climate instability during the last interglacial period recorded in the grip ice core isotope and chemical analyses of the grip ice core from summit central greenland reveal that climate in greenland during the last interglacial period was characterized by a series of severe cold periods which began extremely rapidly and lasted from decades to centuries as the last interglacial seems to have been slightly warmer than the present one its unstable climate raises questions about the effects of future global warming

recumbent folding of divide arches in response to unsteady ice divide migration arches in stratigraphic layers directly under a flow divide raymond bumps are predicted by models of steady ice sheet flow and have been observed in several ice domes here we model the evolution of these layers when a formerly stationary divide migrates rapidly to a new position then again becomes stationary leaving the arched layers in a flank position as they are then carried downstream with the flow these abandoned arches can develop into recumbent folds these folds can occur over a wide range of divide migration speeds the shearing flow that produces these recumbent folds also distributes the folded layers over a wide distance downstream from the original divide location if the divide offset is abrupt pre cores or material lines comprising core relative isochrones can be used to quickly identify portions of an abandoned raymond bump that would be overturned at any future ice core site downstream if as appears to be the case in greenland the divide is never stable long enough to produce a mature arch folds of this type would not occur the most likely place to find such folds might be the flank of an ice ridge bounded by unsteady ice streams

electrical conductivity measurements from the gisp2 and grip greenland ice cores the direct current electrical conductivity of glacial ice depends on its acidity1 3 and can also indicate changes in climate as ice formed in cold dusty periods has a high concentration of alkaline dust1 4 5 which significantly reduces the conductivity6 7 compared to warmer less dusty periods here we present electrical conductivity records for the greenland ice sheet project 2 gisp2 and greenland ice core project grip ice cores drilled 28 km apart to enable direct comparison of the results the upper parts of both records are consistent with previous evidence from other greenland cores4 8 12 for a stable greenland climate during the holocene and a series of warm events punctuating the last glacial period however there is a significant discrepancy between the two records in the bottom 10 of the cores calling into question recent reports of climate variability in the last interglacial4 8 and the penultimate glaciation8 at this stage it is too early to say what exactly is causing the discrepancy although ice flow may have introduced some discontinuities into the records further work will be necessary to establish how much climatic information it will eventually be possible to extract from the lower parts of the two cores

sensitivity of the ice divide position in greenland to climate change model calculations of depth age relations for deep ice cores in central greenland are sensitive to stability of the ice divide position in addition the folding of layers observed in the deep ice could be instigated by divide migration changing the velocity and particle paths of ice flow we use simple steady state calculations to show that lateral divide migration of between 10 km and 50 km and elevation change of approximately 100 m is likely on glacial interglacial time scales enough to affect model dating the ice divide location appears to be most sensitive to the position of the ice sheet margins by contrast the ice divide elevation is most sensitive to the accumulation rate the temperature profile and the ice stiffness profile

textures and fabrics in the grip ice core a comprehensive study of textures and fabrics has been carried out on the greenland ice core project grip ice core crystal sizes and c axis orientations have been measured on thin sections with conventional techniques yielding new information on the growth rotation and recrystallization of ice crystals in the greenland ice sheet normal grain growth is found to persist to a depth of 700 m in the core where the onset of polygonization due to increasing strain prevents a further increase in grain size in the holocene ice smaller crystals are observed in the wisconsin ice larger crystals are found in the eemian ice and the crystal size is found to vary with climatic parameters in these periods this dependence which probably results from variable impurity content in the ice persists to a depth of 2930 m coarse grained ice probably resulting from rapid growth of crystals at comparatively high temperatures is found in the lowest 100 m of the core the data on c axis orientations reveal a steady evolution of the fabric from random near the surface to a strong single maximum in the lower part of the ice sheet a significant strengthening is not observed at the holocene wisconsin transition the fabric development indicates that vertical compression at the ice divide is the main mode of deformation down to a depth of 2850 m the evolution toward a single maximum fabric hardens the ice against vertical compression but softens it against simple shear evidence of simple shear deformation is clearly observed between 2850 m and 2950 m depth stretched fabrics in coarse grained ice in the lowest 100 m could be due to tensional stresses this ice is unlikely to be undergoing any significant horizontal deformation at the present time

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

crystal size variations in eemian age ice from the grip ice core central greenland continuous measurements of ice crystal size have been carried out on an 80 m sequence between 2790 and 2870 m depth in the grip ice core from central greenland the ice in this interval is at present considered to originate from the eemian interglacial period the record reveals that the crystal size in ice older than 100 000 yr is highly dependent on climatic conditions at the time of snowfall this dependence shows up as a strong correlation between delta 18 o values and crystal size throughout the eemian as well as a negative correlation between crystal size and several soluble and insoluble impurities although high resolution impurity records are available from selected parts of the eemian ice the study is not conclusive on which impurities are most effective in slowing grain growth it is shown that the normal grain growth process commonly observed in the upper few hundred metres of polar ice sheets does not yield grain sizes compatible with observed ones at this depth in the ice sheet even in those parts of the eemian ice where impurity drag effects are not present polygonization of crystals within the ice sheet and the nucleation and rapid growth of new grains at relatively high temperatures in the lowest part probably play an important role in producing the observed grain size variations the relevance of possible flow disturbances of the grip eemian climatic record for the results presented is discussed briefly

possible displacement of the climate signal in ancient ice by premelting and anomalous diffusion the best high resolution records of climate over the past few hundred millennia are derived from ice cores retrieved from greenland and antarctica the interpretation of these records relies on the assumption that the trace constituents used as proxies for past climate have undergone only modest post depositional migration many of the constituents are soluble impurities found principally in unfrozen liquid that separates the grain boundaries in ice sheets this phase behaviour termed premelting is characteristic of polycrystalline material here we show that premelting influences compositional diffusion in a manner that causes the advection of impurity anomalies towards warmer regions while maintaining their spatial integrity notwithstanding chemical reactions that might fix certain species against this prevailing transport we find that under conditions that resemble those encountered in the eemian interglacial ice of central greenland from about 125 000 to 115 000 years ago impurity fluctuations may be separated from ice of the same age by as much as 50 cm this distance is comparable to the ice thickness of the contested sudden cooling events in eemian ice from the grip core

stochastic perturbation of divide position perturbation of divide position is considered by a linearization about the vialov nye solution and also about related solutions with 0 1 relief relaxation times of one sixteenth the fundamental thickness accumulation rate time scale are found for the vialov nye configuration while substantial basal topography can halve the rate of relaxation steady divide position is most sensitive to anti symmetric accumulation rate distributions near the divide but transient divide motion is most strongly excited by anti symmetric accumulation rate variations halfway between the margin and the divide relaxation times for the antarctic peninsula divide position are estimated to be around 200 years while the larger greenland ice sheet has a divide position relaxation time of around 600 years modelling accumulation rate as a white noise process permits analysis of divide perturbation as a stochastic ornstein uhlenbeck process where the standard deviation of the response is proportional to the standard deviation of the forcing if observed accumulation rate variability in the antarctic peninsula were antisymmetric about the divide it would be sufficient to force the divide position to fluctuate with standard deviation 10 20 times the depth of the ice sheet there appears to be sufficient noise to cause raymond bumps to be spread significantly more data on the statistical variation of accumulation with position are needed random forcing will increase the complexity of any fold structures created in the divide region and in particular the number of such structures intersecting any borehole

oxygen and hydrogen isotopic ratios of water in precipitation beyond paleothermometry paleoclimatologists face a dilemma no sedimentary proxy is a pure recorder of quantitative climate information yet climate modelers and policy makers increasingly seek quantitative comparisons between instrumentally documented possibly anthropogenic climate changes and those produced naturally in the past in the second edition of his influential book bradley 1999 p 6 discussed the calibration of proxy records to learn past climate changes calibration involves using modern climatic records and proxy materials to understand how and to what extent proxy materials are climate dependent it is assumed that the modern relationships observed have operated unchanged throughout the period of interest the principle of uniformitarianism in other words one relates the characteristics of sediment to climate at different places for one time or at a place for short times and then uses that relation plus characteristics of older sediments to estimate the climatic conditions that produced those sedimentary characteristics bradley 1999 then extensively discussed the difficulties in applying this methodology in a complex world with imperfect recorders nonetheless the goal of using calibrated proxies for quantitative as well as qualitative paleoclimatic reconstruction is clear a prominent recent use of calibrated paleoclimatic data is the assessment of whether the probably anthropogenic warming of the latter 20th century moved beyond the band of natural variability of the prevailing climate bradley 2000 combined recent instrumental records with several longer proxy based reconstructions of surface temperature including that of mann et al 1999 based on this composite data set bradley 2000 argued that temperatures in the late 20th century were unique in the context of the entire millennium the proxy records were primarily based on tree ring data but included isotopic and major element geochemistry of corals and occurrence of melt layers and isotopic ratios of water in ice cores however broecker 2001 questioned the basis for the reconstruction of

physical properties crystalline textures and c axis fabrics of the siple dome antarctica ice core the quality of the ice core from siple dome west antarctica varied widely with significant fracturing below 400 m bubbly ice persisted to the ice rock interface at 1004 m and constituted the brittle zone the core has undergone minimal relaxation and has remained brittle and prone to fracturing more than 5 years after it was drilled this behavior is attributed to unrelieved stresses from kamb and bindschadler ice streams former ice streams c and d flanking the dome melt layers were identified sporadically throughout the core as were inclined layers tilted at angles that occasionally exceeded 10 structurally the ice was characterized by extensive recrystallization including grain size changes from 0 074 cm 2 at 59 m to 50 cm 2 at 992 m and major transitions in c axis fabrics unusual fabrics included vertical c axis clusters superimposed on vertical girdles that may reflect vertical compression acting in conjunction with horizontal tension the sudden appearance of a shear type fabric at 700 800 m appears closely linked to the occurrence of abundant tephra particles embedded in the ice the occurrence of dispersed sediment in the bottom 2 m is attributed to freeze on of basal meltwater

physical and structural properties of the greenland ice sheet project 2 ice core a review substantial data sets have been collected on the relaxation characteristics density grain size c axis fabrics and ultrasonic velocities of the greenland ice sheet project 2 gisp2 core to its contact with bedrock at 3053 4 m changes in all these properties paralleled closely those found in cores from byrd station antarctica and dye 3 greenland density increased progressively with depth to a maximum of 0 921 mg m3 at about 1400 m at which depth the ice became bubble free below about 2000 m in situ densities began to decrease in response to increasing ice sheet temperatures since drilling much of the ice core has undergone significant volume expansion relaxation due to microcracking and the exsolving of enclathratized gases especially in the brittle ice zone between 650 and 1400 m grain size increased linearly to about 1000 m thereafter remaining fairly constant until the younger dryas event at 1678 m where a twofold to threefold decrease in grain size occurred these grain size changes were accompanied by a progressive clustering of crystal c axes toward the vertical including a small increase in c axis concentration across the younger dryas holocene boundary increased dust levels in the wisconsin ice have contributed to the maintenance of a fine grained texture which with its strong vertical c axis fabric persisted to nearly 3000 m however beginning at about 2800 m layers of coarse grained ice intermixed with the much finer grained matrix ice are observed below 3000 m the ice became very coarse grained this change attributed to annealing recrystallization at elevated temperatures in the ice sheet was accompanied by a dispersed or ring like redistribution of the c axes about the vertical ultrasonic measurements of vertical and horizontal p wave velocities made at 10 m intervals along the entire length of the gisp2 core fully confirmed the results of the crystallo optical observations a return to fine grained ice coincided with the first appearance of brown silty ice 13 m above bedrock bedrock material consisted of 48 cm of till including boulders and cobbles overlying gray biotite granite comprising the true bedrock there is evidence that disturbed structure in the gisp2 cores begins little more than 70 of the way through the ice sheet this disturbance increases with depth until it becomes large enough to cast suspicion on features lasting centuries or more in the bottom 10 of the ice sheet

high resolution palynological record off the iberian margin direct land sea correlation for the last interglacial complex abstract we present high resolution pollen dinocyst and isotopic data for the last interglacial complex from marine core md952042 southwestern margin of the iberian peninsula 37 48 n 10 10 w 3148 m direct land sea correlation from this core indicates that during this period north atlantic sea surface temperatures were in phase with iberian climate our palynological analysis suggests a younger dryas like event at the marine isotope stage mis 6 5 transition the analysis also indicates that the eemian spans from the lightest isotopic values of mis 5e ca 126 ky bp to the heavier isotopic values towards the mis 5e 5d transition therefore the eemian is not entirely equivalent to mis 5e pollen analysis identifies four climatic phases of low amplitude during the eemian a mediterranean climate in southwestern europe is gradually replaced by oceanic conditions the middle of the eemian is characterized by an increase in precipitation on the land and ocean associated with a slight cooling this seems to be the result of a displacement of the polar front as far south as southern europe during this period after the eemian three relatively short climatic phases on land melisey i st germain ia and montaigu cold event occurred contemporaneously with three shifts of sea surface temperatures the montaigu event first identified in terrestrial pollen sequences is therefore also recorded in core md952042 on the basis of pollen dinocyst and planktonic isotopic data our results also show that the warm periods of mis 5 are not characterized by similar climatic conditions on land

advanced microstructural characterization of four east antarctic firn ice cores the microstructures and microchemistry of four us international trans antarctic scientific expedition ice cores were examined at three depths 30 60 90m each using scanning electron microscopy including electron backscattered patterns and energy dispersive spectroscopy eds in order to assess the relationship between chemical and physical properties the physical characteristics grain size porosity density internal surface volume and crystallographic orientation at the four sites were inhomogeneous as expected on the basis of differences in the moisture content of deposited snow and the accumulation rate at their respective locations evidence of shallow subgrain boundary formation and trends in internal surface volume having implications for the study of firn densification and ice sheet modeling were also found chemical characterization revealed that site specific variations in particulate concentration and source could accurately be determined using eds analysis it was also found that the combination of elements predominant within the sample controls the morphology and microstructural location of the impurities

history of the greenland ice sheet paleoclimatic insights abstract paleoclimatic records show that the greenland ice sheet consistently has lost mass in response to warming and grown in response to cooling such changes have occurred even at times of slow or zero sea level change so changing sea level cannot have been the cause of at least some of the ice sheet changes in contrast there are no documented major ice sheet changes that occurred independent of temperature changes moreover snowfall has increased when the climate warmed but the ice sheet lost mass nonetheless increased accumulation in the ice sheet s center has not been sufficient to counteract increased melting and flow near the edges most documented forcings and ice sheet responses spanned periods of several thousand years but limited data also show rapid response to rapid forcings in particular regions near the ice margin have responded within decades however major changes of central regions of the ice sheet are thought to require centuries to millennia the paleoclimatic record does not yet strongly constrain how rapidly a major shrinkage or nearly complete loss of the ice sheet could occur the evidence suggests nearly total ice sheet loss may result from warming of more than a few degrees above mean 20th century values but this threshold is poorly defined perhaps as little as 2 c or more than 7 c paleoclimatic records are sufficiently sketchy that the ice sheet may have grown temporarily in response to warming or changes may have been induced by factors other than temperature without having been recorded

stochastic resonance in the north atlantic large abrupt changes between warm and cold modes of north atlantic climate exhibit spectral power at 1500 years yet some climatic changes were linked to outburst floods and other events that are unlikely to have been truly periodic we hypothesize that a weak periodic forcing has combined with noise from ice sheet related events to cause the observed mode switches this stochastic resonance hypothesis predicts a recurrence pattern between warmings that is distinct from the predictions of simple periodic and stochastic models and at least some other models the ice isotopic data from central greenland ice cores are consistent with the stochastic resonance hypothesis but not with other models we have tested we thus support arguments for the existence of a periodicity of 1500 years in the north atlantic climate system and for the importance of ice sheet events in forcing north atlantic changes

the role of deep ocean circulation in setting glacial climates the glacial cycles of the pleistocene involve changes in the circulation of the deep ocean in important ways this review seeks to establish what were the robust patterns of deep sea water mass changes and how they might have influenced important parts of the last glacial cycle after a brief review of how tracers in the modern ocean can be used to understand the distribution of water masses i examine the data for biogeochemical circulation rate and conservative tracers during glacial climates some of the robust results from the literature of the last 30 years include a shoaled version of northern source deep water in the atlantic expanded southern source water in the abyss and deep ocean salt rather than heat stratification of the last glacial maximum lgm deep sea and several lines of evidence for slower overturning circulation in the southern deep cell we combine these observations into a new idea for how the ocean atmosphere system moves from interglacial to glacial periods across a single cycle by virtue of its influence on the melting of land based ice around antarctica cooling north atlantic deep water nadw leads to a cold and salty version of antarctic bottom water aabw this previously underappreciated feedback can lead to a more stratified deep ocean that operates as a more effective carbon trap than the modern helping to lower atmospheric co 2 and providing a mechanism for the deep ocean to synchronize the hemispheres in a positive feedback that drives the system to further cooling

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

conditions for bubble elongation in cold ice sheet ice highly elongated bubbles are sometimes observed in ice sheet ice elongation is favored by rapid ice deformation and opposed by diffusive processes we use simple models to show that vapor transport dominates diffusion except possibly very close to the melting point and that latent heat effects are insignificant elongation is favored by larger bubbles at pore close off but is nearly independent of bubble compression below close off the simple presence of highly elongated bubbles indicates only that a critical ice strain rate has been exceeded for significant time and provides no information on possible disruption of stratigraphic continuity by ice deformation

the 2016 south alboran earthquake m w 6 4 a reactivation of the ibero maghrebian region abstract on 25 january 2016 an earthquake of magnitude m w 6 4 occurred at the southern part of the alboran sea between southern spain and northern morocco this shock was preceded by a foreshock m w 5 1 and followed by a long aftershock sequence focal mechanism of main shock has been estimated from slip inversion of body waves at teleseismic distances solution corresponds to left lateral strike slip motion showing a complex bilateral rupture formed by two sub events with most energy propagating along a plane oriented n30 e plane dipping to the nw relocation of larger events of the aftershock series show two alignments of epicentres in ne sw and nne ssw direction that intersect at the epicentre of the main shock we have estimated the focal mechanisms of the largest aftershocks from moment tensor inversion at regional distances we have obtained two families of focal mechanisms corresponding to strike slip for the nne ssw alignment and thrusting motion for the ne sw alignment among the faults present in the area the al idrisi fault or fault zone may be a good candidate for the source of this earthquake the study of coulomb failure stress shows that it is possible that the 2016 earthquake was triggered by the previous nearby earthquakes of 1994 m w 5 8 and 2004 m w 6 3 the possible seismic reactivation of the central part of the ibero maghrebian region is an open question but it is clear that the occurrence of the 2016 earthquake confirms that from 1994 the seismicity of central part of imr is increasing and that focal mechanism of largest earthquakes in this central part correspond to complex ruptures or zone of fault