geographical distribution of transitional vgps evidence for non zonal equatorial symmetry during the matuyama brunhes geomagnetic reversal abstract paleomagnetic records of the matuyama brunhes polarity reversal obtained recently from the northern equatorial and southern latitudes of the atlantic sector display a distinct symmetry with respect to the equator the virtual geomagnetic pole vgp paths from the mid latitude sites in the northern and southern hemispheres dsdp site 609 and core v16 58 are nearly coincident tracking northward through the americas the vgp path from the equatorial site odp site 664 tracks northward through eastern asia nearly antipodal to the other two paths modeling results indicate that superimposing an h 1 3 geometry onto a reversing axial dipole explains the sense of equatorial symmetry exhibited by these three records these results are consistent with the bulk of the data available from other records of this polarity transition suggesting that the transitional vgps during this reversal tend to fall along two distinct longitudinal bands the grouping of transitional vgps suggests that there is a strong geographical control over the reversal process an apparent correlation between the longitudinal bands of transitional vgps and the location of the major flux concentrations in the radial component of the historical geomagnetic field at the core mantle boundary 1 suggests that the processes which cause these flux concentrations may have influenced the geometry of the matuyama brunhes transitional field

longitudinally confined geomagnetic reversal paths from non dipolar transition fields it has long been thought that conditions at the boundary between the core and mantle influence the earth s magnetic field but the supporting evidence is rather indirect1 3 recent palaeomagnetic results suggesting that there are persistent preferred longitudinal paths for the virtual geomagnetic pole vgp during reversals4 would provide the first direct evidence of the solid mantle s influence on the core although their statistical significance has been disputed5 6 the results are potentially exciting because the preferred paths lie close to the pacific rim where the present geomagnetic secular variation changes character2 7 here we present a simple model based on an extension of a previous theory8 that produces reversals with vgp paths confined within relatively narrow longitude bands despite the transition field having a substantially non dipolar structure thus although longitude bias of the vgp paths is definitive evidence for core mantle interaction simple vgp paths are not evidence of near dipolar transition fields

compaction induced inclination shallowing in synthetic and natural clay rich sediments experiments using single clay slurries consisting of acicular or equidimensional magnetite mixed with either kaolinite illite montmorillonite or chlorite were conducted to determine the effect of compaction on inclination a natural marine sediment and a reconstructed marine sediment made up of the clays used in our single clay experiments were also studied a laboratory postdepositional remanence imparted to the slurries by stirring had its inclination shallowed by compaction for both the single clay samples as seen previously anson and kodama 1987 and the natural and reconstructed marine sediments suggesting that our single clay sediments are valid analogues of natural sediments magnetic intensity also decreased by 30 50 during compaction the greatest decrease occurring for samples with the steepest initial inclination altering the ph of a slurry so that the magnetite would have a negative surface charge and be repelled by clay particles did not affect a slurry s inclination shallowing behavior suggesting that electrostatic attraction between magnetite and clay is probably not a major contributor to inclination shallowing however most of the inclination shallowing observed in our experiments occurs at low pressures when void ratios are high comparison with a clay microfabric compaction study mcconnachie 1974 suggests that volume loss at these pressures is accommodated by reorientation of clay particles or clay particle aggregates clay domains since pore spaces are large at these low pressures inclination shallowing is probably caused by magnetite particles either being attached to clay particles perhaps by van der waals forces or having been incorporated into clay domains alternating field demagnetization of compacted samples which contain magnetite with a range in grain sizes supports this model indicating that the smallest magnetic grains are slightly more shallowed by compaction than larger grains our results suggest that a sediment s clay content may be an important contributor to inclination shallowing

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

high resolution record of the upper olduvai transition from po valley italy sediments support for dipolar transition geometry abstract a detailed record of the upper olduvai polarity transition composed of 100 transitional directions has been obtained from the crostolo section in northern italy a careful examination of the rock magnetic properties of the sediments using standard paleomagnetic techniques thermomagnetic scanning electron microscopy microprobe x ray diffraction and mossbauer analysis shows that greigite is the main magnetic carrier of the remanence the correlation of a shift in the magnetic record to a small scale sedimentary feature observed in the section indicates that the magnetization was acquired at deposition or very shortly after the virtual geomagnetic pole vgp path is largely confined along a great circle over north and south america 90 west of the site and consists of three stages first the vgp moves to southern mid latitudes then after a period of standstill it comes back to almost true north and finally the south pole is reached in a third step although different from a record from the southern indian ocean this path is virtually identical to those obtained for the same transition from north pacific deep sea cores and partly coincides with a north atlantic record which suggests that a dipolar component may be present in the transitional field during the upper olduvai reversal a review of the recently obtained records of various transitions shows that in more than two thirds of the cases the vgp paths are similarly confined along a meridian over the americas or antipodal to them irrespective of the sampling site and of the sense of the transition although not deterministic this tendency suggests that a similar dipolar component might be present in the transitional fields of other reversals of different ages

compaction and inclination shallowing in deep sea sediments from the pacific ocean progressive downcore shallowing of the remanent inclination has been observed in a 120 m section of marine sediments at deep sea drilling project site 578 in the northwest pacific representing the past 6 5 m y near the top of the section the average inclination corresponds to the expected geocentric axial dipole value of 53 but shallows down section by about 6 to 8 northward translation of the pacific plate accounts for only about a quarter of the inclination shallowing moreover no inclination shallowing was observed at neighboring site 576 which has considerably lower sedimentation rates and the last 5 m y are represented by a 26 m sedimentary section we correlate the inclination shallowing at site 578 with an average decrease in porosity of 3 4 the porosities in the top 26 m at site 576 are slightly higher than at site 578 and show no definite trend downhole we interpret these results to suggest that both the downhole inclination shallowing and decrease of porosity in site 578 are caused by sediment compaction compaction does not play a significant role for the section at site 576 due to its much shorter length

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

how the geomagnetic field vector reverses polarity a highly detailed record of both the direction and intensity of the earth s magnetic field as it reverses has been obtained from a miocene volcanic sequence the transitional field is low in intensity and is typically non axisymmetric geomagnetic impulses corresponding to astonishingly high rates of change of the field sometimes occur suggesting that liquid velocity within the earth s core increases during geomagnetic reversals

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

gulf stream transport and mixing processes via coherent structure dynamics the gulf stream has been characterized as either a barrier or blender to fluid transfer a duality relevant to gyre scale climate adjustment however previous characterization depended on relatively sparse lagrangian in situ observations the finite time lyapunov exponent ftle is calculated from satellite altimetry to identify lagrangian coherent structures lcs in the gulf stream region these lcs provide dense sampling of flow and capture distinct regions associated with mixing independent observations of ocean color contain similar flow dependent structures providing verification of the method and highlighting transport and mixing processes that influence sea surface temperature and chlorophyll among other water properties diagnosed lcs support the existing bower kinematic model of the gulf stream but also highlight novel behavior of comparable importance these include vortex pinch off and formation of spiral eddies clearly identified by lcs and which may be explained by considering changes to flow topology and the dynamics of shear flow instability at both small and large rossby number such processes seen though lcs may further enable validation of climate models the spatial distribution of these intermittent processes is characterized in terms of the criticality of jet dynamics with respect to rossby wave propagation and whether the jet is in an unstable or wave maker regime the generation and connectivity of hyperbolic trajectories in the flow appears to play an important role in governing large scale transport and mixing across the gulf stream

the matuyama brunhes and upper jaramillo transitions recorded in a loess section at weinan north central china abstract the paleomagnetic records of the matuyama brunhes mb and upper jaramillo uj polarity reversals have been obtained from a loess section at weinan shaanxi province in china an investigation of the rock magnetic properties using stepwise irm acquisition hysteresis loops thermomagnetic analysis and x ray diffraction on magnetic extracts identifies pseudo single domain magnetite as the carrier of the main fraction of the remanence associated with maghemite and hematite the total duration of the directional change based upon independent loess accumulation rates is about 5000 yr for the mb reversal and about 3200 yr for the uj reversal these values are consistent with previous estimates obtained from marine or lacustrine sediments apart from a few intermediate points observed on north america mb and in the pacific uj the transitional virtual geomagnetic pole vgp paths are situated in two longitudinal bands over eastern asia and australia mb and the americas uj the bulk of the uj record and the part of the mb record over the americas are not consistent with the hypothesis of a record arising from smoothing of pre and post transitional directions 1 a reappraisal of the statistical approach of mcfadden et al 2 shows that the longitudinal distribution of the records of reversals is consistent with the hypothesis of genuine geographically preferred sectors for the paths independent of site longitude with a probability higher than 0 45 when the two weinan records are considered thus these results obtained from wind blown loess sustain the hypothesis of two preferential longitudinal bands for reversing transitional vgps suggested from the study of marine and lacustrine sediments

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

on the origin of inclination shallowing in redeposited sediments we studied the detrital remanent magnetization of laboratory redeposited lake sediments previously shown to be excellent recorders of the paleomagnetic field the redeposited and original natural sediments have indistinguishable stabilities to alternating fields af demagnetization and remanence intensities indicating that their remanences reside in the same magnetic particles thermomagnetic behavior hysteresis parameters and relatively high af stabilities of the detrital and anhysteretic remanences are essentially identical throughout the sediment column suggesting uniform distributions of submicron magnetite grains the declination is accurately recorded but inclination shallowing averaging 18 o is observed for routinely hand stirred rapidly redeposited sediments however sonically disaggregated sediments settle more slowly with anomalous shallowing averaging 7 o decreasing upward in the redeposited column these results indicate that thorough disaggregation of the sediment reduces the magnitude of the inclination shallowing probably by more efficiently isolating the magnetic grains from the matrix thus diminishing the dominance of the gravitational over geomagnetic aligning torques during deposition the progressive decrease of the inclination shallowing upward in the sediment column might be due to a parallel upward increase in the fraction of isolated magnetic particles which undergo brownian motion and alignment by the geomagnetic field the dominance of gravitational over geomagnetic aligning torques during deposition might be a cause for inclination shallowing in some coarser sediments in contrast inclination shallowing would be minimized for slow deposition of isolated submicron magnetite grains during natural sedimentation

palaeomagnetic constraints on the geometry of the geomagnetic field during reversals palaeomagnetic records of the path of the pole during reversals of the earth s magnetic field provide a test of the hypothesis that dipolar or low order axisymmetric components of the field dominate during reversals multiple records of reversals during the past 12 myr show no simple or consistent geographical pattern although a more robust analysis of the transitional field awaits a greater number of well distributed sampling sites the present data are not inconsistent with the simplest models in which a field reminiscent of the non dipole component of the present day field becomes dominant

absolute inclination values from deep sea sediments a reexamination of the cretaceous pacific record new data are used to examine anomalously shallow paleomagnetic inclinations derived from previous analyses of cretaceous dsdp sediments of the central western pacific ocean while viscous remanent magnetizations vrms are shown to produce a similar effect the bias toward shallow values is still present with thoroughly demagnetized data superimposed upon this bias is a trend towards maximum flattening at an expected inclination of approximately 55 degree with the flattening falling to zero at the equator and pole flattening due to sediment compaction or inclination error is proposed to explain these observations

paleomagnetism of a polarity transition in the lower triassic chug water formation wyoming a total of 156 oriented specimens taken at stratigraphic intervals of 3 to 20 cm have been collected from the red peak member of the chugwater formation at three localities approximately 25 km southeast of dubois wyoming the stratigraphic interval containing the polarity transition in the remanent magnetic field is about 1 0 m all specimens were thermally demagnetized at 500 550 620 and 660 c and show directions of magnetization of normal reversed and transitional character as well as a remarkable decrease of the remanent intensity in the transition zone the section sampled is divided into five intensity periods two periods recording the dipolar field one recording a dipolar field of decreasing intensity one recording a dipolar field of increasing intensity and a period of low intensity recording the field present during the 180 directional swing a virtual geomagnetic pole vgp path obtained for a n r polarity transition makes a fairly smooth transit with the path confined to a sector of longitude between 85 w and 110 e the reversal s onset is first indicated by a slow movement of the vgp across northern latitudes the vgp then lingers to two positions 60 n and 40 n 35 e to 70 e and 20 n and 40 n 335 e to 5 e suggesting brief periods of field stability or nondipolar field dominance the onset of the transition appears to be more rapid than the recovery the vgp path observed at a northern hemisphere site is located about 90 from the site the pole position derived from stable normal and reversed portions of this study 115 5 e 46 4 n 95 4 1 dp 2 2 dm 4 2 lies approximately in the center of the main group of poles reported for other triassic units

esr dating of ancient flints natural radioactivity can produce measurable amounts of radiation damage in rocks and glasses and the gradual accumulation of this damage with time provides a built in clock that can be used as a dating tool for geology and archaeology although a studied technique for estimating this damage involves measurements of stored thermoluminescent tl activity1 4 several authors have reported the direct observation of natural radiation damage by esr5 7 we describe here esr measurements on a damage signal which we have found to be present without exception in a wide variety of natural flints the point defect responsible for the signal is identified and the samples are calibrated with an artificial damage simulation preliminary age determinations are presented for several natural flints flint artefacts heated in antiquity will have their clock reset to zero and measurements on one sample suggest that the esr technique may have applications in the study of early cultures

longitudinal confinement of geomagnetic reversal paths as a possible sedimentary artefact the positions of the virtual geomagnetic pole vgp during a large number of reversals of the earth s magnetic field seem to show a remarkable confinement to longitudes over the americas or to antipodal longitudes1 2 although it has been argued that this confinement is statistically insufficiently constrained3 it has also been pointed out4 that the same bands of longitude appear in other geophysical observations such as the pattern of fluid motion in the outer core and regions of higher seismic velocities in the lower mantle suggesting a causal relationship here we show that longitudinal confinement of vgps can arise from the smoothing of non antipodal stable directions just before and after a geomagnetic reversal because of the filtering effect of the remanence acquisition process in sediments the origin of this non antipodality is still uncertain and must remain speculative until more reversal records become available that include sufficiently long pre and post transitional intervals

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