geomagnetic field intensity between 70 000 and 130 000 years b p from a volcanic sequence on la reunion indian ocean abstract a detailed paleointensity study was made of a sequence of 70 successive lava flows of the piton des neiges volcano on the island of la reunion indian ocean radiometric dating brackets the age of this sequence between 130 3 ka and 72 3 ka rock magnetic investigations show that titanomagnetites in the pseudo single domain range are the main magnetic carrier of the natural remanent magnetisation nrm over 350 samples were used for paleointensity determinations carried out with the thellier method in vacuum or in an argon atmosphere of these 89 samples yielded reliable results with within flow scatter often lower than 20 these results indicate that the geomagnetic field intensity has varied at la reunion between 13 and 65 t during the period of time explored the average value 42 t is higher than the present field at la reunion 35 t the results from the upper part of the section are consistent with previous results obtained for the 82 98 ka period also at la reunion 1 and document a broad low around 95 ka not associated with large directional changes on a larger geographic scale the paleointensity values from la reunion are significantly higher than those obtained from mount etna 2 precise comparison is however difficult because of the lack of detail in the etna results in the lower part of the section a marked intensity low coinciding with significant deviation from the dipole field direction is observed at 115 ka and could correspond to the end of the blake event

late quaternary geomagnetic secular variation from historical and 14c dated lava flows on hawaii a paleomagnetic record of geomagnetic paleosecular variation psv is constructed for the last 4400 years based on 191 sites in historical and 14c dated lava flows from mauna loa kilauea and hualalai volcanoes on the island of hawaii the features of this new record are similar to those recorded by sediments from lake waiau near the summit of mauna kea volcano but overall mean inclinations for the lava flows 31 to 33 depending on window size are nearer the expected dipole field value 35 than is that for the sediments 27 divergence of the inclination records with increasing age suggests that the lake waiau values at depths below 2 m have been affected by compaction related inclination shallowing although magnetic terrain effects cannot be ruled out the rate of psv indicated by the record presented here is highly variable 20 century and a pronounced shift in inclination from 25 to 40 occurred between 1030 and 975 years b p paleomagnetic directions from undated materials can be correlated with our calibrated curve but the resolution is largely dependent on the psv rate and data densities for both the reference and unknown directions the upper part of the puna basalt 18 lava flows previously sampled for paleomagnetism along the northern wall of kilauea s caldera uwekahuna bluff was likely deposited sometime between 1030 and 750 years b p but the lowest two flows beneath the uwekahuna ash 2100 years b p are correlated with an age of 3034 years b p paleomagnetic data for 54 lava flows of the ka u basalt exposed in the northwest wall of mauna loa s summit caldera mokuaweoweo indicate that they probably accumulated over a relatively short time interval 200 years and are assigned to a 1000 to 1199 year b p time window the mean of ages within this window is 1030 years b p but mapping and other 14c dates indicate that these summit overflows are probably closer to 1200 years b p in age

palaeosecular variation in central mexico over the last 30 000 years the record from lavas summary 13 lava flows of known age ages from i4c dating which have been erupted in the last 30000 years have been studied to determine the palaeosecular variation of the geomagnetic field in central mexico samples were taken from two different monogenetic volcanic fields the michoacan guanajuato volcanic field six sites and the chichinautzin formation seven sites both part of the transmexican volcanic belt the lavas were studied in detail using rock magnetic methods magnetic susceptibility at room temperature low temperature susceptibility behaviour hysteresis loops curie temperatures combined with reflected light microscopy in order to deduce their magnetic mineralogy and the domain states of the magnetic minerals the magnetic carriers are titanomagnetites which show differing degrees of high temperature deuteric oxidation and seem to be predominantly pseudo single domain psd though in many cases are probably a mixture of domain states mean palaeomagnetic directions and palaeointensity values using shaw and thellier techniques were obtained using several specimens from each flow our data seem to indicate a sharp easterly swing in declination about 5000 years ago which is also observed in lake sediments from central mexico the calculated values of the virtual dipole moment vdm range from 3 1 to 14 9 x a m2 our data indicate that the virtual dipole moment seems to have increased gradually in magnitude over the last 30 kyr with a peak at about 9000 years bp these are features that have been observed in other parts of the globe and are probably caused by variations in the dipole part of the geomagnetic field

preliminary paleointensity measurements and detailed magnetic analyses of basalts from the skalamaelifell excursion southwest iceland among the many excursions reported in the late brunhes the skalamaelifell excursion recorded in 22 flows in three hills of assumed late wisconsin basalts in the active rift zone of the reykjanes peninsula is one of the most reliable it is characterized by a single well defined transitional direction d 260 i 16 paleointensity pi measurements using the thellier method as modified by coe in a vacuum with partial thermoremanent magnetization checks were made on six samples from the excursion flows for comparison 10 samples from the adjacent and probably nearly contemporaneous normally magnetized flows were also measured because of sample scarcity four of the excursion samples and seven of the normal samples had been already af demagnetized to 20 mt in iceland the samples however were chosen because of their high coercivity and lack of viscous remanent magnetization they retained 70 95 of their natural remanent magnetization nrm after 20 mt demagnetization and their remanence directions were stable during the 20 mt demagnetization and during 2 6 years of storage following it saturation magnetization js measurements suggest that the reason for the low nrm of the excursion samples was a paleofield one fourth to one tenth that of the normal basalts the mean pi from four excursion samples is 4 3 0 6 t whereas the mean pi of the five normal samples considered reliable is 30 9 t the quality of the nrm thermoremanent magnetization trm data correlated well with the reversibility of the js t curves the pi differences agree in general with those predicted by the nrm js data especially when comparing the means of the two groups which average the effects of different grain sizes a prior partial alternating field af demagnetization of one sample in each of two pairs from the same cores caused the pi to be underestimated by only 10 continuous af demagnetization during the nrm trm measurements of one sample in another pair that had nonlinear trm trm plots even in the undemagnetized sample caused the pi to be overestimated by 40 these results combined with other studies suggest that the effect of partial af demagnetization on high coercivity samples depends on when the demagnetization is done i e only prior to or during the nrm trm measurements and probably more importantly the thermochemical stability of the sample if correct this increases the usefulness for pi determinations of old hard to recollect partially demagnetized collections the very low pi of 4 t is similar to that found for excursions and transitions in iceland and other places however o its virtual geomagnetic pole lies 90 from a north south vertical plane through the sites which rules out a purely axisymmetric model for the excursion the low normal intensities are similar to those found in the chaine des puys france and elsewhere for the period 10 0000 50 000 years b p given its very low pi and a direction only 70 from complete reversal the skalamaelifell excursion is most likely the result of nondipole fields and an unusually weak dipole field such as that observed in france for the 10 000 year interval 35 000 45 000 years b p surrounding the laschamp excursion

latest pleistocene and holocene geomagnetic paleointensity on hawaii geomagnetic paleointensity determinations from radiocarbon dated lava flows on the island of hawaii provide an estimate of broad trends in paleointensity for holocene time and offer a glimpse of intensity variations near the end of the last glacial period when the data from hawaii are compared with others worldwide the intensity of the geomagnetic field seems to have been reduced from the holocene average by about 35 percent between 45 000 and 10 000 years ago a long term reduction of this magnitude is compatible with reported increases in the production rate of cosmogenic nuclides during the same interval

broad trends in geomagnetic paleointensity on hawaii during holocene time paleointensity determinations have been obtained from 22 basaltic lava flows on the island of hawaii using the thelliers method radiocarbon dating indicates that these flows erupted at intervals ranging from about 200 to 1000 years and results of the experiments provide an estimate of broad trends in geomagnetic paleointensity during holocene time in the vicinity of hawaii most of the samples were obtained from quickly cooled flow margins and as a consequence typically contain two titanomagnetite populations only two of the 79 samples analyzed failed to yield an estimate of the paleofield demonstrating that paleointensities can be obtained from such samples if carefully selected virtual dipole moments calculated for the flows are compared with a published curve of dipole field intensity that was constructed using worldwide archeomagnetic data the large nondipole fields that were previously postulated for the vicinity of hawaii are confirmed and the present data indicate that they were present from about 5000 years b p to perhaps as recently as the past 200 years the data indicate however that these nondipole sources must have been virtually absent between about 12 000 and 5000 years b p as they are at the present time

the shaw method of palaeointensity determinations and its application to recent volcanic rocks summary our new version together with previous versions of the shaw method has been compared by applying them to recent volcanic samples for which we have well determined palaeointensity values although the earlier modifications to the shaw method give a linear portion to the nrm trm diagram of all the specimens about half of the calculated palaeointensities deviate by more than 20 per cent from the expected values this may be caused by alteration of the magnetic minerals during the laboratory heating or by chemical alteration prior to the laboratory experiments the new modification uses a double heating of samples above the curie temperature in the laboratory after heating the specimen twice in the same field the measured intensity from the second heating is compared with the laboratory field intensity if the difference is larger than the experimental error then we assume that the anhysteretic remanent magnetization arm correction is not applicable and the sample is therefore rejected this technique provides a powerful criterion with which to select samples using this selection technique palaeointensities are generally within 10 per cent difference of the expected values with a success rate of about 40 per cent

palaeointensity study of the hawaiian 1960 lava implications for possible causes of erroneously high intensities summary it is known that the hawaiian 1960 lava sometimes yields higher palaeointensities than the igrf 36 2 t in order to clarify the causes we have performed a comprehensive investigation of palaeointensity measurements on 19 cores from the lava according to various rock magnetic analyses they are classified into three groups with degrees of the deuteric oxidation in coe s version of the thellier method a significantly larger mean palaeointensity of 49 0 9 6 t n 17 was observed similar to previous studies these palaeointensities showed a dependence on the oxidation indices which can be explained by thermochemical remanent magnetization tcrm acquisition during the natural cooling stage of the lava although it is generally difficult to distinguish the tcrm from thermoremanent magnetization trm a lowtemperature demagnetization ltd treatment on nrm and trm in the thellier experiment seems to allow us to detect the influence of the tcrm on nrm another possible tool for tcrm detection is the double heating technique dht of the shaw method combined with ltd ltd dht shaw method it gave a better average of 39 4 7 9 t n 9 though the success rate was much lower most of the inappropriate results were screened by a combination of the double heating test the arm correction and the ltd treatment if two outliers are further excluded the average is improved to be 35 7 3 3 t n 7 which agrees well with the expected intensity the results of both methods emphasize the importance of taking and measuring samples from the parts of various oxidation degrees in a lava flow

dating recent hawaiian lava flows using paleomagnetic secular variation hawaiian paleomagnetic secular variation sv is defined from samples at 67 sites on lava flows of known age paleomagnetic directions range through 40 of inclination and 30 of declination angular dispersion within sites is commonly dating precision is limited by dispersion of 4 5 among sites of apparently similar age the main dispersion sources are in 14 c dates 3 0 intra flow deformations 2 0 and local magnetic anomalies 1 5 results from 68 sites on undated flows show that 95 of kilauea9s surface is younger than 1 000 yr a hiatus in volcano growth 1 000 1 500 yr ago coincides with the filling of a large caldera averaging of available data yields an sv reference curve which is fairly reliable to 1 500 yr ago contains gaps and ambiguities 1 500 3 000 yr ago and remains highly uncertain 3 000 6 000 yr ago the sv dating method can be precise but is limited by the need for a calibrated sv history it can be a powerful correlation tool even if a history is unavailable

a late quaternary geomagnetic secular variation record from lake waiau hawaii and the question of the pacific nondipole low a continuous paleomagnetic record for the last 13 000 years was obtained from the sediments of lake waiau located near the summit of mauna kea volcano on the island of hawaii the secular variation sv of the geomagnetic field was studied using spectral analysis and precession analysis of the magnetic vector the hypothesis that there has been anomalously low geomagnetic sv in the central pacific was tested using angular dispersion analysis the inclination record has a mean value of 27 that is much lower than the 35 predicted by the geocentric axial dipole hypothesis the spectrum obtained from spectral analysis shows major inclination peaks at periodicities of approximately 3600 and 7900 years and major declination peaks at periodicities of approximately 1100 2000 3300 and 7800 years the approximately 8000 and 3500 year periodicities shown in both inclination and declination spectrums may represent the dipole and nondipole variation respectively the approximately 1100 and 2000 year periodicities are probably derived from either the sources that affect declination only or the offsets between the core sections in addition bauer plots of the magnetic vector are dominated by clockwise motion the angular dispersion calculated from lake sediments is 5 9 with respect to the mean virtual geomagnetic pole vgp and 9 1 with respect to the geographic pole both values are much lower than the 13 5 expected from paleosecular variation model g which separates the angular dispersion into components from the dipole and the quadruple families our results support the hypothesis that an anomalously low secular variation is a characteristic feature of the central pacific furthermore this geomagnetic behavior persists on time scales of 104 years regional anomalies in temperature topography or electrical conductivity at the core mantle boundary are inferred to cause the low sv

high paleointensities of the geomagnetic field from thermomagnetic studies on rift valley pillow basalts from the mid atlantic ridge nineteen pillow basalts dredged within the rift valley of the mid atlantic ridge at 36 8on were studied by the thellier stepwise heating method in order to determine the paleointensity of the geomagnetic field when they erupted on to the sea floor previously reported fission track ages are 2 000 to 6 000 years for the youngest rocks mainly olivine basalts and 10 000 to 100 000 years for the others mainly plagioclase basalts and pyroxene basalts all but three pillow basalts meet the conditions commonly considered as indicative of quite reliable paleointensity estimates stability of the direction of nrm during its thermal demagnetization constant ratio of nrm trm natural remanent magnetization to thermoremanent magnetization over 50 or more of the original nrm intensity 80 to 94 for 11 specimens and reproducibility of low temperature partial trm ptrm however strong field thermomagnetic measurements indicate that 11 of these 16 samples display a significant increase in curie temperature 15 to 80oc during the paleointensity experiments below 250oc notwithstanding the linearity of the nrm trm plot in this temperature interval this alteration probably due to low temperature oxidation of the specimens seems typical of young pillow basalts and may result in paleointensity estimates which are too high this result shows that excellence of remanence tests nrmtrm linearity and ptrm stability does not ensure the absence of chemical changes during the thellier experiments and therefore the validity of the paleointensity obtained the assumption that reliable paleointensities are obtained when the curie point increase is 10oc or less led to the selection of five of our specimens all from the youngest group their mean paleointensity is 64 2 20 5 xt standard deviation and the corresponding virtual axial dipole moment vadm is 11 5 3 7 x 1022 a m 2 given the variations of the vadm of the earth s magnetic field over the last 6 000 years as established from archeomagnetic studies our paleointensity results suggest that the latest eruptions on the inner floor of the rift valley at 36 8on occurred 1 500 1 000 years ago

paleomagnetic field intensity recorded in submarine basaltic glass from the east pacific rise the last 69 ka paleointensity determinations were obtained from basaltic glass recovered from the northern east pacific rise using the thelliers 1959 method as applied by pick and tauxe 1993a the samples are up to a69 ka old based on their distance from the ridge axis and u series disequilibria measurements the results show a general increase in paleointensity of the field from 69 ka to the present with values ranging from 50 higher than the present field to as low as 26 of the present dipole field the average dipole moment is 8 32 1022 am which is similar to the present axial dipole moment the results are in general agreement with other absolute and relative paleointensity records

paleointensity high at 9000 years ago from volcanic rocks in japan six paleointensities were determined from young lava flows of fuji and oshima volcanoes whose ages are around 8000 to 15000 years b p two lava flows which have well defined ages at 8970 390 years b p and 9260 430 years b p give large paleointensities of 67 12 and 65 6 t respectively another lava flow of less accurate age also gives large paleointensity of 63 6 t the new paleointensity data are consistent with the trend of averaged paleointensity variation in japan for the last 9000 years they form a peak of paleointensity high at around 9000 years b p comparable to the well documented one at 2000 years b p this suggests that there exists a dominant component with a characteristic period of about 7000 years in paleointensity variation large paleointensity at about 8000 10000 years ago is also reported in france and iceland paleointensity variation averaged every 1000 years for the last 14000 years is quite similar in europe and japan the sine curve like variation of paleointensity observed in both europe and japan probably reflects the nature of the dipole moment if we consider much difference of longitude between the two regions

paleointensities for 10 22 ka from volcanic rocks in japan and new zealand abstract eight volcanic rocks from japan and new zealand in the age range 10 50 ka b p were studied using the thellier method paleointensity experiment and yielded five successful results for 10 22 ka samples from the shikotsu kuttara and daisen volcanoes in japan were taken from pyroclastic flows both welded and non welded and a pumice fall deposit one of the two new zealand rocks is a rhyolite lava from the taupo volcanic zone four sites out of seven located on rocks that are not usually subjected to thellier s experiment yielded successful results which indicates that volcanic products such as pyroclastic flows and rhyolite lavas are as good a material as basalt and andesite lavas for application of the paleointensity experiment one paleointensity from new zealand at 9 9 ka is very large at about 100 t and provides support for the paleointensity high found in europe and japan 1 2 the other four paleointensities for 14 22 ka are smaller than the present day value and this agrees with the idea of a broad paleointensity minimum for the period 10 50 ka that was suggested by mcelhinny and senanayake 3 it is also noted that the variation curve of absolute paleointensity from the volcanic rocks is reasonably similar to the relative paleointensity curves from marine and lake sediments which were summarized by tauxe 4

holocene secular variation in western north america paleomagnetic record from fish lake harney county oregon an 8 9 m composite section of lake sediment from fish lake steens mountain southeastern oregon has provided a high resolution record of holocene geomagnetic secular variation in western north america the section is composed of overlapping core segments with stratigraphic control based on six tephra layers and other thin distinct bands and with age control from 37 radiocarbon dates the 455 paleomagnetic samples possess a strong and stable magnetic remanence which is carried by pseudo single domain magnetite paleomagnetic records for individual core segments show low scatter and high serial correlation and where the segments overlap there is excellent agreement between records the azimuthal orientation of the composite section was recovered from the known direction of the geomagnetic field at the time of the climactic eruption of mount mazama tephra which is present in fish lake both internal and external evidence including the correspondence with an archaeomagnetic record from the southwestern united states support the conclusion that the fish lake record represents the history of geomagnetic field behavior during at least the last 10 000 years b p analysis of that record reveals that the average geomagnetic direction can not be distinguished from that of a geocentric axial dipole field and that the overall circularity of the field was clockwise but the detailed behavior was quite complex angular standard deviations of both field directions and virtual geomagnetic poles differ significantly from the values predicted from theoretical models of secular variation but are consistent with those of other holocene records when the record from fish lake is compared to the proposed type curve for east central north america there is excellent agreement for the past 6000 years provided the curves are offset by about 400 years this offset corresponds to a rate of westward drift of about 0 062 yr beyond about 6000 radiocarbon years ago good agreement between the two curves occurs when there is no offset suggesting that the field was standing rather than drifting during this time

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

new and revised 14c dates for hawaiian surface lava flows paleomagnetic and geomagnetic implications radiocarbon dates have been obtained for 30 charcoal samples corresponding to 27 surface lava flows from the mauna loa and kilauea volcanoes on the island of hawaii the submitted charcoal was a mixture of fresh and archived material preparation and analysis was undertaken at the nerc radiocarbon laboratory in glasgow scotland and the associated suerc accelerator mass spectrometry facility the resulting dates range from 390 years b p to 12 910 years b p with corresponding error bars an order of magnitude smaller than previously obtained using the gas counting method the new and revised 14c data set can aid hazard and risk assessment on the island the data presented here also have implications for geomagnetic modelling which at present is limited by large dating errors

late quaternary paleomagnetic field secular variation from two minnesota lakes detailed paleomagnetic records from the wet sediments of two minnesota lakes are the basis for estimating the local secular variation of the earth s magnetic field during late quaternary time results from lake st croix cover the interval 0 9500 years b p and results from kylen lake cover the interval 4000 14 000 years b p paleomagnetic data were recovered from two replicate cores in each lake by sampling the sediments at 3 to 10 cm intervals analysis of the potential errors in the results indicates that all cores were correctly oriented in the vertical plane but that some azimuthal offsets of individual core segments did occur various methods for correcting the occasional azimuthal offsets are described the final paleomagnetic data sets from each lake show a high degree of internal consistency these results also show close correspondence 1 between the two lakes 2 with the known historic magnetic field variation and 3 with paleomagnetic results from dated archeological material and lava flows from the western united states rock magnetic analysis of the lake sediments indicates that the natural remanent magnetization is the result of a physical remanence acquisition process detrital remanent magnetization active at or near the sediment water interface there is no evidence for a systematic inclination error in the results statistical analysis of vector time series determined from the paleomagnetic data indicates that both the field vector and virtual geomagnetic pole vgp distributions are non fisherian and essentially axial dipolar when averaged over 10 000 years spectral analysis of the scalar inclination and declination suggests that the field behavior is separable into a dominant low frequency dipolelike waveform period of 9500 years and a higher frequency band irregular waveform fundamental period of 2400 years with multiples at 1200 800 and 600 years that may represent the nondipole field this irregular waveform is interpreted as resulting from zonal drift of the nondipole field past minnesota on the basis of waveform morphology and vgp circularity zonal drift of the nondipole field plus a low frequency dipole variation can account for almost all of the observed paleomagnetic field variation

determination of geomagnetic palaeointensities in vacuum summary specimens with low and intermediate curie points from six hawaiian historic basalt flows were used for the determination of the intensity of the historic geomagnetic field by the thelliers method the specimens were heated either in air or in a vacuum of lo torr the regional intensity of the earth s magnetic field at the time of extrusion of these rocks is known from direct observations the palaeointensities determined in vacuum from four low and intermediate curie point flows are correct within the uncertainty caused by the local field anomalies whereas those determined in air from the same four flows are of lesser quality and accuracy unaltered submarine basalts have similarly low curie points and thus may also be amenable to palaeointensity determination in vacuum the behaviour of the remaining two flows which had higher curie points was more erratic and they yielded less accurate palaeointensities regardless of whether they were determined in air or vacuum the thelliers technique thellier thellier 1959 is one of the most widely used methods for the determination of palaeointensities of the earth s magnetic field from rocks it consists of comparing the thermal spectrum of the natural remanent magnetization nrm of an igneous rock with that of an artifically induced thermoremanent magnetization trm in the same specimen it is assumed that the nrm is a trm acquired during the initial cooling of the rock and that its original thermal spectrum remained unchanged throughout the history of the rock in practice these conditions are not easily met because of two reasons first the nrm acquired by the rock at the time of its formation may be different from that measured in the laboratory because of viscous decay or the superimposition on the nrm of secondary components of magnetization in some cases however this problem may be overcome by thermal cleaning which is an integral part of this technique second when rocks are heated they undergo alterations which affect the chemical composition size and shape of their magnetic mineral grains the effect of these changes may be such that the nrm and trm are produced in two magnetically different materials thus rendering the determination of palaeointensity unreliable because oxidation is the change most likely to occur when heating is carried out in air many authors have recommended the use of high curie temperature rocks which

geomagnetic field intensity and inclination records from hawaii and the reunion island geomagnetic implications abstract we report on new paleointensity and inclination records obtained in hawaii from 386 samples drilled in 137 subaerial flows of the hsdp2 long basaltic core that we have combined in a composite record with the other hawaiian results to produce a unique and accurate lava record of absolute geomagnetic field intensity and inclination at hawaii for the last 405 kyr these data are considered together with published results from la reunion island for about the same time period in order to compare them to model results in hawaii when recognized excursional periods are omitted the average vadm vdm value is about 8 2 8 4 1022 am2 and the inclination is on the average 29 8 i e about 6 shallower than the expected value at this location gad value in the reunion island we selected published results in which both inclination and intensities values are obtained using modern methods and strict criteria the average vadm vdm is 9 4 2 3 8 7 2 3 1022 am2 and the average inclination value when transitional data are ignored is 44 4 2 4 about 7 steeper than the gad value these results compared with model intensities predicted by dynamo solutions that incorporate lateral variations in core mantle boundary heat flow derived from seismic tomography are too high for both localities but they decrease as the amplitude of thermal boundary anomalies increases with strong boundary heat flow the model inclination anomaly at hawaii fits with the data but at la reunion model inclination anomalies have the wrong sign this comparison indicates that strong boundary heat flow anomalies give the best fit to the data and that future improvements will be obtained by proportionally increasing both the rayleigh number and the amplitude of heat flow variations

holocene paleointensity of the island of hawai i from glassy volcanics this study presents new high quality paleointensity records and 14c radiocarbon age determinations from the island of hawai i during the holocene previous studies on hawai i use experimental methods and statistical selection criteria that may produce inaccurate geomagnetic field strength estimates additional high quality paleointensity results can be used to evaluate the existing hawaiian dataset and investigate holocene geomagnetic field behavior new paleointensity sites from 22 lava flows were calculated using the izzi thellier laboratory technique and a strict set of selection criteria rapidly cooled glassy volcanic material was collected for all sites isotopic age determinations range from 270 10 000 years before present nine new 14c ages are also presented as part of this study the median intensity for the 22 flows is 47 5 t with a median absolute deviation uncertainty of 5 6 t substantially greater than the present day field strength at hawai i 36 t these new results are comparable to previously published data from this location and are consistent with global paleointensity models there is no evidence of an intensity spike at 3 000 years before present as seen in the levant and elsewhere previously published data vary in intensity by experimental technique relative to data using glassy material and strict selection criteria non thellier type data are biased low a result of these techniques estimating intensity from possibly non single domain magnetic carriers thellier thellier data are biased high the reasons for which remain unclear as no cooling rate effect was demonstrated and we were unable to reproduce the high bias with different selection criteria

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

absolute paleointensity from hawaiian lavas younger than 35 ka abstract paleointensity studies have been conducted in air and in argon atmosphere on nine lava flows with radiocarbon ages distributed between 3 3 and 28 2 ka from the mauna loa volcano in the big island of hawaii determinations of paleointensity obtained at eight sites depict the same overall pattern as the previous results for the same period in hawaii although the overall average field intensity appears to be lower since the present results were determined at higher temperatures than in the previous studies this discrepancy raises questions regarding the selection of low versus high temperature segments that are usually made for absolute paleointensity the virtual dipole moments are similar to those displayed by the worldwide data set obtained from dated lava flows when averaged within finite time intervals the worldwide values match nicely the variations of the sint 200 synthetic record of relative paleointensity and confirm the overall decrease of the dipole field intensity during most of this period the convergence between the existing records at hawaii and the rest of the world does not favour the presence of persistent strong non dipole components beneath hawaii for this period

relative paleointensity of the earth s magnetic field from marine sedimentary records a global perspective abstract to assess the potential of marine sediments as a source of relative paleointensity information we have undertaken an investigation of selected box cores stored in the scripps core repository we present here new results from two north atlantic box cores magnetic remanence in the sediments is carried by magnetite and is univectorial during alternating field and thermal demagnetization the median destructive field is 25 30 mt normalization by anhysteretic remanent magnetization yields relative paleointensity data that are highly concordant within and between cores age control provided by oxygen isotope data from both cores allows comparison of these relative paleointensity data with contemporaneous archeomagnetic data from czechoslavakia both data sets indicate a peak in paleofield intensity at around 8 10 ka this peak is less prominent in box core data from the western pacific and global averages of archeomagnetic and lava flow data however the peak is evident in data estimating changes in production of radiocarbon which is controlled in part by changes in the dipole moment of the geomagnetic field the excellent agreement between production variations predicted by the changes in dipole moment and those observed suggests that long term variations in atmospheric radiocarbon are dominated by geomagnetic field variations

mechanisms of remanent magnetization acquisition in marl and limestone alternations case study upper cretaceous chron 31 30 sopelana basque country abstract the marl and limestone m l alternations of the cretaceous tertiary section in sopelana in the basque country provide a good example of two chemical magnetizations which have been acquired at different times a detailed study was conducted on a core which contained an unusual paleomagnetic record in the vicinity of chron 30 r the magnetic polarity switches far more rapidly than expected from the reversal time scale a 10 cm sampling interval provided on average six samples in each m l couplet that was identified from a curve of carbonate content the lithostratigraphic nrm pattern is related to the lithology the intervals three to four m l couplets along which the contrast in the carbonate content between the limestone beds and the marly layers is low 20 are characterized in normal polarity intervals by the expected nrm in the limestone beds and a reverse or blurred nrm in the marly layers the dominant magnetic carrier in this complex zone is diagenetic hematite whereas titanomagnetite was identified below this interval the characteristics of the detrital minerals indicate an erosion of a mature landscape dominance of weathered ilmenite probably related to deformation in the pyrenees diagenetic hematite is characterized by two families the first microgranular size only tenths of a micrometre is present in all the samples and is interpreted as a very early chemical remanent magnetization crm acquired in the oxic zone that probably extended over a few decimetres below the sediment surface the second is restricted to the marly layers of intervals with large carbonate content contrasts in the m l couplets the grains in this family are larger very fine some micrometres in size and carry a reverse or blurred polarity in a normal polarity zone it is proposed that this chemical remagnetization was acquired during the circulation of oxygenated fluids the timing for this process is at least 2 m y after deposition and more likely 7 m y the second diagenetic family grew at least 90 m below the seafloor and its distribution shows that the marly layers in intervals with large contrasts in the carbonate content between the marls and the limestones were preferential drains along which circulation was favoured this suggests that although the porosity was still high 40 60 the diagenetic evolution in the m l alternations was already being expressed by different physical properties limestone beds and marly layers in intervals with low carbonate content contrasts in the m l couplets were impermeable to fluid circulation and the early diagenetic crm signal has been preserved in contrast marls in intervals with a large carbonate content contrast in the m l couplets were sites of later diagenesis and their early crm was overprinted by another magnetization

secular variation of the intensity of the geomagnetic field in mexico during the first millennium bce in western eurasia the first millennium bce is characterized by the fastest secular variation of the earth magnetic field observed over the last millennia and by a geomagnetic anomaly centered on the middle east on the global scale the variation of the dipolar field during this period remains poorly constrained because of the lack of data in other geographical areas here we presented 23 new mean archaeointensity data on ceramic sherds dated between 1500 bce and 200 ce from chalcatzingo archaeological site in central mexico archaeointensities were determined using the classical thellier thellier protocol with corrections for trm anisotropy and cooling rate effects our work doubles the number of high quality archaeointensity data in mexico during the considered period using a bayesian approach a new secular variation curve was calculated at mexico city between 1500 bce and 200 ce after selection of mexican archaeointensity data after a period of oscillations of the intensity between 20 and 40 t from 1500 to 300 bce the curve shows a large maximum 65 t in the second century bce the corresponding vadm varied between 4 0 and 11 0 1022 am2 which highlights further that the intensity of the geomagnetic field could vary at regional scale over a larger range as previously thought however this amplitude variation may be overestimated as it does not take into account the fast directional variation observed at this time

geomagnetic intensity and inclination variations at hawaii for the past 98 kyr from core soh 4 big island a new study and a comparison with existing contemporary data abstract reliable thellier thellier paleointensity determinations were obtained from 321 out of 766 examined samples from the upper 200 units 675 m of core soh 4 drilled in the kilauea volcano the main trend of the soh 4 record is a marked decrease from values higher than present in the last 4 kyr to values about 40 smaller from about 20 kyr to the bottom of the sampled interval three main lows coinciding with shallow or negative inclinations observed in the record were identified with an excursion previously documented in hawaii and the mono lake and laschamps events for the last 15 kyr the soh 4 intensities are largely in agreement with the worldwide distribution of volcanic data in the 15 75 kyr interval the general lack of volcanic data makes comparison with the soh 4 data difficult the soh 4 record is similar to a composite record of paleointensity largely based on a recent north atlantic sedimentary paleointensity stack napis 75 the soh 4 record is also generally consistent with a record of geomagnetic field intensity obtained from a study of 36 cl in the grip ice core calculated with the assumption that the production rate of 36 cl is controlled only by the geomagnetic field one pronounced discrepancy between the different records is observed around 50 kyr bp when the sedimentary and cosmogenic based records document high intensities which are not observed in soh 4 these high intensities are also absent in a recent record based on the analyses of sea floor magnetic anomalies the histogram of vadms obtained from soh 4 is different from that obtained from the sedimentary or 36 cl based records and is more skewed than the distribution given by kono and tanaka for the last 2 5 million years

calibration of the c 14 timescale over the past 30 000 years using mass spectrometric u th ages from barbados corals uranium thorium ages obtained by mass spectrometry from corals raised off the island of barbados confirm the high precision of this technique over at least the past 30 000 years comparison of the u th ages with 14c ages obtained on the holocene samples shows that the u th ages are accurate because they accord with the dendrochronological calibration before 9 000 yr bp the 14c ages are systematically younger than the u th ages with a maximum difference of 3 500 yr at 20 000 yrbp the u th technique thus provides a way of calibrating the radiocarbon timescale beyond the range of dendrochronological calibration

absolute paleointensity between 60 and 400 ka from the kohala mountain hawaii abstract magnetic experiments including thermal demagnetization of the natural remanent magnetization nrm mineralogical studies and paleointensity measurements have been conducted on ten lava flows with ages between 60 ka and 400 ka from the kohala mountain hawaii of the samples 62 were subjected to double heating paleointensity experiments under vacuum in total 35 of the specimens did not exhibit significant magnetomineralogical changes during heating and met all the criteria for successful determinations of absolute paleointensity a technique of corrections 1 was attempted for samples that exhibited changes in their ability to acquire partial thermoremanent magnetization ptrm during heating but did not show acquisition of chemical remanence crm this procedure doubled the rate of success with consistent results between the uncorrected and the corrected data from within the same flows the successful paleointensity estimates obtained for 8 lava flows are found to be in good agreement with previous absolute paleointensities obtained from other areas the results are also consistent with the synthetic curve sint 200 of relative paleointensity obtained for the past 200 000 years from deep sea sediment cores 2 there is thus no reason to infer the presence of large non dipole fields in the vicinity of hawaii overall the geomagnetic intensity appears to have the same variability for at least the past 200 ka

geomagnetic field intensity at hawaii for the last 420 kyr from the hawaii scientific drilling project core big island hawaii four hundred twenty five new paleointensity thellier thellier determinations out of 545 analyzed samples have been obtained from core hsdp which penetrates about 1000 meters 208 flows of the mauna loa and mauna kea volcanic series encompassing the last 420 kyr rock magnetic investigations identify pseudo single domain magnetite as the main magnetic mineral inclinations are shallower than expected from a geocentric dipole field but are consistent with data from other geographical regions at the same latitude the inclination record reveals three episodes of negative inclination whose interpolated age correlates well with that of known geomagnetic events the paleointensity record from the mauna loa sequence is not very detailed and does not allow precise comparison with other data in the 0 50 kyr interval the record from the mauna kea sequence on the contrary is very detailed and documents relatively short lived episodes of low and high field strength from 15 to 60 t the average virtual dipole moment 8 7 3 0 10 22 a m 2 is not significantly different from the value reported by kono and tanaka 1995 for the last 2 5 myr a comparison with other data from hawaii and other geographical regions is described in detail there are no drastic changes in paleointensity with the inclination anomaly in agreement with previous results from hawaii but in contrast with most published results which however consider data from polarity transition spectral analysis of a particularly detailed portion of the record between 420 and 326 kyr documents significant periodicities at 36 8 5 and 4 ka in the inclination record but not in the intensity record suggesting that changes in time of the inclination are to a certain extent independent from those of the intensity

geomagnetic field secular variation in pacific ocean a bayesian reference curve based on holocene hawaiian lava flows abstract hawaii is an ideal place for reconstructing the past variations of the earth s magnetic field in the pacific ocean thanks to the almost continuous volcanic activity during the last 10 000 yrs we present here an updated compilation of palaeomagnetic data from historic and radiocarbon dated hawaiian lava flows available for the last ten millennia a total of 278 directional and 66 intensity reference data have been used for the calculation of the first full geomagnetic field reference secular variation sv curves for central pacific covering the last ten millennia the obtained sv curves are calculated following recent advances on curve building based on the bayesian statistics and are well constrained for the last five millennia while for older periods their error envelopes are wide due to the scarce number of reference data the new bayesian sv curves show three clear intensity maxima during the last 3000 yrs that are accompanied by sharp directional changes such short term variations of the geomagnetic field could be interpreted as archaeomagnetic jerks and could be an interesting feature of the geomagnetic field variation in the pacific ocean that should be further explored by new data

late pleistocene geomagnetic excursion in icelandic lavas confirmation of the laschamp excursion in 1980 kristjansson and gudmundsson 1 reported a late glacial geomagnetic excursion in three hills in the reykjanes peninsula iceland with shallow negative inclinations and westerly declinations they named it the skalamaelifell excursion more extensive field work has identified the same excursional paleomagnetic direction declination 258 inclination 15 at four additional outcrops in a 10 10 km area in the reykjanes peninsula the excursion lavas are olivine tholeiites with similar petrography and chemical compositions paleointensity determinations by the thellier method average 4 2 0 2 t for 8 samples more than an order of magnitude weaker than the present geomagnetic field in iceland together these results suggest extrusion of the excursion lavas in a very brief span of time probably less than a few hundred years kar dating of the excursion lavas gives a mean age for 19 determinations of 42 9 7 8ka 2 compilation of thirty kar ages of the laschamp and olby flows by three laboratories yield a new age for the laschamp excursion in france of 46 6 2 4ka 2 the age of the excursion in southwestern iceland is statistically indistinguishable from the laschamp excursion at the 95 confidence level and both have very low paleointensities therefore we suggest that the laschamp and olby flows in france and the skalamaelifell units of iceland recorded essentially the same geomagnetic excursion differences in the virtual paleomagnetic poles vgps of these excursions may be due to 1 the probable non dipole character of the geomagnetic field during the excursion 2 rapid geomagnetic secular variation and possible small age differences of the extrusive rocks in france and iceland and or 3 crustal magnetic anomalies which might dominate the local geomagnetic field during the excursion at either or both locations

paleosecular variation and the average geomagnetic field at 20 latitude we assembled a new paleomagnetic directional data set from lava flows and thin dikes for four regions centered on 20 latitude hawaii mexico the south pacific and reunion we investigate geomagnetic field behavior over the past 5 myr and address whether geographical differences are recorded by our data set we include inclination data from other globally distributed sites with the 20 data to determine the best fitting time averaged field taf for a two parameter longitudinally symmetric zonal model values for our model parameters the axial quadrupole and octupole terms are 4 and 6 of the axial dipole respectively our estimate of the quadrupole term is compatible with most previous studies of deviations from a geocentric axial dipole gad field our estimated octupole term is larger than that from normal polarity continental and igneous rocks and oceanic sediments but consistent with that from reversed polarity continental and igneous rocks the variance reduction compared with a gad field is 12 and the remaining signal is attributed to paleosecular variation psv we examine psv at 20 using virtual geomagnetic pole vgp dispersion and comparisons of directional distributions with simulations from two statistical models regionally the hawaii and reunion data sets lack transitional magnetic directions and have similar inclination anomalies and vgp dispersion in the pacific hemisphere hawaii has a large inclination anomaly and the south pacific exhibits high psv the deviation of the taf from a gad contradicts earlier ideas of a pacific dipole window and the strong regional psv in the south pacific contrasts with the generally low secular variation found on short timescales the taf and psv at hawaii and reunion are distinct from values for the south pacific and mexico demonstrating the need for time averaged and paleosecular variation models that can describe nonzonal field structures investigations of zonal statistical psv models reveal that recent models are incompatible with the empirical 20 directional distributions and cannot fit the data by simply adjusting relative variance contributions to the psv the 20 latitude data set also suggests less psv and smaller persistent deviations from a geocentric axial dipole field during the brunhes

low intensity of the geomagnetic field in early jurassic time from a large collection of jurassic continental tholeiites cropping out in europe and africa we selected 90 samples for paleointensity determinations the samples were carefully selected to avoid any secondary magnetizations especially viscous magnetization use of the thellier method reveals that magnetic property changes due to heating begin often at quite low temperatures but fortunately without modifying noticeably their natural remanent magnetization thermoremanent magnetization ratio twenty eight well clustered paleointensity estimates were obtained from two european dikes that were emplaced during early jurassic time the kerforne dike at brenterc h in brittany northwestern france and the messejana dike on the iberian peninsula spain and portugal virtual dipole moments calculated from both magmatic units are similar and only about one third of present day values these new data lend support to the recently postulated low dipole moment of the mesozoic geomagnetic field

paleointensity of the earth s magnetic field and k ar dating of the louchadiere volcanic flow central france new evidence for the laschamp excursion we report paleointensity results of the earth s magnetic field from a late pleistocene lava flow louchadiere central france which recorded an intermediate geomagnetic field direction 5 sites mean declination 114 1 inclination 58 2 k 130 95 6 7 new k ar age determinations confirm that this flow is contemporaneous with the laschamp and olby flows and that this excursion occurred around 45ka ago using the thellier double heating method reliable paleointensities have been obtained for ten samples from three different sites providing an average field strength of 12 9 t 3 3 this low value and previous results of the laschamp excursion from france and iceland confirm that the earth s magnetic field was in an intermediate state during the laschamp excursion

paleomagnetic secular variation and relative paleointensity during the holocene in south china huguangyan maar lake revisited the scarcity of reliable paleomagnetic secular variation psv records from east asia especially from low latitude regions impedes better understanding of global psv mechanisms here we report on a radiocarbon dated holocene psv record from a composite 6 7 m long core collected from the high sedimentation rate huguangyan maar lake hml in subtropical tropical south china rock magnetic results demonstrate that the natural remanent magnetization resides in single domain magnetite alternating field demagnetization experiments at 1 cm spacing on u channel samples reveal six distinct inclination highs at 7 500 bce 5 100 bce 4 600 bce 3 600 3 400 bce 1 600 1 200 bce and 600 800 ce three inclination lows at 4 800 bce 600 300 bce and 1 000 1 300 ce and three eastward declination trends at 3 600 3 200 bce 2 600 2 400 bce and 400 bce to 200 ce the similarity between the hml psv record and other independently dated records from east asia and geomagnetic field models corroborates the robustness of our age model and holocene psv record strikingly centennial to millennial scale psv features of the hml are comparable within age uncertainties with other holocene records from europe north america and canada suggesting that such directional patterns are likely to be hemispheric in scale although relative paleointensity data of hml are affected by environmental factors e g organic matter diagenesis the record still provides a regionally important new psv reference curve whose conspicuous features may serve as stratigraphic markers for east asian paleorecords

geomagnetic paleointensities from excursion sequences in lavas on oahu hawaii paleomagnetic data demonstrating three late tertiary excursions in the direction of the geomagnetic field recorded in sequences of basaltic lavas on the island of oahu hawaii were published by r r doell and g b dalrymple in 1973 we have determined geomagnetic paleointensities by the thelliers method for 14 lavas from the three sites during these experiments considerable difficulty was encountered because of the presence of titanomaghemite in many lavas and the contamination of natural remanent magnetization by lightning in many others moreover we often observed the production of spurious high temperature chemical remanent magnetization during the thellier experiments an analysis of this particularly troublesome problem is presented two of the sites showed low paleointensities associated with angular departures of the paleomagnetic field direction from that of a geocentric axial dipole which suggests that these excursions represent aborted reversals or fragments of reversals at the third site however the paleointensity did not become low as the field diverged this excursion may reflect the variation of a large nondipole source near hawaii

holocene paleosecular variation from dated lava flows on maui hawaii ow inclination and low secular variation seem to be present for at least the past 5 myear in central pacific the period covering the past 10 kyear is crucial to the study of the field variability and to fill the gap between historical field measurements and long term paleomagnetic records we have conducted a paleomagnetic study of 13 sites of basaltic lava flows from maui island with 14c ages between 10 3 and 0 2 ka two other sites dated at 45 and 730 ka were also sampled eight to 10 samples from each site were demagnetized using thermal treatment and companion specimens from the same samples were demagnetized by alternating fields af thermomagnetic and hysteresis measurements indicated that magnetite 575 c in fine grains was the dominant magnetic carrier although in many cases we also observed a low temperature phase which is likely carried by titanomagnetite with low titanium content the existence of relatively high coercivities associated with these two mineralogical phases generated overlapping components which could not be properly isolated using af demagnetization successful results were obtained after thermal demagnetization for 13 sites with a mean inclination of 36 9 4 the mean inclination inc 36 3 of the eleven sites younger than 10 5 ka is also very close to the value 37 of the geocentric axial dipole gad at the site latitude but the angular dispersion of the virtual geomagnetic poles vgps about the spin axis 9 with n 13 and 6 7 with n 11 is significantly lower than the predictions of the models of paleosecular variation at this latitude the inclination variations for the past 10 kyear are in excellent agreement with the very detailed dataset which has previously been obtained from the big island of hawaii the mean inclination of all volcanic records n 132 is 1 8 lower than expected but this is likely caused by the lack of records between 5 and 7 ka b p we note also that the inclinations from lake waiau sediments big island are shallower than those of the volcanic records mauna kea hawaii and were thus probably affected by compaction we infer that there is no striking evidence for an inclination anomaly under hawaii during this period being aware of the need for additional records covering at least the 5 7 ka b p time interval the absence of a systematic deviation going beyond the dispersion of the inclinations obtained for the past millions of years neither pleads for a significant long term persistent anomaly however all studies report a low dispersion of the vgps which must reflect low secular variation

geomagnetic field intensity over the last 42 000 years from core soh 4 big island hawaii a record of the absolute geomagnetic field intensity spanning the last 42 kyr has been obtained from 100 lava flows 187 successful thellier experiments recovered in the top 468 m of core from the soh 4 well on the island of hawaii assuming a linear extrusion rate between the present and the 42 kyr date obtained with a refined k ar technique this corresponds to an average of one flow every 420 years rock magnetic analysis identifies low ti content and high ti content magnetites in 75 and 25 of the samples respectively as the main carriers of magnetization the low ti magnetites are very stable while some transformations occur upon heating in the other samples but both groups allow reliable paleointensity determinations the directional record documents inclinations consistent with a geocentric dipole field for the last 16 kyr and between 38 and 42 kyr between 16 and 38 kyr shallower and in some cases negative inclinations were observed because of the high extrusion rate and the exceptional percentage of reliable intensity determinations the record of the geomagnetic field intensity over the last 42 kyr has a resolution comparable to the best sedimentary records the geomagnetic field intensity at hawaii appears to have varied between 18 and 79 t and is characterized by large fluctuation with peak to peak amplitudes of 20 25 t the observed values are quite consistent with other values obtained from hawaii for the last 31 kyr mankinen and champion 1993a b tanaka and kono 1991 coe et al 1978 the mean intensity over the explored time interval 45 t is slightly higher than the present value of the geomagnetic field in hawaii

continuous geomagnetic field models for the past 7 millennia 1 a new global data compilation 1 a global data set of archeomagnetic and paleomagnetic data covering the past 7000 years has been compiled it consists of 16 085 results of inclination 13 080 of declination and 3188 of intensity for the time span 5000 bc to 1950 ad declination and inclination data come partly from existing databases and partly from original literature a new global compilation of intensity data for the millennial scale is included data and dating uncertainties are discussed as we attempted to obtain an internally coherent data set the global distribution of the data is very inhomogeneous in both time and space all the data are compared to predictions from the previous 3000 year global model cals3k 1 this collection of data will be useful for global secular variation studies and geomagnetic field modeling although southern hemisphere data are still underrepresented in particular we will use it in a further study to update and extend the existing global model cals3k 1 the huge increase in data compared to the previous compilation will result in significant changes from current models as we might have missed some suitable data we encourage the reader to notify us about any data that have not been included yet and might fit in as improving our global millennial scale models remains our aim for the future the data files described in this paper are available from the earthref digital archive erda at http earthref org cgi bin erda cgi n 331

the effects of aliasing and lock in processes on palaeosecular variation records from sediments studies of sedimentary records of palaeointensity variation report periods as long as 50 kyr archaeointensity data show geomagnetic periods of 2 kyr with large ampli tudes sampling of the sedimentary records can be as coarse as 8 kyr so the apparent long periods could be caused by aliasing the sedimentary lock in process could smooth the record and remove short periods thereby preventing aliasing from occurring we examine possible effects of aliasing by creating a 100 kyr long synthetic sequence of palaeointensity variation with a similar spectrum to that of archaeomagnetic data from the last 12 kyr and resampling at longer intervals with no lock in smoothing aliasing produces spurious energy in the spectra at long periods when smoothing by the sedimentation process is applied the amplitudes of the aliased peaks are reduced but still cause significant spurious long period energy in the spectra for some sedi mentation rates we restrict our analysis to palaeointensity data but similar problems may also exist for coarsely sampled directional data to avoid aliasing we recommend a maximum sampling interval of 2 kyr

time averaged paleomagnetic field at the equator complete data and results from the galapagos islands ecuador 1 we present here the complete paleomagnetic laboratory results from a collection of approximately 1500 oriented cores from all 16 of the galapagos islands ecuador collected by allan cox in 1964 1965 but nearly all previously unpublished the islands are located in the eastern pacific ocean within 1 4 of latitude from the equator and range in age from historically erupted to 3 ma mostly determined by published k ar and 3he isotopic dating the number of sites collected on each island ranges from 1 to 28 for a total of 186 after combining duplicate site mean directions 149 are used for an overall mean direction and 8 represent excursions and one reversal path divided by geomagnetic polarity chron 110 site means are brunhes or jaramillo normal polarity 27 are matuyama reversed polarity and 12 are gauss both polarities we have completed the magnetic cleaning that was commenced in the late 1960s secondary mostly viscous magnetizations were nearly all removed by alternating field demagnetization at 10 mt we have used the so called blanket cleaning method generally at 10 mt all sites were in basalt flows and gave good paleomagnetic results none was rejected in toto and only a few core specimens were magnetically unsatisfactory nearly all sites had eight independently oriented cores and within site angular standard deviations of directions range from 1 to 8 we used both fisher and bingham statistics to analyze the data and found that many of the direction populations are strongly elongate along the paleomagnetic meridian while the corresponding virtual pole vgp populations are essentially circularly distributed the paleomagnetic poles calculated as the means of vgps are as follows brunhes and jaramillo north latitude 86 9 east longitude 245 1 and 95 confidence radius a95 1 9 matuyama latitude 87 2 longitude 158 2 and a95 3 8 gauss latitude 83 0 longitude 204 7 and a95 7 0 these paleomagnetic poles do not differ significantly from one another but the brunhes and jaramillo combined pole is significantly near sided with respect to the galapagos as is the overall mean pole which is at latitude 87 1 and longitude 227 6 with a95 1 7 omitting the excursion and reversal path data the overall angular standard deviation of vgps is 11 7 with lower and upper 95 confidence limits of 10 8 and 12 7 respectively in good agreement with previously published values for near equatorial sampling latitudes

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

geomagnetic field control of 14c production over the last 80 ky implications for the radiocarbon time scale we have used a recent estimate of the geomagnetic dipole field strength over the last 80 ky to examine the extent to which changes in geomagnetic field intensity have affected the radiocarbon time scale the magnetic record was converted into a 14c production signal which was then used to produce a synthetic atmospheric 14c record using a two box model for the carbon cycle this calibration suggests that the geomagnetic field has been the main factor governing the production of cosmogenic 14c atoms over the period of time considered and that 14c changes due to other causes such as climatic changes have been of lesser importance the results indicate that the radiocarbon ages are 2 to 3 ky younger than true age in the interval 18 40 ky b p in good agreement with recent u th and 14c age comparisons moreover the results predict that a negligible difference between true and 14c ages should be observed around 45 50 ky

determination of domain structure in igneous rocks by alternating field and other methods abstract af alternating field demagnetization arm anhysteretic remanent magnetization and strong field hysteresis properties of a large collection of mostly continental igneous rocks are reported here the collection included rocks whose magnetic carriers were believed from previous work to be of one of three types md multidomain sd psd single domain pseudo single domain or a bimodal mixture of md grains e g discrete opaques and sd psd material e g silicate inclusions two series of subaerial basalts with a full range of deuteric oxidation classes included examples of all three classes of behaviour sd psd rocks have relatively hard inflected af decay curves decay rate initially increasing then decreasing md rocks have soft exponential like decay curves and bimodal rocks have a combination of these characteristics relative hardnesses of normalized decay curves of remanences acquired in weak intermediate and strong fields the lowrie fuller test are also distinctively different for the three classes and the results support the theory developed in an accompanying paper 1 that lowrie fuller characteristics are an expression of the shapes of decay curves the lowrie fuller test although its result can be expressed as a numerical parameter is not capable of fine scale classification of domain structure or grain size the shape of the arm induction curve does have a quasi continuous variation with grain size however the parameter ar j rs initial anhysteretic susceptibility normalized to saturation remanence which is easily measured with standard paleomagnetic instrumentation is potentially useful for magnetic granulometry although ar itself was not diagnostic of grain size

a potpourri of pristine moon rocks including a vhk mare basalt and a unique augite rich apollo 17 anorthosite analysis of previously unstudied apollo lithic fragments continues to yield surprising results among this year s samples is a small anorthosite fragment 76504 18 the first pristine anorthosit e found from apollo 17 this unique lithology strongly resembles the main type of apollo anorthosites ferroan anorthosites but 76504 18 has a far higher ratio about 9 of high ca pyroxene to low ca pyroxene higher na in its plagioclase higher contents of incompatible elements such as ree and a higher eu al ratio assuming that 76504 18 is a cumulate with less than 45 trapped liquid its parent melt probably had a negative eu anomaly in all these respects 76504 18 seems more likely than other ferroan anorthosites to be closely related to typical mare basalts apparently this anorthosite was among the latest to form by plagioclase flotation above a primordial magmasphere typical mare basalt source regions probably accumulated at about the same time or even earlier another previously unstudied fragment 14181c is a vhk very high potassium basalt that is similar in most respects to typical aluminous apollo 14 mare basalt but has a k la ratio of 1050 this lithology probably formed after a normal apollo 14 mare basaltic melt partially assimilated granite new data for siderophile elements in apollo 12 mare basalts indicate that only the lowest of earlier data are trustworthy as being free of laboratory contamination

paleointensity applications to timing and extent of eruptive activity 9 10 n east pacific rise placing accurate age constraints on near axis lava flows has become increasingly important given the structural and volcanic complexity of the neovolcanic zone at fast spreading ridges geomagnetic paleointensity of submarine basaltic glass sbg holds promise for placing quantitative age constraints on near axis flows in one of the first extensive tests of paleointensity as a dating tool or temporal marker we present the results of over 550 successful sbg paleointensity estimates from 189 near axis 4 km sites at the east pacific rise 9 10 n paleointensities range from 6 to 53 t and spatially correspond to the pattern expected from known temporal variations in the geomagnetic field samples within and adjacent to the axial summit trough ast have values approximately equal to or slightly higher than the present day samples out to 1 3 km from the ast have values higher than the present day and samples farther off axis have values lower than the present day the on axis samples 500 m from the ast provide a test case for using models of paleofield variation for the past few hundred years as an absolute dating technique results from samples collected near a well documented eruption in 1991 1992 suggest there may be a small negative bias in the paleointensity estimates limiting resolution of the dating technique possible explanations for such a bias include local field anomalies produced by preexisting magnetic terrain anomalously high magnetic unblocking temperatures leading to a small cooling rate bias and or the possibility of a chemical remanence produced by in situ alteration of samples likely to have complicated thermal histories paleointensity remains useful in approximating age differences in young flows and a clear along axis paleointensity contrast near 9 50 n is suggestive of a 150 200 year age difference paleointensity values of off axis samples are generally consistent with rough age interpretations based on side scan data furthermore spatial patterns in the paleointensity suggest extensive off axis flow emplacement may occur infrequently with recurrence intervals of 10 20 kyr results of a stochastic model of lava emplacement show that this can be achieved with a single distribution of flows with flow size linked to time between eruptions

age dependent variation in the magnetization of seamounts the magnetization of pacific seamounts shows a surprisingly large variation when viewed as a function of time similar to normal upper oceanic crust the basalts that form the external shell of volcanic seamounts have a high average magnetization immediately after formation this high magnetization decays rapidly with time resulting in a factor of 5 reduction in intensity in less than 500 000 years because submarine volcanoes appear to consist of the same basalts that form the upper part of the seafloor low temperature oxidation of the magnetic mineral titanomagnetite is assumed to be responsible for the reduction in magnetization for both types of crust some large oceanic volcanoes are of sufficient volume that the upper capping rocks erupt subaerially and submerge completely due to thermal subsidence only after several million years these capping rocks can be locally abundant i e hawaii are exposed to a different post formation environment and can have a magnetic mineralogy that is different from submarine volcanoes in spite of these differences the magnetization of subaerially erupted rocks of the hawaii emperor chain also may show a similar rapid reduction in intensity with age finally seamounts of cretaceous age show a factor of 2 higher average magnetization when compared to tertiary seamounts similar to the previously observed increased magnetization of cretaceous seafloor basalts recovered from deep sea drilling project ocean drilling project cores this increase in cretaceous magnetization appears to be reflected in the increased amplitude of magnetic anomalies in some but not all of the ocean basins prior and subsequent to the cretaceous normal superchron the processes potentially responsible for the elevated cretaceous magnetization values include a high geomagnetic field intensity large scale variations in whole rock major element geochemistry of the extrusive rocks or a systematic acquisition of a secondary magnetization during the cretaceous normal superchron

palaeointensity in the pelagic realm marine sediment data compared with archaeomagnetic and lake sediment records summary four box cores collected from the ontong java plateau during the eurydice expedition have been used to make relative geomagnetic palaeointensity measurements rock magnetic measurements on the sediments show that they are characterized by a uniform magnetic mineralogy and that they are suitable for relative intensity estimates these are obtained by normalizing the nrm by an arm imparted in a low dc bias field the palaeoceanographic event known as the preservation spike is used to establish a crude time scale for the record so that it may be compared with other data from the same region and also with global palaeointensity estimates the marine sediment data are quite similar to australian intensity data from lake sediments and archaeomagnetic sources but as might be expected exhibit some obvious differences from the global record