synchronization of late glacial vegetation changes at crystal lake illinois usa with the north atlantic event stratigraphy abstract late glacial 17 11 cal ka bp pollen records from midwestern north america show similar vegetation trends however poor dating resolution wide interval pollen counts and variable sedimentation rates have prevented the direct correlation with the north atlantic event stratigraphy as represented in the greenland ice core records thus preventing the understanding of the teleconnections and mechanisms of late quaternary events in the northern hemisphere the widespread occurrence of late glacial vegetation and climates with no modern analogs also hinders late glacial climate reconstructions a high resolution pollen record with a well controlled age model from crystal lake in northeastern illinois reveals vegetation and climate conditions during the late glacial and early holocene intervals late glacial crystal lake pollen assemblages dominated by picea mariana and fraxinus nigra with lesser amounts of abies and larix suggest relatively wet climate despite fluctuations between colder and warmer temperatures vegetation changes at crystal lake are coeval with millennial scale trends in the ngrip ice core record but major shifts in vegetation at crystal lake lag the ngrip record by 300 400 yr this lag may be due to the proximity of the laurentide ice sheet the ice sheet s inherent slowness in response to rapid climate changes and its effect on frontal boundary conditions and lake effect temperatures

holocene environments of the central great plains multi proxy evidence from alluvial sequences southeastern nebraska abstract pollen plant macrofossils phytoliths carbon isotopes and alluvial history from sediments exposed along the south fork of the big nemaha river southeastern nebraska usa provide an integrated reconstruction of changes in holocene vegetation climate and fluvial activity from 9000 to 8500 uncalibrated 14 c yr bp climate became more arid and the floodplain and alluvial fans in the main valley aggraded rapidly upland deciduous forest declined and prairie attained its holocene dominance from 8500 to 5800 yr bp upland forest elements disappeared and even riparian trees were sparse under dry climatic conditions alluvial fans continued to aggrade but aggradation in the main valley was interrupted by a stable episode 7000 yr bp from 5800 to 3100 yr bp riparian forests returned to prominence and droughts were intermittent alluviation was slower and punctuated by two major episodes of channel incision and terrace formation in the main valley aggradation on alluvial fans slowed and finally ceased near the end of this period during a short dry interval from 3100 to 2700 yr bp riparian trees except elm disappeared and prairie and weedy species became more abundant this interval is represented by the organic roberts creek member and the alluvial setting was a slightly incised meandering channel belt habitats became similar to presettlement conditions during the last 2700 yr bp weedy taxa dominate modern samples reflecting widespread disturbance alluvial fans and terrace surfaces were stable during the last 2500 years but episodes of floodplain aggradation were punctuated by incision of the main channel

the magnitude of error in conventional bulk sediment radiocarbon dates from central north america abstract although the carbon reservoir problem with bulk sediment radiocarbon dates from lakes has long been recognized many synoptic studies continue to use chronologies derived from such dates for four sites in central north america we evaluate chronologies based on conventional radiocarbon dates from bulk sediment versus chronologies based on accelerator mass spectrometry ams radiocarbon dates from terrestrial plant macrofossils the carbon reservoir error varies among sites and temporally at individual sites from 0 to 8000 yr an error of 500 2000 yr is common this error has important implications for the resolution of precise event chronologies

vegetation and environment in eastern north america during the last glacial maximum abstract knowledge of the vegetation and environment of eastern north america during the last glacial maximum lgm is important to understanding postglacial vegetational and biogeographic dynamics assessing climate sensitivity and constraining and evaluating earth system models our understanding of lgm conditions in the region has been hampered by low site density problems of data quality particularly dating and the possibility that lgm vegetation and climate lacked modern analogs in order to generate improved reconstructions of lgm vegetation and environment we assembled pollen and plant macrofossil data from 21 and 17 well dated lgm sites respectively all sites have assemblages within the lgm timespan of 21 000 1500 calendar yr bp based on these data we prepared maps of isopolls macrofossil presence absence pollen analogs biomes inferred mean january and july temperatures and mean annual precipitation for the lgm tundra and open picea dominated forest grew along the laurentide ice sheet with tundra predominantly in the west in the east pinus dominated vegetation mainly p banksiana with local p resinosa and p strobus occurred extensively to 34 n and possibly as far south as 30 n picea glauca and a now extinct species p critchfieldii occurred locally picea dominated forest grew in the continental interior with temperate hardwoods quercus carya juglans liriodendron fagus ulmus growing locally near the lower mississippi valley at least as far north as 35 n picea critchfieldii was the dominant species in this region the florida peninsula was occupied by open vegetation with warm temperate species of pinus eastern texas was occupied by open vegetation with at least local quercus and picea extensive areas of peninsular florida and the continental interior had vegetation unmatched by any modern pollen samples the paleovegetational data indicate more extensive cooling in eastern north america at the lgm than simulated by either the ncar ccm0 or ccm1 climate models the occurrence of cool temperate conifers and hardwoods as far north as 34 35 n however indicates less severe cooling than some previous reconstructions paleoclimate inferences for the lgm are complicated by lowered atmospheric co 2 concentrations which may be responsible for the open nature and dominance of conifers in lgm vegetation

the younger dryas cold interval as viewed from central greenland abstract greenland ice core records provide an exceptionally clear picture of many aspects of abrupt climate changes and particularly of those associated with the younger dryas event as reviewed here well preserved annual layers can be counted confidently with only 1 errors for the age of the end of the younger dryas 11 500 years before present ice flow corrections allow reconstruction of snow accumulation rates over tens of thousands of years with little additional uncertainty glaciochemical and particulate data record atmospheric loading changes with little uncertainty introduced by changes in snow accumulation confident paleothermometry is provided by site specific calibrations using ice isotopic ratios borehole temperatures and gas isotopic ratios near simultaneous changes in ice core paleoclimatic indicators of local regional and more widespread climate conditions demonstrate that much of the earth experienced abrupt climate changes synchronous with greenland within thirty years or less post younger dryas changes have not duplicated the size extent and rapidity of these paleoclimatic changes

pattern of southern laurentide ice sheet margin position changes during heinrich stadials 2 and 1 abstract with concerns over rising global temperatures ice sheet stability and implications for sea level rise the responses of former ice sheets to past climate change provide useful insights into linkages between the atmosphere cryosphere and oceans one example is the behavior of the laurentide ice sheet lis during heinrich stadials times of cooler sea surface temperature sst in the north atlantic using existing cosmogenic surface exposure ages varve chronologies radiocarbon ages and optically stimulated luminescence ages from the southern lis margin this report tests the hypothesis that the southern side of the lis advanced during times of surface cooling of the north atlantic this test reconstructs the timing of local glacial maxima and the times immediately pro and preceding them from multiple areas and compares them to the timing of heinrich stadial 2 hs2 26 6 23 6 ka and to the timing of heinrich stadial 1 hs1 19 3 15 3 ka during hs2 southern sector lobes new england region huron erie lobe lake michigan lobe green bay lobe and the chippewa lobe advanced towards their maximum position except for the des moines lobe for which there is no applicable data during hs1 these same lobes experienced substantial retreat but the des moines lobe advanced thus the southern laurentide margin behaved differently during the two heinrich stadials this pattern may be attributed to differing east west temperature gradients across north america and associated changes in atmospheric circulation

deglacial hydroclimate of midcontinental north america abstract during the last deglaciation temperatures over midcontinental north america warmed dramatically through the bolling allerod underwent a cool period associated with the younger dryas and then reverted to warmer near modern temperatures during the early holocene however paleo proxy records of the hydroclimate of this period have presented divergent evidence we reconstruct summer relative humidity rh across the last deglacial period using a mechanistic model of cellulose and leaf water 18o and d combined with a pollen based temperature proxy to interpret stable isotopes of sub fossil wood midcontinental rh was similar to modern conditions during the last glacial maximum progressively increased during the bolling allerod peaked during the younger dryas and declined sharply during the early holocene this rh record suggests deglacial summers were cooler and characterized by greater advection of moisture laden air masses from the gulf of mexico and subsequent entrainment over the mid continent by a high pressure system over the laurentide ice sheet these patterns help explain the formation of dark colored cumulic horizons in many great plains paleosol sequences and the development of no analog vegetation types common to the midwest during the last deglacial period likewise reduced early holocene rh and precipitation correspond with a diminished glacial high pressure system during the latter stages of ice sheet collapse

identifying the pollen of an extinct spruce species in the late quaternary sediments of the tunica hills region south eastern united states late quaternary fluvial deposits in the tunica hills region of louisiana and mississippi are rich in spruce macrofossils of the extinct species picea critchfieldii the one recognized plant extinction of the late quaternary however the morphology of p critchfieldii pollen is unknown presenting a barrier to the interpretation of pollen spectra from the last glacial of north america to address this issue we undertook a morphometric study of picea pollen from tunica hills morphometric data together with qualitative observations of pollen morphology using apotome fluorescence microscopy indicate that picea pollen from tunica hills is morphologically distinct from the pollen of p glauca p mariana and p rubens measurements of grain length corpus width and corpus height indicate that picea pollen from tunica hills is larger than the pollen of p mariana and p rubens and is slightly larger than p glauca pollen we argue that the morphologically distinctive tunica hills picea pollen was probably produced by the extinct spruce species p critchfieldii these morphological differences could be used to identify p critchfieldii in existing and newly collected pollen records which would refine its paleoecologic and biogeographic history and clarify the nature and timing of its extinction in the late quaternary

rapid time transgressive and variable responses to early holocene midcontinental drying in north america the end pleistocene to early holocene drying of the north american midcontinent is an important case study for understanding regional drought its drivers and its impacts this paper synthesizes pollen 13c eolian and paleohydrological proxies to document the spatial and temporal patterns in the onset and rate of drying the timing of onset ranged from 14 to 6 ka with most sites drying between 10 and 8 ka calendar years ago the dominant spatial pattern is time transgressive with interior great plains sites beginning to dry before peripheral sites this time transgressive trend can be explained as a region wide drying that caused ecotones to shift eastward or could indicate a progressive shift in atmospheric drying from west to east one third of sites responded rapidly to drying a cluster of rapid responses occurred ca 8 ka the combination of onsets between 14 and 6 ka and rapid responses at 8 ka strongly suggests that 1 the combination of high summer insolation laurentide ice sheet retreat and lake agassiz drainage drove midcontinental drying and 2 drying accelerated with laurentide ice sheet collapse at 8 4 ka local factors strongly mediate the timing and rate of site responses to regional drying local responses to future drying in the great plains also should be highly variable which challenges impact assessments of future climate changes

jurassic paleogeography of the tian shan an evolution driven by far field tectonics and climate abstract the strongly intracontinental tian shan region in central asia represents a key area to understand the long term evolution of continents in general and asia in particular if its paleozoic and cenozoic geodynamics are well understood its mesozoic evolution remains poorly constrained in order to decipher the paleogeographic and large scale tectonic evolution of the tian shan area during the jurassic we compiled detailed field analyses of sedimentary rocks acquired within and around the chinese tian shan region together with previously published data we present three paleogeographical maps corresponding to the late early early middle jurassic late middle early late jurassic and late jurassic early cretaceous transition periods we provide a large scale picture of the jurassic paleogeographic and climatic evolution of the tian shan region and discuss the geological evolution of the range together with the possible driving mechanisms during the early to early middle jurassic the topographic evolution of the tian shan range was dominated by progressive planation of late paleozoic to early mesozoic relief locally interrupted by short lived tectonic uplift throughout the region contemporaneous sedimentation was characterized by alluvial to lacustrine strata deposited under humid conditions during this period recurrent limited deformation events associated with strike slip and compressive tectonics occurred that cannot be explained by far field effect of the qiangtang collision but could instead be associated to the coeval subduction related extension affecting the caspian turan domains during the late middle to early late jurassic the planation of the paleozoic early mesozoic tian shan range then continued a shift to more semi arid conditions during the late jurassic is also recorded in the sedimentary series all over the region at that time few evidences of deformation exists in the tian shan or within the caspian turan domains we propose that the late middle early late jurassic corresponded to a period of relative tectonic quiescence in the area finally the late jurassic early cretaceous transition was marked by a tectonic reactivation leading to the inversion of the yarkand fergana basin and to localized relief building in the tian shan this renewed transpressive deformation phase could be mainly related to the coeval accretion of the helmand block to the south west and possibly to the onset of the accretion of the lhasa block along the southwestern margin of eurasia finally this period was also characterized by the climax of aridification which played a major role on the emplacement of extensive alluvial fan systems in the basins surrounding the range

quaternary paleoecology of the lower mississippi valley copyright c 1996 elsevier science b v all rights reserved in 1938 clair a brown published his classic paleobotanical discoveries from the tunica hills of southeastern louisiana indicating ice age plant migrations of more than 1100km brown collected fossils of both boreal trees such as white spruce picea glaucar and southern coastal plain plants from deposits mapped as the port hickey prairier river terrace by harold n fisk subsequent revisions of terrace mapping radiocarbon dating and paleoecological analysis reconciled browns conceptual and stratigraphic mixingr of these two ecologically incompatible fossil plant groups an older terrace 2 of sangamonian to altonian ager contains the warm temperate assemblage a younger terrace 1 of farmdalian woodfordian and holocene ager includes full glacial and late glacial remains of both boreal and cool temperate plants and a warm temperate suite of plants dates from the holocene interglacial new plant fossil localities with radiocarbon chronologies are now available from within the lower mississippi valley of missouri and arkansas as well as from the adjacent ozark plateaus the interior low plateaus of kentucky and tennessee and the bordering blufflands of tennessee mississippi and louisiana these studies demonstrate that glacial and interglacial patterns of vegetation have been influenced by regional changes in climate glacial runoff and regime of the mississippi river

paleoecology of central kentucky since the last glacial maximum pollen grains and spores plant macrofossils and sponge spicules from a 7 2 m sediment core from jackson pond dating back to 20 000 yr b p are the basis for new interpretations of vegetational limnological and climatic changes in central kentucky during the full glacial interval 20 400 to 16 800 yr b p upland vegetation was closed spruce forest with jack pine as a subdominant aquatic macrophyte and sponge assemblages indicate that the site was a relatively deep open pond with low organic productivity during late glacial time 16 800 to 11 300 yr b p spruce populations continued to dominate while jack pine declined and sedge increased as the vegetation became a more open taiga like boreal woodland between 11 300 and 10 000 yr b p abundances of spruce and oak pollen oscillated reciprocally possibly reflecting the younger dryas oscillation as boreal taxa underwent a series of declines and increases at the southern limit of their ranges before becoming extirpated and replaced by deciduous forest in the early holocene 10 000 to 7300 yr b p a mesic deciduous woodland developed it was replaced by xeric oak hickory forest during the middle holocene between 7300 and 3900 yr b p grass increased after 3900 yr b p indicating that the presettlement vegetation mosaic of mixed deciduous forest and prairie the kentucky barrens became established in central kentucky after the hypsithermal interval sponge spicules increased in number during the holocene reflecting reduced water depths in the pond sediment infilling as well as climatic warming and the expansion of fringing shrub thickets increased nutrient and habitat availability for freshwater sponges

transient simulation of last deglaciation with a new mechanism for bolling allerod warming we conducted the first synchronously coupled atmosphere ocean general circulation model simulation from the last glacial maximum to the bolling allerod ba warming our model reproduces several major features of the deglacial climate evolution suggesting a good agreement in climate sensitivity between the model and observations in particular our model simulates the abrupt ba warming as a transient response of the atlantic meridional overturning circulation amoc to a sudden termination of freshwater discharge to the north atlantic before the ba in contrast to previous mechanisms that invoke amoc multiple equilibrium and southern hemisphere climate forcing we propose that the ba transition is caused by the superposition of climatic responses to the transient co 2 forcing the amoc recovery from heinrich event 1 and an amoc overshoot

deposition times in the northeastern united states during the holocene establishing valid priors for bayesian age models abstract age depth relationships in sedimentary archives such as lakes wetlands and bogs are non linear with irregular probability distributions associated with calibrated radiocarbon dates bayesian approaches are thus well suited to understanding relationships between age and depth for use in paleoecological studies bayesian models for the accumulation of sediment and organic matter within basins combine dated material from one or more records with prior information about the behavior of deposition times yr cm based on expert knowledge well informed priors are essential to good modeling of the age depth relationship but are particularly important in cases where data may be sparse e g few radiocarbon dates or unclear e g age reversals coincident dates age offsets outliers and dates within a radiocarbon plateau here we assessed holocene deposition times using 204 age depth models obtained from the neotoma paleoecology database www neotomadb org for both lacustrine and palustrine environments across the northeastern united states these age depth models were augmented using biostratigraphic events identifiable within pollen records from the northeastern united states during the holocene and late pleistocene deposition times are significantly related to depositional environment palustrine and lacustrine sediment age and sediment depth spatial variables had non significant relationships with deposition time when site effects were considered the best fit model was a generalized additive mixed model that relates deposition time to age stratified by depositional environment with site as a random factor the best fit model accounts for 63 3 of the total deviance in deposition times the strongly increasing accumulation rates of the last 500 1000 years indicate that gamma distributions describing lacustrine deposition times 1 08 18 28 and palustrine deposition times 1 23 22 32 for the entire holocene may be insufficient for bayesian approaches since there is strong variation in the gamma parameters both in the most recent sediments and throughout the holocene time averaged gamma distributions for lacustrine 1 35 19 64 and palustrine samples 1 40 20 72 show lower overall deposition times but variability remains the variation in gamma parameters through time may require the use of multiple gamma distributions during the holocene to generate accurate age depth models we present estimates of gamma parameters for deposition times at 1000 yr intervals the parameters generated in this study can be used directly within bacon to act as bayesian priors for sedimentary age models

pre holocene and holocene pollen records of vegetation history from the florida peninsula and their climatic implications abstract pre holocene sediments may be found in sinkhole lakers or filled sinkholes in the florida peninsula lake tulane in south central florida yielded a core of 18 5 m under 22 7 m of water the core extends from the present to ca 50 000 yr b p it is age controlled by twenty radiocarbon dates the pre holocene shows alternating peaks of pinus pine dominated and quercus ambrosia oak ragweed dominated vegetation the pine peaks correlate in detail with heinrich events h1 to h5 this demonstrates linkage between continental and oceanic climate events the pine peaks are thought to link with periods of relatively high precipitation and warm gulf water during ice advances oak ragweed peaks are thought to indicate aridity when the gulf and north atlantic surface waters were cooled by melwater from retreating ice sheets lake grizelle is afilled sinkhole with several climatic episodes recorded in pollen counts which are separated from the holocene by a hiatus marked by thick sand it is representative of a category of fossil lakes in florida which promise to contain long and perhaps interglacial pollen records sheelar lake records the end of the wisconsin and the assembling of a species rich forest of mesic trees such as fagus beech close to or south of their present southern geographic limits this forest is believed to be ancestral to the mesic deciduous forest which invaded eastern north america in the next 3000 yr

a new greenland ice core chronology for the last glacial termination 1 we present a new common stratigraphic timescale for the north greenland ice core project ngrip and grip ice cores the timescale covers the period 7 9 14 8 kyr before present and includes the bolling allerod younger dryas and early holocene periods we use a combination of new and previously published data the most prominent being new high resolution continuous flow analysis cfa impurity records from the ngrip ice core several investigators have identified and counted annual layers using a multiparameter approach and the maximum counting error is estimated to be up to 2 in the holocene part and about 3 for the older parts these counting error estimates reflect the number of annual layers that were hard to interpret but not a possible bias in the set of rules used for annual layer identification as the grip and ngrip ice cores are not optimal for annual layer counting in the middle and late holocene the timescale is tied to a prominent volcanic event inside the 8 2 kyr cold event recently dated in the dye 3 ice core to 8236 years before a d 2000 b2k with a maximum counting error of 47 years the new timescale dates the younger dryas preboreal transition to 11 703 b2k which is 100 150 years older than according to the present grip and ngrip timescales the age of the transition matches the gisp2 timescale within a few years but viewed over the entire 7 9 14 8 kyr section there are significant differences between the new timescale and the gisp2 timescale the transition from the glacial into the bolling interstadial is dated to 14 692 b2k the presented timescale is a part of a new greenland ice core chronology common to the dye 3 grip and ngrip ice cores named the greenland ice core chronology 2005 gicc05 the annual layer thicknesses are observed to be log normally distributed with good approximation and compared to the early holocene the mean accumulation rates in the younger dryas and bolling periods are found to be 47 2 and 88 2 respectively

ecosystem development following deglaciation a new sedimentary record from devils lake wisconsin usa abstract processes and rates of ecosystem development can be reconstructed using lacustrine sedimentary sequences but this approach often requires records that contain the start of primary succession most lakes in the upper midwestern u s were formed by glaciers at the end of the last ice age approximately 11 700 cal yr bp devils lake wisconsin is a rare example of a lake from this region whose sediments extend into the pleistocene and may include the last glacial maximum sediment magnetic geochemical pollen and charcoal records were generated from a 10 m core whose basal sediments may be 28 000 years old together with a previously published pollen record these proxies combine to reveal a history of long term climatic vegetative and geologic change during the late pleistocene to holocene we identify six sedimentary units that indicate a series of consecutive events rather than a predictable trajectory of ecosystem development at the site productivity in the lake was low during the late pleistocene and increased during the holocene as reflected by the sediment lithology which shows a sudden shift from glacial vivianite rich and organic poor clastic dominated sediments to holocene diatomaceous sapropels several important processes initiated around 17 000 cal yr bp including the onset of organic matter accumulation and fire in the terrestrial ecosystem however the post glacial landscape was not devoid of vegetation because pollen assemblages indicate that terrestrial vegetation likely a spruce tundra survived near the site a switch to a hardwood forest period during the holocene also led to a change in the fire regime with increased frequency of burning aquatic ecosystem productivity lagged terrestrial ecosystem productivity throughout the record nutrient cycling as recorded by sedimentary 15n was variable but not directional and appeared to be correlated with climate conditions early in the record and terrestrial ecosystem processes later in the record throughout the holocene magnetic mineral concentration decreased as productivity increased and the source of magnetic material shifted from almost exclusively lithogenic to approximately 50 derived from soil or biogenic sources magnetic grain size ambrosia pollen percentages and charcoal concentration increased and 15n decreased in the most recent part of the record due to anthropogenic influence in the region including agricultural activities

mapped plant macrofossil and pollen records of late quaternary vegetation change in eastern north america abstract macrofossil presence absence maps and isopoll maps in 3000 year intervals show how ranges and abundance maxima for 12 eastern north american tree and shrub taxa picea p glauca p mariana larix laricina abies balsamea tsuga canadensis pinus strobus p resinosa p banksiana betula papyrifera b alleghaniensis b series humiles have changed from the last glacial maximum to the present the macrofossil maps corroborate patterns shown by the isopoll maps and provide spatial detail and taxonomic resolution the macrofossils confirm the inference from pollen data that unglaciated southeastern north america was cooler during the last glacial maximum 18 and 15 ka than simulated by the cohmap experiments with the ncar ccm0 general circulation model the geographic distribution of macrofossil occurrences during the late glacial 12 and 9 ka indicate that migration lag of boreal species did not occur at regional to subcontinental scales and that pollen assemblages lacking modern analogs resulted from climate gradients lacking modern analogs early holocene 9 and 6 ka macrofossil maps show rapid northward expansion of tree species ranges into deglaciated regions the data also show modest contractions of northern range limits of temperate species and expansions of southern range limits of boreal species in response to cooling trends during the late holocene 3 and 0 ka comparison of modern macrofossil maps of nine of these taxa with corresponding range maps confirm that the macrofossils record the geographic ranges accurately comparison of the modern macrofossil maps with maps of tree growing stock volume shows that for some taxa abies tsuga macrofossil occurrences were most frequent in regions of maximum tree abundance comparison of modern isopoll maps with the modern range and growing stock volume maps indicate that in contrast to the macrofossil data the pollen data provide poorer resolution of range limits for most taxa but better indications of abundance maxima and minima within the ranges

vegetation history in central kentucky and tennessee usa during the last glacial and deglacial periods abstract knowledge about vegetation dynamics during the last glacial and deglacial periods in southeastern north america is under constrained owing to low site density and problematic chronologies new pollen records from two classic sites anderson pond tn and jackson pond ky supported by ams 14 c age models span 25 2 13 7 ka and 31 0 15 4 ka respectively a transition from pinus dominance to picea dominance is recorded at jackson pond ca 26 2 ka coincident with heinrich event h2 anderson and jackson ponds record a transition from conifer to deciduous tree dominance 15 9 and 15 4 ka respectively marking the development of no analog vegetation characterized by moderate to high abundances of picea quercus carya ulmus fraxinus ostrya carpinus cyperaceae and poaceae and preceding by 2000 yr the advent of similar no analog vegetation in glaciated terrain to the north no analog vegetation developed as a time transgressive south to north pattern mediated by climatic warming sporormiella abundances are consistently low throughout the jackson and anderson pond records suggesting that megafaunal abundances and effects on vegetation varied regionally or possibly that the sporormiella signal was not well expressed at these sites additional records with well constrained chronologies are necessary to assess patterns and mechanisms of vegetation dynamics during the last glacial and deglacial periods

pollen representation source area and basin size toward a unified theory of pollen analysis abstract the concepts of pollen source area and of production and dispersal biases in pollen representation are quantified by means of a simple theoretical model source areas and relative pollen representation are shown to depend on basin size according to functions that describe the amount of pollen remaining airborne at increasing distances from single pollen sources the form of these functions is determined by physical processes standard formulas for elevated sources do not apply but the integrated form of sutton s equation for particle dispersal from a ground level source gives useful approximations applicable to pollen transport over a forest canopy simulations using this equation yielded source areas that increased realistically with basin size showed substantial differences between source areas for pollen grains with different deposition velocities and predicted that lighter pollen grains should become better represented with increasing basin size all of these predictions are qualitatively consistent with present knowledge of the characteristics of pollen assemblages in different depositional environments the model further allows parameters that can be estimated by statistical calibration methods to be predicted from underlying physical quantities this extension suggests procedures for testing the theory with quantitative data on surface pollen and forest composition preliminary results showed reasonable agreement between estimated and predicted values of dispersal indices for the most abundant taxa in pollen spectra from the northern midwestern united states

quantitative interpretation of fossil pollen spectra dissimilarity coefficients and the method of modern analogs abstract dissimilarity coefficients measure the difference between multivariate samples and provide a quantitative aid to the identification of modern analogs for fossil pollen samples how eight coefficients responded to differences among modern pollen samples from eastern north america was tested these coefficients represent three different classes 1 unweighted coefficients that are most strongly influenced by large valued pollen types 2 equal weight coefficients that weight all pollen types equally but can be too sensitive to variations among rare types and 3 signal0to noise coefficients that are intermediate in their weighting of pollen types the studies with modern pollen allowed definition of critical values for each coefficient which when not exceeded indicate that two pollen samples originate from the same vegetation region dissimilarity coefficients were used to compare modern and fossil pollen samples and modern samples so similar to fossil samples were found that most of three late quaternary pollen diagrams could be reconstructed by substituting modern samples for fossil samples when the coefficients indicated that the fossil spectra had no modern analogs then the reconstructed diagrams did not match all aspects of the originals no modern analogs existed for samples from before 9300 yr b p at kirchner marsh minnesota and from before 11 000 yr b p at wintergreen lake michigan but modern analogs existed for almost all holocene samples from these two sites and brandreth bog new york

late quaternary vegetation climate and fire history of the southeast atlantic coastal plain based on a 30 000 yr multi proxy record from white pond south carolina usa the patterns and drivers of late quaternary vegetation dynamics in the southeastern united states are poorly understood due to low site density problematic chronologies and a paucity of independent paleoclimate proxy records we present a well dated 15 accelerator mass spectrometry 14 c dates 30 000 yr record from white pond south carolina that consists of high resolution analyses of fossil pollen macroscopic charcoal and sporormiella spores and an independent paleotemperature reconstruction based on branched glycerol dialkyl tetraethers between 30 000 and 20 000 cal yr bp open pinus picea forest grew under cold and dry conditions elevated quercus before 26 000 cal yr bp however suggest warmer conditions in the southeast before the last glacial maximum possibly corresponding to regionally warmer conditions associated with heinrich event h2 warming between 19 700 and 10 400 cal yr bp was accompanied by a transition from conifer dominated to mesic hardwood forest sporormiella spores were not detected and charcoal was low during the late glacial period suggesting megaherbivore grazers and fire were not locally important agents of vegetation change pinus returned to dominance during the holocene with step like increases in pinus at 10 400 and 6400 cal yr bp while charcoal abundance increased tenfold likely due to increased biomass burning associated with warmer conditions low intensity surface fires increased after 1200 cal yr bp possibly related to the establishment of the mississippian culture in the southeast

observation of a large density dropout across the magnetic field at 600 km altitude during the 6 7 april 2000 magnetic storm 1 the first republic of china satellite rocsat 1 at 600 km altitude observed two groups of equatorial spread f esf plasma depletion structures located in either side of the midnight meridian during the 6 7 april 2000 magnetic storm recovery phase in the group of postmidnight esf irregularities a large density dropout was observed in two consecutive rocsat orbits this large density dropout structure is similar to the density depletion event observed by defense meteorological satellite program dmsp f9 spacecraft during the 13 14 march 1989 magnetic storm greenspan et al 1991 except that the rocsat orbit traverses the magnetic field in the east west direction the postmidnight group of depletions together with the large density dropout was observed to drift westward with the background plasma at 170 m s 1 we examine the properties of the large density dropout through the plasma data taken from the drift meter dm and retarding potential analyzer rpa sensors in three consecutive orbits the apparent widening of the density depletion is explained by the different locations of the satellite within the depletion region the extreme low density level inside the upwelling depletion indicates that of the nighttime f peak passed over the rocsat orbit at 600 km the existence of no ions inside the dropout is consistent with such an elevation of the f peak

vegetation history since the last glacial maximum in the ozark highlands usa a new record from cupola pond missouri abstract the timing and drivers of vegetation dynamics and formation of no analog plant communities during the last deglaciation in the unglaciated southeastern us are poorly understood we present a multi proxy record spanning the past 19 800 years from cupola pond in the ozarks mountains consisting of replicate high resolution pollen records 25 ams radiocarbon dates and macrofossil charcoal and coprophilous spore analyses full glacial pinus and picea forests gave way to no analog vegetation after 17 400 yr bp followed by development of quercus dominated holocene forests with late holocene rises in pinus and nyssa vegetation transitions replicated in different cores are closely linked to hemispheric climate events rising quercus abundances coincide with increasing northern hemisphere temperatures and co 2 at 17 500 yr bp declining pinus and picea at 14 500 yr bp are near the bolling allerod onset and rapid decline of fraxinus and rise of ostrya carpinus occur 12 700 yr bp during the younger dryas the cupola no analog vegetation record is unusual for its early initiation 17 000 yr bp and for its three vegetation zones representing distinct rises of fraxinus and ostrya carpinus sporormiella was absent and sedimentary charcoal abundances were low throughout suggesting that fire and megaherbivores were not locally important agents of disturbance and turnover the cupola record thus highlights the complexity of the late glacial no analog communities and suggests direct climatic regulation of their formation and disassembly

holocene environments reconstructed from plant macrofossils in stream deposits from southeastern nebraska usa plant macrofossils from sediments exposed in cutbanks of small streams on the great plains yield details of the history of the vegetation and climate and potentially of the hydrologic behaviour of the streams during the past 9000 radiocarbon years five zones are recognized by visual inspection macrofossils in zone 1 c 9000 to c 8500 bp indicate that upland deciduous forest was sparsely present but that prairie weedy and wetland habitats were also well established zone 2 c 8500 to c 5800 bp is sparsely represented but upland trees disappeared riparian trees and wetland plants were sparse and weeds and prairie taxa fluctuated from low to high values this interval was probably the warmest driest part of the holocene zone 3 c 5800 to c 3100 bp had a rich riparian forest along with well developed prairies disturbed habitats and wetlands this period was more moist but had intermittent droughts zone 4 c 3100 to c 2700 bp has peaks in many prairie and weedy elements whereas riparia

a global perspective on last glacial maximum to holocene climate change while the abrupt climate events of the last deglaciation are well defined in ice core records from the polar regions of both hemispheres their manifestation elsewhere is less well constrained here we compile 104 high resolution paleoclimate records to characterize the timing and spatial pattern of climate change during the last deglaciation this compilation indicates relatively concurrent timing of the last glacial maximum lgm peak glacial conditions and the altithermal peak interglacial conditions in the northern 22 1 4 3 ka and 8 0 3 2 ka and southern 22 3 3 6 ka and 7 4 3 7 ka hemispheres suggesting the hemispheres were synchronized by greenhouse gases local insolation and or northern hemisphere induced ocean circulation changes the magnitude of the glacialeinterglacial temperature change increases with latitude reflecting the polar amplification of climate change with a likely minimum global mean cooling of ww4 9 c during the lgm relative to the altithermal empirical orthogonal function eof analysis of 71 records spanning 19e11 ka indicates that two modes explain 72 of deglacial climate variability eof1 61 of variance shows a globally near uniform pattern with its principal component pc1 strongly correlated with changes in atmospheric co2 e of2 11 of variance exhibits a bipolar seesaw pattern between the hemispheres with its principal component pc2 resembling changes in atlantic meridional overturning circulation strength eof analysis of 90 records from 15 to 11 ka indicates that northern and southern modes of climate variability characterize the younger dryas bolling allerod interval these modes dominate at the higher latitudes of each hemisphere and exhibit a complex interaction in the tropics the magnitude of the younger dryas climate anomaly cooler drier increases with latitude in the northern hemisphere with an opposite pattern warmer wetter in the southern hemisphere reflecting a general bipolar seesaw climate response global mean temperature decreased by w0 6 c during the younger dryas therefore our analysis supports the paradigm that while the younger dryas was a period of global climate change it was not a major global cooling event but rather a manifestation of the bipolar seesaw driven by a reduction in atlantic meridional overturning circulation strength

a chronological framework for the holocene vegetational history of central minnesota the steel lake pollen record abstract paleorecords from minnesota and adjacent areas have often been used to evaluate large scale climatic processes in the mid continent of north america however most of these records are compromised by chronological flaws making problematic any comparisons with climatic interpretations based on other records e g gisp2 in greenland we report here a high resolution pollen record with a secure chronology constrained by 26 14 c dates on terrestrial macrofossils from steel lake central minnesota about 11 2 ka cal bp 11 000 calibrated yr before present the late glacial picea forest near steel lake was succeeded abruptly by pinus banksiana and or resinosa the pinus forest began to open 9 4 ka cal bp with the expansion of prairie taxa and a pine parkland or savanna prevailed until about 8 ka cal bp when quercus replaced pinus to become the dominant tree in the prairie areas for 4500 yr the close chronological control permits the correlation of key vegetational changes with those at other reliably dated sites in the eastern dakotas and in minnesota suggesting that the abrupt decline of the spruce forest was time transgressive from southwest to northeast during 2000 yr and that the development of prairie was time transgressive in the same direction over 2600 yr correlation of key pollen horizons at steel lake with those in the high resolution pollen profiles of elk lake ca 50 km northwest of steel lake suggests that the well known elk lake varve chronology for the early holocene is about 1000 yr too young

a model of pollen source area for an entire lake surface a model of pollen deposition on the surface of an entire basin is developed to estimate pollen source area and results are compared with those for a point at the center of a basin i c prentice 1985 quaternary research 23 76 86 1988 vegetation history pp 17 42 kluwer academic this model is more appropriate for approximating the source area of pollen in lake sediment since mixing in lake water and focusing of sediment redistribute pollen originally deposited over the entire surface in general the pollen source radius for the entire basin surface is 10 30 smaller than the source radius for a point at the center the difference in the source radius is more profound for heavier pollen types such as spruce and sugar maple than for lighter types such as oak and ragweed the average pollen input to the entire surface is more strongly influenced by nearby pollen sources than pollen deposition at the center the pollen record from a lake may therefore provide different spatial resolution than the record from a bog of similar radius

pollen and macrofossils from wisconsinan interstadial sediments in northeastern georgia pollen and plant macrofossil analyses and dating both tams and conventional 14 c of wetland sediments from the nodoroc site in the piedmont region of northeastern georgia identified two depositional episodes of interstadial ca 26 000 22 000 yr b p and mid to late holocene 3600 yr b p age interstadial pollen assemblages were dominated by pinus and quercus with carya picea and abies also present in significant amounts the interstadial assemblages had lower pinus and higher quercus pollen percentages than apparently contemporaneous assemblages from sites in the southern appalachians and coastal plain interstadial macrofossils included two needle morphotypes of pinus subgenus pinus possibly indicating local occurrence of two pinus species one of the needle morphotypes is consistent with pinus banksiana but other species cannot be ruled out comparison of conventional and tams 14c dates illustrates problems in obtaining accurate age models from wetlands and ponds characterized by shallow or fluctuating water levels low sedimentation rates and vegetation growing on the depositional surface