plate tectonics may control geomagnetic reversal frequency 1 the discovery of the reversals of earth s magnetic field and the description of plate tectonics are two of the main breakthroughs in geophysics in the 20th century we claim that these two phenomena are correlated and that plate tectonics controls long term changes in geomagnetic reversal frequency more precisely geological intervals characterized by an asymmetrical distribution of the continents with respect to the equator are followed by intervals of high reversal frequency we speculate that the distribution and symmetry of mantle structures driving continental motions at the surface influence the equatorial symmetry of the flow within the core and thus change the coupling between the dipolar and quadrupolar modes which controls the occurrence of reversals

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

simulated geomagnetic reversals and preferred virtual geomagnetic pole paths summary the question of whether virtual geomagnetic poles vgps recorded during reversals and excursions show a longitudinal preference is a controversial one amongst palaeomagnetists one possible mechanism for such vgp clustering is the heterogeneity of heat flux at the core mantle boundary cmb we use 3 d convection driven numerical dynamo models with imposed non uniform cmb heat flow that show stochastic reversals of the dipole field we calculate transitional vgps for a large number of token sites at the earth s surface in a model with a simple heat flux variation given by a y22 harmonic the vgp density maps for individual reversals differ substantially from each other but the vgps have a tendency to fall around a longitude of high heat flow the mean vgp density for many reversals and excursions shows a statistically significant correlation with the heat flow in a model with an imposed heat flux pattern derived from seismic tomography we find maxima of the mean vgp density at american and east asian longitudes roughly consistent with the vgp paths seen in several palaeomagnetic studies we find that low latitude regions of high heat flow are centres of magnetic activity where intense magnetic flux bundles are generated they contribute to the equatorial dipole component and bias its orientation in longitude during reversals the equatorial dipole part is not necessarily dominant at the earth s surface but is strong enough to explain the longitudinal preference of vgps as seen from different sites

long term variations in palaeointensity we compile a dataset of reliable palaeointensity estimates based both on published work and on new data from basaltic glass the basaltic glass data more than double the number of reliable thellie

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

interpretation of regional aeromagnetic data by the scaling function method the case of southern apennines italy a complex aeromagnetic anomaly in southern apennines italy is analysed and interpreted by a multiscale method based on the scaling function we use multiscale methods allowing analysis of a potential field along ridges which are lines defined by the position of the extrema of the field at the considered scales the method developed and applied in this paper is based on the study of the scaling function of the total magnetic field it allows recovering of source parameters such as depth and structural index the studied area includes a pleistocene volcanic structure mt vulture whose intense dipolar anomaly is superimposed on a longer wavelength regional anomaly the interpretation of ridges of the modulus of the analytic signal at different altitude ranges allows recognition of at least three distinct sources between about 5 km and 20 km depth their interpretation is discussed in light of borehole data and other geophysical constraints a reasonable geological model for these sources indicates the presence of intrusions probably linked to the past activity of mt vulture

convection in the earth s core driven by lateral variations in the core mantle boundary heat flux moving core fluid maintains an isothermal core mantle boundary cmb so lateral variations in the cmb heat flow result from mantle convection such variations will drive thermal winds even if the top of the core is stably stratified these flows may contribute to the magnetic secular variation and are investigated here using a simple non magnetic numerical model of the core the results depend on the equatorial symmetry of the boundary heat flux variation large scale equatorially symmetric es heat flux variations at the outer surface of a rapidly rotating spherical shell drive deeply penetrating flows that are strongly suppressed in stratified fluid smaller scale es heat flux variations drive flows less dominated by rotation and so less inhibited by stratification equatorially anti symmetric flux variations drive flows an order of magnitude less energetic than those driven by es patterns but due to the nature of the coriolis force are less suppressed by stratification the response of the rotating core fluid to a general cmb heat flow pattern will then depend strongly on the subadiabatic temperature profile imposing a lateral heat flux variation linearly related to a model of seismic tomography in the lowermost mantle drives flow in a density stratified fluid that reproduces some features found in flows inverted from geomagnetic data

can heterogeneous core mantle electromagnetic coupling control geomagnetic reversals abstract an analytical model is developed for the electromagnetic torques exerted on the mantle by the poloidal magnetic field of the core interacting with a laterally heterogeneous conducting layer at the base of the mantle torques due to changes in both orientation and intensity of a dipolar poloidal field are included contrary to earlier suggestions our calculations predict that the trajectory of the magnetic pole during a polarity reversal is not strongly affected by heterogeneity in mantle electrical conductivity

paleomagnetic volcanic data and geometric regularity of reversals and excursions mostly on the basis of paleomagnetic sedimentary data it has been suggested that maps of virtual geomagnetic poles vgps corresponding to directions of the magnetic field at each site tend to fall along american and asian longitudes during reversals and excursions such geometric regularity in transitional fields may indicate that the core and mantle are dynamically coupled however studies of paleomagnetic lava data have thus far failed to show any pattern in transitional fields in this paper we examine a paleomagnetic lava database covering reversals and excursions which have occurred over the last 20 myr volcanic eruptions occur sporadically thus we normalize the data to account for the fact that reversal and excursional events at the various sites are recorded by different numbers of intermediate directions but we prefer not to use averaging methods of previous investigators who discarded or combined directions when they appeared to be similar we find that volcanic data give intermediate vgps which tend to fall along american and asian longitudes roughly consistent with the sedimentary data this result is not an apparent artifact arising from the nonuniform geographic distribution of volcanic sites provided the appropriate polarity is assigned to intermediate vgps we find that icelandic vgps tend to fall along asian longitudes other patterns in the data for example latitudinal clustering of vgps or distinguishing longitudinal preferences of excursions from reversals are not resolved however it appears that in general transitional fields are nondipolar

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

absence of preferred longitude sectors for poles from volcanic records of geomagnetic reversals according to a recent compilation of sedimentary records from the past 12 myr the reversing geomagnetic field displays a marked long term longitudinal organization which seems to be correlated with the thermal structure of the lower mantle1 however the crucial palaeomagnetic observation that intermediate virtual geomagnetic poles are preferentially confined to longitudes over the americas or antipodal to this sector2 3 might be an artefact caused by distortion and smoothing of the geomagnetic signal by sediments4 6 here we present a compilation of 400 intermediate poles from 121 volcanic records of excursions and reversals less than 16 myr old sampled at various longitudes and latitudes which does not confirm previous inferences1 and reinforces doubts4 6 about the ability of sediments to properly record a rapidly varying field in contrast to virtual poles from excursions those from reversals are uniformly distributed in longitude which indicates that the reversing field is statistically axisymmetrical thus we conclude that there is no evidence for control of transitional fields by the temperature distribution in the lowermost mantle

preferred vgp paths during geomagnetic polarity reversals symmetry considerations the reversing geomagnetic field has been said to have virtual geomagnetic poles vgps confined to a pair of longitudes close to where magnetic flux is concentrated at the core surface today at approximately 90 w and 110 e this can be explained if flux remains concentrated on the same longitudes throughout the transition the vgp path then depends on site position reversal sense r n or n r and sense of flux migration pole or equator ward for transitional fields which remain antisymmetric about the equator n r transitions with poleward flux migration give western longitude vgp paths for sites in the ne and sw quadrants and eastern longitude vgp paths for sites in the nw and se quadrants sites on quadrant boundaries record rapid variable vgp positions data from the last matuyama brunhes reversal are broadly consistent with poleward flux migration

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

time independent and time dependent behaviour of high latitude flux bundles at the core mantle boundary 1 using a numerical dynamo model we examine whether quasi stationary high latitude flux bundles which are observed in the geomagnetic field over the last 300 years and possibly over much longer time periods result from thermal core mantle interactions we do so by imposing lateral heat flux variations at the core mantle boundary based upon the dominant pattern of seismic heterogeneity in the lowermost mantle we find that the time averaged field then has a similar arrangement of flux bundles to that observed during the last few hundred years however the field is time dependent and this arrangement of flux bundles is not a feature of the field at all times

secular variation in numerical geodynamo models with lateral variations of boundary heat flow we study magnetic field variations in numerical models of the geodynamo with convection driven by nonuniform heat flow imposed at the outer boundary we concentrate on cases with a boundary heat flow pattern derived from seismic anomalies in the lower mantle at a rayleigh number of about 100 times critical with respect to the onset of convection the magnetic field is dominated by the axial dipole component and has a similar spectral distribution as earth s historical magnetic field on the core mantle boundary cmb the time scales of variation of the low order gauss coefficients in the model agree within a factor of two with observed values we have determined the averaging time interval needed to delineate deviations from the axial dipole field caused by the boundary heterogeneity an average over 2000 years the archeomagnetic time scale is barely sufficient to reveal the long term nondipole field the model shows reduced scatter in virtual geomagnetic pole positions vgps in the central pacific consistent with the weak secular variation observed in the historical field longitudinal drift of magnetic field structures is episodic and differs between regions westward magnetic drift is most pronounced beneath the atlantic in our model although frozen flux advection by the large scale flow is generally insufficient to explain the magnetic drift rates there are some exceptions in particular equatorial flux spot pairs produced by expulsion of toroidal magnetic field are rapidly advected westward in localized equatorial jets which we interpret as thermal winds

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

statistical assessment of preferred transitional vgp longitudes based on palaeomagnetic lava data summary we examine the statistical significance of a recent report indicating that palaeomagnetic lava data recording reversals and excursions over the past 20 myr yield transitional virtual geomagnetic poles vgps tending to fall along american and asian longitudes using a bootstrap analysis of spherical densitygrams we find that this result is neither a sensitive function of nor heavily dependent upon particular transitional records previous analyses used 2 and kuiper tests of longitudinal uniformity of transitional vgps however the bootstrap analysis presented here takes such statistical tests one step further showing specifically that american and asian longitudes are preferred with significance at about the 95 per cent confidence level furthermore provided the appropriate polarity convention is used we confirm that icelandic transitional vgps tend to fall along asian longitudes these results are indicative of non dipolar transitional fields whose structure is governed by some form of core mantle coupling

mechanisms of land sea interactions the distribution of metals and sedimentary organic matter in sediments of a river dominated mediterranean karstic estuary this paper examines disposal of metals and the origin characteristics and distribution of sedimentary organic matter som in a mediterranean karstic estuary in the north eastern adriatic this environment offers a real time small model system for studies of geochemical processes in microtidal mediterranean estuaries that are infilling with sediments and classified as river dominated disequilibrium estuaries the results have shown that the longitudinal distribution of heavy metals in sediments follows the sedimentation dynamics and deposition pattern of river borne clay mineral particles the highest concentration of metals was found in the restricted upper part of the estuary characterized by rapid deposition of clay particles and terrestrial sedimentary organic matter and decreases toward the open sea the vertical distribution of metals in sediment cores depends on the prevailing ph and eh conditions significant increases of the concentrations of metals in the uppermost strata are the result of recent anthropogenic inputs the share of the terrestrial component in

thermal core mantle interactions maps of the magnetic field at the core mantle boundary for 1715 1980 reveal static features in the field and an absence of westward drift from much of the core mantle boundary the static features suggest that flow in the core is coupled to the mantle we examine different coupling mechanisms and predict lateral inhomogeneity at the core mantle boundary comparison with models of the lower mantle and core mantle boundary derived seismically and from the geoid is encouraging though their resolution is currently inadequate for a full comparison to be made with the much better resolved magnetic field

time dependent mapping of the magnetic field at the core mantle boundary we consider the problem of constructing a time dependent map of the magnetic field at the core mantle boundary we use almost all the available data from the last 300 years to produce two maps one for the period 1690 1840 and the other for 1840 1990 we represent the spatial dependency of the field using spherical harmonics the time dependency using a cubic b spline basis and seek the smoothest solutions compatible with the observations particular attention must be paid to the effects of the crustal field in the data we argue that for observations from permanent magnetic observatories the most efficient strategy is to use first differences of annual means for satellite data the most efficient strategy is simply to limit the number of data used so as to minimize any tendency to map the crustal field into the core field the resulting model fits the observatory data better than any previous model yet has less power in the secular variation than previous models demonstrating that very simple models fit the data the resulting time dependent field map exhibits much of the same structure in the field and its secular variation identified in earlier studies

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

the role of the earth s mantle in controlling the frequency of geomagnetic reversals a series of computer simulations of the earth s dynamo illustrates how the thermal structure of the lowermost mantle might affect convection and magnetic field generation in the fluid core eight different patterns of heat flux from the core to the mantle are imposed over the core mantle boundary spontaneous magnetic dipole reversals and excursions occur in seven of these cases although sometimes the field only reverses in the outer part of the core and then quickly reverses back the results suggest correlations among the frequency of reversals the duration over which the reversals occur the magnetic field intensity and the secular variation the case with uniform heat flux at the core mantle boundary appears most earth like this result suggests that variations in heat flux at the core mantle boundary of the earth are smaller than previously thought possibly because seismic velocity anomalies in the lowermost mantle might have more of a compositional rather than thermal origin or because of enhanced heat flux in the mantle s zones of ultra low seismic velocity