atmospheric methane during the last four glacial interglacial cycles rapid changes and their link with antarctic temperature 1 atmospheric methane ch4 recorded in antarctic ice cores represents the closest ice proxy available for greenland temperature changes beyond times when greenland climate records are available the record over four climatic cycles from the vostok ice core offers the opportunity to study the phase relationship between greenland and antarctic climate changes through detailed ch4 profiles combining american and french analytical efforts we have improved the time resolution of the existing ch4 record from petit et al 1999 spectral analyses reveal high and low frequency variability including a strong precessional component the phase relationship between ch4 and the antarctic temperature proxy d shows a systematic lag of ch4 versus temperature by 1100 200 years on long timescales 50 400 kyr and a more complex behavior over shorter timescales i e 25 kyr suggesting that dansgaard oeschger type of climatic variability and associated interhemispheric linkage are robust features of late quaternary climate

climate correlations between greenland and antarctica during the past 100 000 years the ice cores recovered from central greenland by the grip1 2 and gisp23 projects record 22 interstadial warm events during the part of the last glaciation spanning 20 105 kyr before present the ice core from vostok east antarctica records nine interstadials during this period4 5 here we explore links between greenland and antarctic climate during the last glaciation using a high resolution chronology derived by correlating oxygen isotope data for trapped o2 in the gisp2 and vostok cores we find that interstadials occurred in east antarctica whenever those in greenland lasted longer than 2 000 years our results suggest that partial deglaciation and changes in ocean circulation are partly responsible for the climate teleconnection between greenland and antarctica ice older than 115 kyr in the gisp2 core shows rapid variations in the 18o of o2 that have no counterpart in the vostok record the age depth relationship and thus the climate record in this part of the gisp2 core appears to be significantly disturbed

strong hemispheric coupling of glacial climate through freshwater discharge and ocean circulation the climate of the last glacial period was extremely variable characterized by abrupt warming events in the northern hemisphere accompanied by slower temperature changes in antarctica and variations of global sea level it is generally accepted that this millennial scale climate variability was caused by abrupt changes in the ocean thermohaline circulation here we use a coupled ocean atmosphere sea ice model to show that freshwater discharge into the north atlantic ocean in addition to a reduction of the thermohaline circulation has a direct effect on southern ocean temperature the related anomalous oceanic southward heat transport arises from a zonal density gradient in the subtropical north atlantic caused by a fast wave adjustment process we present an extended and quantitative bipolar seesaw concept that explains the timing and amplitude of greenland and antarctic temperature changes the slow changes in antarctic temperature and its similarity to sea level as well as a possible time lag of sea level with respect to antarctic temperature during marine isotope stage 3

n2o and ch4 variations during the last glacial epoch insight into global processes 1 greenhouse gas measurements along polar ice cores provide important insight into the former composition of the atmosphere its natural variations and the responses to fast climatic changes in the past we present high resolution nitrous oxide n2o and methane ch4 records measured along two ice cores from central greenland covering part of marine isotope stages 3 and 4 in the last glacial epoch the n2o data confirm the hypothesis that n2o shows variations in phase to fast climatic changes observed in the northern hemisphere the so called dansgaard oeschger d o events variations exist not only for events with a long duration 1500 years and more but also for the shorter ones the comparison with ch4 unveils interesting differences between the response of ch4 and n2o to d o events while the average amplitudes of ch4 oscillations associated with d o events are similar to those of the northern hemisphere summer insolation the magnitude of the n2o concentration change instead correlates with the duration of the d o events the records give further insight into the timing of concentration changes at the beginning of d o events they show that for long lasting events the n2o concentration starts to increase before both the sharp increase in the ch4 concentration and the temperature reconstructed for greenland

on the limits of antarctic and marine climate records synchronization lag estimates during marine isotopic stages 5d and 5c 1 north atlantic sediment records md95 2042 greenland greenland ice core project grip and antarctica byrd and vostok ice core climate records have been synchronized over marine isotopic stage 3 mis 3 64 to 24 kyr b p shackleton et al 2000 the resulting common timescale suggested that md95 2042 18obenthic fluctuations were synchronous with temperature changes in antarctica dice or 18oice records in order to assess the persistency of this result we have used here the recent greenland northgrip ice core covering the last glacial inception we transfer the antarctic vostok gt4 timescale to northgrip 18oice and md95 2042 18oplanktonic records and precisely quantify all the relative timing uncertainties during the rapid warming of dansgaard oeschger 24 md95 2042 18obenthic decrease is in phase with 18oplanktonic decrease and therefore with northgrip temperature increase but it takes place 1700 1100 years after the antarctic warming thus the present study reveals that the results obtained previously for mis 3 cannot be generalized and demonstrates the need to improve common chronologies for marine and polar archives

phase relationships between millennial scale events 64 000 24 000 years ago a core recovered on the iberian margin off southern portugal can be correlated with greenland ice cores using oxygen isotope variability in planktonic foraminifera which closely matches the ice core records of temperature over greenland our age model identifies the base of every interstadial between 64 000 and 24 000 years ago and uses the greenland ice core project grip timescale the oxygen isotope signal in benthic foraminifera on this grip based timescale is quite different from the planktonic record and resembles the temperature record over antarctica when this is synchronized with greenland using the record of methane in the atmospheric air in the polar ice cores we interpret the benthic record as indicating significant fluctuations in ice volume during millennial events and we suggest that antarctic temperature changed as a function of ice volume

a novel method to study the phase relationship between antarctic and greenland climate 1 a classical method for understanding the coupling between northern and southern hemispheres during millennial scale climate events is based on the correlation between greenland and antarctic ice core records of atmospheric composition here we present a new approach based on the use of a single antarctic ice core in which measurements of methane concentration and inert gas isotopes place constraints on the timing of a rapid climate change in the north and of its antarctic counterpart we applied it to the marine isotope stage mis 5d c transition early in the last glaciation 108 ky bp our results indicate that the antarctic temperature increase occurred 2 ky before the methane increase which is used as a time marker of the warming in the northern hemisphere this result is in agreement with the bipolar seesaw mechanism used to explain the phase relationships documented between 23 and 90 ky bp blunier and brook 2001

high resolution record of northern hemisphere climate extending into the last interglacial period high resolution record of northern hemisphere climate extending into the last interglacial period

the classic marine isotope substage 5e since its identi cation nearly fty years ago marine iso tope stage 5 mis 5 has been placed onto absolute time scaleson the basis of three independent approaches cesare emiliani who set up the isotope stages emiliani 1955 depended onuranium series dating of the sediments a method that today isregarded as not generally capable of yielding useful precisionor accuracy broecker and van donk 1970 pioneered the ap proach of correlating to radiometrically dated marine coral ter races thishasbeenmuchaidedinrecentyearsbyimprovementsin the precision and accuracy of these age determinations thathave owed from the development of thermal ionization massspectrometry tims for uranium series dating edwards

modeling the densification of polar firn including heat diffusion application to close off characteristics and gas isotopic fractionation for antarctica and greenland sites 1 modeling the densification of polar firn is essential to estimate variations of close off characteristics density close off depth delta age in relation with past climate parameters temperature and accumulation rates furthermore the air composition in the firn is modified by gravitational and thermal fractionation and stable isotope measurements of permanent gases like nitrogen or argon can provide information on the amplitude of these fractionations in this work we propose a new model coupling firn densification and heat diffusion in addition to the determination of the firn thickness and gas ice age differences the model allows a reconstruction of the time evolution of the temperature for different sites in antarctica vostok and greenland gisp2 and therefore the evolution of gravitational and thermal isotopic fractionations in firn under present day conditions the modeled profiles are in good agreement with the available temperature measurements in firn for sites with low accumulation rates such as vostok the results show the existence of temperature gradients in the firn column even when no rapid climatic changes occur the comparison of the modeled 15n results to measurements allows to better constrain the 18o temperature relationship used to infer the surface temperature history and for gisp2 the model validates the long term borehole based temperature

millennial scale variability of deep water temperature and δ18odw indicating deep water source variations in the northeast atlantic 0 34 cal ka bp 1 paired measurements of mg ca and 18occ calcite 18o in benthic foraminifera from a deep sea core recovered on the iberian margin md99 2334k 37 48 n 10 10 w 3 146 m have been performed in parallel with planktonic 18occ analyses and counts of ice rafted debris ird the synchrony of temperature changes recorded in the greenland ice cores and in north atlantic planktonic 18occ allows the proxy records from md99 2334k to be placed confidently on the gisp2 time scale this correlation is further corroborated by ams 14c dates benthic mg ca measurements in md99 2334k permit the reconstruction of past deep water temperature tdw changes since 34 cal ka bp calendar kiloyears before present using these tdw estimates and parallel benthic 18occ measurements a record of deep water 18o 18odw has been calculated results indicate greatly reduced tdw in the deep northeast atlantic during the last glaciation until 15 cal ka bp when tdw warmed abruptly to near modern values in parallel with the onset of the bolling allerod interstadial subsequently tdw reverted to cold glacial values between 13 4 and 11 4 cal ka bp in parallel with the younger dryas cold reversal and the h0 ice rafting event similar millennial scale tdw changes also occurred during the last glaciation indeed throughout the last 34 cal ka millennial 18odw and tdw changes have remained well coupled and are linked with ird pulses coincident with heinrich events 3 2 1 and the younger dryas when transitions to lower tdw and 18odw conditions occurred in general millennial tdw and 18odw variations recorded in md99 2334k describe an alternation between colder low 18odw and warmer high 18odw conditions which suggests the changing local dominance of northern sourced north atlantic deep water nadw versus southern sourced antarctic bottom water aabw the observed similarity of the tdw and gisp2 18oice records would therefore suggest a common component of variability resulting from the coupling of nadw formation and greenland climate a link between greenland stadials and the incursion of cold low 18odw aabw in the deep northeast atlantic is thus implied which contributes to the relationship between greenland climate and the millennial benthic 18occ signal since 34 cal ka bp

age scale of the air in the summit ice implication for glacial interglacial temperature change the air occluded in ice sheets and glaciers has in general a younger age defined as the time after its isolation from the atmosphere than the surrounding ice matrix because snow is first transformed into open porous firn in which the air can exchange with the atmosphere only at a certain depth firn ice transition the pores are pinched off and the air is definitely isolated from the atmosphere the firn ice transition depth is at around 70 m under present climatic conditions at summit central greenland the air at this depth is roughly 10 years old due to diffusive mixing whereas the ice is about 220 years old this results in an age difference between the air and the ice of 210 years this difference depends on temperature and accumulation rate and did thus not remain constant during the past we used a dynamic firn densification model to calculate the firn ice transition depth and the age of the ice at this depth and an air diffusion model to determine the age of the air at the transition past temperatures and accumulation rates have been deduced from the 18o record using time independent functions we present the results of model calculations of two paleotemperature scenarios yielding a record of the age difference between the air and the ice for the greenland ice core project grip and the greenland ice sheet project two gisp2 ice cores for the last 100 000 years during the holocene the age difference stayed rather stable around 200 years while it reached values up to 1400 years during the last glaciation for the colder scenario the model results are compared with age differences obtained independently by matching corresponding climate events in the methane and 18o records assuming a very small phase lag between variations in the greenland surface temperature and the atmospheric methane the past firn ice transition depths are compared with diffusive column heights obtained from 15n of n2 measurements the results of this study corroborate the large temperature change of 20 to 25 k from the coldest glacial to holocene climate found by evaluating borehole temperature profiles

a minimum thermodynamic model for the bipolar seesaw 1 the simplest possible model is proposed to explain a large fraction of the millennial climate variability measured in the isotopic composition of antarctic ice cores the model results from the classic bipolar seesaw by coupling it to a heat reservoir in this thermal bipolar seesaw the heat reservoir convolves northern time signals with a characteristic timescale applying the model to the data of grip and byrd we demonstrate that maximum correlation can be obtained using a timescale of about 1000 1500 years higher correlations are obtained by first filtering out the long term variability which is due to astronomical and greenhouse gas forcing and not part of the thermal bipolar seesaw the model resolves the apparent confusion whether northern and southern climate records are in or out of phase synchronous or time lagged

changes in the oceanic 13c 12c ratio during the last 140 000 years high latitude surface water records detailed records of the carbon and oxygen isotopic ratios of neogloboquadrina pachyderma are compared between nine high latitude sediment cores from the northern and southern hemispheres covering the last 140 000 yrs the strong analogies between the 13c records permit to define a 13c stratigraphic scale with three clear cut transitions simultaneous with the oxygen isotopic transitions 65 125 kyrs 54 65 kyrs and 21 13 kyrs the 13c records of n pachyderma in the high latitude cores which follow the changes in 13c of the surface water tco2 near areas of deep water formation present trends similar to the benthic foraminifera 13c records in cores v19 30 and m12 392 although amplitudes of the isotopic shifts are different this implies that a large part of the observed variations represents global changes in the carbon distribution between biosphere and ocean the 13c12c ratios of n pachyderma in the north atlantic cores display larger regional variations at 18 kyrs b p than at present to explain these differences we have plotted the 18 kyrs b p 13c values of n pachyderma from 17 cores distributed n of 40 n comparison with published surface water temperature distribution at 18 kyrs b p indicates that a strong divergent cyclonic cell centered approximatively 55 n and 15 w was active during most of the last ice age maximum this hydrology analogous to the present weddell sea explains the published evidences of bottom water formation if located on the northern flank of the gyre and the strong polar front on the southern flank probable location of intermediate water formation

abrupt temperature changes in the western mediterranean over the past 250 000 years a continuous high resolution western mediterranean sea surface temperature sst alkenone record spanning the past 250 000 years shows that abrupt changes were more common at warming than at cooling during marine isotope stage mis 6 sst oscillated following a stadial interstadial pattern but at lower intensities and rates of change than in the dansgaard oeschger events of mis 3 some of the most prominent events occurred over miss 5 and 7 after prolonged warm periods of high stability climate during the whole period was predominantly maintained in interglacial interstadial conditions whereas the duration of stadials was much shorter

16 c rapid temperature variation in central greenland 70 000 years ago variations in the29n2 28n2 ratio of air bubbles trapped in polar ice cores and their relation to variations of the 18o 16o of the ice allow past surface temperature variations and ice age gas age differences to be determined high resolution measurements of29n2 28n2 in dansgaard oeschger event 19 around 70 000 years before the present in ice from central greenland show that at the beginning of the event the ice age gas age difference was 1090 100 years with the use of a combined firn densification temperature and gas diffusion model the 18oice temperature coefficient was determined to be 0 42 0 05 per mil per kelvin this coefficient implies a mean surface temperature change of 16 0 kelvin between 14 3 and 18 1 kelvin which differs substantially from values derived from borehole temperatures and modern spatial 18oice surface temperature correlations

estimation of temperature change and of gas age ice age difference 108 kyr b p at vostok antarctica air trapped in ice core bubbles provides our primary source of information about past atmospheres air isotopic composition 15n 14n and 40ar 36ar permits an estimate of the temperature shifts associated with abrupt climate changes because of isotope fractionation occurring in response to temperature gradients in the snow layer on top of polar ice sheets a rapid surface temperature change modifies temporarily the firn temperature gradient which causes a detectable anomaly in the isotopic composition of nitrogen and argon the location of this anomaly in depth characterizes the gas age ice age difference age during an abrupt event by correlation with the d or 18o anomaly in the ice we focus this study on the marine isotope stage 5d 5c transition 108 kyr b p a climate warming which was one of the most abrupt events in the vostok antarctica ice isotopic record petit et al 1999 a steplike decrease in 15n and 40ar 4 from 0 49 to 0 47 possibly a gravitational signal due to a change in firn thickness is preceded by a small but detectable 15n peak possibly a thermal diffusion signal we obtain an estimate of 5350 300 yr for age close to the model estimate of 5000 years obtained using the vostok glaciological timescale our results also suggest that the use of the present day spatial isotope temperature relationship slightly underestimates but by no more than 20 15 the vostok temperature change from present day at that time which is in contrast to the temperature estimate based on borehole temperature measurements in vostok which suggests that antarctic temperature changes are underestimated by up to 50 salamatin et al 1998

quantification of rapid temperature change during do event 12 and phasing with methane inferred from air isotopic measurements the description of rapid climatic changes during the last glacial period at high northern latitudes has been largely documented through greenland ice cores that are unique climatic and environmental records however greenland ice isotopic records are biased temperature proxies and it is still a matter of debate whether changes in the high latitudes lead or lag rapid changes elsewhere we focus here on the study of the mid glacial dansgaard oeschger event 12 45 ky bp associated to a large d 18 oice change in the grip greenland ice core project ice core we use combined measurements of ch4 d 15 n and d 40 ar in entrapped air associated with a recently developed firn densification and heat diffusion model to infer i the phasing between methane and temperature increases and ii the amplitude of the temperature change our method enables us to overcome the difficulty linked with rapid accumulation change in quantifying the temperature change we obtain a 12f2 5 8c temperature increase at the beginning of do event 12 thus confirming that the conventional use of water isotopes in the greenland ice cores largely underestimates the actual amplitude of rapid temperature change in central greenland in agreement with previous studies methane and temperature increase are in phase at the sampling resolution of that part of our profile 90 years d 2004 elsevier b v all rights reserved

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

paleosalinity and δ18o a critical assessment in paleoceanography a traditional application of oxygen isotope ratios in fossil carbonate from deep sea cores concerns the reconstruction of paleotemperatures recently isotopic data have been increasingly employed in the reconstruction of paleosalinities as well this application involves a number of basic assumptions a critical review of which is presented here as the calculation of paleosalinity from 18o residuals assumes constant and linear surface water salinity s 18o relationships we investigate this basic assumption for areas with important and variable freshwater budget and or sea ice influences we show that it is particularly unwarranted to assume such temporally invariant linearity in temperate and subpolar seas because of the effects of freezing and melting more critically we argue that the determinant of many regions s 18o is advection not the local water balance so that the basic 18o of seawater in many regions of the world ocean would likely have changed in a manner somewhat different from that expected from the ice volume record although the differences may not be significant for the gross reconstruction of such features as past sea level we believe they do potentially bias paleosalinity reconstructions mostly toward high salinities to fully exploit the potential of oxygen isotope records as quantitative paleosalinity indicators both the spatial and temporal variabilities need to be constrained hence we recommend that 1 for any time slice of specific interest to paleoceanographic and paleoclimatological studies a coupled ocean atmosphere isotope model or at least an ocean isotope model driven by a paleoclimate isotope model should be used to define the distribution of the base mixing end member 18o of sea water 2 much further regional scale research is needed on flux rates and isotopic compositions of the various terms in the freshwater budget as well as on the variability in the various advective terms the latter variability may be assessed from reconstructions of past seaice distributions and of volume fluxes isotopic compositions of outflow from marginal seas and through sea straits passages

a continuous record of temperature evolution over a sequence of dansgaard oeschger events during marine isotopic stage 4 76 to 62 kyr bp 1 our knowledge of the temperature evolution over greenland during dansgaard oeschger events do is currently qualitatively described through the water isotopic profile using two independent paleothermometry methods one based on air isotopic measurements and the other on the combined measurements of water isotopes dd and d 18 o we show a complete and quantitative reconstruction of temperature at the northgrip site over the period 76 to 62 kyr bp do 18 19 and 20 we confirm that the associated warmings are larger than those conventionally depicted by the water isotopes 11 c 16 c and 11 c for do 18 19 and 20 secondly we demonstrate that the relationship between temperature and d 18 o varies rapidly during the last glacial period even over a do finally our temperature reconstruction over do 19 agrees well with that predicted from simple climate models linking the do to iceberg discharges 2004 a continuous record of temperature evolution over a sequence of dansgaard oeschger events during marine isotopic stage 4 76 to 62 kyr bp geophys res lett 31 l22211 doi 10 1029 2004gl021193 2 the last glacial cycle was characterized by millenial scale climate fluctuations that have been documented in the north atlantic region through numerous marine and continental records bond et al 1997 genty et al 2003 the grip and gisp2 summit ice cores dansgaard et al 1993 grootes et al 1993 and the newly northgrip northgrip members 2004 ice core exhibit 25 do during the last glacial period these events are characterized by rapid e g in less than 100 years and large up to 16 c lang et al 1999 warmings over greenland 3 ice cores have already provided a wealth of information on do in greenland through the water isotopes for temperature changes chemical records for atmospheric circulation and analysis of air bubbles for changes in greenhouse gases concentration however temperature reconstruction from water isotopes is subject to large biases mainly due to i the seasonality of the precipitation i e periods without precipitation will not have their temperature recorded fawcett et al 1997 and ii changes in the oceanic source of greenland snow boyle 1997 whereas the latter can be estimated from the combined measurement of dd and d 18 o through the deuterium excess parameter d dd a 8 d 18 o we need additional information to account for the influence of seasonality and possible variation of the vertical atmospheric temperature profile the isotopic composition of the snow depends on the condensation temperature 4 one elegant way to overcome those difficulties is to use the isotopic composition of the air trapped in ice a method based on the thermal diffusion of gases nitrogen and argon which allows estimates of the amplitude of rapid temperature increases severinghaus and brook 1999 lang et al 1999 leuenberger et al 1999 this method gives directly access to the local mean surface temperature and is therefore not affected by seasonality the vertical temperature profile in the atmosphere nor the source temperature of the precipitation it confirms that the conventional temperature to water isotopes relationship based on the spatial slope a s the dd 18 o ice temperature slope calculated from present day surface data underestimates greenland temperature change cuffey et al 1995 the temporal slope a t dd 18 o ice dt at a given site between two different climates is lower than a s by up to a factor of 2 we have recently refined this approach using a sophisticated firnification and heat diffusion model goujon et al 2003 to quantify the temperature increase associated with large do 12 landais et al 2004 5 here we go beyond the simple quantification of the temperature increase a detailed set of d 15 n and d 40 ar measurements over do 18 19 and 20 provides us with strong constraints on the complete temperature scenario between 76 and 62 kyr bp this period roughly corresponds to marine isotope stage 4 with rapid ice sheet growth shackleton 1987 we have also measured a continuous dd profile in the ice that combined with the existing d 18 o record allows us to account for the source temperature effects this complete study makes possible the precise estimation of the temperature change over a sequence of do 6 the d 15 n and d 40 ar profiles figure 1 show a sharp peak corresponding to each warming rapid surface warm geophysical research letters vol 31 l22211

evidence of sudden rupture of a large asperity during the 2008 mw7 9 wenchuan earthquake based on strong motion analysis 1 we investigate the rupture process of the 12 may 2008 wenchuan earthquake using the records of 26 strong motion stations located 20 120 km from the seismic fault and with a good azimuthal coverage the wave velocity model required to conduct this analysis has been validated against aftershocks for which the point source hypothesis is a very good approximation the inversion of the main shock rupture process confirms the slip distribution and the average rupture velocity 3 km s previously determined however a very peculiar behavior is clearly resolved by the extensive data set used in this study the major slip area of the wenchuan earthquake located at 20 50 km north east of the epicenter is shown to break almost simultaneously 25 s after earthquake initiation this implies that slip in this part of the fault cannot be understood by simple stress release at the rupture front a more likely interpretation is the presence of a strong asperity which could break only when it was completely surrounded by stress increase resulting in a delayed but brutal rupture

hydrological impact of heinrich events in the subtropical northeast atlantic reconstructing the impact of heinrich events outside the main belt of ice rafting is crucial to understanding the underlying causes of these abrupt climatic events a high resolution study of a marine sediment core from the iberian margin demonstrates that this midlatitude area was strongly affected both by cooling and advection of low salinity arctic water masses during the last three heinrich events these paleoclimatic time series reveal the internal complexity of each of the last three heinrich events and illustrate the value of parallel studies of the organic and inorganic fractions of the sediments