relative paleointensity in sediments a pseudo thellier approach we present a method for normalizing sedimentary records for estimating relative paleointensity of the geomagnetic field similar to that successfully used to obtain absolute paleointensity from thermally blocked remanences it has the advantages that it is more effective in removing unwanted viscous remanence thereby improving agreement among various records and that it allows the estimation of the uncertainty in the relative paleointensity calculated

a 2 14 myr astronomically tuned record of relative geomagnetic paleointensity from the western philippine sea 1 we present a 2 14 myr astronomically tuned relative geomagnetic paleointensity record from the western philippine sea pseudosingle domain titanomagnetite is the only magnetic mineral identified and variations in titanomagnetite concentration fall well within the accepted limits for relative paleointensity variations no significant temporally persistent periodicities are observed in wavelet analyses of the paleointensity time series or in the rock magnetic parameters used for relative paleointensity normalization this suggests that our paleointensity record is largely free of rock magnetic or lithological artefacts and that it represents a reliable record of geomagnetic behavior with no evidence for modulation of the field at earth orbital periods the paleointensity record is highly coherent with the sint 800 global paleointensity stack for the last 800 kyr and with a coeval record from the west caroline basin our record confirms that it is normal for the geomagnetic field to undergo dynamic changes within polarity intervals with relatively frequent collapses of the field to low paleointensities and concomitant deviations away from the stable field direction we do not observe an asymmetrical sawtooth form in our paleointensity record which might suggest that previously observed asymmetrical sawtooth paleointensities result from rock magnetic artefacts also we do not observe a persistent 100 kyr inclination periodicity in contrast to the suggestion that geomagnetic field directions are modulated by orbital eccentricity good agreement between our paleointensity record and the coeval west caroline basin record provides the beginning of a detailed view of geomagnetic field behavior between 0 8 and 2 14 ma

comment on a relative geomagnetic paleointensity stack from ontong java plateau sediments for the matuyama by yvo s kok and lisa tauxe

geomagnetic field intensity and reversals during the past four million years a long and continuous record of the geomagnetic field intensity shows that intensity variations are dominated by two modes major episodes of field regeneration prevail on timescales of a few thousand years immediately after most reversals whereas stable polarity states are characterized by a slow 0 5 myr relaxation process reversals can be seen as the consequence of a progressive degradation of the dipole field

relative geomagnetic field intensity across the jaramillo subchron in sediments from the california margin ocean drilling program leg 167 two relative paleointensity records of the jaramillo subchron were obtained during ocean drilling program leg 167 at sites 1010 and 1021 on the california margin the two sites are 1300 km apart and differ in sediment composition and deposition rate the demagnetization of the natural remanent magnetizations nrm of discrete samples revealed a vertical drilling overprint at both sites which could be removed by alternating field demagnetization at 20 mt for the sediments from site 1010 this was confirmed by a positive reversal test on the characteristic remanent magnetization of the discrete samples across the jaramillo subchron rock magnetic criteria for determining relative paleointensities are fulfilled for the samples from site 1010 because of variations in the rockmagnetic properties two short intervals had to be excluded from relative paleointensity determinations at site 1021 normalization of the nrm at several demagnetization steps with various magnetic parameters yielded similar results only for site 1010 and contributions from viscous remanent magnetization could not be detected in the sediments from this site using the pseudo thellier method of tauxe et al 1995 the relative paleointensity data of site 1021 show variations of up to 50 between normalizations with different magnetic parameters and therefore had to be excluded from further analysis several fine scale intensity features are detectable in the record of hole 1010c displaying a maximum in relative paleointensity shortly above the lower jaramillo transition a prominent local minimum in the middle of the subchron and a gradual decrease toward the upper jaramillo reversal a comparison of this record from the california margin with several published records of relative paleointensity across the jaramillo subchron with different sedimentation rates and sediment composition shows reasonable agreement between the paleointensity patterns but differences in amplitudes this suggests an influence of postdepositional magnetization processes which were not removed during normalization

rock magnetic signature of gas hydrates in accretionary prism sediments sediments from two ocean drilling program leg 146 sites from the cascadia margin of western north america have magnetic properties indicating diagenesis of magnetic minerals associated with the presence of gas hydrates two indices combining coercivity remanence and susceptibility parameters djh jrs js hcr hc and ds jrs k hcr when combined with thermo magnetic data can be diagnostic of these changes at site 892 ds values are distinctly higher and more scattered above the bottom simulating seismic reflector bsr which marks the base of the hydrate stability zone within the hydrate stability zone at site 892 djh shows two trends an increase from about 50 mbsf to the bsr at 73 mbsf corresponding to an expected increase in hydrate concentration near the bsr and a second increase upwards from 50 mbsf to peak values at less than 21 mbsf associated with hydrate recovered in cores above 19 mbsf at site 889 890 djh increases downhole to about 285 mbsf substantially below the bsr at 225 mbsf high djh sediments within a low cl zone at this site have magnetic mineralogies which are dominated by fine grained magnetic sulfides whereas sediments from above and below this zone are characterised by magnetite magnetic sulfide mixes this trend at site 889 890 is consistent with an interpretation based on pore water geochemistry low cl and bottom water temperature that a fossil gas hydrate zone extended downwards to about 295 mbsf during the last glacial the observed changes in magnetic properties can be attributed to steps in the reduction series from magnetite through sd greigite to pyrite or to overgrowth of sd greigite to md size diagenetic growth of magnetic iron sulfides greigite and or pyrrhotite has been reported in other accretionary wedge sediments thermal demagnetization of multi component isothermal remanent magnetization mirm indicates the presence of a low coercivity magnetic mineral with an unblocking temperature tub between 310 and 350 c high jrs k ratios suggest that the low coercivity low unblocking temperature mineral is predominantly greigite rather than pyrrhotite a low to medium coercivity mineral with tub ca 580 c magnetite is also present in varying amounts hydrate apparently controls the presence of greigite by incorporating h2s shown to be present as a hydrate phase together with methane in hydrate recovered at site 892 release of h2s below the base of the hydrate layer allows overgrowth of greigite grains to md size or the conversion of some of the greigite to pyrite

sedimentary records of relative paleointensity of the geomagnetic field theory and practice sediments have proved irresistible targets for attempts at determining the relative variations in the earth s magnetic field because of the possibility of long and continuous sequences that are well dated and have a reasonable global distribution the assumption underlying paleointensity studies using sedimentary sequences is that sediments retain a record reflecting the strength of the magnetic field when they were deposited early theoretical work suggested that because the time required for an assemblage of magnetic particles in water to come into equilibrium with the ambient magnetic field was quite short no dependence on magnetic field was expected nonetheless a number of experiments showed that sedimentary magnetizations varied in accordance with the field albeit not always in a simple linear fashion experiments in which the sediments were stirred in the presence of a field to simulate bioturbation showed a reasonably linear relationship with the applied field and these results spurred the hope that variations in the earth s magnetic field might indeed be recoverable from appropriate sedimentary sequences examination of existing paleointensity data sets allows a few general conclusions to be drawn it appears that sedimentary sequences can and do provide a great deal of information about the variations in relative paleointensity of the earth s magnetic field the dynamic range of sedimentary data sets is comparable to those acquired from thermal remanences moreover when compared directly with such independent measures of magnetic field variations as beryllium isotopic ratios and thermally blocked remanences there is considerable agreement among the various records when viewed over timescales of hundreds to thousands of years relative paleointensity data sets from more than a few thousand kilometers apart bear little resemblance to one another suggesting that they are dominated by nondipole field behavior when viewed over timescales of a few tens of thousands to hundreds of thousands of years however the records show coherence over large distances at least thousands of kilometers and may reflect changes in the dipole field on the basis of a sequence spanning the brunhes and matuyama chrons the magnetic field has oscillated with a period of about 40 ka for the last few hundred thousand years but these oscillations are not clear in the record prior to about 300 ka thus they are probably not an inherent feature in the geomagnetic field and the correspondence of the period of oscillation to that of obliquity is probably coincidence

the top olduvai polarity transition at odp site 983 iceland basin abstract the composite section obtained from odp site 983 has been sampled in the vicinity of the top olduvai polarity transition using the u channel technique the polarity transition occurs in a normalized remanence paleointensity low with post reversal recovery in paleointensity but no systematic decay in paleointensity in the matuyama chron above the reversal the directional change associated with the reversal extends over about one meter depth with a virtual geomagnetic pole vgp path which indicates a number of loops over canada and the north atlantic prior to a path over the americas to a vgp cluster in the south atlantic off argentina the path then leads to a vgp cluster over india then a loop over the caribbean and transits through the atlantic to high southerly latitudes vgp clusters in the transition path are observed at positions which are broadly similar to those seen in the top jaramillo transition at the same site may imply a memory effect in the geodynamo spanning successive reversals of the same sense

a rock and paleomagnetic study of a holocene lava flow in central mexico abstract magnetic measurements of the tres cruces lava flow ca 8500 years bp central mexico show the presence of two remanence carriers a ti rich titanomagnetite with a curie temperature between 350 and 400 c and a ti poor magnetite with a curie temperature close to 580 c magnetic changes after heating indicate that the titanomagnetite exsolves into magnetite and ilmenite when the sample is heated to 580 c paleointensity estimates with the thellier and thellier method thellier e thellier o 1959 sur l intensite du champ magnetique terrestre dans le passe historique et geologique ann geophysique 15 285 376 were only successful up to temperatures of 350 to 400 c this temperature corresponds with the curie temperature of the titanomagnetite which is probably pseudo single or multi domain therefore the paleointensities should be interpreted with caution the magnetic composition changes after 580 c heating may explain the large variations in previous paleointensity determinations for the tres cruces rocks gonzalez s sherwood g bohnel h schnepp e 1997 palaeosecular variation in central mexico over the last 30 000 years the record from lavas geophys j int 130 201 219 using the shaw method shaw j 1974 a new method of determining the magnitude of the palaeomagnetic field application to five historic lavas and five archaeological samples geophys j r astr soc 39 133 141

a database for the matuyama brunhes magnetic reversal abstract we present a paleomagnetic database designated mbd97 comprising sedimentary and lava data for the matuyama brunhes reversal transition field using criteria regarding suitability of the rock samples measurement methods and temporal resolution of the data we consider 62 different studies of the matuyama brunhes reversal of these only 11 satisfy our selection criteria four lava sites one loess site three shallow marine sedimentary sites and three deep ocean sedimentary sites we discuss our reasons for accepting or disguarding each candidate data set we investigate the distribution of transitional virtual geomagnetic poles vgps they fall along american and asian australian longitudes the spatial and temporal complexity of the data indicate that the transitional field was almost certainly nonaxisymmetric and nondipolar the directional transition of the matuyama brunhes reversal took 2300 5000 years during which time the surface intensity fell to 10 20 of its pre and post transitional value we do not find any systematic bias due to site distribution or inclination shallowing although the errors in the data do vary over the duration of the reversal mbd97 is a small database but it has a fairly good geographic distribution and is suitable for studies of the matuyama brunhes transition nonetheless it is clear that further analysis of transitions will benefit from a continued program of data collection and analysis the data in mbd97 are listed

is earth s magnetic field reversing abstract earth s dipole field has been diminishing in strength since the first systematic observations of field intensity were made in the mid nineteenth century this has led to speculation that the geomagnetic field might now be in the early stages of a reversal in the longer term context of paleomagnetic observations it is found that for the current reversal rate and expected statistical variability in polarity interval length an interval as long as the ongoing 0 78 myr brunhes polarity interval is to be expected with a probability of less than 0 15 and the preferred probability estimates range from 0 06 to 0 08 these rather low odds might be used to infer that the next reversal is overdue but the assessment is limited by the statistical treatment of reversals as point processes recent paleofield observations combined with insights derived from field modeling and numerical geodynamo simulations suggest that a reversal is not imminent the current value of the dipole moment remains high compared with the average throughout the ongoing 0 78 myr brunhes polarity interval the present rate of change in earth s dipole strength is not anomalous compared with rates of change for the past 7 kyr furthermore there is evidence that the field has been stronger on average during the brunhes than for the past 160 ma and that high average field values are associated with longer polarity chrons there is no evidence from recent millennial scale time varying paleofield models to indicate that the field is entering a polarity transition nevertheless it remains a reasonable supposition that the magnetic field will eventually reverse even though the time scale is unpredictable a more immediate concern is that ongoing secular variation in the magnetic field may be expected to moderate the current high dipole strength on centennial to millennial time scales it would not be surprising if it dropped substantially returning closer to the average without necessarily reversing this could have important consequences for space weather and also highlights the need for improved understanding of the impact of geomagnetic field strength on the production rates of cosmogenic isotopes that are used to estimate past solar variability

saw toothed pattern of relative paleointensity records and cumulative viscous remanence several studies of relative paleointensity data derived from marine sediments spanning the last 4 myr display an asymmetrical saw toothed pattern polarity reversals of the geomagnetic field are associated with low points in paleofield intensity preceded by a long term progressive decay of the field and followed by a rapid post transitional recovery 1 3 since similar behavior is observed in far flung sites whose rock magnetic records are different it is argued that the world wide saw toothing represents geomagnetic field behavior 3 we present an alternative explanation calling on the effect of hard viscous remanence

a relative geomagnetic paleointensity stack from ontong java plateau sediments for the matuyama we present a stack of geomagnetic paleointensity estimates for the matuyama chron 0 778 2 582 ma sedimentary magnetizations from 4 ocean drilling program odp cores from the ontong java plateau ojp are analyzed with thermal techniques thellier thellier experiments are used to determine magnetic properties and indicate that a thermal treatment to 250 c can be used for paleointensity estimates the correlated records form a coherent ojp stack which is dated at reversal boundaries a coherent feature with two other data sets spanning the matuyama chron is the occurrence of a 150 kyr cyclicity however the time control of the ojp stack is not accurate except for the 10 tie points at the reversals this could at least in part explain why the three matuyama paleointensity records show no other coherent frequencies

non steady state magnetic mineral reduction chemical lock in and delayed remanence acquisition in pelagic sediments abstract magnetic hysteresis data collected from pelagic sediment sites in the western equatorial pacific ocean define a progressive deepening of modern redox boundaries with decreased organic carbon input paleomagnetic data from the site with the highest organic carbon shows marked changes in the natural remanent magnetization at the modern fe redox boundary suggesting a type of chemical lock in process in which some magnetic grains form in situ above the boundary and freely orient in the field the depth of chemical lock in suggests paleomagnetic data will be smoothed on time scales ranging from less than 40 kyr to more than 425 kyr the higher values are compatible with models calling for a delayed remanence acquisition to account for the saw tooth pattern observed in some sedimentary paleointensity records

asymmetrical saw tooth pattern of the geomagnetic field intensity from equatorial sediments in the pacific and indian oceans abstract a record of relative paleointensity from marine sediments in the equatorial indian ocean spanning the last 4 ma completes a previous dataset from ocean drilling program odp leg 138 sites in the equatorial pacific 1 the timescale was defined with a precision better than 20 kyr with different and independent methods most of the features present in the record from the equatorial pacific are confirmed the overall saw tooth pattern is observed across every field reversal and the short term fluctuations superimposed on the slow intensity decrease preceding the reversals appear to be similar at both sites the global character of these features reinforces their interpretation in terms of changes in dipole field intensity and provides new important constraints on models of the geodynamo a synthetic curve of field intensity changes during the last 4 ma is proposed as a first paleointensity timescale for future stratigraphic studies

relative paleointensity across the last geomagnetic reversal from sediments of the atlantic indian and pacific oceans paleointensity records from marine sediments at three locations in the atlantic indian and pacific oceans show coherent and reproducible signals across the brunhes matuyama reversal the low field susceptibility signals are predominantly anticorrelated between the indo pacific and the atlantic sites while the normalization of the natural remanent magnetization by any rock magnetic parameter yield identical results we deduce that in these cases climatic components do not induce first order effects in the determination of relative paleointensity the results establish the worldwide character of the triangular pattern displayed by the decay and recovery phases of the field intensity variations across the brunhes matuyama reversal

asymmetry in growth and decay of the geomagnetic dipole revealed in seafloor magnetization abstract geomagnetic intensity fluctuations provide important constraints on time scales associated with dynamical processes in the outer core padm2m is a reconstructed time series of the 0 2 ma axial dipole moment adm after smoothing to reject high frequency variations padm2m s average growth rate is larger than its decay rate the observed asymmetry in rates of change is compatible with longer term diffusive decay of the adm balanced by advective growth on shorter time scales and provides a potentially useful diagnostic for evaluating numerical geodynamo simulations we re analyze the padm2m record using improved low pass filtering to identify asymmetry and quantify its uncertainty via bootstrap methods before applying the new methodology to other kinds of records asymmetry in distribution of axial dipole moment derivatives is quantified using the geomagnetic skewness coefficient s g a positive value indicates the distribution has a longer positive tail and the average growth rate is greater than the average decay rate the original asymmetry noted by ziegler and constable 2011 is significant and does not depend on the specifics of the analysis a long term record of geomagnetic intensity should also be preserved in the thermoremanent magnetization of oceanic crust recovered by inversion of stacked profiles of marine magnetic anomalies these provide an independent means of verifying the asymmetry seen in padm2m we examine three near bottom surveys a 0 to 780 ka record from the east pacific rise at 19 s a 0 to 5 2 ma record from the pacific antarctic ridge at 51 s and a chron c4ar c5r 9 3 11 2 ma record from the ne pacific all three records show an asymmetry similar in sense to padm2m with geomagnetic skewness coefficients s g 0 results from padm2m and c4ar c5r are most robust reflecting the higher quality of these geomagnetic records our results confirm that marine magnetic anomalies can carry a record of the asymmetric geomagnetic field behavior first found for 0 2 ma in padm2m and show that it was also present during the earlier time interval from 9 3 11 2 ma

geomagnetic paleointensities at hawaii between 3 9 and 2 1 ma preliminary results abstract thellier thellier experiments have been conducted on 454 samples from different sequences of the waianae and koolau volcanoes in oahu hawaii the sampled interval as determined by k ar dating spans from 3 9 to 2 1 ma results from 89 samples from 41 flows meet restrictive reliability criteria and provide the first paleointensity estimates from hawaii for the sampled period with good temporal resolution on this time interval the vadm shows ample fluctuations from a minimum of 2 3 1022 am2 to a maximum of 11 5 1022 am2 about an average value of 7 0 2 3 1022 am2 suggesting that fluctuations of the geomagnetic field strength comparable in amplitude to the time averaged value may be a characteristic of the field during both the reverse mammoth chron and the normal chron between the mammoth and the kaena the field strength appears to increase with time

palaeozoic geography of the alpine realm new palaeomagnetic data from the northern greywacke zone eastern alps the palaeozoic units of alpine europe are generally accepted as having been part of the northern margin of gondwana throughout the cambro ordovician by permo carboniferous times northern gondwana had collided with laurasia forming pangaea but the palaeogeographic affinities of the alpine units in intervening times and whether they remained an integral part of gondwana or formed part of the armorican terrane assemblage ata remains unclear in order to resolve this problem a palaeomagnetic study of late silurian and mid devonian rocks from the kitzbuhel alps of the northern greywacke zone eastern alps part of the norian bosnian terrane or proto alpine terrane has been carried out three ancient directions of magnetisation have been identified in this study the youngest component b is identified in the most deformed rocks and represents a remagnetisation event of alpine age and indicates rotation of the region with respect to stable europe since the remagnetisation event the other two directions are interpreted to be primary in origin and late silurian component c and mid devonian component d in age both components are identified as high unblocking temperature directions of magnetisation and stereographic projection of site mean directions from each age shows a clear small circle distribution after bedding correction which is related to local and small scale vertical axis block rotations both data sets pass the inclination only fold test demonstrating the primary nature of these magnetisations the resulting mean inclination values obtained for the silurian and devonian rocks are 65 95 1 7 and 42 95 4 6 using the negative polarity option these yield palaeolatitudes for the southern proto alpine terrane of 47 s 2 3 and 2 1 upper and lower 95 confidence level during the silurian and 24 s 3 7 and 3 4 during mid devonian times in terms of palaeogeography therefore these new results indicate that in mid palaeozoic times the proto alps were located significantly further south than and were independent of the ata the relationship with northern gondwana is more complex given the uncertainty of the apw path for gondwana palaeobiogeographic indicators however support the interpretation that the proto alps formed an independent microplate rather than having remained part of the north gondwana margin

acquisition of viscous remanent magnetization abstract viscous remanent magnetization vrm has been frequently suggested as a possible source of magnetic anomalies in ocean crust and as an important contributor of natural remanent magnetization in some volcanic and sedimentary rocks most references to vrm assume a linear dependence with log t yet long term experimental data on vrm while scarce often disagree with this simple notion here we investigate the angular dependence grain size dependence initial state dependence and time t dependence of vrm acquisition we observe a non linear log t dependence of vrm acquisition over all time intervals 104 s for all the samples regardless of various experimental conditions echoing earlier studies we also find that vrm acquisition is strongly dependent on the initial magnetic state of the samples samples in a thermally demagnetized initial state were most susceptible to vrm acquisition when vrm is produced parallel to the initial thermal remanence vrm acquisition is negligible vrm acquisition is more prevalent for magnetites in single domain and multidomain md grains than in the pseudo single domain range in md magnetites frequent removal and restoration of samples for the purpose of zero field remanence measurement apparently increases the efficiency of vrm acquisition

a geomagnetic paleointensity stack between 0 8 and 3 0 ma from equatorial pacific sediment cores 1 we have conducted a paleomagnetic study of six sediment cores taken from the equatorial pacific three from the west caroline basin and three from the manihiki plateau in order to make a relative paleointensity stack during the matuyama and late gauss chrons the age of the bottom of the cores ranges from 1 2 to 3 0 ma the sediments show little downcore changes in the proxies of magnetic grain size and mineralogy and are hence suitable for estimating relative paleointensity the age of the cores is controlled by correlating variations in magnetic concentration magnetic susceptibility and or anhysteretic remanent magnetization arm to target oxygen isotope 18o curves all cores from the manihiki plateau show an upward decrease of the natural remanent magnetization nrm intensity normalized by arm and isothermal remanent magnetization irm with a decrease in sedimentation rate such a trend was removed before being converted to relative paleointensity this observation implies that a sedimentation rate change can affect relative paleointensity estimation relative paleointensity records from the six cores coincide well with each other within uncertainty of age we constructed a stacked curve between 0 8 and 3 0 ma the equatorial pacific paleointensity stack epapis 3ma after adjusting age the number of cores stacked is three to four throughout the record quasiperiodic paleointensity lows occur in the epapis 3ma and corresponding paleointensity lows can be found in previously published records with some shifts in age at least parts of the paleointensity minima seem to be accompanied by geomagnetic excursions the asymmetric sawtooth pattern of paleointensity variations is not observed at the matuyama gauss boundary and thereafter in our record a spectral analysis shows that 100 kyr orbital eccentricity frequency may exist in paleointensity variations during the matuyama and late gauss chrons

magnetostratigraphic calibration of southern ocean diatom datums from the eocene oligocene of kerguelen plateau ocean drilling program sites 744 and 748 ocean drilling program holes 744a and 748b represent key sections for calibration of southern ocean eocene oligocene biostratigraphic zonations sites 744 and 748 were above the carbonate compensation depth throughout this time interval and contain good planktonic foraminiferal calcareous nannofossil and diatom biostratigraphic records in particular the southern ocean diatom biostratigraphic zonation for the oligocene critically hinges on calibration of these two holes previous low resolution magnetostratigraphic studies at these sites were hampered by limited sampling and technical difficulties which prompted our high resolution reinvestigation of the magnetostratigraphy magnetic polarity zonations for holes 744a and 748b were constructed after inspection of vector component plots at 1 cm stratigraphic intervals from continuous u channel measurements the magnetizations are generally stable and a robust polarity stratigraphy has been obtained for both holes the increased resolution of our study and identification of persistent secondary overprints which were not recognised in previous studies suggests that the previously published interpretations need to be revised our magnetostratigraphic interpretations for both holes are constrained by foraminiferal and calcareous nannofossil datums as well as by sr isotope ages we have calibrated four diatom datums which are synchronous at the two studied sites to the geomagnetic polarity timescale including the first occurrence fo of lisitzinia ornata 27 8 ma the fo of rocella vigilans var b 27 8 ma the fo of cavitatus jouseanus 30 9 ma and the fo of rhizosolenia oligocaenica 33 8 ma the synchroneity of these datums suggests that diatom biostratigraphy has considerable potential for palaeogene biostratigraphic correlation in the southern ocean although the ages of some datums are obscured by an unconformity in hole 744a our age model from hole 748b suggests age estimates for the last common occurrence of rocella vigilans var a 29 0 ma the fo of rocella vigilans var a 30 0 ma and the fo of rhizosolenia antarctica 33 2 ma it should also be noted that the last occurrence of the calcareous nannofossil chiasmolithus altus occurs in chron c8r rather than c8n in our revised magnetostratigraphic interpretation which indicates that this datum is not diachronous between low and mid latitudes as had previously been suggested significant unconformities are documented in both holes in the middle oligocene and in the middle late oligocene respectively which probably resulted from periods of enhanced circumpolar deep water circulation

a paleomagnetic power spectrum we construct a power spectrum of geomagnetic dipole moment variations or their proxies that spans the period range from some tens of million down to about 100 years empirical estimates of the spectrum are derived from the magnetostratigraphic time scale from marine sediment relative paleointensity records and from a time varying paleomagnetic field model for the past 7 kyr the spectrum has the most power at long periods reflecting the influence of geomagnetic reversals and in general decreases with increasing frequency decreasing period the empirical spectrum is compared with predictions from simple models discrepancies between the observed and predicted spectra are discussed in the context of i changes in reversal rate ii overall average reversal rate iii cryptochrons iv the time taken for a reversal to occur and v long term paleosecular variations and average estimates of the field strength and variance from other sources 2005 elsevier b v all rights reserved

a sedimentary paleomagnetic record of the matuyama chron from the western antarctic margin odp site 1101 a high resolution paleomagnetic record for part of the matuyama chron 0 7 2 1 ma is reported for ocean drilling program site 1101 leg 178 off the antarctic peninsula pacific margin a rock magnetic investigation of 62 discrete samples revealed that the natural remanent magnetization nrm is carried by pseudo single domain magnetite progressive alternating field demagnetization of 83 m of u channels provided a polarity stratigraphy down to the olduvai subchron two geomagnetic events preceding the jaramillo subchron were identified including the cobb mountain polarity interval the bulk magnetic parameters vary by more than a factor of 20 over the entire time interval but by less than a factor of 6 over the 0 7 1 1 ma interval this latter interval was selected for paleointensity determinations which were carried out by normalizing the nrm by the anhysteretic remanent magnetization arm direct comparison of the site 1101 paleointensity record with other curves available for the same time interval suggests a geomagnetic origin for features present in the record a more quantitative comparison was achieved by means of a jackknife test performed on nine records of relative paleointensity over the 0 95 1 1 ma interval this test yielded no outlier for the period considered confirming the geomagnetic character of the records we have constructed a lowresolution stack revealing some of the characteristic paleointensity features of the jaramillo subchron fl 2001 elsevier science b v all rights reserved

intensity of the earth s magnetic field since precambrian from thellier type palaeointensity data and inferences on the thermal history of the core summary we present a compilation of palaeointensity data obtained by the thellier method from magmatic rocks up to 3 5 gyr old no apparent very long term variation occurs from present to early precambrian the overall data are compatible with a gravitationally powered dynamo model in which the inner core is some 4 0 gyr old however palaeointensity data are still too incomplete to dismiss the possibility of a more complex field history with a geodynamo first thermally driven then gravitationally powered despite the limited number of data currently available large differences in the dipole field moment are clearly documented with a time scale of the order of 50 100myr the changes in intensity can be due to repetitive instabilities of the thermal boundary layer at the base of the mantle

reply to comment on saw toothed variations of relative paleointensity and cumulative viscous remanence testing the records and the model by l meynadier j p valet y guyodo and c richter

magnetization of the la palma seamount series implications for seamount paleopoles paleopoles determined from seamount magnetic anomalies constitute the major data source for the pacific apparent polar wander path but relatively little is known about the processes of remanence acquisition in seamounts since magnetic anomalies reflect both natural remanence nrm and the induced field it is important first to assess whether the nrm is likely to represent an original field direction and second to constrain the magnitude of the induced component to this end we present paleomagnetic data from an uplifted subaerially exposed section through a seamount on la palma canary islands the pliocene seamount series of la palma comprises a 6 km sequence of alkalic extrusives and intrusives which includes all lithologies likely to be volumetrically important in seamounts the structural tilt of the seamount series allows separation of early thermal or chemical remanence from magnetization components acquired after tilting e g viscous remanence the nrm provides a poor indication of the original magnetization direction although the characteristic magnetization of many la palma samples is compatible with the original pretilt direction hydrothermal alteration has resulted in the production of ti poor magnetite and an increasing contribution of hematite with increasing degree of alteration more importantly well defined magnetization directions which deviate from any reasonable geomagnetic direction at la palma can be attributed to hydrothermal alteration in a different polarity than prevalent during the original magnetization based on a comparison of the magnitude of low stability components of magnetization and laboratory acquisition of viscous remanence and previous estimates of the induced magnetization we conclude that viscous and induced magnetization probably account for 15 25 of the total magnetization of seamounts the resulting paleopole bias is a function of the polarity and paleolatitude of the seamount and ranges from 4o to 16 o for cretaceous seamounts in the pacific

long τ vrm and relative paleointensity estimates in sediments geomagnetic paleointensity measurements from sedimentary records can be severely affected by viscous remanent magnetization vrm we present a method for determining varying amounts of long term vrm acquired during the present polarity interval using the typically non linear relationship between acquisition of artificial magnetization and demagnetization of nrm the non linear parts are to be avoided for paleointensity determinations but here we focus on their use for indicators of long relaxation time vrm the method which does not require determining paleointensity values suggests correlations with paleoclimate curves and age dependent growth of vrm furthermore it appears that the long vrm acquired during the pleistocene is accompanied by short effects detected in the laboratory environment 1999 elsevier science b v all rights reserved

deciphering records of geomagnetic reversals polarity reversals of the geomagnetic field are a major feature of the earth s dynamo questions remain regarding the dynamical processes that give rise to reversals and the properties of the geomagnetic field during a polarity transition a large number of paleomagnetic reversal records have been acquired during the past 50 years in order to better constrain the structure and geometry of the transitional field in addition over the past two decades numerical dynamo simulations have also provided insights into the reversal mechanism yet despite the large paleomagnetic database controversial interpretations of records of the transitional field persist they result from two characteristics inherent to all reversals both of which are detrimental to an ambiguous analysis on the one hand the reversal process is rapid and requires adequate temporal resolution on the other hand weak field intensities during a reversal can affect the fidelity of magnetic recording in sedimentary records this paper is aimed at reviewing critically the main reversal features derived from paleomagnetic records and at analyzing some of these features in light of numerical simulations we discuss in detail the fidelity of the signal extracted from paleomagnetic records and pay special attention to their resolution with respect to the timing and mechanisms involved in the magnetization process records from marine sediments dominate the database they give rise to transitional field models that often lead to overinterpret the data consequently we attempt to separate robust results and their subsequent interpretations from those that do not stand on a strong observational footing finally we discuss new avenues that should favor progress to better characterize and understand transitional field behavior

geomagnetic palaeointensities and astrochronological ages for the matuyama brunhes boundary and the boundaries of the jaramillo subchron palaeomagnetic and oxygen isotope records from odp site 983 we have measured relative geomagnetic palaeointensity proxies palaeomagnetic directions and 18 o for the 700 1100 ka interval from odp site 983 gardar drift north atlantic where mean sedimentation rates are ca 13 cm kyr 1 the age model was generated by matching the benthic 18 o data to the ice volume model and confirmed by tuning the precessional components of both signals for the matuyama brunhes boundary mbb and the boundaries of the jaramillo subchronozone the duration of the polarity reversal process defined by virtual geomagnetic polar latitudes of less than 45 is ca 5 kyr whereas the generally accepted astrochronological estimates for the boundaries of the jaramillo subchronozone lie within the polarity transitions as recorded at site 983 the astrochronological age for the matuyama brunhes polarity transition 780 ka is ca 5 kyr older than the onset of this transition at site 983 775 ka the polarity reversals lie within palaeointensity lows with abrupt recovery of palaeointensity post reversal there is no progressive sawtooth decrease in palaeointensity within the jaramillo subchronozone or between the top of the jaramillo and the mbb but rather within polarity chrons several short intervals of low palaeointensity which sometimes coincide with high amplitude secular variation orbital 100 and 41 kyr periods are present in the palaeointensity record as they are not obviously attributable to climate lithology in these records they may be a feature of the geomagnetic field itself

paleointensity variations across the last geomagnetic reversal at la palma canary islands spain a detailed volcanic record of absolute paleointensity has been obtained for the period encompassing the last geomagnetic reversal from a succession of 69 distinct lava flows in two adjacent sections on the island of la palma the correlation between the two sections relies on the positions of the brunhes matuyama boundary and on the flow chronology obtained by a refined potassium argon k ar dating technique several pretransitional flows exhibit very shallow paleomagetic inclinations but they cannot be considered as really transitional paleointensity experiments conducted on 288 specimens provided 168 successful determinations 14 were obtained after correction for 50 lava flows that were considered to represent 45 distinct units the evolution of the paleofield intensity can be constructed by assuming uniform extrusion rates between three dated lava flows the period preceding the transition is characterized by large fluctuations with typical peak to peak amplitudes between 20 and 40 t which decrease down to 7 t prior to the transition the 100 kyr long interval following the reversal is associated with field intensities as high as 60 t and depict a strong and rapid restoration of the dipole field the virtual dipole moments vdms have been compared with sedimentary vdms generated from the record of relative paleointensity from ocean drilling program site 851 to which have been added non dipole components similar to the present geomagnetic field a reasonable fit can be made between the two curves which implies variations in the eruption rates but not incompatible with the uncertainties in the k ar age determinations thus these results can support but do not fully demonstrate the existence of asymmerical saw toothed variations in field intensity