the topographic torque on a bounding surface of a rotating gravitating fluid and the excitation by core motions of decadal fluctuations in the earth s rotation general expressions with potential applications in several areas of geophysical fluid dynamics are derived for all three components of the contribution made by the geostrophic part of the pressure field associated with flow in a rotating gravitating fluid to the topographic torque exerted by the fluid on a rigid impermeable bounding surface of any shape when applied to the earth s liquid metallic core which is bounded by nearly spherical surfaces and can be divided into two main regions the torosphere and polosphere the expressions reduce to formulae given previously by the author thereby providing further support for his work and that of others on the role of topographic coupling at the core mantle boundary in the excitation by core motions of earth rotation fluctuations on decadal time scales they also show that recent criticisms of that work are vitiated by mathematical and physical errors contrary to these criticisms the author s scheme for exploiting earth rotation and other geophysical data either real or simulated in computer models in quantitative studies of the topography of the core mantle boundary cmb by intercomparing various models of a motions in the core based on geomagnetic secular variation data and b cmb topography based on seismological and gravity data has a sound theoretical basis the practical scope of the scheme is of course limited by the accuracy of real data but this is a matter for investigation not a priori assessment

density log of a 181 m long ice core from berkner island antarctica a 181 m long ice core was drilled at 79 36 51 s 45 43 28 w near the summit of berkner island antarctica 886 m a s l berkner island is located between the filchner and ronne ice shelves and the ice near the summit shows little lateral flow the density of the ice core was measured every 3 mm along its length using attenuation of a gamma ray beam which gave an absolute accuracy of 2 as expected there is a general density increase with depth the maximum densities of 900 kg m 3 being reached just above 100 m depth comparison with the electrical conductivity method ecm shows density variations with the same wavelength as the annual signals which can be seen in the ecm log higher acidity during summer in the shallowest part of the core the density of winter layers is higher than that of summer layers a relationship which is reversed at greater depth we assume that the densification rates for the two types of firn are different similar density phenomena were observed on ice cores from greenland showing that such phenomena are not a local effect

geomagnetic secular variation impulses the secular variation of the earth s internal magnetic field changes on time scales of the order of a year such impulses occurred in 1969 and probably in 1913 they provide constraints on deep mantle conductivity which appears to be rather low and on the motions of the top of the fluid core which are probably dominated by westward drift present data favour the existence of upwelling of core material at the core mantle boundary the impulses can be correlated with extrema in the earth s rotation rate and interpreted in terms of electromagnetic core mantle coupling

eight glacial cycles from an antarctic ice core the antarctic vostok ice core provided compelling evidence of the nature of climate and of climate feedbacks over the past 420 000 years marine records suggest that the amplitude of climate variability was smaller before that time but such records are often poorly resolved moreover it is not possible to infer the abundance of greenhouse gases in the atmosphere from marine records here we report the recovery of a deep ice core from dome c antarctica that provides a climate record for the past 740 000 years for the four most recent glacial cycles the data agree well with the record from vostok the earlier period between 740 000 and 430 000 years ago was characterized by less pronounced warmth in interglacial periods in antarctica but a higher proportion of each cycle was spent in the warm mode the transition from glacial to interglacial conditions about 430 000 years ago termination v resembles the transition into the present interglacial period in terms of the magnitude of change in temperatures and greenhouse gases but there are significant differences in the patterns of change the interglacial stage following termination v was exceptionally long 28 000 years compared to for example the 12 000 years recorded so far in the present interglacial period given the similarities between this earlier warm period and today our results may imply that without human intervention a climate similar to the present one would extend well into the future

core scale radioscopic imaging a new method reveals density calcium link in antarctic firn abstract a new radioscopic imaging technique has been developed to measure firn density in unprecedented resolution and accuracy even when the porosity is low or the geometry of a core or piece of core is not perfect the technique is based on an x ray microfocus computer tomograph ice ct designed especially for ice core applications applied on an archive piece of the antarctic firn core b32 drilled in dronning maud land in 1998 the obtained density profile shows a strong correlation with the calcium ion concentration as found previously in greenland given the impurity density relationship found previously in greenland our result suggests both improved accuracy of the new density measurements and an impurity density relationship with a similar magnitude in greenland to that on the antarctic plateau our measurements provide first evidence that the impurity density relationship is a universal feature of polar firn and that the calcium ion concentration can serve as a proxy to describe quantitatively the effect of the impurities on densification

unveiling the anatomy of termination 3 using water and air isotopes in the dome c ice core east antarctica abstract each glacial interglacial transition of the quaternary occurs in a different orbital context leading to various timing for the deglaciation and sequence of high vs low latitudes events termination 3 250 kyears before present ka is an unusual deglaciation in the context of the last 9 deglaciations recorded in the old epica dome c edc antarctic ice core it exhibits a three phase sequence two warming phases separated by a small cooling the last phase suggesting a particularly rapid temperature increase we present here new high resolution 15n and deuterium excess d excess data from the edc ice core to provide a detailed temperature change estimate during this termination then we combined the d and 18o to discuss the relationship between high and low latitude changes through the d excess we also provide the high vs low latitude sequence of events over this deglaciation without chronological uncertainty using low latitude ice core proxies in agreement with previous studies based on speleothem analyses we show that the first phase of termination 3 256 249 ka is associated with small heinrich like events linked to changes in itcz position monsoon activity and teleconnections with antarctica in a context of minimum northern hemisphere insolation this leads to a rather strong antarctic warming as observed in the 15n record in contrast to the relatively small d increase the second warming phase occurs during the rise of the northern hemisphere insolation with a large heinrich like event leading to the characteristic antarctic warming observed in the 15n and d increase as for the more recent terminations

an estimate of average lower mantle conductivity by wavelet analysis of geomagnetic jerks it has recently been proposed that geomagnetic jerks observed at the earth s surface could be viewed as singularities in the time behavior of the geomagnetic field with a regularity of about 1 5 when wavelet analyzed such a signal should have suffered some distortion when diffusing from the core mantle boundary cmb through the conducting mantle assuming that the upper mantle is an insulator and given the electromagnetic time constant of the mantle we compute the distortion that a pure singularity introduced at the cmb suffers as it traverses the mantle we compute this distortion through its effects on the so called ridge functions extracted from the wavelet transform of the signal this distortion is very similar to the small but significant one that we observe in real data we therefore speculate that jerks must have been pure singularities at the base of the mantle and infer an average estimate for the mantle electromagnetic time constant from the way the signal is distorted by fitting the synthetic ridge functions to the experimental ones assuming for example a thickness of 2000 km for a uniform lower conducting mantle we find an electrical conductivity smaller than 10 s m 1 this value is in reasonable agreement with values derived from high pressure experiments for a silicate mantle

accumulation rates in dronning maud land antarctica as revealed by dielectric profiling measurements of shallow firn cores the european programme for ice coring in antarctica includes a comprehensive pre site survey on the inland ice plateau of dronning maud land antarctica the german glaciological programme during the 1997 98 field season was carried out along a 1200 km traverse on amundsenisen and involved sampling the snow cover in pits and by shallow firn cores this paper focuses on the accumulation studies the cores were dated by dielectric profiling and continuous flow analysis distinct volcanogenic peaks and seasonal signals in the profiles served to establish a depth time scale the eruptions of krakatoa tambora an unknown volcano kuwae and el chichon are well documented in the ice variations of the accumulation rates over different times were inferred from the depth time scales a composite record of accumulation rates for the last 200 years was produced by stacking 12 annually resolved records according to this accumulation rates decreased in the 19th century and increased in the 20th century the recent values are by no means extraordinary as they do not exceed the values at the beginning of the 19th century variations in accumulation rates are most probably linked to temperature variations indicated in 18o records from amundsenisen

lower mantle conductivity anomalies estimated from geomagnetic jerks 1 the occurrence epochs and duration periods of the 1969 1978 and 1991 geomagnetic jerks are estimated for 65 geomagnetic observatories by applying a statistical time series model to monthly means of the geomagnetic eastward component it is found for every jerk that the occurrence epochs around south africa and south pacific ocean are delayed several years from those in other regions moreover it is found for the 1969 and 1978 jerks that the duration times around south africa which are defined as the lengths of the duration periods are longer than those in other regions the distribution of these anomalous regions of the jerks i e south africa and south pacific ocean may imply the existence of higher conductivities in the lower mantle beneath these regions which possibly relates to the fact that the velocity of a seismic s wave in the d layer beneath these regions is slower than that in other regions the mantle conductivities beneath south africa south pacific ocean and other regions are estimated through a numerical simulation the magnetic diffusion equation in the mantle is solved for an abrupt magnetic change generated at the core mantle boundary comparing the results obtained by the simulation with those obtained by the data analysis it is plausible that the mantle conductivities beneath south africa and south pacific ocean are higher than that beneath other regions although there is an ambiguity in the estimated magnitudes of the mantle conductivity

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

petrology and k ar ages of dredged samples from laysan island and northampton bank volcanoes hawaiian ridge and evolution of the hawaiian emperor chain summary the hawaiian emperor volcanic chain is made up of at least 107 indentifiable shield volcanoes chase and others 1970 bargar and jackson 1974 clague and others 1980 and is the longest linear island and seamount chain in the pacific fig 1 beginning at the active volcanoes kilauea and mauna loa on the island of hawaii this chain of volcanoes extends west northwestward along the hawaiian ridge for a distance of 3 500 km where it turns northward and continues another 2 300 km as the emperor seamounts ending near the intersection of the aleutian and kurile trenches it is now thought that the hawaiian emperor chain was formed as the pacific lithospheric plate moved first northward and then northwestward relative to the hawaiian hot spot a more or less stable melting anomaly in the asthenosphere and that the bend in the chain reflects a major change in pacific plate motion several mechanisms have been proposed to explain the hawaiian and other hot spots including a propagating fracture betz and hess 1942 jackson and wright 1970 diapiric upwelling along a line of structural weakness green 1971 mcdougall 1971 thermal morgan 1972a 1972b or chemical anderson 1975 plumes of gravitationally unstable material rising from the deep mantle shear melting coupled with thermal feedback shaw 1973 and geographically stabilized by gravitational sinking of refractory residua shaw and jackson 1973 and extrusion induced by lateral motion along plate boundaries handschumacher 1973 despite the variety of dynamic models proposed there has yet to be devised a definitive experiment to test any of them all these mechanisms are however encompassed by the general or kinematic hot spot hypothesis which when used in the broadest sense implies neither mechanism nor excess heat

geomagnetic jerks as chaotic fluctuations of the earth s magnetic field 1 the geomagnetic field is chaotic and can be characterized by a mean exponential time scale after which it is no longer predictable it is also ergodic so time analyses can substitute the more difficult phase space analyses taking advantage of these two properties of the earth s magnetic field a scheme of processing global geomagnetic models in time is presented to estimate fluctuations of the time scale here considering that the capability to predict the geomagnetic field is reduced over periods of geomagnetic jerks we propose a method to detect these events over a long time span this approach considers that epochs characterized by relative minima of fluctuations in time scale i e those periods when a geomagnetic field is less predictable are possible jerk occurrence dates we analyze the last 400 years of the geomagnetic field covered by the gufm1 model to detect minima of fluctuations i e epochs characterized by low values of the time scale most of the well known jerks are confirmed through this method and a few others have been suggested finally we also identify some short periods when the field is less chaotic more predictable than usual naming these periods as steady state geomagnetic regime to underline their opposite behavior with respect to jerks

observing and modeling the influence of layering on bubble trapping in polar firn interpretation of ice core trace gas records depends on an accurate understanding of the processes that smooth the atmospheric signal in the firn much work has been done to understand the processes affecting air transport in the open pores of the firn but a paucity of data from air trapped in bubbles in the firn ice transition region has limited the ability to constrain the effect of bubble closure processes here we present high resolution measurements of firn density methane concentrations nitrogen isotopes and total air content that show layering in the firn ice transition region at the west antarctic ice sheet wais divide ice core site using the notion that bubble trapping is a stochastic process we derive a new parameterization for closed porosity that incorporates the effects of layering in a steady state firn modeling approach we include the process of bubble trapping into an open porosity firn air transport model and obtain a good fit to the firn core data we find that layering broadens the depth range over which bubbles are trapped widens the modeled gas age distribution of air in closed bubbles reduces the mean gas age of air in closed bubbles and introduces stratigraphic irregularities in the gas age scale that have a peak to peak variability of 10 years at wais divide for a more complete understanding of gas occlusion and its impact on ice core records we suggest that this experiment be repeated at sites climatically different from wais divide for example on the east antarctic plateau

microstructure dependent densification of polar firn derived from x ray microtomography the densification of dry polar snow and firn results in a continuous increase of density with depth accompanied by significant density fluctuations within seasonal layers density measurements of high spatial resolution reveal a persistent minimum of density fluctuations in the vicinity of the snow firn transition 0 55 0 65 g cm 3 in firn core records in this study we give an explanation for the fluctuation minimum by applying a new method of x ray microtomography to obtain three dimensional 3 d structural data of a greenland firn core at 13 different depths between 10 and 78 m a set of 16 samples of 40 cm total length for each depth interval was measured a reconstructed firn segment of 40 cm covers 1 2 years of snow accumulation using digital image analysis techniques different structural parameters are estimated including 3 d pore and particle sizes and specific surface areas it is shown that the densification rates of snow and firn layers consisting of coarse particles are much higher than those consisting of fine particles within the same depth interval this causes a density crossing of fine and coarse grained layers with a minimum of density variations at the crossover point this crossing over implies that formerly dense layers in the seasonal density signal are not of the same origin as dense layers in the deeper part of the firn column and that the seasonal density signal will totally change shape with depth it is speculated that in coarse and fine grained firn the dominant mechanism of densification acts over different regimes of density

firn model intercomparison experiment firnmice evolution of cold dry snow and firn plays important roles in glaciology however the physical formulation of a densification law is still an active research topic we forced eight firn densification models and one seasonal snow model in six different experiments by imposing step changes in temperature and accumulation rate boundary conditions all of the boundary conditions were chosen to simulate firn densification in cold dry environments while the intended application of the participating models varies they are describing the same physical system and should in principle yield the same solutions the firn models all produce plausible depth density profiles but the model outputs in both steady state and transient modes differ for quantities that are of interest in ice core and altimetry research these differences demonstrate that firn densification models are incorrectly or incompletely representing physical processes we quantitatively characterize the differences among the results from the various models for example we find depth integrated porosity is unlikely to be inferred with confidence from a firn model to better than 2 m in steady state at a specific site with known accumulation rate and temperature firn model intercomparison experiment can provide a benchmark of results for future models provide a basis to quantify model uncertainties and guide future directions of firn densification modeling

firn air δ15n in modern polar sites and glacial interglacial ice a model data mismatch during glacial periods in antarctica the phase lag between atmospheric composition air bubbles and temperature water isotopes can be quantified from ice cores provided that the age difference between entrapped air and the surrounding air can be correctly estimated this difference depends on the lock in depth lid when air no longer mixes with the atmosphere the lid can be estimated from firnification models or from the air isotopic composition 15n and 40ar both methods give consistent results for greenland and one coastal site in antarctica byrd new firn measurements in greenland northgrip and antarctica berkner island bas depot dome c confirm that firnification models correctly reproduce the present lid over a large range of surface conditions however a systematic mismatch is observed for the last glacial maximum lgm in east antarctic sites vostok dome c dome f questioning the model s validity here we use new 15n measurements from two coastal antarctic sites kohnen station and law dome providing depth estimates again distinct from firnification model calculations we show that this discrepancy can be resolved by revising the estimate of past accumulation rates 15n measurements can therefore help to constrain past accumulation rate and improve ice core dating

a geomagnetic jerk for the early 1990 s abstract records from geomagnetic observatories in europe are showing indications of another impulse in the third time derivative of the east component around 1991 these impulses otherwise known as jerks have occurred in the past at about 1925 1969 and 1978 and possibly also 1900 and 1913 they are believed to be internal in origin and have implications for the understanding of the fluid motions in the core which create them or for lower mantle conductivity or both

worldwide character of the 1991 geomagnetic jerk a recent examination of the geomagnetic element annual mean values for the european magnetic observatories has shown the existence of a sudden change in the secular acceleration at about 1991 in this work the space time character of the 1991 impulse is investigated and its worldwide character is evidenced an evaluation of the amplitude spatial distribution is given for the y component analysing data relative to 74 magnetic observatories the external effects were removed from the annual mean data and successively the y secular variation has been evaluated from the corrected annual means moreover the worldwide character of the intensity of the step in the second time derivative is compared with the character of the previous well known jerk events of 1969 and 1978

physical properties of the wais divide ice core the wais west antarctic ice sheet divide deep ice core was recently completed to a total depth of 3405 m ending 50 m above the bed investigation of the visual stratigraphy and grain characteristics indicates that the ice column at the drilling location is undisturbed by any large scale overturning or discontinuity the climate record developed from this core is therefore likely to be continuous and robust measured grain growth rates recrystallization characteristics and grain size response at climate transitions fit within current understanding significant impurity control on grain size is indicated from correlation analysis between impurity loading and grain size bubble number densities and bubble sizes and shapes are presented through the full extent of the bubbly ice where bubble elongation is observed the direction of elongation is preferentially parallel to the trace of the basal 0001 plane preferred crystallographic orientation of grains is present in the shallowest samples measured and increases with depth progressing to a vertical girdle pattern that tightens to a vertical single maximum fabric this single maximum fabric switches into multiple maxima as the grain size increases rapidly in the deepest warmest ice a strong dependence of the fabric on the impurity mediated grain size is apparent in the deepest samples

magnetic flux expulsion from the core as a possible cause of the unusually large acceleration of the north magnetic pole during the 1990s 1 the north magnetic pole nmp has been drifting in a north northwesterly direction since the 19th century both local surveys and geomagnetic models derived from observatory and satellite data show that the nmp suddenly accelerated during the 1990s its speed increased from about 15 km yr in 1989 to about 60 km yr in 2002 after which it started to decrease slightly using a green s function we show that this acceleration is mainly caused by a large negative secular variation change in the radial magnetic field at the core surface under the new siberian islands this change occurs in a region of the core surface where there is a pair of secular variation patches of opposite polarities which we suggest could be the signature of a so called polar magnetic upwelling of the type observed in some recent numerical dynamo simulations indeed a local analysis of the radial secular variation and magnetic field gradient suggests that the secular variation change under the new siberian islands is likely to be accompanied by a significant amount of magnetic diffusion in agreement with such a mechanism we thus hypothesize that the negative secular variation change under the new siberian islands that produced the nmp acceleration could result from a slowdown of the polar magnetic upwelling during the 1990s we finally note that the nmp drift speed is determined by such a combination of factors that it is at present not possible to forecast its future evolution

layering of surface snow and firn at kohnen station antarctica noise or seasonal signal the density of firn is an important property for monitoring and modeling the ice sheet as well as to model the pore close off and thus to interpret ice core based greenhouse gas records one feature which is still in debate is the potential existence of an annual cycle of firn density in low accumulation regions several studies describe or assume seasonally successive density layers horizontally evenly distributed as seen in radar data on the other hand high resolution density measurements on firn cores in antarctica and greenland showed no clear seasonal cycle in the top few meters a major caveat of most existing snow pit and firn core based studies is that they represent one vertical profile from a laterally heterogeneous density field to overcome this we created an extensive dataset of horizontal and vertical density data at kohnen station dronning maud land on the east antarctic plateau we drilled and analyzed three 90 m long firn cores as well as 160 one meter long vertical profiles from two elongated snow trenches to obtain a two dimensional view of the density variations the analysis of the 45 m wide and 1 m deep density fields reveals a seasonal cycle in density however the seasonality is overprinted by strong stratigraphic noise making it invisible when analyzing single firn cores our density dataset extends the view from the local ice core perspective to a hundred meter scale and thus supports linking spatially integrating methods such as radar and seismic studies to ice and firn cores

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

in situ measurements of antarctic snow compaction compared with predictions of models 1 we describe in situ measurements of the compaction of antarctic snow at three different sites in antarctica the rate of compaction was measured hourly over various depth intervals for up to two years these measurements show that compaction at each of the sites occurs through slow viscous deformation of the snowpack with no significant contribution from sudden collapse of weak layers the measured rates of compaction at the coldest site exhibit a strong seasonality consistent with a temperature dependent sintering mechanism having activation energy of 70 kj mol 1 at the two warmer sites activation energies of 80 and 120 kj mol 1 provide slightly better agreement with the observations published models of snow compaction underestimate the temperature sensitivity a good match to our observations is provided by a semi empirical model based on rate equations for lattice diffusion nabarro herring creep of material around pores combined with normal grain growth this model also provides a theoretical basis for a widely used empirical model of snow compaction the rate coefficient for lattice diffusion inferred from our measurements is considerably higher than published values however and other creep mechanisms cannot be ruled out

seasonal and interannual variations of firn densification and ice sheet surface elevation at the greenland summit seasonal and interannual variations in surface elevation at the greenland summit are modeled using a new temperature dependent formulation of firn densification and are compared with elevations from european remote sensing satellite ers 1 2 radar altimetry the rate constant and activation energy usually set as constants in the arrhenius type relation are strongly temperature dependent based on measurements of crystal growth rates a multiplicative factor in the densification rate accounts for differences between densification and grain growth rates and is chosen to match the modeled and measured density profiles from 0 to 40 m the stronger temperature dependence produces a significant seasonal cycle in the densification rate in the upper firn much of the densification and consequent surface lowering occur within 3 months in late spring early summer followed by a build up from accumulation modeled elevation changes using automatic weather station measurements of temperature and accumulation and modeled precipitation agree well with observations the respective seasonal amplitudes are 18 and 25 cm peak to peak with minima in mid summer the minimum elevation in 1995 is driven mainly by a temporary accumulation decrease and secondarily by warmer temperatures increased compaction driven by a summer warming trend decreases the modeled elevation 1992 99 by 20 cm but accumulation increases in latter years dominate the overall 4 2 cm a 1 trend

the layered evolution of fabric and microstructure of snow at point barnola central east antarctica snow fabric defined as the distribution of the c axis orientations of the ice crystals in snow is poorly known so far only one study exits that measured snow fabric based on a statistically representative technique this recent study has revealed the impact of temperature gradient metamorphism on the evolution of fabric in natural snow based on cold laboratory experiments on polar ice sheets snow properties are currently investigated regarding their strong variability in time and space notably because of their potential influence on firn processes and consequently on ice core analysis here we present measurements of fabric and microstructure of snow from point barnola east antarctica close to dome c we analyzed a snow profile from 0 to 3 m depth where temperature gradients occur the main contributions of the paper are 1 a detailed characterization of snow in the upper meters of the ice sheet especially by providing data on snow fabric and 2 the study of a fundamental snow process never observed up to now in a natural snowpack namely the role of temperature gradient metamorphism on the evolution of the snow fabric snow samples were scanned by micro tomography to measure continuous profiles of microstructural properties density specific surface area and pore thickness fabric analysis was performed using an automatic ice texture analyzer on 77 representative thin sections cut out from the samples different types of snow fabric could be identified and persist at depth snow fabric is significantly correlated with snow microstructure pointing to the simultaneous influence of temperature gradient metamorphism on both properties we propose a mechanism based on preferential grain growth to explain the fabric evolution under temperature gradients our work opens the question of how such a layered profile of fabric and microstructure evolves at depth and further influences the physical and mechanical properties of snow and firn more generally it opens the way to further studies on the influence of the snow fabric in snow processes related to anisotropic properties of ice such as grain growth mechanical response electromagnetic behavior

geomagnetic disturbance fields an analysis of observatory monthly means summary this work quantifies the extent to which disturbance phenomena contribute to the observed geomagnetic field on the time scale of months to years a deterministic approach was adopted in which geomagnetic monthly means were analysed in the time domain by presupposing the time dependence of the external and induced field variations which we assume to vary as the aa geomagnetic index we arrived at an estimate of the relative amplitude of the disturbance to each component of the field at a number of observatories in all some 58000 geomagnetic monthly means from 59 observatories operating this century were analysed special attention was paid to the selection and validation of the data as otherwise the numerical computations would have been unduly compromised the results consist of 177 estimates of the relative amplitude of disturbance one for each of three orthogonal components north x east y and vertically down 2 at each observatory the disturbance to the x component was found to be consistently negative over the whole of the earth s surface with an intensification in auroral regions the disturbances to the z component were found to be smaller than that for x except in high latitudes mean disturbances to the y component were smaller still results were in general consistent with the dipole field of the magnetospheric ring current aligned with though of opposite polarity to the earth s main field typical amplitudes of the mean disturbance field from month to month were of the order 10nt the results can be used to estimate the variation of the disturbance field subtracting this from both monthly and annual means yields an improved estimate of the field originating in the core and its secular variation some examples are presented

on the impact of impurities on the densification of polar firn abstract understanding polar firn densification is crucial for reconstructing the age of greenhouse gas concentrations extracted from ice cores and for the interpretation of air in ice as a dating tool or as a climate proxy firn densification is generally modeled as a steady burial and sintering process of defined layers where the structure of the layering is maintained along the whole firn and ice column however available high resolution density data as well as firn air samples question this picture and point to a lack of understanding of firn densification based on analysis of high resolution density and calcium concentration records from antarctic and greenland ice cores we show for the first time that also impurities may have a significant impact on the densification analysis of firn cores shows a correlation between density and the calcium ion ca concentration and this correlation increases with depth the existence of this relationship is independent of the local climatic conditions at the core sites analyzed the strong positive correlation between the density and the logarithm of ca concentration indicates that impurities induce softening and lead to faster densification over a wide range of concentrations in one core the impurity effect manifests itself so strongly that the density develops a seasonal cycle closely following the seasonal cycle of ca our results clearly show that the structure of the firn layering changes with depth and suggest that the increased variability in density observed in deep firn recently described as a universal feature of polar firn may arise from the influence of ca and or other impurities the impurity effect is likely to have direct implications on our understanding of glacial firn densification and on glacial gas age estimates

a local intermittency measure lim approach to the detection of geomagnetic jerks abstract in the present work we investigate the temporal distributions of the geomagnetic jerks occurring in the last three decades of the 20th century using a new method of analysis based on the wavelet transform the local intermittency measure lim this method which allows us to single out intermittent structures within a given time series was applied to geomagnetic monthly means time series of the eastward magnetic field component y of 44 worldwide observatories we study the bimodal distribution of the jerk occurrence in time for the 1978 and 1991 events and point out a local regional jerk in south africa and in the pacific ocean which happened around 1986 the worldwide character of the 1999 geomagnetic jerk is also investigated integrating the lim analysis with the traditional straight line fit

consistently dated records from the greenland grip gisp2 and ngrip ice cores for the past 104 ka reveal regional millennial scale δ18o gradients with possible heinrich event imprint abstract we present a synchronization of the ngrip grip and gisp2 ice cores onto a master chronology extending back to 104 ka before present providing a consistent chronological framework for these three greenland records the synchronization aligns distinct peaks in volcanic proxy records and other impurity records chemo stratigraphic matching and assumes that these layers of elevated impurity content represent the same instantaneous event in the past at all three sites more than 900 marker horizons between the three cores have been identified and our matching is independently confirmed by 24 new and previously identified volcanic ash tephra tie points using the reference horizons we transfer the widely used greenland ice core chronology gicc05modelext to the two summit cores grip and gisp2 furthermore we provide gas chronologies for the summit cores that are consistent with the gicc05modelext timescale by utilizing both existing and new gas data ch4 concentration and 15n of n2 we infer that the accumulation contrast between the stadial and interstadial phases of the glacial period was 10 greater at summit compared to at ngrip the 18o temperature proxy records from ngrip grip and gisp2 are generally very similar and display synchronous behaviour at climate transitions the 18o differences between summit and ngrip however changed slowly over the last glacial interglacial cycle and also underwent abrupt millennial to centennial scale variations we suggest that this observed latitudinal 18o gradient in greenland during the glacial period is the result of 1 relatively higher degree of precipitation with a pacific signature at ngrip 2 increased summer bias in precipitation at summit and 3 enhanced rayleigh distillation due to an increased source to site distance and a potentially larger source to site temperature gradient we propose that these processes are governed by changes in the north american ice sheet nais volume and north atlantic sea ice extent and or sea surface temperatures sst on orbital timescales and that changing sea ice extent and ssts are the driving mechanisms on shorter timescales finally we observe that maxima in the summit ngrip 18o difference are roughly coincident with prominent heinrich events this suggests that the climatic reorganization that takes place during stadials with heinrich events possibly driven by a southward expansion of sea ice and low ssts in the north atlantic are recorded in the ice core records

jerks abound an analysis of geomagnetic observatory data from 1957 to 2008 we present a two step method for the removal of external field signals and the identification of geomagnetic jerks in magnetic observatory monthly mean data providing quantitative uncertainty estimates on jerk occurrence times and amplitudes with minimal a priori information we apply the method to the complete time series of x y and z components at up to 103 observatory locations in the period of 1957 2008 we find features fitting the definition of jerks in individual components to be frequent and not globally contemporaneous identified regional jerks have no consistent occurrence pattern and the most widespread in any given year is identified at 30 of observatories worldwide whilst we identify jerks throughout the period of study relative peaks in the global number of jerk occurrences are found in 1968 71 1973 74 1977 79 1983 85 1989 93 1995 98 and 2002 03 with the suggestion of further poorly sampled events in the early 1960s and late 2000s the mean uncertainties on individual jerk occurrence times and amplitudes are found to be 0 3 yrs and 2 1 nt yr2 respectively for all field components jerk amplitudes suggest possible periodic trends across europe and north america which may be related to the 6 yr periods detected independently in the secular variation and length of day

on core surface flows inferred from satellite magnetic data abstract satellite data allows the magnetic field produced by the dynamo within the earth s core to be imaged with much more accuracy than previously possible with only ground based data changes in this magnetic field can in turn be used to make some inferences about the core surface flow responsible for them in this paper we investigate the improvement brought to core flow computation by new satellite data based core magnetic field models it is shown that the main limitation now encountered is no longer the now high accuracy of those models but the non modelled secular variation produced by interaction of the non resolvable small scales of the core flow with the core field and by interaction of the partly resolvable large scales of the core flow with the small scales of the core field unfortunately masked by the crustal field we show how this non modelled secular variation can be taken into account to recover the largest scales of the core flow in a consistent way we also investigate the uncertainties this introduces in core flows computed with the help of the frozen flux and tangentially geostrophic assumptions it turns out that flows with much more medium and small scales than previously thought are needed to explain the satellite data based core magnetic field models it also turns out that a significant fraction of this flow unfortunately happens to be non recoverable being either non resolvable because too small scale or invisible because in the kernel of the inverse method even though it produces the detectable non modelled secular variation applying this to the magsat 1980 to orsted 2000 field changes leads us to conclude that a flow involving at least strong retrograde vortices below the atlantic hemisphere some less resolved prograde vortices below the pacific hemisphere and some poorly resolved and partly non resolvable polar vortices is needed to explain the 1980 2000 satellite era average secular variation the characteristics of the fraction of the secular variation left unexplained by this flow are also discussed

glacio chemical study spanning the past 2 kyr on three ice cores from dronning maud land antarctica 2 seasonally resolved chemical records the ammonium calcium and sodium concentrations from three intermediate depth ice cores drilled in the area of dronning maud land east antarctica have been investigated since all measurements were performed by a high resolution continuous flow analysis system for the first time seasonal signals of chemical trace species could be obtained from the interior of central antarctica over a period of approximately 2 millennia although the elevation as well as the accumulation rate differ between the drilling sites similar values were obtained by comparing mean concentrations spanning the last 900 years however a distinct lack of intersite correlation was found on decadal timescales despite a noticeable accumulation change apparent in one core no significant concentration change of all three species has occurred all the measured ions show clear seasonal signals over the whole records while the sea salt related component sodium peaks simultaneously with calcium the maximum ammonium concentration occurs in the snow with a time lag of 2 months after the sea salt peak more than 60 of the calcium concentration can be attributed to an ocean source elevated sodium concentrations were found within this millennium compared to mean values of the whole records but the spatially varying shape of the increase suggests that a possible climatic signal is biased by local deposition effects

geomagnetic secular variation of bransfield strait western antarctica from analysis of marine crossover data summary tracking the secular variation of the geomagnetic field in the past is severely limited in some cases by factors relating to the remoteness of the sites this is maximal in the antarctic where the remote location and severe climate lead to logistic limitations that make it difficult to keep a continuous record of magnetic field variations from the magnetic information available from historical marine expeditions it is possible to infer this time varying component from the comparison of readings at crossovers this study focuses on this technique discusses the impact of the different error sources and proposes a simple mathematical procedure to infer average secular variation rates the result is validated by comparing it with local data from the arctowski and livingston magnetic observatories sited in the area additionally using a high quality data set from a local area in the neighbourhood of deception island we have detected a systematic distribution in its secular variation this dichotomy has been interpreted in terms of a volcano magnetic signal this fact and the nature of its principal mechanisms are analysed and discussed

geomagnetic jerks in the polar regions 1 the occurrence of geomagnetic jerks over the arctic and antarctic regions is here investigated maps of geomagnetic secular acceleration over the polar regions are produced from the cm4 and chaos models and the occurrence of geomagnetic jerks is associated with jumps in secular acceleration the obtained results confirm that in antarctica geomagnetic jerks systematically follow geomagnetic jerks in the arctic region with a time delay from one to three years evidence is found of an abrupt change in secular acceleration in both polar regions around 1985 suggesting that the 1985 local jerk could actually be a worldwide event combining our results with the results previously obtained on the occurrence of a geomagnetic jerk at low mid latitudes around 2003 we support the hypothesis of a global extension of an event occurred at the beginning of the 21st century

magnitude and temporal evolution of dansgaard oeschger event 8 abrupt temperature change inferred from nitrogen and argon isotopes in gisp2 ice using a new least squares inversion abstract polar temperature is often inferred from water isotopes in ice cores however non temperature effects on 18 o are important during the abrupt events of the last glacial period such as changes in the seasonality of precipitation the northward movement of the storm track and the increase in accumulation these effects complicate the interpretation of 18 o as a temperature proxy here we present an independent surface temperature reconstruction which allows us to test the relationship between 18 o ice and temperature during dansgaard oeschger event 8 38 2 thousand yrs ago using new 15 n and 40 ar data from the gisp2 ice core in greenland this temperature reconstruction relies on a new inversion of inert gas isotope data using generalized least squares and includes a robust uncertainty estimation we find that both temperature and 18 o increased in two steps of 20 and 140 yrs with an overall amplitude of 11 80 1 8 c between the stadial and interstadial centennial mean temperature the coefficient d o 18 d t changes with each time segment which shows that non temperature sources of fractionation have a significant contribution to the 18 o signal when measured on century averaged values we find that d o 18 d t 0 32 0 06 c which is similar to the glacial holocene value of 0 328 c

temporal variations of strength and location of the south atlantic anomaly as measured by rxte abstract the evolution of the particle background at an altitude of 540 km during the time interval between 1996 and 2007 is studied using the particle monitor of the high energy x ray timing experiment on board nasa s rossi x ray timing explorer a special emphasis of this study is the location and strength of the south atlantic anomaly saa the size and strength of the saa are anti correlated with the 10 7 cm radio flux of the sun which leads the saa strength by 1 year reflecting variations in solar heating of the upper atmosphere the location of the saa is also found to drift westwards with an average drift rate of about 0 3 yr following the drift of the geomagnetic field configuration superimposed to this drift rate are irregularities where the saa suddenly moves eastwards and where furthermore the speed of the drift changes the most prominent of these irregularities is found in the second quarter of 2003 and another event took place in 1999 we suggest that these events are previously unrecognized manifestations of the geomagnetic jerks of the earth s magnetic field

redistributing earth s mass since the end of the last glaciation postglacial rebound of earth9s crust has resulted in a net transfer of mass from the equatorial regions to high latitudes this small mass transfer has been monitored for 25 years but a report by cox and chao suggests that since 1998 the mass transfer has changed direction in their perspective cazenave and nerem discuss possible mechanisms that may have caused this change

extending comprehensive models of the earth s magnetic field with orsted and champ data summary a new model of the quiet time near earth magnetic field has been derived using a comprehensive approach which includes not only pogo and magsat satellite data but also data from the orsted and champ satellites the resulting model shows great improvement over its predecessors in terms of completeness of sources time span and noise reduction in parameters with its well separated fields and extended time domain of 1960 to mid 2002 the model is able to detect the known sequence of geomagnetic jerks within this frame and gives evidence for an event of interest around 1997 because all sources are coestimated in a comprehensive approach intriguing north south features typically filtered out with other methods are being discovered in the lithospheric representation of the model such as the s atlantic spreading ridge and andean subduction zone lineations in addition this lithospheric field exhibits significantly less noise than previous models as a result of improved data selection the f region currents through which the satellites pass are now treated as lying within meridional planes as opposed to being purely radial results are consistent with those found previously for magsat but an analysis at orsted altitude shows exciting evidence that the meridional currents associated with the equatorial electrojet likely close beneath the satellite besides the model a new analysis technique has been developed to infer the portion of a model parameter state resolved by a particular data subset this has proven very useful in diagnosing the cause of peculiar artefacts in the magsat vector data which seem to suggest the presence of a small misalignment bias in the vector magnetometer

investigation of a secular variation impulse using satellite data the 2003 geomagnetic jerk abstract observatory monthly means provide an excellent opportunity to study the temporal changes of the magnetic field at a given location unfortunately the uneven distribution of the present observatory network makes it difficult to determine the global field change pattern recently we have developed an approach to extract satellite monthly means at a regular network of virtual observatories at 400 km altitude based on champ magnetic measurements using monthly means for 2001 2005 from those virtual observatories we investigate the space time structure of the short period variation of the earth s magnetic field by means of a spherical harmonic expansion followed by a separation into external magnetospheric and internal part this allows for the first time to study the secular variation globally and directly from satellite magnetic data analyzing the time series of the magnetic field at the virtual observatories as well as those of the spherical harmonic expansion coefficients we detect a secular variation impulse an abrupt jump in the second time derivative of the magnetic field in the champ satellite data during the first months of the year 2003 the jerk occurred simultaneously in the northern and southern hemispheres in a rather limited area near 90 e with maximum jerk strength at about 30 latitude a region also characterized by a strong secular acceleration second time derivative of the magnetic field we show that the 2003 geomagnetic jerk is not worldwide in occurrence and that there is an evidence for this event in the length of day variation

detection of secular acceleration pulses from magnetic observatory data abstract geomagnetic secular variation sv models for the epochs before the space era are based on magnetic observatory data which represent relatively rough and noisy time series due to magnetic storms anthropogenic spikes and gaps these models are often strongly regularized in time so that fast variations in the sv are smoothed out however recent studies show that at least some of the geomagnetic jerks observed at the earth s surface emanate from increasing and decreasing phases of secular acceleration sa pulses at the core surface the latter ones are direct manifestation of the dynamic processes taking place in the liquid core they were first detected from satellite data which are both of higher quality and more homogeneous in terms of geographical coverage than ground data herein we attempt to carry out similar studies based on observatory data available for a longer period the proposed method of sv modeling and recognition of sa pulses relies on a new technique of processing time series based on fuzzy mathematics comparison with the sv modeling results derived from satellite data shows their high conformity with the proposed method stability and reliability of the sa pulse recognition are demonstrated by the examples of well studied sa pulses in 2006 2009 and 2012 moreover several new sa pulses around 1996 1999 2002 and 2014 are discovered as a result of the new approach application to multi observatory data analysis the latter provides a basis for applying the method to older historical data and investigate sa pulses and geomagnetic jerks further back in time

a geomagnetic jerk for the end of the 20th century abstract the series of magnetic measurements at some european observatories give some hint of a new geomagnetic jerk around 1999 the geomagnetic impulses would present the remarkable and intriguing property to occur with a frequency of one per decade in the last third of the 20th century the geomagnetic jerks have been proposed to be indicators which anticipate the changes in the earth s rotation rate if this statement is of general validity the 1992 jerk should be followed by an acceleration of the earth rotation which would take place now and along the same lines we might predict a new deceleration in less than 10 yr

impurity controlled densification a new model for stratified polar firn a new densification model which simulates the effect of impurities on the densification of polar firn is presented the classical densification models of herron and langway 1980 and pimienta and barnola barnola and others 1991 are modified by assuming that the activation energy for deformation is reduced by the impurities motivated by recent observations the impurity effect is formulated on an empirical basis using the seasonally varying ca2 ion concentration excellent agreement between simulated and measured high resolution density profiles confirms the new approach the same parameterization applies for greenland and antarctica despite the one order of magnitude difference in impurity concentration the new models allow us for the first time to simulate the density layering in firn down to the firn ice transition our results emphasize the importance of impurities and density layering for the air entrapment and for dating gas records of deep ice cores in particular for glacial climate conditions where the impurity concentrations are 10 100 fold higher than in modern firn

signal from noise in geomagnetic field modelling denoising data for secular variation studies summary we analyse the residuals between a continuous geomagnetic field model of the internal field c3fm and observed secular variation a large part of the residual variations correlate closely with the dst index suggesting an origin from unmodelled external field variations removal of this signal enhances the resolution of fine scale detail in secular variation this is useful in considering the phenomenology of geomagnetic jerks the residual variations between different observatories show even better correlation suggesting the possibility of the construction of a proxy for the dst index notable cross correlation is also seen between the residuals and dst index with a lag of about 55 months