10 couplets of planar sands, granules, and climbing ripples with up-valley paleocurrent indicators overlain by massive silt. An interglacial organic silt, occurring within the sequence, indicates at least two flood events are associated with an earlier glaciation, and at least three flood events are associated with a later glaciation which postdates the organic silt. A minimum age for the floods is provided by a glass fission track age of 560,000 ± 80,000 yr on the GI tephra, which occurs 8 m above the flood beds. A maximum age of 780,000 yr for the floods is based on normal magnetic polarity of the sediments. These age constraints allow us to correlate the flood events to the early-middle Pleistocene. And further, the outburst floods indicate extensive glaciation of the Yukon-Tanana Upland during the early-middle Pleistocene, likely representing the most extensive Pleistocene glaciation of the area.  相似文献   

11.
Loess and dune sands that mantle volcanic rocks on the northwest flank of Mauna Kea volcano consist predominantly of fine-grained pyroclasts of the alkalic Laupahoehoe Volcanics produced by explosive eruptions. The loess is divided into lower and upper units, separated by a well-developed paleosol, while older and younger dune sands are separated by loess. Four interstratified tephra marker horizons aid in regional stratigraphic correlation. Radiocarbon ages of charcoal fragments within the loess, U-series ages of rhizoliths in the dune sand, and K/Ar ages and relative stratigraphic positions of lava flows provide a stratigraphic and temporal framework. The lower loess overlies lava flows less than 103,000 ± 10,000 K/Ar yr old, and14C dates from the paleosol developed at its top average ca. 48,000 yr. Loess separating the dune sand units ranges from ca. 38,000 to 25,00014C yr old; the youngest ages from the upper loess are 17,000–18,00014C yr B.P. Dips of sand-dune foreset strata, isopachs on the upper loess, and reconstructed isopachs representing cumulative thickness of tephra associated with late-Pleistocene pyroclastic eruptions suggest that vents upslope (upwind) from the sand dunes were the primary source of the eolian sediments. Average paleowind directions during the eruptive interval (ca. 50,000–15,000 yr B.P.), inferred from cinder-cone asymmetry, distribution of tephra units, orientation of dune foreset strata, and the regional pattern of loess isopachs, suggest that Mauna Kea has remained within the trade-wind belt since before the last glaciation.  相似文献   

12.
Sediments from Rapid Lake document glacial and vegetation history in the Temple Lake valley of the Wind River Range, Wyoming over the past 11,000 to 12,000 yr. Radiocarbon age determinations on basal detrital organic matter from Rapid Lake (11,770 ± 710 yr B.P.) and Temple Lake (11,400 ± 630 yr B.P.) bracket the age of the Temple Lake moraine, suggesting that the moraine formed in the late Pleistocene. This terminal Pleistocene readvance may be represented at lower elevations by the expansion of forest into intermontane basins 12,000 to 10,000 yr B.P. Vegetation in the Wind River Range responded to changing environmental conditions at the end of the Pleistocene. Following deglaciation, alpine tundra in the Temple Lake valley was replaced by a Pinus albicaulis parkland by about 11,300 14C yr B.P. Picea and Abies, established by 10,600 14C yr B.P., grew with Pinus albicaulis in a mixed conifer forest at and up to 100 m above Rapid Lake for most of the Holocene. Middle Holocene summer temperatures were about 1.5°C warmer than today. By about 5400 14C yr B.P. Pinus albicaulis and Abies became less prominent at upper treeline because of decreased winter snowpack and higher maximum summer temperatures. The position of the modern treeline was established by 3000 14 C yr B.P. when Picea retreated downslope in response to Neoglacial cooling.  相似文献   

13.
The age of the Rockland tephra, which includes an ash-flow tuff south and west of Lassen Peak in northern California and a widespread ash-fall deposit that produced a distinct stratigraphic marker in western North America, is constrained to 565,000 to 610,000 yr by 40Ar/39Ar and U-Pb dating. 40Ar/39Ar ages on plagioclase from pumice in the Rockland have a weighted mean age of 609,000 ± 7000 yr. Isotopic ages of spots on individual zircon crystals, analyzed by the SHRIMP-RG ion microprobe, range from ∼500,000 to ∼800,000 yr; a subpopulation representing crystal rims yielded a weighted-mean age of 573,000 ± 19,000 yr. Overall stratigraphic constraints on the age are provided by two volcanic units, including the underlying tephra of the Lava Creek Tuff erupted within Yellowstone National Park that has an age of 639,000 ± 2000 yr. The basaltic andesite of Hootman Ranch stratigraphically overlies the Rockland in the Lassen Peak area and has 40Ar/39Ar ages of 565,000 ± 29,000 and 565,000 ± 12,000 yr for plagioclase and groundmass, respectively. Identification of Rockland tephra in ODP core 1018 offshore of central California is an important stratigraphic age that also constrains the eruption age to between 580,000 and 600,000 yr.  相似文献   

14.
We present 10Be exposure ages from moraines in the Delta River Valley, a reference locality for Pleistocene glaciation in the northern Alaska Range. The ages are from material deposited during the Delta and Donnelly glaciations, which have been correlated with MIS 6 and 2, respectively. 10Be chronology indicates that at least part of the Delta moraine stabilized during MIS 4/3, and that the Donnelly moraine stabilized ∼ 17 ka. These ages correlate with other dates from the Alaska Range and other regions in Alaska, suggesting synchronicity across Beringia during pulses of late Pleistocene glaciation. Several sample types were collected: boulders, single clasts, and gravel samples (amalgamated small clasts) from around boulders as well as from surfaces devoid of boulders. Comparing 10Be ages of these sample types reveals the influence of pre/post-depositional processes, including boulder erosion, boulder exhumation, and moraine surface lowering. These processes occur continuously but seem to accelerate during and immediately after successive glacial episodes. The result is a multi-peak age distribution indicating that once a moraine persists through subsequent glaciations the chronological significance of cosmogenic ages derived from samples collected on that moraine diminishes significantly. The absence of Holocene ages implies relatively minor exhumation and/or weathering since 12 ka.  相似文献   

15.
K---Ar dates for muscovites and biotites in granitoid rocks and hydrothermal ore deposits of the northeastern parts of the plutons making up the Triassic Carabaya batholith, underlying the axial Cordillera Oriental of northern Puno Department, southeastern Peru, are markedly variable and mutually discordant. Steep transverse gradients are defined in the apparent ages of both micas, which decrease systematically from SW to NE, delimiting a ca. 25-km-wide, longitudinal zone of anomalously young Mesozoic to Paleocene dates. Age minima of 37±1 Ma are attained in three of the four studied transects. 40Ar/39Ar step-heating analyses of selected micas confirm the occurrence of a thermal disturbance, and modeling of the spectra suggests that argon loss in muscovites attains at least ca. 75% in the northeastern part of the zone. A single K-feldspar spectrum yielded a minimum at 31 Ma, and apatite fission-track age cluster at ca. 31 and 18.5 Ma. The affected granitoid rocks generally display little megascopic evidence of tectonism, but microscopic deformational fabrics increase in intensity with apparent decreasing K---Ar age, paralleling a marked increase in alkali feldspar ordering. Secondary fluid inclusions trapped within the microfabrics reveal that the plutonic rocks were penetrated by a homogeneous H2O---CO2---CH4---NaCl fluid at ca. 300–400°C and 0.7–2 kbar. This fluid is implicated in the degassing of the rocks. These diverse data are interpreted as evidence for a major, but moderate-temperature (400°C) and brief, tectono-thermal event at ca. 37±1 Ma (biotite closure temperature)—i.e., at the Eocene-Oligocene boundary. The K-feldspar 40Ar/39Ar data and the Oligocene fission-track dates may record the later stages in the event, whereas the Miocene fission-track dates are tentatively ascribed to a distinct Neogene episode. Essentially identical geochronological and petrological relationships have been documented in the Cordillera Real of northwestern Bolivia by McBride et al. (1987), permitting the delimination of a disturbed belt paralleling the South American plate boundary and more than 450 km long. The tectono-thermal domain, which we term the Zongo-San Gabán Zone, constituted the foreland boundary of the Andean orogen in the vicinity of the Arica Deflection during the late Eocene Incaic orogeny. This regional thermal event, which involved the basement, appears to have resulted from compressional or, in some segments, transpressional tectonics.  相似文献   

16.
Pollen diagrams from Joe and Niliq Lakes date to ca. 28,000 and 14,000 yr B.P., respectively. Mesic shurb tundra grew near Joe Lake ca. 28,000 to 26,000 yr B.P. with local Populus populations prior to ca. 27,000 yr B.P. Shrub communities decreased as climate changed with the onset of Itkillik II glaciation (25,000 to 11,500 yr B.P.), and graminoid-dominated tundra characterized vegetation ca. 18,500 to 13,500 yr B.P. Herb tundra was replaced by shrub Betula tundra near both sites ca. 13,500 yr B.P. with local expansion of Populus ca. 11,000 to 10,000 yr B.P. and Alnus ca. 9000 yr B.P. Mixed Picea glauca/P. mariana woodland was established near Joe Lake ca. 6000 yr B.P. These pollen records when combined with others from northern Alaska and northwestern Canada indicate (1) mesic tundra was more common in northwestern Alaska than in northeastern Alaska or northwestern Canada during the Duvanny Yar glacial interval (25,000 to 14,000 yr B.P.); (2) with deglaciation, shrub Betula expanded rapidly in northwestern Alaska but slowly in areas farther east; (3) an early postglacial thermal maximum occurred in northwestern Alaska but had only limited effect on vegetation; and (4) pollen patterns in northern Alaska and northwestern Canada suggest regional differences in late Quaternary climates.  相似文献   

17.
The extent of glaciation in northwestern Alaska, the source of sediment supply to the Chukchi shelf and slope, and the movement of sea ice and icebergs across the shelf during the last glacial maximum (LGM) remain poorly constrained. Here we present geophysical and geological data from the outer Chukchi margin that reveal a regionally extensive, heavily ice-scoured surface ∼ 5-8 m below the modern seafloor. Radiocarbon dating of this discrete event yields age estimates between 10,600 and 11,900 14C yr BP, indicating the discharge event occurred during the Younger Dryas. Based on mineralogy of the ice-rafted debris, the icebergs appear to be sourced from the northwestern Alaskan margin, which places important constraints on the ice extent in northern Alaska during the LGM as well as existing circulation models for the region.  相似文献   

18.
At Lago Buenos Aires, Argentina, 10Be, 26Al, and 40Ar/39Ar ages range from 190,000 to 109,000 yr for two moraines deposited prior to the last glaciation, 23,000–16,000 yr ago. Two approaches, maximum boulder ages assuming no erosion, and the average age of all boulders and an erosion rate of 1.4 mm/103 yr, both yield a common estimate age of 150,000–140,000 yr for the two moraines. The erosion rate estimate derives from 10Be and 26Al concentrations in old erratics, deposited on moraines that are >760,000 yr old on the basis of interbedded 40Ar/39Ar dated lavas. The new cosmogenic ages indicate that a major glaciation during marine oxygen isotope stage 6 occurred in the mid-latitude Andes. The next five youngest moraines correspond to stage 2. There is no preserved record of a glacial advance during stage 4. The distribution of dated boulders and their ages suggest that at least one major glaciation occurred between 760,000 and >200,000 yr ago. The mid-latitude Patagonian glacial record, which is well preserved because of low erosion rates, indicates that during the last two glacial cycles major glaciations in the southern Andes have been in phase with growth and decay of Northern Hemisphere ice sheets, especially at the 100,000 yr periodicity. Thus, glacial maxima are global in nature and are ultimately paced by small changes in Northern Hemisphere insolation.  相似文献   

19.
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.  相似文献   

20.
Improved chronological control on the penultimate advance of the Cordilleran Ice Sheet in northwest Canada (the Reid glaciation) is required for a better understanding of late Quaternary palaeoclimatic and palaeoenvironmental change in eastern Beringia. However, reliable dating of glaciation events beyond the last glacial maximum is commonly hindered by a lack of directly dateable material. In this study we (i) provide the first combined minimum and maximum age constraint on the Reid glaciation at Ash Bend, its reference locale in the Stewart River valley, northwestern Canadian Cordillera, using single-grain optically stimulated luminescence dating of quartz; and (ii) compare the timing of the Reid glaciation with other penultimate ice sheet advances in the region with the aim of establishing improved glacial reconstructions in eastern Beringia. We obtain ages of 158 ± 18 ka and 132 ± 18 ka for glaciofluvial sands overlying and underlying the Reid till, respectively. These ages indicate that the Reid advance, at its reference locale, occurred during MIS 6. This precludes an earlier MIS 8 age, and suggests that the Reid advance may have been synchronous with the Delta glaciation of central Alaska, and is likely correlative with the Mirror Creek glaciation in southern Yukon.  相似文献   

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