stacking paleointensity and oxygen isotope data for the last 1 5 myr piso 1500 coupled relative geomagnetic paleointensity rpi and oxygen isotope records are used to construct rpi and oxygen isotope stacks for the last 1 5 myr the coupled correlations are accomplished using the match algorithm lisiecki l e and lisiecki p a 2002 application of dynamic programming to the correlation of paleoclimate records paleoceanography 17 1049 doi 10 1029 2001pa000733 to simultaneously correlate isotope and rpi records the simultaneous match reduces the degree of freedom associated with correlations using rpi or oxygen isotope records alone the overall compatibility of rpi and oxygen isotopes indicates a dominant global but independent component in both signals the local wavelet power spectrum lwps for the rpi stack indicates little significant orbital power although the accompanying oxygen isotope stack has all the lwps characteristics expected of a high resolution oxygen isotope record containing orbital frequencies the piso 1500 stack represents a new stratigraphic template that can be used to correlate among marine sediment records and link them to polar ice cores via variations in cosmogenic nuclide production scaling the stack to values for virtual axial dipole moment vadm indicate maxima at 15 1022 am2 and minima that imply a threshold of 2 5 1022 am2 below which values are associated with either polarity reversals or magnetic excursions

10be signature of the matuyama brunhes transition from the heqing paleolake basin abstract we present a new 10be record spanning the matuyama brunhes m b reversal from the heqing paleolake basin in southern china it provides a robust proxy for past geomagnetic variations that can be compared with paleomagnetic records the m b transition is identified as a pronounced maximum of authigenic 10be 9be ratio between 768 6 and 778 5 ka that is consistent with all other available 10be proxy records from marine ice and loess archives however it is offset by approximately 0 60 cm depth from the magnetic signature of the polarity transition recorded in the same sediments we attribute this offset to the lock in effect of the remanence acquisition process which is similar to the phenomenon that exists in marine sediments we suggest that after eliminating the climatic hydrological signal the heqing sediments may be used as an archive for atmospheric 10be production rate changes the combination of magnetic remanence measurements and the cosmogenic 10be allows for a more precise geochronology of geomagnetic polarity reversals

10be evidence for delayed acquisition of remanent magnetization in marine sediments implication for a new age for the matuyama brunhes boundary abstract fluxes of the meteoric cosmogenic radionuclide 10 be vary with changes in the incoming cosmic rays modulated by geomagnetic field intensity variations the variability in the 10 be flux can be used to synchronize ice cores as well as marine sediments by comparison with the relative paleointensity variations of the geomagnetic field however lock in of the paleomagnetic signal at some depth below the sediment water interface in marine sediments through acquisition of a post depositional remanent magnetization pdrm adds uncertainty to synchronization despite the long history of such studies the magnitude of the pdrm lock in depth remains controversial in this article we present clear evidence for a downward offset of the paleointensity minimum relative to the 10 be flux anomaly at the matuyama brunhes m b geomagnetic polarity boundary which we interpret to result from a 15 cm pdrm lock in depth this lock in depth indicates that up to several tens of thousands years of age offset probably occurs when a paleomagnetic record is used for dating marine sediments and the age of the m b boundary should be revised to ca 10 kyr younger which is consistent with a younger ice core derived age of 770 6 ka 2 this cosmogenic age tuning strategy will contribute to refining paleomagnetic based age models for marine sediments and identifying of lead lag relationships for global abrupt environmental changes

geomagnetic cosmogenic and climatic changes across the last geomagnetic reversal from equatorial indian ocean sediments abstract high resolution records of beryllium 10be production and relative paleointensity have been obtained across the matuyama brunhes m b reversal from the equatorial indian ocean maldives area both magnetic and geochemical analyses were performed from the same discrete samples to avoid any artificial depth offset the authigenic 10be concentrations were normalized with respect to 9be in order to correct for potential environmental effects while the relative paleointensity was derived from the remanent magnetization intensity after accounting for changes in magnetic concentration within the sediment the relative paleointensity and the 10be 9be records are both characterized by large deviations which culminate in the middle of the reversal in contrast to most previous studies and despite relative high deposition rate 4 7 cm ka we observed a perfect synchronism between the 10be 9be peak the lowest value of relative paleointensity and the switch in direction which indicates that bioturbation and post depositional processes did not affect the magnetic record this leaves no ambiguity for the stratigraphic position of the reversal located within marine isotopic stage 19 as revealed by the planktonic 18o record from the same core the magnetic data depict a two phase process with a precursory event preceding the rapid polarity switch while only the second phase is present in the 10be record similarly to other low latitude records from the indonesian area using an orbitally tuned age model we obtain an age of 772 ka 5 ka for the middle of the transition while the precursory event occurred almost 20 ka before we believe that the bimodal distribution emerging from the compilations of the ages of the m b reversal results from the succession of these two events microtektites from the australasian impact were found at 0 6 m below the transition 790 ka 5 ka b p and confirm that this large event occurred 12 ka prior to the polarity transition the distribution of tektite abundance was used to deconvolve the 10be 9be signal the results confirm that the beryllium changes are concentrated during the transitional period thus likely in presence of a multipolar geomagnetic field or in the vicinity of a geomagnetic pole that favored the penetration of cosmic rays and consequently increased the 10be production the absence of 10be during the precursor indicates that the present site and the indonesian ones were far away from a geomagnetic pole and that interlatitudinal atmospheric mixing was limited the geomagnetic pole positions above the indonesian sites during the precursor would thus be incompatible with the corresponding inclined dipolar field during this period and suggest the dominance of low degree harmonics

paleointensity variation across the matuyama brunhes polarity transition observations from lavas at punaruu valley tahiti 1 we have conducted a paleointensity study of the matuyama brunhes m b polarity transition recorded in 34 successive lava flows of punaruu valley on tahiti a reversed polarity is obtained from the lower part of the record major directional changes are derived from the middle part of the record and a normal polarity is recorded in the upper part of the record these paleomagnetic directions and five 40ar 39ar ages yielding a weighted mean of 771 8 1 ka indicate that 30 lava flows recorded the geomagnetic field across the m b transition the 215 specimens from 32 flows were subjected to the double heating technique of the shaw method combined with low temperature demagnetization ltd dht shaw method yielding 73 successful results from 18 flows for the reversed polarity period just prior to the major directional changes paleointensity shows an oscillation like variation between 3 and 38 t corresponding to virtual dipole moments vdms between 0 9 1022 and 9 6 1022 am2 for the major directional changes a weak paleointensity of 5 t is obtained which gives a vdm of 1 0 1022 am2 for the normal polarity period paleointensities are 14 21 t giving vdms of 3 5 5 2 1022 am2 for the reversed polarity period just prior to the major directional changes a linear relationship with a correlation coefficient of 0 96 is recognized on the diagram of vdm versus virtual geomagnetic pole latitude this linear relationship may be a precursory feature of the geodynamo at the onset of the m b transition

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

dating crystalline groundmass separates of altered cretaceous seamount basalts by the 40ar 39ar incremental heating technique abstract alteration of submarine basalts compromises geochronology using conventional k ar and 40 ar 39 ar techniques to help overcome these problems we re evaluated the potential of groundmass dating techniques incremental heating on acid leached groundmass samples following an overnight bakeout at 200 c and using high resolution heating schedules eliminated most of the low temperature alteration effects in the submarine basalts studied more than 75 of the groundmass analyses n 32 display accurate age plateaus consisting of 30 70 of the total amount of 39 ar k released more than 50 of the analyses have plateau ages concordant with their total fusion ages implying minor or proportional loss of radiogenic 40 ar and 39 ar k overall we could show a high degree of coherence between ages of groundmass separates and comagmatic phenocrysts this suggests that the dating of aphyric basalts which previously has proven problematic can be accomplished with increasing confidence as well adding these rock types to the list of datable submarine basalts significantly enhances our ability to understand the eruptive history of linear submarine volcanic chains

evidence of a partitioned dynamo reversal process from paleomagnetic recordings in tahitian lavas 1 lavas erupted at the society hotspot during the matuyama brunhes m b polarity reversal record transitional field behavior containing two tight paleodirectional groups that when averaged are antipodal at the 95 confidence level and thus correlate to antipodal clustered virtual geomagnetic poles vgps the occurrence of these observations data obtained from two published records of the m b transition from distinct sections of a succession of flows on tahiti is associated with a time when the strength of the axial dipole was significantly reduced one of the clusters was recorded by lavas that were not erupted in succession suggesting that significant time had passed during this volcanic activity time spent during the formation of the antipodal cluster is unknown although it resides in the same location as vgp clusters associated with four other transitional events obtained from society hotspot lavas calculated vgps at the society hotspot for both polarities of the averaged historic field less the axial dipole term are found in the cluster locations these findings offer strong evidence for a two tiered dynamo process in which nearly the entire axial dipole component undergoes both demise and regeneration quasi independently from that of the remainder of the field the pattern of which is tied to long held physical conditions of the lower most mantle

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

identification of the short lived santa rosa geomagnetic excursion in lavas on floreana island galapagos by 40ar 39ar geochronology a set of closely related basaltic lava flows supersite ga x on floreana island in the galapagos archipelago has a published record of an excursional or transitional direction virtual geomagnetic pole located at 153 1 e 54 2 s with 95 5 0 and a geomagnetic field strength 1 1 10 22 am 2 that is only 14 of the strength of the modern magnetic field 7 8 10 22 am 2 the very large age uncertainty of previous dating of a lava flow g43 from this set however has prevented placing this event in the geomagnetic polarity time scale here we report highly reproducible and precise 40 ar 39 ar ages on the lava flow that indicate that the distinct geomagnetic excursion is 925 7 4 6 ka 2 n 6 mean square of weighted deviates 1 23 this shows that this dramatic weakening of the geomagnetic field is associated with the santa rosa excursion instead of the matuyama brunhes polarity reversal our high precision 40 ar 39 ar ages for floreana provide evidence for the global significance of the santa rosa excursion

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

new 40ar 39ar age progression for the louisville hot spot trail and implications for inter hot spot motion 1 in this study we present 42 new 40ar 39ar incremental heating age determinations that contribute to an updated age progression for the louisville seamount trail louisville is the south pacific counterpart to the hawaiian emperor seamount trail both trails representing intraplate volcanism over the same time interval 80 ma to present and being examples of primary hot spot lineaments our data provide evidence for an age progressive trend from 71 to 21 ma assuming fixed hot spots this makes possible a direct comparison to the hawaiian emperor age progression and the most recent absolute plate motion apm model wk08g of wessel and kroenke 2008 we observe that for the louisville seamount trail the measured ages are systematically older relative to both the wk08g model predictions and hawaiian seamount ages with offsets ranging up to 6 myr taking into account the uncertainty about the duration of eruption and magmatic succession at individual louisville volcanoes these age offsets should be considered minimum estimates as our sampling probably tended to recover the youngest lava flows these large deviations point to either a contribution of inter hot spot motion between the louisville and hawaiian hot spots or to a more easterly location of the louisville hot spot than the one inferred in the wk08g model both scenarios are investigated in this paper whereby the more eastern hot spot location 52 0 s 134 5 w versus 52 4 s 137 2 w reduces the average age offset but still results in a relatively large maximum offset of 3 7 myr when comparing the new ages to the apm models s04p s04g by steinberger et al 2004 that attempt to compensate for the motion of hot spots in the pacific hawaii or globally hawaii louisville reunion and walvis the measured and predicted ages are more in agreement showing only a maximum offset of 2 3 myr with respect to the s04g model at face value these more advanced apm models which consider both plate and hot spot motions therefore provide a better fit to the new louisville age data the fit is particularly good for seamounts younger than 50 ma a period for which there is little predicted motion for the louisville hot spot and little inter hot spot motion with hawaii however discrepancies in the louisville age distance record prior to 50 ma indicate there is an extra source of inter hot spot motion between louisville and the other pacific hot spots that was not corrected for in the global s04g model finally based on six new 40ar 39ar age dates the 169 w bend in the louisville seamount trail seems to have formed at least 3 myr before the formation of the hawaiian emperor bend the timing of the most acute parts of both bends thus appears to be asynchronous which would require other processes e g plume motions than a global plate motion change between 50 and 47 ma to explain these two observations

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

magnetic excursions in the late matuyama chron olduvai to matuyama brunhes boundary from north atlantic iodp sites the sedimentary record of late matuyama magnetic excursions between the end of the olduvai subchron and the matuyama brunhes boundary has relied on north atlantic odp sites 983 984 and individual recordings at odp site 1021 from the california margin and core md972143 from the west philippine sea north atlantic iodp sites u1305 u1306 u1307 eirik drift off se greenland and site u1308 central atlantic provide new records from u channel samples and discrete 8 cm3 cubic samples collected from multiple holes drilled at each site the results confirm the existence of the intra jaramillo punaruu bjorn gardar and gilsa excursions and possibly the kamikatsura excursion these excursions are not recorded at all sampled sites although they do correspond to excursions recorded at odp sites 983 984 and to relative paleointensity rpi minima recorded at all studied sites for some records including that of the bjorn excursion 1251 ka antipodal magnetization directions yield virtual geomagnetic poles vgps that reach high northern latitudes implying that the excursion involved reversal of the main axial dipole field on the other hand excursions are often partially and inadequately recorded attributable to a combination of the brevity of geomagnetic excursions bioturbation and filtering of the magnetization signal by the drm detrital remanent magnetization acquisition process non uniform sedimentation rates at millennial centennial scales and possibly unrecognized drilling sampling disturbance

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

post depositional remanent magnetization lock in depth in precisely dated varved sediments assessed by archaeomagnetic field models accurate and precise chronologies are needed to evaluate the existence and effect of a post depositional remanent magnetization lock in process on sedimentary palaeomagnetic records here we present lock in modelling results of two palaeomagnetic records from varved lake sediments lakes kalksjon and gyltigesjon in sweden by using model predictions based on archaeomagnetic data we used the c 14 wiggle match dating technique to improve the precision of the kalksjon varve chronology in the period between 3000 and 2000 cal bp which is characterized by pronounced palaeomagnetic secular variation this method allowed us to infer an age model with uncertainties of 20 years 95 4 probability range furthermore we compared the palaeomagnetic record of kalksjon to gyltigesjon which has a corresponding 14c wiggle matched chronology the ages of palaeomagnetic features derived from the wiggle matched varve chronologies are older than those predicted by the archaeomagnetic models lock in modelling was performed with different filter functions to explain the temporal offset and the amplitude of the lake sediment palaeomagnetic data the analyses suggest that a linear lock in function with lock in depths the depth below which no more natural magnetic remanence is acquired that range between 30 and 80 cm in kalksjon and 50 and 160 cm in gyltigesjon are most appropriate to explain the data these relatively deep lock in depths in sediments without a bioturbated mixed zone can be attributed to the relatively high organic contents and low density of the lake sediments which contribute to a thick unconsolidated upper zone of the sediment sequence in which re alignment of magnetic particles can take place c 2014 elsevier b v all rights reserved

regionally consistent western north america paleomagnetic directions from 15 to 35 ka assessing chronology and uncertainty with paleosecular variation psv stratigraphy abstract new paleomagnetic secular variation psv data from fish lake utah usa along with previously published regional records allow us to build an independently dated western north america psv stack wnam17 from about 35 to 15 ka that quantifies dating and paleomagnetic uncertainties in february 2014 we recovered a composite 11 m long sediment record from fish lake utah that retrieved glacially influenced and post glacial sediments computed tomography ct scans and magnetic susceptibility were used to build a precise depth scale while radiocarbon dating of terrestrial macrofossils and tephrostratigraphy provide tight age control glacially influenced sediments have well defined characteristic remanent magnetizations with all maximum angular deviation values

constraining in situ cosmogenic nuclide paleo production rates using sequential lava flows during a paleomagnetic field strength low abstract the geomagnetic field prevents a portion of incoming cosmic rays from reaching earth s atmosphere during magnetic reversals and excursions the field strength can decrease by up to 90 relative to the modern day value during such anomalies cosmic ray bombardment to earth s atmosphere increases as evident from atmospheric 10be anomalies recorded in sediment and ice cores however how the flux of cosmic rays to earth s surface varies during such geomagnetic anomalies is not well constrained we measured fossil cosmogenic 3he in olivine from the tops of two pairs of 40ar 39ar age dated tahitian lava flows that erupted during the matuyama brunhes reversal precursor event we corrected these raw values for the diffusive loss of helium caused by heating from the overlying flow with a diffusion model using cooling rates and maximum temperature conditions based on field measurements of active lava flows from kilauea hawaii we assume the maximum temperature suggested by field measurements and thus present a limiting case for the highest diffusive loss corrections and thus the highest paleo production rates based on paleomagnetic field strength reconstructions and scaling factor models the upper limits of the corrected in situ 3he paleo production rates 100 23 144 35 atoms g 1 a 1 are in agreement with those expected during the period of a geomagnetic field strength low when these flow tops were exposed however the more plausible contact temperatures

constraining the age of the last geomagnetic reversal from geochemical and magnetic analyses of atlantic indian and pacific ocean sediments we studied four marine sediment records of the matuyama brunhes geomagnetic reversal from four sites located in the indian atlantic and western pacific oceans the results combine paleomagnetic cosmogenic nuclide beryllium be 10 and oxygen isotope analyses that were performed on the same samples in order to avoid any stratigraphic bias the three records from the equatorial indian ocean and north atlantic ocean did not reveal any offset between the authigenic be 10 be 9 ratio be ratio peak and the interval of low relative paleointensity rpi that characterizes the reversal the lower and upper limits of transitional directions are also concomitant with the increased atmospheric be 10 production that accompanied the geomagnetic dipole intensity decrease in contrast the record from western equatorial pacific ocean sediments was found 18 cm below the be ratio changes as a result of late magnetization acquisition at all four sites maximum be 10 production occurred at the same period soon after the maximum of marine isotope stage 19 mis 19 and therefore indicates its synchronous worldwide character such features are effectively observed from the be ratio signals and their relationship with the magnetic transition interval which further confirms the synchronous character of the transition taking all dating uncertainties 1 6 ka between sites and 5 ka for the age model into consideration our records suggest a mean reversal age of 772 4 6 6 ka the age of the transition in the atlantic ocean record is closer to 774 ka but this difference is within the limit of significance

re evaluation of the ages of 40ar 39ar sanidine standards and supereruptions in the western u s using a noblesse multi collector mass spectrometer abstract the rewards from resolving the tempo of magmatic paleobiologic paleoclimatic and tectonic processes at better than the per mil level of precision have spurred a quest within the geochronology community to improve the precision accuracy and calibration of several chronometers including the 40 ar 39 ar variant of the k ar clock a new generation of multi collector mass spectrometers for 40 ar 39 ar geochronology provides the means to address these issues as well as to evaluate the ages of 40 ar 39 ar standards and key marker horizons deposited by large volume caldera forming supereruptions in the western u s at unprecedented temporal resolution we highlight the utility of a 5 collector noblesse mass spectrometer that has a sufficiently high mass resolving power to distinguish 36 ar h 35 cl from a 36 ar h 35 cl 12 c 3 sum peak at m e 36 maintains a remarkably low and stable background and thus yields 40 ar 39 ar dates of exceptionally high precision we have calibrated a reference gas and developed a simple analytical routine involving one peak hop both of which are used to evaluate detector bias correction factors because the 40 ar 39 ar chronometer is a relative dating technique that relies on calibration against mineral standards of known age precise and accurate determination of the ages of these standards and verification of isotopic composition and homogeneity is essential we present results from both total fusion and incremental heating experiments on single crystals of the widely used alder creek acs and fish canyon fcs sanidine standards several recent independent measurements of acs using multi collector mass spectrometers including those presented herein favor an age of 1 1864 0 0003 0 0012 ma 95 confidence analytical full external uncertainty that is 1 younger than the widely used value obtained by nomade et al 2005 using a single collector mass spectrometer incremental heating experiments on single crystals of fcs most commonly do not yield plateau ages instead many crystals appear to be affected by ar loss or excess ar our findings obtained using ion counters independently confirm those recently obtained using a multiple faraday detectors or a combination of faraday and ion counting detectors thus careful consideration must be given to using fcs in future high precision 40 ar 39 ar calibrations of the geologic timescale notwithstanding 40 ar 39 ar dating of sanidine using the 5 collector noblesse can now provide leverage critical to interpreting the large range of u pb zircon dates commonly found in rhyolitic lavas and tuffs

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

simulations of a time varying non dipole field during geomagnetic reversals and excursions abstract following brown et al 2007 reversals and excursions have been simulated by decreasing the axial dipole field to zero while leaving up a time varying non dipolar field similar to the cal7k model korte m constable c g 2005 continuous geomagnetic models for the past 7 millennia ii cals7k geochem geophys geosyst 6 2 doi 10 1029 2004gc000801 we also envisaged the situation in which the total field changes sign we first investigated the relationship between the non axial dipole structure of the historical and present field snapshots and the virtual geomagnetic poles vgps the accumulation of vgps in the vicinity of large flux concentrations in south atlantic and australia is constrained by the equatorial dipole only a few sites directly above the anomalies are associated with vgps in their vicinity therefore it seems delicate to associate recurring clusters of vgps during reversals with magnetic flux concentrations at the cmb the individual vgps resulting from simulations of reversals show all kinds of trajectories some being very simple with longitudinal confinement while others are complex with alternances of slow and fast changes as well as loopings of vgps preference for a specific longitude frequently happens in individual records but this is not systematic a direct consequence is that records of different reversals from a global array of sites are effectively dominated by preferred longitudinal bands thus a time varying non dipolar transitional field can generate anisotropy of the vgp distribution smearing of the signal by sedimentary records considerably amplifies the longitudinal confinement of the vgps overall these simulations account for most characteristics reported in detailed reversal records and thus strongly support the concept of a non dipolar transitional field we also simulated field excursions by assuming either a vanishing dipole followed by its return to initial polarity or by a short episode with the opposite polarity preceding its restoration the model accounts for the different characteristics inherent to the volcanic and sedimentary records of the recent excursions the simple structure of sedimentary vgp paths dominated by a large loop between northern and southern latitudes partly results from smearing of the signal therefore these records do not fully describe the complexity of the excursional field and remain exclusively sensitive to very low degree harmonics most detailed studies of excursions exhibit reversed directions which cannot exist without a very short dipolar episode of opposite polarity

matuyama brunhes reversal and kamikatsura event on maui paleomagnetic directions 40 ar 39 ar ages and implications abstract eighty nine basaltic lava flows from the northwest wall of haleakala caldera preserve a concatenated paleomagnetic record of portions of the matuyama brunhes m b reversal and the preceding kamikatsura event as well as secular variation of the full polarity reversed and normal geomagnetic field they provide the most detailed volcanic record to date of the m b transition the 24 flows in the transition zone show for the first time transitional virtual geomagnetic poles vgps that move from reverse to normal along the americas concluding with an oscillation in the pacific ocean to a cluster of vgps east of new zealand and back finally to stable polarity in the north polar region all but one of the 16 kamikatsura vgps cluster in central south america the full polarity flows with 40ar 39ar ages spanning a total of 680 kyr pass a reversal test and give an average vgp insignificantly different from the rotation axis with standard deviation consistent with that for other 0 5 ma lava flows of similar latitude precise 40ar 39ar dating consisting of 31 incremental heating experiments on 12 transitional flows yields weighted mean ages of 775 6 1 9 and 900 3 4 7 ka for the m b and kamikatsura transitional flows respectively this matuyama brunhes age is 16 kyr younger than ages for m b flows from the canary islands tahiti and chile that were dated using exactly the same techniques and standards suggesting that this polarity transition may have taken considerably longer to complete and been more complex than is generally believed for reversals

evidence from lava flows for complex polarity transitions the new composite steens mountain reversal record summary geomagnetic polarity transitions may be significantly more complex than are currently depicted in many sedimentary and lava flow records by splicing together paleomagnetic results from earlier studies at steens mountain with those from three newly studied sections of oregon plateau flood basalts at catlow peak and poker jim ridge 70 90 km to the southeast and west respectively we provide support for this interpretation with the most detailed account of a magnetic field reversal yet observed in volcanic rocks forty five new distinguishable transitional t directions together with 30 earlier ones reveal a much more complex and detailed record of the 16 7 ma reversed r to normal n polarity transition that marks the end of chron c5cr compared to the earlier r t n t n reversal record the new record can be described as r t n t n t r t n the composite record confirms earlier features adds new west and up directions and an entire large n t r t segment to the path and fills in directions on the path between earlier directional jumps persistent virtual geomagnetic pole vgp clusters and separate vgps have a preference for previously described longitudinal bands from transition study compilations which suggests the presence of features at the core mantle boundary that influence the flow of core fluid and distribution of magnetic flux overall the record is consistent with the generalization that vgp paths vary greatly from reversal to reversal and depend on the location of the observer rates of secular variation confirm that the flows comprising these sections were erupted rapidly with maximum rates estimated to be 85 120 m ka 1 at catlow and 130 195 m ka 1 at poker jim south paleomagnetic poles from other studies are combined with 32 non transitional poles found here to give a clockwise rotation of the oregon plateau of 11 4 5 6 with respect to the younger columbia river basalt group flows to the north and 14 5 4 6 with respect to cratonic north america 95 per cent confidence interval

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

the matuyama brunhes boundary interval 500 900 ka in north atlantic drift sediments summary ocean drilling program odp sites 984 and 983 located on north atlantic sediment drifts provide high resolution records across the matuyama brunhes boundary at odp site 984 bjorn drift iceland basin the mean sedimentation rate in the 500 900 ka interval is 12 cm kyr 1 based on an age model derived by matching the planktonic and benthic oxygen isotope records to an ice volume model the matuyama brunhes polarity transition at site 984 as defined by virtual geomagnetic polar vgp latitudes 50 has an apparent duration of 7 kyr with a mid point at 773 5 ka compared with 772 5 ka at neighbouring site 983 outside the polarity transition at both sites 984 and 983 excursions in vgp latitudes to values 20 at 540 590 and 670 ka correspond to troughs in the palaeointensity record new u channel palaeomagnetic data across the matuyama brunhes boundary for working and archive halves of core sections from three holes at both sites 984 and 983 augment data published by channell lehman 1997 and are compared with back to back 1 cm 3 discrete samples clusters of vgps in the south atlantic and northeast asia in both u channel and discrete sample records imply that polarity transition fields have characteristics similar to the modern non axial dipole nad field

strain accumulation during basal accretion in continental collision a case study from the suretta nappe eastern swiss alps abstract stacked crystalline basement nappes are a typical feature in the core of many orogens the suretta nappe exposed in eastern switzerland consists of briancon derived crustal slices which were assembled in a south dipping subduction zone during the alpine orogenic cycle the nappe contains post variscan rocks of the rofna porphyry complex rpc and permo mesozoic cover sequences which are ideal to study the strain evolution during alpine nappe formation we present new structural cross sections finite strain analyses and a retrodeformation for the frontal part of the suretta nappe the overall geometry of the suretta nappe is the result of two main deformation phases 1 eocene top to the nnw directed thrusting and folding ferrera phase which is overprinted by 2 backfolding and backshearing niemet beverin phase we suspect that this backshearing was caused by the buoyant rise of light continental crust beneath the suretta nappe and by frictional resistance at the top contact of the suretta nappe which had been accreted to the upper plate in the course of nappe stacking in the lower and interior parts of the suretta nappe weakly to undeformed boudins are generally surrounded by l tectonites indicating wsw ene stretching foliated equivalents reveal a plane strain deformation state the upper part of the suretta nappe which was strongly affected by backshearing shows flattening strain strain data were used to reconstruct the palinspastic evolution of the suretta nappe at two distinct time slots prior to niemet beverin backfolding the suretta nappe is characterized by thrust faults and large scale detachment folds before the onset of ferrera phase nappe stacking jurassic normal faults and the rpc intrusion shape trigger the localization of subsequent deformation our structural study confirms a complex polyphase evolution for the suretta nappe which might be characteristic for crystalline basement nappes in continental collision zones elsewhere

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

geomagnetic paleointensity and directional secular variation at ocean drilling program odp site 984 bjorn drift since 500 ka comparisons with odp site 983 gardar drift normalized remanence relative geomagnetic paleointensity records for the last 500 kyr can be matched between two iceland basin ocean drilling program odp sites 984 and 983 and correlated with other high resolution records directional secular variation is however not easily correlated between sites due to inadequate recording at these mean sedimentation rates 12 15 cm kyr of the characteristic high frequency variability of the directional record both sites record the iceland basin event at 186 189 ka in which the characteristic magnetization component rotates through 180 and back coincident with a paleointensity low that lasted about 3 kyr other brunhes chron geomagnetic excursions appear to be manifest as intervals of higher amplitude secular variation during lows in paleointensity there is a tendency for paleointensity lows to correlate with peak interglacials in the oxygen isotope records however this does not translate into a correlation between paleointensity and percent carbonate or between paleointensity and magnetic concentration or grain size parameters

ar ar ages from transitionally magnetized lavas on la palma canary islands and the geomagnetic instability timescale 1 a detailed study of 43 lava flows comprising two stratigraphic sequences exposed along the north and south walls of barranco de los tilos on the island of la palma canary islands reveals a complex temporally segmented record of geodynamo behavior that contains no less than three distinct geomagnetic events the matuyama brunhes m b reversal is recorded in five transitionally magnetized lava flows from the north tn section the isochrons obtained from three of the lower four m b lavas are defined by 14 incremental heating experiments that together with a previous age determination yielded a weighted mean of 798 4 6 2 ka all uncertainties 2s in addition a 780 3 10 3 ka isochron was determined for the overlying transitionally magnetized flow indicating that it was erupted during a distinctly younger portion of the transition near the base of the south ts section one finds a sequence of weakly magnetized flows associated with virtual geomagnetic pole vgp positions in the southwest indian ocean between latitudes 56 s and 65 s suggesting instability of the geomagnetic field beyond that of typical secular variation 40 ar 39 ar isochrons from three of these flows defined by 11 separate incremental heating experiments gave a weighted mean of 822 2 8 7 ka this anomalous field behavior recorded 24 11 kyr prior to the m b reversal may coincide with an event featured in several marine sediment records directly above two normal polarity flows 40 ar 39 ar isochrons of 751 9 18 1 ka and 675 0 15 7 ka are nine transitionally magnetized lavas having magnetization directions associated with low to midlatitude vgps spanning 23 60 n these flows are then capped by a single flow possessing normal polarity based on 12 incremental heating experiments 40 ar 39 ar isochrons of five of these nine lavas along with the uppermost flow gave a weighted mean age of 580 2 7 8 ka for this period of transitional to normal field behavior from these same transitional lavas quidelleur et al 1999 reported three unspiked k ar ages with a weighted mean of 602 24 ka and proposed a new event called the la palma excursion however the 40 ar 39 ar age presented here is three times more precise than the k ar age and is indistinguishable at the 95 confidence level from the 40 ar 39 ar age of a lava from the snake river plain idaho that originally defined the big lost event transitional field behavior of similar age observed in astronomically dated marine cores further establishes that the big lost event recorded at la palma was indeed global in extent rigorous temporal and geomagnetic constraints for several additional periods of geomagnetic field instability during the last several million years will comprise a geomagnetic instability timescale that can be factored confidently into models of the dynamo process index terms 1560 geomagnetism and paleomagnetism time variations secular and long term 1520 geomagnetism and paleomagnetism magnetostratigraphy 1035 geochemistry geochronology 1513 geomagnetism and paleomagnetism geomagnetic excursions keywords argonargon dating lavas excursion paleomagnetism reversal

natural remanent magnetization acquisition in bioturbated sediment general theory and implications for relative paleointensity reconstructions we present a general theory for the acquisition of natural remanent magnetizations nrm in sediment under the influence of a magnetic torques b randomizing torques and c torques resulting from interaction forces dynamic equilibrium between a and b in the water column and at the sediment water interface generates a detrital remanent magnetization drm while much stronger randomizing torques may be provided by bioturbation inside the mixed layer these generate a so called mixed remanent magnetization mrm which is stabilized by mechanical interaction forces during the time required to cross the surface mixed layer drm is lost and mrm is acquired at a rate that depends on bioturbation intensity both processes are governed by a mrm lock in function the final nrm intensity is controlled mainly by a single parameter that is defined as the product of rotational diffusion and mixed layer thickness divided by sedimentation rate this parameter defines three regimes 1 slow mixing 10 with mrm acquisition and full drm randomization and 3 intermediate mixing because the acquisition efficiency of drm is larger than that of mrm nrm intensity is particularly sensitive to in case of mixed regimes generating variable nrm acquisition efficiencies this model explains 1 lock in delays that can be matched with empirical reconstructions from paleomagnetic records 2 the existence of small lock in depths that lead to drm preservation 3 specific nrm acquisition efficiencies of magnetofossil rich sediments and 4 some relative paleointensity artifacts

complexity in matuyama brunhes polarity transitions from north atlantic iodp odp deep sea sites abstract integrated ocean drilling program iodp expedition 303 to the north atlantic provided 16 records of the matuyama brunhes polarity transition mbt based on u channel and discrete samples from holes drilled at three sites sites u1304 u1305 and u1306 that have mean brunhes sedimentation rates of 16 18 cm kyr the mbt occurs during the transition from marine isotope stage mis 19c to mis 18e with mid point at 773 ka and a transition duration of 8 kyr combining the new mbt records including one new record for the top jaramillo with previously published north atlantic mbt records odp sites 983 984 and 1063 yields a total of more than 20 high sedimentation rate polarity transition records the mbt yields a repetitive pattern of transitional field states as virtual geomagnetic poles vgps move from high southern latitudes to loop over the pacific group in ne asia and transit into the mid latitude south atlantic before reaching high latitudes in the northern hemisphere the vgps for the top jaramillo transition feature a loop over the pacific then a ne asia group before transit over the indian ocean to high southerly latitudes the north atlantic mbt records described here contrast with longitudinally constrained vgp paths for the mbt indicating that relatively low sedimentation rate 4 cm kyr records of the mbt are heavily smoothed by the remanence acquisition process and do not adequately represent the mbt field the vgps at the mbt and top jaramillo as measured in the north atlantic have similarities with excursion iceland basin vgp paths and were apparently guided by maxima in downward vertical flux similar to those seen in the modern non dipole nd field implying longevity in nd features through time

measurement of the degree of smoothing in sediment paleomagnetic secular variation records an example from late quaternary deep sea sediments of the bermuda rise western north atlantic ocean abstract replicate paleomagnetic secular variation psv records for the last 15 000 yr have been recovered from two deep sea sediment gravity cores on the bermuda rise western north atlantic ocean the records have been correlated using several different parameters and dated by correlation to a nearby piston core which has a high resolution 14 c chronology the psv records are systematically subdued with respect to the expected local magnetic field variability during the holocene when sedimentation rates were about 10 cm kyr during the late pleistocene when sedimentation rates were a factor of 2 3 higher the psv variability was significantly larger we attribute this to smoothing of the psv records by a depositional post depositional remanence drm pdrm acquisition process with a 10 20 cm lock in interval the degree of psv smoothing was evaluated by comparing the vgp angular dispersion and vector spectral content of the bermuda rise cores with high resolution psv records from north america and great britain these results suggest a 50 reduction in the drm pdrm recording of vector variability during the holocene on the bermuda rise with most of the psv reduction occurring for features of less than 2000 yr in duration we can reproduce the subdued psv pattern with a mathematical model that simulates the site drm pdrm remanence acquisition process finally the replicate gravity cores document that psv records may be reproducible on a local scale even though they only retain a low pass filtered record of true magnetic field variability

paleosecular variation during sequential geomagnetic reversals from hawaii abstract paleomagnetic directions measured from 175 flows in four volcanic sequences on the island of o ahu hawaii cover about 0 3 m y of geomagnetic field changes and include detailed records of the successive gilbert gauss 3 57 ma and lower mammoth reversals 3 33 ma the dominant pattern of the directional changes is the presence of large inclination variations with increasing amplitude and steep values during the transitional periods the morphology of these recurring features during different transitions is reminiscent of the characteristics of the non dipole field inferred from studies of paleosecular variation in the pacific these observations are consistent with transitional mechanisms yielding the emergence of non dipole components with characteristics similar to those during periods of stable polarity the three different records of the lower mammoth reversal show that clusters of transitional pole positions are not reproducible between parallel sections and are thus linked to local eruptions and not to long lived states of the transitional field

dynamics of the earth magnetic field in the 10 75 kyr period comprising the laschamp and mono lake excursions new results from the french chaine des puys in a global perspective abstract we report here on a new paleomagnetic directions and intensities and coupled k ar and 40 ar 39 ar analysis of 35 different flows emplaced in the chaine des puys during the 75 to 10 kyr interval which contains the mono lake and laschamp excursions there is a remarkable agreement between the new set of absolute volcanic intensities and published sedimentary glopis 75 and cosmogenic 10 be and 36 cl records the laschamp and mono lake excursions are clearly revealed by a very significant intensity drop at 41 2 1 6 ka and 34 2 1 2 ka respectively the duration of the laschamp excursion is 1500 yr and about 640 yr when the drop of paleointensity or the directional change are considered respectively the intensity drop at the mono lake is twice as short in the 7 ka interval separating the two excursions the field intensity recovers to almost non transitional values the rate of decrease of the field intensity during these excursions attains 18 nt yr for the laschamp and even greater value 33 nt yr for the mono lake this figure is for the laschamp excursion similar to the present field intensity decrease in the last two centuries so that one may wonder whether such a high rate of change may be characteristic of an impending geomagnetic event reversal or excursion we suggest that the name auckland excursion should be used for the present day called mono lake

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

a paleomagnetic power spectrum we construct a power spectrum of geomagnetic dipole moment variations or their proxies that spans the period range from some tens of million down to about 100 years empirical estimates of the spectrum are derived from the magnetostratigraphic time scale from marine sediment relative paleointensity records and from a time varying paleomagnetic field model for the past 7 kyr the spectrum has the most power at long periods reflecting the influence of geomagnetic reversals and in general decreases with increasing frequency decreasing period the empirical spectrum is compared with predictions from simple models discrepancies between the observed and predicted spectra are discussed in the context of i changes in reversal rate ii overall average reversal rate iii cryptochrons iv the time taken for a reversal to occur and v long term paleosecular variations and average estimates of the field strength and variance from other sources 2005 elsevier b v all rights reserved

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

a simple model for geomagnetic field excursions and inferences for palaeomagnetic observations abstract we explore simple excursion scenarios by imposing changes on the axial dipole component of the holocene geomagnetic field model cals10k 2 and investigate implications for our understanding of palaeomagnetic observations of excursions our findings indicate that globally observed directions of fully opposing polarity are only possible when the axial dipole reverses linearly decaying the axial dipole to zero and then reestablishing it with the same sign produces a global intensity minimum but does not produce fully reversed directions globally reversing the axial dipole term increases the intensity of the geomagnetic field observed at earth s surface across the mid point of the excursion which results in a double dip intensity structure during the excursion only a limited number of palaeomagnetic records of excursions contain such a double dip intensity structure rather the maximum directional change is coeval with a geomagnetic field intensity minimum

age of matuyama brunhes boundary constrained by u pb zircon dating of a widespread tephra abstractthe youngest geomagnetic polarity reversal the matuyama brunhes boundary mbb provides an important datum plane for sediments ice cores and lavas its frequently cited age of 780 ka is based on orbital tuning of marine sedimentary records and is supported by 40 ar 39 ar dating of hawaiian lavas using recent age calibrations challenging this age how ever are reports of younger astrochronological ages based on oxygen isotope stratigraphy of high sedimentation rate marine records and cosmogenic nuclides in marine sediments and an antarctic ice core here we present a u pb zircon age of 772 7 7 2 ka from a marine deposited tephra just below the mbb in a forearc basin in japan u pb dating has a distinct advantage over 40 ar 39 ar dating in that it is relatively free from assumptions regarding stan dardization and decay constants this u pb zircon age coupled with a newly obtained oxygen isotope chronology yields an mbb age of 770 2 7 3 ka our mbb age is consistent with those based on the latest orbitally tuned marine sediment records and on an antarctic ice core we provide the first direct comparison between orbital tuning u pb dating and magnetostratig raphy for the mbb fulfilling a key requirement in calibrating the geological time scale introduction

incremental heating of bishop tuff sanidine reveals preeruptive radiogenic ar and rapid remobilization from cold storage abstract accurate and precise ages of large silicic eruptions are critical to calibrating the geologic timescale and gauging the tempo of changes in climate biologic evolution and magmatic processes throughout earth history the conventional approach to dating these eruptive products using the 40ar 39ar method is to fuse dozens of individual feldspar crystals however dispersion of fusion dates is common and interpretation is complicated by increasingly precise data obtained via multicollector mass spectrometry incremental heating of 49 individual bishop tuff bt sanidine crystals produces 40ar 39ar dates with reduced dispersion yet we find a 16 ky range of plateau dates that is not attributable to excess ar we interpret this dispersion to reflect cooling of the magma reservoir margins below 475 c accumulation of radiogenic ar and rapid preeruption remobilization accordingly these data elucidate the recycling of subsolidus material into voluminous rhyolite magma reservoirs and the effect of preeruptive magmatic processes on the 40ar 39ar system the youngest sanidine dates likely the most representative of the bt eruption age yield a weighted mean of 764 8 0 3 0 6 ka 2 analytical full uncertainty indicating eruption only 7 ky following the matuyama brunhes magnetic polarity reversal single crystal incremental heating provides leverage with which to interpret complex populations of 40ar 39ar sanidine and u pb zircon dates and a substantially improved capability to resolve the timing and causal relationship of events in the geologic record

the alpine geodynamic evolution of penninic nappes in the eastern central alps the eastern central alps consist of several pennine nappes with different tectonometamorphic histories the tectonically uppermost units oceanic avers bundnerschiefer continental suretta and tambo nappes oceanic vals bundnerschiefer show cretaceous early tertiary w directed thrusting with associated blueschist facies metamorphism related to subduction of the pennine units beneath the austroalpine continental crust this event caused eclogite facies metamorphism in the underlying continental adula nappe the gross effect was crustal thickening the tectonically lower continental simano nappe is devoid of any imprint from this event in the course of continent continent collision high t metamorphism and n directed movements occurred both affected the whole nappe pile more or less continuously from amphibolite to greenschist facies conditions crustal thinning commenced during the regional temperature peak a final phase is related to differential uplift under retrograde p t conditions further thinning of the crust was accommodated by e to ne directed extensional deformation

notes on geochronologic and chronostratigraphic units there is no need to regard the term geochronology as a redundant synonym of either geologic time scale or radiometric dating this term may reasonably be used in reference to all principles and methods employed in geology to determine the relative and numerical ages of all kinds of rocks the term chronostratigraphy would best be restricted to the process of defining the formal subdivisions of the geologic time scale geochronologic unit is best used as a general category term it includes aurichronologic units spans of time defined operationally by golden spikes geochronometric units spans of time defined theoretically in terms of absolute numerical age before present and biochronologic units spans of time defined theoretically by paleobiological events a chronostratigraphic unit is a set of material existing stratified rock that was formed during a given span of time geochronologic unit therefore a geochronologic unit must be defined before the corresponding chronostratigraphic unit can even be conceptualized chronostratigraphic units are generally not isochronous bodies of rock because owing to the unconformities that exist over any appreciable geographic area their upper and lower material boundaries are not everywhere synchronous thus particular systems series and stages are material stratigraphic units of geographically variable time value within the maximum temporal limits of the corresponding periods epochs and ages geochronologic boundaries that have been formally ratified serve a conventional function and will rarely if ever need to be redefined we can continue to discover and debate important events in earth history without insisting that such events must be used to constantly redefine the boundaries of our global temporal pigeonholes

dependence of the duration of geomagnetic polarity reversals on site latitude an important constraint on the processes governing the geodynamo the flow in the outer core responsible for generating earth s magnetic field is the duration of geomagnetic polarity reversals that is how long it takes for earth s magnetic field to reverse1 it is generally accepted that earth s magnetic field strength drops to low levels during polarity reversals and the field direction progresses through a 180 change while the field is weak1 the time it takes for this process to happen however remains uncertain with estimates ranging from a few thousand up to 28 000 years here i present an analysis of the available sediment records of the four most recent polarity reversals these records yield an average estimate of about 7 000 years for the time it takes for the directional change to occur the variation about this mean duration is not random but instead varies with site latitude with shorter durations observed at low latitude sites and longer durations observed at mid to high latitude sites such variation of duration with site latitude is predicted by simple geometrical reversal models in which non dipole fields are allowed to persist while the axial dipole decays through zero and then builds in the opposite direction and provides a constraint on numerical dynamo models

mechanical decoupling of high pressure crustal units during continental subduction the mechanics of the transition from continental subduction toward upper crustal nappe stacking is still poorly understood and is studied here through a 2d thermal and strength numerical modelling of a subducted passive margin geological observations in the core of most mountain belts show the piling up of several hp lt upper crustal units that are most likely related to the detachment of upper crustal units from the subducted continental margin and to the subsequent stacking of the detached units at depths the adula unit lepontine dome central alps switzerland is a long and thin upper crustal unit and is used here as a natural case study as it provides a well documented example of these units 2d thermal modeling shows that two steps successive in time characterized the burial history of the passive margin undergoing continental subduction 1 an increase in the margin strength due to an increase in the confining pressure during the first million years of the margin subduction and 2 the progressive heating of the subducted margin from the overlying lithosphere leads to a decrease in the margin strength due to thermal weakening which progressively counter balances the increase in confining pressure two strength gradients develop within the subducted lithosphere 1 along the slab the strength decreases with increasing burial depth and 2 perpendicular to the slab the strength increases with depth due to an inverse temperature distribution the detachment of hp lt continental units from the subducted margin could occur when the slab strength becomes lower than the applied net stress this allows the detachment of ductile weakened thin and long upper crustal units the thickness and length of the detached crustal units are controlled by the following parameters in order of their importance subduction dip angle crustal rheology mantle heat flux and subduction velocity the comparison of our model results with the geometry and pt conditions of the adula unit yields an estimate of the alpine subduction dip angle at the time of deformation and metamorphism

odp site 1063 bermuda rise revisited oxygen isotopes excursions and paleointensity in the brunhes chron an age model for the brunhes chron of ocean drilling program odp site 1063 bermuda rise is constructed by tandem correlation of oxygen isotope and relative paleointensity data to calibrated reference templates four intervals in the brunhes chron where paleomagnetic inclinations are negative for both u channel samples and discrete samples are correlated to the following magnetic excursions with site 1063 ages in brackets laschamp 41 ka blake 116 ka iceland basin 190 ka pringle falls 239 ka these ages are consistent with current age estimates for three of these excursions but not for pringle falls which has an apparent age older than a recently published estimate by 28 kyr for each of these excursions termed category 1 excursions virtual geomagnetic poles vgps reach high southerly latitudes implying paired polarity reversals of the earth s main dipole field that apparently occurred in a brief time span 2 kyr in each case several times shorter than the apparent duration of regular polarity transitions in addition several intervals of low paleomagnetic inclination low and negative in one case are observed both in u channel and discrete samples at 318 ka mis 9 412 ka mis 11 and in the 500 600 ka interval mis 14 15 these category 2 excursions may constitute inadequately recorded category 1 excursions or high amplitude secular variation

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

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

exploring the influence of the non dipole field on magnetic records for field reversals and excursions summary we have used the model cals7k 2 to explore the possible influence of the time varying non dipole components of the geomagnetic field during field reversals and excursions our findings suggest that non dipole components could add significant structure to the field during the reversal and excursion processes globally the main polarity reversal is variable in duration and rapid reversals on subdecadal timescales are seen for a small number of locations the model generates variable reversal paths however there is a longitudinal preference both spatially and more weakly temporally directional reversal features are not globally synchronous some polarity changes finish before they start elsewhere global intensity variations however appear more coherent we also find support for the idea that field intensity changes occur some time before and after the major directional changes of the reversal large excursions appear naturally when the axial dipole has been reduced to 20 per cent for the whole time period however they are not globally synchronous or uniform

the c27n c26r geomagnetic polarity reversal recorded in the west greenland flood basalt province how complex is the transitional field 1 we present a paleomagnetic record of the c27n c26r 60 9 ma geomagnetic polarity reversal gpr sampled in two separate lava sequences in the vaigat formation west greenland flood basalt province the two profiles distanced 16 km apart consist of 45 and 35 sampled lava flows of which 31 and 23 respectively record transitional field directions a prominent feature of each profile is the presence of successive flows with similar paleomagnetic directions which based on geological geochemical and paleomagnetic evidences are interpreted to represent flow fields i e lavas erupted in a short period of time rather than periods when the geomagnetic field stood still multimodel photogrammetry shows that the typical flow length of vaigat formation picritic lavas is 2 km which explains why flow to flow correlation of paleomagnetic directions between the two profiles 16 km apart is not possible the two profiles therefore record different and independent time series of the same gpr allowing us to cast light on some of the questions that have arisen from single records particularly noteworthy is the fact that the two high resolution paleomagnetic records share no general trends clearly indicating that the c27n c26r transitional field was more complex and faster changing than can be resolved by either record in addition we obtained stable polarity paleomagnetic data from both pretransitional c27n and posttransitional c26r yielding a paleomagnetic pole latitude 64 8 n longitude 141 5 e n 14 k 19 8 a95 9 2 that passes reversal test

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

dynamical similarity of geomagnetic field reversals volcanic records of the reversals of the geomagnetic field can be well matched under the assumption of a common reversal duration and imply that the reversal process comprises three phases a precursor a fast polarity switch and a rebound the properties of which have remained unchanged for about 180 million years the geomagnetic variations that prevail during magnetic field reversals convey important information on the nature of the geodynamo that drives earth s magnetic field in this paper jean pierre valet and co authors show that the ten most detailed volcanic records of geomagnetic reversals have a common duration as well as common dynamical characteristics they propose that the reversing process has remained the same for at least the past 180 million years characterized by three phases a precursor event a 180o polarity switch and then a rebound no consensus has been reached so far on the properties of the geomagnetic field during reversals or on the main features that might reveal its dynamics a main characteristic of the reversing field is a large decrease in the axial dipole and the dominant role of non dipole components1 2 3 other features strongly depend on whether they are derived from sedimentary or volcanic records only thermal remanent magnetization of lava flows can capture faithful records of a rapidly varying non dipole field but because of episodic volcanic activity sequences of overlying flows yield incomplete records here we show that the ten most detailed volcanic records of reversals can be matched in a very satisfactory way under the assumption of a common duration revealing common dynamical characteristics we infer that the reversal process has remained unchanged with the same time constants and durations at least since 180 million years ago we propose that the reversing field is characterized by three successive phases a precursory event a 180 polarity switch and a rebound the first and third phases reflect the emergence of the non dipole field with large amplitude secular variation they are rarely both recorded at the same site owing to the rapidly changing field geometry and last for less than 2 500 years the actual transit between the two polarities does not last longer than 1 000 years and might therefore result from mechanisms other than those governing normal secular variation such changes are too brief to be accurately recorded by most sediments

paleomagnetic reconstruction of the global geomagnetic field evolution during the matuyama brunhes transition iterative bayesian inversion and independent verification abstract the earth s magnetic field changed its polarity from the last reversed into today s normal state approximately 780 000 years ago while before and after this so called matuyama brunhes reversal the earth magnetic field was essentially an axial dipole the details of its transitional structure are still largely unknown here a bayesian inversion method is developed to reconstruct the spherical harmonic expansion of this transitional field from paleomagnetic data this is achieved by minimizing the total variational power at the core mantle boundary during the transition under paleomagnetic constraints the validity of the inversion technique is proved in two ways first by inverting synthetic data sets from a modeled reversal here it is possible to reliably reconstruct the gauss coefficients even from noisy records second by iteratively combining four geographically distributed high quality paleomagnetic records of the matuyama brunhes reversal into a single geometric reversal scenario without assuming an a priori common age model the obtained spatio temporal reversal scenario successfully predicts most independent matuyama brunhes transitional records therefore the obtained global reconstruction based on paleomagnetic data invites to compare the inferred transitional field structure with results from numerical geodynamo models regarding the morphology of the transitional field it is found that radial magnetic flux patches form at the equator and move polewards during the transition our model indicates an increase of non dipolar energy prior to the last reversal and a non dipolar dominance during the transition thus the character and information of surface geomagnetic field records is strongly site dependent the reconstruction also offers new answers to the question of existence of preferred longitudinal bands during the transition and to the problem of reversal duration different types of directional variations of the surface geomagnetic field continuous or abrupt are found during the transition two preferred longitudinal bands along the americas and east asia are not predicted for uniformly distributed sampling locations on the globe similar to geodynamo models with cmb heatflux derived from present day lower mantle heterogeneities a preference of transitional vgps for the pacific hemisphere is found the paleomagnetic duration of reversals shows not only a latitudinal but also a longitudinal variation even the paleomagnetically determined age of the reversal varies significantly between different sites on the globe the described bayesian inversion technique can easily be applied to other high quality full vector reversal records also its extension to inversion of secular variation and excursion data is straightforward

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

10 be evidence for the matuyama brunhes geomagnetic reversal in the epica dome c ice core every few hundred thousand years earth s north and south magnetic poles swap places for the first time a record of one of these geomagnetic reversals has been found in an ice core because the strength of the field is very low during a reversal the intensity of cosmic rays in the earth s atmosphere is increased this leads to an increase in the quantity of 10be which like 14c is formed by cosmic rays in the atmosphere deposited in the ice it is this 10be enhancement in the ice that has been detected an ice core drilled at dome c antarctica is the oldest ice core so far retrieved1 on the basis of ice flow modelling and a comparison between the deuterium signal in the ice with climate records from marine sediment cores the ice at a depth of 3 190 m in the dome c core is believed to have been deposited around 800 000 years ago2 offering a rare opportunity to study climatic and environmental conditions over this time period however an independent determination of this age is important because the deuterium profile below a depth of 3 190 m depth does not show the expected correlation with the marine record2 here we present evidence for enhanced 10be deposition in the ice at 3 160 3 170 m which we interpret as a result of the low dipole field strength during the matuyama brunhes geomagnetic reversal which occurred about 780 000 years ago if correct this provides a crucial tie point between ice cores marine cores and a radiometric timescale

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