dual role of microcracks toughening and degradation one of the methods of improving the mechanical properties of ceramics is to introduce a defective structure that acts as a restraint for the propagation of cracks in the present study a detailed investigation was carried out by introducing a defective structure in rock to determine if there is any improvement in properties similar to ceramics granite was chosen for the investigation and the microcracks were introduced by a heating and cooling cycle uniaxial compression tests have shown that granite thermally treated to 200 c shows the highest strength and the strength of granite treated to 400 c is comparable to that of unheated granite both ultrasonic images and acoustic emission monitoring show that for thermally treated samples the stress induced microcrack and macrocrack nucleation and their growth are retarded the variations in mechanical properties are explained based on the concept of toughening and degradation uniaxial compression tests on unheated and thermally treated granite samples have clearly established the dual role of microcracks which operate in the toughening and degradation mechanisms

alluvial charcoal in the sigatoka valley viti levu island fiji abstract charcoal concentrations in alluvial sediments throughout the sigatoka and adjacent catchments western viti levu island fiji were located sampled and where possible dated the earliest date 5579 5052 cal year bp almost certainly predates human arrival and represents a natural fire perhaps associated with drought conditions during an el nino event the next three dates are clustered around the time just after initial human arrival in fiji 2900 2700 cal year bp and could mean that humans transited the area 40 km inland within this time the next date 1520 1260 cal year bp is not associated with any other the final five dates are likewise clustered around the time of the ad 1300 event and are believed to mark the time when permanent inland upland settlement was established in this part of viti levu

on the mechanism of formation of depositional remanent magnetization 1 coagulation of particles into aggregates during their settling in an aqueous solution is numerically simulated with regard to brownian motion van der waals and stokes s forces gravitation and magnetostatic interactions clusters obtained have a fractal structure with the average fractal dimension d 1 83 magnetic grains do not group until their concentration exceeds at least a few percent the deposition process obeys a scaling principle the sizes of clusters arriving at the bottom of a basin do not change if the product of the basin depth h and the concentration of initial material c0 is constant attempts at numerical simulations of laboratory redeposition experiments are made good agreement between numerical simulations and experimental results by van vreumingen 1993 demonstrates that the modeling algorithm is based on reasonable physical assumptions the magnetization of a flocculating suspension is defined by at least seven parameters which characterize magnetic and nonmagnetic particles as well as the aqueous medium this multiparametric dependence hinders estimations of paleofield intensity by the redeposition method because it is practically impossible to reproduce natural conditions in the laboratory flocculation influences the magnetization intensity of the settling suspension at concentrations c0 typical for redeposition experiments or natural sedimentation in lakes and shallow seas flocculation is of minor importance for deep oceanic regions because of their extremely low sedimentation rate however factors like small scale turbulence and biotic processes are not taken into account by the model and may require modification of these conclusions also a simple model of pdrm acquisition based on elastic and plastic properties of sediment slurry is proposed

new advances for paleomagnetic studies of sediment cores using u channels accurate and detailed paleomagnetic data can be obtained from continuous measurements of u channels extracted from sedimentary cores using a small diameter pass through magnetometer and high resolution signal response pick up coils the spatial resolution is not significantly lower than that obtained by measuring standard paleomagnetic samples and features on the order of 3 5 cm can be confidently resolved we propose a method to analyze the demagnetization data based on the angular dispersion of the directions between successive demagnetization steps and the downcore changes in the rate of intensity decrease this technique allows us to distinguish very rapidly between intervals with well identified characteristic magnetic components and zones characterized by a more complex behavior during demagnetization it is also possible to directly measure the magnetic susceptibility and to induce and measure most rock magnetic properties along the u channels thus all parameters necessary for a complete paleomagnetic study of sediment cores can be confidently obtained directly from u channels without extracting individual samples finally this equipment makes possible the measurement and complete demagnetization of up to 10 single samples at a time in less than one hour by using techniques similar to those developed for the u channels

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

the matuyama brunhes transition recorded from lake tecopa sediments california abstract the paleomagnetic record of the matuyama brunhes reversal in the sediment of ancient lake tecopa provided the first evidence that the geomagnetic field during transition is not dipolar we resampled the section to increase the time resolution of the record our new data show that the characteristic direction could not be completely isolated by af demagnetization the transitional directions obtained from thermal demagnetization are thus quite different from those of the previous study but the primary conclusion that the field was non dipolar remains the vgp paths lying close to the site longitude is consistent with an axisymmetric transition field

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

α β phase transition in quartz monitored using acoustic emissions summary it is usually suggested that thermal cracking in a quartz bearing rock results from the anomalously high volumetric expansion coefficients of quartz e g simmons cooper 1978 it has also been recognized that thermal expansion mismatch and mineral anisotropy contribute to thermal cracking in materials that consist of a polycrystalline aggregate composed of several anisotropic minerals even in the absence of a temperature gradient friedrich wong 1986 experiments investigating thermal cracking in rocks commonly involve imaging and quantitative stereology of crack populations induced in rocks treated to various peak temperatures e g friedrich johnson 1978 homand etienne troalan 1984 atkinson mcdonald meredith 1984 meredith atkinson 1985 here we report on acoustic emission experiments that monitor the process of thermal cracking as it occurs during heating supported by measurements of crack surface area pore fluid permeability porosity and surface conductivity carried out on rock samples treated to various peak temperatures the acoustic emission measurements show a strong peak of microcracking at the phase transition temperature for quartz 573 c superimposed upon a background of microcracking due to thermal expansion there is also a clear peak of microcracking at higher temperatures 800 c that can be attributed to oxidation dehydroxylation reactions of hornblende and chlorite measurements of fluid permeability pore surface area porosity and electrical conductivity made on samples that have been heat treated to various maximum temperatures show increases associated with a major episode of cracking in the 500 600 c temperature range indicating that the new cracks form a well interconnected network this has been confirmed by sem and optical microscopy these results have implications for the electrical conductivity of the continental crust providing a mechanism enabling the high pore fluid connectivity needed to explain zones of high electrical conductivity at depth providing that cracks opened in this way remain open at the high pressures existing at depth it should be recognized however that th se measurements are limited in their direct application since they were obtained under initially dry conditions at laboratory pressures

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

can heterogeneous core mantle electromagnetic coupling control geomagnetic reversals abstract an analytical model is developed for the electromagnetic torques exerted on the mantle by the poloidal magnetic field of the core interacting with a laterally heterogeneous conducting layer at the base of the mantle torques due to changes in both orientation and intensity of a dipolar poloidal field are included contrary to earlier suggestions our calculations predict that the trajectory of the magnetic pole during a polarity reversal is not strongly affected by heterogeneity in mantle electrical conductivity

common evolution of mechanical and transport properties in thermally cracked westerly granite at elevated hydrostatic pressure increasing the damage and crack porosity in crustal rocks can result in significant changes to various key physical properties including mechanical strength elastic and mechanical anisotropy and the enhancement of transport properties using a non interactive crack effective medium nic theory as a fundamental tool we show that elastic wave dispersion can be inverted to evaluate crack density as a function of temperature and is compared with optically determined crack density further we show how the existence of embedded microcrack fabrics in rocks also significantly influences the fracture toughness kic of rocks as measured via a suite of tensile failure experiments chevron cracked notch brazilian disk finally we include fluid flow in our analysis via the gueguen and dienes crack porosity permeability model using the crack density and aspect ratio recovered from the elastic wave velocity inversion we successfully compare permeability evolution with pressure with the laboratory measurements of permeability

a late holocene record of human impact from the southwest coast of new caledonia a late holocene vegetation record is presented from the southwest coast of new caledonia lac saint louis is a freshwater swamp at 3 m a s l adjacent to the river coulee delta pollen analysis charcoal analysis radiocarbon dating and stratigraphic analyses have been used to reconstruct the vegetation and sedimentary history of the swamp the sediment record commences at 6000 bp and reflects rapid flood plain development associated with postglacial sea level rise this rapid accumulation of sediment ceased around 5500 bp and the site became a freshwater swamp from 5500 to 3000 bp the pollen record reflects several aspects of this coastal landscape coastal forest the swamp surface and the adjacent mangrove zone at 3000 bp charcoal levels increase significantly in conjunction with abrupt changes in the pollen record mangrove pollen declines and pollen indicative of coastal forest is replaced by pollen indicative of coastal savanna these changes coincide with the commencement of the archaeological record in new caledonia and mirror similar changes in other late holocene records elsewhere on the island and are interpreted as representing human impact yam terraces surrounding lac saint louis may have been built around 2000 bp based on mineral magnetic measurements and charcoal accumulation the decline in mangrove vegetation adjacent to site may be the result of a late holocene fall in sea level increased sedi ment accumulation along the coastal foreshore as a result of forest clearance in the river coulee s catch ment however is also considered

demise of the rapid field change hypothesis at steens mountain the crucial role of continuous thermal demagnetization abstract rapid continuous thermal demagnetization in a triaxe vibrating sample magnetometer disproves an earlier hypothesis that an episode of extraordinarily fast field change is recorded in the remanent magnetization of a single lava flow that cooled during a geomagnetic polarity reversal the original evidence for this hypothesis was the distinctive pattern of variation in direction with vertical position after stepwise demagnetization to 500 c but continuous thermal demagnetization now proves that this flow actually carries consistent high temperature remanence parallel to that of the flow immediately below why does continuous thermal demagnetization succeed when conventional thermal and alternating field demagnetization fail to isolate this direction detailed investigation of the demagnetization trajectories and underlying rock magnetic properties suggest that normal polarity secondary remanent magnetization acquired both at room temperature in today s polarity chron and during modest reheating in a normal field during cooling of the overlying flow is responsible for the misleading pattern of directions alteration during ordinary thermal demagnetization evidently promotes the overprint to higher blocking temperatures so that it masks the primary component whereas the rapid heating 40 c min during continuous demagnetization is fast enough to demagnetize the normal overprint before this masking can happen

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

environmental change in the late pleistocene and later holocene at wanda site soroako south sulawesi indonesia abstract this paper reports the pollen analysis of an 8 m core from a mire at 445 m altitude and 2 s latitude on ultramafic soils near the western shore of lake matano south sulawesi the area is almost undisturbed by humans and the record is believed to cover about 50 75 000 yr b p with some breaks in the record a section from 17 000 to around 5500 yr b p and the last 1000 years of the record may be missing or disturbed the results show that montane forest grew around the site continuously through the late pleistocene with a maximum increase in higher altitude taxa from 35 000 to after 17 000 yr b p the time of glacial maxima elsewhere also in this period increases in grass and casuarina indicate possible drier conditions the holocene sections record lower altitude forest with increased taxonomic diversity including secondary species the results show that vegetation in the region has been sensitive to climatic change the pleistocene ecology being consistent with a temperature change of about 2 5 3 5 c with a phase up to 50 drier than present the site climate was possibly affected by shelf exposure in the gulf of bone due to lower sea levels however the tropical forest demonstrates overall long term stability in which changes in dominance may reflect minor shifts in disturbance and tree longevity a distinctive record of fine charcoal occurs throughout the deposit this shows that the forest has never been continuously fire free for a very long time the cause of the fires may be natural as no evidence for human occupation was discerned

imaging slow failure in triaxially deformed etna basalt using 3d acoustic emission location and x ray computed tomography we have deformed basalt from mount etna italy in triaxial compression tests under an effective confining pressure representative of conditions under a volcanic edifice 40 mpa and at a constant strain rate of 5 similar to 10 6 s 1 despite containing a high level of pre existing microcrack damage etna basalt retains a high strength of 475 mpa we have monitored the complete deformation cycle through contemporaneous measurements of axial strain pore volume change compressional wave velocity change and acoustic emission ae output we have been able to follow the complete evolution of the throughgoing shear fault without recourse to any artificial means of slowing the deformation locations of ae events over time yields an estimate of the fault propagation velocity of between 2 and 4 mm s 1 we also find excellent agreement between ae locations and post test images from x ray microtomography scanning that delineates deformation zone architecture

detailed paleomagnetic records during the brunhes matuyama geomagnetic reversal and a direct determination of depth lag for magnetization in marine sediments abstract detailed paleomagnetic records were studied from homoclinal bathyal siltstone deposited during the brunhes matuyama geomagnetic reversal outcropping in the boso peninsula central japan the records are based on three continuous sediment cores from three sections within a distance of 14 km the stratigraphic horizons were correlated using a volcanic ash bed which occurs about 2 m above the brunhes matuyama reversal the sedimentation rate according to the oxygen isotope stratigraphy for the three sections decreases eastward and is 3 70 2 20 and 1 83 m ka 1 the depth lag of magnetization in the sediments was calculated directly as 42 cm based on the differences in the relative position of the key ash layer in the correlated virtual geomagnetic pole vgp paths for the three sections vgp movement during the geomagnetic reversal is characterized by short intervals of very rapid motion between periods of relative stability the last swing of the vgp from the southern hemisphere to the northern hemisphere in the brunhes matuyama geomagnetic reversal took only 38 yr intensities of the remanent magnetization normalized by anhysteretic remanent magnetization indicate that the intensity of the geomagnetic field decreased to one fifth for about 5 4 ka including the geomagnetic reversal the movement of the vgp longitude is 0 36 yr 1 westward and this westward drift can be related to the rotation of the non dipole field

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

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

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

linking permeability to crack density evolution in thermally stressed rocks under cyclic loading 1 to improve our understanding of the complex coupling between circulating fluids and the development of crack damage we performed flow through tests on samples of etna basalt and westerly granite that were cyclically loaded by deviatoric stresses the basalt was naturally microfractured while the relatively crack free westerly granite was thermally pretreated to 500 c and 800 c to generate microcrack damage samples were repeatedly loaded and then unloaded under deviatoric stress paths and ultimately to failure permeability and water volume content were measured throughout the loading history together with the differential stress permeability decreases at low differential stresses and increases at intermediate differential stresses up to a steady value at failure we use water volume content as a proxy for fluid storage and show that both permeability and storage evolve with damage and evolution of crack density we use crack models to represent the evolution of permeability as a function of loading state and are able to independently link it to the observed evolution of deformability used as an independent measure of crack density

link between geomagnetic reversal paths and secular variation of the field over the past 5 myr palaeomagnetic records provide information about the behaviour of the geomagnetic field during reversals1 2 existing records are incompatible with transitional field configurations that are either entirely dipolar or entirely zonal dependent only on latitude 3 4 recent compilations5 8 have indicated that the transi tional paths of virtual geomagnetic poles vgps for the past few reversals are located preferentially within two antipodal longitudinal bands suggesting that simple but non zonal field configurations dominate during reversals here i point out that one of the longitudinal bands coincides with that expected from the reversal of a non axial dipole field exactly like that present today the other requires only a sign change in the non axial dipole terms of today s field evidence for persistent non zonal contributions to the field has generally9 13 but not always14 13 been regarded as not statistically significant in the light of poor data distributions i show here that a non zonal bias similar to that observed in reversal data is evident in data on secular variation of the field over the past 5 myr even after normalization according to site locations these results suggest that the time averaged field does indeed contain persistent but not constant non zonal contributions

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

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

behaviour of the geomagnetic field during the matuyama brunhes polarity transition abstract the ability to derive gauss coefficients up to and including degree 3 and their variation through a geomagnetic polarity transition is studied using simulated palaeomagnetic data it is concluded that for a specified distribution of palaeomagnetic sites reasonable estimates of the behaviour of the coefficients can be derived even when uncertainties in the data and in the compilation of contemporaneous records are considered published palaeomagnetic records of the matuyama brunhes transition are then used as basis for deriving the variation of the gauss coefficients over a 32 kyear period encompassing the reversal individual records are interpolated to uniform time intervals of 0 5 kyear and put on to a common time scale by correlating between sites the variation in the latitude of vgp s through the reversal relative palaeointensity data are scaled by the geocentric axial dipole field intensity for 2000 at each site and the gauss coefficients derived by a matrix inversion employing singular value decomposition the derived variation with time of the gauss coefficients suggests that over the time span of the data the dipole and non dipole fields have approximately equal intensities plots of the variation of the surface vertical magnetic field through the reversal suggest that immediately prior to the reversal a large patch of reverse flux appears in the southern hemisphere this may subsequently have been responsible for the weakening of the vertical field leading into the reversal a similar patch of reverse flux is observed some 20 15 kyear prior to the actual reversal and may be associated with an observed excursion in vgps at several sites

magnetic anomalies of lava fields in the canary islands possible consequences for paleomagnetic records abstract measurements of the total magnetic field have been performed above the surface of 12 lava flows in the canary islands of la palma and tenerife directions can differ by up to 15 and intensity changes reach 20 the mean flow directions do not coincide and the mean flow virtual dipole moments vdms differ by more than 10 the deviations are linked to topographic features at the surface of the flows and should persist during emplacement of the next overlying flow such effects can thus limit the resolution of paleomagnetic records of detailed geomagnetic features and the field intensity variations above the flows and between the mean flow values could explain a significant part of the scatter inherent to many paleointensity studies historical field variations have been recorded from the same flows and are in agreement with the european curve for the same period however there is also some correlation with the present field which suggest that the underlying magnetized bodies could have affected the paleomagnetic records biases induced by the crustal field would increase considerably during periods of reduced field intensity like during reversals and induce erroneous records samplings performed over large distances within each lava flow would provide the best way to average out the contribution of the anomalies in any case such effects should not be neglected in paleomagnetic studies

assessing the timing of greigite formation and the reliability of the upper olduvai polarity transition record from the crostolo river italy 1 the crostolo river section in italy yielded a detailed record of the upper olduvai geomagnetic polarity transition that has been used to support the hypothesis of deep mantle control on the transitional geomagnetic field the paleomagnetic record is carried by the authigenic iron sulphide greigite which was interpreted to have formed shortly after deposition our detailed scanning electron microscope investigations indicate the presence of at least 3 generations of pyrite which usually forms with greigite as a precursor this suggests that the total magnetization is a complex composite that produced a smoothed record of transitional field behaviour

the matuyama brunhes reversal at tecopa basin southeastern california revisited again abstract numerous records of the matuyama brunhes geomagnetic transition have been obtained from paleomagnetic studies because few of the reversal records are of acceptable reliability however the exact behavior of the field during the transition has remained enigmatic to provide confirmation of one of the more reliable records we have re examined the transition at two sites 150 m apart in lake sediments of tecopa basin southeastern california the two sites are geographically very close to that of valet et al 10 who previously obtained a record from that site indicating that the transitional field was non dipolar and axisymmetric the matuyama brunhes reversal is recorded differently at each of our two sites and at that of valet et al 10 zones of mixed polarities and or intermediate directions occur at all three sites but they differ greatly in polarity character thickness and stratigraphic position it appears that all three sites have provided mutually contradictory records of the transition it is unlikely therefore that any of the records is acceptable for establishing the nature of the transition at this locality obliteration of the transition is apparently the result of acquisition of a stable normal polarity overprint that appears to consist of two remanence components one acquired during post depositional compaction and dewatering and one during later sediment diagenesis

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

sea surface temperature estimates for the tropical western pacific during the last glaciation and their implications for the pacific warm pool twenty deep sea sediment cores from the western pacific between 30 n and 30 s provide evidence of sea surface temperature sst changes throughout the tropics and subtropics glacial ssts were estimated using the modern analog technique mat applied to planktonic foraminifers and planktonic foraminiferal 18o changes we used 18o to identify the last glacial maximum the mat method differs from the traditional transfer function approach in that it utilizes a global coretop database and estimates paleotemperature by finding analogs from the modern coretop samples in addition the mat approach appears to be less susceptible than the transfer function technique to biases introduced by carbonate dissolution our results indicate that tropical ssts differed by less than 2 c from present away from the tropics 30 n and 30 s ssts were at least 3 c cooler our results differ from those of previous studies in the western pacific by using a set of well preserved high sedimentation rate cores from shallow regions the results of this study clearly indicate that a western pacific warm pool existed during the last glacial maximum lgm providing a heat and moisture source for a walker circulation cell similar to that of today we propose that a steeper lapse rate existed during the last glacial maximum and that this can explain at least part of the discrepancy between marine and terrestrial temperature estimates adjacent to new guinea for the lgm

dispersion and anisotropy of elastic waves in cracked rocks 1 rocks in the earth s crust contain variable amount of cracks depending on the deviatoric and confining stress levels pore pressure and temperature conditions crack damage results in effects that have been investigated for a long time in particular a decrease in elastic wave velocity and the development of anisotropy in this paper we focus on cracked rocks and develop a method to calculate both the elastic wave anisotropy and the dispersion in a fluid saturated cracked rock we show that analytic expressions can be derived for both properties in terms of the crack density tensor components two fundamental theoretical tools are used in this purpose the first one is the effective medium theory and the second one is the anisotropic poroelastic theory using both it is possible from first principles to predict anisotropy and dispersion results are shown to be valid up to a total crack density of 0 5 the failure threshold appears close to a total crack density of 1 between 0 5 and 1 the model still gives reasonably good results anisotropy can increase up to 60 and dispersion up to 30 depending on the crack distribution these results point out important differences that could exist in fluid saturated rocks between laboratory high frequency data and field low frequency results

palaeomagnetic location of the jaramillo subchron and the matuyama brunhes transition in the castlecliffian stratotype section wanganui basin new zealand abstract magnetostratigraphy is described for the lower part of the castlecliff section in wanganui basin new zealand based on analysis of multiple samples from 37 horizons the castlecliff section is the stratotype of the middle pleistocene castlecliffian stage and comprises eleven unconformity bound sequences which accumulated at inner shelf depths during interglacials a range of rock magnetic experiments undertaken on representative lithologies within the section show that the bulk of the remanence in all specimens is carried by ferrimagnetic mineral close to magnetite in composition in the stable single domain or pseudo single domain size range however at some sites post depositional chemical changes appear to have produced small amounts of another ferrimagnetic mineral chiefly within the superparamagnetic single domain size range resulting in hard secondary components of magnetization nevertheless at most sites the primary component of remanence was clearly identified by stepwise demagnetization four magnetozones are identified and these can be correlated to the magnetic polarity time scale using existing biostratigraphic constraints the section sampled spans the interval from below the jaramillo subchron to above the matuyama brunhes boundary the matuyama brunhes transition extends over 10 5 m of section within the upper westmere siltstone virtual geomagnetic pole positions describe an equatorial excursion in an otherwise far sided transition path these represent the first published data on the occurrence of the matuyama brunhes boundary on land in new zealand

southern hemisphere westerly wind changes during the last glacial maximum paleo data synthesis changes in the strength and position of southern hemisphere westerly winds during the last glacial cycle have been invoked to explain both millennial and glacial interglacial climate fluctuations however neither paleo models nor paleodata agree on the magnitude or even the sign of the change in wind strength and latitude during the most studied glacial period the last glacial maximum lgm compared to the recent past this paper synthesizes paleo environmental data that have been used to infer changes in lgm winds data compilations are provided for changes in terrestrial moisture dust deposition sea surface temperatures and ocean fronts and ocean productivity and existing data on southern hemisphere ocean circulation changes during the lgm are summarized we find that any hypothesis of lgm wind and climate change needs to provide a plausible explanation for increased moisture on the west coast of continents cooler temperatures and higher productivity in the subantarctic zone and reductions in agulhas leakage around southern africa our comparison suggests that an overall strengthening an equatorward displacement or no change at all in winds could all be interpreted as consistent with observations if a single cause related to the southern westerlies is sought for all the evidence presented then an equatorward displacement or strengthening of the winds would be consistent with the largest proportion of the observations however other processes such as weakening or poleward shifts in winds a weakened hydrological cycle extended sea ice cover and changed buoyancy fluxes cannot be ruled out as potential explanations of observed changes in moisture surface temperature and productivity we contend that resolving the position and strength of westerly winds during the lgm remains elusive based on data reconstructions alone however we believe that these data reconstructions of environmental conditions can be used in conjunction with model simulations to identify which processes best represent westerly wind conditions during the lgm

how does chinese loess become magnetized despite nearly three decades of paleomagnetic research on the extensive chinese loess deposits a convincing explanation has yet to be developed for how chinese loess becomes magnetized to address this problem we conducted re deposition experiments using weakly consolidated holocene chinese loess that was disaggregated in the laboratory and compare our results with published paleomagnetic data we simulated a depositional remanent magnetization drm associated with dry deposition of eolian sediments and a post depositional remanent magnetization pdrm in which the sediment was water saturated after deposition the simulated drm faithfully records the declination of the applied field but with systematic inclination flattening addition of minor water slightly improves recording of the applied field but inclination flattening persists reliable recording of the applied field occurs for pdrm simulation in water saturated sediment our synthesis of paleomagnetic data from chinese loess indicates that time averaged paleomagnetic directions are often indistinguishable from the expected geocentric axial dipole gad field but in many cases inclinations are shallower than for a gad field we conclude that the chinese loess is magnetized by a combination of drm and pdrm mechanisms with water content providing the dominant control on which mechanism aligns the detrital mineral fraction where pedogenesis causes neoformation of magnetic minerals an additional chemical remanent magnetization crm will occur the magnetization of chinese loess therefore appears to be controlled by a complex time varying combination of drm pdrm and crm mechanisms

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

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

magnetic properties of sedimentary greigite fe3s4 an update greigite fe3s4 is an authigenic ferrimagnetic mineral that grows as a precursor to pyrite during early diagenetic sedimentary sulfate reduction it can also grow at any time when dissolved iron and sulfide are available during diagenesis greigite is important in paleomagnetic environmental biological biogeochemical tectonic and industrial processes much recent progress has been made in understanding its magnetic properties greigite is an inverse spinel and a collinear ferrimagnet with antiferromagnetic coupling between iron in octahedral and tetrahedral sites the crystallographic c axis is the easy axis of magnetization with magnetic properties dominated by magnetocrystalline anisotropy robust empirical estimates of the saturation magnetization anisotropy constant and exchange constant for greigite have been obtained recently for the first time and the first robust estimate of the low field magnetic susceptibility is reported here the curie temperature of greigite remains unknown but must exceed 350 c greigite lacks a low temperature magnetic transition on the basis of preliminary micromagnetic modeling the size range for stable single domain behavior is 17 200 nm for cubic crystals and 17 500 nm for octahedral crystals gradual variation in magnetic properties is observed through the pseudo single domain size range we systematically document the known magnetic properties of greigite at high ambient and low temperatures and with alternating and direct fields and illustrate how grain size variations affect magnetic properties recognition of this range of magnetic properties will aid identification and constrain interpretation of magnetic signals carried by greigite which is increasingly proving to be environmentally important and responsible for complex paleomagnetic records including widespread remagnetizations

on the persistence of geomagnetic flux lobes in global holocene field models abstract to investigate the longevity and robustness of high latitude flux patches in the geomagnetic field at the core mantle boundary we present time dependent models of the geomagnetic field for the past 7000 years our models use the same data set as previously used for time dependent archaeomagnetic field modelling but constrained with additional priori models from time averages of field models covering the last 150 400 and 3000 years we find that the data are consistent with flux patches existing in both north and south hemispheres for the past 7000 years and that the northern hemisphere patches at least have highly dynamic behaviour simple averaging of the historical field may not be appropriate to obtain a characteristic time averaged model of the field for comparison with other geophysical observables our results should inform geodynamo studies of thermal core mantle coupling and of possible long term structure in the geomagnetic field

is earth s magnetic field reversing abstract earth s dipole field has been diminishing in strength since the first systematic observations of field intensity were made in the mid nineteenth century this has led to speculation that the geomagnetic field might now be in the early stages of a reversal in the longer term context of paleomagnetic observations it is found that for the current reversal rate and expected statistical variability in polarity interval length an interval as long as the ongoing 0 78 myr brunhes polarity interval is to be expected with a probability of less than 0 15 and the preferred probability estimates range from 0 06 to 0 08 these rather low odds might be used to infer that the next reversal is overdue but the assessment is limited by the statistical treatment of reversals as point processes recent paleofield observations combined with insights derived from field modeling and numerical geodynamo simulations suggest that a reversal is not imminent the current value of the dipole moment remains high compared with the average throughout the ongoing 0 78 myr brunhes polarity interval the present rate of change in earth s dipole strength is not anomalous compared with rates of change for the past 7 kyr furthermore there is evidence that the field has been stronger on average during the brunhes than for the past 160 ma and that high average field values are associated with longer polarity chrons there is no evidence from recent millennial scale time varying paleofield models to indicate that the field is entering a polarity transition nevertheless it remains a reasonable supposition that the magnetic field will eventually reverse even though the time scale is unpredictable a more immediate concern is that ongoing secular variation in the magnetic field may be expected to moderate the current high dipole strength on centennial to millennial time scales it would not be surprising if it dropped substantially returning closer to the average without necessarily reversing this could have important consequences for space weather and also highlights the need for improved understanding of the impact of geomagnetic field strength on the production rates of cosmogenic isotopes that are used to estimate past solar variability

elastic wave velocities and permeability of cracked rocks abstract cracks play a major role in most rocks submitted to crustal conditions mechanically cracks make the rock much more compliant they also make it much easier for fluid to flow through any rock body relying on fracture mechanics and statistical physics we introduce a few key concepts which allow to understand and quantify how cracks do modify both the elastic and transport properties of rocks the main different schemes which can be used to derive the elastic effective moduli of a rock are presented it is shown from experimental results that an excellent approximation is the so called non interactive scheme the main consequences of the existence of cracks on the elastic waves is the development of elastic anisotropy due to the anisotropic distribution of crack orientations and the dispersion effect due to microscopic local fluid flow at a larger scale macroscopic fluid flow takes place through the crack network above the percolation threshold two macroscopic fluid flow regimes can be distinguished the percolative regime close to the percolation threshold and the connected regime well above it experimental data on very different rock types show both of these behaviors

a comparison of late quaternary forest changes in new caledonia and northeastern australia a pollen record from lake xere wapo southeast new caledonia is the longest continuous terrestrial record to be recovered from the tropical southwest pacific and reveals a series of millennial scale changes in vegetation over the last 130 000 yr a comparison of the lake xere wapo record with the key northeast australian record of lynch s crater reveals regional patterns of change from 120 000 to 50 000 yr ago the vegetation around lake xere wapo alternated between rainforest and maquis with fire an important disturbance factor in the last 50 000 yr fire is almost absent from the record and the vegetation assumes a character unprecedented in the preceding 100 000 yr dominated by dacrydium and podocarpus pollen the most compelling aspect of the comparison with lynch s crater is that the much discussed araucaria decline at around 45 000 yr ago in northern queensland is matched by a similar decline in the lake xere wapo record d 2005 university of washington all rights reserved

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

ultrasonic velocities acoustic emission characteristics and crack damage of basalt and granite acoustic emissions ae compressional p shear s wave velocities and volumetric strain of etna basalt and aue granite were measured simultaneously during triaxial compression tests deformation induced ae activity and velocity changes were monitored using twelve p wave sensors and eight orthogonally polarized s wave piezoelectric sensors volumetric strain was measured using two pairs of orthogonal strain gages glued directly to the rock surface p wave velocity in basalt is about 3 km s at atmospheric pressure but increases by 50 when the hydrostatic pressure is increased to 120 mpa in granite samples initial p wave velocity is 5 km s and increases with pressure by 20 the pressure induced changes of elastic wave speed indicate dominantly compliant low aspect ratio pores in both materials in addition etna basalt also contains high aspect ratio voids in triaxial loading stress induced anisotropy of p wave velocities was significantly higher for basalt than for granite with vertical velocity components being faster than horizontal velocities however with increasing axial load horizontal velocities show a small increase for basalt but a significant decrease for granite using first motion polarity we determined ae source types generated during triaxial loading of the samples with increasing differential stress ae activity in granite and basalt increased with a significant contribution of tensile events close to failure the relative contribution of tensile events and horizontal wave velocities decreased significantly a concomitant increase of double couple events indicating shear suggests shear cracks linking previously formed tensile cracks

fracture in westerly granite under ae feedback and constant strain rate loading nucleation quasi static propagation and the transition to unstable fracture propagation new observations of fracture nucleation are presented from three triaxial compression experiments on intact samples of westerly granite using acoustic emission ae monitoring by conducting the tests under different loading conditions the fracture process is demonstrated for quasi static fracture under ae feedback load a slowly developing unstable fracture loaded at a slow constant strain rate of 2 5 10 6 s and an unstable fracture that develops near instantaneously loaded at a fast constant strain rate of 5 10 5 s by recording a continuous ultrasonic waveform during the critical period of fracture the entire ae catalogue can be captured and the exact time of fracture defined under constant strain loading three stages are observed 1 an initial nucleation or stable growth phase at a rate of 1 3 mm s 2 a sudden increase to a constant or slowly accelerating propagation speed of 18 mm s and 3 unstable accelerating propagation in the 100 ms before rupture the high level of ae activity as seen on the continuous record prevented the location of discrete ae events a lower bound estimate of the average propagation velocity using the time to rupture and the existing fracture length suggests values of a few m s however from a low gain acoustic record we infer that in the final few ms the fracture propagation speed increased to 175 m s these results demonstrate similarities between fracture nucleation in intact rock and the nucleation of dynamic instabilities in stick slip experiments it is suggested that the ability to constrain the size of an evolving fracture provides a crucial tool in further understanding the controls on fracture nucleation

brittle creep in basalt and its application to time dependent volcano deformation time dependent brittle deformation is a fundamental and pervasive process operating in the earth s upper crust its characterization is a pre requisite to understanding and unraveling the complexities of crustal evolution and dynamics the preferential chemical interaction between pore fluids and strained atomic bonds at crack tips a mechanism known as stress corrosion allows rock to fail under a constant stress that is well below its short term strength over an extended period of time a process known as brittle creep here we present the first experimental measurements of brittle creep in a basic igneous rock a basalt from mt etna volcano under triaxial stress conditions results from conventional creep experiments show that creep strain rates are highly dependent on the level of applied stress and can be equally well fit by a power law or an exponential law with a 20 increase in stress producing close to three orders of magnitude increase in creep strain rate results from stress stepping creep experiments show that creep strain rates are also influenced by the imposed effective confining pressure we show that only part of this change can be attributed to the purely mechanical influence of an increase in effective pressure with the remainder interpreted as due to a reduction in stress corrosion reactions the result of a reduction in crack aperture that restricts the rate of transport of reactive species to crack tips overall our results also suggest that a critical level of crack damage is required before the deformation starts to accelerate to failure regardless of the level of applied stress and the time taken to reach this point the experimental results are discussed in terms of microstructural observations and fits to a macroscopic creep law and compared with the observed deformation history at mt etna volcano c 2011 elsevier b v all rights reserved

influence of dilatancy on the frictional constitutive behavior of a saturated fault zone under a variety of drainage conditions we use numerical simulations to investigate how fault zone dilatancy and pore fluid decompression influence fault strength and friction constitutive behavior dilatant hardening can change the frictional response and the effective critical stiffness kcr which determines the transition from stable to unstable sliding in velocity weakening fault zones we study the frictional shear strength response to numerical velocity stepping experiments and show that when the duration of pore fluid decompression is long compared to the time necessary for frictional evolution as dictated by rate and state friction both the effective critical slip distance dc and the effective shear strength direct effect a are increased we investigate the role of fault zone permeability k dilatancy coefficient varepsilon and the magnitude of shearing velocity of the fault zone vlp and compare results using the dieterich and ruina state evolution laws over the range from k 10 15 to 10 21 m2 dc increases from 25 m to 2 mm and a increases from 0 15 to 5 mpa we vary varepsilon from 10 5 to 10 3 and the size of the velocity perturbation from 3x to 1000x and find large increases in the values of dc and a which may lead to inhibition of unstable sliding our results indicate that spatial variations with either depth or lateral extent in varepsilon and k could result in significant changes in the drainage state in fault zones such variation may lead to spatial variation of the nucleation and propagation of earthquakes based upon the drainage capabilities of the fault zone

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

a successful rock magnetic technique for correctng paleomagnetic inclination shallowing case study of the nacimiento formation new mexico the accuracy of an inclination shallowing correction technique was tested on the remanence of the paleocene nacimiento formation the correction assumes that changes in paleomagnetic inclination during deformation will be directly related to changes in remanence anisotropy the remanence anisotropy is also a function of the magnetic anisotropy of the individual magnetic grains the nacimiento formation was selected for this test because its inclination is shallow by 7 to 8 and previous paleomagnetic studies indicate that it has a primary remanence with a well constrained age and that magnetic overprints are minimal tectonics are an unlikely explanation for its shallow inclination these conditions allow the inclination corrected remanence to be compared to a well defined paleomagnetic pole to determine the accuracy of the technique the characteristic remanence from 20 nacimiento sites was isolated by alternating field and thermal demagnetization and was corrected using the anisotropy of anhysteretic remanence measured for three to five samples per site individual particle anisotropy was determined by drying a magnetic separate epoxy mixture in a 35 50 mt magnetic field laboratory compaction of disaggregated nacimiento material was also used to yield an effective individual magnetic particle anisotropy the two techniques resulted in similar values for individual particle anisotropy and similar inclination shallowing corrections both corrections were successful and the inclination corrected formation mean direction was indistinguishable from the direction predicted by north america s paleocene paleopole the inclination shallowing correction technique reported here should be applied routinely in paleomagnetic studies provided that the remanence is a primary depositional remanence that has been affected only by syndepositional or early postdepositional processes e g compaction

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

geomagnetic intensity variations over the past 780 kyr obtained from near seafloor magnetic anomalies knowledge of past variations in the intensity of the earth s magnetic field provides an important constraint on models of the geodynamo a record of absolute palaeointensity for the past 50 kyr has been compiled from archaeomagnetic and volcanic materials and relative palaeointensities over the past 800 kyr have been obtained from sedimentary sequences but a long term record of geomagnetic intensity should also be carried by the thermoremanence of the oceanic crust here we show that near seafloor magnetic anomalies recorded over the southern east pacific rise are well correlated with independent estimates of geomagnetic intensity during the past 780 kyr moreover the pattern of absolute palaeointensity of seafloor glass samples from the same area agrees with the well documented dipole intensity pattern for the past 50 kyr a comparison of palaeointensities derived from seafloor glass samples with global intensity variations thus allows us to estimate the ages of surficial lava flows in this region the record of geomagnetic intensity preserved in the oceanic crust should provide a higher time resolution record of crustal accretion processes at mid ocean ridges than has previously been obtainable

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

confounding influence of magnetic fabric on sedimentary records of a field reversal recent compilations of geomagnetic reversal records have generated a controversy as to whether the geomagnetic field is geographically biased during polarity transitions at present there is general agreement that the virtual geomagnetic poles recorded from cenozoic sediments preferentially lie over the americas or the antipodal longitude yet such a preference is not statistically established however it is intriguing that the claimed preferred paths lie 90 away from the site longitude although this may be partly inherent in the very poor geographical distribution of the sites we prefer not to rely on fortuitous coincidences several authors have argued that sedimentary palaeomagnetic records may be modified by artefacts linked to the acquisition of magnetization here we report that in two sedimentary records of the upper olduvai reversal from confidence hills california the declinations of the remanent magnetization recorded during the reversal are similar to the directions of the maximum horizontal axes of the ellipsoids of magnetic anisotropy this supports the idea that at times of low geomagnetic intensity such as during a reversal factors other than the geomagnetic field influence the orientation of elongated grains

a selective procedure for absolute paleointensity in lava flows 1 we present a compilation of experiments of absolute paleointensity using double heating protocols on very recent lava flows from hawaii la reunion the canary islands and santorini the existence of a sharp distribution of grain sizes carried by a single mineralogical phase always yielded successful determinations of paleointensity that deviate by less than 10 from the actual field value thus a rapid decrease of at least 70 of the initial magnetization over a narrow range of temperatures prior to the curie point combined with a unique mineralogical phase define an optimal situation for obtaining reliable estimates of absolute paleointensity consequently we suggest that stepwise standard thermal demagnetization of companion specimens should be routinely performed prior to paleointensity experiments not only do these measurements provide important information about the characteristic magnetization but they indicate which samples are appropriate for paleointensity experiments which increases the success rate to almost 100

time dependent mapping of the magnetic field at the core mantle boundary we consider the problem of constructing a time dependent map of the magnetic field at the core mantle boundary we use almost all the available data from the last 300 years to produce two maps one for the period 1690 1840 and the other for 1840 1990 we represent the spatial dependency of the field using spherical harmonics the time dependency using a cubic b spline basis and seek the smoothest solutions compatible with the observations particular attention must be paid to the effects of the crustal field in the data we argue that for observations from permanent magnetic observatories the most efficient strategy is to use first differences of annual means for satellite data the most efficient strategy is simply to limit the number of data used so as to minimize any tendency to map the crustal field into the core field the resulting model fits the observatory data better than any previous model yet has less power in the secular variation than previous models demonstrating that very simple models fit the data the resulting time dependent field map exhibits much of the same structure in the field and its secular variation identified in earlier studies

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

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

vegetation change in the coastal lowland rainforest at avai o vuna swamp vava u kingdom of tonga avai o vuna swamp a small coastal wetland in vava u kingdom of tonga produced a 4500 year pollen and sediment record results are 1 a mid holocene sea level highstand is confirmed for tonga between about 4500 and 2600 14 c yr b p marine clay contains pollen from mangroves rhizophora mangle coastal forest trees barringtonia asiatica and cocos nucifera and rainforest trees alphitonia rhus hedycarya and calophyllum 2 microscopic charcoal first appeared at 2600 14 c yr b p coincident with the arrival of polynesians 3 cocos pandanus excoecaria macaranga and elaeocarpaceae pollen reflects the establishment of a mixed coastal lowland rainforest in the last 2500 years 4 the loss of hedycarya elaeocarpus calophyllum and guettarda and the reduction of terminalia and taxa in the papilionaceae family by about 1000 years ago may be due to habitat destruction and the loss of dispersal capabilities of some species through the extinction of the two largest pigeons in tonga d 2005 university of washington all rights reserved

absolute paleointensity and reversal records from the waianae sequence oahu hawaii usa abstract paleointensity experiments were performed on samples from four parallel sections of the waianae volcanics of oahu hawaii that document 0 4 myr of geomagnetic field variations the records of the three successive gilbert gauss lower and upper mammoth reversals confirm that large oscillations of directions precede or follow the reversals which are similar to waveforms generated by paleosecular variation with their amplitude being considerably amplified by the decrease of the dipole determinations of absolute paleointensity were attempted on 546 samples we only selected data which were obtained from segments covering a large part of the remanent magnetization more than 70 on the average and without concave up arai diagrams this procedure limited the success rate to 14 but provided consistent and reliable paleointensities in addition to other time intervals the results document the field variations surrounding the lower mammoth transition a weak field period dominated before the reversal then the transition was initiated by a transit from normal to reverse polarity followed by a short restoration of the field intensity in reverse polarity a second episode of a very weak field was accompanied by a return to positive inclinations before definitely reaching the reverse polarity a strong and apparent rapid recovery of the dipole following the completion of the reversal culminated at a value of 16 1022 am2 similar to field intensities reported for the other detailed volcanic records of reversals studied so far the asymmetry between the pre and the post reversal phases indicates the importance of field regeneration to initiate a new stable polarity interval

a late quaternary geomagnetic secular variation record from lake waiau hawaii and the question of the pacific nondipole low a continuous paleomagnetic record for the last 13 000 years was obtained from the sediments of lake waiau located near the summit of mauna kea volcano on the island of hawaii the secular variation sv of the geomagnetic field was studied using spectral analysis and precession analysis of the magnetic vector the hypothesis that there has been anomalously low geomagnetic sv in the central pacific was tested using angular dispersion analysis the inclination record has a mean value of 27 that is much lower than the 35 predicted by the geocentric axial dipole hypothesis the spectrum obtained from spectral analysis shows major inclination peaks at periodicities of approximately 3600 and 7900 years and major declination peaks at periodicities of approximately 1100 2000 3300 and 7800 years the approximately 8000 and 3500 year periodicities shown in both inclination and declination spectrums may represent the dipole and nondipole variation respectively the approximately 1100 and 2000 year periodicities are probably derived from either the sources that affect declination only or the offsets between the core sections in addition bauer plots of the magnetic vector are dominated by clockwise motion the angular dispersion calculated from lake sediments is 5 9 with respect to the mean virtual geomagnetic pole vgp and 9 1 with respect to the geographic pole both values are much lower than the 13 5 expected from paleosecular variation model g which separates the angular dispersion into components from the dipole and the quadruple families our results support the hypothesis that an anomalously low secular variation is a characteristic feature of the central pacific furthermore this geomagnetic behavior persists on time scales of 104 years regional anomalies in temperature topography or electrical conductivity at the core mantle boundary are inferred to cause the low sv

a late quaternary record of environmental change and human impact from new caledonia a late quaternary vegetation record is presented from a lowland site on the leeward southwest coast of new caledonia plum swamp is a backswamp on the plum river at around 10 m a s l just within the ultramafic terrain that dominates the southern third of new caledonia pollen analysis charcoal analysis radiocarbon dating and stratigraphic analysis are employed to reconstruct the vegetation and sedimentary history of the valley the vegetation record commences at around 20 000 yr bp and shows that from this time up until the late holocene the valley was forested the greater representation of taxa more prevalent at higher altitudes during the late glacial suggests a response to cooling and an enrichment of the lowland forest with more montane elements the late glacial transition between 14 000 and 9000 yr bp is a period of instability within the valley when vegetation is disturbed by fire forest recovers after this and there is increasing stability until the arrival of people in the late holocene at around 3000 yr bp the initial colonisers of new caledonia had a profound effect upon the vegetation of the valley converting the lowland forest to a stunted and species poor maquis on the ultramafic sediments and melaleuca woodland on the non ultramafic substrates however sedimentary processes are relatively unchanged during this time dramatic geomorphic change does occur in the catchment though during the latter part of the 20th century as a consequence of mining q 2001 elsevier science b v all rights reserved

global changes in intensity of the earth s magnetic field during the past 800kyr recent advances in palaeomagnetic and dating techniques have led to increasingly precise records of the relative intensity of the earth s past magnetic field at numerous field sites the compilation and analysis of these records can provide important constraints on changes in global magnetic field intensity and therefore on the earth s geodynamo itself a previous compilation for the past 200 kyr integrated 17 marine records into a composite curve1 with the geomagnetic origin of the signal supported by an independent analysis of 10be production made on different cores2 the persistence of long term features in the earth s magnetic intensity or the existence of long term periodic changes cannot however be resolved in this relatively short time span here we present the integration of 33 records of relative palaeointensity3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 into a composite curve spanning the past 800 kyr we find that the intensity of the earth s dipole field has experienced large amplitude variations over this time period with pronounced intensity minima coinciding with known excursions in field direction reflecting the emergence of non dipole components no stable periodicity was found in our composite record and therefore our data set does not support the hypothesis that the earth s orbital parameters have a direct and strong influence on the geodynamo

evidence suggesting extremely rapid field variation during a geomagnetic reversal abstract large systematic variations in direction of high temperature remanence as a function of vertical position occur in a basalt flow from the miocene volcanic sequence at steens mountain oregon that has provided a detailed record of a geomagnetic reversal these may have been caused by an impulsive change in the transitional field as the flow cooled during the reversal if this is true the short time required for the flow to cool by conduction and acquire its thermoremanent magnetization implies astonishingly high rates of change of the geomagnetic field at least 3 and 300 gammas per day

on the structure and origin of major glaciation cycles 1 linear responses to milankovitch forcing time series of ocean properties provide a measure of global ice volume and monitor key features of the wind driven and density driven circulations over the past 400 000 years cycles with periods near 23 000 41 000 and 100 000 years dominate this climatic narrative when the narrative is examined in a geographic array of time series the phase of each climatic oscillation is seen to progress through the system in essentially the same geographic sequence in all three cycles we argue that the 23 000 and 41 000 year cycles of glaciation are continuous linear responses to orbitally driven changes in the arctic radiation budget and we use the phase progression in each climatic cycle to identify the main pathways along which the initial local responses to radiation are propagated by the atmosphere and ocean early in this progression deep waters of the southern ocean appear to act as a carbon trap to stimulate new observations and modeling efforts we offer a process model that gives a synoptic view of climate at the four end member states needed to describe the system s evolution and we propose a dynamic system model that explains the phase progression along causal pathways by specifying inertial constants in a chain of four subsystems solutions to problems involving systems of such complexity are not born full grown like athena from the head of zeus rather they evolve slowly in stages each of which requires a pause to examine data at great lengths in order to guarantee a sure footing and to properly choose the next step victor p starr

the steens mountain oregon geomagnetic polarity transition 2 field intensity variations and discussion of reversal models we carried out an extensive paleointensity study of the 15 5 0 3 m y miocene reversed to normal polarity transition recorded in lava flows from steens mountain south central oregon one hundred eighty five samples from the collection whose paleodirectional study is reported by mankinen et al this issue were chosen for paleointensity investigations because of their low viscosity index high curie point and reversibility or near reversibility of the strong field magnetization curve versus temperature application of the thellier stepwise double heating method was very successful yielding 157 usable paleointensity estimates corresponding to 73 distinct lava flows after grouping successive lava flows that did not differ significantly in direction and intensity we obtained 51 distinguishable complete field vectors of which 10 are reversed 28 are transitional and 13 are normal the record is complex quite unlike that predicted by simple flooding or standing nondipole field models it begins with an estimated several thousand years of reversed polarity with an average intensity of 31 5 8 5 t about one third lower than the expected miocene intensity this difference is interpreted as a long term reduction of the dipole moment prior to the reversal when site directions and intensities are considered truly transitional directions and intensities appear almost at the same time at the beginning of the transition and they disappear simultaneously at the end of the reversal large deviations in declination occur during this approximately 4500 1000 year transition period that are compatible with roughly similar average magnitudes of zonal and nonzonal field components at the site the transitional intensity is generally low with an average of 10 9 4 9 t for directions more than 45 away from the dipole field and a minimum of about 5 t the root mean square of the three field components x y and z are of the same order of magnitude for the transitional field and the historical nondipole field at the site latitude however a field intensity increase to pretransitional values occurs when the field temporarily reaches normal directions which suggests that dipolar structure could have been briefly regenerated during the transition in an aborted attempt to reestablish a stationary field changes in the field vector are progressive but jerky with at least two and possibly three large swings at astonishingly high rates each of those transitional geomagnetic impulses occurs when the field intensity is low less than 10 t and is followed by an interval of directional stasis during which the magnitude of the field increases greatly for the best documented geomagnetic impulse the rapid directional change corresponds to a vectorial intensity change of 6700 2700 nt yr 1 which is about 15 50 times larger than the maximum rate of change of the nondipole field observed during the last centuries the occurrence of geomagnetic impulses seems to support reversal models assuming an increase in the level of turbulence within the liquid core during transitions the record closes with an estimated several thousand years of normal polarity with an average intensity of 46 7 20 1 t agreeing with the expected miocene value however the occurrence of rather large and apparently rapid intensity fluctuations accompanied by little change in direction suggests that the newly reestablished dipole was still somewhat unstable

geomagnetic dipole moment and 10be production rate intercalibration from authigenic 10be 9be for the last 1 3 ma 1 a synthesis of authigenic 10be 9be ratio records constructed from sedimentary sequences deposited over the last 300 ka in the north east atlantic and over the 500 1300 ka bp interval in the west equatorial pacific ocean is compared with proxies of the geomagnetic moment variations over the same time intervals these proxies consist in 1 relative paleointensity records obtained from the same sediments 2 a seafloor magnetization record derived from magnetic anomalies and 3 an absolute virtual dipole moment vdm record constructed from an updated database of absolute paleointensities the authigenic 10be 9be ratio records document the occurrence of the paleomagnetic events excursions and reversals reported for these time intervals by paleomagnetic studies reconstructions of 10be 9be derived magnetic moment records based on theoretical and empirical relationships between 10be 9be ratio and absolute vdm allow establishing 10be derived vdm records that exhibit the same overall structures as the geomagnetic proxy records

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

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

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

the van zijl jurassic geomagnetic reversal revisited we have collected two new detailed records of what could be the second oldest well documented reversal the van zijl jurassic 180 ma reversal recorded in the thick basalt sequences of the karoo large igneous province in lesotho and south africa sections yielded 10 and 8 independent transitional paleomagnetic directions respectively over two continuous 130 m and 160 m sequences the corresponding vgp reversing paths share a number of consistent features and are integrated with a third record at bushmen s pass from prevot et al 2003 to yield a single reversal path with 1 a large directional jump with no transitional poles between 45 s and 45 n in the first main phase of the reversal 2 a large elongated loop multiple hairpin to the se with poles going down to 20 latitudes in the second phase of the reversal and 3 a rebound before finally settling in normal polarity we have measured relative paleo intensities on a set of carefully selected samples intensity was low prior to the directional changes and recovery to the following normal polarity was progressive transitional intensity is 80 to 90 lower than full polarity values there is no significant increase in intensity at the time of transitional directional clusters our results are consistent with a weak non dipolar reversing field with faster and larger directional secular variation as the equator is crossed by vgps they tend to vindicate the notion of a short duration of the directional reversal though one cannot draw global conclusions from a record limited to essentially a single location

influence of macro fractures and micro fractures on permeability and elastic wave velocities in basalt at elevated pressure fractures are ubiquitous on all scales in crustal rocks the investigation of fractures and their influence on physical and transport properties of rocks is therefore essential for understanding of many key problems in seismology volcanology and rock engineering in crystalline rocks pore water is primarily stored in and migrates through networks of cracks and fractures at all scales it is therefore essential to know how fluid flow in such fracture networks responds to the elevated pressures found at depth here we report results from an investigation of changes in fluid permeability and associated changes in p wave and s wave velocities at elevated effective pressure for intact macro fractured and micro fractured samples of seljadur basalt in all cases permeability decreases and both wave velocities increase with increasing effective pressure permeability decreases were smallest in the intact material from approximately 10 19 m 2 to 3 x 10 20 m 2 intermediate in the micro fractured material from approximately 5 x 10 17 m 2 to 1 x 10 17 m 2 and largest in the macro fractured material from approximately 3 x 10 15 m 2 to 9 x 10 19 m 2 for a material containing both micro fractures and macro fractures the closure of macro fractures dominated the permeability reduction at low pressure with the closure of micro fractures exerting an increasing influence at higher pressure c 2010 elsevier b v all rights reserved

matuyama brunhes transition recorded in lava flows of the chilean andes evidence for dipolar fields during reversals the transitional behavior exhibited by earth9s magnetic field during polarity reversals holds the key to understanding the complexities of the field interpretation of the severely limited reversal data set however remains controversial discussion centers around the nature of pole paths during transitions and their possible relation to other geophysical phenomena paleomagnetic data from volcan tatara san pedro in the central chilean andes lat 36 s long 71 w provide the first record of the matuyama brunhes reversal from south america and only the third transitional record from the southern hemisphere a stratigraphic section of ten lava flows yielded intermediate pole positions that center in australia the mean pole is 16 8 s 133 0 e k ar analyses of two of these flows provided dates of 768 8 ka and 763 14 ka ages coincident with the established matuyama brunhes boundary the clustering of these transitional poles strengthens the hypothesis that earth9s magnetic field retains a strong dipole component during reversals

a long vegetation history from lowland irian jaya indonesia abstract this paper reports the pollen analysis of a 10 m core from a mire at 780 m altitude and 2 s latitude on ultrabasic soils on a northern coastal range of new guinea the area is almost undisturbed by humans and the record is believed to cover about 60 000 yr b p the results show that montane forest grew around the site continously through the late pleistocene with a distinct increases in higher altitude taxa from 25 000 to 10 500 yr b p the time of glacial maxima elsewhere the invasion of the site by lower altitude forest which commenced at 10 500 yr b p was reversed after a few hundred years and was not finally completed until about 7000 yr b p the results show that vegetation in the region has been sensitive to climatic change the pleistocene ecology being consistent with a temperature change of about 3 4 c times of change agree with other tropical areas even though the site climate was probably affected by changing sea levels however the tropical forest demonstrates overall long term stability in which changes in dominance may reflect minor shifts in disturbance and tree longevity a distinctive record of fine charcoal occurs after 10 900 yr b p but not at any level prior to that time this shows that the forest was continously fire free for a very long time the charcoal is probably an indication of anthropogenic disturbance which may correlate with the spread of agricultural innovation in lowland new guinea an increase in secondary species after 7000 yr b p may be a result of minor anthropogenic activity or the result of warmer conditions and shorter tree life in the lower altitude forests

pleistocene vegetation of the humid tropics of northeastern queensland australia abstract the humid tropics area of northeastern queensland has provided a substantial and detailed history of rainforest and rainforest savanna interactions from the pollen analysis of a number of sites on the volcanic atherton tableland through the latter part of the quaternary period a recent extension of the record to close to the base of the quaternary from examination of an offshore core provides a broad regional and temporal context for evaluation of the true significance of late quaternary changes it is apparent that the major sustained change in the vegetation of the region occurred relatively recently probably within the last 140 000 yr and that this change was time transgressive it involved the replacement of extensive moist rainforest by open eucalypt woodland and is considered to have been most likely caused by the burning activities of aboriginal people some aspects of the record from this region can contribute to the history of rainforest on a global scale but other features of it re inforce the unique nature of the development of australian vegetation patterns

some aspects of the late tertiary geomagnetic field in iceland summary in 1972 78 the late n d watkins and others carried out ajoint field programme of geological mapping in the mio pliocene flood basalts of iceland including sampling for k ar dating and palaeomagnetic research the major part of the palaeomagnetic sampling is represented by 2462 lavas in five long composite sections through the lava pile this paper deals with various statistical properties of this data set it is concluded that geomagnetic reversals occur more frequently than is assumed in the current ocean floor polarity time scale there is no evidence for significant asymmetries between normal and reverse polarity states of the field neither as regards chron lengths secular variation or virtual dipole moment magnitude intensities of remanence in these lavas are shown to be well approximated in terms of a hyperbolic distribution the latitude distribution of virtual magnetic poles can be fitted with a bingham function having k 4 5 and low latitude poles do not occur preferentially in any particular longitude interval

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

relating seismic velocities thermal cracking and permeability in mt etna and iceland basalts we report simultaneous laboratory measurements of seismic velocities and fluid permeability on lava flow basalt from etna italy and columnar basalt from seljadur iceland measurements were made in a servo controlled steady state flow permeameter at effective pressures from 5 80 mpa during both increasing and decreasing pressure cycles selected samples were thermally stressed at temperatures up to 900 degrees c to induce thermal crack damage acoustic emission output was recorded throughout each thermal stressing experiment at low pressure 0 10 mpa the p wave velocity of the columnar seljadur basalt was 5 4 km s while for the etnean lava flow basalt it was only 3 0 3 5 km s on increasing the pressure to 80 mpa the velocity of etnean basalt increased by 45 60 whereas that of seljadur basalt increased by less than 2 furthermore the velocity of seljadur basalt thermally stressed to 900 degrees c fell by about 2 0 km s whereas the decrease for etnean basalt was negligible a similar pattern was observed in the permeability data permeability of etnean basalt fell from about 7 5 x 10 16 m 2 to about 1 5 x 10 16 m 2 over the pressure range 5 80 mpa while that for seljadur basalt varied little from its initial low value of 9 x 10 21 m 2 again thermal stressing significantly increased the permeability of seljadur basalt whilst having a negligible effect on the etnean basalt these results clearly indicate that the etnean basalt contains a much higher level of crack damage than the seljadur basalt and hence can explain the low velocities 3 4 km s generally inferred from seismic tomography for the mt etna volcanic edifice c 2005 elsevier ltd all rights reserved

tropical vegetational change in the late pleistocene of new caledonia the south eastern plateau of new caledonia 22 s preserves remnants of a deep regolith of ultramafic soils which is extensively gullied or eroded to form large alluvial fans dated sediments from four closed basins on the plaine des lacs ca 220 m altitude show that organic muds were accumulating prior to 30 000 yr b p over the next 5000 14 000 years limonitic clays infilled two basins as a result of slope instability in their catchment resistant laterite and bog iron surfaces became exposed during the period 30 000 15 000 yr b p by erosional events that seem to have been of greater magnitude than any in the earlier pleistocene two sites lake emeric and lake suprin were chosen for pollen analysis during the phases of organic deposition of these lakes nothofagus forests collapsed several times and were replaced by gymnostoma maquis apparently as a result of fires these pleistocene fires were associated with some inorganic deposition presumably due to the substantial loss of primary forest cover regeneration of forest usually followed after an estimated gap of several centuries thus fire was able to cause forest retreats of the same order of magnitude as that caused by massive fires today the low nutritional status of the soils or lateritic crusts means that as now forest had difficulty reclaiming sites human activity is unknown prior to 3000 years ago so the major erosion is presumed to have had natural causes thus many of the wide ranging changes are the result of natural instability in the sub tropics and some may be much older than hitherto thought climate change towards periods of dry fire prone conditions may have been an underlying cause in the holocene some climatic factor possibly cyclonic storms seems to prevent organic sedimentation switching from cyclone free to cyclone prone at the end of the pleistocene may reflect temperature change in sea surfaces in the region

the role of the earth s mantle in controlling the frequency of geomagnetic reversals a series of computer simulations of the earth s dynamo illustrates how the thermal structure of the lowermost mantle might affect convection and magnetic field generation in the fluid core eight different patterns of heat flux from the core to the mantle are imposed over the core mantle boundary spontaneous magnetic dipole reversals and excursions occur in seven of these cases although sometimes the field only reverses in the outer part of the core and then quickly reverses back the results suggest correlations among the frequency of reversals the duration over which the reversals occur the magnetic field intensity and the secular variation the case with uniform heat flux at the core mantle boundary appears most earth like this result suggests that variations in heat flux at the core mantle boundary of the earth are smaller than previously thought possibly because seismic velocity anomalies in the lowermost mantle might have more of a compositional rather than thermal origin or because of enhanced heat flux in the mantle s zones of ultra low seismic velocity

new evidence for extraordinarily rapid change of the geomagnetic field during a reversal palaeomagnetic results from lava flows recording a geomagnetic polarity reversal at steens mountain oregon suggest the occurrence of brief episodes of astonishingly rapid field change of six degrees per day the evidence is large systematic variations in the direction of remanent magnetization as a function of the temperature of thermal demagnetization and of vertical position within a single flow which are most simply explained by the hypothesis that the field was changing direction as the flow cooled