climate instability during the last interglacial period recorded in the grip ice core isotope and chemical analyses of the grip ice core from summit central greenland reveal that climate in greenland during the last interglacial period was characterized by a series of severe cold periods which began extremely rapidly and lasted from decades to centuries as the last interglacial seems to have been slightly warmer than the present one its unstable climate raises questions about the effects of future global warming

meltwater discharge to the skagerrak kattegat from the baltic ice lake during the younger dryas interval abstract diatom data from the skagerrak kattegat show that large amounts of meltwater were discharged into the kattegat skagerrak from the baltic ice lake during the younger dryas interval strong meltwater discharge greatly freshened surface water salinity in the kattegat and areas along the swedish west coast and possibly changed the directions of sea surface salinity gradients from north south to east west or northwest southeast it resulted in a markedly stratified water column in salinity in the kattegat which complicates the environmental interpretation based on different types of microfossils the meltwater influence on the large area of the skagerrak during the younger dryas was however restricted along the norwegian coast where it flowed into the norwegian sea

the role of the wind evaporation sea surface temperature wes feedback as a thermodynamic pathway for the equatorward propagation of high latitude sea ice induced cold anomalies abstract the role of the wind evaporation sea surface temperature wes feedback in the propagation of the high latitude cooling signal to the tropical oceans using the ncar atmospheric community climate model ccm3 coupled thermodynamically to a slab ocean model som is studied abruptly imposed additional northern hemispheric sea ice cover equivalent to the last glacial maximum lgm 18 kyr bp in the model causes a northern hemisphere wide cooling as well as the generation and amplification of an anomalous cross equatorial meridional sst dipole associated with a southward migration of the intertropical convergence zone itcz stabilizing within a period of 5 yr in experiments where the wes feedback is switched off explicitly by modifying the sensible and latent heat flux bulk aerodynamic formulations over the oceans in ccm3 imposed northern hemispheric sea ice also results in widespread northern cooling at the same rate as the unmodified run suggesting that the wes feedback is not essential in th

a revised 10 4 c magnitude of the abrupt change in greenland temperature at the younger dryas termination using published gisp2 gas isotope data and air thermal diffusion constants abstract we revisit the portion of nature 391 1998 141 devoted to the abrupt temperature increase reconstruction at the younger dryas preboreal transition the original estimate of 5 to 10 c abrupt warming is revised to 10 4 c the gas isotope data from the original work were employed combined with recently measured precise air thermal diffusion constants geochim cosmochim acta 67 2003a 345 j phys chem 23a 2003b 4636 the new constants allow a robust interpretation of the gas isotope signal in terms of temperature change this was not possible at the time of the original work when no air constants were available three quasi independent approaches employed in this work all give the same result of a 10 c warming in several decades or less the new result provides a firm target for climate models that attempt to predict future climates

the geochemistry of major and selected trace elements in a forested peat bog kalimantan se asia and its implications for past atmospheric dust deposition abstract biogeochemical processes in a forested tropical peat deposit and its record of past atmospheric dust deposition were assessed using the vertical distribution of lithophilic and plant essential elements in a dated core profile from borneo se asia peat formation started 22 120 14c yr before present bp and ca mg mass ratios of the solid peat and very low ash contents indicate a strongly ombrotrophic character throughout the deposit implying that most of the inorganic fraction has been supplied exclusively by atmospheric inputs concentration profiles of mn sr and ca suggest a very minor influence of chemical diagenesis in the underlying sediments silicon ca mg p s and k show a strong and extended zone of enrichment in the top 200 cm of the profile indicating that biological accumulation mechanisms are much more extensive than in temperate peat bogs in the lower core sections where the element distribution is dominated solely by past atmospheric deposition average al ti ratios are similar to the upper continental crust ucc whereas fe is slightly enriched and si is strongly depleted this condition favors highly weathered tropical soil dust as the main inorganic mineral source significant correlation of al fe si s ca and ti with the lithophilic elements y and zr suggests that the distribution of these elements is controlled by sources of atmospheric mineral dust the ca mg ca k and mg k ratios of the collected rainwater samples are similar to the global average of continental rainwater and suggest a continental character for the site this is supported by the similarity of the average concentration of br mg ca and s to that in temperate continental and maritime bogs in switzerland and scotland the concentration profiles of si fe al and ti show distinct peaks within the profile implying enhanced dust deposition reduced rates of peat accumulation or possibly both owing to climatic changes during the holocene enhanced dust deposition between 10 830 and 9060 14c yr bp is tentatively interpreted as a younger dryas like event with dust fluxes of 10 8 mg m2 yr the variations in al ti and fe ti profiles suggest that mineral dust sources have been changing constantly during the holocene with local sources being dominant between 7820 and 9500 14c yr bp and long range transport derived most likely from china being important during the late pleistocene and early holocene and from 7820 14c yr bp to the present

the younger dryas termination and north atlantic deep water formation insights from climate model simulations and greenland ice cores results from the gisp2 and grip ice cores show that the termination of the younger dryas yd climate event in greenland was a large and extremely fast climate change a reinitiation of north atlantic deep water formation following a shutdown and its associated winter release of heat to the atmosphere has been suggested as the most likely cause of this climate transition to test this idea two general circulation model experiments using genesis have been completed for yd time 12 000 calendar years ago one with low heat flux in the nordic seas 10 w m deep water shutdown and one with high nordic sea heat flux 300 w m active deep water formation comparison of greenland climate differences between these experiments with the ice core records shows that when deep water is turned on much of the yd termination warming is achieved the increase in precipitation is underestimated because of a model tendency to overestimate summertime precipitation which obscures the dominantly wintertime response to the specified forcing the winter storm track shift toward greenland contributes much of the climate change at the yd termination

palaeotemperatures still exist in the greenland ice sheet the temperature distribution through the greenland ice sheet at the dye 3 borehole is a record of the past climatic changes in the arctic the numerical model of the temperature distribution now presented reproduces the observed temperature distribution within 0 03 k and shows that the basal ice is still cooled 5 k by the cold ice age climate the results suggest a mean ice age temperature of 32 2 c which is 12 k colder than the present temperature and a precipitation rate 50 25 of the present rate calculations of a more detailed temperature history through the present inter glacial period reveal evidence of the ad 1920 50 maximum the little ice age and the atlantic period

younger dryas style cooling events at glacial terminations i vi at odp site 658 associated benthic δ13c anomalies constrain meltwater hypothesis high resolution planktonic and epibenthic stable isotope records from ocean drilling program site 658 off northwest africa provide a basis for a detailed study of glacial terminations i vi during the last 650 000 years the duration of the terminations was about one half to one quarter of an orbital precession cycle 5800 10 700 years when its amplitude was high at low amplitudes the terminations lasted longer than half an obliquity cycle 29 000 years marked climatic rebounds similar to the younger dryas each with a duration of 1000 2500 years subdivided all six terminations into distinct steps a b and c important parts of the deglacial steps were as brief as 700 1000 years the speed of climatic change suggests that special associations existed between orbital forcing and inherent instability of the ice sheets in harmony with published models the more rapid pulses of glacial meltwater incursions to the northern north atlantic led to one or more brief short term shut downs of north atlantic deep water formation this process is reflected by pronounced benthic 13c minima that precisely coincide in most cases with the end of the 18o deglaciation steps and immediately terminate with succeeding younger dryas style cooling events thus we conclude that the rebounds resulted from a short term antecedent estuarine circulation regime in the north atlantic except for the younger dryas itself which succeeded the allerod with its well ventilated atlantic deepwater circulation and hence continues to be an enigma

paleoclimate and paleoceanography over the past 20 000 yr in the mediterranean sea basins as indicated by sediment elemental proxies abstract marine sediments record paleoenvironmental changes over time through variations in major and trace element concentrations the main objective of this paper is to review such changes in the mediterranean sea basins over the last 20 ka using the inorganic chemistry and mineralogy of marine sediment records elemental ratio proxies that are mostly used are discussed and the uncertainties involved in using them for paleoclimate and paleoceanographic reconstructions are evaluated the focus on the mediterranean region is based on the sensitivity of this region to global climate changes due to its semi enclosed nature the elemental ratios that have been particularly useful for reconstructing terrigenous inputs into the mediterranean have been ti al and zr al ratios as proxies for eolian dust input and mg al k al and rb al ratios as proxies for fluvial input redox sensitive elements e g u mo v co ni cr have provided reliable reconstructions of oxygen conditions at the time of deposition some of these elements are also particularly susceptible to post depositional remobilization and record diagenetic processes instead of the original environmental signatures regarding productivity fluctuations most of the paleoproductivity reconstructions are based on the abundance of barite and ba excess algorithms the biogeochemistry of ba is however not fully understood and mechanisms for barite precipitation in the water column are not yet known two case studies are presented the eastern mediterranean sapropel s1 deposited between 10 8 and 6 1 cal ka bp and the westernmost mediterranean paleoclimate record over the last 20 ka

late weichselian glacier retreat in kongsfjorden west spitsbergen svalbard the chronology of late weichselian to holocene deglaciation of kongsfjorden west spitsbergen has been reconstructed based on the geomorphic and stratigraphic record of ice retreat relative sea level relationships and 14c dating of associated marine organic materials the seaward extent of glacial drift and fjord bathymetry constrain a secure reconstruction for the ice sheet near the mouth of the fjord at ca 13 000 yr b p but do not rule out the possibility that more extensive glaciation was achieved earlier during the late weichselian regional shoreline relations rates of emergence and radiocarbon dating of foraminifera deposited just above till indicate that deglaciation occurred in two steps one beginning during or just prior to the late weichselian marine limit phase at 13 000 12 000 yr b p and another beginning at 10 000 9500 yr b p the fjord was completely deglaciated by 9440 130 yr b p a period of stable relative sea level began 10 700 yr b p and ended between 10 000 and 9500 yr b p which we take to indicate renewed glacial loading during the younger dryas glacier readvance within kongsfjorden at this time was diminutive suggesting that most of the younger dryas ice sheet growth was confined to the eastern part of the archipelago and or to the barents shelf the two step deglaciation of kongsfjorden occurred during intervals of accelerated global ice sheet melting and rapid oceanic and atmospheric warming in more temperate latitudes of the circum atlantic region this coincidence most likely resulted from contemporaneous increases in the poleward transport of oceanic heat

the arid humid transition in the sahara and the sahel during the last deglaciation at the time of the last glacial maximum the sahara and sahel regions of north africa were extremely dry new records of rainfall show that during the subsequent deglaciation the transition from arid to humid conditions in these regions occurred synchronously in two main steps comparison with other records of palaeoclimate in europe and the north atlantic ocean shows that certain common factors controlled changes in ocean and atmosphere dynamics during the deglaciation

glacial millennial scale fluctuations in central african precipitation recorded in terrigenous sediment supply and freshwater signals offshore cameroon abstract evidence of rapid climatic oscillations like those observed in the greenland ice cores and sediments from high latitudes of the northern atlantic have been recognized in the pulses of terrigenous material to continental margin sediments off cameroon fe ca ratios used as a parameter to quantify the relative proportions of terrigenous fluxes versus marine carbonate monitor the variability of the west african monsoon they reveal the history of abrupt changes in precipitation over western and central africa during the past 52 kyr these rapid changes are particularly pronounced during the last glacial period and occur at timescales of a few thousand years stable oxygen isotope 18o records of globigerinoides ruber pink show high negative values reflecting periods of high monsoon precipitation the fe ca pattern is very similar to the dansgaard oeschger cycles from the greenland ice cores the good correspondence between the warm interstadials of the dansgaard oeschger cycles from the gisp2 ice core records and the high pulses of fe ca sedimentation in our core suggest a strong teleconnection between the low latitude african climate and the high latitude northern hemisphere climate oscillations during the last glacial this climatic link is probably vested in the west african monsoonal fluctuation that alters tropical sea surface temperatures thermohaline circulations and in turn net export of heat from the south to the north atlantic coupled with the variability of the low latitude southeast trade winds

sea ice switch mechanism and glacial interglacial co2 variations 1 a physical mechanism is proposed for the glacial interglacial variations in the rate of vertical mixing and in sea ice cover in the southern ocean such variations were postulated without an explanation by toggweiler 1999 and stephens and keeling 2000 who used them to explain the glacial interglacial co2 variations in the physical mechanism explored here initially given by gildor and tziperman 2001b changes in the stratification of the southern ocean due to the cooling of north atlantic deep water nadw during glacial maxima reduce the rate of vertical mixing of the surface water with the deep water the changed temperature of the nadw arriving to the southern ocean and the reduced vertical mixing there also increase the southern ocean sea ice cover during glacial maxima these vertical mixing and sea ice cover changes are shown to be a natural result of the sea ice switch mechanism of the glacial cycles a box model of the climate system is used to demonstrate the above physical mechanism and its effect on the atmospheric co2 because of the uncertainties in the exact dependence of the vertical mixing on vertical stratification it is impossible to quantify the exact separate contribution of reduced vertical mixing and larger sea ice cover to the glacial co2 variations the co2 variations are not essential to the existence of the glacial cycles in the sea ice switch mechanism yet they are shown to amplify the glacial interglacial variability consistent with the view of the role of co2 deduced from proxy observations

cryosphere ocean interactions at the margin of the laurentide ice sheet during the younger dryas chron se baffin shelf northwest territories cores hu82 034 057 and hu84 035 008 resolution basin se baffin shelf contain 200 and 450 cm respectively of sediment that spans the younger dryas chron in both cores the interval is bracketed by 14c dates on foraminifera or molluscs these sites were close to the margin of the late wisconsin foxe ice sheet as it flowed toward the labrador sea prior to 11 ka both cores record moderate to high accumulations of foraminifera relatively high del 18o values in planktonic foraminifera and low values of detrital carbonate the diatom and percent opal records imply occasional seasonally open water conditions during part of the younger dryas chron both the diatom and opal analyses imply a shutoff of biogenic silica production suggesting surface water conditions affected by increased sea ice and or reduced nutrients in addition the younger dryas interval is marked by an increase in coarse sand and detrital carbonate a decrease in total organic carbon and foraminifera and high rates of sediment accumulation the inferred environment during the younger dryas is ice proximal in hu82 034 057 the foraminifera and other data suggest a change in conditions during the middle part of the younger dryas chron

from faulted blocks to megamullions and megaboudins tethyan heritage in the structure of the western alps along the grenoble pelvoux viso transect in the western alps we schematize the geometrical relationships between 1 ductile marly calcareous sedimentary covers and 2 their more rigid substratum this substratum may be the pre alpine granitic gneissic basement the triassic dolomites or even the jurassic ophiolites basement mullions a few kilometer size are characteristic of the external part of the western alps arc dauphine zone they strongly contrast with the dolomitic or ophiolitic boudins also a few kilometer size in the inner part of the arc piedmont zone the dauphine megamullions formed and then became accentuated during the oligocene and then the neogene tectonic phases which correspond to the last major alpine contractions at relatively shallow structural levels in the sedimentary cover upright or inclined folds were associated with slaty or strain slip cleavage development and low grade metamorphism the corresponding stretching directions locally subvertical trend normal to the regional fold and mega mullions axes and to the mountain belt itself the piedmont megaboudinage took place during the earlier major eocene alpine phase which but for some exeptions did not affect the dauphine zone in the very ductile schistes lustres that surround the dolomitic and ophiolitic megaboudins generalized intrafolial recumbent folding was associated with blueschist metamorphism the deformation characteristic of a deep structural level was a strong flattening within a subhorizontal plane and a preferential elongation parallel to the fold axes owing to regional horizontal festooning those axes trend normal oblique or even parallel to the piedmont zone i e to the incipient mountain belt restored initial trends coeval megaboudinage was probably omnidirectional chocolate tablet pattern the eocene piedmont structure has not been really obliterated during the later alpine deformations leading to megamullions development in the dauphine zone we summarize the very numerous stratigraphical sedimentological and tectonic field observations which lead us to regard each dauphine megamullion as directly inherited from a previous pre alpine faulted block each piedmont megaboudin has a similar origin the corresponding paleofaults are in fact normal and or strike slip faults of truly tethyan age their synsedimentary offset occurred either during the liassic middle jurassic passive margin rifting evolution of the southern edge of the european continent or during the late jurassic opening up of the ligurian oceanic tethys we analyze how those tethyan faults partly induced the style of the alpine collisional structures according to the regional characters of the successive synmetamorphic deformations i e to their localization in the western alps mountain belt

the mutual climatic range method of palaeoclimate reconstruction based on insect fossils new applications and interhemispheric comparisons abstract the mutual climatic range mcr method of palaeoclimate reconstruction has been employed in europe for the last decade a quantitative calibrated method mcr has many advantages over qualitative methods more recent applications deal with eastern and central north america and the method is also being developed for desert and arctic faunas the climate envelopes for north american beetles have been compiled using a 25 km gridded north american climate database that pairs climate parameters with modern collection sites modern tests of the reliability of the mcr method for north american species yielded similar results to prior european tests linear regressions of predicted on observed values yielded equations used to calibrate the mcr estimates work is under way to develop mcr estimates of mean annual precipitation for fossil assemblages from the desert southwest where moisture conditions may play a more important role in determining beetle species ranges an examination of british and north american mean july temperature reconstructions during the late wisconsinan glacial interval compares and contrasts three sets of records the north american records show no indication of the younger dryas cooling that is clearly marked in records from northwest europe the mcr method adds vigour to our reconstructions and allows us to compare between regions and with other palaeoenvironmental methods

diatom and vegetation responses to late glacial and early holocene climate changes at lake estanya southern pyrenees ne spain abstract we investigate lake estanya s diatom and pollen records from the late glacial lg to the early holocene eh in order to compare limnological and vegetation responses to common climate forcing the biotic changes recognized in this study largely agree with the hydrological evolution of the lake described previously for the same period the diatom record shows high sensitivity to fluctuations in both lake level and salinity concentration as a consequence of climate shifts in addition vegetation results indicate that the area could have played an important role as regional vegetation refuge shallow lake conditions during the last glacial maximum lgm were punctuated by relatively deeper freshwaters between 19 3 and 18 6 cal kyr bp and at 18 0 cal kyr bp as recorded by diatom shifts a subsequent increasing aridity trend coinciding with the mystery interval mi affected the diatom accumulation rates which dropped to its minimum values between 17 2 and 14 7 cal kyr bp particularly dry and cold conditions during the lgm and mi are supported by the largest values of steppic pollen taxa of the whole sequence which account for up to 40 however relatively high values of betula during the heinrich event 1 suggest a plausible regional vegetation refuge abrupt cooling and warming episodes within the lg triggered remarkable ecological threshold crossings in the diatom communities especially during the stadial interstadial episodes at this point the vegetation reflects the onset of warm conditions during the bolling allerod with the partial substitution of betula by marcescent and evergreen quercus which probably indicates the arrival of temperate taxa to the area and the likely migration of birch to higher altitudes the younger dryas stadial shows a complex ecological response diatoms are very poorly preserved but aquatic taxa reach their highest values an increase in marcescent quercus during this cold stage lends further support to the hypothesis that this is a regional vegetation refuge low lake levels recorded during the eh affected the development and preservation of diatom communities a delay in the onset of humid conditions for the eh is also supported by the vegetation composition characterized by the maximum expansion of juniperus

an investigation of rapid warm transitions during mis2 and mis3 using greenland ice core data and the climber 2 model in the search for a better understanding of the dominant mechanisms of the earth s climate system we present a study of the rapid warm climate transitions into the dansgaard oeschger events as seen in the ice cores from the greenland ice sheet we present a continuous o record from the grip core with a resolution of 5 yrs until 50 kyrs bp and 20 yrs until 100 kyrs bp these data are compared with other high resolution records i e the gisp2 chemistry record 25 yrs until 50 kyrs bp and the grip ca record 3 yrs until 100 kyrs bp all records have been transformed to the gisp2 meese sowers time scale the high resolution records are separated into interstadials and stadials defined by the grip and gisp2 o records we examine in detail the transitions into the dansgaard oeschger events and propose a scenario for the changes taking place in the different ice core records during the approximately 50 yrs the transitions takes the main difference from previous studies is in the much higher resolution data sets available until 50 kyrs bp previous high resolution studies by taylor and others 1997 have focused on only the yd pb transition

a calendar age estimate of the younger dryas holocene boundary at krakenes western norway the younger dryas holocene transition yd h in the sediments of krakenes lake western norway is well marked both lithologically and palaeobiologically at 756 5 cm in the investigated core a ser

a reevaluation of the evidence for a younger dryas climatic reversal in the tropical andes abstract a review of the evidence for a younger dryas equivalent climatic reversal in the tropical andes shows that a number of hypotheses need to be reassessed assumptions of the el abra cold and humid phase in colombia are compromised by insufficient dating control periods of glacier advance in peru and bolivia lack sufficient dating control to be correlated with the european younger dryas only glaciers on the quelccaya ice cap in peru may have advanced shortly before the onset of the european climatic reversal but even there a correlation is questionable a late glacial advance in the chimborazo area in ecuador is considerably older than previously suggested the same glacier advance has been dated to earlier than 12 000 bp in other areas of ecuador the emerging picture suggests two phases of glacier advances in the tropical andes 13 000 12 000 bp and 9000 10 000 bp both may have been caused by increasing precipitation unequivocal evidence for a cold younger dryas event in the tropical andes has yet to emerge

orbital and suborbital scale changes in the east asian summer monsoon since the last deglaciation the variability of the east asian summer monsoon easm has far reaching effects on the global climate system and the environment and full understanding of the variability and dynamics of the easm contributes to predictions of its future behavior here we present a well dated mineralogical and total organic carbon record from a saline inland lake in northern china which provides a robust archive of the easm evolution since 16 0 cal ka bp our record reveals a series of rapid and frequent millennial scale climatic fluctuations during the last deglaciation these fluctuations are documented by changes in the abundances of mirabilite bloedite and gypsum which appear to record the oldest dryas the bolling allerod warm period and the younger dryas the peak easm moisture occurred in the early and middle holocene which was punctuated by a prominent and abrupt weak monsoon interval that occurred synchronously with the 8 2 cal ka bp cold event this moisture maximum was terminated at 6 9 5 9 cal ka bp

evidence for an early glacial maximum in the french vosges during the last glacial cycle the last glacial maximum is observed to have occurred between 18 000 and 20 000 years ago throughout northern europe here we present evidence for the surprising result that in the region of the french vosges the regional ice sheet advanced during the early part of the last glacial cycle and this expansion was considerably larger than that which occurred during the last glacial maximum from these results we conclude that the glacial maximum may not have occurred at the same time worldwide contrary to what has usually been assumed1

the configuration sensitivity and rapid retreat of the late weichselian icelandic ice sheet abstract the fragmentary glacial geological record across the icelandic continental shelf has hampered reconstruction of the volume extent and chronology of the late weichselian ice sheet particularly in key offshore zones marine geophysical data collected over the last two decades reveal that the ice sheet likely attained a continental shelf break position in all sectors during the last glacial maximum though its precise timing and configuration remains largely unknown within this context we review the available empirical evidence and use a well constrained three dimensional thermomechanical model to investigate the drivers of an extensive late weichselian icelandic ice sheet its sensitivity to environmental forcing and phases of deglaciation our reconstruction attains the continental shelf break across all sectors with a total ice volume of 5 96 105 km3 with high precipitation rates being critical to forcing extensive ice sheet flow offshore due to its location astride an active mantle plume a relatively fast and dynamic ice sheet with a low aspect ratio is maintained our results reveal that once initial ice sheet retreat was triggered through climate warming at 21 8 ka bp marine deglaciation was rapid and accomplished in all sectors within c 5 ka at a mean rate of 71 gt of mass loss per year this rate of ice wastage is comparable to contemporary rates observed for the west antarctic ice sheet the ice sheet subsequently stabilised on shallow pinning points across the near shelf for two millennia but abrupt atmospheric warming during the bolling interstadial forced a second dramatic collapse of the ice sheet onshore with a net wastage of 221 gt a 1 over 750 years analogous to contemporary greenland rates of mass loss geothermal conditions impart a significant control on the ice sheet s transient response particularly during phases of rapid retreat insights from this study suggests that large sectors of contemporary ice sheets overlying geothermally active regions such as siple coast antarctica and ne greenland have the potential to experience rapid phases of mass loss and deglaciation once initial retreat is initiated

surface temperature in nw europe during the younger dryas agcm simulation compared with temperature reconstructions during the younger dryas yd the climate in nw europe returned to near glacial conditions to improve our understanding of climate variability during this cold interval we compare an agcm simulation of this climate performed with the echam model with temperature reconstructions for nw europe based on geological and paleoecological records maps for the mean winter summer and annual temperature are presented the simulated winters are consistent with reconstructions in the northern part of the study area a strong deviation is noted in ireland and england where the simulation is too warm by at least 10 c it appears that the n atlantic was cooler than prescribed in the yd simulation including a southward expansion of the sea ice margin the comparison for the summer shows a too warm continental europe in the simulation supposedly these anomalously warm conditions are caused by the agcm s response to the prescribed increased summer insolation the region of maximum summer cooling is similar in both the simulation and reconstruction i e s sweden we suggest that this is due to the local cooling effect of the scandinavian ice sheet compared to the present climate a considerable increase of the annual temperature range is inferred especially for regions close to the atlantic ocean

evidence from western north america for rapid shifts in climate during the last glacial maximum the estancia basin in southwestern united states contains evidence for strong and rapid pulsations in the supply of moisture brought into the region during the last ice age the pulses were recorded during episodes of stream discharge that spread plumes of fresh water laden with quartz sand over the saline lake the largest pulses in stream discharge lasted only a few decades were organized into cycles that were spaced approximately 200 to 250 and 2000 years apart and were of sufficient magnitude to freshen and maintain the lake at its maximum recorded elevation

tidal wetland record of holocene sea level movements and climate history abstract paleontological geochemical and lithological indicators of former marine and nonmarine influence are correlated between 13 cores from a tidal wetland wolfe glade on the southeast coast of delaware bay twenty new radiocarbon dates are used to establish the chronostratigraphy of marsh facies sediment iron content weight loss on ignition and remains of plants diatoms and foraminifera known to tolerate specific tidal inundation cycles are used to describe cored facies originating from supratidal intertidal and subtidal salt marsh subenvironments pollen assemblages are used to infer local holocene climate fluctuations and to reconstruct paleoenvironments during marsh evolution the record of paleoclimatology suggests the atlantic chron was warm with variable humidity in the region whereas the subboreal was dominated by cool and humid conditions the subatlantic was warmer and initially dryer with a cool humid phase prior to about 1 ka returning to warmer humid conditions in the last several centuries local relative sea level movements are characterized by five rapid short term episodes when the rate of sea level rise accelerated relative to the rate of marsh aggradation these episodes are recorded by transgressive facies contacts at 5 3 0 2 ka 4 4 0 2 ka 3 25 0 2 ka and 1 8 0 2 ka sidereal years an earlier transgressive facies contact dated 6 9 0 2 ka is probably not the product of a true sea level movement sea level movements are recorded throughout the marsh as palustrine or high marsh peats or peaty overlain by lower intertidal or subtidal marine deposits several features suggest that these episodes are local relative transgressions produced by short term accelerations in the rate of sea level rise relative to marsh aggradation the contemporaneity and apparent suddenness of marine inundations the sequence of facies indicating marine drowning the presence of similar events in marshes elsewhere in delaware bay and the marsh wide extent of indicative facies transitions we propose that the rapid and frequent sea level movements observed in wolfe glade are the result of surges and relaxations in the gulf stream and associated spin up and partial collapse of the north atlantic gyre in response to winds generated by changes in the north atlantic atmospheric thermal gradient associated with holocene climate fluctuations

sea ice as the glacial cycles climate switch role of seasonal and orbital forcing a box model of the coupled ocean atmosphere sea ice and land ice climate system is used to study glacial interglacial oscillations under seasonally and orbitally varying solar forcing the dominant 100 kyr oscillation in land ice volume has the familiar sawtooth shape of climate proxy records and to zeroth order it does not depend on the seasonal and milankovitch forcing the sea ice controls via its albedo and insulating effects the atmospheric moisture fluxes and precipitation that enable the land ice sheet growth this control and the rapid growth and melting of the sea ice allow the sea ice to rapidly switch the climate system from a growing ice sheet phase to a retreating ice sheet phase and to shape the oscillation s sawtooth structure a specific physical mechanism is proposed by which the insolation changes act as a pacemaker setting the phase of the oscillation by directly controlling summer melting of ice sheets this mechanism is shown to induce deglaciations during periods of lower summer insolation superimposed on the 100 kyr are the linear milankovitch forced frequencies of 19 23 and 41 kyr the transition from 41 kyr glacial cycles to 100 kyr cycles one million years ago may be explained as being due to the activation of the sea ice switch at that time this would be the case if sea ice extent was more limited during the warmer climate of the early pleistocene

vegetation response to climatic oscillations during the last glacial interglacial transition in northern japan abstract a high resolution decade time resolution analysis of pollen from ayameshitsugen mire 750 m a s l hokkaido japan focused on vegetation response to climate change during the last glacial interglacial transition the succession analyzed covers the 14 050 8450 cal bp interval pollen records indicate that the area around the study site was occupied by boreal betula picea abies forest during the late glacial period picea began to decrease at 11 600 cal bp and disappeared at 11 000 cal bp temperate forest elements began to increase about 100 years after the beginning of the decrease in picea changing pollen assemblages over the early holocene 11 600 8450 cal bp interval indicate that vegetation has not been in equilibrium with climate since the onset of holocene conditions higher abundance of ulmus and juglans than at present characterizes early holocene vegetation a distinct cooling event coinciding with the younger dryas stade in the region only occurred 12 700 12 450 cal bp the stade was less pronounced than in the north atlantic region and a gradual increase in picea and betula during the stade indicates gradual climatic amelioration in the region mire vegetation did not show a simultaneous response to the onset of holocene warming

evidence for seasonal low salinity surface waters in the gulf of mexico over the last 16 000 years oxygen isotopic analyses of individual orbulina universa from orca basin core en32 pc6 document the presence of low salinity surface waters in the northern gulf of mexico over the past 16 kyr isotopic data from an interval immediately following the younger dryas event indicate the rapid decrease in 18o values at the conclusion of the younger dryas was due to a year round return of meltwater to the gulf of mexico data indicate periodic or seasonal low salinity waters existed over the region of the orca basin prior to the initiation of the meltwater spike estimates suggest o universa grew its shell in salinities at least 4 5 below ambient since o universa may have calcified deep in the mixed layer during periods of low salinity surface salinities could have been even lower comparison of the average of individual o universa oxygen isotopic values with data from multiple shell samples of white gs ruber from the same core samples demonstrates that the two species record similar values during the late holocene in contrast o universa records lower oxygen isotopic values during the late glacial deglacial intervals possibly due to differences in seasonal distribution or shell ontogeny between the two species

reconstructing annual and seasonal climatic responses from volcanic events since a d 1270 as recorded in the deuterium signal from the greenland ice sheet project 2 ice core a 200 m ice core at the gisp 2 site summit greenland provides a 720 year record of volcanic eruptions identified by pronounced increases in so42 in relation to the background concentration deuterium hydrogen isotope ratios d measured in the same core provide a proxy climate record temperature used to examine the response of the climate system in greenland to these eruptions a group of 34 known volcanic events and a subgroup of the six largest volcanic events are isolated and examined for both groups significant negative cooler isotopic excursions occur beginning the year the volcanic event is detected in the ice core cooling is significant for 1 to 2 years the maximum isotopic excursion averages 4 0 7 c for all 34 events and 10 1 8 c for the subgroup of largest events for individual events the response in the isotope signal to volcanic events is highly variable but occurs at most 1 year before so42 is detected in the core examination of subannual isotope data suggests that peak winter isotope values are lowered more than peak summer values we hypothesize that this result is flawed by posted positional alteration of the isotopic profile by vapor diffusion to test this hypothesis we use numerical deconvolution to account for the smoothing effects of vapor diffusion in the firn analysis of the deconvolved data shows greater summer than winter cooling a result more consistent with instrumental temperature data

a fast and easy method to derive highest resolution time series datasets from drillcores and rock samples abstract we present a fast method to obtain highest resolution greyvalue datasets from digital core images these datasets are processed using a software package developed by us artificially introduced trends in the datasets are corrected disturbances i e cracks voids are eliminated to reduce the high level noise in data we have developed a vector filter that effectively removes such noise application of the method to odp drillcores from the santa barbara basin allowed exact correlation of two drillcores from the same basin the influence of the termination of the last glacial cold period on the flux rates in the basin are reflected in the greyvalue datasets we found a transitional period of about 3000 yr between the end of the last glacial cold period and the onset of post glacial climatic oscillation in the basin fourier analyses show that the high resolution greyvalue time series record climatic cycles with frequencies as high as 1 cycle yr varves

monitoring termination ii at high latitude anomalies in the planktic foraminiferal record microfaunal studies of planktic foraminifera carried out on 21 sediment cores from the norwegian greenland sea ngs reveal the spatial and lateral distribution as well as meltwater implication of today s non polar subpolar species beella megastoma earland previous findings are verified in that this foraminifera is characteristic only of the deglaciation phases of termination ii iii and vi and not the ensuing interglacial optima thus rendering this species a meltwater indicator its distribution is restricted to cores from the central i e more pelagic part of the ngs covering an area as far north as 77 latitude a detailed investigation of termination ii indicates that b megastoma first appeared in the southwest of the ngs at 131 ka and then about 6 kyr later in the eastern and northern parts of the ngs for the entire duration b megastoma always coincided with the deposition of distinct ice rafted detritus ird suggesting the presence of drifting icebergs during this period two different oceanographic models each with a two stepped evolution of the post saalian surface water circulation are proposed to account for this time transgressive character the mechanism of brine formation as possible oceanic phenomenon forcing atlantic water northwards is suggested for being responsible for the occurrence of b megastoma in the ngs during early termination ii the presence of b megastoma always ceased with the culmination of the interglacial optimum oxygen isotopic substage 5 51 eemian when the subpolar foraminiferal fauna reached highest abundances and a general lack of ird is observed

resolving the younger dryas event through borehole thermometry meltwater influxes may partly explain the low oxygen isotope values measured in the dye 3 and camp century ice cores this has led to speculation that greenland may not have cooled during the younger dryas and underlined the need for independent checks of the oxygen isotope record using optimal control methods and heat flow modeling the author makes a valiant but ultimately futile attempt to distinguish the younger dryas event in the ice sheet temperatures measured at dye 3 south greenland the author discusses the prospects for attempting the same in the new summit boreholes in central greenland how that will require more accurate temperature measurements a coupled thermo mechanical model and a refined uncertainty analysis he concludes by discussing how borehole temperature analysis may improve the climate histories determined from ice cores

the abrupt termination of the younger dryas climate event previous studies on two deep greenland ice cores have shown that a long series of climate oscillations characterized the late weichselian glaciation in the north atlantic region1 and that the last glacial cold period the younger dryas ended abruptly 10 700 years ago2 here we further focus on this epoch defining event and present detailed heavy isotope and dust concentration profiles which suggest that in less than 20 years the climate in the north atlantic region turned into a milder and less stormy regime as a consequence of a rapid retreat of the sea ice cover a warming of 7 c in south greenland was completed in about 50 years

a sediment record of cyclic bottom current variability from the northwest weddell sea cores and 3 5 khz sub bottom profiles collected from the continental slope and rise near 64 s 52 w off the tip of the antarctic peninsula reveal transport and depositional processes in a high energy environment a large channel runs downslope from the shelf break the remaining slope area is smooth and shows no acoustic penetration except locally in the lee of topography sediments on the slope and rise were mainly deposited by suspension from a 150 to 300 m thick nepheloid layer periodic resuspension and winnowing by bottom currents has produced coarse lags of ice rafted debris throughout these deposits and has also produced an elongated field of mudwaves running parallel to the base of the slope these low amplitude asymmetric features were formed by currents flowing from west to east parallel to the local slope short gravity cores show a compositional and textural cyclicity developed between two distinct lithologies intervals of poorly sorted sediment barren of fauna and with mud dominated size distributions are interpreted to result from a cold climate and low bottom current strength production of weddell sea bottom water probably lessened during these times and there was a greater duration of sea ice cover these sediments are interbedded with winnowed and better sorted deposits comprising fine sands and silts that contain abundant planktonic foraminifera interpreted as implying warmer climatic conditions and stronger current activity a varied calcareous benthic foraminiferal assemblage also indicates that these intervals lay above the ccd during deposition renewed and invigorated production of bottom water is implied contemporaneous with a shorter duration of sea ice cover petrographic evidence indicates an antarctic peninsula source throughout the cycles are considered to be the result of climatic fluctuations occurring over the timespan of glacial interstadials although confirmation of this timescale awaits acquisition of an absolute chronology

new evidence for episodic post glacial sea level rise central great barrier reef australia abstract we present an extensive database of 364 radiocarbon dates from coastal and marine sediments of the central great barrier reef gbr shelf of which 110 are previously unpublished the elevation data have been reduced to a common datum australian height datum ahd and the various sources of error have been assessed using modern lithological and biological relationships with sea level the elements of the radiocarbon database have been converted into sea level indicators the upper bound of the assembled dataset corresponds to a best estimate sea level curve and the full dataset provides a narrow envelope for sea level rise on the gbr shelf for the last 11 12 kyr not including hydro isostatic crustal flexing the envelope is consistent with episodic rise in post glacial sea levels the rising post glacial sea level probably included stillstands or minor falls at ca 45 m ahd at ca 10 5 kyr b p 5 m 7 8 kyr b p 2 m ca 6 kyr b p and 1 7 m 5 5 kyr b p there is evidence for a significant fall in sea level between stillstands at 11 m 8 5 kyr b p and 17 m 8 2 kyr b p stillstand durations apparently ranged between the vertical spread in the derived sea level data is very wide the use of shell material for dating seems unreliable and prone to large and unpredictable errors data from bulk mangrove muds appear reliable for determination of ancient sea level but may at times result in sea level being placed up to 4 m below the true level in situ biogenic carbonates such as preserved oyster beds and coral micro atolls are the most reliable indicators of sea level position while deposits of mangrove mud give a useful first order approximation of ancient sea levels caution should be used in drawing sea level curves from few data points we conclude that the post 11 12 kyr b p relative rise in sea level was episodic on the central gbr continental margin more data are required to define clearly sea level change up to ca 20 m at 9 kyr b p

glacial and climatic events in iceland reflecting regional north atlantic climatic shifts during the pleistocene holocene transition abstract this paper presents a summary of the evidence for glacial and climatic changes during the late pleistocene early holocene transition in iceland the deglaciation during the bolling allerod event was interrupted by a short lived older dryas glacial advance a biostratigraphical record from northern iceland shows significant climate warming in late allerod when mean july temperatures were at least as warm as those of today an abrupt cooling marked the beginning of the younger dryas event it was characterised by a cold and stable polar climate and an extensive glaciation before the postglacial warming of climate set in the icelandic paleoclimatic record is discussed in the light of climatic oscillations recorded from the grip ice core from the greenland inland ice and with reference to major shifts in the oceanic front systems recorded in the troll 8903 marine sediment core from the north sea the vedde ash gives a unique opportunity to address the chronological problems and correlate event stratigraphies of the different proxies it is concluded that the icelandic record of glacial and climatic changes during the late pleistocene early holocene transition largely reflects the climatic development in the north atlantic region

late pleistocene climatic events reflected in the caspian sea geological history based on drilling data abstract the analysis of seismic acoustic profiles and drilling data from the northern caspian showed the climatic events are quite distinguishable in the upper pleistocene sedimentary sequence the climatic events of the first half of late pleistocene mis 5 resulted in the onset of two warm water transgressive basins late khazarian and hyrcanian ones though cooler than at the late khazarian transgression the climate during the hyrcanian time was attributable to interglacial one as the glacial stage mis 4 approached its maximum under conditions of a dry and cold climate the hyrcanian sea basin regressed the atelian regression of the caspian sea corresponded to mis 4 stage and to the initial phases of the mis 3 interstadial warming the development of the global interstadial warming led to a considerable increase in the surface runoff from the catchment and resulted in the rising of the atelian lake level and the onset of the first phase of the khvalynian transgression the sea level rising was interrupted at the time of maximum cooling and aridization at mis 2 and resumed when the ice sheet was decaying the conspicuous climatic events known as warm phases of bolling and allerod promoted the ice sheet melting along with thawing of permafrost the latter having been widespread in the volga drainage basin all the above contributed to the khvalynian transgression the chocolate clays were accumulated in the volga estuary as well as in depressions of the pre khvalynian topography phases of a noticeable cooling known as the oldest older and younger dryas marked by a decrease in the runoff volume from the caspian drainage basin are correlated with regressive stages in the khvalynian basin history the khvalynian came to its end at the time of the first sharp warming that resulted in the rise of the caspian level and is generally taken as marking the pleistocene holocene boundary the mangyshlakian regression is dated to the holocene and was essentially a response of the caspian sea to the increase in the climate continentality during the boreal period

rapid switch like sea ice growth and land ice sea ice hysteresis 1 rapid and extensive growth of sea ice cover was suggested to play a major role in the sea ice switch mechanism for the glacial cycles as well as on shorter millennial scales gildor and tziperman 2000 this mechanism also predicts a hysteresis between sea ice and land ice such that land ice grows when sea ice cover is small and withdraws when sea ice cover is more extensive the switch like sea ice growth and the hysteresis were previously demonstrated using a simple highly idealized box model in this work we demonstrate a switch like sea ice behavior as well as the sea ice land ice hysteresis using a coupled climate model that is continuous in the latitudinal dimension it is shown that the switch like sea ice growth occurs when the initial meridional atmospheric temperature gradient is not too strong it is also shown that the meridional extent to which sea ice grows in a switch like manner is not affected by the intensity of the thermohaline circulation which does however influence the climate cooling that is needed to trigger such rapid sea ice growth

mccall glacier record of arctic climate change interpreting a northern alaska ice core with regional water isotopes abstract we explored modern precipitation and ice core isotope ratios to better understand both modern and paleo climate in the arctic paleoclimate reconstructions require an understanding of how modern synoptic climate influences proxies used in those reconstructions such as water isotopes therefore we measured periodic precipitation samples at toolik lake field station toolik in the northern foothills of the brooks range in the alaskan arctic to determine 18 o and 2 h we applied this multi decadal local precipitation 18 o temperature regression to 65 years of mccall glacier also in the brooks range ice core isotope measurements and found an increase in reconstructed temperatures over the late 20th and early 21st centuries we also show that the mccall glacier 18 o isotope record is negatively correlated with the winter bidecadal north pacific index npi climate oscillation mccall glacier deuterium excess d excess 2 h 8 18 o values display a bidecadal periodicity coherent with the npi and suggest shifts from more southwestern bering sea moisture sources with less sea ice lower d excess values to more northern arctic ocean moisture sources with more sea ice higher d excess values northern ice covered arctic ocean mccall glacier moisture sources are associated with weak aleutian low al circulation patterns and the southern moisture sources with strong al patterns ice core d excess values significantly decrease over the record coincident with warmer temperatures and a significant reduction in alaska sea ice concentration which suggests that ice free northern ocean waters are increasingly serving as terrestrial precipitation moisture sources a concept recently proposed by modeling studies and also present in greenland ice core d excess values during previous transitions to warm periods this study also shows the efficacy and importance of using ice cores from arctic valley glaciers in paleoclimate reconstructions

climate induced variations in productivity and planktonic ecosystem structure from the younger dryas to holocene in the cariaco basin venezuela a high resolution molecular organic geochemical study of sediments in the anoxic cariaco basin indicates significant changes in primary productivity and planktonic community structure associated with the transition from the younger dryas to the holocene variations in climate conditions over the past 12 14c kyr have induced large scale changes in upwelling intensity which directly affected levels of primary productivity as reflected in accumulation rates of bulk productivity proxies concentrations and accumulation rates of sterol and alkenone biomarkers have been used to identify how productivity changes affected the structure of the planktonic ecosystem a shift in the dominant primary producer from diatoms younger dryas to coccolithophores holocene is identified if productivity and ecosystem variations like those identified in the tropical upwelling zone of the cariaco basin region occur throughout the tropical oceans they have the potential to affect global climate through perturbations in the biogeochemical cycle of carbon

geoarchaeological and paleohydrological evidence for a clovis age drought in north america and its bearing on extinction abstract at the murray springs clovis site in southeastern arizona stratigraphic and geomorphic evidence indicates that an abnormally low water table 10 900 yr b p was followed soon thereafter by a water table rise accompanied by the deposition of an algal mat the black mat that buried mammoth tracks clovis artifacts and a well this water table fluctuation correlates with pluvial lake fluctuations in the great basin during and immediately following clovis occupation of that region many elements of pleistocene megafauna in north america became extinct during the dry period oxygen isotope records show a marked decrease in 18o correlated with the younger dryas cold dry event of northern europe which ended 10 750 yr b p essentially the same time as the water table began to rise in southeastern arizona clovis hunters may have found large game animals easier prey when concentrated at water holes and under stress if so both climate and human predation contributed to pleistocene extinction in america

abstract effects of bioturbation on reservoir sandstone porosity and permeability studies of outcrop samples from the upper cretaceous book cliffs utah

beryllium 7 and lead 210 in the atmosphere and surface snow over the greenland ice sheet in the summer of 1989 the concentrations of 7be and 210pb were measured in surface air and fresh and aging snow samples from summit 72 20 n 38 45 w and dye 3 65 10 n 44 45 w greenland during june and july 1989 the aerosol concentrations of these radionuclides showed rapid variations at both sites but were nearly twice as high on average at summit concentrations in the 16 fresh snowfall events that were sampled also showed wide variability but the averages were the same at the two sites the apparent difference in air snow fractionation and the lack of coherence in the concentration in air time series between the two sites indicate previously unsuspected complexity in atmospheric dynamics over the ice sheet improved understanding of atmospheric processes and how the results of those processes are recorded in snow and ice is crucial for full interpretation of the information about past atmospheric chemistry and climate contained in the snow and ice of glaciers around the world

increased ablation at the margin of the greenland ice sheet under a greenhouse effect climate increased ablation under a greenhouse effec t climate is calculated by an energy balance model for two sites at the margin of the greenland ice sheet nordbogletscher south greenland and qaman lrssop sermia west greenland the change in summer ablation is nearly linear with change in summer temperature with gradients of 0 43 and 0 57 m water a i deg i for nordbogletscher and qamanarssup sermia respectively however the increase in ablation rate must be less in the higher parts of the ice sheet a future climatic warming will therefore cause a rapid retreat of the ice sheet margin and a steeper ice sheet profile

the nature and timing of faunal change in the british isles across the pleistocene holocene transition the abrupt change in climate during the last deglaciation is reflected in the british isles by a transition from periglacial to temperate faunas in contrast to the coleopteran record terrestrial

evidence for global teleconnections in a late pleistocene speleothem record of water balance and vegetation change at sudwala cave south africa abstract understanding the global response to millennial scale climatic events is essential to our comprehension of climatic teleconnections and projection of future change however the extent and nature of their expression in areas of the southern hemisphere is often viewed as equivocal here we report uranium thorium dating of speleothem formations sampled at sudwala cave in the north eastern lowveld region of south africa s summer rainfall zone srz the growth intervals of multiple formations alongside a detailed chronology and multi proxy analysis of two periods of growth 40 35 ka and 13 8 12 8 ka in a stalagmite sc1 provide information regarding key fluctuations in the palaeoclimatic and palaeoenvironmental conditions during the late pleistocene high resolution stable isotope trace element and micro raman analysis are used alongside petrographic investigation to provide a detailed assessment of the climatic conditions associated with the onset and termination of growth in sc1 the combined raman and petrographic analysis represents a rare approach enabling the identification of aragonite calcite shifts both within and across growth intervals and diagenetic events potentially significantly influencing the recorded signal and often resulting in the major loss of chemical information consequently the identification of this post depositional chemical alteration could become a crucial prerequisite in speleothem palaeoclimatology particularly in areas prone to aragonite speleothem deposition susceptible to calcite conversion such as cave sites hosted by dolomitic karst systems the multiple proxies used in this study highlight the complex forcing relationships between climatically related environmental change and local cave conditions on speleothem precipitation contesting a common paradigm by associating drier conditions at sudwala with the initiation of speleothem growth the growth interval identified in stalagmite sc1 during the late deglaciation 13 85 12 79 ka coincides convincingly with both the southern hemispherically forced antarctic cold reversal 14 1 12 8 ka and the younger dryas 12 9 11 5 ka of northern hemispheric origin identifying southern africa as a vital location for the investigation of the hemispheric to global expression of the millennial scale fluctuations of the last deglaciation

total volume and temporal variation of meltwater from last glacial maximum inferred from sea level observations at barbados and tahiti abstract it is very important to determine the temporal variation of meltwater volume from the last glacial maximum lgm for studying climate changes and for constraining mantle rheology sea level observations have been obtained at barbados at the lgm and in late glacial phase and tahiti in late glacial phase and suggest that two events of rapid sea level rise existed at about 14 000 and 11 000 calendar years before present we examine the total meltwater volume and temporal variation of meltwater volume by comparing sea level observations and predictions at barbados and tahiti comparison between observations and predictions for barbados indicates that the total meltwater volume from the lgm is 103 117 m for eustatic sea level rise the predicted sea level variations at tahiti based on the ice model satisfying observations for barbados are however inconsistent with late glacial sea level observations at tahiti thus two events of rapid sea level rise are not supported from the comparison between observations and predictions at tahiti more in situ observational data are required for the period from the lgm to late glacial phase in order to study the relationship between rapid sea level rise and climate change

the holocene younger dryas transition recorded at summit greenland analysis of ice from dye 3 greenland has demonstrated that the transition between the younger dryas and holocene climate periods occurred over a 40 year period a near annually resolved multiparameter record of the transition recorded in the gisp2 core from summit greenland shows that most of the transition occurred in a series of steps with durations of about 5 years some climate proxies associated with mid latitude sources appear to have changed about 15 years before climate proxies associated with more northern regions changes in atmospheric water vapor are likely to have played a large role in the climate transition

evidence for younger dryas age cooling on the north pacific coast of america abstract a review of the palynological evidence from the olympic peninsula of washington state north to the alaska panhandle strongly supports the existence of a climatic oscillation similar in timing and effect to the younger dryas cooling 11 10 ka bp of europe and eastern north america the evidence includes many late glacial pollen peaks of mountain hemlock tsuga mertensiana an indicator of cool and moist climate reversals from forest to non arboreal vegetation and paleoclimate analysis using pollen climate transfer functions on the queen charlotte islands evidence of cooler ocean waters based on fossil foraminifera in cores from the continental shelf also supports an interpretation of a younger dryas age climatic reversal on the other hand geological evidence of glacier readvances during the younger dryas chronozone is weak and poorly dated although more and better constrained ams dates are needed to confirm the timing of the pacific northwest cold oscillation results so far point to maximum cooling and increased moisture between ca 10 7 10 ka bp followed by rapid warming in the early holocene additional late glacial sites need to be investigated in detail to confirm the geographical pattern of vegetation and climate change during this interval which is best expressed in hypermaritime and maritime climate regions similar to the younger dryas event around the north atlantic these results suggest that the search for causal mechanisms to drive the younger dryas cooling cannot be limited to events in the north atlantic region but should focus on possible hemispheric or global processes

paleoclimatic changes on the southern tibetan plateau over the past 19 000 years recorded in lake pumoyum co and their implications for the southwest monsoon evolution abstract the paleoclimatic changes in the southern region on the tibetan plateau tp after the lgm were studied by multi proxy analysis including pollen ca carbonates toc 13 c toc plant macrofossils and sands with sediment cores from lake pumoyum co close to the east end of the himalayas the following paleoclimatic changes characteristic of the southern tp were suggested by comparing with the paleoclimatic records from various regions within the southwest sw monsoon domain on the tp soon after the lgm deglaciation in the southern tp had already started at 18 5 ka bp and more actively increased after ca 16 5 ka bp with the hydrological circulation oscillating in ca 1200 2000 and 4000 year periods an abrupt increase in humidity under the reduced sw monsoon intensity occurred at 15 0 ka bp as part of further deglaciation processes subsequently the climate amelioration in the southern region started from 15 0 ka bp a few centuries earlier than other regions of the tp and then was further intensified by synchronizing with the bolling allored b a event over the b a period the sw monsoon intensity in the southern region almost continuously and sharply increased unlike many regions on the tp the southern region experienced almost no or substantially small impact of the younger dryas yd event and also other major dry cold reversals such as heinrich 1 event the 8 2 kyr event and an abrupt spell occurring between 5 and 4 ka bp the monsoon intensity in the southern region after the yd event started to rapidly increase from 11 4 ka bp and reached a maximum between 10 8 and 10 0 ka bp after the lgm which lagged about 1200 years behind the maximum insolation in the northern hemisphere from ca 9 6 ka bp the gradual decrease in humidity in the southern tp started and continued until 4 2 ka bp it s aridification trend was characterized by three stepwise transition observed at 9 6 7 7 and 5 5 ka bp however the aridification in the southern tp was rather weak and the climatic conditions were sustained fairly humid and comparably stable the humidity between 4 2 and 2 5 ka bp essentially increased despite the weakened sw monsoon intensity at that time whereas from 2 5 ka bp to the present the climatic conditions became most arid and instable during the holocene as a consequence the holocene climate optimum in the southern region lasted from 11 4 ka bp until 2 5 ka bp and was marked by the earliest start and the latest end timings within the sw monsoon domain on the tp the comparison of the start timings from various paleoclimatic records indicated that the first sw monsoon on the tp after the yd event generated at 11 4 ka bp in the southern region subsequently according to the degree of the recovery of positive vegetation albedo feedback system in each region the monsoon probably extended from the southern tp over the entire tp taking 3000 4000 years based on these results the possible key factors and mechanisms responsible for the paleoclimatic changes in the southern region and also the possible processes as to the evolution of the sw monsoon over the tp are discussed

the age and origin of the younger dryas climate event in greenland ice cores 230th 234u and 14c dating of barbados corals has extended the calibration of 14c years b p to calendar years b p beyond the 9200 year tree ring series bard et al 1990 this now permits the conversion of 14c chronozones which delimit major climate shifts in western europe to calendar years the younger dryas chronozone defined as 11 000 to 10 000 14c years b p corresponds to 13 000 to 11 700 calendar years b p this calibration affects the interpretation of an intensely studied example of the younger dryas climate event the 18o anomaly between 1785 and 1793 m in dye 3 ice core the end of the 18o anomaly in dye 3 ice core has been dated by measurements of 14c in air bubbles andree et al 1984 1986 and by annual layer counting hammer et al 1986 the older 14c dates fall out of the range of the tree ring calibration series but can now be calibrated to calendar years using the barbados 230th 234u calibration the 14ccorrected age for the end of the 18o event is 10 300 400 calendar years b p compared to the annual layer counting age of 10 720 150 years b p thus the younger dryas event in the dye 3 ice core ends in the preboreal chronozone 11 700 to 10 000 calendar years b p and is not correlative with the end of the younger dryas event identified in pollen records marking european vegetation changes the end of the dye 3 18o event is however correlative with the end of meltwater pulse ib fairbanks 1989 marking a period of intense deglaciation with meltwater discharge rates exceeding 13 000 km yr

water isotopes as tools to document oceanic sources of precipitation the isotopic composition of precipitation in deuterium oxygen 18 and oxygen 17 depends on the climatic conditions prevailing in the oceanic regions where it originates mainly the sea surface temperature and the relative humidity of air this dependency applies to present day precipitation but also to past records which are extracted for example from polar ice cores in turn coisotopic measurements of deuterium and oxygen 18 offer the possibility to retrieve information about the oceanic origin of modern precipitation as well as about past changes in sea surface temperature and relative humidity of air this interpretation of isotopic measurements has largely relied on simple rayleigh type isotopic models and is complemented by lagrangian back trajectory analysis of moisture sources it is now complemented by isotopic general circulation models igcm in which the origin of precipitation can be tagged we shortly review published results documenting this link between the oceanic sources of precipitation and their isotopic composition we then present experiments performed with two different igcms the giss model ii and the lmdz model we focus our study on marine water vapor and its contribution to precipitation over antarctica and over the andean region of south america we show how igcm experiments allow us to relate climatic conditions prevailing in the oceanic source of precipitation to its isotopic composition such experiments support at least qualitatively the current interpretation of ice core isotopic data in terms of changes in sea surface temperature additionally we discuss recent studies clearly showing the added value of oxygen 17 measurements key points link between the isotopic content of precipitation and its oceanic origin application of this method to polar ice cores andean precipitation potential of combining deuterium excess and oxygen 17 excess 2013 american geophysical union all rights reserved

east asian monsoon variation and climate changes in jeju island korea during the latest pleistocene to early holocene a 4 96 m long sediment core from the hanon paleo maar in jeju island korea was studied to investigate the paleoclimatic change and east asian monsoon variations during the latest pleistocene to early holocene 23 000 9000 cal yr bp high resolution toc content magnetic susceptibility and major element composition data indicate that jeju island experienced the coldest climate around 18 000 cal yr bp which corresponds to the last glacial maximum lgm further these multi proxy data show an abrupt shift in climatic regime from cold and arid to warm and humid conditions at around 14 000 cal yr bp which represents the commencement of the last major deglaciation after the last major deglaciation the toc content decreased from 13 300 to 12 000 cal yr bp and from 11 500 to 9800 cal yr bp thereby reflecting the weakening of the summer monsoon the lgm in jeju island occurred later in comparison with the chinese loess plateau such a disparity in climatic change events between central china and jeju island appears to be caused by the asynchrony between the coldest temperature event and the minimum precipitation event in central china and by the buffering effect of the pacific ocean

tree rings and ice cores reveal c 14 calibration uncertainties during the younger dryas the younger dryas interval during the last glacial termination was an abrupt return to glacial like conditions punctuating the transition to a warmer interglacial climate despite recent advances in the layer counting of ice core records of the termination the timing and length of the younger dryas remain controversial also a steep rise in the concentration of atmospheric radiocarbon at the onset of the interval recorded primarily in the cariaco basin has been difficult to reconcile with simulations of the younger dryas carbon cycle here we discuss a radiocarbon chronology from a tree ring record covering the late glacial period that has not been absolutely dated we correlate the chronology to ice core timescales using the common cosmic production signal in tree ring 14c and ice core 10be concentrations the results of this correlation suggest that the cariaco record may be biased by changes in the concentration of radiocarbon in the upper ocean during the early phase of the younger dryas climate reversal in the cariaco basin this bias in the marine record may also affect the accuracy of a widely used radiocarbon calibration curve over this interval our tree ring based radiocarbon record is easily reconciled with simulated production rates and carbon cycle changes associated with reduced ocean ventilation during the younger dryas attaching a floating tree ring chronology to ice core records that cover the abrupt younger dryas cold interval during the last glacial termination provides a better estimate of the onset and duration of the radiocarbon anomaly the chronology suggests that marine records may be biased by changes in the concentration of radiocarbon in the ocean which may affect the accuracy of a popular radiocarbon calibration program during this interval

a reinterpretation of scottish hummocky moraine and its significance for the deglaciation of the scottish highlands during the younger dryas or loch lomond stadial the aim of this paper is to demonstrate that much of the hummocky moraine present within the northern part of the lochlomond readvance ice cap formerly situated in the north west scottish highlands may be interpreted as suites of ice front moraines deposited during active decay these landforms can be used to reconstruct ice cap decay whichleads to important insights into the shrinking form of the ice cap and associated environmental conditions evidence has been collected from 10803 airphotographs and from detailed field survey it is presented at three spatial scales

millennial scale climate changes manifest milankovitch combination tones and hallstatt solar cycles in the devonian greenhouse world sub milankovitch rhythmic features in sedimentary records have been reported from throughout geological time however their origin remains enigmatic in particular during so called greenhouse periods in earth s history to better understand such short term climatic changes we sampled two 3 m thick intervals of early devonian hemipelagic carbonate at 1 cm resolution in the pod barrandovem section czech republic greenhouse conditions prevailed during early devonian times and the chosen resolution enables the detection of millennial scale climate change as recorded by elemental abundances we used a previously published astrochronology for the section to transform the studied series from the stratigraphic into the time domain spectral analysis of the time calibrated log transformed ti records reveal obliquity and precession cycles confirming the applied astrochronology additional spectral peaks with periods of 2 3 2 7 6 8 and 10 12 k y appear in both records furthermore a 1 5 k y periodicity close to the pleistocene dansgaard oeschger oscillation is also identified but only in the record with higher accumulation rate 3 5 cm k y bi coherence spectra reveal that the 6 8 and 10 12 k y periodic components are combination tones of milankovitch cycles we infer the shorter 2 5 k y periodicity to be the result of solar forcing related to the hallstatt cycle these new observations strengthen the case for an external origin of millennial scale features

younger dryas early holocene transition in the south eastern iberian peninsula insights from land snail shell middens the land snail sphincterochila candidissima from archeological records in villena se spain was studied isotopically to estimate the younger dryas yd early holocene transition in the western mediterranean live collected individuals exhibited body 21 8 1 6 and shell 5 8 1 4 d 13 c values typical of a c3 plant diet probably combined with carbonate ingestion calculations of a carbon flux balance mixing model suggest that living specimens experienced similar metabolic rates with comparable ratio of input and output fluxes of bicarbonate from the snail hemolymph all fossil shells showed comparable d 13 c values among each other but values were 2 higher than living specimens this may be explained by higher water stress at the yd holocene transition or by the suess effect shell d 18 o values averaged 1 3 0 8 for living individuals 0 5 0 8 for holocene 8 4 10 2 cal ka bp specimens and 0 4 0 6 for yd 12 0 12 4 cal ka bp snails an oxygen flux balance mixing model suggests that yd shells precipitated during relative humidity rh values of 79 82 after which rh increased gradually reaching maximum values of 87 88 at 8 4 8 6 cal ka bp and from there rh eventually declined to present values of 82 comparisons with other snail data suggest that the xerophilous sphincterochila records different environmental signatures fro other contemporaneous taxa copyright 2013 john wiley sons ltd

data model comparison of the younger dryas event the younger dryas cooling event is well established in the north atlantic region through numerous climate proxy records although the climatological controls vary from site to site it is considered to have taken place between about 10 000 and 11 000 bp radiocarbon years 11 500 13 000 calendar years ago outside the north atlantic region climate proxy records and chronology commonly become problematic because of weaker signals and less dating control in addition to this evidence oceanic records reveal conflicting evidence for younger dryas forcing mechanisms and the timing of events we compare proxy evidence with the results from an ocean general circulation model coupled to the energy moisture balance atmospheric model the model results reveal a global pattern and regional magnitude which generally agree with temperature changes interpreted from paleoclimate reconstructions the model also supports the general duration of global cooling of the younger dryas although proxy data can be controversial outside of the north atlantic region the authors believe that there is enough evidence to support the younger dryas event on a global scale they also recognize however that more concrete evidence is needed before the question can be unequivocally answered

paleoceanographic development in the sw barents sea during the late weichselian early holocene transition abstract the late weichselian early holocene variability of the north atlantic current has been studied with focus on the zonal component of this meridional transport during the transition from glacial to interglacial conditions the investigated sediment core is from 409 m water depth in the sw barents sea eight accelerator mass spectrometry ams 14c dates show that the core covers the last 20 000 cal yr b p with a centennial scale resolution during late weichselian early holocene planktic foraminiferal assemblages were analyzed using the 100 m size fraction and foraminiferal planktic and benthic 13c and 18o isotopes were measured furthermore a range of physical and chemical analyses has been carried out on the bulk sediment samples four time periods have been identified which represent the varying oceanographic conditions in ingoydjupet a glacial trough located off the north coast of norway in the sw barents sea 1 the late glacial before ca 15 000 cal yr b p influenced by the nearby ice sheets with high amounts of sea ice or iceberg transported detritus 2 the late oldest dryas stadial and the bolling allerod interstadial ca 15 000 12 700 cal yr b p with cold surface water conditions influenced by the collapse of the nearby ice sheets high amounts of sea ice or iceberg transported detritus and melt water and weak subsurface inflow of atlantic water 3 the younger dryas cold stadial 12 700 11 650 cal yr b p with low primary productivity and extensive sea ice cover and 4 the preboreal and early holocene 11 650 6800 cal yr b p cal yr b p with strong influx of atlantic water into the area near absence of ice rafted debris and generally ameliorated conditions in both surface and bottom water masses as seen from a high flux of foraminifera and increased marine primary production

retreat velocity of the north atlantic polar front during the last deglaciation determined by 14 c accelerator mass spectrometry the glacial oscillations which dominate the quaternary global climate are closely related to changes in incoming solar radiation but the details of palaeoclimatic spectra suggest that other parameters usually referred to as feedback have significantly increased the climatic response to extraterrestrial forcing1 the last deglaciation appears to be one of the more characteristic examples of nonlinearity because its abruptness strongly contrasts with the smooth shape of the associated solar radiation maximum dated at 11 000 yr bp in addition the presence of the younger dryas cold event concomitant with the solar radiation maximum could be interpreted as a transitory phenomenon caused by the rapid meltwater influx linked to the first phase of the ice sheet s disintegration2 3 here using accelerator mass spectrometry we date precisely the movements of the polar front during the whole period of glacial retreat 15 000 8 000 yr bp our results show that the polar front has moved between 35 n and 55 n in 1 000 yr for the earliest retreat 12 500 12 000 bp and moved almost instantaneously that is 400 yr during the two abrupt climatic changes associated with the younger dry as cold event

isotope signature of the younger dryas and two minor oscillations at gerzensee switzerland palaeoclimatic and palaeolimnologic interpretation based on bulk and biogenic carbonates oxygen and carbon isotope ratios in the carbonate of benthic ostracodes pseudocandona marchica and molluscs pisidium ssp were measured across the transitions bordering the younger dryas chronozone in littoral lacustrine cores from gerzensee switzerland the specific biogenic carbonate records confirm the major shifts already visible in the continuous bulk carbonate oxygen isotope record 18occ if corrected for their vital offsets oxygen isotope ratios of pisidium and juvenile p marchica both formed in summer are almost identical to 18occ this bulk carbonate is mainly composed of encrustations of benthic macrophythes chara ssp also mainly produced during summer adult p marchica which calcify in winter show consistently higher 18o larger shifts across both transitions and short positive excursions compared with the summer forms especially during early preboreal despite such complexity the 18o of adult p marchica probably reflects more accurately the variations of the 18o of former lake water because during winter calcification temperatures are less variable and the water column isotopically uniform the difference between normalised 18o of calcite precipitated in winter to that formed in summer can be used to estimate the minimum difference between summer and winter water temperatures in general the results indicate warmer summers during the late allerod and early preboreal compared with the younger dryas altogether the isotopic composition of lake water 18ol and of the dissolved inorganic carbonate 13cdic reconstructed from adult pseudocandona marchica as well as the seasonal water temperature contrasts indicate that the major shifts in the 18o of local precipitation at gerzensee were augmented by changes of the lake s water balance with relatively higher evaporative loss occurring during the allerod compared with the younger dryas it is possible that during the early preboreal the lake might even have been hydrologically closed for a short period we speculate that such hydrologic changes reflect a combination of varying evapotranspiration and a rearrangement of groundwater recharge during those climatic shifts

the oxygen and carbon isotopic signatures of biogenic carbonates in gerzensee switzerland during the rapid warming around 14 685 years bp and the following interstadial abstract the stable isotope signature of ostracods molluscs and charophyte remains from the late glacial section of a shallow core from lake gerzensee switzerland is analyzed along with the bulk carbonate isotope composition in a multi proxy study aiming to document the biotic responses to the first strong warming ca 14 6 ka ago after full glacial conditions the main goal of our contribution is to understand the climatic significance of the oxygen isotope variations in and between the different carbonate species and ideally provide a quantitative estimate of the oxygen isotopic composition of meteoric precipitation which then could be translated to mean temperature estimates corrected for the respective vital offsets the different carbonates show almost identical oxygen isotope ratios for the time preceding and after the rapid transition from greenland climate stages gs2 to gi1 indicating low and seasonally constant water temperatures at the sediment water interface for this period in the following the difference between cold season and warm season carbonates increases gradually pointing to a summer winter temperature difference of roughly 10 k at the end of gi1 we conclude that this gradual water temperature increase is independent of climate and is mainly due to sedimentation shallowing the sediment water interface eventually accentuated by a gradual decrease of gerzensee s water level during gi1 corrected for the isotope fractionation induced by the long term trend of such water temperature change the higher resolved 18o record from the bulk carbonates allows calculation of the presumed oxygen isotope ratio of former lake water 18ol which shows striking similarity to the record from ammersee introducing a tentative hydrological correction close to the present day offset between 18ol and the oxygen isotopes in meteoric precipitation 18op we propose a quantitative 18op and mean air temperature record for the gerzensee region

glaciation of scotland during the younger dryas stadial a review considerable research effort in recent years has refined our knowledge of the landforms and sedimentary sequences generated by glaciers of the last cold episode in scotland the younger dryas and the chronology of their formation these recent advances have benefited from new technologies such as high resolution digital terrain models numerical ice sheet simulations and cosmic ray surface exposure dating this review presents a summary of recent younger dryas research in scotland particularly focusing on studies from the last two decades the findings of these investigations are compared with those of earlier workers and critically are appraised in terms of their relevance to five key aspects of younger dryas glaciation growth of the ice cap its geometry dynamics mode of decay and likely stadial climate minor differences aside the consensus view appears to favour a relatively thick but areally restricted central ice cap surrounded by satellite ice fields at its centre the ice cap was probably thickest during the first 500 years of the stadial after which it most likely thinned as the climate became increasingly arid outlet glaciers were dynamic only in marginal areas where they produced abundant moraine sequences during deglaciation but throughout much of the stadial central areas of the ice cap effected only minimal landscape modification deglaciation took place by active retreat under a rapidly warming climate overall the stadial was dominated by much colder and drier conditions than present and probably experi enced greater intra annual temperature ranges copyright 2009 john wiley sons ltd