intercalibration of astronomical and radioisotopic time the 40ar 39ar radioisotopic dating technique is one of the most precise and versatile methods available for dating events in earth s history but the accuracy of this method is limited by the accuracy with which the ages of neutron fluence monitors dating standards are known calibrating the ages of standards by conventional means has proved difficult and contentious the emerging astronomically calibrated geomagnetic polarity time scale apts offers a means to calibrate the ages of 40ar 39ar dating standards that is independent of absolute isotopic abundance measurements seven published 40ar 39ar dates for polarity transitions nominally ranging from 0 78 to 3 40 ma are based on the fish canyon sanidine standard and can be compared with apts predictions solving the 40ar 39ar age equation for the age of the fish canyon sanidine that produces coincidence with the apts age for each of these seven reversals yields mutually indistinguishable estimates ranging from 27 78 to 28 09 ma with an inverse variance weighted mean of 27 95 0 18 ma normalized residuals are minimized at an age of 27 92 ma indicating the robustness of the solution

why are geomagnetic excursions not always recorded in sediments constraints from post depositional remanent magnetization lock in modelling abstract many geomagnetic excursions have been documented during the brunhes chron however high resolution paleomagnetic studies of sediments often provide evidence of few if any excursions we have investigated the reasons for this observation by modelling the post depositional remanent magnetization pdrm lock in process a cubic lock in function produces more rapid lock in compared to linear and exponential functions proposed in the literature we therefore used a cubic function to model best case scenarios for the quality of the paleomagnetic record when a high frequency geomagnetic input signal is convolved with the sediment lock in function for a wide range of sedimentation rates even for a lock in depth of 10 cm where 95 of the pdrm is locked in 5 cm below the surface mixed layer an input signal containing abundant excursions with 1 kyr duration is smoothed so drastically at low sedimentation rates 1 3 cm kyr that no excursions are recorded excursions are further attenuated by increasing the lock in depth as well as by sampling at discrete stratigraphic intervals the pdrm process acts as a low pass filter for high frequency secular variation signals even at moderate sedimentation rates 10 cm kyr we have also modelled the effects of modulation of sedimentation rate by a climatic forcing function e g insolation even though the average sedimentation rate might be substantial pdrm recording becomes much less reliable in intervals where sedimentation rates are lowest in these cases some excursions are reliably recorded in detail while others may not be recorded at all our modelling suggests that in order to consistently detect the presence of geomagnetic excursions it is ideal to work with sediments that maintain minimum sedimentation rates above 10 cm kyr if the normal spectrum of geomagnetic field behaviour contains abundant excursions failure to document excursions in many apparently high resolution analyses probably results from pdrm lock in within relatively slowly deposited sediments or to unrecognized intervals of slow sedimentation in environments with higher average sedimentation rates

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

coupled deep water flow and climate variability in the middle pleistocene north atlantic abrupt changes in the strength of the atlantic meridional overturning circulation comprising northward flow of warm water and a cold southward return flow are implicated in abrupt climate changes in the late pleistocene a sensitive place to assess this circulation is in the cold return flow of deep western boundary currents here in records of flow speed and isotopic composition of surface and bottom waters from a deep western boundary current location near the northern source of north atlantic deep water we show both orbital and millennial scale coupling between deep ocean flow and climate in the middle pleistocene 0 75 0 87 ma when the boundary conditions in terms of the mean state and amplitude of climate change were different from more recent periods the coupling appears as a phased series of events initiated by reduced vertical density gradients and initial ventilation of deep waters the occurrence of these events in interglacials during the middle pleistocene suggests that the millennial scale climate variability in the north atlantic was more pronounced at that time than previously thought this demonstrates that given the right boundary conditions rapid climate shifts can also occur during relatively warm climate conditions

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

insolation values for the climate of the last 10 million years new values for the astronomical parameters of the earth s orbit and rotation eccentricity obliquity and precession are proposed for paleoclimatic research related to the late miocene the pliocene and the quaternary they have been obtained from a numerical solution of the lagrangian system of the planetary point masses and from an analytical solution of the poisson equations of the earth moon system the analytical expansion allows the direct determination of the main frequencies with their phase and amplitude numerical and analytical comparisons with the former astronomical solution ber78 are performed so that the accuracy and the interval of time over which the new solution is valid can be estimated the corresponding insolation values have also been computed this analysis leads to the conclusion that the new values are expected to be reliable over the last 5 ma in the time domain and at least over the last 10 ma in the frequency domain

orbital modulation of the earth s magnetic field intensity more than 20 years ago on the basis of data from a pacific sediment core it was suggested that geomagnetic field intensity may vary with the earth s orbital obliquity centred on a period of 41 kyr as a result of the effect of obliquity on precessional forces in the earth s core1 it had also been proposed that precession plays an important role in the energy budget of the earth s geodynamo2 but subsequent analyses indicated that the energy available from precession is at least an order of magnitude less than that required to drive the geodynamo3 here however we report a spectral analysis of sedimentary records of relative geomagnetic palaeointensity from two north atlantic sites which shows significant power both at orbital eccentricity 100 kyr and obliquity 41 kyr the eccentricity power is also present in bulk magnetic properties such as susceptibility and is therefore attributable to lithological variations controlled by eccentricity driven climate change the obliquity power however is not apparent in bulk magnetic properties and seems to be a property of the geomagnetic field itself thus providing evidence for the orbital forcing of geomagnetic field intensity

reconciling astrochronological and 40ar 39ar ages for the matuyama brunhes boundary and late matuyama chron when five matuyama brunhes m b boundary records from the north atlantic are placed on isotope age models produced by correlation of the 18o record directly or indirectly to an ice volume model the m b boundary lies consistently at the young end of marine isotope stage 19 with a mean age for the midpoint of the reversal of 773 1 ka standard deviation 0 4 kyr 7 kyr younger than the presently accepted astrochronological age for this polarity reversal 780 781 ka two recently proposed revisions of the age of the 40ar 39ar fish canyon sanidine fcs standard to 28 201 0 046 ma and 28 305 0 036 ma would adjust 40ar 39ar ages applicable to the m b boundary and other reversals and excursions back to 1 2 ma to ages older than the new astrochronological ages by 8 24 kyr the variables used to construct the ice volume models cannot account for the discrepancy the fcs standard age that best fits the astrochronological ages is 27 93 ma which is within the uncertainty associated with the commonly used value of 28 02 0 16 ma but younger than the recently proposed fcs ages the edc2 and edc3 age models in the dome c antarctic ice core yield ages of 771 7 ka and 766 4 ka respectively for the 10be flux peak that denotes the paleointensity minimum at the reversal boundary implying that the edc2 rather than edc3 age model is consistent with the observations from marine sediments at least close to the m b boundary

geomagnetic dipole strength and reversal rate over the past two million years independent records of relative magnetic palaeointensity from sediment cores in different areas of the world can be stacked together to extract the evolution of the geomagnetic dipole moment1 2 and thus provide information regarding the processes governing the geodynamo so far this procedure has been limited to the past 800 000 years 800 kyr ref 3 which does not include any geomagnetic reversals here we present a composite curve that shows the evolution of the dipole moment during the past two million years this reconstruction is in good agreement with the absolute dipole moments derived from volcanic lavas which were used for calibration we show that at least during this period the time averaged field was higher during periods without reversals but the amplitude of the short term oscillations remained the same as a consequence few intervals of very low intensity and thus fewer instabilities are expected during periods with a strong average dipole moment whereas more excursions and reversals are expected during periods of weak field intensity we also observe that the axial dipole begins to decay 60 80 kyr before reversals but rebuilds itself in the opposite direction in only a few thousand years

a new rock magnetic approach to selecting sediments for geomagnetic paleointensity studies application to paleointensity for the last 4000 years the hypothesis that the ratio of detrital remanent magnetization to anhysteretic remanent magnetization drm arm for sediment samples is a measure of relative geomagnetic paleointensity is critically evaluated by two distinct approaches one approach is a detailed rock magnetic examination of the implicit assumptions of the drm arm method and the construction of a selection process by which to identify sediments that conform to requirements satisfying these assumptions sediments are uniform with respect to drm arm ratio if they contain magnetite in the 1 15 ym particle size range as the predominant magnetic mineral and have variations in magnetite content of less than 20 30 times the minimum concentration the drm arm ratios of these sediments should provide estimates of relative geomagnetic paleointensity relative particle size variations in magnetite are detected with a plot of anhysteretic susceptibility xarm versus low field susceptibility x and the size range 1 15 m is approximately identified by high field hysteresis parameters a rock magnetic evaluation of leboeuf lake sediments with these techniques indicates that these sediments are suitable for a relative plaeointensity study the second approach to evaluating the drm arm ratio as a measure of relative paleointensity is direct comparison with absolute paleointensity data a comparison between the leboeuf lake estimates and thellier thellier results from the western united states supports the conclusion that suitable sediments can record geomagnetic paleointensity fluctuations

sedimentary records of relative paleointensity of the geomagnetic field theory and practice sediments have proved irresistible targets for attempts at determining the relative variations in the earth s magnetic field because of the possibility of long and continuous sequences that are well dated and have a reasonable global distribution the assumption underlying paleointensity studies using sedimentary sequences is that sediments retain a record reflecting the strength of the magnetic field when they were deposited early theoretical work suggested that because the time required for an assemblage of magnetic particles in water to come into equilibrium with the ambient magnetic field was quite short no dependence on magnetic field was expected nonetheless a number of experiments showed that sedimentary magnetizations varied in accordance with the field albeit not always in a simple linear fashion experiments in which the sediments were stirred in the presence of a field to simulate bioturbation showed a reasonably linear relationship with the applied field and these results spurred the hope that variations in the earth s magnetic field might indeed be recoverable from appropriate sedimentary sequences examination of existing paleointensity data sets allows a few general conclusions to be drawn it appears that sedimentary sequences can and do provide a great deal of information about the variations in relative paleointensity of the earth s magnetic field the dynamic range of sedimentary data sets is comparable to those acquired from thermal remanences moreover when compared directly with such independent measures of magnetic field variations as beryllium isotopic ratios and thermally blocked remanences there is considerable agreement among the various records when viewed over timescales of hundreds to thousands of years relative paleointensity data sets from more than a few thousand kilometers apart bear little resemblance to one another suggesting that they are dominated by nondipole field behavior when viewed over timescales of a few tens of thousands to hundreds of thousands of years however the records show coherence over large distances at least thousands of kilometers and may reflect changes in the dipole field on the basis of a sequence spanning the brunhes and matuyama chrons the magnetic field has oscillated with a period of about 40 ka for the last few hundred thousand years but these oscillations are not clear in the record prior to about 300 ka thus they are probably not an inherent feature in the geomagnetic field and the correspondence of the period of oscillation to that of obliquity is probably coincidence

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

the distinction between geomagnetic excursions and reversals summary two recent studies of the geomagnetic eld in the last 1 myr have found 14 excursions large changes in direction lasting 5 10 kyr each six of which are established as global phenomena by correlation between diierent sites the older picture of the geomagnetic eld enjoying long periods of stable polarity may not therefore be correct instead the eld appears to suier many dramatic changes in direction and concomitant reduction in intensity for 10 20 per cent of the time during excursions the eld may reverse in the liquid outer core which has timescales of 500 yr or less but not in the solid inner core where the eld must change by diiusion with a timescale of 3 kyr this timescale is consistent with the remarkably uniform duration of well dated excursions the disparity of dynamical timescales between the inner and outer cores a factor of 10 is consistent with the 10 excursions between full reversals

sawtooth variation in magnetic intensity profiles and delayed acquisition of magnetization in deep sea cores abstract recent records of normalised intensity of natural remanent magnetization nrm obtained from deep sea sediments document an asymmetrical saw tooth pattern associated with geomagnetic reversals this observation may reflect a fundamental aspect of the geodynamo time evolution or alternatively delay in acquisition of the nrm it is demonstrated here that a simple model in which a large fraction of the magnetic grains acquire nrm at deposition time while remaining grains reorientate or acquire magnetization after deposition is able to produce a saw tooth profile strikingly similar to that observed interestingly this mechanism of nrm acquisition does not damp out high frequency oscillations of the field such as those observed in the deep sea records exhibiting the saw tooth profile it provides an accurate record of field directions except for periods of very low field intensity

paleomagnetic directions and 40ar 39ar ages from the tatara san pedro volcanic complex chilean andes lava record of a matuyama brunhes precursor the same volcanic sequence overlain by flows with normal polarity the 40 ar 39 ar incremental heating experiments on lavas within the two sections provide nine independent age determinations and yield a weighted mean of 791 7 3 0 ka 2s for the paleomagnetic transition the sections are linked by geological mapping the precise radioisotopic dating and geochemical correlations alternating field and thermal demagnetization studies rock magnetic analyses and petrographic observations indicate that the magnetization is primary and carried by titanomagnetite the polarity change is characterized by a jump from reverse poles to a quasi stationary cluster of virtual geomagnetic poles over australia followed by a jump to normal polarity latitudes magnetization of these lavas is thus consistent with either a brief period when the field was dominated by a subequatorial dipole or a more complex nondipolar field that may reflect the influence of a long lived regional lower mantle control over a weakened dynamo the quebrada turbia lavas are circa 16 kyr older than those dated by exactly the same methods which record a later more complex portion of the reversal at haleakala volcano maui moreover the 792 ka radioisotopic age of these chilean lavas is older than most astronomical estimates for the matuyama brunhes reversal suggesting that this section may in fact record a precursor to the actual field reversal that is expressed by low paleointensities in more than a dozen well studied marine sediment cores index terms 1535 geomagnetism and paleomagnetism reversals process timescale magnetostratigraphy 1030 geochemistry geochemical cycles 0330 9360 information related to geographic region south america 9604 information related to geologic time cenozoic keywords paleomagnetism reversal

a comparison of different techniques for relative paleointensity parallel relative paleointensity records that are supposed to display the same geomagnetic variations often show discrepancies we compared various techniques of normalization with different parameters involving alternating fields a f and thermal demagnetization of sediment samples spanning the same period from the indian and the pacific oceans the results are basically unchanged using any technique there is also no causal relationship between changes in rock magnetic properties and mismatch between the two records thus discrepancies between parallel records do not depend on methods for relative paleointensity and any technique can be used with confidence provided that the characteristic component has been properly isolated and the sediments are magnetically homogeneous complications are more likely due to subtle variations in the sediment remanent magnetization acquisition these results point to the importance of sedimentological lithology and physical properties factors in controling the process of magnetization

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

saw toothed pattern of relative paleointensity records and cumulative viscous remanence several studies of relative paleointensity data derived from marine sediments spanning the last 4 myr display an asymmetrical saw toothed pattern polarity reversals of the geomagnetic field are associated with low points in paleofield intensity preceded by a long term progressive decay of the field and followed by a rapid post transitional recovery 1 3 since similar behavior is observed in far flung sites whose rock magnetic records are different it is argued that the world wide saw toothing represents geomagnetic field behavior 3 we present an alternative explanation calling on the effect of hard viscous remanence

a magnetostratigraphic reassessment of correlation between chinese loess and marine oxygen isotope records over the last 1 1 ma abstract we present a magnetic polarity based paleoclimatic correlation between chinese loess and marine oxygen isotope stage mis records over the last 1 1 ma this revised land ocean comparison indicates that inconsistent occurrence of the matuyama brunhes boundary which lies in mis 19 in marine sediments and occurs either at the base lower part of loess l8 or the uppermost part of soil s8 in chinese loess is due to regional and or local climate variability across the chinese loess plateau and does not need to be attributed to lock in effects of loess remanence acquisition associated with pedogenesis this result implies that the geomagnetic polarity boundaries in chinese loess in general are reliable chronostratigraphic time markers therefore astronomically tuned timescales for chinese loess that presume a downward shift of polarity boundaries due to a time lag in remanence acquisition may have overestimated ages of certain loess paleosol units

stability and dynamics of serpentinite layer in subduction zone abstract the hydrous phyllosilicate serpentines have a strong influence on subduction zone dynamics because of their high water content and low strength at shallow and intermediate depths in the absence of data newtonian rheology of serpentinites has been assumed in numerical models yet experimental data show that serpentine rheology is best described by a power law rheology recently determined in subduction zone conditions hilairet n et al 2007 high pressure creep of serpentine interseismic deformation and initiation of subduction science 318 5858 1910 1913 using a simple 1d model of a serpentinized channel and as opposed to previous models in this power law rheology we examine the influence of channel thickness temperature and subduction angle on serpentine flow driven by density contrast serpentinization degree with the surroundings at temperatures of 200 500 c relevant to intermediate depths a fully serpentinized channel is unlikely to be thicker than 2 3 km for channel thicknesses of 2 km upward velocities are comparable to those using a constant viscosity of 10 18 pa s the velocity profile using power law rheology shows shear zones at the edges of the channel and a low strain rate region at its centre consistent with the frequent observation of weakly deformed hp rocks upward velocities estimated for channels 1 to 3 km thick are comparable to the serpentinization rates for maximum estimates of fluid velocities within shear zones in the literature competition between the upward flow and serpentinization may lead to intermittent behavior with alternating growth periods and thinning by exhumation at shallower levels the thickness allowed for a channel may be up to 8 10 km if the rheology has a higher dependence on stress we therefore propose that the exhumation of hp oceanic units in serpentinite channels is organized in two levels the deepest and fastest motion being driven by density contrast with the surrounding mantle and the shallowest circulation being driven by forced return flow the thicknesses estimated here for serpentinized layers at intermediated depths are similar to the precision of seismic studies the deepest serpentinite channel may thus be difficult to detect by seismic methods but it will have a strong influence on the mechanical coupling between the slab and mantle wedge

astronomical calibration of the matuyama brunhes boundary consequences for magnetic remanence acquisition in marine carbonates and the asian loess sequences we have compiled 19 records from marine carbonate cores in which the matuyama brunhes boundary mbb has been reasonably well constrained within the astronomically forced stratigraphic framework using oxygen isotopes by correlation of the 18o data to a timescale based on astronomical forcing we estimate astronomical ages for each of the mbb horizons in all but one record the mbb occurs within stage 19 most magnetostratigraphic sections in asian loess place the mbb within a loess interval since loess deposition is presumed to be associated with glacial intervals loess horizons should correspond to even numbered oxygen isotope stages a glacial age for the mbb is at odds with the results presented here which firmly place the mbb within interglacial stage 19 inconsistency among the many loess sections and between the loess and the marine records suggests that the magnetic interpretation of loess sections may be more complicated than hitherto supposed the mean of the stage 19 age estimates for the mbb is 777 9 1 8 n 18 inclusion of the single stage 20 age results in a mean of 778 8 2 5 n 19 the astronomical age estimate of the mbb compares favorably with an unweighted mean of 778 2 3 5 n 10 from a compilation of 40ar 39ar results of transitional lava flows combining the two independent data sets yields a grand mean of 778 0 1 7 n 28 the new compilation shows virtually no trend in placement of the mbb within isotope stage 19 as a function of sediment accumulation rate we interpret this to mean that the average depth of remanence acquisition is within a few centimeters of the sediment water interface separating the cores into two geographic regions an indo pacific caribbean ipc group and an atlantic group results in a significant difference in the position of the mid point of the reversal with respect to the astronomical time scale the data presented here suggest a difference of several thousand years between the two regions this observation could be caused by systematic differences between the two regions in sedimentation rate within the interval of interest systematic differences in remanence acquisition or by genuine differences in the timing of the directional changes between the two regions

improvements in long core measurement techniques applications in palaeomagnetism and palaeoceanography we report on new developments in the long core measurement techniques using u channels in connection with new small access cryogenic magnetometers with these new instruments u channel samples allow us to measure a wide range of magnetic parameters along entire core length with a spatial resolution and accuracy virtually equivalent to that obtained using discrete cubic samples an additional advantage is that the sediment disturbance when sampling is significantly less when using u channels we present comparisons of results from both continuous and discrete measurements and examine the effectiveness of minimal deconvolution using various techniques examples of continuous measurements of low field susceptibility nrm arm irm hrc from north atlantic deep sea cores show that it is possible to generate logs of combination of parameters related to rock magnetic characteristics of the sediments useful in palaeoceanography in particular the continuous method allows us to rapidly detect core regions with fast changing magnetic parameters useful for correlation between cores and also for understanding changes in palaeo environmental conditions the fast scan also allows us to detect regions of the core where the rock magnetic parameters are suitable or not suitable for palaeomagnetic studies related to changes in the geomagnetic field such as the relative changes in palaeointensity

expansion of subarctic water masses in the north atlantic and pacific oceans and implications for mid pleistocene ice sheet growth 1 past surface ocean circulation changes associated with the mid pleistocene transition 0 9 0 6 ma were reconstructed in the northern north atlantic odp 983 and the northwest pacific odp 882 using proxies for subarctic subpolar water mass distributions c37 4 alkenone and sea surface temperature u37 both sites experienced a secular expansion of subarctic waters from 1 15 ma spanning both glacial and interglacial intervals after 0 9 ma low c37 4 at site 983 records a northward retreat of subarctic waters during interglacials in the atlantic while continued high glacial c37 4 indicate extensive subarctic waters during glacial maxima associated with the development of the larger late pleistocene ice sheets in contrast a secular decline in c37 4 occurred at site 882 from 0 9 to 0 5 ma marking a more gradual retreat of subarctic conditions in the pacific it is proposed that the expansion of subarctic waters between 1 15 and 0 9 ma exerted negative feedbacks to the moisture supply to the ice sheet source regions and may account for the apparent delayed ice sheet response to atmosphere ocean circulation changes associated with the mid pleistocene transition that began as early as 1 2 ma

a precursor to the matuyama brunhes transition field instability as recorded in pelagic sediments abstract the period some 20 25 kyr just prior to the most recent generally recognized geomagnetic field polarity transition the matuyama to brunhes reversal appears to be marked by significant geomagnetic variability manifested as pronounced oscillations in intensity we compare several previously published paleomagnetic records with new high resolution paleomagnetic data obtained from five pelagic marine sites north atlantic dsdp hole 609b equatorial atlantic odp hole 665a and western equatorial pacific odp holes 803b 804c and 805b using standard rock magnetic normalization for all of the samples as well as a thellier thellier method on the sediments of hole 804c we consistently find a decrease in paleointensity dip of 1 approximately 15 kyr prior to the matuyama to brunhes transition in the five new records as well as in the previously published records despite sedimentation accumulation rates sar that range from 11 cm kyr to 1 cm kyr these sequences yield paleointensity curves that are broadly similar in form even at the lowest sars the intensity of the pre reversal low dip1 appears to be of the same magnitude as that of the transition itself dip2 of 1 in some of the records a directional excursion to nearly full normal polarity accompanies dip1 and remains after alternating field af and or thermal demagnetization whereas in other records the directional changes vanish with demagnetization and appear to be caused by overprinting a viscous remanent magnetization vrm contribution to nrm was identified in two of the records and until removed by af or thermal demagnetization was found to blur the double dip nature of the paleointensity profiles into an apparent single dip and also resulted in an apparent but erroneous sawtooth like post transitional sudden increase in paleointensity after appropriate normalization the magnitude of the post transitional recovery was much reduced the magnetic directions of three of the new records after cleaning and adjusting the stable declinations to either 0 normal or 180 reverse map to vgp positions lying in the pacific the directional variations however are far less consistent than the intensity variations the confirmed global existence of this dip so closely preceding a major reversal invites questions about its relation to the reversal itself the apparent normal character of this interval can also present problems for magnetostratigraphical interpretations based on coarse or incomplete sampling by mimicking the brunhes matuyama reversal some 15 kyr earlier in the section than its true location

phase equilibrium geobarometers for silicic rocks based on rhyolite melts part 2 application to taupo volcanic zone rhyolites constraining the pressure of crystallisation of large silicic magma bodies gives important insight into the depth and vertical extent of magmatic plumbing systems however it is notably difficult to constrain pressure at the level of detail necessary to understand shallow magmatic systems in this study we use the recently developed rhyolite melts geobarometer to constrain the crystallisation pressures of rhyolites from the taupo volcanic zone tvz as sanidine is absent from the studied deposits we calculate the pressures at which quartz and feldspar are found to be in equilibrium with melt now preserved as glass the quartz 1 feldspar constraint of gualda and ghiorso contrib mineral petrol 168 1033 doi 10 1007 s00410 014 1033 3 2014 we use glass compositions matrix glass and melt inclusions from seven eruptive deposits dated between 320 and 0 7 ka from four distinct calderas in the central tvz and we discuss advantages and limitations of the rhyolite melts geobarometer in comparison with other geobarometers applied to the same eruptive deposits overall there is good agreement with other pressure estimates from the literature amphibole geobarometry and h2o co2 solubility models one of the main advantages of this new geobarometer is that it can be applied to both matrix glass and melt inclusions regardless of volatile saturation the examples presented also emphasise the utility of this method to filter out spurious glass compositions pressure estimates obtained with the new rhyolite melts geobarometer range between 250 to 50 mpa with a large majority at 100 mpa these results confirm that the tvz hosts some of the shallowest rhyolitic magma bodies on the planet resulting from the extensional tectonic regime and thinning of the crust distinct populations with different equilibration pressures are also recognised which is consistent with the idea that multiple batches of eruptible magma can be present in the crust at the same time and can be tapped simultaneously by large eruptive events

asymmetrical saw tooth pattern of the geomagnetic field intensity from equatorial sediments in the pacific and indian oceans abstract a record of relative paleointensity from marine sediments in the equatorial indian ocean spanning the last 4 ma completes a previous dataset from ocean drilling program odp leg 138 sites in the equatorial pacific 1 the timescale was defined with a precision better than 20 kyr with different and independent methods most of the features present in the record from the equatorial pacific are confirmed the overall saw tooth pattern is observed across every field reversal and the short term fluctuations superimposed on the slow intensity decrease preceding the reversals appear to be similar at both sites the global character of these features reinforces their interpretation in terms of changes in dipole field intensity and provides new important constraints on models of the geodynamo a synthetic curve of field intensity changes during the last 4 ma is proposed as a first paleointensity timescale for future stratigraphic studies

40ar 39ar age constraints for the jaramillo normal subchron and the matuyama brunhes geomagnetic boundary our mid pleistocene 4oar 39ar age recalibration of the geomagnetic polarity timescale is nearly in accord with the oxygen isotope climate record calibration of the astronomical timescale proposed by johnson 1982 and shackleton et al 1990 4oar 39ar ages of a normally magnetized rhyolite dome in the valles caldera northern mexico yielded a weighted mean age of 1 004 0 019 ma a k ar age of 0 909 0 019 ma for this rock by doell and dalrymple 1966 was the linchpin for the recognition and calibration of the jaramillo normal subchron jns other 4oar 39ar ages from the vailes caldera and 4oar 39ar ages of ivory coast tektites indicate that the jns began at about 1 11 ma and ended before 0 92 ma probably near 0 97 ma the matuyama brunhes boundary occurred between 0 79 ma and 0 76 ma on the basis of 4oar 39ar sanidine ages from 1 three reversely magnetized rhyolite domes of the valles caldera 0 793 0 018 ma 0 794 0 007 ma and 0 812 0 023 ma and pumice 0 789 0 006 ma from the reversely magnetized oldest toba tuff of sumatra and 2 pumice 0 764 0 005 ma and 0 757 0 009 ma from the lower and upper units of the normally magnetized bishop tuff the age of the boundary may be close to 0 77 ma as deduced from rates of sedimentation in ancient lake bonneville utah increasingly aware that the polarity of earth s magnetic field has changed repeatedly through geologic time and isotopic dating of these polarity reversals resulted in a geomagnetic polarity time scale gpts for the late cenozoic this combined paleomagnetic and isotopic research played a pivotal role in confirming plate tectonic theory glen 19821 which revolutionized the earth sciences continual refinement of isotopic ages which define the gpts has increased the usefulness of geomagnetic polarity reversals as marker hori zons in many types of geologic studies we present 4oar 39ar data and ages for some key volcanic units that support changes in the isotopic age chronology of the gpts using volcanic rocks erupted near the time of the geologically youngest two geomagnetic polarity reversals figure 1 the older of the two reversals originally called by doell and dalrymple 1966 the jaramillo event is now named the jaramillo normal subchron jns it is recorded in middle pleistocene rocks as a brief episode of normal polar ity within the late matuyama chron the younger of the two called the matuyama brunhes boundary mbb is regis tered in middle pleistocene rocks worldwide and consists of

astronomic timescale for the pliocene atlantic δ18o and dust flux records of ocean drilling program site 659 high resolution benthic oxygen isotope and dust flux records from ocean drilling program site 659 have been analyzed to extend the astronomically calibrated isotope timescale for the atlantic from 2 85 ma back to 5 ma spectral analysis of the 18o record indicates that the 41 kyr period of earth s orbital obliquity dominates the pliocene record this is shown to be true regardless of fundamental changes in the earth s climate during the pliocene however the cycles of sahelian aridity fluctuations indicate a shift in spectral character near 3 ma from the early pliocene to 3 ma the periodicities were dominantly precessional 19 and 23 kyr and remained strong until 1 5 ma subsequent to 3 ma the variance at the obliquity period 41 kyr increased the timescale tuned to precession suggests that the pliocene was longer than previously estimated by more than 0 5 m y

deconvolution of u channel paleomagnetic data near geomagnetic reversals and short events 1 the deconvolution scheme of oda and shibuya 1996 designed for half core data from the pass through magnetometer aboard the joides resolution has been adapted for treatment of u channel data the program was tested using u channels from ocean drilling program odp site 1090 a significant increase in resolution of the u channel data can be achieved by deconvolution resulting in data comparable with that obtained by continuous discrete sampling large amplitude changes of the magnetization vector revealed by the deconvolution treatment are in good agreement with the results obtained on discrete samples extracted from the same u channels application of this procedure to u channel data from odp sites 983 and 984 over a short time interval around 1 255 ma permitted enhanced definition of the so called bjorn geomagnetic event

extension of the astronomically calibrated polarity time scale to the miocene pliocene boundary the early pleistocene to late pliocene astronomically calibrated time scale of shackleton et al 1 and hilgen 2 is extended to the miocene pliocene boundary this is done by correlating the detailed record of caco3 cycles in the trubi and the lower part of the overlying narbone formation rossello composite section sicily to the astronomical record using 1 inferred phase relations between these caco3 cycles and the corresponding orbital cycles of precession and eccentricity and 2 calibration points provided by the previously established astronomical calibration of sapropelitic layers which occur in the topmost part of the caco3 record 2 this correlation allows all small scale caco3 cycles in the rossello composite section to be coded after the correlative precession peak and to be dated astronomically with an accuracy of 1 ky in combination with the available high resolution magnetostratigraphy it further allows the construction of an astronomically calibrated geomagnetic polarity time scale for the major part of the gilbert and gauss chrons this new time scale gives ages for the reversal boundaries of the kaena 3 04 3 11 ma and mammoth 3 22 3 33 ma the gilbert gauss chron boundary 3 58 ma and the reversal boundaries of the cochiti 4 18 4 29 ma nunivak 4 48 4 62 ma sidufjall 4 80 4 89 ma and thvera 4 98 5 23 ma these ages are significantly older than the ages given by conventional time scales age discrepancies gradually increase with increasing age the age of 5 32 ma for the miocene pliocene boundary as defined by the base of the trubi on sicily is almost 0 5 m y 9 5 older than the presently accepted age of 4 86 ma 3 these considerable discrepancies with the conventional time scale can either be explained by an consistent error in the decay constants used in k ar radiometric dating or more likely by diffusional loss of radiogenic argon

high resolution record of the matuyama brunhes transition constrains the age of javanese homo erectus in the sangiran dome indonesia a detailed paleomagnetic study conducted in the sangiran area java has provided a reliable age constraint on hominid fossil bearing formations a reverse to normal polarity transition marks a 7 m thick section across the upper tuff in the bapang formation the transition has three short reversal episodes and is overlain by a thick normal polarity magnetozone that was fission track dated to the brunhes chron this pattern closely resembles another high resolution matuyama brunhes mb transition record in an osaka bay marine core in the sangiran sediments four successive transitional polarity fields lie just below the presumed main mb boundary their virtual geomagnetic poles cluster in the western south pacific partly overlapping the transitional virtual geomagnetic poles from hawaiian and canary islands lavas which have a mean 40ar 39ar age of 776 2 ka thus the polarity transition is unambiguously the mb boundary a revised correlation of tuff layers in the bapang formation reveals that the hominid last occurrence and the tektite level in the sangiran area are nearly coincident just below the upper middle tuff which underlies the mb transition the stratigraphic relationship of the tektite level to the mb transition in the sangiran area is consistent with deep sea core data that show that the meteorite impact preceded the mb reversal by about 12 ka the mb boundary currently defines the uppermost horizon yielding homo erectus fossils in the sangiran area

an ice core perspective on the age of the matuyama brunhes boundary abstract two intervals of enhanced 10be flux thought to be associated with periods of low dipole intensity and identified as the matuyama brunhes transition and a precursor event have been observed in the bottom section of the epica dome c ice core the peaks span 764 776 ka and 788 798 ka on the new edc3 chronology with a stated absolute age uncertainty of 6 ka 2 this chronology uses orbital tuning of atmospheric oxygen 18 18oatm to correct for anomalies in ice flow in the bottom 500 m of the core an additional 28 18oatm data points have been measured to improve resolution and verify the accuracy of the tuning and the stated timescale uncertainty both the dating of the increased 10be and that relative to climatic records are compared to paleointensity records found in orbitally tuned marine sediments the mid point of the 10be peak associated with the m b is approximately 10 ka younger than the age determined radioisotopically from lavas with transitional orientations taking into account recent revisions to the 40ar 39ar dating standard and improved precision climatic constraints on the edc3 agescale make an error of this magnitude in the ice chronology implausible this age difference however is consistent with recent modeling suggesting that directional changes are spatially asynchronous and may precede the dipole intensity minimum in some locations although formally less precise than the published age from astrochronologically dated marine sediments ice core ages are potentially more accurate because they are not subject to lock in depth uncertainties

towards absolute calibration of sedimentary paleointensity records abstract using relative paleointensity estimates derived from twelve globally distributed pelagic sediment cores we assess whether they record a signal consistent with that expected from a dominant geocentric axial dipole the cores span the matuyama brunhes boundary and we normalize the observations by supposing that at the time the direction reverses the intensity low reflects only the non axial dipole contribution to the field we further assume that this non axial dipole contribution to the field is invariant with geographic location from absolute paleointensity compilations we estimate its size to be about 7 5 t this supplies the calibration for the axial dipole signal away from the extreme low in intensity the data predict the dipole field variation with latitude with similar accuracy to that observed in absolute paleointensity records and show similar behavior when transformed to virtual axial dipole moments

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

relative geomagnetic paleointensity from the jaramillo subchron to the matuyama brunhes boundary as recorded in a mediterranean piston core piston core lc07 located west of the sicily strait in the mediterranean sea unambiguously records the matuyama brunhes m b and the upper jaramillo polarity reversals with similar average sediment accumulation rates sars for the brunhes chron 2 29 cm kyr and late matuyama chron c1r 1r 2 19 cm kyr we report a relative paleointensity record for the interval spanning the m b boundary down into the jaramillo subchron which is unique in the mediterranean because existing records from this basin cover only the last 80 kyr the average sar in core lc07 is used to translate the depth related paleointensity record to the time domain the ratio of anhysteretic remanent magnetization to low field magnetic susceptibility is climatically sensitive and is used to tune the age model this correlation produces a good fit to the global ice volume model derived for summer insolation at 65 n with this age model a paleointensity minimum in association with the m b boundary has a duration of about 4 5 kyr while the directional change has a duration of 6 3 kyr a second paleointensity minimum of similar duration is found about 16 kyr below the m b boundary this feature precursor or dip in the literature has previously been recognized at the same time interval in many marine records which reinforces the validity of our age model other relative paleointensity minima are found within chron c1r 1r and within the uncertainties of the respective age models these minima coincide with those observed from the few published coeval paleointensity records in particular there is good correspondence between the ages of minima at about 0 92 and 0 89 ma which probably correlate with two geomagnetic excursions santa rosa and kamikatsura respectively that have been recorded in lava flows and dated using the 40 ar 39 ar technique in contrast a recently dated excursion at 0 83 ma from la palma seems to correspond to a paleointensity maximum this observation is opposite to that expected and this excursion needs to be confirmed in contrast to some recently published paleointensity records spectral analysis of the lc07 record does not reveal identification of significant power at the orbital obliquity frequency fl 2002 elsevier science b v all rights reserved

cenozoic sediment accumulation on drifts in the northern north atlantic the northern north atlantic sediment drifts have a much greater areal extent than has previously been indicated the northern north atlantic extends from the east greenland to european continental shelves and from the charlie gibbs fracture zone to the greenland scotland ridge within this region are seven major sediment drifts containing some of the best clues for pre quaternary bottom water circulation in the north atlantic feni drift is the oldest in the region originating near the eocene oligocene boundary it was followed by accumulation of bjorn gardar hatton and snorri drifts from the late early to middle miocene eirik drift may have started to accumulate in the late miocene and gloria drift in the early pliocene through analysis of the cenozoic sediment mass age distribution and the change in area of seafloor for the study area through time it can be concluded that there is a deficit of oligocene to miocene sediment in the region this could be explained by sediment cycling the erosion of older sediment to produce young sediment the mass of pliocene to quaternary sediment is much larger than that of the older sediment and may have been derived primarily through the erosion of eocene and oligocene sediment by deepwater currents the sediment drift portion of the total regional sediment mass increased significantly three times during the cenozoic each of these drift growth phases lasted between 3 and 4 m y the first two growth phases from the late eocene to early oligocene and from the early to middle miocene were accompanied by northward shifts of depocenters to the newly forming sediment drifts the third and largest growth phase occurred between 7 and 3 ma and may have been preceded by the initiation of accumulation on eirik drift 8 7 ma this most recent growth phase was accompanied by high apparent accumulation rates over southern gardar drift as it expanded to the east erosion rates over reykjanes ridge may have been higher during the mid pliocene than in the quaternary there are two main sites where dense water could have formed and flowed south through the rockall trough to start the accumulation of feni drift if the atmosphere were cool enough dense water could have formed in the norwegian sea or on the faeroe shelf dense water may also have formed on the rockall plateau due to salinity increase through evaporation in an arid climate as an alternative to the hypothesis of arctic ocean water overflowing the iceland faeroe ridge in the early middle miocene it is suggested that dense water formed along shallow segments of the iceland faeroe ridge and then flowed into the south iceland basin to begin accumulation of bjorn gardar and snorri drifts the initiation of sediment drift formation in the south iceland basin may have been caused by a decreased rate of deep water production combined with a higher rate of detrital input due to a middle miocene uplift event along the iceland faeroe ridge a second uplift event along the greenland scotland ridge and a decreased production of northern component water in the early pliocene may be responsible for increased sediment accumulation in bjorn gardar and eirik drifts

correlation of paleointensity variation records in the brunhes matuyama polarity transition interval abstract paleomagnetic records of the 0 78 ma brunhes matuyama b m polarity reversal interval described from high resolution 8 11 cm kyr deep sea sediments from western equatorial pacific odp sites 767b and 769a show two successive and marked decreases in paleointensity of about 15 kyr wavelength each and a shift from full reversed to fuli normal polarity directions in only 2 kyr associated with the uppermost paleointensity decrease in contrast transition records from deep sea sediments with lower sedimentation rates tend to show simpler paleointensity variation with more complex patterns of directional change for example the b m transition record described from deep sea sediment core v16 58 from the southern indian ocean we suggest that the twin 10 be peaks reported in v16 58 correspond to increased production of this cosmogenic isotope associated with the double paleointensity dips recorded in odp 767b 769a the low resolution v16 58 paleomagnetic record can be accounted for by modeling of the geomagnetic field history in odp 767b at a nominal recording rate of 1 cm kyr and using an exponential lock in process filter with a half fixing depth of 16 cm the double paleointensity dips become a single broad magnetization low and the change in paleomagnetic directions is offset by about 16 cm below the actual level of the polarity transition similar to what we infer for v16 58 the low resultant magnetizations will be especially prone to overprinting which if in the present field will tend to give an americas vgp path the main features of the 10 be profile will be unaffected by the remanence lock in process and consequently retain their usefulness for correlation as a proxy of geomagnetic dipole intensity variation

age and duration of the matuyama brunhes geomagnetic polarity reversal from 40ar39ar incremental heating analyses of lavas abstract constraints on the timing of geomagnetic polarity reversals have come mainly from k ar or more recently 40 ar 39 ar age determinations of lavas or their k rich phenocrysts that erupted prior or subsequent to particular geomagnetic events we have obtained 40 ar 39 ar isochron ages using incremental heating techniques on groundmass separates phenocryst poor whole rock samples or plagioclase from eight basaltic to andesitic lavas that erupted during the matuyama brunhes m b polarity transition at four geographically dispersed sites these eight lavas range from 784 6 7 1 ka to 770 8 5 2 ka 1 errors the weighted mean 778 7 1 9 ka gives a high precision age that is remarkably consistent with revised astronomical age estimates for the m b polarity transition 6 12 13 despite uncertainties in absolute calibration of 40 ar 39 ar ages relative to mineral standards used as neutron fluence monitors our age determinations are consistent with five other 40 ar 39 ar studies focused on the m b transition these results confirm that the earlier k ar based geomagnetic polarity time scale underestimated the age of the m b reversal by about 6 none of the eight isochron ages are distinguishable from one another at the 95 confidence level however we are tantalizingly close to testing for the duration of this reversal one lava at the base of a sequence of transitionally magnetized flows in chile and the uppermost lava in a similar sequence on maui are only just indistinguishable in age at the 95 confidence level and preserve different magnetic orientations we suggest that the 12 kyr difference in age represents an upper limit for the duration of the reversal and is similar to the period of low magnetic field intensity associated with records of the m b reversal from deep sea sediment cores together with the short duration 2 kyr of the directional reversal observed in several different marine sediment sections our data suggest that reversal of the field s direction could have occurred at slightly different times depending on the position of the recording site

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

millennial scale iceberg surges after intensification of northern hemisphere glaciation iceberg discharges from continental ice sheets are widely believed to have exerted a great influence on global climate but an iceberg discharge regime in early glacial periods after intensification of northern hemisphere glaciation nhg remains largely unclear here we present high resolution rock magnetic records during the period from 2 1 to 2 75 ma after intensification of nhg reconstructed from the subpolar north atlantic although the establishment of the middle pliocene chronology of north atlantic sediments is often a serious problem we overcame it based on findings concerning the properties of magnetic susceptibility and natural gamma radiation reconstructed rock magnetic records indicate that millennial scale iceberg surges were dominant features in the early glacial periods additionally the millennial scale iceberg surges occurred within glacial stages during intervals when ratios of global oxygen isotope stack from benthic foraminifera lr04 18o stack surpassed approximately 3 5 these are comparable to the climatic and environmental changes in pleistocene glacial periods as represented by last glacial dansgaard oeschger cycles suggesting that continental ice sheets have oscillated and calved icebergs in a similar manner since intensification of nhg

diffuse spectral reflectance as a proxy for percent carbonate content in north atlantic sediments diffuse reflectance records from fern drift in the north atlantic faithfully record sediment percent carbonate a high resolu tion reflectance based age model for these sediments derived from an orbitally tuned age model for western equatorial atlantic ceara rise sediments was generated by spectral frequency mapping power spectra of the fern drift record indicate statistically significant sub milankovitch cyclicity at 7 6 8 4 and 4 8 6 1 kyr we infer that these 8 and 5 kyr cycles document a linkage between north and equa torial atlantic climate given our ability to correlate these records these climate cycles influence atlantic basin carbonate prior to the intensification of northern hemisphere glaciation and thus must arise from some portion of the climate system other than the dynamics of large ice sheets the presence of these peaks which could be related to equatorial clipped precession implies a possible non linear response to milankovitch forcing abstract diffuse reflectance records from feni drift in the north atlantic faithfully record sediment percent carbonate a high resolu tion reflectance based age model for these sediments derived from an orbitally tuned age model for western equatorial atlantic ceara rise sediments was generated by spectral frequency mapping power spectra of the feni drift record indicate statistically significant sub milankovitch cyclicity at 7 6 8 4 and 4 8 6 1 kyr we infer that these 8 and 5 kyr cycles document a linkage between north and equa torial atlantic climate given our ability to correlate these records these climate cycles influence atlantic basin carbonate prior to the intensification of northern hemisphere glaciation and thus must arise from some portion of the climate system other than the dynamics of large ice sheets the presence of these peaks which could be related to equatorial clipped precession implies a possible non linear response to milankovitch forcing

searching for single domain magnetite in the pseudo single domain sedimentary haystack implications of biogenic magnetite preservation for sediment magnetism and relative paleointensity determinations 1 magnetic hysteresis measurements of sediments have resulted in widespread reporting of pseudo single domain like magnetic properties in contrast the ideal single domain sd properties that would be expected to be responsible for high quality paleomagnetic records are rare determining whether sd particles are rare or common in sediments requires application of techniques that enable discrimination among different magnetic components in a sediment we apply a range of such techniques and find that sd particles are much more common than has been reported in the literature and that magnetite magnetofossils the inorganic remains of magnetotactic bacteria are widely preserved at depth in a range of sediment types including biogenic pelagic carbonates lacustrine and marine clays and possibly even in glaci marine sediments thus instead of being rarely preserved in the geological record we find that magnetofossils are widespread this observation has important implications for our understanding of how sediments become magnetized and highlights the need to develop a more robust basis for understanding how biogenic magnetite contributes to the magnetization of sediments magnetofossils also have grain sizes that are substantially smaller than the 1 15 m size range for which there is reasonable empirical support for relative paleointensity studies the different magnetic response of coexisting fine biogenic and coarser lithogenic particles is likely to complicate relative paleointensity studies this issue needs much closer attention despite the fact that sediments have been subjected to paleomagnetic investigation for over 60 years much remains to be understood about how they become magnetized

high resolution 10be and paleomagnetic recording of the last polarity reversal in the chiba composite section age and dynamics of the matuyama brunhes transition we present new magnetic direction and relative paleointensity and beryllium isotope 10 be 9 be and 10 be 9 be ratio results covering the last geomagnetic reversal i e the matuyama brunhes transition mbt from the chiba composite section cbcs east central japan the very high sedimentation rates 90 cm ka of the studied site a candidate site for the global boundary stratotype section and point gssp of the lower middle pleistocene boundary allow the acquisition of a dataset of exceptional resolution coupled cosmogenic 10 be and magnetic results measured on the same samples demonstrate that the magnetization acquisition conserved the timing of rapid geomagnetic features allowing accurate paleomagnetic interpretations a polarity switch ps capturing most of the angular deviation occurring between 771 9 and 773 9 ka was followed by a directional instability rebound ic1 until 768 5 ka this sequence with 5 4 ka duration was characterized by a weak dipole field as low as 2 3 0 3 10 22 am 2 four rapid episodes of directional instabilities preceding and following the ps ic1 phase completed the reversal sequence the asymmetry observed between the long term dipole decay and sharp recovery and rapid oscillations 1 ka demonstrate the complex transitional field behavior during dipole moment low before reestablishment of the full polarity state our observations reinforce the fact that most reversal records do not integrate the full field behavior associated with the geodynamo action although this poses problems for understanding the underlying physical processes that produce reversals it does not hamper stratigraphic correlations among most geologic records

threshold behavior of millennial scale variability in deep water hydrography inferred from a 1 1 ma long record of sediment provenance at the southern gardar drift 1 integrated ocean drilling program iodp site u1314 of the north atlantic is a critical sedimentary archive record of subpolar deep water from the southern gardar drift for which we derived an age model of orbital resolution for the last 1 8 ma this chronology combined with high resolution cm scale x ray fluorescence core scanning measurements of major elements allows tracking changes in terrigenous provenance during the last 1 1 ma low potassium to titanium k ti ratios reflect enhanced transport of basalt derived titanomagnetites during warm climate intervals while high k ti ratios indicate a dominance of acidic sediment sources typical for glacial and stadial events changes in k ti and magnetic concentration at site 1314 are coeval with fluctuations in smectite content and grain size data from nearby piston cores suggesting that the provenance changes are mainly controlled by variable flow of the iceland scotland overflow water an important branch of north atlantic deep water furthermore k ti variations on orbital time scales show a striking similarity to the deep sea 13c record from odp site 607 pervasive features of the k ti time series during and after the mid pleistocene transition are suborbital changes similar to dansgaard oeschger and bond oscillations that appear to be strongly amplified during ice growth phases when global benthic 18o was within the range of 4 1 4 6 per mil the strong increase in variability of sediment provenance and subsequently deep hydrography at benthic 18o values below 4 1 suggests that the extent of glaciations and therefore sea level corresponding to this value constitutes an important physical threshold that was persistent at least for the last 1 1 ma

relative geomagnetic paleointensity across the jaramillo subchron and the matuyama brunhes boundary analysis of old paleomagnetic data and new mineral magnetic data from a sediment core from the central equatorial pacific ocean indicates that this core can be used for determination of relative geomagnetic paleointensity these data indicate that during the jaramillo subchron there was a gradual decrease in paleointensity from the onset of the subchron to its termination the data from the subchron also display high amplitude fine scale features that may be coherent over the scale of an ocean basin furthermore there was a rapid rise in paleointensity immediately after the termination of the matuyama brunhes polarity transition these observations support the hypothesis that an asymmetrical saw tooth pattern in paleointensity may be a common characteristic of polarity transitions however a better understanding of the high amplitude features is needed before definitive conclusions can be drawn about the overall behavior of the field the coherence of fine scale features across such large distances does not support the suggestion that the asymmetrical sawtooth pattern is an artifact of viscous remanent remagnetization

relative paleointensity of the geomagnetic field during brunhes chron recorded in north pacific deep sea sediment cores orbital influence abstract relative paleointensity records of the geomagnetic field during the brunhes chron were obtained from two sediment cores in the north pacific near the hess rise the cores taken below the carbonate compensation depth ccd showed an oxidized environment and accompany little paleoclimatically induced lithological changes homogeneity of magnetic mineralogy and magnetic grain size which are prerequisites for sedimentary paleointensity were thoroughly examined using various techniques including low temperature magnetometry room temperature hysteresis parameters and s ratio isothermal remanent magnetization irm was used as a normalizer of the natural remanent magnetization nrm the appropriateness of the normalization was checked by the absence of correlation between the normalized intensity nrm irm and the normalizer irm the relative paleointensity records obtained are generally similar to the datasets from the east and central equatorial pacific spectral analysis showed significant power at the orbital eccentricity 100 kyr frequency in the relative intensity record but not for the normalizer this suggests orbital influence on the geomagnetic field intensity but the possibility of paleoclimatic contamination through rock magnetic changes should be examined further using cores from different sedimentary regime

variable remanence acquisition efficiency in sediments containing biogenic and detrital magnetites implications for relative paleointensity signal recording widespread geological preservation of biogenic magnetite makes it important to assess how such particles contribute to sedimentary paleomagnetic signals we studied a sediment core from the south china sea that passes the strict empirical criteria for magnetic uniformity used in relative paleointensity studies such assessments are based routinely on bulk magnetic parameters that often fail to enable identification of mixed magnetic mineral assemblages using techniques that enable component specific magnetic mineral identification we find that biogenic and detrital magnetites occur in approximately equal concentrations within the studied sediments we analyzed normalized remanence signals associated with the two magnetite components to assess whether co occurring biogenic and detrital magnetites record geomagnetic information in the same way and with the same efficiency paleomagnetic directions for the two components have no phase lag which suggests that the biogenic and detrital magnetites acquired their magnetizations at equivalent times however we find that the biogenic magnetite is generally 2 4 times more efficient as the detrital magnetite in contributing to the natural remanent magnetization nrm despite their approximately equal magnetic contributions variations in the concentration and efficiency of remanence acquisition of the two components suggest that a significant part of the nrm is controlled by nongeomagnetic factors that will affect relative paleointensity recording we recommend that methods suited to the detection of variable recording efficiency associated with biogenic and detrital magnetites should be used on a routine basis in relative paleointensity studies

rapid changes in the mechanism of ocean convection during the last glacial period high amplitude rapid climate fluctuations are common features of glacial times the prominent changes in air temperature recorded in the greenland ice cores1 2 are coherent with shifts in the magnitude of the northward heat flux carried by the north atlantic surface ocean3 4 changes in the ocean s thermohaline circulation are a key component in many explanations of this climate flickering5 here we use stable isotope and other sedimentological data to reveal specific oceanic reorganizations during these rapid climate change events deep water was generated more or less continuously in the nordic seas during the latter part of the last glacial period 60 to 10 thousand years ago but by two different mechanisms the deep water formation occurred by convection in the open ocean during warmer periods interstadials but during colder phases stadials a freshening of the surface ocean reduced or stopped open ocean convection and deep water formation was instead driven by brine release during sea ice freezing these shifting magnitudes and modes nested within the overall continuity of deep water formation were probably important for the structuring and rapidity of the prevailing climate changes

paleomagnetic secular variation study of ar ar dated lavas flows from tacambaro area central mexico possible evidence of intra jaramillo geomagnetic excursion in volcanic rocks more than 350 oriented paleomagnetic cores were obtained for rock magnetic and paleomagnetic analysis from radiometrically dated 40 ar 39 ar magmatic rocks occurring in the southern segment jorullo and tacambaro areas of the michoacan guanajuato volcanic field in the trans mexican volcanic belt most of the lavas 37 stem from monogenetic volcanoes dated at less than 4 ma two additional sites were sampled from the plutonic basement dated at 33 30 ma primary remanences carried by low ti titanomagnetites allowed to determining 34 reliable site mean directions of mostly normal 27 but also reversed 7 polarities the mean directions of these two populations are antipodal and suggest neither major vertical axis rotations with respect to the north america craton nor tilting in the region for the last 4 ma rotation and flattening of the inclination parameters being less than 5 9 3 8 and 0 1 3 9 respectively the corresponding paleomagnetic pole obtained for pliocene pleistocene times is plat 83 4 plon 2 4 n 32 a95 2 7 virtual geomagnetic poles also contribute to the time averaged field global database and to the paleosecular variation psv investigations at low latitudes from lavas for the last 5 ma showing a geomagnetic dispersion value that is in agreement with available psv models when comparing the magnetic polarities and corresponding radiometric ages of the studied sites with the cenozoic geomagnetic polarity time scale gpts a good correlation is observable this finding underscores the suitability of data obtained on lavas in central mexico for contributing to the gpts furthermore the detection of short lived geomagnetic features seems possible since the possible evidence of intra jaramillo geomagnetic excursion could be documented for the first time in these volcanic rocks

relative geomagnetic paleointensity and δ18o at odp site 983 gardar drift north atlantic since 350 ka abstract at odp site 983 relative geomagnetic paleointensity and planktic and benthic 18o records have been acquired for the last 350 kyr the mean sedimentation rate in this interval is 11 3 cm kyr magnetic properties and hysteresis ratios indicate that pseudo single domain magnetite is the remanence carrier volume susceptibility k anhysteretic arm and isothermal irm remanence values vary by a factor of 3 4 well within the criteria usually cited for paleointensity studies natural remanent magnetization nrm is normalized by arm and irm to acquire the paleointensity proxy arithmetic means of nrm arm and nrm irm calculated for five demagnetization steps in the 25 45 mt range constitute the relative paleointensity estimates some paleointensity lows particularly those at 40 120 and 188 ka are associated with directional excursions of the field especially the event at 188 ka referred to here as the iceland basin event that constitutes a short lived polarity reversal for the last 200 kyr the records can be correlated with other high resolution paleointensity records such as those from the labrador sea mediterranean somali basin and sulu sea implying that the millennial scale features are globally synchronous a labeling system for paleointensity features is proposed that ties prominent highs and lows to oxygen isotope stages

magnetic paleointensity stratigraphy and high resolution quaternary geochronology successes and future challenges abstract magnetic paleointensity stratigraphy is used to detect variations in the strength of earth s ancient magnetic field paleointensity studies have demonstrated that a dominantly dipolar geomagnetic signal can be recorded in a globally coherent manner in different types of sediments and in non sedimentary archives including ice core records and marine magnetic anomaly profiles the dominantly dipolar nature of geomagnetic paleointensity variations provides a global geophysical signal that has come to be widely used to date quaternary sediments despite the many successful applications of paleointensity assisted chronology the mechanisms by which sediments become magnetized remain poorly understood and there is no satisfactory theoretical foundation for paleointensity estimation in this paper we outline past successes of sedimentary paleointensity analysis as well as remaining challenges that need to be addressed to place such work on a more secure theoretical and empirical foundation we illustrate how common concepts for explaining sedimentary remanence acquisition can give rise to centennial to millennial offsets between paleomagnetic and other signals which is a key limitation for using paleointensity signals for geochronology our approach is intended to help non specialists to better understand the legitimate uses and limitations of paleointensity stratigraphy while pointing to outstanding problems that require concerted specialist efforts to resolve

paleoenvironmental and climatostratigraphic insights for marine isotope stage 19 pleistocene at the montalbano jonico succession south italy abstract quantitative analyses on pollen and ostracods were performed on the montalbano jonico succession mjs through marine isotope stages mis 18 21 in order to acquire details of paleoenvironmental changes and climate pattern during mis 19 which includes the matuyama brunhes paleomagnetic boundary mbb while the mjs does not record the mbb which is the main criterion for defining the early middle pleistocene boundary it is a candidate global boundary stratotype section and point gssp for the ionian stage and middle pleistocene subseries because of its excellent exposure continuous deposition and astronomically tuned record between mis 37 and mis 16 the new 40ar 39ar age of 773 9 1 3 ka for volcaniclastic layer v4 allows the age model for the succession to be refined ostracod assemblages show significant paleodepth fluctuations at the glacial interglacial scale in agreement with the pollen distality index pinus vs caryophyllaceae amaranthaceae and ephedra which also suggests higher values during mis 19 pollen analysis provides supplementary paleoclimate data that are valuable to reconstruct temperature variation during the identified phases a major expansion of steppic vegetation at 790 ka in the uppermost mis 20 correlates with a significant cold and arid northern hemisphere climate phase as documented by a concurrent peak both in north atlantic ice rafted debris and mediterranean dust records climate conditions recorded in mis 19 3 were similar to those of the present interglacial whereas a dry climate phase marks mis 19 2 at 771 84 ka slightly above the v4 layer millennial scale climate variability occurred during mis 19 as evidenced by two abrupt short term moderately dry pollen events at 783 5 ka and 774 8 ka that could be related to a period of reduced geomagnetic field intensity through the mbb transition additional environmental chronological events accompanied this stratigraphic interval i the volcaniclastic layers v3 and v4 radiometrically dated at 801 2 19 5 ka and 773 9 1 3 ka respectively ii the co occurrence of maximum flooding and thermal maximum in agreement with the previously recorded co occurrence of the neopycnodonte community and tropical subtropical mesopelagic teleostean bonapartia pedaliota close to the increase of quercus and iii the maximum depth md between 777 3 and 773 25 ka as recorded by outer shelf upper slope macroinvertebrate fauna close to the highest value of pollen distality index in mis 19 results enhance the understanding of orbital suborbital paleoenvironmental change during the mid pleistocene and particularly of the climate pattern during mis 19 and across the mbb

asynchronous records of brunhes matuyama reversal in marine sediments and chinese loess review and discussion abstract geomagnetic polarity reversals are generally considered to occur synchronously around the world and are commonly used as time markers to correlate events between different sediment archives they also play an indispensable role for understanding the mechanisms which generate the earth s geomagnetic field nevertheless in the case of the most recent reversal the brunhes matuyama b m reversal 780 ka comparison of paleomagnetic studies in chinese loess paleosol sequences versus marine cores reveal a marked discrepancy in timing of this event this timing offset has led to uncertainties in correlating the chinese loess paleosol sequences with other global climate records this paper briefly reviews the history of study on the b m reversal both in marine and chinese loess sediments and reviews several contentious explanations of the aforementioned b m boundary offset finally the most recent progress of paleogeomagnetic field variation studies using cosmogenic 10 be in chinese loess is introduced with the purpose of suggesting a potential way to address this b m boundary timing offset issue

deciphering records of geomagnetic reversals polarity reversals of the geomagnetic field are a major feature of the earth s dynamo questions remain regarding the dynamical processes that give rise to reversals and the properties of the geomagnetic field during a polarity transition a large number of paleomagnetic reversal records have been acquired during the past 50 years in order to better constrain the structure and geometry of the transitional field in addition over the past two decades numerical dynamo simulations have also provided insights into the reversal mechanism yet despite the large paleomagnetic database controversial interpretations of records of the transitional field persist they result from two characteristics inherent to all reversals both of which are detrimental to an ambiguous analysis on the one hand the reversal process is rapid and requires adequate temporal resolution on the other hand weak field intensities during a reversal can affect the fidelity of magnetic recording in sedimentary records this paper is aimed at reviewing critically the main reversal features derived from paleomagnetic records and at analyzing some of these features in light of numerical simulations we discuss in detail the fidelity of the signal extracted from paleomagnetic records and pay special attention to their resolution with respect to the timing and mechanisms involved in the magnetization process records from marine sediments dominate the database they give rise to transitional field models that often lead to overinterpret the data consequently we attempt to separate robust results and their subsequent interpretations from those that do not stand on a strong observational footing finally we discuss new avenues that should favor progress to better characterize and understand transitional field behavior

benthic δ18o records in the north atlantic over the last glacial period 60 10 kyr evidence for brine formation four high resolution benthic isotope records from north atlantic and south norwegian sea cores are compared for the past 60 kyr the benthic records show light 18o peaks simultaneously to or following the heinrich events the amplitude of these light peaks varies between 1 for the most northern core enam93 21 and 0 35 for the deep north atlantic cores the high 18o gradient between the north atlantic cores and the pacific core v19 30 suggests a regional influence of 18o depleted meltwater to deeper depth during periods of iceberg discharges the most likely process for transferring 18o depleted meltwater to depth is sea ice formation induced by brine formation during heinrich events 4 and 3 this mecanism was of major importance compared to open ocean convection in the southern norwegian sea and at the northern boundaries of the atlantic leading to intermediate to deep water formation other 18o excursions appearing in all the records point to the contribution of both brine formation and global changes in the 18o of seawater associated with the heinrich event periods

geomagnetic palaeointensities and astrochronological ages for the matuyama brunhes boundary and the boundaries of the jaramillo subchron palaeomagnetic and oxygen isotope records from odp site 983 we have measured relative geomagnetic palaeointensity proxies palaeomagnetic directions and 18 o for the 700 1100 ka interval from odp site 983 gardar drift north atlantic where mean sedimentation rates are ca 13 cm kyr 1 the age model was generated by matching the benthic 18 o data to the ice volume model and confirmed by tuning the precessional components of both signals for the matuyama brunhes boundary mbb and the boundaries of the jaramillo subchronozone the duration of the polarity reversal process defined by virtual geomagnetic polar latitudes of less than 45 is ca 5 kyr whereas the generally accepted astrochronological estimates for the boundaries of the jaramillo subchronozone lie within the polarity transitions as recorded at site 983 the astrochronological age for the matuyama brunhes polarity transition 780 ka is ca 5 kyr older than the onset of this transition at site 983 775 ka the polarity reversals lie within palaeointensity lows with abrupt recovery of palaeointensity post reversal there is no progressive sawtooth decrease in palaeointensity within the jaramillo subchronozone or between the top of the jaramillo and the mbb but rather within polarity chrons several short intervals of low palaeointensity which sometimes coincide with high amplitude secular variation orbital 100 and 41 kyr periods are present in the palaeointensity record as they are not obviously attributable to climate lithology in these records they may be a feature of the geomagnetic field itself

seven thousand year duration for a geomagnetic excursion constrained by 230thxs 1 the duration of geomagnetic excursions may provide important information about the geodynamo it has proven difficult however to assess the exact duration of both excursions and reversals we combine high resolution paleomagnetic records from marine sediments with measurements of the 230thxs flux to the seafloor in order to better constrain the duration of the iceland basin excursion ibe we find the duration of the ibe to be 7 8 kyr implying that it is considerably longer than previous estimates for the laschamp excursion 2 kyr and more similar in duration to that of full polarity reversals there may not be therefore a characteristic timescale for transitional field configurations that distinguishes excursions from reversals

millennial scale climate instability during the early pleistocene epoch climate proxy records of the past 100 000 years show that the earth s climate has varied significantly and continuously on timescales as short as a few thousand years 1 7 similar variability has also recently been observed for the interval 340 500 thousand years ago8 these dramatic climate shifts expressed most strongly in the north atlantic region may be linked to and possibly amplified by alterations in the mode of ocean thermohaline circulation4 5 6 7 8 9 here we use sediment records of past iceberg discharge and deep water chemistry to show that such millennial scale oscillations in climate occurred over one million years ago this was a time of significantly different climate boundary conditions not only was the early pleistocene epoch generally warmer but global climate variations were governed largely by changes in earth s orbital obliguity our results suggest that such millennial scale climate instability may be a pervasive and long term characteristic of earth s climate rather than just a feature of the strong glacial interglacial cycles of the past 800 000 years

paleointensity variations across the last geomagnetic reversal at la palma canary islands spain a detailed volcanic record of absolute paleointensity has been obtained for the period encompassing the last geomagnetic reversal from a succession of 69 distinct lava flows in two adjacent sections on the island of la palma the correlation between the two sections relies on the positions of the brunhes matuyama boundary and on the flow chronology obtained by a refined potassium argon k ar dating technique several pretransitional flows exhibit very shallow paleomagetic inclinations but they cannot be considered as really transitional paleointensity experiments conducted on 288 specimens provided 168 successful determinations 14 were obtained after correction for 50 lava flows that were considered to represent 45 distinct units the evolution of the paleofield intensity can be constructed by assuming uniform extrusion rates between three dated lava flows the period preceding the transition is characterized by large fluctuations with typical peak to peak amplitudes between 20 and 40 t which decrease down to 7 t prior to the transition the 100 kyr long interval following the reversal is associated with field intensities as high as 60 t and depict a strong and rapid restoration of the dipole field the virtual dipole moments vdms have been compared with sedimentary vdms generated from the record of relative paleointensity from ocean drilling program site 851 to which have been added non dipole components similar to the present geomagnetic field a reasonable fit can be made between the two curves which implies variations in the eruption rates but not incompatible with the uncertainties in the k ar age determinations thus these results can support but do not fully demonstrate the existence of asymmerical saw toothed variations in field intensity

saw toothed variations of relative paleointensity and cumulative viscous remanence testing the records and the model a recent model kok and tauxe 1996a that attempts to explain asymmetrical sawtooth patterns of relative paleointensity from sediments valet and meynadier 1993 as the consequence of cumulative long term viscosity effects has been tested theoretically and experimentally a mathematical description of the model demonstrates that only a sharp distribution of relaxation times would be able to match the paleomagnetic record which implies that all the magnetic grains would be identical this has direct consequences for the distribution of the blocking temperatures tb which would not exceed 260 c and for the coercivities of the natural remanent magnetization nrm this tb spectrum derived from the model is inconsistent with the unblocking temperatures of the nrm and with the tb spectrum of the experimental thermoremanent magnetization trm of the sediment that recorded the sawtooth new results including thermal demagnetization and thellier thellier experiments across brunhes matuyama confirm the overall stability of the original records the stability of the older part of the record 2 to 4 ma has also been strengthed the sawtooth is unchanged between 20 mt and 65 mt and thermal demagnetization in a neutral atmosphere at 300 c and 450 c duplicates the results obtained by alternation field a f demagnetization thus viscous processes at least within the framework of a cumulative viscous model and for the existing data sets are not responsible for saw toothed paleointensity records

mudwaves on the gardar sediment drift ne atlantic small amplitude 15 m mudwaves were mapped on the gardar sediment drift using the hydrosweep ds multibeam echosounder system on the r v maurice ewing improved data processing techniques allowed the swath mapped bathymetry to image features smaller than the nominal resolution 14 m of hydrosweep mudwave heights wavelengths and orientations were determined from the hydrosweep data and in conjunction with 3 5 khz profiles these mudwave fields were analyzed with regard to present models of mudwave formation mudwave crest orientation was clockwise to mean bottom flow direction and migration was to the right of measured currents for two areas on the drift this is consistent with models developed on large amplitude mudwaves using mudwave orientation and migration direction in a third area bottom current directions were inferred

relative geomagnetic field intensity and reversals for the last 1 8 my from a central equatorial pacific core we have made a new analysis of core kk78o30 from the central equatorial pacific this core has recorded the last 4 reversals mineral magnetic analysis establishes that these sediments meet the current criteria for reliable relative paleointensity determinations normalisation with arm irm and yields very similar results there is no evidence for a saw tooth pattern such as the one obtained by valet and meynadier 1993 and meynadier et al 1994 for the last 4 million years rather the average field intensity record appears to display more or less symmetric peaks and troughs superposed to a broad high between reversals

millennial to submillennial scale features of the matuyama brunhes geomagnetic polarity transition from osaka bay southwestern japan 1 a high resolution magnetic record of the matuyama brunhes mb reversal transition was obtained from homogeneous marine clay in a 1700 m core from osaka bay the transition stretches over a core length of 6 4 m and yields four short reversal episodes two predate the main mb boundary mbb and two postdate it we made diatom analyses of sediments to estimate relative sea level changes and constructed an age model by correlation with the astronomically calibrated marine oxygen isotope curve the age model shows the short reversal episodes with durations of 1 kyr occurred at 4 5 kyr intervals within stage 19 and the mbb occurred about 5 kyr after the sea level highstand corresponding to isotopic event 19 3 it also shows the paleointensity minimum of 15 20 of the postreversal mean occurred at about 6 kyr before the mbb and another minimum at about 1 kyr after four virtual reverse to normal polarity boundaries found in this study suggest different ages for the mbb could be observed in lava flow studies some millennial to submillennial scale transition features found in this study are compatible with the previous numerical simulations of geomagnetic field reversal

a detailed paleomagnetic record between 2 1 and 2 75 ma at iodp site u1314 in the north atlantic geomagnetic excursions and the gauss matuyama transition 1 this study investigated the detailed geomagnetic field variation between 2 1 and 2 75 ma from a sediment core iodp site u1314 with high sedimentation rate 10 cm kyr and good age control characteristic remanent magnetization directions were well resolved by stepwise alternating field demagnetization as a proxy of relative paleointensity natural remanent magnetization nrm normalized by anhysteretic remanent magnetization arm was used after testing that the influence of magnetic interaction in arm is negligible as a result the following features of the geomagnetic field in the studied period have been revealed during the transition of the gauss matuyama g m reversal and the reunion subchron the paleointensity decreased to the value lower than 20 of the average intensity in the whole studied interval in addition to these lows eight paleointensity lows were found associated with large directional changes that satisfy the definition of a geomagnetic excursion four of these have ages close to ages reported for geomagnetic excursions in prior studies whereas the other four excursions have not previously been observed in our results we confirm that the g m transition occurred in marine isotope stage 103 even if we consider the shift in depth due to the lock in process of magnetic particles the temporal variation in paleointensity showed asymmetric behavior associated with the g m transition with a gradual decrease prior to the transition and a rapid recovery after the transition

saw toothed pattern of sedimentary paleointensity records explained by cumulative viscous remanence abstract the relative paleointensity of the earth s magnetic field from odp site 851 has been characterized by progressive decay towards polarity reversals followed by sharp recovery of pre reversal values 1 we resampled the gilbert gau reversal boundary of this deep sea core and show that during demagnetization this saw toothed pattern disappears further the recently published cumulative viscous remanence model 2 using the herewith obtained paleointensity record and constraints from thermal treatment replicates the saw tooth of 1 implying that it is of non geomagnetic origin