palaeozoic reefs and bioaccumulations climatic and evolutionary controls the geological record contains a fascinating diversity of reefs and shell accumulations as my other biosedimentary structures their facies characterization requires careful observation at outcrop and sample scale and in thin section to provide information about the global geometries fabrics and textures respectively this collection of papers encompasses the breadth of sedimentary geometries and facies displayed by palaeozoic reefs shell accumulations and transitional composite deposits the definition of reefs and shell concentrations has given rise to variations in nomenclature the papers in this volume cover specific problems regarding the nomenclature and facies characterization of reefs shell accumulations and transitional composite deposits however rather than attempt a complete revision of terms the authors have touched on some of the important issues at this stage of development in the field the main climatic environmental and evolutionary factors that controlled the palaeozoic development of shell accumulation and reefs

how many paleointensity determinations are required from a single lava flow to constitute a reliable average 1 statistical analysis has been undertaken to investigate the minimum number of paleointensity estimates required to allow certainty at the 95 confidence limit that the mean is within 10 of the implied true mean for an entire lava flow the data used for the analysis was provided by previous paleointensity studies which used three different methods thellier coe shaw and microwave thellier on 19 or more samples from three different basalt lava flows the results strongly suggest that future paleointensity studies need to produce many more estimates per lava flow than is currently the norm in order to ensure that the calculated mean paleointensity does not differ significantly from the true flow mean furthermore they indicate that a low measured dispersion for a small number of estimates is not a safeguard against misrepresentation of the flow mean paleointensity finally we provide quantitative evidence that paleointensity studies should always attempt to maximise vertical coverage of lava flows to avoid biasing of the mean

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

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

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

age and archaeological implications of xitle volcano southwestern basin of mexico city the pedregal lavas are fresh well exposed basaltic flows erupted from the xitle scoria and cinder cone in the southwestern part of the basin of mexico these lavas cover an area of 70 km 2 and were emplaced over pyramids and other buildings e g cuicuilco and copilco archaeological sites today a part of mexico city including the national university is built on the flows initial strombolian activity produced an ash fallout layer which was immediately followed by effusive emplacement of lava flows the xitle cone grew on the north facing slope of ajusco volcano and lava flowed down to the n ne until it reached the basin floor more than 30 radiocarbon dates have been obtained by several workers on charcoal samples from beneath the lava and several ages for the eruption have been proposed from these dates most dated samples were not directly produced by xitle s eruption but instead are artifacts of human activity that predates the eruption thus these ages mostly about 2000 bp are older than the eruption a new age of 1670 35 years bp ad 245 315 obtained on charcoal samples collected just beneath the lavas is favored for the xitle eruption these samples originated by ignition of vegetation during the emplacement of hot scoriaceous tephra the new age is within the classic period of mesoamerican archaeology whereas the earlier reported ages are at the end of the preclassic the new age carries important implications for the timing of population shifts within the basin of mexico q 2000 elsevier science b v all rights reserved

analysis of long term variations in the geomagnetic poloidal field intensity and evaluation of their relationship with global geodynamics summary the 1167 published cooling unit cu palaeointensity estimates contained in the 400 10 ma portion of the pint global database were rigorously filtered according to accurate age determinations palaeodirectional reliability recognition of polarity and the method of palaeointensity acquisition the remaining 865 estimates group 1 were further filtered to ensure self consistency reducing the data set to 425 estimates group 2 group 1 and 2 data were clustered into temporally and or spatially distinct rock suites rs enabling each part of the record to be assessed for potential biasing by overrepresentation of palaeosecular variation psv the record was segmented according to the distribution of the data rather than using arbitrary time windows to ensure quasi consistent behaviour within each segment differences between these segments clearly indicate that a significant long timescale 10 7 and 10 8 yr variation of the mean geomagnetic poloidal field intensity gpfi occurred during the 400 10 ma period and hence that changing lowermost mantle conditions affect the capacity of the geodynamo to generate a poloidal field both the mean dipole moment and its standard deviation appear to be a function of the range of values each cu may adopt at one particular time this range is itself controlled by the variation of the maximum limit of dipole moment while the value of the minimum limit remains relatively constant tentative support is provided for the recent suggestion that psv may have been reduced during the cretaceous normal superchron cns though more data are needed in the range 120 60 ma to confirm this no conclusive evidence was found to support the suggestion that the gpfi record may be biased towards low or high values by palaeointensity determinations obtained using methods that do not adopt ptrm checks indeed offsets caused by unreliable data in well represented parts of the record are likely to be random and cancel one another out when gpfi variation is analysed at a sufficiently high resolution to allow comparisons with the geomagnetic polarity reversal frequency rf it is not possible to confirm whether the two parameters are anticorrelated decoupled or related in some more complex way however it is clear that gpfi and rf are definitely not positively correlated as has been previously suggested the present database documents sharp increases in gpfi around the onset times of the two recognized superchrons itself implying an anticorrelation the implications for geodynamo and mantle modelling of both an anticorrelation and a decoupling of the geomagnetic parameters are discussed briefly a generic geodynamic model is proposed to explain the relationship between observed longterm changes in gpfi and global geodynamic processes the model predicts that changes in gpfi result from a chain of geodynamic processes extending from crust to core beginning with plate reorganizations at the surface and culminating in increases in the vigour of outer core convection supercontinents are transient surface expressions of such geodynamic processes and provide the potential to test the generic model four time stages are proposed to describe the major long term changes in gpfi since the early devonian 400 350 350 250 250 175

palaeointensity record through the lower mammoth reversal from the waianae volcano hawaii the normal to reverse lower mammoth reversal 3 33 ma has been recorded in several sequences of lava from the waianae volcano the island of oahu hawaii 137 samples from 29 flows from the pu u paheehee section have been the subject of a palaeointensity study using the microwave technique duplicate sister samples from the directional study of herrero bervera and valet earth planet sci lett 171 1999 139 148 were used microwave demagnetisation was carried out on all samples and the directions compared to the published flow mean directions microwave palaeointensity experiments were carried out on all accepted samples using the 8 2 ghz and 14 ghz microwave systems the perpendicular applied field palaeointensity method and a coe analogue method were used eighty four samples from 24 flows gave acceptable palaeointensity results the results indicate that the geomagnetic field was low mean 5 9 1 3 t n 7 prior to the transitional directions during the first stage of the reversal the field remains low results however could only be obtained from three transitional flows the field then strongly recovers with very high intensity 70 t and reversed direction after this the intensity decreases before the field becomes transitional again for a cluster of four flows the field does not reduce as much as previously rather it is about twice the pre reversal intensity for the final section of reversed flows the intensity is more than twice the pre reversal mean value 15 1 5 9 t n 7 whilst some similarities are seen between this reversal and other reversals of different ages and locations there is not enough data at present to say whether there is any systematic behaviour

controls on the explosive emplacement of diamondiferous kimberlites new insights from hypabyssal and pyroclastic units in the diavik mine canada abstract kimberlites are mantle derived magmas that either crystallise as hypabyssal intrusions erupt explosively after rapid ascent to the surface or less commonly form lava lakes and flows thereby creating texturally distinct kimberlite units efforts to fully understand the processes responsible for the explosive eruption of kimberlite magmas have been hindered by the widespread alteration and crustal contamination of most volcaniclastic kimberlites to address this issue we have undertaken a detailed petrographic and mineral chemical study of fresh i e minimally altered pyroclastic and hypabyssal kimberlites hk from the ca 55 56 ma a154 north and south kimberlite pipes in the diavik mine lac de gras canada these localities host exceptionally fresh kimberlites and are therefore ideally suited to this study kimberlite emplacement at a154 north and south initiated with the intrusion of hypabyssal kimberlite external dykes and was followed by the explosive formation of kimberlite pipes and volcaniclastic kimberlite infill subsequent kimberlite magmas intruded the volcaniclastic kimberlite units forming multiple cross cutting internal dykes the studied volcaniclastic units feature abundant rounded magmaclasts and massive textures suggestive of primary deposits these units are classified as pyroclastic kimberlites pk pyroclastic and hypabyssal kimberlite units at diavik exhibit subtle mineral compositional differences samples from both internal hk units and pk units feature identical compositions for liquidus olivine rims mg 90 5 0 1 and 90 7 0 2 respectively with a marginally lower mg of 90 2 0 2 in olivine rims from the external hk dykes similarly early formed chromite compositions are the same for internal hk and pk units cr 79 1 3 4 and 78 3 5 7 mg 60 0 1 3 and 60 0 2 2 but differ in the external hk units cr 86 9 2 7 mg 52 8 1 9 the internal hk and pk units also exhibit lower carbonate contents than the internal hk units these compositional differences indicate that the external dykes were probably derived from slightly different primitive melt compositions to those parental to the internal hk and pk units spinel evolutionary trends from chromite to magnesian ulv spinel magnetite mum compositions are indistinguishable in internal hk and pk samples fe3 47 2 5 8 and 49 7 9 3 cr 25 7 11 0 and 17 0 14 0 these results demonstrate that the primitive melt compositions and early magmatic evolution processes are identical for the internal kimberlite units regardless of whether the kimberlite melts erupted explosively or were emplaced as shallow intrusions however magmaclasts in the pk units contain higher abundances of phlogopite

a comparison of thermal and microwave palaeomagnetic techniques using lava containing laboratory induced remanence summary a series of laboratory experiments has been carried out using lava from hawaii and australia containing laboratory induced remanence to compare thermal and microwave palaeomagnetic techniques three samples of hawaiian lava containing a known laboratory induced thermal remanence trm were subjected to microwave palaeointensity experiments using the liverpool 8 2 ghz microwave system conventional modified thellier palaeointensity experiments were carried out on the same three lava samples this time containing a known microwave induced thermoremanent magnetisation tmrm both palaeointensity techniques produced the correct intensity value demonstrating the equivalence of trm and tmrm no indications of alteration during the experiment were evident during the microwave experiments whereas alteration was pronounced after heating to 350 c in the thellier experiments demonstrating that alteration during microwave palaeointensity analysis is reduced when compared to thermal techniques in a second set of experiments six samples three hawaiian and three australian lava samples were given a laboratory induced full trm and a secondary perpendicular partial trm ptrm at lower temperatures sister samples were then demagnetised using thermal and microwave methods analysis of the orthogonal vector plots ovps revealed little difference in separating the two components for five out of the six samples for one sample microwave demagnetisation did not clearly distinguish between the two components with the primary component not being resolved until 74 per cent of the moment had been demagnetised compared to just 4 per cent with thermal demagnetisation for this one sample it appears that the microwave unblocking spectra is not equivalent to the thermal unblocking spectra probably due to a combination of microwave demagnetisation processes occurring comparison of the intensity of magnetisation plots during demagnetisation shows that the relationship between temperature and microwave power appears more logarithmic than linear this study demonstrates the validity of the microwave palaeointensity technique and as long as the primary component of magnetisation can be isolated from multicomponent remanence then the microwave palaeointensity technique can be applied

critical analysis of the holocene palaeointensity database in central america impact on geomagnetic modelling abstract thanks to its rich archaeological heritage central america is a key region to recover the past secular variation of the geomagnetic field this article presents 13 new palaeointensity data on epiclassic 650 900 ce pottery sherds from central mexico archaeointensities were determined using the thellier thellier protocol with anisotropy and cooling rate corrections average results between 25 and 42 t reveals a fast secular variation in the second half of the first millennium ce but are not in agreement with global geomagnetic models that predict a higher geomagnetic field strength to check the reasons of this discrepancy we compiled all intensity data over the last millennia published in central america the bayesian curve calculated from 194 data covering the last 4 millennia highlights a rapid succession of oscillations of the geomagnetic field strength between 20 and 80 t but a critical analysis of the dataset shows a large influence of data quality 74 of them having a poor cooling unit consistency and experimental quality the small number of specimens per cooling unit and the anisotropy correction absent or incorrectly made increase the scatter between data whereas the absence of cooling rate correction biases the dataset towards higher palaeointensity discarding these data results in a lower secular variation by removing most extreme values and several intensity oscillations the weaknesses of the dataset are likely the main reason of the limitations of global models in central america pending the acquisition of new high quality data archaeomagnetic dating seems premature in central america

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

an automated superconducting magnetometer and demagnetizing system summary a novel 3 axis superconducting magnetometer has been constructed using just one squid superconducting quantum interference device sensor the magnetometer has been placed in a large mu metal shield together with a non magnetic oven and 3 axis af demagnetizer once a sample is entered into the system a microprocessor controller executes a predetermined measuring and demagnetizing remagnetizing sequence the system was originally designed automatically to determine field magnitude values from ceramics and igneous rocks but has proved itself to be a useful general purpose palaeomagnetic instrument

archaeomagnetic paleointensity in the american southwest during the past 2000 years abstract an archaeomagnetic paleointensity study was carried out on pottery samples from the hohokam anasazi and mogollon cultures of the american southwest the thellier thellier paleointensity experiment was used on 187 specimens from 77 different sherds derived from 23 archaeological sites interpretations were made for 127 specimens from 54 sherds about a two thirds success rate the median value for the quality factor was 12 the results were corrected for magnetic anisotropy owing to a fabric in the pottery using an easy plane model of magnetization the anisotropy of the hard axis relative to the easy plane of magnetization averaged 31 the correction factor for the paleointensity was typically 5 4 and systematically increased the paleointensities because the laboratory heatings were applied within the easy plane the sample average paleointensities were compiled along with other data from north america the hohokam data were kept as a separate set because of uncertainties concerning the hohokam chronology curves of the virtual axial dipole moment vadm variation for the hohokam and non hohokam data sets were derived using a moving window smoothing technique the hohokam and non hohokam curves show good agreement regardless of which hohokam chronology is used relative paleointensity records from north american lake sediments are also congruent with the archaeointensity results despite the scatter in the archaeomagnetic data the correlation of these records suggests that the secular variation pattern is being recovered the field strength at 300 bc is 30 40 higher than at present a low from 250 to 1250 ad is 10 15 weaker than at present and a peak at 1500 ad is about 30 higher than at present the low from 250 to 1250 ad in comparison with global paleointensity compilations suggests a substantial non dipole field in north america at that time the rate of change of the archaeomagnetic vadm is similar to rates observed today

magnetic fabric and flow direction in basaltic pahoehoe lava of xitle volcano mexico abstract we sampled five basaltic lava flow units from xitle volcano mexico city to study the variation of anisotropy of magnetic susceptibility within their cooling boundaries we find that the mean maximum susceptibility parallels the geologically inferred flow direction in the units that were emplaced on a steeper slope whereas for those on a negligible slope the mean intermediate susceptibility points in the flow direction we propose however that the maximum susceptibility always points in the direction of local movement and that a change in slope produces a deviation of the local motion from that of the unit as a whole the axis of susceptibility closest to the geologically inferred flow direction usually plunges upflow in the basal part of the flow unit comprising an imbrication which clearly marks the flow azimuth of the lava thus the scenario of emplacement may influence the results in a predictable way we suggest that the degree of anisotropy could bear a direct relationship to either the viscosity of the lava the morphology of the flows or both based on a comparison with lavas from azufre argentina and ko olau o ahu volcanoes also we suggest that the shape of the susceptibility ellipsoid may be related to the degree of internal deformation of the lava flows we also compare the two methods currently available to calculate regions of confidence around the mean principal susceptibilities

microwave palaeointensities from a recent mexican lava flow baked sediments and reheated pottery microwave palaeointensity pi estimates have been produced from samples from the 1670 yr old xitle lava flow mexico seventeen out of 19 experiments were successful producing high quality estimates and a mean pi of 58 3 7 9 5 wt within errors of that expected from global data for the time period the dispersion is high but could be reduced to 5 5 wt by using stricter selection criteria previous data obtained by the thellier coe method using samples from the same lava flow were less successful and of much lower quality producing a higher mean of 67 9 7 9 8 wt 25 out of 65 samples this difference is most probably caused by the presence of multi domain particles producing concave up arai plots combined with alteration processes affecting the thellier coe experiments at higher temperatures pi calculations restricted to the initial part of an arai plot therefore tend to overestimate the correct values in addition to lava samples microwave pis were also determined from pottery fragments recovered from the contact zone between the lava flow and the underlying baked sediments which were also studied these materials also provided good quality results with a higher mean pi of 66 8 7 7 1 wt which is still statistically indistinct from that produced by the microwave analysis of the lava samples nevertheless these samples seem to be less suitable for pi determinations because they are characterised by larger magnetic grains and apparently more prone to thermally induced alteration than the lava samples we conclude that in the case of the xitle lava the microwave pi method is superior to the conventional thellier coe method in many respects and that the results produced by the latter method even when satisfying strict acceptance criteria may be unreliable however we also draw attention to the fact that microwave produced pi determinations though of high technical quality may still be prone to inaccuracy when rock magnetic parameters indicate that the material is likely to be a poor pi recorder as for the pottery sediment and some of the lava samples described here 2003 elsevier b v all rights reserved

variations in the geomagnetic dipole moment over the last 12 000 years summary an analysis has been made of archaeointensity data for the past 12 000 years there are 3243 results from diverent areas of the world covering the past 12 000 years of these 2203 are from the european region and 1040 are from the rest of the world the archaeointensity data set analysed in the present study is almost three times larger than that used by mcelhinny senanayake 1982 although there is no major diverence between our global data and the earlier data the data for the non european region have been improved and we now have a data set for asia

refined 3600 years palaeointensity curve for mexico abstract our knowledge of the earth s magnetic field intensity changes over the past few thousand years is still limited because of the uneven spatial and temporal distribution of data which also includes the americas the present study reports 41 new palaeointensity data covering the past 3600 years which together with 38 previously published data of similar quality are used to construct a palaeointensity secular variation curve for central mexico these new data are an important contribution to the global intensity database and will also improve the application of palaeomagnetic dating in mexico which is of interest because of the many holocene monogenetic volcanoes within the trans mexican volcanic belt and of its archaeological heritage the most conspicuous feature of the new intensity curve is the rapid increase between 400 and 250 bce from about 45 to 65 t other relative intensity highs of 45 55 t were found around 1600 bce 370 ce and 1200 ce and relative intensity lows of 30 35 t around 1350 bce and 700 ce comparing the virtual axial dipole moment of our intensity curve with global vadm vdm dipole estimates indicates that non dipole components in central mexico have been observed at several times from 1600 bce to 1000 ce while the dipole field was prevalent from 1000 to 1800 ce a comparison of our results with selected data from southern usa shows a significant difference between the two regions which indicates an important nondipole feature of the magnetic field during the last few thousands of years and or undetected problems in the age palaeointensity data accuracy

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

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

absolute palaeointensity study of the mono lake excursion recorded by new zealand basalts abstract one of the geomagnetic excursions recorded in basalts of the auckland volcanic field new zealand has recently been correlated unequivocally with the mono lake excursion making it the only confirmed record from the southern hemisphere this record is also exceptional in occurring in five separate monogenetic basaltic volcanoes absolute palaeointensity determinations using the microwave technique based on a comprehensive suite of samples of the auckland basalts recording the excursion show that the geomagnetic field in new zealand at the time of the mono lake excursion was reduced to about 14 t i e to 30 of its normal value this result confirms previous estimates based on more limited sampling in addition it provides a comparison of the microwave and ldt dht shaw methods of measuring palaeointensity which give results that are statistically indistinguishable the palaeointensities determined from the five different volcanoes are also indistinguishable though the palaeodirection data suggest that a very small segment of the vgp path may have been recorded this study confirms the reliability of palaeointensity and palaeodirection determinations from these particular new zealand basalts which together with their definitive 40ar 39ar ages establishes this record of the mono lake excursion as one of the best documented consequently there is significant potential in searching for records of this excursion elsewhere in the pacific region for use as a stratigraphic marker in studies of recent volcanism and palaeoclimate reconstructions

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

archaeomagnetic intensity results from california and ecuador evaluation of regional data we present new archaeointensity data for southeastern california v33 n v115 w 50 1500 yr bp and northwestern south america ecuador 2 4 s 80 7 w 4000 5000 yr bp these results represent the only data from california as well as the oldest archaeointensity data now available in northwestern south america in comparing our results to previously published data for the southwestern united states and northwestern south america we note that significant scatter in the existing data makes comparisons and interpretations difficult we undertake an analysis of the sources of data scatter including age uncertainty experimental errors cooling rate differences magnetic anisotropy and field distortion and evaluate the effects of scatter and error on the smoothed archaeointensity record by making corrections where possible and eliminating questionable data scatter is significantly reduced especially in south america but is far from eliminated however we believe the long period fluctuations in intensity can be resolved and differences between the southwestern and south american records can be identified the southwest data are distinguished from the south american data by much higher virtual axial dipole moment values from v0 600 yr bp and by a broad low between v1000 1500 yr bp comparisons to global paleofield models reveal disagreements between the models and the archaeointensity data in these two regions underscoring the need for additional intensity data to constrain the models in much of the world 2002 elsevier science b v all rights reserved

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

an assessment of the reliability of palaeointensity results obtained from the cretaceous aged suhongtu section inner mongolia china abstract here we present microwave palaeointensity results from 89 sister samples from the study of zhu et al zhu r pan y he h qin h ren s 2008 palaeomagnetism and 40 ar 39 ar age from a cretaceous volcanic sequence inner mongolia china implications for the field variation during the cretaceous normal superchron phys earth planet int 169 59 75 who carried out thellier palaeointensity analysis as part of their integrated palaeomagnetic and 40 ar 39 ar dating study of cretaceous lava from the suhungtu section inner mongolia china additionally a comprehensive rock magnetic investigation has been carried out in order to determine the mineralogy and hence the validity of assuming that the remanence is a thermal remanent magnetisation trm the microwave results are of apparent high quality and give flow mean palaeointensity estimates ranging from 13 to 49 t corresponding to virtual dipole moment vdm estimates ranging from 2 5 to 8 9 10 22 am 2 and an overall mean vdm of 5 5 1 9 10 22 am 2 for the 24 flows aged 110 6 0 1 ma when the microwave results using the perpendicular applied field method with partial microwave thermal remanence pt m rm and pt m rm tail checks are compared to those obtained with the thellier method coe version with ptrm but not tail checks and heating in argon atmosphere differences are seen at the sample flow and palaeomagnetic unit level however the overall means and spread in palaeointensity estimates are consistent some discrepancy is due to the differing sized sample sets and sample inhomogeneity but discrepancy is also interpreted to be due to the differing protocols methodology plus the subjectivity in interpretation considering only those results that are consistent to within 20 the spread in palaeointensity estimates remains there is substantial rock magnetic evidence from progressive heating in air and argon experiments both showing irreversible thermomagnetic behaviour as well as looking at samples under the scanning electron microscope to suggest that maghaemite is present albeit to varying degrees in many of the samples alteration therefore occurred in nature and it is likely that the remanence will have been affected to differing degrees potentially causing underestimates in palaeointensity maghaemite is interpreted to be a remanence carrier where a component of remanence remains after heating to 580 c and loss of ptrm acquisition capacity is found on heating no correlation was found between the estimated palaeointensity microwave or thellier datasets and amount of high temperature remanence or any rock magnetic parameter this could suggest that the palaeointensity estimates are reliable or as seems likely more than one factor such as methodology protocol interpretation chemical remanent magnetisation crm contamination and the geomagnetic field are influencing the palaeointensity estimates in an attempt to remove the influence of palaeointensity protocol and methodology the palaeointensity datasets were reduced to consist only of those results mutually consistent to 20 n 22 no obvious correlation was again seen between the inferred maghaemite contribution and palaeointensity estimate there is therefore no clear evidence for the biasing of results due to crm contamination assuming a simple relationship between biasing and maghaemite contribution and it can thus be inferred that the geomagnetic field was a major influencing factor further studies are however needed to fully elucidate the extent and result of each possible influencing factor and ultimately the reliability of palaeointensity estimates from rocks of this type

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