a high resolution paleointensity stack of the past 14 to 68 ka from black sea sediments abstract detailed paleo and mineral magnetic analyses of a sediment composite record from the southeastern black sea yielded a high resolution well dated paleointensity record though hampered by some larger hiatuses in some cores and contaminated by diagenetically formed greigite the paleomagnetic composite record obtained from the preserved primary detrital magnetite phase reflects a highly dynamic geomagnetic field during the last glacial period relative variations of paleointensity inferred from the sediments magnetisations were converted into a record of the virtual axial dipole moment vadm lowest vadm values are linked with the laschamp 0 50 10 22 a m 2 at 41 0 ka the norwegian greenland sea 1 5 10 22 a m 2 at 64 5 ka and the mono lake 3 0 10 22 a m 2 at 34 5 ka geomagnetic excursions the fully reversed field during the laschamp excursion exhibits a vadm of 2 0 10 22 a m 2 which is more than 25 of the present day axial dipole moment 7 628 10 22 a m 2 rates of change calculated from the black sea vadm record also give some information on how to assess the global decay of the present day geomagnetic field which is significantly enhanced in the area of the south atlantic anomaly comparison with provided c 14 and 10 be records confirm partly in the very detail the non linear anti correlation of geomagnetic field intensity and the production of cosmogenic radionuclides in the earth s upper atmosphere however discrepancies in the timing of lows and highs in the compiled records points out that the combination of different data sets from different archives remains a challenge

soil water flux in the southern great basin united states temporal and spatial variations over the last 120 000 years the disposal of hazardous and radioactive waste in arid regions requires a thorough understanding of the occurrence of soil water flux and recharge soil water chemistry and isotopic data are presented from three deep vadose zone boreholes 230 m at the nevada test site located in the great basin geographic province of the southwestern united states to quantify soil water flux and its relation to climate the low water contents found in the soils significantly reduce the mixing of tracers in the subsurface and provide a unique opportunity to examine the role of climate variation on recharge in arid climates tracing techniques and core data are examined in this work to reconstruct the paleohydrologic conditions existing in the vadose zone well beyond the timescales typically investigated stable chloride and chlorine 36 profiles indicate that the soil waters deep in the vadose zone range in age from approximately 20 000 to 120 000 years secondary chloride bulges that are present in two of the three profiles support the concept of recharge occurring at or near the last two glacial maxima when the climate of the area was considerably wetter and cooler the stable isotopic composition of the soil water in the profiles is significantly more depleted in heavy isotopes than is modern precipitation suggesting that recharge under the current climate is not occurring at this arid site past and present recharge appears to have been strongly controlled by surface topography with increased incidence of recharge where runoff from the surrounding mountains may have been concentrated the data obtained from this detailed drilling and sampling program shed new light on the behavior of water in thick vadose zones and in particular show the sensitivity of arid regions to the extreme variations in climate experienced by the region over the last two glacial maxima

the late pleistocene palaeoclimatic record of north atlantic deep sea sediments revealed by mineral magnetic measurements eight cores of north atlantic deep sea sediment are correlated using simple rapid and non destructive measurements of magnetic susceptibility and isothermal remanence this stratigraphy is demonstrably related to late pleistocene palaeoclimatic variations by comparison with the oxygen isotope and microfossil records of these cores fluctuations in the concentration dependent magnetic properties of the cores largely reflect their lithological composition consisting of climatically modulated cycles of marl glacial horizons and nannofossil chalk interglacial horizons high magnetic concentration characterises glacial horizons which correspond to periods when carbonate productivity was severely restricted and ice rafted detrital inputs were prolific coincident with the migration south of the north atlantic polar front to within 3 of latitude north of the coring site in contrast interglacial horizons are characterised by low magnetic concentration and high carbonate content of the sediment consequent on the retreat of the polar front and the re establishment of coccolith floras in its wake during these periods ratios formed between individual magnetic parameters are independent of magnetic concentration and are thus free of any climatic influences wrought by carbonate oscillations in this study however such ratios exhibit a definite response to palaeoclimatic variations attributable to differences in the composition and grain size of the magnetic mineral assemblage of glacial vis a vis interglacial horizons of the cores these differences in mineral magnetic composition are substantially confirmed by mossbauer effect studies of selected samples representing peak glacial and interglacial horizons of one core such glacial interglacial contrasts in the concentration mineralogy and grain size of the magnetic assemblage most probably result from variations in the source and flux density of detritral inputs induced by changes in climate x ray diffraction studies of the same core indicate that variations in the mineralogy of the detrital fraction of the sediment are consistent with those of the magnetic assemblage together these data point to the conclusion that a subsidiary low latitude possibly aeolian detrital flux supplements high latitude ice rafted input in glacial horizons of the cores the low latitude flux however is also identifiable in interglacial horizons independent evidence suggests that this low latitude aeolian flux may be derived from the sahel region of north africa and transported to the coring site by harmattan winds associated with saharan dust outbreaks

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

a new rock magnetic approach to selecting sediments for geomagnetic paleointensity studies application to paleointensity for the last 4000 years the hypothesis that the ratio of detrital remanent magnetization to anhysteretic remanent magnetization drm arm for sediment samples is a measure of relative geomagnetic paleointensity is critically evaluated by two distinct approaches one approach is a detailed rock magnetic examination of the implicit assumptions of the drm arm method and the construction of a selection process by which to identify sediments that conform to requirements satisfying these assumptions sediments are uniform with respect to drm arm ratio if they contain magnetite in the 1 15 ym particle size range as the predominant magnetic mineral and have variations in magnetite content of less than 20 30 times the minimum concentration the drm arm ratios of these sediments should provide estimates of relative geomagnetic paleointensity relative particle size variations in magnetite are detected with a plot of anhysteretic susceptibility xarm versus low field susceptibility x and the size range 1 15 m is approximately identified by high field hysteresis parameters a rock magnetic evaluation of leboeuf lake sediments with these techniques indicates that these sediments are suitable for a relative plaeointensity study the second approach to evaluating the drm arm ratio as a measure of relative paleointensity is direct comparison with absolute paleointensity data a comparison between the leboeuf lake estimates and thellier thellier results from the western united states supports the conclusion that suitable sediments can record geomagnetic paleointensity fluctuations

fossil bacterial magnetite in deep sea sediments from the south atlantic ocean the magnetostratigraphy of deep sea sediment cores has proven to be a powerful and high resolution method of dating and comparing sedimentary records in contrast to this widespread application relatively few attempts have been made to analyse and identify the mineralogy grain size and origin of the carrier of the remanent magnetization1 2 it is widely assumed that dominant ferrimagnetic minerals in deep sea sediments are of lithogenic origin that is that they derive from oceanic or continental detritus volcanic ashfalls micrometeorites hydrothermal precipitation or chemical mineralization processes a very different view was taken by kirschvink and lowenstam3 who suggested that fossil remnants of magnetite mineralizing organisms in particular magnetotactic bacteria might play an important part in the magnetization of sediments deposited in certain detritus poor aquatic systems we have used rock magnetic diagnostic methods to characterize the magnetic phases in deep sea sediments from the south atlantic these phases were then extracted and studied with the electron microscope our results indicate that fossil bacterial magnetite is the main carrier of the natural remanent magnetization in sediments ranging in age from quarternary to eocene different types of bacterial magnetosomes were detected most of them occurring in chains of identical particles and nearly all of them falling into the theoretically determined4 size range of single domain magnetitite

a comparison of different magnetic methods for determining the relative grain size of magnetite in natural materials some results from lake sediments abstract a new rapid method for identifying relative grain size variations in magnetic involves the parameter anhysteretic susceptibility arm i e specific arm obtained in a 1 oe steady field which is particularly sensitive to the single domain sd and small pseudo single domain psd grains of the finer magnetite fraction a second parameter low field susceptibility is relatively more sensitive to the coarser magnetite fraction larger psd and smaller multidomain md grains we can then obtain a measure of the ratio of coarse to fine grain magnetite for large numbers of samples by plotting arversus a simple idealized model based on sized magnetite samples is proposed to explain the use of the armversus plot for detecting relative grain size changes in the magnetic content of natural materials the sediments of three lakes that contain magnetite or a similar magnetic carrier and have a wide range of values of arm and are used to test the model the model is used to interpret the magnetic variations observed and the interpretations are supported by high field hysteresis measurements of the same sediments the combination of the high field hysteresis method of day et al 1 and the arm vs method is a powerful technique allowing the rapid identification of both the relative grain size and domain state for large numbers of samples containing magnetite the armvs method should be used as an intial means of identifying distinct groups of samples the high field hysteresis method should then be applied to a few representative samples from each group to confirm the initial interpretation

10be and 26al production rates deduced from an instantaneous event within the dendro calibration curve the landslide of kofels otz valley austria surface exposure dating requires the knowledge of cosmogenic nuclide production rates when determining time integrated production rates the exposure ages of the calibration samples need to be accurately known the landslide of kofels austria is very well suited for this purpose it is the largest landslide in the crystalline alps of austria dating back to 7800 100 years bc ams 14c dating of buried wood which is well within the 14c dendro calibration curve exposed quartz veins were sampled from the tops of large boulders from the toe of the landslide for analysis of 10be and 26al to calculate sea level high geomagnetic latitude 60 open sky radionuclide production rates corrections were applied for altitude and latitude for shielding by surrounding mountains for sample geometry vegetation and snow cover and for sample thickness the production rates for an exposure age of 10 000 years are 5 75 0 24 10be atoms yr g sio2 and 37 4 1 9 26al atoms yr g sio2 a 26al 10be ratio of 6 52 0 43 can be calculated the influence of the geomagnetic field on these production rates has been estimated using two different geomagnetic field records our production rates should be a good approximation for the use of surface exposure dating between about 5000 and 13 000 years bp

paleointensity of the earth s magnetic field during the laschamp excursion and its geomagnetic implications the reversed paleomagnetic direction of the laschamp and olby flows represents a specific feature of the geomagnetic field this is supported by paleomagnetic evidence showing that the same anomalous direction was recorded at several distinct sites including scoria of the laschamp volcano to examine this anomalous geomagnetic fluctuation we studied the paleointensity of the laschamp and olby flows using the thellier method twenty five samples were selected for the paleointensity experiments and from seven we obtained reliable results because the paleointensity results of the olby and laschamp flows as well as laschamp scoria are very similar they can be represented by a single mean paleointensity f 7 7 t considering that this low paleointensity is less than 1 6 of the present geomagnetic field and is more characteristic of transitional behavior our results suggest that the paleomagnetic directions of the laschamp and olby flows were not acquired during a stable reversed polarity interval a more likely explanation is that the laschamp excursion represents an unsuccessful or aborted reversal

paleoenvironmental implications of rock magnetic properties of late quaternary sediment cores from the eastern equatorial atlantic rapid and nondestructive rock magnetic measurements of late quaternary sediment cores from the senegal continental slope and the sierra leone rise in the eastern equatorial atlantic correlate strongly with the specmap oxygen isotope master curve and carbonate content the rock magnetic paleoclimatic relationship results from differences in the concentration accumulation rate grain size and composition of magnetic minerals between glacials and interglacials at both sites magnetic mineral assemblages of glacial episodes are characterized by higher concentrations and accumulation rates of magnetic minerals by coarser grained ferrimagnetic minerals e g magnetite and by a lower ratio of ferrimagnetic to antiferromagnetic e g goethite hematite minerals by serving as a sensitive tracer for various deep sea sedimentary components rock magnetic properties can provide insights into changing sediment sources and fluxes and therefore are a useful addition to more traditional chemically and mineralogically based paleoceanographic techniques

limestones under salt decay tests assessment of pore network dependent durability predictors three portuguese limestones two grainstones semi rijo and moca creme commercial types and a travertine were subjected to salt crystallisation test en 12370 1999 grainstones specimens showed higher weight loss than the travertine ones results are discussed using parametric and nonparametric statistics in relation to pore space characteristics that have been considered durability predictors evaluated at the macroscopic water imbibition and microscopic mercury injection porosimetry levels several of the pore space parameters indicate differences between grainstones and travertine but these differences are much lower than the differences in weight loss results of the durability dimensional estimator dde seem to present a discrepancy with higher values in the rock type having lower weight loss travertine additionally weathering patterns of travertine samples show marked irregularities a feature that is not assessed by weight loss these irregularities are attributed to heterogeneous spatial distribution of detritic components a feature that is not assessed by pore space parameters comparing the grainstones moca creme showed higher values of mass loss and several durability predictors suggested noteworthy differences in relation to semi rijo however these differences are much lower than differences in weight loss calculated crystallisation pressures are even higher for semi rijo it is proposed that petrographical aspects of moca creme namely heterogeneity related to the presence of bioclasts and veinlets could contribute to explain the higher weathering susceptibility of this rock type these results highlight the need to include petrographical features in assessments of durability and the usefulness of visual description on the characterization of salt weathering

periodicities of palaeomagnetic intensity and palaeoclimatic variations a walsh spectral approach abstract the walsh power spectral technique has been applied to the non sinusoidal palaeoclimatic thegloborotalia menardii climatic curve and palaeomagnetic intensity variation records of the last 1 2 myr the analysis reveals in the case of palaeomagnetic variations statistically significant peaks at 90 confidence level centered around 150 60 and 24 kyr the palaeoclimatic spectrum however indicates only two significant peaks of 60 and 24 kyr the identical periodicities in both the phenomena may suggest a causal link between the geomagnetic intensity variations and climatic changes

a compartmental mixing cell approach for the quantitative assessment of groundwater dynamics in the otway basin south australia abstract transient groundwater fluxes as calculated by a groundwater flow model modflow are used as input into a compartmental mixing cell cmc model to simulate the transport of hydrochemical and isotopic species in regional groundwater systems good agreement between modeled and measured concentrations of solutes in groundwater provides greater confidence in estimating aquifer parameters for the flow model the cmc model is currently developed for multi layered groundwater systems and allows for both recharge to the uppermost aquifer and upward or downward leakage between individual aquifers the combination of the cmc approach and modflow allows chemical and hydraulic data to be analysed simultaneously to allow for improved conceptualization of the groundwater system the use of transient fluxes reflects the temporal scale and variability of the chemical tracers as well as the temporal variability of hydraulic conditions the model is applied to a transect in the gambier embayment of the otway basin in south australia to quantify mixing between two main aquifer systems this groundwater system is hydraulically connected to the ocean in the west and as a result would have been affected by sea level changes over the last 30 000 yr radiocarbon simulations helped to quantify a major zone of downward leakage to the confined aquifer with modeled rates of leakage ranging 2 9 mm yr

a dynamical model of high latitude convection derived from superdarn plasma drift measurements 1 a discrete set of climatological patterns of high latitude ionospheric convection are derived using line of sight plasma drift data from the super dual auroral radar network superdarn the patterns are derived independently for the northern hemisphere and southern hemisphere and for varying solar wind interplanetary magnetic field imf and dipole tilt angle conditions by interpolating between discrete patterns a dynamical model of convection is obtained which can uniquely specify the high latitude electrostatic potential distribution for a wide range of solar wind imf and dipole tilt parameter values accounting for solar wind velocity dependencies in convection leads to better resolving the large scale convection pattern as compared to previous statistical models based on superdarn data it is shown that the mesoscale features of the climatological model compare favorably to the features seen in instantaneous patterns of convection observed with superdarn comparison of the model to other statistical or empirical models derived from ground and space based measurements shows good agreement with most models

eastern australian geomagnetic field intensity over the past 14000 yr summary two north eastern australian volcanic crater lake cores have been used to obtain relative intensity estimates for the geomagnetic field arm imparted in a low dc bias field has been used as a normalizing parameter the intensity fluctuations in the two lakes are in excellent agreement with each other and with south eastern australian archaeointensity data over their coeval time spans this strongly suggests that the same sources are influencing the geomagnetic secular variation throughout eastern australia at this time the relative intensity records go back to about 14000yr bp thereby extending currently available recent australian intensity records by some 7000 yr

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

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

tyrrhenian sea tephrochronology of the oxygen isotope record for the past 60 000 years abstract a chronology of the marine oxygen isotope record of the past 60 ka is proposed by correlating eight marine ash layers with terrestrial volcanic deposits dated by the 14c and or k ar methods detailed oxygen isotope analyses were made on the planktonic foraminifera of three cores collected in the central tyrrhenian sea the origin of the ash layers was determined by major and trace elements analyses then the major pyroclastic terrestrial deposits were sampled and analysed in order to provide land sea correlations two ash layers occurred during the warm event that followed the first step of the deglaciation and which was dated at 12 6 0 5 ka b p another ash layer is present near the last glacial maximum that we have dated at 17 5 0 3 ka b p during isotopic stage 3 five ash layers were recognized and dated which allowed us to assign ages to the isotopically light oxygen peaks warmer interstadials at 31 1 5 ka and 33 4 1 6 ka denekamp at 37 5 2 ka hengelo and successively at 49 2 ka 52 4 2 2 ka 57 2 2 ka and 60 5 2 2 ka moershoofd complex

a preliminary palaeomagnetic study of the holocene and late wurmian sediments of lac du bouchet haute loire france summary lac du bouchet is a small maar type lake located at 45 n 4 e in central france the bottom sediments have been sampled down to a depth of 6 m with mackereth type corers a chronology has been established from pollen analytical and diatom zonations and by radiocarbon age determinations throughout the period covered which runs from 22 000 years ago through to the present time declination and inclination variations consist of oscillations of up to about 30 amplitude no abnormally large amplitude excursions in direction are recorded the main contribution of this study is the extension of the european secular variation sv records to the interval from 13000 bp years before present to 22000 bp the sediments deposited through this interval ancient dryas intc wurm originated from the erosion of the internal walls of the volcano spectrum analyses of the declination and inclination curves from 13000 to 22000 bp reveal peaks at periods of 3000 4000 1800 1150 and 700 yr cross spectrum analysis shows strong coherence of the two signals at the longest of these periods the phase relationship of all these peaks indicates clockwise rotation of the palaeomagnetic vector possibly suggesting westward drifting sources although the palaeomagnetic sv record through the holocene 0 10300 bp is compressed into 1 7m of highly organic sediments the principal features exhibited by the type curves established for uk can be identified and the resulting magnetic ages are consistent with the palynology the sense of rotation of the magnetic vector is biased equally in both senses the late glacial 10300 bp to 13000 bp is characterized by oscillating climatic conditions recent dryas allerod and bolling which resulted in poorly resolved and probably discontinuous palaeomagnetic sv records which show a strong bias to anti clockwise rotation all ages are given in radiocarbon years

north south asymmetry of the high latitude thermospheric density imf by effect previous studies have established that the y component of the interplanetary magnetic field imf by plays a role in the north south asymmetry of the high latitude plasma convection and wind the effect of the positive negative imf by in the northern hemisphere resembles the effect that the negative positive imf by would have in the southern hemisphere in this study we demonstrate that the imf by effect can also contribute to the hemispheric asymmetry of the thermospheric density we use high accuracy air drag measurements from the challenging minisatellite payload champ satellite and supermag ae index during the period 2001 2006 to examine the response of the high latitude thermospheric density to geomagnetic activity our statistical analysis reveals that the density response at 400 km is greater in the southern hemisphere under positive imf by conditions and greater in the northern hemisphere under negative imf by conditions the results suggest that the imf by effect needs to be taken into account in upper atmospheric modeling for an accurate description of high latitude densities during periods of enhanced geomagnetic activity

normalized remanence in sediments of the western equatorial pacific relative paleointensity of the geomagnetic field piston cores from the ontong java plateau collected during the eurydice expedition have been used to estimate regional geomagnetic paleointensity rock magnetic measurements of the sediments show that 1 the remanence is carried by stable pseudo single domain magnetite 2 the grain sizes and concentrations are relatively uniform with respect to the normalization technique and 3 susceptibility is an adequate parameter for normalizing remanent intensity in order to determine relative variations in the intensity of the paleomagnetic field we therefore have measured the remanent intensity both natural and demagnetized to 15 mt and susceptibility of archive quarters of three cores at centimeter intervals using an automated through bore magnetometer a further test of the normalized intensity records is provided by the coherence function between the normalized remanence and the susceptibility of the cores one of our cores displays high coherence over a broad range of frequencies while for two others coherence is insignificantly different from zero at the 95 level of confidence oxygen isotopic records paleontological datum levels and the brunhes matuyama boundary provide an independent temporal framework for inter core comparison when the two cores which pass our acceptance criteria are placed on a common time scale the coherence between the normalized intensity records is significantly different from zero at well above the 95 level of confidence suggesting some common input signal over the same band of frequencies furthermore there is a great deal of similarity between the records from the ontong java plateau and a record from core rc10 167 taken some 3000 km to the north the distribution of intensities obtained from these sedimentary records has the same shape as the absolute virtual dipole moment data obtained from lava flows 0 5 ma with a slightly smaller standard deviation the inter core agreement within a single region the similarity of records from 3000 km apart and the general correspondence of statistical character of these sediments with lava flow data all argue that the normalized intensity records presented here reflect variations in the intensity of the ancient geomagnetic field spectral analysis of the paleointensity time series shows that there a is significant enhancement of variance concentrated in frequencies around 33 thousand years over the period 0 350 ka this feature is however lacking for the period 350 700 ka

absolute paleointensities between 60 and 160 kyear bp from mount etna sicily abstract paleointensity measurements were conducted on 17 distinct lava flows from mount etna sicily in order to obtain records of absolute paleointensities for the period covering the last 60 160 kyear a complementary aspect of this study was to improve and extend the calibration of relative paleointensities obtained for the same interval from sedimentary sequences based on absolute values of virtual axial dipole moments seventy three samples have been subjected to double heating thellier coe experiments under vacuum and care was taken to perform additional studies dealing with the magnetic mineralogy and to scrutinize the effects of heating upon magnetic minerals potassium argon k ar datings provided precise ages in most cases absolute paleointensities were obtained with success for ten lavas flows the succession of the virtual axial geomagnetic dipoles vadm oscillate between extrema centered around a mean value of 6 3 2 6 10 22 a m 2 this result is in good agreement with the sedimentary record published for the same period and confirms that there has been an appreciable decrease in geomagnetic field intensity between 80 and 150 kyear

paleointensity of the geomagnetic field during the last 80 000 years high resolution records of the relative paleointensity of the geomagnetic field have been obtained from five marine cores three duplicate records were used to estimate the regional coherency of the data within a single area tyrrhrenian sea while the two others document the field variations in the eastern mediterranean and the southern indian ocean careful investigations of distinct rock magnetic parameters have established the downcore uniformity of the sediments in terms of magnetic mineralogy and grain sizes the time depth control was provided by oxygen isotopes and small scale variations in the deposition rates were constrained by means of tephrachronology the synthetic curve calculated from the mediterranean records provides a continuous record of the intensity variations during the last 80 000 years 80 kyr which correlates well with the sparse volcanic data available for the period 0 40 kyr the fact that identical behavior is seen in both data sets and that they also compare quite well with results from a core collected in the pacific ocean establishes the truly dipolar character of these variations the dipole field moment is characterized by large scale changes as shown by the existence of pronounced drops at 39 and 60 kyr alternating with periods of higher intensity the record suggests a periodic nature for these intensity variations however the period studied is not sufficiently long to state this conclusively these results demonstrate the potential of sediments for such studies and constitute a first step towards obtaining a global paleointensity record over a long period of time

extension of the lac du bouchet palaeomagnetic record over the last 120 000 years a high resolution profile of the palaeomagnetic variations recorded in the upper 20 m of the lac du bouchet sedimentary sequence has been reconstructed from several parallel mackereth and livingstone type cores inter core correlations were established by comparing individual core profiles of magnetic susceptibility individual records of af cleaned directions were then stretched to a common depth scale and merged the mean record was then filtered and smoothed in order to reduce the background noise accelerator radiocarbon determinations and pollen analyses show that the sequence covers the last interglacial glacial cycle back to ca 120 kyr b p and allow to perform a depth to time transformation there is a noted absence of widely divergent directions back to 90 kyr b p it is estimated that the recorded signal a pdrm signal represents the geomagnetic input signal attenuated by about 19 it follows that the excursions reported for the laschamp and olby lavas if they existed at all must have been of rather short duration two durable excursions appear in narrow diameter sections at the base of three independent cores b c and d while a reverse to normal transition is possibly documented in the lowermost meter core d this part of the sequence is dated from the recognition of major climatic events related to the 18 o isotopic stages defined in the oceanic record between 116 and 95 kyr this introduces the possibility of new excursions at ca 104 and 95 kyr i e immediately after the recovery of normal polarity documenting the end of the blake event

complex behavior of the geomagnetic field during three successive polarity reversals 11 12 m y b p detailed paleomagnetic records of both the directional and relative field intensity changes occurring during three sequential polarity reversals have been obtained from marine clays of serravallian tortonian boundary age in the island of zackinthos greece the three reversals are characterized by a complicated geometrical and temporal structure periods of rapid changes of the geomagnetic field vector alternate with periods of near stationarity different episodes of collapse and recovery of the field intensity and the presence of rebounds have been documented overall the records obtained here are reminiscent of volcanic records and appear much more complex and quite different from those previously obtained from upper tortonian marine clays in crete in the same geographical area the bearing of these results on some of the published reversal models is discussed and it is also suggested that the greater complexity of the zackinthos records might be related to the fact that they are exactly coeval with the last reversal spurt documented by reversal frequency analyses

archaeomagnetic investigations of geomagnetic secular variations bulgarian archaeomagnetic directions and intensities show a satisfactory agreement with those for ukraine and paris bauer plots of declination against inclination show both clockwise and anticlockwise looping through the last eight millennia the necessity for accurate dating of samples studied is stressed

effect of imf by on thermospheric composition at high and middle latitudes 1 numerical experiments 1 magnetic storms and their effects on the thermosphere and ionosphere have been studied for many years yet there are many aspects of the thermospheric and ionospheric responses that are not understood the purpose of this paper is to show how the high latitude composition depends on the sign of the imf by component using controlled simulations with a global first principles model because the high latitude convection and neutral wind systems are strongly controlled by the imf by component it seems likely that the compositional response that is driven by high latitude forcing should also be sensitive to the by component to date no first principles modeling has been performed to test the idea of imf by effects on composition numerical experiments using model simulations provide insight into this important scientific question since the thermospheric compositional response to the convection patterns for different imf bz and by can be studied in isolation in a model in this paper we use a first principles model to determine the effect of the imf by component on the compositional response of the high latitude thermosphere we show for the first time that a clockwise rotation of the potential pattern resulting from a change from by negative to by positive drives a corresponding rotation in the wind neutral density and composition distributions by control of thermospheric composition has been invoked in the literature to explain an apparent variability in the effectiveness of auroral activity in causing thermospheric storm effects at middle latitudes as observed in global images of the far ultraviolet fuv oi 130 4 nm emission from the de 1 auroral imager however the effect in the simulations presented here is opposite from that suggested by earlier work based on de data indicating another explanation must be sought for the de results these simulations are highly relevant for interpreting data being provided by more modern uv imaging instruments on the dmsp timed and image satellites

relative variations in geomagnetic intensity from sedimentary records the past 200 000 years abstract eighteen records of relative paleointensity from marine sediments have been compiled in order to retrace the global changes in geomagnetic intensity during the past 200 kyr before dealing with any other source of noise we focused on problems inherent to the depth versus time correlations the accuracy and resolution of dating mostly deduced from isotopic stratigraphy were identified as significant factors of distortion we found also that different time scales have been used by authors after readjustment with respect to the same time scale the records were stacked together and the statistical significance of the results tested by a bootstrap technique seventeen records shown to be coherent were combined into a synthetic curve defined as sint 200 other stacks combining various subsets of data were found to be identical to sint 200 except for changes in dispersion in fact the dispersion around sint 200 could be reduced further by improving the correlations without violating the stratigraphy a procedure which at least for some records emphasizes the need for more detailed depth time correlations another characteristic emerging from the results is that paleointensity features as short as 10 kyr are observed between 120 200 ka and can thus in principle be resolved however subsequent refinement is still necessary for this period which is documented by fewer records than the following 120 kyr finally we compared sint 200 with the records from the labrador sea and the lac du bouchet ldb both of which rely mostly on 14c chronology the two records show satisfactory agreement with sint 200 when transferred with respect to the age model established by correlating their 18o labrador and susceptibility features ldb with the 18o reference record the offset of 7 kyr at 40 ka measured in both cases between the 14c and 18o ages is higher than the value deduced from a recent model of a geomagnetic field control of 14c production 1 but is not inconsistent with recent u th dates

new age for the skalamaelifell excursion and identification of a global geomagnetic event in the late brunhes chron article ar 39 ar geochronology paleomagnetism sixteen lava flows sampled from isolated hills including skalamaelifell hill on the reykjanes peninsula in southwest iceland record excursional geomagnetic field directions that correspond to virtual geomagnetic poles vgps grouped at 12 s 250 e in the eastern pacific ocean the vgps are similar to those obtained in earlier studies of these lavas for which the name skalamaelifell excursion was previously assigned based on unspiked k ar dating of nine samples from six hills it has been accepted for more than two decades that the transitionally magnetized lavas reflect the 41 ka laschamp excursion however nine new 40 ar 39 ar incremental heating experiments on four of the transitionally magnetized lavas give an age of 91 13 ka moreover 238 u 230 th isochrons determined for two of these lava flows yield a weighted mean age of 96 10 ka 2 the weighted meanof the

the blake magnetic polarity episode in cores from the mediterranean sea abstract palaeomagnetic and oxygen isotope studies on samples taken from five cores in the mediterranean area are described directions of the stable remanent magnetisation show the presence of an anomalous reversed zone during the transition between the oxygen isotope substages 5e and 5d this position corresponds to the age of about 117 kyr b p and thus to the blake polarity episode the duration of this episode is estimated to be about 6000 years the study of the anisotropy of the magnetic susceptibility shows no indication of sedimentological disturbances in recorded sections the blake episode therefore represents a stratigraphic marker which may be used to link the continental and marine palaeoclimatic and sedimentary records

11 million years of oligocene geomagnetic field behaviour summary an 11 million year long record of the oligocene geomagnetic field has been obtained from pelagic sediments of dsdp hole 522 an average sample spacing of 4 cm yielded approximately one specimen per 4 to 8 kyr the rock magnetics are remarkably consistent across the entire interval previous work demonstrated a magnetic mineralogy dominated by magnetically stable magnetite the natural remanent magnetism nrm carries an oligocene polarity timescale that is in excellent agreement with the oligocene reversal record as determined from marine magnetic anomalies mmas including many of the so called crypto chrons normalized nrm intensities from the undisturbed portions of the record yield a time series of variations with features consistent with a number of other palaeointensity time series derived from both sedimentary and lava sequences these features include consistent major decreases in palaeointensity dips at reversal boundaries and occasional dips between reversal boundaries that could correspond to lineated tiny wiggles in the mma records the data set suggests that the overall field strength was 40 per cent higher in the first half of the oligocene when the average reversal frequency was 1 6 myr l than in the second half when the reversal frequency was 4myr there is also a weak dependence of average field strength on length of polarity interval finally in the three cores suited to spectral analysis of coherent polarity and relative intensity independent of lithological contamination there is a persistent ca 30 50 ka periodicity in the variations of the relative intensity suggesting that the geomagnetic field pulses at about this frequency not only during the brunhes as demonstrated by tauxe shackleton 1994 but in the oligocene as well

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