accelerator mass spectrometer ages for the younger dryas event in atlantic canada abstract in atlantic canada bulk sediment dates for the onset of a late glacial cooling widely regarded as the younger dryas event are highly variable ranging from 14 300 270 to 10 800 100 yr b p we present the first accelerator mass spectrometer ams 14c dates from atlantic canada at or close to the boundaries of this event from six sites in new brunswick and nova scotia the mean of five dates places the onset of this cooling at ca 10 770 yr b p the inclusion of a sixth perhaps anomalously old date changes the mean to 10 880 yr b p the termination averages three dates ca 10 000 yr b p these dates place the timing of the younger dryas event in atlantic canada closer in line with the traditional chronozone boundaries of 11 000 and 10 000 yr b p in northwest europe

moisture and temperature changes associated with the mid holocene tsuga decline in the northeastern united states abstract a decline of hemlock tsuga populations at ca 5 5 ka thousands of calibrated radiocarbon years before 1950 ad stands out as the most abrupt vegetation change of the holocene in north america but remains poorly understood after decades of study recent analyses of fossil pollen have revealed a concurrent abrupt oak quercus decline and increases in the abundance of beech fagus and pine pinus on cape cod in eastern massachusetts but the replacement of drought tolerant oaks by moisture sensitive beeches appears inconsistent with low lake levels in the region at the same time the oak and beech changes are also limited to coastal areas and the coastal inland differences require an explanation here we develop a new lake level reconstruction from deep pond cape cod by using a transect of sediment cores and ground penetrating radar gpr profiles to constrain the past elevations of the sandy littoral zone of the pond the reconstruction shows that a series of multi century episodes of low water coincide with the abrupt hemlock and oak declines and interrupt subsequent phases of hemlock recovery the lake level variations equal precipitation deficits of 100 mm superimposed on a holocene long moisture increase of 400 mm however because moisture deficits do not easily explain the oak and beech changes we also evaluate how the climate preferences of the regional vegetation changed over time by matching the fossil pollen assemblages from deep pond with their modern equivalents reconstructions of the precipitation requirements of the vegetation correlate well even in detail with the lake level record r 0 88 at deep pond and indicate close tracking of effective moisture precipitation minus evapotranspiration by the vegetation despite the abrupt species declines which could have decoupled climate and vegetation trends reconstructions of the temperature preferences of the vegetation indicate that coastal sites may have cooled by 0 5 2 5 c after ca 5 5 ka while inland sites warmed by 0 5 1 c the change in coastal temperature preferences agrees with sea surface cooling in the western atlantic ocean of 1 c consequently the persistence of low hemlock abundance after 5 5 ka in the northeast u s may have resulted from oceanic changes that produced multi century droughts and thus delayed the post decline recovery of hemlock populations

thermodynamic analysis of ocean circulation abstract calculating a streamfunction as function of depth and density is proposed as a new way of analyzing the thermodynamic character of the overturning circulation in the global ocean the sign of an overturning cell in this streamfunction directly shows whether it is driven mechanically by large scale wind stress or thermally by heat conduction and small scale mixing it is also shown that the integral of this streamfunction gives the thermodynamic work performed by the fluid the analysis is also valid for the boussinesq equations although formally there is no thermodynamic work in an incompressible fluid the proposed method is applied both to an idealized coarse resolution three dimensional numerical ocean model and to the realistic high resolution ocean circulation and climate advanced model occam it is shown that the overturning circulation in occam between the 200 and 1000 m depth is dominated by a thermally indirect cell of 24 sverdrups 1 sv 106 m3 s 1 forced by ekman pumping in th

the representation of ocean circulation and variability in thermodynamic coordinates the ocean s circulation is analyzed in absolute salinity sa and conservative temperature q coordinates it is separated into 1 an advective component related to geographical displacements in the direction normal to sa and q isosurfaces and 2 into a local component related to local changes in sa q values without a geographical displacement in this decomposition the sum of the advective and local components of the circulation is equivalent to the material derivative of sa and q the sum is directly related to sources and sinks of salt and heat the advective component is represented by the advective thermohaline streamfunction c advaq after removing a trend the local component can be represented by the local thermohaline streamfunction c locaq here c locaq can be diagnosed using a monthly averaged time series of sa and q from an observational dataset in addition c advaq and c locaq are determined from a coupled climate model the diathermohaline streamfunction c diaaq is the sum of c advaq and c locaq and represents the nondivergent diathermohaline circulation in sa q coordinates the diathermohaline trend resulting from the trend in the local changes of sa and q quantifies the redistribution of the ocean s volume in sa q coordinates over time it is argued that the diathermohaline streamfunction provides a powerful tool for the analysis of and comparison among ocean models and observation based gridded climatologies

younger dryas interval and outflow from the laurentide ice sheet a boxmodel of the great lakes is used to estimate meltwater flow into the north atlantic between 8000 and 14 000 calendar years b p controls on the model include the oxygen isotopic composition of meltwaters and lake waters as measured in the shells of ostracodes outflow rates are highest when oxygen isotopic values of the lake waters are most negative denoting a maximum glacial meltwater component flow rates reach maximum values before the onset of the younger dryas and after it ends these maxima appear to be correlative with the major meltwater pulses mwp 1a and 1b although the resumption of north atlantic deep water formation may be tied to the reduction in ice sheet melting neither the onset nor the end of the younger dryas as recorded in the greenland ice sheet project gisp2 records appear tied to maxima in meltwater outflow from the laurentide ice sheet

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

response of interior north america to abrupt climate oscillations in the north atlantic region during the last deglaciation several broad scale climatic oscillations during the last deglaciation are well documented in regions around the north atlantic ocean this paper reviews empirical evidence for these deglacial climatic oscillations from non coastal north america and discusses implications for testing climatic simulations and for understanding the cause and transmittal mechanisms paleoclimatic interpretation of oxygen isotope records from several small sites in the eastern great lakes region indicates a classic deglacial climatic sequence that is comparable with records from europe and greenland the climatic events as interpreted from crawford lake oxygen isotopes include the bolling allerod boa warming at 12 700 14c bp a warm boa at 12 500 10 920 14c bp an intra allerod cold period shortly before 11 000 14c bp a cold younger dryas yd climate reversal at 10 920 10 000 14c bp the holocene warming at 10 000 14c bp a brief preboreal oscillation pb at 9650 14c bp and an early holocene cold event at 7500 14c bp 8200 cal bp some of these events were also evident from changes in upland and aquatic vegetation and sediment lithology the pronounced yd climatic reversal has been documented from pollen records along the ecotones at this time and from glacier readvances in the great lakes region along the rocky mountains from alberta to colorado the yd event is indicated by alpine glacier advance and or shift in timberline vegetation in minnesota and upper new york early holocene climatic instability is also suggested by oxygen isotope records and or varve thickness the regional variations in evidence for the yd and other events in north america suggest that climatic oscillations may have different expressions in paleo records depending on geographic location and characteristics of a particular site the extent and magnitude of these climatic oscillations across north america suggest that these oscillations are an expression of climatic change that was probably widespread rather than locally induced by a nearby glacier the location of these sites implies that climatic signals were likely carried over the northern hemisphere through the atmosphere as indicated by general circulation models we hypothesize that the lack of evidence for a cold yd in interior north america west of the great lakes region and east of the rocky mountains was caused by the trapping of cold arctic air mass north of the laurentide ice sheet and by uninterrupted northward expansion of warm caribbean air this strongly contrasted climate also produced non analogous biological assemblages

deglaciation basin formation and post glacial climate change from a regional network of sediment core sites in ohio and eastern indiana abstract many paleoclimate and landscape change studies in the american midwest have focused on the late glacial and early holocene time periods 16 11 ka but little work has addressed landscape change in this area between the last glacial maximum and the late glacial 22 16 ka sediment cores were collected from 29 new lake and bog sites in ohio and indiana to address this gap the basal radiocarbon dates from these cores show that initial ice retreat from the maximal last glacial ice extent occurred by 22 ka and numerous sites that are 100 km inside this limit were exposed by 18 9 ka post glacial environmental changes were identified as stratigraphic or biologic changes in select cores the strongest signal occurs between 18 5 and 14 6 ka these midwestern events correspond with evidence to the northeast suggesting that initial deglaciation of the ice sheet and ensuing environmental changes were episodic and rapid significantly these changes predate the onset of the bolling postglacial warming 14 8 ka as recorded by the greenland ice cores thus deglaciation and landscape change around the southern margins of the laurentide ice sheet happened 7 ka before postglacial changes were felt in central greenland

the amphi atlantic distribution of the younger dryas paleoclimatic oscillation abstract interest in significant climatic fluctuations affecting large geographic areas but having a time range that is too brief to adduce the milankovitch radiation cycles has focused on the allerod younger dryas event which is a well established phenomenon of western europe manifested by glacial and vegetational reconstructions it is also shown by the foraminiferal evidence for shifts in the polar front in the north atlantic recent pollen and lithostratigraphic investigations in the maritime provinces of eastern canada and in the till plains of west central ohio on opposite sides of the late glacial spruce forest indicate that the event also left a record in eastern north america experiments with an atmospheric general circulation model with the temperature of north atlantic surface waters depressed to full glacial levels lowered summer temperatures in upwind periglacial eastern north america by 2 c just as far inland as they did downwind in western europe where the paleoecological record of cooling is clear a leading hypothesis for the cause for the younger dryas cooling appeals to the influx of tabular icebergs released from arctic ice shelves around the wasting scandinavian and siberian ice sheets during the allerod warming the resulting cooling of the north atlantic waters during the younger dryas could be facilitated or precipitated by the abrupt influx of glacial meltwater derived from almost the entire southern margin of the laurentide ice sheet and emerging from the gulf of st lawrence as brought about by a major shift in drainage direction from the mississippi river to the st lawrence river about 10 8 ka bp a supplemental influx of fresh water may have come from the contemporaneous drainage of the baltic ice lake the diversion of laurentide meltwater to the atlantic was caused by retreat of an ice lobe in northwestern ontario north of minnesota allowing the huge glacial lake agassiz in saskatchewan and manitoba to drain into the great lakes and more than double the inflow of meltwaters in the north atlantic thereby diverting the gulf stream and permitting the influx of icebergs from the north to cool the ocean surface minor readvance of the ice lobe about 10 ka bp temporarily diverted the lake agassiz drainage to the south with the final retreat of the ice the drainage again went to the east but by this time the ice sheets were much diminished and the regional climate had warmed enough so that the effects of the meltwater flux on the gulf stream were muted and the ice sheets bordering the north atlantic had retreated onto the land and no longer delivered enough icebergs to cool the surface waters the younger dryas interval thus was terminated and the warming manifested in the allerod was renewed

geomorphic expression of abrupt climate change in southwestern north america at the glacial termination eolian and subaqueous landforms composed of gypsum sand provide geomorphic evidence for a wet episode at the termination of glacial climate in southwestern north america drying of pluvial lake estancia central new mexico occurred after ca 12 000 14c yr b p thereafter eolian landforms on the old lake floor constructed of gypsum sand were overridden by rising lake water modified by subaqueous processes and organized into beach ridges along the lake s eastern shore preservation of preexisting eolian landforms in the shallow lake suggests abupt changes in lake level and climate available radiocarbon ages suggest that the final highstand recorded by beach ridges may have developed during the younger dryas yd stade the beach ridges provide information about lake surface area which was 45 of the lake area reached during the maximum highstands of the late pleistocene a similar proportional response has been reported for yd climate changes outside the north atlantic region

temperature variations in the southern great lakes during the last deglaciation comparison between pollen and gdgt proxies abstract our understanding of deglacial climate history in the southern great lakes region of the united states is primarily based upon fossil pollen data with few independent and multi proxy climate reconstructions here we introduce a new well dated fossil pollen record from stotzel leis oh and a new deglacial temperature record based on branched glycerol dialkyl glycerol tetraethers brgdgts at silver lake oh we compare these new data to previously published records and to a regional stack of pollen based temperature reconstructions from stotzel leis silver lake and three other well dated sites the new and previously published pollen records at stotzel leis are similar but our new age model brings vegetation events into closer alignment with known climatic events such as the younger dryas yd brgdgt inferred temperatures correlate strongly with pollen based regional temperature reconstructions with the strongest correlation obtained for a global soil based brgdgt calibration r2 0 88 lending confidence to the deglacial reconstructions and the use of brgdgt and regional pollen stacks as paleotemperature proxies in eastern north america however individual pollen records show large differences in timing rates and amplitudes of inferred temperature change indicating caution with paleoclimatic inferences based on single site pollen records from 16 0 to 10 0ka both proxies indicate that regional temperatures rose by 10 c roughly double the 5 c estimates for the northern hemisphere reported in prior syntheses change point analysis of the pollen stack shows accelerated warming at 14 0 1 2ka cooling at 12 6 0 4ka and warming from 11 6 0 5ka into the holocene the timing of bolling allerod b a warming and yd onset in our records lag by 300 500 years those reported in syntheses of temperature records from the northern mid latitudes this discrepancy is too large to be attributed to uncertainties in radiocarbon dating and correlation between pollen and brgdgt temperature reconstructions rules out vegetation lags as a cause however the yd termination appears synchronous among the brgdgt record regional pollen stack and northern hemisphere stack the cause of the larger and lagged temperature changes in the southern great lakes relative to northern hemisphere averages remains unclear but may be due to the effects of continentality and ice sheet extent on regional climate evolution

late glacial to early holocene climate changes from a central appalachian pollen and macrofossil record abstract a late glacial to early holocene record of pollen plant macrofossils and charcoal has been obtained from two cores from browns pond in the central appalachians of virginia an ams radiocarbon chronology defines the timing of moist and cold excursions superimposed on the overall warming trend from 14 200 to 7500 14 c yr b p this site had cold moist conditions from ca 14 200 to 12 700 14 c yr b p with warming at 12 730 11 280 and 10 050 14 c yr b p a decrease in deciduous broad leaved tree taxa and pinus strobus haploxylon pollen simultaneous with a reexpansion of abies denotes a brief cold reversal from 12 260 to 12 200 14 c yr b p a second cold reversal inferred from increases in montane conifers is centered at 7500 14 c yr b p the cold reversals at browns pond may be synchronous with climate change in greenland and northwestern europe warming at 11 280 14 c yr b p shows the complexity of regional climate responses during the younger dryas chronozone

vertical heat transport by ocean circulation and the role of mechanical and haline forcing abstractvertical transport of heat by ocean circulation is investigated using a coupled climate model and novel thermodynamic methods using a streamfunction in temperature depth coordinates cells are identified by whether they are thermally direct flux heat upward or indirect flux heat downward these cells are then projected into geographical and other thermodynamic coordinates three cells are identified in the model a thermally direct cell coincident with antarctic bottom water a thermally indirect deep cell coincident with the upper limb of the meridional overturning circulation and a thermally direct shallow cell coincident with the subtropical gyres at the surface the mechanisms maintaining the thermally indirect deep cell are investigated sinking water within the deep cell is more saline than that which upwells because of the coupling between the upper limb and the subtropical gyres in a broader thermohaline circulation despite the higher salinity of its sinking water the deep cell tr

environment and paleoecology of a 12 ka mid north american younger dryas forest chronicled in tree rings abstract until now availability of wood from the younger dryas abrupt cooling event yde in n america ca 12 9 to 11 6 ka has been insufficient to develop high resolution chronologies for refining our understanding of yde conditions here we present a multi proxy tree ring chronology ring widths events evidenced by microanatomy and macro features stable isotopes from a buried black spruce forest in the great lakes area liverpool east site spanning 116 yr at ca 12 000 cal yr bp during this largely cold and wet period the proxies convey a coherent and precise forest history including frost events tilting drowning and burial in estuarine sands as the laurentide ice sheet deteriorated in the middle of the period a short mild interval appears to have launched the final and largest episode of tree recruitment ultimately the tops of the trees were sheared off after death perhaps by wind driven ice floes culminating an interval of rising water and sediment deposition around the base of the trees although relative influences of the continental ice sheet and local effects from ancestral lake michigan are indeterminate the tree ring proxies provide important insight into environment and ecology of a n american yde boreal forest stand

expanded response surfaces a new method to reconstruct paleoclimates from fossil pollen assemblages that lack modern analogues pollen based paleoclimatic interpretations of late glacial to early holocene climates 17 9 ka in midwestern north america are hampered by samples that lack modern analogues unresolved questions include the magnitude and direction of temperature seasonality i e were these climates more or less seasonal than present and the temporal changes in precipitation central to the no analogue problem is the truncation of modern pollen climate relationships for abundant late glacial taxa such as fraxinus here we present a new method called the expanded response surface ers method developed to reconstruct climates from no analogue pollen assemblages and applied to a high resolution late glacial pollen record from crystal lake illinois to test hypotheses about late glacial climates the key assumptions central to the ers method are 1 plant species and pollen abundances follow symmetrical unimodal distributions along climatic gradients 2 taxa with truncated distributions in modern climate space occupy a subset of their fundamental niche and 3 expansion of truncated distributions by mirroring around the distributional mode recovers the portion of the fundamental niche not realized in the modern climate space with the ers method we expanded modern pollen climate relationships by mirroring pollen abundances for each taxon around a mode defined with respect to four climate axes mean winter temperature mean summer temperature mean winter precipitation and mean summer precipitation the ers method reconstructed past temperatures and precipitation during the height of no analogue conditions 14 160 12 370 cal yr bp for 37 of the crystal lake samples where techniques that employed only modern observational data found matches for only 13 of the fossil samples the total climate space of the expanded taxa set allowed analogue matches under more seasonal than present climates with higher than present precipitation the ers climate reconstructions for the height of no analogue conditions indicated cooler than present summer and winter temperatures similar to present seasonal range in temperatures higher than present winter precipitation and similar to present summer precipitation these results thus suggest that high moisture availability helped drive the formation of the midwestern no analogue communities with high fraxinus nigra abundances but do not show higher than present temperature seasonality notwithstanding the higher than present insolation seasonality at this time during the no analogue late glacial interval picea mariana f nigra and larix stands probably grew on low lying poorly drained soils in the crystal lake region whereas abies picea glauca quercus and ostrya carpinus grew on upland positions with better soil drainage

absolute dating of the late glacial period in switzerland using annually laminated sediments abstract results of a high resolution late glacial ams 14 c chronology from rotsee central switzerland have given evidence that the atmospheric radiocarbon concentration was not constant between 13 000 and 9500 yr b p this resulted in three marked phases of constant radiocarbon age at 12 700 at 10 000 and at 9500 yr b p new results of a late glacial varve chronology from soppensee central switzerland suggest that the younger two phases of constant 14 c age each had a duration of ca 400 calendar years the length of the late glacial chronozones has been calculated on the basis of these replicate varve counts and a comparison with their estimated duration in radiocarbon years shows that the estimated duration of the chronozones bolling ca 800 calendar yr vs 1000 14 c yr and younger dryas ca 900 calendar yr vs 1000 14 c yr agree with the expected time span whereas the estimated duration of the allerod chronozone ca 400 calendar yr vs 1000 14 c yr is substantially shorter than expected furthermore a tentative comparison of varve ages and 14 c ages suggests that the varve chronology is more than 1000 yr offset toward older ages from the radiocarbon chronology during the late glacial period

holocene climatic change in the northern midwest pollen derived estimates mapping of holocene pollen data in the midwestern united states has revealed several broadscale vegetational changes that can be interpreted in climatic terms these changes include 1 the early holocene northward movement of the spruce dominated forest and its later southward movement after 3000 yr b p and 2 the eastward movement of the prairie forest border into southwestern wisconsin by 8000 yr b p and its subsequent westward retreat after 6000 yr b p when certain basic assumptions are met multiple regression models can be derived from modern pollen and climate data and used to transform the pollen record of these vegetational changes into quantitative estimates of temperature or precipitation to maximize the reliability of the regression equations we followed a sequence of procedures that minimize violations of the assumptions that underlie regression analysis reconstructions of precipitation during the holocene indicated that from 9000 to 6000 yr b p precipitation decreased by 10 to 25 over much of the midwest while mean july temperature increased by 0 5 to 2 0 c at 6000 yr b p precipitation was less than 80 of its modern values over parts of wisconsin and minnesota after 6000 yr b p precipitation generally increased while mean july temperature decreased in the north and increased in the south the time of the maximum temperature varies within the midwest and is earlier in the north and later in the south

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

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

a high resolution speleothem record of climatic variability at the allerod younger dryas transition in missouri central united states stable isotopic analysis and 234 u 230 th mass spectrometry dating of a single calcite stalagmite from onondaga cave east central missouri reveal climatic variability between 13 200 and 12 400 yr bp coincident with the aller d younger dryas transition in the gisp2 greenland ice cores n 18 o values of speleothem calcite mark decreasing temperatures between 13 200 and 13 000 yr bp while more rapidly decreasing n 13 c values record a sharp shift toward c3 vegetation at approximately 13 100 yr bp because of uncertainties in the chronological control of both this speleothem and the gisp2 record and the brevity of this speleothem record v800 years direct comparison of the two sequences is difficult the interval preserved by this speleothem may span the inter aller d cold period iacp alone both the iacp and the start of the younger dryas or only the first half of the younger dryas in any case these data demonstrate that the climate of the north american interior deteriorated significantly and rapidly during the interval associated with the aller d younger dryas transition fl 2001 elsevier science b v all rights reserved

episodes of low dissolved oxygen indicated by ostracodes and sediment geochemistry at crystal lake illinois usa low dissolved oxygen during the summer and early fall controls profundal continental ostracode distribution in crystal lake mchenry county illinois favoring cypria ophthalmica and physocypria globula at water depths from 6 to 13 m these species also thrived in the lake s profundal zone from 14 165 to 9600 calendar year before present cal yr b p during the late bolling allerod and younger dryas chronozones and early holocene characterized by sand cemented tubules large aquatic gastropod shells and littoral ostracode valves thin 1 6 cm tempestite deposits punctuate thicker deposits of organic gyttja from 16 080 to 11 900 cal yr b p the succeeding 2300 yr 11 900 9600 cal yr b p lack tempestites and reconstructed water depths were at their maximum deposition of marl under relatively well oxygenated conditions occurred during the remainder of the holocene until the arrival of europeans when the lake returned to a pattern of seasonally low dissolved oxygen such conditions are also indicated in the lake sediment by the speciation of phosphorus high concentrations of organic carbon and abundant iron and manganese occluded to mineral grains initial low dissolved oxygen was probably caused by the delivery of dissolved p and fe in shallow groundwater the chemistry of which was influenced by spodosol pedogenesis under a spruce forest the triggering may have been regionally warm and wet conditions associated with retreat of the lake michigan lobe south central laurentide ice sheet

late glacial allerod younger dryas buried organic deposits nova scotia canada a contribution to the north atlantic seaboard programme of igcp 253 termination of the pleistocene abstract numerous deposits of organic shallow pond sediments and wetland peat accumulations buried by minerogenic sediments have been discovered throughout nova scotia they are interpreted as representing a late glacial climatic oscillation correlative with the allerod younger dryas event of europe and the north atlantic ocean the organic deposits began forming during the warm or warming interval following deglaciation and often record a transition to colder conditions the climatic reversion recorded in many deposits began about 10 8 ka bp and continued or sustained cooling culminated in the burial of the organic deposits by minerogenic sediments indicative of solifluction and mass wasting processes some deposits provide strong evidence for regeneration or rejuvenation of local glaciers as the organic deposits are not as susceptible as lake sediments to contamination by old carbon they provide a more reliable chronological framework for deglaciation and late glacial vegetational history palynological evidence shows that pioneer herb tundra communities colonized some areas shortly after 13 ka bp willow and birch shrubs followed soon after spruce woodlands had migrated into the region prior to 10 8 ka bp but had not yet reached the northeast mainland and cape breton island cooling after 10 8 ka bp decimated tree populations and favoured a return to shrub and herb communities the record in the buried deposits was then truncated by deposition of minerogenic sediments and lake sediment sequences from deeper basins with continuous sedimentation are required to complete the record

climatic solar oceanic and geomagnetic influences on late glacial and holocene atmospheric 14c 12c change late glacial and holocene 14c12c ratios of atmospheric co2 vary in magnitude from a few per mil for annual decadal pertubations to more than 10 for events lasting millennia a data set illuminating 10 to 104 yr variability refines our understanding of oceanic climatic versus geomagnetic or solar forcing of atmospheric 14c12c ratios most of the variance in the holocene atmospheric 14c12c record can be attributed to the geomagnetic millennia time scale and solar century time scale influence on the flux of primary cosmic rays entering the atmosphere attributing the observed atmospheric 14c12c changes to climate alone leads to ocean circulation and or global wind speed changes incompatible with proxy records climate ocean related 14c redistribution between carbon reservoirs while evidently playing a minor role during the holocene may have perturbed atmospheric 14c12c ratios measurably during the late glacial younger dryas event first order corrections to the radiocarbon time scale 12 000 30 000 14c yr b p are calculated from adjusted lake sediment and tree ring records and from geomagnetically defined model 14c histories paleosunspot numbers 100 9700 cal yr b p are derived from the relationship of model 14c production rates to sunspot observations the spectral interpretation of the 14c12c atmospheric record favors higher than average solar activity levels for the next century minimal evidence was found for a sun weather relationship

correspondence regarding atmospheric changes in north america during the last deglaciation from dune wetland records in the midwestern united states by wang h stumpf a j miao x lowell t v 2012 quaternary science reviews 58 124 134

age verification of the lake gribben forest bed and the younger dryas advance of the laurentide ice sheet analysis of nine wood samples from the lake gribben forest bed near lake gribben michigan yielded a combined age of 10 025 100 14c years bp which confirms and refines prior age estimates for the bed the stratigraphic position of these samples below a prograding ice contact fan indicates the time that a glacial margin reached the southern edge of the lake superior basin geomorphic tracing and correlation of associated deposits indicate that a contemporaneous margin extended almost 1000 km from duluth minnesota across the lake superior basin to north bay ontario along the southern shore of lake superior ice margin expansion began during and ended at the close of the younger dryas a surging glacier system would not produce a nearly linear moraine system across both a major basin lake superior and a major upland abitibi upland therefore we attribute this advance of the laurentide ice sheet to climatic forcing of the younger dryas event

ecosystem response to lateglacial and early holocene climate oscillations in the great lakes region of north america abstract fossil pollen plant macrofossils gastropods and elemental and stable isotope geochemistry in a sediment core from twiss marl pond southern ontario canada were used to document climate oscillations during the last glacial interglacial transition 13 000 8500 14 c bp and understand their ecological effects chronology was provided by ams 14 c dating and regional pollen correlation oxygen isotope 18 o results from mollusc shells chara encrustations and bulk carbonates show a classic climate sequence of a warm bolling allerod boa at 12 500 10 920 14 c bp a cold younger dryas yd at 10 920 10 000 14 c bp the holocene warming at 10 000 14 c bp a brief preboreal oscillation pb at 9650 14 c bp and a possible gerzensee killarney g k cooling shortly before 11 000 14 c bp clay sediments at the base of the core contain high herb and shrub pollen and abundant arctic alpine plant macrofossils indicating a treeless tundra with severe soil erosion in watershed during the boa warm period authigenic marl began to be deposited and picea woodland became established the establishment of picea woodland after peaks of 18 o and of carbonate accumulation suggests a lagged response of upland vegetation to boa warming in contrast the occurrence of warmth loving aquatics najas flexilis and typha latifolia at that time indicates sensitive responses of aquatic plants the yd cooling is indicated by a 1 5 negative excursion in 18 o an increase in minerogenic matter and higher concentrations of erosion derived elements al na k ti and v pollen data show no forest transformation in response to yd cooling which is attributed to the insensitive nonecotonal vegetation at that time however more openings in the forests and increased erosion in the watershed are indicated by a slight increase of herb pollen high concentrations of erosion elements and a pediastrum peak the onset of the holocene was marked by an abrupt increase of 2 in 18 o and the replacement of picea woodland by pinus dominated forest the picea recurrence at 9650 14 c bp demonstrates sensitive response of ecotonal vegetation to the pb climate oscillation which is also indicated by 0 4 negative excursion of 18 o these new results suggest the importance of multiproxy records for reliable paleoclimate reconstruction reevaluation and revised chronologies of previously published sites gage street and nichols brook in the eastern great lakes region show their major 18 o shifts correlative to the yd and pb oscillations as documented from twiss marl pond and nearby crawford lake the sequence and magnitude of climatic oscillations from these sites match in detail with records from the atlantic seaboard suggesting that these oscillations are an expression of broad scale probably global climate change rather than local meltwater induced climate cooling

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

a tree ring chronology and paleoclimate record for the younger dryas early holocene transition from northeastern north america spruce and tamarack logs dating from the younger dryas and early holocene yd eh 12 9 11 3k cal a bp were found at bell creek in the lake ontario lowlands of the great lakes region north america a 211 year tree ring chronology dates to 11 755 11 545 cal a bp across the yd eh transition a 23 year period of higher year to year ring width variability dates to around 11 650 cal a bp infers strong regional climatic perturbations and may represent the end of the yd tamarack and spruce were dominant species throughout the yd eh interval at the site indicating that boreal conditions persisted into the eh in contrast to geographical regions immediately south and east of the lowlands but consistent with the great lakes interior lowlands this infers that bell creek was at the eastern boundary of a boreal ecotone perhaps a result of its lower elevation and the non analog dynamics of the laurentide ice sheet this finding suggests that the ecotone boundary extended farther east during the yd eh transition than previously thought

can the ocean s heat engine control horizontal circulation insights from the caspian sea we investigate the role of the ocean s heat engine in setting horizontal circulation using a numerical model of the caspian sea the caspian sea can be seen as a virtual laboratory a compromise between realistic global models which are hampered by long equilibration times and idealized basin geometry models which are not constrained by observations we find that increases in vertical mixing drive stronger thermally direct overturning and consequent conversion of available potential to kinetic energy numerical solutions with water mass structures closest to observations overturn 0 02 0 04 x 106m3 s sv representing the first estimate of caspian sea overturning our results also suggest that the overturning is thermally forced increasing in intensity with increasing vertical diffusivity finally stronger thermally direct overturning is associated with a stronger horizontal circulation in the caspian sea this suggests the ocean s heat engine can strongly impact broader horizontal circulations in the ocean

the global response to younger dryas boundary conditions in an agcm simulation geological evidence points to a global younger dryas yd climatic oscillation during the last glacial present interglacial transition phase a convincing mechanism to explain this global yd climatic oscillation is not yet available nevertheless a profound understanding of the mechanism behind the yd climate would lead to a better understanding of climate variability

isotopic evidence for younger dryas aridity in the north american midcontinent determining the impact of the younger dryas yd climate event on the unglaciated north american midcontinent has proved difficult due to a scarcity of suitable paleoclimate proxies here we present a well dated carbon isotope 13c record from a buried soil sequence in southwestern missouri which reveals a large isotopic excursion during the yd chronozone in this region of the modern prairie forest border the 13c signature of soil organic matter is a reliable indicator of past climatic change because 13c values are controlled primarily by the relative abundance of c3 and c4 plants which is tied to the environmental setting between ca 13 200 and 11 900 yr ago the abundance of c4 grasses increased by upwards of 50 of the total biomass indicating expansion of grassland most likely driven by increased aridity during this period environmental gradients in the midcontinent must have been very steep because at the same time that a c4 rich prairie existed in southwestern missouri spruce forests grew in iowa illinois and ohio

the age of water and ventilation timescales in a global ocean model abstract the age of water in the world ocean is studied using a passive age tracer introduced into a global ocean model additional information is derived from a transient dye tracer that tracks the time dependent spreading of surface waters into the model ocean interior of particular interest is the nature of ocean ventilation over the 10 100 yr timescale as well as the simulated age of deep and bottom water masses in the upper model levels young water is found to correspond with regions of convergence and downwelling in the surface ekman layer upwelling and convection are both shown to age the upper ocean by entraining older waters into the surface mixed layer in the deep model levels water age varies greatly between oceans with young water found in convectively active regions in the north atlantic and in the ross and weddell seas and old water found in the deep north pacific the oldest water mass mixture located at 2228 m depth in the western pacific ocean is dated at 1494 years made

terrestrial sensitivity to abrupt cooling recorded by aeolian activity in northwest ohio usa article i nfo article history received 7 may 2010 available online 21 february 2011 optically stimulated luminescence dated sand dunes and pleistocene beach ridges in northwest ohio are used to reconstruct landscape modification more than 5000 yr after deglaciation four of the osl ages 13 3 11 1 ka cluster around the younger dryas cold event five ages 10 8 8 2 ka cluster around the preboreal one young age 0 9 0 7 ka records more recent aeolian activity and one age of 15 1 13 1 ka dates a barrier spit in lake warren in northwest ohio both landscape instability recorded by aeolian activity and a vegetation response recorded by pollen are coeval with the younger dryas however the climate conditions during the preboreal resulting in aeolian activity are not recorded in the available pollen records from this we conclude that aeolian dunes and surfaces susceptible to deflation are sensitive to cooler drier episodes of climate and can complement pollen data younger dryas and preboreal aged aeolian activity in northwestern ohio coincides with aeolian records elsewhere in the great lakes region east of the prairie forest ecotone

intensity gradients and reversals in late glacial environmental change in east central north america abstract radiocarbon dated pollen sites from the allegheny plateau east and the till plains west document significant vegetational and climatic change between 15 000 and 9000 14 c bp south of the great lakes although limited by the problems of no analog vegetation pollen transfer functions tf and response surfaces rs quantify the climatic changes including a cold oscillation chronologically equivalent to the european younger dryas phase i before ca 13 000 14 c bp conditions across the region were similar january 20 to 16 c july 12 c rs or 19 c tf mean annual precipitation 80 to 90 cm year 1 phase iia between ca 13 000 and 11 000 14 c bp n s and e w climate gradients were initiated till plains temperatures increased by ca 8 c in january and by 8 c rs or 2 3 c tf in july plateau increases for january were 3 6 c and for july were 8 c rs or 1 2 c tf precipitation generally increased somewhat phase iib ca 11 000 to 10 000 14 c bp gradients became less pronounced the western sites show a climate reversal january drop of ca 5 to 2 c july drop to ca 1 2 c becoming more like the plateau sites which show a brief small fluctuation this is followed by an abrupt 10 to 20 cm year 1 reduction in precipitation january warming of ca 3 c and continued cooler julys phase iii after 10 000 14 c bp gradients similar to those of today developed the changes in phases iib and iii are probably linked to the drainage history of the great lakes and via that history to the changes on the european continent

the anatomy of a climatic oscillation vegetation change in eastern north america during the younger dryas chronozone century scale climate changes reshaped circulation patterns over the north atlantic and adjacent regions during the last glacialto interglacial transition here we show that vegetation across eastern north america shifted dramatically at the beginning and end of the younger dryas chronozone ydc 12 900 11 600 cal yr b p when changes in ocean circulation rapidly cooled and then warmed the north atlantic sea surface on both the site specific scale and the continental scale vegetation changed only gradually during the millennia before 15 000 13 000 cal yr b p and after 11 000 9000 cal yr b p the ydc but climate changes ca 12 900 and 11 600 cal yr b p altered the vegetation on both spatial scales within centuries plant associations changed and some taxa rapidly migrated hundreds of kilometers 300 km within b100 yr in limited regions near the north atlantic coast abrupt cooling ca 12 900 cal yr b p resulted in a return to earlier vegetation types elsewhere however the vegetation patterns during the ydc were distinct from those of both earlier and later intervals they indicate abrupt non reversing seasonal temperature changes that were probably related to atmospheric circulation changes during the ydc rather than to the direct influence of north atlantic sea surface temperatures atmospheric circulation patterns during the ydc were unique within the last 21 000 yr because of a unique combination of climate controls insolation ice sheet extent and atmospheric composition were significantly different from their full glacial states even when the north atlantic returned to near full glacial conditions the ydc vegetation patterns demonstrate 1 rapid ecological responsiveness to abrupt climate change and 2 spatially varied patterns of ydc climate change r 2002 elsevier science ltd all rights reserved

temperature salinity structure of the north atlantic circulation and associated heat and freshwater transports abstractthis study investigates the circulation structure and relative contribution of circulation components to the time mean meridional heat and freshwater transports in the north atlantic using numerical results of a high resolution ocean model that are shown to be in excellent agreement with the observations the north atlantic circulation can be separated into the large scale atlantic meridional overturning circulation amoc that is diapycnal and the subtropical and subpolar gyres that largely flow along isopycnal surfaces but also include prominent gyre scale diapycnal overturning in the subtropical mode water and labrador sea water integrals of the meridional volume transport as a function of potential temperature and salinity s yield streamfunctions with respect to and to s and heat functions these argue for a significant contribution to the heat transport by the southward circulation of north atlantic deep water at 26 5 n the isopycnic component of the subtropical gyre is colder and fr

a late pleistocene holocene noble gas paleotemperature record in southern michigan 1 noble gas temperatures ngts and 14c derived ages in groundwaters of the michigan basin reveal a ground temperature of 1 c toward the end of the last glacial maximum lgm suggesting that groundwater recharge occurred under the laurentide ice sheet lis cover in addition to the general warming observed since the lgm the ngt record indicates an abrupt warming event between 12 8 and 11 1kyrs bp correlative to the bolling allerod boa warm phases ice sheet linked changes in freshwater delivery to the north atlantic together with changes in the north atlantic deep water nadw circulation are possible causes of such abrupt climate shifts in northeastern us pleistocene waters yielding the lowest ngts have the highest 18o and d values suggesting an atmospheric circulation pattern distinct from today with a stronger moisture component from the gulf of mexico possibly due to the presence of the lis which weakened the pacific westerly flow

younger dryas climatic reversal in northeastern usa ams ages for an old problem abstract late glacial macrofossils from a 10 m core from alpine swamp new jersey were radiocarbon dated using accelerator mass spectrometry ams the arrival of the first trees to the area following deglaciation is indicated by maximum percentages of spruce pollen and a date of 12 290 440 yr b p on a single spruce needle subsequent spread of deciduous hardwoods was followed by the expansion of boreal taxa including spruce picea fir abies larch larix laricina paper birch betula papyrifera and alder alnus three ams dates on paper birch seeds and a spruce needle during this boreal expansion indicate that it took place between 11 000 and 10 000 yr b p the timing of this vegetational shift and its correlation with late glacial pollen stratigraphy from many sites in southern new england indicate that a climatic reversal correlative with the younger dryas characterized the north atlantic seaboard of the united states

14c ams wiggle matching of raised bog deposits and models of peat accumulation abstract high resolution accelerator mass spectrometer ams 14 c dates of selected plant macrofossils from the raised bog engbertsdijksvenen eastern netherlands show century scale wiggles analogous to the radiocarbon calibration curve we used three relative time scales viz based on depth mass and pollen concentration respectively to match the peat ams dates to the calibration curve this procedure is repeated for one conventionally dated core for each relative time scale realistic confidence intervals are calculated depth appears to be the best time scale for certain stratigraphical units of a core this justifies using depth for wiggle matching conventionally dated mire cores published by others even when 14 c errors are larger our evidence shows four major sources of 14 c variation for mire deposits compared to treerings 1 dating error due to sample composition this includes a reservoir effect demonstrated for many bulk peat samples 2 hiatuses causing a sudden leap of peat 14 c age 3 changing accumulation rates apparent from a break in the slope of the peat 14 c ages 4 sampling error these results shed doubt on the assumed continuities in theoretical peat accumulation models both mire stratigraphy and changing accumulation rates can be explained in qualitative hydrological terms more evidence for a previously discovered reservoir effect in bog deposits is presented though the physical biological mechanisms remain uncertain

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

climatic effects of glacial lake agassiz in the midwestern united states during the last deglaciation stable isotope and pollen analyses of a sediment core from deep lake minnesota provide new information on the climatic effects of glacial lake agassiz in minnesota and insights into the cause of the prominent picea recurrence during the younger dryas in the southern great lakes region bulk carbonate 18o exhibited large fluctuations between 12 0 and 9 1 ka dates are in calendar years throughout this paper unless indicated otherwise probably reflecting the effects of glacial lake agassiz superimposed on climatic warming related to large scale climatic controls in particular a 3 decrease in 18o 11 2 10 2 ka interrupted the 18o enrichment of 1 from 12 0 to 11 2 ka and 3 5 from 10 2 to 9 1 ka this 18o decrease coincided with the expansion of lake agassiz we interpret this decrease as a result of decreased summer temperature and increased precipitation derived from the cold and isotopically light meltwaters of lake agassiz during this 18o decline pinus pollen continued to increase at the expense of picea pollen at deep lake as at other minnesota sites providing evidence that climatic cooling induced by lake agassiz did not cause a reversal to a picea dominated vegetation the absence of such a vegetational response implies that the prominent picea recurrence during the younger dryas in the southern great lakes region was not caused solely by climatic cooling due to increased flux of meltwater from lake agassiz into the great lakes instead the picea recurrence might have been driven primarily by the westward penetration of the younger dryas cooling

the impact of turbulence and convection on transport in the southern ocean volume transport in the presence of fully resolved convection and turbulence is investigated in a reentrant channel model of the southern ocean the response of the meridional overturning and zonal transport to variations in wind and buoyancy forcing is quantified our simulations show two overturning cells a buoyancy driven lower cell and a wind driven upper cell the lower overturning cell is much larger in meridional extent and magnitude than the upper cell the mean component of the overturning transport is smaller than the fluctuating component in the lower overturning cell indicating that transport is dominated by eddies and or turbulent convective flow in contrast the upper cell can be primarily described as a mean flow indicating minimal eddy compensation both cells strengthen with increasing winds with the upper cell being more sensitive to increasing wind intensity than the lower cell scaling for the mean upper overturning is also derived and matches previous theories which predict a linear sensitivity of the upper cell to wind zonal transport remains insensitive to increasing wind stress suggesting that the system is eddy saturated when turbulence and eddies are resolved these results suggest that fine scale flows control zonal transport and abyssal meridional overturning in the southern ocean while wind stress drives the upper meridional overturning transport highlighting the importance of accurately characterizing turbulence and convection in large scale models

pronounced variations in fagus grandifolia abundances in the great lakes region during the holocene mesic tree species such as fagus grandifolia and tsuga canadensis experienced multiple abundance declines in eastern north america during the last 8000 years but the causes remain unclear this paper presents a new sub centennial record of holocene vegetation fire and sedimentological changes at spicer lake in to test hypotheses about the role of fire and hydrological variations on shifts in vegetation composition four pollen zones are reported abies picea forests 15 11 8 ka bp pinus dominated mixed forest 11 8 10 6 ka bp transitional mixed forest 10 6 6 8 ka bp and deciduous forest characterized by the expansion and high variability of f grandifolia after 6 8 ka bp macroscopic charcoal indicates five to seven fires between 6 1 and 4 4 ka bp and no fires between 4 4 and 2 ka bp despite several large declines in f grandifolia and more fires after 1 8 ka bp likely linked to declining f grandifolia abundances after 1 1 ka bp six peaks in mineralogenic sediments are suggestive of hydro

isopycnal mixing in ocean circulation models abstract a subgrid scale form for mesoscale eddy mixing on isopycnal surfaces is proposed for use in non eddy resolving ocean circulation models the mixing is applied in isopycnal coordinates to isopycnal layer thickness or inverse density gradient as well as to passive scalars temperature and salinity the transformation of these mixing forms to physical coordinates is also presented

rapid ice margin fluctuations during the younger dryas in the tropical andes abstract radiocarbon dated lacustrine sequences in peru show that the chronology of glaciation during the late glacial in the tropical andes was significantly out of phase with the record of climate change in the north atlantic region fluvial incision of glacial lake deposits in the cordillera blanca central peru has exposed a glacial outwash gravel radiocarbon dates from peat stratigraphically bounding the gravel imply that a glacier advance culminated between 11 280 and 10 990 14c yr b p rapid ice recession followed similarly in southern peru ice readvanced between 11 500 and 10 900 14c yr b p as shown by a basal radiocarbon date of 10 870 14c yr b p from a lake within 1 km of the quelccaya ice cap by 10 900 14c yr b p the ice front had retreated to nearly within its modern limits thus glaciers in central and southern peru advanced and retreated in near lockstep with one another the younger dryas in the peruvian andes was apparently marked by retreating ice fronts in spite of the cool conditions that are inferred from the 18o record of sajama ice this retreat was apparently driven by reduced precipitation which is consistent with interpretations of other paleoclimatic indicators from the region and which may have been a nonlinear response to steadily decreasing summer insolation

impact of mesoscale eddy transfer on heat uptake in an eddy parameterizing ocean model abstractusing a set of experiments with an eddy parameterizing ocean model it is found that the strength of the atlantic meridional overturning circulation amoc intensifies with the decrease of

labrador sea boundary currents and the fate of the irminger sea water abstract the general circulation of the labrador sea is studied with a dataset of 53 surface drifters drogued at 15 m and several hydrographic sections done in may 1997 surface drifters indicate three distinct speed regimes fast boundary currents a slower crossover from greenland to labrador and a slow eddy dominated flow in the basin interior mean eulerian velocity maps show several recirculation cells located offshore of the main currents in addition to the cyclonic circulation of the labrador sea above the northern slope of the basin the surface drifters have two preferential paths one between the 1000 m and 2000 m isobaths and the other close to the 3000 m isobath the vertical shear estimated from ctd data supports the presence of two distinct currents around the basin one current more baroclinic flows between the 1000 m and 2000 m isobaths the other one more barotropic flows above the lower continental slope the irminger sea water carried by the boundary currents is altered as it tr

the 8 2 ka abrupt climate change event in brown s lake northeast ohio many northern hemisphere paleoclimatic records including ice cores speleothems lake sediments ocean cores and glacier chronologies indicate an abrupt cooling event about 8200 cal yr bp a new well dated series of sediment cores taken from brown s lake a kettle in northeast ohio shows two closely spaced intervals of loess deposition during this time period the source of loess is uncertain however it is likely from an abandoned drainage and former glacial lake basin located to the north of the stagnant ice topography that gave rise to the kettle lake strong visual stratigraphy loss on ignition data and sediment grain size analyses dated with 3 ams radiocarbon dates place the two intervals of loess deposition between 8950 and 8005 cal yr bp the possibility of a two phase abrupt climate change at this time is a finding that has been suggested in other research this record adds detail to the spatial extent and timing as well as possible structure of the 8 2 ka abrupt climate change event

sedimentologic and palynologic records of the last deglaciation and holocene from ballston lake new york continuous pollen and sediment records from two 8 5 m long cores document late pleistocene and holocene sedimentation and vegetation change in the ballston lake basin eastern new york state pebbles at the base of both cores and the geomorphology of the watershed reflect the presence of the mohawk river in the basin prior to 12 900 70 cal yr b p ballston lake formed at the onset of the younger dryas yd by an avulsion of the mohawk river the transition from clay to gyttja with low magnetic susceptibility ms low bulk density and high organic carbon indicates rapid warming and increased lake productivity beginning 11 020 cal yr b p ms measurements reveal that the influx of magnetic particles associated with pre holocene clastic sedimentation ceased after 10 780 cal yr b p the pollen record is subdivided into six zones bl1 12 920 to 11 020 cal yr b p is dominated by boreal forest pollen bl2 11 020 to 10 780 cal yr b p by pine pinus forest pollen bl3 10 780 to 5290 cal yr b p by hemlock tsuga and mixed hardwood pollen bl4 5290 to 2680 cal yr b p by mixed hardwood pollen bl5a 2680 cal yr b p to 1030 cal yr b p by conifer and mixed hardwood pollen and bl5b 1030 cal b p to present by increasing ragweed ambrosia pollen a 62 decrease in spruce picea pollen in 320 cal years during bl1 reflects rapid warming at the end of the yd holocene pollen zones record more subtle climatic shifts than occurred at the end of the yd one of the largest changes in the holocene pollen spectra began 5300 cal yr b p and is characterized by a marked decline in hemlock pollen this has been noted in other pollen records from the region and may record preferential selection of hemlock by a pathogen or parasites

reply to comments by curry et al 2013 on atmospheric changes in north america during the last deglaciation from dune wetland records in the midwestern united states

vertical heat transport in eddying ocean models 1 the effect of mesoscale eddies on the vertical heat transport of the ocean is examined using two eddy resolving numerical models the global heat transport by the mean flow and diffusion are both downwards and are balanced by an upward eddy heat flux mean and eddy advective heat fluxes dominate the subpolar regions while diffusive flux is important primarily in the subtropics in the subtropical abyss the mean advective heat flux is balanced by a combination of eddy and diffusive fluxes and the classical munk type advective diffusive heat balance must be modified the munk and wunsch 1998 expression for the vertical turbulent diffusivity over estimates the diffusivity by as much as a factor of four near the base of the main thermocline this implies that the mixing required to close the meridional overturning circulation determined by munk and wunsch 1998 may be an over estimate due to the neglect of mesoscale eddies

effects of glacial meltwater in the giss coupled atmosphereocean model 1 north atlantic deep water response varying magnitudes of freshwater input through the st lawrence are added to different versions of the giss coupled atmosphere ocean model the results show a generally linear response in percentage north atlantic deep water nadw reduction to the volume of water added without any obvious threshold effects when the estimated best guess freshwater input for the allerod younger dryas interval is added only small reductions in nadw production occur in less than a century with complete cessation requiring more than 300 years if the ocean circulation was weaker in the allerod preceding the younger dryas somewhat larger percentage reductions occur and it would take 150 200 years for a complete shutdown the freshening is aided by a positive feedback associated with the reduction in evaporation relative to precipitation in the north atlantic amounting to some 20 30 of the added freshwater runoff when the freshwater input is stopped after complete nadw shutdown nadw production does not resume with complete nadw cessation cooling eventually occurs throughout the northern hemisphere including regions where it is not thought to have arisen during the younger dryas and the globe cools by 1 c due primarily to the sea ice albedo feedback while in many regions the observed cooling is reproduced the cooling in greenland is much less than is estimated with only 50 nadw cessation cooling is primarily around the north atlantic no rapid warming is seen in response to freshwater cessation the sensitivity displayed here for nadw reduction due to freshwater input from the st lawrence is roughly similar to that associated with increased precipitation accompanying global warming in the next century so a similar sensitivity may arise with younger dryas freshwater inputs from other suspected locations greenland europe

palynology after y2k understanding the source area of pollen in sediments pollen grains preserved in lake and bog sediment provide a record of past vegetation that has been an important source of information about climate and land cover during the quaternary period yet from the beginning questions have been raised about the source area of pollen in sediment interpretation has been hampered by the lack of well developed theory treating the relationship between the spatial distribution of trees on the landscape and the percentages of pollen in sediment within the past decade however new theory models and empirical data show how heterogeneous vegetation is represented by pollen the distinction between local and regional pollen is explained by the prentice sugita dispersal deposition models which predict how the ratio of regional to local pollen changes with lake size sugita s model simulating a landscape with heterogeneous vegetation predicts the size of the relevant source area the area of vegetation reflected in between lake variations in pollen loading while demo

vertical eddy fluxes in the southern ocean the overturning circulation of the southern ocean has been investigated using eddying coupled ocean sea ice models the circulation is diagnosed in both density latitude coordinates and in depth density coordinates depth density coordinates follow streamlines where the antarctic circumpolar current is equivalent barotropic capture the descent of antarctic bottom water follow density outcrops at the surface and can be interpreted energetically in density latitude coordinates wind driven northward transport of light water and southward transport of dense water are compensated by standing meanders and to a lesser degree by transient eddies consistent with previous results in depth density coordinates however wind driven upwelling of dense water and downwelling of light water are compensated more strongly by transient eddy fluxes than fluxes because of standing meanders model realizations are discussed where the wind pattern of the southern annular mode is amplified in density latitude coordinates meridional fluxes because of transient eddies can increase to counter changes in ekman transport and decrease in response to changes in the standing meanders in depth density coordinates vertical fluxes because of transient eddies directly counter changes in ekman pumping

the chronology of the last deglaciation implications to the cause of the younger dryas event it has long been recognized that the transition from the last glacial to the present interglacial was punctuated by a brief and intense return to cold conditions this extraordinary event referred to by european palynologists as the younger dryas was centered in the northern atlantic basin evidence is accumulating that it may have been initiated and terminated by changes in the mode of operation of the northern atlantic ocean further it appears that these mode changes may have been triggered by diversions of glacial meltwater between the mississippi river and the st lawrence river drainage systems we report here accelerator mass spectrometry ams radiocarbon results on two strategically located deep sea cores one provides a chronology for surface water temperatures in the northern atlantic and the other for the meltwater discharge from the mississippi river our objective in obtaining these results was to strengthen our ability to correlate the air temperature history for the northern atlantic basin with the meltwater history for the laurentian ice sheet

patterns of holocene environmental change in the midwestern united states abstract four pollen sequences along a transect from north central iowa to southeast wisconsin reveal the distribution of prairie and forest during the holocene and test the use of pollen isopolls in locating the holocene prairie forest border prairie was dominant in central iowa and climate was drier than present from about 8000 to 3000 yr b p during the driest part of this period in central iowa 6500 5500 yr b p mesic forest prevailed in eastern iowa and wisconsin suggesting conditions wetter than at present prairie replaced the mesic forest about 5400 yr b p in eastern iowa but did not extend much farther east mesic forests were replaced in southern wisconsin and northern illinois about 5400 yr b p by xeric oak forests this change from mesic to xeric conditions at 5400 yr b p was widespread and suggests that the intrusion of drier pacific air was blocked by maritime tropical air from the gulf of mexico until the late holocene in this area

isopycnal averaging at constant height part i the formulation and a case study simple eulerian averaging of velocities density and tracers at constant position is the most natural way of averaging however eulerian averaging gives incorrect watermass distributions and properties as well as spurious diabatic circulations such as the deacon cell instead of averaging at constant height averaging along isopycnals removes such fictitious mixing and diabatic circulations such isopycnal averaging is normally performed at constant latitude that is averaging along isopynals as they heave up and down as a result height information is lost and the sea surface becomes much warmer or lighter than with simple eulerian averaging in fact averaging can be performed along arbitrarily aligned surfaces this study considers a particular case in which isopycnal averaging is performed at constant height thus this new isopycnal averaging follows isopycnals as they meander horizontally at constant z height information is now retained at the cost of losing latitudinal information the advantage of this averaging is that it avoids the problem of giving a surface that is too warm associated with this new isopycnal averaging a vertical transport streamfunction in r z space can be defined in analogy to the conventional meridional overturning streamfunction in y r space here in part i this constant height isopycnal averaging is explained and illustrated in an idealized zonal channel model in part ii the relationship between the two different isopycnal averagings and the eulerian mean eddy flux divergence is explored

temporal variability of the atlantic meridional overturning circulation at 26 5 n the vigor of atlantic meridional overturning circulation moc is thought to be vulnerable to global warming but its short term temporal variability is unknown so changes inferred from sparse observations on the decadal time scale of recent climate change are uncertain we combine continuous measurements of the moc beginning in 2004 using the purposefully designed transatlantic rapid climate change array of moored instruments deployed along 26 5 n with time series of gulf stream transport and surface layer ekman transport to quantify its intra annual variability the year long average overturning is 18 7 5 6 sverdrups sv range 4 0 to 34 9 sv where 1 sv a flow of ocean water of 106 cubic meters per second interannual changes in the overturning can be monitored with a resolution of 1 5 sv

stable isotope o and c and pollen trends in eastern lake erie evidence for a locally induced climatic reversal of younger dryas age in the great lakes basin a cool period from about 11000 to 10 500 bp 11 to 10 5 ka is recognized in pollen records from the southern great lakes area by the return of picea and abies dominance and by the persistence of herbs the area of cooling appears centred on the upper great lakes a high resolution record ca 9 mm y from a borehole in eastern lake erie reveals in the same time interval this pollen anomaly isotope evidence of meltwater presence a 3 per mil shift in 18o and a 1 1 per mil shift in 13c increased sand and reduced detrital calcite content all suggesting concurrent cooling of lake erie the onset of cooling is mainly attributed to the effect of enhanced meltwater inflow on the relatively large upstream main lake algonquin during the first eastward discharge of glacial lake agassiz termination of the cooling coincides with drainage of lake algonquin and is attributed to loss of its cooling effectiveness associated with a substantial reduction in its surface area it is hypothesized that the cold extra inflow effectively prolonged the seasonal presence of lake ice and the period of spring overturn in lake algonquin the deep mixing would have greatly increased the thermal conductive capacity of this extensive lake causing suppression of summer surface lakewater temperatures and reduction of onshore growing degree days alternatively a rapid flow of meltwater buoyed on sediment charged denser lakewater may have kept the lake surface cold in summer other factors such as wind shifted pollen deposition and possible effects from the younger dryas north atlantic cooling could have contributed to the great lakes climatic reversal but further studies are needed to resolve their relative significance

continental biogenic species in the greenland ice core project ice core tracing back the biomass history of the north american continent ammonium nitrate and organic acid records from the greenland ice core project deep ice core are discussed all species have a continental biogenic source that is situated predominantly on the north american continent for species deposited in summit central greenland the record therefore can be used to trace back the biomass history of the north american continent difficulty in the interpretation of these records arises from their unknown transfer behavior in a more alkaline atmosphere which characterizes glacial time periods compared to interglacial stadials this may have implications not only for weak acids such as formate and acetate but also possibly for the transport and incorporation of hno3 into aerosols whereas ammonium is probably not affected by the alkalinity change of the atmosphere our approach is to compare samples with similar h concentrations throughout the record from the records we infer several significant fluctuations in the extent of the laurentide ice sheet during the last glaciation we find evidence against the occurrence of an extensive younger dryas event in north america

evidence of late glacial permafrost in newfoundland abstract structures interpreted as ice wedge pseudomorphs are developed in postglacial sediments at several locations in coastal areas of western central and eastern newfoundland the ice wedge casts indicate that permafrost was prevalent in exposed coastal locations following late wisconsinan deglaciation and marine regression due to isostatic uplift formation of the ice wedge casts indicates that the mean annual temperatures were colder than at present along the newfoundland coast and that snow pack depths were less and snow cover less persistent the environment following marine regression was marked by cold windswept conditions which permitted the development of permafrost in the coastal regions much of the interior of newfoundland remained covered by glacial ice and consequently did not support subaerial permafrost direct numerical dating of the ice wedge casts is not possible at present however the geomorphic settings and stratigraphic positions of the features indicate that they developed subsequent to latest wisconsinan glacial retreat and followed regression from the postglacial marine limit permafrost formation preceded climatic warming and establishment of arboreal vegetation in the early holocene the time of formation of the ice wedges therefore is constrained between the initiation of postglacial marine regression ca 12 300 11 000 bp uncal and the initiation of climate amelioration ca 10 400 bp uncal this chronological range corresponds to the combined duration of the initial postglacial cooling episode and the subsequent younger dryas cold climate event documented in palaeoenvironmental records from newfoundland as lasting from shortly before ca 11 200 to ca 10 400 bp