secular variation of the geomagnetic field intensity during the past 2000 years in japan abstract we report the results of paleointensity determination with volcanic rocks of izu oshima and fuji volcano japan samples used for the present study were basaltic rocks from nine lava flow units of oshima volcano which erupted between ad 590 and 1950 and aokigahara lava flow derived from fuji volcano in ad 864 demagnetization and several rock magnetic experiments show that the samples have stable single component remanences which are carried by magnetite or titanomagnetite with the psd magnetic domain state thellier s method was used for the paleointensity experiments and reliable determinations for 10 eruptive units were obtained from 58 samples in total the unit mean values generally show good consistency with the previous archaeointensity and paleointensity results allowing us to construct a reference curve of the field intensity variation for the past 2000 yr in japan the reference curve has a general trend of decreasing intensity since ad 500 showing two peaks of the field strength around ad 500 and 1400 and two troughs around ad 1200 and 1700 since around ad 800 the field intensity predictions for japan by the inversion spherical harmonic models show good consistency with the reference curve suggesting that the peak and the troughs in this period have probably been produced mainly by non dipole sources the geocentric axial dipole variation extracted from the reference curve with the inversion model shows good agreement with the global averages of the virtual axial dipole moment except that our reference curve yields a significant peak around ad 500 this suggests that a temporal strengthening of the dipole moment might have occurred over a few hundred years around ad 500

intensity of the geomagnetic field in western europe over the past 2000 years new data from ancient french pottery 1 we studied 14 groups of french pottery fragments dated between the 4th and 16th centuries the potsherds were analyzed using the thellier and thellier 1959 method revised by coe 1967 intensity values were corrected for thermoremanent magnetization trm anisotropy and cooling rate dependence of trm acquisition we first analyzed modern ceramics produced following ancient techniques and fired in a wood burning kiln inside of which field intensity was measured the recovered mean intensity is within 3 of the expected value which proves the reliability of our experimental procedure thermal experiments carried out at rapid and slow cooling rates clearly indicate that the cooling rate correction is critical in archeointensity studies our data indicate that large variations in intensity occurred in france over the last 2000 years two relative maxima in intensity are observed one between the 8th and 10th centuries and the second between the 14th and 15th centuries similarities are observed between the archeointensity data from france and ukraine yielding some evidence for eastward drift of geomagnetic sources between western and eastern europe from a d 800 to a d 1700 we also show that the dipole moment evolution proposed by mcelhinny and senanayake 1982 and yang et al 2000 for the last two millennia is likely biased toward higher values mainly because of the absence of correction for the cooling rate dependence of trm acquisition in most published archeointensity studies we finally underline a possible relationship valid at least in western europe between changes in direction and intensity of the geomagnetic field

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

geomagnetic intensity spike recorded in high resolution slag deposit in southern jordan article i nfo in paleomagnetism periods of high field intensity have been largely ignored in favor of the more spectacular directional changes associated with low field intensity periods of excursions and reversals hence questions such as how strong the field can get and how fast changes occur are still open in this paper we report on data obtained from an archaeometallurgical excavation in the middle east designed specifically for archaeomagnetic sampling we measured 342 specimens from 72 samples collected from a 6 1 m mound of well stratified copper production debris at the early iron age 12th 9th centuries bce site of khirbat en nahas in southern jordan seventeen samples spanning 200 yr yielded excellent archaeointensity results that demonstrate rapid changes in field intensity in a period of overall high field values the results display a remarkable spike in field strength with sample mean values of over 120 t compared to the current field strength of 44 t a suite of 13 radiocarbon dates intimately associated with our samples tight control of sample location and relative stratigraphy provide tight constraints on the rate and magnitude of changes in archaeomagnetic field intensities

cyclic rapid warming on centennial scale revealed by a 2650 year stalagmite record of warm season temperature 1 a 2650 year bc665 ad1985 warm season mjja may june july august temperature reconstruction is derived from a correlation between thickness variations in annual layers of a stalagmite from shihua cave beijing china and instrumental meteorological records observations of soil co2 and drip water suggest that the temperature signal is amplified by the soil organism co2 system and recorded by the annual layer series our reconstruction reveals that centennial scale rapid warming occurred repeatedly following multicentenial cooling trends during the last millennia these results correlate with different records from the northern hemisphere indicating that the periodic alternation between cool and warm periods on a sub millennial scale had a sub hemispherical influence

on the persistence of geomagnetic flux lobes in global holocene field models abstract to investigate the longevity and robustness of high latitude flux patches in the geomagnetic field at the core mantle boundary we present time dependent models of the geomagnetic field for the past 7000 years our models use the same data set as previously used for time dependent archaeomagnetic field modelling but constrained with additional priori models from time averages of field models covering the last 150 400 and 3000 years we find that the data are consistent with flux patches existing in both north and south hemispheres for the past 7000 years and that the northern hemisphere patches at least have highly dynamic behaviour simple averaging of the historical field may not be appropriate to obtain a characteristic time averaged model of the field for comparison with other geophysical observables our results should inform geodynamo studies of thermal core mantle coupling and of possible long term structure in the geomagnetic field

paleomagnetic and rock magnetic studies of the sakurajima 1914 and 1946 andesitic lavas from japan a comparison of the ltd dht shaw and thellier paleointensity methods abstract paleomagnetic and rock magnetic measurements have been made on samples from the 1914 and 1946 sakurajima andesitic lava flows in order to investigate the suitability of andesite to record both paleodirection and paleointensity the lava flows were split in to three sites each of which consists of two or three sub sites a range of rock magnetic experiments have been made on multiple samples per flow and the samples can be split into two broad rock magnetic categories i and ii paleodirection was determined by stepwise thermal demagnetization and the mean direction determined from both flows is very close to that expected from igrf 10 paleointensity was determined using the ltd dht shaw technique and the thellier method on sister samples of the same cores sixty four out of 72 ltd dht shaw paleointensity experiments were successful and there are no differences between the sub sites seventy out of 71 thellier paleointensity experiments were successful however for sub sites in two out of the three sites there are large differences between the mean paleointensities when all paleointensities are normalized to the expected value from igrf 10 the mean is 0 98 0 11 for the ltd dht shaw method and 1 12 0 13 for the thellier method these means are statistically distinguishable and this is related to the different rock magnetic properties of the samples if only category i samples are considered the ltd dht shaw mean is 1 05 0 07 compared with the thellier mean of 1 08 0 08 in contrast there is a much larger spread in the means determined from category ii samples 0 94 0 11 from the ltd dht shaw experiments and 1 15 0 15 from the thellier experiments this spread could be caused by magnetic interactions between magnetic domains assuming high temperature oxidation of titanomagnetite grains during natural cooling we suggest a scenario that can qualitatively explain the difference between the ltd dht shaw and thellier paleointensity results

intensity of the geomagnetic field near loyang china between 500 bc and ad 1900 there are remarkably few data on the intensity of the geomagnetic field in early china new results reported here indicate that the total intensity of the magnetic field of the earth in the region near loyang lat 34 42 n long 112 40 e has changed considerably during the past 2 400 yr 500 bc ad 1900 the maximum value reached about 1 700 yr ago is as much as 54 higher than the present one

long term variations in palaeointensity we compile a dataset of reliable palaeointensity estimates based both on published work and on new data from basaltic glass the basaltic glass data more than double the number of reliable thellie

changes in remanence coercivity and domain state at low temperature in magnetite abstract submicron magnetite crystals with mean sizes of 0 037 0 10 and 0 22 m undergo major changes in hysteresis properties and domain states in crossing the verwey transition t v 120 k the 0 037 m crystals are single domain sd both in the cubic phase at room temperature t 0 and in the monoclinic phase below t v the 0 10 and 0 22 m crystals have a mixture of sd and two domain 2d states at room temperature t 0 but mainly sd structures below t v in agreement with micromagnetic calculations coercive force h c increases on cooling through t v by a factor 3 5 in the submicron magnetites and 40 in a 1 3 mm single crystal because of the high crystalline anisotropy and magnetostriction of monoclinic magnetite as a result domain walls and sd moments are so effectively pinned below t v that all remanence variations in warming or cooling are reversible however between 100 k and t 0 remanence behavior is variable saturation remanence sirm produced in monoclinic magnetite at 5 k drops by 70 100 in warming across t v with minor recovery in cooling back through t v ultimate levels at 5 k of 23 37 for the submicron crystals and 3 for the 1 3 mm crystal in contrast sirm produced in the cubic phase at 300 k decreases 5 35 submicron or 95 1 3 mm during cooling from 300 to 120 k due to continuous re equilibration of domain walls but there is little further change in cooling through t v itself however the submicron magnetites lose a further 5 15 of their remanence when reheated through t v these irreversible changes in cycling across t v and the amounts of the changes have potential value in determining submicron magnetite grain sizes the irreversibility is mainly caused by 2d sd transformations on cooling through t v which preserve or enhance remanence while sd 2d transformations on warming through t v cause remanence to demagnetize

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

paleointensity of the geomagnetic field recovered on archaeomagnetic sites from france abstract twenty new paleointensity determinations have been obtained from archaeomagnetic sites from france time intervals investigated range from 65 to 380 ad and from 1370 to 1700 ad two hundred forty two samples of tiles and bricks were treated with the classical thellier method for paleointensity determination rock magnetic studies identify magnetite substituted maghemite and hematite as the main magnetic minerals as is usual with archaeomagnetic samples especially tiles magnetic anisotropy was observed in all samples the effect of thermoremanent magnetization trm anisotropy upon the paleointensity values was corrected after the determination of the tensor of trm anisotropy in each sample these determinations allow corrections of the field strength obtained which are usually lower than 20 the effect of the cooling rate on the trm intensity acquired was also determined and seems relatively low on our samples most frequently eight to 10 samples per site were used to calculate the mean intensity standard deviations are typically lower than 7 of the average value during the roman period small variations of the field intensity in paris are observed between 62 and 70 t while during the more recent period the intensity of the field decreases from 62 to 52 t an analysis of the paleointensity data available for the last 2000 years obtained from sites in western europe was performed using a weighing factor which takes into account the number and type of the samples studied as well as the technique used for the paleointensity determination this weighing factor clearly shows that most of the existing data might not be reliable the analysis performed indicates that more data is needed especially from the time interval 500 to 1400 ad before it would be possible to determine a reference curve of the time variations of the archaeomagnetic intensity of the field in western europe

are there connections between the earth s magnetic field and climate understanding climate change is an active topic of research much of the observed increase in global surface temperature over the past 150 years occurred prior to the 1940s and after the 1980s the main causes invoked are solar variability changes in atmospheric greenhouse gas content or sulfur due to natural or anthropogenic action or internal variability of the coupled ocean atmosphere system magnetism has seldom been invoked and evidence for connections between climate and magnetic field variations have received little attention we review evidence for correlations which could suggest such causal or non causal connections at various time scales recent secular variation 10 100 yr historical and archeomagnetic change 100 5000 yr and excursions and reversals 10 3 10 6 yr and attempt to suggest mechanisms evidence for correlations which invoke milankovic forcing in the core either directly or through changes in ice distribution and moments of inertia of the earth is still tenuous correlation between decadal changes in amplitude of geomagnetic variations of external origin solar irradiance and global temperature is stronger it suggests that solar irradiance could have been a major forcing function of climate until the mid 1980s when anomalous warming becomes apparent the most intriguing feature may be the recently proposed archeomagnetic jerks i e fairly abrupt 100 yr long geomagneticfieldvariationsfoundatirregularintervalsoverthepastfewmillennia usingthearcheologicalrecordfromeuropetothe middle east these seem to correlate with significant climatic events in the eastern north atlantic region a proposed mechanism involves variations in the geometry of the geomagnetic field f i tilt ofthe dipole to lower latitudes resulting in enhanced cosmic ray induced nucleation of clouds no forcing factor be it changes in co2 concentration in the atmosphere or changes in cosmic ray flux modulatedbysolaractivityandgeomagnetism orpossiblyotherfactors canatpresentbeneglectedorshowntobetheoverwhelming single driver of climate change in past centuries intensive data acquisition is required to further probe indications that the earth s and sun s magnetic fields may have significant bearing on climate change at certain time scales 2006 elsevier b v all rights reserved

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

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

toward an optimal geomagnetic field intensity determination technique 1 paleointensity determinations based on double heating techniques in field zero field cooling zero field in field cooling and two in field steps with opposite laboratory fields are generally considered to be functionally interchangeable producing equally reliable paleointensity estimates to investigate this premise we have developed a simple mathematical model we find that both the zero field first and in field first methods have a strong angular dependence on the laboratory field parallel orthogonal and anti parallel while the two in field steps method is independent of the direction of the laboratory produced field contrary to common practice each method yields quite different outcomes if the condition of reciprocity of blocking and unblocking temperatures is not met even with marginal 10 tails of partial thermoremanence our calculations suggest that the zero field first method with the laboratory produced field anti parallel to the natural remanence nrm is the most robust paleointensity determination technique when the intensity of the lab induced field is smaller than ancient field however the zero field first method with the laboratory field parallel to the nrm is the optimum approach when the intensity of the lab induced field is larger than the ancient field by far the best approach however is to alternatethe infield zerofield iz steps with zerofield infield zi steps

the effect of remanence anisotropy on paleointensity estimates a case study from the archean stillwater complex paleomagnetism of archean rocks potentially provides information about the early development of the earth and of the geodynamo precambrian layered intrusive rocks are good candidates for paleomagnetic studies such complexes are commonly relatively unaltered and may contain some single domain magnetite armored by silicate mineral grains however layered intrusives often have a strong petrofabric that may result in a strong remanence anisotropy magnetic anisotropy can have particularly disastrous consequences for paleointensity experiments if the anisotropy is unrecognized and if its effects remain uncorrected here we examine the magnetic anisotropy of an anorthosite sample with a well developed magmatic foliation the effect of the sample s remanence fabric on paleointensity determinations is significant paleointensities estimated by the method of thellier and thellier range from 17 to 55 t for specimens magnetized in a field of 25 t we describe a technique based on the remanence anisotropy tensor to correct paleointensity estimates for the effects of magnetic fabric and use it to estimate a paleointensity for the stillwater complex mt usa of 32 t adjusted for the effects of slow cooling

geomagnetic field intensity behavior in the middle east between 3000 bc and 1500 bc 1 an archeointensity study was carried out on 14 sites of syrian baked clay artifacts archeologically dated between 2500 bc and 1600 bc using an experimental protocol involving high temperature magnetization measurements well defined mean intensity values were derived for 13 different sites with three to nine results obtained at the fragment level per site results of similar ages are coherent and the new data set is in good agreement with previous archeointensity results obtained from the same region all together these data allow one to refine the evolution of the geomagnetic field intensity in the middle east during the third and the second millennium bc in particular they show the occurrence of three periods of rather sharp intensity increase at 2600 bc 2200 bc and 1600 bc possibly at the times of climatic cooling in the eastern north atlantic further suggesting a connection between the earth s magnetic field and multi decadal climatic events

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

geomagnetic field hemispheric asymmetry and archeomagnetic jerks abstract we investigate the origin of the so called archeomagnetic jerks detected in the french archeomagnetic record over the past three millennia although only very large scale global archeomagnetic field models are currently available we show that the occurrence of archeomagnetic jerks is intimately linked to what we define as most eccentric events i e periods of time when a simple description of the geomagnetic field in terms of an eccentric dipole reveals the center of this eccentric dipole to strongly move away from the earth s center from the behavior of the much better known historical field we interpret the evolution of the center of the eccentric dipole as reflecting the production and gathering of flux patches at the core mantle boundary within preferential hemispheres archeomagnetic jerks would thus correspond to episodes of maximum geomagnetic field hemispheric asymmetry such most eccentric events could also provide an explanation for some of the properties previously reported in the long term paleomagnetic field

archaeomagnetic results on three early iron age salt kilns from moyenvic france variations of the earth fs magnetic field during the first millennium bc in western europe remain poorly constrained especially archaeointensity changes three salt kilns moa mob and moc sampled in moyenvic lorraine eastern france have been studied to provide new reference data each kiln has been dated by radiocarbon to originate from the early iron age or hallstatt period between viii and vth century bc rock magnetic experiments and hysteresis results suggest the predominance of pseudo single domain psd ti poor magnetite archaeomagnetic directions obtained by thermal and alternating field demagnetizations have high mean inclination close to 70 and declination between 19 and 31 a first set of classical thellier thellier experiments was conducted on 46 samples with a laboratory field almost parallel to the direction of the characteristic remanent magnetization chrm only 24 of these specimens present a linear nrm trm plot for other specimens nrm trm plots are concave up with positive ptrm checks the very large dispersion observed between the determined palaeointensity values suggests some artefacts have not been fully recognized a second set of thellier experiments was conducted on 34 sister specimens with the laboratory field applied quasi perpendicular to the chrm in these cases mineralogical evolutions during heating and chemical remanent magnetization acquisitions have been clearly recognized despite positive ptrm checks the concave up shapes of nrm trm plots appear mainly due to mineralogical alteration rather than to the presence of psd md grains for the entire set of samples the success rate of the palaeointensity determinations is very low with 80 per cent of the samples rejected nevertheless reliable mean archaeointensities have been obtained for two of the three kilns moa 80 1 14 5 et and mob 86 6 6 9 et at the latitude of paris the high field strength and the archaeomagnetic directions determined combined with previous published data provide further evidence for important changes of the earth magnetic field in europe during the first half of the first millennium bc these large variations of the geomagnetic field during the iron ages indicate that archaeomagnetism is highly suitable for dating of structures from this period

secondary magnetizations cast doubt on exceptional palaeointensity values from the lousetown creek lavas of nevada usa summary renewed palaeomagnetic investigation of pliocene lavas of the lousetown creek formation of nevada usa has given reason to question the conclusions of earlier palaeofield direction and palaeofield intensity studies of these lavas contrary to the suggestion of heinrichs 1967 the lavas do not record a complete transition of the geomagnetic field from reversed to normal polarity rather it is found that lavas recording an intermediate direction for the palaeomagnetic field overlie lavas recording a normal polarity direction the presence of large secondary magnetizations of random direction is widespread they might account for the unusually high transitional palaeointensity obtained for one of the lavas by shaw 1977

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

tracking a non dipole geomagnetic anomaly using new archaeointensity results from north east china summary a collection of ceramics and samples collected from north east china with ages ranging from 1000 to 7000 years have been investigated using a modified version of the shaw palaeointensity techniques shaw 1974 rolph shaw 1985 in which only partial nrms and trms pnrms and ptrms with blocking temperatures t above 300 c are used after pre selection of samples by mineral magnetic analysis a secular variation curve obtained from this study is quite consistent with previous results from other areas of china wei et af 1987 tang et af 1991 as well as with the global model of mcelhinny senanayake 1982 comparison of the chinese results with contemporaneous results from greece aitken el af 1989 has allowed us to track the movement of a large non dipole anomaly as it drifts westwards

testing the izzi protocol of geomagnetic field intensity determination 1 a new paleointensity determination protocol the izzi method was recently proposed the izzi technique combines the aitken in field zero field iz and coe zero field in field zi methods the izzi protocol of paleointensity method was experimentally tested showing a strong angular dependence resulting from the undemagnetized portions of partial thermoremanent magnetization ptrm tails the izzi method is better than the conventional techniques aitken coe and thellier in three respects 1 it can easily detect the angular dependence 2 it provides a quantitative estimate for the consistency of the outcome between iz and zi step and 3 it is quicker because the extra ptrm tail check step is unnecessary

anisotropic approximations for mudrocks a seismic laboratory study an experimental technique for determining the in situ elastic properties of mudrocks with horizontal alignments in the microstructure is used to study the accuracy of a set of three nested scalar anisotropic approximations for transversely isotropic ti media each subsequent approximation adds one more velocity parameter and includes the previous as a special case these approximations are convenient and robust because of their close relationship to standard geophysical measurements there exists no good theory to predict the effects of an imposed stress on the elasticity of mudrocks in this study the tensor of elastic moduli of a single test specimen of mudrock subjected to an anisotropic stress field is determined from ultrasonic group velocity measurements involving pointlike transducers the mechanical characterization performed at constant pore pressure is accompanied by detailed microscopic observations and analysis the method was used to obtain accurate elastic constants for five well defi

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

application of copper slag in geomagnetic archaeointensity research 1 paleointensity and archaeointensity studies since the 1950s have produced numerous geomagnetic intensity data for the last seven millennia as a consequence of different experiments and materials there is a complex and internally inconsistent picture of the geomagnetic field behavior in this study we present data using a recently developed experimental design on a heretofore unexploited recording medium copper slag deposits our results based on hundreds of specimens from various archaeometallurgical sites of the southern levant demonstrate the applicability of copper slag material for archaeointensity studies in addition to frequently exhibiting good experimental behavior slag has further advantages such as dense multilayer deposits and in cases embedded charcoals which open the door to data sets with excellent age control and resolution the data presented here augment the high quality database from the middle east and support previously observed periods of rapid change of the intensity of the geomagnetic field

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

archaeomagnetic study of five mounds from upper mesopotamia between 2500 and 700 bce further evidence for an extremely strong geomagnetic field ca 3000 years ago the distribution of archaeomagnetic data in eastern europe and the near and middle east shows a remarkable gap in turkey this study presents the first archaeomagnetic results from five different mounds in southeast turkey the northern part of mesopotamia the rock magnetic experiments indicate that in the majority of the samples the dominant magnetic carrier is magnetite which is stable to heating to temperatures of 700 1c in general the demagnetization diagrams are single component and all five sets display well defined characteristic magnetizations and clustered directions for the period between 2500 and 700 bce the declinations are between 3501 and 201 while inclinations are in the range of 49 641 the directional results are compared with the global geomagnetic field models cals7k 2 arch3k cst 1 and cals3k 4 and the data from the archaeomagnetic database geomagia50v2 the results are coherent with both the data and the models except for two nearcontemporaneous sets dating 2000 bce which are offset to the east by more than 201 with respect to cals7k 2 archaeointensity measurements were made using the microwave and conventional thermal thellier methods applied to five sets of samples four furnaces and a mud brick wall these yielded comparable and intriguing results while those from the furnaces are slightly higher than the cals7k 2 model and in agreement with the geomagia50v2 and the middle east data the results from the mud brick wall suggest a high intensity of 100 8 mt 17 7 10 22 am 2 a t 1000 bce this result is

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

a continuous multimillennial ring width chronology in yamal northwestern siberia remains of subfossil siberian larch trees in the holocene deposits of the yamal peninsula western siberia have been collected in order to develop a continuous multimillennium tree ring width chronology this work has produced a calendar age dated 4000 year 2000 bc to ad 1996 series from these data summer temperature variability in this region has been estimated on annual to multidecadal timescales radiocarbon dating of selected older material shows that the oldest subfossil wood is 9400 years old and the dates of the sampled material are generally distributed evenly through time it will therefore be possible to develop a tree ring chronology for more than nine millennia an initial assessment of long term e uctuations in yamal summer warmth has been realized through the reconstruction of tree line dynamics using a combination of dendrochron ological absolute dated material and less precisely radiocarbon dated older subfossils

palaeomagnetic intensities from 14 c dated lava flows on the big island hawaii 0 21 kyr thellier thellier experiments were carried out on 216 lava samples collected by the usgs on the big island 35 individual flows from the kilauea mauna loa and hualalai volcanoes are represented and independent radiocarbon dating of the flows yields absolute ages ranging from 290 to 20 240 yrs old the palaeomagnetic analysis was carried out at the laboratoire des sciences du climat et de l environnement in gif sur yvette france in two custom built large capacity furnaces that have been specifically designed to minimise oxidation the temperature steps were adapted to accommodate the characteristic loss of magnetisation at low temperatures seen in the curie balance results and the use of half size samples allowed secondary experiments to be carried out where necessary the strict picrit 03 selection criteria were rigorously applied to the data and a high success rate of 53 has been achieved on a sample level the flow averaged results almost double the existing 14c dated palaeointensity dataset for this time window and confirm a period of high intensity over the past 4 kyr preceded by a period in which the dipole moment was weaker however the values attained in this study are on average higher than previously published data reliability of these values is discussed

three millennia of directional variation of the earth s magnetic field in western europe as revealed by archeological artefacts directional secular variation of the geomagnetic field over the last 2000 years has been defined in western europe from numerous archeomagnetic studies however the number of archeomagnetic results for older periods is much more limited for this reason we present new data obtained from fired archeological structures found in two french sites loupiac and aspiran dated within the first millennium b c latest bronze earliest iron age transition 850 700 b c and iron age 525 475 b c respectively from a compilation of archeomagnetic results from western europe great britain italy and france and northern africa tunisia we propose a directional secular variation curve for western europe that covers the entire first millennium b c this curve exhibits a large clockwise motion with rapid changes during the first half of the millennium while the last four centuries b c are characterized by weak variations

importance of cooling rate dependence of thermoremanence in paleointensity determination 1 the practical effect of cooling rate on the magnitude of thermal remanent magnetization trm is experimentally tested using both natural and synthetic magnetites for synthetic and natural single domain sd magnetites trm intensity increases as the cooling rate decreases because a longer exposure time for slower cooling offers more chances to achieve an equilibrium magnetization multidomain md magnetites have the opposite response trm decreases as the cooling rate increases because slower cooling allows more time to achieve lower magnetization because of self demagnetization for pseudo single domain psd magnetite the effect of cooling rate on the remanence intensity appears to be insignificant intermediate in behavior between sd and md for a sd trm differences are restricted to lower temperatures but diminish near the curie point strongly indicating a serious problem in a conventional treatment of cooling rate correction on the practical side merely applying trm differences as a correction can overcorrect the true paleointensity another important significance of the present study is that a cooling rate difference alone can cause a slight nonlinearity in an arai plot for a sd

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

geomagnetic field variations in western europe from 1500 bc to 200 ad part ii new intensity secular variation curve abstract in order to extend the secular variation curve svc of archaeointensity in western europe to the first millennium bc we studied 24 kilns and hearths in place two displaced hearths and six sets of pottery sherds from french archaeological sites archaeological artefacts radiocarbon and dendrochronology dated the acquisition of the thermoremanent magnetization trm carried by the studied objects rock magnetism experiments suggest that the main carrier of the magnetization is a ti poor titanomagnetite archaeointensity was determined by the thellier thellier classical protocol with ptrm checks a strict criteria set was applied to select only the most reliable results with linear nrm trm diagrams 55 of total specimens this study demonstrates that pottery sherds with two trms give reliable archaeointensities in the low temperature interval if the nrm trm diagram is adequately adjusted eighteen new mean archaeointensities 14 corrected from the anisotropy of trm and 16 from cooling rate were computed the comparison with previously published western europe paleointensities show a strong dispersion between data primarily due to their variable quality western europe data were weighted following the archaeointensity protocol the number of specimens per site and the type of studied materials in order to better highlight the secular variation of archaeointensity during the first millennium bc the svc built with sliding windows of 160 years shifted every 50 years presents at paris a maximum of 90 t around 800 bc and a minimum of 60 t around 250 bc these archaeointensity maximum and minimum correspond to cusps of the geomagnetic field direction in western europe this new curve is consistent with mesopotamian and eastern europe data the archaeointensity secular variation in western europe predicted by global geomagnetic models cals3k 4 arch3k 1 and arch3k cst 1 is smoother than our svc we used our directional dataset herve et al 2013 to build a new western europe vgps and vdms mean curves comparison with the predictions given by the global models points out a possible persistent non dipole fields effect over europe between 1000 bc and 600 500 bc finally we note that the strong variations of intensity of the geomagnetic field with a mean decrease rate per century close to 6 t will be useful for archaeomagnetic dating purposes

an integrated archeomagnetic and c14 study on pre columbian potsherds and associated charcoals intercalated between holocene lacustrine sediments in western mexico geomagnetic implications 1 the firm relationship between archeomagnetic samples and their absolute ages which is usually determined or estimated using a variety of methods archaeological context thermoluminiscense c14 etc is crucial in any archaeomagnetic investigation in this paper we report 18 successful from 65 analyzed archaeointensity determinations of high technical quality performed on specimens obtained from three fragments of pre columbian potsherds these are unambiguously correlated with charcoal samples as they were found together in three thin lacustrine sedimentary layers in western mexico moreover new radiometric results were obtained in this study from all charcoal samples beta analytic yielding ages of 1490 40 1510 40 and 1580 40 bp in addition detailed magnetic measurements were carried out on all studied ceramic fragments these rock magnetic experiments which included low field susceptibility versus temperature hysteresis and first order reversal curve measurements indicate that the main magnetic carrier is ti poor titanomagnetite the average archeointensity values per ceramic fragments obtained in this study are very similar in all three cases 35 2 1 3 t n 5 36 8 1 6 t n 6 and 37 2 3 4 t n 7 this corresponds to an average intensity of 36 4 1 1 t and a virtual axial dipole moment value of 8 1 0 2 1022 am2 which is slightly lower than the present field strength corresponding at an age interval between 640 and 430 a d although we detected some relationship between the earth s magnetic field intensity and multidecadal climatic events such observations can only be considered tentative at this stage

microwave palaeointensities from holocene age hawaiian lavas investigation of magnetic properties and comparison with thermal palaeointensities abstract sixteen c 14 dated hawaiian surface lava flows spanning 0 4 5 ka have been investigated using the microwave perpendicular applied field palaeointensity technique and classical thermal thellier thellier palaeointensity technique in parallel the overall microwave experimental success rate is 63 compared to 59 in the thermal experiments nineteen percent of the microwave results are deemed to be first class compared to 52 of the thermal results high palaeointensities and a large amount of within flow variation are seen in the microwave results with reliable palaeointensity estimates ranging from 25 10 to 82 94 t these variations do not appear to be systematically related to variations in rock magnetic properties such as mineralogy grain size texture or oxidation state the flow means from the microwave and thermal studies agree for 9 of the 13 flows that can be directly compared and the average difference between the two techniques is not significantly different from zero neither experimental technique produces mean palaeointensities systematically lower or more precise than the other suggesting that the differences seen are a reflection of natural within flow variation rather than experimental technique

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

strength of the geomagnetic field in the cretaceous normal superchron new data from submarine basaltic glass of the troodos ophiolite 1 we present here new paleointensity data from 39 sampling sites collected from the quenched margins of pillow lavas and dikes exposed within the troodos ophiolite similar to 92 ma formed during the cretaceous normal superchron cns a period of approximately 40 million years when the geomagnetic field reversed extremely infrequently if at all monte carlo simulations suggest that a minimum of 25 estimates are necessary for a reasonably robust estimate for the average field strength our data suggest a dipole strength equivalent to the present field or nearly twice the post cns average the mean and standard deviation of the dipole moment 81 43 zam 2 z 10 21 from the 57 data points compiled here agree remarkably well with those predicted from the long paleointensity record derived from dsdp site 522 the new data set for the cns suggests a picture of a strong and stable field during the period of time when it stopped reversing moreover the similarity of the cns data with the present geomagnetic field suggests that it is presently in a state of unusual polarity stability

does earth s magnetic field secular variation control centennial climate change abstract we obtained new archeointensity data from french faience potsherds dated from the 17th to 19th century these results further document the occurrence of sharp changes in geomagnetic field secular variation in western europe over the past three millennia the intensity variation curve shows several maxima whose rising parts appear to coincide in time with the occurrence of cooling events documented in this region from natural and historical data this coincidence suggests a causal link between enhanced secular variation of the geomagnetic field and climate change over centennial time scales challenging the role of solar forcing as the sole factor provoking these climatic variations we propose that the archeomagnetic jerks described by gallet et al 1 y gallet a genevey v courtillot on the possible occurrence of archeomagnetic jerks in the geomagnetic field over the past three millennia earth planet sci lett 214 2003 237 242 may engage the mechanism for centennial climate change

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

eight thousand years of geomagnetic field intensity variations in the eastern mediterranean 1 twenty new intensity determinations of the ancient geomagnetic field have been obtained from groups of potsherds and brick fragments from syria these artifacts archeologically well dated from 6000 b c to approximately a d 1200 have been analyzed using the thellier and thellier 1959 method as modified by coe 1967 intensity values have been corrected for the effects of anisotropy of thermal remanent magnetization and cooling rate our results indicate that field intensities were moderate in syria from 6000 b c to 3500 b c with values of 30 40 t there was a significant increase in intensity by a factor of 2 from 3500 b c to 700 b c which was interrupted by a moderate decrease between 2550 b c and 1750 b c during more recent periods our results show an intensity minimum approximately a d 200 and a maximum around the tenth century comparison with different data sets from the eastern mediterranean and central asia shows that geomagnetic field intensity variations were consistent at this large regional scale at least over the last 5 millennia

globally strong geomagnetic field intensity circa 3000 years ago abstract high fidelity geomagnetic field intensity determination was carried out using 191 baked fragments collected from 20 kilns or hearths with ages ranging between 1200 bc and ad 1725 in south korea geomagnetic field intensity variation displayed three narrow minima at 800 700 bc ad 700 and ad 1600 and two maxima at 1200 1100 bc and ad 1000 1100 in most time intervals virtual axial dipole moment vadm variation is confined within 20 of the present vadm however geomagnetic field intensity circa 3000 yr ago is nearly 40 larger than the present value such high vadms circa 3000 yr ago are in phase with those in other longitudinal bands in northern hemisphere centered at 5e france 30e the middle east and 200e hawaii although strong geomagnetic field intensity circa 3000 yr ago is globally synchronous the highest vadm occurs at slightly different time intervals in different locations hence it is possible that the globally strong geomagnetic field intensity circa 3000 yr ago reflects the migration of persistent hemispheric flux in northern hemisphere or an episode of geomagnetic field hemispheric asymmetry

evidence for rapid geomagnetic field intensity variations in western europe over the past 800 years from new french archeointensity data abstract the number of reliable archeointensity determinations obtained from western europe for the past millennium remains limited moreover the large scatter between different datasets available is puzzling the present study analyzed 31 new groups of baked clay ceramic or brick fragments sampled in france 29 groups and in belgium 2 groups these groups contain several fragments collected from different artefacts and are precisely dated principally from historical constraints between the xiiith and the xixth centuries additionally we re evaluated 14 intensity values that we previously obtained from the same time period the fragments were analyzed using two different thermal methods 1 the in field zero field iz or the izzi version of the classical thellier and thellier method and 2 the triaxe protocol that involves high temperature magnetization measurements data were corrected for the anisotropy of thermoremanent magnetization trm and the dependence of trm acquisition on the cooling rate was taken into account in the different protocols archeointensity data obtained on twin specimens sampled from the same fragment and using both experimental techniques generally show a good agreement i e within 5 at the fragment and at the site level all retained site level averaged intensity results 43 of 45 groups have standard deviations of less than 5 t furthermore groups of approximately the same age have very consistent archeointensity altogether the data presented herein recover a detailed and smoothed geomagnetic field intensity variation curve characterized by two peaks in intensity the first during the second half of the xivth century and the second around ad 1600 followed by a significant decreasing trend in intensity during most the xviith and xviiith centuries this evolution does not satisfactorily fit with the expected intensity values for france derived from geomagnetic field models relying on a different evolution of the axial dipole moment our results lead us to propose that the axial dipole moment decreased from ad 1600 to the end of the xviiith century then slightly increased up to ad 1850 before decreasing again to present day

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