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

palaeomagnetic records of the brunhes matuyama polarity transition from odp leg 124 celebes and sulu seas summary palaeomagnetic records of the brunhes matuyama geomagnetic polarity transition were obtained from deep sea sediments of odp leg 124 in the celebes and sulu seas advanced piston core apc samples with high magnetization intensities 2 200 ma m1 and high sedimentation rates 8 4 10 cm kyr1 were recovered from this cruise rock magnetic measurements revealed the carrier of the remanence to be nearly pure magnetite in the pseudo single domain range pass through measurements at intervals of 5 mm on apc cores across the brunhes matuyama polarity transition for holes 767b 769a and 769b were deconvolved with the magnetometer sensor response using the abic minimizing method discrete samples were also taken from the polarity transition zones and subjected to either thermal or alternating field stepwise demagnetization the results were generally consistent with the pass through data after the deconvolution results from the three holes are in good agreement particularly those from the two holes 100 m apart at site 769 the transitional vgp paths from these two holes show two small loops near new zealand before the equator is crossed the vgps continue to swing eastwards to the north atlantic and then move to the northeastern margin of the pacific ocean the positions of the vgp loops obtained from site 769 are dierent from the vgp clusters obtained from both the known volcanic records and the sediment records at the boso peninsula and the north atlantic sites with high sedimentation rates such a dierence may imply the predominance of the non dipole field during the transition relatively stable mid high northern latitude vgps are recognized on all three cores just after the reversal lasting about 4000 years the plot of relative intensity versus vgp latitude for the sediment records from the north atlantic dsdp hole 609b shows a remarkable similarity with our records similar patterns were also obtained for the plot of the palaeointensity versus vgp latitude for the la palma volcanic lava records these results may suggest the existence of a metastable state of the geodynamo producing a zonal component just after the brunhes matuyama boundary which may have played a role in the change of the field intensity

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

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

link between geomagnetic reversal paths and secular variation of the field over the past 5 myr palaeomagnetic records provide information about the behaviour of the geomagnetic field during reversals1 2 existing records are incompatible with transitional field configurations that are either entirely dipolar or entirely zonal dependent only on latitude 3 4 recent compilations5 8 have indicated that the transi tional paths of virtual geomagnetic poles vgps for the past few reversals are located preferentially within two antipodal longitudinal bands suggesting that simple but non zonal field configurations dominate during reversals here i point out that one of the longitudinal bands coincides with that expected from the reversal of a non axial dipole field exactly like that present today the other requires only a sign change in the non axial dipole terms of today s field evidence for persistent non zonal contributions to the field has generally9 13 but not always14 13 been regarded as not statistically significant in the light of poor data distributions i show here that a non zonal bias similar to that observed in reversal data is evident in data on secular variation of the field over the past 5 myr even after normalization according to site locations these results suggest that the time averaged field does indeed contain persistent but not constant non zonal contributions

a southern hemisphere record of the matuyama brunhes polarity reversal the authors present a record of the matuyama brunhes 0 73 ma polarity transition from a southern hemisphere deep sea sediment core v16 58 46 degree s 30 degree e the transition is recorded across at least 30 sm of section and is defined by a nearly 180 degree change in directions which occurs during a low in the relative intensities an increase in the sup 10 be sup 9 be ratios associated with the intensity low suggests that the relative intensities may document a decrease in the strength of the geomagnetic field the virtual geomagnetic pole vgp path is neither clearly near or far sided but tracks approximately 120 degree west of the site longitude considered with other matuyama brunhes transition records this southern hemisphere record supports previous interpretations that the matuyama brunhes transitional field was not dipolar instead the v16 58 transition together with other records of this reversal recently obtained from this longitudinal sector suggests that nonzonal terms which are symmetric about the equator were important during this transitional field

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 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

history of earth s magnetic field and possible connections to core mantle boundary processes the core mantle boundary cmb sets boundary conditions for processes occurring within the core thus the history of the geomagnetic field is intimately connected with the history of the cmb information about one can often provide information about the other it is a simple matter to separate sources of external origin from those of internal origin but there is no unique way to specify the location of the internal sources indirect arguments suggest that sources within earth s outer core strongly dominate the field for spherical harmonic terms with degree less than about 14 and that crustal sources play an increasing role in harmonics with higher degrees building on the pioneering studies of roberts and scott and of backus in the 1960s the velocity field of the core fluid at the top of the outer core can be estimated from geomagnetic secular variation data however even with appropriate assumptions about location of the magnetic field source there is a serious nonuniqueness in inversion of the secular variation data to velocity field because magnetic field lines are not changed by movement of conductive fluid parallel to those field lines additional assumptions about the flow are therefore required sometimes leading to quite different estimates in the pattern of fluid flow the resulting velocity field estimates appear to be sensitive to conditions e g small lateral variations in temperature at the cmb consideration of mantle dynamics suggests that large changes in the cmb conditions probably occur on a 10 to 100 m y timescale secular variations with periods shorter than a million years but longer than several years almost certainly originate from processes operating in the outer core unfortunately there is not yet consensus as to what those processes are longer period variations in the paleomagnetic record including nonstationarities in the rates of magnetic field reversals in paleointensities and in paleosecular variation may reflect changes in cmb conditions because these boundary conditions are controlled primarily by mantle dynamics there have been several speculations regarding causal links between changes in earth s lithosphere and changes in earth s magnetic field finally the evidence linking lateral variations at the cmb to perceived systematics in polarity transition data is found to be intriguing but insufficient

do virtual geomagnetic poles follow preferred paths during geomagnetic reversals virtual geomagnetic poles vgps recorded in sediments during reversals of the earth s magnetic field show an apparent preference for two antipodal sectors of longitude1 3 not only in records of the same reversal from different sites but also in records of different reversals if preferred bands really have persisted from one reversal to the next this would imply that the mantle exerts a significant control over the reversal process4 here we analyse the available database of reversal records from the past 12 myr using a statistical test specifically designed to test the hypothesis of two preferred antipodal longitudinal bands our analysis shows that the records taken as a group of independent observations do show an overall preference for two antipodal longitudinal bands however the site longitudes are also strongly grouped5 and a comparison of the transitional vgp longitudes with site longitudes shows an unlikely grouping under the hypothesis of a genuine geographical preference for transitional vgps we conclude that it is premature to accept the hypothesis of mantle control over the core during geomagnetic reversals

core flow instabilities and geomagnetic storms during reversals the steens mountain impulsive field variations revisited abstract the origin of the large variations in magnetization direction recorded in two different lava flows at steens mountain oregon is discussed the time constant for lava flow cooling and the amplitude of the variations imply extremely large variation rates for the ambient field four orders of magnitude above the present day secular variation two different mechanisms in the liquid outer core namely turbulence and inertial waves that may produce such variations are investigated energetic considerations indicate that the observed impulsive variations are unlikely to be of internal core origin on the other hand it is shown that during reversals when the dipolar field is reduced by a factor ten magnetic storm perturbations at the earth s surface are enhanced by a factor two with respect to their present amplitude it follows that intense magnetic storms can significantly contribute to ambient field changes during reversals the amplitude of the impulsive variations recorded at steens mountain is shown to be consistent with the effect of a strong magnetic storm occurring during cooling of a flow the direction of the variations is also shown to be consistent with this mechanism if the dipolar field contributes significantly to the internal field during the impulsive change

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

the upper olduvai geomagnetic field reversal from death valley california a fold test of transitional directions high resolution records of the upper olduvai geomagnetic field reversal were obtained from two localities within steeply dipping lacustrine sediments of the confidence hills southern death valley california the difference in bedding attitude between the two localities allowed us to perform a fold test of both transitional and non transitional paleomagnetic directions this is the first positive fold test obtained for transitional directions from any geomagnetic field reversal in addition variations in lithology allowed a comparison of transitional records from different sedimentary environments alternating field and thermal demagnetization reveals that most samples have either single or simple two component magnetizations the low coercivity low blocking temperature component is most likely held by large multidomain grains of magnetite and aligns with the present day field when uncorrected for bedding orientation the high coercivity component is held primarily by single and pseudo single domain titanomagnetite as determined from standard magnetic studies and electron microscopy this component is interpreted as a primary magnetic signature based on the presence of stratigraphically bound reversals in the tilt corrected data and positive reversal tests short term field variations can be correlated between the two records and differences are readily explained by the recording process inherent to the two sedimentary environments the sampling site is located within one of the preferred bands of transitional virtual geomagnetic poles vgp s 1 3 yet the transitional vgp paths from both localities are clustered within two primary longitudinal bands which are approximately 90 away from the sampling site longitude one lies over the atlantic ocean 10 50 w and the other antipodal to this 130 170 e these bands are also inconsistent with eight previous studies of the upper olduvai transition 4 9 3 10 when all reported studies of this reversal are examined however no global consistency exists instead the transitional vgp s are better grouped in a sampling site reference frame

the last two geomagnetic polarity reversals recorded in high deposition rate sediment drifts high resolution records of geomagnetic polarity transitions are rare owing to the difficulty of preserving in the geological record details of a process with a duration of only a few thousand years lavas can record the instantaneous field on cooling but lava records are themselves discontinuous and therefore polarity transition fields are seldom captured sedimentary records are more continuous but typical deep sea sedimentation rates of a few cm kyr 1are too low to record transition fields adequately here we report transition records from north atlantic drift deposits with high sedimentation rates 12 cm kyr 1 at the ocean drilling program sites 983 and 984 virtual geomagnetic polar vgp paths are similar within and between sites for the last two geomagnetic reversals attesting to a memory effect spanning back to back reversals the complex vgp paths feature two vgp clusters one located in northeastern asia and the other in the south atlantic ocean off south america and provide a link between the longitudinally constrained transitional vgps often recorded in sediments1 2 which represent highly smoothed renditions of transition fields and the scattered transitional vgps3 and vgp clusters4 observed in lava records

numerical models of the geodynamo and observational constraints 1 the past few years have seen the emergence of a large number of numerical simulations of the geodynamo in parallel both new and old geomagnetic archeomagnetic and paleomagnetic observations have been interpreted as actual geomagnetic features and used as constraints for dynamo models naturally model predictions should be tested against actual characteristics of the geomagnetic field despite huge differences sometimes in excess of a billion between the values of parameters used in the simulations and those estimated for the earth it is intriguing that many available simulations succeed in producing largely axial dipolar magnetic fields with weaker nondipolar structures in agreement with the first order characteristics of the geomagnetic field yet when considering finer characteristics there are significant differences and failures to actually produce a number of fundamental characteristic features in this presentation we first review numerical results obtained to date then we attempt to summarize which field characteristics derived from observational data sets can be considered robust on the basis of simple criteria used to evaluate the degree of confidence that can be placed in each datum we sort presumably characteristic geomagnetic features into three categories robust controversial and unlikely we conclude that numerical models should be illustrated with a number of key predictions averaged over at least 10 dipole diffusion times these predictions should be tested against the subset of robust observations only controversial observations should await additional confirmation

the robustness of seismic moment and magnitudes estimated using spectral analysis we present an assessment of how microseismic moment magnitude mw e stimates vary with the method and parameters used to calculate seismic moment this is an important topic for operators and regulators who require good magnitude estimates when monitoring induced seismicity it is therefore imperative that these parties know and understand what errors exist in given magnitude values something that is poorly reported this study concentrates on spectral analysis techniques and compares mw computed in the time and frequency domains using recordings of mw 1 5e vents at cotton valley east texas the maximum discrepancy between mw estimated using the different methods is 0 6 units a significant variation by adjusting parameters in the mw calculation we find that the radiation pattern correction term can have the most significant effect on m w generally up to 0 8 units following this investigation we make a series of recommendations for estimating microseismic m w using spectral methods noise should be estimated and removed from recordings and an attenuation correction should be applied the spectral level can be measured by spectral fitting or taken from the low frequency level significant factors in obtaining reliable microseismic mw estimates include using at least four receivers recording at 1000 hz and making radiation pattern corrections based on focal mechanism solutions not average values

ice core records of biomass burning tracers levoglucosan and dehydroabietic vanillic and p hydroxybenzoic acids and total organic carbon for past 300 years in the kamchatka peninsula northeast asia abstract we successfully detected biomass burning tracers including levoglucosan and vanillic p hydroxybenzoic and dehydroabietic acids in an ice core 153 m long ca 300 years old taken from ushkovsky ice cap altitude 3903 m the kamchatka peninsula northeast asia concentrations of total organic carbon toc were also determined in the ice core levoglucosan which is produced by pyrolysis of cellulose and hemicellulose and thus is a general tracer of biomass burning showed sporadic peaks in the years of 1705 1759 1883 1915 1949 and 1972 with the largest peak in 1949 however its concentrations did not show a systematic increase in the last century although the concentration peaks seemingly corresponded to the higher ambient temperatures in the northern high latitudes in contrast dehydroabietic acid a specific tracer of the pyrolysis of conifer resin showed a gradual increase from the early 1900s to 1990s with a significant peak in 1970 contributions of dehydroabietic acid to toc also showed an increasing trend for the 20th century similarly vanillic and p hydroxybenzoic acids presented higher concentrations in the last half century with sporadic peaks in 1705 1759 and 1949 this study showed that general biomass burning tracers such as levoglucosan have been sporadically transported over the glacier of the kamchatka peninsula in contrast the ice core record of dehydroabietic acid indicated that fires of boreal conifer forest have more frequently and increasingly occurred in far east and siberia during the last century and transported to the northwestern pacific the present study demonstrates that organic tracers of biomass burning preserved in ice core could provide historical records of biomass burning and boreal forest fires

sampling bias in vgp longitudes i derive probability densities for virtual geomag netic pole vgp longitudes for a general statistical model of local magnetic field variations i show that even for very simple statistically homogeneous models of secular varia tion the distribution of vgp longitudes is peaked 90 o away from the sampling longitude thus when sites are distri buted unevenly a non uniform overall distribution of vgp longitudes is to be expected analysis of the recent geomag netic field indicates that this bias can be significant particu laxly when random errors in paleomagnetic data are allowed for it is possible that some of the deviations from unifor mity seen in recent compilations of paleomagnetic reversal and secular variation data are a statistical artifact resulting from the distortion of the vgp transformation and the non uniformity of paleomagnetic sampling sites

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

fidelity and time resolution of the magnetostratigraphic record in siwalik sediments high resolution study of a complete polarity transition and evidence for cryptochrons in a miocene fluviatile section summary the surai khola section in southwest nepal a 5000 m continuously exposed record of fluvial sedimentation since middle miocene was revisited for high resolution magnetostratigraphy in sequences with expected cryptochrons and reversals of the geomagnetic field polarity intervals with durations of a few tens of thousands of years are recorded as zones of stable palaeomagnetic directions polarity transitions are recorded as zones with complex demagnetization behaviour of specimens in the sedimentary column almost antiparallel palaeoremanence directions residing in diverent haematite phases in the same specimens could generally not be separated properly by thermal demagnetization divering demagnetization paths for neighbouring specimens during a reversal suggest that measured transitional directions are not true geomagnetic field directions but rather are generated by the superposition of variable amounts of at least two almost antiparallel components of magnetization accompanying studies of recent river sand deposits demonstrate that these sediments acquire a true depositional remanent magnetization drm with considerable inclination errors and scattered directions for individual specimens

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

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

transitional field behavior during the gilbert gauss and lower mammoth reversals recorded in lavas from the wai anae volcano o ahu hawaii using a portable fluxgate magnetometer we identified two polarity transitions one a reversed to normal r n in a 600 m section called pu u heleakala and second a normal to reversed n r in a 300 m section known as pu u kamai leunu of superposed basalt flows on the western flank of wai anae volcano one of the two volcanoes that compose the hawaiian island of o ahu volcanic stratigraphic studies suggest that the r n reversal corresponds to the gilbert gauss polarity boundary and the n r reversal correlates with the lower mammoth polarity transition we drilled an average of six to eight samples from each of the lava flows 36 flows from the heleakala and 29 from the kamai leunu spanning the two sections lightning induced secondary magnetization is common in some flows requiring detailed alternating field demagnetization to remove it and to isolate the primary directions thermal demagnetization proved to be ineffective and if employed could lead to incorrect identification of lightning dominated remanence as primary our laboratory experiments indicate that we have successfully identified one transition zone on each section composed of at least seven to eight transitional lava flows the transitional vgps of the gilbert gauss reversal are located in northwestern africa sri lanka and borneo whereas for the lower mammoth reversal the transitional vgps are located in northern africa and in the southern atlantic these observations are not in agreement with the hypothesized antipodal longitudinal bands over the americas and the western pacific rim the two transitions are characterized by periods of high negative inclinations corresponding to vgps in africa about 180 away from the site longitude the obtained vgp paths could be explained by a transitional field affected by anomalous geomagnetic fields that exist beneath the pacific hemisphere these recurrent nonaxisymmetric components in the transitional fields suggest that lateral variations in lower mantle properties influence the geodynamo

confounding influence of magnetic fabric on sedimentary records of a field reversal recent compilations of geomagnetic reversal records have generated a controversy as to whether the geomagnetic field is geographically biased during polarity transitions at present there is general agreement that the virtual geomagnetic poles recorded from cenozoic sediments preferentially lie over the americas or the antipodal longitude yet such a preference is not statistically established however it is intriguing that the claimed preferred paths lie 90 away from the site longitude although this may be partly inherent in the very poor geographical distribution of the sites we prefer not to rely on fortuitous coincidences several authors have argued that sedimentary palaeomagnetic records may be modified by artefacts linked to the acquisition of magnetization here we report that in two sedimentary records of the upper olduvai reversal from confidence hills california the declinations of the remanent magnetization recorded during the reversal are similar to the directions of the maximum horizontal axes of the ellipsoids of magnetic anisotropy this supports the idea that at times of low geomagnetic intensity such as during a reversal factors other than the geomagnetic field influence the orientation of elongated grains

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

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

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

geomagnetic field stability reversal events and excursions abstract it has been suggested that there are several reverse subchrons in the brunhes and this suggestion is gaining some acceptance however such a suggestion has fundanmental implications for the reversal process because it implies that the reverse polarity state has been substantially less stable than the normal polarity state during the brunhes chron analysis of the latest polarity reversal timescale confirms that there is no reason to reject the hypothesis of a common stability for the two polarity states the cretaceous superchron appears to be a time when the reversal rate slowed to the point where the reversal process actually halted so does not indicate any difference in the stability of the two polarity states after examining the data we found no convincing evidence for even a single reversal event during the brunhes and we conclude that the only reversal excursions that are sufficiently well documented for the brunhes are the laschamp and the blake further we note that the question of whether of large excursions represent aborted reversals is unresolved so excursion data should not at this stage be grouped with transition polarity data finally we note that most of the time the nearly axially symmetric normal and reverse polarity states appear to be remarkably stable