the palisades is a key reference site for the middle pleistocene of eastern beringia new evidence from paleomagnetics and regional tephrostratigraphy abstract the palisades in central alaska is one of the most prominent exposures of quaternary sediments on the yukon river perennially frozen silt and sand at the palisades are presently thought to preserve paleoenvironmental records from the holocene to marine isotope stage mis 8 and beneath a major unconformity the earliest pleistocene 2 ma we present new paleomagnetic and tephrochronologic constraints that substantially revise the age of the sediments at the palisades we describe 15 new tephra beds including five beds below the prominent pal tephra that correlate to known tephra with independent age control from other sites in eastern beringia these five known tephra include chester bluff tephra which is present in east central alaska and the yukon and the newly named alyeska pipeline and taylor highway tephra from central alaska all are constrained to the middle pleistocene paleomagnetic transects from the base of the bluff to the mis 5e forest bed yield normal polarity with the exception of a brief reversal event between old crow tephra 124 10 ka and the mis 5e forest bed that is likely the first documentation of the blake paleomagnetic event in alaskan loess the detailed tephrostratigraphy and paleomagnetic data collectively suggest that most of the sedimentary record at the palisades is middle pleistocene in age the palisades thus preserves a rare record of late to middle pleistocene paleoenvironments with multiple regionally distributed tephra beds

old crow tephra a new late pleistocene stratigraphic marker across north central alaska and western yukon territory old crow tephra is the first extensive pleistocene tephra unit to be documented in the northwestern part of north america it has a calc alkaline dacitic composition with abundant pyroxene plagioclase and fe ti oxides and minor hornblende biotite apatite and zircon thin clear bubble wall fragments are the dominant type of glass shard this tephra can be recognized by its glass and phenocryst compositions as determined by x ray fluorescence microprobe and instrumental neutron activation techniques it has an age between the limits of 60 000 and 120 000 yr set by 14c and fission track measurements respectively old crow tephra has been recognized in the koyukuk basin and fairbanks region of alaska and in the old crow lowlands of the northern yukon territory some 600 km to the east northeast the source vent is unknown but these occurrences considered in relation to the distant locations of potential quaternary volcanic sources demonstrate the widespread distribution of this tephra and underscore its importance as a regional stratigraphic marker

the application of icp ms methods to tephrochronological problems pearce nicholas westgate j a perkins w t preece s j 2004 the application of icp ms methods to tephrochronological problems applied gechemistry 19 3 pp 289 322 rae2008

stratigraphic context of old crow tephra holitna lowland interior southwest alaska abstract a thick deposit of old crow tephra was discovered in a bluff exposure along the middle holitna river near the kulukbuk hills 61 20 n latitude 157 10 w longitude in interior southwest alaska this locality is the southwesternmost known deposit of old crow tephra in alaska thickness and grain size data from this site support a source volcano in the eastern aleutian arc pleistocene stratigraphic sequences in the lowland are dominated by upward fining eolian sand sheet deposits and loess separated by organic silt these deposits record at least two episodes of regional glaciation and an intervening nonglacial period marine oxygen isotope stage 3 stage 5 or both old crow tephra crops out near the top of the lower upward fining eolian unit indicating that the ash erupted near the end of an interval of periglacial eolian sedimentation the sequence of eolian deposits that contain old crow tephra probably accumulated during the latter part of marine oxygen isotope stage 6 whereas the overlying eolian sequence formed during the last glaciation stage 2 this stratigraphic position is consistent with other stratigraphic contexts for the tephra and with fission track and thermoluminescence ages of ca 140 000 10 000 yr b p

formal ratification of the quaternary system period and the pleistocene series epoch with a base at 2 58 ma in june 2009 the executive committee of the international union of geological sciences iugs formally ratified a proposal by the international commission on stratigraphy to lower the base of the quaternary system period to the global stratotype section and point gssp of the gelasian stage age at monte san nicola sicily italy the gelasian until then had been the uppermost stage of the pliocene series epoch the base of the gelasian corresponds to marine isotope stage 103 and has an astronomically tuned age of 2 58 ma a proposal that the base of the pleistocene series epoch be lowered to coincide with that of the quaternary the gelasian gssp was also accepted by the iugs executive committee the gssp at vrica calabria italy which had hitherto defined the basal boundary of both the quaternary and the pleistocene remains available as the base of the calabrian stage age now the second stage of the revised pleistocene in ratifying these proposals the iugs has acknowledged the distinctive qualities of the quaternary by reaffirming it as a full system period correctly complied with the hierarchical requirements of the geological timescale by lowering the base of the pleistocene to that of the quaternary and fully respected the historical and widespread current usage of both the terms quaternary and pleistocene copyright 2009 john wiley sons ltd

vegetation and paleoclimate of the last interglacial period central alaska the last interglacial period is thought to be the last time global climate was signi cantly warmer than present new stratigraphic studies at eva creek near fairbanks alaska indicate a complex last interglacial record wherein periods of loess deposition alternated with periods of soil formation the eva forest bed appears to have formed about the time of or after deposition of the old crow tephra dated to 160 to 120 ka and is therefore correlated with the last interglacial period pollen macrofossils and soils from the eva forest bed indicate that boreal forest was the dominant vegetation and precipitation may have been greater than present around fairbanks during the peak of the last interglacial period a new compilation of last interglacial localities indicates that boreal forest was extensive over interior alaska and yukon territory boreal forest also extended beyond its present range onto the seward and baldwin peninsulas and probably migrated to higher elevations now occupied by tundra in the interior comparison of last interglacial pollen and macrofossil data with atmospheric general circulation model results shows both agreement and disagreement model results of warmer than present summers are in agreement with fossil data however numerous localities with boreal forest records are in con ict with model reconstructions of an extensive cool steppe in interior alaska and much of yukon territory during the last interglacial 2000 elsevier science ltd all rights reserved

a late middle pleistocene marine isotope stage 6 vegetated surface buried by old crow tephra at the palisades interior alaska abstract a 40 cm thick primary bed of old crow tephra 131 11 ka an important stratigraphic marker in eastern beringia directly overlies a vegetated surface at palisades west on the yukon river in central alaska analyses of insect bryophyte and vascular plant macrofossils from the buried surface and underlying 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 directly overlying 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 reworking from older deposits rather than the local presence of trees the revised glass fission track age for old crow tephra places its deposition closer to the time of the last interglaciation than earlier age determinations but stratigraphy and paleoecology of sites with old crow tephra indicate a late marine isotope stage 6 age regional permafrost degradation and associated thaw slumping are responsible for the close stratigraphic and paleoecological relations between old crow tephra and last interglacial deposits at some sites in eastern beringia

the full glacial environment of the northern seward peninsula alaska reconstructed from the 21 500 year old kitluk paleosol abstract paleoenvironmental conditions are reconstructed from soils buried under volcanic ash ca 21 500 years ago on the seward peninsula soil development was minimal reflecting the continuous regional deposition of loess which originated from river floodplains and the exposed chukchi shelf cryoturbated soil horizons ice wedges and ice lens formation indicate a permafrost environment and mean annual temperatures below 6 to 8 c shallow active layers average 45 cm minimal evidence for chemical leaching of soils and the presence of earthen hummocks indicate a cold and seasonally dry climate neither steppe nor polar desert soils are appropriate analogues for these zonal soils of loess covered central beringia no exact analogues are known however soils underlying dry tundra near the arctic coast of northern yakutia russia and under moist nonacidic tundra of the alaskan north slope have properties in common with the buried soils

tephrochronology magnetostratigraphy and mammalian faunas of middle and early pleistocene sediments at two sites on the old crow river northern yukon territory canada abstract alluvial and lacustrine sediments exposed beneath late pleistocene glaciolacustrine silt and clay at two sites along the old crow river northern yukon territory are rich in fossils and contain tephra beds surprise creek tephra szt occurs in the lower part of the alluvial sequence at crh47 and little timber tephra ltt is present near the base of the exposure at crh94 surprise creek tephra has a glass fission track age of 0 17 0 07 ma and little timber tephra is 1 37 0 12 ma all sediments at crh47 have a normal remanent magnetic polarity and those near ltt at crh94 have a reversed polarity in agreement with the geomagnetic time scale small mammal remains from sediments near ltt support an early pleistocene age but the chronology is not so clear at crh47 because of the large error associated with the szt age determination tephrochronological and paleomagnetic considerations point to an mis 7 age for the interglacial beds just below szt at crh47 and at chester bluffs in east central alaska but mammalian fossils recovered from sediments close to szt suggest a late irvingtonian age therefore older than mis 7 further studies are needed to resolve this problem

thermoluminescence properties of a deglacial marine sequence from cumberland sound arctic canada sedimentologic and geochronologic implications abstract thermoluminescence tl analysis of glacial marine and postglacial sediments in core 85027 029 from cumberland sound baffin island shows that the tl signal can be used to differentiate glacial marine sediments deposited extremely close 4 to 10 5 yrs

cryostratigraphy of the klondike muck deposits west central yukon territory four late quaternary cryostratigraphic units are recognized in the unconsolidated valley bottom deposits of the klondike area yukon territory three of the units in ice rich loessal sediments of pre wisconsinan or wisconsinan age collectively compose the king solomon formation they are overlain by a holocene organic unit the units are distinguished by their cryostratigraphic characteristics and oxygen isotope ratios of included ground ice the basal unit is the last chance creek member a pre late wisconsinan deposit containing preserved ice wedges 18o 28 to 26 d 225 to 209 the overlying quartz creek member a late wisconsinan unit is dominated by organic rich loess massive ice is noticeably absent although the sediments are ice rich the isotopic composition of ice in this unit is characteristic of full glacial conditions 18o 32 to 29 d 234 to 257 an abrupt change to warmer and wetter conditions at the end of glaciation prior to the holocene is recorded by the i

an 18 million year record of vegetation and climate change in northwestern canada and alaska tectonic and global climatic correlates abstract we reconstruct long term vegetation paleoclimatic trends spanning the last 18 million years in alaska yukon and far western northwest territories twenty one average percentage spectra for pollen and spores are assembled from eight surface subsurface sections the sections are dated independently or by correlation pollen and spore ratios indicate the direction of change in vegetation and climatic parameters growing season temperature test tree canopy density cest and paludification at study sites pest a global warm peak ca 15 ma is shown by the abundance of thermophilous taxa including fagus and quercus a temperature decline immediately following 15 ma parallels climatic reconstructions based on marine oxygen isotopes subsequent declines correlate to the messinian event and the onset of late pliocene pleistocene glaciation after 7 ma herbs and shrubs become more important elements of the palynological assemblages suggesting a more continental colder drier climate however a late pliocene warm interval is evident vegetation climatic changes during the early to late miocene show synchrony with and are most economically attributable to global events after 7 ma vegetation climate change is attributed primarily to latest miocene to pleistocene uplift of the alaska range and st elias mts the continuing influence of global climatic patterns is shown in the late pliocene warm interval despite uplift to the south the opening of the bering strait ca 3 ma may have moderated the climate in the study area

stratigraphy and palaeoclimatic significance of late quaternary loess palaeosol sequences of the last interglacial glacial cycle in central alaska abstract loess is one of the most widespread subaerial deposits in alaska and adjacent yukon territory and may have a history that goes back 3 ma based on mineralogy and major and trace element chemistry central alaskan loess has a composition that is distinctive from other loess bodies of the world although it is quartz dominated central alaskan loess was probably derived from a variety of rock types including granites metabasalts and schists detailed stratigraphic data and pedologic criteria indicate that contrary to early studies many palaeosols are present in central alaskan loess sections the buried soils indicate that loess sedimentation was episodic or at least rates of deposition decreased to the point where pedogenesis could keep ahead of aeolian input as in china loess deposition and pedogenesis are likely competing processes and neither stops completely during either phase of the loess soil formation cycle loess deposition in central alaska took place before and probably during the last interglacial period during stadials of the mid wisconsin period during the last glacial period and during the holocene an unexpected result of our geochronological studies is that only moderate loess deposition took place during the last glacial period our studies lead us to conclude that vegetation plays a key role in loess accumulation in alaska factors favouring loess production are enhanced during glacial periods but factors that favour loess accumulation are diminished during glacial periods the most important of these is vegetation boreal forest serves as an effective loess trap but sparsely distributed herb tundra does not thus thick accumulations of loess should not be expected where tundra vegetation was dominant and this is borne out by modern studies near the treeline in central alaska much of the stratigraphic diversity of north american loess including that found in the central lowlands the great plains and alaska is explained by a new model that emphasizes the relative importance of loess production factors versus loess accumulation factors

interglacial deposits at birch creek northeast interior alaska abstract a 12 m sequence of lake sediment and peat in a 45 m high exposure on birch creek northeast alaska contains pollen of picea betula alnus and populus and wood of picea and populus this sequence which may represent 10 000 yr of more of accumulation is beyond the limit of radiocarbon dating it lies between two units of loess the underlying loess lies above the old crow tephra recently dated at 149 000 13 000 yr b p the lake sediments probably were deposited during the last interglaciation isotope substage 5e and subsequently buried by wisconsinan loess analogs for the ancient lake may be deep long lived thaw lakes that are present in the modern landscape when birch creek is correlated with other sites across nonglaciated alaska and northwest canada there appears to be a common interglacial signal in sediments overlying the old crow tephra

luminescence chronology of late pleistocene loess paleosol and tephra sequences near fairbanks alaska abstract thermoluminescence tl and infrared stimulated luminescence irsl sediment dating methods have been applied to paleosol and tephra bearing loess sequences younger than marine oxygen isotope stage mis 7 at three important sites tl ages indicate the development of significant paleosols 75 000 and 30 000 yr ago in the loess sequence at the gold hill site relatively minor soil development occurred 70 000 and 48 000 yr ago like the 75 000 yr old soil the 30 000 yr old soil is apparently of global extent and consistent in timing with inferred warm intervals elsewhere e g greenland europe western and central china at birch hill replicate tl dating of primary loess combined with two earlier tl results from the same site and with an earlier mean fission track glass shard age of 140 000 10 000 yr for the associated old crow tephra yield a more precise numeric age of 142 300 6600 yr for this alaska yukon chronostratigraphic marker ash bed three of the tl ages at the halfway house site are difficult to interpret but combined with other evidence they indicate 1 the upper 5 6 m of loess from halfway house is not part of the gold hill loess equivalent to pre mis 5 age as long thought by t l pewe but rather is much younger 2 the regionally significant variegated tephra found in the fairbanks and klondike areas and previously thought to be older than mis 5 has an age of 77 800 4100 yr late mis 5

the last interglaciation in alaska stratigraphy and paleoecology of potential sites at least 20 localities in alaska contain deposits that may provide information on the last interglaciation oxygen isotope substage 5e these widely dispersed localities include river bluffs coastal bluffs and terraces elevated marine shorelines lake basins and artificial excavations most of the inferred interglacial deposits contain macrofossils or pollen that are older than the range of radiocarbon dating and commonly indicate climate as warm as or warmer than the present at a few localities evidence for deep thaw of permafrost also indicates a warm paleoclimate at eight localities the old crow tephra occurs at or below organic deposits that may represent substage 5e the tephra occurs beneath conspicuous organic deposits at fairbanks the yukon palisades and holitna lowland and directly above a peat bed at hogatza mine at birch creek halfway house ky 11 and imuruk lake the tephra occurs within a paleosol or organic deposit but other organic horizons that more likely indicate interglacial conditions occur at higher stratigraphic levels the varied stratigraphic relations of the old crow tephra suggest that it may have been deposited close to the boundary between isotope substages 6 and 5 which is dated at about 130 ka in the marine record and between 132 and 140 ka on land these age relations suggests that the tephra may have been deposited about 135 5 ka validating the recent fission track age determination of 140 10 ka for this deposit six coastal localities contain deposits of probable interglacial age and these commonly are associated with evidence for eustatic sea levels higher than those of the present beach and sublittoral sediments of the pelukian transgression occur up to 12 m asl along the northwest coast of alaska and are correlative with barrier island and lagoonal sediments on the alaskan arctic coastal plain both sets of deposits commonly contain extralimital mollusks and microfauna that indicate marine water slightly warmer than present and suggest that seasonal sea ice did not extend south of bering strait during the last interglacial as it does today farther south elevated marine terrace deposits on amchitka island contain marine invertebrates that indicate a climate warmer than at present peat horizons in coastal exposure at goose bay and coastal terraces at lituya bay contain pollen spectra that suggest forests like those of the present day and spruce macrofossils exposed on baldwin peninsula indicate boreal forest more extensive than at present sediments from several lakes in northwestern alaska may contain continuous records of the last interglaciation a major warm interval possibly isotope substage 5e has been identified in a core from squirrel lake by a peak in picea pollen that indicates forest extension beyond present limits similar pollen records are potentially available from two maars which formed in the cape espenberg area more than 125 ka terrestrial organic deposits thought to record the last interglaciation occur interstratified with marine and glaciogenic sediments in the nushagak lowland of southwest alaska and on baldwin peninsula in kotzebue sound extensive exposures along the copper and nenana rivers may also contain organic deposits that record the last interglaciation

identification of last interglacial deposits in eastern beringia a cautionary note from the palisades interior alaska last interglacial sediments in unglaciated alaska and yukon eastern beringia are commonly identified by palaeoe cological indicators and stratigraphic position 2 5 m above the regionally prominent old crow tephra 124 10 ka we demonstrate that this approach can yield erroneous age assignments using data from a new exposure at the palisades a site in interior alaska with numerous exposures of last interglacial sediments tephrochronology stratigraphy plant macrofossils pollen and fossil insects from a prominent wood rich organic silt unit are all consistent with a last interglacial age assignment however six 14 c dates on plant and insect macrofossils from the organic silt range from non finite to 4 0 14 c ka bp indicating that the organic silt instead represents a holocene deposit with a mixed age assemblage of organic material in contrast wood samples from presumed last interglacial organic rich sediments elsewhere at the palisades in a similar stratigraphic position with respect to old crow tephra yield non finite 14 c ages given that local permafrost thaw since the last interglaciation may facilitate reworking of older sediments into new stratigraphic positions minimum constraining ages based on 14 c dating or other methods should supplement age assignments for last interglacial sediments in eastern beringia that are based on palaeoecology and stratigraphic association with old crow tephra copyright 2011 john wiley sons ltd

thermoluminescence dating applied to a thin winter varve of the late glacial south thompson silt south central british columbia the partial bleach r procedure of thermoluminescence tl dating of unheated sediments has been applied to the 2 4 m sized feldspar dominated grains from two components of a 10 11 ka bp glaciolacustrine silt a previous tl study of the 4 11 m sized feldspars from a 12 cm thick summer layer of this varved deposit did not produce the correct age here it is shown that the 2 4 m feldspar grains from a contiguous 5 mm thick clay rich winter varve give a satisfactory tl apparent age of 14 2 2 3 ka on the other hand and consistent with the previous results the 2 4 m feldspar grains from the thicker summer layer yield an incorrect high apparent age of 55 13 ka these results have implications for general tl dating of waterlaid sediments

permafrost response to last interglacial warming field evidence from non glaciated yukon and alaska abstract we present stratigraphic observations from three sites in eastern beringia ch ijee s bluff in northern yukon and nearby exposures on the old crow river the palisades on the yukon river in alaska and placer mining exposures at thistle creek in west central yukon which provide insight into the response of permafrost to regional warming during the last interglaciation chronology is based on the presence of old crow tephra an important regional stratigraphic marker that dates to late marine isotope stage 6 supplemented by paleoecology and non finite 14c ages on wood rich organic silts old crow tephra overlies several relict ice wedges at the palisades and thistle creek indicating that permafrost at these sites did not thaw completely during the last interglaciation prominent deposits of last interglacial wood rich organic silt are present at multiple sites in eastern beringia and probably represent accumulations of reworked forest vegetation due to thaw slumping or deposition into thermokarst ponds or depressions consistent stratigraphic relations between these deposits old crow tephra and ice wedge pseudomorphs at our three study sites and at least six other sites in eastern beringia suggest that thaw of shallow permafrost was widespread during the last interglaciation limited stratigraphic evidence suggests that thaw was probably on the order of meters rather than 10s of meters the ubiquity of shallow permafrost degradation during the last interglaciation suggests that current ground warming may foreshadow widespread near surface thaw under even modest future warming scenarios however the persistence of relict pre last interglacial ice wedges highlights the potential for the regional antiquity of discontinuous permafrost and provides compelling field evidence for the long term resilience of deep permafrost during sustained periods of warmer than present climate

middle to late pleistocene ice extents tephrochronology and paleoenvironments of the white river area southwest yukon abstract sedimentary deposits from two middle to late pleistocene glaciations and intervening non glacial intervals exposed along the white river in southwest yukon canada provide a record of environmental change for much of the past 200 000 years the study sites are beyond the marine isotope stage mis 2 glacial limit near the maximum regional extent of pleistocene glaciation non glacial deposits include up to 25 m of loess peat and gravel with paleosols pollen plant and insect macrofossils large mammal fossils and tephra beds finite and non finite radiocarbon dates and twelve different tephra beds constrain the chronology of these deposits tills correlated to mis 4 and 6 represent the penultimate and maximum pleistocene glacial limits respectively the proximity of these glacial limits to each other compared to limits in central yukon suggests precipitation conditions were more consistent in southwest yukon than in central yukon during the pleistocene conditions in mis 5e and 5a are recorded by two boreal forest beds separated by a shrub birch tundra that indicate environments as warm or warmer than present a dry treeless steppe tundra dominated by artemisia frigida upland grasses and forbs existed during the transition from late mis 3 to early mis 2 these glacial and non glacial deposits constrain the glacial limits and paleoenvironments during the middle to late pleistocene in southwest yukon

uranium isotopes and dissolved organic carbon in loess permafrost modeling the age of ancient ice abstract the residence time of ice in permafrost is an indicator of past climate history and of the resilience and vulnerability of high latitude ecosystems to global change development of geochemical indicators of ground ice residence times in permafrost will advance understanding of the circumstances and evidence of permafrost formation preservation and thaw in response to climate warming and other disturbance we used uranium isotopes to evaluate the residence time of segregated ground ice from ice rich loess permafrost cores in central alaska activity ratios of 234 u vs 238 u 234 u 238 u in water from thawed core sections ranged between 1 163 and 1 904 due to contact of ice and associated liquid water with mineral surfaces over time measured 234 u 238 u values in ground ice showed an overall increase with depth in a series of five neighboring cores up to 21 m deep this is consistent with increasing residence time of ice with depth as a result of accumulation of loess over time as well as characteristic ice morphologies high segregated ice content and wedge ice all of which support an interpretation of syngenetic permafrost formation associated with loess deposition at the same time stratigraphic evidence indicates some past sediment redistribution and possibly shallow thaw among cores with local mixing of aged thaw waters using measures of surface area and a leaching experiment to determine u distribution a geometric model of 234 u 238 u evolution suggests mean ages of up to 200 ky bp in the deepest core with estimated uncertainties of up to an order of magnitude evidence of secondary coatings on loess grains with elevated 234 u 238 u values and u concentrations suggests that refinement of the geometric model to account for weathering processes is needed to reduce uncertainty we suggest that in this area of deep ice rich loess permafrost ice bodies have been preserved from the last glacial period 10 100 ky bp despite subsequent fluctuations in climate fire disturbance and vegetation radiocarbon 14 c analysis of dissolved organic carbon doc in thaw waters supports ages greater than 40 ky bp below 10 m doc concentrations in thaw waters increased with depth to maxima of 1000 ppm despite little change in ice content or cryostructures these relations suggest time dependent production of old doc that will be released upon permafrost thaw at a rate that is mediated by sediment transport among other factors

the last interglacial to glacial transition togiak bay southwestern alaska an 18 m high coastal bluff at togiak bay northwestern bristol bay southwestern alaska exposes marine lacustrine fluvial glacial volcanic and organic deposits that record the 50 000year long transition from the peak of the last interglaciation to the early wisconsin glaciation the base of the section is dominated by stratified sand and silt extending up to 4 3 m above sea level marine diatoms are present and pollen assemblages are characterized by relatively high percentages of picea alnus and betula and low percentages of poaceae and cyperaceae the marine sediment was probably deposited during the peak of marine oxygen isotope stage ois 5e an infrared stimulated luminescence irsl age of 151 000 13 000 yr from near the base of the exposure is permissive of this correlation the marine sand and silt are overlain by 0 8 m of peaty silt with diatoms that record a transition from marine to lacustrine conditions during this interval poaceae and cyperaceae dominate the pollen assemblages and picea and shrubs are nearly absent suggesting that herb tundra occupied the landscape this interval probably encompasses ois 5d on the basis of the herb tundra and an irsl age of 119 000 10 000 yr from 60 cm below the marine lacustrine transition the organic mud is overlain by 3 1 m of stratified sand and organic silt that apparently record shallowing of the lake reappearance of spruce and shrubs dois 5c and subsequent deepening of the lake dois 5b followed by aggradation of a floodplain dois 5a which was dry at the time basaltic lava buried the site thermoluminescence analyses on lava baked sediment indicate that the eruption occurred 70 000 10 000 yr ago sometime thereafter but prior to 53 600 14 c yr b p an outlet of the ahklun mountains ice cap advanced over the site and deposited 7 m of bouldery ice contact drift the sedimentary sequence contains at least four tephra beds major and trace element chemistry provide a basis for correlating two of the tephras with tephra beds at nearby sites the tephras luminescence ages and correlations with marine isotope stages provide the geochronological control to place the togiak bay section into a global context the site serves as an important new reference section for late pleistocene paleoenvironmental change in eastern beringia

middle pleistocene mis 7 to holocene fossil insect assemblages from the old crow basin northern yukon canada abstract the old crow basin northern yukon provides a key record of environmental change in northwestern north america from the late middle pleistocene through the early holocene site chronologies are based on the presence of the old crow tephra oct 124 10 ka at three sites and provide a stratigraphic framework for observations ecological affinities of fossil insects indicate that pleistocene environments were dominated by dry tundra and steppe tundra forest insect species are relatively rare even during the last interglaciation mis 5e and it is only by considering the total insect assemblages that interglacial beds can be recognized last interglacial insects from these sites indicate a relatively wet and warm tundra environment with evidence for sparse forest vegetation rare taxa with strong affinities to warmer temperatures are present and could suggest a warmer than modern climate early holocene deposits yield a greater abundance of forest insects relative to mis 5 or 7 interglacial deposits fossil insect assemblages reflect the position of the sites above the arctic circle and in contrast to central yukon sites steppe insects are less common during cold stages and forest insects are less common during warm stages these data suggest overall that the contrast between cold and warm stages was less pronounced than in other regions of yukon and alaska and may indicate influence of persistent large regional lakes during the pleistocene

the variegated vt tephra a new regional marker for middle to late marine isotope stage 5 across yukon and alaska abstract the variegated vt tephra likely sourced from the eastern aleutian arc has a geographic distribution that places it amongst the most widespread tephra beds in eastern beringia first identified in the fairbanks area of interior alaska it has been identified at eight additional sites ranging from togiak bay in southwestern alaska to the klondike area of west central yukon correlation of these occurrences is established through the equivalence of glass major and trace element geochemistry fe ti oxide geochemistry stratigraphy and independent age data in yukon and alaska vt tephra has a minimum bulk tephra volume estimate of 32 km3 previous age estimates for vt tephra have varied ranging from a glass fission track age of 125 30 ka to a weighted mean thermoluminescence tl age of 77 8 4 1 ka from bracketing ages on loess a new infrared stimulated luminescence irsl age of 106 10 ka paleoenvironmental data and several tl and irsl ages from togiak bay suggest that the time of deposition is more likely between these previous age estimates post marine isotope stage mis 5e but underlying a prominent soil likely associated with mis 5c placing it within late mis 5d

on the eruption age and provenance of the old crow tephra abstract tephrochronology is used to correlate and reconstruct geographically disparate sedimentary records of changing environment climate and landscape throughout geologic time single tephra layers represent isochronous markers across broad regions thus accurate and precise radiometric constraints on the timing of eruption are critical to their utility the old crow tephra is found throughout eastern beringia and represents the largest preserved pleistocene ashfall event in the region despite its volume and significance as a stratigraphic marker the provenance of this tephra is debated and the interpreted eruption age of marine isotope stage mis 5 at 125 ka has vacillated to investigate provenance and eruption age we develop a geochemical fingerprint for the old crow tephra via titanomagnetite geochemistry and zircon crystallization cooling age via coupled u pb u th and u th he zircon geochronology our results indicate that old crow oxides are geochemically distinct from the commonly assumed source caldera system at the emmons lake volcanic center elvc zircon crystals from the old crow tephra range in age from proterozoic to pleistocene with concordant zircon u pb u th and u th he dates on the youngest population of grains suggesting crystallization in their parent magma and in turn eruption at 202 9 9 5 ka we discuss strengths and shortcomings of our radiogenic datasets in light of this result and review the far reaching implications of a change in old crow eruption age

150 000 years of loess accumulation in central alaska abstract the halfway house site in interior alaska is arguably the most studied loess deposit in northwestern north america the site contains a complex paleomagnetic and paleoenvironmental record but has lacked the robust chronologic control that would allow its full potential to be exploited detailed reexamination of stratigraphy paleomagnetics and tephrostratigraphy reveals a relatively complete marine isotope stage mis 6 to holocene record constrained by the old crow 124 10 ka vt 106 10 ka sheep creek klondike ca 80 ka dominion creek 77 8 ka and dawson ca 30 2 cal ka bp tephras we show two well developed paleosols formed during marine isotope stages mis 5e and 5a while mis 5c and 5b are either poorly represented or absent the new tephrostratigraphy presented here is the most complete one to date for the late pleistocene and indicates mis 5 sediments are more common than previously recognized a magnetic excursion within the sediments is identified as the post blake excursion 94 1 7 8 ka providing independent age control and adding to the increasing body of evidence that alaskan loess is a detailed recorder of variations of the earth s magnetic field over time a high resolution magnetic susceptibility profile placed into this new chronostratigraphic framework supports the hypothesis that wind intensity is the main variable controlling fluctuations in susceptibility correlation of the susceptibility record to global marine 18o records is complicated by highly variable accumulation rates we find the lowest rates of accumulation during peak warm and cold stages while abrupt increases are associated with periods of transition between marine isotope sub stages building on previous accumulation models for alaska surface roughness is likely a leading variable controlling loess accumulation rates during transitions and peak cold periods but the negligible accumulation during mis 5e and 5a suggests that loess production was exceedingly low negating the role of surface roughness this interplay of variables leads to optimal conditions for loess accumulation during transitions between isotope stages and to a somewhat lesser extent stadials and interstadials

dawson tephra a widespread 29 ka marker bed in a marine core from patton seamount off the alaska peninsula and its potential marine terrestrial correlation

zeroing of the tl signal in sediment undergoing fluvioglacial transport an example from austerdalen western norway abstract study of levels of natural tl in sediment undergoing fluvial transport in austerdalen norway shows that bleaching does occur but does not follow a regular pattern this is attributed to the incorporation of older unbleached material into the sediment load as a result of riverbank erosion levels of residual tl are high enough to raise doubts about the accuracy of fine grain tl dating of fluvioglacial sediments which have experienced only a short distance of transport

characterization identity distribution and source of late cenozoic tephra beds in the klondike district of the yukon canada a large number of distal silicic tephra beds have been preserved in the late cenozoic deposits of the klondike region yukon territory forty one tephra samples representing twelve distinctive beds are detailed in this study they range in composition from basaltic andesite to high silica rhyolite and were deposited during the late pliocene to late wisconsinan time interval seven tephra beds are derived from volcanoes in the wrangell volcanic field and four come from the more distant eastern aleutian arc alaska peninsula region but the source of the single andesitic tephra is unknown the widespread and well known old crow and sheep creek tephra beds have been identified in the klondike district but all the other tephra units are characterized in detail for the first time the ages of most tephra beds are poorly constrained but will undoubtedly become better known with the application of recently developed glass fission track methods hence prospects are favourable for the eventual development

late quaternary paleoenvironmental records from the chatanika river valley near fairbanks alaska abstract perennially frozen deposits are considered as excellent paleoenvironmental archives similar to lacustrine deep marine and glacier records because of the long term and good preservation of fossil records under stable permafrost conditions a permafrost tunnel in the vault creek valley chatanika river valley near fairbanks exposes a sequence of frozen deposits and ground ice that provides a comprehensive set of proxies to reconstruct the late quaternary environmental history of interior alaska the multi proxy approach includes different dating techniques radiocarbon accelerator mass spectrometry ams 14 c optically stimulated luminescence osl thorium uranium radioisotope disequilibria 230 th u as well as methods of sedimentology paleoecology hydrochemistry and stable isotope geochemistry of ground ice the studied sequence consists of 36 m thick late quaternary deposits above schistose bedrock main portions of the sequence accumulated during the early and middle wisconsin periods the lowermost unit a consists of about 9 m thick ice bonded fluvial gravels with sand and peat lenses a late sangamon mis 5a age of unit a is assumed spruce forest with birch larch and some shrubby alder dominated the vegetation high presence of sphagnum spores and cyperaceae pollen points to mires in the vault creek valley the overlying unit b consists of 10 m thick alternating fluvial gravels loess like silt and sand layers penetrated by small ice wedges osl dates support a stadial early wisconsin mis 4 age of unit b pollen and plant macrofossil data point to spruce forests with some birch interspersed with wetlands around the site the following unit c is composed of 15 m thick ice rich loess like and organic rich silt with fossil bones and large ice wedges unit c formed during the interstadial mid wisconsin mis 3 and stadial late wisconsin mis 2 as indicated by radiocarbon ages post depositional slope processes significantly deformed both ground ice and sediments of unit c pollen data show that spruce forests and wetlands dominated the area the macrofossil remains of picea larix and alnus incana ssp tenuifolia also prove the existence of boreal coniferous forests during the mid wisconsin interstadial which were replaced by treeless tundra steppe vegetation during the late wisconsin stadial unit c is discordantly overlain by the 2 m thick late holocene deposits of unit d the pollen record of unit d indicates boreal forest vegetation similar to the modern one the permafrost record from the vault creek tunnel reflects more than 90 ka of periglacial landscape dynamics triggered by fluvial and eolian accumulation and formation of ice wedge polygons and post depositional deformation by slope processes the record represents a typical wisconsin valley bottom facies in central alaska

tephrochronology of late cenozoic loess at fairbanks central alaska nineteen distinctive distal tephra beds from the gold hill loess in the fairbanks region central alaska are divided into two groups based on their color mineralogy glass shard morphology and the major and trace element composition of their glass shards this dichotomy in tephra composition is believed to reflect the two source regions namely the eastern aleutian arc alaska peninsula region and the wrangell volcanic field despite the fragmentary nature of the preserved loess record which contains many local unconformities these tephra beds can be used to link the numerous loess exposures in the fairbanks region in their proper time sequence establishing thereby a comprehensive reliable time stratigraphic framework because these distal tephra beds are related to large magnitude volcanic eruptions given the long distance to the nearest possible source vent they will also enable correlation of the late cenozoic sedimentary sequence at fairbanks to those of adjacent regions of alaska and the yukon tephra beds with bubble wall or chunky glass shards have been dated by the isothermal plateau fission track method results demonstrate that the gold hill loess spans a long interval of time from late pliocene time to the last interglacial about 125 ka however the age of the basal part of this formation is not well constrained due to the apparent absence of tephra beds paleomagnetic studies suggest that loess deposition in the fairbanks region began about 3 ma

the regional and temporal significance of primary aeolian magnetic fabrics preserved in alaskan loess abstract the anisotropy of magnetic susceptibility ams data presented herein underscore the resolving power magnetic anisotropy has in non lithified poorly consolidated sediments we turn our attention to primary aeolian sedimentary magnetic fabrics preserved along two coeval loess profiles in central alaska at the halfway house hh3 and gold hill steps ghs2 sites we first demonstrate that published preliminary results along hh1 and hh2 are reproducible moreover the new sample suite the first ever at ghs collected at a higher temporal resolution and statistical confidence permits a better evaluation of the significance of the inferred paleowind directions and their temporal variability the data demonstrate that the resolvability of the magnetic lineation is independent of both the eccentricity of the ams ellipsoid and the strength of the magnetic foliation furthermore the orientations and temporal variability of the most significant magnetic lineations are not artifacts of the data filtering process developed to isolate them we conclude that the temporal variations of inferred paleowind directions along hh3 track paleoclimatic glacial interglacial cycles as well as provide additional constraints to the geochronology of the loess deposit along ghs2 the limited coeval primary magnetic fabrics suggest an influence from a local microclimate not present at the hh site complicating the interpretation of small scale variability in paleowind directions however the overall average paleowind directions are similar to those inferred along hh3 confirming a regional spatial consistency of past surface air circulation

origin of last glacial loess in the western yukon tanana upland central alaska usa loess is widespread over alaska and its accumulation has traditionally been associated with glacial periods surprisingly loess deposits securely dated to the last glacial period are rare in alaska and paleowind reconstructions for this time period are limited to inferences from dune orientations we report a rare occurrence of loess deposits dating to the last glacial period 19 ka to 12 ka in the yukon tanana upland loess in this area is very coarse grained abundant coarse silt with decreases in particle size moving south of the yukon river implying that the drainage basin of this river was the main source geochemical data show however that the tanana river valley to the south is also a likely distal source the occurrence of last glacial loess with sources to both the south and north is explained by both regional synoptic scale winds from the northeast and opposing katabatic winds that could have developed from expanded glaciers in both the brooks range to the north and the alaska range to the south based on a comparison with recent climate modeling for the last glacial period seasonality of dust transport may also have played a role in bringing about contributions from both northern and southern sources

vegetation buried under dawson tephra 25 300 14c years bp and locally diverse late pleistocene paleoenvironments of goldbottom creek yukon canada paleoecological research at goldbottom creek in the klondike region of yukon territory nw canada documents an in situ riparian grassy meadow that was buried during the winter or early spring by dawson tephra near the onset of marine isotope stage mis 2 ca 25 300 14 c years bp analyses of vascular plant macrofossils bryophytes pollen insects and paleosols from the riparian meadow contrast with evidence for well drained upland steppe tundra habitats obtained from fossil arctic ground squirrel middens within the same valley the mesic valley bottom vegetation consisted of grasses deschampsia caespitosa alopecurus sedges carex horsetail equisetum cf palustre diverse bryophytes and few forbs upland habitats with dry loessal soils along the valley slopes contained graminoids elymus festuca kobresia myosuroides sage artemisia frigida and diverse steppe and tundra forbs phlox hoodii plantago cf canescens anemone patens var multifida bistorta vivipara draba these paleoecological data highlight the effect of topographic position and moisture on substrates and their control on local scale habitat variability this study represents the first recorded in situ riparian surface from the unglaciated pleistocene refugium of beringia and provides well documented evidence for local habitat heterogeneity and ecosystem structure within the mammoth steppe biome other radiocarbon dated paleoecological data from our study sites indicate that full glacial steppe tundra habitats in west central yukon territory were established during the later stages of the mis 3 interstadial by 29 000 14 c years bp the diverse data obtained through integration of multiple paleoecological methods

stratigraphic isotopic and mineralogical evidence for an early holocene thaw unconformity at mayo yukon territory ice rich glaciolacustrine sediments near mayo yukon territory reveal a thaw unconformity in the form of truncated ice wedges and abrupt changes in cryotexture the unconformity has been radiocarbon dated at 8870 200 years bp which is within the holocene period of optimal climatic conditions in northern yukon and the mackenzie delta area reported by other workers analysis of the mineralogy of the sediments indicates that the material above the unconformity is enriched in minerals that are the products of a more intense weathering environment than those deeper in the profile oxygen isotope ratios of ground ice in the sediments suggest the presence of two genetically distinct ice units above and below the unconformity an average rate of upward permafrost growth in this area of 0 1 0 2 mm year 1 is calculated for the period since the climatic optimum

an extensive middle to late pleistocene tephrochronologic record from east central alaska abstract chester bluff cb in yukon charley rivers national preserve ycnp alaska contains an extensive middle to late pleistocene tephra record within thick loess deposited on a terrace the regionally prominent old crow oc tephra 140 10 ka gi tephra 560 80 ka of fairbanks the preido hill prh tephra of the klondike and vt tephra 77 8 4 1 ka of both the klondike and fairbanks were found at cb the presence of these tephra beds will aid in correlating disparate paleoenvironmental records across eastern beringia there are 15 new tephra beds 11 from the wrangell volcanic field wvf and or hayes volcano one from the aleutian arc alaska peninsula aaap and three unclassified beds cb is normally magnetized indicating the entire sequence is of bruhnes age

origin of sulfide inclusions in cumulate xenoliths from nograd gomor volcanic field pannonian basin north hungary south slovakia abstract we provide new information about the evolution of the lithosphere beneath the nograd gomor volcanic field ngvf northern pannonian basin based on sulfide inclusions in cumulate origin ultramafic xenoliths the clinopyroxene rich cumulate xenoliths representing the lower crust and upper mantle underwent metasomatic alteration which resulted in formation of amphiboles we have carried out a detailed petrographic observation on sulfide inclusions using reflected light microscope and analyzed numerous back scattered electron images of the most typical sulfide blebs based on the petrographic study only rounded elongated or negative crystal shaped single inclusions occurring randomly in clinopyroxene and amphibole and rarely in olivine and spinel have been selected for detailed electron microprobe analysis the size of these single inclusions ranges from 3 to 75 m in diameter the sulfide blebs consist mostly of pyrrhotite and minor chalcopyrite pentlandite and cubanite pyrrhotite which is the major phase in all the inclusions is ni poor max 6 1 wt chalcopyrite is deficient in cu content 28 9 wt pentlandite and cubanite show regular compositions and were identified only in one xenolith the bulk compositions of sulfide blebs show a tight compositional range and are rich in fe compared to those in type i peridotite xenoliths from the same volcanic field geochim cosmochim acta 59 1995 3917 falus gy 2000 geochemical significance of sulfide inclusions of cr diopsidic xenoliths of alkaline basalts occurring in the carpathian pannonian region msc thesis department of petrology and geochemistry eotvos university budapest the sulfide blebs studied likely formed from a sulfide melt coexisting with a silicate melt the latter melt was the source of host clinopyroxene rich cumulates the sulfide blebs mineralogically pyrrhotite experienced a high and low temperature evolution producing further sulfide phases chalcopyrite pentlandite cubanite present in the blebs

age of sheep creek tephra pleistocene in central alaska from thermoluminescence dating of bracketing loess the age of the sheep creek tephra sct a widespread marker ash bed in eastern alaska and western yukon territory has been ambiguous and controversial we have obtained three reliable thermoluminescence age estimates from bracketing loess near fairbanks that imply a deposition age of about 190 000 20 000 yr for sct three of six loess samples near and closely bracketing the sct beds near fairbanks yielded younger age estimates 117 000 and 135 000 yr most likely based on field aspects because of reworking and contamination by translocated grains the new reliable age assignment of 190 000 yr confirms independent stratigraphic evidence of a pre last interglaciation age and stratigraphic evidence from one site upper eva creek that sct is older than the more widespread 140 000 yr old old crow tephra the sct age also has implications for regional correlations of glacial and nonglacial deposits in particular it supports the stratigraphic and geomorphic interpretation that the delta glaciation in the east central alaska range and the reid glaciation in western yukon territory are older than the last interglaciation isotope substage 5e

comparison of thermoluminescence and radiocarbon age determinations from late pleistocene alluvial deposits near sydney australia abstract radiocarbon and thermoluminescence tl age determinations have been obtained for a large pleistocene alluvial terrace on the nepean river near sydney new south wales australia the deposit was laid down by a braided river system prior to the last glacial maximum six thoroughly pretreated samples of charcoal and degraded wood buried within gravels at the base of the terrace yielded apparent 14 c ages of 37 000 42 000 yr b p these compare favorably with four tl determinations that gave apparent ages of 41 000 47 000 yr b p for the same deposit adjustment of the 14 c ages to take geomagnetic effects into account further improves the correlation between these two independent dating techniques in addition 14 c and tl correctly identified a reworked portion of the fine grained alluvial overburden as being substantially younger than the main body of the terrace these results augur well for the usefulness of tl age determinations of certain alluvial deposits

effectiveness of natural zeroing of the thermoluminescence in sediments the last exposure to sunlight of detrital feldspars and quartz grains in quaternary sediments can be dated directly by thermoluminescence tl methods but the effectiveness of zeroing of the light sensitive tl signal by daylight varies widely depending upon details of the sedimentation processes to document this variability and to help develop objective criteria useful for future sample selection and laboratory procedures i have collected and analyzed several eolian fluvial marine deltaic and lacustrine samples from less than 100 year old deposits as well as suspensions from sediment plumes also underwater spectral irradiance was measured during recovery of suspensions tl properties of polyrnineralic feldspar dominated 2 4 m and 4 11 m size fractions were measured in the laboratory unexpectedly some light sensitive tl was found to remain in all samples however the light sensitive tl was more effectively zeroed in some samples a proximal loess a sandy braided channel deposit tidal flat and upper delta front muds and thin clayey lacustrine laminae than in others a silty braided channel deposit submarine fan muds and silt bearing lacustrine laminae for the first group the minimum relict paleodose obtained with the partial bleach technique is about 0 4 0 3 gy corresponding to a relict tl age estimate of about 200 years for these sediments thus similar sediments excluding fluvial deposits as young as about 2 ka can be accurately and routinely dated without significant correction for a zero point error if one uses the partial bleach technique coupled with integrated optical bleaching energy fluences of less than 1 j cm 2 and wavelengths only above about 550 nm also some simple optical bleaching tests are reported that have a potential use in sample selection and in provenance studies of some surficial detrital sediments finally a simple mathematical model is presented that helps account for the gross features of the observed bleaching response curves

late pliocene to late pleistocene environments preserved at the palisades site central yukon river alaska abstract the palisades site is an extensive silt loam bluff complex on the central yukon river preserving a nearly continuous record of the last 2 myr volcanic ash deposits present include the old crow oct 140 000 yr sheep creek sct 190 000 yr pa 2 02 myr ec ca 2 myr and mining camp ca 2 myr tephras two new tephras pal and pau are geochemically similar to the pa and ec tephras and appear to be comagmatic the pa tephra occurs in ice wedge casts and solifluction deposits marking the oldest occurrence of permafrost in central alaska three buried forest horizons are present in association with dated tephras the uppermost forest bed occurs immediately above the oct the middle forest horizon occurs below the sct the lowest forest bed occurs between the ec and the pa tephras and correlates with the dawson cut forest bed plant taxa in all three peats are common elements of moist taiga forest found in lowlands of central alaska today large mammal fossils are all from common late pleistocene taxa those recovered in situ came from a single horizon radiocarbon dated to ca 27 000 14 c yr b p the incongruous small mammal assemblage in that horizon reflects a diverse landscape with both wet and mesic environments

evidence of quaternary climatic variations in a sequence of loess and related deposits at birch creek alaska implications for the stage 5 climatic chronology abstract a 45 m outcrop of quaternary sediments on birch creek near circle alaska reveals a record of fluctuating environmental conditions that probably spans several glacial interglacial cycles from base to top the deposits are forested floodplain warm colluvium with ice wedges cold forest soil warm loess cold paleosol containing old crow tephra oct cool to warm loess cold lacustrine very warm loess cold and modern forest soil warm resolution of the paleoclimatic history associated with the oct event is critical to understanding the nature of stage 5 in the western north american arctic application of recent age estimates for the oct tephra ca 140 000 yr bp to the birch creek section would indicate that either i the tephra paleosol dates from the 6 5 5 transition a strongly developed glacial interval occurred within stage 5 and the overlying very warm interval occurred in 5 3 or 5 1 or ii the tephra was deposited during a non milankovitch warming event late in stage 6 a paleoclimate chronology provides an alternative interpretation iii in which the tephra paleosol corresponds to stage 6 or even stage 7 the overlying loess to stage 6 and the lake sediments to all or part of stage 5 but the oct is older than 140 000 yr bp chronologies ii and iii imply a very warm beginning to stage 5 consistent with paleoclimate model simulations and data from other regions

applications and limitations of thermoluminescence to date quaternary sediments abstract the thermoluminescence tl method dates sediment by quantifying time dependent dosimetric properties of mineral grains sunlight exposure of the sediment during transportation and deposition or heating resets the tl clock to time zero after burial of the mineral grains ionizing radiation from the decay of radioisotopes imparts a tl signal this accumulated tl signal is proportional to the equivalent dose ed which is a measure of radiation exposure during the burial period a tl age estimate is determined from the ratio of the ed to the environmental radiation dose rate the calculation of an ed is influenced by the susceptibility of sediments to optical bleaching the emission spectra of minerals and possible long and short term fading there are three basic methods for determining an ed regeneration total and partial bleach the regeneration technique may yield under estimates because of sensitivity changes with laboratory bleaching the latter two techniques are most useful for well and partially light bleached sediments respectively a number of sediments can be dated by the tl method eolian and a variety of terrestrial sediments yield apparently accurate tl age estimates though there are possible complications associated with pedogenesis it appears that some fluvial sediment can be accurately dated by tl but little is known about the efficiency of optical bleaching for fluvial depositional systems till and glacier proximal deposits are inadequately light bleached to be dated by tl sediments deposited distal from the glacier margin which are at least partially bleached are preferred for dating future research is needed to understand better the relationship between sedimentary environments and the extent of light bleaching also studies should concentrate on dating feldspar extracts from well dated sequences to more fully ascertain the temporal limitations of the method only by understanding the interplay of sedimentology tl dosimetry and environmental radiation can the tl method be effectively applied to the myriad of depositional environments spanning the late to middle pleistocene

late pliocene dawson cut forest bed and new tephrochronological findings in the gold hill loess east central alaska thick loess deposits in the fairbanks region of interior alaska are a rich source of information on past climates and environments during the late cenozoic and the numerous distal silicic tephra beds preserved within them offer the potential for good chronological control the dawson cut forest bed lies in the lower part of this loess cover plant macrofossils consist of picea glauca picea mariana betula papyrifera and populus balsamifera but no pinus these fossils together with the abundance and size of spruce remains pollen tree ring characteristics and 13 c values of spruce wood demonstrate that the boreal forest represented by the dawson cut forest bed was similar to the modern boreal forest of central alaska warming conditions during the early part of the dawson cut interglaciation initiated thawing of permafrost and melting of ground ice as evidenced in the presence of ice wedge casts and major erosion of the lower gold hill loess tephrochronological magnetostratigraphic and glass fission track dating studies in the fairbanks area and at the palisades site on the yukon river in central alaska suggest an age for the dawson cut forest bed of ca 2 ma hence the northern boreal forest of northwestern north america as we know it today has a long history that probably extends back to at least 2 ma

thermoluminescence dating of late devensian loesses in southern england scattered across southern england are many isolated deposits of loess like material1 a few such as that at pegwell bay in kent are highly calcareous and unweathered but most have been reworked by fluvial or colluvial processes there is good stratigraphical evidence for a few pre devensian loesses also in kent but dating of more recent loess has so far been based on indirect evidence much work has been done on the pegwell bay loess as it is the most extensive truly aeolian loessic deposit in britain kerney2 compared the late devensian deposits in the isle of thanet and at pegwell bay with similar deposits in holland and belgium where radiocarbon dates have been obtained for interstadial deposits correlation of the east kent deposits with these in northern europe indicates that the loesses in kent were formed between 30 000 and 14 000 yr ago i report here dates for six of the more recent deposits in southern britain from the scilly isles to kent the dates have been obtained on the loess itself using a recently developed thermoluminescence tl dating technique3 4 and confirm the ages as being late devensian

full glacial upland tundra vegetation preserved under tephra in the beringia national park seward peninsula alaska the nature of the full glacial vegetation of beringia has been the subject of a great deal of investigation and debate here we present a reconstruction of an intact example of the full glacial upland vegetation of part of the northern seward peninsula at one point in time the area was blanketed by more than 1 m of tephra ca 18 000 14c bp ca 21 500 cal bp and the former land surface was preserved in the permafrost the discovery of the land surface provides a unique opportunity to study a fossil ecosystem preserved in situ macrofossils were used to reconstruct the vegetation growing at several sites on the buried land surface the macrofossil assemblages indicate a vegetation characterized by graminoids and forbs with the occasional occurrence of salix arctica the vegetation was dominated by kobresia myosuroides other sedges carex and grasses with a ne scale mosaic related to snow accumulation and moisture availability overall the vegetation was a closed dry herb rich tundra grassland with a continuous moss layer growing on calcareous soil that was continuously supplied with loess nutrient renewal by loess deposition was probably responsible for the relatively fertile vegetation and the occurrence of a continuous mat of acrocarpous mosses good physiognomic analogues can be suggested but no exact modern vegetational analogues have been found probably because the full glacial environment and climate with loess deposition do not occur today 2000 elsevier science ltd all rights reserved

lithostratigraphy and limiting optical ages of the pleistocene fill in fraser river valley near clinton south central british columbia the fraser river valley near clinton contains a thick sediment fill that is presently incised down to bedrock the sequence approximately 500 m thick is generally upward fining and consists of up to 100 m of glacigenic debris flow diamicton and glaciofluvial deltaic gravel and sand at the base overlain by about 350 m of glaciolacustrine sedi ments and minor diamicton which is in turn capped by several metres of till the sequence is interpreted to represent i valley aggradation in response to glaciation followed by ii the impoundment of the valley by sediment and or ice and the formation of a large proglacial lake s and finally iii overriding of the valley fill by glaciers this glacial advance sequence can be readily correlated with previously studied units situated immediately to the north most of which have been associated with the last glaciation however the age of these units has been based only on strati graphic relations we introduce limiting optical ages from a widespread glaciolacustrine unit that show that the glacial advance sequence was formed at the earliest during the penultimate okanagan centre glaciation oxygen isotope stage 4 ca 74 59 ka but almost certainly during the last fraser glaciation oxygen isotope stage 2 ca 24 12 ka it could not have been deposited during oxygen isotope stage 6 or during an older glaciation

on the nature and origin of muck deposits in the klondike area yukon territory organic rich muck deposits which blanket auriferous gravels in the klondike area yukon territory comprise two principal stratigraphic units i a silty late pleistocene deposit and ii holocene organics lying unconformably on the silt the deposits are found predominantly in valley bottoms and if undisturbed are normally perennially frozen field and laboratory investigations of particle size mineralogy and morphology as well as organic matter and sedimentary structures indicate that the silt is both primary massive and redeposited bedded loess by weight 87 medium and coarse silt and fine sand radiocarbon dates indicate that the loess was deposited during late wisconsinan mcconnell glaciation beginning after 27 000 14c years bp the loess was likely derived from the floodplain of the yukon river during periods of low flow turf in growth position and organic matter in the silt similar to that of loessal grasslands near kluane lake suggest a grassland environment for the area during

seasonality of the late pleistocene dawson tephra and exceptional preservation of a buried riparian surface in central yukon territory canada abstract the late pleistocene dawson tephra was deposited by one of the largest quaternary eruptions in northwestern north america its distribution is known sparsely from sites near the source area in southwestern alaska and central yukon territory where more than 20 occurrences are documented in the klondike region dawson tephra erupted about 25 300 14 c yr bp near the onset of the last glaciation and provides a stratigraphic marker across eastern beringia we report radiocarbon ages paleobotanical data and cryostratigraphic observations from a new dawson tephra locale at goldbottom creek in the klondike region of yukon territory which collectively indicate that the eruption occurred in the late winter or early spring multiple fining upward tephra rich ice beds are interpreted as remnants of surface icings which presently are common in the region during spring a buried in situ riparian meadow preserved below the icing and tephra consists of abundant tufted hair grass deschampsia caespitosa with interspersed horsetails equisetum cf palustre and mosses detrital plant remains and preserved in situ grass inflorescences entombed in the icing had expelled their fruits consistent with a late season surface when the icing was active the extraordinary thickness of dawson tephra in central yukon likely reflects reworking of a winter deposited tephra by snow melt in the spring following the eruption indicating that the primary thickness may be overestimated at valley bottom sites winter deposition of the tephra may have in part minimized the terrestrial ecological impacts of the eruption on zonal steppe tundra vegetation through the retransportation of tephra from hillslopes to the riparian areas where the tephra became incorporated into local fluvial systems

climate and seismicity in the higher central himalaya during 20 10 ka evidence from the garbayang basin uttaranchal india abstract in the trans himalaya the trans himadri fault thf controlled the extent of past glaciers the rivers were impounded by glacial debris resulting in the formation of lakes that recorded regional climatic and seismic events one such lake formed at garbayang where the upper 28 m lacustrine succession spanned an age range of 20 10 ka magnetic susceptibility and sedimentological data suggest that the region experienced high amplitude low frequency millennial scale climate changes during the period 20 3 and 13 2 ka followed by low amplitude high frequency changes from 13 2 11 1 ka a sudden drop in magnetic susceptibility and elemental concentration of layers dated at 11 12 ka suggests cooling associated with the younger dryas occurrence of faulted strata seismites during 20 17 and 14 13 ka suggests recent activation of the thf this is noteworthy as the thf has been considered dormant since its last reactivation at 11 ma further the luminescence dates suggest that radiocarbon ages on organic carbon are grossly overestimated due to a hard water effect caused by the dominance of limestone in the lake catchment this implies that earlier radiocarbon based palaeoenvironmental reconstruction in the region will need revalidation

old crow tephra across eastern beringia a single cataclysmic eruption at the close of marine isotope stage 6 abstract old crow tephra is the largest and most widespread quaternary eruption presently known in eastern beringia its major and trace element geochemistry fe ti oxides and stratigraphic and paleoecological context indicate that it is the result of a single cataclysmic eruption the proximal region may well have experienced tephra fallout from small eruptions just prior to or after the old crow event but there is no evidence to indicate that the distal area was affected we recalculate the glass fission track age at 124 10 ka which coupled with stratigraphic and paleoenvironmental reconstructions indicates that deposition occurred prior to development of the last interglacial boreal forest which suggests a latest marine isotope stage mis 6 age the bulk tephra volume erupted is estimated by three different approaches that are in broad agreement at 200 km 3 but this result must be considered as tentative given the poor controls on definition of isopachs over such a large area the source caldera although presently unrecognized is located in the eastern aleutian arc possibly at or near the emmons lake volcanic center

single grain osl dating of glaciofluvial quartz constrains reid glaciation in nw canada to mis 6 abstract 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

paleoenvironmental reconstruction of the last glacial maximum inferred from insect fossils from a tephra buried soil at tempest lake seward peninsula alaska abstract sediments and vegetation dated 21 570 cal yr bp were buried under tephra on the northern seward peninsula this buried surface has yielded plant macrofossils in growth position as well as numerous insect fossils excellently preserved in permafrost it appears that many of the insects were buried alive by the volcanic ash the species composition and ecological affinities of this fossil fauna are typical of alaskan late pleistocene steppe tundra environments the assemblages are dominated by the weevil lepidophorus lineaticollis one of the most common species in eastern beringian pleistocene fossil assemblages many other members of the ancient steppe tundra insect community are preserved in these assemblages including the pill beetle morychus sp and weevils of the genus coniocleonus in alaska most of these species but not all of them survived the pleistocene holocene environmental transition but are restricted today to relict patches of steppe like vegetation faunal diversity is low in spite of the recovery of more than 1000 individual insects and mites including more than 600 beetles this reflects the small number of species adapted to the cold dry environments of the lgm in eastern beringia they represent an ecosystem which no longer exists

tl dating studies of tephra loess and lacustrine sediments abstract a brief review is presented of my efforts to improve the accuracy of dating unheated sediments by tl and of the development of a new tephrochronometer specific applications to known age deposits are outlined as are studies of the depositional environment of subaqueous sediments removal of anomalous fading in loess by storage at 75 c for 8 days is demostrated throughout for unheated sediments the preferential use of the partial bleach r gamma or r beta technique is emphasized especially in situations where the growth curves are sublinear contrary to a widespread misconception sublinearity does not invalidate the partial bleach method furthermore examples are given of tl sensitivity or efficiency changes occurring with the regeneration technique changes that are variable and seemingly sample dependent

a 3 m y record of pliocene pleistocene loess in interior alaska many distal tephra beds exist in the thick fossiliferous loess deposits near fairbanks interior alaska isothermal plateau fission track ages determined on glass shards from tephra beds in conjunction with tephrostratigraphic and magnetostratigraphic techniques indicate that loess deposition began in the late pliocene an antiquity previously unsuspected hence there is the opportunity now to reconstruct a detailed well dated record of environmental changes in interior alaska during the past 3 m y

the blue emission of k feldspar coarse grains and its potential for overcoming tl age underestimation abstract this paper presents preliminary tl ages on k feldspar coarse grains from raised shallow marine sediments of nw europe and southern italy comparisons between tl ages using different optical filters suggest the use of the blue transmitting corning 7 59 filter allows measurement of an apparently stable tl component in k feldspar which could overcome the problem of age underestimation in the 75 300 ka range

continuous late quaternary proxy climate records from loess in beringia abstract loess deposits in eastern beringia contain continuous proxy records of the effects of past climatic change on terrestrial landscapes at high latitudes variations of environmental magnetism and sedimentology of high latitude loess deposits indicate that the timing and pattern of responses to local variations in wind intensity storminess and pedogenesis in eastern beringia closely resemble the pattern of global climate change during the late quaternary deduced from studies of marine and ice core records the age of paleoclimatic fluctuations permafrost features volcanic ash horizons buried forest layers and paleosols and other features of the eastern beringian loess record can be determined using a variety of quaternary dating methods tephrochronologic correlations between the loess record and the glacial history of eastern beringia indicate the delta glaciation occurred during marine isotope stage 6 several other middle and late quaternary glaciations across eastern beringia can be tephrochronologically tied to the loess record and appear to have been in phase with episodes of global cooling recorded in deep sea records

old crow tephra found at the palisades of the yukon alaska abstract a 20 cm thick volcanic ash layer at the palisades along the yukon river is geochemically correlated with the old crow tephra a buried organic and wood rich layer above the old crow tephra probably records the last interglaciation the recognition of the old crow tephra provides the first chronologic information on the age of the thick sequence of pleistocene sediments at the palisades an important ice age mammal site in central alaska and a key locakity for the reconstruction of the history of the yukon river and the last interglaciation in central alaska

thermoluminescence dating tests for lacustrine glaciomarine and floodplain sediments from western washington and british columbia to help further develop reliable procedures for accurate thermoluminescence tl dating of quaternary waterlaid sediments we tested tl dating procedures on sediment types rarely examined six glaciolacustrine samples three samples of glaciomarine drift and eight samples of floodplain deposits we used the partial bleach r beta gamma technique applied to fine silt polymineral grains results from our younger known age glaciolacustrine sediments confirm earlier observations that only the clayey laminae are generally suitable for tl dating a clayey lamina older than ca 140 150 ka produced no age underestimation and this result ca 300 ka suggests that such older lake sediments are probably suitable for tl dating two proximal samples of glaciomarine drift produced large tl age overestimates whereas the single distal glaciomarine drift sample yielded an expected age 177 38 ka suggesting that follow up studies are warranted our results for eight flood plain samples confirm that zeroing of li

extending the geochronometry of arctic lake cores beyond the radiocarbon limit by using thermoluminescence reconstructing detailed paleoclimatic paleoenvironmental histories from pleistocene sediments in the arctic is an ongoing challenge because of the difficulty of measuring accurate timescales beyond the range of radiocarbon 14c dating important archives of such histories are arctic lakes that exist outside the known extent of the last major glaciations to develop reliable geochronometry of sediment cores from arctic and other lakes we applied thermoluminescence tl sediment dating methods to 24 samples 8 reported earlier but some reanalyzed here selected from cores from squirrel and joe lakes in northwestern alaska a sequence of 23 14c ages from the upper parts of these cores permitted tests of the zeroing assumption of tl dating that all fine silt grains were exposed to daylight before final burial we performed multiple tl tests on most samples to isolate stable tl signals and measured tl emissions spectra to help characterize samples tl ages generally agree with the 30 kyr age depth trend derived from 14c dating and extend the timescale apparently to 180 ka for squirrel lake and to at least 60 ka for joe lake an anomalously old tl result from squirrel lake is attributed to failure of the tl zeroing assumption within that sediment horizon in the lower joe lake core we assign a timing of 45 ka to 50 ka for the beginning of an apparently climatic warming interval these results identify and quantify the main tl sources of tl age inaccuracy and the preferred tl protocols for accurate tl sediment dating in northern high latitude lake deposits they also provide the first direct chronology beyond the 14c dating range for these lakes

thermoluminescence dating of periods of loess deposition and soil formation in normandy the least incomplete records of pleistocene climate change come from the study of deep sea sediments in which the stratigraphic record of oxygen isotope variations reflecting terrestrial ice volume is the primary correlation medium on continents the available record is far less complete because many of the characteristics of glaciation are erosional rather than depositional the areas of loess deposition may provide the best deposits in which to develop relatively complete regional stratigraphies that may be compared with the deep sea record1 2 in this study we use thermoluminescence tl dating of loess and of soil developed on loess to test this model we have confined ourselves to the last complete glacial cycle within which the reproducibility of the age determinations is similar to the time resolution of most deep sea core records