invariant and changing transitional field configurations in a sequence of geomagnetic reversals in the past few years the increasing number of detailed records of geomagnetic transitions has provided new and important information about the behaviour of the earth s magnetic field during a reversal surprisingly however no attempt has been made to obtain information about the evolution of the transitional field characteristics over a long period of time we report here the results of a palaeomagnetic study of four different geomagnetic reversals three of which are successive recorded at different stratigraphical levels of two upper tortonian marine clay sections in western crete greece this sequence represents 1 5 myr of geomagnetic field history the results obtained suggest that the process of reversal from either starting polarity may remain invariant for a long period of time then change rather abruptly between successive reversals

brunhes matuyama polarity transition in three deep sea sediment cores the brunhes matuyama polarity transition was studied in three deep sea sediment cores obtained from the mid northern and equatorial pacific ocean rates of sedimentation vary from 0 7 to 1 1 cm k a 1 the cores were sampled across the transition in 4 mm slices and each level was further subdivided into three specimens inclinations above and below the transition interval are in close agreement with that of an axial dipole field estimates of transition duration based on directional change range from 4900 to 8500 years the transition can be described by a longitudinally confined portion in each of the v g p paths but is complicated by broad loops in all three records while fine scale sampling reveals considerable detail the notable increase in within level dispersion observed in the transition and the apparent differences in v g p paths in nearby cores from the same region indicate that factors other than the geomagnetic field may contribute to these transition records

age dating and the orbital theory of the ice ages development of a high resolution 0 to 300 000 year chronostratigraphy using the concept of orbital tuning a continuous high resolution deep sea chronostratigraphy has been developed spanning the last 300 000 yr the chronology is developed using a stacked oxygen isotope stratigraphy and four different orbital tuning approaches each of which is based upon a different assumption concerning the response of the orbital signal recorded in the data each approach yields a separate chronology the error measured by the standard deviation about the average of these four results which represents the best chronology has an average magnitude of only 2500 yr this small value indicates that the chronology produced is insensitive to the specific orbital tuning technique used excellent convergence between chronologies developed using each of five different paleoclimatological indicators from a single core is also obtained the resultant chronology is also insensitive to the specific indicator used the error associated with each tuning approach is estimated independently and propagated through to the average result the resulting error estimate is independent of that associated with the degree of convergence and has an average magnitude of 3500 yr in excellent agreement with the 2500 yr estimate transfer of the final chronology to the stacked record leads to an estimated error of 1500 yr thus the final chronology has an average error of 5000 yr

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

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

polarity transitions and excursions of the geomagnetic field the nature of geomagnetic field behavior during polarity transitions is one of the most hotly debated issues in modern geophysics dynamo action in the earth s fluid outer core is known to be responsible for generating the earth s main magnetic field however the mechanism for polarity transition is still unknown polarity transitions are a fundamentally important property of the geomagnetic field but because they have not taken place in historic time it is necessary to turn to the paleomagnetic record to understand the mechanism by which they occur

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

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

geomagnetic excursions and paleointensities in the matuyama chron at ocean drilling program sites 983 and 984 iceland basin 1 we report natural remanent magnetization nrm directions and geomagnetic paleointensity proxies for part of the matuyama chron 0 9 2 2 ma interval from two sites located on sediment drifts in the iceland basin at ocean drilling program sites 983 and 984 mean sedimentation rates in the late matuyama chron are 15 9 and 11 5 cm kyr 1 respectively for the older part of the record 1 2 ma oxygen isotope data are too sparse to provide the sole basis for age model construction the resemblance of the volume susceptibility record and a reference 18o record led us to match the two records to derive the age models this match based on site 983 984 susceptibility is consistent with available site 983 984 benthic 18o data paleointensity proxies were derived from the slope of the nrm versus anhysteretic remanent magnetization plot for alternating field demagnetization in the 30 60 mt peak field range paleointensity lows correspond to polarity reversals at the limits of the jaramillo olduvai cobb mountain and reunion subchrons and to seven excursions in nrm component directions magnetic excursions defined here by virtual geomagnetic polar latitudes crossing the virtual geomagnetic equator are observed at 932 1048 1115 1190 1215 cobb mountain subchron 1255 1472 1480 1567 1575 gilsa subchron and 1977 ka the results indicate that geomagnetic directional excursions associated with paleointensity minima are a characteristic of the matuyama chron and probably of polarity chrons in general

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

time variations in geomagnetic intensity 1 after many years spent by paleomagnetists studying the directional behavior of the earth s magnetic field at all possible timescales detailed measurements of field intensity are now needed to document the variations of the entire vector and to analyze the time evolution of the field components a significant step has been achieved by combining intensity records derived from archeological materials and from lava flows in order to extract the global field changes over the past 12 kyr a second significant step was due to the emergence of coherent records of relative paleointensity using the remanent magnetization of sediments to retrace the evolution of the dipole field a third step was the juxtaposition of these signals with those derived from cosmogenic isotopes contemporaneous with the acquisition of records new techniques have been developed to constrain the geomagnetic origin of the signals much activity has also been devoted to improving the quality of determinations of absolute paleointensity from volcanic rocks with new materials proper selection of samples and investigations of complex changes in magnetization during laboratory experiments altogether these developments brought us from a situation where the field changes were restricted to the past 40 kyr to the emergence of a coherent picture of the changes in the geomagnetic dipole moment for at least the past 1 myr on longer timescales the field variability and its average behavior is relatively well documented for the past 400 myr section 3 gives a summary of most methods and techniques that are presently used to track the field intensity changes in the past in each case current limits and potential promises are discussed the section 4 describes the field variations measured so far over various timescales covered by the archeomagnetic and the paleomagnetic records preference has always been given to composite records and databases in order to extract and discuss major and global geomagnetic features special attention has been devoted to discussing the degree of confidence to be put in the data by considering the integration of multiple data sets involving different techniques and or materials

short polarity intervals within the matuyama transitional field records from hydraulic piston cored sediments from the north atlantic abstract detailed sampling of two short magnetozones within the matuyama chronozone recorded at dsdp site 609 49 86 n 335 77 e confirms that one the cobb mountain subchronozone 1 12 ma is a very short full normal polarity interval and that the other the older interval is a record of a geomagnetic excursion which occurred at approximately 1 55 ma the cobb mountain subchron lasted approximately 25 000 years one third the duration of the jaramillo subchron the normal polarity interval is bounded by two transition zones which document an antisymmetry in the sequence of directions in the reverse to normal and normal to reverse polarity transitions we interpret the antisymmetry as reflecting a dependence upon the sense of the reversal without significant changes in the relative contributions of non dipole terms the polarity interval recorded at 1 55 ma lasted only 8 800 years with what may be regarded as full polarity directions observed across only 3 cm of stratigraphic section this feature is interpreted as an excursion of the geomagnetic field and appears to be correlative with the gilsa subchron similarities between the transition bounding these two magnetozones suggest that these features occur as the result of the same process or triggering mechanisms in the earth s outer core

a southern hemisphere record of the matuyama brunhes polarity reversal the authors present a record of the matuyama brunhes 0 73 ma polarity transition from a southern hemisphere deep sea sediment core v16 58 46 degree s 30 degree e the transition is recorded across at least 30 sm of section and is defined by a nearly 180 degree change in directions which occurs during a low in the relative intensities an increase in the sup 10 be sup 9 be ratios associated with the intensity low suggests that the relative intensities may document a decrease in the strength of the geomagnetic field the virtual geomagnetic pole vgp path is neither clearly near or far sided but tracks approximately 120 degree west of the site longitude considered with other matuyama brunhes transition records this southern hemisphere record supports previous interpretations that the matuyama brunhes transitional field was not dipolar instead the v16 58 transition together with other records of this reversal recently obtained from this longitudinal sector suggests that nonzonal terms which are symmetric about the equator were important during this transitional field

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

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

magnetostratigraphic dating of river terraces rapid and intermittent incision by the yellow river of the northeastern margin of the tibetan plateau during the quaternary up to 23 paleosols have been identified in loess sequences that overlie fluviatile sediments on seven terraces of huang he the yellow river and daxia he in the linxia basin at the northeastern margin of the tibetan plateau the magnetostratigraphic record in the oldest sequence appears to be fairly complete and spans the entire quaternary it includes evidence for the jaramillo and olduvai normal polarity subchrons within the matuyama chron as well as the cobb mountain and stage 54 formerly gilsa events age constraints within the brunhes are provided by 14c and thermoluminescence optical stimulated luminescence dating the magnetostratigraphic chronology of the quaternary sediments indicates that terraces were formed at about 1 66 1 4 1 1 0 6 0 12 0 055 and 0 01 ma the terraces may represent climatically induced pauses in long term tectonically induced incision by the rivers on the basis of terrace heights and ages incision rates were highest from 1 66 to 1 4 ma 1 m kyr decreased from 1 4 to 0 12 ma 0 1 m kyr and increased from 0 12 ma to present 0 75 m kyr if river incision is attributed solely to uplift of the tibetan plateau then this uplift appears to have been rapid as well as intermittent following the first evidence of the existence of huang he in this area at about 1 75 ma however the effects of uplift cannot be fully separated from incision caused by changes in climate copyright 1997 by the american geophysical union

seismic bearing capacity of a strip footing on rock slopes most of the seismic calculations currently used for the evaluation of seismic bearing capacity are formulated in terms of a linear mohr coulomb failure criterion however experimental evidence sho

mechanism for geomagnetic polarity reversals recent studies of old magnetic observations1 and the seismic velocity of the lower mantle2 suggest that at least part of the geomagnetic secular variation is driven by temperature anomalies in the solid mantle3 in particular a patch of flux of opposite sign to that expected for a dipole field occurs beneath southern africa it has intensified throughout the twentieth century by a process that is believed to be flux expulsion associated with a particularly hot part of the lowermost mantle and is drifting westwards a similar patch lies beneath south america here i show that the present fall in the dipole moment is directly related to the intensification and southward movement of these patches and suggest the fall occasionally leads to polarity reversal the almost linear increase in the frequency of polarity reversals that has occurred since the cretaceous quiet interval is attributed to the growth of the hot patch of mantle presently under southern africa

palaeomagnetic study of a subaerial volcanic ridge sao jorge island azores for the past 1 3 myr evidence for the cobb mountain subchron volcano flank instability and tectonomagmatic implications we present a palaeomagnetic study on 38 lava flows and 20 dykes encompassing the past 1 3 myr on s jorge island azores archipelagonorth atlantic ocean the sections sampled in the southeastern and central western parts of the island record reversed and normal polarities respectively they indicate a mean palaeomagnetic pole 81 3 degrees n 160 7 degrees e k 33 and a95 3 4 degrees with a latitude shallower than that expected from geocentric axial dipole assumption suggesting an effect of non dipolar components of the earth magnetic field virtual geomagnetic poles of eight flows and two dykes closely follow the contemporaneous records of the cobb mountain subchron odp dsdp programs and constrain the age transition from reversed to normal polarity at ca 1 207 0 017 ma volcano flank instabilities probably related to dyke emplacement along an nnwsse direction led to southwestward tilting of the lava pile towards the sea two spatially and temporally distinct dyke systems have been recognized on the island the eastern is dominated by nnwsse trending dykes emplaced before the end of the matuyama chron whereas in the central western parts the eruptive fissures oriented wnwese controlled the westward growth of the s jorge island during the brunhes chron both directions are consistent with the present day regional stress conditions deduced from plate kinematics and tectonomorphology and suggest the emplacement of dykes along pre existing fractures the distinct timing and location of each dyke system likely results from a slight shift of the magmatic source

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

a detailed record of normal reversed polarity transition obtained from a thick loess sequence at jiuzhoutai near lanzhou china summary a record of normal reversed polarity transition has been obtained from a 4 m thickness of loess exposed at a section near lanzhou china magnetostratigraphic studies suggest it may represent a reversal bounding the onset of a reversed polarity zone within the jaramillo normal subchron the natural remanent magnetization consists of two components a low coercivity i 20 mt low unblocking temperature i 300 c component of viscous origin and a high coercivity 20 mt high unblockingtemperature 25g700 c component carrying the characteristic remanence mineral magnetic analyses confirmed the presence of magnetite its low temperature oxidation products and haematite each contributing to the remanence properties grain size and concentration showed limited variations and there was little evidence for the presence of the ultrafine magnetic phase commonly associated with palaeosol formation pedogenic processes appeared negligible and their effects unimportant with detrital processes dominating the mineralogy and most probably the acquisition of the characteristic remanence the reversal record was characterized by the decay and recovery of the geocentric axial dipole term with large directional swings occurring during periods of reduced relative palaeofield intensity the virtual geomagnetic poles traced a complex path exhibiting no particular geographical confinement relative palaeofield intensity determinations were insensitive to the choice of normalization parameter and showed a distinctive asymmetry striking similarities were observed with the matuyama jaramillo reversal record obtained from the same section rolph 1993 and the steens mountain reversal record prcvot et al 1985 lending further support for the existence of unusually high post transitional field intensities

dating transitionally magnetized lavas of the late matuyama chron toward a new 40ar 39ar timescale of reversals and events the k ar based geomagnetic polarity timescale was constructed using data from lavas and tuffs that bracketed but rarely dated the transitions between polarity intervals subsequent 40ar 39ar dating indicated that the ages of some polarity transitions had been underestimated by about 6 although the accepted ages of the polarity chron boundaries have increased their precise temporal definition remained uncertain we have taken a different approach and used incremental heating techniques to obtain 18 new 40ar 39ar ages from basaltic lavas within flow sequences at punaruu valley tahiti and haleakala volcano hawaii these lavas record transitional paleomagnetic directions corresponding to four mid pleistocene polarity reversals or events three lavas from punaruu valley previously thought to record the cobb mountain normal polarity subchron cmns gave a mean age of 1 105 0 005 ma indicating that they were erupted about 76 kyr after the cmns this period of transitional field behavior is designated the punaruu event in addition seven new 40ar 39ar ages from the punaruu valley indicate that the jaramillo normal polarity subchron jns lasted about 67 kyr starting at 1 053 0 006 ma and ending 0 986 0 005 ma this agrees with astronomical estimates but conflicts with jns ages proposed by spell and mcdougall 1992 and izett and obradovich 1994 on the basis of 40ar 39ar dating of rhyolite domes in the valles caldera indistinguishable 40ar 39ar ages of seven lavas including one from punaruu valley and six from haleakala that record broadly similar intermediate paleodirections suggest that the kamikatsura event occurred at 0 886 0 003 ma moreover these data indicate that the kamikatsura event occurred 20 40 kyr after another geomagnetic event most probably taking place at 0 92 ma we designate this earlier field behavior the santa rosa event adopting its name from that of a transitionally magnetized rhyolite dome which happened to figure prominently in the original definition of the end of the jns in the 1968 study of doell et al 1968 the discovery of these new short lived polarity events during the matuyama reversed chron suggests that the 400 kyr period between 1 18 and 0 78 ma experienced no less than 7 and perhaps more than 11 attempts by the geodynamo to reverse this newly determined higher frequency of geomagnetic activity illustrates vividly the importance of obtaining precise age control directly from transitionally magnetized rocks

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

depth of post depositional remanence acquisition in deep sea sediments a case study of the brunhes matuyama reversal and oxygen isotopic stage 19 1 although post depositional magnetization pdrm in deep sea sediments appears to be acquired during the earliest stages of sediment compaction the natural variability of the pdrm lock in depth in deep sea sediments is poorly understood and as yet unquantified here we consider variations in the relative stratigraphic positions of oxygen isotopic interglacial stage 19 1 and the brunhes matuyama b m chronozone reversal for eight deep sea sediment cores results from a similar study of the displacement between a widespread microtektite layer and the b m boundary are also included 1 the pdrm lock in depth and the temporal relationships between the b m and stage 19 1 datums can be determined from the offsets between the paleomagnetic and the isotopic and microtektite stratigraphies for cores with sedimentation rates greater than 1 cm kyr a the depth offset between the paleomagnetic and isotopic datums is a linear function of sedimentation rate a simple model r 2 0 77 demonstrates that 1 pdrm acquisition occurs 16 cm below the sediment surface and 2 the b m reversal occurs 6 kyr 4 2 kyr after the stage 19 1 datum and the stage 19 1 datum occurs 9 kyr 4 3 kyr after the deposition of the australasian microtektite strewnfield this example argues against simple geophysical models linking geomagnetic field reversals to climate change or impact events the b m boundary is anomalously deep 30 50 cm in very low accumulation rate sediments 1 cm kyr 1 and this may reflect the unusual physical properties of these sediments a review of the geotechnical literature suggests that very low accumulation rate sediments have sufficient time to develop enhanced interparticle rigidity structural strength which inhibits early compaction and hence pdrm acquisition

numerical correlation of recent paleomagnetic records in two lake tahoe cores abstract recent paleomagnetic fluctuations are smooth and well correlated in two short cores collected 28 km apart in lake tahoe cross correlation analysis of the unit vector time series data shows that the characteristic angular difference between the two records is only 11 for the past 3000 years this compares favorably with the characteristic angular differences of 7 8 between adjacent samples within each core taken separately the correspondingly high values of pairwise between cores and serial within core correlation are significant at 99 99 confidence these data collected and previously studied qualitatively by a s goldstein show that the sediment in lake tahoe can provide reliable recent paleosecular variation records the paucity of fine scale paleomagnetic correlation between other cores from that basin is due likely to stratigraphic rather than rock magnetic difficulties

the secular variation of earth s magnetic field models of the magnetic field at the core mantle boundary at selected epochs from 1715 0 to 1980 0 reveal novel features in the field at the core these suggest that the flow of core fluid is coupled to the mantle and that magnetic diffusion is significant

paleomagnetic record at odp site 980 feni drift rockall for the past 1 2 myrs 1 a detailed record of variations in the direction and intensity of the geomagnetic field over the past 1 2 myrs has been obtained from an 120 m thick sedimentary section cored at odp site 980 on the feni drift north atlantic the record has high resolution due to high mean sedimentation rates 11 3 cm kyr for the brunhes chron and 5 5 cm kyr for the matuyama chron paleomagnetic measurements every 1 cm along u channel samples and high quality isotopic age control the iceland basin event is manifest by virtual geomagnetic poles crossing the equator at 190 ka the base of the recovered section lies immediately below the cobb mountain subchronozone that occurs within marine isotope stage 35 normalized remanence data can be correlated to paleointensity records from odp sites 983 and 984 700 km to the nw from site 980 and with lower resolution paleointensity data from the pacific ocean differences between the site 980 paleointensity record and the pacific records are attributed to variable sedimentation rates variable quality age control and inadequacies in the normalization procedure used to derive the paleointensity proxies

high resolution stratigraphic correlation of benthic oxygen isotopic records spanning the last 300 000 years abstract in order to compare the response of different oceanographic regions to global climate change very detailed stratigraphic techniques are required the global signal of ice volume changes recorded in the oxygen isotopic composition of foraminifera can provide the tool for developing the necessary high resolution stratigraphy in order to evaluate the resolution of a stratigraphy based on detailed isotopic records two techniques are used to correlate a set of benthic oxygen isotope records from seven piston cores taken in the north and south atlantic the indian and the equatorial and north pacific oceans the first technique was modified from the graphic correlation procedure of shaw 1964 this procedure requires the identification of isotopic events that are correlated from core to core detailed correlations for intervals of the cores between events are provided by a series of straight line segments connecting all common events the second technique developed by martinson et al 1982 uses inverse procedures to define a continuous non linear mapping function that correlates the isotopic records the mapping function maximizes the correlation coefficient between data sets being compared the techniques are independent in that they rely on different criteria for correlating the data series stratigraphic correlations obtained by these procedures are in excellent agreement the mean difference between the correlations is on the order of the sampling intervals of each core and when corrected for sedimentation rates suggests that benthic isotope records from the suite of seven cores can be correlated to a resolution of 2000 to 4000 yrs

a quantitative comparison of two paleomagnetic records of the cobb mountain subchron from north atlantic deep sea sediments we present a new paleomagnetic record of the cobb mountain subchron obtained from deep sea sediments cored at ocean drilling program site 647 in the southern labrador sea the details of the transitional field behavior documented by this record appear to be very similar to those recorded in a previously published record of this subchron obtained at deep sea drilling project dsdp site 609 in the north atlantic clement and kent 1987 we used a quantitative correlation technique martinson et al 1982 to establish statistically the degree of similarity between these two records and thereby constrain the spatial variability in these transitional fields the error in the alignments is reduced significantly by aligning records of virtual geomagnetic pole positions rather than directions indicating that these records document such large scale changes in the fields that we can not distinguish them from dipolar changes given the proximity of these two sites these replicate records of the cobb mountain subchron provide evidence that deep sea sediments are capable of providing high resolution records of geomagnetic field behavior a reexamination of the sequence of polarity transitions recorded at dsdp site 609 in light of these results suggests the presence of two preferred transitional field configurations the field appears to change from one configuration to the other for several reversals and then back to the original configuration suggesting that a geographical influence on the reversal process persists through this sequence the variability in these reversal records provides insights into the response of the geodynamo to this geographical influence

a detailed record of the lower jaramillo polarity transition from a southern hemisphere deep sea sediment core a detailed record of the lower jaramillo reversed to normal polarity transition was obtained from a southern hemisphere deep sea sediment core latitude 35 91 e longitude 59 97 e the record consists of over 850 samples taken across 140 cm of section the transition itself is recorded across approximately 70 cm and is represented by more than 475 specimens from about 160 levels giving intermediate directions the transition is identified by a nearly 180 shift from directions in good agreement with a reversed axial dipole field to those closely aligned with a normal axial dipole field for the core site latitude the inclinations shallow gradually early in the reversal and pass through very steep negative values 80 late in the transition the declinations show little appreciable variation until the inclinations have moved through the near vertical and then slowly approach values in agreement with a normal polarity field an intensity low accompanies the directional change during which the intensity drops to less than 15 of the maximum values observed in this sample interval the intensity fluctuation spans a wider interval than the directional change decreasing prior to any systematic change in the directions and then increasing to pre transition levels by the same depth at which the directions have stabilized the vgp path constructed for this reversal is longitudinally constrained to a certain extent between 140 and 230 for intermediate vgp latitudes and is roughly centered 120 from the site longitude this path is therefore a far sided vgp path in hoffman s 1977 terminology assuming a constant sedimentation rate 67m ma through the jaramillo subchron the duration of the transition is estimated to be 11 200 years to 4 500 years depending on the criteria for the directional change whereas the associated intensity variation occurred over 15 000 to 20 000 years considered together with records of the most recent reversal matuyama brunhes in light of current transitional field models this record strongly suggests that the lower jaramillo transitional field was dominated by different harmonics than the matuyam brunhes transitional field

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

equatorial and mid latitude records of the last geomagnetic reversal from the atlantic ocean abstract three records of the matuyama brunhes reversal have been obtained from odp deep sea cores distributed along the west african margin in the atlantic ocean these studies in addition to the record from dsdp site 609b provide a latitudinal transect extending from the equator to 50 n simulations of natural smoothing by post depositional processes show that characteristic features of a transitional field geometry dominated by axisymmetry should be preserved in the records especially at the equator the results do not seem to favour the hypothesis that axisymmetrical terms would dominate during this transition

age calibrated relative paleointensity for the last 1 5 myr at iodp site u1308 north atlantic integrated ocean drilling program iodp site u1308 central north atlantic records paleomagnetic directional and relative paleointensity rpi variations for the last 1 5 myr in 110 m of the sediment sequence at a mean sedimentation rate of 7 3 cm kyr a detailed benthic oxygen isotope record was combined with rpi to produce an integrated high resolution magneto isotopic stratigraphy for site u1308 apart from the well known polarity reversals in this interval the punaruu excursion is recorded at 1092 ka and the cobb mountain subchron in the 1182 1208 ka interval the paleointensity proxies are determined as slopes of nrm versus arm and nrm versus armaq arm acquisition with linear correlation coefficients to monitor the quality of the linear fit the rpi record for site u1308 is compared with the three other paleointensity records one from the western equatorial pacific and two from the north atlantic that cover the same time interval and have accompanying oxygen isotope records the match protocol of lisiecki and lisiecki 2002 is used to optimize the correlation of paleointensity records beginning with the original published age models for each record the match routine is used to optimize the rpi correlations to site u1308 with checks to ensure compatibility with oxygen isotope records squared wavelet coherence wtc indicates significant improvement in rpi and oxygen isotope correlations after matching each rpi record to site u1308 particularly for periods 10 kyr the level of coherence for the atlantic rpi records and the lower resolution pacific record implies synchronous global variability at scales 10 kyr that can be attributed to the axial dipole geomagnetic field

sedimentary records of reversal transitions magnetization smoothing artefact or geomagnetic field behaviour sedimentary records of reversals for the past 12 million years exhibit preferred longitudes of their vgp paths this observation may be related to long term aspects of core mantle dynamics however it has been suggested that sedimentary vgp paths could be an artefact due to smoothing which mixes non antipodal pre and post transitional directions we show here that smoothing over unrealistically long time scales is required to generate intermediate directions of magnetization which are purely mixtures of pre and post transitionals directions in sediments whose remanence is primarily depositional drm or post depositional remanence pdrm sedimentary records of transitions showing vgp paths confined to the planes of immediately pre and post transitional vgps have however been observed these can be explained by non axisymmetric bias in the non reversing field gubbins and bloxham 1987 constable 1992 which plays a role in reversals therefore provided that magnetization directions are properly cleaned to give single component directions the preferential longitudes of reversal vgp paths will be due to field behaviour and not to an artefact of the magnetization process if however mixtures of primary magnetization acquired at the time of the formation of the rock and secondary magnetization acquired later are measured then artefacts will be generated

the 17 july 1998 tsunami papua new guinea evidence and initial interpretation abstract a m w 7 1 earthquake that occurred at 08 49 hours 17 july 1998 utc 6 49 pm local time off the north coast of papua new guinea generated a locally destructive tsunami more than 2189 people died and virtually no structures were left standing along 19 km of coast a fast moving wall of sand laden water left fishing nets and other detritus in trees up to 17 5 m above sea level concrete was stripped to the reinforcing metal and some trees were ripped out and carried more than a kilometre back into sissano lagoon the site is in an active sedimentary basin the aitape trough with 4500 m thickness of neogene sediments between the bewani fault zone and the wewak trench there is some debate about the form of tsunami propagation for this event however we present evidence for subsidence of 30 40 cm on the landward side of the spit fronting sissano lagoon furthermore the area may have subsided up to three times this century in situ stumps of drowned trees in the lagoon record at least one earlier event the low angle harvard university cmt solution m w 7 1 depth 6 0 km 10 landward dip on fault with northward displacement is consistent with the tectonic setting and pattern of aftershocks elastic modelling of the energy release suggests 2 m horizontal displacement over 600 km 2 40 cm subsidence landward and 60 cm uplift seaward convergent flow of the displaced water into the area of subsidence focused wave energy on the coast and generated the locally high wave the record of tsunami at sissano lagoon indicates that the spit fronting the lagoon is unsafe for habitation there is potential for similar coseismic coastal subsidence to focus tsunami in other areas with similar tectonic settings

comment on the lau basin cobb mountain records by abrahamsen and sager abstract the cobb mountain subchron is one of the few short polarity intervals for which transition records have been obtained both from a wide geographical distribution and from different types of paleomagnetic recorders these records exhibit vgp paths that are remarkably similar given the wide geographic distribution of these sites the similarity in vgp paths is most simply interpreted as the presence of very large scale if not dipolar symmetries in the transitional fields only the records from western california and the lau basin exhibit vgp paths that are markedly different from the others new dates for the punaruu lavas suggest that they recorded a new geomagnetic cryptochron rather than the cobb mountain singer b s hoffman k a chauvin a coe r s 1999 dating transitionally magnetized lavas of the late matuyama chron toward a new 40 ar 39 ar timescale of reversals and events j geophys res 104 679 693 this new age indicates a revised correlation of some of the other california records whose ages agree better with the punaruu cryptochron a re examination of the lau basin records reveals that much of the difference between these and the other records results from a bias introduced when the paleomagnetic data were smoothed reanalysis of these records shows that the lau basin vgp paths are very similar to those obtained from western pacific and the north atlantic

11 million years of oligocene geomagnetic field behaviour summary an 11 million year long record of the oligocene geomagnetic field has been obtained from pelagic sediments of dsdp hole 522 an average sample spacing of 4 cm yielded approximately one specimen per 4 to 8 kyr the rock magnetics are remarkably consistent across the entire interval previous work demonstrated a magnetic mineralogy dominated by magnetically stable magnetite the natural remanent magnetism nrm carries an oligocene polarity timescale that is in excellent agreement with the oligocene reversal record as determined from marine magnetic anomalies mmas including many of the so called crypto chrons normalized nrm intensities from the undisturbed portions of the record yield a time series of variations with features consistent with a number of other palaeointensity time series derived from both sedimentary and lava sequences these features include consistent major decreases in palaeointensity dips at reversal boundaries and occasional dips between reversal boundaries that could correspond to lineated tiny wiggles in the mma records the data set suggests that the overall field strength was 40 per cent higher in the first half of the oligocene when the average reversal frequency was 1 6 myr l than in the second half when the reversal frequency was 4myr there is also a weak dependence of average field strength on length of polarity interval finally in the three cores suited to spectral analysis of coherent polarity and relative intensity independent of lithological contamination there is a persistent ca 30 50 ka periodicity in the variations of the relative intensity suggesting that the geomagnetic field pulses at about this frequency not only during the brunhes as demonstrated by tauxe shackleton 1994 but in the oligocene as well

alder creek sanidine acs 2 a quaternary 40ar 39ar dating standard tied to the cobb mountain geomagnetic event accurate 40ar 39ar dating of quaternary and holocene samples requires a standard neutron fluence monitor of appropriate age this standard should be isotopically homogenous at the scale of analysis and should be well intercalibrated with other standards several such standards have been proposed including sanidine from the 1 19 ma alder creek rhyolite acs turrin et al 1994 turrin b d donnely nolan j m hearn b c jr 1994 40ar 39ar ages from the rhyolite of alder creek california age of the cobb mountain normal polarity subchron revisited geology 22 251 254 this sanidine is satisfactorily homogeneous well intercalibrated with fish canyon sanidine fcs renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 and is linked to the astronomically calibrated magnetic polarity time scale since the alder creek rhyolite lava is the type occurrence of the cobb mountain event cm we collected 408 kg of this rock from which we anticipate being able to extract more than 8 kg of 425 m sanidine crystals analyses of sanidine phenocryts from the acs 2 from three separate irradiations and two distinct grain sizes yield ages ranging from 1 190 0 004 to 1 194 0 003 ma1 with a weighted mean age of 1 193 0 001 ma mswd 0 74 n 225 based on fcs 28 02 ma renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 the value of the intercalibration factor renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 is rfcsacs 2 0 04229 0 00012 n 8 identical with that of the original acs measurements 0 04229 0 00012 renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 the age of acs 2 is in agreement with the recent astronomical calibration of the cm event 1 215 to 1 190 ma channell j e t mazaud a sullivan p turner s raymo m e 2002 geomagnetic excursions and paleointensities in the matuyama chron at ocean drilling program sites 983 and 984 iceland basin j geophys res 107 6 10 1029 2001jb000491 the apparent absence of xenocrysts and excess argon suggests that multigrain analyses are feasible and therefore the amount of material can be tailored to experimental requirements aliquots from various grain sizes will be available upon request zircon is also potentially available which may be of interest to u th he researchers