dust a diagnostic of the hydrologic cycle during the last glacial maximum dust concentrations in ice of the last glacial maximum lgm are high in ice cores from greenland and antarctica the magnitude of the enhancements can be explained if the strength of the hydrologic cycle during the lgm was about half of that at present this notion is consistent with a large decrease 5 c in ocean temperature during the lgm as recently deduced from measurements of strontium and calcium in corals

the paleoclimatic record provided by eolian deposition in the deep sea the geologic history of wind the mineral component of pelagic sediment is brought to the deep sea by transport in the wind extraction and analysis of this dust allows estimation of the past aridity of the eolian source region via flux determinations and of the intensity of the transporting winds from grain size data these two parameters the grain size and mass flux of dust vary independently there are three significant sources of modern dust eastern and central asia northwest africa and arabia all in the northern hemisphere as the rainfall associated with the intertropical convergence zone is an effective barrier to southerly transport of dust the northern hemisphere is an order of magnitude more dusty than is the southern an asymmetry that has characterized most of the cenozoic eolian flux data show that most of the northern hemisphere was more arid during glacial maxima with 3 to 5 times as much dust transported during glacial stages than during interglacials only northwestern south america varied in the opposite sense the periodicity of quaternary variation in both eolian flux and eolian grain size data is strongly influenced by the milankovitch cycles of orbital variability wind intensities vary on a shorter time scale than the general 100 kyr cycles of glaciation and general aridity eolian grain sizes display forcing at precessional 19 and 23 kyr tilt 41 kyr and at approximately 30 kyr periodicities as a result the generalization that winds are uniformly stronger during glacial times is not valid whole cenozoic records show that the largest change in dust flux an order of magnitude increase occurred in the northern hemisphere and reflects continental drying associated with the late pliocene onset of northern hemisphere glaciation southern hemisphere eolian records show no sign of paleoclimatic changes in the late pliocene the most important change in cenozoic atmospheric circulation was a severalfold reduction in wind intensity that occurred at the time of the paleocene eocene boundary before then latest cretaceous and paleocene winds were essentially as strong as those of the late cenozoic this shift appears to be one of several climatic responses to a change in the nature and amount of global heat transport at about 55 ma

late glacial stage and holocene tropical ice core records from huascaran peru two ice cores from the col of huascaran in the north central andes of peru contain a paleoclimatic history extending well into the wisconsinan wurm glacial stage and include evidence of the younger dryas cool phase glacial stage conditions at high elevations in the tropics appear to have been as much as 8 to 12 c cooler than today the atmosphere contained about 200 times as much dust and the amazon basin forest cover may have been much less extensive differences in both the oxygen isotope ratio 18o 8 per mil and the deuterium excess 4 5 per mil from the late glacial stage to the holocene are comparable with polar ice core records these data imply that the tropical atlantic was possibly 5 to 6 c cooler during the late glacial stage that the climate was warmest from 8400 to 5200 years before present and that it cooled gradually culminating with the little ice age 200 to 500 years before present a strong warming has dominated the last two centuries

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

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

mineralogy of atmospheric microparticles deposited along the greenland ice core project ice core the mineralogical composition of atmospheric dust particles changes along the greenland ice core project grip ice core in phase with other paleoatmospheric records showing that climatic changes affect the location the relative extent and the surface characteristics of the dust source areas fifty grip ice samples from the eemian to the last glacial maximum were analyzed using a scanning electron microscope coupled with x ray energy dispersive system microprobe more than 20 mineral types or groups were identified and their relative abundance was evaluated a different mineralogical composition was systematically observed in cold and warm time periods quartz illite chlorite micas and feldspars prevail in the mineral assemblage associated with cold stages or phases marine isotope stage mis 4 mis 2 cold part of mis 3 fast variations while kaolinite and fe hydr oxides dominate in warm episodes mis 5e5 warm parts of fast variations the first dust mineral assemblage is related mainly to the mechanical weathering prevailing in arid and cold mid high latitude areas while the second dust mineral assemblage is related mainly to deep chemical weathering in warm and humid low latitude areas k c and o q ratios may be assumed as indicative of the relative importance of low latitude versus arid and mid high latitude source areas it is worthwhile to mention that the first warmer part of eemian 5e5 differs in dust mineralogy from the second part 5e4 5e1 and that also the fast variations are accompanied by clear mineralogical changes of the dust microparticles the changes of the atmospheric dust mineral background are possibly related to changes of the atmosphere dynamics meridian pressure gradient polar front shift storm track changes etc and or to changes in the relative extent of the deflation source areas

major features and forcing of high latitude northern hemisphere atmospheric circulation using a 110 000 year long glaciochemical series the greenland ice sheet project 2 glaciochemical series sodium potassium ammonium calcium magnesium sulfate nitrate and chloride provides a unique view of the chemistry of the atmosphere and the history of atmospheric circulation over both the high latitudes and mid low latitudes of the northern hemisphere interpretation of this record reveals a diverse array of environmental signatures that include the documentation of anthropogenically derived pollutants volcanic and biomass burning events storminess over marine surfaces continental aridity and biogenic source strength plus information related to the controls on both high and low frequency climate events of the last 110 000 years climate forcings investigated include changes in insolation of the order of the major orbital cycles that control the long term behavior of atmospheric circulation patterns through changes in ice volume sea level events such as the heinrich events massive discharges of icebergs first identified in the marine record that are found to operate on a 6100 year cycle due largely to the lagged response of ice sheets to changes in insolation and consequent glacier dynamics and rapid climate change events massive reorganizations of atmospheric circulation that are demonstrated to operate on 1450 year cycles changes in insolation and associated positive feedbacks related to ice sheets may assist in explaining favorable time periods and controls on the amplitude of massive rapid climate change events explanation for the exact timing and global synchroneity of these events is however more complicated preliminary evidence points to possible solar variability climate associations for these events and perhaps others that are embedded in our ice core derived atmospheric circulation records

relationships between structural units in the tormes gneiss dome nw iberian massif spain geometry structure and kinematics of contractional and extensional variscan deformation situated in the inner zone of the iberian massif the tormes gneiss dome is composed of two units with different lithological contents and metamorphic evolution the upper unit consists of a thick sequence of low to high grade metasediments ranging in age from late proterozoic to silurian the lower unit is a high grade metamorphic complex composed mostly of granitic orthogneisses and minor amounts of metasediments four variscan deformations are distinguished at deep structural levels the most prominent d1 ductile structures are recumbent anticlines with ne vergence cored by orthogneisses and separated by narrow synclines these recumbent folds grade upward into less flattened and ne vergent steeper structures the overall structure is that of a large scale stacking of orthogneissic slices underlying a shortened and thickened sedimentary sequence that formed a huge orogenic wedge in this region during the heterogeneous and ductile d2 deformation the rheological behaviour of the orthogneisses and metasediments became similar the vertical d2 shortening associated with a strong top to the se shearing in a large scale subhorizontal shear zone folded the prior sw dipping structures developing sw vergent folds with axes close to nw se l2 mineral and stretching lineations d2 corresponds to post collisional crustal thinning following d1 crustal thickening the d3 and d4 late structures are much more localized and occurred under retrograde conditions but have a significant effect on the final geometry of the metamorphic complex this sequence of contractional and extensional deformative events permits a tectonic interpretation in the framework of the dynamic wedge theory based on the evolution in the time of the stress configuration applied to a portion of the crust

ice age paleotopography a gravitationally self consistent theory of postglacial relative sea level change is used to infer the variation of surface ice and water cover since the last glacial maximum lgm the results show that lgm ice volume was approximately 35 percent lower than suggested by the climap reconstruction and the maximum heights of the main laurentian and fennoscandian ice complexes are inferred to have been commensurately lower with respect to sea level use of these ice age boundary conditions in atmospheric general circulation models will yield climates that differ significantly from those previously inferred on the basis of the climap data set

deposits and soils of the past 130 000 years at the desert loess transition in northern china the desert loess transitional zone in north central china has long been thought sensitive to quaternary climatic change however reconstruction of quaternary climates in this area has been hindered by incompleteness of geological sections here we report the analytical results of two recently found sand loess soil sections both sections have thick eolian deposits from the last interglacial glacial cycle and can be correlated with one another field observations thermoluminescence dating and other laboratory analyses show that the last interglacial period produced three paleosols and two intercalated loess layers loess from the last glacial period is interbedded with three sand horizons that represent desert extension the expansion and contraction of desert in northern china may have been forced by the east asia monsoon

timescales for dust variability in the greenland ice core project grip ice core in the last 100 000 years the calcium representing dust concentration record of the last 100 000 years from the greenland ice core project grip ice core shows a huge dynamic range factor 100 the relationship between dust concentrations and temperature represented by the oxygen isotope ratio is not a simple one as has often been assumed a rapid alternation factor of 5 10 between low concentrations during the dansgaard oeschger interstadial periods and high levels in colder periods is superimposed on a long term trend encompassing a further factor of 5 10 within climate periods there is only a very weak relationship between ca concentration and temperature previous authors biscaye et al 1997 have suggested that the most likely source for the increased dust is eastern asia for the first time we consider each possible cause of both rapid and slow increases from source to deposition we suggest that to account for the size and rapidity of the fast changes significantly higher wind speed in the source area is required although changes in atmospheric residence time could also play a role for the slower long term variability changes in transport speed or possibly route are probably also involved changes in the size of the source area could give some change on longer time periods the probable importance of changes in source area wind speed almost simultaneous with greenland temperature changes confirms that climatic parameters in high and low latitudes were strongly coupled through the atmosphere during glacial climatic changes this adds to evidence that the atmospheric circulation system underwent almost instantaneous large scale changes during the last glacial period

the size distribution of microparticles from selected segments of the greenland ice core project ice core representing different climatic periods the size distribution of insoluble microparticles dust in the particle size interval 0 4 6 0 m radius has been measured in more than 1400 samples from the greenland ice core project grip deep ice core from summit greenland the samples were taken from ice core segments representing 32 climatic periods including ice from the eem interglacial and ice from below the eemian ice the mean dust volume distributions from the climatic periods are compared and the relationship of total dust mass to the concentration of ca2 and to the stable isotopic composition 18o is investigated the dust volume distributions are found to be lognormal in the size interval 0 4 2 0 m with nearly identical shapes changes of the lognormal part of the volume distributions are found to be connected to the total dust mass in a systematic way the total dust mass is correlated to the ca2 concentration and data do not show any enrichment in ca2 from exposed continental shelves due to lower sea levels during the last glacial maximum the total dust mass is strongly connected to 18o the volume distribution of the particles in the size interval 2 0 6 0 m is found to be almost the same in most periods the exceptions are the cold periods in the last part of the last glacial period where the volume of these particles is higher than in most periods and the periods from the eemian and just above the eemian where the volume of these particles is lower the volume distributions of both the eemian warm periods and the cold eemian events are different from the distributions in ice from both below and above the eemian ice climate changes appear to have modified the processes of production transport modification and deposition of the dust aerosol in the same way over the last 120 000 years or more

greenland ice core signal characteristics an expanded view of climate change the last millenium of earth history is of particular interest because it documents the environmental complexities of both natural variability and anthropogenic activity we have analyzed the major ions contained in the greenland ice sheet project 2 gisp 2 ice core from the present to 674 a d to yield an environmental reconstruction for this period that includes a description of nitrogen and sulfur cycling volcanic emissions sea salt and terrestrial influences we have adapted and extended mathematical procedures for extracting sporadic e g volcanic events secular trends and periodicities found in the data sets finally by not assuming that periodic components signals were stationary and by utilizing evolutionary spectral analysis we were able to reveal periodic processes in the climate system which change in frequency turn on and turn off with other climate transitions such as that between the little ice age and the medieval warm period

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

the gisp2 δ18o climate record of the past 16 500 years and the role of the sun ocean and volcanoes measured 18o 16o ratios from the greenland ice sheet project 2 gisp2 ice core extending back to 16 500 cal yr b p provide a continuous record of climate change since the last glaciation high resolution annual 18o 16o results were obtained for most of the current millennium a d 818 1985 and record the medieval warm period the little ice age and a distinct 11 yr 18o 16o cycle volcanic aerosols depress central greenland annual temperature 1 5 c maximally and annual 18o 16o for about 4 yr after each major eruptive event on a bidecadal to millennial time scale the contribution of solar variability to holocene greenlandic temperature change is 0 4 c the role of thermohaline circulation change on climate problematic during the holocene is more distinct for the 16 500 10 000 cal yr b p interval analogous to 14c age calibration terminology we express time in calibrated cal yr b p a d 1950 0 cal yr b p the oldest dryas bolling older dryas allerod younger dryas sequence appears in great detail bidecadal variance in 18o 16o but not necessarily in temperature is enhanced during the last phase of lateglacial time and the younger dryas interval suggesting switches of air mass transport between jet stream branches the branched system is nearly instantaneously replaced at the beginning of the bolling and holocene at 14 670 and 11 650 cal yr b p respectively by an atmospheric circulation system in which 18o 16o and annual accumulation initially track each other closely thermodynamic considerations of the accumulation rate temperature relationship can be used to evaluate the 18 o 16o temperature relationship the gisp2 ice layer count years of major gisp2 climate transitions also support the use of coral 14c ages for age calibration

paleoclimatic variability inferred from the spectral analysis of greenland and antarctic ice core data paleoclimate variations occur at various time scales between a few centuries for the heinrich events and several hundreds of millenia for the glacial to interglacial variations the recent ice cores from greenland greenland ice core project and greenland ice sheet project 2 and antarctica vostok span at least one glacial oscillation and provide many opportunities to investigate climate variations with a very fine resolution the joint study of cores from both hemispheres allows us to distinguish between the sources of variability and helps to propose mechanisms of variations for the different time scales involved the climate proxies we analyze are inferred from 18o and d for temperature and chemical species such as calcium for the joint behavior of the major ions in the atmosphere which yield an estimate of the polar circulation index those data provide time series of climatic variables from which we extract the information on the dynamics of the underlying system we used several independent spectral analysis techniques to reduce the possibility of spurious results those methods encompass the multitaper spectral analysis singular spectrum analysis maximum entropy method principal component analysis minimum bias spectral estimates and digital filter reconstructions our results show some differences between the two hemispheres in the slow variability associated with the astronomical forcing common features found in the three ice core records occur on shorter periods between 1 and 7 kyr the holocene also shows recurrent common patterns between greenland and antarctica we propose and discuss mechanisms to explain such behavior

anatomy of a dansgaard oeschger warming transition high resolution analysis of the north greenland ice core project ice core large and abrupt temperature oscillations during the last glacial period known as dansgaard oeschger do events are clearly observed in the greenland ice core record here we present a new high resolution chemical 2 mm and stable isotope 20 mm record from the north greenland ice core project ngrip ice core at the onset of one of the most prominent do events of the last glacial do 8 observed 38 000 years ago the unique subannual resolution ngrip record provides a true sequence of change during a do warming with detailed annual layer counting of very high depth resolution geochemical measurements used to determine the exact duration of the transition the continental ions indicative of long range atmospheric loading and dustiness from east asia are the first to change followed by the snow accumulation the moisture source conditions and finally the atmospheric temperature in greenland the sequence of events shows that atmospheric and oceanic source and circulation changes preceded the do warming by several years

simulated airborne particle size distributions over greenland during last glacial maximum polar ice cores indicate that the deposition of dust from the atmosphere was strongly enhanced during last glacial maximum lgm the concentration of dust in the ice sheets and in the overlaying atmosphere are not proportional to each other but are dependent among other things on the relative magnitudes of dry and wet deposition which change with climate observed dust particle size distributions in the greenland ice sheet are shifted toward larger particles during lgm by applying common theories for particle removal processes we show that the airborne particle size distributions over greenland probably remained the same in the two different climates this leads to the conclusion that the airborne dust concentration was even higher during lgm than indicated by the enhancement in deposition flux we suggest a lgm pre industrial current climate aerosol ratio including the soluble fraction over greenland of about 90 125 by mass and 75 100 by number

the role of burial diagenesis in hydrocarbon destruction and h2s accumulation upper jurassic smackover formation black creek field mississippi organic inorganic interactions during burial of the smackover formation at black creek field mississippi have resulted in nearly complete destruction of hydrocarbons the formation has been buried to a depth of 6 km has experienced temperatures of over 200 degrees c and presently contains 78 h 2 s 20 co 2 and 2 ch 4 three distinct stages of burial diagenesis correspond to three phases of organic matter maturation pre oil window diagenesis was dominated by precipitation of prebitumen calcite cement diagenesis in the oil window was characterized by precipitation of saddle dolomite and anhydrite in water filled layers and by formation of solid bitumen in the oil column diagenesis in the gas window was dominated by thermochemical sulfate reduction tsr resulting in hydrocarbon destruction anhydrite dissolution large amounts of h 2 s co 2 and s generation and postbitumen calcite cementation during tsr anhydrite reacted with h 2 s to produce s which in turn reacted with ch 4 to generate more h 2 s in a self reinforcing cycle the lack of metal cations to stabilize h 2 s as metal sulfides availability of sufficient sulfate to generate h 2 s and a closed system to prevent h 2 s from escaping resulted in the continuation of the tsr cycle until nearly all hydrocarbons were consumed in mississippi concentration of h 2 s is nearly zero in smackover hydrocarbon reservoirs that have experienced temperatures of 120 degrees c for more than 50 m y suggesting that tsr is not a kinetic time dependent process high h 2 s concentrations initiate at temperatures above 140 degrees c and increase with temperature indicating that tsr is a thermodynamic phenomenon reported high h 2 s concentrations at low temperatures 80 120 degrees c from other locations may be explained by the following processes 1 migration of h 2 s into these reservoirs 2 high geothermal gradients or local thermal perturbations in the past 3 a biochemical origin for the h 2 s or 4 exposure of these reservoirs to temperatures greater than 150 degrees c and a rapid uplift in black creek field burial cementation and pressure solution resulted in total destruction of porosity and permeability in limestone reservoirs but not in dolomite reservoirs which still possess up to 20 porosity and 100 md permeability secondary porosity was not created as a result of hydrocarbon migration abundant co 2 derived during hydrocarbon destruction resulted in calcite cementation rather than carbonate dissolution late secondary porosity development in carbonates may be related to acids generated by metal sulfide precipitation

dust sources and deposition during the last glacial maximum and current climate a comparison of model results with paleodata from ice cores and marine sediments mineral dust aerosols in the atmosphere have the potential to affect the global climate by influencing the radiative balance of the atmosphere and the supply of micronutrients to the ocean ice and marine sediment cores indicate that dust deposition from the atmosphere was at some locations 2 20 times greater during glacial periods raising the possibility that mineral aerosols might have contributed to climate change on glacial interglacial time scales to address this question we have used linked terrestrial biosphere dust source and atmospheric transport models to simulate the dust cycle in the atmosphere for current and last glacial maximum lgm climates we obtain a 2 5 fold higher dust loading in the entire atmosphere and a twenty fold higher loading in high latitudes in lgm relative to present comparisons to a compilation of atmospheric dust deposition flux estimates for lgm and present in marine sediment and ice cores show that the simulated flux ratios are broadly in agreement with observations differences suggest where further improvements in the simple dust model could be made the simulated increase in high latitude dustiness depends on the expansion of unvegetated areas especially in the high latitudes and in central asia caused by a combination of increased aridity and low atmospheric co2 the existence of these dust source areas at the lgm is supported by pollen data and loess distribution in the northern continents these results point to a role for vegetation feedbacks including climate effects and physiological effects of low co2 in modulating the atmospheric distribution of dust

air mass trajectories to summit greenland a 44 year climatology and some episodic events the seasonal variation in atmospheric transport patterns to summit greenland is examined using a 44 year record of daily 10 day isobaric back trajectories at the 500 hpa level over 24 000 modeled trajectories are aggregated into distinct patterns using cluster analysis ten day trajectories reaching summit are longest during winter with 67 extending upwind westward as far back as asia or europe trajectories are shortest during summer with 46 having 10 day origins over north america during all seasons a small percentage 3 7 of trajectories originate in west asia europe and follow a meridional path over the arctic ocean before approaching summit from the northwest trajectories at the 700 hpa level tend to be shorter than at 500 hpa with many of the 700 hpa trajectories from north america tracking over the north atlantic and approaching summit from the south the long range transport climatology for summit is similar to a year round climatology prepared for dye 3 located 900 km to the south davidson et al 1993b an analysis of several aerosol species measured at summit during summer 1994 reveals examples of the usefulness and also the limitations of using long range air trajectories to interpret chemical data

continuous dust concentration profile of pre holocene ice from the greenland ice sheet project 2 ice core dust stadials interstadials and the eemian in this paper we present and discuss the pre holocene dust profile of the greenland ice sheet project 2 gisp2 ice core retrieved from central greenland measured using laser light scattering lls from ice and compare it with the electrical conductivity method and oxygen isotope profiles we describe how we dated the pre holocene ice using the annual dust peaks observed with lls starting from the accepted chronology to 2250 m and obtained a chronology in substantial accord with the gas age dating of the ice we also show that the gisp2 ice core penetrated the eemian and that a record of a 10 2 kyr penultimate glaciation is present in the gisp2 core

asian provenance of glacial dust stage 2 in the greenland ice sheet project 2 ice core summit greenland samples of dust from the greenland ice sheet project 2 gisp2 ice core summit greenland dated within marine isotope stage 2 between 23 340 and 26 180 calendar years b p around the time of the coldest local last glacial temperatures have been analyzed to determine their provenance to accomplish this we have compared them with approximately coeval aeolian sediments mostly loesses sampled in possible source areas psas from around the northern hemisphere the 5 m grain size fraction of these samples was analyzed on the basis that it corresponds to the atmospheric dust component of that time and locale which was sufficiently fine grained to be transported over long distances on the basis of comparison of the clay mineralogy and sr nd and pb isotope composition with ice dust and psas and assuming that we have sampled the most important psas we have determined that the probable source area of these gisp2 dusts was in eastern asia the dust was not derived from either the midcontinental united states or the sahara two more proximal areas that have been suggested as potential sources based on atmospheric circulation modeling except for a brief period during an interstadial when dust transport was exceptionally low for glacial times and had a mineralogical composition indicative of a slightly more southern provenance the source area of the dust did not change significantly during times of variably higher fluxes of dust with larger mean grain size or lower fluxes of dust with smaller mean grain size this includes the high dust period that correlates with the heinrich 2 period of major iceberg discharge into the north atlantic variable wind strengths must therefore be invoked to account for these abrupt and significant changes in dust flux and grain size

the mineral dust aerosol cycle during the last glacial maximum the canadian centre for climate modelling and analysis general circulation model gcm is used to investigate the factors influencing the mineral dust distribution at the time of the last glacial maximum lgm simulated deposition rates are used in conjunction with existing paleoclimate data to infer likely characteristics of the mineral dust conditions at that time and identify contributions to the pattern of enhanced dustiness of the lgm relative to the present indicated by ice core and loess data maximal consistency between the gcm the climate long range investigation mapping and prediction climap data set and the ice core data requires approximately a 15 to 30 fold increase in dust production globally during the last glacial maximum relative to the preindustrial holocene in particular regional 10 to 50 fold and 20 fold lgm increases are indicated in eastern asia and south america respectively these implied source enhancements are quite insensitive to the details of the dust model analysis of the contributing factors suggests an increase in fine particle availability in the source regions

230th 234u and 36cl dating of evaporite deposits from the western qaidam basin china implications for glacial period dust export from central asia the western qaidam basin contains numerous hydrologically closed lakes and playas we have measured uranium thorium and chlorine radioisotopes in sediments from drill cores at two of these gasikule lake and dalangtan dry playa in order to date cycles of high and low lake level the sediment chronologies from the two sub basins are generally concordant and the u th ages indicate transitions from high to low lake levels at 302 56 ka at 138 6 ka and at 16 3 2 2 ka these ages indicate that high lake stands in the qaidam basin were terminated at the end of continental glacial maxima this result is unexpected because previously the large eolian dust fluxes out of central asia during glacial maxima have been considered to be associated with increased aridity in the region not increased humidity the lacustrine evidence combined with previous data from ice cores suggests instead that the persistence of strong winds may have played a more important role in enhanced dust fluxes than did variations in dust availability due to changes in aridity

lake evolution in the tengger desert northwestern china during the last 40 000 years climatic changes inferred from lacustrine deposits and lake level fluctuations in northwestern and central china are mainly based on paleoclimatic records from the tibetan plateau while there is still a lack of data relating to the semiarid arid desert regions of inner mongolia in the tengger desert different paleolake levels at baijian hu are documented by six paleoshorelines and stratified lake carbonates the highest lake level occurred ca 39 000 14c yr b p and prevailed over about 16 000 km2 from sediment structure geochemical composition and ostracods we infer humid cool conditions until 23 000 14c yr b p in the northern badanjilin desert at gaxan nur sogu nur high lake levels can be deduced from mollusc bearing paleobeaches and lake carbonates which have been dated to 34 000 14c yr b p and indicate a lake that covered some 32 000 km2 after ca 20 000 yr b p the climate became dry with increased eolian activity and decreasing lake levels reestablishment of wet conditions occurred ca 13 000 yr b p the holocene is represented by stratified lake deposits that alternate with fluvial and eolian deposits indicating a longterm oscillating trend toward arid conditions the existence of widespread freshwater lakes during the late pleistocene indicates a semihumid climate without an accompanying fall in temperature to arctic conditions

late pleistocene and holocene aeolian dust deposition in north china and the northwest pacific ocean abstract rates of dust deposition in the north pacific and the central loess plateau of china show a poor correlation during the last 30 000 yr although in both cases the dust was derived from deserts in northern china loess accumulation rates were high at the time of the last glacial maximum while dust flux to the north pacific was low at this time maximum dust flux to the north pacific is recorded in the mid holocene but holocene rates of loess accumulation have been low these differences are shown to reflect the different nature of the wind conditions responsible for dust transport to the loess plateau and the north pacific dust transport responsible for formation of the loess occurred mainly in the lower 1 5 km of the atmosphere and was accomplished by cold low level northwesterly winds associated with cold waves emanating from the mongolian anticyclone at present such conditions occur quite frequently in winter but during the coldest phases of the pleistocene they probably also occurred in the spring and summer long range dust transport from northern china to the north pacific requires that the dust is lifted to high levels above 3 km where it can be incorporated into the strong mid latitude westerlies strong vertical motions of the atmosphere occur most frequently in the spring when the mongolian anticyclone begins to weaken and frontal depressions move across the deserts of northern china such frontal depressions were probably more frequent during the climatic optimum of the mid holocene and during previous interglacials changes in continental aridity have probably played a minor role in controlling the magnitude of the dust flux to the north pacific

a compilation of western european terrestrial records 60 8 ka bp towards an understanding of latitudinal climatic gradients terrestrial records of past climatic conditions such as lake sediments and speleothems provide data of great importance for understanding environmental changes however unlike marine and ice core records terrestrial palaeodata are often not available in databases or in a format that is easily accessible to the non specialist as a consequence many excellent terrestrial records are unknown to the broader palaeoclimate community and are not included in compilations comparisons or modelling exercises here we present a compilation of western european terrestrial palaeo records covering entirely or partially the 60 8 ka intimate time period the compilation contains 56 natural archives including lake records speleothems ice cores and terrestrial proxies in marine records the compilation is limited to include records of high temporal resolution and or records that provide climate proxies or quantitative reconstructions of environmental parameters such as temperature or precipitation and that are of relevance and interest to a broader community we briefly review the different types of terrestrial archives their respective proxies their interpretation and their application for palaeoclimatic reconstructions we also discuss the importance of independent chronologies and the issue of record synchronization the aim of this exercise is to provide the wider palaeo community with a consistent compilation of high quality terrestrial records to facilitate model data comparisons and to identify key areas of interest for future investigations we use the compilation to investigate western european latitudinal climate gradients during the deglacial period and despite of poorly constrained chronologies for the older records we summarize the main results obtained from nw and sw european terrestrial records before the lgm

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

observation of atmospheric and climate dynamics from a high resolution ice core record of a passive tracer over the last glaciation here we present a temporal analysis of diluted calcium from the greenland ice core project summit ice core over a length corresponding to the duration of the last glacial period 92 8 ka our analysis suggests that this signal can be split into two distinct temporal components whose dynamics are characterized by respective timescale regimes timescales longer than approximately 200 years show a strong negative correlation with the temperature proxy 18 o signal this we interpret as a result of the temperature dependence of processes governing source area efficiency at faster timescales 200 years correlation with the temperature proxy is significantly weakened while its variability or mean absolute deviation is negatively correlated with the 18 o this we believe is a sign of the turbulent nature or storminess of the atmosphere during the glacial climate furthermore this component of the signal displays intermittency and multifractality supporting our interpretation of an observation of atmospheric dynamics this then provides a signal of atmospheric and climate dynamics back to 92 ka

continuous record of microparticle concentration and size distribution in the central greenland ngrip ice core during the last glacial period 1 a novel laser microparticle detector used in conjunction with continuous sample melting has provided a more than 1500 m long record of particle concentration and size distribution of the ngrip ice core covering continuously the period approximately from 9 5 100 kyr before present measurements were at 1 65 m depth resolution corresponding to approximately 35 200 yr particle concentration increased by a factor of 100 in the last glacial maximum lgm compared to the preboreal and sharp variations of concentration occurred synchronously with rapid changes in the 18o temperature proxy the lognormal mode of the volume distribution shows clear systematic variations with smaller modes during warmer climates and coarser modes during colder periods we find 1 7 m diameter during lgm and 1 3 m during the preboreal on timescales below several 100 years and the particle concentration exhibit a certain degree of independence present especially during warm periods when generally is more variable using highly simplifying considerations for atmospheric transport and deposition of particles we infer that 1 the observed changes of in the ice largely reflect changes in the size of airborne particles above the ice sheet and 2 changes of are indicative of changes in long range atmospheric transport time from the observed size changes we estimate shorter transit times by roughly 25 during lgm compared to the preboreal the associated particle concentration increase from more efficient long range transport is estimated to less than one order of magnitude

changes in atmospheric circulation and ocean ice cover over the north atlantic during the last 41 000 years high resolution continuous multivariate chemical records from a central greenland ice core provide a sensitive measure of climate change and chemical composition of the atmosphere over the last 41 000 years these chemical series reveal a record of change in the relative size and intensity of the circulation system that transported air masses to greenland defined here as the polar circulation index pci and in the extent of ocean ice cover massive iceberg discharge events previously defined from the marine record are correlated with notable expansions of ocean ice cover and increases in pci during stadials without discharge events ocean ice cover appears to reach some common maximum level the massive aerosol loadings and dramatic variations in ocean ice cover documented in ice cores should be included in climate modeling

changes in continental and sea salt atmospheric loadings in central greenland during the most recent deglaciation model based estimates by fitting a very simple atmospheric impurity model to high resolution data on ice accumulation and contaminant fluxes in the gisp2 ice core we have estimated changes in the atmospheric concentrations of soluble major ions insoluble particulates and 10 be during the transition from glacial to holocene conditions for many species changes in concentration in the ice typically overestimate atmospheric changes and changes in flux to the ice typically underestimate atmospheric changes because times of increased atmospheric contaminant loading are also times of reduced snowfall the model interpolates between the flux and concentration records by explicitly allowing for wet and dry deposition processes compared to the warm preboreal that followed we estimate that the atmosphere over greenland sampled by snow accumulated during the younger dry as cold event contained on average four seven times the insoluble particulates and nearly seven times the soluble calcium derived from continental sources and about three times the sea salt but only slightly more cosmogenic 10 be

primary aerosol sea salt and soil dust deposited in greenland ice during the last climatic cycle comparison with east antarctic records ion chromatography data of ca mg cl and ca and coulter counter particle measurements are used to study the cycle of marine and continental primary aerosol reaching greenland in relation to climatic changes over the last 150 kyr a detailed comparison between greenland dome summit and antarctic vostok records provides new insight on a potential link between northern and southern patterns ca is a good indicator of continental input and is mainly emitted as caco3 an attempt is made to estimate the contribution of aluminosilicates using the concentration of insoluble particles greater than 0 5 m in diameter the relative abundance of non sea salt mg and ca and of aluminosilicate shows that the calcium content of continental background aerosol over greenland was much higher during the glacial age the neutralization capacity of carbonaceous aerosol is estimated the inverse relationship between 18o and continental input as well as the response of this input to the rapid climatic variations that have occurred during the second part of the glacial age are discussed in terms of source and transport modification in relation to the presence and the extent of the great laurentide ice cap the corresponding vostok profiles strongly suggest that some of the phenomena observed at high northern latitudes are of global concern the marine component of na nam is a good tracer of sea salt aerosol similarly to continental input it shows an inverse but more linear relationship with 18o the sensitivity of greenland and antarctic marine input to climate variations of small and large amplitude is compared and a corresponding estimation is made for the aeolian contribution the respective influence of atmospheric circulation and the water vapor cycle is discussed the chlorine to marine sodium weight ratio increases with temperature from values very close to the bulk seawater ratio during the last glacial maximum 18 20 kyr b p to values significantly higher during the holocene and warm eemian the corresponding excess of chloride hcl is discussed in terms of atmospheric transport taking into account the role of atmospheric acidity on sea salt fractionation processes owing to postdepositional phenomena similar vostok data must be considered cautiously nevertheless aerosol fractionation seems to have been much more important over the vostok site except during glacial extrema

regional variability of ice core dust composition and provenance in greenland 1 mineralogical and isotopic composition sr and nd of six dust samples obtained from six widely spread ice coring sites in greenland were analyzed in order to investigate the regional geographic variability of dust provenance we show that long range transport from eastern asian deserts provides mineral dust with essentially the same composition to all elevated interior sites dye 3 site a grip and northgrip while most material deposited at sites located closer to the edge and at lower altitude hans tausen and renland derives from proximal source regions no contribution from other sources is apparent at any of the interior sites from the mineralogical and isotopic composition of the dust samples each of which represents several decades of dust deposition during the 17th 18th century these results provide additional evidence that african and north american deserts do not play a significant role in the dust deposited over greenland which has implications for ice core record interpretation and atmospheric dust transport model validation

link between european and north atlantic abrupt climate changes over the last glaciation 1 sub millennial timescale climate variations correlated with those in the north atlantic area are identified in the loess sequence from nussloch germany exceptionally developed over the interval between about 38 000 and 18 000 years ago the dansgaard oeschger do events 8 to 2 as well as some less pronounced warming episodes recorded in the greenland ice have their counterpart in the nussloch loess stratigraphy and in the variations of the loess grain size index gsi the north atlantic heinrich events 3 and 2 are expressed at nussloch as maxima of the gsi record the similarity of the fine variations in the nussloch loess and the greenland ice records points to an intimate link of climate changes in western europe with those in the north atlantic

paleoclimatic tracers an investigation using an atmospheric general circulation model under ice age conditions 2 water isotopes the linear relationship observed between the water isotopic contents of precipitation and surface air temperatures leads to the use of the water isotopes h218o and hdo in paleoclimatology applied to the measurements of the isotopic content of paleowaters like groundwaters and deep ice cores this relationship is used to infer paleotemperatures however this interpretation of paleo isotopic contents is only valid if the isotope temperature relationship is not affected by climate change to address this problem we have developed a water isotope modeling inside an atmospheric general circulation model agcm and performed simulations of both the present day and last glacial maximum lgm climatic conditions agcms are indeed the only appropriate tools able to account the whole complexity of the atmospheric circulation for the present day climate preliminary results for january were presented by joussaume et al 1984 and are complemented by new simulations performed for both february and august climatic conditions with a higher resolution version of the model model results are well corroborated by observations they also exhibit some effects of the atmospheric circulation on the isotopic fields for the simulated lgm climate the model results compare well with paleoclimatic data of water isotopic contents except for a higher than observed spatial variability the overall patterns of the simulated 18o temperature relationship for the lgm climate are practically unchanged which tends to comfort the use of water isotopes in paleoclimatology however concerning the deuterium excess i e the relationship between oxygen 18 and deuterium the model results are not sufficiently valid to allow a discussion of the use of deuterium excess in paleoclimatology

correlation between climate events in the north atlantic and china during the last glaciation episodes of massive iceberg release heinrich events 1 3 into the north atlantic ocean during the last glaciation were associated with recurring episodes of unusually cold north atlantic surface water bond cycles 4 and cold air temperatures over greenland dansgaard oeschger events 5 6 four of the youngest of these cold events have also been reported in climate records from sites outside the north atlantic region7 but until now the entire suite has been identified only in north atlantic marine sediments greenland ice core records and tentatively in french lake sediments8 here we examine grain size data from chinese loess and intercalated accretionary palaeosols of last glacial age for evidence of similar climate signals remote from the north atlantic region we see grain size maxima with ages that match those of the last six heinrich events which we interpret as an indication of the changing strength of the east asian winter monsoon which largely controls the transport and deposition of central asian aeolian dust thus it seems that these heinrich events have left their signature in the chinese loess record this is consistent with simulations of the glacial climate9 which imply that the climates of the north atlantic and china were linked by the effect of westerly winds