sub ice topography and meteorite finds near the allan hills and the near western ice field victoria land antarctica we have investigated the glaciological causes of large meteorite concentrations on blue ice fields west and southwest of the allan hills in antarctica a sub ice topography map for the area was prepared from data of a radio echo sounding survey the map reveals a mesa type paleosurface it was formed prior to and modified by glacial processes during the initial stage of glaciation of antarctica ice flow toward mawson glacier north of the allan hills is largely confined to a n s trending depression between the allan hills ice field and the near western ice field blue ice at the margins of the ice stream flows over the mesas on both sides of this depression meteorites entrained in blue ice are uncovered by sublimation and ablation we present arguments showing that currently almost all of the blue ice flowing into the allan hills ice field is sublimated leaving meteorites on the ice surface we present evidence that windblown meteorites are trapped by snow bridges across crevasses resulting in a near surface meteorite concentration near the ice ridges of the allan hills ice field nevertheless most meteorites are exposed for only a short period of time to the atmosphere before they are blown by the wind across the ice toward the western foot of the allan hills from where they are slowly carried northward to mawson glacier and the ross sea

geomagnetic field for 0 3 ka 2 a new series of time varying global models 1 steadily increasing numbers of archeomagnetic and paleomagnetic data for the holocene have allowed development of temporally continuous global spherical harmonic models of the geomagnetic field extending present and historical global descriptions of magnetic field evolution the current work uses various subsets of improved data compilations details of which are given in a companion paper by donadini et al 2009 and minor modifications of standard modeling strategies using temporally and spatially regularized inversion of the data and cubic spline parameterizations for temporal variations to produce five models with enhanced spatial and temporal resolution for 0 3 ka spurious end effects present in earlier models are eliminated by enforcing large scale agreement with the gufm1 historical model for 1650 1990 a d and by extending the model range to accommodate data older than 3 ka age errors are not considered as a contribution to data uncertainties but are included along with data uncertainties in an investigation of statistical uncertainty estimates for the models using parametric bootstrap resampling techniques we find common features but also significant differences among the various models indicating intrinsic uncertainties in global models based on the currently available holocene data model cals3k 3 based on all available archeomagnetic and sediment data without a priori quality selection currently constitutes the best global representation of the past field the new models have slightly higher dipole moments than our previous models virtual axial dipole moments vadms calculated directly from the data are in good agreement with all corresponding model predictions of vadms these are always higher than the spherical harmonic dipole moment indicating the limitations of using vadms as a measure of geomagnetic dipole moments

reconstructing holocene geomagnetic field variation new methods models and implications reconstructions of the holocene geomagnetic field and how it varies on millennial timescales are important for understanding processes in the core but may also be used to study long term solar terrestrial relationships and as relative dating tools for geological and archaeological archives here we present a new family of spherical harmonic geomagnetic field models spanning the past 9000 yr based on magnetic field directions and intensity stored in archaeological artefacts igneous rocks and sediment records a new modelling strategy introduces alternative data treatments with a focus on extracting more information from sedimentary data to reduce the influence of a few individual records all sedimentary data are resampled in 50 yr bins which also means that more weight is given to archaeomagnetic data during the inversion the sedimentary declination data are treated as relative values and adjusted iteratively based on prior information finally an alternative way of treating the sediment data chronologies has enabled us to both assess the likely range of age uncertainties often up to and possibly exceeding 500 yr and adjust the timescale of each record based on comparisons with predictions from a preliminary model as a result of the data adjustments power has been shifted from quadrupole and octupole to higher degrees compared with previous holocene geomagnetic field models we find evidence for dominantly westward drift of northern high latitude high intensity flux patches at the core mantle boundary for the last 4000 yr the new models also show intermittent occurrence of reversed flux at the edge of or inside the inner core tangent cylinder possibly originating from the equator less

azimuthal flows in the earth s core and changes in length of day at millennial timescales summary variations in the length of day lod reconstructed from ancient records of eclipses contain an oscillating component with a periodicity of roughly 1500 yr a part of the time dependent variations observed in archaeomagnetic field models consist of episodic eastward and westward motions with a similar periodicity using standard inversion techniques we obtain time dependent azimuthal flows at the surface of the core that can explain parts of the archaeomagnetic secular variation and with amplitude and characteristic timescale of variation consistent to explain the lod signal on this basis we argue that exchanges of angular momentum between core flows and the mantle are responsible for the observed millennial oscillations in the earth s rotation rate however we do not obtain a detailed match between the observed changes in lod and the prediction based on simple flow profiles inside the core we argue that this is due to the presence of time dependent azimuthal flows that have an important shear in the direction of the rotation axis such flows are interpreted as oscillations in thermal and magnetic winds

the time dependence of intense archeomagnetic flux patches 1 the long term temporal behavior of intense geomagnetic flux patches at the core mantle boundary and the relation with lower mantle lateral heterogeneity are under debate we apply an algorithm to detect centers of intense flux patches and track their time evolution in a recent archeomagnetic field model in order to study the kinematics of such intense magnetic flux patches on millennial timescale we find that most intense flux patches appear near the edge of the tangent cylinder quasi stationary periods with small oscillations of patches occur more than drifts detailed comparison of the archeomagnetic patches behavior with that seen in numerical dynamos with tomographic heat flux boundary conditions suggests that core mantle thermal coupling could be the cause of a statistical preference for some longitudes on the long term which does not exclude significant time spent away from the preferred longitudes this could explain the roughly coincident locations of high latitude patches in the historical geomagnetic field with that of the time average paleomagnetic field together with the much weaker patches intensity in the latter alternating eastward and westward drifts are also observed the drifts are more westward than eastward especially in the southern hemisphere indicating that the time average zonal core flow may also be driven by core mantle thermal coupling an average patch lifetime of 300 years is found which we hypothesize may indicate the vortex lifetime in the outer core

time average and time dependent parts of core flow abstract we infer fluid motions below the core mantle boundary by inverting geomagnetic secular variation data over a 150 years assuming helical geostrophic flow we obtain snapshot images of core flow at 5 year intervals which we combine to give time average and time dependent parts of the motion over this time interval the most prominent time average flow structure is a large anti cyclonic vortex in the southern hemisphere beneath the atlantic and indian oceans the time average zonal core flow outside the inner core tangent cylinder is significantly westward in the southern hemisphere but nearly zero in the northern hemisphere westward polar vortices occur inside the tangent cylinder in both hemispheres particularly in the north in terms of mantle versus core origins mantle driving appears to be responsible for the mid latitude asymmetry in the zonal core flow whereas core driving appears to be responsible for the flow at high latitudes we also compare changes in the core s angular momentum calculated from our time dependent core flow with changes in the mantle s angular momentum derived from decade scale length of day variations and find adequate agreement a fit of our time dependent core flow to a torsional oscillations model yields dominant periods of 110 and 53 years

continuous geomagnetic field models for the past 7 millennia 2 cals7k 1 we present two continuous global geomagnetic field models for recent millennia cals3k 2 covering the past 3000 years and cals7k 2 covering 7000 years from 5000 bc to 1950 ad the models were determined by regularized least squares inversion of archeomagnetic and paleomagnetic data using spherical harmonics in space and cubic b splines in time they are derived from a greatly increased number of paleomagnetic directional data compared to previous efforts and for the first time a significant amount of archeointensity data is used in this kind of global model allowing the determination of evolution of geomagnetic dipole strength while data accuracy and dating uncertainties remain a limitation reliable low resolution global models can be obtained the results agree well with previous results from virtual axial dipole moment vadm studies from archeomagnetic intensity data apart from a systematic offset in strength a comparison of model predictions with the previous 3000 year model cals3k 1 gives general agreement but also some significant differences particularly for the early epochs the new models suggest that the prominent two northern hemisphere flux lobes are more stationary than cals3k 1 implied extending considerably the time span of stationary flux lobes observed in historical models between 5000 bc and 2000 bc there are time intervals of weak dipole moment where dipole power is exceeded by low degree nondipole power at the core mantle boundary model coefficients and evaluation code can be obtained from the earthref digital archive erda together with animations and snapshots plots for every 100 years at http www earthref org detailed urls for the different material are listed in appendix a

emergence and evolution of the south atlantic anomaly revealed by the new paleomagnetic reconstruction shawq2k abstract the south atlantic anomaly saa is one of the most outstanding features of the present geomagnetic field thus a good knowledge of the saa is clue for a better understanding of the dynamical behavior of the geomagnetic field to achieve this goal paleomagnetic data are crucial since they provide the unique way to investigate past changes in the earth s magnetic field here we present a new global geomagnetic field reconstruction the shawq2k model which is based on a critical revision of the global archeomagnetic and volcanic dataset the new model provides an improved description of the geomagnetic field during the last 2 millennia and yields surprising outcomes about the emergence and development of the saa it shows that the reversed flux patch observed at the core mantle boundary and linked to the saa emerged in the southern hemisphere from at least 950 ad this patch moved westward from the equator to southern latitudes being clearly isolated after 1400 ad in addition since 1550 ad a second reversed flux patch moving northeastward is observed in the north atlantic the new data now available for the southern hemisphere coming from africa and south america together with the use of an appropriated weighting scheme in the modeling process have improved our understanding of past geomagnetic field behavior and showed new evolutionary aspects of the saa

geomagnetic evidence for fluid upwelling at the core mantle boundary summary previous studies both geomagnetic and seismic have been unable to show conclusively whether or not there is fluid upwelling at the core mantle boundary here a new method is developed in which an attempt is made to invert geomagnetic secular variation data measured at the earth s surface for a frozen flux purely toroidal core mantle boundary cmb velocity field under the assumption that the mantle is electrically insulating and flux is frozen in at the cmb these data have previously been inverted for the core mantle boundary radial secular variation from which the appropriate fit between model and data is known two different main field models were used to assess the effect of uncertainty in its radial component at the cmb the conclusions were the same in both cases frozen flux purely toroidal motions provide a poor fit a statistical test allows very firm rejection of the hypothesis that the residuals are not significantly larger whereas there is no statistical difference between the residuals of inversions for radial secular variation and frozen flux velocity fields at the cmb if upwelling and down welling is included the inherent non uniqueness in the velocity field obtained is not of concern since only their statistical properties are utilized and no physical significance is attached to the flows obtained

reconstructing the holocene geomagnetic field abstract knowledge of the holocene evolution of earth s magnetic field is important for understanding geodynamo processes in the core is necessary for studying long term solar terrestrial relationships and can provide useful age constraints for archeologicaland stratigraphic applications continuous time varying global field models based on archeo and paleomagnetic data are useful tools in this regard we use a comprehensive data compilation and recently refined modelling strategies to produce cals10k 1b the first time varying spherical harmonic geomagnetic field model spanning 10 ky the model is an average obtained from bootstrap sampling to take account of uncertainties in magnetic components and ages in the data and hence has version number 1b instead of 1 this model shows less spatial and temporal resolution than earlier versions for 0 3 ka and particularly aims to provide a robust representation of the large scale field at the core mantle boundary cmb we discuss the geomagnetic dipole evolution and changes in holocene magnetic field morphology at the cmb as shown by the new reconstruction the results are compatible with earlier models cals3k 3 and cals3k 4 for 0 3 ka but reveal some clear deficiencies in the 0 7 ka cals7k 2 model prior to 3 ka cals10k 1b is able to resolve mobile and structurally evolving high latitude radial field flux lobes at the cmb in both hemispheres as well as persistent non zonal structure in the 10 ky average contributions to the average field from time varying structures in the equatorial indonesian australian region are particularly striking

the steady part of the secular variation of the earth s magnetic field the secular variation of the earth s magnetic field results from the effects of magnetic induction in the fluid outer core and from the effects of magnetic diffusion in the core and the mantle adequate observations to map the magnetic field at the core mantle boundary extend back over three centuries providing a model of the secular variation at the core mantle boundary here we consider how best to analyze this time dependent part of the field we propose that the first step should be to seek the steady core flow that best fits the field maps isolating what we call the steady secular variation the part of the secular variation explained by steady core flow to calculate steady core flow over long time periods we introduce an adaptation of our earlier method of calculating the flow in order to achieve greater numerical stability we perform this procedure for the periods 1840 1990 and 1690 1840 and find that well over 90 of the variance of the time depedent field can be explained by simple steady core flow the core flows obtained for the two intervals are broadly similar to each other and to flows determined over much shorter recent intervals although we can explain a large part of the signal with these flows none of them provides an adequate explanation of the secular variation in particular the fit to the original observations from which the field models were derived is inadequate we examine the residual secular variation the part of the signal that remains we argue that it is dominated by the effects of nonsteady flow and magnetic diffusion we do not find any compelling evidence for torsional oscillations as the dominant ingredient of this nonsteady part of the secular variation

fluid flow near the surface of earth s outer core maps of the fluid flow at the core surface are important for a number of reasons foremost they may provide some insight into the workings of the geodynamo and may place useful constraints on geodynamo models from the flow the force balance at the top of the core can at least in part be deduced the flow can provide short term predictions of the secular variation the flow is important in understanding changes in the length of day and constraints on lateral temperature variations and topography at the core mantle boundary may be derived unlike the case of mantle convection only very small lateral variations in core density are required to drive the flow these density variations are too small by several orders of magnitude to be imaged seismically so instead we use the geomagnetic secular variation to infer the flow despite considerable recent progress in mapping the core flow substantial differences exist between maps produced by different researchers here we examine the possible underlying reasons for these differences paying particular attention to the inherent problems of nonuniqueness we focus on the aspects of the flow which do seem to be well determined and discuss their geophysical implications

improving geomagnetic field reconstructions for 0 3 ka global geomagnetic field reconstructions on millennial time scales can be based on comprehensive paleomagnetic data compilations but especially for older data these still suffer from limitations in data quality and age controls as well as poor temporal and spatial coverage here we present updated global models for the time interval 0 3 ka where additions to the data basis mainly impact the south east asian alaskan and siberian regions we summarize recent progress in millennial scale modelling documenting the cumulative results from incremental modifications to the standard algorithms used to produce regularized time varying spherical harmonic models spanning 1000 bc to 1990 ad from 1590 to 1990 ad gauss coefficients from the historical gufm1 model supplement the paleomagnetic information in addition to absolute paleointensities calibrated relative paleointensity data from sediments are now routinely included iterative data rejection and recalibration of relative intensity records from sediments ensure stable results bootstrap experiments to generate uncertainty estimates for the model take account of uncertainties in both age and magnetic elements and additionally assess the impact of sampling in both time and space based on averaged results from bootstrap experiments taking account of data and age uncertainties we distinguish more conservative model estimates cals3k nb representing robust field structure at the core mantle boundary from relatively high resolution models cals3k n for model versions n 3 and 4 we assess the impact of newly available data and modifications to the modelling method by comparing the previous cals3k 3 the new cals3k 4 and the conservative new model cals3k 4b we conclude that with presently available data it is not feasible to produce a model that is equally suitable for relatively high resolution field predictions at earth s surface and robust reconstruction of field evolution avoiding spurious structure at the core mantle boundary cmb we presently consider cals3k 4 the best high resolution model and recommend the more conservative lower resolution version for studies of field evolution at the cmb

on the possible occurrence of archaeomagnetic jerks in the geomagnetic field over the past three millennia abstract archaeomagnetism can provide a high resolution full vector description of the earth s magnetic field for the past several thousand years we analyse the bulk of archaeomagnetic data both direction and intensity obtained recently in western europe and the eastern mediterranean covering the past three millennia we demonstrate a remarkable coincidence between sharp cusps in geomagnetic field direction and intensity maxima two clear ones at ad 200 and 1400 two presently less well constrained at 800 bc and ad 800 these sharp changes may constitute a new feature of geomagnetic secular variation archaeomagnetic jerks with time characteristics intermediate between geomagnetic jerks and magnetic excursions

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

continuous global geomagnetic field models for the past 3000 years abstract several global geomagnetic field models exist for recent decades but due to limited data availability models for several centuries to millennia are rare we present a continuous spherical harmonic model for almost 3 millennia from 1000 b c to 1800 a d based on a dataset of directional archaeo and paleomagnetic data and axial dipole constraints the model named continuous archaeomagnetic and lake sediment geomagnetic model for the last 3k years cals3k 1 can be used to predict both the field and secular variation comparisons and tests with synthetic data lead to the conclusion that cals3k 1 gives a good general large scale representation of the geomagnetic field but lacks small scale structure due to the limited resolution of the sparse dataset in future applications the model can be used for comparisons with additional new data for that time span for better resolved regions the agreement of data with cals3k 1 will provide an idea about the general compatibility of the data with the field and secular variation in that region of the world for poorly covered regions and time intervals we hope to iteratively improve the model by comparisons with and inclusion of new data animations and additional snapshot plots of model predictions as well as the model coefficients and a fortran code to evaluate them for any time can be accessed under http www mahi ucsd edu cathy holocene holocene html the whole package is also stored in the earthref digital archive at http www earthref org

observed periodicities and the spectrum of field variations in holocene magnetic records abstract in order to understand mechanisms that maintain and drive the evolution of the earth s magnetic field a characterization of its behavior on time scales of centuries to millennia is required we have conducted a search for periodicities in holocene sediment magnetic records by applying three techniques multitaper spectral estimation wavelet analysis and empirical mode decomposition when records are grouped according to their geographical locations we find encouraging consistency amongst the observed periods especially in nearby inclination records no evidence was obtained for discrete globally observed periods rather we find a continuous broadband spectrum with a slope corresponding to a power law with exponent of 2 3 0 6 for the period range between 300 and 4000 yr this is consistent with the hypothesis that chaotic convection in the outer core drives the majority of secular variation

eastward and westward drift of the earth s magnetic field for the last three millennia abstract we analyse the secular variation captured by the archaeomagnetic field model cals7k 2 in an effort to determine episodes of eastward and westward motions of earth s magnetic field at the core mantle boundary cmb over the past 3000 yr the direction amplitude and geographical distribution of these motions are described we find that the clearest azimuthal motions are observed at mid to high latitudes in the northern hemisphere where both eastward and westward motions occur these azimuthal motions correspond to displacements and distortions of the two main quasi stationary high latitude magnetic flux patches similar motions are not observed in the southern hemisphere although this may be a consequence of the poorer data coverage there the globally averaged drift for the past 1000 yr has been westward since 1400 ad but eastward between 1000 ad and 1400 ad in the broad region of the cmb under europe the times of transition in the direction of the mean azimuthal motion coincide with the times at which archaeomagnetic jerks have been reported our results suggest that these are caused by a relatively rapid

a 2000 year geomagnetic field record from the gulf of papua abstract a high resolution holocene record of geomagnetic field direction and intensity in the gulf of papua was constructed using paleomagnetic data from eight piston cores collected in the region the chronology of the cores was based largely on radiocarbon dates from benthic foraminifera correlation of seismic reflectors imaged in subbottom data between nearby cores was consistent with the chronostratigraphic framework based on radiocarbon dates and provided additional confidence of the age model in other regions where cores sampled different depositional settings associated with the three dimensional prograding lobes reflectors could not be confidently correlated between cores furthermore radiocarbon ages from the bottoms of trigger cores and tops of co located piston cores revealed varying amounts of overpenetration of the piston core below the seafloor the relative paleointensity and a general eastward trend in the declination and steepening of the inclination in our new record are generally compatible with the closest 4300 km existing records our record does not agree well with global field models cals3k and pfm9k likely reflecting the lack of existing data contributing to field models in the region

the evolution of the core surface flow over the last seven thousands years 1 we analyze the core surface flow over the last seven thousands years derived from cals7k 2 a continuous model of the geomagnetic field and its secular variation for the period 5000 bc to 1950 ad this model relies on indirect observations of the geomagnetic field through the sampling of natural archives lake sediments lavas and archaeological artifacts an extensive comparison between gufm1 a historical field model spanning the interval 1590 to 1990 and cals7k 2 is carried out while both are similar in their descriptions of the large scale core field their agreement compares poorly in terms of secular variation the differences are primarily caused by the different data quality used to build these models such a comparison is essential for assessing the reliability of the results of flow inversion we seek instantaneous core flow solutions that are purely toroidal and tangentially geostrophic for which we can reasonably neglect the effect of magnetic diffusion our flow solutions suggest that the core surface flow undergoes different regimes of zonal flow direction with epochs of mainly westward flow alternating with epochs of mainly eastward flow the episodes of eastward and westward flow are robust as they are obtained for purely toroidal and tangentially geostrophic flow the changes in the large scale zonal flow direction seem to recur with a typical period of between 540 and 1050 years variations in thermal and magnetic winds occurring on timescales comparable with the convective overturn of the liquid outer core could account for the observed zonal flow variations

variation in the composition of apatite through the merensky cyclic unit in the western bushveld complex microprobe analysis of interstitial apatite from a 100 m section through the merensky reef of the bushveld complex reveals that this stratigraphic section marks a transition from cl rich to more f rich compositions in the critical and main zones respectively apatite compositions across the section vary by more than 80 mole percent in the chlorapatite component the trend of decreasing cl concentration with stratigraphic height is not regular and cl contents in apatites from some samples may vary as much as 50 mole percent la 2 o 3 and ce 2 o 3 tend to be enriched in the more melanocratic orthocumulate parts of the section sro concentrations of apatite show an increase with stratigraphic position a trend which is opposite that observed in coexisting cumulus plagioclase the data are consistent with mineralogic and textural evidence for an influx of cl rich vapor into the cumulus section which has affected the cl f variations through the lower portion of the bastard unit this influx occurred above the crystallization temperature of apatite 1 000 degrees 1 050 degrees c it is suggested that much of the modal variation seen in the section may be the result of shifts in liquidus field boundaries caused by gain or loss of volatile components from the melt

continuous geomagnetic field models for the past 7 millennia 1 a new global data compilation 1 a global data set of archeomagnetic and paleomagnetic data covering the past 7000 years has been compiled it consists of 16 085 results of inclination 13 080 of declination and 3188 of intensity for the time span 5000 bc to 1950 ad declination and inclination data come partly from existing databases and partly from original literature a new global compilation of intensity data for the millennial scale is included data and dating uncertainties are discussed as we attempted to obtain an internally coherent data set the global distribution of the data is very inhomogeneous in both time and space all the data are compared to predictions from the previous 3000 year global model cals3k 1 this collection of data will be useful for global secular variation studies and geomagnetic field modeling although southern hemisphere data are still underrepresented in particular we will use it in a further study to update and extend the existing global model cals3k 1 the huge increase in data compared to the previous compilation will result in significant changes from current models as we might have missed some suitable data we encourage the reader to notify us about any data that have not been included yet and might fit in as improving our global millennial scale models remains our aim for the future the data files described in this paper are available from the earthref digital archive erda at http earthref org cgi bin erda cgi n 331