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

the magnetic properties of baked clays and their implications for past geomagnetic field intensity determinations summary burnt clays provide a vital source of information about the archaeomagnetic field but their magnetic properties and the dependency of these on thermal history are diverse complex and poorly understood here we attempt to shed light on this problem through the investigation of artificial clay samples prepared from three different clay types repeatedly heated in known magnetic field to two different temperatures 400 and 700 c combined rock magnetic and x ray analyses were carried out to obtain information about the mineralogical content and magnetic properties of the diverse raw and heated clays and also their evolution during the course of multiple heating cooling treatments the magnetic behaviour of the three clay types evolved significantly during the course of being repeatedly heated to both 400 and 700 c phyllosilicates containing iron substitutions in their matrix apparently played an important role in supplying iron oxides during the heatings and the iron oxides themselves underwent progressive oxidation the samples heated to the higher temperature exhibited more ideal magnetic behaviour but even those heated only to 400 c achieved magnetic stabilization after multiple heatings after 10 reheating treatments samples heated to both peak temperatures yielded reliable palaeointensity results and a grand mean intensity value fa 48 57 1 19 twhichdiffersbylessthan2 t orabout3percent fromtheknownintensity of the inducing field the results confirm that the thermoremanent magnetization produced as a result of multiple heatings even to moderate temperature in the antiquity can give reliable palaeointensity determination they also highlight that materials from repeatedly used baked clay structures kilns hearths etc may be far more appropriate for archaeointensity study than singly baked clay structures destruction layers floor plasters etc

evidence of electro seismic phenomena abstract ground based observations of electromagnetic precursors to large earthquakes have been reported in japan iran chile greece and the soviet union ionospheric disturbances measured by soviet satellites have been claimed to be seismic precursors in addition observations of very low frequency anomalies measured by geos satellites have been reported by french researchers to confirm reports of electromagnetic precursors to earthquakes a series of low frequency broadband radio frequency receivers were deployed along the san andreas fault system this system of monitors has recorded electromagnetic anomalies occurring just before earthquakes between 1983 and 1986 these anomalies are of two types and are presumably caused by separate effects both of which may be related to stress induced changes in the crust just before fault rupture the first of these types of anomalies appears to be an interruption in the propagation of radio waves radio depressions lasting for many hours before an earthquake the second type is similar to those in previously published reports and consists of short duration broadband radio emissions given the results reported by other researchers and the results from our own experiments we believe that electromagnetic data may be useful as a tool for predicting earthquakes

investigating the reasons for the failure of palaeointensity experiments a study on historical lava flows from mt etna italy summary a refined palaeointensity experiment accompanied by rock magnetic studies has been carried out on six lava flows from 1910 and 1928 from mt etna the purpose of the study was to try to understand why these very young basaltic flows are generally unable to provide a correct estimate of the magnitude of the ambient magnetic field during flow cooling susceptibility versus temperature curves and ore microscopic studies show that 3 types of magnetic minerals phases h m and l are present in these flows some samples containing a single largely dominant magnetic phase while others contain a mixture of several phases phase h i s a thermally stable near magnetite phase resulting from titanomagnetite oxyexsolution phase l is a thermally stable titanomagnetite with a curie temperature of approximately 200 c phase m is a titanomagnetite phase of curie temperature between 450 490 c which is unstable at temperatures above 400 c in addition to the usual reliability checks of the thellier method nrm trm linearity ptrm checks our palaeointensity experiments included additional heating allowing determination of the md or psd sd character of each ptrm and determination of crm or transdomain remanences possibly acquired during heating from the 28 samples studied 20 provide a linear nrm trm plot over about 1 4 or more of total nrm however only six of them all containing near magnetite as a single phase display positive ptrm checks nevertheless these six samples yield a mean palaeointensity of about 52 t which exceeds the real field palaeomagnitude 42 t by some 25 per cent the reasons for this almost total failure of palaeointensity experiments are diverse for samples with a dominant l phase ptrms present a behaviour typical of large md grains with as much as 1 3 of remanence with unblocking temperatures exceeding the blocking range no crm is acquired yet a remanence does develop during heating in a field followed by cooling in zero field we suggest that this remanence is a transdomain remanence resulting from domain rearrangements these two observations are in conflict with some of the basic requirements of the thellier method phase m seems chemically stable up to 400 c but ptrm changes start at lower temperatures this thermal instability and the development of a significant transdomain remanence seem to be the causes of the failure of palaeointensity experiments the reason for the rather large and quite unexpected error in the average palaeointensity provided by the samples containing only a near magnetite phase h may lie in the fact that the low to medium temperature ptrms which represent a significant fraction of the total trm seem to be carried by small md particles in conclusion several modifications of the thellier method are proposed

different mechanisms of magnetisation recorded in experimental fires archaeomagnetic implications abstract we present here the archaeomagnetic and rock magnetic results obtained from the experimental recreation of prehistoric fires on a clayish soil substratum by using a system of thermocouples that allows the control of temperatures and heat penetration with depth the magneto mineralogical transformations as well as the ability to acquire magnetic remanences are studied a detailed set of rock magnetic analyses carried out both on surface and depth with a millimetre scale resolution shows the creation of post burn magnetite over the entire surface of the hearth thus promoting a distinguishable magnetic enhancement on the basis of a detailed control of temperatures interesting differences between areas centre vs periphery were observed and two different mechanisms of magnetisation have been recorded the central area which exceeded 600 c acquired a thermal remanent magnetisation trm and the periphery which was heated up to 300 c recorded a thermo chemical tcrm remanent magnetisation the most noticeable mineralogical changes are focused to the first centimetre of the central area where single domain magnetite has been created the implications of these results for archaeomagnetic and palaeointensity studies as well as their utility for the detection of fire in archaeological sites are discussed

a reliable absolute palaeointensity determination obtained from a non ideal recorder reliable measurements of the intensity of the palaeomagnetic field are notoriously difficult to obtain the approach generally taken is to produce multiple estimates per rock unit and assume that those which meet certain minimum standards of technical quality will combine to produce an accurate mean however here using results produced from a package of 20th century basaltic lava flows from mount etna in sicily we demonstrate that this approach can fail in this case the application of typical sets of selection criteria actually introduces bias into the mean determination that we measure we demonstrate that this is caused by two types of non ideal behaviour acting in combination the first is a result of the multidomain grains that the samples contain and the second is caused by differences in the natural and laboratory cooling rates we discuss means of avoiding these sources of error in future palaeointensity studies performed on ancient rocks we also develop a new more general reliability criterion which is effective here and which we argue should be applied wherever possible in future palaeointensity studies in conjunction with standard criteria it requires two distinct types of materials which both produce some good quality palaeointensity measurements and uses their range of overlap to constrain the true palaeointensity both the application of this criterion and general reliability considerations require that future palaeointensity studies should measure many samples per cooling unit and that these should be as diverse in terms of their rock magnetic properties and cooling histories as possible

reliable absolute palaeointensities independent of magnetic domain state knowledge of palaeointensity variation is required for determining the full vector variation of the geomagnetic field as a function of geological time this provides essential constraints for numerical geodynamo models to date most palaeointensity determination methods are laborious characterised by rather low success rates and demand substantial processing time the rocks under investigation must obey stringent criteria to yield faithful palaeointensities the magnetic particles must be single domain the natural remanent magnetisation must be a thermoremanent magnetisation and during the successive heating steps in the laboratory no chemical alteration should occur here we describe a new method that allows all magnetic domain states to be processed i e it does not require single domain particles the method makes use of the linearity of partial thermoremanent magnetisation ptrm with the applied laboratory field multiple specimens are used so that every sample is exposed only once to a laboratory field warranting that all samples experienced the same magnetic history through the limited number of thermal steps alteration effects are reduced as well the laboratory ptrm and natural remanent magnetisation nrm of the specimens are oriented parallel to minimise the effects of high temperature tails that affect multidomain minerals the ptrm acquisition temperature is selected below the temperature at which chemical alteration sets in and above the temperature trajectory where secondary viscous nrm components occur the procedure requires a lower number of steps than any other palaeointensity method reducing significantly the total time needed per rock unit we propose to name the new protocol multispecimen parallel differential ptrm method i t provides the correct answer to 5 for artificial samples and natural rocks containing multidomain magnetic particles that were given a laboratory trm of known intensity application to the paricutin september december 1943 lava flow three sites 7 specimens per site yields a weighted mean of 45 9 1 25 t within uncertainty margin of the expected value of 45 0 t

partial anhysteretic remanence and its anisotropy applications and grainsize dependence the distribution of remanence coercivities in a rock sample can be obtained by differentiating alternating field af demagnetization curves of anhysteretic remanent magnetization arm however significant errors can be introduced by grain interactions and by the noise inherent in differentiation we have found that 1 this error can be significantly reduced by measuring partial arm s parm s and 2 parm patterns show a strong empirical correlation with grain size parm s are imparted by exposing a sample to a small direct magnetic field in the presence of a large decreasing af the direct field is switched on over a specified range of af s thus magnetizing a fraction of the magnetite grains in the corresponding coercivity range we have further found in both synthetic and natural sediments that measurement of the anisotropy of parm enables us to detect the preferred orientations of distinct subpopulations of magnetite particles related to the size and shape of the grains

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

the earth s dynamic surface an overview debate about the relative roles of catastrophic v continuous processes of landform evolution is as old as the discipline of earth science itself over the last 10 years or so research in the earth sciences has focussed strongly on the earth s surface and particularly in terms of quantifying rates of processes this research parallels developments in geomorphology and sedimentology in the quantification of surface processes since the 1950s and 1960s these surface processes are the manifestation of the large scale interaction of climate and tectonics operating over a wide range of spatial and temporal scales thus recent research had required integration of the historically distinct subjects of geomorphology sedimentology climatology and tectonics partly as a cause and partly as a consequence of this integration there have been many recent developments in quantitative modelling and both laboratory and field based analytical tools together these have provided new insights into absolute and relative rates of denudation and the factors that control the many dynamic processes involved one of the outstanding issues concerns the balance between tectonics climate and denudation and in particular the limiting effects of one on the others and the nature of dynamic feedback mechanisms the fact that processes can be considered catastrophic or continuous depending on the timescale of observation or interest can hinder the predictability of models depending on how they are formulated certain conditions may lead to a steady state situation in which denudation balances tectonic uplift leading to a more or less constant topography steady state topography means that detailed study of present day landforms can provide important insights into the nature of surface processes back in time such assumptions underpin debates in geomorphology relating to the process form linkage and the understanding of characteristic forms in the landscape alternatively the recognition of non steady state situations and a clearer understanding of why these situations occur provide the key for resolving the climate tectonics landscape evolution feedback loop the transition between the two states will reflect the process response time and therefore the transitory state may provide a clearer picture of the time lag of topographic response to changes in the rates of climate change and tectonic forcing however the response time is not necessarily constant and may have changed considerably at key points in the past such as the evolution of plants on land in the palaeozoic and the acceleration of human activity within the landscape in the holocene in terms of denudation physical erosion and chemical and mechanical weathering there are clearly catastrophic processes such as landsliding which operate discretely and on short timescales and more continuous processes such as chemical weathering which can be considerably more protracted the distinction between discrete catastrophe and continuous modification depends also on the time and spatial scales of interest these considerations also impact directly on the questions of if and how steady state topography can be achieved how the processes controlling this state can be quantified and resolved what causes departures from a steady state condition and how topography reflects the coupling between denudation climate and tectonics some of the key current research areas in the world are tectonically active regions such as new zealand southern alps taiwan and olympic mountains usa however the link between tectonics and denudation is complicated in these convergent zones e g willett et al 2001 as there is a significant horizontal component to the deformation and additionally climatic variations often produce marked asymmetry in denudation which itself then feedbacks into the isostatic component of vertical motion in practice this research field necessarily involves a broad range of disciplines including field geologists geomorphologists structural geologists geochemists climatologists and geophysical modellers these researchers address the observational constraints on

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

assessment of the usefulness of lithic clasts from pyroclastic deposits for paleointensity determination 1 paleomagnetic and rock magnetic measurements were carried out on lithic clasts found within pyroclastic deposits to assess their potential for paleointensity determinations the use of multiple lithologies in a single paleointensity determination would provide confidence that the result is not biased by alteration within one lithology lithic clasts were sampled from three historically active volcanoes lascar in the chilean andes mt st helens united states and vesuvius italy at lascar triple heating paleointensity experiments allow development of new selection criteria for lithic clasts found within pyroclastic deposits using these criteria the lascar data yield a mean paleointensity of 24 3 1 3 t 1 n 26 which agrees well with the expected value of 24 0 t this indicates that pyroclastic rocks have promise for paleointensity determinations pyroclastics however still suffer from the range of problems associated with conventional paleointensity experiments on lava flows samples from mt st helens are strongly affected by multidomain md behavior which results in all samples failing to pass the paleointensity selection criteria at vesuvius md grains magnetic interactions and chemical remanent magnetizations contributed to failure of all paleointensity experiments rock magnetic analyses allow identification of the causes of failure of the paleointensity experiments however in this study they have not provided adequate preselection criteria for identifying pyroclastics that are suitable for paleointensity determination

estimating baking temperatures in a roman pottery kiln by rock magnetic properties implications of thermochemical alteration on archaeointensity determinations summary absolute past geomagnetic field intensity determinations requiring laboratory heating are labourious and the success rate is rather low mostly because of induced thermochemical magnetic mineral alterations archaeomagnetic intensity determinations are mainly limited to displaced ceramics produced in kilns in this study the suitability of an in situ baked structure is investigated different magnetic properties of baked material taken from the combustion chamber wall and floor of a roman pottery kiln with variable colouring are examined in dependence on the distance to the combustion chamber the temperature distribution is re constructed based on rock magnetic experiments after stepwise heating the rock magnetic temperature estimates agree fairly well with a mathematical heat conduction model demonstrating the penetration of heat into the combustion chamber wall the rock magnetic results show that blackish and greyish coloured kiln parts that had been in close contact with the fuel during ancient kiln operation are not suitable for intensity determinations although sufficiently baked they strongly alter during laboratory heating and new remanence carrying minerals are formed the brownish coloured material at a distance 65 80 mm away from the combustion chamber seems to be most suitable as its magnetic properties remain nearly unchanged during laboratory heating rock magnetic and modelled temperature estimates for this material consistently indicate ancient baking temperatures of about 600 c the model demonstrates that cooling takes longer in the inner parts of the combustion chamber wall retarded cooling affects the blocking temperatures and hence the strength of the thermoremanent magnetization the variability of cooling rates should be taken into account when investigating archaeointensities of specimens cut from large samples or of samples taken from different parts of a kiln

environmental magnetism principles and applications 1 in environmental magnetism rock and mineral magnetic techniques are used to investigate the formation transportation deposition and postdepositional alterations of magnetic minerals under the influences of a wide range of environmental processes all materials respond in some way to an applied magnetic field and iron bearing minerals are sensitive to a range of environmental processes which makes magnetic measurements extremely useful for detecting signals associated with environmental processes environmental magnetism has grown considerably since the mid 1970s and now contributes to research in the geosciences and in branches of physics chemistry and biology and environmental science including research on climate change pollution iron biomineralization and depositional and diagenetic processes in sediments to name a few applications magnetic parameters are used to routinely scan sediments but interpretation is often difficult and requires understanding of the underlying physics and chemistry thorough examination of magnetic properties and of the environmental processes that give rise to the measured magnetic signal is needed to avoid ambiguities complexities and limitations to interpretations in this review we evaluate environmental magnetic parameters based on theory and empirical results we describe how ambiguities can be resolved by use of combined techniques and demonstrate the power of environmental magnetism in enabling quantitative environmental interpretations we also review recent developments that demonstrate the mutual benefit of environmental magnetism from close collaborations with biology chemistry and physics finally we discuss directions in which environmental magnetism is likely to develop in the future

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

thellier paleointensity theory and experiments for multidomain grains 1 we extend theories of thermoremanent magnetization trm and partial trm ptrm in multidomain md grains to model thermal demagnetization and ptrm acquisition steps in thellier paleointensity determination because of the interleaving of zero field and in field heating cooling steps to increasing temperatures the initial state for any step is complex and theoretical modeling is more intricate than for ptrm production or thermal demagnetization separately at low to moderate temperature t trm lost exceeds ptrm regained causing convex down arai plots sagging below the ideal single domain sd line at moderate to high t ptrm acquisition outweighs trm loss as t approaches tcurie ptrm gain exactly equals trm loss and the arai plot becomes ideal when ptrms are produced perpendicular to the original trm and measured directly rather than by differencing field on and field off results there is less deviation from ideality at low to moderate t our theory agrees semiquantitatively with results for parallel and perpendicular ptrms for large md magnetites 135 m smaller md magnetites 6 and 20 m have less curved arai plots and the smallest magnetites 0 6 and 1 m have almost linear plots positive ptrm checks demonstrate that curved arai plots of md grains are reproducible unlike curved plots for rocks that alter physicochemically in the thellier experiment while negative ptrm tail checks indicate undemagnetized ptrm residuals low temperature demagnetization improves linearity only slightly practical applications of this work include using the predicted threshold t below which no net ptrm is produced in a parallel thellier experiment to screen data used for paleointensity fits straight line fits through low and medium t points in arai plots of md 135 m grains overestimated the paleofield by as much as 100 and for small pseudo single domain psd 0 6 and 1 m grains overestimated by about 25 however by using linear segments of medium to high t data with f values 0 5 it may be possible to obtain reasonable paleointensity estimates even for larger psd 6 and 20 m and md grains middle to high t fits for 0 6 m grains gave paleointensities within 4 of the correct value utilizing essentially the entire data set f 0 9 perpendicular data always gave superior linear fits orienting samples with their natural remanent magnetizations perpendicular to the laboratory field is therefore recommended for rocks containing psd and md grains however double heatings are preferable to single heatings because they allow ptrm tail checks to be carried out

the effects of secondary mineral formation on coe type paleointensity determinations theory and simulation thellier type experiments are the most widely applied approaches for determining the absolute paleointensities of earth s magnetic field one major problem however is that specimens are prone to thermal alteration due to the intensive thermal treatment during experiments linear arai plots with acceptable partial thermal remanent magnetization ptrm checks have been considered as evidence for the absence of or negligible effects of thermal alteration and as reliable indicators of high quality paleointensity estimates however by simulating the coe variant of the thellier method on assemblages of single domain sd magnetite particles it is demonstrated that new magnetic minerals which form during thermal treatments can result in linear concave up or concave down arai plots depending on the magnetic properties of both the primary and secondary magnetic phases among this range of behavior pseudoideal arai plots which are linear with acceptable ptrm check statistics would lead to paleointensity underestimates it is further demonstrated that ptrm checks are proportional to the degree of underestimate with a magnetic granulometry dependency for sd particles due to the complexity of this dependency ptrm check statistics are only comparable when specimens have similar magnetic properties this suggests that a universal threshold for ptrm check statistics is not likely to be effective since the criteria of linearity and low ptrm check statistics are insufficient to guarantee the fidelity of the estimates auxiliary rock magnetic methods such as temperature dependent hysteresis parameters and anhysteretic remanent magnetization are highly recommended to identify the presence of alteration

new high precision full vector archaeomagnetic data from a roman kiln in merida spain abstract this study presents new high precision age and full vector archaeomagnetic data from a kiln excavated in the roman archaeological site of mitreo s house merida badajoz spain the age of the kiln was obtained by the stratigraphic method and by the ceramological study of the italic and sigillata pottery found with a very precise age date of 55 15 years ad rock magnetic experiments pointed towards magnetite as the main carrier of remanence and highly reversible thermomagnetic curves suggested that the studied samples were suitable for archaeoeointensity determinations paleomagnetic experiments including thermal and stepwise alternating field demagnetization yielded the following mean direction for the kiln declination d 0 6 inclination i 54 0 k 481 95 2 5 archaeointensityexperiments with the thellier coe protocol on 28 samples yielded successful determinations in 27 cases anisotropy factors between 0 90 and 1 04 were obtained from anisotropy of thermoremanent magnetization atrm experiments a mean anisotropy corrected archaeointensity value f 56 3 5 5 t was obtained the geomagnetic model sha dif 14 k was used for an archaeomagnetic dating yielding a chronological interval between 40 bce and 150 ce this low age resolution when compared with the pottery based age data is related to the behavior of the geomagnetic field in the iberian peninsula during the roman period which does not allow to differentiate well between results corresponding to those centuries however it is also related to the fact that the iberian archaeomagnetic dataset in the analysed time range is highly scattered a high quality full vector data with a very precise age have been obtained which contribute to improve the iberian secular variation curve and geomagnetic databases

nonlinear thermoremanence acquisition and implications for paleointensity data abstract in paleointensity studies thermoremanence is generally regarded as a linear function of ambient magnetic field at low fields comparable to that of the present day earth we find pronounced nonlinearity at low fields for a class of materials with silicate hosted magnetite that otherwise perform well in paleointensity experiments we model this nonlinearity with narrow size ranges of large acicular single domain grains which are most likely in a vortex state i e nonuniformly magnetized sometimes labeled pseudosingle domain simple trm theory predicts that even certain single domain particles will also exhibit a nonlinear response saturating in fields as low as the earth s such behavior although likely to be rare may bias some paleointensity estimates the bias is especially pronounced when the laboratory field is higher than the ancient field fortunately the fundamental assumption that thermoremanence is proportional to applied field can and should be routinely checked at the end of successful paleointensity experiments by adding two extra heating steps

extended and revised archaeomagnetic database and secular variation curves from bulgaria for the last eight millennia the efforts of geophysicists to describe geomagnetic field behaviour in the past lead to creation of different geomagnetic field models on the other hand the established regional palaeosecular variations of geomagnetic elements are increasingly used for dating purposes in archaeology both of these goals can be achieved if sufficient amounts of long archaeomagnetic data sets exist for different geographical regions the accumulation of archaeomagnetic determinations began at the middle of the last century parallel with the progressive development of experimental methodology and acceptance criteria the presence of great number of old determinations requires their critical assessment the important question about the reliability of the associated dating intervals should be also re assessed all this requires the continuous refinement and extension of the accumulated databases this paper presents the last synthesis of bulgarian archaeomagnetic database and the local palaeosecular variation curves obtained using a statistical treatment based on bayesian approach rencurve software the rock magnetic characteristics of the newly included non published results are summarized

first order symmetry of weak field partial thermoremanence in multi domain md ferromagnetic grains 2 implications for thellier type palaeointensity determination two models derived in very different ways but both incorporating the symmetry constraints outlined in the previous paper are developed for use in simulating thellier type palaeointensity experiments these are then tested against experimental data which endorse them both as powerful tools they predict that non ideal behaviour due to md grain content is sensitive not only to the protocol employed and the vector difference between the partial thermoremanent magnetisations ptrms imparted in the laboratory and the natural remanent magnetisation nrm but also to the spacing of the temperature steps and the type of ptrm imparted during the remagnetisation stages consistent with experimental studies the models predict that ptrm checks may fail despite no alteration occurring and that any double stage experimental protocol will produce non ideal results to some degree provided that md grains are present a single heating perpendicular or high angle single heating protocol is predicted to remove non ideal effects provided that the grains obey the symmetry constraints perfectly however this remains to be experimentally tested and does not allow for the use of ptrm checks five straightforward recommendations for future palaeointensity studies to follow are outlined these are designed to minimise non ideal behaviour due to md grains as far as possible while retaining the capacity to detect and reject those samples which will nonetheless produce significant errors

multiple specimen absolute paleointensity determination an optimal protocol including ptrm normalization domain state correction and alteration test abstract a recent proposal of a multiple specimen technique promises to be a viable alternative to the classical thellier thellier method of absolute paleointensity determination however to exploit the full potential of the multiple specimen approach a thorough understanding of its theoretical foundation and a detailed experimental verification of its implicit assumptions is required here the validity of the multiple specimen technique is studied on a collection of synthetic samples covering grain sizes ranging from single domain sd over intermediate pseudo single domain psd to multidomain md the experimental data indicate that the multiple specimen method in its present form systematically overestimates paleointensity for intermediate psd to md particle sizes this finding is investigated theoretically by a statistical theory of weak field thermoremanence and quantified by a phenomenological thermoremanence model based on this theoretical framework and on the new experimental evidence an extended version of the multiple specimen technique is designed which is more reliable in the critical domain state range the new measurement scheme improves normalization and quantifies the psd and md overestimate which then can even be corrected for furthermore the proposed measurement scheme includes a thermal repeat measurement to assess the effect of alteration upon the accuracy of the final paleointensity result the new technique is verified experimentally for the synthetic samples investigated

are ceramics and bricks reliable absolute geomagnetic intensity carriers a detailed rock magnetic and archeointensity study was carried out on materials baked by a western mexican artisan following traditional techniques to produce faithful reproductions of archeological pieces of the michoacan region western mesoamerica the field strength at the site 41 0 0 5 lt was measured with a fluxgate magnetometer and the temperature of the furnace during the baking process was monitored continually by means of a thermocouple placed in the middle of the baking cavity rock magnetic experiments performed on the raw material clay and paste and on in situ prepared baked ceramics and bricks included measurement of thermomagnetic curves susceptibility and strong field magnetization versus temperature first order reversal curves forc anisotropy of magnetic susceptibility ams and anisotropy of thermoremanent magnetization a trm magnetite and probably hematite are present in the samples as carriers of the remanence hysteresis ratios suggest that the samples fall in the pseudo single domain grain size region which may indicate a mixture of multidomain and a significant amount of single domain grains ceramic pieces and brick fragments were subjected to the thellier coe archeointensity method and to an alternative paleointensity experiment with a triaxe magnetometer in order to check whether they are faithful recorders of the local geomagnetic field strength mean raw intensity of sample m1 pottery overestimates a 7 the expected site intensity while those corresponding to the brick samples lq1 and lq2 underestimate it 15 brick sample lnq shows a slightly lower intensity 7 but agrees with the expected site intensity within the experimental uncertainty the intensity retrieved from the volcanic fragment also included closely reproduces the expected intensity after a trm and cooling rate corrections all mean raw values move closer to the expected intensity measurement of temperatures at different parts inside the kiln bottom and upper parts of both central and peripheral parts revealed the existence of significant thermal gradients similar to those observed in ovens from other localities different cooling rates are then expected in a single oven the scatter in the intensity determinations observed in this study retrieved from pieces elaborated together in the same oven could arise from this differentiated cooling rate within the oven and thus to an inappropriate cooling rate correction in the archeointensity protocol as this situation was probably reproduced in the baking of ancient ceramic artifacts a better knowledge of the temperature distribution inside these types of kiln would be desirable in order to choose the appropriate cooling rate correction 2011 elsevier b v all rights reserved

effect of multi cycle heat treatment and pre history dependence on partial thermoremanence ptrm and ptrm tails abstract we test two fundamental assumptions embedded in thellier experiments the initial state dependence and the effect of multi cycle heat treatment we observe that the magnitude of partial thermoremanent magnetizations ptrms imparted on an initial state of thermal demagnetization is larger than those of ptrms in the presence of a trm when the field used to impart ptrm is equal in magnitude and parallel to that used to produce trm a multi cycle thellier analysis on coarse grained magnetites progressively produces more intense ptrms and progressively erases more of the ptrm tails both pre history and multi cycle dependence will likely enhance the non linear features of the arai plot for coarse grained magnetites

detecting multidomain magnetic grains in thellier palaeointensity experiments the thellier technique is the most commonly used method for absolute palaeointensity studies it is based on thellier s laws stating that partial thermoremanent magnetisations ptrms are independent and additive which however only are true for non interacting single domain sd grains for multidomain md grains individual ptrms are not independent and thellier palaeointensity experiments on rocks with a significant proportion of md grains may therefore lead to erroneous palaeofield estimates by introducing a ptrm tail check in the coe version of the thellier technique j geophys res 72 1967 3247 we present a simple method to detect and discard such rocks the usefulness of the ptrm tail check is exemplified by thellier palaeointensity experiments performed on two palaeomagnetic sample collections with different magnetomineralogy miocene baked sediments from french massif central are shown to be ideal for thellier studies with the remanent magnetisation residing in thermally stable sd grains for palaeocene eocene basaltic lavas from faeroe islands 64 of otherwise acceptable palaeointensity estimates are discarded due to the failure of ptrm tail checks indicating the presence of significant concentrations of md grains 2001 elsevier science b v all rights reserved

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

factors determining magnetic enhancement of burnt clay from archaeological sites abstract the magnetic susceptibility of a collection of 212 archaeological samples of burnt clay of different origin burnt soil ovens and bricks taken from 60 archaeological sites from bulgaria were studied in order to determine factors which may have affected their magnetic enhancement the highest magentic susceptibility values were obtained for bricks and the lowest for burnt soils suggesting that the firing temperature and the duration of burning are important conditions for determining magnetic enhancement specific spatial pattern of the low field magnetic susceptibility was also observed characterized by separation of sites situated in northern and southern parts of the country divided by the west east trending mountain chain stara planina maximum susceptibility values are obtained for burnt soils in southern bulgaria and minima in northern bulgaria in contrast burnt clay plasters and bricks show the opposite picture it is supposed that the observed pattern is due to differences in the outcropped geological formations and variable climate conditions between the two regions which determine different iron supply during burning of initial clay material results from the thermomagnetic analyses show that the dominant ferrimagnetic mineral in all burnt clay materials studied is magnetite titanomagnetite with low ti content and different grain sizes high values of frequency dependent magnetic susceptibility indicate the presence of significant amount of fine grained d

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

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

a 400 ka tephrochronological framework for central america from lake peten itza guatemala sediments highlights dating 400 ka paleoclimate record of neotropics revision and new eruptive volumes for large central american eruptions age models for peten itza sediments linking lacustrine ash inventory to eruptions from central america and mexico abstract lake peten itza northern guatemala lies within a hydrologically closed basin in the south central area of the yucatan peninsula and was drilled under the auspices of the international continental scientific drilling program icdp in 2006 at 16 55 n latitude the lake is ideally located for study of past climate and environmental conditions in the neotropical lowlands because of its great depth 160 m lake peten itza has a record of continuous sediment accumulation that extends well into the late pleistocene a key obstacle to obtaining long climate records from the region is the difficulty of establishing a robust chronology beyond 40 ka the limit of 14c dating tephra layers within the lake peten itza sediments however enable development of age depth relations beyond 40 ka ash beds from large magnitude pleistocene to holocene silicic eruptions of caldera volcanoes along the central american volcanic arc cava were found throughout drill cores collected from lake peten itza these ash beds were used to establish a robust chronology extending back 400 ka we used major and trace element glass composition to establish 12 well constrained correlations between the lacustrine tephra layers in lake peten itza sediments and dated deposits at the cava source volcanoes and with their marine equivalents in eastern pacific ocean sediments the data also enabled revision of eight previous determinations of erupted volumes and masses and initial estimates for another four eruptions as well as the designation of source areas for 14 previously unknown eruptions the new and revised sedimentation rates for the older sediment successions identify the interglacial of mis5a between 84 and 72 ka followed by a stadial between 72 and 59 ka that corresponds to mis4 we modified the age models for the lake peten itza sediment sequences extended the paleoclimate and paleoecological record for this neotropical region to 400 ka and determined the magnitude and timing of cava eruptions