timing of the antarctic cold reversal and the atmospheric co2increase with respect to the younger dryas event the transition from the last glacial to the holocene is a key period for understanding the mechanisms of global climate change ice cores from the large polar ice sheets provide a wealth of information with good time resolution for this period however interactions between the two hemispheres can only be investigated if ice core records from greenland and antarctica can be synchronised accurately and reliably the atmospheric methane concentration shows large and very fast changes during this period these variations are well suited for a synchronisation of the age scales of ice cores from greenland and antarctica here we confirm the proposed lead of the antarctic cold reversal on the younger dryas cold event the antarctic cooling precedes the younger dryas by at least 1 8 kyr this suggests that northern and southern hemispheres were in anti phase during the younger dryas cold event a further result of the synchronisation is that the long term glacial interglacial increase of atmospheric co 2 was not interrupted during the younger dryas event and that atmospheric co 2 changes are not necessarily dominated by changes in the north atlantic circulation

what do 14c changes across the gerzensee oscillation gi 1b event imply for deglacial oscillations high resolution studies were performed on late glacial sediments from a small lake in western denmark with respect to lithology geochemistry stable isotopes pollen stratigraphy and radiocarbon dating on terrestrial macrofossils one purpose was to detect the so called gerzensee oscillation or the gi 1b event in the later part of the allerod warm period and to describe the environmental impact of this short cooling the other aim was to test the hypothesis that considerable 14c changes occur over this time which can be related to ocean ventilation thermohaline circulation changes we find that the gi 1b event had a major impact on both terrestrial and limnic ecosystems large vegetation changes increased soil erosion and lowered aquatic production by correlations to events in the grip ice core and 14c patterns in the cariaco basin we also transformed our 14c dated record into calendar years to calculate 14c values the 14c dates show that the gi 1b event both preceded and was part of the 11 400 11 300 14c yr bp radiocarbon plateau and was followed by the 11 000 10 900 14c yr bp plateau thus the later part of the event coincides with a distinct age decline this delayed age drop 14c rise in relation to the hypothetical triggering mechanism behind the event decreased ocean ventilation could be explained by redeposited macrofossils at the onset of gi 1b this phenomenon also seen at the onset of younger dryas may also reflect increased soil erosion and redeposition at the start of cold periods the independent cariaco basin record however implies that the very end of the cool gi 1b event is related to a distinct rise in 14c likewise the 10be record from gisp2 shows a distinct rise in the middle of the event precluding decreased solar forcing as the trigger of the climate event but making it likely that high cosmic ray flux low solar activity may be the cause of the rising atmospheric 14c content we thus conclude that the 14c changes over the gerzensee oscillation gi 1b being one of several coolings during the last termination does not seem to be related to ocean ventilation changes the reason behind this lack of coincidence between rising 14c and a fairly distinct northern hemisphere cooling may be due to the fact that the oceanic changes during some of these coolings are too subtle to give an atmospheric 14c imprint or that an anti phase relationship between the two hemispheres blurs the 14c signal or finally that a partly unknown mechanism may lie behind such coolings copyright 2000 john wiley sons ltd

radiocarbon dating the devensian lateglacial in britain new evidence from llanilid south wales radiocarbon dates are described from a section through lateglacial and early flandrian sediments at llanilid mid glamorgan south wales comparisons between age determinations on the alkali soluble humic and alkali insoluble humin organic fractions from 12 biostratigraphic horizons reveal the extent of contamination by both older and younger carbon residues the llanilid time scale suggests that for the lateglacial the earliest organic sediments date from around 13 200 yr bp the early interstadial juniperus maximum occurred at ca 12 400 12 500 yr bp with a marked decline some 200 years later the main betula phase lasted only from ca 11 700 to 11 400 yr bp and the end of the interstadial occurred around 11 100 yr bp the beginning of the flandrian dates from ca 10 000 yr bp the juniperus maximum occurred approximately 200 years later the expansion of birch woodland began around 9600 yr bp while the first hazel arrived in the area at ca 9300 yr bp these age determinations are discussed in the context of radiocarbon dates from comparable biostratigraphic horizons in western britain and the dating of lateglacial events in the ocean core records from the north atlantic

rapid changes in ocean circulation and atmospheric radiocarbon a latitude depth coupled global ocean ice atmosphere model is extended to include a simple biosphere component a physically reasonable adjustment of runoff into the north atlantic is invoked to achieve a transient response to glacial meltwater perturbations which closely resembles the younger dryas climate event we then investigate the evolution of the isotopic ratio of atmospheric radiocarbon 14c due to the rapid changes of deep ocean circulation when the north atlantic branch of the conveyor belt circulation is interrupted the oceanic uptake of radiocarbon is reduced resulting in an increase of atmospheric 14c by about 35 the reduction of ventilation in the north atlantic is partly compensated by an increase of the 14c ratios of the biosphere the southern ocean and the upper ocean above 1000 m depth a plateau of the 14c year calendar year relation can be generated at the time of the rapid reinitiation of deep ocean ventilation which begins coincident with the major temperature increase and lasts for about 60 years it is hence significantly shorter than that found by analyzing tree rings during the termination of younger dryas longer than 400 years a sensitivity study reveals that the duration of the plateau depends strongly on the transient evolution of the gas exchange rate and can increase to 150 300 years if changes of pco2 or sea ice coverage are taken into account

variations of atmospheric 14 c concentrations over the allerood younger dryas transition highly variable atmospheric radiocarbon concentrations are a distinct feature during the last deglaciation the synchronisation of two high resolution ams 14c dated records lake go ci z and a floating late weichselian glacial varve chronology at the allerod younger dryas transition allowed us to assess in detail atmospheric 14c changes between late allerod and early preboreal the combined data set shows a drastic rise in 14c during the first 200 years or so of younger dryas and the two following about 500 year long 14c plateaux model experiments which included variations in the geomagnetic field atmospheric co2 variations and a drastic reduction in north atlantic deep water flux at the onset of younger dryas allowed to reproduce the distinct rise in 14c during the first 200 years of younger dryas fairly well also the drop in 14c at the younger dryas holocene boundary seems reasonably explained by changes in north atlantic deep water circulation however the reason behind the anomalous behaviour of the 14c signal in the middle of younger dryas remains an open question

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

deglacial changes in ocean circulation from an extended radiocarbon calibration temporal variations in the atmospheric concentration of radiocarbon sometimes result in radiocarbon based age estimates of biogenic material that do not agree with true calendar age this problem is particularly severe beyond the limit of the high resolution radiocarbon calibration based on tree ring data which stretches back only to1 2 about 11 8 kyr before present bp near the termination of the younger dryas cold period if a wide range of palaeoclimate records are to be exploited for better understanding the rates and patterns of environmental change during the last deglaciation extending the well calibrated radiocarbon timescale back further in time is crucial several studies attempting such an extension using uranium thorium dated corals3 4 5 and laminae counts in varved sediments6 7 8 9 show conflicting results here we use radiocarbon data from varved sediments in the cariaco basin in the southern caribbean sea to construct an accurate and continuous radiocarbon calibration for the period 9 to 14 5 kyr bp nearly 3 000 years beyond the tree ring based calibration a simple model compared to the calculated atmospheric radiocarbon concentration and palaeoclimate data from the same sediment core suggests that north atlantic deep water formation shut down during the younger dryas period but was gradually replaced by an alternative mode of convection possibly via the formation of north atlantic intermediate water

late glacial climatic oscillations as recorded in swiss lake sediments regional pollen assemblage zones for the late glacial period of the swiss plateau are introduced and defined they include four major zones artemisia juniperus hippophae betula pinus paz with several subzones pollen and oxygen isotope analyses on lacustrine sediments from several lakes in the area reveal four distinct phases of climatic oscillation in the time period of 13 000 9500 yr bp the first oscillation termed the aegelsee fluctuation occurs shortly before 12 000 yr bp and varve counts suggest its duration was ca 100 yr it is characterised by a short decrease in the oxygen isotopes as well as a short increase in nap associated with a depression in birch pollen values the second oscillation which occurs in the 18o record shortly before the deposition of the laacher see tephra ca 11 000 yr bp is termed the gerzensee fluctuation it occurs during a pine dominated phase and its vegetational effects cannot be determined palynologically the most prominent regressive phase is the younger dryas biozone ca 10 700 10 000 yr bp characterised by an increase in heliophilous nap and low 18o values the younger dryas biozone can often be subdivided palynologically into two parts a first part rich in grasses and juniper and a second part with higher filipendula and birch values during the preboreal biozone another distinct oscillation is evidenced only in the oxygen isotope ratios comparison of the swiss oxygen isotope profiles with the greenland dye 3 record suggests that not only the three major shifts in the 18o curves but also the minor ones are closely comparable suggesting some common climatic control

possible solar forcing of century scale drought frequency in the northern great plains a 2100 yr decadal resolution salinity and aridity proxy record of lacustrine ostracode shell mg ca ratios from a closed basin lake in the northern great plains shows statistically significant periodicities of 400 200 130 and 100 yr these periodicities are similar to the three principal solar oscillation periods 420 218 143 yr as inferred from the atmospheric radiocarbon record suggesting strong solar forcing of century scale drought frequency with a fundamental 400 yr period and its 2nd 3rd and 4th harmonics our proxy record correlates visually and statistically cross spectral analysis with the atmospheric 14 c record solar proxy and the gisp2 greenland ice sheet project two 18 o record climate proxy showing that solar minima are in phase with drought periods in the northern great plains and cold periods in greenland this spectral similarity together with phase correlation indicates a possible teleconnection of century scale global climate fluctuations through common solar forcing

the preboreal oscillation around the nordic seas terrestrial and lacustrine responses the occurrence of an early preboreal climatic cooling oscillation pbo in lacustrine and glacial records from northwest europe iceland and greenland is reviewed and documented the often subtle response of the proxy records to this oscillation in combination with its short duration make it difficult to detect owing to its chronostratigraphic position between the 10 000 9900 and 9600 9500 14 c plateaux c 11 300 11 150 calendar yr bp it is also difficult to 14 c date with precision we find that the vegetation response to the pbo varies between sites and regions in contrast to the pioneer vegetation in iceland and southern sweden the expanding birch pine forest in germany denmark was more susceptible to deteriorating growing conditions the combined lacustrine tree ring and glacial records imply that the pbo was characterised by cool and humid conditions throughout northwestern and central europe this is documented by vegetation changes decreased aquatic production increased soil erosion increased 2 h and 13 c content in tree rings readvances or stillstands of the ice sheet in norway and finland and ingression of brackish water into the baltic icelandic proxy records from lake sediments and glacial moraines imply cooler conditions than during the previous preboreal period but not as extreme as during the younger dryas greenland records suggest that the early preboreal was characterised by ice readvances as an effect of cool climate and increased precipitation in relation to the younger dryas it was not until the end of the pbo that climate was warm enough to melt the land based ice sheet this preboreal oscillation found on both sides of the nordic seas is interpreted as an effect of increased freshwater forcing on the thermohaline circulation in the nordic seas which is implied by a simultaneous and distinct rise in the atmospheric 14 c 12 c ratio a slow down of the thermohaline circulation may tempor arily have pushed the polar front further south 1997 by john wiley sons ltd preboreal oscillation thermohaline circulation lacustrine records glacial records chronology

a 43 ka record of paleoenvironmental change in the central american lowlands inferred from stable isotopes of lacustrine ostracods we present a continuous ostracod isotope 18o and 13c record from lake peten itza peten guatemala in the northern lowland neotropics that spans the last 43 cal ka bp variations in oxygen and carbon isotopes closely follow lithologic variations which consist of alternating gypsum and clay deposits that were deposited under relatively dry and wet climate respectively during the last glacial period the greatest 18o and 13c values coincide with gypsum deposited during lake lowstands under arid climate conditions that were correlated previously with north atlantic heinrich events in contrast interstadials and the entirety of the last glacial maximum 24 19 cal ka bp are marked by clay deposition and lower 18o and 13c values reflecting higher lake levels and relatively moister climate isotope results and pollen data along with independently inferred past water levels show the early deglacial period 19 15 cal ka bp was the time of greatest aridity and lowest lake stage of the past 43 ka this period occurred during heinrich stadial 1 hs 1 when an extensive tropical megadrought has been postulated stager et al 2011 heinrich stadial 1 is represented by two episodes of gypsum precipitation and high 18o and 13c values in peten itza interrupted by an intervening period of lower 18o and 13c and clay deposition centered on 17 cal ka bp the two periods of inferred maximum cold and or arid conditions at 17 5 and 16 1 cal ka bp coincide approximately with two pulses of ice rafted debris ird recorded off southern portugal bard et al 2000 at 15 cal ka bp coinciding with the start of the bolling allerod period 18o and 13c decrease and gypsum precipitation ceases indicating a transition to warmer and or wetter conditions gypsum precipitation resumed while 18o and 13c increased at the start of the younger dryas at 13 1 cal ka bp and continued until 10 4 cal ka bp near the onset of the holocene precipitation changes during the last glacial period in the northern hemisphere neotropics were closely linked with freshwater forcing to the high latitude north atlantic and sensitive to changes in the location of meltwater input climate was coldest driest when meltwater directly entered the high latitude north atlantic permitting sea ice expansion and weakening of atlantic meridional overturning circulation amoc which resulted in a more southerly position of the intertropical convergence zone itcz upon deglaciation when meltwater was directed to the gulf of mexico at 17 ka and during the bolling allerod period 15 13 ka precipitation increased in the northern hemisphere neotropics as north atlantic sea ice retreated and the itcz shifted northward results from lake peten itza offer some support for the meltwater routing hypothesis of clark et al 2001

the greenland ice sheet project 2 depth age scale methods and results the greenland ice sheet project 2 gisp2 depth age scale is presented based on a multiparameter continuous count approach to a depth of 2800 m using a systematic combination of parameters that have never been used to this extent before the ice at 2800 m is dated at 110 000 years b p with an estimated error ranging from 1 to 10 in the top 2500 m of the core and averaging 20 between 2500 and 2800 m parameters used to date the core include visual stratigraphy oxygen isotopic ratios of the ice electrical conductivity measurements laser light scattering from dust volcanic signals and major ion chemistry gisp2 ages for major climatic events agree with independent ages based on varve chronologies calibrated radiocarbon dates and other techniques within the combined uncertainties good agreement also is obtained with greenland ice core project ice core dates and with the specmap marine timescale after correlation through the 18 o of o 2 although the core is deformed below 2800 m and the continuity of the record is unclear we attempted to date this section of the core on the basis of the laser light scattering of dust in the ice

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

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

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

monitoring fracture propagation in a soft rock neapolitan tuff using acoustic emissions and digital images sudden and unexpected collapses of underground cavities below the city of naples italy represent a serious safety hazard the collapses occur due to the detachment of large blocks from the cavity roofs walls and pillars often a long time after the original quarry excavation has been completed it is recognised that existing discontinuities e g fractures play an important role in the failure process by inducing local stress concentrations and reducing the overall material strength the larger fractures which ultimately lead to collapse occur through interaction propagation and coalescence of these discontinuities this paper presents recent results of experiments carried out on natural dry specimens of neapolitan finegrained tuff tuff to investigate the mechanisms involved in sample failure a better understanding of fracture development and rock bridge behaviour is gained through a combination of ae and photographic monitoring in an experimental program considering samples with artificial pre existing heterogeneities which simulate the in situ discontinuities for a range of rock bridge geometries the mechanisms and timing of different stages of the failure process are identified and characterised the results show that in general a classical description of failure for samples without artificial flaws or with only a single flaw is followed 1 crack closure 2 linear stress strain response and crack initiation with stable crack growth 3 crack damage and unstable crack growth leading to failure for samples with two artificial pre existing flaws the third phase is split into two parts and failure of the sample occurs only after both the unstable propagation of external wing cracks and coalescence of the internal cracks in the bridge in terms of the timing and duration of each phase it is seen that phases 1 and 2 have little dependence on the flaw configuration but phase 3 seems to depend directly on this in particular the angle in rock bridge between the inner tips of the pre existing flaws plays a key role phase 3 is shorter for 120 than for 105 these differences by bridge rotation which appears to take longer than the simpler mode of failure for 120 it has only been possible to determine the time ranges of interest using the ae signatures whilst the photographs allow the fracture geometry evolution to be described additionally the frequency character of ae events is investigated and shown to have significant potential for characterisation of ae source types and thus failure processes

modelling the concentration of atmospheric co2 during the younger dryas climate event the younger dryas yd dated between 12 7 11 6 ky bp in the grip ice core central greenland is a distinct cold period in the north atlantic region during the last deglaciation a popular but controversial hypothesis to explain the cooling is a reduction of the atlantic thermohaline circulation thc and associated northward heat flux as triggered by glacial meltwater recently a ch4 based synchronization of grip 18o and byrd co2 records west antarctica indicated that the concentration of atmospheric co2 coatm2 rose steadily during the yd suggesting a minor influence of the thc on coatm2 at that time here we show that the coatm2 change in a zonally averaged circulation biogeochemistry ocean model when thc is collapsed by freshwater flux anomaly is consistent with the byrd record cooling in the north atlantic has a small effect on coatm2 in this model because it is spatially limited and compensated by far field changes such as a warming in the southern ocean the modelled southern ocean warming is in agreement with the anti phase evolution of isotopic temperature records from grip northern hemisphere and from byrd and vostok east antarctica during the yd 13c depletion and po4 enrichment are predicted at depth in the north atlantic but not in the southern ocean this could explain a part of the controversy about the intensity of the thc during the yd potential weaknesses in our interpretation of the byrd co2 record in terms of thc changes are discussed

an event stratigraphy for the last termination in the north atlantic region based on the greenland ice core record a proposal by the intimate group it is suggested that the grip greenland ice core should constitute the stratotype for the last termination based on the oxygen isotope signal in that core a new event stratigraphy spanning the time interval from ca 22 0 to 11 5 k grip yr bp ca 19 0 10 0 k 14 c yr bp is proposed for the north atlantic region this covers the period from the last glacial maximum through termination 1 of the deep ocean record to the pleistocene holocene boundary and encompasses the last glacial late glacial of the traditional northwest european stratigraphy the isotopic record for this period is divided into two stadial episodes greenland stadials 1 gs 1 and 2 gs 2 and two interstadial events greenland interstadials 1 gi 1 and 2 gi 2 in addition gi 1 and gs 2 are further subdivided into shorter episodes the event stratigraphy is equally applicable to ice core marine and terrestrial records and is considered to be a more appropriate classificatory scheme than the terrestrially based radiocarbon dated chronostratigraphy that has been used hitherto 1998 john wiley sons ltd

the chronology of the last deglaciation implications to the cause of the younger dryas event it has long been recognized that the transition from the last glacial to the present interglacial was punctuated by a brief and intense return to cold conditions this extraordinary event referred to by european palynologists as the younger dryas was centered in the northern atlantic basin evidence is accumulating that it may have been initiated and terminated by changes in the mode of operation of the northern atlantic ocean further it appears that these mode changes may have been triggered by diversions of glacial meltwater between the mississippi river and the st lawrence river drainage systems we report here accelerator mass spectrometry ams radiocarbon results on two strategically located deep sea cores one provides a chronology for surface water temperatures in the northern atlantic and the other for the meltwater discharge from the mississippi river our objective in obtaining these results was to strengthen our ability to correlate the air temperature history for the northern atlantic basin with the meltwater history for the laurentian ice sheet

temperature accumulation and ice sheet elevation in central greenland through the last deglacial transition we present a combined heat and ice flow model constrained by measurements of temperature in the greenland ice sheet project 2 gisp2 borehole and by the gisp2 18o record and depth age scale which determines a history of temperature accumulation rate and ice sheet elevation for the past 50 000 years in central greenland important results are that the temperature increase from average glacial to holocene conditions was large approximately 15 c with a 20 c warming from late glacial to holocene that the average accumulation rate during the last glacial maximum between 15 and 30 kyr b p was 5 5 to 7 cm yr 1 approximately 25 of the modern accumulation rate that long term 500 1000 years averaged accumulation rate and temperature have been inversely correlated during the most recent 7 millennia of the holocene and that the greenland ice sheet probably thickened during the deglacial transition the inverse correlation of accumulation rate and temperature in the mid and late holocene suggests that the greenland ice sheet is more prone to volume reduction in a warmed climate than previously thought and demonstrates that accumulation rate is not a reliable proxy for temperature the elevation history of the ice sheet is poorly constrained by the model and independent evidence is needed we also present a simple estimate of the response time for thinning of the interior region of an ice sheet due to retreat of its margins this was approximately 1900 years for central greenland during deglaciation