hot spot activity and the break up of pangea abstract the mesozoic and cenozoic positions of the continents that formed pangea in the triassic jurassic were derived from paleomagnetic and intraplate volcanic data paleoclimatic observations such as reef and fossil flora distribution and geological observations major hot spots helped to determine the longitudinal position of pangea and to construct a model of plate motion during the pangean break up the position of the northern part of pangea was constrained using iceland and jan mayen hot spots the iceland hot spot was traced from its present day position to greenland in the paleocene to baffin bay in the late cretaceous to the alpha ridge in the early cretaceous to the chukchi borderland in the middle late jurassic to franz joseph land in the early jurassic the yenisei khatanga trough in the middle triassic and finally to west siberia in the late permian early triassic the hot spot activity is expressed by eastern and western greenland volcanics the siberian trap basalts and perhaps by alpha ridge and chukchi borderland volcanics the chukchi borderland volcanics are related to the early stage of the opening of the canadian basin the position of the southern part of pangea was constrained using the bouvet hot spot this hot spot was tracked from its present day position to western antarctica in the early cretaceous late jurassic and to south africa in the early jurassic late triassic this hot spot activity produced the ferrar and karoo volcanics the model of plate motion obtained agrees with other data on intraplate volcanics which are also related to hot spots at the time of the opening of the central atlantic the cape verde and canary island hot spots were located along the ocean s spreading axis the long lasting location of hot spot and associated mantle upwelling plumes could help to explain forces driving the tethys transit plates opening of the ligurian ocean and eastern mediterranean the european hot spots are related to the several mesozoic phases of rifting for example the rhine graben hot spot may have been located at the rifting axis in the north sea several authors have already discussed the contribution of hot spots to the opening of the south atlantic and indian oceans mantle plumes associated with hot spots play an active role in rifting and initial phases of spreading the lithospheric displacement caused by upwelling combined with sometimes remote collisional forces on the other side of continental plate may result in compression and basin inversion

postglacial vegetation history of the mackenzie river basin abstract three radiocarbon dated cores from small lakes provide pollen records of postglacial vegetation change in the central mackenzie river basin these data are combined with previously published records to reconstruct vegetation development from central alberta to the mackenzie delta the paleoecological data demonstrate that a continuous corridor of herb and shrub dominated vegetation extended between beringia and the great plains of north america from deglaciaiton until about 10 000 yr b p the initial vegetation was replaced by spruce forests between 10 000 and 8500 yr b p significant vegetation change continued until approximately 500 yr b p when the muskeg dominated landscape typical of the present subarctic was established the initial herb and shrub vegetation was likely a reprise of conditions which existed during earlier pleistocene deglaciations these conditions may have provided a favorable environment for animals adapted to nonforested vegetation and promoted migration southward from beringia and northward from the great plains during the last deglaciation and earlier pleistocene deglaciations the sequence of postglacial vegetation change in the mackenzie basin likely reflects a combination of climatic change edaphic change and plant migration patterns and interactions

late quaternary vegetation history of the fishhook bend area porcupine river alaska ped pond is a lake lying in a pleistocene scour channel of the porcupine valley the valley is considered to be a possible path of late quaternary species migrations between interior alaska and northwest canada pollen in the oldest lake sediments ca 13 000 years old indicates that shrub birch and willow probably occurred locally in a sparsely vegetated herb dominated landscape populus juniperus and typha latifolia increased about 10 000 years ago suggesting particularly warm temperatures during the early holocene picea glauca which is presently common in the surrounding forests first arrived near ped pond about 8500 years ago this arrival date together with existing evidence from other sites indicates that spruce expanded rapidly across far eastern beringia 8000 9000 years ago other boreal forest taxa picea mariana betula papyrifera and alnus spp became established locally between 7000 and 8000 years bp a date of ca 7500 years bp at ped pond suggests that the alnus rise in northeast

paleoenvironmental analyses of an organic deposit from an erosional landscape remnant arctic coastal plain of alaska abstract the dominant landscape process on the arctic coastal plain of northern alaska is the formation and drainage of thaw lakes lakes and drained thaw lake basins account for approximately 75 of the modern surface expression of the barrow peninsula the thaw lake cycle usually obliterates lacustrine or peat sediments from previous cycles which could otherwise be used for paleoecological reconstruction of long term landscape and vegetation changes several possible erosional remnants of a former topographic surface that predates the formation of the thaw lakes have been tentatively identified these remnants are characterized by a higher elevation a thick organic layer with very high ground ice content in the upper permafrost and a plant community somewhat atypical of the region ten soil cores were collected from one site and one core was intensively sampled for soil organic carbon content pollen analysis and 14c dating the lowest level of the organic sediments represents the earliest phase of plant growth and dates to ca 9000 cal bp palynological evidence indicates the presence of mesic shrub tundra including sedge birch willow and heath vegetation and microfossil indicators point to wetter eutrophic conditions during this period carbon accumulation was rapid due to high net primary productivity in a relatively nutrient rich environment these results are interpreted as the local response to ameliorating climate during the early holocene the middle holocene portion of the record contains an unconformity indicating that between 8200 and 4200 cal bp sediments were eroded from the site presumably in response to wind activity during a drier period centered around 4500 cal bp the modern vegetation community of the erosional remnant was established after 4200 cal bp and peat growth resumed during the late holocene carbon accumulation rates cars were greatly reduced in response to the combined effects of declining productivity associated with climatic cooling and increased nutrient stress as paludification and permafrost aggradation sequestered mineral nutrients

geomagnetic quiet day selection based on published literature and the response to a questionnaire sent to geomagnetic field ionospheric and magnetospheric researchers several methods of choosing periods of quiet conditions based on geomagnetic records as well as other observed parameters have been identified caveats with respect to using geomagnetic indices to select quiet periods include the following 1 geomagnetic disturbances are strongly local even if the data from all available observatories indicate quiet behavior there is the distinct possibility that some other location not sampled may be disturbed 2 geomagnetic indices are convenient but imperfect indicators of geomagnetic activity indices based on a quiet day reference level have uncertainties comparable to the threshold value for quiet conditions indices representing average conditions during a 24 hr ut day may not be appropriate 3 geomagnetic activity does not fully reflect the range of possible factors that influence the ionosphere or magnetosphere

a 130 0 year history of vegetation and environmental change at jan lake east central alaska pollen stomates and macrofossils were analysed from a 3 63 m sediment core at jan lake eastcentral alaska in order to improve spatial resolution of patterns of vegetation history in this region the chronology was based on 19 ams 14c dates on plant macrofossils and concentrated pollen zone jan i 12400 11 600 yr bp records a unique herb tundra pollen assemblage particularly high in chenopodia ceae adding supporting evidence for a spatial mosaic of lateglacial vegetation in east central alaska unexpected pollen of picea betula and alnus in lateglacial sediments appears to arise from reworking of older pollen bearing deposits according to a radiocarbon date of 22180yr bp on a pollen sample shrub betula tundra dominates zone jan il 11 600 10000 yr bp with indications of climatic amelioration tree species populus picea glauca betula papyrifera began to arrive in zone jan illa 9200 6600 yr bp and the modern boreal forest including alnus and picea mariana became established during zone j

average electron precipitation patterns and visual aurora characteristics during geomagnetic quiescence the pattern of polar region electron precipitation during geomagnetically quiet periods i e mostly under the positive imf bz condition is examined on the basis of measurements from the defense meteorological satellite program dmsp f2 f3 and f4 satellites the auroral electron precipitation during quiet times consists of two distinct types that differ from those found in active times the high latitude part of the precipitation region has average electron energies lower than 500 ev and has bursty spatial structures but the average energies of the low latitude part are generally greater than 500 ev with smoother spatial structures the boundary between the hard and soft regions is called the transition boundary it is found that during quiet times the average location of the poleward boundary of electron precipitation is at particularly high geomagnetic latitudes about 82 to 84 in the morning noon and evening sectors and at about 81 to 82 in the midnight sector the equatorward edge is at about 70 in the morning noon and evening sectors and about 69 in the midnight sector the latitudinal width of the region of auroral electron precipitation is unusually large indicating a widening of the auroral oval in periods of geomagnetic quiescence dmsp auroral imaging data further reveal that there are no bright discrete optical auroras in the region of low average energy electron precipitation faint stable auroral arcs are sometimes seen in the equatorial region of the high average energy electron precipitation the location of the poleward boundary is affected by the magnitude of the northward interplanetary magnetic field imf component however the location of the equatorward and transition boundaries of precipitation are not related to the positive imf bz magnitude these results indicate that the unusually high latitude of the poleward boundary of precipitation is the consequence of a very small number of geomagnetic flux lines interconnected with the imf during a northward imf condition the results also imply that the size of the polar cap is controlled inversely by the magnitude of the northward bz component the insensitivity of the transition and equatorward precipitation boundaries to the northward imf bz can be used to imply that the boundary dynamics are dominated by internal magnetospheric processes and not by direct solar magnetospheric interactions

chronology and glass chemistry of tephra and cryptotephra horizons from lake sediments in northern alaska usa holocene tephrostratigraphy in alaska provides independent chronology and stratigraphic correlation in a region where reworked old holocene organic carbon can significantly distort radiocarbon chronologies here we present new glass chemistry and chronology for holocene tephras preserved in three alaskan lakes one in the eastern interior and two in the southern brooks range tephra beds in the eastern interior lake sediment core are correlated with the white river ash and the hayes tephra set h 4200 3700 cal yr bp while an additional discrete tephra bed is likely from the aleutian arc alaska peninsula cryptotephras non visible tephras found in the brooks range include the informally named ruppert tephra 2700 2300 cal yr bp and the aniakchak caldera forming event ii tephra cfe ii 3600 cal yr bp a third underlying brooks range cryptotephra is chemically indistinguishable from the aniakchak cfe ii tephra 4070 3760 cal yr bp and is likely to be from an earlier eruption of the aniakchak volcano

a late wisconsin and holocene vegetation history from the central brooks range implications for alaskan palaeoecology abstract five pollen diagrams reveal late wisconsin and holocene vegetation changes in the walker lake alatna valley region of the central brooks range approximately 100 km west of the area studied by d a livingstone 1955 ecology 36 587 600 new insights into the vegetation history of this region are provided by calculations of pollen influx and by the use of linear discriminant analysis to separate picea glauca and p mariana pollen three major pollen zones are identified 1 a basal herb zone characterized by high percentages of cyperaceae gramineae salix and artemisia and low total pollen influx 2 a shrub betula zone with increased total pollen influx and very high percentages of betula pollen predominantly in the size range of b nana and b glandulosa and 3 and alnus zone dominated by alnus pollen lakes currently within the boreal forest or near tree line show relatively high percentages of picea pollen in the alnus zone several striking vegetation changes occurred between ca 10 000 and 7000 yr b p between ca 11 000 and 10 000 yr b p populus balsamifera pollen percentages as great as 30 indicate that this species was present at low elevation sites near walker lake these populations declined abruptly ca 10 000 yr ago and have never regained prominence about 8500 yr b p picea glauca pollen reached 10 15 indicating the arrival of p glauca in or near the study area p glauca populations evidently decreased ca 8000 yr ago when picea pollen percentages and influx fell to low values about 7000 yr b p alnus pollen percentages and influx rose sharply as alder shrubs became established widely picea once more expanded ca 5000 yr ago but these populations were dominated by p mariana rather than p glauca which increased slowly at this time and may still be advancing northward some vegetation changes have been remarkably synchronous over wide areas of interior alaska and probably reflect responses of in situ vegetation to environmental changes but others may reflect the lagged responses of species migrating into new areas

holocene development of boreal forests and fire regimes on the kenai lowlands of alaska several studies have noted a relationship between vegetation type and fire frequency yet despite the importance of ecosystem processes such as fire the long term relationships between disturbance climate and vegetation type are incompletely understood we analysed pollen plant macrofossils and sedimentary charcoal from three lakes within the kenai lowlands to determine postglacial relationships between disturbance climate and vegetation for the boreal forest of southwest alaska an herb tundra was established in the lowlands following deglaciation by 13 000 cal bp salix alnus and probably betula kenaica expanded in the area after 10 700 cal bp followed by picea glauca by 8500 cal bp picea mariana became established by 4600 cal bp the early holocene was probably the driest time during the postglacial as determined by aquatic plant macrofossils and climate models lake levels reached near modern conditions by at least 8000 cal bp mean fire intervals mfi were longest during the shrubherb tundra phase 1389 65 yr decreased after expansion of b kenaica salix and populus 779 49 yr and picea glauca 819 41 yr and increased again with the arrival of p mariana 1309 66 yr unlike previous studies our data demonstrate the highest fire frequencies during the early to mid holocene and less frequent fire during the late holocene when p mariana forests dominated the lowlands early holocene forests of p glauca and b kenaica existed in summers that were longer and drier than today while the increasingly wetter and cooler climates of the late holocene probably hindered forest fire around paradox lake perhaps because of less frequent summer drought

the shift of the auroral electron precipitation boundaries in the dawn dusk sector in association with geomagnetic activity and interplanetary magnetic field on the basis of the auroral precipitating electron data along the dawn dusk meridian from a defense meteorological satellite program satellite dmsp f2 we show that the electron precipitation region extends poleward often to the geomagnetic latitudes 85 from the average oval location during quiet periods namely during periods of a large positive bz component this result may be interpreted as an indication that only a small amount of geomagnetic flux interconnects with the northward imf resulting in a contracted open region the polar cap the control of the location of the poleward boundary by the north south component of the imf is also statistically examined the rate of latitudinal movement is about 0 4 to 0 8 per 1 nt for positive imf bz values and about 0 7 to 1 1 per 1 nt for negative imf bz values the shift of the equatorward boundary is not obvious however for positive imf bz values as geomagnetic activity increases both the poleward and equatorward boundaries shift toward lower latitudes the equatorward shift of the poleward boundary is greater than that of the equatorward boundary resulting in a thinner electron precipitation region during disturbed periods than during quiet periods

dated wood from alaska and the yukon implications for forest refugia in beringia abstract postulations on the existence of forest refugia in parts of beringia during the last glacial have been in large part based on ambiguous evidence existing data on radiocarbon dated and identified fossil wood and macrofossils from alaska and northwest canada are synthesized here and are augmented by results of palynological studies in an effort to show the persistence of some and total extinction of other tree and large shrub species possible dispersal routes taken by species that reinvaded beringia in postglacial times are also reconstructed from the fossil record macrofossil and pollen evidence when combined with climatic factors makes cottonwood a good candidate for survival during the last glacial larch and aspen are also candidates though the evidence for them is less positive pollen and macrofossils of alder are very scarce in deposits of the last glacial age and if it survived at all it was probably in very isolated vegetatively reproducing clones shrub birch may have been present in beringia but tree birch probably was reintroduced during the holocene spruce also appears to have been absent in alaska from about 30 000 to 11 500 yr ago and probably reinvaded beringia from a refugium south of the laurentide ice sheet

temporal and spatial variations of the polar cap dimension inferred from the precipitation boundaries we examine temporal and spatial variations of the distance between the electron precipitation boundaries across the polar cap the defense meteorological satellite program satellite data within a few hours of the dawn dusk and noon midnight meridians are used it is found that changes in the distance between the precipitation boundaries across the polar region in both the dawn dusk and noon midnight sectors tend to follow grossly changes of the ae index on the basis of the simultaneous isee 3 data it is found that the increase of the distance begins at about the time when the southward turning of the interplanetary magnetic field reaches the magnetosphere but prior to the corresponding increase of the ae index the period of the maximum distance approximately coincides with the period of the maximum ae value the decrease of the distance begins at about the same time as the substorm activity begins to subside but it continues well after the ae index has reduced to very small values these findings are discussed in terms of changes of the total open magnetic flux by assuming that the precipitation boundaries coincide approximately with the boundaries of the open field line region

late quaternary history of tundra vegetation in northwestern alaska pollen analysis of a new core from joe lake indicates that the late quaternary vegetation of northwestern alaska was characterized by four tundra and two forest tundra types these vegetation types were differentiated by combining quantitative comparisons of fossil and modern pollen assemblages with traditional qualitative approaches for inferring past vegetation such as the use of indicator species although imprecisely dated the core probably spans at least the past 40 000 yr a graminoid salix tundra dominated during the later and early portions of the glacial record the middle glacial interval and the transition from glacial to interglacial conditions are characterized by a graminoid betula salix tundra a populus forest betula shrub tundra existed during the middle potion of this transition being replaced in the early holocene by a betula alnus shrub tundra the modern picea forest shrub tundra was established by the middle holocene these results suggest that the composition of modem tundra communities in northwestern alaska developed relatively recently and that throughout much of the late quaternary tundra communities were unlike the predominant types found today in northern north america although descriptions of vegetation variations within the tundra will always be restricted by the innate taxonomic limitations of their herb dominated pollen spectra the application of multiple interpretive approaches improves the ability to reconstruct the historical development of this vegetation type

ionospheric and birkeland current distributions for northward interplanetary magnetic field inferred polar convection a new stable system of birkeland and ionospheric currents has been added to the existing region 1 region 2 auroral electrojet system when the interplanetary magnetic field imf is directed northward these currents exist poleward of the region 1 system across the polar region and have been called the northward imf bz nbz current system a companion paper iijima et al this issue describes in detail the birkeland currents magnetic signature a w shaped pattern in the sunward antisunward direction and the nbz system dependence on the various components of the imf the companion study used mag sat magnetometer data from 146 orbits over the southern polar regions from november 1979 to january 1980 data from two days november 11 1979 and january 8 1980 have been analyzed in great detail with regard to the nbz w signature of birkeland currents in addition a fourier technique has been used in an analysis of the disturbance component parallel to the main field to derive the ionospheric current system during those two days the nbz ionospheric current system has shown a similar w shaped pattern with antisunward current implying sunward convection over the magnetic pole and with return currents on either side multicell patterns emerge from the nbz ionospheric patterns and an asymmetry appears to be caused by the imf by component as well suggestions are given for multicell patterns over the polar regions for northward imf during an imf by sign change

climate change and arctic ecosystems 1 vegetation changes north of 55 n between the last glacial maximum mid holocene and present a unified scheme to assign pollen samples to vegetation types was used to reconstruct vegetation patterns north of 55 n at the last glacial maximum lgm and mid holocene 6000 years b p the pollen data set assembled for this purpose represents a comprehensive compilation based on the work of many projects and research groups five tundra types cushion forb tundra graminoid and forb tundra prostrate dwarf shrub tundra erect dwarf shrub tundra and low and high shrub tundra were distinguished and mapped on the basis of modern pollen surface samples the tundra forest boundary and the distributions of boreal and temperate forest types today were realistically reconstructed during the mid holocene the tundra forest boundary was north of its present position in some regions but the pattern of this shift was strongly asymmetrical around the pole with the largest northward shift in central siberia 200 km little change in beringia and a southward shift in keewatin and labrador 200 km low and high shrub tundra extended farther north than today at the lgm forests were absent from high latitudes graminoid and forb tundra abutted on temperate steppe in northwestern eurasia while prostrate dwarf shrub erect dwarf shrub and graminoid and forb tundra formed a mosaic in beringia graminoid and forb tundra is restricted today and does not form a large continuous biome but the pollen data show that it was far more extensive at the lgm while low and high shrub tundra were greatly reduced illustrating the potential for climate change to dramatically alter the relative areas occupied by different vegetation types

holocene glaciation of alaska and adjoining yukon territory canada abstract holocene glacial fluctuations between arctic central interior and southern maritime alaska are broadly synchronous this synchrony is evident from a review of work in 11 study areas with varying numbers of glaciers 3 100 glacier types subpolar cirque temperate valley tidewater surging and dating methodologies the pattern is temporally and spatially consistent with vegetation data and generally with climatic deteriorations culminating in the little ice age early holocene glacial advances are confirmed for 7 6 5 8 lichen ka bp at 4 of 97 glaciers in the brooks range advances also occurred by 5 85 and 5 7 ka bp at separate glacier tongues in the northern alaska range at least by 6 5 ka bp in lituya bay and possibly also in the malaspina and aleutian islands areas a sharp increase in glacier activity began by 4 4 ka bp in the arctic and glacier bay areas and by about 3 ka bp major glacier advances probably were initiated or well underway throughout alaska during holocene glacial maxima ice filled glacier and lituya bays as well as fjords of prince william sound and massive moraines buried forests in the northeastern st elias mountains area minor recession is recorded directly or implied in most of alaska about 2 ka bp or locally through the following millennium subsequently distinct but locally weak advances occurred by about 1 2 ka bp in most areas again followed by intervals of glacier recession most pervasive and ubiquitous were the little ice age advances initiated about 700 bp these generally culminated in two major advances between about 400 and 50 bp in the arctic those of a d 1600 dominated those of the late 1800s in the gulf of alaska those of the middle 1700s or late 1800s dominated large scale asynchronous advances and retreats of calving tidewater glaciers have been recorded during the twentieth century but most such movements are only indirectly related to climate

holocene thermal maximum in the western arctic 0 180 w the spatio temporal pattern of peak holocene warmth holocene thermal maximum htm is traced over 140 sites across the western hemisphere of the arctic 0 180 w north of 60 n paleoclimate inferences based on a wide variety of proxy indicators provide clear evidence for warmer than present conditions at 120 of these sites at the 16 terrestrial sites where quantitative estimates have been obtained local htm temperatures primarily summer estimates were on average 1 6 0 8 c higher than present approximate average of the 20th century but the warming was time transgressive across the western arctic as the precession driven summer insolation anomaly peaked 12 10 ka thousands of calendar years ago warming was concentrated in northwest north america while cool conditions lingered in the northeast alaska and northwest canada experienced the htm between ca 11 and 9 ka about 4000 yr prior to the htm in northeast canada the delayed warming in quebec and labrador was linked to the residual laurentide ice sheet which chilled the region through its impact on surface energy balance and ocean circulation the lingering ice also attests to the inherent asymmetry of atmospheric and oceanic circulation that predisposes the region to glaciation and modulates the pattern of climatic change the spatial asymmetry of warming during the htm resembles the pattern of warming observed in the arctic over the last several decades although the two warmings are described at different temporal scales and the htm was additionally affected by the residual laurentide ice the similarities suggest there might be a preferred mode of variability in the atmospheric circulation that generates a recurrent pattern of warming under positive radiative forcing unlike the htm however future warming will not be counterbalanced by the cooling effect of a residual north american ice sheet

latitudinal electron precipitation patterns during large and small imf magnitudes for northward imf conditions we demonstrate that there are distinct differences in the electron precipitation patterns or the polar cap size geomagnetic activity and field aligned currents in the highest latitude region for small and large imf bz values when the imf bz component is positive first during periods of weakly northward imf there is a distinct area in the highest latitude region in which the electron precipitation is absent except for the polar rain by contrast during strongly northward imf the entire polar region is often filled with burst type soft electron precipitations second geomagnetic disturbances and field aligned current intensitites in the highest latitude region 80 in geomagnetic latitude are less during a weak imf bz condition than those during a strongly northward imf bz condition geomagnetic activity in the auroral zone for both conditions is absent or very weak

late quaternary vegetational change in the kotzebue sound area northwestern alaska abstract two sediment cores from kaiyak and squirrel lakes in northwestern alaska yielded pollen records that date to ca 39 000 and 27 000 yr b p respectively between 39 000 and 14 000 yr b p the vegetation around these lakes was dominated by gramineae and cyperaceae with some salix and possibly betula nana glandulosa forming a local shrub component of the vegetation betula pollen percentages increased about 14 000 yr b p indicating the presence of a birchdominated shrub tundra alnus pollen appeared at both sites between 9000 and 8000yr b p and picea pollen mostly p mariana arrived at squirrel lake about 5000 yr b p the current foresttundra mosaic around squirrel lake was established at this time whereas shrub tundra existed near kaiyak lake throughout the holocene when compared to other pollen records from north western north america these cores 1 represent a meadow component of lowland beringian tundra between 39 000 and 14 000 yr b p 2 demonstrate an early holocene arrival of alnus in northwestern alaska that predates most other alnus horizons in northern alaska or northwestern canada and 3 show an east to west migration of picea across northern alaska from 9000 to 5000 yr b p

extremely high latitude auroras over 1600 defense meteorological satellite program north polar images from the winters 1972 1973 1974 1975 were studied for the occurrence of auroral arcs at corrected geomagnetic latitudes greater than 80 auroras at these latitudes are designated extremely high latitude auroras and are categorized by the local time sectors in which they occur and their relationship to discrete oval arcs a data base of 300 cases of cap auroras was formed for correlation studies with geomagnetic activity indices and solar wind parameters by far the greatest number of extremely high latitude auroras occur in the morning sector and appear to be expansions of the system of morning oval arcs poleward the remaining categories include evening arcs expanding into the cap single sun aligned arcs and midnight sector oval auroras expanding into the cap except for the last category the auroras occur for similar magnetospheric conditions moderate values of kp and dst but comparatively low values of ae they also occur preferentially for high solar wind speed and for positive bz while far less clear their location in the cap shows a dependence on by morning evening arcs occur with greater frequency for negative positive by

late quaternary pollen records from the kobuk and noatak river drainages northwestern alaska abstract pollen diagrams from joe and niliq lakes date to ca 28 000 and 14 000 yr b p respectively mesic shurb tundra grew near joe lake ca 28 000 to 26 000 yr b p with local populus populations prior to ca 27 000 yr b p shrub communities decreased as climate changed with the onset of itkillik ii glaciation 25 000 to 11 500 yr b p and graminoid dominated tundra characterized vegetation ca 18 500 to 13 500 yr b p herb tundra was replaced by shrub betula tundra near both sites ca 13 500 yr b p with local expansion of populus ca 11 000 to 10 000 yr b p and alnus ca 9000 yr b p mixed picea glauca p mariana woodland was established near joe lake ca 6000 yr b p these pollen records when combined with others from northern alaska and northwestern canada indicate 1 mesic tundra was more common in northwestern alaska than in northeastern alaska or northwestern canada during the duvanny yar glacial interval 25 000 to 14 000 yr b p 2 with deglaciation shrub betula expanded rapidly in northwestern alaska but slowly in areas farther east 3 an early postglacial thermal maximum occurred in northwestern alaska but had only limited effect on vegetation and 4 pollen patterns in northern alaska and northwestern canada suggest regional differences in late quaternary climates

late quaternary vegetational history of the black river region in northeastern alaska two pollen diagrams describe the previously unknown vegetational history of the black river and little black river drain ages of northeastern alaska tundra grew throughout much of the region between ca 19 800 and 15 000 years bp prior to 18 000 years bp the vegetation was dominated perhaps by xeric cyperaceae communities but apparently between ca 18 000 and 15 000 years bp a more diverse herb tundra characterized the vegetation herbaceous species continued to be important in the vegetation until ca 9700 years bp although betula nana glandulosa and salix species became more common after 15 000 years bp at ca 9700 years bp a major change in the vegetation occurred with the spread of populus probably p balsamifera b nana glandulosa and salix possibly resulting in a mosaic of shrub tundra and populus gallery forest picea glauca migrated into the black river region ca 7500 years bp followed by alnus ca 7200 years bp and picea mariana ca 6000 years bp herb zone records from the black rive

low and high frequency climate variability in eastern beringia during the past 25 000 yearsthis article is one of a series of papers published in this special issue on the theme polar climate stability network we present new temperature and precipitation reconstructions for the past 25 000 cal years bp from across eastern beringia based on a network of pollen diagrams an updated modern pollen calibration database and an improved methodology using the modern analogue technique mat time series show july temperatures were around 4 c lower during full glacial and january temperatures were about 2 c lower than present annual temperatures rose beginning around 16 000 cal years bp reaching a maximum around 12 000 cal year bp the warming was more rapid in southern beringia annual precipitation varied by 250 mm during the past 25 000 cal years bp maps of reconstructed precipitation patterns show increasingly drier conditions since 12 000 cal years bp that vary regionally suggesting holocene atmospheric circulation changes at multiple time and space scales orbitally forced seasonality changes during the late glacial and early holocene resulted in reversed seasonal temperature reconstructions due to me

a late pleistocene glacial chronology for the southern brooks range stratigraphic record and regional significance radiocarbon dates from 11 measured sections in the koyukuk region provide a chronology of the last pleistocene glaciation glaciers were advancing strongly by 24 000 yr ago they built moraines near the south flank of the brooks range retreated briefly about 22 000 to 20 000 yr b p then readvanced at least one more time into their terminal zones glaciation was accompanied by alluviation of the koyukuk and kobuk drainage systems and by periglacial processes that resemble those taking place today farther north and at higher altitudes moraines near the south flank of the range were being revegetated by 13 500 yr b p a strong final readvance into end moraine belts of some northern valleys occurred about 13 000 to 12 500 yr ago and this event may be synchronous with less extensive glacier readvances in upper valleys of the koyukuk region upper valleys were largely deglaciated by 11 800 yr b p dated stratigraphic sections from the northern alaska range show similar ages for initiation and close of glaciation and also suggest a possible interstadial episode about 20 000 yr ago scanty records of fluctuations during ice wastage probably reflect the general scarcity of datable wood peat and organic soils between about 19 500 and 13 500 yr ago the brooks range and alaska range chronologies closely approximate glacial successions determined else where in eastern beringia and in siberia advance and retreat of glaciers throughout this region evidently were associated with widespread climatic changes that also controlled the late wisconsin history of the laurentide ice sheet the itkillik ii and late itkillik phases of former usage are part of a single glaciation that was entirely separate from the preceding itkillik i ice advance for this reason the local term walker lake glaciation is here extended to the last major glaciation of the entire southern brooks range and use of itkillik phases should be discontinued the term itkillik glaciation is hereby restricted to the next older ice advance in accord with its original definition cold and dry conditions during the last glaciation of eastern beringia are indicated by 1 relatively small mountain glaciers 2 slight 200 m depression of glaciation limits below modern values 3 periglacial features indicating severe frost action on slopes with little protective plant cover 4 widespread accretion of eolian sand 5 low pollen influx rates 6 scarcity of radiocarbon datable organic remains and 7 general absence of carbonaceous paleosols plant growth may have been much more restricted than generally believed with relatively low capacity to support grazing animals and human hunting bands

climate and vegetation in northeastern alaska 18 000 yr b p present changing abundances of taxa in the pollen record of northeastern alaska contain a climate signal and may be compared with gcm simulations of paleoclimates cold dry conditions indicated by full glacial pollen spectra are in broad agreement with model simulations successive expansion of betula and salix and later populus during the deglacial period 14 000 11 000 yr b p are likely correlated with the crossing of temperature thresholds determined largely by the size and position of the laurentide ice a degree of asynchroneity between sites probably reflects differential site responses to regional effects plus inaccuracies of radiocarbon dating the early holocene was probably the warmest period of the present interglacial as summer climate was influenced by a strong positive insolation anomaly picea glauca expanded in majorr river valleys ca 9000 yr ago and slightly later at upland sites in the brooks and alaska ranges at some sites a juniperus subzone which suggests particularly warm and dry conditions occurs between ca 10 000 and 8000 yr b p early mid holocene expansions of alnus 8500 7000 yr b p and picea mariana 7500 5000 yr b p suggest the subsequent development of moister conditions which have continued to the present temporal patterns in picea treeline position during the holocene are different in northern alaska northwestern canada and central canada in alaska the treeline is currently at its furthest holocene extent in northwestern canada the treeline has retreated since the mid holocene and in central canada a mid holocene advance and retreat occurred such differences between regions suggest a complex response of this taxon to climatic controls and underscore the need to interpret vegetation changes in relation to regional scale climate phenomena if the pollen chronology is calibrated with reference to u th dates the relationship of vegetational events to the insolation maximum at ca 10 000 yr b p is significantly altered the changed chronology also affects the interpretation of pollen records in relation to gcm simulations which are partly keyed to calendar years