the 9th generation international geomagnetic reference field summary the international association of geomagnetism and aeronomy has recently released the 9th generation international geomagnetic reference field the latest version of a standard mathematical description of the earth s main magnetic field used widely in studies of the earth s deep interior its crust and its ionosphere and magnetosphere the coefficients were recently finalized at the xxiii general assembly of the international union of geophysics and geodesy held at sapporo in japan in 2003 july the igrf is the product of a huge collaborative effort between magnetic field modellers and the institutes involved in collecting and disseminating magnetic field data from satellites and from observatories and surveys around the world

dextral shear along the eastern margin of the colorado plateau a kinematic link between laramide contraction and rio grande rifting ca 75 13 ma abstract kinematic data associated with both laramide age and style and rio grande rift related structures show that the latest cretaceous to neogene interaction between the colorado plateau and the north american craton was dominantly coupled with a component of dextral shear consistent with earlier studies minor fault data in this study yielded results of varied kinematics inverted to a common northeast oriented hemisphere the mean trend of kinematic shortening associated with laramide age structures is documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6

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

secular variation of the earth s magnetic field and application to paleomagnetic dating of historical lava flows in chile abstract the recent geomagnetic secular variation is mainly characterized by the large growth of the south atlantic magnetic anomaly during the last three centuries first documented by the geomagnetic model gufm1 jackson et al 2000 we report new paleomagnetic results directions and paleointensities from several sites in two well dated lava flows in chile the 1835 ad eruption of the osorno volcano and the 1751 ad eruption of the llaima volcano in addition paleointensities were obtained on 14 samples from bricks of shelters built along the main road across the andes from santiago chile to mendoza argentina in 1770 5 ad the results confirm the high reliability of the global geomagnetic model gufm1 for the last three centuries with a large amplitude of the secular variation in inclination 20 and intensity 25 t results from three 14 c dated volcanic units in the time interval 1400 1750 ad indicate that more paleomagnetic results in well dated lava flows are necessary to improve the robustness of existing global geomagnetic models at this stage precise paleomagnetic or archeomagnetic dating in south america using global models should be restricted to the last 3 centuries to illustrate the potential of paleomagnetic dating in region and time interval with very large geomagnetic secular variation we report paleomagnetic data from several sites in historical lava flows 1700 1900 ad from the antuco llaima and villarrica volcanoes that permit to refine the ages of the major historical effusive volcanic events

geomagnetic and mineral magnetic characterization of the anthropocene geomagnetic and mineral magnetic data provide geological indices that are both independent of human impact i e geomagnetic and respond to human induced environmental impact i e mineral magnetic we provide the first discussion of such magnetic events for help in defining the anthropocene within the holocene a potential geomagnetic marker for the anthropocene is the low dipole latitude at c 2700 cal a bp which is associated with distinct palaeosecular variation features in northerly mid to high latitude sites mineral magnetic records from lake and marine sediments identify major deforestation and soil delivery events from catchment systems in many parts of the world during the last 4000 years in europe clusters of these events occur around both 2600 cal a bp and ad 1100 the former coinciding with a low in geomagnetic field dipole latitude and peak intensity mineral magnetic records in peats and lake sediments can reflect particulate pollution from fossil fuel burning the expansion of major coal burning began c ad 1800 in western europe and eastern north america but around ad 1900 this expanded due to more widely distributed coal use and this event is the most clear mineral magnetic marker for the base of the anthropocene

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

first archaeomagnetic field intensity data from ethiopia africa 1615 12 ad abstract first archaeointensity determinations have been obtained from ethiopia seven bricks 34 specimens from the dabsan archaeological remains were subjected to archaeointensity determination by means of classical thellier thellier experiment including tests for magnetic anisotropy and magnetic cooling rate dependency the age of the dabsan palace is well controlled by historical information between 1603 when land grants were conceded to the jesuits and the catholicism was established as the official religion in ethiopia and the age of the palace foundation in 1626 27 successful archaeointensity determinations were obtained in 27 specimens from five individual bricks revealing an average field value of 33 5 1 1 t which is 11 26 lower than expected values from global geomagnetic models based on historical and archaeomagnetic data global models for 1615 ad predict a low in central southern africa related to past location of the present southern atlantic anomaly saa our results suggest that the field intensity in central africa may have been slightly lower than global model predictions this would indicate that the low could be probably more extended towards central eastern africa or more intense than previously considered further data from this region are especially welcome to delineate the evolution of the saa

archeointensity in northeast brazil over the past five centuries abstract this study presents the first archeointensity results from northeast brazil obtained from 14 groups of architectural brick fragments sampled in the city of salvador bahia state 13 s 38 5 w and dated between the middle of the xvith century and the beginning of the xixth century the dating is ascertained by historical documents complemented by archeological constraints yielding in all cases age uncertainties of less than 50 years analyses were carried out using two experimental protocols 1 the zero field in field version of the classical thellier and thellier method as proposed by coe tt zi including partial thermoremanent magnetization ptrm and ptrm tail checks and 2 the triaxe procedure involving continuous high temperature magnetization measurements both trm anisotropy and cooling rate effects were taken into account for the intensity determinations the cooling rate effect was further explored for the tt zi protocol using three increasing slow cooling times 5 h 10 h and 25 h between 450 c and room temperature following archeological constraints the slowest cooling time was retained in our study yielding decreases of the raw intensity values by 4 to 14 for each fragment a mean intensity was computed and retained only when the data obtained from all specimens between 2 and 6 satisfied a coherence test at 5 a total of 57 fragments 183 specimens was considered for the computations of site mean intensity values with derived standard deviations of less than 8 of the corresponding means when separately computed using the two experimental techniques the site mean intensity values always agree to within 5 a good consistency is observed between intensity values of similar or close ages which strengthen their reliability our data principally show a significant and continuous decrease in geomagnetic field intensity in northeast brazil between the first half of the xviith century and the xxth century one result dated to the second half of the xvith century further suggests that the geomagnetic field intensity reached a maximum around 1600 ad this evolution is in good agreement with that expected in the city of salvador from the available global geomagnetic field models however the accuracy of these models appears less well constrained between 1550 ad and 1650 ad

implications of jurassic cretaceous and proterozoic piercing lines for laramide oblique slip faulting in new mexico and rotation of the colorado plateau discussion and reply discussion cather 1999 1 provided a useful compilation of stratigraphic data relevant to tests of models for possible dextral offset along the east side of the colorado plateau during the laramide orogeny his paper allowed the reader to judge whether this independent data set is consistent with

olivine spinel transitions experimental and thermodynamic constraints and implications for the nature of the 400 km seismic discontinuity the sequence of high pressure phase transitions in olivine is traditionally used as a model for seismic velocity variations in the 200 to 650 km depth interval in a mantle of peridotitic bulk composition it has been proposed that the observed seismic velocity increase at 400 km depth is too sharp and of too small a magnitude to be attributable to the phase change and that the upper mantle must thus be chemically stratified with the 400 km discontinuity being due either to a combination of phase changes in a layer of pyroxene garnet rich piclogite composition or to a chemical boundary between such a piclogite layer and an overlying peridotitic layer using available calorimetric thermoelastic and synthesis data and their associated experimental uncertainties we have derived internally consistent high pressure phase relations for the mg2sio4 fe2sio4 join we find that the divariant transition which is generally regarded as occurring over a broad depth interval for mantle olivine compositions is in fact extremely sharp the seismic discontinuity corresponding to the transition in mg0 9fe0 1 2sio4 should occur over a depth interval isothermal of about 6 km at a depth of approximately 400 km the sharpness of this transition is quite insensitive to uncertainties in the constraining calorimetric thermoelastic and synthesis data in addition we have computed seismic velocity profiles for a model mantle consisting of pure olivine of mg0 9fe0 1 2sio4 composition comparison of these computed profiles to those derived from recent seismic studies indicates that the magnitude of the observed velocity increase at 400 km depth is consistent with a mantle transition zone composed of about 60 70 olivine we conclude that there is no need to infer the existence of pyroxene garnet rich compositions such as eclogite or piclogite in the transition zone since an upper mantle of homogeneous olivine rich peridotitic composition is consistent with the available seismic velocity data

on the use of calibrated relative paleointensity records to improve millennial scale geomagnetic field models current millennial scale time varying global geomagnetic field models suffer from a lack of intensity data compared to directional data because only thermoremanently magnetized material can provide absolute information about the past field strength the number of archeomagnetic artifacts that can provide such data diminishes rapidly prior to 3000 b c sediment cores provide time series of declination and inclination and of variations of magnetization the latter can reflect relative geomagnetic field variations if suitably normalized we propose a calibration technique based on predictions from global models and use the cals7k 2 model to calibrate relative paleointensity records from 22 globally distributed locations and assess whether they reflect actual field variations all except a few contain useful information for 0 to 7 ka and could be used to improve the existing models using synthetic data from a numerical dynamo simulation we show that with the existing directional data the distribution of intensity data has an important influence on model quality intensity data from a broad range of latitudes seem particularly important this study opens the possibility of extending global time varying geomagnetic field models further back in time than the current 7 kyr interval

geomagnetic secular variation as recorded in british lake sediments and its application to archaeomagnetic studies abstract lake sediments can play an important role in understanding and reconstructing temporal characteristics of the geomagnetic field as they potentially offer near continuous high resolution archives of magnetic information extending throughout the holocene to date lake sediment geomagnetic data has typically been excluded from british archaeomagnetic secular variation curves svc due to uncertainties with the acquisition of magnetisation by lake sediments this paper presents the argument that with regards to british datasets the real problem lies with poor chronological control and sets out to illustrate that with british archaeomagnetic data some progress has been made the results indicate that it is not currently possible to resolve secular variation on a time scale of 100 years from published british lake sediment data but more success has been made with data from archaeological sediments this level of detail has been considered necessary for the incorporation of lake sediment data into the british archaeomagnetic dataset as the ability to resolve short term geomagnetic changes is critical for the integration of any dataset into the british svc as the latter is predominantly employed to date archaeological architecture and artefacts requires that it has the ability to resolve changes over timescales relevant to human lifetimes using currently available data this retrospective critique confirms that in archaeological sediments depositional and thermoremanent magnetisation can record the same direction over the same time interval

new constraints on the most significant paleointensity change in western europe over the last two millennia a non dipolar origin over the last years new evidences of several short lived regional maxima of the geomagnetic field intensity at various times and locations have been defined these features have important implications both for geomagnetic field modeling and for earth s dynamo simulations however the nature extent and underlying causes of these variations are still poorly understood the major constraint for an adequate description of these important features is the absence of continuous detailed records and the current limited availability of precisely dated geomagnetic field recorders in this context archeological materials from superimposed strata covering long sequences of occupation provide a powerful tool to investigate the temporal variability of geomagnetic field strength at decadal and centennial time scales in this work we report the archeomagnetic study of 79 potteries from southeastern spain collected in 14 different stratigraphic units the chronological framework of the studied collection ranging from the 9th to the 12th centuries is based on historical archeological data such as written sources and well established typological and archeometric documentation on ceramics found on reference contexts in the area additionally two radiocarbon dates obtained from two different stratigraphic units confirm the proposed chronological intervals from classical thellier and thellier experiments including partial thermoremanent magnetization ptrm checks and trm anisotropy and cooling rate corrections height new high quality mean intensities were derived the new data provide an improved description of the sharp abrupt intensity decay that took place in western europe after the 800 ad intensity maximum the most significant geomagnetic field intensity feature observed in europe over the last two millennia the new results confirm that several rapid intensity changes with rates higher than 10 t century took place in western europe during the recent history of the earth the comparison between the regional curve of western europe and the sha dif 14k global field model predictions suggests that the 800 ad event observed in europe is probably controlled by non dipolar geomagnetic sources

earth s magnetic field is probably not reversing the geomagnetic field has been decaying at a rate of 5 per century from at least 1840 with indirect observations suggesting a decay since 1600 or even earlier this has led to the assertion that the geomagnetic field may be undergoing a reversal or an excursion we have derived a model of the geomagnetic field spanning 30 50 ka constructed to study the behavior of the two most recent excursions the laschamp and mono lake centered at 41 and 34 ka respectively here we show that neither excursion demonstrates field evolution similar to current changes in the geomagnetic field at earlier times centered at 49 and 46 ka the field is comparable to today s field with an intensity structure similar to today s south atlantic anomaly saa however neither of these saa like fields develop into an excursion or reversal this suggests that the current weakened field will also recover without an extreme event such as an excursion or reversal the saa like field structure at 46 ka appears to be coeval with published increases in geomagnetically modulated beryllium and chlorine nuclide production despite the global dipole field not weakening significantly in our model during this time this agreement suggests a greater complexity in the relationship between cosmogenic nuclide production and the geomagnetic field than is commonly assumed

high fidelity paleointensity determination from historic volcanoes in japan 1 providing high fidelity paleointensity estimates is pivotal to identify temporal fluctuation of ancient geomagnetic field despite abundance of available paleointensity data in japan only dozen of them satisfy modern standards with systematic alteration checks high fidelity paleointensity estimates were obtained from historic andesitic lavas collected from mt aso ojodake 700 bc mt kirishima iwoyama ad 1768 ohachi ad 1235 and mt sakurajima an ei ad 1779 nabeyama ad 764 766 variation of geomagnetic field intensity is distinctively different from the prediction of global models arch3k 1 cals3k 3 cals3k 4 cals7k 2 and cals10k 1b for the past 4000 years in japan the compilation of high fidelity thellier data set in japan showed two obvious high intensities at ad 590 765 and ad 1330 1435 and one low intensity at 700 bc these time intervals with anomalous high low intensities are nearly identical to three of the four potential archeomagnetic jerks recognized from the european archeomagnetic data implying that the archeomagnetic jerks were global or at least northern hemispheric features to improve the poor representation of regional geomagnetic field variation with respect to the global model prediction more high fidelity paleointensity determinations are required in east asia

global geomagnetic field models for the past 3000 years transient or permanent flux lobes psvmod1 0 is a compilation of globally distributed palaeodirectional data from archaeomagnetic artefacts lava flows and lake sediments at 24 sites evaluated at 100 year intervals from 1000 bc to ad 1800 we estimate uncertainty in these measures of declination and inclination by comparison with predictions from standard historical models in time intervals of overlap and use the 100 year samples and their associated uncertainties to construct a sequence of minimum structure global geomagnetic field models global predictions of radial magnetic field at the coremantle boundary cmb as well as inclination and declination anomalies at the earth9s surface provide an unprecedented view of geomagnetic secular variations over the past 3000 years and demonstrate a consistent evolution of the field with time resolution of the models is poorest in the southern hemisphere where only six of the 24 sites are located several with incomplete temporal coverage low flux regions seen in the historical field near the north pole are poorly resolved but the northern hemisphere flux lobes are clearly visible in the models these lobes are not fixed in position and intensity but they only rarely venture into the pacific hemisphere the pacific region is seen to have experienced significant secular variation a strong negative inclination anomaly in the region like that seen in 0 5 ma models persists from 1000 bc until ad 1000 and then gradually evolves into the smaller positive anomaly seen today on average between 1000 bc and ad 1800 the non axial dipole contribution to the radial magnetic field at the core mantle boundary is largest in the north central pacific and beneath central asia with clear non zonal contributions at the earth9s surface average inclination anomalies are large and negative in the central pacific and most positive slightly to the east of central africa inclination anomalies decrease with increasing latitude average declinations are smallest in equatorial regions again with strong longitudinal variations largest negative departures are centred over australia and eastern asia secular variation at the earth9s surface is quantified by standard deviation of inclination and declination about their average values and at the cmb by standard deviation in radial magnetic field all three show significant geographical variations but appear incompatible with the idea that secular variation in the pacific hemisphere is permanently attenuated by greatly enhanced conductivity in d 00 beneath the region

secular variation of the geomagnetic dipole during the past 2000 years we have constructed a very simple model of a time varying geocentric dipole based on the archeomagnetic records obtained at four widely separated sites on the globe for the past 2 ka the predictions of the model in terms of directional variations have been tested against actual archeomagnetic data from 12 sites distributed over the globe they were also compared with the hongre et al 1998 time varying spherical harmonic model and with the cals7k 2 model by korte et al 2005 we find that the misfits between the predictions of our simple dipole model and the data are equivalent to those of the spherical harmonic models for the european sites and not strikingly larger for the rest of the world many discrepancies can be accounted for by uncertainties inherent to the archeomagnetic records which along with the small number and poor geographical distribution of sites leads us to conclude that the present state of the database does not allow the extraction of secular variations described by terms going beyond degree 2 it appears also that dipole tilt could be responsible for the main part of the secular variation associated with time constants exceeding 102 years in a second step we used the paleointensity records contained in the same database to construct the curve depicting the variations of the true dipole moment the present decrease of the dipole did not begin prior to 1000 years ago and the dipole was actually increasing from 0 until a d 500 the dipole moment of cals7k is lower than the present estimate probably due to large repartition of energy to higher harmonics to minimize the misfit between the inversion and the data the tilt and strength of the dipole can predict the dipole field at any site and were used to derive the contribution of the nondipole field to values of paleointensity at paris during the past 2 ka the results show that the archeomagnetic jerks are associated with various configurations depending on the phase relationship between the nondipolar and dipolar parts of the field

paleomagnetic study of el metate shield volcano michoacan mexico confirms its monogenetic nature and young age 1250 ce in a recent study chevrel et al 2016a b radiocarbon dated the oldest lava flow of the voluminous 9 2 km3 el metate shield volcano michoacan mexico at cal 1250 1260 ce and proposed that its eruption was monogenetic in origin with twelve younger lava flows emplaced during a short period of only 35 years but certainly 275 years in order to test this hypothesis we undertook a detailed paleomagnetic study of five lava flows from el metate to check the consistency of their paleomagnetic directions additionally a group of representative specimens was treated with the double heating thellier experiment using the izzi protocol for paleointensity determination flow mean paleomagnetic directions obtained for four of the flows are indistinguishable and discordant directions were obtained from the site of the 5th flow measured probably due to the tilting of the sampled block after remanence acquisition mean paleodirections and intensities were used for paleomagnetic dating applying the global paleosecular variation model sha dif 14k the resulting age range for the eruption is 1150 1290 ce which overlaps with the range previously determined by the 14c method by chevrel et al 2016a accepting the 14c age of the oldest flow as the maximum age the age range would be reduced to 1250 1290 ce which strongly supports the hypothesis of a monogenetic nature of the el metate eruption

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

archaeomagnetic research in the andean highlands abstract eleven archaeomagnetic samples were collected from several sites near porco bolivia as part of the proyecto arqueologico porco potosi these samples were collected in order to 1 explore the availability of appropriate sample materials in the bolivian andes and 2 assess the feasibility of pursuing reference curve research for this region in addition this research explored the archaeomagnetic dating application of reference curves constructed from the global geomagnetic field model gufm1 jackson et al 2000 which covers the period from ad 1590 1990 this model is particularly applicable to our project since many of the project sites are late prehispanic pre ad 1532 and early historic ca ad 1532 1825 in age of the 11 samples three appeared to contain an insufficient amount of ferromagnetic minerals to provide reliable results while eight samples produced acceptable to good results the data from the eight good samples were compared to a reference curve for the period of ad 1600 2000 that was calculated for this project from the global field model gufm1 the archaeomagnetic dates obtained for two of the samples agreed with archaeological expectations while those for four other samples marginally or greatly postdated the archaeological age expectations two additional samples produced significantly different directions and could not be dated against the reference curve it is likely that the age discrepancies for four of the samples stem from inaccuracies in the archaeomagnetic data the archaeological age expectations the calculated reference curve or some combination of these factors

holocene geomagnetic paleointensities a blind test of absolute paleointensity techniques and materials through several decades of research absolute paleointensity estimates have been obtained from a wide range of rocks with varying ages covering the last 3 45 ga years these paleointensity data are crucial to study the past geodynamo and the geological evolution of the earth s deep interior however paleointensity data are often difficult to interpret and on going discussions concerning the past geomagnetic field invariably focus on data reliability and which kind of material and method is best suited for absolute paleointensity experiments in this paper we blind test paleointensity methods and materials using the new geomagia50 database that contains all available experimental information about published paleointensity estimates covering the last 50 000 years our analysis is built on simple comparisons of results obtained from different materials and methodologies to investigate their possible influence on the paleointensity estimate we also study the effect of various numbers of samples and dating accuracies we focus on paleointensity estimates from the last 12 000 years which includes the holocene epoch the advantage of this period is that data have been obtained from a wide array of methodological techniques as well as a wide selection of materials including both natural ones e g lava flows and various archaeological artifacts our main observations are i that well fired archaeological materials bricks potteries clays and ceramics show the best correlation with the rest of the dataset which we interpret to suggest that these data are the most reliable and ii although paleointensity data obtained from lava flows are slightly more scattered there is no evidence for paleointensity data from lavas to be significantly lower or higher than other data irrespective of the methodology c 2007 elsevier b v all rights reserved less

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

directional changes of the geomagnetic field in west africa insights from the metallurgical site of korsimoro abstract this work shows the first archeomagnetic directions from western africa measured on 32 iron smelting kilns dated between 650 and 1800 ad the archeological excavation of the vast metallurgical site of korsimoro established the existence of four distinct iron smelting techniques the time frame of each technique could be clearly determined with radiocarbon dating many of the kilns investigated in this study could also be dated individually with residual charcoals found in their inside the results indicate that the inclination of the field changed gradually from shallow normal to shallow reversed during 800 to 1300 ad and then went back to shallow normal around 1600 1700 ad the declination was instead stable around 10 e between 800 and 1400 ad thereafter it started changing towards north this trend correlates well with available secular variation curves from the balkan and from spain and indicates that the field variation in west africa was similar to the one in europe

paleogeographic and paleotectonic setting of laramide sedimentary basins in the central rocky mountain region in the rocky mountain region between central montana and central new mexico sedimentologically isolated nonmarine basins were produced by basement deformation during the laramide orogeny within the area formerly occupied by a broad late cretaceous foreland basin in which laterally continuous marine facies had accumulated previously laramide structures of varying trend and scale reflect heterogeneity of crustal strain caused by shear between the continental lithosphere and an underlying subhorizontal slab of subducted oceanic lithosphere laramide basins include perimeter basins along the cratonic periphery of the arcuate laramide province axial basins along a north south intramontane trend and ponded basins located farther west closer to the overthrust belt twelve specific stratigraphic and sedimentologic criteria for the onset duration and termination of laramide deformation allow the chronology of basin development to be inferred independently for each basin maastrichtian initiation of laramide deformation was approximately synchronous throughout the laramide province but termination of laramide deformation was systematically diachronous from north to south between early and late eocene time widespread eocene erosion surfaces truncate syntectonic laramide sequences and are overlain by largely volcanic and volcaniclastic post laramide strata of eocene age in the north and oligocene age in the south fluvial depositional systems draining toward the great plains were dominant in perimeter and axial basins but ponded basins were occupied at times by large lakes that served as regional sediment traps paleocene drainages from ponded basins also led eastward toward the continental interior but partly interconnected lakes that developed within several ponded basins by mid eocene time were either closed hydrologically or else they drained westward into the tyee paleoriver of the pacific northwest

palaeomagnetic study of a subaerial volcanic ridge sao jorge island azores for the past 1 3 myr evidence for the cobb mountain subchron volcano flank instability and tectonomagmatic implications we present a palaeomagnetic study on 38 lava flows and 20 dykes encompassing the past 1 3 myr on s jorge island azores archipelagonorth atlantic ocean the sections sampled in the southeastern and central western parts of the island record reversed and normal polarities respectively they indicate a mean palaeomagnetic pole 81 3 degrees n 160 7 degrees e k 33 and a95 3 4 degrees with a latitude shallower than that expected from geocentric axial dipole assumption suggesting an effect of non dipolar components of the earth magnetic field virtual geomagnetic poles of eight flows and two dykes closely follow the contemporaneous records of the cobb mountain subchron odp dsdp programs and constrain the age transition from reversed to normal polarity at ca 1 207 0 017 ma volcano flank instabilities probably related to dyke emplacement along an nnwsse direction led to southwestward tilting of the lava pile towards the sea two spatially and temporally distinct dyke systems have been recognized on the island the eastern is dominated by nnwsse trending dykes emplaced before the end of the matuyama chron whereas in the central western parts the eruptive fissures oriented wnwese controlled the westward growth of the s jorge island during the brunhes chron both directions are consistent with the present day regional stress conditions deduced from plate kinematics and tectonomorphology and suggest the emplacement of dykes along pre existing fractures the distinct timing and location of each dyke system likely results from a slight shift of the magmatic source

two thousand years of geomagnetic field direction over central europe revealed by indirect measurements summary supplemented by 32 new directions the hungarian archaeomagnetic data set now consists of 217 archaeologically dated directions ranging in age from 300 bc to 1800 ad from this data set reference curves of secular variation of the geomagnetic field direction were computed using hierarchical modelling and curve estimation by moving average technique thanks to some of the new data the gap in the earlier reference curves around 500 ad has now been filled up for comparison s sake directional records of comparable length from central europe were also processed by the same curve building method for this purpose all dated directional data declination and inclination with statistics were drawn from the geomagia50 database for france germany the ukraine and moldavia bulgaria and italy and transferred via their virtual geomagnetic poles to a reference point of their respective countries the resulting reference curves including those for hungary show more or less similar temporal behaviour to the corresponding cals7k 2 model curves also available from the geomagia50 database but significant deviations from the low order cals7k 2 predictions are also discernible owing to the likely presence of additional higher order complications in the regional field therefore the regional field and its secular variation are suggested to be approximated by the reference rather than the predicted curves at any other location within the study area the direction of the regional field can be obtained by spatial interpolation from the reference curves as illustrated by isogonic and isoclinic maps shown for selected times local time series of interpolated directions for other central european countries lacking reference curves might serve as master curves for magnetic dating

large geomagnetic field anomalies revealed in bronze to iron age archeomagnetic data from tel megiddo and tel hazor israel abstract geomagnetic field measurements from the past few centuries show heightened secular variation activity in the southern hemisphere associated with the south atlantic anomaly saa it is uncertain whether geomagnetic anomalies at a similar scale have existed in the past owing to limited coverage and uncertainties in the paleomagnetic database here we provide new evidence from archaeological sources in the levant suggesting a large positive northern hemisphere anomaly similar in magnitude to the saa during the 9th 8th centuries bce called levantine iron age anomaly we also report an additional geomagnetic spike in the 8th century the new dataset comprises 73 high precision paleointensity estimates from ca 3000 bce to 732 bce and five directional measurements between the 14th and the 9th centuries bce well dated pottery and cooking ovens were collected from twenty archaeological strata in two large contemporaneous stratigraphical mounds tells in israel tel megiddo and tel hazor the new data are combined with previously published data and interpreted automatically using the pmagpy thellier gui program the tel megiddo and tel hazor data sets demonstrate excellent internal consistency and remarkable agreement with published data from mesopotamia syria the data illustrate the evolution of an extreme geomagnetic high that culminated in at least two spikes between the 11th and the 8th centuries bce iron age in the levant the paleomagnetic directional data of the 9th century bce show positive inclination anomalies and deviations of up to 22 from the averaged geocentric axial dipole gad direction from comparison of the levantine archaeomagnetic data with igrf model for 2015 we infer the levantine iron age anomaly between the 10th and the 8th centuries bce is a local positive anomaly the eastward extent of the anomaly is currently unknown

geomagnetic field for 0 3 ka 1 new data sets for global modeling 1 paleomagnetic and archeomagnetic records are used in both regional and global studies of earth s magnetic field we present a description and assessment of five newly compiled data sets also used in the companion paper by korte et al 2009 to produce a series of time varying spherical harmonic models of the geomagnetic field for the last 3000 years data are drawn from our compilation of lake sediment records and from the online database geomagia50v2 the five selections are available from the earthref digital archive at http earthref org cgi bin erda cgi n 944 data are grouped according to the source of material and we conducted separate assessments of reliability for archeomagnetic artifacts and lava flows the arch3k dat data set and for sediments sed3k dat the overall number of data is 55 greater than in previous compilations constrained data sets were selected using different criteria for each group winnowing of archeological data was based on uncertainties supplied by the original data providers the lake sediment data assessment relied on preassigned age uncertainties and one or more of the following comparisons with archeomagnetic data from the same region regional consistency among several lakes and consistency with global archeomagnetic models we discuss relative merits of a larger unconstrained data set or a smaller possibly more reliable one the constrained data sets eliminate a priori up to 35 of the available data in each case and rely on potentially subjective assessments of data quality given the limited data available our analyses indicate that iterative rejection of a small number 1 1 5 of outlying data during global field modeling is a preferable approach specific regional comparisons among the models and data support the conclusion that korte et al s outlier free cals3k 3 model based on all available measurements from sediments and archeological artifacts currently provides the best global representation of the 0 3 ka field the arch3k 1 model provides a better fit to the denser european archeomagnetic data and may be better in that region

on the anisotropy of secular variation deduced from paleomagnetic volcanic data we seek to establish whether or not secular variation the rate at which the magnetic field is changing in time is a function of field direction is secular variation anisotropic from a large number of paleomagnetic lava data many of which record polarity transitions we find that vectors from stratigraphically adjacent flows are most least correlated during nontransitional transitional periods since volcanic activity is unrelated to and therefore uncorrelated with magnetic secular variation this relationship indicates that secular variation is enhanced during transitions perhaps by a factor of 2 thus secular variation is anisotropic being a function of the deviation of field direction from an axial dipole despite this spatial dependence we find little convincing evidence that transitional fields exhibit persistent recurring quasi stationary states which manifest themselves in terms of midlatitude virtual geomagnetic poles vgps as has sometimes been hypothesized on the other hand we identify tentatively some relative quiescence in secular variation during midtransition when vgps fall near the equator numerous subsets of the database are examined for consistency and statistical errors are estimated by bootstrap analyses statistical models of secular variation need to incorporate the information content in serial correlations like those analyzed here from stratigraphie sections if the lava data are to be fully exploited

variations in the geomagnetic dipole moment over the last 12 000 years summary an analysis has been made of archaeointensity data for the past 12 000 years there are 3243 results from diverent areas of the world covering the past 12 000 years of these 2203 are from the european region and 1040 are from the rest of the world the archaeointensity data set analysed in the present study is almost three times larger than that used by mcelhinny senanayake 1982 although there is no major diverence between our global data and the earlier data the data for the non european region have been improved and we now have a data set for asia

solar activity reconstructed over the last 7000 years the influence of geomagnetic field changes 1 the long term solar activity as manifested by sunspot number has been recently reconstructed on multi millennium time scales by s k solanki et al 2004 from the measured concentration of 14 c in tree rings the exact level of the reconstructed solar activity depends however on independently evaluated data of the geomagnetic dipole strength variations recently a new series of the palaeomagnetic dipole moment reconstruction for the last 7000 years has been presented by m korte and c g constable 2005a on the basis of a thorough analysis of global samples the new palaeomagnetic series yields a systematically lower dipole moment in the past compared to the earlier geomagnetic reconstructions we have revised the earlier sunspot activity reconstruction since 5000 bc using the new geomagnetic data series and found that it is roughly consistent with the previous results during most of the period although the revised sunspot number values are in general higher nonetheless it is confirmed with the new palaeomagnetic series that the sun spends only 2 3 of the time in a state of high activity similar to the modem episode this strengthens the conclusion that the modem high activity level is very unusual during the last 7000 years

paleointensity estimates from historic and modern hawaiian lava flows using glassy basalt as a primary source material abstract published paleointensity estimates derived from lavas extruded in known fields are highly variable and rarely recover the expected field strength within an accuracy of better than 10 inconsistent results on modern volcanic rocks lend even greater uncertainty to intensity experiments performed on lava flows emplaced during periods of unknown geomagnetic field strength the majority of published paleointensity data are collected from the slowly cooled massive centers of lava flows where the magnetic grains are more likely to be multi domain and produce non ideal experimental results glassy volcanic material found on subaerial lava flow tops and in sub aqueous and subglacial environments however is rapidly cooled and therefore most likely of all volcanic materials to behave as single domain particles demanded by neel theory we present a new paleointensity study of historic and modern hawaiian lavas and test the viability of subaerially emplaced glassy basaltic material as an accurate recorder of magnetic field intensity six of eight lava flows sampled on the big island of hawaii 1843 1859 1935 1950 1960 1990 c e produce well behaved arai plots and recover an average intensity to within 2 7 t of the expected field strength or better than 8 accuracy we apply very strict selection criteria including a minimum of three specimens per site to prevent extraneous field estimates from affecting the final results individual volcanic glass results from the 1960 c e lava flow have a much lower variance than published data from the same volcanic unit glassy materials should therefore be collected wherever possible as they allow recovery of geomagnetic field strength with unprecedented accuracy

mesozoic stratigraphic constraints on laramide right slip on the east side of the colorado plateau regional eastward trending piercing lines defined by stratigraphic truncations and depositional pinchouts in jurassic and cretaceous strata in north central new mexico allow 5 to 20 km of right slip between the eastern margin of the colorado plateau and the craton to the east in laramide late cretaceous paleogene and younger time estimates of 60 170 km of laramide right slip by previous workers were based largely on 1 the amount of presumed crustal shortening across thrust and reverse faults in the wyoming province north of the plateau or 2 offset of piercing lines defined by a proterozoic regionally subhorizontal isobaric metamorphic surface that intersects preexisting structures and steep stratigraphic contacts major right slip inferred on the basis of these metamorphically defined piercing lines in proterozoic rocks must therefore predate the laramide orogeny and cannot be used to support a model postulating large scale laramide right slip restoration of 60 170 km of right slip creates lateral offset of the north south extent of the jurassic todilto formation producing an untenable paleodepositional basin geometry

geomagnetic intensity variations for the past 8 kyr new archaeointensity results from eastern china abstract in this study we have carried out paleointensity experiments on 918 specimens spanning the last 7 kyr including pottery fragments baked clay and slag collected from shandong liaoning zhejiang and hebei provinces in china approximately half of the specimens yielded results that passed strict data selection criteria and give high fidelity paleointensities the virtual axial dipole moments vadms of our sites range from 2 10 22 to 13 10 22 am 2 at 2250 bce our results suggest a paleointensity low of 2 10 22 am 2 which increases to a high of 13 10 22 am 2 by 1300 bce this rapid less than 1000 yrs six fold change in the paleointensity may have important implications for the dynamics of core flow at this time our data from the last 3 kyr are generally in good agreement with the arch3k 1 model but deviate significantly at certain time periods from the cals3k 4 and cals10k 1b model which is likely due to differences in the data used to constrain these models at ages older than 3 ka where only the cals10k 1b model is available for comparison our data deviate significantly from the model combining our new results with the published data from china and japan we provide greatly improved constraints for the regional model of eastern asia when comparing the variations of geomagnetic field in three global representative areas of eastern asia the middle east and southern europe a common general trend of sinusoidal variations since 8 ka is shown likely dominated by the dipole component however significant disparities are revealed as well which we attribute to non dipolar components caused by movement of magnetic flux patches at the core mantle boundary

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

application of the multispecimen palaeointensity method to pleistocene lava flows from the trans mexican volcanic belt abstract despite the many advances recently made in palaeointensity pi studies more high quality pi data are required for a better understanding of the full vector evolution of the ancient geomagnetic field in this study we applied the recently proposed multispecimen parallel differential ptrm ms method for absolute pi determination to rocks older than those studied hitherto with this method fifty one lava flows from within the trans mexican volcanic belt which are generally younger than 1 ma returned 32 new pi estimates rock magnetic investigations revealed that in most cases the remanence was carried by ti poor titanomagnetites of pseudo single domain magnetic structure although titanomagnetites with higher ti contents and additional contributions of titano haematite were also occasionally present comparisons with reconstructions of the global dipole moment reveal that the pi estimates from this study were on average around 30 higher than expected other mainly thellier type pi data from mexico were also observed to be high relative to global records which could indicate that persistent non dipole features might be responsible for the higher than expected results however the paucity of available data obscures the significance of this observation and the balance of evidence rather suggests an artificial biasing of most measurements towards high values our results seem to corroborate results from previous studies on historical lava flows and synthetic samples in which domain state effects were found to cause overestimates of the pi by up to 30 in the ms method we expect that the degree of the overestimate in the majority of these new ms results is no larger than what might be expected from thellier experiments performed on samples with a similar given degree of multidomain behaviour however seven of the lava flows which were studied here using the ms method were subjected to a preceding demagnetisation step of 200 c in order to erase viscous remanences and these may have produced results which were further biased towards higher intensities this additional bias is expected from theoretical considerations and may represent a challenge to future application of the ms method to older rock units it could however potentially be avoided in future studies by performing measurements of the sample moment at temperatures higher than the blocking temperature of any overprint

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

paleomagnetic secular variation and relative paleointensity during the holocene in south china huguangyan maar lake revisited the scarcity of reliable paleomagnetic secular variation psv records from east asia especially from low latitude regions impedes better understanding of global psv mechanisms here we report on a radiocarbon dated holocene psv record from a composite 6 7 m long core collected from the high sedimentation rate huguangyan maar lake hml in subtropical tropical south china rock magnetic results demonstrate that the natural remanent magnetization resides in single domain magnetite alternating field demagnetization experiments at 1 cm spacing on u channel samples reveal six distinct inclination highs at 7 500 bce 5 100 bce 4 600 bce 3 600 3 400 bce 1 600 1 200 bce and 600 800 ce three inclination lows at 4 800 bce 600 300 bce and 1 000 1 300 ce and three eastward declination trends at 3 600 3 200 bce 2 600 2 400 bce and 400 bce to 200 ce the similarity between the hml psv record and other independently dated records from east asia and geomagnetic field models corroborates the robustness of our age model and holocene psv record strikingly centennial to millennial scale psv features of the hml are comparable within age uncertainties with other holocene records from europe north america and canada suggesting that such directional patterns are likely to be hemispheric in scale although relative paleointensity data of hml are affected by environmental factors e g organic matter diagenesis the record still provides a regionally important new psv reference curve whose conspicuous features may serve as stratigraphic markers for east asian paleorecords

implications of jurassic cretaceous and proterozoic piercing lines for laramide oblique slip faulting in new mexico and rotation of the colorado plateau regional isopach patterns and pinch out data for nine stratigraphic units of jurassic and cretaceous age are either permissive or supportive of significant dextral slip of laramide age along the eastern colorado plateau boundary in new mexico the best constrained dextral offset estimates are for the sand hill nacimiento fault system 20 33 km and the previously published 13 km estimate for the defiance monocline which together yield a cumulative offset of 33 46 km mesozoic stratigraphic constraints for other laramide fault systems are less precise and typically provide only maximum limits for possible dextral offsets because of widely spaced control points and broad areas of tertiary erosion these less precise constraints allow as much as 40 60 km of laramide dextral slip along what is now the rio grande rift and as much as 110 km across the entire breadth of the laramide deformed zone in central and northern new mexico well documented dextral offsets of proterozoic lithologies and structures across the tusas picuris fault system 15 km and picuris pecos fault 37 km probably represent minimum laramide displacements because of the need to account for sinistral components related to other deformations these displacements when combined with dextral offsets along sand hill nacimiento and defiance structures yield a minimum dextral offset of 85 km for laramide structures in northern new mexico this minimum dextral offset is approximately equivalent to the amount of laramide crustal shortening on and northward of the colorado plateau a result that argues against nearby euler pole locations for laramide rotation of the colorado plateau relative to cratonic north america geological constraints allow 0 to 3 of laramide clockwise rotation of the colorado plateau additional clockwise plateau rotation during late cenozoic development of the rio grande rift was 1 to 1 5 geological constraints thus indicate that clockwise rotation of the colorado plateau from combined laramide and rio grande rift deformations was between about 1 and 4 5

cenozoic sedimentation and paleotectonics of north central new mexico implications for initiation and evolution of the rio grande rift the rio grande rift is one of the major late cenozoic continental rifts of the world sharing most geophysical geochemical and geological characteristics with other rifts cenozoic evolution of the rift was synchronous with lithospheric plate interactions along and under the western north american margin paleocene eocene laramide primarily amagmatic compression related to flat slab subduction oligocene intermediate to silicic volcanism related to collapse of the slab miocene to present rifting related to complex plate interactions overprinted on the previous history cenozoic paleogeographic and paleotectonic characteristics are consistent with a passive mantle mode of rifting north central new mexico provides a unique opportunity to constrain models for rift initiation and evolution it is one of the few locations within any rift where excellently exposed pre rift and syn rift basin fill has been studied thoroughly enough to allow detailed paleogeographic reconstruction for almost the entire cenozoic cenozoic paleogeography for the study area is summarized as follows 1 eocene 58 37 ma a single amagmatic sedimentary basin el rito galisteo trended northwest southeast with laramide basement uplifts on three sides 2 early to late oligocene 37 28 ma intermediate magmatism with volcaniclastic aprons derived from the san juan and ortiz cerrillos volcanic fields and residual laramide uplifts 3 late oligocene early miocene 28 21 ma initiation of bimodal volcanism with widely dispersed volcaniclastic aprons derived from the predominantly silicic san juan and latir volcanic fields 4 early to middle miocene 21 15 ma continued volcaniclastic dispersal from silicic volcanic centers concurrent with initiation of block faulting to form half grabens internal drainage and erosion of phanerozoic strata and precambrian basement 5 middle to late miocene 15 8 ma continued deepening of half grabens widespread exposure of precambrian terranes formation of complex depositional environments in basin centers and continued bimodal volcanism 6 late miocene to present 8 0 ma concentration of extension in central grabens linked by accommodation zones major bimodal volcanism for example taos plateau and jemez mountains regional uplift and integration of rio grande drainage dispersal patterns petrofacies analysis k ar ages and chemical analyses of volcanic clasts provide details concerning three primary volcanic centers 1 san juan mountains 27 29 m y high k andesite and rhyodacite 2 latir volcanic field 25 28 m y high k andesite and rhyolite and 3 the previously unrecognized servilleta plaza center 22 23 m y latite high k andesite and rhyodacite which may have been a southern extension of the latir field these three volcanic centers provided detritus to the following units respectively 1 esquibel mbr of los pinos fm upper abiquiu fm middle picuris fm bishops lodge mbr of tesuque fm and volcaniclastic units in the northern albuquerque basin zia and abiquiu 2 cordito mbr of los pinos fm and uppermost abiquiu fm and 3 chama el rito mbr of tesuque fm and upper picuris fm use of petrofacies 1 esquibel 2 cordito and 3 plaza simplifies the chaos of stratigraphic nomenclature and promotes regional correlations of poorly exposed units

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

the 10th generation international geomagnetic reference field abstract the international association of geomagnetism and aeronomy iaga on 12 december 2004 released the 10th generation international geomagnetic reference field igrf the latest version of a standard mathematical description of the earth s main magnetic field and used widely in studies of the earth s deep interior its crust ionosphere and magnetosphere the coefficients were finalised by a task force of iaga the igrf is the product of a large collaborative effort between magnetic field modellers and the institutes involved in collecting and disseminating magnetic field data from satellites and observatories around the world

archaeointensity determinations from italy new data and the earth s magnetic field strength variation over the past three millennia summary three kilns and a collection of baked bricks from italian archaeological sites have been studied for archaeointensity determination using the thellier method as modified by coe all sites are dated based on archaeological information and their ages range from 500 to 800 ad and 1500 to 1700 ad rock magnetic studies identify magnetite ti magnetite and hematite as the main magnetic minerals and magnetic susceptibility versus temperature shows a good thermal stability of the samples the intensity results have been corrected for anisotropy of the thermoremanent magnetization and cooling rate effects the new data together with 136 previously published results are used to estimate the variation of the earth s magnetic field over the past three millennia the time distribution of the italian absolute intensity data is irregular with the majority of determinations concentrated during the last four centuries while older periods are very poorly covered most of the data come from volcanic rocks and show significant discrepancies all data have been compared with archaeointensity results from europe and regional and global models predictions this work indicates the need to obtain more high quality archaeointensity results in particular for the periods older than 200 bc and between 200 and 1000 ad in order to determine a robust italian intensity secular variation curve that in combination with directional data could be used for archaeomagnetic dating

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

rocky mountain foreland uplifts products of a rotating stress field or strain partitioning stress tensor analysis of minor faults near the structural range fronts of diversely oriented laramide foreland uplifts in the central rocky mountains wyoming indicates that paleo 1 was horizontal and oriented nearly perpendicular to uplift trends trend directions for 1 vary from east northeast in the north trending laramie range to northeast in the northwest trending bighorn mountains to north at east trending casper mountain and in the east trending central owl creek mountains although these compression directions are compatible with models invoking temporally changing stress directions during the laramide orogeny the apparent similarity in timing of uplift in these ranges suggests that the computed stress tensors may not reflect regional stress conditions recent stress determinations in transpressional regimes suggest that strain partitioning may be an important factor in considering local stress patterns in the vicinity of mountain belt structures oblique to regional trends in light of these studies caution should be used in interpreting regional paleostress from structures measured close to the tectonic fronts of laramide uplifts

archeoint an upgraded compilation of geomagnetic field intensity data for the past ten millennia and its application to the recovery of the past dipole moment 1 this paper presents a compilation of intensity data covering the past 10 millennia archeoint this compilation which upgrades the one of korte et al 2005 contains 3648 data and incorporates additional intensity and directional data sets a large majority of these data 87 were acquired on archeological artifacts and the remaining 13 correspond to data obtained from volcanic products the present compilation also includes important metadata for evaluating the intensity data quality and providing a foundation to guide improved selection criteria we show that 50 of the data set fulfill reasonable reliability standards which take into account the anisotropic nature of most studied objects potsherds the stability of the magnetization and the data dispersion the temporal and geographical distributions of this sub data set are similar to those of the main data set with 72 of the data dated from the past three millennia and 76 obtained from western eurasia approximately half of the selected intensity data are associated with at least an inclination value to constrain the axial and full dipole evolution over the past three millennia requires that we avoid any overrepresentation of the western eurasian data we introduce a first order regional weighting scheme based on the definition of eight widely distributed regions of 30 width within which the selected data are numerous enough the regional curves of virtual axial dipole moments vadm and of mixed vadm virtual dipole moments vdm averaged over sliding windows of 200 years and 500 years testify for strong contributions from either equatorial dipole or nondipole components the computation of global vadm and mixed vadm vdm variation curves assuming an equal weight for each region yields a dipole evolution marked by a distinct minimum around 0 b c a d followed by a maximum around the third fourth century a d a second minimum is present around the eighth century a d this variation pattern is compatible with the one deduced from earlier more

persistent high paleosecular variation activity in southern hemisphere for at least 10 000 years abstract direct observations of the geomagnetic field show that secular variation is strong in the atlantic hemisphere and comparatively reduced in the pacific region the dipole has been decaying since at least 1840 ad driven by growth and migration of reverse flux patches in the southern hemisphere we investigate whether anything like this modern pattern of geomagnetic secular variation persists and can be detected in global paleomagnetic field models synthesis of results from two new time varying spherical harmonic models shows that geographically distinct geomagnetic secular variation extends to at least 10 000 bp the models use the same database but differ in methodology leading to some regional differences in results consistent large scale surface features include strong average fields in the northern hemisphere and weaker fields with greater overall variability in the south longitudinal structure is present with weaker average fields in the western pacific than in the east and prominent negative inclination anomalies extending beneath indonesia across africa and to brazil but weaker anomalies in the central pacific marginally positive inclination anomalies occur west of the americas paleosecular variation activity peaks at high southern latitudes and there is a pattern of reduced activity at equatorial and mid latitudes beneath the pacific although the dipole has exhibited both growth and decay over the interval 0 10 000 bp our results show that geomagnetic paleosecular variation is preferentially focused in similar geographic regions to secular variation seen in the modern field

paleomagnetic rock magnetic and geochemical study of the 1755 tsunami deposit at boca do rio algarve portugal abstract a multiproxy study combining paleomagnetic rock magnetic magnetic susceptibility anhysteretic remanent magnetization arm isothermal remanent magnetization irm anisotropy of magnetic susceptibility ams and geochemical xrf data was carried out on trench sediments collected in the area of boca do rio algarve south portugal known to have been stricken by a tsunami wave related to the 1755 lisbon earthquake the magnetic mineral assemblage is dominated by multidomain ferrimagnetic grains with a significant contribution 5 9 of ultra fine magnetic particles related to the presence of pedogenetic magnetite the variable amount of this post depositional magnetite plays a role on the characteristic remanent magnetization chrm of the sediment the primary magnetization recorded at t1 located farthest from the coast is representative of the paleosecular variation of the earth s magnetic field and a comparison of the results with the regional model for the geomagnetic field in europe scha dif 3k was used to constrain the erosional signature of the tsunami layer the result indicates that less than eighty years could be missing in the sedimentary column the eroded sediment thickness 27 40 cm was incorporated in the tsunami layer that is characterized by a low magnetic content and a large increase in the ca content representing allochthonous material sedimentological geochemical magnetic and ams data show that the signature of the tsunami layer reflects at least two waves with an erosive capacity that is controlled by the distance from the shore

holocene break up and reestablishment of the petermann ice tongue northwest greenland abstract over the last decade two major calving events of the petermann ice tongue in northwest greenland have led to speculation on its future stability and contribution to further greenland ice sheet mass loss however it has been unclear if these events are anomalous or typical within the context of limited historical observations we extend the historical record of the floating ice tongue using the stratigraphy of petermann fjord sediments to provide a longer term perspective computed tomography ct scans x ray fluorescence xrf scans ice rafted debris ird counts and the magnetic properties of specific particle size fractions constrain changes in depositional processes and sediment sources at our core sites allowing for reconstructions of past behavior of the petermann ice tongue radiocarbon dating of foraminifera 210pb and paleomagnetic secular variation psv provide age control and help to address uncertainties in radiocarbon reservoir ages a floating ice tongue in petermann fjord formed in late glacial time as petermann glacier retreated from an advanced grounded position this paleo ice tongue broke up during the early holocene when high northern latitude summer insolation was higher than present after gradual regrowth of the ice tongue associated with regional cooling the ice tongue reached its historical extent only within the last millennium little or no ice tongue was present for nearly 5000 years during the middle holocene when decadal mean regional temperatures are estimated to be 0 8 2 9 c higher than preindustrial 1750 ce and seasonal sea ice in the lincoln sea was reduced this pre historical behavior shows that recent anthropogenic warming may already be in the range of ice tongue instability and future projected warming increases the risk of ice tongue break up by the mid 21st century

scaling time integrated in situ cosmogenic nuclide production rates using a continuous geomagnetic model abstract spatial and temporal variations in earth s magnetic field affect the corresponding time integrated distribution of in situ cosmogenic nuclide cn production rates these effects can be quantified by the effective vertical cutoff rigidity r c a measure of the energy required for primary cosmic rays to penetrate the geomagnetic field and interact with the atmosphere at a given location recent cn production rate scaling models are based on atmospheric cosmic ray measurements parameterized using r c estimates derived from detailed modern geomagnetic field representations which include both dipole and non dipole field contributions however published methods for quantifying time integrated geomagnetic effects on cn production rate scaling rely on various geocentric dipolar approximations to r c driven by separate records of paleomagnetic pole position and paleointensity therefore applying dipolar paleomagnetic records spanning millennial time scales which explicitly ignore past non dipole effects to scaling models derived using modern geomagnetic field representations may lead to systematic errors in any calculated results a recently published continuous geomagnetic model covering the last 7 kyr cals7k 2 m korte and c g constable continuous geomagnetic field models for the past 7 millennia 2 cals7k 2005 geochem geophys geosyst 6 q02h16 doi 10 1029 2004gc000801 may allow reduction of such errors by bridging the gap between detailed modern geomagnetic and simplified paleomagnetic models we have developed a new model framework describing temporal and spatial variation in r c for 0 7 ka and earlier based on cals7k 2 which explicitly accounts for non dipole field effects while attempting to mitigate systematic scaling biases scaling factors derived using the new r c framework predict significant longitudinal variability in time integrated cn production while predictions using dipolar geomagnetic approximations do not one can test these predictions using in situ cosmogenic 14 c in situ 14 c in quartz due to its short half life 5 73 kyr 14 c attains secular equilibrium between production and decay after approximately 25 kyr of exposure at which point its measured concentration is only a function of its integrated average production rate initial in situ 14 c results from samples at secular equilibrium from 38 n and 3 5 km in tibet and eastern california are consistent with the longitudinal variability predicted by the new framework

variations in the geomagnetic dipole moment during the holocene and the past 50 kyr all absolute paleointensity data published in peer reviewed journals were recently compiled in the geomacia50 database based on the information in geomagia50 we reconstruct variations in the geomagnetic dipole moment over the past 50 kyr with a focus on the holocene period a running window approach is used to determine the axial dipole moment that provides the optimal least squares fit to the paleointensity data whereas associated error estimates are constrained using a bootstrap procedure we subsequently compare the reconstruction from this study with previous reconstructions of the geomagnetic dipole moment including those based on cosmogenic radionuclides be 10 and c 14 this comparison generally lends support to the axial dipole moments obtained in this study our reconstruction shows that the evolution of the dipole moment was highly dynamic and the recently observed rates of change 5 per century do not appear unique we observe no apparent link between the occurrence of archeomagnetic jerks and changes in the geomagnetic dipole moment suggesting that archeomagnetic jerks most likely represent drastic changes in the orientation of the geomagnetic dipole axis or periods characterized by large secular variation of the non dipole field this study also shows that the holocene geomagnetic dipole moment was high compared to that of the preceding similar to 40 kyr and that similar to 4 10 22 am 2 appears to represent a critical threshold below which geomagnetic excursions and reversals occur c 2008 elsevier b v all rights reserved less

time dependent mapping of the magnetic field at the core mantle boundary we consider the problem of constructing a time dependent map of the magnetic field at the core mantle boundary we use almost all the available data from the last 300 years to produce two maps one for the period 1690 1840 and the other for 1840 1990 we represent the spatial dependency of the field using spherical harmonics the time dependency using a cubic b spline basis and seek the smoothest solutions compatible with the observations particular attention must be paid to the effects of the crustal field in the data we argue that for observations from permanent magnetic observatories the most efficient strategy is to use first differences of annual means for satellite data the most efficient strategy is simply to limit the number of data used so as to minimize any tendency to map the crustal field into the core field the resulting model fits the observatory data better than any previous model yet has less power in the secular variation than previous models demonstrating that very simple models fit the data the resulting time dependent field map exhibits much of the same structure in the field and its secular variation identified in earlier studies

new archaeomagnetic data recovered from the study of roman and visigothic remains from central spain 3rd 7th centuries summary new archaeomagnetic results from four heated combustion structures recovered from two archaeological sites in central spain are reported they have been dated by archaeological evidence and in two cases by radiocarbon dating rock magnetic experiments indicate low coercivity magnetic phases such as magnetite and thermally stable maghaemite as the main carriers of the remanent magnetization haematite has been observed in poorly heated baked clays archaeomagnetic directions have been obtained from either alternating field or thermal demagnetization experiments performed on 57 specimens coming from 46 independently oriented samples the four well defined archaeomagnetic directions obtained are in good agreement with previous archaeomagnetic data and with recent regional and global field models they define the beginning of easterly declination drift that was initiated around 350 400 ad and culminated around 800 850 ad and delineate the maximum in inclination that took place around 600 650 ad in addition classical thellier thellier experiments including thermal remanent magnetization anisotropy and cooling rate corrections were conducted on 23 specimens only 13 specimens corresponding to well defined single component behaviour gave reliable results new mean archaeointensities have been obtained for two of the four studied structures vbk1 64 2 5 0 t and vbt1 62 4 2 6 t the new data suggest that two relative intensity maxima occurred in western europe around 320 and 630 ad being of lower magnitude that observed in eastern europe

palaeoenvironmental change and glacier fluctuations in the high tian shan mountains during the last millennium based on sediments from lake ala kol kyrgyzstan abstract a suite of limnological proxies measured in clastic sediments of the glacier fed lake ala kol ne kyrgyzstan document palaeoenvironmental change and glacier fluctuations in the central tian shan over the last millennium rock magnetic and geochemical data provide supporting evidence for three glacier expansion episodes occurring in 1200 1400 ce 1500 1850 ce and 1900 1950 ce and glacier retreat periods during 1000 1200 ce and the intervening periods the maximal glacier extent as inferred from high lf si inc coh ca inc coh and low arm30 arm is recorded between 1500 and 1850 ce and corresponds to cold and wet conditions during the little ice age in the high central tian shan periods of glacier waxing and waning recorded at lake ala kol present a close correspondence to ice accumulation rate in the northern tibetan plateau suggesting that precipitation changes are the main driver of glacier fluctuations in central tian shan during the last millennium although temperature may have exerted a more complex influence on centennial timescales

formation of the rocky mountains western united states a continuum computer model one hypothesis for the information of the rocky mountain structures in late cretaceous through eocene time is that plate of oceanic lithosphere was underthrust horizontally along the base of the north american lithosphere the horizontal components of the motion of this plate are known from paleomagnetism and the edge of the region of flat slab can estimated from reconstructed patterns of volcanism new techniques of finite element modeling allow prediction of the thermal and mechanical effects of horizontal subduction on the north american plate a model that has a realistic temperature dependent rheology and a simple plane layered initial condition is used to compute the consequences of horizontal underthrusting in the time interval 75 million to 30 million years before present successful prediction of this model include i the location amount and direction of horizontal shortening that has been inferred from laramide structures ii massive transport of lower crust from southwest to northeast iii the location and timing of the subsequent extension in metamorphic core complexes and the rio grande rift and iv the total area eventually involved in basin and range style extension in a broad sense this model has predicted the belt of laramide structures the transport of crust from the coastal region to the continental interior the subsequent extension in metamorphic core complexes and the rio grande rift and the geographic region of late tertiary basin and range extension its principal defects are that i many events are predicted about 5 million to 10 million years too late and ii the wave of crustal thickening does not travel far enough to the east reasonable modifications to the oceanic plate kinematics and rheologies that were assumed may correct these defects the correspondence of model predictions to actual geology is already sufficiently close to show that the hypothesis that horizontal subduction caused the laramide orogeny is probably correct the rocky mountain thrust and reverse faults formed in an environment of east west to northeast southwest compressive stress that was caused by the viscous coupling between the oceanic plate and the base of the north american crust nonuniform crustal thickening by simple shear transport also caused relative uplifts therefore this model is consistent with both of the range forming mechanisms that have been inferred 1 a new proposal that arises from this simulation is that horizontal subduction also caused the subsequent extensional basin and range taphrogeny by stripping away the mantle lithosphere so that the crust was exposed to hot asthenosphere after the oceanic slab dropped away

archeomagnetic secular variation from korea implication for the occurrence of global archeomagnetic jerks abstract an archeomagnetic paleosecular variation psv was first defined in korea using baked materials collected from 26 kilns or hearths with ages ranging from 1100 bc to ad 1790 variations of geomagnetic declination and inclination from the korean peninsula are distinctively different from the prediction of a global model cals3k 3 or cals7k 2 for the past 3500 yr in particular a distinctive offset in magnitude and phase is noticeable between the observations and predictions at 745 bc ad 300 and ad 1400 1700 a bi plot of magnetic declination versus inclination displays three cusps at the corresponding time intervals these time intervals are nearly identical to or at least overlap with three of the four potential archeomagnetic jerks suggested by gallet et al 2003 from the european archeomagnetic data a comparison of the psv curves for neighboring countries regions revealed that european archeomagnetic jerks at 800 bc ad 200 ad 800 and ad 1400 were all preserved in east asia suggesting that the archeomagnetic jerks were global or at least northern hemispheric features

fall in earth s magnetic field is erratic earth s magnetic field has decayed by about 5 per century since measurements began in 1840 directional measurements predate those of intensity by more than 250 years and we combined the global model of directions with paleomagnetic intensity measurements to estimate the fall in strength for this earlier period 1590 to 1840 a d we found that magnetic field strength was nearly constant throughout this time in contrast to the later period extrapolating to the core surface showed that the fall in strength originated in patches of reverse magnetic flux in the southern hemisphere these patches were detectable by directional data alone the pre 1840 model showed little or no evidence of them supporting the conclusion of a steady dipole up to 1840

geomagnetic field intensity how high can it get how fast can it change constraints from iron age copper slag article the intensity of the geomagnetic field varies over different time scales yet constraints on the maximum intensity of the field as well as for its maximum rate of change are inadequate due to poor temporal resolution and large uncertainties in the geomagnetic record the purpose of this study is to place firm limits on these fundamental properties by constructing a high resolution archaeointensity record of the levant from the 11th century to the early 9th century bce a period over which the geomagnetic field reached its maximum intensity in eurasia over the past 50 000 years we investigate a 14 c dated sequence of ten layers of slag material which accumulated within an ancient industrial waste mound of an iron age copper smelting site in southern israel depositional stratigraphy constrains relative ages of samples analyzed for paleointensity and 14 c dates from different horizons of the mound constrain the age of the whole sequence the analysis yielded 35 paleointenisty data points with accuracy better than 94 and precision better than 6 covering a period of less than 350 years most probably 200 years we construct a new high resolution quasi continuous archaeointensity curve of the levant that displays two dramatic spikes in geomagnetic intensity each corresponding to virtual axial dipole moment vadm in excess of 200 zam 2