a 2 14 myr astronomically tuned record of relative geomagnetic paleointensity from the western philippine sea 1 we present a 2 14 myr astronomically tuned relative geomagnetic paleointensity record from the western philippine sea pseudosingle domain titanomagnetite is the only magnetic mineral identified and variations in titanomagnetite concentration fall well within the accepted limits for relative paleointensity variations no significant temporally persistent periodicities are observed in wavelet analyses of the paleointensity time series or in the rock magnetic parameters used for relative paleointensity normalization this suggests that our paleointensity record is largely free of rock magnetic or lithological artefacts and that it represents a reliable record of geomagnetic behavior with no evidence for modulation of the field at earth orbital periods the paleointensity record is highly coherent with the sint 800 global paleointensity stack for the last 800 kyr and with a coeval record from the west caroline basin our record confirms that it is normal for the geomagnetic field to undergo dynamic changes within polarity intervals with relatively frequent collapses of the field to low paleointensities and concomitant deviations away from the stable field direction we do not observe an asymmetrical sawtooth form in our paleointensity record which might suggest that previously observed asymmetrical sawtooth paleointensities result from rock magnetic artefacts also we do not observe a persistent 100 kyr inclination periodicity in contrast to the suggestion that geomagnetic field directions are modulated by orbital eccentricity good agreement between our paleointensity record and the coeval west caroline basin record provides the beginning of a detailed view of geomagnetic field behavior between 0 8 and 2 14 ma

microwave backscatter modeling of erg surfaces in the sahara desert the sahara desert includes large expanses of sand dunes called ergs these dunes are formed and constantly reshaped by prevailing winds previous study shows that saharan ergs exhibit significant radar backscatter spl sigma spl deg modulation with azimuth angle f we use spl sigma spl deg measurements observed at various incidence angles and f from the nasa scatterometer nscat the seawinds scatterometer the ers scatterometer escat and the tropical rainfall measuring mission s precipitation radar to model the spl sigma spl deg response from sand dunes observations reveal a characteristic relationship between the backscatter modulation and the dune type i e the number and orientation of the dune slopes sand dunes are modeled as a composite of tilted rough facets which are characterized by a probability distribution of tilt with a mean value and small ripples on the facet surface the small ripples are modeled as cosinusoidal surface waves that contribute to the return signal at bragg angles only longitudinal and transverse dunes are modeled with rough facets having gaussian tilt distributions the model results in a spl sigma spl deg response similar to nscat and escat observations over areas of known dune types in the sahara the response is high at look angles equal to the mean tilts of the rough facets and is lower elsewhere this analysis provides a unique insight into scattering by large scale sand bedforms

comment on a relative geomagnetic paleointensity stack from ontong java plateau sediments for the matuyama by yvo s kok and lisa tauxe

a comparison of relative paleointensity records of the matuyama chron for the period 0 75 1 25 ma abstract a comparison of 15 records of relative geomagnetic paleointensity spanning the time interval 0 75 1 25 ma has been performed after synchronizing the records at the paleointensity minima corresponding to geomagnetic reversals filtering and smoothing of the records indicated that after synchronization paleointensity features with wavelengths larger than about 20 ky can be matched from one record to another although age offsets can still be observed in several records further synchronization of the records using intensity minima permitted construction of a composite stack of these records this stack provides useful information about the long term evolution of the geomagnetic field intensity over this period of time and confirms the existence of a rapid recovery of the field intensity after each reversal analysis of the standard deviation and jackknife calculation suggest that improvement in the stack accuracy and resolution could still be achieved with better constrained age models

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

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

a geomagnetic record over the last 3 5 million years from deep tow magnetic anomaly profiles across the central indian ridge high resolution records of the geomagnetic field intensity over the last 4 myr provided by paleomagnetic analyses of marine sediments have shown the occurrence of short lived low field intensity features associated with excursions or short polarity intervals in order to evaluate the ability of marine magnetic anomalies to record the same geomagnetic events we have collected six deep tow 500 m above the seafloor and several sea surface magnetic anomaly profiles from the central indian ridge across the brunhes matuyama and gauss chrons i e from the ridge axis to anomaly 2a after removal of topography latitude and azimuth effects we converted distances into time sequences using well dated polarity reversal anomalies as tie points we calculated the average signal to test the robustness of the short wavelength anomalies the resulting stacked profile is very similar to stacked sea surface and downward continued profiles from the central indian ridge the east pacific rise and the pacific antarctic ridge our results suggest that in addition to polarity reversals to previously suggested geomagnetic events subchrons or excursions within the brunhes and matuyama chrons a new small scale magnetic anomaly likely generated by several closely spaced geomagnetic field intensity variations represent the major contributor to the detailed shape of recent marine magnetic anomalies in investigated areas we observe a dense succession of microanomalies that are correlated excursions ontong java 1 and 2 and gilsa is found after the olduvai chron the near bottom results support the existence of three geo magnetic features between the gauss matuyama boundary and olduvai they also suggest three geomagnetic events during the c2a i n subchron within the gauss chron this study emphasizes the potential of deep tow magnetic surveys in detecting fluctuations in geo magnetic field intensity and in particular short lived excursions a poorly constrained part of the geomagnetic field temporal variation spectrum

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

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

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

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

stochastic simulation of volcanic tremor from ruapehu abstract the most common volcanic tremor produced by ruapehu is a continuous signal with a dominant frequency of about 2 hz this signal has a sharply peaked spectrum and an autocorrelation function with a high degree of coherence even for lags of over 20 seconds these characteristics strongly suggest that the cause of this tremor is a single resonator probably a fluid filled cavity resonating in an organ pipe mode the stochastic simulation of such a resonator uses the equation of motion of a simple harmonic oscillator which applies to an organ pipe fundamental resonance with either the characteristics of the oscillator or the forcing function containing a random element a white noise forcing function which would be appropriate for excitation of the cavity by a high pressure gas input gave good agreement with the observed spectra and autocorrelation functions another possible model used an oscillator with a damping factor which varied randomly and was sometimes negative so oscillations built up rather than decayed this also gave a reasonable simulation of ruapehu tremor the third excitation model used a poisson process in which during each time interval there was a certain probability of applying a fixed impulse to the resonator it was found that the impulses had to be frequent i e several times a second to match the characteristics of ruapehu tremor it has been suggested that tremor is composed of a succession of low frequency b type earthquakes the results of this simulation show that at ruapehu tremor could be produced by a resonator with positive feedback just sustaining oscillation or by a resonator excited by external impulses the most promising model for low frequency earthquakes describes them as the result of a major external disturbance of the resonator

real time seismic amplitude measurement rsam a volcano monitoring and prediction tool seismicity is one of the most commonly monitored phenomena used to determine the state of a volcano and for the prediction of volcanic eruptions although several real time earthquake detection and data acquisition systems exist few continuously measure seismic amplitude in circumstances where individual events are difficult to recognize or where volcanic tremor is prevalent analog seismic records provide a quick visual overview of activity however continuous rapid quantitative analysis to define the intensity of seismic activity for the purpose of predicing volcanic eruptions is not always possible because of clipping that results from the limited dynamic range of analog recorders at the cascades volcano observatory an inexpensive 8 bit analog to digital system controlled by a laptop computer is used to provide 1 min average amplitude information from eight telemetered seismic stations the absolute voltage level for each station is digitized averaged and appended in near real time to a data file on a multiuser computer system raw realtime seismic amplitude measurement rsam data or transformed rsam data are then plotted on a common time base with other available volcano monitoring information such as tilt changes in earthquake activity associated with dome building episodes weather and instrumental difficulties are recognized as distinct patterns in the rsam data set rsam data for domebuilding episodes gradually develop into exponential increases that terminate just before the time of magma extrusion mount st helens crater earthquakes show up as isolated spikes on amplitude plots for crater seismic stations but seldom for more distant stations weather related noise shows up as low level long term disturbances on all seismic stations regardless of distance from the volcano implemented in mid 1985 the rsam system has proved valuable in providing up to date information on seismic activity for three mount st helens eruptive episodes from 1985 to 1986 may 1985 may 1986 and october 1986 tiltmeter data the only other telemetered geophysical information that was available for the three dome building episodes is compared to rsam data to show that the increase in rsam data was related to the transport of magma to the surface thus if tiltmeter data is not available rsam data can be used to predict future magmatic eruptions at mount st helens we also recognize the limitations of rsam data two examples of rsam data associated with phreatic or shallow phreatomagmatic explosions were not preceded by the same increases in rsam data or changes in tilt associated with the three dome building eruptions

long term secular variation in geomagnetic field inclination during brunhes chron recorded in sediment cores from ontong java plateau abstract a paleomagnetic study has been conducted on three gravity cores obtained from the ontong java plateau ojp in the western equatorial pacific in order to investigate long term secular variations during the brunhes chron the cores consist of nanofossil foraminiferal ooze and covers the last 500 000 800 000 years with average sedimentation rates of 6 9 m m y downcore changes of magnetic concentration represented by magnetic susceptibility are a factor of six or less and variations in magnetic grain size and mineralogy are estimated to be small from the hysteresis parameters ratio of anhysteretic remanent magnetization arm to saturation isothermal remanent magnetization sirm and s ratio these results indicate that the sediments are rock magnetically homogeneous in general and are hence suitable for relative paleointensity estimation i chose sirm as a normalizer for relative paleointensity the three cores could be precisely correlated with each other using relative paleointensity variations depth to age conversion was carried out by tying to the sint 800 paleointensity stack guyodo and valet 1999 the ages are consistent with the oxygen isotope stratigraphy available for one of the three cores inclination variations were obtained after alternating field demagnetization large inclination anomaly about 7 in average was observed as has been suggested to occur in the western equatorial pacific by some studies the inclination records of the three cores were integrated into a single record based on the correlation by the relative paleointensity long term inclination variations of the order of 10 000 100 000 years were recognized the inclinations tend to deflect toward negative when relative paleointensity is low the variations are similar in general to the record from the west caroline basin yamazaki and oda 2002 which supports their model that long term inclination variation is controlled by relative strength of the geocentric axial dipole gad and persistent non dipole components

volcanic tremors at deception island south shetland islands antarctica abstract deception island 62 43 s 60 57 w is an active volcano located at the spreading centre of the bransfield strait backarc on the southwestern side of the scotia sea region antarctica since 1986 the spanish antarctic national program has supported geophysical and geological surveys addressed to the study of seismic and volcanic activities in the area the present volcanotectonic activity is restricted to significant seismic activity and several fumarole fields numerous earthquakes with magnitudes ranging from 0 1 to 2 2 have been recorded most of them distributed along the main fractures of the island which coincide with the regional structural trend of the bransfield strait back arc several types of volcanic tremors have also been recorded all of them with high spectral stability contents with few and well defined spectral peaks the location of the stations that record tremors the correlation of seismic noise with the tremors and the geological characteristics of deception island together suggest that the tremors are generally associated with geothermal noise originated in the uppermost ducts of the fumarolian system if we associate the surficial ducts to harmonic oscillators it is observed that the motion is basically unidirectional as is expected in the degasification of an aquifer taking the main observed frequencies and applying the classical mechanical expressions of organ pipes we obtain a duct length which agrees with the length of a water column necessary to maintain the equilibrium as indicated by the application of the clausius clapeyron equation

climatic influence in nrm and 10be derived geomagnetic paleointensity data abstract one can determine geomagnetic paleointensities from natural remanent magnetizations nrm and by inverting production rates of cosmogenic isotopes such as 10 be and 14 c recently two independently derived 200 kyr stacks y guyodo j p valet relative variations in geomagnetic intensity from sedimentary records the past 200 000 years earth planet sci lett 143 1996 23 36 m frank b schwarz s baumann p w kubik m suter a mangini a 200 kyr record of cosmogenic radionuclide production rate and geomagnetic field intensity from 10 be in globally stacked deep sea sediments earth planet sci lett 149 1997 121 129 were compared and the good agreement was suggested to validate the use of sedimentary cores for studies both compilations use mainly the astronomically forced and climatically controlled oxygen isotope stratigraphy to date and synchronize the sedimentary records while this very curve has several coherent features with the supposedly pure geomagnetic records an nrm relative paleointensity record which was included in the conventional paleointensity stack shows correspondence with climatic features which is explained by an inadequacy in the normalization technique therefore it is possible that the extraction of the pure paleointensity signal from marine sediments has not always been accomplished

geomagnetic field variations between chrons 33r and 19r 83 41 ma from sea surface magnetic anomaly profiles abstract sea surface magnetic profiles exhibit coherent short wavelength micro anomalies or tiny wiggles superimposed to the main anomalies due to reversals in this study we investigate the nature and distribution of these tiny wiggles on oceanic crust formed during the 42 myr long period following the cretaceous normal superchron to this end we compute stacks of anomaly profiles from different areas in the indian and the pacific oceans using a simple method based on upward continuation we demonstrate that the tiny wiggles are consistent worldwide although their patterns exhibit different resolutions at different spreading rates they are therefore confidently ascribed to past fluctuations of the geomagnetic dipole moment a high resolution record of these fluctuations is obtained by selecting and stacking profiles from areas with the highest spreading rates modeling the micro anomalies as short magnetic polarity intervals yields durations for these intervals generally shorter than 10 kyr likely too short to be indeed true subchrons moreover the number of detected tiny wiggles clearly depends on the spreading rate these results support geomagnetic intensity fluctuations as being the cause of most tiny wiggles as also suggested by recent magnetostratigraphic data the tiny wiggles are uniformly distributed within chrons indicating that paleointensity fluctuations are neither inhibited after nor enhanced before a reversal beyond a blind zone of about 10 km corresponding to 80 to 250 kyr depending on the spreading rate for which the anomalies due to reversals prevent the detection of tiny wiggles most tiny wiggles probably represent a filtered record of a uniform secular variation regime as suggested by their uniform spatial distribution over the whole investigated period

multiple rapid polarity swings during the matuyama brunhes transition from two high resolution loess paleosol records 1 two high resolution magnetic records of the matuyama brunhes m b transition have been obtained from xifeng and baoji loess paleosol sequences in the chinese loess plateau the results of stepwise thermal and alternating field af demagnetizations confirm that both treatments are effective in isolating characteristic remanent magnetization chrm from specimens of loess paleosol sediments which suffer from weak pedogenesis and that af demagnetization clearly fails to isolate chrm from specimens of paleosol sediments which have undergone strong pedogenesis on the basis of virtual geomagnetic pole latitudes crossing the virtual geomagnetic equator the m b transition involves at least 15 rapid polarity swings rock magnetic experiments and anisotropy of low field magnetic susceptibility show that high frequency polarity swings during the m b transition cannot be attributed to rock magnetic and or sedimentary causes extrapolating a constant accumulation rate between the jaramillo matuyama and m b boundary ages the m b transition interval of both the xifeng and baoji sections is estimated to have a duration of more than 10 kyr durations of each short polarity interval are estimated to range from about 0 3 to 5 6 kyr our new results combined with previous paleomagnetic observations from lavas and marine sediments suggest that multiple rapid polarity swings occurred during the m b transition

geomagnetic field intensity during the last 5 myr ltd dht shaw palaeointensities from volcanic rocks of the society islands french polynesia summary on the basis of the thellier palaeointensities reported so far it has been thought that the time averaged virtual dipole moment vdm and virtual axial dipole moment vadm for the last few million years are almost the same as the present geomagnetic dipole moment 8 1022 a m2 this estimate has been called into question however because recent studies have revealed that the thellier method occasionally overestimates palaeointensities by as much as twice the true values in contrast a recently developed palaeointensity technique namely the double heating technique of the shaw method combined with low temperature demagnetization the ltd dht shaw method can yield expected field intensities from samples which give unreliable palaeointensities using the thellier method therefore we have measured absolute palaeointensities from 0 5 4 6 ma volcanic rocks from the society islands french polynesia mainly using the ltd dht shaw method as a result 195 out of 361 samples passed the selection criteria some of which are compared with additional results obtained with coe s version of the thellier method in the thellier experiments 18 out of 40 samples passed the criteria giving palaeointensities both consistent and inconsistent with the ltd dht shaw results these samples are characterized mostly by two segmented arai diagrams if we take the ltd dht shaw palaeointensities 24 reliable site means are available and give a mean vadm of 3 64 2 10 1022 a m2 this is nearly half of the mean of the 0 5 ma thellier data selected from the latest palaeointensity database 7 46 3 10 1022 a m2 n 458 as well as the present dipole moment the ltd dht shaw palaeointensities newly determined in this study suggest that the present day field is so strong that it may not be typical of the past geomagnetic field

grain size dependent paleointensity results from very recent mid oceanic ridge basalts 1 we report paleointensity analyses on a suite of four samples from two axial zero age mid oceanic ridge basalt flows from the east pacific rise paleointensity experiments have been performed on several profiles each consisting of a batch of small millimeter scale subsamples going from the rapidly cooled glassy margin to the interior the coe version of the thellier double heating procedure was used with back checks performed after every other heating step most of the samples show very good behavior i e constant ratio between the lost and acquired magnetization through a large temperature range quality factor usually above 10 and positive checks which lead to unambiguous paleointensity determinations paleointensities obtained on glasses reported here and in a previously published study are very consistent and reproducible and in agreement with expected in situ values of earth s magnetic field intensity however results found within the crystalline part of the samples show values that are considerably up to 50 higher than expected these variations seem to be correlated with the cooling history of the samples and appear to be universal since all samples exhibit the same intriguing pattern an extensive study of magnetic properties allows us to link this incongruent behavior to 1 the presence of multidomain effects and 2 the cooling rate difference between laboratory experiments and in situ cooling

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

the santa rosa event 40ar 39ar and paleomagnetic results from the valles rhyolite near jaramillo creek jemez mountains new mexico the jaramillo event was originally defined by doell and dalrymple in 1966 on the basis of k ar ages from sanidine in the normally transitionally and reversely magnetized rhyolite domes named cerro del abrigo cerro santa rosa i and cerro santa rosa ii respectively that erupted following collapse of the archetypal valles caldera new mexico we have collected new paleomagnetic data from the three domes and new 40 ar 39 ar laser fusion and furnace incremental heating experiments on sanidine crystals from the cerro santa rosa i rhyolite step wise alternating field and thermal demagnetization techniques applied to 52 samples from seven sites indicate that the original paleomagnetic results of doell and dalrymple science 152 1966 1060 1061 are valid cerro del abrigo is normally magnetized whereas the santa rosa i dome is transitional with an inclination of 363 toward the east and the santa rosa ii dome is of reversed polarity twenty five laser fusion experiments on sanidine crystals from the the cerro santa rosa i dome together with the saddle shaped spectrum obtained by incrementally heating the sanidine in a furnace reveal that this rhyolite contained a small but significant component of excess argon prior to eruption our preferred age of 936 8 ka 2c for the santa rosa i rhyolite is based on the concordant laser fusion isochron and incremental heating plateau ages this age is significantly older than was inferred on the basis of earlier laser fusion 40 ar 39 ar results that suggested a trapped component characterized by a 40 ar 36 ar ratio lower than the atmospheric value of 295 5 our new age distinguishes the santa rosa i dome by 65 kyr from the termination of the jaramillo normal subchron 1001 10 ka and by 37 kyr from basaltic lavas at haleakala and tahiti which record the kamikatsura event 899 6 ka moreover this determination is a factor of three more precise than the best previous estimate for a proposed geomagnetic event in this time period the cerro santa rosa i rhyolite dome once intimately linked with the end of the jaramillo event and the acceptance of plate tectonic theory now defines a highly resolved feature the santa rosa event in a terrestrial geomagnetic reversal time scale that is consistent with the global record of magnetic field intensity from marine sediments fl 2002 elsevier science b v all rights reserved

geomagnetic moment instability between 0 6 and 1 3 ma from cosmonuclide evidence 1 the reliability of paleomagnetic records as proxies of the geomagnetic field intensity is still a matter of controversy since volcanic materials hardly provide continuous records and marine sediments are suspected to carry a remanence biased by post depositional realignments and or by overprints such long standing debate emphasizes the need for the development of methods independent from paleomagnetism to decipher geomagnetic intensity variations high resolution measurements of authigenic 10be 9be along with a detailed sedimentary record of directional and relative paleointensity variations evidence over the 0 6 1 3 ma time interval frequent and recurrent excursions or short events in the late matuyama and the early brunhes epochs among which two brunhes matuyama reversal precursors and an intra jaramillo excursion the results of this study confirm the idea of a highly unstable geomagnetic field as suggested by paleomagnetic evidences

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

deep volcanic tremor and magma ascent mechanism under kilauea hawaii deep harmonic tremor originating at depths around 40 km under kilauea was studied using records accumulated since 1962 at the hawaii volcano observatory of the u s geological survey the deep source of the tremor was determined by onset times and confirmed by the relative amplitude across the island wide network of seismometers the period of tremor was conclusively shown to be determined by the source effect and not by the path or station site effect because the period would change considerably in time but maintained uniformity across the seismic net during the tremor episode the tremor appeared to be primarily composed of p waves we interpret the observed period and amplitude in terms of the stationary crack model of aki et al 1977 and find that the seismic moment rates for deep tremors are considerably larger than those for shallow tremors suggesting more vigorous transport for the former we propose a kinematic source model which may be more appropriate for deep tremor according to this model a measurable quantity called reduced displacement is directly proportional to the rate of magma flow a systematic search for deep tremor episodes was made for the period from 1962 through 1979 and the amplitude period and duration of the tremor were tabulated we then constructed a cumulative reduced displacement plot over the 18 year period the result shows a generally steady process which does not seem to be significantly affected by major eruptions and large earthquakes near the surface the total magma flow estimated from the reduced displacement is however one order of magnitude smaller than that estimated by swanson 1972 it may be that most channels transport magma aseismically and only those with strong barriers generate tremor

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

a relative geomagnetic paleointensity stack from ontong java plateau sediments for the matuyama we present a stack of geomagnetic paleointensity estimates for the matuyama chron 0 778 2 582 ma sedimentary magnetizations from 4 ocean drilling program odp cores from the ontong java plateau ojp are analyzed with thermal techniques thellier thellier experiments are used to determine magnetic properties and indicate that a thermal treatment to 250 c can be used for paleointensity estimates the correlated records form a coherent ojp stack which is dated at reversal boundaries a coherent feature with two other data sets spanning the matuyama chron is the occurrence of a 150 kyr cyclicity however the time control of the ojp stack is not accurate except for the 10 tie points at the reversals this could at least in part explain why the three matuyama paleointensity records show no other coherent frequencies

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

evidence for the kamikatsura and santa rosa excursions recorded in eolian deposits from the southern chinese loess plateau 1 a detailed paleomagnetic investigation of a loess paleosol sequence in baoji southern chinese loess plateau reveals two distinct geomagnetic excursions accompanied by inferred relative paleointensity minima in the late matuyama chron rock magnetic experiments demonstrate that the sediments of the excursion intervals in loess unit l9 have the same magnetic properties as those from surrounding normal and reversed polarity intervals assuming constant sedimentation rate between the jaramillo matuyama and matuyama brunhes magnetic polarity boundaries the midpoint ages and durations for these two excursions are estimated at 0 89 ma and 16 kyr and 0 92 ma and 7 kyr respectively in addition these two events are separated by an interval with fully reversed polarity that had a duration of 16 kyr these estimates are supported by paleoclimatologically inferred age constraints the two excursions are probably correlated with the kamikatsura and santa rosa excursions respectively

reassessing the age of atapuerca td6 spain new paleomagnetic results we carry out a new paleomagnetic study across stratigraphic layer td7 of gran dolina site atapuerca and reassess the existing absolute ages thermoluminescence electron spin resonance obtained at this site the new and more detailed magnetostratigraphic record of the matuyama brunhes boundary combined with absolute ages enables further constraint on the age of the underlying human bearing level or aurora stratum paleomagnetism of the studied silts and sandstones reveals stable dual polarity characteristic remanent magnetization directions a new normal magnetic polarity short interval is found at several consecutive sampling localities in the upper part of td7 providing a new ante quem date for the human bearing sediments of 0 9 ma possibly formed during mis 25

paleomagnetic field intensity derived from non sd testing the thellier izzi technique on md slag and a new bootstrap procedure abstract experimental techniques to determine paleomagnetic field intensity are based on a theoretical framework that is valid only for single domain sd ferromagnetic particles yet most of the available materials exhibit distinctly non sd properties designing the optimal paleointensity methodology for non sd is therefore a fundamental challenge in paleomagnetism the objective of this study is to experimentally test the izzi thellier absolute paleointensity method on small md recorders the test has two purposes 1 to describe the characteristic non sd patterns occurring in arai plots and 2 to identify the optimal approach in interpreting non sd behavior we carried out paleointensity experiments on 40 specimens from 4 synthetic re melted slag samples with identical magnetic properties mineralogy texture and non sd state produced under different field intensities we ran three batches of izzi experiments using different conditions that allow for a detailed characterization of the non sd behavior we find that the curvature of the arai plot is systematically dependent on the angle and the proportion between the field used in the paleointensity experiment btrm and the field in which the nrm was acquired bnrm straight line arai plot occur when the two fields are parallel and equal and seems to always give the true slope convex curves occur when btrm is parallel and significantly stronger than bnrm concave curves occur in all the other cases and yield two end case slopes that are always different than the true slope in addition zigzagged patterns increase with the angle the proportion between btrm and bnrm we test the accuracy of the best fitting line approach and conclude that best fitting line in curved plots cannot provide robust paleointensity estimates yet the two end case slopes in concave curves provide adequate constraints for the true value we introduce a new procedure to calculate a 95 confidence interval of the paleointensity from curved plots using bootstrap statistics we substantiate the new procedure by conducting two independent tests the first uses synthetic re melted slag produced under known field intensities 3 sd samples and 4 non sd samples the second compares paleointensity determinations from archeological slag samples of the same age 34 sd samples and 10 non sd samples the two tests demonstrate that the bootstrap technique may be the optimal approach for non ideal dataset

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

wavelet analysis of deep tow magnetic profiles modeling the magnetic layer thickness over oceanic ridges 1 the interpretation of marine magnetic anomalies usually consists either in determining the magnetization distribution assuming the source geometry and magnetization direction or in determining the magnetic layer thickness assuming the magnetization direction and intensity in this paper we introduce a new technique that allows modeling of the thickness of the magnetic source layer with very few a priori assumptions about the magnetization the magnetic layer is assumed to be made of a series of bodies each having a constant unknown magnetization and an unknown size this technique is based upon the application of the continuous wavelet transform recently introduced for the interpretation of potential field data as a multipole decomposition we present applications to synthetic data to one deep tow magnetic profile recorded across the juan de fuca ridge jdf and to three deep tow magnetic profiles recorded across the central indian ridge cir our results confirm that despite significant source thickness variations 100 1200 m across the cir measured magnetic anomalies mostly reflect past geomagnetic field intensity fluctuations however we show that within the axial region of high magnetization thickness variations have a significant contribution to short wavelength variations of deep tow magnetic signals 100 nt

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

padm2m a penalized maximum likelihood model of the 0 2 ma palaeomagnetic axial dipole moment summary we present a new time varying model for palaeomagnetic axial dipole moment padm for the past 2 myr and compare it with earlier virtual axial dipole moment vadm reconstructions which have been based on stacking and averaging scaled relative palaeointensity records the padm is derived from both absolute and relative palaeointensity data and constructed using a new penalized maximum likelihood pml approach to recover a cubic b spline representation of axial dipole field variations on million year timescales the pml method is explicitly intended to reduce bias in estimating the true axial dipole moment that arises in average vadm reconstructions we apply the pml method to a set of 96 032 published data 1800 palaeointensities from igneous rocks 3300 archaeointensities and 86 relative palaeointensity time series of variable lengths and resolutions two models are discussed padm2mp is a trial model based on a subset of the nine longest available sedimentary records padm2m uses a comprehensive data set 76 records 81 446 data 10 records were eliminated and is our preferred model padm2m has a lower mean than existing vadm reconstructions but shows similarities in long period variability some differences in timing amplitude and resolution of certain features can be attributed to variations in age assignments others result from our more comprehensive data set and a reduction in bias attributable to pml modelling padm2m has an average axial dipole moment over 0 2 ma of 5 3 1022 am2 with a standard deviation of 1 5 1022 am2 the brunhes chron average 6 2 1022 am2 is higher than for earlier epochs of matuyama 4 8 1022 am2 as seen in some previous studies the power spectrum for our model agrees with previous estimates of the global palaeomagnetic power spectrum for frequencies up to about 102 myr 1 we see no distinctive evidence in the power spectrum for orbital forcing of geodynamo behaviour

multipolarization microwave scattering model for sahelian grassland a coherent scattering formulation is developed for radar remote sensing of sahelian grassland this african vegetation is mainly composed of annual grass and shrubs in the proposed procedure first a temporal model for generation of grass and shrub structures which includes important realistic botanical information is implemented because we develop a coherent scattering model preserving the relative position of plant elements in a statistical manner as accurately as possible is very important shrubs are reproduced using cylindrical elements which represent trunks branches and thin green stems that function as leaves for these shrubs their crown shape is highly irregular but for the most part can be encompassed in an ellipsoidal or cylindrical volume on the other hand the grass is represented as a set of cylindrical stalks and blade leaves the scattered power from each grass element is added because multiple scattering among adjacent elements can be neglected at microwave frequencies we calculate the soil scattering using the integral equation method and neglect the soil volume scattering which may become significant for dry soil condition at high incidence angles backscatter statistics are acquired via a monte carlo simulation over a large number of realizations the accuracy of the model is verified using measured data acquired by the c band environmental satellite advanced synthetic aperture radar instrument at different incident angles

temporal trends of magnetic dissolution in the pelagic realm gauging paleoproductivity abstract magnetic hysteresis data from the western equatorial pacific suggest that the preservation of magnetic remanence in pelagic carbonates is controlled by paleoproductivity hysteresis excursions 100 k y intervals are linked here to greater magnetic dissolution associated with increased glacial organic carbon supply changes in the redox boundary coupled to glacial terminations result in anomalous magnetic features which are preserved in the sedimentary record these high frequency variations are superimposed on a longer term signal of magnetic dissolution which may reflect decreased productivity since the miocene

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

a geomagnetic paleointensity stack between 0 8 and 3 0 ma from equatorial pacific sediment cores 1 we have conducted a paleomagnetic study of six sediment cores taken from the equatorial pacific three from the west caroline basin and three from the manihiki plateau in order to make a relative paleointensity stack during the matuyama and late gauss chrons the age of the bottom of the cores ranges from 1 2 to 3 0 ma the sediments show little downcore changes in the proxies of magnetic grain size and mineralogy and are hence suitable for estimating relative paleointensity the age of the cores is controlled by correlating variations in magnetic concentration magnetic susceptibility and or anhysteretic remanent magnetization arm to target oxygen isotope 18o curves all cores from the manihiki plateau show an upward decrease of the natural remanent magnetization nrm intensity normalized by arm and isothermal remanent magnetization irm with a decrease in sedimentation rate such a trend was removed before being converted to relative paleointensity this observation implies that a sedimentation rate change can affect relative paleointensity estimation relative paleointensity records from the six cores coincide well with each other within uncertainty of age we constructed a stacked curve between 0 8 and 3 0 ma the equatorial pacific paleointensity stack epapis 3ma after adjusting age the number of cores stacked is three to four throughout the record quasiperiodic paleointensity lows occur in the epapis 3ma and corresponding paleointensity lows can be found in previously published records with some shifts in age at least parts of the paleointensity minima seem to be accompanied by geomagnetic excursions the asymmetric sawtooth pattern of paleointensity variations is not observed at the matuyama gauss boundary and thereafter in our record a spectral analysis shows that 100 kyr orbital eccentricity frequency may exist in paleointensity variations during the matuyama and late gauss chrons

a microwave scattering model for layered vegetation a microwave scattering model was developed for layered vegetation based on an iterative solution of the radiative transfer equation up to the second order to account for multiple scattering within the canopy and between the ground and the canopy the model is designed to operate over a wide frequency range for both deciduous and coniferous forest and to account for the branch size distribution leaf orientation distribution and branch orientation distribution for each size the canopy is modeled as a two layered medium above a rough interface the upper layer is the crown containing leaves stems and branches the lower layer is the trunk region modeled as randomly positioned cylinders with a preferred orientation distribution above an irregular soil surface comparisons of results obtained using this model with measurements from deciduous and coniferous forests show good agreement at several frequencies for both like and cross polarizations

a sedimentary paleomagnetic record of the matuyama chron from the western antarctic margin odp site 1101 a high resolution paleomagnetic record for part of the matuyama chron 0 7 2 1 ma is reported for ocean drilling program site 1101 leg 178 off the antarctic peninsula pacific margin a rock magnetic investigation of 62 discrete samples revealed that the natural remanent magnetization nrm is carried by pseudo single domain magnetite progressive alternating field demagnetization of 83 m of u channels provided a polarity stratigraphy down to the olduvai subchron two geomagnetic events preceding the jaramillo subchron were identified including the cobb mountain polarity interval the bulk magnetic parameters vary by more than a factor of 20 over the entire time interval but by less than a factor of 6 over the 0 7 1 1 ma interval this latter interval was selected for paleointensity determinations which were carried out by normalizing the nrm by the anhysteretic remanent magnetization arm direct comparison of the site 1101 paleointensity record with other curves available for the same time interval suggests a geomagnetic origin for features present in the record a more quantitative comparison was achieved by means of a jackknife test performed on nine records of relative paleointensity over the 0 95 1 1 ma interval this test yielded no outlier for the period considered confirming the geomagnetic character of the records we have constructed a lowresolution stack revealing some of the characteristic paleointensity features of the jaramillo subchron fl 2001 elsevier science b v all rights reserved

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

volcanic earthquakes and their relationship to eruptions at ruapehu and ngauruhoe volcanoes abstract crustal earthquakes near ruapehu and ngauruhoe fall into two classes each of which can be subdivided on the one hand there are high frequency events 3 hz with sharp well defined phases mainly concentrated beneath ruapehu crater lake low frequency events the low frequency events resemble minakami s b type and explosion earthquakes but sometimes occur where no vent exists and rather deeper than his formal definition difficulty in applying minakami s classification rigorously and the fact that low frequencies may be due to abnormal attenuation of higher frequencies along the path rather than to their suppression or absence at the source has led to reclassification of earthquakes near the volcanoes into two broad groups tectonic and volcanic the former includes all high frequency earthquakes and those discrete events in which dominant low frequencies are due to path effects the latter includes multiple and emergent events which show evidence of prolonged or repetitive source mechanism dominant low frequencies are ascribed to occurrence in heat weakened material and high frequencies to instantaneous source mechanisms operating in competent rock the term volcano tectonic describes tectonic earthquakes within some arbitrary distance of a volcano at ngauruhoe and ruapehu volcanic earthquakes accompany explosive vent clearing eruptions subsequent open vent degassing and ash emission however although often powerful and prolonged usually occurs without earthquakes such activity is however frequently accompanied by volcanic tremor at ruapehu under closed vent conditions when lake temperature is low low frequency earthquakes up to magnitude ml 3 4 have occurred without any eruption five types of phreatic eruptions are identified at ruapehu each having a distinctive seismic pattern the three most explosive types appear to be generated by a chain reaction process and all involve flashing of water to steam the first by failure of the roof with little precursory seismicity after a closed vent period during which lake temperature decreases the second after prolonged heating of the lake and much preliminary volcanic tremor interpreted as due to rising magma and the third under open vent conditions in the wake of one of the two preceding types a fourth probably occurs in wet sediments near the base of the lake as a result of upward migration of hot gas and a fifth aseismic or accompanied by very weak volcanic tremor is associated with convective overturn within crater lake

normalized remanence in sediments of the western equatorial pacific relative paleointensity of the geomagnetic field piston cores from the ontong java plateau collected during the eurydice expedition have been used to estimate regional geomagnetic paleointensity rock magnetic measurements of the sediments show that 1 the remanence is carried by stable pseudo single domain magnetite 2 the grain sizes and concentrations are relatively uniform with respect to the normalization technique and 3 susceptibility is an adequate parameter for normalizing remanent intensity in order to determine relative variations in the intensity of the paleomagnetic field we therefore have measured the remanent intensity both natural and demagnetized to 15 mt and susceptibility of archive quarters of three cores at centimeter intervals using an automated through bore magnetometer a further test of the normalized intensity records is provided by the coherence function between the normalized remanence and the susceptibility of the cores one of our cores displays high coherence over a broad range of frequencies while for two others coherence is insignificantly different from zero at the 95 level of confidence oxygen isotopic records paleontological datum levels and the brunhes matuyama boundary provide an independent temporal framework for inter core comparison when the two cores which pass our acceptance criteria are placed on a common time scale the coherence between the normalized intensity records is significantly different from zero at well above the 95 level of confidence suggesting some common input signal over the same band of frequencies furthermore there is a great deal of similarity between the records from the ontong java plateau and a record from core rc10 167 taken some 3000 km to the north the distribution of intensities obtained from these sedimentary records has the same shape as the absolute virtual dipole moment data obtained from lava flows 0 5 ma with a slightly smaller standard deviation the inter core agreement within a single region the similarity of records from 3000 km apart and the general correspondence of statistical character of these sediments with lava flow data all argue that the normalized intensity records presented here reflect variations in the intensity of the ancient geomagnetic field spectral analysis of the paleointensity time series shows that there a is significant enhancement of variance concentrated in frequencies around 33 thousand years over the period 0 350 ka this feature is however lacking for the period 350 700 ka

radar signatures of sahelian surfaces in mali using envisat asar data this paper presents an analysis of ensivat advanced synthetic aperture radar data acquired over a sahelian region located in mali west africa the considered period is 2004 2005 and includes two rainy seasons emphasis is put on two scansar modes namely the global monitoring gm and the wide swath ws modes characterized by spatial resolutions of about 1 km and 150 m respectively results show that the ws mode offers better performance in terms of radiometric resolution radiometric stability and speckle reduction than the gm mode the latter is more appropriate for studies at large scale 10 times 10 km in both modes pronounced angular and temporal signatures are observed for most soil surfaces and azimuthal effects are observed on markedly orientated rocky surfaces in contrast polarization differences vv hh are small during the dry season except on flat loamy soil surfaces finally a relationship is observed between the normalized ws backscattering signal at hh polarization and the surface soil moisture of sandy soils

relative paleointensity rpi and oxygen isotope stratigraphy at iodp site u1308 north atlantic rpi stack for 1 2 2 2 ma narpi 2200 and age of the olduvai subchron 2015 elsevier ltd integrated ocean drilling program iodp site u1308 49 53 n 24 14 w water depth 3871 m provides a record of relative paleointensity rpi and benthic stable isotope stratigraphy back to 3 2 ma the record since 1 5 ma was published previously and here we present the interval from 1 5 ma to 3 2 ma early pleistocene late pliocene the benthic oxygen isotope record in this interval is correlated to marine isotope stage mis 51 to km2 with an apparent hiatus that removed part of the interval spanning mis 104 g2 2 6 2 65 ma at the gauss matuyama boundary the mean sedimentation rate for the 1 5 3 2 ma period is 8 5 cm kyr the age model was built by correlation of the benthic oxygen isotope record to a global stack lr04 apart from the expected polarity reversals three magnetic excursions are recorded punaruu in mis 31 32 at 1092 ka gilsa in mis 54 55 at 1584 ka and a newly recognized excursion labeled porcupine after the nearby porcupine abyssal plain in mis g6 g7 at 2737 ka the ages of polarity reversals at site u1308 on the lr04 time scale are consistent with the current geomagnetic polarity timescale gpts with the exception of the base of the olduvai subchron that occurred in mis 73 corresponding to 1925 ka on the lr04 time scale 25 kyr younger than in the current gpts the rpi record at site u1308 is calibrated using the oxygen isotope age model and combined with four other north atlantic records to obtain a north atlantic rpi stack for 1 2 2 2 ma narpi 2200 that is compared with published rpi stacks epapis sint 2000 and padm2m for 2 2 3 2 ma the site u1308 rpi record is compared with a rpi record from north atlantic iodp site u1314 and with the pacific epapis stack the mean sedimentation rates of the north atlantic sites in narpi 2200 are greater by about an order of magnitude than most of the records incorporated in other stacks the comparison of epapis and narpi 2200 yields an apparent lag for narpi 2200 relative to epapis attributed the epapis age model constructed by correlation of magnetic concentration parameters a proxy for carbonate percentage to a calibrated oxygen isotope record the long rpi record from site u1308 yields a very similar mean value for the brunhes and matuyama virtual axial dipole moments 7 05 1022 am2 implying no polarity bias in the strength of the main geomagnetic dipole in contrast to interpretations from sint 2000 and padm2m the results strengthen the case that rpi can be used to improve global stratigraphic correlation for sites with mean sedimentation rates up to several decimeters kyr

maghemite to magnetite reduction across the fe redox boundary in a sediment core from the ontong java plateau influence on relative palaeointensity estimation and environmental magnetic application summary during reduction diagenesis production of dissolved fe2 by reduction of ferric oxide starts at the fe redox boundary the associated magnetic property changes may influence palaeomagnetic and environmental magnetic records of marine sediments however this has not been evaluated thoroughly in this study using a gravity core from the ontong java plateau we document in detail rock magnetic changes across the fe redox boundary and investigate their influence on relative palaeointensity estimation and on a magnetic proxy for the proportion of terrigenous biogenic magnetic minerals the magnetic mineral assemblage above the fe redox boundary is characterized by a component with a mean coercivity of 100 mt in isothermal remanent magnetization irm component analyses and low s ratios s 0 1t low temperature irm measurements and mossbauer spectroscopy indicate that the degree of maghemitization is higher above the fe redox boundary from these observations we conclude that reduction of maghemite to magnetite occurs at the fe redox boundary and we infer that a maghemite skin on magnetic grains is lost across the boundary relative palaeointensity variations obtained by normalizing nrm intensity with sirm agree well with regional and global palaeointensity stacks which suggests that relative palaeointensity estimation is not significantly affected by the fe redox boundary temporal variations of the ratio of anhysteretic remanent magnetization susceptibility and saturation irm arm sirm coincide with the regional pattern across the ontong java plateau it is also possible to estimate variations in the proportion of terrigenous to biogenic components using the arm sirm ratio across the fe redox boundary

analysis of 11 myr of geomagnetic intensity variation we have conducted a detailed exploratory analysis of an 11 million year long almost continuous record of relative geomagnetic paleointensity from a sediment core acquired on deep sea drilling project leg 73 at site 522 in the south atlantic we assess the quality of the paleointensity record using spectral methods and conclude that the relative intensity record is minimally influenced by climate variations isothermal remanence is shown to be the most effective normalizer for these data although both susceptibility and anhysteretic remanence are also adequate statistical analysis shows that the paleointensity variations follow a gamma distribution and are compatible with predictions from modified paleosccular variation models and global absolute paleointensity data when subdivided by polarity interval the variability in paleointensity is proportional to the average and further the average is weakly correlated with interval length spectral estimates for times from 28 77 until 22 74 ma when the reversal rate is about 4 myr 1 are compatible with a poisson model in which the spectrum of intensity variations is dominated by the reversal process in the frequency range 1 50 myr 1 in contrast between 34 7 and 29 4 ma when the reversal rate is about 1 6 myr 1 the spectra indicate a different secular variation regime the magnetic field is stronger and more variable and a strong peak in the spectrum occurs at about 8 myr 1 this peak may be a reflection of the same signal as recorded by the small variations known as tiny wiggles seen in marine magnetic anomaly profiles

seismicity associated with the 1995 1996 eruptions of ruapehu volcano new zealand narrative and insights into physical processes abstract seismicity associated with the 1995 1996 eruptions of ruapehu reflects processes associated with the intrusion of magma to shallow depth in the volcanic edifice its subsequent eruption and changes in the volcanic plumbing system resulting from the eruptions the sequence consisted of two distinct periods of eruptive activity the first lasting from september 17 1995 until late october or early november 1995 and the second from june 16 1996 until late july 1996 immediately prior to and during the early stages of the 1995 eruptions seismicity was similar to that recorded at ruapehu during the previous 25 years with the exception of the occurrence between september 17 and 25 1995 of sub 1 hz tremor which is thought to represent magmatic intrusion in early october a new pattern of seismicity was established with the disappearance of the 2 hz tremor resonator source and a change to wideband 2 10 hz tremor and wideband volcanic earthquakes this change coincided with the ejection of the last vestiges of crater lake and the change in eruptive style from phreatomagmatic to magmatic seismicity associated with the 1996 eruptions was very similar to that which accompanied the later part of the 1995 activity implying the same eruption processes and that no significant changes had occurred in the volcanic plumbing system during the intervening period geochemical geodetic and seismic precursors to the 1995 eruptions were both minor and inconsistent highlighting the difficulty in forecasting these eruptions furthermore deep volcano tectonic earthquakes were extremely rare throughout the 1995 and 1996 eruptive sequences suggesting that stresses associated with magmatic intrusion were minor this most likely resulted from the existence of either an open or a ductile pathway from the deep magma source to the surface prior to the eruptions and because the volume of magma intruded and subsequently erupted was relatively small 3

seismicity at white island volcano new zealand a revised classification and inferences about source mechanism the classification of earthquakes at white island volcano new zealand has been revised to address problems in existing classification schemes to better reflect new data and to try to focus more on source processes seismicity generated by the direct involvement of magmatic or hydrothermal fluids are referred to as volcanic and that generated by fault movement in response to stresses caused by those fluids regional stresses thermal effects and so on are referred to as volcano tectonic spasmodic bursts form a separate category as we have insufficient information to classify them as volcanic or volcano tectonic volcanic seismicity is divided into short duration long period volcanic earthquakes long duration volcanic earthquakes and harmonic and non harmonic volcanic tremor while volcano tectonic seismicity is divided into shallow and deep volcano tectonic earthquakes harmonic volcanic tremor is related to sub surface intrusive processes while non harmonic volcanic tremor originates close to active craters at shallow depth and usually occurs during eruptive activity short duration long period volcanic earthquakes come from a single source close to the active craters but originate deeper than non harmonic volcanic tremor and are not related to eruptive activity long duration volcanic earthquakes often accompany larger discrete eruptions the waveform of these events consists of an initial low frequency part from a deep source and a later cigar shaped part of mixed frequencies from a shallow crater source

superparamagnetism and reduction diagenesis in pelagic sediments enhancement or depletion superparamagnetic grains ultra fine d 0 05 m particles unable to retain a remanence at room temperature have come under increased scrutiny as indicators of the diagenetic and authigenic history of sedimentary rocks in marine sediments fine grained magnetite with large surface area to volume is thought to be preferentially removed during magnetic mineral reduction on the basis of new magnetic hysteresis low temperature and geochemical studies of pelagic sediments from the western equatorial pacific ocean it is proposed that reduction processes increase rather than deplete the ultra fine magnetic grain population at the modern fe redox boundary enhanced superparamagnetism coincides with a coarsening of remanence carrying grains superparamagnetism also tracks proposed temporal changes in magnetite reduction caused by climatically driven fluctuations in organic carbon corg supply together with hysteresis characteristics changes in superparamagnetism may help the identification of paleointensity artifacts resulting from non steady state magnetic mineral reduction

strengths and weaknesses of multi year envisat asar backscatter measurements to map permanent open water bodies at global scale abstract the mapping of water bodies at global scale has been undertaken primarily using multi spectral optical earth observation data limitations of optical data associated with non uniform and temporally variable spectral signatures suggested investigating alternative approaches towards a more consistent and reliable detection of water bodies multi year 2005 2012 observations of sar backscattered intensities at moderate resolution from the envisat advanced synthetic aperture radar asar instrument were used in this study to generate an indicator of open permanent water bodies sar wbi for the year 2010 time frame and for all land surfaces excluding antarctica and the greenland ice sheet a first map of potential water bodies with a spatial resolution of 150 m was obtained with a global detection algorithm based on a set of thresholds applied to multi temporal metrics of the sar backscatter temporal variability tv and minimum backscatter mb local refinements were then used to reduce systematic commission and omission errors 4 6 of the total area mapped due to the similarity of tv and mb over open water bodies and other land surface types primarily in cold and arid environments the refinement rules are here explained by means of a detailed signature analysis of the sar backscatter in such environments the accuracy of the sar wbi was 80 when compared against 2078 manually interpreted footprints with a size of 150 150 m2 omission errors were primarily observed along coast and shorelines whereas commission errors were associated with i ephemeral water bodies ii seasonally inundated areas and iii an incorrect choice of the local refinement

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

a stacked record of relative geomagnetic paleointensity for the past 270 kyr from the western continental rise of the antarctic peninsula paleomagneticandrockmagnetic investigations were carriedout onfourgravity coresrecoveredfrom the westerncontinental rise of the antarctic peninsula during the sedano ii cruise of rv ogs explora the studied cores each about 6 5 m long were collected at a depth of 3700 4100 m below the sea level on the distal gentle side of sediment drift 7 and consist of very fine grained sediments spanning through various glacial interglacial cycles detailed analysis of the paleomagnetic and rock magnetic data allowed toreconstruct relative paleointensity rpi records nrm20 mt arm20 mt for eachcore weestablished arefined agemodel for the studied sequences by correlating individual sedano rpi curves to the global rpi stack sint 800 y guyodo j p valet global changes in intensity of the earth s magnetic field during the past 800 kyr nature 399 1999 249 252 the individual normalized sedano rpi records are in mutual close agreement they were thus merged in a rpi stacking curve spanning the last 270 kyr and showing a low standard deviation this study also points out that rpi records may provide a viable tool to date otherwise difficult to datesedimentarysequences suchasthosedepositedalongperi antarctic margins thenewrpichronologyindicatesthat thesampled sedimentary sequenceisyoungerthanpreviouslythought andallowsanew high resolutioncorrelation tooxygenisotope stages furthermore we recognized variations in the rock magnetic parameters that appear to be climatically driven with changes in therelative proportionoftwomagnetic mineralpopulationswithdistinctcoercivities rockmagnetic andlithological trendsobserved in the sedano cores indicate that during the climatic cycles of the late pleistocene this sector of the peri antarctic margin was subjected to subtle yet identifiable environmental changes confirming a relatively higher instability of the west antarctic ice sheet with respect to the east antarctic counterpart

volcanic tremors at etna a model for hydraulic origin spectral analysis of volcanic tremors at etna reveals 2 characteristic features 1 the escaping vapor and gas streams drive the magma in random turbulent motion generating a seismic wave emission according toa f n exp m f k wherea amplitude f frequency n m k parameters 2 sudden transients in the boundary conditions of the piping system generate hydraulic pressure pulses which in the seismogram appear as wave packages their monochromatic frequency content suggests that their origin is associated with strong resonance phenomena it is conclusive that volcanic eruptions may be distinguished and classified according to different eigenfrequencies of the fissure and pipe network involved in the volcanic activity furthermore positions of active dykes may be traced following the amplitude distributions associated with pronounced spectral lines in the tremor spectra

reply to comment by j p valet and l meynadier on a relative geomagnetic paleointensity stack from ontong java plateau sediments for the matuyama

relative variations in geomagnetic intensity from sedimentary records the past 200 000 years abstract eighteen records of relative paleointensity from marine sediments have been compiled in order to retrace the global changes in geomagnetic intensity during the past 200 kyr before dealing with any other source of noise we focused on problems inherent to the depth versus time correlations the accuracy and resolution of dating mostly deduced from isotopic stratigraphy were identified as significant factors of distortion we found also that different time scales have been used by authors after readjustment with respect to the same time scale the records were stacked together and the statistical significance of the results tested by a bootstrap technique seventeen records shown to be coherent were combined into a synthetic curve defined as sint 200 other stacks combining various subsets of data were found to be identical to sint 200 except for changes in dispersion in fact the dispersion around sint 200 could be reduced further by improving the correlations without violating the stratigraphy a procedure which at least for some records emphasizes the need for more detailed depth time correlations another characteristic emerging from the results is that paleointensity features as short as 10 kyr are observed between 120 200 ka and can thus in principle be resolved however subsequent refinement is still necessary for this period which is documented by fewer records than the following 120 kyr finally we compared sint 200 with the records from the labrador sea and the lac du bouchet ldb both of which rely mostly on 14c chronology the two records show satisfactory agreement with sint 200 when transferred with respect to the age model established by correlating their 18o labrador and susceptibility features ldb with the 18o reference record the offset of 7 kyr at 40 ka measured in both cases between the 14c and 18o ages is higher than the value deduced from a recent model of a geomagnetic field control of 14c production 1 but is not inconsistent with recent u th dates

a volcanic tremor monitoring system abstract the digitising system installed at the chateau observatory in the north island of new zealand to monitor ruapehu and ngauruhoe volcanoes is used to digitise the seismic records from three seismometers and to calculate the power spectra of volcanic tremor in real time with the smoothed power spectra being recorded every 5 minutes on magnetic disk the 5 minute averages are later processed to remove the effects of earthquakes and interference before hourly averages are calculated this system has a better dynamic range than visual chart records and analysis of the data requires much less labour

soil moisture mapping over the central part of the tibetan plateau using a series of asar ws images abstract the wide swath ws and global monitoring gm modes of the advanced synthetic aperture radar asar provide for monitoring purposes a good compromise between the spatial coverage spatial resolution and revisit time similar imaging modes will be a part of the sentinel 1 sar sensing system a consequence of operating at the wider swath is however the rather large view angle range about 16 43 which needs to be corrected for if biophysical variables are to be retrieved from these data sets a soil moisture retrieval algorithm based on the integral equation method iem scattering model is presented that explicitly simulates the backscatter as a function of the incidence angle instead of normalizing this algorithm is applied to 150 asar ws scenes collected from april 2005 to september 2007 over the central part of the tibetan plateau we selected three of these images covering the study area at different view angles during the winter season for the iem roughness derivation subsequently soil moisture is retrieved from the entire o data sets via inversion of the iem model with the obtained roughness parameters as input the retrievals are validated against soil moisture measured from july 2006 to september 2007 at four sites representative for a 10 10 km 2 area further the uncertainties following from assumptions made for the development of the algorithm are also quantified

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

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

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

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