variability of the north atlantic thermohaline circulation during the last interglacial period studies of natural climate variability are essential for evaluating its future evolution greenland ice cores suggest that the modern warm period the holocene has been relatively stable for the past 9 000 years much less is known about other warm interglacial periods which comprise less than 10 of the climate record during the past 2 5 million years here we present high resolution ocean sediment records of surface and deep water variables from the bermuda rise spanning the last interglacial period about 118 000 127 000 years ago in general deep water chemical changes are coincident with transitions in surface climate at this site the records do not show any substantial fluctuations relative to the much higher variability observed during the preceding and subsequent cool climates the relatively stable interglacial period begins and ends with abrupt changes in deep water flow we estimate using 230 th measurements to constrain the chronology that transitions occur in less than 400 years

u th ages obtained by mass spectrometry in corals from barbados sea level during the past 130 000 years the study of the sea level record during the last glacial cycle has primarily proceeded indirectly by means of oxygen isotope measurements on foraminifera from deep sea sediments1 2 the direct approach of dating sea level indicators stagnated during the past decade mainly because the samples required to complete our knowledge of the past glaciations are below the present day sea level3 7 using the 14c ages of acropora palmata samples collected by drilling offshore the island of barbados fairbanks8 presented the first detailed chronology for the last deglaciation this radiocarbon chronology is limited to the past 30 kyr because of the short half life of 14c 5 730 yr we must therefore rely on other dating methods to obtain information for the whole last glacial cycle during the past four years 230th 234u dating of corals by thermal ionization mass spectrometry has been shown to be significantly more precise and accurate than the classical counting method9 14 we have used this technique to measure u th ages in coral samples from the barbados collection and from subaerially exposed outcrops see also ref 15 here we present results bearing on the sea level record for the past 130 000 years we conclude that the last deglaciation started 3 000 years earlier than previously thought and confirm that there were two surges in melt water8 at about 11 kyr and 14 kyr bp before present

origin of the northern atlantic s heinrich events as first noted by heinrich 1988 glacial age sediments in the eastern part of the northern atlantic contain layers with unusually high ratios of ice rafted lithic fragments to foraminifera shells he estimated that these layers are spaced at intervals of roughly 10 000 years in this paper we present detailed information documenting the existence of the upper five of these layers in odp core 609 from 50 n and 24 w their ages are respectively 15 000 radiocarbon years 20 000 radiocarbon years 27 000 radiocarbon years about 40 000 years and about 50 000 years we also note that the high lithic fragment to foram ratio is the result of a near absence of shells in these layers although we are not of one mind regarding the origin of these layers we lean toward an explanation that the heinrich layers are debris released during the melting of massive influxes of icebergs into the northern atlantic these sudden inputs may be the result of surges along the eastern margin of the laurentide ice sheet

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 hierarchical structure of glacial climatic oscillations interactions between ice sheet dynamics and climate abrupt climatic oscillations around the north atlantic have been identified recently in greenland ice cores as well as in north atlantic marine sediment cores the good correlation between the dansgaard oeschger events in the ice and the heinrich events in the ocean suggests that climate in the north atlantic region underwent several massive reorganizations in the last glacial period a characteristic feature of these events seems to be their hierarchical structure every 7 to 10 thousand years when the temperature is close to its minimum the ice sheet undergoes a massive iceberg discharge this heinrich event is then followed by an abrupt warming then by several other oscillations each one lasting between one and two thousand years these secondary oscillations do not have a clear signature in marine sediments but constitute most of the dansgaard oeschger events found in the ice here we use a simplified model coupling an ice sheet and an ocean basin in order to illustrate how the interactions between these two components can lead to such a hierarchical structure the ice sheet model exhibits internal oscillations composed of ice sheet growing phases and basal ice melting phases that induce massive iceberg discharges these massive fresh water inputs in the ocean stop for a moment the thermohaline circulation enhancing the temperature contrast between low and high latitudes just after this event the thermohaline circulation restarts and an abrupt warming of high latitude regions is observed for some parameter values these warmer temperatures have in turn some influence on the ice sheet inducing secondary oscillations similar to those found in paleoclimatic records although the mechanism presented here may be too grossly simplified it nevertheless underlines the potential importance of the coupling between ice sheet dynamics and oceanic thermohaline circulation on the structure of the climatic records during the last glacial period

a continuum mixture model of ice stream thermomechanics in the laurentide ice sheet 1 theory we employ a continuum mixture framework to incorporate ice streams in a three dimensional thermomechanical model of the laurentide ice sheet the ice mass is composed of a binary mixture of sheet ice which deforms by viscous creep and stream ice which flows by sliding and or sediment deformation at the bed dynamic and thermal evolutions are solved for each component in the mixture with coupling rules to govern transfer between flow regimes we describe two different transfer mechanisms 1 creep exchange the nourishment of ice streams by viscous creep inflow from the surrounding ice sheet and 2 bed exchange the activation growth and deactivation of ice streams perpetrated by transfers of bed area between flow constituents this paper develops the underlying mixture theory we express the governing equations for mass momentum and energy balance in a form suitable for direct incorporation in existing numerical models of ice thermomechanics a companion paper in this issue explores mixture and ice stream behavior in applications with the laurentide ice sheet

rapid changes of glacial climate simulated in a coupled climate model abrupt changes in climate termed dansgaard oeschger and heinrich events have punctuated the last glacial period approximately 100 10 kyr ago but not the holocene the past 10 kyr here we use an intermediate complexity climate model to investigate the stability of glacial climate and we find that only one mode of atlantic ocean circulation is stable a cold mode with deep water formation in the atlantic ocean south of iceland however a warm circulation mode similar to the present day atlantic ocean is only marginally unstable and temporary transitions to this warm mode can easily be triggered this leads to abrupt warm events in the model which share many characteristics of the observed dansgaard oeschger events for a large freshwater input such as a large release of icebergs the model s deep water formation is temporarily switched off causing no strong cooling in greenland but warming in antarctica as is observed for heinrich events our stability analysis provides an explanation why glacial climate is much more variable than holocene climate

binge purge oscillations of the laurentide ice sheet as a cause of the north atlantic s heinrich events ice rafted debris in sediment cores from the north atlantic suggests that the laurentide ice sheet lis periodically disgorged icebergs in brief but violent episodes which occurred approximately every 7 000 years here i propose that heinrich events i e what these episodes are called were caused by free oscillations in the flow of the laurentide ice sheet which arose because the floor of hudson bay and hudson strait is covered with soft unconsolidated sediment that forms a slippery lubricant when thawed the proposed heinrich event cycle has two phases the growth phase occurs when the sediment is frozen and the lis is stranded immobile on a rigid bed the volume of the lis slowly grows during this phase at a rate dictated by snow accumulation the purge phase occurs when the basal sediment thaws and a basally lubricated discharge pathway i e an ice stream such as those which occur in west antarctica today developes through hudson strait the volume of the lis rapidly equilibrates to the reduced basal friction during this phase by dumping icebergs into the labrador sea the periodicity t k sl2g 2 7000 years of the proposed heinrich event cycle is a function of the thermal conductivity and diffusivity of ice k and respectively the atmospheric sea level temperature sl in degrees celsius and the excess geothermal heat flux defined by g g k where is the atmospheric lapse rate and g is the geothermal heat flux agreement between the predicted t and the apparent periodicity implied by the marine record is the main virtue of the free oscillation mechanism i propose an alternative mechanism in which heinrich events are forced by periodic variations in external climate is implausible because periodic atmospheric temperature perturbations are strongly attenuated with depth in an ice sheet

evidence of zn isotopic fractionation in a soil plant system of a pristine tropical watershed nsimi cameroon abstract the objective of this study is to determine for the first time the zinc zn isotopic composition of different soil horizons parent rocks litter and plants from a single pristine watershed three soil profiles from nsimi zoetele site south cameroon africa have been investigated the 66zn 66zn 66zn 64zn sample 66zn 64zn jmc 3 0749l 1 103 measured in the soils and rocks range from 0 05 to 0 64 in a typical hillslope soil profile developed in situ from a granitic parent rock we observe two important and systematic trends i a weathering regime in the saprolite horizon that involves strong zn depletion with no or slight fractionation compared to the fresh igneous rock and ii a shallower weathering regime with significant depletion in heavy zn isotopes in the most superficial soil horizons in contrast the soil profile of the swamp zone does not exhibit isotopic fractionation between the soil and the parent rock six plants and two litter samples were analyzed for their zn isotopic compositions 66zn varied from 0 75 to 0 91 among all samples plant roots and shoots are generally enriched in heavy isotopes 66zn of 0 42 to 0 76 relative to the litter layer 0 12 to 0 25 and most superficial soils similar to the controlled plant growth study of weiss et al weiss d j mason t f d zhao f j kirk g j d coles b j and horstwood m s a 2005 isotopic discrimination of zinc in higher plants new phytologist 165 703 710 negative 66zn values were obtained for tree leaves the most aerial part of the tree based on biomass data for the tropical forest vegetation is on average enriched in light isotopes compared to the litter and the superficial soil horizons which are the nutrients reservoirs due to the age of these soils and the complexity of pedogenetic processes it is difficult to explain the isotopic composition of the soils at present however the transformation of the saprolite horizon into a ferruginous horizon is associated with an enrichment in light zn isotopes this observation may be due to the adsorption of zn onto clays surfaces the uptake of zn by roots in the soils leads to an enrichment in heavy isotopes once the zn is in the xylem it will be transported within the plant during this transport both diffusion and active uptake of heavy isotopes by cells out of the xylem favour the mobility of light isotopes to the most aerial parts of the plants the difference in zn isotopic composition between the deep horizons and the superficial horizons of soils suggests that zn isotopes can be used to discriminate the source of zn and other metals leaving the catchment by river water

sensitivity experiments with a model of the ice age cycle the response to milankovitch forcing abstract we address the question of the extent to which a simple one dimensional time dependent climate model is able to explain the fluctuations of continental ice volume which have occurred during the last quarter of the pleistocene epoch the model includes explicit ice physics based on the glen flow law and explicit glacial isostatic adjustment based on a realistic viscoelastic field theory when the model is forced by the milankovitch variations of insulation it delivers ice volume fluctuations which are very similar to those implied by the specmap 18o record in particular the model appears capable of explaining the observed correlation between eccentricity and ice volume variations the dominance of the 105 year period in the power spectrum of the ice volume time series and the thickness and latitudinal extents of the continental ice sheets which existed at wurm wisconsin maximum 18 000 years ago

abrupt increase in greenland snow accumulation at the end of the younger dryas event the warming at the end of the last glaciation was characterized by a series of abrupt returns to glacial climate the best known of which is the younger dryas event1 despite much study of the causes of this event and the mechanisms by which it ended many questions remain unresolved1 oxygen isotope data from greenland ice cores2 4 suggest that the younger dryas ended abruptly over a period of about 50 years dust concentrations2 4 in these cores show an even more rapid transition 20 years this extremely short timescale places severe constraints on the mechanisms underlying the transition but dust concentrations can reflect subtle changes in atmospheric circulation which need not be associated with a large change in climate here we present results from a new greenland ice core gisp2 showing that snow accumulation doubled rapidly from the younger dryas event to the subsequent preboreal interval possibly in one to three years we also find that the accumulation rate change from the oldest dryas to the bo11ing allerod warm period was large and abrupt the extreme rapidity of these changes in a variable that directly represents regional climate implies that the events at the end of the last glaciation may have been responses to some kind of threshold or trigger in the north atlantic climate system

tracing the metal dynamics in semi arid soils near mine tailings using stable cu and pb isotopes abstract the dynamics of cu and pb were studied in eight soil profiles luvisols chernozems located at various distances from the tailing dams near a closed cu pb zn ag mine at kombat namibia by a combination of isotope and elemental studies leaching techniques and speciation modelling tailings soils and bedrocks were analyzed for the bulk cu and pb concentrations for their chemical forms using the sequential extraction procedure and for the isotope compositions of cu 65cu and pb 206pb 207pb 208pb 206pb in spite of the differences in the contamination of the studied profiles by tailing dust both soil types exhibit characteristic trends in the behaviour of metals and distribution of their isotopes while most luvisols exhibit an increase in the metal content towards depth in the profile chernozems exhibit the opposite trend decrease in metal content with depth or similar contents in the whole profile luvisols were mostly characterized by changing 65cu values as a function of depth the value of 65cu decreases with depth to the bt horizon and then the 65cu value increases with increasing depth chernozems mostly yielded uniform cu concentrations and isotope compositions in all the soil horizons clearly contrasting with the isotopically lighter compositions of the carbonate bedrocks positive 65cu values in the tailing material affect the surface of the soil especially soil sampled in its vicinity in most luvisols and chernozems the isotope composition of pb exhibits similar patterns with a low 206pb 207pb ratio on the surface of the profile and with a slight increase to depth the low 206pb 207pb values at the surface of the profiles are caused by dust from the tailings the elemental and isotopic patterns of both contaminated and uncontaminated soil types reflect not only the distance from the tailing dam but also pedogenetic processes in the profiles and geochemical behaviour of both contaminants the copper elemental and isotope patterns reflect the formation of more mobile species in the surface environment and movement of solutions and colloids and precipitation onto secondary minerals in luvisols plant uptake of lighter isotopes in chernozems can influence the cu isotope patterns in these soil types the limited movement of colloids and higher sorption capacity of chernozems is manifested in the isotope compositions of cu and pb in their profiles

late pleistocene ice age scenarios based on observational evidence ice age scenarios for the last glacial interglacial cycle based on observations of boyle and keigwin concerning the north atlantic thermohaline circulation and of barnola et al concerning atmospheric co sub 2 variations derived from the vostok ice cores are herein analyzed northern hemisphere continental ice sheets are simulated with an energy balance model ebm that is asynchronously coupled to vertically integrated ice sheets models based on the glen flow law the ebm includes both a realistic land sea distribution and temperature albedo feedback and is driven with orbital variations of effective solar insolation with the addition of atmospheric co sub 2 and ocean heat flux variations but not in their absence a complete collapse is obtained for the eurasian ice sheet but not for the north american ice sheet we therefore suggest that further feedback mechanisms perhaps involving more accurate modeling of the dynamics of the mostly marine based laurentide complex appears necessary to explain termination i 96 refs 12 figs 2 tabs

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

a thermomechanical model of ice flow in west antarctica this study uses a three dimensional thermo mechanical model to investigate the internal flow dynamics of the west antarctic ice sheet wais the model allows ice thickness flow and temperature to interact freely its domain is prescribed as that of the present day grounded wais realistic present day climatic and topographical boundary conditions are employed the analysis of a series of experiments pays particular attention to the location and dynamics of concentrations of ice flow ice streams underlying topographic troughs are crucial in determining the strength and location of these concentrations of flow the flow pattern generated by subglacial troughs is made more distinct by the inclusion of ice flow temperature coupling the inclusion of sliding leads to the generation of limit cycles in the ice flow they are concentrated around the present day ice streams b and c of the siple coast and have a period of 5 to 10 ky there appears to be competition between several preferred ice flow pathways in this area the two end members of the flow regime are a strong ice stream c with a weakened ice stream a b complex and strong ice streams a and b with a dormant ice stream c ice streams appear to require ice discharges above a certain threshold in order to maintain frictional heat generation and fast flow individual ice streams can therefore interact through changes in catchment area size a reduction in catchment area reduces the volume of ice entering a stream and can cause stagnation as the amount of frictional heating falls

isotopic trends and background fluxes of atmospheric lead in northern europe analyses of three ombrotrophic bogs from south sweden isotopic trends and background fluxes of atmospheric lead in northern europe analyses of three ombrotrophic bogs from south sweden

simulation of abrupt climate change induced by freshwater input to the north atlantic ocean temperature records from greenland ice cores1 2 suggest that large and abrupt changes of north atlantic climate occurred frequently during both glacial and postglacial periods one example is the younger dryas cold event broecker3 speculated that these changes result from rapid changes in the thermohaline circulation of the atlantic ocean which were caused by the release of large amounts of melt water from continental ice sheets here we describe an attempt to explore this intriguing phenomenon using a coupled ocean atmosphere model in response to a massive surface flux of fresh water to the northern north atlantic of the model the thermohaline circulation weakens abruptly intensifies and weakens again followed by a gradual recovery generating episodes that resemble the abrupt changes of the ocean atmosphere system recorded in ice and deep sea cores4 the associated change of surface air temperature is particularly large in the northern north atlantic ocean and its neighbourhood but is relatively small in the rest of the world

sensitivity of a greenland ice sheet model to ice flow and ablation parameters consequences for the evolution through the last climatic cycle sensitivity experiments are conducted to test the influence of poorly known model parameters on the simulation of the greenland ice sheet by means of a three dimensional numerical model including the mechanical and thermal processes within the ice two types of experiments are performed steady state climatic conditions and simulations over the last climatic cycle with a climatic forcing derived from the grip record the experiments show that the maximum altitude of the ice sheet depends on the ice flow parameters deformation and sliding law coefficients geothermal flux and that it is low when the ice flow is fast on the other hand the maximum altitude is not sensitive to the ablation strength and consequently during the climatic cycle it is driven by changes in accumulation rate the ice sheet extension shows the opposite sensitivity it is barely affected by ice flow velocity and the ice covered area is smaller for large ablation coefficients for colder climates when there is no ablation the ice sheet extension depends on the sea level an interesting result is that the variations with time of the altitude at the ice divide summit do not depend on the parameters we tested the present modelled ice sheets resulting from the climatic cycle experiments are compared with the present measured ice sheet in order to find the set of parameters that gives the best fit between modelled and measured geometry it seems that compared to the parameter set most commonly used higher ablation rate coefficents must be used

climate sensitivity to continental ice sheet size and configuration abstract a version of the ncar community climate model ccm has been used to carry out a study of climate sensitivity to the size and distribution of continental ice sheets by comparing two perpetual season fixed sea surface temperature sst winter simulations of the climate at a glacial maximum the two simulations differ only in the size and configuration of the continental ice sheet boundary conditions these conditions represent minimum and maximum ice sheet extent for a glacial maximum as presently constrained by the geologic evidence all other boundary conditions are as specified by climap 1981 for winter surface conditions during the last glacial maximum lgm the ice sheets specified by climap 1981 used by many previous researchers to simulate ice age climate conditions are intermediate in size between the minimum and maximum ice sheets used for the simulations described here the maximum case simulation resulted in unexpected positive temperature anomalies over the northern hemisphere

climatology glacial hiccups the climate instability of glacial times probably resulted from abrupt switches in ocean circulation a computer model simulation provides the first glimpse of the dynamics involved

effect of isostatic rebound on modelled ice volume variations during the last 200 kyr deformation of the lithosphere under an ice load can be approximated using the hypothesis of local damped isostasy this simple formulation has been systematically compared with a three dimensional model of the crust mantle system for simple ice load scenarios with a period in the range 20 100 kyr the comparison enables us to introduce the concepts of effective upper mantle density and effective relaxation time into the isostatic model for the response of the earth to the ice sheets these parameters depend on the earth model considered ice sheet size and the period of the load cycle the local damped isostasy model has been implemented in the louvain la neuve climate model to assess the impact of isostasy on continental ice volume variations for the last 200 kyr results suggest that isostasy acts as a negative feedback for ice volume during the glaciation process and acts as a positive feedback during the deglaciation moreover taking isostasy into account is necessary to simulate variations in northern hemisphere ice volume during isotopic stage 3 lastly the use of effective mantle density and effective relaxation time improves the model performance regarding specmap reconstructions c 2001 elsevier science b v all rights reserved

intermediate focus earthquakes under south shetland islands antarctica this study is based on data from five field surveys 1992 1996 of seismic monitoring on deception island south shetland islands antarctica in the 1994 95 and 1995 96 surveys earthquakes were recorded by a seismic array in previous surveys data were collected by a high dynamic range short period station the analysis of the events shows evidence of intermediate focus seismicity 50 h 100 km related to the underplating zone of the south shetland plate because of poor station coverage we had to use unconventional techniques to locate the events such as zero lag cross correlation method that provide information about apparent velocity and back azimuth ray tracing procedure and particle motion patterns these criteria allowed us to identify 15 earthquakes that could be considered as intermediate focus events for which some focal parameters were estimated we conclude that the subduction zone between the drake plate and south shetland microplate has a moderate level of intermediate depth microseismicity

impact of the north american ice sheet orography on the last glacial maximum eddies and snowfall the present work evaluates the influence of the north american ice sheet orography on the last glacial maximum lgm 21 000 years ago atmospheric circulation and snowfall in the northern mid latitudes focusing on the north atlantic sector three atmospheric general circulation model experiments are analysed a control and an lgm climate simulation and an lgm run with flat ice sheets over northern north america this ice sheet orography affects lee cyclogenesis over north america and forces differences in stationary waves and therefore in the baroclinicity of the mean flow as a result the atlantic storm track is reinforced in the flat ice sheet experiment compared to the lgm one this in turn has a profound impact on snowfall over northern europe implying a coupling between the two ice sheets

the marine record of deglaciation from the continental margin off nova scotia continental margin sediments off nova scotia accumulate at high rates up to 360 cm kyr 1 and contain a history of millennial scale environmental changes which are dominated by the proximity of the laurentide ice sheet during the latest quaternary using stable isotope ratios of oxygen accelerator mass spectrometer radiocarbon dating micropaleontology and sedimentology we document these changes in six piston cores ranging in water depth from 450 to 4300 m we find that maximum 18o in n pachyderma occurred about 15 ka and preceded the maximum abundance of this species in these cores by 1000 years between 13 and 14 ka we find a second peak in abundance of n pachyderma minimum 18o and two pulses of ice rafting the sediment lithology supports terrestrial studies which indicate that there was a general withdrawal of ice beyond the upper paleozoic and mesozoic red beds by 14 ka in southeastern canada so the ice rafting events between 13 and 14 ka probably reflect ice stream activity in the st lawrence valley the younger dry as event is recognized as a peak in abundance of n pachyderma and ice rafting dated as 11 3 ka but meltwater discharge to the gulf of st lawrence was either too small or occurred over too long a time to leave a distinct 18o minimum off nova scotia at 7 1 ka in the middle of holocene warming we find a third peak in abundance of n pachyderma and another 18o minimum but no ice rafting we interpret these data as evidence of a late occurring meltwater event which if correct could have originated in the great lakes in the labrador ungava region or in both the final millennial scale phenomenon off nova scotia is the onset of neoglaciation marked by increased ice rafting and increased n pachyderma beginning about 5 kyr ago

ice sheet growth and high latitudes sea surface temperature the response of the lln 2 d climate model to the insolation and co2 forcings during the eemian interglacial is compared to reconstructions obtained from deep sea cores drilled in the norwegian sea and in the north atlantic both reconstructions and modeling results show a decrease of sea surface temperature sst in the higher latitudes 70 75 n zonal belt for the model and the norwegian sea for the proxy records associated with a more moderate cooling at lower latitudes 50 55 n and north atlantic at the middle of isotopic substage 5e several millenia before the beginning of continental ice sheet growth such a comparison between the simulated sst and ice volume of the northern hemisphere has been extended to the whole last glacial interglacial cycle the influence of the insolation forcing on sst and the shortcomings of the model due to its zonal character are discussed

refining the pre industrial atmospheric pb isotope evolution curve in europe using an 8000 year old peat core from nw spain abstract pb pollution has existed for several millennia and remains relevant today by using peat cores as environmental archives it is possible to reconstruct this long history on a regional scale this is a significant contribution to the findings from ice core records the only other archive recording purely atmospheric additions without information that allows linking and comparison between sites regionally within europe and elsewhere our ability to make coherent global models of the natural pb cycle and anthropogenic forcing of this cycle is limited in this respect the characteristics of the pb pre pollution aerosol ppa are important to define globally we characterize for the first time a ppa in southern europe with pb 0 78 0 86 g g 1 net pb accumulation rates of 0 032 0 030 mg m 2 y 1 and a 206 pb 207 pb signature of 1 25470 0 02575 this ppa pb isotope signature is more radiogenic than that found thus far in western and northern europe spain is a historically important mining site using three isotope plots and a pool of potential pb isotope signatures a detailed source appointment of both natural and anthropogenic pb sources was made we found evidence of saharan aridification and its termination 4400 bp and or agricultural signals and strong local control from rock and soil of the pb ppa human impact is first recorded at 3210 bp but does not exceed 50 of deposited pb until 3005 bp mines in se spain dominate early pb pollution history at this site during the rise of roman rule contributions come from mines in n nw and sw spain with no strong indication of other european mining activities in medieval and industrial times local contributions to the peat bog are reduced

seismic noise correlations to image structural and mechanical changes associated with the mw 7 9 2008 wenchuan earthquake we locate temporal changes of seismic wave speed and scattering properties of the crust associated with the 2008 mw7 9 wenchuan earthquake to that end we analyze ambient seismic noise records from 2007 until the end of 2008 in the 1 3 s period band and in the 12 20 s period band measured in a region that covers the southern two thirds of the fault activated during the earthquake to locate the changes we use a refined imaging procedure based on the sensitivity of scattered waves to weak perturbations this inverse method uses the radiative transfer approximation to describe the intensity of the noise correlation coda our results show that the largest structural changes are observed northeast of the wenchuan epicenter around the fault zone where aftershocks are distributed the spatial and temporal characteristics of the behavior of the crust at depth around the wenchuan earthquake suggest a postseismic and geology dependent signature of the middle crust we also observe a clear seasonal signature within the sichuan basin at depth that we relate to the heavy rainfalls during the monsoon season this seasonality can also be observed at the surface in form of a waveform decoherence our study highlights that seismic velocity changes and waveform decoherence are independent measurements that are sensitive to different parameters and can thus give complementary information in our study we also point out the value of studying the evolution of the changes at different times in the coda

glaciation and deglaciation mechanisms in a coupled two dimensional climate ice sheet model a two dimensional model which links the atmosphere the mixed layer of the ocean the sea ice the continents the ice sheets and their underlying bedrock has been used to test the milankovitch theory over the last glacial interglacial cycle it was found that the orbital variations alone can induce in such a system feed backs sufficient to generate the low frequency part of the climatic variations over the last 122 kyear these simulated variations at the astronomical time scale are broadly in agreement with ice volume and sea level reconstructions independently obtained from geological data imperfections in the simulated climate were the insufficient southward extent of the ice sheets and the too small hemispheric cooling during the last glacial maximum these deficiencies were partly remedied in a further experiment gallee and others in press by using the time dependent co2 atmospheric concentration given by the vostok ice core in addition to the astronomical forcing for this second experiment the main mechanisms and feedbacks responsible for the glaciation and the deglaciation in the model are discussed here

deposition and migration of atmospheric pb in soils from a forested silicate catchment today and in the past strengbach case evidence from 210pb activities and pb isotope ratios abstract 210 pb activities pb isotope ratios and elemental abundances of leachates from soils were determined to identify the origin of atmospheric pb and characterize its migration in mineralogically and chemically different soil systems of a forested silicate catchment leaching experiments were performed to separate the leachable mobile reservoir of the soil system mainly containing atmosphere derived pb from that of the residual bedrock reservoir pb isotope data allow recognition of an evolution in the soil leachates with increasing depth pointing to at least two major pb sources surface samples collected in 2001 show a pb isotopic signature which is comparable with that of particulate matter pm collected in 1995 and 2005 2006 in an urban environment close to the catchment and hence corresponds to a very recent isotopic signature of the atmosphere the more radiogenic pb isotopic signature in soil leachates from 30 to 40 cm depth corresponds to older airborne pb probably derived from historical mining and smelting close to the catchment 210 pb inventories confirm trace element data and indicate that the soils are important sinks of atmospheric pb in the uppermost part 40 cm of the soil profile the migration speeds derived from these inventories for the different depths 0 5 cm y 1 0 2 cm y 1 1 6 cm y 1 suggest that the atmospheric pb needs less than 150 years to reach the depth of 40 cm thus the pb enrichments above this depth might be related to a period of local mining activities which ceased in the mid 20th century the soil solutions are not in isotopic equilibrium with the corresponding soil leachates soil solutions from 5 to 60 cm depth have pb isotopic compositions similar to those of stream and spring waters and soil leachates from 10 to 20 cm depth they carry a pb with current atmospheric pb isotopic compositions the presence of anthropogenic pb in spring waters might suggest that some of it migrated rather quickly through the soils

terminating the 100 kyr ice age cycle we report a simulation of the most recent 100 000 year glaciationdeglaciation cycle of the late pleistocene ice age a simulation that delivers an ice sheet chronology that is in close accord with that inferred from the geological record our analyses are performed with a reduced model of the climate system that incorporates significant improvements to the representation of both climate forcing and mass balance response in a previously described theory based upon a coupled one level energy balance model ebm and vertically integrated ice sheet model ism the theory fully incorporates the influences of orbital insolation forcing glacial isostatic adjustment and variations in the atmospheric concentrations of greenhouse gases it correctly predicts the main geographical regions of the northern hemisphere that were glaciated at last glacial maximum 21 000 years ago as well as the abrupt termination of the glacial epoch that occurred subsequently the latter feature of the ice age cycle is obtained without the need to incorporate unconstrained and therefore controversial physical processes into the model a limitation of all previous attempts to understand this global scale climate cycle our analyses suggest that the radiative impact on surface glaciation due to the changing atmospheric concentration of co2 is critical to the ability of the model to deliver a synthetic history of glaciation and deglaciation that is in accord with inferences based upon surface geological and geomorphological evidence with the incorporation of this influence model predicted ice thickness distributions at last glacial maximum lgm are very similar to those of the recently described ice 4g reconstruction that was based upon the inversion of postglacial relative sea level histories

new precipitation and accumulation maps for greenland annual total precipitation and the annual accumulation on the greenland ice sheet are evaluated and presented in two maps the maps are based on accumulation measurements of 25 1 pits and cores obtained from the upper accumulation zone and precipitation measurements made at 35 meteorological stations in the coastal region to construct the accumulation map the annual precipitation was split into solid and liquid precipitation components annual total precipitation exceeding 2500 mm w e occurs on the southeastern tip of greenland while the minimum precipitation is estimated to occur on the northeastern slope of the ice sheet the mean annual precipitation for all of greenland is 340 mm w e the largest annual accumulation of about 1500 mm w e is found on the glaciers in the southeastern corner of greenland while the smallest accumulation is found on the northeastern slope of the ice sheet west of danmarkshavn the mean accumulation on the greenland ice sheet is estimated at 310 mm w e the regional difference in accumulation is examined with respect to the 850 hpa m bar level circulation the present surface topography is found to play an important role in determining regional accumulation on the ice sheet

irregular glacial interstadials recorded in a new greenland ice core the greenland ice sheet offers the most favourable conditions in the northern hemisphere for obtaining high resolution continuous time series of climate related parameters profiles of 18o 16o ratio along three previous deep greenland ice cores1 3 seemed to reveal irregular but well defined episodes of relatively mild climate conditions interstadials during the mid and late parts of the last glaciation but there has been some doubt as to whether the shifts in oxygen isotope ratio were genuine representations of changes in climate rather than artefacts due to disturbed stratification here we present results from a new deep ice core drilled at the summit of the greenland ice sheet where the depositional environ ment and the flow pattern of the ice are close to ideal for core recovery and analysis the results reproduce the previous findings to such a degree that the existence of the interstadial episodes can no longer be in doubt according to a preliminary timescale based on stratigraphic studies the interstadials lasted from 500 to 2 000 years and their irregular occurrence suggests complexity in the behaviour of the north atlantic ocean circulation

simulation of the last glacial cycle by a coupled sectorially averaged climate ice sheet model 2 response to insolation and co2 variations a two dimensional climate model which links the northern hemisphere atmosphere ocean mixed layer sea ice and continents has been asynchronously coupled to a model of the three main northern ice sheets and their underlying bedrock the coupled model has been used to test the influence of several factors including snow surface albedo over the ice sheets in producing plausible ice age simulations using astronomically derived insolation and co2 data from the vostok ice core the impact of potentially important processes such as the water vapor transport clouds and deep sea circulation was not investigated in this study after several sensitivity experiments designed to identify the main mechanism governing surface temperature and ice accumulation the model is first run with ice sheet feedback by forcing it only with the astronomical insolation over the past 122 kyr large variations of ice volume are simulated between 122 and 55 kyr b p with a rapid latitudinal extension of the north american and eurasian ice sheets starting at 120 kyr b p the simulated last glacial maximum is at 19 kyr b p the model is able to simulate deglaciation as well the simulated evolution of the three northern ice sheets is generally in phase with geological reconstructions the major discrepancy between the simulation and paleoclimate reconstructions lies in the underestimation of temperature variations linked with an underestimation of the ice sheet extent and an excess in the prescribed co2 concentration sensitivity experiments show that ablation is more important to the ice sheet response than snow precipitation variations in the model a key mechanism in the deglaciation after the last glacial maximum appears to be the aging of snow which decreases its albedo the other factors which play an important role are in decreasing level of importance the ice sheet altitude insolation taiga cover and ice sheet extent a final set of experiments addresses the effects of co2 on the simulated climate of the last glacial maximum and on a new long term experiment covering the last 122 kyr this last experiment is made by forcing the model with both insolation and co2 variations this additional forcing improves the temperature and ice volume results despite the limitations inherent to the present modeling approach the sensitivity experiments performed can provide insight into the relative importance of possible mechanism responsible for the building and melting of huge ice sheets during the last glacial interglacial cycle

quantifying the role of biosphere atmosphere feedbacks in climate change coupled model simulations for 6000 years bp and comparison with palaeodata for northern eurasia and northern africa the lmd agcm was iteratively coupled to the global biome1 model in order to explore the role of vegetation climate interactions in response to mid holocene 6000 y bp orbital forcing the sea surface temperature and sea ice distribution used were present day and co2 concentration was pre industrial the land surface was initially prescribed with present day vegetation initial climate anomalies differences between agcm results for 6000 y bp and control were used to drive biome1 the simulated vegetation was provided to a further agcm run and so on results after five iterations were compared to the initial results in order to identify vegetation feedbacks these were centred on regions showing strong initial responses the orbitally induced high latitude summer warming and the intensification and extension of northern hemisphere tropical monsoons were both amplified by vegetation feedbacks vegetation feedbacks were smaller than the initial orbital effects for most regions and seasons but in west africa the summer precipitation increase more than doubled in response to changes in vegetation in the last iteration global tundra area was reduced by 25 and the southern limit of the sahara desert was shifted 2 5 n north to 18 n relative to today these results were compared with 6000 y bp observational data recording forest tundra boundary changes in northern eurasia and savana desert boundary changes in northern africa although the inclusion of vegetation feedbacks improved the qualitative agreement between the model results and the data the simulated changes were still insufficient perhaps due to the lack of ocean surface feedbacks

baseline determination of the atmospheric pb sr and nd isotopic compositions in the rhine valley vosges mountains france and the central swiss alps abstract the pb sr and nd isotopic compositions of biomonitors lichen moss bark and soil litter from different regions in the rhine valley as well as of

sensitivity of the lln climate model to the astronomical and co2 forcings over the last 200 ky the louvain la neuve climate model there referred to as the lln 2 d model has been used extensively to simulate the northern hemisphere ice volume under both the insolation and co2 forcings the period analysed here covers the last 200 ky first sensitivity analyses to constant co2 concentration were performed the model was accordingly forced by insolation changes only the co2 concentration being kept constant to respectively 210 250 and 290 ppmv results show that the simulated ice volume variations are comparable to the geological reconstructions only when the co2 concentration is low 210 ppmv and that the sensitivity of the simulated northern hemisphere ice volume to co2 is not constant through time second three co2 reconstructions were used to force the lln 2 d model in addition to insolation results show 1 a better agreement with the specmap oxygen isotope time series in particular as far as the amplitude of the signal is concerned and 2 that the simulated northern hemisphere ice volume is not very sensitive to the slight differences between these three reconstructions

pb concentrations and isotope ratios of soil o and c horizons in nord trondelag central norway anthropogenic or natural sources abstract soil o and c horizon samples n 752 were collected at a sample density of 1 site 36 km 2 in nord trondelag and parts of sor trondelag c 25 000 km 2 and analysed for pb and three of the four naturally occurring pb isotopes 206 pb 207 pb and 208 pb in a hno 3 hcl extraction soil o and c horizons are decoupled in terms of both pb concentrations and pb isotope ratios in the soil c horizon the grong olden culmination a continuous exposure of the precambrian crystalline basement across the general grain of the caledonian orogen is marked by a distinct 206 pb 207 pb isotope ratio anomaly no clear regional or even local patterns are detected when mapping the pb isotope ratios in the soil o horizon samples variation in the isotope ratios declines significantly from the soil c to the o horizon on average pb concentrations in the o horizon are four times higher and the 206 pb 207 pb isotope ratio is shifted towards a median of 1 15 in comparison to 1 27 in the c horizon it is demonstrated that natural processes like weathering in combination with plant uptake need to be taken into account in order to distinguish anthropogenic input from natural influences on pb concentration and the 206 pb 207 pb isotope ratio in the soil o horizon

sudden changes in north atlantic circulation during the last deglaciation sudden changes in the flow of warm atlantic surface waters into the norwegian sea occurred frequently during the last deglaciation typically involving shifts in sea surface temperature of 5 c in fewer than 40 years these led to equally large and rapid changes in atmospheric temperatures and to shifts in atlantic deep thermo haline circulation and ice sheet melting rates

a 3 d model for the antarctic ice sheet a sensitivity study on the glacial interglacial contrast on the longer climatic time scales changes in the elevation and extent of the antarctic ice sheet have an important role in modulating global atmospheric and oceanographic processes and contribute significantly to world wide sea levels in this paper a 3 d time dependent thermomechanical model for the entire ice sheet is presented that is subsequently used to examine the effects of glacial interglacial shifts in environmental boundary conditions on its geometry the model takes into account a coupled ice shelf grounding line dynamics basal sliding and isostatic bed adjustment and considers the fully coupled velocity and temperature fields ice flow is calculated on a fine mesh 40 km horizontal grid size and 10 layers in the vertical for grounded and floating ice and a stress transition zone in between at the grounding line where all stress components contribute in the effective stress in the flow law there is free interaction between ice sheet and ice shelf so that the entire geometry is internally generated a simulation of the present ice sheet reveals that the model is able to yield realistic results a series of sensitivity experiments are then performed in which lower temperatures reduced accumulation rates and lower global sea level stands are imposed either singly or in combination by comparing results of pairs of experiments the effects of each of these environmental changes can be determined in agreement with glacial geological evidence we found that the most pronounced changes show up in the west antarctic ice sheet configuration they appear to be essentially controlled by variations in eustatic sea level whereas typical glacial interglacial changes in temperature and ice deposition rates tend to balance one another these findings support the hypothesis that the antarctic ice sheet basically follows glacial episodes in the northern hemisphere by means of sea level teleconnections grounding occurs more readily in the weddell sea than in the ross sea and long time scales appear to be involved it may take up to 30 40000 years for these continental shelf areas to become completely grounded after an initial stepwise perturbation in boundary conditions according to these reconstructions a steady state antarctic ice sheet may contribute some 16 m to global sea level lowering at maximum glaciation

modelling of last glacial maximum ice sheets using different accumulation parameterizations we use a three dimensional thermomechanical ice sheet model previously tested on the greenland ice sheet to reconstruct last glacial maximum lgm ice sheets we compare the effects on the results of the ice sheet model of three different accumulation parameterization schemes in the first and second schemes lgm precipitation is computed from the present precipitation taking and not taking into account moisture transport in the third scheme lgm precipitation and surface temperatures are computed using outputs of an atmospheric global circulation model agcm treated in anomaly mode results are compared to the last reconstruction of the northern hemisphere ice sheets peltier 1994 computed using global rebound rates in a visco elastic model of the earth s crust the first two accumulation parameterizations do not give satisfactory reconstructions of the lgm ice sheets since they are unable to compute realistic lgm climatic conditions the third method gives very satisfactory results which leads us to conclude that the best way to obtain realistic lgm climatic conditions is to use agcm outputs

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

role of orbitally induced changes in tundra area in the onset of glaciation the link between glacial interglacial cycles and changes in insolation due to variations in the earth s orbital parameters is well established1 4 but of the attempts to simulate incipient glaciation using three dimensional general circulation models gcms driven by orbital forcing alone5 10 only one10 has been successful gcm experiments7 11 show that reduced summer insolation 115 000 years ago during an interglacial to glacial climate shift produces sufficient high latitude cooling to cause expansion of tundra at the expense of boreal forest11 which in turn can induce more cooling11 14 here we show using a global climate model that the increase in surface albedo under snow covered conditions that results from a biome model estimate11 of tundra expansion 115 000 years ago is sufficient to induce glaciation over extreme northeastern canada if the additional cooling from this estimated tundra expansion induces further expansion then widespread glaciation occurs at latitudes above 65 n these results suggest that the climate feedback from high latitude tundra expansion might have contributed to the onset of the most recent glaciation

a comparison of different ways of dealing with isostasy examples from modelling the antarctic ice sheet during the last glacial cycle the bedrock isostatic response exerts a strong control on ice sheet dynamics and is therefore always taken into account in ice sheet models this paperreviews the various methods normally used in the ice sheet modeling community to deal with the bedrock response and compares these with a moresophisticated full earth model each of these bedrock treatments five in total are coupled with a three dimensional thermomechanical ice sheet model underthe same forcing conditions to simulate the antarctic ice sheet during the last glacial cycle the outputs of the simulations are compared on the basis of thetime dependent behavior for the total ice volume and the mean bedrock elevation during the cycle and of the present rate of uplift over antarctica thiscomparison confirms the necessity of accounting for the elastic bending of the lithosphere in order to yield realistic bedrock patterns it furthermoredemonstrates the deficiencies inherent to the diffusion equation in modeling the complex deformation within the mantle nevertheless when characteristicparameters are varied within their range of uncertainty differences within one single method are often of the same order as those between the variousmethods this overview finally tries to point out the main advantages and drawbacks of each of these methods and to determine which one is mostappropriate depending on the specific modeling requirements

ice sheet mass balance during the last glacial maximum in the framework of the paleoclimate modelling intercomparison project pmip simulations of the last glacial maximum lgm have been performed more than 10 different atmospheric general circulation models agcms have been used with the same boundary conditions sea surface temperatures prescribed by climap 1981 ice sheet reconstruction provided by peltier 1994 change in insolation and reduced co2 content one of the major questions is to investigate whether the simulations of the lgm are in equilibrium with the prescribed ice sheet reconstruction to answer this question we have used two different approaches first we analyze the results of a sel of lgm simulations performed with different versions of the laboratoire de meteorolo gie dynamique lmd agcm and study the hydrologic and snow budgets over the laurcntide and fennoscandian ice sheets second we use the agcm outputs to force an ice sheet model in order to investigate its ability to maintain the ice sheets as reconstructed by climap 1981 or peltier 1994

timing of the last glacial maximum from observed sea level minima during the last glacial maximum ice sheets covered large areas in northern latitudes and global temperatures were significantly lower than today but few direct estimates exist of the volume of the ice sheets or the timing and rates of change during their advance and retreat here we analyze four distinct sediment facies in the shallow tectonically stable bonaparte gulf australia each of which is characteristic of a distinct range in sea level to estimate the maximum volume of land based ice during the last glaciation and the timing of the initial melting phase we use faunal assemblages and preservation status of the sediments to distinguish open marine shallow marine marginal marine and brackish conditions and estimate the timing and the mass of the ice sheets using radiocarbon dating and glacio hydroisostatic modelling our results indicate that from at least 22 000 to 19 000 calendar years before present land based ice volume was at its maximum exceeding today s grounded ice sheets by 52 5 x 10 exp 6 cu km a rapid decrease in ice volume by about 10 percent within a few hundred years terminated the last glacial maximum at 19 000 250 years

evidence for massive discharges of icebergs into the north atlantic ocean during the last glacial period sediments in the north atlantic ocean contain a series of layers that are rich in ice rafted debris and unusually poor in foraminifera1 here we present evidence that the most recent six of these heinrich layers deposited between 14 000 and 70 000 years ago record marked decreases in sea surface temperature and salinity decreases in the flux of planktonic foraminifera to the sediments and short lived massive discharges of icebergs originating in eastern canada the path of the icebergs clearly marked by the presence of ice rafted detrital carbonate can be traced for more than 3 000 km a remarkable distance attesting to extreme cooling of surface waters and enormous amounts of drifting ice the cause of these extreme events is puzzling they may reflect repeated rapid advances of the laurentide ice sheet perhaps associated with reductions in air temperatures yet temperature records from greenland ice cores appear to exhibit only a weak corresponding signal moreover the 5 10 000 yr intervals between the events are inconsistent with milankovitch orbital periodicities raising the question of what the ultimate cause of the postulated cooling may have been

modelling northern hemisphere ice volume over the last 3 ma the northern hemisphere ice sheet volumes over the last 3 ma were simulated using the lln 2 d model the forcings were both insolation and co2 concentration different atmospheric co2 scenarios were used because of a lack of co2 reconstruction over this remote past with constant co2 concentrations the simulated ice volume does not show any gradual increase as recorded in the marine sediments moreover its spectrum changes according to the chosen value of co2 when co2 is fixed at 220 ppmv the simulated ice volume is dominated by the similar to 100 ka period if a pre industrial co2 concentration of 280 ppmv is used the simulation is dominated by the similar to 41 ka period by using a linearly decreasing co2 concentration going from 320 ppmv at 3 ma bp to 200 ppmv at present the simulated changes in the power spectrum are in agreement with those obtained from the sedimentary records roughly before 1 ma bp the dominating period is similar to 41 ka afterwards the similar to 100 ka period becomes dominant this transition is accompanied by a gradual increase of the ice volume potential mechanisms for this transition are discussed using cyclic co2 fluctuations over the last 0 6 ma does not change significantly the spectral characteristics of the simulated ice volume but amplifies the amplitude of its variations c 1998 elsevier science ltd all rights reserved

the biosphere a homogeniser of pb isotope signals abstract rock soil and plant terrestrial moss european mountain ash leaves mountain birch leaves bark and wood and spruce needles and wood samples collected at 3 km intervals along a 120 km long transect 40 sites cutting the city of oslo norway were analysed for their pb concentration and pb isotope ratios a general decrease in 206 pb 207 pb 208 pb 207 pb and 206 pb 208 pb ratios with a characteristic low variability in all plant materials and the plant derived o horizon of soil profiles compared to rocks and mineral soils is observed along the transect it is demonstrated that minerogenic and biogenic sample materials belong to two different spheres the lithosphere and biosphere and that geochemical processes determining their chemical and isotopic compositions differ widely background variation for both sample materials needs to be established and documented at the continental and global scale before the anthropogenic influence on the geochemistry of the earth s surface can be reliably estimated

simulated influence of carbon dioxide orbital forcing and ice sheets on the climate of the last glacial maximum a coupled atmosphere ocean sea ice model is used to investigate the climate of the last glacial maximum 21 000 years ago and the relative climate forcing effects of atmosphere co2 the earth s orbital parameters and ice sheet albedo tropical temperatures are found to be 2 2 c less than today s slightly colder than indicated by the climap palaeoclimate reconstruction this result is consistent with a low to medium climate sensitivity to radiative perturbations temperatures are colder still in the northern north atlantic region owing to a weakening and shallowing of the thermohaline circulation a sensitivity analysis suggests that changes in ocean circulation since the last glacial maximum have not contributed directly to the global mean temperature change since that time

comment on the biosphere a homogeniser of pb isotope signals by c reimann b flem a arnoldussen p englmaier t e finne f koller and o nordgulen reimann et al 2008 recently published a study on pb isotope signature along a 120 km long transect cutting the city of oslo based on concentration but also isotope data they misinterpret pb concentration of the biosphere in rural places and explain these large enrichments of pb as being due to natural processes the study ignores numerous previous studies either on local regional or global scales see reviews by shotyk and le roux 2005 and callender 2003 and references therein which clearly demonstrate that anthropogenic pb emitted in the atmosphere from different sources leaded gasoline coal burning metallurgy etc was and is dispersed worldwide the study also ignores work on norway by the steinnes and colleagues group harmens et al 2008 steinnes et al 2005a steinnes et al 2005b and aberg et al 2004 and measurements and modelling by the emep network www emep int emep 2005 the study also neglects numerous works on preanthropogenic pb deposition rate and isotopic signature using continental archives of atmospheric deposition like peat bogs shotyk et al 1998 klaminder et al 2003 kylander et al 2005 and le roux et al 2005 these studies have shown that preanthropogenic pb atmospheric deposition rate and its pb isotopic signature is regionally defined but also that those signals are negligible compared to past 2 ka and recent pb atmospheric fluxes table 1

geomorphological reconstruction of the labrador sector of the laurentide ice sheet abstract the labrador sector formed one of three major domes of the laurentide ice sheet during the last wisconsinan glaciation reconstruction of its evolution is important because it modulated atmospheric and oceanic circulation over 1 100 ka time scales this paper reports new geomorphological evidence of ice flow which is used to reconstruct glimpses of the behaviour of the labrador sector during its evolution flow evidence indicates a southern position 51 n of the labrador ice divide presumed to be the usual location when the ice sheet was close to its maximum extent a previously inferred dramatic surge of labrador ice across hudson strait and onto baffin island at 9 9 9 4 ka is verified seemingly contradictory evidence regarding deglaciation in northern quebec has been analysed and a reconstruction developed that solves this problem deglacial retreat of the labrador ice sheet was highly asymmetric with rapid retreat of the southern margin northwards migration of the centre of mass by 500 km and slow retreat along the northern margin as it shrank its thermal regime changed from cold centred to entirely cold based and it finally fragmented into many residual ice caps and glaciers this dynamic and asymmetric behaviour may be due to destabilisation of the labrador ice sheet triggered by heinrich event ice streaming through hudson strait

the co 2 induced thickening thinning of the greenland and antarctic ice sheets as simulated by a gcm ccmi and an ice sheet model scaling analysis shows that the mean thickness of an ice sheet depends on the product of two poorly known quantities the ice viscosity and the net snow accumulation rate we adjust the viscosity of an ice sheet in order to get a consistent value of this product for the present day ice sheet volume and area given the net snow accumulation rate calculated by an atmospheric general circulation model gcm we then hold this artificial rheology constant in further numerical experiments we hope that in doing so we can partially compensate for systematic gcm errors in simulating the snow accumulation rate and therefore thickening thinning of ice sheets will depend mostly on the tendency in the net accumulation change rather than on its absolute value using this approach the response of the greenland and antarctic ice sheets to doubling co2 concentration is simulated and the horizontal distribution of possible thickening thinning of polar ice obtained we find that initially the region of thickening ice is close to the area of increased snowfall rate but later it significantly changes under the influence of internal ice flow dynamics the sea level changes predicted by our experiments agree with some empirical estimates the sensitivity experiment with assigned basal sliding does not show significant changes in the large scale ice topography meaning for example that there is no indication of a possible disintegration of the west antarctic ice sheet at the same time the regional thickening thinning of ice and consequently the sea level change depends strongly on processes at the ice sheet bottom