geomagnetic field intensity and reversals during the past four million years a long and continuous record of the geomagnetic field intensity shows that intensity variations are dominated by two modes major episodes of field regeneration prevail on timescales of a few thousand years immediately after most reversals whereas stable polarity states are characterized by a slow 0 5 myr relaxation process reversals can be seen as the consequence of a progressive degradation of the dipole field

the age and duration of the matuyama brunhes transition from new k ar data from la palma canary islands and revisited 40ar 39ar ages abstract paleomagnetic investigations conducted on lava sequences from la palma canary islands yielded several independent records of the matuyama brunhes transition mbt seven k ar ages of flows sampled across the mbt suggest a duration between 0 and 11 kyr and provide a weighted mean age of 786 3 ka analytical uncertainty only for the transition this value is significantly older than the previously admitted age of 779 2 ka derived from volcanic or sedimentary sequences when the 1 uncertainty on the calibration standard used in the present study is considered our age for the transition recorded at la palma becomes 786 8 ka we have recalculated the 40ar 39ar determinations from the previous studies using recent revisited ages for 40ar 39ar standards this yields a global dataset of 23 k ar and 40ar 39ar determinations our best estimate for the mbt age derived from this compilation is 789 2 ka analytical error or 789 8 ka total error within this 8 kyr uncertainty which could be considered as a minimum estimate this age remains compatible with recent determinations made by the astronomical time scale however the present study suggests that better control of the lock in depth as well as better quantification of uncertainties involved in the tuning of 18o records to orbital forcing models are needed to improve further the astronomical polarity time scale derived from sedimentary records in addition absolute dating by k ar and 40ar 39ar will gain in accuracy when uncertainty determinations on standards will be significantly improved

timing of quaternary geomagnetic reversals and excursions in volcanic and sedimentary archives abstract we review the ages of quaternary magnetic polarity reversals and magnetic excursions improvements in radioisotopic 40ar 39ar dating of reversals and excursions reflect the use of multi collector mass spectrometers along with ultra low backgrounds laser based incremental heating methods and adoption of 28 201 ma for the fish canyon sanidine fcs standard age kuiper et al 2008 these improvements have brought 40ar 39ar ages of reversals and excursions into close correspondence with astrochronological ages based on the correlation of 18o data to lr04 lisiecki and raymo 2005 and in a few cases correlation to ice core chronologies ages of two polarity reversals the matuyama brunhes boundary and base olduvai are at odds with ages given in popular geomagnetic polarity timescales sedimentary records of the laschamp and iceland basin excursion are numerous and widespread and attest to global manifestation and global synchronicity of these excursions records of other quaternary excursions are less numerous the blake excursion 115 ka has been recorded in north atlantic sediments chinese loess indian ocean volcanics and spanish speleothems the santa rosa excursion 932 ka has been recorded in north atlantic and phillipine basin sediments as well as in volcanic rocks from new mexico and galapagos the punaruu excursion 1115 ka has been observed in sediments from the north atlantic and california margin as well as in volcanic rocks on tahiti uniformity of individual excursion ages using astrochronological 40ar 39ar and u th methods argues for synchronous global manifestation of excursions strengthening the usefulness of magnetic excursions and reversals in quaternary stratigraphy

the steens mountain oregon geomagnetic polarity transition 2 field intensity variations and discussion of reversal models we carried out an extensive paleointensity study of the 15 5 0 3 m y miocene reversed to normal polarity transition recorded in lava flows from steens mountain south central oregon one hundred eighty five samples from the collection whose paleodirectional study is reported by mankinen et al this issue were chosen for paleointensity investigations because of their low viscosity index high curie point and reversibility or near reversibility of the strong field magnetization curve versus temperature application of the thellier stepwise double heating method was very successful yielding 157 usable paleointensity estimates corresponding to 73 distinct lava flows after grouping successive lava flows that did not differ significantly in direction and intensity we obtained 51 distinguishable complete field vectors of which 10 are reversed 28 are transitional and 13 are normal the record is complex quite unlike that predicted by simple flooding or standing nondipole field models it begins with an estimated several thousand years of reversed polarity with an average intensity of 31 5 8 5 t about one third lower than the expected miocene intensity this difference is interpreted as a long term reduction of the dipole moment prior to the reversal when site directions and intensities are considered truly transitional directions and intensities appear almost at the same time at the beginning of the transition and they disappear simultaneously at the end of the reversal large deviations in declination occur during this approximately 4500 1000 year transition period that are compatible with roughly similar average magnitudes of zonal and nonzonal field components at the site the transitional intensity is generally low with an average of 10 9 4 9 t for directions more than 45 away from the dipole field and a minimum of about 5 t the root mean square of the three field components x y and z are of the same order of magnitude for the transitional field and the historical nondipole field at the site latitude however a field intensity increase to pretransitional values occurs when the field temporarily reaches normal directions which suggests that dipolar structure could have been briefly regenerated during the transition in an aborted attempt to reestablish a stationary field changes in the field vector are progressive but jerky with at least two and possibly three large swings at astonishingly high rates each of those transitional geomagnetic impulses occurs when the field intensity is low less than 10 t and is followed by an interval of directional stasis during which the magnitude of the field increases greatly for the best documented geomagnetic impulse the rapid directional change corresponds to a vectorial intensity change of 6700 2700 nt yr 1 which is about 15 50 times larger than the maximum rate of change of the nondipole field observed during the last centuries the occurrence of geomagnetic impulses seems to support reversal models assuming an increase in the level of turbulence within the liquid core during transitions the record closes with an estimated several thousand years of normal polarity with an average intensity of 46 7 20 1 t agreeing with the expected miocene value however the occurrence of rather large and apparently rapid intensity fluctuations accompanied by little change in direction suggests that the newly reestablished dipole was still somewhat unstable

a high resolution marine sedimentary record of geomagnetic intensity during the brunhes chron abstract this investigation of paleointensity employs data from four cores taken from two sites in the sulu sea during leg 124 of the ocean drilling program these sediments accumulated at a rate of about 10 cm ky yet they provide strong stable magnetizations a combination rarely found in the deep sea environment this study examines the interval of the last glacial cycle since 130 ka in most detail using paleomagnetic measurements from discrete samples to explore a variety of normalization parameters paleomagnetic remanence and susceptibility measurements taken aboard ship at odp site 769 extend the paleointensity record to the brunhes matuyama boundary after normalization by susceptibility the shipboard intensity data compare well with the more detailed measurements from discrete samples in the intervals for which such data are available and so complete a brunhes age 0 0 78 ma history of paleointensity for the sulu sea that record shows a number of short duration decreases in paleointensity that appear to be useful stratigraphic markers these results also indicate that the average geomagnetic dipole moment during the brunhes chron may have been considerably lower than the present value

geomagnetic changes across the last reversal recorded in lava flows from la palma canary islands two adjacent sequences of 41 and 21 individual lava flows have been sampled in the island of la palma although no detailed correlation can be established between the two sections they both encompassed the matuyama brunhes m b reversal stepwise alternating field af and thermal demagnetization in air and in vacuum accompanied by investigations of the rock magnetic properties allowed us to identify several lava flows with the same signature and thus most likely associated with very short episodes of intense volcanic activity the intermediate virtual geomagnetic pole vgp positions do not cluster within any preferred geographic areas the polarity change is followed by a rebound or an excursion with a cluster of vgps over northeastern india whether or not this feature is truly associated with the transition or correlated with the delta event 0 69 ma will hopefully be resolved by further dating paleointensity experiments were performed on 60 samples using the modified thellier method coe 1967 corrections for magnetomineralogical changes induced by heating were applied using the technique of partial thermoremanent magnetization ptrm checks valet et al 1994b with a significant increase in the success rate from 38 to 58 twenty four independent estimates of the paleofield range between 3 9 and 46 0 t the field intensity was reduced to at least one tenth of the present day value during the transition low transitional field intensities are also observed during the rebound or excursion the lava flows preceding the polarity change show evidence for a significant drop in the dipole field intensity while the posttransitional flows exhibit high field intensities these features are compatible with the asymmetrical saw tooth pattern recently observed in sedimentary records meynadier et al 1994 the characteristics of the m b transition emerging from a compilation of the four available volcanic records are not inconsistent with the hypothesis of rapidly time varying nondipole transitional components favored by the decrease of the dipole and its subsequent recovery in the opposite polarity state

relative paleointensity of the geomagnetic field during the brunhes chron abstract we conducted a paleomagnetic study of four sediment cores taken from different regions in the low latitudes of the pacific ocean in order to recover relative paleointensity variations during the brunhes chron three cores have good age control from oxygen isotope stratigraphy we extensively examined rock magnetic uniformity by the iso thermal remanent magnetization irm acquisition and the alternating field demagnetization of the anhysteretic remanent magnetization arm and the irm only rock magnetically homogeneous sediments were used biogenic magnetites were found in all but one core and their dissolution in early diagenesis was not significant the four cores showed coherent variations of the remanent intensity normalized by the arm implying that these variations represent the behavior of the geomagnetic field long term paleointensity variations during the brunhes chron are characterized by an asymmetric pattern rapid increases in the intensity occurred just after the brunhes matuyama transition in the mid brunhes from about 500 to 400 ka and during the last 100 ka and gradual decreases between them the first one supports the idea of rapid regeneration of the geomagnetic field after polarity reversals 1 the other two may have been triggered by very short polarity reversals

relative paleointensity and environmental magnetism since 1 2 ma at iodp site u1305 eirik drift nw atlantic abstract integrated ocean drilling program iodp expedition 303 to the north atlantic in 2004 recovered rapidly deposited deep sea sediments at iodp site u1305 on eirik drift located south of greenland at 3460 m water depth along the path of the western boundary under current wbuc about 200 m of sediment was sampled with u channels from the composite section providing a continuous record of paleomagnetic field directions and relative paleointensity rpi variations covering the past 1 2 myr the age model based on an oxygen isotope record is consistent with the fit of the rpi record to a calibrated template and indicates higher sedimentation rates during interglacials relative to glacial epochs magnetite grain size and concentration proxies indicate higher concentrations of magnetite with larger grain sizes during interglacials enhanced interglacial deposition can be attributed to a combination of elevated entrainment of terrigenous detritus into the wbuc due to glacial retreat on continents flanking the upstream path of the wbuc east greenland and iceland and of increased bottom current wbuc vigor leading to elevated transport and deposition at the site during interglacials this pattern is opposite to observations of flow of the antarctic circum polar current acc in the south indian ocean which suggests an inter hemispheric antiphase in marine circulation at the milankovitch scale with strong circulation in the deep north atlantic when the acc is weak and vice versa

absolute paleointensity and magnetomineralogical changes determinations of absolute paleointensity are often hampered by magnetomineralogical changes produced within samples of lava flows during successive heatings such changes which directly affect the capacity for a sample to acquire a partial thermoremanent magnetization ptrm are reflected by negative ptrm checks a deviation between the original ptrm and a repeated measurement after heating at a higher temperature because the deviations indicated by the ptrm checks can provide a direct estimate of the effects to the ptrms we suggest that they can be used to correct ptrms the correction method we present requires additional measurements in particular ptrm checks performed after each ptrm and specific diagrams to detect acquisition of chemical remanence performing multiple successive ptrm checks can also provide information on the blocking temperatures of the alteration product these experiments conducted on lava flows from different localities show that the magnetomineralogical changes mainly involved grains with blocking temperatures lower than the last heating step the corrections have been tested on recent and historic lava flows from several localities we have also compared results on samples from the same lava flows that were heated in air and in vacuum the mean paleointensity values were found to lie within less than 10 from the expected field intensity at each site the corrections resulted in the recovery of reliable paleointensity determinations for about twice as many specimens heated in vacuum and 65 additional data for specimens heated in air successive heatings at the same temperature in air are often accompanied by a time dependent oxidation of the natural remanent magnetization which is responsible for paleointensity values lower than the expected field a problem that is not related to the corrections we conclude that corrections can be used with fairly high confidence to the ptrms performed in vacuum and in air

asthenospheric viscosity inferred from correlated land sea earthquakes in north east japan the viscosity of the earth s mantle has been estimated from studies of post glacial rebound1 2 post seismic deformations of the ground following large earthquakes3 4 and aftershock sequences5 7 here we derive a value for the viscosity of the asthenosphere from a correlation found in the historical catalogue of subduction induced seismicity between the intraplate land and interplate sea earthquakes in north east japan the correlation persists since the time of reliably reported earthquakes in ad 1600 land events precede sea events by 36 yr with a mean distance between land sea pairs of 200 km because of the visocoelastic coupling of the lithosphere to the asthenosphere a plausible mechanism to explain the correlation is stress migration governed by the viscosity of the asthenosphere large land shocks generate diffuse like stress pulses which sweep past and unlock the thrust fault in the subduction zone thus triggering the sea events the correlation time and distance provide a measure of the speed of diffusion 5 6 km yr 1 and hence an estimate of the viscosity 7 1018 pa s

upper plate splay fault earthquakes along the arakan subduction belt recorded by uplifted coral microatolls on northern ramree island western myanmar burma abstract upper plate structures that splay out from the megathrusts are common features along major convergent plate boundaries however their earthquake and tsunami hazard potentials have not yet received significant attention in this study we identified at least one earthquake event that may have been produced by an upper plate splay fault offshore western myanmar based on u th ages of uplifted coral microatolls this event is likely an earthquake that was documented historically in c e 1848 with an estimated magnitude between 6 8 and 7 2 based on regional structural characteristics such magnitude is consistent with the observed co seismic uplift amount of 0 5 m although these events are smaller in magnitude than events produced by megathrusts they may produce higher earthquake and tsunami hazards for local coastal communities due to their proximity our results also indicate that earthquake events with co seismic uplift along the coast may not necessarily produce a flight of marine terraces therefore using only records of uplifted marine terraces as megathrust earthquake proxies may overlook the importance of upper plate splay fault ruptures and underestimate the overall earthquake frequency for future seismic and tsunami hazards along major subduction zones of the world

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

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

sedimentation rates in the makarov basin central arctic ocean a paleomagnetic and rock magnetic approach three long sediment cores from the makarov basin have been subjected to detailed paleomagnetic and rock magnetic analyses investigated sediments are dominated by normal polarity including short reversal excursions indicating that most of the sediments are of brunhes age in general the recovered sediments show only low to moderate variability in concentration and grain size of the remanence carrying minerals estimations of relative paleointensity variations yielded a well documented succession of pronounced lows and highs that could be correlated to published reference curves however together with five accelerator mass spectrometry c 14 ages and an incomplete be 10 record still two different interpretations of the paleomagnetic data are possible with long term sedimentation rates of either 1 3 or 4 cm kyr 1 however both models implicate highly variable sedimentation rates of up to 10 cm kyr 1 and abrupt changes in rock magnetic parameters might even indicate several hiatuses

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

mid brunhes magnetic excursions in marine isotope stages 9 13 14 and 15 286 495 540 and 590 ka at north atlantic iodp sites u1302 3 u1305 and u1306

paleoclimatic context of geomagnetic dipole lows and excursions in the brunhes clue for an orbital influence on the geodynamo the hypothesis of an influence of the astronomical precession on the geodynamo energy budget was recently reappraised by theoreticians paleomagnetic indications of such an influence remain controversial because reconstructions of paleointensity variations from sediments are suspected to be contaminated by lithological paleoclimatically induced influences three sets of complementary indicators are however available 1 records of the direction of magnetization in sediments 2 records of magnetic anomalies of the deep sea floor basalts and 3 records of production variations of cosmogenic isotopes from sediment and ice cores these records confirm the genuine geomagnetic origin of paleointensity lows and their narrow link with excursions or short reversals recorded in various materials and often dated by radiometric methods the analysis of these time series and their comparison with 18o records of the paleoclimate suggest that such globally recorded geomagnetic dipole lows have preferentially occurred in the context of interglacial or transitional paleoclimates at the time of low or decreasing obliquity the dominant periods extracted with the complex continuous wavelet transform technique range from 40 to 125 ka further suggesting a link with orbital parameters these results encourage future efforts of research to improve the precision the resolution and the dating of the time series of geomagnetic dipole low in order to better decipher orbital signatures and understand their origin an important implication of this topic is that the next geomagnetic dipole low should be related with the present interglacial

a discrete domain description of multiphase flow in porous media rugged energy landscapes and the origin of hysteresis we propose a discrete domain model to describe mesoscale many pore immiscible displacements in porous media we conceptualize the porous medium and fluid system as a set of weakly connected multistable compartments the overall properties of the system emerge from the small scale compartment dynamics our model aims at capturing the rugged energy landscape of multiphase porous media systems emphasizing the role of metastability and local equilibria in the origin of hysteresis under two phase displacements the system behaves hysteretically but our description does not rely on past saturations turning points or drainage imbibition labels we characterize the connection between micrometastability and overall system behavior and elucidate the different nature of pressure controlled and rate controlled immiscible displacements in porous media

mono lake or laschamp geomagnetic event recorded from lava flows in amsterdam island southeastern indian ocean summary we report a survey carried out on basalt flows from amsterdam island southeastern indian ocean in order to check the presence of intermediate directions interpreted as belonging to a geomagnetic field excursion within the brunhes epoch completing this palaeomagnetic record with palaeointensity determinations and radiometric dating because the palaeomagnetic sampling was done over a few hours during the resupply of the french scientific base martin du viviers by the marion dufresne vessel we could collect only 29 samples from four lava flows the directional results corroborate the findings by watkins nougier normal polarity is found for two units and an intermediate direction with associated virtual geomagnetic poles vgps close to the equator for the other two units a notable result is that these volcanic rocks are well suited for absolute palaeointensity determinations 50 per cent of the samples yields reliable intensity values with high quality factors an original element of this study is that we made use of the thermomagnetic criterion ptrm tail test of shcherbakova et al to help in the interpretation of the palaeointensity measurements doing this only the high temperature intervals beyond 400 c were retained to obtain the most reliable estimate of the strength of the ancient magnetic field however not applying the ptrm tail test does not change the flow mean values significantly because the samples we selected by conventional criteria for estimating the palaeointensity carry only a small proportion of their remanence below 400 c the normal units yield virtual dipole moments vdm of 6 2 and 7 7 10 22 am 2 and the excursional units yield values of 3 7 and 3 4 10 22 am 2 these results are quite consistent with the other thellier determinations from brunhes excursion records all characterized by a decrease of the vdm as the vgp latitude decreases 40 ar 39 ar isotopic age determinations provide an estimate of 26 15 and 18 9 kyr for the transitional lava flows which could correspond to the mono lake excursion however the large error bars associated with these ages do not exclude the hypothesis that this event is the laschamp

geomagnetic secular variation we analyse over 175000 magnetic observations from an interval spanning 1695 1980 to produce a sequence of maps of the magnetic field at the core mantle boundary we find that even the earlier data enable us to determine reliable maps we produce these maps at approximately 60 year intervals through the eighteenth and nineteenth centuries and at 10 year intervals in the twentieth century this span of maps is long enough to render straightforward the distinction between static and drifting features in the field we observe that some features show no sign whatsoever of drift over the entire 285 year time interval although others drift westwards in particular we observe that the secular variation is very low beneath the pacific ocean but beneath southern africa and the south atlantic ocean we observe rapid secular variation we interpret the morphology of the static field in terms of a simple model of the dynamo and conjecture that interactions between the core and the mantle are an important element of the process as part of the static field we identify four main concentrations of flux two in each hemisphere at high latitudes these features largely account for the earth s axial dipole moment we find unequivocal evidence that magnetic flux has not remained frozen over the time span of our models much of the diffusive behaviour that we identify is associated with the formation of a pair of flux spots a core spot beneath southern africa early in this century nevertheless we are able to construct maps that satisfy a set of necessary conditions for frozen flux and use these maps to construct maps of the core surface fluid flow based on the steady flow hypothesis although we find no strong evidence against the steady flow hypothesis we do find some grounds on which to doubt the validity of the flow maps

paleointensity across the reunion event in ethiopia abstract paleointensity experiments have been conducted on a volcanic series of 33 flows which recorded the geomagnetic reunion event successive determinations of absolute paleointensity from 20 flows indicate that the reunion event would actually be characterized by two successive periods of low field intensity with a total duration not in excess of 30 kyr the first period has a mean paleointensity of 12 t and shows significant deviations from initial reverse polarity but no signs of fully normal polarity it is followed by field recovery then a decrease prior to the second directional episode the second period has a mean paleointensity of 17 t and is associated with full normal directions and thus likely a dominantly dipolar configuration these results are compatible with a simple earthlike magnetic field model with a large decrease in dipole field strength which yields local emergence of relatively stronger non dipole components field characteristics from this configuration would be different at different locations and could explain the variability of the paleomagnetic records collected around the world we propose that the absence of field recovery following the event is responsible for not leading to a durable polarity interval

relative palaeointensity records from the ontong java plateau summary rock magnetic palaeomagnetic and oxygen isotopic results are presented from core rndb 75p which was recovered from the ontong java plateau ojp a high degree of uniformity in magnetic properties characterized by relatively small changes in concentration and grain size in the upper 4 m of the core combined with a lack of coherence between the normalized remanence and rock magnetic data suggests that the natural remanence normalized by saturation remanence reflects variations in relative palaeointensity of the geomagnetic field the record from rndb 75p replicates other ontong java records spanning the last 400ka and extends the record back to some 700ka spectral analysis of the ontong java record suggests periodic behaviour in the relative palaeointensity record with a dominant period of between 30 and 40ka which appears not be be an artefact of lithologic variability this dominant period lies between functions describing climatic precession and obliquity changes in the earth s orbit comparison of the normalized remanence record with astronomical precession 26 ka period however is much more favorable none the less tuning the palaeointensity record to that of astronomical precession appears inconsistent with existing isotopic age constraints derived from the specmap time scale based on these data we must choose between assuming that the earth s orbit controlled ice volume inherent in the specmap time scale and assuming that the earth s magnetic field is driven by astronomical precession the former assumption has a substantial theoretical and observational base and we prefer to interpret the data presented here as suggesting that the earth s orbit has not played a detectable role in the modulation of the magnetic field plots of saturation remanence and magnetic susceptibility are very sensitive to quite subtle changes in magnetic grain size a slight shift within the pseudo singledomain grain size range toward the multidomain or superparamagnetic field was detected at about 4m in rndb 75p this change in grain size may reflect a diagenetic alteration of the magnetite such as dissolution and may be related to the phenomenon responsible for the loss of magnetic remanence at depth detected in other cores from the region

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