the use of magnetic field excursions in stratigraphy abstract the use of magnetic field excursions in stratigraphy is difficult primarily because the excursion field is complex and not dominantly dipolar in contrast with a reversal which is a global event an excursion can be evidenced at one location but not another although this does not by itself rule out the use of excursions in stratigraphy it does limit the geographic area over which they may be correlated we recommend somewhat conservatively that excursions can be used to correlate between sedimentary cores separated by angular distances of less than 30 on earth s surface correlation between cores separated by more than 45 should not be attempted

evolution of the geomagnetic field prior to the matuyama brunhes transition radiometric dating of a 820 ka excursion at la palma we present cassignol technique k ar dating of lava flows from la palma canary islands spain that bracket the matuyama brunhes transition an age of 821 13 ka obtained for a transitionally magnetized flow ls118 provides the first volcanic evidence for a geomag netic excursion occurring about 40 kyr prior to the transition this interval has been successfully correlated with intensity minima present in high resolution deep sea records before the matuyama brunhes transition this study along with the growing number of well dated excursions reported for the brunhes and matuyama chrons shows that the occurrence of excursions is a common feature of the evolution of the geomagnetic field during stable polarity intervals however the presence of two successive excursional states within 40 kyr prior to the actual reversal conjugated with moderate averaged palaeointensity in this interval as deduced from deep sea records could suggest that favorable conditions for the reversal to occur existed since about 820 ka the present study highlights the importance of detailed chronological constraints for our understanding of the transition of the geomagnetic field from a stable polarity to a reversal state

relative geomagnetic intensity of the field during the last 140 ka relative magnetic field palaeointensities have been obtained from three marine cores in the somali basin western indian ocean the correlation between the records based on changes in the low field susceptibility and the inclination is extremely detailed a time versus depth correlation has been established from the 18o record the remanence intensity was normalized with respect to the anhysteretic remanent magnetization with special attention to changes in grain sizes coercivities and mineralogy the synthetic record of relative palaeointensity has been extended to cover the last 140 ka the quasi cyclic pattern observed during the last 80 ka confirms the results previously obtained in the mediterranean sea and thereby establishes its dominantly dipolar character between 140 and 80 ka the field intensity exhibits a much longer cycle spectral analysis of the intensity records shows the presence of two dominant periodicities but the length of the signal is too short to document the presence of a stationary process

geomagnetic and mineral magnetic characterization of the anthropocene geomagnetic and mineral magnetic data provide geological indices that are both independent of human impact i e geomagnetic and respond to human induced environmental impact i e mineral magnetic we provide the first discussion of such magnetic events for help in defining the anthropocene within the holocene a potential geomagnetic marker for the anthropocene is the low dipole latitude at c 2700 cal a bp which is associated with distinct palaeosecular variation features in northerly mid to high latitude sites mineral magnetic records from lake and marine sediments identify major deforestation and soil delivery events from catchment systems in many parts of the world during the last 4000 years in europe clusters of these events occur around both 2600 cal a bp and ad 1100 the former coinciding with a low in geomagnetic field dipole latitude and peak intensity mineral magnetic records in peats and lake sediments can reflect particulate pollution from fossil fuel burning the expansion of major coal burning began c ad 1800 in western europe and eastern north america but around ad 1900 this expanded due to more widely distributed coal use and this event is the most clear mineral magnetic marker for the base of the anthropocene

late pleistocene paleomagnetic secular variation from the sea of galilee israel 1 stable magnetization of laminated lacustrine lisan formation show typical paleomagnetic secular variation and a possible geomagnetic field excursion 14c ages from the top and assumed 0 9 mm yr deposition rate constrain the recorded section between 29 28 ka and 22 23 ka samples were recovered from two trenches at the southwestern shore of the sea of galilee a graben within the dead sea transform system the angle between the two site mean directions is 21 but retilting the beds to horizontal reduces it to 7 indicating pre folding probably depositional magnetization aberrations of field directions at one locality where vgps deviate 40 from the geographic north and low nrm intensities between 29 28 ka and 25 24 ka are contemporaneous with the mono lake excursion if the correlation is correct the new data support the hypothesis that the excursion was global justifying its utilization as a chronological datum

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

roles of tropical sst anomalies in modulating the western north pacific anomalous cyclone during strong la nina decaying years this study re explores the roles of tropical sst anomalies in modulating the anomalous cyclone over the indo western pacific during strong la nina decaying years via a series of numerical experiments the results suggest that central to eastern equatorial pacific cooling western north pacific wnp warming and indian ocean cooling all contribute to the indo western pacific cyclonic wind anomalies the wnp sst anomalies play a role via a rossby wave type response as the positive sst anomalies in the wnp decay the local forcing effect decays as well the indian ocean remote forcing seems to play an important role during boreal summer via cold kelvin wave induced cyclonic shear and boundary layer convergence during strong la nina decaying years the negative sst anomalies in the equatorial central to eastern pacific can maintain and persist into summer such cooling could induce anomalous walker circulation with ascending branch over the maritime continent to philippines and descending branch over central to eastern pacific which links the wnp climate to the equatorial central and eastern pacific sst anomalies the convective heating over the wnp induced remotely by central to eastern pacific cooling in turn forces atmospheric rossby wave to its west which may amplify the anomalous cyclone over the indo western pacific

volcanic records of the laschamp geomagnetic excursion from mt ruapehu new zealand abstract we present palaeodirectional records of the laschamp geomagnetic excursion from lavas on mt ruapehu new zealand fourteen lava flows on the northwestern and southern flanks of mt ruapehu with 40ar 39ar weighted mean plateau ages that range from 46 3 2 0 to 39 9 1 4 ka were studied the youngest and older flows carry a normal polarity magnetization however six flows dated between 46 3 2 0 and 42 7 1 8 ka record excursional directions three of these flows record southerly palaeomagnetic declinations and negative inclinations that agree well with a published laschamp record from the auckland volcanic field avf together the avf and mt ruapehu lavas currently represent the only volcanic records of the laschamp excursion outside the chaine des puys region france thus they make an important contribution to the global set of laschamp excursion records virtual geomagnetic pole vgp groups for the new zealand and french records early in the excursion are compatible with a dipole dominated field that rotated to an equatorial orientation while simultaneously decaying in strength in contrast younger excursional flows from france and new zealand yield separate vgp groups which suggest either that the field had a nondipolar morphology in this later phase or that the vgp groups were not synchronous 40ar 39ar ages for the mt ruapehu record are on average slightly older than published northern hemisphere ages and from the relative palaeointensity minimum in the glopis sedimentary stack although few individual ages differ significantly at the 2 level the spread suggests an overall excursion duration that is longer than the currently accepted 1500 years this age spread may result from excess ar in magmas at the time of the eruption biasing the results to slightly older ages or from non synchronous excursional field behaviour at near antipodal locations or possibly a precursory phase prior to the main excursion

marine magnetic anomalies evidence that tiny wiggles represent short period geomagnetic polarity intervals since the 1960 s the geomagnetic polarity time scale which is based on marine magnetic anomalies has become fundamentally important in geochronology despite the importance of marine magnetic anomaly records there has been longstanding uncertainty about the meaning of the smallest anomalies observed in these records small amplitude short wavelength anomalies are frequently observed in marine magnetic anomaly records across fast spreading oceanic crust s 50 mm yr the origin of these small scale anomalies referred to as tiny wiggles has remained controversial over the last 30 years tiny wiggles have been interpreted to represent either short period polarity intervals or largescale fluctuations in the ancient field intensity we present palaeomagnetic evidence from a sedimentary record from the north pacific ocean which demonstrates that two short but clearly resolvable polarity zones in addition to a probable geomagnetic excursion occur within chron c5n 2n 9 92 10 95 ma where three tiny wiggles have been reported on marine magnetic anomaly profiles relative palaeointensity data indicate that the field collapsed prior to and during the reversals and during the excursion but that it recovered to higher field intensities within the polarity intervals before collapsing to low values at the succeeding polarity transition this indicates that some tiny wiggles represent real short period geomagnetic polarity intervals while others may represent geomagnetic excursions the existence of such short polarity intervals confirms the predictions of statistical analyses of geomagnetic reversal frequency and indicates that tiny wiggles represent the maximum resolution of geomagnetic polarity intervals in marine magnetic anomaly records fl 2000 elsevier science b v all rights reserved

the variation of intensity of earth s magnetic field with time abstract a new and larger database is established to assess the variation of paleointensity with time it is shown that submarine basaltic glass probably contains a grain growth chemical remanent magnetization in magnetite intensity estimates from these samples are used to establish a lower bound estimate on intensities statistical tests on various subsets of the database are used to establish a more reliable database for finding long term trends of the paleointensity we can obtain a reasonable estimate for intensity versus time for the cenozoic but not for the mesozoic or late paleozoic the distributions of data as a function of time also provide valuable information in particular the two well documented superchrons exhibit different intensity properties which suggests that there is not a simple correlation between reversal rate and intensity

rapid directional changes associated with a 6 5 kyr long blake geomagnetic excursion at the blake bahama outer ridge geomagnetic excursions are recognised as intrinsic features of the earth s magnetic field high resolution records of field behaviour captured in marine sedimentary cores present an opportunity to determine the temporal and geometric character of the field during geomagnetic excursions and provide constraints on the mechanisms producing field variability we present here the highest resolution record yet published of the blake geomagnetic excursion 125 ka measured in three cores from ocean drilling program odp site 1062 on the blake bahama outer ridge the blake excursion has a controversial structure and timing but these cores have a sufficiently high sedimentation rate 10 cm ka to allow detailed reconstruction of the field behaviour at this site during the excursion palaeomagnetic measurements of the cores reveal rapid transitions 230th excess measurements to assess variations in the sedimentation rates through the sections of interest this provides an age and duration for the blake excursion with greater accuracy and with constrained uncertainty we date the directional excursion as falling between 129 and 122 ka with a duration for the deviation of 6 5 1 3 kyr the long duration of this interval and the fully reversed field suggest the existence of a pseudo stable reversed dipole field component during the excursion and challenge the idea that excursions are always of short duration

geomagnetic excursions knowns and unknowns geomagnetic excursions are short lived episodes when earth s magnetic field deviates into an intermediate polarity state understanding the origin frequency amplitude duration and field behavior associated with excursions is a forefront research area within solid earth geophysics recent advances in excursion research are summarized here and key further research is suggested to resolve major unanswered questions improving the global distribution of excursion records particularly from the southern hemisphere obtaining high resolution sedimentary excursion records with good age control from sites with sedimentation rates 10 cm kyr obtaining volcanic excursion records coupled with high precision geochronology and estimating excursion duration with high chronological precision will all facilitate hypothesis testing concerning the deep earth dynamics that generate geomagnetic excursions

the character of the field during geomagnetic reversals it is well established that the geomagnetic field which is predominantly dipolar at the earth s surface has alternated between normal and reverse polarity through geologic time figure 1 evidence of this behavior is preserved in the ancient magnetism of rocks the remanence of radio metrically dated lava flows for example shows that the field has reversed at least 23 times over the past 5 million years figure 2 reversals are not periodic it is apparent in figure 2 that lengths of recent polarity intervals range from lo4 to lo6 years and that the intervals follow a very irregular pattern this behavior contrasts sharply with that of the sun s field which switches polarity every 1 1 years paleomagnetic observations discussed in more detail below show that a geomagnetic reversal takes thousands of years to complete what happens to the geomagnetic field during the transitional period does the entire field just rotate from one orientation to the other does the field disappear

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

evidence for the blake event recorded at the eemian archaeological site of caours france a palaeomagnetic study of the last interglacial calcareous tufa sequence at the archaeological site of caours northern france identified a geomagnetic excursion that we interpret as the blake event earlier palaeontological molluscs mammals and geochemical proxy studies at this site allowed recognition of full interglacial conditions prevailing during the deposition of the tufa sequence the tufa sequence and associated palaeolithic levels have been dated to the eemian interglacial by a set of tims u th measurements on calcitic concretions average 123 3 ka by previous correlations of the blake event with the eemian sensu stricto as defined in the netherlands pollenzones at neumark nord 2 germany and rutten the netherlands it has been shown that the continental eemian starts after the peak of marine isotope stage mis 5e the identification of the blake event at caours implies a post mis 5e peak age for all four levels of the palaeolithic occupation in perspective of the time lag between the mis 5e peak and the beginning of the eemian identified in other studies it is very likely that during the main occupation of caours a significant barrier was in place between north western france and great britain in the form of the english channel it is possible that the chronological position of the last interglacial environments in northwestern europe in relation to sea level change is a key factor behind the apparent absence of last interglacial palaeolithic sites in great britain

ultrahigh resolution marine magnetic anomaly profiles a record of continuous paleointensity variations a distinctive pattern of small scale marine magnetic anomalies 25 100 nt amplitude 8 25 km wavelength tiny wiggles is superimposed on the more generally recognized seafloor spreading pattern between anomalies 24 and 27 in the indian ocean by normalizing and stacking multiple profiles it is demonstrated that this pattern of tiny wiggles is a high resolution recording of paleodipole field behavior between chrons c24 and c27 the pattern of tiny wiggles between anomalies 26 and 27 is compared to an ultrafast spreading 82 mm yr half rate profile from the southeast pacific where a similar signal is observed confirming the paleodipole field origin of the anomalies two basic models are considered in which the tiny wiggles are attributed either to short polarity intervals or to paleointensity fluctuations we conclude that tiny wiggles are most likely caused by paleointensity fluctuations of the dipole field and are a ubiquitous background signal to most fast spreading magnetic profiles the implications of this study are that 1 tiny wiggles may provide information on the temporal evolution of the geomagnetic dynamo 2 the small scale anomalies observed in the jurassic quiet zones may be due to paleointensity fluctuations 3 tiny wiggles are potential time markers in large regions of uniform crustal polarity such as the cretaceous quiet zones and 4 much of the variance in anomaly profiles normally attributed to crustal eraplacement processes particularly at fast and ultrafast spreading rates is actually due to intensity variations in the paleomagnetic field

global changes in intensity of the earth s magnetic field during the past 800kyr recent advances in palaeomagnetic and dating techniques have led to increasingly precise records of the relative intensity of the earth s past magnetic field at numerous field sites the compilation and analysis of these records can provide important constraints on changes in global magnetic field intensity and therefore on the earth s geodynamo itself a previous compilation for the past 200 kyr integrated 17 marine records into a composite curve1 with the geomagnetic origin of the signal supported by an independent analysis of 10be production made on different cores2 the persistence of long term features in the earth s magnetic intensity or the existence of long term periodic changes cannot however be resolved in this relatively short time span here we present the integration of 33 records of relative palaeointensity3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 into a composite curve spanning the past 800 kyr we find that the intensity of the earth s dipole field has experienced large amplitude variations over this time period with pronounced intensity minima coinciding with known excursions in field direction reflecting the emergence of non dipole components no stable periodicity was found in our composite record and therefore our data set does not support the hypothesis that the earth s orbital parameters have a direct and strong influence on the geodynamo

high resolution record of the laschamp geomagnetic excursion at the blake bahama outer ridge summary geomagnetic excursions are brief deviations of the geomagnetic field from behaviour expected during normal secular variation the laschamp excursion at 41ka was one such deviation previously published records suggest rapid changes in field direction and a concurrent substantial decrease in field intensity associated with this excursion accurate dating of excursions and determination of their durations from multiple locations is vital to our understanding of global field behaviour during these deviations we present here high resolution palaeomagnetic records of the laschamp excursion obtained from two ocean drilling program odp sites 1061 and 1062 on the blake bahama outer ridge odp leg 172 high sedimentation rates 30 40cmkyr 1 at these locations allow determination of transitional field behaviour during the excursion palaeomagnetic measurements of discrete samples from four cores reveal a single excursional feature across an interval of 30cm associated with a broader palaeointensity low we determine the age and duration of the laschamp excursion using a stratigraphy linked to the 18 o record from the greenland ice cores this chronology dates the laschamp excursion at the blake ridge to 41 3ka the excursion is characterized by rapid transitions less than 200yr between stable normal polarity and a partially reversed polarity state the palaeointensity record is in good agreement between the two sites revealing two prominent minima the first minimum is associated with the laschamp excursion at 41ka and the second corresponds to the mono lake excursion at 35 5ka we determine that the directional excursion during the laschamp at this location was no longer than 400yr occurring within a palaeointensity minimum that lasted 2000yr the laschamp excursion at this location is much shorter in duration than the blake and iceland basin excursions

lower mantle convection and geomagnetism the statistical properties of recent marine magnetic anomaly time scales covering the period from 165 ma to the present are analyzed via the methods presented in the companion paper mcfadden this issue the data no longer support the hypothesis that during the last 86 ma there were two periods separated by a discontinuity at about 45 ma when the properties were stationary instead there is strong evidence that the rate of reversals decreased in an approximately linear manner with time from 165 ma until it reached zero resulting in the cretaceous normal polarity interval at about 86 ma ago the process of reversals began again and the rate of reversals increased linearly with time there is a slight indication that the rate of reversals reached a maximum about 10 ma ago and is again decreasing analysis of the reversal sequences shows there is no good evidence for the concept of differences in the relative stabilities of the two polarity states nor need such a concept be invoked to explain long intervals without reversals such as the cretaceous normal polarity interval quite clearly there has been some form of thermal perturbation at the base of the mantle with a characteristic time of the order of 108 years our interpretations require convection deep within the mantle and suggest that the temperature at the base of the mantle attains a maximum during long magnetic quiet intervals

middle pleistocene magnetostratigraphy and susceptibility stratigraphy data from a carbonate aeolian system mallorca western mediterranean abstract this study shows that successions of pleistocene carbonate aeolian deposits can be placed successfully in a geochronologic framework using magnetostratigraphic and susceptibility stratigraphic analysis supplemented by luminescence dating studies of wave cut platforms and biostratigraphic evidence the investigated aeolian system covers a significant part of southernmost mallorca and is exposed in impressive coastal cliff sections at the study site at els bancals the aeolian system has a maximum thickness of 16 m and is composed of alternating dark red colluvial deposits and greyish red aeolian dune and sand sheet deposits forming seven cyclostratigraphic units each cyclostratigraphic unit represents landscape stabilisation colluviation and soil formation followed by dunefield development when marine carbonate sand was transported far inland by westerly or north westerly winds the aeolian system is located on top of a wave cut marine platform 12 14 m a s l this platform probably formed during a sea level highstand in marine isotope stage mis 11 427 364 ka and renewed marine activity probably later in mis 11 is indicated by the formation of beach deposits two sections at els bancals were sampled for a paleomagnetic study additional samples were taken to detect variations in magnetic susceptibility ms the characteristic remanent magnetisation has been recovered for the most part of the succession in spite of diagenetic overprinting there is evidence for two probably three reversal polarity excursions possible connected to the levantine cr1 and cr0 biwa iii episodes if this correlation is correct the sampled succession represents a time interval in the middle pleistocene between ca 410 and ca 260 ka this age estimate is supported by the ms study and by luminescence dates of 333 70 ka aeolianite from lower part of the succession and 275 23 ka aeolianite from the top of the succession the nature of the succession suggests deposition during alternating warm and moist colluvial deposition soil formation and cold dry and windy conditions dunefield formation the susceptibility signal can be correlated with the insolation signal at 65 n suggesting that environmental variation on mallorca was linked to orbitally forced climate change and it seems that aeolian activity and dunefield formation were linked to glacial or stadial periods

a new geomagnetic polarity time scale for the late cretaceous and cenozoic we have constructed a magnetic polarity time scale for the late cretaceous and cenozoic based on an analysis of marine magnetic profiles from the world s ocean basins this is the first time since heirtzler et al 1968 published their time scale that the relative widths of the magnetic polarity intervals for the entire late cretaceous and cenozoic have been systematically determined from magnetic profiles a composite geomagnetic polarity sequence was derived based primarily on data from the south atlantic anomaly spacings in the south atlantic were constrained by a combination of finite rotation poles and averages of stacked profiles fine scale information was derived from magnetic profiles on faster spreading ridges in the pacific and indian oceans and inserted into the south ariantic sequence based on the assumption that spreading rates in the south atlantic were smoothly varying but not necessarily constant a time scale was generated by using a spline function to fit a set of nine age calibration points

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

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

high resolution record of geomagnetic secular variation from late pleistocene lake lisan sediments paleo dead sea we measured geomagnetic secular variation in lake lisan sediments paleo dead sea more than 1500 oriented samples were collected from a 27 3 m section of alternating aragonite and detritus laminae in the dead sea basin ranging in age from 67 to 32 ka the natural remanent magnetization nrm is carried by titanomagnetite in the detrital laminae whereas the aragonite is diamagnetic the nrm is very stable and was acquired several hundred years after deposition the mean direction of 878 horizons is dd 005o id 45o 95d 1o d 22 we observed three modes of directional geomagnetic variation as a function of and by inference time very rapid inter sample changes slow variation in mean direction and inclination shallowing of about 1o m the overall rate of change in direction is 0 57 0 57o year not significantly different from zero for about 83 of the record the rate of change is less than 1o year and comparable to historical values high rates of change are observed more frequently in the lisan than in historical records and peak rates are up to ten times faster a smoothed curve resulting in a maximum rate of change of 0 66o year and a mean 0 10 0 10o year may be a more realistic representation of the field behavior no reverse nrms were observed but geomagnetic field excursions may be present where the vgps deviate by more than 40o from the geographic north at about 52 and 41 ka the latter may represent the laschamp event 1998 elsevier science b v all rights reserved

a detailed palaeomagnetic record for the last interglacial period abstract a palaeomagnetic investigation of a late quaternary loess palaeosol sequence in central china has revealed two episodes of anomalous geomagnetic field behaviour in the lower and upper parts of soil complex s1 which spans the last interglacial interval comparison of the magnetic susceptibility record with the deep sea oxygen isotope stratigraphy provides a chronology which has been used to estimate the age and duration of these episodes the age of the younger episode has been estimated at 89 75 ka comparable with the age of a geomagnetic event recognised from sediments in the norwegian greenland sea and arctic ocean the older episode is associated with a broad low in relative palaeointensity which has been assigned to the time interval 130 110 ka this is consistent with existing age estimates of the so called blake event in contrast to a published record of the blake event from the loess at xining which shows three short episodes of almost full reversed polarity in the current record the blake event is represented by a loop in the vgp path which passes through shallow southern latitudes although both episodes occur within the same part of soil complex s1 the difference between the two records implies a greater degree of smoothing of the geomagnetic signal in the huanxian record this is probably a reflection of the lower accumulation rate at huanxian and may also indicate a greater time constant for the remanence aquisition at this site

paleomagnetic tomography of the core mantle boundary abstract analysis of updated global paleomagnetic data indicates that over the last 5 million years the normal and reverse polarity states are discernibly different this difference which can be characterized by a higher ratio of the axial quadrupole to dipole field for the reverse polarity relative to the normal polarity is attributed to lateral variations in temperature or composition at or near the core mantle boundary although several mechanisms are considered thermal electric effects associated with core mantle topography and or with chemical variations in d the seismic layer approximately 150 km thick at the base of the mantle are judged to be the most likely sources of the observed polarity asymmetry this asymmetry should not necessarily be expected to give rise to systematic trends in polarity transitions

paleomagnetism and the nature of the geodynamo records of direct observations of the earth s magnetic field cover less than a ten millionth ofthe known lifetime of the field thus our knowledge of several geomagnetic phenomena critical to our understanding of the geodynamo must come firom the paleomagnetic record a combination of substantial advances during the past decade or so both in dynamo theory previously the domain of the mathematician and in paleomagnetism previously the domain of the geologist has led to provocative models of the earth s magnetic field and a better understanding of the geodynamo

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

revisions to the age of the brunhes matuyama boundary and the pleistocene geomagnetic polarity timescale the currently referenced age of 730 ka for the brunhes matuyama b m geomagnetic polarity boundary mankinen and dalrymple 1979 is based largely on k ar and paleomagnetic data from postcollapse rhyolites in the valles caldera new mexico doell and dalrymple 1966 doell et al 1968 dating of sanidine from valles rhyolites by the 40a 39ar laser fusion technique indicates that samples critical to defining the b m boundary are up to 90 ka older than previously thought the new data combined with earlier results yield an age for the b m boundary of 780 10 ka 40ar 39ar ages from other samples suggest the jaramillo normal polarity subchron extended from 1010 to 915 ka

two geomagnetic excursions during the brunhes chron recorded in chinese loess palaeosol sediments summary detailed palaeomagnetic integrated with rock magnetic studies have been carried out on a loess palaeosol sequence in baoji shaanxi province southern chinese loess plateau for most samples stepwise thermal demagnetization revealed two well defined magnetization components a low temperature component ltc which was isolated between 100 and 200 c is consistent with the present geomagnetic field direction a high temperature component htc which was isolated between 250 and 620 680 c shows normal reversed or transitional polarities our new magnetostatigraphy revealed two distinct geomagnetic excursions recorded in loess unit of l5 and palaeosol unit of s7 respectively and the matuyama brunhes m b polarity boundary in loess unit of l8 rock magnetic experiments demonstrated that the specimens from the excursion zones have the same magnetic properties as those from the brunhes normal or matuyama reversed polarity zones measurements of anisotropy of magnetic susceptibility ams showed that the sediments have primary sedimentary fabrics based on the palaeoclimatological and magnetostratigraphic age models the middle brunhes excursion in loess l5 is dated at 413 433 ka and the early brunhes excursion is estimated to occur 23 33 ka after the m b reversal comparing with previously reported geomagnetic excursions in the brunhes chron the middle brunhes excursion l5 is likely global for the early brunhes excursion s7 we need further studies to examine its global occurrence

paleomagnetism of ar ar dated lava flows from the ceboruco san pedro volcanic field western mexico evidence for the matuyama brunhes transition precursor and a fully reversed geomagnetic event in the brunhes chron 1 we report a detailed paleomagnetic and rock magnetic study of 17 independent lava flows belonging to the trans mexican volcanic belt 175 oriented samples were collected in the ceboruco san pedro volcanic field these sites were previously dated by means of a state of the art 40ar 39ar geochronological method and span from 819 to 2 ka rock magnetic experiments which included continuous susceptibility and hysteresis measurements point to simple magnetic mineralogy in most cases the remanence is carried by ti poor titanomagnetite of pseudosingle domain magnetic structure fourteen flows give normal magnetic polarities while two are reversely magnetized only one cooling unit yields intermediate paleodirections the paleodirections of the flow dated at 819 25 ka correspond to a vgp latitude of 18 n this anomalous field behavior apparently recorded prior to the matuyama brunhes m b reversal may coincide with the transitionally magnetized lavas on la palma canary islands 40ar 39ar age of 822 2 8 7 ka and with an event featured in several marine sediment records thus this geomagnetic event defined as m b precursor is probably global in extent two independent lava flows dated at 623 91 and 614 16 ka yield reverse paleodirections age uncertainties make it difficult to claim the discovery of a new geomagnetic event it is possible that these lavas erupted during the worldwide observable big lost event 40ar 39ar age of 580 2 7 8 ka which has probably been longer and more complex than it is generally believed for geomagnetic excursions

geodynamo reversal frequency and heterogeneous core mantle boundary heat flow abstract we analyze more than two hundred and forty magnetic polarity reversals and excursions over forty million years of time from numerical dynamos with heterogeneous boundary heat flux comparisons between a dynamo with uniform boundary heat flux dynamos with boundary heat flux patterns consisting of a single spherical harmonic and dynamos with tomographic heat flux patterns proportional to the large scale lateral variation of seismic shear wave velocity in the lower mantle reveal that boundary heat flux heterogeneity tends to increase the average frequency of polarity reversals relative to uniform boundary conditions particularly if the heterogeneity increases the average equatorial heat flux non axisymmetric spherical harmonic degree l 1 and 2 boundary heterogeneity and the seismically derived tomographic heterogeneity produce comparable reversal and excursion frequencies in our models suggesting that geomagnetic polarity reversal rates may be relatively insensitive to the non axisymmetric planform of core mantle boundary heat flux in contrast the average polarity reversal frequency in our models is quite sensitive to the total boundary heat flow and to the total heat flow at the equator with non reversing behavior at low heat flow and frequent reversals at high heat flow conditions reversal frequency also increases with the amplitude of the boundary heterogeneity although it is less sensitive to this parameter our results suggest that geomagnetic superchrons may correspond to times with reduced core mantle boundary heat flow either globally or equatorially and conversely periods with high paleomagnetic reversal frequency may correspond to times with increased heat flow at the core mantle boundary

inconsistent palaeomagnetic recording of the blake event in chinese loess related to sedimentary environment three loess sequences located east of the liupan mountain region china were studied to obtain records of the blake polarity event palaeomagnetic records from the appropriate stratigraphic interval of these three sequences however record the blake event poorly in one case and not at all in the other two cases it is important to understand why these loess sequences miss the blake event whereas others record it quite well we conclude that regional variations in the sedimentary environment over the chinese loess plateau are responsible for the inconsistent recording of the blake event the palaeomagnetic records suggest a threefold subdivision of the chinese loess plateau into 1 the central and southern portions of the part of the plateau that lies east of the liupan mountains where the primary record of the blake event has been severely smoothed or wiped out by pedogenic processes during palaeosol s1 formation 2 the northern margin of the loess plateau to the east of the liupan mountains where frequent severe dust storms have resulted in discontinuous records with a high probability of missing the blake event and 3 the region west of the liupan mountains where loess sequences are deposited rapidly and nearly continuously with only minor pedogenesis and are thus capable of recording short events like the blake

evidence for geomagnetic excursions recorded in brunhes and matuyama chron lavas from the trans mexican volcanic belt 1 this study presents paleomagnetic data from 59 independent lava flows from the trans mexican volcanic belt tmvb with ages from 6 4 ma to recent 52 being younger than 1 ma and 11 new 40ar 39ar age determinations most remanence carriers are ti poor titanomagnetite of pseudosingle domain magnetic structure nine lavas contain small amounts of titanomaghemite and four lavas additional titano hematite paleosecular variation of lava flows younger than 1 7 ma is consistent with latitude dependent model g and also in agreement with other pleistocene paleomagnetic data from the tmvb the directional record of brunhes and matuyama chrons lavas was correlated to the geomagnetic polarity timescale and there is evidence for at least four geomagnetic excursions one lava flow dated at 592 20 ka has a fully reversed paleodirection and most likely erupted during the big lost excursion another fully reversed flow dated at 671 12 ka gives new volcanic evidence for the delta stage 17 excursion this excursion is supported by a reversed intermediate direction of another flow from a different volcanic field but of very close age of 673 10 ka from the matuyama age lavas one flow with normal polarity magnetization dated at 949 37 ka could either be related to the kamikatsura or the santa rosa excursion and a normal polarity flow dated at 1628 56 ka could have been emplaced during the gilsa excursion the results presented here confirm in one case but disagree in four cases with results presented in two previous studies of the same lava flows and interpreted as geomagnetic excursions

the mono lake excursion recorded in phonolitic lavas from tenerife canary islands paleomagnetic analyses and coupled k ar and ar ar dating abstract we present a coupled paleomagnetic dating investigation conducted on three different lava flows from the island of tenerife canary islands spain erupted during the mono lake excursion mle paleomagnetic analyses consist in zero field demagnetizations af and or thermal and of thellier and thellier experiments using the picrit 03 set of criteria to select reliable intensity determinations one of the flows is characterized by a direction largely deviated from the one expected from an axial geocentric dipole gad field its paleointensity value is very low 7 8 t the two other sites are characterized by inclinations slightly shallower than the gad value and by low intensity values about 12 and 21 t present value 38 t the three k ar ages combined with two 40 ar 39 ar ages range from 32 0 to 33 2 ka and they are not statistically distinguishable from one another it therefore appears that these lavas have recorded the mle the only excursion in this time interval confirming its brief duration shorter than the minimum age uncertainties available the mean age is younger but within the uncertainties consistent with the age of the 10 be peak and of the marine intensity low when reported in the most recent ice age model these new results are the first ones with radiometric dating produced from the northern hemisphere combined with existing cosmogenic marine and volcanic paleomagnetic data these results are discussed in terms of dating and geometry of the earth magnetic field during the excursion

transitional field clusters from uppermost oligocene volcanic rocks in the central walker lane western nevada abstract three sections of the candelaria hills volcanic sequence west central nevada appear to have recorded parts of two transitional field records or reversal excursions paleomagnetic data and 40 ar 39 ar laser fusion sanidine age estimates for pyroclastic rocks and associated flows show that these rocks recorded the unusual field behavior at about 25 7 ma and about 23 8 ma fifteen sites yield northeast declination moderate to shallow negative inclination mean directions and 16 sites yield west to southwest declination moderate negative inclination directions both populations of site mean directions representing a total of 12 independent eruptive units are highly discordant to a time averaged late tertiary field direction and neither can be explained by a geologically reasonable magnitude of vertical axis rotation virtual paleomagnetic poles vgps estimated from the directional data lie at low to intermediate latitudes 29 of the 31 flows at intermediate latitudes

paleomagnetic evidence from cape verde islands basalts for fully reversed excursions in the brunhes chron abstract in this study we present paleomagnetic data from two lava sequences on santo antao cape verde islands the tarrafal and agua nova profiles from which 63 and 43 lava flows were sampled respectively previous 40ar 39ar ages have constrained the two profiles to the brunhes chron which is in accordance with the normal polarity of the majority of the flows some individual lava flows as well as flow sequences with virtual geomagnetic poles deviating more than 45 from the geographic north pole are interpreted to represent geomagnetic excursions based on interpretation of the directional data three excursions are recorded in the tarrafal profile t i t ii and t iii and four in the agua nova profile ag i ag ii ag iii and ag iv both 40ar 39ar results and paleomagnetic directional data indicate that an excursion defined by eight flows in the tarrafal profile t i is the same as one recorded in six flows in the agua nova profile ag i this excursion which passes the reversal test with classification c is the first geomagnetic excursion in the brunhes chron to pass the reversal test the 40ar 39ar ages suggest that this excursion could represent chryptochron c1n 1 0 493 0 504 ma the only chryptochron or tiny wiggle identified in the marine magnetic anomaly record for the brunhes chron the remaining excursions may not yet be unambiguously correlated between the two profiles another excursion recorded by two flows in the top of the tarrafal profile t iii also displays completely reversed polarity

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

evolution of the geomagnetic reversal rate since 160 ma is the process continuous two main interpretations have been advanced for the evolution of the geomagnetic reversal rate since about 160 ma these interpretations have quite different implications for earth history and associated events and processes our own interpretation is that gradually and continuously decreased from its value of about 41 2 per million years at about 160 ma to zero at about 118 ma then at about 83 ma began to recover again gradually and continuously until reaching its present value of about 41 2 per million years the cretaceous superchron occurred from about 118 to about 83 ma when the reversal rate was zero recently gallet and hulot 1997 suggested an alternative interpretation where 1 was constant from 160 ma to about 130 ma at which time the process was rapidly and strongly perturbed this perturbation led to a discontinuous increase in the mean length of intervals and caused the cretaceous superchron the process then gradually recovered from the strong perturbation with the mean length gradually decreasing until the reversal process again became stationary at about 25 ma this interpretation is predicated on being constant from 160 to 130 ma whereas our interpretation requires a gradual increase in over this time interval consequently a new statistic is suggested to distinguish between the null hypothesis that the reversal process is stationary against the specific alternative that there is a trend in the mean length application of this statistic shows that the data require a trend toward larger values of in the reversal process going from 160 to 130 ma and that this trend continues strongly to the beginning of the superchron thus the addition of a discontinuity at 130 ma is neither required nor supported by the data furthermore application of this statistic shows that the trend toward smaller values of from 83 to 25 ma appears to continue from 83 ma to at least 12 ma certainly there is no requirement for a change in the style of process at 25 ma overall the gallet and hulot interpretation seems untenable with the available data despite this the gallet and hulot interpretation has great value in that it is inherently testable and our ability to reject it has increased our knowledge about the reversal process

sawtooth paleointensity and reversals of the geomagnetic field abstract valet and meynadier valet j p meynadier l 1993 noted an intriguing sawtooth intensity pattern in sequences drilled during leg 138 of the ocean drilling program the pattern is an almost linear decrease in intensity during stable polarity times followed by a rapid recovery or increase in intensity immediately after a reversal they further noted an apparent correlation between the size of the intensity of the recovery and the length of the subsequent polarity chron and inferred that reversals are a consequence of a progressive degradation of the dipole field in that a strong dipole field inhibits a future reversal this has significant potential implications for dynamo theory however their hypothesis implies that the nonstationarity in reversal rate during the cenozoic is controlled by inhibition and so their hypothesis is immediately testable using the available reversal chronology analysis of the chronology shows that the hypothesis is not sustainable

a stochastic model for palaeomagnetic field variations summary regeneration of the earth s magnetic field by convection in the liquid core produces a broad spectrum of time variation relative palaeointensity measurements in marine sediments provide a detailed record over the past 2 myr but an explicit reconstruction of the underlying dynamics is not feasible a more practical alternative is to construct a stochastic model from estimates of the virtual axial dipole moment the deterministic part of the model drift term describes time averaged behaviour whereas the random part diffusion term characterizes complex interactions over convective timescales we recover estimates of the drift and diffusion terms from the sint2000 model of valet et al and the padm2m model of ziegler et al the results are used in numerical solutions of the fokker planck equation to predict statistical properties of the palaeomagnetic field including the average rates of magnetic reversals and excursions a physical interpretation of the stochastic model suggests that the timescale for adjustments in the axial dipole moment is set by the dipole decay time d w e obtain d 29kyr from the stochastic models which falls within the expected range for the earth s core we also predict the amplitude of convective fluctuations in the core and establish a physical connection to the rates of magnetic reversals and excursions chrons lasting longer than 10 myr are unlikely under present day conditions however long chrons become more likely if the diffusion term is reduced by a factor of 2 such a change is accomplished by reducing the velocity fluctuations in the core by a factor of 2 which could be attributed to a shift in the spatial pattern of heat flux from the core or a reduction in the total core heat flow

paleointensity of the geomagnetic field during the last 80 000 years high resolution records of the relative paleointensity of the geomagnetic field have been obtained from five marine cores three duplicate records were used to estimate the regional coherency of the data within a single area tyrrhrenian sea while the two others document the field variations in the eastern mediterranean and the southern indian ocean careful investigations of distinct rock magnetic parameters have established the downcore uniformity of the sediments in terms of magnetic mineralogy and grain sizes the time depth control was provided by oxygen isotopes and small scale variations in the deposition rates were constrained by means of tephrachronology the synthetic curve calculated from the mediterranean records provides a continuous record of the intensity variations during the last 80 000 years 80 kyr which correlates well with the sparse volcanic data available for the period 0 40 kyr the fact that identical behavior is seen in both data sets and that they also compare quite well with results from a core collected in the pacific ocean establishes the truly dipolar character of these variations the dipole field moment is characterized by large scale changes as shown by the existence of pronounced drops at 39 and 60 kyr alternating with periods of higher intensity the record suggests a periodic nature for these intensity variations however the period studied is not sufficiently long to state this conclusively these results demonstrate the potential of sediments for such studies and constitute a first step towards obtaining a global paleointensity record over a long period of time

late pleistocene geomagnetic excursion in icelandic lavas confirmation of the laschamp excursion in 1980 kristjansson and gudmundsson 1 reported a late glacial geomagnetic excursion in three hills in the reykjanes peninsula iceland with shallow negative inclinations and westerly declinations they named it the skalamaelifell excursion more extensive field work has identified the same excursional paleomagnetic direction declination 258 inclination 15 at four additional outcrops in a 10 10 km area in the reykjanes peninsula the excursion lavas are olivine tholeiites with similar petrography and chemical compositions paleointensity determinations by the thellier method average 4 2 0 2 t for 8 samples more than an order of magnitude weaker than the present geomagnetic field in iceland together these results suggest extrusion of the excursion lavas in a very brief span of time probably less than a few hundred years kar dating of the excursion lavas gives a mean age for 19 determinations of 42 9 7 8ka 2 compilation of thirty kar ages of the laschamp and olby flows by three laboratories yield a new age for the laschamp excursion in france of 46 6 2 4ka 2 the age of the excursion in southwestern iceland is statistically indistinguishable from the laschamp excursion at the 95 confidence level and both have very low paleointensities therefore we suggest that the laschamp and olby flows in france and the skalamaelifell units of iceland recorded essentially the same geomagnetic excursion differences in the virtual paleomagnetic poles vgps of these excursions may be due to 1 the probable non dipole character of the geomagnetic field during the excursion 2 rapid geomagnetic secular variation and possible small age differences of the extrusive rocks in france and iceland and or 3 crustal magnetic anomalies which might dominate the local geomagnetic field during the excursion at either or both locations

absolute geomagnetic paleointensity after the cretaceous normal superchron and just prior to the cretaceous tertiary transition 1 we report a detailed rock magnetic and thellier paleointensity study from autlan volcanic succession in western mexico the whole rock k ar age obtained for these lavas 67 4 1 2 ma together with the magnetostratigraphy results indicates that the lava flows were erupted during chron 30n just prior to cretaceous tertiary transition thirty nine samples from eight cooling units were preselected for paleointensity experiments on the basis of their low viscosity index stable remanent magnetization and close to reversible continuous thermomagnetic curves high quality determinations were obtained for 18 individual samples from four independent lava flows the mean virtual dipole moment vdm obtained in this study is 4 9 0 6 x 10 22 a m 2 which is significantly lower than the present geomagnetic field strength but in reasonably good accord with other comparable quality determinations between 35 and 80 ma our data are not significantly different from almost all published paleointensities between 5 and 90 ma moreover rigorously selected worldwide paleointensity data in the same interval better fit trend 2 of heller et al 2002 a careful examination of high quality paleointensity data from cretaceous indicates that there is no simple relation between magnitude of earth s magnetic field and reversal rate

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

evidence for a new geomagnetic reversal from lava flows in idaho discussion of short polarity reversals in the brunhes and late matuyama polarity chrons k ar ages and paleomagnetic data for basalt samples from a new core hole site e at the idaho national engineering laboratory inel indicate that the age of the reversed polarity event recorded in snake river plain lavas is older than 465 50 ka 1000 years before present reported previously by champion et al 1981 nine basalt flows eight with normal polarity and one with reversed polarity were recognized in the site e core hole the flows above and below the reversed flow have ages of 491 80 ka and 580 93 ka respectively the inclination of the paleomagnetic field direction of the reversed flow at site e agrees with the inclination of reversed flows elsewhere at inel which have an age of 565 14 ka these reversed flows were previously thought to be correlative with the emperor event we suggest that this polarity event is an older event which we name the big lost reversed polarity subchronozone and subchron a review of data documenting short reversal records from volcanic and sedimentary rocks shows that there is evidence for eight polarity subchrons in the brunhes and two besides the jaramillo in the late matuyama these 10 short subchrons begin to indicate the many short events that cox 1968 hypothesized must exist if polarity interval lengths have a poisson distribution these events are true subchrons not excursions and may or may not have low associated paleointensities although low field strengths might explain why the reversal process aborts the mean sustained polarity interval length since late matuyama chron time is 90 000 years the similarity of this number with the 105 year period of the earth s orbital eccentricity suggests anew that linkage between geomagnetic paleoclimatic and possible underlying earth orbital parameters should be evaluated

high resolution paleomagnetic secular variations and relative paleointensity since the late pleistocene in southern south america abstract paleomagnetic inclination declination and relative paleointensity were reconstructed from the sediments of laguna potrok aike in the framework of the international continental scientific drilling program icdp potrok aike maar lake sediment archive drilling project pasado here we present the u channel based full vector paleomagnetic field reconstruction since 51 2 ka cal bp the relative paleointensity proxy rpi was built by normalising the natural remanent magnetisation with the anhysteretic remanent magnetisation using the average ratio at 4 demagnetisation steps part of the chrm interval nrm arm 10 40 mt a grain size influence on the rpi was removed using a correction based on the linear relationship between the rpi and the median destructive field of the natural remanent magnetisation mdf nrm the new record is compared with other lacustrine and marine records and stacks from the mid to high latitudes of the southern hemisphere revealing consistent millennial scale variability the identification of the laschamp and possibly the mono lake geomagnetic excursions and a direction swing possibly associated to the hilina pali excursion at 20 ka cal bp nonetheless a global scale comparison with other high resolution records located on the opposite side of the earth and with various dipole field references hint at a different behaviour of the geomagnetic field around southern south america at 46 ka cal bp

40ar 39ar dating links albuquerque volcanoes to the pringle falls excursion and the geomagnetic instability time scale abstract incremental heating of basaltic lava groundmass from the albuquerque volcanoes new mexico and plagioclase from ash d at pringle falls oregon yield 40ar 39ar isochron ages of 218 14 and 211 13 ka respectively sediments in which ash d was deposited and the eight lava flows of the albuquerque volcanoes display excursional paleomagnetic data with virtual geomagnetic poles vgps in the southern hemisphere and together with the statistically indistinguishable 40ar 39ar dates establish that both sites record the pringle falls excursion this excursion is also recorded by the 227 8 ka mamaku ignimbrite new zealand and by high deposition rate sediments at odp site 919 in the north atlantic ocean that are dated astrochronologically at 209 207 ka we propose that the names albuquerque and jamaica excursion be abandoned and that a radioisotopic age of 211 13 ka be adopted for the pringle falls excursion which is one of five globally expressed well documented excursions in marine sediment cores or dated by 40ar 39ar methods that took place from 220 to 30 ka together with at least five other well dated excursions between 730 and 520 ka some ten excursions define the geomagnetic instability time scale gits for the bruhnes chron eight of these excursions have been dated using 40ar 39ar methods if the temporal clustering of excursions in the gits is not a sampling artefact this suggests that the geodynamo is not intrinsically unstable but rather that instability in the flow pattern of the outer core fluid flow causes the main dipole field to be considerably weakened such that it becomes unstable with a 200 to 300 ka recurrence interval excursions are an important part of dynamo behavior over geologic time scales that in addition to reversals of the main dipole field need to be fully considered when assessing whether theoretical and numerical simulations of the dynamo produce earth like results

paleomagnetic evidence for the gothenburg geomagnetic excursion during the pleistocene holocene transition recorded in the paleo danyang lake eastern china abstract this paper aims to identify the gothenburg geomagnetic excursion as a stratigraphic marker and presents the results of a paleomagnetic investigation of deposits within the southern part of the paleo danyang lake area of eastern china these paleomagnetic data provide evidence for the gothenburg geomagnetic excursion at 12 494 to 13 081 cal year bp at depths of 6 5 7 2 m within the paleo lake we also analyzed the spatial distribution of localities that both record and do not record the excursion event and the estimated timing of this excursion event in east asia the presence of uninterrupted sedimentation at a high depositional rate during the pleistocene holocene transition appears to be the key factor in preserving evidence for this excursion in the localities that record this event the non existence of the excursion appears to be the result of low sedimentation rates under sampling and the presence of sedimentary hiatuses associated with erosion the fact that these types of excursions can be correlated between sedimentary cores separated by angular distances of less than 30 on earth s surface means that the gothenburg excursion can be used to correlate between lacustrine and marine deposits in east asia and potentially further afield as well as being useful for chronostratigraphic purposes comparisons between cave stalagmite 18o records and the polarities of the lacustrine sediments also indicate that the timing of the gothenburg geomagnetic excursion is consistent with an abrupt decrease in temperature since the last deglaciation although the meaning of this correlation requires further research

link between excursions and paleointensity inferred from abnormal field directions recorded at la palma around 600 ka abstract an excursion of the geomagnetic field characterized by strongly abnormal directions associated with a field intensity as low as 7 t has been identified in a lava flow sequence from la palma canary islands the age of 602 12 ka obtained by k ar dating using the cassignol technique for a transitional flow represents our best estimate for the age of the occurrence of this episode of excursional directions this age is concordant with a minimum observed in the global sint800 composite record derived from worldwide deep sea records of relative paleointensity we show that a geomagnetic field model composed of a present day like non dipole field superimposed on an axial dipole reduced by a factor of 2 compared to its present value can account for excursional directions associated with low paleointensity in some areas while most other parts of the globe display apparently normal polarity directions we next generate through linear inversion a synthetic field model constrained by the actual la palma data and a set of directions at 30 sites distributed worldwide based on an axial dipole field with half present day intensity the energy spectrum of this synthetic field model is compatible with most of the non dipole part of the present day field and in that sense is earth like i e represents an acceptable configuration of the past geomagnetic field at 600 ka the model shows that very anomalous excursional directions covering 1 5 of the earth s surface may co exist with apparently normal directions elsewhere the new data and modeling strongly support the hypothesis that excursions are an integral aspect of secular variation there may be a continuum between normal sv and field reversals primarily controlled by the ratio of axial dipole field intensity relative to the remaining non axial dipole part

cretaceous geomagnetic paleointensities thellier experiments on pillow lavas and submarine basaltic glass from the ontong java plateau 1 we present new thellier paleointensity results on pillow lavas and pillow rim submarine basaltic glass sbg from 120 ma basalts erupted on the ontong java plateau all pillow lavas have suffered low temperature alteration and are unsuitable for thellier paleointensity experiments in contrast pillow rim sbg is virtually impermeable to seawater and therefore has suffered little or no low temperature alteration out of 21 selected sbg samples with magnetic moment larger than 10 10 am2 18 samples from 12 chilled sbg margins yielded paleointensity estimates of good technical quality rock magnetic measurements show the primary carrier of remanence is low ti titanomagnetite occurring in a continuous range of compositions starting from nearly pure magnetite as suggested by a recent study magnetic hysteresis parameters were monitored during the entire paleointensity experiment and a drastic change of parameters was observed above 400 440 c which is in accordance with the partial thermoremanent magnetization checks failing significantly above the same temperature we suggest that paleointensity estimates obtained below this critical temperature represent true determination of the paleomagnetic field strength for several pillow margins two to three samples were measured and in such cases the data agree well altogether we obtain reliable estimates from 12 sbg pillow rims yielding a mean paleointensity of 27 1 13 4 t and a corresponding virtual axial dipole moment of 5 8 2 8 1022 am2 this value is slightly higher than whole rock data from similar aged subaerial lava flows the paleosecular variation of the intensity estimates is slightly larger than previously reported for paleointensity determinations for the cretaceous normal superchron

millennial scale instability in the geomagnetic field prior to the matuyama brunhes reversal changes in the earth s magnetic field have global significance that reach from the outer core extending out to the uppermost atmosphere paleomagnetic records derived from sedimentary and volcanic sequences provide important insights into the geodynamo processes that govern the largest geomagnetic changes polarity reversals but dating uncertainties have hindered progress in this understanding here we report a paleomagnetic record from multiple lava flows on tahiti that bracket the matuyama brunhes m b polarity reversal 771 thousand years ago our high precision 40 ar 39 ar ages constrain several rapid and short lived changes in field orientation up to 33 000 years prior to the m b reversal these changes are similar to ones identified in other less well dated lava flows in maui chile and la palma that occurred during an extended period of reduced field strength recorded in sediments we use a simple stochastic model to show that these rapid polarity changes are highly attenuated in sediment records with low sedimentation rates this prolonged 33 000 year period of reduced field strength and increased geomagnetic instability supports models that show frequent centennial to millennial scale polarity changes in the presence of a strongly weakened dipole field

brief reversed polarity interval during the cretaceous normal polarity superchron a striking feature of the geomagnetic time scale is the 34 m y period of normal polarity field bias known as the cretaceous normal polarity superchron new paleomagnetic data from three deep sea drilling project cores sites 317a 402a and 463 indicate that a brief interval of reversed polarity occurred during the superchron this reversed polarity interval may help refine past motions of the pacific plate and aid resolution of the statistical structure of the geomagnetic reversal chronology

the relative stabilities of the reverse and normal polarity states of the earth s magnetic field recent analyses of the geomagnetic reversal sequence have led to different conclusions regarding the important question of whether there is a discernible difference between the properties of the two polarity states the main differences between the two most recent studies are the statistical analyses and the possibility of an additional 57 reversal events in the cenozoic these additional events occur predominantly during reverse polarity time but it is unlikely that all of them represent true reversal events nevertheless the question of the relative stabilities of the polarity states is examined in detail both for the case when all 57 events are included in the reversal chronology and when they are all excluded it is found that there is not a discernible difference between the stabilities of the two polarity states in either case inclusion of these short events does however change the structure of the non stationarity in reversal rate but still allows a smooth non stationarity only 7 of the 57 short events are pre 38 ma but the evidence suggests that this is a real geomagnetic phenomenon rather than degradation of the magnetic recording or a bias in observation this could be tested by detailed magnetostratigraphic and oceanic magnetic surveys of the paleogene and late cretaceous overall it would appear that the present geomagnetic polarity timescale for 0 160 ma is probably a very good representation of the actual history and that different timescales and additional events now represent only changes in detail

fundamental transitions in the geodynamo as suggested by paleomagnetic data abstract analysis of the reversal chronology shows that the reversal process effectively ceases during the cretaceous superchron the geodynamo appears to have two basic states a reversing states and a non reversing state this must reflect a fundamental transition in the geodynamo process at the onset and termination of the superchron we suggest that this transition and indeed the long term variations in reversal rate may reflect a change from a predominantly oscillatory dynamo to a steady one which might for example be caused by a change in the relative roles of the and effects in 2 dynamos if so this should probably have been reflected in reduced secular variation during the superchron unfortunately data are not available to test this prediction directly but we use the surface integral of the angular standard deviation of paleomagnetic field directions as a proxy this shows a significantly reduced secular variation during the superchron as predicted

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

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

petrogenesis of mid proterozoic granitic magmas examples from central and west texas abstract circa 1 1 ga granitic magmatism in texas was manifested as two compositional groups 1 the 1 12 ga red bluff granitic suite in west texas and 2 1 12 1 07 ga granites of the llano uplift of central texas both suites share some characteristics typical of anorogenic granites e g potassium and iron rich bulk compositions fe rich hydrous silicates emplacement conditions involving low oxygen fugacities and water contents and exhibit similar isotopic characteristics however rock associations mineral chemistries and trace element compositions of the two suites are distinct and no single petrogenetic model for the two suites is possible the red bluff granitic suite includes cogenetic syenites quartz syenites and granites transitional ferrobasaltic dikes are also present in contrast syenitic and mafic rocks are not associated with the llano granites the llano granites contain biotite and calcic amphibole with lower fe fe mg ratios compared to those occurring in the red bluff rocks alkali amphiboles e g arfvedsonite occur in the red bluff granites but not in the llano granites the red bluff granitoids are characterized by high feo t mgo ratios high na2o k2o high concentrations of hfse and rare earth elements ree and other features typical of a type within plate granites e g the pikes peak batholith ppb the llano granites are geochemically distinct with generally higher p2o5 and sr lower na2o feo t mgo zr y and ree and much lower ta and nb nd isotopic data overlap between the two granite suites and have juvenile signatures however trace element data suggest different petrogeneses for the two suites the red bluff suite is interpreted as having a direct derivation from mantle sources via extended fractional crystallization of basaltic parental magmas with minor crustal assimilation the llano granites appear to represent anatectic melts derived from slightly older juvenile crustal sources some melts underwent fractional crystallization controlled by feldspar and accessory minerals the petrology and geochemistry of 1 1 ga granites in texas indicate that they should not be considered as part of a single anorogenic magmatic event the red bluff granitic suite was emplaced into a shelf sequence north of the grenville front within a broad zone characterized by mild extension in contrast llano granites are late stage intrusions emplaced into multiply deformed and metamorphosed crust south of the grenville front during or after waning stages of grenville orogenesis

asymmetry in the reversal rate before and after the cretaceous normal polarity superchron abstract the geomagnetic reversal chronology shows a decreasing reversal rate from about 160 ma to about 118 ma a cessation in the reversal process the cretaceous normal polarity superchron until about 83 ma and then an increasing until recent times visually the rate of decrease in leading into the superchron appears larger than the rate of increase in following the superchron statistical tests developed here confirm the visual perception of an asymmetry in the rate of change in this asymmetry could be caused by a relatively rapid breakdown of d with thermal energy being advected away followed by a slower thickening of the layer through thermal conduction however in the absence of good thermal models as to how different core mantle boundary conditions affect the geodynamo there are probably many apparently viable speculations as to the origin of the observed rate asymmetry

is the pringle falls excursion a product of geomagnetic field behaviour or an artefact of sedimentation processes insights from anisotropy of magnetic susceptibility ams analyses summary it is well known that the earth s magnetic field changes both in direction and magnitude as a function of time the magnitudes of those changes and the timescales over which those changes take place however are still a matter of some debate in particular the record of geomagnetic excursions obtained from sedimentary sequences has been suspected to introduce some noise in the analyses of geomagnetic changes due to disturbances associated with the deposition of the sediments the range of possible disturbances is large and includes both natural and human induced factors in this work we show that it is possible to assess the extent to which the geomagnetic record might have been influenced by fluctuations in the conditions of deposition of the sediments by combining the measurement of the anisotropy of magnetic susceptibility with remanence measurements the case of study focuses in the geomagnetic record of the geomagnetic excursion known as the pringle falls event preserved in different locations within the same basin the various petrofabric signals obtained from the different sections sampled are shown to be independent of the variations of the geomagnetic field the results presented therefore suggest that in at least some cases the observed flips in the orientation of the remanence are actual features of the geomagnetic field rather than being associated to fluctuations in the conditions of deposition of the sediments

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

the laschamp geomagnetic field excursion recorded in icelandic lavas abstract we sampled 28 lava flows and a tephra layer dated at about 40 kyear at the reykjanes peninsula iceland 10 flows and the tephra recorded what has been originally referred to as the skalamaelifell geomagnetic field excursion the age of this excursion 42 9 7 8 ka is statistically indistinguishable from the laschamp excursion 40 4 2 0 ka rock magnetic investigations show that the main remanence carriers are titano magnetites with different degrees of oxidation one excursional flow exhibits partial self reversal behavior however it could be shown by continuous thermal demagnetization that its paleodirection is unaffected we subjected 52 samples from 16 flows to thellier type paleointensity determinations reliable paleointensity data were obtained for 10 of the 29 sites in the beginning of the excursion virtual geomagnetic poles vgps in the southeast pacific are recorded these sites are characterized by paleointensities of 4 5 t about 1 10 of the intensity of the normal polarity flows which ranges from 27 4 t to 59 3 t towards the end of the excursion vgps are found in north africa at these sites paleointensity has already regained about half of its original value 19 9 2 4 t a comparison of the paleointensity data with the results of previous studies gives a very consistent picture as all records show almost identical intensity values during the skalamaelifell excursion a tentative stratigraphic relationship between 25 sites prior to during and after the skalamaelifell excursion was established by comparing them with virtual geomagnetic poles vgps from different marine sedimentary records only vgps from four flows could not be matched unambiguously to those of the marine records our results support the theory that the geomagnetic field during the laschamp excursion likely had a simple transitional field geometry at least during the onset of the excursion the data are best explained by a decrease of the axial dipole field and a substantial transitional equatorial dipole field that was accompanied by a considerably reduced non dipole field

self reversal of chemical remanent magnetization on the transformation of maghemite to haematite summary self reversed chemical remanent magnetization crm has been observed in haematite formed on heating maghemite which has been produced by the dehydration of acicular crystals of synthetic lepidocrocite 1 to 2 pm in length our experimental evidence suggests that self reversal of haematite remanence only occurs when the parent maghemite is still blocked at the temperature of its transformation to haematite when the transformation temperature is above the blocking temperature of the parent maghemite and it is unblocked the resulting haematite remanence is normally magnetized it is suggested that the strong dependence on remanent state supports exchange control of the self reversal process we propose that the self reversal is probably a general feature of the maghemite to haematite transition and the significance of the source lepidocrocite in our experiments is that it produces maghemite of a suitable grain size so that much of it remains blocked at the elevated temperatures required to make the transformation to haematite occur over the short time scale of the laboratory experiments in nature transformation of maghemite to haematite will occur at much lower temperatures due to the much longer time scales involved and self reversal of the resulting crm may occur over a much less restricted grain size range of parent maghemite