effet de la vitesse de refroidissement sur l intensite de l aimantation thermoremanente etude experimentale consequences theoriques the experiments reported here compare intensity and thermal resistance of two thermal remanent magnetizations trm successively acquired by the same sample under a magnetic field but at two different cooling speeds the samples one with a base of cro2 powder and the others derived from oceanic basalts were representative of the different types of structure domains characteristic of magnetic grains the experimental procedure established aimed at limiting the various factors of uncertainty mineralogical and structural modifications related to heating inhomogeneity of the sample temperature at crucial stages of each experiment effects of undesirable magnetizations acquired concurrently with the trm the main conclusion of this study is that the trm intensity increases as a function of time of cooling independently of the types of structure domains carried by the magnetic grains the greatest effect is with the cro2 monodomain the trm obtained between 130 and 100 c during a cooling period of 120

a comparison of archaeointensity results from chinese ceramics using thellier s and shaw s palaeointensity methods summary a collection of chinese ceramics covering the time period 2700 7500 bp has been investigated using adaptations of both the thellier thellier 1959 and shaw 1974 palaoeintensity techniques several rock magnetic techniques were used to investigate the magnetic properties of the samples and on the basis of these measurements some samples were rejected because of their poor magnetic behaviour anisotropy of magnetic susceptibility ams thermo remanent magnetization trm and anhysteretic remanent magnetization arm were investigated based on the relationship between arm and trm anisotropy modified arms were used for anisotropy correction twenty samples were studied using the shaw method and 34 were studied using the thellier method an acceptable agreement between the results from the two methods was found general features of the secular variation of geomagnetic intensity determined from the present study are consistent with those of the global model of mcelhinny senanayake 1982

potbellies wasp waists and superparamagnetism in magnetic hysteresis because the response of a magnetic substance to an applied field depends strongly on the physical properties of the material much can be learned by monitoring that response through what is known as a magnetic hysteresis loop the measurements are rapid and quickly becoming part of the standard set of tools supporting paleomagnetic research yet the interpretation of hysteresis loops is not simple it has become apparent that although classic single domain pseudo single domain and multidomain loops described in textbooks occur in natural samples loops are frequently distorted having constricted middles wasp waisted loops or spreading middles and slouching shoulders potbellies such complicated loops are often interpreted in oversimplified ways leading to erroneous conclusions the physics of the problem have been understood for nearly half a century yet numerical simulations appropriate to geological materials are almost unavailable in this paper we discuss results of numerical simulations using the simplest of systems the single domain superparamagnetic sd sp system examination of the synthetic hysteresis loops leads to the following observations 1 wasp waisting and potbellies can easily be generated from populations of sd and sp grains 2 wasp waisting requires an sp contribution that saturates quickly resulting in a steep initial slope and potbellies require low initial slopes the sp contribution approaching saturation at higher fields the approach to saturation is dependent on volume hence the cube of grain diameter therefore there is a very strong dependence of hysteresis loop shape on the assumed threshold size 3 we were unable to generate potbellies using an sp sd threshold size as large as 30 nm and wasp waists cannot be generated using a threshold size as small as 8 nm the occurrence of both potbellies and wasp waists in natural samples is consistent with a room temperature threshold size of some 15 nm 5 4 simulations using a threshold size of 15 20 nm with populations dominated by sp grain sizes that is with a small number of sd grains produce synthetic hysteresis loops consistent with measured hysteresis loops and transmission electron microscopic observations from submarine basaltic glass 5 simulations and measurements using two populations with distinct coercivity spectra can also generate wasp waisted loops a relatively straightforward analysis of the resulting loops can distinguish the latter case from wasp waisting resulting from sp sd behavior

determination of geomagnetic archaeomagnitudes from clay pipes archaeomagnitude determinations of a selection of clay pipes dateable to ad 1645 10 as well as studies of pottery samples from the same site and of the same age have been made values of the magnitude of the ancient magnetic field banc were obtained from two pottery sherds two pipe bowls and three pipe stems the values from the sherds and bowls agree within 2 and compare well with the average value of the magnitude of the magnetic field for the seventeenth century as determined by other archaeomagnetic studies however the pipe stems give values of banc which are significantly less than those from the bowls and pottery we have not yet been able to explain this and thus we suggest that reliable archaeomagnitude determinations can be made from the bowls of clay pipes but not from the stems nevertheless this result provides a new source of material for investigating variations in the geomagnetic field strength over the past 400 yr clay pipes have been manufactured in england since the end of the sixteenth century in the firing process some pipes were broken and disposed of without ever having been smoked one such collection discovered at rainford lancashire in 1978 consisted of a series of discrete dumps including pipes kiln debris and a small collection of contemporary used earthenware sherds the internal consideration of the dumps suggested a very short period of activity and archaeologists p davey personal communication ascribe all the material to the period 1645 10 yr with such well dated material we set out to check whether or not reliable archaeomagnitudes could be obtained from the pipes

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

on the suitability of baked clay for archaeomagnetic studies as deduced from detailed rock magnetic studies summary extensive rock magnetic investigations have been carried out on baked clays from four kilns two from bulgaria and two from switzerland found in archaeological sites of different age knowledge of the magnetic characteristics of the grains responsible for the archaeomagnetic signal enables us to determine which baked clays have the stablest magnetization and why this is so this is important in directional studies but even more so in painstaking palaeointensity studies that require a very careful evaluation of the suitability of the burnt clay material the proposed rock magnetic experiments enable the identification of the carriers responsible for the remanence and an adequate interpretation of the experimental results connected with the palaeointensity evaluation the experimental methods employed are illustrated with the particular results obtained from each of the four kilns studied the preliminary elucidation of the magnetic mineralogy of the archaeological samples helps first by obtaining a more reliable palaeointensity result and secondly by explaining some of the discrepancies in the palaeodirectional results examples of successful and failed palaeointensity experiments are given in relation to the magnetic properties previously established for each oven the burnt clay materials in this present study satisfy the essential condition of carrying a thermoremanence in spite of that it is shown that there are many factors that can produce undesirable magnetic properties and thus restrict the suitability of these materials for archaeomagnetic analysis the most important factors influencing the magnetic behaviour during magneto diagnostic experiments are the degree of heating in antiquity the initial composition of the unbaked material and the burial conditions the large difference in heating temperatures within a particular archaeological feature is a major cause of variation in magnetic behaviour amongst individual specimens and so preventing a successful pre selection of specimens for palaeointensity experiments nevertheless the study has shown a very good coincidence between the determined rock magnetic characteristics and the success rate in palaeointensity evaluation

full vector archaeomagnetic records from anatolia between 2400 and 1350 bce implications for geomagnetic field models and the dating of fires in antiquity abstract anatolia as one of the busiest crossroads of ancient civilizations provides an ideal platform for archaeomagnetic studies previous results from the middle east have suggested the occurrence of a strong peak in geomagnetic intensity at 1000 bce associated with dramatic field strength variations that could require a radical rethinking of geodynamo theory the behavior of the field in the centuries preceding this peak remains poorly constrained however here we present the results of full vector archaeomagnetic experiments performed on 18 sets of samples from three archaeological sites belonging to assyrian trade colony and hittite periods associated rock magnetic analyses showed that the major magnetic carrier is magnetite chemically stable up to 700 c and the magnetic mineral assemblage is composed mostly of non interacting psd grains the directional results are compared with existing data and with the most recent global geomagnetic field models pfm9k 1b and sha dif 14k the directions are in remarkably good agreement with sha dif 14k which is based on archaeomagnetic and lava flow data together with our earlier results from anatolia we triple the existing database of directions for the 700 year long period 2200 1500 bce over a large region from greece to azerbaijan and from moldavia ukraine to egypt three archaeointensity methods thermal izzi thellier microwave thellier and the multi specimen protocol msp produced virtual axial dipole moment estimates 9 0 10 9 10 22 am 2 that are somewhat higher than contemporaneous regional and global data and model predictions suggesting that the field was already substantially stronger than today more than 800 years prior to the reported peak in addition to constraining geomagnetic variability our data also allow us to assign relative dates to inferred fire events in the assyrian trade colony period sites this allows us to conclude that the fire events at the largest site kultepe were not all contemporaneous with one another and with the abandonment of the site as has been previously hypothesized

estimate of the magnetic anisotropy effect on the archaeomagnetic inclination of ancient bricks abstract the magnetic fabric of 59 bricks coming from 5 ancient kilns has been studied by measuring the anisotropy of magnetic susceptibility ams and the anisotropy of isothermal airm anhysteretic aarm and thermal atrm remanent magnetization the bricks are characterized by a well developed magnetic fabric that matches their flat shape the shape of the anisotropy ellipsoids is in almost all cases oblate with the maximum and intermediate axes lying parallel to the large face of the brick and the minimum axis perpendicular to it the directions of the principal axes are almost the same irrespectively of the type of anisotropy measured whereas the degree of anisotropy of the airm aarm and atrm is much higher than the ams as the bricks lie horizontally within the kiln the planar magnetic fabric results in an inclination shallowing of the archaeomagnetic direction with respect to that of the earth s magnetic field at the time of their last cooling estimation of this effect on the grounds of atrm measurements yields a shallowing that varies from 4 to 10 for individual samples such inclination difference may significantly bias archeomagnetic dating for the case of the canosa late roman kiln it leads to a dating error of more than two centuries

the steens mountain oregon geomagnetic polarity transition 2 field intensity variations and discussion of reversal models we carried out an extensive paleointensity study of the 15 5 0 3 m y miocene reversed to normal polarity transition recorded in lava flows from steens mountain south central oregon one hundred eighty five samples from the collection whose paleodirectional study is reported by mankinen et al this issue were chosen for paleointensity investigations because of their low viscosity index high curie point and reversibility or near reversibility of the strong field magnetization curve versus temperature application of the thellier stepwise double heating method was very successful yielding 157 usable paleointensity estimates corresponding to 73 distinct lava flows after grouping successive lava flows that did not differ significantly in direction and intensity we obtained 51 distinguishable complete field vectors of which 10 are reversed 28 are transitional and 13 are normal the record is complex quite unlike that predicted by simple flooding or standing nondipole field models it begins with an estimated several thousand years of reversed polarity with an average intensity of 31 5 8 5 t about one third lower than the expected miocene intensity this difference is interpreted as a long term reduction of the dipole moment prior to the reversal when site directions and intensities are considered truly transitional directions and intensities appear almost at the same time at the beginning of the transition and they disappear simultaneously at the end of the reversal large deviations in declination occur during this approximately 4500 1000 year transition period that are compatible with roughly similar average magnitudes of zonal and nonzonal field components at the site the transitional intensity is generally low with an average of 10 9 4 9 t for directions more than 45 away from the dipole field and a minimum of about 5 t the root mean square of the three field components x y and z are of the same order of magnitude for the transitional field and the historical nondipole field at the site latitude however a field intensity increase to pretransitional values occurs when the field temporarily reaches normal directions which suggests that dipolar structure could have been briefly regenerated during the transition in an aborted attempt to reestablish a stationary field changes in the field vector are progressive but jerky with at least two and possibly three large swings at astonishingly high rates each of those transitional geomagnetic impulses occurs when the field intensity is low less than 10 t and is followed by an interval of directional stasis during which the magnitude of the field increases greatly for the best documented geomagnetic impulse the rapid directional change corresponds to a vectorial intensity change of 6700 2700 nt yr 1 which is about 15 50 times larger than the maximum rate of change of the nondipole field observed during the last centuries the occurrence of geomagnetic impulses seems to support reversal models assuming an increase in the level of turbulence within the liquid core during transitions the record closes with an estimated several thousand years of normal polarity with an average intensity of 46 7 20 1 t agreeing with the expected miocene value however the occurrence of rather large and apparently rapid intensity fluctuations accompanied by little change in direction suggests that the newly reestablished dipole was still somewhat unstable

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

paleointensity during periods of rapid reversal a case study from the middle jurassic shamrock batholith western nevada paleointensity is one of the least deter mined parameters in geomagnetism yet this information has the potential to address many fundamental geophysical problems that bear on the evolution of the earth s core and lower mantle we test two important hy potheses that affect our understanding of how paleointensity has changed in earth s past 1 the mesozoic dipole low mdl hypoth esis and 2 the inverse relationship between geomagnetic polarity reversal rate and paleo intensity we report paleointensity results determined using both the coe modified thellier method and the izzi protocol on the medium grained hornblende biotite granite of the middle jurassic shamrock batholith in the southern singatse range western nevada usa previous zircon u pb dating gives an age of batholith emplacement at ca 165 8 0 4 ma this age estimate is coincident with a period of unusually high reversal frequency 10 reversals ma in the middle jurassic remanence in the shamrock batholith is held primarily by exsolved submicron magnetite inclusions in plagioclase the labora tory un blocking temperature range for the well de fined thermal remanent magnetization is nar row between 540 and 575 c and median destructive fields are between 40 and 50 mt thus we infer that the age of rema nence ac quisition is approximated by the u pb age estimate for pluton emplacement two sets of quality criteria were used to assess the paleo intensity results the first set yielded an 87 success rate while the second set using more stringent criteria resulted in only a 20 suc cess rate mean paleointensity values for the shamrock batholith using the loose and strict quality criteria are 34 0 6 4 t n 13 and 33 6 9 6 t n 3 respectively with errors reported at one standard deviation after correction for both remanence anisot ropy and cooling rate the mean values that pass loose and strict criteria are respec tively 17 9 2 7 t and 17 5 2 6 t the uncorrected and corrected paleointensity values for the strict estimates yield respec tive mean virtual dipole moments vdms of 62 7 18 zam 2 z 10 21 and 32 7 4 9 zam 2 the uncorrected value is within error of the vdms reported for plagioclase grains isolated from ca 160 ma basalts from ocean drilling program site 801 however these values have an absolute difference ranging from 6 to 20 zam 2 while the corrected vdm value is more consistent with the esti mated average jurassic field strength of 29 zam 2 using a recent estimate for the long term stable field strength of 42 zam 2 our corrected values add further credence to both the mdl and the inverse relationship between geomagnetic polarity reversal rate and dipole moment hypotheses

an updated and homogeneous world secular variation data base 1 smoothing of the archaeomagnetic results the available original archaeomagnetic world data on intensity and direction have been compiled and sorted according to geographical region then after correcting them to correspond to a single site associated with each region called a world site they have been processed using the gaussian statistic for the intensity f data and bivariate statistic for the declination d and inclination i data to obtain smoothed values at regular time intervals we include a discussion of the statistical methods used we have limited the data to the last 20 centuries because the archaeomagnetic data alone do not allow us to go back further in the past we have also excluded archaeomagnetic studies at world sites where the number of data is not sufficient to perform a statistical analysis we obtained results at nine world sites for which the values of at least two of the three above quoted parameters have been calculated at 25 year intervals from the beginning of our era the results for each world site with their associated errors are given and discussed the potential of this new set of world data for improving our knowledge of the global terrestrial magnetic field tmf during the past 20 centuries is sketched these results will be completed in a second paper by analysing in the same way the data supplied for volcanic rocks and lake sediments

a new method for measuring palaeomagnetic intensities because measurements of palaeomagnetic intensities henceforth palaeointensities are usually time consuming and the success rate only poor say 30 50 many palaeointensity values are based on as few as three experimental results in an attempt to improve on this a technique has been developed using automated equipment which is faster and which can detect alteration of a sample during measurement allowing the experiment to be terminated and another sample to be tried the technique described here has been used on a collection of ancient bricks and the results show the palaeofield at cambridge england to have decreased from about 49 t in ad 1511 to 42 t in ad 1758 in good agreement with results using shaw s method

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

paleointensity of the earth s magnetic field recorded by two late quaternary volcanic sequences at the island of la reunion indian ocean a paleomagnetic study has been undertaken on two late quaternary volcanic sequences from the piton de la fournaise volcano island of la reunion in order to provide absolute paleointensities of the earth s magnetic field from a site in the southern hemisphere the pattern of secular variation recorded by both sequences clearly indicates that the extrusion rate of the lava flows was not constant with time a detailed investigation of the magnetic properties of the samples was carried out in parallel with paleointensity experiments which were performed with the thellier method samples with minimum overprint composed by single domain or pseudo single domain magnetite showing good thermal stability and a narrow blocking temperature spectrum were the most successful during thellier s experiments values of the paleofield strength from 19 to 55 mt have been obtained on 23 flows among the 30 which were sampled few variations 7 5 9 9 1022 a m2 are observed in the virtual dipole moments vdms determined on the youngest sequence 5 12 ka those values are within the range of fluctuations of the worldwide variations of vdms for this period on the other hand data from the oldest sequence 82 98 ka are the first paleointensity values obtained in that time period and the observed vdms extend from 4 1 to 8 8 1022 a m2 these results suggest that for this time interval the field did not depart from its average behavior

experiments on trm intensity dependence on cooling rate palaeointensity studies have an important role to play in understanding the origin and time variation of the geomagnetic field however theoretical considerations indicate that the intensity of magnetization recorded by a rock or archaeological sample will be dependent on the rate at which it cooled as well as field magnitude experiments on synthetic magnetite and titanomagnetites of varying grain sizes indicate that for acicular magnetite at low concentration slower cooled trm is stronger than faster cooled trm but for all other samples slower cooled trm is weaker this is the predicted behaviour however the magnitude of the effect is much greater than the predicted kinetic effects and indicates that the magnetization of sub microscopic magnetite rich inclusions in the titanomagnetites strongly controls the trm intensity of the matrix

fluid evolution in a volcanic hosted epithermal carbonate base metal gold vein system alto de la blenda farallon negro argentina alto de la blenda is a 6 6 ma intermediate sulphidation epithermal vein system in the farallon negro volcanic complex which also hosts the 7 1 ma porphyry cu au deposit of bajo de la alumbrera the epithermal vein system is characterised by a large extent and continuity 2 km 400 m open to depth 6 m maximum width and an average gold grade of 8 g t the vein is best developed within an intrusion of a fine grained equigranular monzonite interpreted as the central conduit of a stratovolcano whose extrusive activity ended prior to porphyry cu au emplacement at bajo de la alumbrera which is in turn cut by minor epithermal veins the alto de la blenda vein consists predominantly of variably mn rich carbonates and quartz with a few percent of pyrite sphalerite galena and other sulphide and sulphosalt minerals four phases of vein opening hydrothermal mineralisation and repeated brecciation can be correlated between different vein segments stages 2 and 3 contain the greatest fraction of sulphide and gold they are separated by the emplacement of a polymictic breccia containing clasts of quartz feldspar porphyry as well as basement rocks fluid inclusions in quartz related to stages 2 to 4 are liquid rich with 2 4 wt nacl eq they homogenise between 160 and 300 c with very consistent values within each assemblage vapour inclusions are practically absent in the epithermal vein quartz fragments in the polymictic breccia contain inclusions of intermediate to vapour like density and similar low salinity 3 wt nacl eq besides rare brine inclusions containing halite laser ablation inductively coupled plasma mass spectrometry la icp ms analyses of epithermal inclusions indicate high concentrations of k fe as sb cs and pb that significantly vary within and through subsequent vein stages careful consideration of detection limits for individual inclusions shows high gold concentrations of 0 5 to 3 ppm dissolved in the ore fluid which contains variably high sulphur concentrations in excess over fe and other chalcophile metals compositional variations are interpreted to reflect cooling and contraction of lower density magmatic fluids at depth like those preserved in porphyry clasts that were mechanically transported up by the polymictic breccia ore mineral precipitation from the magmatic fluid occurred by further cooling and possibly minor mixing with surface derived water leading to sulphide saturation de sulphidation of the magmatic fluid and consequent gold precipitation the absence of flash boiling and or reduction by carbonaceous host rocks has led to relatively modest but constant gold grades in the carbonate base metal gold veins of alto de la blenda

a new contribution to the archaeomagnetic study of a roman pottery kiln from calahorra spain summary results from archaeomagnetic research on a roman pottery kiln are presented in a previous paper pares et al 1992 a collection of samples from the kiln s heating chamber was studied and palaeodirection results were obtained in the same paper dating was performed on the basis of the secular variation reference curve of the inclination at paris in the present study the palaeointensity was evaluated on the basis of the same collection of samples the mean value obtained is 60 9pt a comparison of this value with other available archaeointensity values obtained from neighbouring territories confirms the date estimated by pares et al 1992kthe middle of the 1st century ad

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 theoretical and experimental comparison of the anisotropies of magnetic susceptibility and remanence in rocks and minerals summary susceptibility thermo remanent magnetization trm and isothermal remanent magnetization irm anisotropy ellipsoids have been determined for several rock samples the results indicate that the ellipsoid of initial susceptibility is less anisotropic than the trm and low field irm ellipsoids which are found experimentally to be of identical shape this suggests that palaeomagnetic data for anisotropic rocks may be corrected by using the anisotropy ellipsoid determined from magnetically non destructive low field irm measurements such irm measurements can also be used to obtain anisotropy axes of samples which are inherently anisotropic but which have a susceptibility which is too weak to be accurately measured the results for a series of artificial anisotropic samples containing magnetite particles of different sizes in the range 0 2 90 m were very similar to those for the rocks in contrast a comparison of the susceptibility and irm ellipsoids for anisotropic samples containing particles from a magnetic tape gave very different results in accordance with theory such results imply that susceptibility and irm ellipsoids could be used to determine whether anisotropic rocks contain uniaxial single domain particles magnetization confined to the easy axis or whether the particles are essentially multidomain

the earth s magnetic field in italy during the neolithic period new data from the early neolithic site of portonovo marche italy we present new full geomagnetic field vector results from three neolithic ovens discovered at the archaeological site of portonovo marche italy the discovered structures are a rare example of very well preserved underground ovens from the early neolithic period standard thermal demagnetization procedures were used to isolate the direction of the characteristic remanent magnetization acquired by the baked clay during the ovens last firing the corresponding archaeointensities were determined by the multi specimen procedure msp dsc and show a clear intensity low during the neolithic period both directional and intensity results are of high quality offering the first contribution of full geomagnetic field vector data for this period in italy the new data are compared with other contemporaneous data from europe and with global geomagnetic field models independent archaeomagnetic dating of the three ovens was also performed by means of the scha dif 14k model the obtained results are in excellent agreement with available radiocarbon dates and confirm that all ovens belong to the neolithic these new data importantly enrich our knowledge of the geomagnetic field during the neolithic period that is poorly documented by data not only in italy but also in the whole of europe and show that archaeomagnetic dating can provide precise results even for prehistoric periods

an updated archeomagnetic directional variation curve for france over the past two millennia following 25 years of additional data acquisition abstract nearly 40 years ago emile thellier published an article summarizing the archeomagnetic data he had obtained during his career which had allowed him to recover the main features of the directional variations of the geomagnetic field in france for the last two millennia this database went on to be significantly completed 25 years ago by ileana bucur who had taken over thellier s work on archeomagnetism this forms the current basis of our knowledge of the directional evolution of the geomagnetic field in france since then archeomagnetic studies have been continued at thellier s historical laboratory in saint maur this article presents the directional archeomagnetic data obtained in france over the past 25 years in this same laboratory a total of 528 new data are presented which together with the 170 results obtained on the french territory previously listed in bucur 1994 constitute the french directional database 698 data in all all but two of these data were obtained using the experimental protocol developed by e thellier based on the analysis of large samples and on a magnetic viscosity test that has proved its reliability on numerous occasions the directions from the entire french database have been precisely defined with 95 of the 95 values being less than 1 9 and 50 being less than 0 8 the selection of 286 data with dating uncertainties of 50 years allowed us to compute a new reference directional variation curve for france since the first century bc using sliding windows the variable durations and shifts of which were adapted to the time distribution of the available data and the bivariate extension of the fisher statistics this shows ample and smooth variations fairly similar to those previously determined from a much smaller database the resulting secular variation curve is particularly well suited for use in archeomagnetic dating

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