geomagnetic intensity variation during the last 4000 years abstract thellier type determinations of the ancient geomagnetic intensity are reported for a large number of samples from greece comprising ancient pottery and other archaeologically dated forms of baked clay comparison is made with results for other parts of the world of about the same latitude the near east china and the western u s a these data indicate that in the time range studied variation of the geomagnetic dipole moment is unlikely to have been the dominant influence it is suggested that the westward drift of a non dipole disturbance could have been responsible

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

palaeointensity results for historic lavas from mt etna using microwave demagnetization remagnetization in a modified thellier type experiment summary the bane of conventional thellier type palaeointensity experiments is the thermal alteration of the samples during experimentation however high frequency microwaves can be used to stimulate the magnetic minerals directly thus eliminating the need for the bulk sample to be heated walton et al 1993 the microwave technique has been successfully applied to ceramics shaw et al 1996 1999 and here we present results for historic lavas from mt etna sicily 20 samples were randomly selected from 10 diverent historic flows dating from 1843 to 1983 hysteresis parameters were monitored as a check for alteration during experimentation and minimal alteration was found rock magnetic analysis and af plus microwave demagnetizations were carried out on all samples and microwave intensity analysis was carried out on those samples that were stable to microwave demagnetization 16 in total with one exception all samples gave high quality intensity results two or three subsamples from each sample were investigated and a weighted average calculated to give a single estimate of the field when the intensity values were compared with the sv model based on direct observatory measurements bloxham gubbins 1985 it was found that those samples that contained a high multidomain md component gave intensity values around 20 per cent lower than expected this can be explained by the cooling rate evect dodson mcclelland brown 1980 and or the presence of an md component mcclelland et al 1996 those samples with the lowest numbers of md grains gave field values which within error were the same as those for the model or slightly too high this is compatible with the cooling rate evect

tectonics of the southern portion of the ribeira belt apiai domain abstract the name ribeira belt was originally used in a reference to a broad belt of deformed crust which strikes subparallel to the southeastern brazilian coastline it was originally believed to be neoproterozoic brasiliano panafrican but present investigations indicate that this age seems to be mainly related to the extensive arc related plutonism and tectono metamorphism that affected its most representative formations whereas the time of deposition of the supracrustals would be significantly older in this study the main tectono stratigraphic features of the apiai domain of the ribeira belt are described the lithostratigraphy is reviewed as well the paleogeographic structural geochemical and radiometric data and their implications for the tectonic evolution models for the region the marine sediments of the apiai domain were laid down on the borders of the parana and luis alves cratons and on an intervening mesoproterozoic 1 8 1 1 ga ocean crust they were intensively deformed by ocean closure and collision of the cratonic fragments and they constitute the majority of the local supracrustals designated as the acungui supergroup symmetrical continental passive margin deposits are represented to the northwest by a more proximal facies itaiacoca group probably related to the parana paleocontinent and by the capiru formation to the southeast between them lies the central domain formed by the votuverava group characterized by the remains of gradually deepening carbonatic platform sediments of the lajeado subgroup which grade eastward into distal turbiditic sequences of the ribeira subgroup where ocean floor and immature island arc magmatic rocks are present during the neoproterozoic the apiai domain behaved as a single continental mass into which basic rocks were intruded between 1 1 and 0 85 ga between 0 7 and 0 6 ga the apiai domain evolved into a cordilleran type magmatic arc related to the evolution of a subduction zone possibly situated to the southeast of the luis alves cratonic fragment this subduction caused metamorphism and migmatization of the supra and infra crustal rocks of the region and produced the extensive calc alkaline granitoid plutonism between 0 6 and 0 45 ga the majority of the southeastern continental blocks amalgamated during closure of the adamastor ocean followed by development of a system of regional dextral transcurrent shear zones related to oblique collision and or escape tectonics pull apart basins opened up and were filled with molasse sequences the region was intruded by post tectonic granitoids and then underwent regional cooling

palaeomagnetic investigation of tertiary lava from barrington tops nsw australia using thermal and microwave techniques abstract tertiary lava 53 ma from barrington tops new south wales australia has been studied using conventional thermal and microwave techniques twenty seven flows in two sections were sampled and their rock magnetic characteristics determined on heating the majority of samples exhibited a single curie temperature at around 200 c indicating a titanium rich titanomagnetite some samples exhibited two magnetic phases and a few a single low titanium titanomagnetite phase thermal demagnetisation yielded a mean direction of d 189 5 i 63 6 95 4 3 which corroborates the previous findings of wellman et al geophys j r astron soc 18 1969 371 395 a pilot conventional thellier palaeointensity analysis was unsuccessful mainly due to the samples being highly susceptible to thermo chemical alteration far greater success was achieved 58 using the microwave palaeointensity technique where heating of the bulk sample and hence alteration is vastly reduced palaeointensity estimates range from 3 to 28 t mean 11 5 t which assuming that the remanence is a primary thermal remanent magnetisation indicates a low field intensity a fifth of the present day value in the early tertiary this study demonstrates the applicability of the microwave palaeointensity technique to ancient lava

the cooling rate effect on microwave archeointensity estimates 1 new microwave mw paleointensity data on historical bricks from northeast brazil presented a bias toward higher fields when compared to previous cooling rate corrected double heating paleointensity estimates the same relates to the previously reported values for pottery from southwestern pacific islands a simple theoretical approach suggests that the mw bias in both collections is due to a cooling rate effect on mw estimates we then experimentally corrected the mw cooling rate effect on brazilian fragments increasing the degree of consistency between the previous and new results reducing discrepancies from 25 to 8 results indicate similar experimental behavior between microwave and thermal procedures despite the different ways in which the energy is transferred into the spin system finally they allow cooling times of less than 90 s to be empirically estimated in most of these mw experiments highlighting the need for systematic cooling rate corrections to be applied in similar mw paleointensity studies in the future

archaeological and archaeomagnetic dating at a site from the ager tarraconensis tarragona spain el vila sec roman pottery a very accurate archaeological dating of a roman site in ne spain el vila sec was made based on the typology of pottery artifacts three different phases were identified with activity ranging from the mid 1st century bc to the early 3rd century ad analyses of bricks from kilns at el vila sec produced data on their stored archaeomagnetic vector these data were compared with the secular variation curve for the iberian peninsula and the scha dif 3k regional archaeomagnetic model both the reference curve and the model produced probability distributions for the final period of use for two kilns from the second archaeological phase that were not used during the third phase at a 95 confidence level both time distributions cover a wide chronological range including the presumed archaeological age both the iberian secular variation curve and the scha dif 3k regional model proved to be suitable models for dating the site although on their own they do not produce a single unambiguous solution this archaeomagnetic approach could also be applied to neighbouring archaeological sites that have an imprecise archaeological age

deformation in the vicinity of ice divides calculs numeriques aux elements finis de la variation de la vitesse horizontale avec la profondeur au voisinage d une difference de la glace symetrique isotherme et ou glissement sur un lit plat

timing of terrane accretion in the neoproterozoic eopaleozoic ribeira orogen se brazil abstract new id tims u pb geochronological data of the oriental terrane combined with detailed geological data of other terranes support the presented tectonic evolutionary model for the central segment of the neoproterozoic ribeira belt southeastern brazil that can be subdivided in four major tectonic units a the occidental terrane regarded as the reworked margin of the sao francisco craton to the northwest b the paraiba do sul klippe c the oriental terrane comprising arc complexes and d the cabo frio terrane to the southeast the oriental terrane and the cabo frio terrane were progressively accreted to the reworked continental margin of the sao francisco paleoplate the occidental terrane contains pre 1 8 ga basement slivers interlayered with neoproterozoic passive margin successions andrel ndia group the oriental terrane focus of the work was firstly accreted to the sao francisco margin at ca 580 ma it displays three structural domains thrusted to nw toward the reworked sao francisco margin from west to east a the cambuci domain that is interpreted as the fore arc association b the costeiro domain that contains a distal passive margin succession intruded by the ca 790 and 635 620 ma rio negro arc complexes and c the italva domain that comprises marbles psamitic gneisses and amphibolites of ca 840 and is regarded both as the proximal passive margin of the oriental terrane or a back arc basin syn to late collisional granitoids and three metamorphic episodes ca 605 580 and 550 ma indicate a complex history for the collision i they were also described for the kaoko damara and west congo only the youngest one pan african belts located at occidental africa and are supposed to record the main episode of convergence in western gondwana a second collisional stage at ca 530 ma accreted the cabo frio terrane to the oriental terrane and is possibly related with the closure of a back arc basin located between the oriental terrane and kalahari craton that remained open until the ordovician the effects of the collision ii were also detected at the oriental terrane finally late to post tectonic granites marked the tectonic collapse of the orogenic building between 513 480 ma

a new three axis vibrating sample magnetometer for continuous high temperature magnetization measurements applications to paleo and archeo intensity determinations we have developed a new three axis vibrating sample magnetometer which allows continuous high temperature magnetization measurements of individual cylindrical 0 75 cm3 samples up to 650 c and the acquisition of thermoremanent magnetization trm in any direction and field intensity up to 200 t we propose a fast less than 2 5 h automated experimental procedure adapted from boyd s nature 319 1986 208 209 modified version of the thellier and thellier ann geophys 15 1959 285 376 method which provides continuous intensity determinations over a large typically 300 c temperature interval for each sample this procedure allows one to take into account both the cooling rate dependence of trm acquisition and anisotropy of trm several examples of analyses of ancient magnetization demonstrate the quality and reliability of the data and illustrate the promising potential of this new instrument in paleo and archeomagnetism

time variations in geomagnetic intensity 1 after many years spent by paleomagnetists studying the directional behavior of the earth s magnetic field at all possible timescales detailed measurements of field intensity are now needed to document the variations of the entire vector and to analyze the time evolution of the field components a significant step has been achieved by combining intensity records derived from archeological materials and from lava flows in order to extract the global field changes over the past 12 kyr a second significant step was due to the emergence of coherent records of relative paleointensity using the remanent magnetization of sediments to retrace the evolution of the dipole field a third step was the juxtaposition of these signals with those derived from cosmogenic isotopes contemporaneous with the acquisition of records new techniques have been developed to constrain the geomagnetic origin of the signals much activity has also been devoted to improving the quality of determinations of absolute paleointensity from volcanic rocks with new materials proper selection of samples and investigations of complex changes in magnetization during laboratory experiments altogether these developments brought us from a situation where the field changes were restricted to the past 40 kyr to the emergence of a coherent picture of the changes in the geomagnetic dipole moment for at least the past 1 myr on longer timescales the field variability and its average behavior is relatively well documented for the past 400 myr section 3 gives a summary of most methods and techniques that are presently used to track the field intensity changes in the past in each case current limits and potential promises are discussed the section 4 describes the field variations measured so far over various timescales covered by the archeomagnetic and the paleomagnetic records preference has always been given to composite records and databases in order to extract and discuss major and global geomagnetic features special attention has been devoted to discussing the degree of confidence to be put in the data by considering the integration of multiple data sets involving different techniques and or materials

holocene glacier and climate variations in vestfirðir iceland from the modeling of drangajokull ice cap abstract drangajokull is a maritime ice cap located in northwest vestfir ir iceland drangajokull s evolution is therefore closely linked to atmospheric and ocean variability in order to better constrain the holocene climate and glacier history of vestfir ir we model the past evolution of drangajokull ice cap simulations from 10 ka to present are forced by general circulation model output ice core based temperature reconstructions and sea surface temperature reconstructions based on these 10 thousand year simulations drangajokull did not persist through the holocene we estimate that air temperatures were 2 5 3 0 c higher during the holocene thermal maximum than the local 1960 1990 average simulations support drangajokull s late holocene inception between 2 and 1 ka though intermittent ice likely occupied cirques as early as 2 6 ka drangajokull is primarily a little ice age ice cap it expanded between 1300 and 1750 ce with the most rapid growth occurring between 1600 and 1750 ce the maximum holocene extent of drangajokull occurred between 1700 and 1925 ce despite the lowest late holocene temperatures occurring between 1650 and 1720 ce between 1700 and 1925 ce temperatures were likely 0 6 0 8 c lower than the 1950 2015 reference temperature the modern equilibrium line altitude ela is bracketed by topographic thresholds a 1 c temperature increase from the modern ela would eliminate the ice cap s accumulation area while a reduction of 0 5 c would lead to the rapid expansion of the ice cap across vestfir ir the proximity of drangajokull to topographic thresholds may explain its late inception and rapid expansion during the little ice age

correlation of neoproterozoic terranes between the ribeira belt se brazil and its african counterpart comparative tectonic evolution and open questions

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

comparison of the coe thellier thellier and microwave palaeointensity techniques using high titanium titanomagnetites results from a tertiary basaltic intrusion from the sydney basin new south wales abstract conventional thellier and microwave mw palaeointensity experiments accompanied by rock magnetic tests were conducted on samples from an early tertiary ca 49 ma basaltic intrusion the peats ridge pr basalt from the sydney basin thermomagnetic and low temperature susceptibility measurements suggest that the dominant magnetic mineral is ti enriched titanomagnetite tm65 tm50 and hysteresis results indicate that the majority of samples exhibit psd like behaviour some samples show evidence of alteration during laboratory heating but only at temperatures in excess of 400 c no alteration is observed in the dominant temperature range 70 300 c used for calculation of palaeointensities stepwise thermal demagnetisation studies reveal single component after removal of a small viscous reverse polarity magnetisations in all samples with a mean characteristic remanence chrm direction of d i 197 60 95 5 k 62 coe thellier thellier ctt experiments yielded a high success rate with 68 of samples conforming to standard acceptance criteria microwave mw palaeointensity experiments predominantly using the perpendicular applied field technique produced a greater success rate of 82 mean palaeointensities of 27 1 4 4 t n 21 and 27 4 4 0 t n 28 were obtained from the ctt and mw techniques respectively these means could not be distinguished statistically and the respective quality indicators were similar these results suggest that the new microwave technique for calculating palaeointensities is in this case equivalent to the well established ctt technique this is an intuitive outcome given that rock magnetic analyses suggest that alteration does not occur below the curie temperature of the samples our results therefore provide a positive test for the equivalence of the mw and ctt techniques the virtual dipole moment vdm value for the intrusion is 4 7 0 7 10 22 am 2 suggesting that the dipole field was approximately 59 of the present day value at ca 49 ma this provides a valuable data point from the southern hemisphere for a sparsely covered time period in the global palaeointensity database

a 1500 year record of accumulation at amundsenisen western dronning maud land antarctica derived from electrical and radioactive measurements on a 120 m ice core during the nordic epica pre site survey in dronning maud land in 1997 1998 a 120 m long ice core was retrieved 76 00 s 08 03 w 2400 m above sea level the whole core has been measured using the electric conductivity measurement ecm and dielectric profiling dep techniques and the core chronology has been established by detecting major volcanic eruptions in a nearby shallow core radioactive traces from nuclear tests conducted during the 1950s and 1960s have been identified altogether 13 ecm and dep peaks in the long core are identified as originating from specific volcanic eruptions in addition two peaks of increased total activity are identified in the short core accumulation is calculated as averages over the time periods between these dated events accumulation rate is 62 millimetres w eq yr for the last 181 years 1816 a d to present and 61 mm w eq yr for the last 1457 years 540 a d to present our record shows an 8 decrease in accumulation between 1452 and 1641 a d i e part of the little ice age compared to the long term mean

a flow line model for calculating the surface profile and the velocity strain rate and stress fields in an ice sheet a flow line model is presented for calculating the surface profile and the velocity strain rate and stress fields in an ice sheet with given base elevation profile ice thickness at the dome divide flow law parameters mass balance distribution and convergence divergence conditions along the flow line the model which is based on a quasi similarity hypothesis as regards the horizontal velocity depth profiles accounts for changes along the flow line in the depth distributions of temperature normal stress deviators and possible enhanced flow of deep ice of wisconsin origin a curvilinear coordinate system is applied with horizontal axes along flow lines and surface elevation contours respectively the flow equations are reduced to two differential equations one for the surface elevation profile and the other for a profile function that determines the depth distributions of velocities and strain rates the two equations are coupled through a profile parameter that communicates the influence of velocity profile changes to the surface profile equation it is shown that the variation along the flow line of this parameter should also be considered when deriving flow law parameters from ice sheet flow line data for a symmetric dome explicit expressions are derived for the depth distributions of the vertical velocity strain rates and stresses the strain rate profiles display an inflection about half way down the ice sheet and in the case of isothermal ice have surface values 2 2 times their depth averaged values the depth distribution of the vertical velocity indicates that a relatively thick layer of almost stagnant ice is present at the ice sheet base below a dome

iaga paleointensity database distribution and quality of the data set abstract the aim of this paper is to provide details about the design of the paleointensity database and focus on the distribution of the available data set version 2003 of the paleointensity database comprises 3128 data from 215 references only absolute paleointensity determinations from igneous rocks and baked contacts are considered regardless which method of paleointensity determination was used the entries in the database correspond to cooling units all polarities as well as all units are considered normal reverse or transitional the earth s magnetic field is the only surface trace of the working of the geodynamo and is therefore a very important tool for our understanding of the earth s deep interior especially its variation with geologic time however to study the long term variation of the paleomagnetic field non dipole effects have to be removed in recent years the production rate of paleointensity data has strongly increased and the most reliable methods were more frequently used however the present data set still has an important temporal bias with 35 of the rocks having ages less than 1 ma as well as a strong geographic bias towards the northern hemisphere especially north america and europe for most time intervals secular variation cannot be averaged out neither temporally nor spatially therefore interpretation of the averaged field is very limited and is avoided here for the future a harmonization or a combination of all iaga databases would be desirable furthermore the input of raw data at the specimen level would be useful in order to allow reinterpretation of data with more developed and sophisticated methods based on our increasing understanding of rock magnetism finally new reliable data are very much needed

40ar 39ar dating and preliminary paleointensity determination on a single lava flow from chifeng inner mongolia abstract a precise 40ar 39ar age and paleointensity data for the cretaceous lava flow from chifeng southern inner mongolia northeastern china are presented in this study detailed rock magnetic investigations including the variation of magnetization with temperature low temperature susceptibility and hysteresis loops show that pseudo single domain psd grain size high ti titanomagnetite is the main magnetic mineral in the studied lava flow both the microwave and double heating thellier techniques were used to determine the paleointensity yielding mean flow paleointensities of 15 6 3 2 t and 23 9 8 0 t respectively however the paleointensity results using the microwave technique are of higher quality mean q 12 for microwave compared to q 2 for thellier and yield higher internal consistency for the flow mean 21 standard deviation about the mean for microwave compared to 34 for thellier the microwave paleointensity result 15 6 3 2 t is therefore deemed the more reliable estimate for the paleointensity of the niutoushan lava flow 40ar 39ar age determination on the lava flow is 106 42 0 48 ma 2 relative to ga 1550 biotite 98 79 0 96 ma combining our 40ar 39ar dating and paleointensity results with other published paleointensity data suggests that the intensity of the earth s magnetic field during the middle cretaceous normal superchron cns was weak but variable throughout the whole cns

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

transamazonian orogeny in the southern sao francisco craton region minas gerais brazil evidence for paleoproterozoic collision and collapse in the quadrilatero ferrifero abstract the paleoproterozoic transamazonian orogeny yielded a series of discrete orogens in brazil the present field study in the quadrilatero ferrifero qf indicates that the southern sao francisco craton region of the brazilian highlands preserves a portion of one of these orogens two sets of transamazonian structures occur in this region the first consists of northwest verging folds and thrusts affecting supracrustal sequences it is suggested that this set formed in a fold thrust belt setting shortly after 2 125 ga during the closure of a passive margin basin that had initiated along the margins of a preexisting continental mass at ca 2 5 ga the second set consists of structures defining the prominent dome and keel architecture of the qf this set a consequence of the emplacement of basement domes against supracrustal rocks at 2 095 ga may reflect the consequences of orogenic collapse narrow conglomerate filled intermontane basins may have formed coevally with dome emplacement formation of an ocean basin east of the present day sao francisco craton eventually occurred in late mesoproterozoic in effect the transamazonian orogen of the qf represents the collision and collapse stages of a paleoproterozoic wilson cycle the contractional phase of the transamazonian orogeny probably represents accretion of an offshore arc to the eastern and southeastern margin of the present day sao francisco craton region the arc and an associated suture may be traced in the brasiliano pan african orogen east of the sao francisco craton northwards into the northeastern lobe of the sao francisco craton clearly initial assembly of crustal blocks to form a larger continent involving south america occurred during the paleoproterozoic 2 1 ga post transamazonian rifting of this continent created the basins which were later inverted during the brasiliano assembly of gondwana

advances in archaeomagnetic dating in britain new data new approaches and a new calibration curve abstract archaeomagnetic dating offers a valuable chronological tool for archaeological investigations particularly for dating fired material the method depends on the establishment of a dated record of secular variation of the earth s magnetic field and this paper presents new and updated archaeomagnetic directional data from the uk and geomagnetic secular variation curves arising from them the data are taken from publications from the 1950 s to the present day 422 dated entries derived from existing archaeo and geomagnetic databases are re evaluated and 487 new directions added resulting in 909 entries with corresponding dates the largest collection of dated archaeomagnetic directions from a single country an approach to improving the largest source of uncertainty the independent dating is proposed and applied to the british iron age resulting in 145 directions from currently available databases being updated with revised ages and or uncertainties and a large scale reassessment of age assignments prior to inclusion into the magnetic moments of the past and geomagia50 databases from the significantly improved dataset a new archaeomagnetic dating curve for the uk is derived through the development of a temporally continuous geomagnetic field model and is compared with previous uk archaeomagnetic dating curves and global field models the new model arch uk 1 allows model predictions for any location in the uk with associated uncertainties it is shown to improve precision and accuracy in archaeomagnetic dating and to provide new insight into past geomagnetic field changes

establishing a first archaeointensity record for the sw pacific abstract the dearth of archaeomagnetic intensity data from the southern hemisphere is a limiting factor in evaluating models of global geomagnetic field evolution during the holocene here we present high quality microwave archaeointensity data obtained from 34 ceramic fragments 21 archaeological contexts from the duke of york islands fiji and vanuatu sw pacific complementary thellier type experiments corrected for anisotropy give good agreement with the microwave results the majority of the new data prior to 250 ad exhibit significantly lower intensity than predicted by current global field models cals3k 3 and arch3k for the region with an apparent intensity minimum at 250 bc reaching as low as 50 of the present day field strength between 400 ad and 1500 ad the data are broadly consistent with the global field models but with a 20 higher field between 1200 and 1400 ad these new results therefore imply that the geomagnetic field has a greater range of variability than predicted and that further data from the region are essential to better constrain the global field models the results also demonstrate that establishing an archaeomagnetic reference curve for dating sw pacific ceramic artefacts is feasible which has potentially significant implications for pacific archaeological research

local surface mass balance reconstruction from a tephra layer a case study on the northern slope of mýrdalsjokull iceland most icelandic glaciers show high accumulation rates during winter and strong surface melting during summer although it is difficult to establish and maintain mass balance programs on these glaciers mass balance series do exist for several of the ice caps bjornsson and others 2013 we make use of the frequent volcanic eruptions in iceland which cause widespread internal tephra layers in the ice caps to reconstruct the surface mass balance smb in the ablation zone this method requires information about surface geometry and ice velocity derived from remote sensing information in addition the emergence angle of the tephra layer needs to be known as a proof of concept we utilize a prominent tephra layer of the m rdalsjokull ice cap to infer local smb estimates in the ablation area back to 1988 using tephra layer outcrop locations across the glacier at different points in time it is possible to determine local mass changes loss and redistribution for a large part of the ablation zone without the use of historic elevation models which often are not available

experimental reassessment of the shaw paleointensity method using laboratory induced thermal remanent magnetization 1 artificial thermoremanent magnetization imposed on natural basalt samples was used to check the reliability of shaw s palaeointensity method shaw 1974 kono 1978 rolph and shaw 1985 and to give an indication of the accuracy of paleointensity determinations first thermal remanence was induced in all samples 50 in total in a known field 50 0 1 t this remanence was regarded as the original natural remanent magnetization nrm the samples were divided in to three subsets a b and c and intensity experiments were performed each subset of samples used different thermal remanent magnetization trm acquisition parameters rock magnetic properties on 14 representative subsamples were also measured both before and after the trm acquisition steps to monitor possible magnetochemical alteration it has been demonstrated that intensity estimates by the shaw method give values close to the expected value 50 5 t in all three sets of basalt samples the modified method with rolph and shaw 1985 correction i e using ratios of anhysteretic remanent magnetization given before the trm step arm1 and after arm2 is proved to be a powerful approach to correct the trm capacity changes that result from magnetochemical alteration this is especially so for sample set c where the samples were heated up to 700 c and then were held at this temperature for 40 min during trm acquisition after correction 45 of set c samples gave acceptable results however some samples with strong magnetochemical alteration sometimes invalidated the arm1 arm2 ratio correction and gave wrong intensity estimates monitoring rock magnetic properties is therefore very important when applying the shaw method

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

using internal layers from the greenland ice sheet identified from radio echo sounding data with numerical models spatially extensive internal layers have been traced in airborne radio echo sounding res data collected over greenland during the late 1990s by linking internal layers within individual flight lines at crossover points it is possible to identify spatiallycontinuouslayersthatareinterpretedas isochronoussurfaces severalof thesur vey lines pass over the grip core site and this allows us to use the published gripage depth relationship to accurately date these surfaces two layers with ages of 3891 and 6956years bp have been traced over a large part of north greenland accurately dated and spatially continuous isochrones are valuable for both assimilation within and verifi cation of numerical models forexample comparison of isochronous surfaces from a nu merical simulation with those layers observed in res data can be used to inform the choice of parameters e g rheology and climate history usedto force a numerical model to demonstrate the potential of the res data two layers for north greenlandwere used to determine palaeo accumulation rates the inversion from layer depthto accumulation rate requires a three dimensional velocity field this velocity field is constructed by com biningatwo dimensionalbalance velocity fieldwithanassumedverticalstructureforthe horizontal velocity the isochronous layer derived accumulation rates were compared with the bales and others 2001 rates a larger east west gradient was found across the central ice divide for the derived accumulation rate suggesting a trend in the holocene accumulation rates for this region the layers were also compared with isochronous sur faces derived from simulations of a three dimensional thermodynamic ice sheet model using the isochronous layer derived accumulation rates to force the model improved the matchbetween modelled and observed layers

anisotropic radio wave scattering from englacial water regimes mýrdalsjokull iceland colinear polarized 5 mhz radar profiling data were obtained on m rdalsjokull a temperate glacier in iceland radar transects and therefore polarization planes were aligned approximately parallel transverse and oblique to the ice flow direction echoes from the shallower half to two thirds of the ice were 10 20db stronger on the oblique and longitudinal transects than those on the transverse transects anisotropy as a function of depth is clearly seen at the sites where the transects cross strong scattering on longitudinal transects apparently caused extinction of a radar reflecting layer that was continuously profiled on the transverse transects a radio wave scattering model shows that scattering from a longitudinal water filled conduit parallel to the glacier surface can explain the observed azimuthal variations of the echo we conclude that low frequency mhz radio waves can help to characterize englacial water regimes

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

holocene climatic changes in iceland evidence from modelling glacier length fluctuations at solheimajokull abstract we use a coupled energy balance glacier flow model to reconstruct holocene climate from a 5000 yr record of glacier length fluctuations at solheimajokull an outlet glacier in southern iceland the climatic reconstruction is presented in terms of equilibrium line altitude and is translated to envelopes of possible temperature and precipitation changes the reconstruction is highly resolved between ad 1700 and 1990 the 20th century retreat of the glacier resulted from a warming between ad 1920 and 1940 while the advance since ad 1970 was in response to both a cooling and a precipitation increase after ad 1940 the early 18th century climate of iceland was warm comparable to that of the early 20th century while the coldest period in recent history occurred during the 1780s earlier holocene glacier advances at about 5000 3100 and 1400 yr bp and in the 10th and 14th centuries ad resulted from cooling of 1 2 c and correlate with worldwide glacier expansion atmospheric changes in greenland and changes in ocean circulation north of iceland glacier expansions follow periods when sea ice is extensive in the greenland and barents sea resulting temperature changes mean that natural climatic variability has a large signature in iceland making it difficult to isolate any longer term trends such as an enhanced anthropogenic signal

dipole strength and variation of the time averaged reversing and nonreversing geodynamo based on thellier analyses of single plagioclase crystals 1 single plagioclase crystals separated from lavas can contain minute magnetic inclusions that faithfully record the geomagnetic field while the silicate host limits natural and experimentally induced alteration here we use paleointensity analyses of plagioclase crystals to examine three characteristic geodynamo regimes spanning the last 200 myr an interval of moderate reversal frequency during the paleogene the nonreversing field of the cretaceous normal polarity superchron and a period of very high reversal occurrence during the late jurassic an inverse relationship between reversal rate and intensity is supported furthermore the strength of the time averaged reversing field is more variable than that of the nonreversing field these observations are consistent with an active lower mantle shaping core mantle boundary heat flux and efficiency of the geodynamo on timescales of tens of millions of years

a comparison of archaeointensity results from chinese ceramics using microwave and conventional thellier s and shaw s methods palaeo and archaeointensity determinations of the geomagnetic field are limited by mineralogical alteration that may occur when samples are heated in the laboratory to produce a trm thellier thellier 1959 shaw 1974 by using microwave excitation of the magnetic grains we have been able to create a trm without significantly heating the bulk samples thereby avoiding thermal alteration walton et al 1993 shaw et al 1996 when applied to peruvian ceramics in a manner analogous to the standard thellier method the microwave technique was found to reduce the scatter dramatically in the archaeointensity results shaw et al 1996 in the present study we apply this microwave technique to a collection of chinese ceramics covering the time interval 2700 7500 yr bp these ceramics have already been investigated using adaptations of both thellier s coe 1967 and shaw s rolph shaw 1985 palaeointensity methods yang et al 1993a although an acceptable agreement was found between those two methods the equivalent virtual axial dipole moments vadms were significantly lower than for the global model of mcelhinny senanayake 1982 the present study using the microwave technique with cooling rate correction has provided more consistent vadms and is much closer to the global model

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

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

microwave archaeointensities from peruvian ceramics summary accurate determination of the strength of the ancient geomagnetic field is limited by mineralogical alteration that may occur when samples are heated to produce a thermoremanent magnetization thellier thellier 1959 shaw 1974 by using direct microwave excitation of the magnetic grains we have been able to form a thermoremanent magnetization without significantly heating the bulk sample thus avoiding thermal alteration walton et al 1993 incorporation of this new microwave heating technique with the conventional thellier palaeointensity method has dramatically reduced the scatter of archaeointensity results from peruvian ceramics and clearly defined how the field strength has varied in peru over the past 2000 years the new microwave technique will therefore enhance the feasibility of archaeointensity dating and provide accurate field strength data for geomagnetic modelling

application of a flow model to the ice divide region of devon island ice cap canada le modele d ecoulement en regime permanent de reeh est utilise pour etudier les distributions des contraintes de la vitesse de deformation et la variation de l epaisseur de la couche annuelle avec la profondeur dans la region de la ligne de partage d une calotte glaciaire sur un terrain accidente

magnetic field intensity study of the 1960 kilauea lava flow hawaii using the microwave palaeointensity technique summary it is extremely valuable to study historic lava flows where the geomagnetic field at their time of extrusion is well known in this study two vertical sections 16 m apart have been sampled from the approximately 1 m thick 1960 kilauea lava flow hawaii variations are seen in the rock magnetic and palaeomagnetic properties between and within the two sections indicating that there are small scale lateral and vertical variations in the lava flow the two sections showed dierent responses to microwave palaeointensity analysis section h6001 generally gave ideal linear behaviour on plots of natural remanent magnetization nrm lost against microwave induced thermoremanent magnetization t m rm gained whilst the majority of samples from h6002 showed anomalous two slope behaviour when all plots were interpreted by taking the best fitting line through all points the flow mean intensity for h6001 was 31 6 3 6 mt and that for h6002 was 37 1 6 4 mt compared with the expected intensity of 36 mt additional historic flows need to be studied in order to ascertain whether this behaviour is typical of all lava and whether it is best to always interpret nrm lost t m rm gained plots by taking the line of best fit regardless of shape

time scale for adjustment of glaciers to changes in mass balance the length of time tm over which a glacier responds to a prior change in climate is investigated with reference to the linearized theory of kinematic waves and to results from numerical models we show the following tm may in general be estimated by a volume time scale describing the time required for a step change in mass balance to supply the volume difference between the initial and final steady states the factor f in the classical estimate of m l u where i is glacier length and u is terminus velocity has a simple geometrical interpretation ft is the ratio of thickness change averaged over the full length i to the change at the terminus although both u and f relate to dynamic processes local to the terminus zone the ratio f u and therefore tm are insensitive to details of the terminus dynamics in contrast to conclusions derived from some simplified kinematic wave models a more robust estimate of tm independent of terminus dynamics is given by tm h b where h is a thickness scale for the glacier and b is the mass balance rate negative at the terminus we suggest that tm for mountain glaciers can be substantially less than the 1o2 103 years commonly considered to be theoretically expected

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

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

are systematic differences between thermal and microwave thellier type palaeointensity estimates a consequence of multidomain bias in the thermal results a meta analysis which directly compares 13 datasets of microwave and thermal thellier palaeointensity measurements obtained from igneous rocks is reported the results clearly indicate systematic differences in the measurements made using the two approaches the thermal results tend to be higher often by several tens of percent and associated with slightly noisier arai plots additionally in nearly half of the cases considered higher thermal palaeointensity estimates were reported to be calculated from the lowtemperature portions of concave up arai plots while microwave measurements made on specimens from the same core sample or rock unit were derived from more linear arai plots the most plausible explanation for the bulk of these differences appears to be that non ideal behaviour caused by multidomain vortex state and or interacting ferrimagnetic grains is exaggerated in the results of the thermal experiments over their microwave counterparts a detailed investigation of the individual studies concludes that such effects may be adversely affecting some of the measurements thermal and or microwave in 10 of the 13 datasets considered it is probable that these non ideal effects are enhanced in the thermal results over their microwave counterparts at least in part because of the different thellier type protocols that are used in the two types of study the findings of this study suggest that biasing of absolute palaeointensity estimates by multidomain effects may be much more prevalent than previously thought and that this effect might be responsible for certain discrepancies that have been observed in the geomagia50 database they also clearly indicate that in addition to taking the steps they already do to avoid biasing from secondary overprints and thermally induced alteration future absolute palaeointensity studies of

investigating the ancient martian magnetic field using microwaves abstract the new microwave palaeointensity technique has been used to investigate samples from the martian meteorite nakhla this technique is a promising new way to obtain absolute palaeointensity information regarding the ancient martian magnetic field as recorded by the martian meteorites assuming that a part of the magnetic remanence is of thermal origin and originating on mars the two samples studied yield estimates of 4 t for the martian magnetic field at 1 35 ga