laser probe 40ar 39ar dating studies on sub milligram whole rock basalt samples the age of the steens mountain geomagnetic polarity transition revisited the steens mountain geomagnetic polarity transition is arguably the most comprehensively studied geomagnetic field reversal trapped in igneous rocks over twenty years ago this reversal was dated at 15 5 0 3 ma by the kar method we have now tested the feasibility of carrying out 40ar 39ar dating on 1 mg samples of fine grained whole rock basalt utilizing a laser probe i e on samples 1000 times smaller than have been traditionally analyzed twenty samples taken from lava flows below within and above the reversal n r in steens mountain oregon have been studied by this laser incremental heating technique eighteen of these specimens yield acceptable plateau ages with a weighted mean age of 6 2 0 1 ma standard error on the mean this result is in excellent agreement with the results obtained on two bulk samples of whole rock basalt that average 16 1 0 1 ma an attempt to confirm these ages by fusion of plagioclase laths taken from a few of the individual lava flows was hampered by their low k content 0 2 0 3 our best estimate for the age of six plagioclase laths each 1 mg weight is 16 5 0 5 ma we suggest the techniques utilized herein could be extended to carry out dating of basaltic material of age 1 ma

geomagnetic reversal spurts and episodes of extraterrestrial catastrophism the possibility that the observed 26 32 myr periodicity in the records of extinction events1 and astrobleme ages2 was caused by periodic episodes of physical bombardment of the planetary surface has raised the likelihood that such an astronomical clock also affects geomagnetic reversals3 4 however the detection5 of a comparable 30 myr harmonic in the geomagnetic reversal record has now been found6 to be an accident of data length while the earlier reported7 32 34 myr harmonic of the reversal record may well be an artefact of the data base here we examine the temporal relation between geomagnetic reversals and those catastrophic episodes which occurred during periods of frequent reversals as geomagnetic history comprises long intervals of frequent reversals alternating with intervals of fixed polarity our rationale is that the geomagnetic field should be most susceptible to catastrophism during its relatively unstable periods of frequent reversals we find spurts in the frequency of geomagnetic reversals at approximately 30 myr intervals correlated with catastrophic episodes these spurts can be seen as the geomagnetic signatures of terrestrial catastrophism and we tentatively ascribe them to enhanced core turbulence during episodes of bombardment the absence of reversal spurts during long intervals of fixed polarity is attributed to the failure during these intervals of the geomagnetic dynamo s bi stable oscillations

successive palaeomagnetic reversal records from kauai geomagnetic field reversal is a rapid phenomenon1 2 perhaps taking only a few thousand years knowledge of how the field behaves during these brief and active periods may eventually help to constrain theories of the geodynamo and thus inform us about conditions in the earth s core unfortunately reliable and detailed palaeomagnetic records of transitional field behaviour are few almost entirely from northern hemisphere sites and heavily biased toward reversals of the reverse normal r n sense we report here new data from basalts on the island of kauai hawaii which have recorded successive r n and n r field reversals the sequences of changing field directions for the two reversals are very similar as would be expected if a standing nondipole component controlled the transitional field the data are also consistent with zonal flooding models in which the reversal process depends on the sign of the field other reversal records however cannot be explained by these same models unless the standing field or flooding process changes between some reversals

paleomagnetic data from the coso range california and current status of the cobb mountain normal geomagnetic polarity event two basalt flows which erupted about 1 08 m y ago in the coso range california have normal magnetic polarity and thus provide additional evidence for the cobb mountain normal polarity event a review of available data confirms that this event was of geomagnetic origin a mean age of 1 10 0 02 m y b p for the cobb mountain normal polarity event was found to best fit all available radiometric and geologic data

evidence of a partitioned dynamo reversal process from paleomagnetic recordings in tahitian lavas 1 lavas erupted at the society hotspot during the matuyama brunhes m b polarity reversal record transitional field behavior containing two tight paleodirectional groups that when averaged are antipodal at the 95 confidence level and thus correlate to antipodal clustered virtual geomagnetic poles vgps the occurrence of these observations data obtained from two published records of the m b transition from distinct sections of a succession of flows on tahiti is associated with a time when the strength of the axial dipole was significantly reduced one of the clusters was recorded by lavas that were not erupted in succession suggesting that significant time had passed during this volcanic activity time spent during the formation of the antipodal cluster is unknown although it resides in the same location as vgp clusters associated with four other transitional events obtained from society hotspot lavas calculated vgps at the society hotspot for both polarities of the averaged historic field less the axial dipole term are found in the cluster locations these findings offer strong evidence for a two tiered dynamo process in which nearly the entire axial dipole component undergoes both demise and regeneration quasi independently from that of the remainder of the field the pattern of which is tied to long held physical conditions of the lower most mantle

stratigraphy and remanence acquisition of a palaeomagnetic reversal in alluvial siwalik rocks of pakistan a high resolution record of a palaeomagnetic reversal is documented in miocene alluvial rocks of pakistan we examined lateral variability of lithostratigraphy and palaeomagnetic stratigraphy through the same palaeomagnetic reversal in six correlated sections each section contains one or more palaeomagnetic sites with directions between fully reversed and fully normal the position of the reversal illustrates local relief in the study area and the time transgressive nature of certain stratigraphic units variability in the thickness of the transitional interval indicates contemporaneous variability in sediment accumulation rates different characteristics of palaeomagnetic remanence are associated with the depositional and postdepositional history of these sediments variability in patterns of remanence behaviour is the basis for inferences about post depositional processes we discuss two magnetic parameters that express coherency of palaeomagnetic samples the maximum angle of deviation and the circular standard deviation of particular interest are samples with incoherent palaeomagnetic signals the incoherency of samples is inversely correlated with the thickness of the transitional interval a low rate of sediment accumulation suggested by a thin transitional interval may facilitate a prolonged period of remagnetization through pedogenic or hydrological processes alternatively transition intervals defined by coherent magnetization may be thin as a result of pedogenically induced incoherency

geomagnetic fluctuations during a polarity transition the extensive roza member of the columbia river basalt group washington state has intermediate paleomagnetic directions bracketed by underlying normal and overlying reverse polarity flows a consistent paleomagnetic direction was measured at 11 widely distributed outcrops the average direction has a declination of 189 and an inclination of 5 with greater variation in the inclination rietman 1966 in this study the roza member was sampled in two pasco basin drillcores where it is a single cooling unit and its thickness exceeds 50 m excellent core recovery allowed uniform and dense sampling of the drillcores during its protracted cooling the roza flow in the drillcores recorded part of a 15 5 ma geomagnetic polarity transition the inclination has symmetric quasicyclic intraflow variation while the declination is nearly constant consistent with the results from the outcrops thermal models of the cooling flow provide the timing for remanence acquisition the inclination is inferred to have progressed from 0 to 15 and back to 3 over a period of 15 to 60 years at rates of 1 6 to 0 5 yr because the geomagnetic intensity was probably weak during the transition these apparently high rates of change are not significantly different from present day secular variation these results agree with the hypothesis that normal secular variation persists through geomagnetic transitions the iow amplitude quasicyclical fluctuations of the field over tens of years recorded by roza suggest that the geomagnetic field reverses in discrete steps and that more than 15 60 years were required to complete this reversal

long term polarity stability and strength of the geomagnetic dipole palaeomagnetically observed reversals so far provide the strongest evidence1 that reversals originate from processes of core magnetohydrodynamics mhd thus furnishing a crucial constraint for geomagnetic dynamo modelling the long term variations in reversal frequency favouring2 independent mechanisms for the dynamo and its reversals provide an added constraint here we present evidence for a correlation between polarity superchrons and palaeointensity data and also marine magnetic and mags at anomalies our results indicate that the dipole moment is correlated to the reversal frequency being appreciably greater during the fixed polarity superchrons this new information derived from the reversal record provides fresh insight into the workings of the geodynamo and also explains why the oceanic crust associated with the cretaceous quiet zone carries such a strong thermoremanence

brunhes chron excursion polarity episode recorded during the late pleistocene albuquerque volcanoes new mexico usa summary all basaltic lava flows of the albuquerque volcanoes lat 35 2 n long 253 2 e albuquerque belen basin new mexico record a short excursion polarity episode k ar isotopic age determinations weighted average 155 f 47 ka and evaluation of soil profiles on flow surfaces suggest the late pleistocene circa between 250 and 80 ka as the time of extrusion results from 63 sites in a minimum of eight flows yield a mean direction of d 101 lo z 36 lo ag5 1 2 a 0 7 a 1 2 k 2219 n 8 flows and a corresponding virtual geomagnetic pole position vgp of 354 1 e 20 2 s dp 0 8 dm 1 4 a 0 5 1 4 and vgp angular standard deviation asd 1 8 the unusual magnetization in flows of the albuquerque volcanoes is carried by fine grained single domain sd and or pseudo single domain psd low ti magnetite and is apparently not a function of complex sub solidus alteration of magnetic phases underlying baked soils contain a relatively low coercivity magnetization with directions comprising two groups one is roughly antipodal to the lava remanence the other intermediate between the lava remanence and present field direction because all flows yield statistically indistinguishable directions a field directional path clearly cannot be defined at the precision level of our age determinations the albuquerque feature may correlate with other short polarity episodes or excursions of late pleistocene age e g blake or jamaica though characteristically limited in directional morphology well dated polarity episodes i e sub chrons and excursions recorded in volcanic rocks may provide information on the frequency of significant dynamo instabilities

the blake geomagnetic polarity episode recorded in chinese loess a geomagnetic anomaly consisting of three clearly defined periods of reverse polarity separated by two short intervals of normal polarity has been documented in a loess section near xining western china on the basis of previous studies of chinese loess stratigraphy we correlate this geomagnetic polarity episode with the blake event while more research is needed to improve our understanding of the exact process of remanence acquisition of loess deposits this study shows that loess may provide records of fast geomagnetic events and lends further support to the global nature of the blake geomagnetic event

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

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

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

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

the polarity of the silurian magnetic field indications from a global data compilation silurian palaeomagnetic data from all continents are compiled in an attempt to define the polarity history of the earth s magnetic field during the period data sets from llandovery ludlow and pridoli strata often display evidence of field reversals i e the presence of both normal and reverse polarity magnetization directions despite the fact that the palaeomagnetic sample collections may cover only a limited stratigraphic range in each case palaeomagnetic data from rocks of wenlock age are exclusively of normal magnetic polarity a comparison of the available silurian data with published ordovician palaeomagnetic results suggests that the frequency of magnetic field reversals increased from ordovician to silurian times palaeomagnetic data sets are not yet available from a continuous silurian stratigraphic sequence of any great duration so the silurian magnetostratigraphy is still at a much more preliminary stage than that for the ordovician apart from the wenlock all silurian series have some intervals with frequent

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

absolute paleointensity determinations by using of conventional double heating and multispecimen approaches on a pliocene lava flow sequence from the lesser caucasus abstract we report 28 successful thellier type absolute geomagnetic paleointensity determinations from a pleistocene lava sequence composed of 39 successive flows in the djavakheti highland lesser caucasus georgia additionally multispecimen technique provided the estimation of geomagnetic field strength for 12 independent cooling units paleointensity studies were performed using both thellier type double heating and multispecimen techniques samples selection was mainly based on uni vectorial remanent magnetization thermal stability and domain size of the samples flow mean thellier paleointensity values range from 16 3 5 2 to 71 0 0 3 t while intensities obtained using multispecimen approach vary from17 2 2 3 to 69 3 7 9 t one of the flows is located near a possible discontinuity in the sequence and yields a rather low thellier absolute intensity 16 3 5 2 suggesting a transitional regime and the onset of the matuyama olduvai polarity transition which does not appear on the directional record multispecimen paleointensities from the same flow however yield higher close to present day values which makes untenable the hypothesis of occurrence of transitional field thus the whole sequence was emplaced in a short time between the olduvai chron and 1 73 0 03 ma as suggested by available radiometric and paleomagnetic data caccavari et al 2014

revisiting the jurassic geomagnetic reversal recorded in the lesotho basalt southern africa summary we carried out a detailed and continuous palaeomagnetic sampling of the reversed to normal geomagnetic transition recorded by some 60 consecutive flow units near the base of the lesotho basalt 183 1 ma after alternating field or thermal cleaning the directions of remanence are generally well clustered within flow units in contrast the thermal instability of the samples did not allow us to obtain reliable palaeointensity determinations the geomagnetic transition is incompletely recorded due to a gap in volcanic activity attested to both by eolian deposits and a large angular distance between the field directions of the flows underlying or overlying these deposits the transition path is noticeably different from that reported in a pioneering work carried out in 1962 the most transitional virtual geomagnetic poles vgps are observed after the volcanic hiatus once continents are replaced in their relative position 180 myr the post hiatus vgps cluster over russia however two successive rebounds from that cluster are found with vgps reaching repeatedly the eastern asia coast thus the vgp path is not narrowly constrained in palaeolongitude the decrease in intensity of magnetization as the field deviates from the normal or reversed direction suggests that the decrease in field magnitude during the reversal reached 80 90 per cent we conclude that although the reversal is of a dipole of much weaker moment than that which existed on average during cenozoic time the characteristics of the reversing geodynamo seem to be basically similar

intensity of the earth s magnetic field evidence for a mesozoic dipole low the variation of the strength of the earth s magnetic field throughout geological time is known only poorly large uncertainties spoil indirect paleointensity determinations based on magnetic anomaly amplitude or remanence of sedimentary rocks even when paleointensity data are obtained from direct thermomagnetic studies of magmatic units fundamental problems related to the actual nature of the primary remanence and to the effects of different cooling rates may result in erroneous determinations these uncertainties combined with well known experimental difficulties can explain the large scatter in paleointensity data reported in most previous review papers because experimental difficulties are more rigourously dealt with using the thellier method which also offers some opportunities to minimize some fundamental uncertainties we carried out a compilation of all triassic and younger paleointensity determinations made using this method this selected data set indicates the occurrence of large long term changes of the geomagnetic dipole strength with time the average dipole strength seems to have been approximately constant since late cretaceous time but a dipole strength only one third of the cenozoic value prevailed during most of mesozoic time following a period of larger strength ending possibly during early jurassic times the mesozoic dipole low may be correlated with a cessation of relative motion between the geomagnetic axis and the mantle the terrestrial dynamo seems therefore to exhibit different long term stable states which are probably controlled by the thermal structure in the lowermost mantle

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

rare palaeomagnetic evidence of long term mantle control of the geodynamo and possible role of the nad field in the reversal process the degree to which the lower most mantle influences behavior of the geodynamo has been debated over the past quarter century our analysis of a comprehensive set of 17 cenozoic paleomagnetic transitional field records obtained from lavas in the southern hemisphere provides robust evidence of stable mantle control since the pliocene the records come from a region where given a significantly weakened axial dipole the magnetic field today would be largely controlled by the non axial dipole nad flux patch currently emanating from earth s outer core beneath western australia the paleomagnetic recording sites from west to east include the south indian ocean eastern downloaded from https academic oup com gji advance article abstract doi 10 1093 gji ggz480 5602599 by guest on 17 november 2019 australia new zealand and french polynesia the analyzed records contain from 2 to 25 sequential transitional virtual geomagnetic poles vgps ten of the 17 records supply at least one vgp within a narrow longitudinal band 10 wide between 60 s and the equator centered along longitude 102 4 e that is transitional data from 59 of the cenozoic recordings are found to reside in a region that encompasses a mere 2 8 of the vgp transitional area on earth s surface a robust monte carlo approach applied to this dataset one that takes into account the number of transitional vgps contained in each record finds this result highly improbable p value 0 0006 the present day pattern of vertical flux at the core mantle boundary shows an anomalously strong thin southern hemisphere longitudinal band off the west coast of australia that strikingly coincides with this unusual paleomagnetic finding we conclude with a high degree of confidence that this band of flux has remained virtually unmoved for at least the past 3 myr seemingly independent of the behavior of the axial dipole our findings indicate it has dominated the magnetic field over an area of considerable size during attempts by the geodynamo to reverse polarity

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

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

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

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

on the possibility of recovering paleo diurnal magnetic variations in transitional lava flows i constraints from thermoremanence modelling for an experimental protocol abstract one of the tenets in paleomagnetism is that perturbations of the ground magnetic field due to magnetospheric or ionospheric current systems are too small to leave a detectable paleomagnetic signature in lava flows as suggested by recent work in paleomagnetosphere modelling however external field perturbations may be significantly enhanced during periods of transitional field behavior particularly when the dipole field axis is strongly tilted towards the equator which then leads to an extremely dynamic magnetosphere on the diurnal time scale even for quiet solar wind conditions we here demonstrate that thin rapidly cooled lava flows 50 cm thick with high magnetic blocking temperatures within 50 100 c below the curie temperature indeed have the potential to record such diurnal perturbations further an experimental protocol is suggested to paleomagnetically extract these perturbations our proof of concept is based on numerical modelling of thermoremanence trm acquisition and simulation of thermal demagnetization surfaces for discrete temperature steps in function of vertical position in the flow the trm direction recovered at a given thermal demagnetization step varies with vertical position in the flow and thereby reveals the wave form of the external field variation characteristically the vertical position of a captured signal changes systematically with unblocking temperature which reflects the oblique orientation of cooling isochrons along which the signals are blocked the signals have their largest amplitudes at the maximum unblocking temperatures but decay away at lower temperatures it is by these systmatic trends that external field perturbations if trapped can be paleomagnetically identified and distinguished from a secondary overprint the experimental procedure requires a sample spacing of 1 cm with 1 cm drill cores and small thermal demagnetization steps 15 c at elevated temperatures

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

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

a method for the display and analysis of transitional paleomagnetic data application of a simple coordinate transformation to data from polarity transition and excursion records renders a form of display having strong similarities yet distinct advantages over the use of virtual geomagnetic poles specifically the procedure consists of a single rotation about the east west line so as to align the z axis of the local cartesian system with the normal polarity axial dipole field direction associated with the recording site at the time of the event the method makes possible the redefinition of concepts relevant to transitional field studies in terms of vector directions a more meaningful approach than the use of virtual poles when nondipole field components are significant visually polar stereographic plots in d i space present paleodirectional movement as if one were literally peering down the dipole direction analysis of individual transition records in this rotated directional space makes easy the determination of 1 the angular distance of a given paleodirection from that of the axial dipole field 2 the number of strictly transitional directions contained within a given record and 3 the degree of near sidedness or far sidedness of each paleodirection in addition the comparison of records from distant sites for the purpose of harmonic analysis can be accomplished without the need for additional normalization such comparison plots in d i space display transitional data in a form strongly reminiscent of virtual geomagnetic poles plotted with respect to a common site longitude records of polarity transitions and excursions are presented which illustrate the utility of the proposed approach

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

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

demise of the rapid field change hypothesis at steens mountain the crucial role of continuous thermal demagnetization abstract rapid continuous thermal demagnetization in a triaxe vibrating sample magnetometer disproves an earlier hypothesis that an episode of extraordinarily fast field change is recorded in the remanent magnetization of a single lava flow that cooled during a geomagnetic polarity reversal the original evidence for this hypothesis was the distinctive pattern of variation in direction with vertical position after stepwise demagnetization to 500 c but continuous thermal demagnetization now proves that this flow actually carries consistent high temperature remanence parallel to that of the flow immediately below why does continuous thermal demagnetization succeed when conventional thermal and alternating field demagnetization fail to isolate this direction detailed investigation of the demagnetization trajectories and underlying rock magnetic properties suggest that normal polarity secondary remanent magnetization acquired both at room temperature in today s polarity chron and during modest reheating in a normal field during cooling of the overlying flow is responsible for the misleading pattern of directions alteration during ordinary thermal demagnetization evidently promotes the overprint to higher blocking temperatures so that it masks the primary component whereas the rapid heating 40 c min during continuous demagnetization is fast enough to demagnetize the normal overprint before this masking can happen

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

the van zijl jurassic geomagnetic reversal revisited we have collected two new detailed records of what could be the second oldest well documented reversal the van zijl jurassic 180 ma reversal recorded in the thick basalt sequences of the karoo large igneous province in lesotho and south africa sections yielded 10 and 8 independent transitional paleomagnetic directions respectively over two continuous 130 m and 160 m sequences the corresponding vgp reversing paths share a number of consistent features and are integrated with a third record at bushmen s pass from prevot et al 2003 to yield a single reversal path with 1 a large directional jump with no transitional poles between 45 s and 45 n in the first main phase of the reversal 2 a large elongated loop multiple hairpin to the se with poles going down to 20 latitudes in the second phase of the reversal and 3 a rebound before finally settling in normal polarity we have measured relative paleo intensities on a set of carefully selected samples intensity was low prior to the directional changes and recovery to the following normal polarity was progressive transitional intensity is 80 to 90 lower than full polarity values there is no significant increase in intensity at the time of transitional directional clusters our results are consistent with a weak non dipolar reversing field with faster and larger directional secular variation as the equator is crossed by vgps they tend to vindicate the notion of a short duration of the directional reversal though one cannot draw global conclusions from a record limited to essentially a single location

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

paleomagnetic transition records of the cobb mountain event from sediments of the celebes and sulu seas the cobb mountain cm event is a short normal polarity interval within the matuyama chron just before the jaramillo subchron this event was found in the sediments of site 767 in the celebes sea and site 768 in the sulu sea during odp leg 124 reversal depth and age relation analysis gave ages of 1 11 and 1 10 m y ago for the onset and termination of this event we used discrete samples and high density pass through measurements to study the behavior of the geomagnetic field crossing this event although the two sites are very close to each other the records of the transitional fields were found to be quite different they are also very different from another cm event record from the north atlantic ocean based on the transition data we conclude that the transitional fields for the cm event were non axisymmetric one brief reverse in direction called rebound was found within the onset reversal at site 767 the absence of this feature at site 768 raises an interesting question concerning the source for rebounds

high resolution data of the iceland basin geomagnetic excursion from odp sites 1063 and 983 existence of intense flux patches during the excursion based on discrete samples we report new high resolution records of the 185 kyr iceland basin ib geomagnetic excursion from ocean drilling project odp site 1063 on the bermuda rise sedimentation rate 32 cm kyr 1 and from odp site 983 in the far north atlantic sedimentation rate 18 cm kyr 1 two records from holes 1063a and 1063b are very consistent and provide the highest resolution of the detailed field behaviour during the ib excursion obtained so far inclination records from holes 983b and 983c in the far north atlantic are also very consistent whereas declination anomalies deviate more notably the pseudothellier pt technique was applied along with more conventional palaeointensity proxies nrm arm and nrm to recover relative palaeointensity rpi estimates from hole 1063a and hole 983b as expected these proxies indicate that the field intensity generally dropped at both sites during the ib excursion but also that the history of rpi from the two sites is different vgps from site 1063 indicate that the field at this location experienced some stop and go behaviour between patches of intense vertical flux over north america and the tip of south america areas which coincide fairly well with patches of preferred transitional vgp clustering from reversals and zones of high seismic velocity in the lower mantle changes in rpi at this location were generally gradual possibly due to the proximity of these flux patches and the first period of vgp clustering over north america was accompanied by a conspicuous increase in rpi vgps from site 983 track along a different path and the associated rpi changes are very abrupt and completely synchronous with the onset and termination of the excursion the differing vgp paths from sites 1063 and 983 indicate that the global field structure during the ib excursion was not dominated by a single dipole

variable dispersion of neogene geomagnetic field directions in iceland abstract analysis of the primary paleomagnetic directions in 4000 lava flows in iceland reveals that the magnitude of total virtual geomagnetic pole dispersion has decreased during the last 15 ma this dispersion which is independent of the polarity of the field is in part caused by the occurrence of erratically varying paleofield directions during intermittent periods possibly as long as 100 ka

transitional field behaviour from southern hemisphere lavas evidence for two stage reversals of the geodynamo palaeomagnetic records from australasian basalts provide a rare view of transitional field behaviour as seen from mid southern latitudes these data suggest that a geomagnetic reversal is accomplished in two steps separated by an interval of relative dynamo quiescence a potentially long lived field configuration developed early is observed to be the stepping stone from which both successful and unsuccessful reversal attempts are made several aborted attempts involving the same intermediate field geometry may precede an actual polarity transition

revision of leptite gneisses in the menderes massif a supracrustal metasedimentary origin the pan african basement of the menderes massif comprises a regular lithological succession that reaches a thickness of 8 km the oldest units of which are in ascending order so called leptite gneisses a quartzite sequence transition zone and mica schists new findings suggest that the protoliths for the leptite gneisses proposed to be of volcanic origin in previous studies were predominantly clastic sediments of litharenitic composition geochemical data indicate that the protoliths for the leptite gneisses were of cratonic provenance because they have undergone pan african polymetamorphism and are intruded by approximately 550 ma gneisses of granitic origin it is believed that the time of deposition of their protoliths was late proterozoic relict parageneses indicate that the pan african metamorphism reached granulite facies conditions in the leptite gneisses supracrustal sedimentary origin of these rocks require that these rocks formerly termed leptite and or leptite gneiss should be renamed sillimanite garnet gneiss and or paragneiss

transitional geomagnetic field behavior volcanic records from french polynesia here we report the behavior of the earth s magnetic field for transitions recorded on volcanic islands from polynesia in the south central pacific ocean detailed transitional fields are observed on huahine island the data suggest the record of an excursion n n following the kaena event the jaramillo termination and the brunhes onset are recorded in a volcanic sequence from tahiti island the transitional field is mainly characterized by low intensities while the beginning of the transition appears to be consistent with the zonal model of reversal large deviations from a north south planar distribution of the intermediate directions show that non dipolar fields without preferential axisymmetry control the middle of the transition

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

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

on the possibility of recovering palaeo diurnal magnetic variations in transitional lava flows 2 an experimental case study abstract geomagnetic field variations of external origin may be enhanced during periods of transitional field behaviour particularly when the dipole moment is low in which case they are likely to leave a paleomagnetic signature in rapidly cooled lava flows to test this proposition we have resampled en bloc and studied in fine detail a thin transitional aa flow from a mid miocene lava sequence on gran canaria which was paleomagnetically investigated previously leonhardt r soffel h c 2002 a reversal of the earth s magnetic field recorded in mid miocene lava flows of gran canaria paleointensities journal of geophysical research 107 2299 doi 10 1029 2001jb000949 the flow is characterised by high unblocking temperatures an equatorial vgp position and a very low absolute palaeointensity of 2 t two slabs were cut out of the flow and sampled at 1 cm intervals along four vertical profiles running parallel to each other thermal demagnetisation was performed on two profiles using heating steps as small as 15 c at elevated temperatures the high temperature part of the unblocking spectrum was found to be remarkably constant across the flow as was the curie temperature of 540 c and the negligible anisotropy of magnetic susceptibility the exsolution lamallae observed under the microscope point to deuteric high temperature oxidation having occurred prior to the acquisition of the primary thermoremanent magnetisation while the absolute palaeointensity values vary only little with vertical position the magnetisation directions recovered by thermal demagnetisation vary considerably on average by some 20 at 500 c these large variations can be attributed to an overprint by secondary minerals formed by fluid diffusion around vesicles and low temperature oxidation since the secondary magnetisation recorded transitional directions as well the overprint must have occurred soon after emplacement the directional variations typically decrease in amplitude with increasing blocking temperature which is contrary to what would be expected if pronounced diurnal external field variations were trapped in the flow

the induced earthquake sequence related to the st gallen deep geothermal project switzerland fault reactivation and fluid interactions imaged by microseismicity in july 2013 a sequence of more than 340 earthquakes was induced by reservoir stimulations and well control procedures following a gas kick at a deep geothermal drilling project close to the city of st gallen switzerland the sequence culminated in an ml 3 5 earthquake which was felt within 10 15 km from the epicenter high quality earthquake locations and 3 d reflection seismic data acquired in the st gallen project provide a unique data set which allows high resolution studies of earthquake triggering related to the injection of fluids into macroscopic fault zones in this study we present a high precision earthquake catalog of the induced sequence absolute locations are constrained by a coupled hypocenter velocity inversion and subsequent double difference relocations image the geometry of the ml 3 5 rupture and resolve the spatiotemporal evolution of seismicity a joint interpretation of earthquake and seismic data shows that the majority of the seismicity occurred in the pre mesozoic basement hundreds of meters below the borehole and the targeted mesozoic sequence we propose a hydraulic connectivity between the reactivated fault and the borehole likely through faults mapped by seismic data despite the excellent quality of the seismic data the association of seismicity with mapped faults remains ambiguous in summary our results document that the actual hydraulic properties of a fault system and hydraulic connections between its fault segments are complex and may not be predictable upfront incomplete knowledge of fault structures and stress heterogeneities within highly complex fault systems additionally challenge the degree of predictability of induced seismicity related to underground fluid injections

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

a simple statistical model for geomagnetic reversals the diversity of paleomagnetic records of geomagnetic reversals now available indicate that the field configuration during transitions cannot be adequtely described by simple zonal or standing field models a new model described here is based on statistical properties inferred from the present field and is capable of simulating field transitions like those observed some insight is obtained into what one can hope to learn from paleomagnetic records in particular it is crucial that the effects of smoothing in the remanence acquisition process be separated from true geomagnetic field behavior this might enable us to determine the time constants associated with the dominant field configuration during a reversal

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

upper and lower jaramillo polarity transitions recorded in iodp expedition 303 north atlantic sediments implications for transitional field geometry sediments collected during the integrated ocean drilling program iodp expedition 303 in the north atlantic provide records of polarity transitions and geomagnetic excursions at a high resolution here we investigate polarity transitions at the upper and lower boundaries of the jaramillo subchronozone at sites u1305 u1304 and u1306 the sediments carry strong natural remanent magnetizations nrm with median destructive fields consistent with magnetite as the dominant magnetic carrier both polarity transitions are characterized by low values of relative paleointensity the u1305 record of the lower jaramillo reversal exhibits a marked cluster of virtual geomagnetic poles vgps over southern south america and a secondary accumulation in the region of ne asia north pacific the main south american vgp cluster is also visible at site u1304 which documents a less complex pattern possibly because of a higher degree of smoothing records of the upper jaramillo polarity transition document a vgp loop over the americas followed by north to south motion including a secondary vgp accumulation near india the similarity between several records of the upper jaramillo transition obtained at various sites in the north atlantic is a strong indication that geomagnetic field changes have been faithfully captured results suggest that a transverse possibly dipolar field component has fluctuated during these polarity reversals and that these fluctuations combined with a reduced axial dipole component yielded the observed field at the earth s surface during the polarity transitions the lower jaramillo transition also exhibits vgp clusters in the vicinity of south america and eastern asia but these clusters are shifted relative to the upper jaramillo vgp clusters implying a memory exerted by the upper mantle on polarity transition geometry

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 reversal history the history of geomagnetic polarity reversals in the cenozoic and late mesozoic is well known since the late jurassic oxfordian a continuous record of polarity has been derived for this time interval from the interpretation of oceanic magnetic anomalies most of the polarity chrons in this oceanic record have been verified and dated in coordinated magnetostratigraphic and biostratigraphic studies this has led to the generation of progressively refined and improved geomagnetic reversal time scales that provide a framework for absolute dating of palaeontological zonations by serving as a basis for statistical analysis of reversal frequency they provide information relevant to processes in the earth s core the rate of reversals since the late cretaceous shows a steady increase on which a cyclical variation appears to be superposed a stochastic model for reversals predicts a poisson distribution of polarity interval lengths the polarity time scales contain many fewer short 50 kyr polarity chrons than a poisson distribution and it has been suggested that a gamma renewal process with index greater than unity is a more appropriate statistical model the statistical arguments give no convincing reason for abandoning the model and other physical reasons must be sought to explain the incompleteness of the reversal record the discovery and verification of short chrons in the oceanic record may best be investigated by deep tow magnetometer surveys the reversal history before the late jurassic is not well known magnetostratigraphy in coeval early jurassic sections has not given correlatable records and it has not been possible to compile a definitive polarity sequence evaluation of geomagnetic polarity history for the early mesozoic and the palaeozoic will require unambiguous magnetostratigraphy in well dated sections where verification of the polarity pattern is possible at the fossil zone or stage level

revisiting the initial sites of geomagnetic field impulses during the steens mountain polarity reversal summary we present a new palaeomagnetic investigation of the two sites from the steens mountain oregon volcanic record of a miocene polarity reversal which were supposed to record very fast changes of the geomagnetic field or impulses mankinen et al 1985 prevot et al 1985a b approximately 130 cores were first drilled from the two initial sites belonging to sections a and b in order to obtain at least one detailed and complete vertical sampling of each lava flow thermal analyses of natural remanent magnetization complemented by some alternating field treatments low field thermo magnetic curves microscopic observations and electron probe analyses of key magnetic phases lead us to somewhat different conclusions for the first and the second impulses at site b first impulse we find that the dependence of the remanence direction on the sample vertical position in flow b51 does not imply a directional field change during flow cooling but is better explained by a thermochemical overprinting due to the overlying b50 flow however this conclusion does not challenge the existence of the first impulse because this field change seems to be recorded some 25 m away in flow b51 coe prevot 1989 at a place where it is thick enough for this record not to have been erased by the baking due to b50 regarding the second impulse restudied at site a our new findings are more comprehensively explained by a change in the field direction during cooling of flow a41 2 than by some overprinting using a simple model of flow cooling the angular rate of change of the field is estimated to have been of the order of 2 3 or 250 350 nt per day during the impulse this figure is similar to that previously obtained from site d some 250 m away however the directional paths describing the field change are somewhat different at the two sites new investigations are planned to try to understand the origin of this discrepancy

high resolution sedimentary record of a geomagnetic reversal a detailed study of a geomagnetic reversal shows that during the transition the geomagnetic field undergoes both short and long term fluctuations the first are thought to be linked to the reversal process the second are present with the same time constants both before and during the reversal this suggests that the secular variation at least in its longer period terms is not affected by the reversal of the main field this feature has important consequences for the validity of the frozen flux approximation

long term nongeocentric axial dipole directions and a geomagnetic excursion from the middle pleistocene sediments of the humboldt river canyon pershing county nevada a paleomagnetic record has been obtained from the middle pleistocene sediments of the humboldt river canyon pershing county nevada the age of the sediments is constrained by two tephra layers the 610 000 year old lava creek tephra and the approximately 495 000 year old dibekulewe tephra the magnetization of the sediments is strong and stable and the similarity of data from replicate sections strongly suggests that the data set represents an accurate record of the magnetic field except for the presence of postmagnetization rotations of less than 10 the influence of postmagnetization rotations was eliminated in one of the sections hr1a by rotating the paleomagnetic directions in accordance with the direction of the lava creek welded tuff previously determined from five sampling sites near yellowstone national park after correcting the mean directions we observed nongeocentric axial dipole non gad mean directions of moderate amplitude 19 13 in declination and 22 8 in inclination in the record from the hrla section which may represent as much as 115 000 years of magnetic field behavior in the youngest part of the humboldt river canyon sediments a zone of anomalous directions was recorded which we interpret to be a detailed record of an unconfirmed geomagnetic excursion this record exhibits behavior consistent with that of previously reported polarity transitions because of the widespread nature of the tephras especially the lava creek tephra the prospects are very good for improving estimates of the magnitude and duration of the non gad paleomagnetic directions and also for confirming the excursion

new evidence for extraordinarily rapid change of the geomagnetic field during a reversal palaeomagnetic results from lava flows recording a geomagnetic polarity reversal at steens mountain oregon suggest the occurrence of brief episodes of astonishingly rapid field change of six degrees per day the evidence is large systematic variations in the direction of remanent magnetization as a function of the temperature of thermal demagnetization and of vertical position within a single flow which are most simply explained by the hypothesis that the field was changing direction as the flow cooled