thickness and extent of the subglacial till layer beneath an antarctic paleo ice stream fast flowing ice streams and outlet glaciers currently account for as much as 90 of the discharge from the antarctic and greenland ice sheets although the deformation of subglacial material has been proposed as the mechanism for this rapid motion such sediment is usually hidden under several kilometers of ice marine geophysical records have allowed reconstruction of the three dimensional thickness of the sedimentary bed beneath a large antarctic paleo ice stream for the first time fast flow is indicated by streamlined seafloor lineations that form the surface of a layer of low shear strength unsorted sediment averaging 4 6 m thick rapid motion of the paleo ice stream was a result of subglacial deformation within this layer

origin and causes of the mammoth steppe a story of cloud cover woolly mammal tooth pits buckles and inside out beringia to account for the vastness of the northern arid steppes during glacial episodes i propose the proximate key variable was simply frequent clear skies this hitherto under emphasized point is the hub which best explains many questions low maritime cloud cover best accounts for today s tundra and in a related way the cloudy polar front accounts for the whole of the taiga even during glacial maxima the proximity of the sea to the bering isthmus created intermittent maritime cloud cover this regional cloud cover produced an ecological interruption or buckle of the arid steppe belt while this beringian mesic buckle did not serve as an intercontinental ecological barrier to most steppe adapted species it does seem to have limited the distributions of woolly rhinos camels american kiangs short faced bears badgers and some others at the beginning of the holocene this narrow refugium seems to have been a source of some mesic adapted species which colonized westward into the now tundra vegetation of northern asia and eastward into northern north america this holocene expansion from a limited and regional pleistocene refugium created our present misconceptions about beringia the mid strait mesic ecological conditions were the exception to the more extensive arid adapted communities of the mammoth steppe 2000 published by elsevier science ltd all rights reserved

late wisconsin environments of the bering land bridge abstract late wisconsin paleobotanical and fossil insect data from the central and northern sectors of the bering land bridge indicate widespread mesic shrub tundra environments even during the last glacial maximum vegetation before the last glacial maximum was a birch heath graminoid tundra with few or no steppe elements shrubs were not an important element of the vegetation but were present in small numbers during the interval 20 000 14 000 yr bp land bridge vegetation was dominated by birch graminoid tundra with small ponds containing aquatic plants heaths were relatively unimportant insects from this interval were indicative of arctic climate with drier tundra than during the late glacial during the late glacial interval 14 000 11 000 yr bp land bridge vegetation was dominated by birch heath graminoid tundra with small ponds choked with aquatic plants the insect record indicates open ground habitats dominated by mesic tundra by 11 000 yr bp insect data suggest that summer temperatures on the emergent bering shelf were warmer than present day upland regions in western alaska summer temperatures on the chukchi shelf were warmer than the present day north slope of alaska contrary to previous hypotheses we found no evidence of steppe tundra on the land bridge new accelerator mass spectrometer ams 14c dates show that much of the land bridge was above sea level and thus available for human and animal migration until as late as 11 000 yr bp

ice age megafauna in arctic alaska extinction invasion survival abstract radical restructuring of the terrestrial large mammal fauna living in arctic alaska occurred between 14 000 and 10 000 years ago at the end of the last ice age steppe bison horse and woolly mammoth became extinct moose and humans invaded while muskox and caribou persisted the ice age megafauna was more diverse in species and possibly contained 6 more individual animals than live in the region today megafaunal biomass during the last ice age may have been 30 greater than present horse was the dominant species in terms of number of individuals lions short faced bears wolves and possibly grizzly bears comprised the predator scavenger guild the youngest mammoth so far discovered lived ca 13 800 years ago while horses and bison persisted on the north slope until at least 12 500 years ago during the younger dryas cold interval the first people arrived on the north slope ca 13 500 years ago bone isotope measurements and foot loading characteristics suggest megafaunal niches were segregated along a moisture gradient with the surviving species muskox and caribou utilizing the warmer and moister portions of the vegetation mosaic as the ice age ended the moisture gradient shifted and eliminated habitats utilized by the dryland grazing species bison horse mammoth the proximate cause for this change was regional paludification the spread of organic soil horizons and peat end pleistocene extinctions in arctic alaska represent local not global extinctions since the megafaunal species lost there persisted to later times elsewhere hunting seems unlikely as the cause of these extinctions but it cannot be ruled out as the final blow to megafaunal populations that were already functionally extinct by the time humans arrived in the region

mid late pleistocene glacimarine sedimentary processes of a high latitude deep sea sediment drift antarctic peninsula pacific margin the effects of glaciation on sediment drifts is recognised from marked sedimentary facies variation in deep sea cores taken from the continental rise of the antarctic peninsula pacific margin nineteen sediment cores were visually described logged for magnetic susceptibility and x radiographed about 1000 analyses were performed for grain size clay minerals and biostratigraphy foraminifera nannofossils and diatoms four sediment types associated with distinct sedimentary processes are recognised based on textural compositional analysis 1 hemipelagic mud forms the bulk of the interglacial sediment and accumulated from the pelagic settling of bioclasts and ice rafted wind transported detritus 2 terrigenous mud forms the bulk of the glacial sediment and accumulated from a combination of sedimentary processes including turbidity currents turbid plumes and bottom current reworking of nepheloid layers 3 silty deposits occurring as laminated layers and lenses represent the lateral spillout of low density turbidity currents 4 lastly glacial interglacial gravelly mud layers derive from settling of ice rafted detritus five depositional settings are interpreted within sediment drift 7 each characterised by the dominance interaction of one or several depositional processes the repetitive succession of typical sedimentary facies is inferred to reflect a sequence of four climatic stages glaciation glacial deglaciation and interglacial each one characterised by a distinctive clay mineral assemblage and bioclastic content variations in clay mineral assemblage within interglacial stage 5 core sed 06 suggest minor colder climatic fluctuations possibly correlatable with substages 5a to 5e

tracking late quaternary extinctions in interior alaska using megaherbivore bone remains and dung fungal spores one major challenge in the study of late quaternary extinctions lqes is providing better estimates of past megafauna abundance to show how megaherbivore population size varied before and after the last extinctions in interior alaska we use both a database of radiocarbon dated bone remains spanning 25 0 ka and spores of the obligate dung fungus sporormiella recovered from radiocarbon dated lake sediment cores spanning 17 0 ka bone fossils show that the last stage of lqes in the region occurred at about 13 ka ago but the number of megaherbivore bones remains high into the holocene sporormiella abundance also remains high into the holocene and does not decrease with major vegetation changes recorded by arboreal pollen percentages at two sites the interpretation of sporormiella was enhanced by additional dung fungal spore types e g sordaria in contrast to many sites where the last stage of lqes is marked by a sharp decline in sporormiella abundance in interior alaska our results indicate the continuance of megaherbivore abundance albeit with a major taxonomic turnover including mammuthus and equus extinction from predominantly grazing to browsing dietary guilds this new and robust evidence implies that regional lqes were not systematically associated with crashes of overall megaherbivore abundance

a multidisciplinary study of a late pleistocene arctic ground squirrel urocitellus parryii midden from yukon canada middens nests and caches of late pleistocene arctic ground squirrels urocitellus parryii that are preserved in the permafrost of beringia archive valuable paleoecological data arctic ground squirrels selectively include the plant material placed in middens to account for this selectivity bias we used a multi proxy approach that includes ancient dna adna and macro and microfossil analyses here we provide insight into pleistocene vegetation conditions using macrofossils pollen phytoliths and non pollen palynomorphs and adna collected from one such midden from the yukon territory canada which was formed between 30 740 and 30 380 cal yr bp adna confirmed the midden was constructed by u parryii we recovered 39 vascular plant and bryophyte genera and 68 fungal genera from the midden samples grass and other herbaceous families dominated vegetation assemblages according to all proxies adna data yielded several records of vascular plants that are outside their current biogeographic range while some of the recovered fungi yielded additional evidence for local occurrence of picea trees during glacial conditions we propose that future work on fossil middens should combine the study of macro and microfossils with adna analysis to get the most out of these environmental archives

flow of the west antarctic ice sheet on the continental margin of the bellingshausen sea at the last glacial maximum geophysical data show that during the last glaciation the west antarctic ice sheet wais drained to the continental shelf edge of the bellingshausen sea through a cross shelf bathymetric trough belgica trough as a grounded fast flowing ice stream the drainage basin feeding this ice stream probably encompassed southwestern palmer land parts of southern alexander island and the bryan coast of ellsworth land with an area exceeding 200 000 km2 on the inner continental shelf streamlined bedrock and drumlins mapped by swath bathymetry show that the ice stream was fed by convergent ice flow draining from eltanin bay and bays to the east as well as by ice draining the southern part of the antarctic peninsula ice sheet through the ronne entrance the presence of a paleoice stream in belgica trough is indicated by megascale glacial lineations formed in soft till and a trough mouth fan on the continental margin grounding zone wedges on the inner and midshelf record ice marginal stillstands during deglaciation and imply a staggered pattern of ice sheet retreat these new data indicate an extensive wais at the last glacial maximum lgm on the bellingshausen sea continental margin which advanced to the shelf edge in conjunction with ice sheet reconstructions from the antarctic peninsula and pine island bay this implies a regionally extensive ice sheet configuration during the lgm along the antarctic peninsula bellingshausen sea and amundsen sea margins with fast flowing ice streams draining the wais and antarctic peninsula ice sheet to the continental shelf edge

sediments in arctic sea ice implications for entrainment transport and release despite the arctic sea ice cover s recognized sensitivity to environmental change the role of sediment inclusions in lowering ice albedo and affecting ice ablation is poorly understood sea ice sediment inclusions were studied in the central arctic ocean during the arctic 91 expedition and in the laptev sea east siberian arctic region expedition 1992 results from these investigations are here combined with previous studies performed in major areas of ice ablation and the southern central arctic ocean this study documents the regional distribution and composition of particle laden ice investigates and evaluates processes by which sediment is incorporated into the ice cover and identifies transport paths and probable depositional centers for the released sediment in april 1992 sea ice in the laptev sea was relatively clean the sediment occasionally observed was distributed diffusely over the entire ice column forming turbid ice observations indicate that frazil and anchor ice formation occurring in a large coastal polynya provide a main mechanism for sediment entrainment in the central arctic ocean sediments are concentrated in layers within or at the surface of ice floes due to melting and refreezing processes the surface sediment accumulation in central arctic multi year sea ice exceeds by far the amounts observed in first year ice from the laptev sea in april 1992 sea ice sediments are generally fine grained although coarse sediments and stones up to 5 cm in diameter are observed component analysis indicates that quartz and clay minerals are the main terrigenous sediment particles the biogenous components namely shells of pelecypods and benthic foraminiferal tests point to a shallow benthic marine source area apparently sediment inclusions were resuspended from shelf areas before and incorporated into the sea ice by suspension freezing clay mineralogy of ice rafted sediments provides information on potential source areas a smectite maximum in sea ice sediment samples repeatedly occurred between 81 n and 83 n along the arctic 91 transect indicating a rather stable and narrow smectite rich ice drift stream of the transpolar drift the smectite concentrations are comparable to those found in both laptev sea shelf sediments and anchor ice sediments pointing to this sea as a potential source area for sea ice sediments in the central arctic ocean sea ice clay mineralogy is significantly different from deep sea clay mineral distribution patterns the contribution of sea ice sediments to the deep sea is apparently diluted by sedimentary material provided by other transport mechanisms

lower temperature as the main cause of c4 plant declines during the glacial periods on the chinese loess plateau abstract the distribution of c3 and c4 plants changed in regionally contrasting ways during the last glacial period c4 plant expansion in low latitude africa and america coincided with c4 plant decreases in mesoamerica and the us great plains this c4 plant expansion has been attributed to lower pco2 and increased aridity and the decline in c4 plants is believed to have been caused by increased winter precipitation and lower temperatures however it is still difficult to generalize whether pco2 temperature or aridity was mainly responsible for c3 vs c4 vegetation changes during glacial periods paleoclimate conditions on the chinese loess plateau clp offer opportunities to further evaluate the role of temperature in the c3 vs c4 competition detailed climate records have shown that the clp region was drier and colder during glacial periods with aridity favoring c4 expansion and lower temperatures favoring c3 expansion here we present two high resolution compound specific carbon isotope records of n alkanes from the clp as vegetation biomarkers to estimate the relative abundance of c4 plants during the last two glacial interglacial transitions more negative 13c values during the glacial periods from two clp sites clearly show that in contrast with low latitude sites c4 plants declined this decrease in c4 abundance was caused by lower temperature despite the lower pco2 and increased aridity on the clp during glacial time

recent glacial history and rapid ice stream retreat in the amundsen sea microfossil analyses of core top samples from 20 sites in the eastern and southern amundsen sea delimit three biotic provinces which are related in part to the activity of a large ice stream and modern sea ice distribution in recent sediments on the outer shelf planktonic and calcareous benthic foraminifera are abundant but diatoms are rare here low diatom abundances in sediments despite high abundances in the overlying sea ice are attributed to nondeposition caused by ocean currents pine island bay sediments are almost barren of microfossils even though diatoms are abundant in overlying surface waters we attribute this discrepancy to deposition of exclusively non organic sediments beneath a former extension of the floating terminus of pine island glacier which probably retreated from this area during the last century the polynya which occupies this area during present day summers is probably so recent a feature that little or no biogenic sediment has accumulated recent sediments in the eastern margin province between pine island bay and the outer shelf contain abundant diatoms and arenaceous foraminifera high diatom abundances here probably record the presence of the amundsen sea polynya during the last few millennia the presence of a compact diamicton beneath recent sediments on the outer shelf and in the eastern margin province suggests that grounded ice formerly occupied the amundsen sea shelf with ice streams in deep elongated troughs these ice streams may have retreated up to 200 km during the holocene

the rosa keystone dunes field the geoarchaeology and paleoecology of a late quaternary stabilized dune field in eastern beringia stabilized sand sheets and dunes hold a remarkable amount of information on paleoenvironmental conditions under which late quaternary landscapes evolved in northern subarctic regions we provide the results of a project focused on understanding the development of lowland environments and ecosystems including dunes and sand sheets which were critical habitat for early human occupations in subarctic regions our study area is the rosa keystone dunes field in the shaw creek flats of the middle tanana river basin interior alaska one of the oldest continuously occupied areas in north america 14 000 cal bp to present the disturbance regimes of reactivated dunes and associated forest fire cycles between 12 500 and 8800 cal bp fostered a unique early to mid successional mixed vegetation community including herbaceous tundra shrubs and deciduous trees this environment provided key habitats for large grazers and browsers significant resources for early hunter gatherer populations in central alaska aft

arctic ground squirrels of the mammoth steppe paleoecology of late pleistocene middens 24000 29450 14c yr bp yukon territory canada abstract this paper presents paleoecological analyses of 48 fossil arctic ground squirrel spermophilus parryii middens nests and caches recovered from ice rich loess sediments in the klondike region of west central yukon territory ams radiocarbon dates and stratigraphic association of middens with dawson tephra 25 300 14 c yr bp indicate these paleoecological data reflect the onset of glacial conditions of early marine isotope stage mis 2 and terminal mis 3 24 000 29 450 14 c yr bp plant macrofossils include at least 60 plant taxa including diverse graminoids poa elymus trachycaulus kobresia myosuroides steppe forbs penstemon gormanii anemone patens var multifida plantago cf canescens tundra forbs draba spp bistorta vivipara dwarf shrubs salix cf arctica s cf polaris sage artemisia frigida and rare trees picea mariana many of these taxa identified in the middens represent the first recorded fossils for these plants in eastern beringia and add to our knowledge of the floristic composition of pleistocene vegetation and biogeography in this region fossil beetles include typical members of the eastern beringian steppe tundra fauna lepidophorus lineaticollis and connatichela artemisiae and others suggesting predominantly dry open habitats cache forage selection is suggested by some plant taxa which were particularly frequent and abundant in the middens bistorta vivipara kobresia myosuroides ranunculus spp potentilla erysimum cf cheiranthoides poa carex and draba factors such as proximity of vegetation to burrows and abundance of fruits and seeds per plant were probably important in cache selection glacial conditions enabled arctic ground squirrels to form widespread and dense populations in regions such as the klondike in which they are rare or absent at present this fossil midden record supports previous hypotheses that suggest arctic ground squirrels evolved in and are well adapted to the open steppe tundra vegetation loessal soils and glacial climates of the mammoth steppe biome

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

tomographic imaging of lg and sn propagation in the middle east observations based on relatively limited data recorded by sparsely distributed stations have indicated that regional seismic phase propagation lg and sn is very complex in the middle east accurate characterization of regional seismic wave propagation in this region necessitates the use of a large number of seismic stations we have compiled a large data set of regional and local seismograms recorded in the middle east this data set comprises approximately four years of data from national short period networks in turkey and syria data from temporary broadband arrays in saudi arabia and the caspian sea region and data from gsn mednet and geofon stations in the middle east we have used this data set to decipher the character and pattern of regional seismic wave propagation we have mapped zones of blockage as well as inefficient and efficient propagation for lg pg and sn throughout the middle east two tomographic techniques have been developed in order to objectively determine regions of lithospheric attenuation in the middle east we observe evidence of major increase in lg attenuation relative to pg across the bitlis suture and the zagros fold and thrust belt corresponding to the boundary between the arabian and eurasian plates we also observe a zone of inefficient sn propagation along the dead sea fault system which coincides with low pn velocities along most of the dead sea fault system and with previous observations of poor sn propagation in western jordan our observations indicate that in the northern portion of the arabian plate south of the bitlis suture there is also a zone of inefficient sn propagation that would not have been predicted from prior measurements of relatively low pn velocities mapped high attenuation of sn correlates well with regions of cenozoic and holocene basaltic volcanism these regions of uppermost mantle shear wave attenuation most probably have anomously hot and possibly thin lithosphere

morphology and sedimentary processes on the continental slope off pine island bay amundsen sea west antarctica morphology and sedimentation on the pine island bay continental margin eastern amundsen sea is investigated using swath bathymetry parametric subbottom profiler topas records and sediment cores a major cross shelf trough extends to the continental shelf edge at 114 w through which fast flowing ice drained the west antarctic ice sheet under full glacial conditions the slope adjacent to the trough mouth is 4 whereas to the east and west it is steeper a bulge in bathymetric contours beyond and east of the trough mouth indicates some slope progradation elsewhere seismic reflection profiles indicate major shelf aggradation and minor slope progradation networks of gullies and channels dominate the slope adjacent to pine island bay in front of the trough mouth gullies at the shelf edge and uppermost slope connect directly with channels that continue to the continental rise other gullies connect indirectly with channels further downslope topas records indicate that gully channel systems acted as conduits for coarse grained sediment transfer cores comprise turbidites representing channel deposits and diamicton facies deposited by debris flows sandy turbidites interbedded with hemipelagic muds occur on the rise and abyssal plain the west antarctic ice sheet extended to the shelf edge during full glacials delivering water saturated till and turbid water directly to the continental slope along a line source gully channel systems are probably a product of sediment gravity flows derived from the release of water saturated till and turbid water along the ice sheet margin perhaps augmented subsequently by downslope flow of dense cold and saline water associated with sea ice production during the holocene the gully channel systems enable sediment to bypass the continental slope spatial variations in gully channel system morphology along the amundsen sea continental slope are probably linked to differences in sediment gravity flow processes and the rate and composition of sediment delivered to the shelf edge from the ice sheet cored sediments on the slope and abyssal plain record the widespread presence of turbidite sands indicating that downslope transfer of glacier derived sediments has taken place this supports our interpretation that submarine channels are derived from and are conduits for mass wasting submarine channels on the upper slope continue into the abyssal plain as far north as 67 s they are separated by sediment drifts and sediment waves resulting from the interaction between downslope turbidity current processes and along slope bottom currents similar deep sea sedimentary processes operate along much of west antarctica and the western side of the antarctic peninsula

evidence for rapid retreat and mass loss of thwaites glacier west antarctica thwaites glacier the second largest ice stream in west antarctica drains an area of 166 500 2000 km2 which accumulates 55 5 gt a 1 or 60 6 km3 ice a 1 into the amundsen sea unrestrained by an ice shelf using interferometric synthetic aperture radar insar data collected by the european remote sensing satellites ers 1 and 2 in 1996 an output flux of 71 7 gt a 1 or 77 8 km3 ice a 1 is estimated at the grounding line where ice thickness is deduced from hydrostatic equilibrium a similar flux 70 7 gt a 1 or 76 8 km3 ice a 1 is obtained at a gate located 20 km upstream where ice thickness was measured in 1978 by ice sounding radar total accumulation in between the two gates is 1 6 gt a 1 or 1 8 km3 ice a 1 ice discharge therefore exceeds mass accumulation by 30 15 and thwaites glacier must be thinning and retreating at present the insar data show that the glacier floating ice tongue exerts no back pressure on the inland ice calves into tabular icebergs along a significant fraction of its grounding line and has a grounding line thickness which exceeds a prior calculated limit for stability glacier thinning is confirmed at the coast by the detection of a 1 4 0 2 km retreat of its grounding line between 1992 and 1996 with insar which implies 3 2 0 6 m ice a 1 thinning at the glacier center and less near the sides these results complement the decimeter scale annual surface lowering observed with satellite radar altimetry several hundred km inland of the grounding line the magnitude of ice thinning estimated at the coast however rules out temporal changes in accumulation as the explanation for surface lowering ice thinning must be due to changes in ice flow

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

paleoecology of beringian packrat middens from central yukon territory canada rodent middens from ice rich loess deposits are important new paleoenvironmental archives for eastern beringia plant macrofossils recovered from three middens associated with dawson tephra ca 24 000 14 c yr b p at two sites in yukon territory include diverse graminoids forbs and mosses these data suggest substantial local scale floristic and habitat diversity in valley settings including steppetundra on well drained soils moist streamside meadows and hydric habitats fossil arctic ground squirrel burrows and nesting sites indicate that permafrost active layers were thicker during pleistocene glacial periods than at present on north facing slopes d 2004 university of washington all rights reserved

macrofossil and pollen evidence for full glacial steppe within an ecological mosaic along the bluefish river eastern beringia abstract plant and insect macrofossil assemblages dating to the full glacial late wisconsinan are rare from eastern beringia here we present an assemblage of fossil pollen insect and plant macrofossils recovered from alluvium at the bluefish exposure northern yukon territory nine ams radiocarbon ages place these data between ca 18 880 16 440 14c yr bp 22 313 19 597 cal yr bp these data indicate that xeric steppe rich in bunchgrasses poa and elymus artemisia frigida and diverse forbs was interspersed within a mosaic of local vegetation types including mid rich fens mesic graminoid meadows steppe tundra and herb tundra macrofossils and minor pollen of tundra forbs suggest steppe tundra plant associations within midslope elevations and discontinuous herb tundra on high elevation uplands on exposed bedrock ridges the composition and distribution of local vegetation was dependent on available moisture drainage aspect and elevation compositional and physiognomic similarities can be made with extrazonal steppe dominated dry slopes and high elevation steppe tundra ecotones in central alaska and yukon territory our paleoecological data reflect environments inhabited by the diverse late pleistocene bluefish caves fauna including woolly mammoth horse steppe bison and saiga antelope

distribution of glacial geomorphic features on the antarctic continental shelf and correlation with substrate implications for ice behavior surveys were conducted seaward of all the major drainage outlets of the antarctic ice sheet from the pennell coast north victoria land to marguerite bay antarctic peninsula the results show that the ice sheet extended onto the outer shelf glacial troughs occur offshore of all major glacial outlets where the substrate is crystalline bedrock ice flow tended to follow the structural grain of the bedrock deposited little sediment and eroded the underlying bedrock where ice flowed over relatively soft more easily eroded sedimentary strata the direction of ice flow was more directly offshore and depositional features characterize the sea floor in these areas the signature of the grounded ice consists of till deposits and large scale geomorphic features drumlins occur within the region of contact between crystalline and sedimentary substrates the different geological substrates are interpreted to have exerted a fundamental control on the behavior of past ice sheets the troughs in the areas of bedrock composed of sedimentary substrate are interpreted to have been occupied by relatively fast flowing ice ice streams and the troughs in the areas of crystalline substrate are interpreted to have been occupied by slower moving ice the area between these two zones was characterized by ice acceleration and is marked by drumlins

mutual climatic range reconstructions of seasonal temperatures based on late pleistocene fossil beetle assemblages in eastern beringia abstract the mutual climatic range mcr method of paleoclimate reconstruction has been employed by paleoentomologists for the last decade a quantitative calibrated method mcr has many advantages over qualitative methods the method has now been developed for arctic faunas climate envelopes for beetle species found in eastern beringia have been compiled using a 25 km gridded north american climate database that pairs climate parameters with modern collection sites linear regressions of predicted on observed values yielded equations used to calibrate the mcr estimates this study presents the initial results from the mcr analysis of 46 fossil beetle assemblages from alaska and the yukon territory the assemblages range in age from about 150 000 9200 yr bp isotope stage six assemblages yielded estimates of mean july temperature tmax 4 5 c colder than at present assemblages associated with the old crow tephra oct dated approximately 140 000 yr bp likewise indicated tmax values well below modern levels isotope stage 5e faunas showed tmax values as much as 3 5 c warmer than modern early wisconsinan assemblages showed climatic cooling with tmax levels dropping as low as 6 c colder than at present a mid wisconsinan interstadial period is marked by warming of mean summer temperatures to near modern levels at some sites last glacial maximum assemblages reflect cooling of tmax to a level about 6 4 c colder than at present although most assemblages from this period yielded mean january temperatures above modern levels warming of summer temperatures began at least by 12 500 yr bp and warmer than modern tmax values were recorded from all assemblages between then and 9200 yr bp

late holocene moisture balance variability in the southwest yukon territory canada abstract analyses of sediment cores from marcella lake a small hydrologically closed lake in the semi arid southwest yukon provides effective moisture information for the last 4500 years at century scale resolution water chemistry and oxygen isotope analyses from lakes and precipitation in the region indicate that marcella lake is currently enriched in 18 o by summer evaporation past lake water values are inferred from oxygen isotope analyses of sedimentary endogenic carbonate in the form of algal charophyte stem encrustations a record of the 18 o composition of mean annual precipitation at jellybean lake a nearby evaporation insensitive system provides data of simultaneous 18 o variations related to decade to century scale shifts in aleutian low intensity position the difference between the two isotope records represents 18 o enrichment in marcella lake water caused by summer effective moisture conditions results indicate increased effective moisture between 3000 and 1200 cal bp and two marked shifts toward increased aridity at 1200 and between 300 and 200 cal bp these prominent late holocene changes in effective moisture occurred simultaneously with changes in aleutian low circulation patterns over the gulf of alaska indicated by jellybean lake the reconstructed climate patterns are consistent with the topographically controlled climatic heterogeneity observed in the coastal mountains and interior valleys of the region today

physiological significance of low atmospheric co2 for plant climate interactions abstract methods of palaeoclimate reconstruction from pollen are built upon the assumption that plant climate interactions remain the same through time or that these interactions are independent of changes in atmospheric co 2 the latter may be problematic because air trapped in polar ice caps indicates that atmospheric co 2 has fluctuated significantly over at least the past 400 000 yr and likely the last 1 6 million yr three other points indicate potential biases for vegetation based climate proxies first c 3 plant physiological research shows that the processes that determine growth optima in plants photosynthesis mitochondrial respiration photorespiration are all highly co 2 dependent and thus were likely affected by the lower co 2 levels of the last glacial maximum second the ratio of carbon assimilation per unit transpiration called water use efficiency is sensitive to changes in atmospheric co 2 through effects on stomatal conductance and may have altered c 3 plant responses to drought third leaf gas exchange experiments indicate that the response of plants to carbon depleting environmental stresses are strengthened under low co 2 relative to today this paper reviews the scope of research addressing the consequences of low atmospheric co 2 for plant and ecosystem processes and highlights why consideration of the physiological effects of low atmospheric co 2 on plant function is recommended for any future refinements to pollen based palaeoclimatic reconstructions

geometry and development of glacial continental margin depositional systems in the bellingshausen sea we present multi channel seismic mcs reflection profiles and bathymetry data acquired across a remote and poorly surveyed part of the antarctic continental margin in the bellingshausen and amundsen seas this new information has been combined with published data and used to interpret the style of sedimentation on the continental shelf slope and rise and to describe sedimentation processes which have been active in this region most seismic reflection profiles crossing the continental margin show prograded sequences beneath the outer shelf and upper slope and we infer that the stratal characteristics of these sequences indicate that grounded ice sheets reached the shelf edge during previous glacial times although there are general similarities in stratal geometry on these profiles in detail they reveal significant longitudinal variations in sediment input from the shelf to the upper slope on several profiles we found evidence of mass wasting of the continental slope in the form of slump and debris flow deposits at greater depth turbidity flows bottom currents and coriolis force have controlled the further transportation and deposition of sediment which has resulted in the development of mounds channels and sediment wave fields the distribution and variations in the size and geometry of the mounds reflect sediment input and the relative contribution of these other factors which control sedimentation on the continental rise

late quaternary vegetation changes around lake rutundu mount kenya east africa evidence from grass cuticles pollen and stable carbon isotopes woody subalpine shrubs and grasses currently surround lake rutundu mount kenya multiple proxies including carbon isotopes pollen and grass cuticles from a 755 cm long core were used to reconstruct the vegetation over the past 38 300 calendar years stable carbon isotope ratios of total organic carbon and terrestrial biomarkers from the lake sediments imply that the proportion of terrestrial plants using the c4 photosynthetic pathway was greater during the late pleistocene than in the holocene pollen data show that grasses were a major constituent of the vegetation throughout the late pleistocene and holocene the proportion of grass pollen relative to the pollen from other plants was greatest at the last glacial maximum lgm grass cuticles confirm evidence that c4 grass taxa were present at the lgm and that the majority followed the cold tolerant nadp mec4 subpathway copyright 2003 john wiley sons ltd

late pleistocene paleoecology of arctic ground squirrel urocitellus parryii caches and nests from interior alaska s mammoth steppe ecosystem usa botanical analyses of fossil and modern arctic ground squirrel urocitellus parryii caches and nests have been used to reconstruct the past vegetation from some parts of beringia but such archives are understudied in alaska five modern and four fossil samples from arctic ground squirrel caches and nests provide information on late pleistocene vegetation in eastern beringia modern arctic ground squirrel caches from alaska s arctic tundra were dominated by willow and grass leaves and grass seeds and bearberries which were widespread in the local vegetation as confirmed by vegetation surveys late pleistocene caches from interior alaska were primarily composed of steppe and dry tundra graminoid and herb seeds graminoid cuticle analysis of fossil leaves identified calamagrostis canadensis koeleria sp and carex albonigra as being common in the fossil samples stable carbon isotopes analysis of these graminoid specimens indicated that plants using the c3 photosynthetic pathways were present and functioning with medium to high water use efficiency fossil plant taxa and environments from ground squirrel caches in alaska are similar to other macrofossil assemblages from the yukon territory which supports the existence of a widespread mammoth steppe ecosystem type in eastern beringia that persisted throughout much of the late pleistocene

pleistocene graminoid dominated ecosystems in the arctic abstract we review evidence obtained from analyses of multiple proxies floristics mammal remains paleoinsects pollen macrofossils plant cuticles phytoliths stable isotopes and modeling that elucidate the composition and character of the graminoid dominated ecosystems of the pleistocene arctic the past thirty years have seen a renewed interest in this now extinct biome sometimes referred to as tundra steppe steppe tundra in north american sources while many questions remain converging evidence from many new terrestrial records and proxies coupled with better understanding of paleoclimate dynamics point to the predominance of xeric and cold adapted grassland as the key former vegetation type in the arctic confirming earlier conjectures completed in the 1960s 1980s a variety of still existing species of grasses and forbs played key roles in the species assemblages of the time but their mixtures were not analogous to the tundras of today local mosaics based on topography proximity to the ice sheets and coasts soil heterogeneity animal disturbance and fire regimes were undoubtedly present however inadequate coverage of terrestrial proxies exist to resolve this spatial heterogeneity these past ecosystems were maintained by a combination of dry and cold climate and grazing pressure disturbance by large e g mammoth and horse and small e g ground squirrels mammals some recent studies from eastern beringia alaska suggest that more progress will be possible when analyses of many proxies are combined at local scales

early wisconsinan mis 4 arctic ground squirrel middens and a squirrel eye view of the mammoth steppe abstract fossil arctic ground squirrel spermophilus parryii middens were recovered from ice rich loess sediments in association with sheep creek klondike and dominion creek tephras ca 80 ka exposed in west central yukon these middens provide plant and insect macrofossil evidence for a steppe tundra ecosystem during the early wisconsinan mis 4 glacial interval midden plant and insect macrofossil data are compared with those previously published for late wisconsinan middens dating to 25 2914c ka bp mis 3 2 from the region although multivariate statistical comparisons suggest differences between the relative abundances of plant macrofossils the co occurrence of steppe tundra plants and insects e g elymus trachycaulus kobresia myosuroides artemisia frigida phlox hoodii connatichela artemisiae provides evidence for successive reestablishment of the zonal steppe tundra habitats during cold stages of the late pleistocene arctic ground squirrels were well adapted to the cold arid climates steppe tundra vegetation and well drained loessal soils that characterize cold stages of late pleistocene beringia these glacial conditions enabled arctic ground squirrel populations to expand their range to the interior regions of alaska and yukon including the klondike where they are absent today arctic ground squirrels have endured numerous quaternary climate oscillations by retracting populations to disjunct interglacial refugia during warm interglacial periods e g south facing steppe slopes well drained arctic and alpine tundra areas and expanding their distribution across the mammoth steppe biome during cold arid glacial intervals

the depositional pattern and distribution of glacial interglacial sequences on the antarctic peninsula pacific margin the outer shelf on the antarctic peninsula pacific margin south of 63 30 s is underlain by pliocene pleistocene prograding sequences which have been produced mainly by the action of ice sheets grounded out to the shelf edge at times of glacial maximum most sediment in these sequences has probably been transported to the margin in a deforming basal till which implies deposition on a broad front a line source a representative prograding sequence mapped across an extensive network of multichannel seismic reflection lines has an elongate depocentre on the upper palaeoslope which is consistent with the grounded ice sheet model however it is likely that each sequence recognized on existing multichannel seismic data represents several ice advances depth to surface maps reveal a broad variation along the margin in the amount of progradation reflecting differences in sediment supply the pattern of progradation and the bathymetry of the outer shelf suggest that the main depocentre in the area studied was fed by an ice stream at times of glacial maximum seismic lines across the margin farther to the southwest indicate the existence of other depocentres several broad depositional lobes have probably coalesced to form the extensive outer shelf the present continental slope is smooth and steep and is not cut by major canyons a downslope change in seismic facies and scouring on the uppermost rise probably reflect downslope transitions from slumps to debris flows to turbidity currents these processes are likely to be most active at times of glacial maximum deep drilling data indicate that the rise sediments consist mainly of terrigenous turbidite and ice rafted detritus a marked upward change in seismic facies on the continental rise indicates a change to a higher energy sedimentary regime and appears to correlate with the start of glacial progradation on the shelf

sedimentation on the continental rise west of the antarctic peninsula over the last three glacial cycles abstract the continental rise west of the antarctic peninsula includes a number of large sediment mounds interpreted as contourite drifts cores from six sediment drifts spanning some 650 km of the margin and 4 of latitude have been dated using chemical and isotopic tracers of palaeoproductivity and diatom biostratigraphy interglacial sedimentation rates range from 1 1 to 4 3 cm ka glacial sedimentation rates range from 1 8 to 13 5 cm ka and decrease from proximal to distal sites on each drift late quaternary sedimentation was cyclic with brown biogenic burrowed mud containing ice rafted debris ird in interglacials and grey barren laminated mud in glacials foraminiferal intervals occur in interglacial stages 5 and 7 but not in the holocene processes of terrigenous sediment supply during glacial stages differed meltwater plumes were more important in stages 2 4 turbidity currents and ice rafting in stage 6 the terrigenous component shows compositional changes along the margin more marked in glacials the major oxides al2o3 and k2o are higher in the southwest and cao and tio2 higher in the northeast there is more smectite among the clay minerals in the northeast magnetic susceptibility varies along and between drifts these changes reflect source variations along the margin interglacial sediments show less clear trends and their ird was derived from a wider area downslope processes were dominant in glacials but alongslope processes may have attained equal importance in interglacials the area contrasts with the east antarctic continental slope in the se weddell sea where ice rafting is the dominant process and where interglacial sedimentation rates are much higher than glacial the differences in glacial setting and margin physiography can account for these contrasts

age dating and the orbital theory of the ice ages development of a high resolution 0 to 300 000 year chronostratigraphy using the concept of orbital tuning a continuous high resolution deep sea chronostratigraphy has been developed spanning the last 300 000 yr the chronology is developed using a stacked oxygen isotope stratigraphy and four different orbital tuning approaches each of which is based upon a different assumption concerning the response of the orbital signal recorded in the data each approach yields a separate chronology the error measured by the standard deviation about the average of these four results which represents the best chronology has an average magnitude of only 2500 yr this small value indicates that the chronology produced is insensitive to the specific orbital tuning technique used excellent convergence between chronologies developed using each of five different paleoclimatological indicators from a single core is also obtained the resultant chronology is also insensitive to the specific indicator used the error associated with each tuning approach is estimated independently and propagated through to the average result the resulting error estimate is independent of that associated with the degree of convergence and has an average magnitude of 3500 yr in excellent agreement with the 2500 yr estimate transfer of the final chronology to the stacked record leads to an estimated error of 1500 yr thus the final chronology has an average error of 5000 yr