archeomagnetic study of seven contemporaneous kilns from murcia spain new archeointensity values have been determined from seven spanish kilns sampled at the same archeological site and dated by archeological constraints as between 1100 and 1200 a d the directions of the characteristic remanent magnetization and paleointensities have been obtained from classical thellier experiments conducted on 69 samples collected from the walls of the kilns thermoremanent magnetization trm anisotropy and the cooling rate effect upon trm intensity acquisition have been investigated in all the samples differences between mean archeointensities per site before and after corrections for cooling rate are between 3 and 17 the study of these seven contemporaneous kilns coming from the same archeological site and treated in the same way in the laboratory allows us to discuss the different factors that can cause the dispersion between sites generally observed in archeointensity studies the results confirm on one hand the need for cooling rate corrections in this kind of study and on the other hand the need to obtain several archeointensity determinations per century in order to construct a reliable archeointensity curve finally the new data provide a robust mean intensity 60 3 3 3 t at the latitude of paris for the middle of the xii century in western europe and constrain better the evolution of the geomagnetic field intensity during medieval times in this area

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

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

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

early development and flysch sedimentation in ordovician taconic foreland basin west central newfoundland during the early to late ordovician the taconic foredeep in west central newfoundland evolved from an underfilled to an overfilled state in response to cratonward advance thickening and erosion of the taconic orogen early orogen derived sediment in the foreland basin consisted of middle to lake arenigian deep water mudstones that accumulated on an inner craton facing slope prism uppermost parts of shallow bay and green point formations and correlative units these deposits are interbedded with and overlie passive margin slope sediments in the middle arenigian to early llanvirnian sand from the orogen formed several small sand rich submarine fans lower head formation and correlative units on the lower reaches of the inner slope and basin plain the fans may have been fed by closely spaced rivers draining the orogen as presently occurs in western south america only proximal portions of these fans are now exposed the flysch basin was narrow constricted by the inner slope and the passive ma

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

lithospheric domains and controls on kimberlite emplacement slave province canada evidence from elastic thickness and upper mantle composition 1 we have mapped the deep structure of the slave craton by combining analysis the effective elastic thickness te with data on mantle samples from numerous kimberlites three dimensional mapping of the subcontinental lithospheric mantle sclm using mantle derived xenoliths and xenocrysts in kimberlites has shown that much of the craton is underlain by a strongly layered sclm a highly depleted upper layer low in basaltic components ca al fe is separated from a relatively fertile lower layer by a sharp boundary this boundary lies at 140 150 km depth in the lac de gras area and shallows to 100 km in the northern and southern parts of the craton weak lithosphere te 56 km in the younger eastern part of the craton is separated from the older western part by a zone of steep te gradient parallel to the major locus of kimberlite intrusion which may map the deep extension of the boundary between the two domains another strong te gradient across the kilohigok basin accompanies a marked compositional change in the upper layer of the lithospheric mantle the basin probably marks a major translithospheric fault correlations between te and mantle composition suggest that te is strongly influenced by the rheology of the upper mantle

a catalogue of spanish archaeomagnetic data a total of 58 new archaeomagnetic directions has been determined from archaeological structures in spain together with five previous results they allow the compilation of the first archaeomagnetic catalogue for spain which includes 63 directions with ages ranging between the 2nd century bc and the 20th century ad characteristic remanence directions have been obtained from stepwise thermal and alternating field demagnetization the hierarchical structure has been respected in the calculation of the mean site directions rock magnetic experiments reveal that the main magnetic carrier is magnetite or titanomagnetite with different titanium contents the age estimate of the studied structures is generally well justified by archaeological constraints for six structures the proposed date is also supported by physical methods the data are in close agreement with the french secular variation sv curve this catalogue represents the first step in the construction of a sv curve for the iberian peninsula which will be of much use in archaeomagnetic dating and in modelling of the earth s magnetic field in western europe

recent sediment accumulation rates for the western margin of the barents sea abstract we investigate patterns processes and rates of sediment accumulation within the marginal ice zone of the western barents sea the barents sea is among the most productive of the arctic marginal seas with approximately half of the sediment burial flux derived from marine rather than terrestrial sources sediment accumulation rates were quantified by 210 pb geochronology at 14 stations ranging in water depths from 173 to 503 m along a south north latitudinal gradient of 6 75 81 n the average sediment accumulation rate for all stations is 0 7 0 4 mm yr in general lower sediment accumulation rates are associated with coarser sediment fractions at shallower water depths 200 m where currents remove fine grained sediments and transport material to the deeper regions higher sediment accumulation rates are detected in deeper water stations and are largely associated with specific sedimentation events and or features of the barents seafloor although the c n ratio of surface sediments 0 2 cm from all stations indicates a predominance of sediments of marine origin c n 9 0 1 0 there is no distinct depositional pattern indicating preferential burial in areas more heavily influenced by the marginal ice zone we conclude that a combination of mixed sediment sources large shifts in the location of the marginal ice zone over time and the benthic boundary layer processes obscure the productivity signal preserved in these seafloor sediment deposits of the western barents sea

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

archaeomagnetic secular variation in germany during the past 2500 years summary the german archaeomagnetic data set was supplemented with 35 new directions from ger man sites mainly dating from the past 3000 yr together with archaeomagnetic data from the neighbouring countries of germany a database of 166 reliable archaeomagnetic directions has been compiled for the reference area which is defined by a 500 km radius circle around gottingen the retrieved directions come from well dated archaeological structures and about 40 per cent of the dating relay on natural science methods such as radiocarbon thermolumines cence dendrochronology dating or historical documents from this data set a secular variation sv reference curve has been calculated using a bivariate algorithm which fits a natural cubic spline based on roughness penalty to declination inclination and time simultaneously the error tube surrounding this curve was obtained from bayesian modelling of the experimental errors which can also take stratigraphic information into account the obtained sv reference curve for the past 2500 yr is similar to that from france but also significant differences are seen comparison of the curves does not show a simple westward drift of the sv pattern the german reference curve allows archaeomagnetic dating in the reference area and extends this dating technique to sites situated in middle europe

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

high quality microwave archaeointensity determinations from an early 18th century ad english brick kiln summary samples of bricks from the dogmersfield park post medieval brick kiln hampshire uk have been used to determine the intensity of the geomagnetic field during the early 18th century ad extensive magnetic mineralogical analyses have been carried out revealing weathered zones and multidomain dominated areas within the bricks likewise statistical data treatment has been used to identify mineralogical heterogeneities once the most suitable parts for the study were selected archaeointensity determinations were carried out applying coe s double step variant of the thellier method the 14 ghz microwave system used in this study was developed at liverpool university and the thermal demagnetizer is a commercial instrument the former uses microwaves instead of heat to demagnetize the sample the latter uses heat and was used for comparison data from the microwave system were of very high quality and uncertainties assigned to single measurements are smaller than the intrinsic variability of the archaeointensity stored within the bricks both ptrm and ptrm tail checks rarely fail and even pass when the sample is almost fully demagnetized microwave intensities group around a central value 53 t slightly higher than the present day field intensity and agree well with previous studies and geomagnetic field models thermal results exhibit higher and more scattered values some samples show evidence of laboratory thermal alteration the different behaviour of samples when subjected to microwave and thermal experiments appears to be linked with the magnetic mineralogy high ti titanomagnetite easy microwave demagnetization without alteration is achieved whereas sample alteration is almost unavoidable during thermal demagnetization high ti titanomagnetite seems to be the ideal mineralogy for microwave experiments instead of pure magnetite preferred in conventional thellier experiments the last part of this paper discusses the usefulness of conventional alteration tests applied to microwave experiments it appears that for this technique negative alteration checks can originate from poor reproducibility of the absorbed microwave power

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

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

reliability of geomagnetic paleointensity data the effects of the nrm fraction and concave up behavior on paleointensity determinations by the thellier method abstract to test the reliability of the thellier method for paleointensity determinations we studied six historic lavas from hawaii and two gauss age lava flows from raiatea island french polynesia our aim is to investigate the effects of the nrm fraction and concave up behavior of nrm thermal remanent magnetization trm diagrams on paleointensity determinations for the hawaiian samples the paleointensity results were investigated at both sample and site levels for consistency and confidence in the paleointensity results it is important to measure multiple samples from each cooling unit the results from the raiatea island samples confirm that reliable paleointensities can be obtained from nrm trm diagrams with concave up curvature provided the data are accompanied by successful partial trm ptrm checks and no significant chemical remanent magnetization crm production we conclude that reliable determinations of the paleofield strength require analyses of linear segments representing at least 40 50 of the total nrm this new criterion has to be considered for future studies and for evaluating published paleointensities for calculating average geomagnetic field models using this condition together with other commonly employed selection criteria the observed mean site paleointensities are typically within 10 of the definitive geomagnetic reference field dgrf our new results for the hawaii 1960 lava flow are in excellent agreement with the expected value in contrast to significant discrepancies observed in some earlier studies overestimates of paleointensity determinations can arise from cooling rate dependence of trm acquisition viscous remanent magnetization vrm at elevated temperatures and trm properties of multidomain md particles these outcomes are exaggerated at lower temperature ranges therefore we suggest that provided the ptrm checks are successful and there is no significant crm production it is better to increase the nrm fraction used in paleointensity analyses rather than to maximize correlation coefficients of line segments on the nrm trm diagrams we introduce the factor q n q to assess the quality of the weighted mean paleointensity hw for each cooling unit

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

recent eruptive history of stromboli aeolian islands italy determined from high accuracy archeomagnetic dating 1 the large sample method of archeomagnetic dating was applied to stromboli 127 samples 10 sites yielded paleofield directions with 95 confidence intervals less than 2 coupled with a reference curve for secular variation in western europe these allow accurate dating of volcanic events a lava flow underlying san bartolo village gave a minimum age of ad 100 100 a lava overflow predating the last sector collapse was dated from 1350 60 suggesting the recent occurrence of highly hazardous events lava spatters and a hot avalanche high on the northern flank were emplaced during the xxth century lava spatters on the lower flanks could date the onset of the still ongoing phase from ad 550 50 these results are different from those obtained in a recent study probably because traditional paleomagnetic sampling cannot yield sufficient precision considering the characteristics of the archeomagnetic secular variation curve

holocene paleointensities thellier experiments on submarine basaltic glass from the east pacific rise a complete description of the geomagnetic field requires both paleodirectional and paleointensity data although the paleointensity data base has grown steadily over the last three decades it remains limited in time and space the majority of data are of holocene age and come from europe furthermore it has been difficult to assess the reliability of the paleointensity determinations here we present paleointensity determinations on precisely dated holocene 0 to 3500 years old submarine basaltic glass from the east pacific rise 15 s to 22 s although hysteresis measurements and low temperature isothermal remanent magnetization irm acquisition experiments document a significant contribution of superparamagnetic grains high blocking temperatures above 400 c and curie temperatures between 490 c and 550 c indicate a single domain low ti magnetite as the carrier of the remanent magnetization this notion is further supported by the fact that saturation of remanence is achieved in moderate fields of about 200 300 mt submarine basaltic glass proves to be nearly ideal for paleointensity determinations in that it produces a high success rate for thellier experiments twenty six out of 30 samples resulted in acceptable paleointenisty determinations multiple experiments on splits from the same sample show good reproducibility the paleointensities for zero age glasses correspond precisely with the present field intensity at the site of recovery the results of the remaining samples range from 16 7 to 53 9 t with corresponding virtual axial dipole moments vadm of 3 61 1022 to 11 9 1022 a m2 the intensities vary rapidly with time excluding a westward drifting nondipole component as the source for these fluctuations basaltic glass is frequently recovered in both dredged and drilled material from the ocean floor the availability of submarine basaltic glass throughout the world oceans therefore holds great potential for a better distribution of paleointensity data through time and space