a ce nd isotope study of crustal contamination processes affecting palaeocene magmas in skye northwest scotland twelve138ce 136ce isotope determinations 31 nd isotope analyses and 31 ree profiles are presented for tertiary basic to intermediate igneous rocks from the isle of skye nw scotland the aim of this work is to precisely identify the contamination mechanisms of basic magmas emplaced through old crust and to test the effectiveness of ce isotope analysis as a petrogenetic tool

isotopic and geochemical evidence for magma mixing in the petrogenesis of the coire uaigneich granophyre isle of skye n w scotland field relations petrographic features major and selected trace element compositions and sr and pb isotope characteristics indicate that the coire uaigneich granophyre cug was formed by the mixing of two magmas one of these a liquid of high 87sr 86sr ratio ca 0 731 formed by the anatexis of late precambrian torridonian sediments contained relict quartz and zircon the other liquid was an acid differentiate of basaltic magma and was enriched in incompatible elements such as zr and y the two magmas mixed in the proportions of approximately two parts anatectic melt to one part acid differentiate hydrothermal metamorphism had no significant effect on the bulk chemistry of the cug

simulations of northern hemisphere ice sheet retreat sensitivity to physical mechanisms involved during the last deglaciation a 3 dimensional thermomechanical ice sheet model is used to simulate the evolution of the geometry of northern hemisphere ice sheets through the last deglaciation the ice sheet model is forced by a time evolving climatology provided by the linear interpolation through time of climate snapshots simulated by the lmd5 3 atmospheric general circulation model agcm at different periods of the last deglaciation 21 15 9 6 and 0 kyr bp the agcm is driven by insolation atmospheric co2 content ice sheet configuration and sea surface temperatures although our approach is able to produce the complete continental ice retreat our simulated deglaciation presents a phase lag with reconstructions based on observational evidences this suggests that physical mechanisms related to climate forcing and or ice sheet internal dynamics are not properly represented the influence of millennial scale forcing feedback mechanisms between ice sheet elevation and surface mass balance and parameterization of the ice flow is also tested through a set of sensitivity experiments the rapid variability has a strong impact on the evolution of the ice volume because of nonlinear effects in temperature mass balance relationships fennoscandia appears to be strongly sensitive to the small scale ice sheet instability both ice sheets are to some extent sensitive to an increased basal sliding

the younger dryas and the sea of ancient ice we propose that prior to the younger dryas period the arctic ocean supported extremely thick multi year fast ice overlain by superimposed ice and firn we re introduce the historical term paleocrystic ice to describe this the ice was independent of continental glacier ice and formed a massive floating body trapped within the almost closed arctic basin when sea level was lower during the last glacial maximum as sea level rose and the barents sea shelf became deglaciated the volume of warm atlantic water entering the arctic ocean increased as did the corresponding egress driving the paleocrystic ice towards fram strait new evidence shows that bering strait was resubmerged around the same time providing further dynamical forcing of the ice as the transpolar drift became established additional freshwater entered the arctic basin from siberia and north america from proglacial lakes and meltwater derived from the laurentide ice sheet collectively these forces drove large volumes of thick paleocrystic ice and relatively fresh water from the arctic ocean into the greenland sea shutting down deepwater formation and creating conditions conducive for extensive sea ice to form and persist as far south as 60 n we propose that the forcing responsible for the younger dryas cold episode was thus the result of extremely thick sea ice being driven from the arctic ocean dampening or shutting off the thermohaline circulation as sea level rose and atlantic and pacific waters entered the arctic basin this hypothesis focuses attention on the potential role of arctic sea ice in causing the younger dryas episode but does not preclude other factors that may also have played a role

applicability of la ce systematics to planetary samples ce isotopic compositions in several terrestrial and extraterrestrial materials were determined in order to investigate the applicability of using ce as an isotopic tracer to geological processes owing to the low abundances of 138la and 138ce in nature our measurements of 138ce 140ce ratios of natural samples have relatively large 0 02 errors and the variations in ce isotope ratios were barely resolved a tenuous anticorrelation was observed between ece and end for terrestrial basalts and granites indicating that with some improvement in analytical techniques the ce isotopic composition may prove useful as a tracer for geological processes a very low ece predicted from the ce anomaly in the ree pattern of the khohar l3 chondrite nakamura and masuda 1973 was not observed in this study indicating that the ce anomaly may be caused by terrestrial contamination or alteration a low ece estimated for the source of nakhla is consistent with our previous estimate nakamura et al 1982 of a lree depleted source

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

laser induced ree3 photoluminescence of selected accessory minerals an advantageous artefact in raman spectroscopy abstract laser induced photoluminescence of trivalent rare earth elements rees which is obtained as analytical artefacts in raman spectra of selected accessory minerals was studied spectra of natural titanite monazite ce xenotime y and zircon samples from various geological environments were compared with emission spectra of synthetic flux grown analogues doped with rees the latter is of great importance to identify potentially mistakable bands as either raman or pl signal and to assign them to certain ree centres in the samples investigated various ree centres are excited selectively using 473 514 532 633 and 785 nm laser excitation their assignment was verified by photoluminescence excitation experiments luminescence spectral patterns of zircon and titanite vary in dependence of trace ree concentrations hence reflecting geochemical growth conditions ree artefacts in raman spectra of accessory minerals may be used as fingerprint tool for mineral phase identification the distribution of ree emission intensities revealed by hyperspectral mapping opens up the opportunity to visualise mineral textures complementary to cathodoluminescence imaging techniques

a rapid climatic change at the end of the younger dryas in south sweden palaeoclimatic and palaeoenvironmental reconstructions based on fossil insect assemblages a rapid climatic change at the end of the younger dryas in s sweden palaeoclimatic and palaeo environmental reconstructions based on fossil insect assemblages

the impact of the north atlantic ocean on the younger dryas climate in northwestern and central europe the main results of multiproxy climate reconstructions for the younger dryas and experiments carried out with an atmospheric general circulation model are discussed quantitative temperature inferences for northwestern and central europe show that winter conditions were remarkably extreme with values of 20 30 c below that of today annual temperature ranges with a continental signature 30 34 c were reconstructed for the entire study area compared with today the greatest cooling was experienced in the seaboard area the comparison of the climate reconstructions and simulation experiments indicates that sea ice in the north atlantic ocean played a decisive role in the climate of the study area by cooling the surface air temperatures and by controlling the position of the storm track surface air circulation was westerly even during younger dryas winters a slight northward shift of the mean atlantic sea ice margin during winter may explain the proposed subdivision of the younger drays into a phase of maximum cold and humidity succeeded by a less cold and relatively dry phase 1998 john wiley sons ltd

coupled ocean and atmospheric changes during greenland stadial 1 in southwestern europe abstract paleoclimate reconstructions suggest that the complex variability within the greenland stadial 1 gs 1 over western europe was governed by coupled ocean and atmospheric changes however few works from the north atlantic mid latitudes document both the gs 1 onset and its termination which are often considered as single abrupt transition events here we present a direct comparison between marine alkenone based sea surface temperatures and terrestrial pollen data at very high resolution 28 years mean from the southwestern iberian shelf record d13882 our results reveal a rather complex climatic period with internally changing conditions the gs 1 onset gs 1a 12890 12720 yr bp is marked by a progressive cooling and drying gs 1b 12720 12390 yr bp is the coldest and driest phase gs 1c 12390 12030 yr bp is marked by a progressive warming and increase in moisture conditions gs 1 termination gs 1d 12030 11770 yr bp is marked by rapid switches between cool wet cold dry and cool wet conditions although hydroclimate response was very unsteady throughout the gs 1 and in particular during its termination phase the persistence of an open temperate and mediterranean forest in southwestern iberia during the entire episode suggests that at least some moisture was delivered via the westerlies we propose coupled ocean and atmospheric mechanisms to reproduce these scenaria changes in the strength of the atlantic meridional overturning circulation amoc as well as variations in the north atlantic sea ice growth have favoured the displacement of the polar jet stream s latitudinal position and contributed to a complex spatial pattern and strength of the westerlies across western europe

cathodoluminescence of synthetic doped with rare earth elements and natural anhydrites abstract in order to interpret cathodoluminescence cl spectra of natural anhydrites synthetic crystals were indirectly obtained by precipitation of gypsum from solutions containing doping rare earth elements ree and by transformation into anhydrite by dehydration at 800 c during 3 h analyses by icp ms of several anhydrites obtained from solutions containing 130 ppm of ree clearly showed that light ree lree can be concentrated to up to 2000 3000 ppm in the solid heavy ree hree are less prone to enter the structure and only 255 ppm were found for lu ree such as ce3 pr3 sm3 eu2 gd3 tb3 dy3 and tm3 display strong cl emissions and spectra usually are complex except for three of them with intense emissions in the near uv domain ce3 306 327 nm gd3 314 nm and eu2 386 nm in the near ir domain only nd3 874 nm gives an important luminescence several natural anhydrites were examined for ree cl emissions a sample from the faraday uranium mine bancroft ontario presents mainly a strong ce3 emission with traces of eu2 dy3 sm3 and nd3 in the anhydrite from balazuc france gd3 eu2 and dy3 are the more abundant followed by ce3 and traces of tb3 and sm3 a crystal probably from hall tyrol displays gd3 ce3 er3 tb3 dy3 and sm3 its analysis by icp ms showed that the cl can be induced by only some ppm of these elements even less than 1 ppm for er3 and tb3

data model comparison of the younger dryas event discussion rutter et al 2000 present an interesting overview of global evidence for the younger dryas event yd 11 500 13 000 calendar years ago as inferred from various kinds of proxy data such an overview is important for a better understanding of mechanisms behind rapid climate change moreover rutter et al make an effort to compare these data with a coupled atmosphere ocean model experiment on the yd performed by fanning and weaver 1997 in this experiment the production of north atlantic deep water nadw collapses after the release of a large amount of fresh water in the north atlantic based on the data model comparison rutter et al conclude that the model results reveal a global pattern and regional magnitude which generally agree with temperature changes interpreted from paleoclimatic reconstructions abstract p 811 this implies that the yd temperature change is

water mass exchange between the north atlantic and the norwegian sea during the past 28 000 years the greenland iceland and norwegian gin seas are important regulators of heat transport in the northern hemisphere and of ocean atmosphere co2exchange1 5 rapid changes in the circulation of surface and deep waters in this region may induce nonlinear climatic effects and climate instabilities2 3 here we present carbon and oxygen isotope data that provide a record of circulation changes in the gin seas during and at the termination of the last glacial maximum lgm inflow of nutrient depleted waters from the gin seas to form intermediate waters of the north atlantic resulted in nutrient enrichment of north atlantic deep water and consequent enhanced drawdown of atmospheric co2 contributing to the lower atmospheric during the lgm the onset of deglaciation occurred at a time of low salinity in the gin seas and thus of reduced thermohaline circulation although strong thermohaline circulation was later reinitiated in the north atlantic as deglaciation proceeded it cannot therefore have caused the onset of warming similarly we find that rapid changes in thermohaline circulation cannot account for the transient return to a cooler climate during the younger dryas episode

investigation of the relationship between permafrost distribution in nw europe and extensive winter sea ice cover in the north atlantic ocean during the cold phases of the last glaciation atmospheric model simulations with different extents of sea ice are compared with reconstructed european mean annual temperatures derived from permafrost indicators analysis of the results suggest that during cold phases of the last glacial the southern margin of permafrost in western europe was controlled by the latitude of the winter sea ice margin in the north atlantic ocean in this case reconstructions of permafrost extent in europe may be used to constrain past winter sea ice conditions in the north atlantic ocean accordingly extensive north atlantic sea ice cover southwards to at least 50degreesn is inferred during four phases of the last glaciation 1 early pleniglacial 74 59 cal kyr bp 2 the hasselo stadial 41 5 40 cal kyr bp 3 the lgm 23 19 cal kyr bp and 4 the younger dryas 12 7 11 5 cal kyr bp the extensive sea ice cover for the phase of maximum cold disagrees with recent studies suggesting a relatively warm north atlantic during the lgm while it agrees with the original climap reconstruction moreover the estimate for the younger dryas cooling conflicts with reconstructions based on marine proxy data c 2002 elsevier science ltd all rights reserved

bacterial tetraether membrane lipids in peat and coal testing the mbt cbt temperature proxy for climate reconstruction abstract peatlands are widespread and important natural archives of environmental change here we explore the potential of the recently introduced mbt cbt proxy methylation index and cyclisation ratio of branched tetraethers to estimate past annual mean air temperature mat based on the distribution of bacterially derived branched glycerol dialkyl glycerol tetraether gdgt membrane lipids in peat and coal to this end branched gdgts in an ombrotrophic peat bog from switzerland and three coal deposits of increasing maturity were analysed for the surface of the bog reconstructed annual mat is higher than both measured annual mat and measured in situ pore water temperature changes in the cbt ratio considered a proxy for ph with depth in the bog do not match with present day in situ pore water ph but coincide with a peat stratigraphic boundary this indicates that gdgts down the bog profile are predominantly fossil and not derived from extant biomass the mbt cbt derived annual mat record also shows a large drop at this stratigraphic boundary which likely relates to past change in trophic status of the bog branched gdgts are abundant in an immature lignite vitrinite reflectance r o 0 25 but occur in low amount in a slightly more mature coal r o 0 32 annual mat could be reconstructed for the lignite alone and is higher than other proxy based estimates from approximately the same time and location our results indicate potential for the application of the mbt cbt proxy in peat and immature coals but improved constraints on the effects of different types of peat on branched gdgt distributions as well as improved calibration of mat estimates are needed before the method can be confidently applied

the nature of mis 3 stadial interstadial transitions in europe new insights from model data comparisons 15 abrupt warming transitions perturbed glacial climate in greenland during marine isotope stage 3 mis 3 60 27 ka bp one hypothesis states that the 8 16 degrees c warming between greenland stad

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

comparison of oxygen isotope records from the gisp2 and grip greenland ice cores recent results1 2 from the greenland ice core project grip summit ice core suggest that the climate in greenland has been remarkably stable during the holocene but was extremely unstable for the time period represented by the rest of the core spanning the last two glaciations and the intervening eemian inter glacial here we present the complete oxygen isotope record for the greenland ice sheet project 2 gisp2 core drilled 28 km west of the grip core we observe large rapid climate fluctuations throughout the last glacial period which closely match those reported for the grip core however in the bottom 10 of the cores spanning the eemian interglacial and the previous glacia tion there are significant differences between the two records it is possible that ice flow may have altered the chronological sequences of the stratigraphy for the bottom part of one or both of the cores considerable further work will be necessary to evaluate the likelihood of this and the extent to which it will still be possible to extract meaningful climate information from the lowest sections of the cores

land ice teleconnections of cold climatic periods during the last glacial interglacial transition independent calendar year chronologies are a basic requirement for the establishment of high resolution land ice teleconnections the annually laminated meerfelder maar record provides both an independent chronology established by varve counting and high resolution lithological proxy data for the period of the last glacial interglacial transition these data reveal a series of four periods of climatic deterioration coinciding with negative isotopic deviations in the grip record signal thus demonstrating the synchronicity of environment changes in western germany and temperature shifts in greenland the terrestrial data supports a further sub division of the event stratigraphy based on the grip core by introducing the cold event gi 1c2 between 13 500 and 13 400 calendar years bp multiproxy analyses reveal that the environmental response at meerfelder maar was not linear throughout the lateglacial but was modified by local processes a change in the response of the lake environment to climate deterioration was observed during sub stage gi 1b gerzensee oscillation the only event with gradual rather than abrupt transitions the two fold character of the younger dryas as seen in the grip record is more pronounced in the meerfelder maar record this lithological signal occurred with a delay of 60 years to the grip signal and has been linked to a shift in the catchment it is proposed that the trigger for this shift was a trend towards a more humid second half of the younger dryas

the impact of periglacial processes on palaeolithic sites the case of sorted patterned grounds during the last glaciation western europe witnessed recurrent millennial scale episodes of periglacial climates that had significant impact on palaeolithic site preservation after a review on the genesis and climatic meaning of sorted patterned grounds examples of polygons stripes and stone banked solifluction lobes taken from archaeological contexts are described sorted features in palaeolithic sites imply that caution should be taken in site patterning analysis for reconstructing past human behaviours since spatial alteration of the original anthropogenic pattern took place on low gradients artefact movements remain limited but may increase dramatically on slopes when solifluction adds to sorting processes although these features are widespread in modern periglacial environments archaeological examples remain scarce multiple factors are probably involved including lack of knowledge of periglacial processes and landforms by archaeologists leading to misinterpretation but also more or less complete obliteration of surface features by sedimentary processes upon burial however it is assumed that scarcity of clear anthropogenic patterning in french palaeolithic sites before the late glacial is due for a large part to post depositional disturbance by periglacial processes

a late weichselian stable isotope stratigraphy compared with biostratigraphical data a case study from southern sweden late weichselian lake sediments from a site in southern sweden were analysed for stable carbon and oxygen isotopes as well as plant macrofossils and insect remains by comparison of independent d

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

the loch lomond readvance on the isle of mull scotland glacier reconstruction and palaeoclimatic implications geomorphological mapping of mull provides evidence for an icefield 143 km2 in area flanked by six corrie glaciers with a total area of ca 13 km2 the absence of lateglacial periglacial features shorelines and pollen sites from the area occupied by this readvance together with radiocarbon dating of shell fragments confirm that it occurred during the loch lomond younger dryas stade the thickness of glacigenic deposits within the area of the readvance is attributed to reworking of paraglacial sediments up valley continuity of recessional moraines indicates that the ice remained active and near to equilibrium during retreat consistent with slow warming following the coldest part of the stade reconstructed equilibrium line altitudes elas imply vigorous snow blowing by westerly winds and are consistent with a general southwestwards decline in elas across the scottish highlands an ela of 250 m was calculated for the mull icefield using an ablation accumulation balance ratio abr approach palaeotemperature and palaeoprecipitation estimates were derived by calculating a theoretical regional ela from meteorological data and assuming that the combination of temperature and precipitation implied by the theoretical ela approximates conditions at 250 m on mull during the loch lomond stade the result indicates a mean july sea level temperature of 5 7 0 5 c and a mean annual precipitation at 250 m of ca 2700 3800 mm best estimate 3200 mm indicating higher precipitation totals than at present owing to more vigorous atmospheric circulation copyright 2002 john wiley sons ltd

younger dryas glaciers and climate in the mourne mountains northern ireland here we present evidence to suggest that the mourne mountains northern ireland were last occupied by glaciers during the younger dryas stadial the margins of these glaciers are marked by moraines chronologically constrained to the younger dryas by schmidt hammer exposure dating reconstructions indicate that these glaciers had equilibrium line altitudes elas ranging from 356 33 m a s l to 570 9 m a s l with a mean of 475 36 m a s l elas rise from west to east probably reflecting the contribution of windblown snow and ice to the accumulation of younger dryas glaciers in the western mournes taking this into consideration a mean climatic ela of 529 4 m a s l is calculated for the mountains as a whole assuming a mean annual sea level air temperature of 8 c and an annual temperature range of 34 c degree day modelling suggests that during the younger dryas accumulation at the climatic ela of glaciers in the mournes was 846 990 mm a 1 this suggests increased aridity relative to present and is consistent with other parts of nw europe where reduced precipitation alongside notable cooling is thought to reflect increased north atlantic sea ice extent during the younger dryas

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

the partitioning of chalcophile elements between sediment melts and fluids at 3 gpa 950 1050 c with implications for slab fluids in subduction zones abstract there is growing evidence for the importance of slab sediment components in transferring trace element signatures to the arc mantle the chalcophile elements are of particular interest due to their differing compatibility in sulfide and sulfate phases the stability of which are dependent on subducted sediment redox many studies have shown that chalcophile elements are compatible in saline fluids at shallow levels of arcs f o 2 conditions where po is present at 1000 c dfluid melt ce 4 0 3 dfluid melt mo 319 43 dfluid melt as 669 134 dfluid melt sb 1437 208 dfluid melt pb 73 4 all errors are 1 and in oxidized conditions where anh is present at 1000 c dfluid melt ce 150 48 dfluid melt mo 3 7 1 9 dfluid melt as 2 5 1 7 dfluid melt sb 12 5 1 dfluid melt pb 3 8 1 8 the contrasting behavior of mo and ce in fluids versus melts is consistent with the notion that elevated mo in lavas from the lesser antilles arc is associated with reduced fluids extracting mo from subducted black shales the mo ce ratio is a promising tool in other arcs to infer the role that fluid may or may not play for chalcophile enrichment in the sub arc mantle unlike melts fluids in equilibrium with reduced po bearing sediments can endow the arc mantle in chalcophile elements without concomitant oxidation

dating synthesis and interpretation of palaeoclimatic records of the last glacial cycle and model data integration advances by the intimate integration of ice core marine and terrestrial records cost action es0907 since 2010 the intimate integration of ice core marine and terrestrial records network has been operating as a cost action designated es0907 this paper outlines the accomplishments of the intimate cost action in the context of how the intimate ideas have evolved during the network s twenty year life span and highlights a number of challenges that can guide further work in the second part of the paper the contributions that comprise this intimate special issue are introduced

simulation of the climate of 18 000 years bp results for the north american north atlantic european sector and comparison with the geologic record of north america abstract the large ice sheets in north america and europe and the extensive sea ice cover in the north atlantic at the time of the last glacial maximum must have greatly modified the surface temperature patterns and in turn the location and intensity of the surface winds and jet streams a general circulation model was used to simulate the january and july patterns of temperature precipitation and wind for 18 ka bp boundary conditions for the model consisting of ice sheet location and height sea ice location and sea surface temperature were prescribed from climap 1981 the model results are illustrated and described for the north american north atlantic european sector the jet stream splits around the north american ice sheet and the southern branch strengthens considerably compared to present over the southern portion of the united states the sea ice margin of the north atlantic and the southern edge of the european ice sheet geologic evidence principally from north america of wind temperature and moisture conditions is assessed from sand dune and loess records estimates of snowline depression pollen records and lake level studies the geologic evidence is generally compatible with the model simulation

pingo collapse and paleoclimatic reconstruction long term field studies of contemporary pingo growth collapse and rampart formation along the western arctic coast of canada provide criteria that may be helpful in the identification of pingo ramparts in nonpermafrost environments such criteria include the volume of the ramparts which should approximate that of the enclosed depressions from which the rampart materials were derived peripheral deposits associated with mass wasting streamflow and debris flow casts of dilation crack ice trending across the ramparts and high angle peripheral normal faults the conventional method of correlating the present mean annual air temperature with the present pingo distribution to establish warm side limiting temperatures for paleoclimatic reconstruction is unsound because most pingos in north america and the soviet union commenced growth hundreds to thousands of years ago under mean annual air temperatures that may have differed greatly from those of the present some other factors to be considered in paleo

the atmospheric winter circulation during the younger dryas stadial in the atlantic european sector the younger dryas yd stadial signified an interruption of the warming during the transition from the last glacial to the present interglacial the mechanism responsible for this cooling is still uncertain so valuable information concerning climate variability can be obtained by numerical simulation of the yd climate we performed four experiments on the younger dryas climate with the hamburg atmospheric general circulation model here we use the results of these experiments which differed in prescribed boundary conditions to characterize the atmospheric winter circulation during the yd stadial in the north atlantic european sector the 10 year means of the following variables are presented sea level pressure 500 hpa geopotential heights and 200 hpa winds in addition we used daily values to calculate an index to assess the occurrence of blocking and strong zonal flow and to compute storm tracks our results show that the yd cooling in europe was present with a strong and stable westerly circulation without blocking this is in conflict with an earlier study suggesting frequent easterly winds over nw europe in our experiments the sea ice cover in the north atlantic ocean was the crucial factor forcing this specific yd circulation moreover the jet stream over the north atlantic was strengthened considerably causing an enhanced cyclonic activity over the eurasian continent the yd winter circulation was different from the circulation found in most simulation studies on the last glacial maximum since no glacial anticyclones were present and no split of the jet stream occurred

reduced solar activity as a trigger for the start of the younger dryas it is generally assumed that changes in ocean circulation forced the abrupt climate changes during the late pleistocene including the younger dryas event recently however it was proposed that variations in solar irradiance could have played a much more prominent role in forcing pleistocene climate changes furthermore two physical mechanisms were recently published that explain how relatively small changes in solar irradiance could have had a strong impact on the climate system we discuss the possibility that an abrupt reduction in solar irradiance triggered the start of the younger dryas and we argue that this is indeed supported by three observations 1 the abrupt and strong increase in residual c 14 at the start of the younger dryas that seems to be too sharp to be caused by ocean circulation changes alone 2 the younger dryas being part of an similar to 2500 year quasi cycle also found in the c 14 record that is supposedly of solar origin 3 the registration of the younger dryas in geological records in the tropics and the mid latitudes of the southern hemisphere moreover the proposed two physical mechanisms could possibly explain how the north atlantic thermohaline circulation was perturbed through an increase in precipitation together with iceberg influxes in addition the full magnitude of the younger dryas cooling as evidenced by terrestrial records in europe could be explained we conclude that a solar triggering of the younger dryas is a valid option that should be studied in detail with climate models c 2000 elsevier science ltd and inqua all rights reserved

modern glacier characteristics as a basis for inferring former climates with particular reference to the loch lomond stadial abstract studies of the mass balance of modern glaciers have indicated that the equilibrium line altitude ela is the most critical parameter in the link between glaciers and climate average accumulation on a glacier includes components derived from direct precipitation on the glacier surface as well as wind drifting and avalanching of snow and approximates closely to accumulation at the equilibrium line ablation is most usefully predicted by mean summer temperature and there is a close non linear relationship between accumulation and mean summer temperature at the equilibrium line following discussion of methods of reconstructing former glaciers and calculating their equilibrium line altitudes the application of the above relationships to the loch lomond readvance glaciers of highland britain is considered and an outline of the precipitation pattern and the mean summer temperature of the loch lomond stadial is given

mean july temperatures during the younger dryas in northwestern and central europe as inferred from climate indicator plant species abstract we estimated minimum mean july temperatures in northwestern and central europe during the younger dryas 10 950 10 15014c yr b p from distributions of climate indicator plant species which were reconstructed from 140 pollen and plant macrofossil diagrams paleobotanical records mainly from the central and eastern part of the study area show that the coldest conditions occurred early in the younger dryas before 10 55014c yr b p for this phase mean july temperatures at sea level of around 10 c are suggested for the northern part of the british isles and for ice free scandinavia we estimated a mean july temperature of 12 c for central england the netherlands and northern germany the 13 c mean july isotherm largely based on the modern distribution oftypha latifolia was most probably located in southern england belgium central germany and poland the reappearance of thermophilous elements in the records after 10 55014c yr b p suggests a summer warming at least temporarily of 1 to 2 c in the study area the reconstructed temperatures are comparable with temperature estimates based on beetle data however they appear rather high when compared with estimates based on glaciological evidence

influence of seasonality on glacier mass balance and implications for palaeoclimate reconstructions climates inferred from former glacier geometries in some areas exhibit discrepancies with regional palaeoclimates predicted by general circulation models gcms and modelling of palaeoecological data possibly as a consequence of their differing treatments of climatic seasonality since glacier based climate reconstructions potentially offer an important tool in the calibration of gcms which themselves need validation if used to predict future climate scenarios we attempt to resolve mismatches between these techniques by 1 investigating the influence of seasonality on glacier mass balance and 2 refining the methodology used for the derivation of glacier based palaeoclimates focussing on the younger dryas stadial glaciation of scotland northeast atlantic we show that sea ice amplified seasonality led to a significantly drier climate than has been suggested by glacier based interpretations this was characterised by a relatively short ablation season and the survival of a more substantial winter snowpack we suggest that if palaeoglaciological studies were to account for changes in seasonal temperature and precipitation variability their results would agree more closely with the cold arid northeast atlantic palaeoenvironment predicted by atmospheric modelling and northwest european pollen studies and would therefore provide more accurate constraints for gcm calibration

latest pleistocene glacier advances and post younger dryas rock glacier stabilization in the mt krivaň group high tatra mountains slovakia abstract the timing of glacier disappearance and rock glacier stabilization in the highest massifs of the carpathians is still poorly documented in this paper we establish the extent and chronology of the final stages of glaciation and timing of stabilization of the uppermost rock glaciers in the tatra mountains which are the highest 2654 m a s l northernmost n 49 12 and coldest range in the carpathians on the base of morphostratigraphic principles schmidt hammer tests and glacier equilibrium line altitude ela estimation we assigned moraines and rock glaciers in the mt kriva massif sw part of high tatras to a threefold stratigraphic sequence with glacial stages advances from older to younger popradske pleso pp sucha va ecka i sv i and sucha va ecka ii sv ii landforms attributed to two younger advances sv i and sv ii were sampled for the purpose of cosmogenic 10be surface exposure dating our data show that pp and sv i glacial advances occurred during the oldest dryas climatic downturn the younger advance sv i occurred just before the bolling allerod b a warming at 14 9 0 4 ka at this time small glaciers occupied lower glacial cirques in the study area the youngest sv ii advance was characterized by rock glaciers and small debris covered glaciers confined to the highest cirques the mean exposure age of four relict rock glaciers of this stage is 11 1 0 9 ka but individual rock glacier mean ages spanned between 11 8 and 10 4 ka our research results show that the youngest moraines and rock glaciers in the tatra mountains were formed during the younger dryas yd but the final permafrost melting and rock glacier stabilization period was delayed until the early holocene but no later than 10 4 ka in high elevation cirque bottoms up to 2120 m a s l the yd glaciers and rock glaciers readvanced across land which was ice free as early as 14 8 14 2 ka thus it is likely that during the b a interphase glaciers completely disappeared in the tatra mountains the yd was the last period of glacial periglacial activity in the massif as we investigated one of the highest situated rock glaciers in the tatras up to 2220 m a s l we conclude that all rock glaciers in this massif are relict landforms which developed in response to severe lateglacial climate conditions the lateglacial climate of the tatra mountains is discussed herein on the basis of glacier equilibrium line altitude ela estimation and the vertical distribution of rock glacier belts we argue that paleoclimate significance of the presented geomorphological record may match paleobotanical proxy reconstructions and the results of climate simulations but only if the impact of enhanced temperature seasonality on glacier mass balance and rock glacier activity is taken into account

the reconstruction of past climates using multi proxy evidence an example of the weichselian pleniglacial in northwest and central europe abstract different lines of terrestrial biotic and abiotic evidence are used for large scale climate reconstructions to transform proxy data into climate parameter values standardisation procedures are defined and described as each type of biotic and abiotic proxy provides its own climate parameter a detailed climate reconstruction may be obtained the multi proxy database for palaeoclimate reconstruction is a specially designed computer database that facilitates data processing and management of weichselian climate proxies this paper describes the application of multi proxy evidence to reconstruct the climate across lowland northwest and central europe during the interval 41 38 bp for this period the mean temperature of the warmest month is estimated at 10 to 11 c and the mean temperature of the coldest month ranges from 27 to 20 c the annual range suggests a cold continental setting with an estimated mean annual temperature between 9 and 4 c a north to south transition from dis continuous permafrost to seasonally frozen ground is suggested by periglacial evidence

a chironomid based holocene summer air temperature reconstruction from the swiss alps we developed a quantitative chironomid july air temperature inference model based on surface sediments from 81 swiss lakes and applied it to the holocene subfossil chironomid record of hinterburgsee a small subalpine lake in the northern swiss alps present day mean july air temperature of 11 3 c after smoothing to reduce the high between sample variability of inferred temperatures the reconstruction indicates july air temperatures of 10 4 10 9 c at the end of the younger dryas of 11 9 12 8 c during the early and mid holocene 11500 4000 cal bp and slightly lower temperatures of 11 5 12 0 c during the late holocene 3500 1000 cal bp a warming trend inferred for the past millennium is most likely an artifact of human impact on hinterburgsee s chironomid fauna rather than a genuine temperature signal the most prominent climatological events during the holocene were two periods of lower temperatures at c 10700 10500 cal bp and 8200 7700 cal bp and an abrupt shift to a cooler late holocene cl

lateglacial environmental changes in the netherlands spatial and temporal patterns a contribution to the north atlantic seaboard programme of igcp 253 termination of the pleistocene abstract the history of vegetational changes during the weichselian lateglacial in the netherlands is based primarily on pollen records derived from lacustrine sediments they provide a fairly consistent picture of progressive followed by retrogressive vegetation succession through time problems arise however when we try to match pollen records obtained from terrestrial peat sequences with those obtained from lacustrine deposits this is especially the case at around about 12 000 bp conditions seem to have varied significantly depending on the geomorphological position of individual site terrestrial sites are more prone to hiatuses in response to episodes of pronounced reductions in effective precipitation the spread of pinus from ca 11 300 bp which has been interpreted traditionally as a successive phase in vegetational development coincides with changes in the local vegetation that indicate more intense freeze thaw conditions more continental conditions are assumed implying more severe winters by comparison with the preceding time interval at ca 10 850 bp both vegetation and coleopteran evidence indicate a distinct drop in the average july temperature from 18 15 c to 11 10 c consequently the mean annual temperature was also lowered estimated to between 2 and 5 c and conditions approximated those of permafrost environments simultaneously effective precipitation increased considerably resulting in a pronounced rise in lake levels and large scale floodings of rivers soon after at ca 10 500 bp summer temperatures appear to have risen while effective precipitation declined considerably resulting in a fall in lake levels and periodic exposure of river beds which provided sources for the accumulation of large inland dunes

climate change in the circum north atlantic region during the last deglaciation a survey of new and published palaeoclimate data indicates that both the high and low latitude north atlantic regions were characterized by at least three synchronous periods of abrupt climate change during the last glacial to interglacial transi tion climate model results suggest that changes in the melting history of the laurentide ice sheet may explain much of this nonlinear response of the climate system to astronomical milankovitch forcing

surface wind climate during the younger dryas in europe as inferred from aeolian records and model simulations a comparison of well dated and well documented aeolian records from the younger dryas and model simulations executed with an atmospheric general circulation model agcm shows that during this marked cold interval depositional winds in northwestern and central europe came from westerly quadrants two agcm grid cells representing the netherlands and central poland are compared whereas in the netherlands extensive river dunes were formed in poland the younger dryas is considered a phase of remodeling and limited aeolian activity this regional contrast may be explained by differences in the surface wind climate differences in sediment supply by dissimilar river types and differences in the vegetation cover the model simulations show that wind speeds were higher in the netherlands than in central poland due to the proximity of the ocean aeolian landforms indicate that during the younger dryas in the netherlands depositional winds came from the sw to w whereas in central poland w winds dominated model experiments support these observations it is suggested that aeolian processes primarily took place during the winter season

late weichselian environmental change in southern sweden and denmark a synthesis is presented of the envronmental and climatic variations that are inferred to have occurred in southern sweden up to latitude 59 n and denmark during the weichselian late glacial 14 9 ka bp the chronology and characteristics of the main phases of deglaciation sea level change periglacial activity soil development vegetation cover and climate change are summarised a curve representing the main changes in temperature including quantitative estimates based upon beetle data and using the mutual climatic range method is presented

the impact of cold north atlantic sea surface temperatures on climate implications for the younger dryas cooling 11 10 k the sensitivity of global climate to colder north atlantic sea surface temperatures is in vestigated with the use of the giss general circulation model north atlantic ocean temperatures 18 000 b p resembling those prevalent during the younger dryas were incorporated into the model of the present climate and also into an experiment using orbital parameters and land ice characteristic of 11 000 b p the results show that with both 11 000 b p and present conditions the colder ocean temperatures produce cooling over western and central europe in good agreement with younger dryas paleoclimatic evidence cooling also occurs over extreme eastern north america although the precise magnitude and location depends upon the specification of ocean temperature change in the western atlantic despite the presence of increased land ice and colder ocean temperatures the younger dryas summer air temperatures at northern hemisphere midlatitudes in the model are warmer than those of today due to changes in the orbital parameters chiefly precession and atmospheric subsidence at the perimeter of the ice sheets

laacher see tephra a widespread isochronous late quaternary tephra layer in central and northern europe a late quaternary tephra layer widespread in central and northern europe resulted from explosive plinian and phreatomagmatic eruptions of the laacher see volcano 11 000 yr b p the tephra is distinguished from other late quaternary andesitic rhyolitic airfall tuff layers in northern europe and from basaltic or trachytic tuff deposits in southern europe by its phonolitic composition and abundance of sanidine plagioclase clinopyroxene amphibole and sphene the proximal tephia sequence at laacher see is divided into three main deposits the predominantly plinian deposits of lower and middle laacher see tephra llst and mlst and phreatomagmatic deposits of the upper laacher see tephra ulst the mlst member is further subdivided into beds a b and c1 c2 and c3 the chemical composition of the magma is highly differentiated phonolite in the llst to mlst b sections but mafic phonolite in the mlst c1 to ulst sections all deposits are considered to be isochronous the frequency maximum of 16 radiocarbon datings indicating an eruption about 11 000 50 yr b p distal ash was deposited in three main fans directed to the northeast lst traced up to 1 100 km distance south lst traced up to 600 km and southwest lst traced up to 100 km tephrostratigraphic correlation of the distal ash deposits is based on a the major element composition of glass shards b lithology and c heavy mineral analyses the northeastern fan consists of deposits from llst mlst b and mlst c1 eruptive phases the southern fan comprises mlst a mlst c2 and ulst deposits and the southwestern fan consists exclusively of ash from the ulst eruptive phase northeastern transport of ash during eruptive phases with high plinian eruption columns but southern and southwestern transport of ash along phases of relatively low eruption columns are interpreted in terms of prevailing southwesterly paleowinds at high altitudes tropopause level but northerly winds dominating in the lower atmosphere the laacher see eruption columns were emplaced into an atmosphere vertically zoned with respect to paleowind directions which also explains the near vent shifting of llst mlst b and mlst c1 iso pach axes from east southeast to northeast within the first 20 km of transport

model intercomparison for the present day the mid holocene and the last glacial maximum over western europe 1 the focus of this paper is an evaluation of the performance over western europe of an ensemble of general circulation models gcms used to simulate climates at the present day the mid holocene and the last glacial maximum lgm experiments are considered from the palaeoclimate modeling intercomparison project pmip and from more recent sensitivity studies for the same time slices surface air temperature and precipitation are compared between models and with observed and proxy data sets using evaluation measures such as correlation coefficients and root mean squared errors hierarchical and k means cluster analysis are used to quantitatively assess relative model performance comparisons are made for western europe for a gridded domain between 38n13w south west corner and 65n30e north east corner and for four transects within this domain inter experiment comparisons reveal that the ranges of results from the sensitivity studies are within those of the pmip models there is no month to month or time slice to time slice consistency in the clustering patterns produced although one clear characteristic is that experiments performed with models from the same institution tend to cluster inter model agreement at past time slices is considerably reduced when we consider differences from the present day rather than absolute values and this is particularly the case for mid holocene temperature when the insolation forcing only represents a small departure from modern values statistical comparisons of the models with observed and proxy data sets demonstrate a lack of consistency in model performance between months transects and time slices

biogeochemical and micro facial fingerprints of ecosystem response to rapid late glacial climatic changes in varved sediments of meerfelder maar germany the response of a lacustrine ecosystem to climatic changes from 13 500 to 10 800 bp was studied in a varve dated sediment profile of lake meerfelder maar western germany bulk biogeochemical parameters stable carbon and nitrogen isotopes of sedimentary organic matter and micro facies analysis are used for a detailed investigation of the lake s development from the allerod interstadial al to the younger dryas stadial yd and into the early holocene preboreal varve micro facies reveal rapid changes in composition and seasonal structure of the depositional environment mainly in temporal accordance with changes of terrestrial biozones the respective responses of bulk proxy parameters bi decadal resolution indicate different sensitivities to diverse climatic changes a prominent transition took place within two decades at the al yd boundary 12 690 12 670 bp increased flux of nutrients released from redeposited littoral sediments and shorter yd summer seasons led to an acceleration and concentration of lacustrine primary production with reduced discrimination against 13c high lacustrine primary production was further favored by relatively warm yd summer temperatures at 12 240 varve years bp from 1 year to the next a regular deposition of detrital layers in spring set in which is related to local hydrologic threshold processes amplifying the response to increased snowmelt discharge this distinct change in the middle of the yd is clearly detected by varve micro facies but not equally recorded in organic bulk proxy data especially carbon isotope ratios remain constant and indicate a negligible effect of this process on the productivity of lacustrine algae the yd preboreal transition 11 645 11 585 bp is marked by a characteristic change in micro facies throughout this transition stable carbon isotope ratios strongly decline while the accumulation of opal and organic matter om remained constant this is seen as an increasing importance of spring and autumn for gross primary production at the onset of the holocene

changes in east atlantic deepwater circulation over the last 30 000 years eight time slice reconstructions using 95 epibenthic 13c records eight time slices were reconstructed to trace the distribution of east atlantic deepwater and intermediate water masses over the last 30 000 years our results show that there have been three distinct modes of deepwater circulation near the stage 3 2 boundary the origin of north atlantic deep water nadw was similar to today mode 1 however after late stage 3 the source region of the nadw end member shifted from the norwegian greenland sea to areas south of iceland mode 2 a reduced nadw flow persisted during the last glacial maximum with constant preformed 13c values the nutrient content of nadw increased markedly near the azores fracture zone from north to south probably because of the mixing of upwelled antarctic bottom water aabw from below which then advected with much higher flux rates into the northeast atlantic later the spread of glacial meltwater over the north atlantic led to a marked short term ventilation minimum below 1800 m about 13 500 14c years ago mode 3 the formation of nadw recommenced abruptly north of iceland 12 800 12 500 years ago and reached a volume approaching that of the holocene in the younger dryas 10 800 10 350 years b p another short term shutdown of deepwater formation followed between 10 200 and 9 600 years b p linked to a further major meltwater pulse into the atlantic each renewal of deepwater formation led to a marked release of fossil co2 from the ocean the likely cause of the contemporaneous 14c plateaus over the last 9000 years deepwater circulation varied little from today apart from a slight increase in aabw about 7000 14c years ago it is also shown that the oxygenated mediterranean outflow varied largely independent of the variations in deepwater circulation over the last 30 000 years

mineralogical and geochemical study of the rare earth element mineralization at gakara burundi abstract the rare earth element ree mineralization of gakara burundi has first been discovered in 1936 and has periodically been the subject of geological studies at times when the exploitation of bastnasite ce and monazite ce was economically interesting this study focuses on the establishment of a mineral paragenesis for gakara with special attention to the ree bearing phases to understand the formation history of the deposit the paragenesis can be subdivided into 3 stages primary ore deposition brecciation stage and supergene alteration evidence for fenitization processes i e pinkish red cathodoluminescence of k feldpar brecciation stage and the strong enrichment of light rees in bastnasite and monazite substantiate the hypothesis of a structurally controlled hydrothermal mineralization with a strong carbonatitic affinity this likely confirms the association of the gakara ree deposit with the neoproterozoic alignment of alkaline complexes and carbonatites along the present day western rift it suggests a direct link with a currently unidentified carbonatitic body at depth possibly derived from a predominantly metasomatized lithospheric mantle

indication of deep groundwater flow through the crystalline rocks of southern norway we assess the major aspects of the subsurface thermal regime within crystalline rocks of southern norway results of two dimensional modeling of coupled groundwater flow and heat transfer demonstrate that advective cooling due to groundwater flow is an important factor for the reduction of subsurface temperatures in southwestern norway the bergen and stavanger areas where the normal annual precipitation is high on the western windward side of the scandes mountains on the other hand the influence of groundwater flow on subsurface temperatures is most likely relatively low in southeastern norway the moss area which represents the rain shadow area with light precipitation and is characterized by smoothed landforms therefore where the reduced subsurface temperatures are associated with high atmospheric precipitation and complex surrounding topography deep groundwater flow is most likely present within crystalline rocks in this case the observed low values of the thermal gradient within crystalline rocks can be used as groundwater flow tracers

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

very fast environmental changes at the pleistocene holocene boundary recorded in laminated sediments of lake gościz poland abstract this paper presents a reconstruction of the climatic changes at the pleistocene holocene transition recorded in the annually laminated sediments of lake go ci a z poland this reconstruction is based on pollen stable isotope mineralogical and chemical analyses of sediments made with a time resolution of 1 4 yr it reveals a sequence of abrupt climatic changes which preceded and followed the major warming around 11 500 cal bp our data show that the warming period was preceded by a 30 yr long phase of increased winter and decreased summer precipitation 11 550 11 520 cal bp about 11 520 cal bp a very brief period of wetter conditions occurred simultaneously with the beginning of the major warming however most of the first phase of the major warming 11 520 11 500 cal bp was characterised by a generally dry climate presumably due to lowered winter precipitation our records suggest that in this phase the warming concerned winter seasons mostly in the second phase 11 500 11 460 cal bp the warming also concerned summer seasons also a distinct increase of humidity is noted in this period which caused moistening of soils and despite enhanced evaporation and evapotranspiration inhibited the process of lake level drop the following 70 years 11 460 11 390 cal bp were rather dry which caused a distinct lowering of the lake and probably also of groundwater level in this period maximum biological productivity per unit area in the lake is noted the whole sequence of phases of rather different climates was completed within a time interval 160 years spanning two human lifes only one should stress that the transitions between the consecutive phases were rather abrupt lasting no longer than 10 years

the role of seasonality in abrupt climate change abstract a case is made that seasonality switches dominated by wintertime were instrumental in abrupt climate changes in the north atlantic region during the last glaciation and into the holocene the primary evidence comes from mismatches between mean annual temperatures from greenland ice cores in comparison with snowline changes in east greenland northern europe and north america the most likely explanation is a shutdown or reduction in strength of the conveyor this allows the spread of winter sea ice across the north atlantic thus causing the northern region to experience much colder winters because they mimic the greenland temperature rather than the snowline signal changes in the atlantic intertropical convergence zone and the asian monsoon may also share a winter linkage with greenland thus the paleoclimate record is consistent with the notion that a huge continental sector of the northern hemisphere stretching from greenland to asia was close to an extreme winter threshold during much of the last glaciation winter climate crossed this threshold repeatedly with marked changes in seasonality that may well have amplified and propagated a signal of abrupt change throughout the hemisphere and into the tropics

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

climate and environment during the younger dryas gs 1 as reflected by composite stable isotope records of lacustrine carbonates at torreberga southern sweden climatic and environmental changes during the younger dryas stadial gs 1 and preceding and following transitions are inferred from stable carbon and oxygen isotope records obtained from the sediments of ancient lake torreberga southern sweden event gs 1 is represented in the sediment sequence by 3 5 m of clay containing lacustrine carbonates of various origins comparison of isotopic records obtained on mollusc shells ostracod valves and chara encrustations precipitated during specific seasons of the year supports estimates of relative changes in both lake water and mean annual air temperatures variations in soil erosion rates can also be estimated from a simple isotope mass balance model to separate allochthonous and autochthonous carbonate contributions to the bulk carbonate content of the sediments the well known rapid climatic shifts characterising the last termination in the north atlantic region are clearly reflected in the isotopic data as well as longer term changes within gs 1 following maximum cooling shortly after the allerod younger dryas gi 1 gs 1 transition a progressive warming and a slight increase in aquatic productivity is indicated at the younger dryas preboreal gs 1 pb transition mean annual air temperature rapidly increased by more than 5 c and summer lake water temperature increased by ca 12 c the subsequent preboreal oscillation is characterised by an increase in soil erosion and a slight decrease in mean annual air temperature these results are in harmony with recent findings about large scale climate dynamics during the last termination copyright 1999 john wiley sons ltd

paleoceanographic reconstructions of surface ocean conditions in the greenland iceland and norwegian seas through the last 14 ka based on diatoms prior reconstructions of the last glacial maximum and the holocene did not include detailed paleoclimate data from the greenland iceland and norwegian gin seas which cover sensitive areas for oceanographic boundaries sea ice distribution and deep water formation in order to fill this gap we studied nine well dated sediment cores from this area and reconstructed the paleoceanographic conditions of the surface gin seas at 2000 year intervals since 13 400 bp based on the diatom record results show that a n s extending sea ice free corridor had opened along norway already at 13 400 bp indicating a northward flow of a branch of the north atlantic drift possibly caused by a change in the jet stream flow due to the decreased height of the north american ice sheet a major change of climatic conditions occurred over the gin seas as the insolation anomaly reached its maximum around 9000 bp when the sea ice cover and the oceanic fronts retreated to a northwesterly position along greenland and the ocean temperatures rose the first half of the holocene is recorded as the warmest period during the last 13 400 bp in the gin seas the duration of this climatic optimum decreased both toward the north and toward the margins of the area the second half of the holocene is characterized by a cooling trend in step with the decreasing insolation these results support the proposal of imbrie et al 1992 that the gin seas region acts as an initial responder to insolation changes

the la ce geochronometer a new dating method the isotope 138la 0 089 of natural lanthanum decays by and electron capture to 138ce and 138ba with a long half life we present here the first age determination using la ce dating an internal la ce isochron of a gabbro from the upper zone of the bushveld south africa yields an age of 2 390 480 2 myr based on the partial decay constant 2 58 10 12 yr 1 this age agrees with the sm nd isochron age 2 050 90 2 myr which was obtained from an aliquot of the same sample solution within the uncertainties of 138la half life and isotopic measurement

climate patterns during former periods of mountain glaciation in britain and ireland inferences from the cirque record we map glacial cirques and analyse spatial variability in their altitude and aspect to derive a long term time integrated perspective on climate patterns during former periods of mountain glaciation likely spanning multiple quaternary glaciations in britain and ireland the data reveal that although air temperatures were important exposure to moisture bearing air masses was the key factor in regulating sites of former mountain glacier formation and indicate that during such periods moisture supply was largely controlled by north atlantic westerlies with notable inland precipitation gradients precipitation decreasing inland similar to present day in places trends in cirque altitude may also reflect regional differences in the extent of cirque deepening controlled by the dimensions and dynamics of the glaciers that came to occupy them specifically comparatively deep cirques in coastal locations may reflect the former presence of dynamic fed by moisture from the north atlantic but comparatively small glaciers largely confined to their cirques by contrast decreasing cirque depth further inland may reflect the former presence of larger and or less dynamic ice masses occupying comparatively continental climatic conditions