rock magnetism of the early pliocene trubi formation at eraclea minoa sicily the most important magnetic mineral in the finegrained marls of the early pliocene trubi formation near eraclea minoa is magnetite some goethite is also present the magnetite is of high purity and of primary origin hysteresis and low temperature measurements show that most magnetite grains are single domain the presence of superpara magnetic grains is suspected multi domain grains are rare it is stewn that af demagnetization does not yield correct nrm directions

the upper and lower nunivak sedimentary geomagnetic transitional records from southern sicily van hoof a a m van os b j h and langereis c g 1993 the upper and lower nunivak sedimentary geomagnetic transitional records from southern sicily phys earth planet inter 77 297 313 the detailed paleomagnetic records of the upper and lower nunivak polarity transitions have been determined from pliocene marine marls in southern sicily magnetites are the most important carrier of the remanence in both records the transitions are recorded in two components a low temperature and a high temperature component the two components do not represent the geomagnetic field because the changes in these components take place at lithological boundaries in addition the directional changes do not completely match directional changes of the same transitions recorded in calabria some 250 km away the character of the virtual geomagnetic poles paths is probably caused by smoothing of the stable directions before and after the transitions the directional changes as well as the smoothing mechanism can be explained by the diagenetic magnetite formation model van hoof et al 1992 in which shortly after burial the remanence carried by newly formed secondary magnetites is superposed on the initial remanence carried by primary magnetite

a reversal of the earth s magnetic field recorded in mid miocene lava flows of gran canaria paleointensities 1 an extensive paleointensity study was carried out on an approximately 14 1 myr old reverse to normal transition of the geomagnetic field a total of 188 samples from a volcanic sequence on gran canaria canary islands were subjected to thellier type paleointensity determinations the paleodirectional results of this sequence are reported by leonhardt et al 2002 a modified thellier technique which facilitates the recognition of md tails and the formation of new magnetic remanences with higher blocking temperatures than the actual heating step was used on the majority of the samples we obtained reliable paleointensity results for 35 of the 87 sampled lava flows in general the obtained field intensities are lower than expected for the mid miocene this observation is very likely related to a long term reduction of the field close to transitions very low paleointensities with values 5 t were obtained during an excursion preceding the actual transition and also close to significant changes of the local field directions these are interpreted as nondipolar components becoming dominant for short periods and provoking a rapid change of field directions during the transition 15 successive lava flows recorded similar local field directions corresponding to a cluster of virtual geomagnetic poles close to south america chronologically within this cluster the paleointensity increases from about 9 to 28 t followed by a decrease back to approximately 9 t this variation of paleointensity between lava flows with similar direction indicates independent records of the geomagnetic field and gives strong evidence for a stable transitional state that lasted a significant period

thermal core mantle interactions and time averaged paleomagnetic field recent studies of historical geomagnetic secular variation and lower mantle seismic velocity suggest the earth s magnetic field is controlled by lateral variations in mantle temperature in this paper the average expected magnetic field pattern is compared with the time averaged field obtained from paleomagnetic pole positions from the last 5 m y the historical record suggests that the time averaged inclination anomalies are negative far sided in the atlantic region north of 30 s and the west pacific and positive in the south atlantic extreme north and south pacific and east pacific the pole positions give positive inclination anomalies in the south atlantic and extreme north pacific and negative anomalies elsewhere in agreement with the theory except for the east pacific this suggests that the historical record is not long enough to give an accurate picture of long term secular change in the eastern pacific but that elsewhere the time averaged field of paleomagnetism is predicted satisfactorily the preponderance of far sidedness in the paleomagnetic record is a consequence of the present distribution of temperature at the base of the mantle rather than a fundamental property of the geodynamo and it may therefore change on the long time scale of mantle convection in the historical record declinations average to negative values in the atlantic hemisphere and positive values in the pacific which if typical of longer term behavior could explain the preponderance of positive declinations for reversed polarity which are mainly in the atlantic hemisphere

periodicities of caco3cycles in the pliocene of sicily discrepancies with the quasi periods of the earth s orbital cycles a high resolution caco3 record in combination with a detailed magnetostratigraphy is presented for the rhythmically bedded marly sequence of the pliocene trubi formation on sicily italy magnetostratigraphic age calibration and subsequent spectral analysis of the variatrions in caco3 content yield a remarkably consistent discrepancy between the resulting periodicities and the corresponding quasi periods of the earth s orbital cycles this discrepancy can only be explained by assuming that ages assigned to polarity reversal boundaries in the geomagnetic polarity time scale lack sufficient precision a new polarity time scale is presented for the major part of the gilbert and gauss chron using the radiometrically dated gilbert gauss chron boundary at 3 40 ma as an age reference point and extrapolating an average quasi period of 21 7 kyr of the precession cycle as the periodicity of small scale colour annex caco3 cycles

the upper and lower thvera sedimentary geomagnetic reversal records from southern sicily detailed paleomagnetic records of the upper and lower thvera polarity transitions have been determined from pliocenc marine marls in southern sicily the dominant magnetic mineral is fine grained magnetite the two transitions have vgp paths following a great circle passing south america and the west coast of north america there are strong indications that the vgp paths result from smoothing of the non antipodal stable directions before and after the transitions the upper thvera transitional record is preceded by two excursions and followed by a third one the first excursion is a sedimentary artefact caused by post depositional migration of magnetic minerals and the third one by is caused by weathering of the sediment the upper thvera transition from sicily is compared with the record from calabria about 250 km away the two records show similarities as well as differences both transitions have identical vgp paths but in calabria the transition is recorded lower in the sediment the first and third excursions have different characters and the second one is not present apparently the registration of transitions of the earth s magnetic field in sediments is strongly influenced by smoothing and diagenetic processes after deposition

lower pliocene polarity transitions from the suva marl fiji we present a paleomagnetic record of the lower cochiti polarity transition and new results from the lower and upper nunivak transitions obtained from the lower pliocene suva marl fiji 18 s 178 e this formation has yielded a well defined magnetostratigraphy that combined with 40ar 39ar ages from interbedded tuff beds provides excellent chronostratigraphic control clement et al 1997 we previously presented the upper cochiti and the upper and lower nunivak polarity transition records from this unit clement et al 1995 1996 new results were obtained from oriented block samples spanning these reversals each of these reversals was sampled at duplicate sections separated by 4 to 8 km the agreement between the records obtained from different sections suggests that the paleomagnetic records are not artifacts of local sedimentary disturbances or random degrees of incompletely removed overprints instead the similarities in these and other records of these same reversals from different types of paleomagnetic recorders suggest that they provide information about the behavior of the geomagnetic field as it reverses

microearthquakes in the tyrrhenian sea as revealed by joint land and sea bottom seismographs abstract five independent ocean bottom seismographs obs were operated from 17 to 28 may 1987 in the southeastern tyrrhenian sea at a depth range of 1000 2800 m these were concomitantly run with nine vesuvian observatory and university of calabria temporary digital seismic stations along the calabrian coast together with fifteen permanent seismic stations in southern italy the latter stations also belong to the university of calabria and to the italian national institute of geophysics about 200 local shocks mainly microearthquakes ml

the distinction between geomagnetic excursions and reversals summary two recent studies of the geomagnetic eld in the last 1 myr have found 14 excursions large changes in direction lasting 5 10 kyr each six of which are established as global phenomena by correlation between diierent sites the older picture of the geomagnetic eld enjoying long periods of stable polarity may not therefore be correct instead the eld appears to suier many dramatic changes in direction and concomitant reduction in intensity for 10 20 per cent of the time during excursions the eld may reverse in the liquid outer core which has timescales of 500 yr or less but not in the solid inner core where the eld must change by diiusion with a timescale of 3 kyr this timescale is consistent with the remarkably uniform duration of well dated excursions the disparity of dynamical timescales between the inner and outer cores a factor of 10 is consistent with the 10 excursions between full reversals

a neogene geomagnetic polarity transition record from lavas of the canary islands spain episodic volcanism and or metastable transitional fields summary we present results from a study of a partial magnetic polarity transition record from a 9 7 ma 40ar 39ar dated volcanic section on the island of gomera canary islands spain the record provides results that bear on the debate over whether long term persistent features of field behaviour exist during transitions the sampled section containing more than 20 flows begins after the onset of the transition the nine lowermost flows in the section which are all transitional yield virtually identical directions and similar low paleointensities following the recovery of virtual geomagnetic pole positions vgps to near the spin axis paleointensities progress to very high values transitional directions lay due west reflecting the presence of a non zonal intermediate field vgps for these flows cluster in the west atlantic which according to hoffman would correspond to a zone of fastest p wave propagation in the lower mantle and to a radial flux centre of the present day non dipole field hoffman had noted a similar occurrence at two other locations he found that in a select set of volcanic records vgps tended to group over two areas australia and south america which also corresponded to fast p wave and non dipole field anomalies given the discontinuous and irregular character typical of volcanic sequences it is natural to question whether these clusters are merely artefacts of episodic volcanism or actually reflect standstills of the reversing field we address this question for our record by comparing chemical and magnetomineralogic characteristics of the flows we find that all of the transitional flows have very similar chemical and rock magnetic properties furthermore these flows differ significantly from the overlying reverse flows which give distinctly different values and much greater flow to flow variation from this we conclude that the packet of transitional flows was derived from the same source and had erupted over a relatively short period of time as reflected by a magma source that presumably did not have time to evolve significantly while the vgp cluster in the gomera record is consistent with long term mantle control on transitional fields we cannot conclude that the cluster represents a standstill of the field these results highlight the difficulties of interpreting temporal changes in field behaviour from volcanic sequences in addition they provide a clear incentive for investigating physical characteristics of lavas to assess relative eruptive rates

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

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

long lived transitional states of the geomagnetic field and the two dynamo families palaeomagnetic data from lavas produced at the hawaii and society islands hotspots suggest that one or more specific long lived field configurations recur during successive polarity reversals spanning perhaps several million years the apparent quasistationarity of these states and the relative placement of the two recording sites offer a rare opportunity to explore the fundamental geometrical properties of transitional fields

geographical distribution of transitional vgps evidence for non zonal equatorial symmetry during the matuyama brunhes geomagnetic reversal abstract paleomagnetic records of the matuyama brunhes polarity reversal obtained recently from the northern equatorial and southern latitudes of the atlantic sector display a distinct symmetry with respect to the equator the virtual geomagnetic pole vgp paths from the mid latitude sites in the northern and southern hemispheres dsdp site 609 and core v16 58 are nearly coincident tracking northward through the americas the vgp path from the equatorial site odp site 664 tracks northward through eastern asia nearly antipodal to the other two paths modeling results indicate that superimposing an h 1 3 geometry onto a reversing axial dipole explains the sense of equatorial symmetry exhibited by these three records these results are consistent with the bulk of the data available from other records of this polarity transition suggesting that the transitional vgps during this reversal tend to fall along two distinct longitudinal bands the grouping of transitional vgps suggests that there is a strong geographical control over the reversal process an apparent correlation between the longitudinal bands of transitional vgps and the location of the major flux concentrations in the radial component of the historical geomagnetic field at the core mantle boundary 1 suggests that the processes which cause these flux concentrations may have influenced the geometry of the matuyama brunhes transitional field

evidence for a geomagnetic excursion recorded in the sediments of lac st front france a link with the laschamp excursion anomalous paleomagnetic directions accompanied by low intensities of natural remanent magnetization are recorded at 17 m depth in the sediments cored from lac st front massif central france the rock magnetic properties and the parameters characterizing anisotropy of susceptibility of samples taken from the sediment horizons carrying these anomalous directions spanning some 30 40 cm are identical to those of samples taken from the other stratigraphic intervals carrying a record of normal geomagnetic secular variation therefore the sequence of anomalous directions is interpreted as a geomagnetic excursion furthermore it is concluded that this excursion occurred at a time when the paleointensity field was low the excursion is dated at about 33 14c 37 ka b p by correlating the susceptibility and paleomagnetic logs from lac st front with the corresponding logs from the nearby lac du bouchet sedimentary sequence the excursional directions in lac st front can be averaged out with a triangular window of 20 cm these smoothed excursional directions resemble the directional variations observed in the corresponding lac du bouchet interval this finding indicates that the absence of this excursion in lac du bouchet may be due to a larger smoothing of the geomagnetic signal

dipolar reversal states of the geomagnetic field and core mantle dynamics palaeomagnetic records from lavas suggest that at least two specific inclined dipolar field configurations have dominated the reversal process for the past ten million years these long lived states provide a way to explain directional rebounds aborted reversals and the recording in sediments of what appear to be preferred longitudinal paths of the virtual geomagnetic pole the polar orientations correlate with near radial flux concentrations recognizable when today s field is stripped of its axial dipole and with lower mantle seismic anomalies suggesting a tie to deep earth dynamics

longitudinally confined geomagnetic reversal paths from non dipolar transition fields it has long been thought that conditions at the boundary between the core and mantle influence the earth s magnetic field but the supporting evidence is rather indirect1 3 recent palaeomagnetic results suggesting that there are persistent preferred longitudinal paths for the virtual geomagnetic pole vgp during reversals4 would provide the first direct evidence of the solid mantle s influence on the core although their statistical significance has been disputed5 6 the results are potentially exciting because the preferred paths lie close to the pacific rim where the present geomagnetic secular variation changes character2 7 here we present a simple model based on an extension of a previous theory8 that produces reversals with vgp paths confined within relatively narrow longitude bands despite the transition field having a substantially non dipolar structure thus although longitude bias of the vgp paths is definitive evidence for core mantle interaction simple vgp paths are not evidence of near dipolar transition fields

origin and tectonic significance of the aldrich mountains serpentinite matrix melange northeastern oregon the permian triassic aldrich mountains serpentinite melange consists of a matrix of massive and foliated serpentinite lizardite chrysotile that encases a variety of metamorphic igneous and sedimentary block types serpentinite matrix foliation trends ne sw dips steeply 75 and is parallel to long dimensions of blocks at block matrix contacts the foliation traces block margins the volumetrically most abundant block types are greenstone chert and metagabbro subordinate block types are amphibolite tectonites epiclastic sediments breccia conglomerate sandstone and argillite silicic volcanic rocks silicic plutonic rocks and ultramafic rocks three types of metamorphism are evident from textures and mineral assemblages of blocks and matrix 1 serpentinization and related hydrous alteration confined to matrix ultramafic blocks and margins of mafic blocks 2 lower to middle greenschist and prehnite actinolite facies metamorphism characterized by static non penetrative fabrics of some blocks and 3 dynamothermal epidote amphibolite to upper amphibolite lower granulite metamorphism exhibited by strongly schistose amphibolite tectonites mineral assemblages and mineral chemistry indicate amphibolite tectonites formed at moderate to high temperatures 550 780 c and low to moderate pressures 3 5 kbar most blocks were affected by variable degrees of post metamorphic brittle deformation prior to immersion in serpentinite matrix the melange is interpreted to have formed within a permian to late triassic sediment starved tectonically active forearc fragments of abyssal oceanic crust and early permian ensimatic arc crust were mixed by tectonic and sedimentary processes within the forearc highly mobile serpentinite generated beneath the forearc was protrusively emplaced into the tectonized forearc and mixed with the blocks forming the melange melange generation probably began in the late early permian and ceased by the late triassic

evidence suggesting extremely rapid field variation during a geomagnetic reversal abstract large systematic variations in direction of high temperature remanence as a function of vertical position occur in a basalt flow from the miocene volcanic sequence at steens mountain oregon that has provided a detailed record of a geomagnetic reversal these may have been caused by an impulsive change in the transitional field as the flow cooled during the reversal if this is true the short time required for the flow to cool by conduction and acquire its thermoremanent magnetization implies astonishingly high rates of change of the geomagnetic field at least 3 and 300 gammas per day

high resolution record of the upper olduvai transition from po valley italy sediments support for dipolar transition geometry abstract a detailed record of the upper olduvai polarity transition composed of 100 transitional directions has been obtained from the crostolo section in northern italy a careful examination of the rock magnetic properties of the sediments using standard paleomagnetic techniques thermomagnetic scanning electron microscopy microprobe x ray diffraction and mossbauer analysis shows that greigite is the main magnetic carrier of the remanence the correlation of a shift in the magnetic record to a small scale sedimentary feature observed in the section indicates that the magnetization was acquired at deposition or very shortly after the virtual geomagnetic pole vgp path is largely confined along a great circle over north and south america 90 west of the site and consists of three stages first the vgp moves to southern mid latitudes then after a period of standstill it comes back to almost true north and finally the south pole is reached in a third step although different from a record from the southern indian ocean this path is virtually identical to those obtained for the same transition from north pacific deep sea cores and partly coincides with a north atlantic record which suggests that a dipolar component may be present in the transitional field during the upper olduvai reversal a review of the recently obtained records of various transitions shows that in more than two thirds of the cases the vgp paths are similarly confined along a meridian over the americas or antipodal to them irrespective of the sampling site and of the sense of the transition although not deterministic this tendency suggests that a similar dipolar component might be present in the transitional fields of other reversals of different ages

the upper kaena sedimentary geomagnetic reversal record from southern sicily a detailed paleomagnetic and rock magnetic record of the upper kaena polarity reversal was determined from late pliocene marine marls in southern sicily two separate characteristic magnetization components were distinguished a low temperature lt component between 200 and 50 c and a high temperature ht component between 500 and 580 610 c the two components record the reversal in a slightly different way but both were acquired in an early stage following deposition most probably by authigenic biogenic formation of magnetic minerals the actual r n reversal via e and horizontal inclinations seems to take 3 3 kyr and is followed by a stable normal interval 3 3 kyr and subsequently by a significant excursion 50 60 in declination 3 6 kyr relative intensities were lower during a time span of 5 6 kyr and show a minimum of 10 of the nontransitional values but during the excursion no intensity changes are apparent the vgps of the ht component are confined to a longitude between india and australia those of the lt component oscillate between this same longitude and one passing over north and south america we conclude that most if riot all observed features are largely determined by the remanence acquisition process which is complicated by authigenic formation of magnetic minerals and cyclically fluctuating paleoredox conditions this has not only resulted in considerable smoothing but also in delayed nrm acquisition in specific sedimentary intervals we believe that the observed excursion is a sedimentary artifact and not related to geomagnetic field behavior during transitions we emphasize that many reported geomagnetic features may in fact be due to the sedimentary remanence acquisition process

the gilbert gauss sedimentary geomagnetic reversal record from southern sicily a detailed paleomagnetic record of the gilbert gauss reversal boundary in deep sea marls from southern sicily shows that the remanence characterising the reversal is carried by a high temperature magnetite component a low temperature component is intermediate between the high temperature component and a present day overprint suggesting that it is obscured by weathering the actual reversal record is clearly affected by filtering caused by early diagenetic acquisition of a chemical remanent magnetisation crm intermediate vgps show a general confinement to a longitude over the americas more specifically they are restricted to two clusters a less distinct one off se south america and a very prominent one near ne north america the remarkable coincidence with two hoffmann clusters hoffmann 1992 suggests that even a crm acquisition filter cannot entirely obscure long lived transitional features

evidence from lava flows for complex polarity transitions the new composite steens mountain reversal record summary geomagnetic polarity transitions may be significantly more complex than are currently depicted in many sedimentary and lava flow records by splicing together paleomagnetic results from earlier studies at steens mountain with those from three newly studied sections of oregon plateau flood basalts at catlow peak and poker jim ridge 70 90 km to the southeast and west respectively we provide support for this interpretation with the most detailed account of a magnetic field reversal yet observed in volcanic rocks forty five new distinguishable transitional t directions together with 30 earlier ones reveal a much more complex and detailed record of the 16 7 ma reversed r to normal n polarity transition that marks the end of chron c5cr compared to the earlier r t n t n reversal record the new record can be described as r t n t n t r t n the composite record confirms earlier features adds new west and up directions and an entire large n t r t segment to the path and fills in directions on the path between earlier directional jumps persistent virtual geomagnetic pole vgp clusters and separate vgps have a preference for previously described longitudinal bands from transition study compilations which suggests the presence of features at the core mantle boundary that influence the flow of core fluid and distribution of magnetic flux overall the record is consistent with the generalization that vgp paths vary greatly from reversal to reversal and depend on the location of the observer rates of secular variation confirm that the flows comprising these sections were erupted rapidly with maximum rates estimated to be 85 120 m ka 1 at catlow and 130 195 m ka 1 at poker jim south paleomagnetic poles from other studies are combined with 32 non transitional poles found here to give a clockwise rotation of the oregon plateau of 11 4 5 6 with respect to the younger columbia river basalt group flows to the north and 14 5 4 6 with respect to cratonic north america 95 per cent confidence interval

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

detailed paleomagnetic study of two volcanic polarity transitions recorded in eastern iceland abstract two sequences of 45 and 49 individual lava flows respectively have been sampled in eastern iceland the two sections range in age from 12 09 to 10 21 ma as reported by watkins and walker watkins n walker g p l 1977 magnetostratigraphy of eastern iceland am j sci 277 513 584 and are labelled as profiles c and d stepwise alternating field af and thermal demagnetizations accompanied by investigations of rock magnetic properties indicate that the magnetization is primarily carried by titanomagnetite demagnetization experiments have identified eight and six transitional lavas respectively for these profiles the transitional virtual geomagnetic poles vgps of the r n profile c reversal are located mainly between patagonia and antarctica with a tendency of the virtual poles to move towards west coast of south america subsequently traveling to the northern hemisphere through several discrete steps located in the middle part of south america then to west africa and on to central asia before the poles settle into normal polarity the second and younger vgp path corresponding to profile d is a path characterized by a reverse to normal to reverse r n r motion of the virtual poles this path is characterized by poles located in west antarctica patagonia and the western and eastern part of south america the passage from the southern to the northern hemisphere is also through a discrete sets of steps along the southwestern pacific followed by a rapid motion to the northern siberian region continuing to the western equatorial part of south america before moving on to the central region of asia followed by a motion to the western pacific prior to the final move to the eastern part of antarctica the data from eastern iceland based on highly reliable transitional results as indicated by the demagnetizations and rock magnetic experiments derived from relatively spaced sites and different ages seem to indicate that the eastern icelandic high latitude 66 n profiles studied have several persistent transitional paleofield features that are uniquely observed at those sites particularly when compared to other volcanic records located at lower latitudes

geomagnetic polarity reversals a connection with secular variation and core mantle interaction geomagnetic polarity reversal remains one of nature s most enigmatic phenomena dynamo theory admits solutions in pairs with reversed magnetic fields b and b but detailed calculations are required to understand how the field can change sign theory also admits separate solutions with different symmetry across the equatorial plane the symmetric es quadrupole and the antisymmetric ea dipole solutions which may be important in the reversal process and which offer a simple framework for interpreting small paleomagnetic data sets ordinary secular variation leads to very large changes in the magnetic field over several centuries and could easily develop into full reversal the theory of secular variation which is relatively well developed may therefore help in understanding reversals other clues to geomagnetic reversals come from the sun whose magnetic field reverses every 11 years paleomagnetic data show the earth s magnetic field reverses every million years or so with each transition taking about a thousand years during which the intensity may fall by as much as 1 order of magnitude reversal frequency undergoes a modulation on the long timescale 107 years of mantle convection and there have been two long intervals in the past with no reversals such behavior is typical of a highly nonlinear dynamical system but the very long timescale of changes in reversal frequency and its close proximity to the overturn time of mantle convection suggests some control of the dynamo by the mantle short term phenomena such as change in the length of day and secular variation have been studied extensively for evidence of core mantle interactions and we may draw on this body of evidence in order to understand long term effects three physical mechanisms have been proposed topographic electromagnetic and thermal with the last two being most significant for long term effects symmetries allow the dynamo to generate an ea field with the major component a dipole but lateral variations on the core mantle boundary may lead to magnetic fields with no symmetry reflecting the structure of the boundary anomalies changes in reversal frequency on the mantle convection timescale could arise either from changes in total heat flux from the core to the mantle or from instabilities associated with lateral variations at the core mantle boundary neither mechanism is well understood but the former involves significant changes to the earth s overall heat budget whereas the latter must always arise as a natural consequence of deep mantle convection recent measurements of transition fields show pole paths that lie close to two preferred longitudes near 90 w and 90 e if substantiated the result would provide the first definitive evidence of long term mantle control of the geomagnetic field further evidence suggests that the geomagnetic pole during stable polarity also lies along these two longitudes and that magnetic flux at the core surface tends to concentrate along the same longitudes as does the present field a simple theory is proposed relating changes in the core field to apparent transition paths measured at the earth s surface the model shows that longitude confinement of the transition paths can occur for quite complicated core fields and that surface intensities can drop by 1 order of magnitude on average simply because of the reduction in length scale of the transitional field simple transition paths may be an indication of some organization of flux at the core surface but not of large scale or small amplitude core fields

mechanism for geomagnetic polarity reversals recent studies of old magnetic observations1 and the seismic velocity of the lower mantle2 suggest that at least part of the geomagnetic secular variation is driven by temperature anomalies in the solid mantle3 in particular a patch of flux of opposite sign to that expected for a dipole field occurs beneath southern africa it has intensified throughout the twentieth century by a process that is believed to be flux expulsion associated with a particularly hot part of the lowermost mantle and is drifting westwards a similar patch lies beneath south america here i show that the present fall in the dipole moment is directly related to the intensification and southward movement of these patches and suggest the fall occasionally leads to polarity reversal the almost linear increase in the frequency of polarity reversals that has occurred since the cretaceous quiet interval is attributed to the growth of the hot patch of mantle presently under southern africa

reversal records in marine marls and delayed acquisition of remanent magnetization records of geomagnetic reversals frozen into the magnetic components of sediments provide a means to study the time dependent behaviour of the geomagnetic field during polarity transitions although sedimentary records have the advantage of being readily available and continuous the process by which they acquire a remanent magnetization is still not fully understood1 magnetites which lose their magnetization at relatively high temperatures 580 c are generally considered to carry the primary remanence of the sediment that is to record the palaeomagnetic direction here we describe two reversed to normal transitional records from marine marls in which this high temperature component shows a delayed remanence acquisition relative to a lower temperature component in these samples therefore the high temperature component does not reflect geo magnetic changes during the reversal at least for marine marls detailed palaeomagnetic rock magnetic and geochemical studies are apparently necessary to judge the validity of reversal records

transitional geomagnetic impulse hypothesis geomagnetic fact or rock magnetic artifact a striking feature of the steens mountain oregon geomagnetic polarity reversal is the two maybe three extremely rapid field directional changes 6 degrees per day proposed to account for unusual behavior in direction of remanent magnetization in a single lava flow each of these very fast field changes or impulses is associated with a large directional gap some 90 in the record in order to check the spatial reproducibility of the paleomagnetic signal over distances up to several kilometers we have carried out a paleomagnetic investigation of two new sections b and f in the steens summit region which cover the second and the third directional gap the main result is the description of two new directions which are located between the pre second and post second impulse directions these findings weigh against the hypothesis that the geomagnetic field cause the unusual intraflow fluctuations which now appears to be more ad hoc as an explanation of the paleomagnetic data however the alternative baking hypothesis remains also ad hoc since we have to assume variable rock magnetic properties that we have not yet been able to detect within the flows at the original section steens a and d 1 5 km to the north in addition new results for 22 transitional and normal lava flows in section b are presented that correlate well with earlier results from section a

persistent features of polarity transition records from western north america we report on a high resolution transition record from searles valley california that appears not to be hampered by problems of smoothing or recording breakdown and unlike many other sedimentary records defines a path unconfined in longitude a prominent feature of the record is the steady oscillation of the pole along a swath stretching from west africa across north eurasia to the northwest pacific the endpoints of the swath are associated with a change between steady and rapid field variation consistent with results for an older transition from a nearby volcanic record the transition culminates with poles migrating from west africa to south america midway between the two earlier proposed longitudinal bands this path indicates the reversing field has either a wider spectrum of behavior than recently suggested or is described by a more complicated field geometry than one dominated by an equatorial dipole or strong low order nondipole component from a comparison with existing records from the same general geographic area we address the significance of the swath the lack of longitudinal confinement patterns of steady and rapid field changes and the temporal persistence of these features in relation to the geometry of the reversing field

paleosecular variation during sequential geomagnetic reversals from hawaii abstract paleomagnetic directions measured from 175 flows in four volcanic sequences on the island of o ahu hawaii cover about 0 3 m y of geomagnetic field changes and include detailed records of the successive gilbert gauss 3 57 ma and lower mammoth reversals 3 33 ma the dominant pattern of the directional changes is the presence of large inclination variations with increasing amplitude and steep values during the transitional periods the morphology of these recurring features during different transitions is reminiscent of the characteristics of the non dipole field inferred from studies of paleosecular variation in the pacific these observations are consistent with transitional mechanisms yielding the emergence of non dipole components with characteristics similar to those during periods of stable polarity the three different records of the lower mammoth reversal show that clusters of transitional pole positions are not reproducible between parallel sections and are thus linked to local eruptions and not to long lived states of the transitional field

records of geomagnetic reversals from volcanic islands of french polynesia 1 paleomagnetic study of a polarity transition in a lava sequence from the island of huahine we report the behavior of the earth s magnetic field for a transition recorded in lavas at the volcanic island of huahine in french polynesia south central pacific ocean detailed intermediate fields are observed that record an apparent n n excursion the available k ar ages indicate that most volcanic rocks of the southern part of huahine erupted at a time close to the kaena subchron gaps in the paleomagnetic sampling or a hiatus in the volcanic activity during the kaena event may have led to the partial record of a complete polarity transition missing the fully reversed state nevertheless numerous flows have recorded intermediate fields that somewhat constrain reversal models the low intensities of magnetization associated with the intermediate directions suggest that the transitional field has mainly low intensities the beginning of the transition shows inclination steepening consistent with a zonal model of geomagnetic field reversal based on an axial quadrupole large deviations from a north south planar distribution of the intermediate directions show that non axisymmetrical nondipolar fields are important in the middle of the transition

palaeointensity of west greenland palaeocene basalts asymmetric intensity around the c27n c26r transition abstract palaeomagnetic sampling of the lower part of the palaeocene flood basalts of west greenland covering a composite succession of 1 6 km and close to 350 individual lava flows revealed two polarity zones and a 200 m zone of intermediate directions 40 ar 39 ar ages indicate that the transition is c27n c26r following palaeodirectional and rock magnetic analysis 102 samples from 64 flows from the pre transitional normal polarity zone the zone of intermediate directions and the post transitional reverse polarity zone were chosen for thellier palaeointensity experiments altogether 83 samples yielded reliable palaeointensity estimates the mean intensity of the pre transitional c27n field is 16 1 6 9 t 17 flows which is only twice the mean transitional field strength 9 9 3 7 t 10 flows the post transitional c26r field intensity is generally high with a mean 43 3 15 0 t 21 flows the asymmetric palaeointensity around the c27n c26r transition with a high post transitional field resembles the only other detailed absolute palaeointensity studies of geomagnetic transitions recorded in volcanic sequences in hawaii 4 ma and steens mountain 15 5 ma these new data therefore suggest that a strong post transitional field might be a systematic feature of geomagnetic reversals probably indicating a prolonged period of an unusual state of the geodynamo following polarity transitions

when and why sediments fail to record the geomagnetic field during polarity reversals we present four new records of the matuyama brunhes m b reversal from sediments of the equatorial indian ocean west equatorial pacific and north atlantic oceans with deposition rates between 2 cm kyr and 4 5 cm kyr the magnetic measurements were performed using 8 cc cubic samples and provided well defined reverse and normal polarity directions prior and after the last reversal in three records stepwise demagnetization of the transitional samples revealed a succession of scattered directions instead of a well defined characteristic component of magnetization there is no relationship with changes in magnetic mineralogy magnetic concentration and magnetic grain sizes this behavior could be caused by weakly magnetized sediment however the transitional samples of two cores have almost three orders of magnitude stronger magnetizations than the non transitional samples that yielded unambiguous primary directions in the other two cores moreover a similar proportion of magnetic grains was aligned in all records therefore the large amount of magnetic grains oriented by the weak transitional field did not contribute to improve the definition of the characteristic component we infer that the weakness of the field might not be only responsible assuming that the transitional period is dominated by a multipolar field it is likely that the rapidly moving non dipole components generated different directions that were recorded over the 2 cm stratigraphic thickness of each sample these components are carried by grains with similar magnetic properties yielding scattered directions during demagnetization in contrast the strongly magnetized sediments of the fourth core from the west equatorial pacific ocean were exempt of problems during demagnetization the declinations rotate smoothly between the two polarities while the inclinations remain close to zero this scenario results from post depositional realignment that integrated various amounts of pre and post transitional magnetic directions within each sample the present results point out the impossibility of extracting reliable information on geomagnetic reversals from low medium deposition rate sediments with the current sampling techniques among other features they put in question the relationship between reversal duration and site latitude

paleomagnetic full vector record of four consecutive mid miocene geomagnetic reversals abstract seventy mid miocene lava flows from flood basalt piles near neskaupstadur east iceland were sampled which provide a quasi continuous record of geomagnetic field variations samples were collected along the profile b of watkins and walker watkins n walker g p l 1977 magnetostratigraphy of eastern iceland am j sci 277 513 584 which was extended about 250 m farther down in a neighboring stream bed published radiometric age determinations harrison c mcdougall i watkins n 1979 a geomagnetic field reversal time scale back to 13 0 million years before present earth planet sci lett 42 143 152 range from 12 2 to 12 8 ma for the sampled sequence four reversals were recorded in this profile with 18 transitional lavas found within or between 17 normal and 30 reversed polarity flows the large amount of transitional lavas and the large virtual geomagnetic pole dispersion for stable field directions are noteworthy as such features are commonly observed in icelandic lavas and manifest in a far sidedness of the average vgp the reason for this characteristic which could be related to an anomaly beneath iceland a global field phenomenon local tectonics and or non horizontal flow emplacement is scrutinized non horizontal flow emplacement is likely in volcanic environments particularly if the sampled lavas are located on the paleoslopes of a central volcano from the difference of the observed paleomagnetic mean directions to the expected directions assuming a geocentric axial dipole gad a paleoslope which would explain the observed difference was calculated numerically the obtained dip and dip direction point consistently to a possible volcanic extrusion center of the lavas the determined paleodip however proved to be significantly too high compared to the usual slope of a central volcano suggesting further reasons for deviations from the gad other datasets of this age from europe also show enhanced vgp dispersion suggesting further contributions of geomagnetic origin for this observation basically all reversal paths move across the pacific transitions were identified as belonging to c5an 1r c5ar 3r based on the astronomically tuned neogene timescale lourens l hilgen f j laskar j shackleton n j wilson d 2004 a geological time scale cambridge university press we selected 122 samples for paleointensity measurements using a modified thellier method including tests for alteration and multidomain bias 85 of the measured samples yielded data of sufficient quality to calculate paleointensities for 26 lava flows the average paleointensity for stable field directions was 23 3 t whereas the intensity drops to a minimum of 5 8 t during field transitions the stable field intensities represent only about half of the present day field the saw tooth pattern of intensities which is characterized by a sharp increase of intensity directly after a reversal and then followed by a gradual decrease towards the next reversal was not found in this study

how the geomagnetic field vector reverses polarity a highly detailed record of both the direction and intensity of the earth s magnetic field as it reverses has been obtained from a miocene volcanic sequence the transitional field is low in intensity and is typically non axisymmetric geomagnetic impulses corresponding to astonishingly high rates of change of the field sometimes occur suggesting that liquid velocity within the earth s core increases during geomagnetic reversals

early late pliocene biochronology and surface water temperature variations in the mediterranean a high resolution magnetobiostratigraphy is presented for the early late pliocene in sicily paleomagnetic measurements on samples from 100 stratigraphic levels provide an unprecedented high quality polarity sequence which extends from the upper reversed gilbert to the upper normal gauss subchron the resultant chronology for the early late pliocene provides ages for the following principal mediterranean bioevents 1 fodgloborotalia crassaformis at 3 40 ma 2 prolonged absence ofgloborotalia puncticulata between 3 38 and 3 16 ma 3 loduvigerina rutila at 3 16 ma 4 lodsphaeroidinellopsis seminulina at 3 07 ma 5 lodgloboquadrina altispira at 3 04 ma 6 lodcibicides italicus at 2 96 ma summer and winter surface water temperature estimates for the period 3 50 to 2 92 ma are based on the abundances ofglobigerinoides sacculifer andgloborotalia puncticulata in 111 samples with an average resolution of some 5000 years short term variations in temperature of the surface water and in colour and carbonate content of the sediment are primarily controlled by the equinoxal precession a higher seasonality during deposition of the grey coloured carbonate poor facies suggests that at that time the summer solstice occurred near perihelion whereas an inverse alignment occurred at the time of deposition of white coloured carbonate rich layers this phase relationship between the precession cycle and grey coloured layers is also valid for the sapropels which began to intercalate in the grey coloured layers at 2 94 ma long term changes in surface water temperatures include a warming between 3 38 and 3 18 ma followed by a cooling from 3 18 to 3 03 ma the warming of mediterranean surface waters at 3 38 ma is believed to be associated with the final closure of the isthmus of panama consequent strengthening of the gulf stream injected larger volumes of warm surface water into the north atlantic drift and eastern boundary current and this caused surface water temperatures in the mid to high latitude northeast atlantic and mediterranean to rise the subsequent climatic cooling at 3 18 ma is linked to a first stage of northern hemisphere continental ice growth

preferred reversal paths caused by a heterogeneous conducting layer at the base of the mantle 1 time variations in the geomagnetic field induce secondary electric currents in the mantle which contribute to the magnetic field at the surface we explore the possibility that currents in a conducting layer at the base of the mantle disturb the transition field during magnetic reversals and alter the path of the virtual geomagnetic pole vgp we calculate the magnitude of the magnetic perturbation during a reversal and quantify the influence of this perturbation on the vgp reversal path the calculations show that the vgp reversal path depends on the geographic position of the observation site a histogram of reversal paths obtained using a randomly distributed set of sites reveals a preferred path through the americas when the histogram of reversal paths is evaluated with a limited number of sites based on locations in existing lava and sediment databases we find that the paths are confined to asian and american longitudes which is compatible with reversal path distributions inferred from paleomagnetic records

a detailed record of normal reversed polarity transition obtained from a thick loess sequence at jiuzhoutai near lanzhou china summary a record of normal reversed polarity transition has been obtained from a 4 m thickness of loess exposed at a section near lanzhou china magnetostratigraphic studies suggest it may represent a reversal bounding the onset of a reversed polarity zone within the jaramillo normal subchron the natural remanent magnetization consists of two components a low coercivity i 20 mt low unblocking temperature i 300 c component of viscous origin and a high coercivity 20 mt high unblockingtemperature 25g700 c component carrying the characteristic remanence mineral magnetic analyses confirmed the presence of magnetite its low temperature oxidation products and haematite each contributing to the remanence properties grain size and concentration showed limited variations and there was little evidence for the presence of the ultrafine magnetic phase commonly associated with palaeosol formation pedogenic processes appeared negligible and their effects unimportant with detrital processes dominating the mineralogy and most probably the acquisition of the characteristic remanence the reversal record was characterized by the decay and recovery of the geocentric axial dipole term with large directional swings occurring during periods of reduced relative palaeofield intensity the virtual geomagnetic poles traced a complex path exhibiting no particular geographical confinement relative palaeofield intensity determinations were insensitive to the choice of normalization parameter and showed a distinctive asymmetry striking similarities were observed with the matuyama jaramillo reversal record obtained from the same section rolph 1993 and the steens mountain reversal record prcvot et al 1985 lending further support for the existence of unusually high post transitional field intensities

inclination anomalies from indian ocean sediments and the possibility of a standing nondipole field we report magnetic inclinations measured in deep sea sediments of the equatorial indian ocean which record the behavior of a nondipole component of the time averaged geomagnetic field during the plio pleistocene 0 5 ma the magnitude of the nondipole effect recorded in these sediments appears to depend on polarity state with inclinations showing departures from geocentric axial dipole directions which are small 2 o during normal polarity and larger 5 o during reverse polarity times the overall nondipole effect observed here is consistent with prior spherical harmonic estimates of the paleomagnetic field the polarity bias found agrees in both sense and magnitude with earlier reports of polarity asymmetry in the low degree zonal harmonic fields the presence of this asymmetry supports previous suggestions of the existence of a standing component of the nondipole field which does not invert during reversals of the main field we explore whether the standing field so indicated may have influenced paleomagnetic directions recorded during polarity transitions at other equatorial sites

recurring transitional geomagnetic field geometries evidence from sediments and lavas records of the geomagnetic polarity transitions bounding the nunivak subchron have been obtained from the suva marl fiji 18 2 s 178 5 e the transitional virtual geomagnetic pole vgp positions cluster near southern south america and are remarkably similar to the vgp paths for these same reversals obtained from sicily the lower nunivak vgp path and relative intensity record are also very similar to a lower pliocene reverse to normal r to n polarity transition recorded in lavas on kauai the similarities in these transition records from both lava and sediment records suggests that an inclined near dipolar state of the transitional field recurred in these back to back reversals

inclination shallowing and preferred transitional vgp paths transitional vgp paths recorded in sediments cluster into two antipodal preferred longitude bands that tend to lie 90 away from their site longitudes the latter also being clustered vgp paths obtained from lava flow sequences though much fewer appear not to show these biases suggesting a rock magnetic influence on vgp paths recorded in sediments inclination shallowing of detrital magnetic remanence enhanced under low transitional field strengths is the most likely candidate we illustrate the effects of inclination shallowing by applying a simple shallowing model tan ir f tan ia where ia is the inclination of the magnetic remanence and ia is the inclination of the ambient field with field variation to hypothetical data sets shallowing induced clustering increases as f decreases and becomes extreme as f approaches 0 1 we have used the model to de shallow the available set of transitional vgp sediment records for various values of f the probability that the observations arise from inclination shallowing of a uniform random distribution of paths increases as f decreases when f drops to 0 13 there is a 50 chance of getting at least as much grouping as observed to decide if inclination shallowing is a dominant factor in the clustering we need to know whether such extreme shallowing is widespread in sedimentary records under transitional field conditions field and laboratory redeposition data are not yet adequate to resolve this question

depth of post depositional remanence acquisition in deep sea sediments a case study of the brunhes matuyama reversal and oxygen isotopic stage 19 1 although post depositional magnetization pdrm in deep sea sediments appears to be acquired during the earliest stages of sediment compaction the natural variability of the pdrm lock in depth in deep sea sediments is poorly understood and as yet unquantified here we consider variations in the relative stratigraphic positions of oxygen isotopic interglacial stage 19 1 and the brunhes matuyama b m chronozone reversal for eight deep sea sediment cores results from a similar study of the displacement between a widespread microtektite layer and the b m boundary are also included 1 the pdrm lock in depth and the temporal relationships between the b m and stage 19 1 datums can be determined from the offsets between the paleomagnetic and the isotopic and microtektite stratigraphies for cores with sedimentation rates greater than 1 cm kyr a the depth offset between the paleomagnetic and isotopic datums is a linear function of sedimentation rate a simple model r 2 0 77 demonstrates that 1 pdrm acquisition occurs 16 cm below the sediment surface and 2 the b m reversal occurs 6 kyr 4 2 kyr after the stage 19 1 datum and the stage 19 1 datum occurs 9 kyr 4 3 kyr after the deposition of the australasian microtektite strewnfield this example argues against simple geophysical models linking geomagnetic field reversals to climate change or impact events the b m boundary is anomalously deep 30 50 cm in very low accumulation rate sediments 1 cm kyr 1 and this may reflect the unusual physical properties of these sediments a review of the geotechnical literature suggests that very low accumulation rate sediments have sufficient time to develop enhanced interparticle rigidity structural strength which inhibits early compaction and hence pdrm acquisition

statistical properties of paleomagnetic directions in kerguelen lava flows implications for the late oligocene paleomagnetic field 40 ar 39 ar ages are reported to aid in the calibration of the paleomagnetic results the primary contribution of this report however is a compilation of these new data with those already published in order to describe statistically the characteristics of the paleomagnetic field as recorded by the kerguelen flood basalts in total 258 paleomagnetic directions sampled at 13 stratigraphic sections through the lava pile are available and span an approximately 5 ma window from 25 to 30 ma the composite section represents at least 11 polarity zones that are correlated to the reference geomagnetic polarity timescale our approach is to investigate the average normal and reversed polarity field directions over this 5 ma window we calculated a paleomagnetic pole found to be located at l 85 5n f 189 3 e a95 2 3 k 16 5 n 233 this pole is in close agreement with the coeval paleomagnetic poles obtained from different worldwide places when analyzed in the indo atlantic hot spot reference frame in the statistical analysis we tackle the specific question is the secular variation isotropic this question is directly related to the occurrence of a longitudinal confinement of the virtual geomagnetic poles which is still a matter of debate among paleomagnetists by means of statistical tests we show that the paleomagnetic data from kerguelen agree with an isotropic model for paleosecular

palaeomagnetic constraints on the geometry of the geomagnetic field during reversals palaeomagnetic records of the path of the pole during reversals of the earth s magnetic field provide a test of the hypothesis that dipolar or low order axisymmetric components of the field dominate during reversals multiple records of reversals during the past 12 myr show no simple or consistent geographical pattern although a more robust analysis of the transitional field awaits a greater number of well distributed sampling sites the present data are not inconsistent with the simplest models in which a field reminiscent of the non dipole component of the present day field becomes dominant

the rossello composite a mediterranean and global reference section for the early to early late pliocene a high resolution magnetostratigraphy planktonic foraminiferal biostratigraphy and cyclostratigraphy is presented for the pliocene trubi marls in the punta di maiata section on sicily the integrated stratigraphy of the rossello composite section of hilgen 1 newslett stratigr 17 1987 is thereby completed this composite section provides an unprecedented high quality reference section for the early to early late pliocene containing a continuous sequence ranging from below the thvera subchron into the matuyama chron 4 86 2 45 ma the punta di maiata section extends from the sidufjall subchron of the gilbert into the gauss chron 4 50 3 30 ma linear interpolation between paleomagnetic datum planes in this section yields first order age estimates of 3 72 0 01 and 3 59 0 01 ma for the last common occurrence lco and the actual last occurrence lo of globorotalia margaritae in addition this age of 3 59 ma provides an accurate age for the zanclean piacenzian z p boundary provided the lo of g margaritae is maintained as a criterion to define this boundary in the mediterranean irrespective however of the criterion used the punta di maiata and punta piccola subsections of the rossello composite are at present the most suitable sections to be designated as stratotypes for the z p boundary the global significance of the rossello composite is further strongly enhanced by the establishment of an astronomically calibrated geomagnetic polarity time scale based on the correlation of the trubi sedimentary cycles with the astronomical record

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

an attempt at reconstructing the geomagnetic field at the core mantle boundary during the upper olduvai polarity transition 1 66 myear abstract the palaeomagnetic directions of five published records obtained from sedimentary sections from various parts of the world were used to calculate maps of the radial field at the core mantle boundary cmb during the upper olduvai polarity reversal dated at 1 66 myear the five records were synchronized assuming that major changes in field direction occurred simultaneously at the five sites at the beginning and the end of the polarity transition low order coefficients of the field were determined using the hypothesis that the magnetic energy remained constant during the transition maps of the radial field at the cmb were calculated assuming a perfectly insulating mantle the results of this mapping exercise show a non axisymmetric field at the cmb in the first and last stages of the transition with strong flux moving in the longitudinal bands evidenced as preferred ones for transitional virtual geomagnetic poles vgp paths maps calculated at mid reversal do not show a simple geometry more palaeomagnetic records and more refined modelling techniques are desirable however the results are consistent with the idea that the transitional geodynamo is controlled by the thermal structure of the lower mantle

overprinting of natural magnetic remanence in lake sediments by a subsequent high intensity field there has been considerable debate recently on how well sedimentary rocks record transitional field directions during a polarity reversal1 5 this has not been an easy question to answer because the input geomagnetic signal for a given sedimentary record is not known independently and because the process by which remanent magnetization is acquired by sediments is difficult to duplicate in the laboratory the geomagnetic excursion recorded by sediments outcropping around the shores of mono lake california6 8 offers an unusually good opportunity to examine this question we report here a study of the mono lake excursion mle at a new locality on the southeast shore where although the records satisfy usually applied standards of palaeomagnetic quality it is clear from comparison with records from other localities that the field directions during a time of low field intensity have not been faithfully preserved the distinctive directions of natural remanence in the discrepant part of the record show that the southeast shore sediments were overprinted by the ensuing higher intensity field acting on the still unconsolidated sediment apparently by realigning some magnetic grains that had previously been locked in

the c27n c26r geomagnetic polarity reversal recorded in the west greenland flood basalt province how complex is the transitional field 1 we present a paleomagnetic record of the c27n c26r 60 9 ma geomagnetic polarity reversal gpr sampled in two separate lava sequences in the vaigat formation west greenland flood basalt province the two profiles distanced 16 km apart consist of 45 and 35 sampled lava flows of which 31 and 23 respectively record transitional field directions a prominent feature of each profile is the presence of successive flows with similar paleomagnetic directions which based on geological geochemical and paleomagnetic evidences are interpreted to represent flow fields i e lavas erupted in a short period of time rather than periods when the geomagnetic field stood still multimodel photogrammetry shows that the typical flow length of vaigat formation picritic lavas is 2 km which explains why flow to flow correlation of paleomagnetic directions between the two profiles 16 km apart is not possible the two profiles therefore record different and independent time series of the same gpr allowing us to cast light on some of the questions that have arisen from single records particularly noteworthy is the fact that the two high resolution paleomagnetic records share no general trends clearly indicating that the c27n c26r transitional field was more complex and faster changing than can be resolved by either record in addition we obtained stable polarity paleomagnetic data from both pretransitional c27n and posttransitional c26r yielding a paleomagnetic pole latitude 64 8 n longitude 141 5 e n 14 k 19 8 a95 9 2 that passes reversal test

equatorial and mid latitude records of the last geomagnetic reversal from the atlantic ocean abstract three records of the matuyama brunhes reversal have been obtained from odp deep sea cores distributed along the west african margin in the atlantic ocean these studies in addition to the record from dsdp site 609b provide a latitudinal transect extending from the equator to 50 n simulations of natural smoothing by post depositional processes show that characteristic features of a transitional field geometry dominated by axisymmetry should be preserved in the records especially at the equator the results do not seem to favour the hypothesis that axisymmetrical terms would dominate during this transition

geomagnetic field behavior during the iceland basin and laschamp geomagnetic excursions a simple transitional field geometry we present four new records of the iceland basin excursion ibe and five new records of the laschamp excursion le obtained from rapidly deposited marine sediments in the north atlantic ocean the nordic seas the gulf of mexico the south china sea and the southern indian ocean marked minima in relative paleointensity correspond with the paleomagnetic directional changes associated with all of the excursion records the virtual geomagnetic pole vgp paths of the four ibe records are all similar the vgps move southward over europe and africa reaching the southern hemisphere three reach antarctica and then move to more eastern longitudes before returning northward over australia and east asia describing a large counterclockwise loop the same vgp pattern is observed in other published records the vgp paths observed for the le are similar to those of the ibe however they loop clockwise instead of counterclockwise despite the different sense of looping the marked similarity among the paths for the two excursions suggests that a similar relatively simple geometry dominated the transitional field during both the ibe and the le similar dynamo mechanisms must therefore have been active in the earth s core for both excursions the duration of the excursions is estimated at 2 000 years which supports the suggestion that a difference exists between the mechanisms for excursions and reversals however the coincidence of the longitudinal bands for vgps associated with excursions compared to some reversal paths could also indicate an inherent link between the mechanisms for reversals and excursions

sedimentary records of reversal transitions magnetization smoothing artefact or geomagnetic field behaviour sedimentary records of reversals for the past 12 million years exhibit preferred longitudes of their vgp paths this observation may be related to long term aspects of core mantle dynamics however it has been suggested that sedimentary vgp paths could be an artefact due to smoothing which mixes non antipodal pre and post transitional directions we show here that smoothing over unrealistically long time scales is required to generate intermediate directions of magnetization which are purely mixtures of pre and post transitionals directions in sediments whose remanence is primarily depositional drm or post depositional remanence pdrm sedimentary records of transitions showing vgp paths confined to the planes of immediately pre and post transitional vgps have however been observed these can be explained by non axisymmetric bias in the non reversing field gubbins and bloxham 1987 constable 1992 which plays a role in reversals therefore provided that magnetization directions are properly cleaned to give single component directions the preferential longitudes of reversal vgp paths will be due to field behaviour and not to an artefact of the magnetization process if however mixtures of primary magnetization acquired at the time of the formation of the rock and secondary magnetization acquired later are measured then artefacts will be generated

geodynamo energy source constraints from palaeomagnetic data abstract analyses of palaeo geomagnetic data can provide surprisingly strong constraints upon models of the geodynamo in particular it is shown here from an analysis of the geomagnetic reversal sequence that long term variation in reversal rate does not occur as a result of change in any inherent inhibition or encouragement in the reversal process instead there must be some physical process that actually triggers reversals and this process must derive from an energy source effectively independent of the energy source powering the main geomagnetic field from this two very simple generalised models referred to as the hot blob and cold blob models are suggested data from individual reversal transitions are used to discriminate between these two models the preferred model is that the geodynamo is driven primarily by convection related to freezing of the outer core at the inner core outer core boundary cold blobs descending from the core mantle interface are identified as the instabilities that trigger reversals

dynamical similarity of geomagnetic field reversals volcanic records of the reversals of the geomagnetic field can be well matched under the assumption of a common reversal duration and imply that the reversal process comprises three phases a precursor a fast polarity switch and a rebound the properties of which have remained unchanged for about 180 million years the geomagnetic variations that prevail during magnetic field reversals convey important information on the nature of the geodynamo that drives earth s magnetic field in this paper jean pierre valet and co authors show that the ten most detailed volcanic records of geomagnetic reversals have a common duration as well as common dynamical characteristics they propose that the reversing process has remained the same for at least the past 180 million years characterized by three phases a precursor event a 180o polarity switch and then a rebound no consensus has been reached so far on the properties of the geomagnetic field during reversals or on the main features that might reveal its dynamics a main characteristic of the reversing field is a large decrease in the axial dipole and the dominant role of non dipole components1 2 3 other features strongly depend on whether they are derived from sedimentary or volcanic records only thermal remanent magnetization of lava flows can capture faithful records of a rapidly varying non dipole field but because of episodic volcanic activity sequences of overlying flows yield incomplete records here we show that the ten most detailed volcanic records of reversals can be matched in a very satisfactory way under the assumption of a common duration revealing common dynamical characteristics we infer that the reversal process has remained unchanged with the same time constants and durations at least since 180 million years ago we propose that the reversing field is characterized by three successive phases a precursory event a 180 polarity switch and a rebound the first and third phases reflect the emergence of the non dipole field with large amplitude secular variation they are rarely both recorded at the same site owing to the rapidly changing field geometry and last for less than 2 500 years the actual transit between the two polarities does not last longer than 1 000 years and might therefore result from mechanisms other than those governing normal secular variation such changes are too brief to be accurately recorded by most sediments

longitudinal confinement of geomagnetic reversal paths as a possible sedimentary artefact the positions of the virtual geomagnetic pole vgp during a large number of reversals of the earth s magnetic field seem to show a remarkable confinement to longitudes over the americas or to antipodal longitudes1 2 although it has been argued that this confinement is statistically insufficiently constrained3 it has also been pointed out4 that the same bands of longitude appear in other geophysical observations such as the pattern of fluid motion in the outer core and regions of higher seismic velocities in the lower mantle suggesting a causal relationship here we show that longitudinal confinement of vgps can arise from the smoothing of non antipodal stable directions just before and after a geomagnetic reversal because of the filtering effect of the remanence acquisition process in sediments the origin of this non antipodality is still uncertain and must remain speculative until more reversal records become available that include sufficiently long pre and post transitional intervals

a late pleistocene clockwise rotation phase of zakynthos greece and implications for the evolution of the western aegean arc palaeomagnetic measurements have been carried out on eocene to pleistocene sediments on the ionian island of zakynthos nw greece magnetostratigraphic constraints biostratigraphic analyses of planktonic foraminifera and calcareous nannofossils provide a reliable time frame for these deposits the results show that no significant rotation occurred between 8 11 and 0 77 ma but that zakynthos underwent a 21 6o 7 4o clockwise rotation between 0 77 ma and recent thus our data indicate a rapid rotational event in contrast to continuous rotation since 5 ma as previously postulated laj et al tectonophysics 86 1982 45 67 we speculate this late pleistocene tectonic rotation phase to be linked to rapid uplift in the greek region which results from rebound processes caused by african slab detachment underneath the ionian islands 1999 elsevier science b v all rights reserved