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

geodynamo solar wind and magnetopause 3 4 to 3 45 billion years ago stellar wind standoff by a planetary magnetic field prevents atmospheric erosion and water loss although the early earth retained its water and atmosphere and thus evolved as a habitable planet little is known about earth s magnetic field strength during that time we report paleointensity results from single silicate crystals bearing magnetic inclusions that record a geodynamo 3 4 to 3 45 billion years ago the measured field strength is 50 to 70 that of the present day field when combined with a greater paleoarchean solar wind pressure the paleofield strength data suggest steady state magnetopause standoff distances of 5 earth radii similar to values observed during recent coronal mass ejection events the data also suggest lower latitude aurora and increases in polar cap area as well as heating expansion and volatile loss from the exosphere that would have affected long term atmospheric composition

the influence of salinity and flocculation upon the acquisition of remanent magnetization in some artificial sediments summary a range of redeposition experiments with both magnetite and maghemite grains embedded in kaolinite or quartz has revealed a significant non linear relationship between salinity and the intensity and magnitude of the inclination error of the depositional remanent magnetization drm increasing drm intensities and inclination errors were observed for salinities between 0 1 g l a dramatic reduction in intensity as well as in inclination error appears for salinities above ca 2 g l although there is a negligible effect on intensity of salinity for concentrations above 3 g l a significant inclination error persists over this range of salinities approaching less than 5 at concentrations larger than 30 g l the observed relationships are suggested to reflect mechanisms related to flocculation the results are probably not generally applicable to natural sediments since floc formation in nature usually involves organic compounds

the paleomagnetism of single silicate crystals recording geomagnetic field strength during mixed polarity intervals superchrons and inner core growth 1 the basic features of the geomagnetic reversal chronology of the last 160 million years are well established the relationship between this history and other features of the field however has been elusive the determination of past field strength paleointensity is especially challenging commonly accepted results have come from analyses of bulk samples of lava historic lavas have been shown to faithfully record the past field strength when analyzed using the thellier double heating method data from older lavas however tend to show effects of in situ and laboratory induced alteration here we review an alternative approach single plagioclase crystals can contain minute magnetic inclusions 50 350 nm in size that are potential high fidelity field recorders thellier experiments using plagioclase feldspars from an historic lava on hawaii provide a benchmark for the method rock magnetic data from older lavas indicate that the feldspars are less susceptible to experimental alteration than bulk samples this resistance is likely related to the lack of clays in addition magnetic minerals are sheltered by the encasing silicate matrix from natural alteration that can otherwise transform the well defined thermoremanent magnetization into an irresolute chemical remanent magnetization if there is a relationship between geomagnetic reversal frequency and paleointensity it should be best expressed during superchrons intervals with few or no reversals thellier data sets based on single plagioclase crystals from lavas erupted during the cretaceous normal polarity superchron 83 120 million years ago suggest a strong 12 1022 am2 stable field consistent with an inverse relationship between reversal frequency and paleointensity superchrons may represent times when the pattern of core mantle boundary heat flux allows the geodynamo to operate at peak efficiency as suggested in some numerical models thellier data from single plagioclase crystals formed during times of moderate 10 reversals million years reversal occurrence suggest a weaker and more variable field these paleointensity data together with a consideration of paleomagnetic directions suggest that geomagnetic reversals field morphology secular variation and intensity are related the linkages over tens of millions of years imply a lower mantle control on the geodynamo on even longer timescales the magnetization held by plagioclase and other silicate crystals can be used to investigate the proterozoic and archean geomagnetic field during the onset of growth of the solid inner core data from plagioclase crystals separated from mafic dikes together with directional data from whole rocks indicate a dipole dominated field similar to that of the modern 2 5 2 7 billion years ago older archean rocks are of great interest for paleomagnetic and paleointensity investigations because they may record a time when the compositionally driven convection of the modern dynamo may not have been operating and a solid inner core did not play its current role in controlling the geometry of outer core flow most rocks of this age have been affected by low grade metamorphism investigations using single silicate grains provide arguably our best hope of seeing through secondary geologic events and reading the early history of the geodynamo absolute paleointensity measurements of the oldest rocks on the planet will require the further development of methods to investigate silicate crystals with exsolved magnetic minerals that address the uncertainties posed by thermocrystallization remanent magnetization anisotropy and slow cooling fortunately prior work in rock magnetism together with advances in analytical equipment and techniques provides a solid foundation from which these frontier issues can be approached

a statistical simulation of magnetic particle alignment in sediments s u m m a r y sedimentary magnetizations are fundamental to palaeomagnetism but the mechanisms that control remanence acquisition remain poorly constrained observed sedimentary natural remanent magnetizations are often orders of magnitude smaller than the saturation remanent magnetization of the same sediment which indicates inefficient remanence acquisition we present a statistical model based on the vonmises fisher distribution in which magnetic particle reorientations towards an ambient field are considered without representing the physics of the magnetization acquisition process the results provide insights into the nature of sedimentary magnetizations specifically an assemblage of randomly oriented magnetic particles can acquire a high fidelity palaeomagnetic signal with only small rotations in some cases 1 of particles towards the ambient field direction this demonstrates that the action of a geomagnetic torque on individual magnetic mineral particle orientation may be minor and that a weak directional bias on an assemblage of particles could be responsible for the typically observed inefficiency of sedimentary remanence acquisition additionally we demonstrate that weak fields produce sedimentary magnetizations with larger directional uncertainties for natural sediments however these uncertainties appear to be small enough to allow reliable recording of directional geomagnetic field behaviour during periods with weak fields such as palaeomagnetic reversals and excursions

paleomagnetic recording efficiency of sedimentary magnetic mineral inclusions implications for relative paleointensity determinations this study was supported by the national natural science foundation of china grants 41574060 41574063 and 41722402 and the australian research council grants dp120103952 and dp160100805 g a p is supported by a nerc independent research fellowship ne p017266 1 l t acknowledges support from nsf grant ear1547263

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

field dependence on magnetization of laboratory redeposited deep sea sediments first results abstract a variety of redeposition experiments have been conducted in order to investigate the alignment efficiency of magnetic grains in low field intensities the field direction is reproduced by the magnetic remanence with good fidelity and there is low directional dispersion for samples obtained with field intensities larger than 10 t however there is an increase in dispersion for lower field strengths and a critical threshold is reached at 5 t i e similar to the geomagnetic intensity during reversals below which no coherent signal is observed we notice also that there is a good linear relationship within about 10 between the intensity of magnetization corrected for magnetic concentration and the value of the applied field a column of sediment deposited in the presence of two successive polarities of the field shows downcore changes in direction and intensity which confirm the occurrence of post depositional reorientations about 50 of the magnetization was locked in within 2 cm of sediment and 60 within about 10 cm an analysis of the magnetization components shows that reorientation did not occur exactly along the direction of the post reversal field which suggests that grain mobility is rapidly affected at small depths within the sediment

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

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

detrital magnetization of laboratory redeposited sediments we conducted several redeposition experiments in laboratory using natural and artificial sediments in order to investigate the role of grain size and lithology on sedimentary remanence acquisition the role of grain size was investigated by using sorted sediment from natural turbidites taking advantage of the magnetic grain size distribution within turbidites we compared redeposition experiments performed with coarse magnetic grains taken from the bottom layers of a turbidite with fine grains from the upper layers of the same turbidite in order to document the magnetization acquired for increasing sediment concentrations that is analogous to increasing depth in the sediment column the samples were frozen at temperatures between 5 and 10 c magnetization acquisition behaved similarly in both situations so that little smearing of the palaeomagnetic signal should be linked to grain size variability within this context other series of experiments were aimed at investigating the influence of lithology we used clay or carbonated sediments that were combined with magnetic separates from basaltic rocks or with single domain biogenic magnetite the experiments revealed that the magne tization responded differently with clay and carbonates clay rapidly inhibited alignment of magnetic grains at low concentrations and therefore significant magnetization lock in occurred despite large water contents perhaps even within the bioturbated layer extension of the process over a deeper interval contributes to smear the geomagnetic signal and therefore to alter the palaeomagnetic record in carbonates the magnetization was acquired within a narrow window of 45 50 per cent sediment concentration therefore little smearing of the ge omagnetic signal can be expected finally experiments on carbonate sediments and biogenic magnetite with increasing field intensities indicate that magnetization acquisition is linear with respect to field intensity altogether the results suggest that sediments with dominant carbonate content should be favoured for records of geomagnetic field changes provided that the minor clay fraction does not vary excessively they confirm the advantage of using cultures of magnetotactic bacteria for redeposition experiments

holocene tephrostratigraphy of varved sediment records from lakes tiefer see ne germany and czechowskie n poland a detailed holocene tephrostratigraphic framework has been developed for two predominately varved lake sediment sequences from ne germany lake tiefer see and central n poland lake czechowskie a total of thirteen tephras and cryptotephras of icelandic provenance were detected and chemically fingerprinted in order to define correlatives and to integrate known tephra ages into the sediment chronologies out of these three cryptotephras askja ad1875 askja s and h asseldalen were identified in both records thus allowing a detailed synchronization of developing high resolution palaeoenvironmental proxy data the early holocene saksunarvatn ash layer and the middle holocene lairg b and hekla 4 cryptotephras in lake tiefer see are further important anchor points for the comparison with other high resolution palaeoclimate records in central and northern europe tentative correlations of cryptotephras have been made with a historical basaltic grimsv otn eruption ad890 e ad856 and three late holocene rhyolitic eruptions including the 2 1 ka glen garry and two unknown high silicic cryptotephras of probably icelandic provenance 1 9 cal ka bp

oriented inclusions of magnetite in clinopyroxene source of stable remanent magnetization in gabbros of the messum complex namibia 1 crystallographically oriented and highly elongate magnetite inclusions in clinopyroxene are the dominant source of highly stable remanent magnetization in gabbros of the early cretaceous messum complex namibia rock magnetic properties determined for individual pyroxene crystals indicate a high proportion of single domain magnetite consistent with the observed sizes and shape anisotropy of the magnetite inclusions as in previous studies of similar inclusions these are inferred to have formed by exsolution two arrays of inclusions are regularly present in the messum clinopyroxenes inclined at about 74 consistent with formation at about 800 c deduced from optimization of phase boundary orientations virtual geomagnetic poles from these rocks are consistent with reference data confirming that the magnetization is of thermoremanent origin bipolar magnetizations are recorded at one site as well in individual clinopyroxene crystals suggesting that remanence acquisition upon initial cooling of the gabbro spanned a geomagnetic polarity reversal

constraints on the acquisition of remanent magnetization in fine grained sediments imposed by redeposition experiments the magnetization of sediments is acquired through complex processes involving a large number of physical mineralogical and magnetic parameters despite many attempts the degree to which these processes distort the record of the geomagnetic field as it is archived as a natural remanent magnetization nrm remains poorly documented among many other parameters it is important to evaluate the amount of smoothing inherent to the signal its relation with the field intensity and its variability in the sediment column in order to address these problems we performed new redeposition experiments using carbonate rich ocean drilling program odp site 851 and clay rich odp site 854 sediments we used a dilute solution of gelatin which gels below 20 c thereby allowing mechanical blocking of the magnetic grains we observed two critical results 1 the efficiency of detrital remanent magnetization drm decreases with increasing sediment concentration for a given slurry sediment concentration is defined as c ms ms mh2o where ms and mh2o are the sediment and water mass respectively higher c would then reflect greater compaction lower water content and presumably greater depth in the sediment column this effect reduces drm efficiency nearly to zero for c 50 2 post depositional remanent magnetization pdrm is important for c 50 pdrm is carried by grains covering the entire coercivity spectrum by comparing the mean value of nrm divided by anhysteretic remanent magnetization from the previous magnetostratigraphic study at site 851 with the relevant ratio derived from our redeposition experiments we were able to estimate that pdrm was significant within the depth interval where 44 c 56 if the sediment concentration profile for the uppermost sediment was known at site 851 we could define the transfer function for the deconvolution of the field variations finally the dependence of drm efficiency on c suggests that changes in the thickness of the surface mixed layer would change drm efficiency thus fluctuations in maximum bioturbation depth could possibly cause drm intensity changes regardless of changes in earth s field

depositional remanent magnetization toward an improved theoretical and experimental foundation the first theoretical predictions for the behavior of magnetic particles in water were that sedimentary magnetizations would be fully aligned with the ambient field yet redeposition experiments showed a strong and quasi linear dependence on the external field this empirically observed linearity has served as the fundamental assumption of sedimentary paleointensity studies for decades we present redeposition experiments which suggest instead that the relationship between depositional remanence drm and applied field may frequently be curved for magnetic fields in the range of the earth s numerical simulations using a flocculation model can explain the redeposition data and suggest that drm will be significantly non linear when the flocs are small several microns there is a strong dependence of floc size on salinity particularly in low salinity environments floc size has a profound influence on the efficiency of drm hence low salinity environment may give results with poor reproducibility the size of the floc in which magnetic particles are embedded is not accounted for in current methods of normalization yet is the most important parameter on the bright side however it now seems possible to quantitatively explain paleointensity in sedimentary systems opening the door to absolute paleointensity estimates from sediments whose key parameters of floc size distribution and settling times can be constrained

the role of magnetostatic interactions in sediment suspensions summary the processes that influence a detrital remanent magnetization as well as the physical microscale factors that control formation of a stable post depositional remanent magnetization are still not fully understood previous laboratory studies and statistical numerical approaches have shown the possibility that sediment suspensions can display complex magnetization phenomena such behaviour has been attributed to the effect of magnetostatic interactions in the suspension which could provide one explanation for spurious magnetizations observed in marine sediment cores in laboratory experiments we investigated magnetization decay as a function of time in sediment suspensions produced with varying lithologies and particle concentrations a companion model takes into account the physics of magnetic particle particle interactions brownian motion and hydrodynamic forces to investigate numerically the magnetization behaviour of sediment suspensions when combined the experiments and the numerical models reveal a weak effect of magnetostatic interactions in the natural sediment suspensions which is expressed as an increase in the magnetization decay rate in addition a calculation of effective particle size based on the response of each suspension to brownian motion indicates that the majority of the sedimentary magnetic particles are attached to larger clay particles

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

detrital magnetizations from redeposition experiments of different natural sediments abstract we carried out several experiments with carbonate rich and clay rich sediments that were redeposited in plastic cubes or in 1 meter long cylindrical tubes with the aim of investigating the sensitivity of the depositional remanent magnetization drm to various environmental and physical parameters in contrast to previous studies we did not observe any difference in the drm acquired with or without saline water for either kind of sediment taking advantage of a gelatin that fixes the position of particles in suspension we were able to measure the magnetization of suspended sediments within water filled columns and thus to test the degree of alignment of magnetic grains the magnetization of clay rich sediments was not far from saturation but the magnetization was considerably reduced after deflocculation because the large flocs that favored alignment of magnetic grains were destroyed by the deflocculant similarly the large flocs found at the bottom of the deposition tubes also reveal an efficient magnetic alignment in contrast the weak magnetization of the carbonate rich sediments with small flocs yielded accurate and well grouped magnetization directions that were not sensitive to deflocculation only a fraction of magnetic grains was mechanically oriented by the field in this case because of the weak net magnetic moments of the grains embedded within small flocs the drm was constrained by redeposition in plastic cubes performed without gelatin and with different field intensities the drm intensity of the carbonate rich sediments is linearly related to field strength which attests to their suitability for studies of relative paleointensity tests performed with mud from several stratigraphic levels in different marine sediment cores failed to reveal a significant influence of carbonate content on drm which suggests a relatively minor role of lithological changes on records of relative paleointensity

the effect of remanence anisotropy on paleointensity estimates a case study from the archean stillwater complex paleomagnetism of archean rocks potentially provides information about the early development of the earth and of the geodynamo precambrian layered intrusive rocks are good candidates for paleomagnetic studies such complexes are commonly relatively unaltered and may contain some single domain magnetite armored by silicate mineral grains however layered intrusives often have a strong petrofabric that may result in a strong remanence anisotropy magnetic anisotropy can have particularly disastrous consequences for paleointensity experiments if the anisotropy is unrecognized and if its effects remain uncorrected here we examine the magnetic anisotropy of an anorthosite sample with a well developed magmatic foliation the effect of the sample s remanence fabric on paleointensity determinations is significant paleointensities estimated by the method of thellier and thellier range from 17 to 55 t for specimens magnetized in a field of 25 t we describe a technique based on the remanence anisotropy tensor to correct paleointensity estimates for the effects of magnetic fabric and use it to estimate a paleointensity for the stillwater complex mt usa of 32 t adjusted for the effects of slow cooling

exsolved magnetite inclusions in silicates features determining their remanence behavior submicroscopic needle shaped titanomagnetite inclusions exsolved in silicate minerals commonly occur in mafic intrusive rocks and are protected from alteration by their silicate hosts making them excellent candidates for paleomagnetic studies a suite of samples containing clinopyroxene and plagioclase hosted magnetite inclusions from five geologically diverse sites was examined using magnetic force microscopy to image the inclusions magnetic domain state alternating field demagnetization experiments indicate that some inclusions are more stable recorders than others the two factors controlling the remanence behavior of the inclusions are internal microstructures and inclusion dimensions magnetite ulvospinel unmixing within an inclusion subdivides the original titanomagnetite solid solution into a boxwork structure composed of 103 105 magnetite prisms separated by thin ulvospinel lamellae the conversion of multidomain sized needles into assemblages of interacting single domains increases the coercivity and hence relaxation time of the inclusions and results in a thermochemical magnetic remanence in samples without this exsolution microstructure the inclusions diameters determine coercivity and their magnetization is thermoremanent both styles of high coercivity inclusions successfully record paleomagnetic directions in mesozoic rocks and their ubiquity within silicate minerals clinopyroxene and plagioclase of mafic intrusive rocks indicates their value as chemically and magnetically stable tools for elucidating the ancient magnetic field marine magnetic anomalies and crustal kinematics

magnetic mineral diagenesis abstract reduction oxidation redox reactions occur during burial because sediments contain reactive mixtures of oxidised and reduced components diagenetic chemical reactions represent the approach of all sedimentary components toward equilibrium and control the long term stability of sedimentary iron bearing minerals magnetic minerals are sensitive indicators of sedimentary redox conditions and of changes in these conditions through time with diagenetic effects ranging from subtle to pervasive despite the importance of magnetic mineral diagenesis in paleomagnetism rock magnetism and environmental magnetism and the usefulness of these subjects in the earth and environmental sciences there is no systematic single published treatment of magnetic mineral diagenesis this paper is an attempt to provide such a treatment for the full range of diagenetic environments magnetic mineral diagenesis during early burial is driven largely by chemical changes associated with organic matter degradation in a succession of environments that range from oxic to nitrogenous to manganiferous to ferruginous to sulphidic to methanic where the free energy yielded by different oxidants decreases progressively in each environment in oxic environments the most important diagenetic processes involve surface oxidation of detrital minerals and precipitation of fe3 bearing minerals from solution in ferruginous environments the most reactive detrital and authigenic iron oxides undergo dissolution often mediated by dissimilatory iron reducing bacteria which releases fe2 that becomes available for other reactions the fe2 in solution can diffuse upward where it is oxidised to form new authigenic iron oxyhydr oxide minerals or it can become bioavailable to enable magnetotactic bacteria to biomineralise magnetite generally at the base of the overlying nitrogenous zone alternatively dissimilatory iron reducing bacteria can produce extracellular magnetite within ferruginous environments in sulphidic environments iron bearing detrital mineral assemblages undergo more radical alteration hydrogen sulphide which is a byproduct of bacterial sulphate reduction or of anaerobic oxidation of methane reacts with the fe2 released from iron mineral dissolution or directly with solid iron oxyhydr oxide minerals to form iron sulphide minerals mackinawite greigite and pyrite authigenic growth of ferrimagnetic greigite has important implications for paleomagnetic recording secondary iron sulphide formation can also occur as a result of anaerobic oxidation of methane methane migration through sediments in association with biogenic or thermogenic methane production or in association with gas hydrate dissociation can disrupt the diagenetic steady state and give rise to greigite and monoclinic pyrrhotite formation that remagnetises sediments most of the above described diagenetic processes occur below 50 c with continuing burial above 50 c but at sub metamorphic temperatures magnetic minerals can undergo further thermally induced chemical changes that give rise to a wide range of mineralogical transformations that affect the magnetic record of the host sediment these changes include remagnetisations magnetic analysis can provide much valuable information concerning diagenesis in environmental processes the range of processes discussed in this paper should assist researchers in analysing sediment magnetic properties for which the assessment of diagenetic effects has become a necessary component

effects of clay mineralogy and the electrical conductivity of water on the acquisition of depositional remanent magnetization in sediments new experiments on the interrelationship of clay minerals solution properties and postdepositional remanent magnetization pdrm provide interesting insights into pdrm acquisition we have found that clay mineral content composition and solution conductivity affect pdrm strongly with increasing clay concentration or electrolyte concentration pdrm intensity decreases clay minerals may also be responsible for the inclination shallowing of pdrm different clay minerals behave differently for example montmorillonite diminishes pdrm intensity more rapidly than kaolin anson and kodama 1987 have suggested an electrostatic model to explain their results of a similar study however we cannot explain all of our results by using their electrostatic model to us it seems that colloid coagulation is the key to explaining the results two kinds of forces exist in a colloidal solution electric repulsion and van der waals attraction in low electrolyte concentration solutions repulsion is larger than attraction strong brownian motion is therefore required to enable the particles to cross over the repulsive energy barrier in order to stick to each other but in high electrolyte concentration solutions repulsion becomes relatively small and particles stick together much more easily thus the effective mass of the grain becomes larger clay magnetite but the magnetic moment remains the same and hence the magnetic torque is relatively less effective as a result the magnetic carriers do not align effectively in the direction of the ambient field making the pdrm intensity weaker

responses of mountain ice caps in central iceland to holocene climate change abstract a chronology of holocene fluctuations of small outlet glaciers from the regnabu ajokull ice cap on hrutfell central iceland allows comparison of their sensitivity with the margin of the nearby langjokull icefield to ascertain which frequencies of climatic variability are recorded by adjacent glaciers of different size dating utilised tephra layers in aeolian soils lying on between and beneath moraine ridges key marker isochrones dating from the hekla 4 tephra c 3 8 ka bp to katla a d 1918 provide unequivocal bracketing ages the stratigraphy and geochemical fingerprinting of tephras on younger moraines allows subdivision of little ice age moraines five groups of moraines are identified at c 4 5 5 0 c 3 0 3 5 ka bp c 2 0 2 5 ka bp and from the little ice age at c a d 1700 and in the late 19th early 20th century these represent a neoglacial sequence in which steep small glaciers readvanced to similar positions during what are here termed little ice age type periods liatps in contrast the nearby margins of langjokull show evidence of a late nineteenth century advance only suggesting that this was the holocene maximum for this glacier the contrasting responses of local glaciers and the large icefield are explained by their different sizes and response times so that the preserved moraine record is largely pre conditioned by the glacier type in general the forefields of steep fast responding glaciers contain more complete archives of holocene climatic changes than do the margins of the large icefields

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

full vector model for magnetization in sediments abstract sediments provide a continuous record of past geomagnetic field variations although it is theoretically possible to get both the direction and intensity of the geomagnetic field from sediment records the mechanism is not fully understood previous workers have postulated that flocculation plays an important role in detrital remanent magnetism drm flocs are porous loose and highly fragile aggregates of microscopic clay particles and their behavior in a viscous medium is likely to be different than single particles of magnetic minerals in order to understand the role of flocculation in sediment magnetization we carried out a set of redeposition experiments at different field intensities and a quasi constant field inclination of 45 we present here a simple numerical model of flocculation incorporating both magnetic and hydrodynamic torques to explain the experimental data at small floc sizes drm acquisition is likely to be non linear in field strengths comparable to the earth s but the sediments may be able to record the directions accurately with increasing floc sizes sediments may retain a record of the intensity that is linearly related to the applied field or a direction parallel to the applied field but are unlikely to do both at the same time also the majority of the magnetic particles in the sediments may not be contributing significantly towards the net drm and any bulk normalizing parameter may be unsuitable if the depositional environment has changed over the depositional period

direct imaging of nanoscale magnetic interactions in minerals the magnetic microstructure of a natural finely exsolved intergrowth of submicron magnetite blocks in an ulvospinel matrix is characterized by using off axis electron holography in the transmission electron microscope single domain and vortex states in individual blocks as well as magnetostatic interaction fields between them are imaged at a spatial resolution approaching the nanometer scale the images reveal an extremely complicated magnetic structure dominated by the shapes of the blocks and magnetostatic interactions magnetic superstates in which clusters of magnetite blocks act collectively to form vortex and multidomain states that have zero net magnetization are observed directly

asian monsoon modulation of nonsteady state diagenesis in hemipelagic marine sediments offshore of japan we have identifiedmillennial scale variations inmagneticmineral diagenesis from pacific ocean sediments offshore of japan that we correlate with changes in organic carbon burial that were likely driven by asian monsoon fluctuations the correlation was determined by identifying offsets between the positions of fossil diagenetic fronts and climatically induced variations in organic carbon burial inferred frommagnetic and geochemical analyses episodes of intense monsoon activity and attendant sedimentmagneticmineral diagenesis also appear to correlate with heinrich events which supports the existence of climatic telecommunications between asia and the north atlantic region several lines of evidence support our conclusions 1 fluctuations in down coremagnetic properties and diagenetic pyrite precipitation are approximately coeval 2 localized stratigraphic intervals with relatively strongermagneticmineral dissolution are linked to enhanced sedimentary organic carbon contents that gave rise to nonsteady state diagenesis 3 down core variations in elemental s content provide a proxy for nonsteady state diagenesis that correlate with key records of asianmonsoon variations and 4 relict titanomagnetite that is preserved as inclusions within silicate particles rather than secondary authigenic phases e g greigite dominates the strongly diagenetically altered sediment intervals and are protected against sulfidic dissolution we suggest that such millennial scale environmentalmodulation of nonsteady state diagenesis that creates a temporal diagenetic filter and relict magneticmineral signatures is likely to be common in organic rich hemipelagic sedimentary settingswith rapidly varying depositional conditions ourwork also demonstrates the usefulness of magnetic mineral inclusions for recording important environmentalmagnetic signals

effects of ph and salinity on the intensity of magnetization in redeposited sediments abstract the intensity of magnetization in redeposited sediments was measured as a function of salinity and ph of the sediment water mixture the intensity was relatively low at high salinity or low ph under these conditions inter particle bonds are enhanced which lead to the formation of larger flocs ferromagnetic minerals magnetite in our sediment are incorporated in the flocs consisting mainly of non magnetic minerals and orientation along an applied field is resisted by the drag proportional to the third power of the diameter of the floc higher intensity is recorded at lower salinity and higher ph when inter particle forces are weaker and floc sizes are smaller the increase of magnetization as a function of ph stops at a ph of about 5 which is the pzc of the edge of illite the dominant non magnetic mineral in our sediment above this ph both the edge and face of the illite grains are negative and there is net repulsion which retards flocculation the interactions between clay particles therefore have a larger effect on the intensity of magnetization than interactions between magnetite and clay this suggests that magnetite is always included in a clay magnetite domain and this domain is the smallest unit that is reoriented by a magnetic field

subtropical microfossil indicators from the late pliocene celtic sea abstract the first record of calcareous nannofossils from the upper pliocene st erth beds cornwall is presented the sediments contain the most diverse neogene nannoflora yet recovered from mainland britain nineteen species the abundant occurrence of aragonitic ascidian didemnid spicules is also documented within the nannofossil size fraction the spicules were probably transported into a sheltered basin of mud deposition from offshore hardground areas the presence of diverse nannofossil assemblages together with well preserved ascidian spicules within the st erth beds is postulated to indicate deposition from a subtropical water mass subtropical temperatures 18 24 c in the celtic sea of late pliocene times could have been caused either by a global warming or as a result of the gulf stream spreading waters around cornwall and bathing the celtic coasts of northwest europe

the fidelity of paleomagnetic records carried by magnetosome chains article history received 30 may 2013 received in revised form 19 september 2013 accepted 20 september 2013 available online xxxx editor j lynch stieglitz magnetotactic bacteria mtb which use aligned chains of magnetosomes magnetic crystals as a navi gation tool are found in a wide range of modern day marine river and lacustrine environments and their fossilized remains are being increasing recognized in geological records despite an increasing realization that biogenically derived magnetic particles may play a key role in sedimentary magnetizations little is known about the influence that they may have on the fidelity of paleomagnetic recordings using cultured magnetospirillum magneticum strain amb 1 we have conducted simple 2d i e zero inclination deposi tion experiments to assess the efficiency with which magnetosome chains align along magnetic field lines and the implications that this has for paleomagnetic records our results indicate that the natural rema nent magnetization nrm acquired by deposited mtb is near linear with applied field 0 120 t but that nrm acquisition does not perfectly follow the assumed linear trend and over a six fold increase in applied field only a factor 5 5 increase in nrm intensity is observed i e 9 lower than the assumed trend both anhysteretic remanent magnetization arm and saturation isothermal remanent magnetiza tion sirm normalized relative paleointensities rpis can successfully recover field strength variations under identical situations when the mtb concentration of the initial solution is varied up to a factor 8 nrm carried by mtb responds in the expected fashion i e a doubling of bacteria produces a doubling of nrm intensity both arm and sirm however do not respond to as expected a doubling of con centration corresponds to an increase of 1 76 in sirm intensity and an increase of only 1 63 in arm intensity both are influenced by magnetostatic interactions that arise from the close packing of bacterial cells or subchain interactions within a single bacterial cell first order reversal curve forc diagrams however are too insensitive to indicate the presence of paleomagnetically relevant magnetic interactions remanence based methods i e the arm ratio are more appropriate to detect such interactions

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

are hydrodynamic shape effects important when modelling the formation of depositional remanent magnetization recent conceptual models have attributed the weak depositional remanent magnetizations observed in natural sediments to flocculation processes in the water column magnetic particles included into flocs have not only to rotate themselves into alignment with the geomagnetic field but also the larger particles to which they are attached making remanence acquisition an inefficient process alignment is hindered further when the magnetization vectors of the particles in any given floc partially cancel reducing the overall magnetic torque existing numerical simulations of flocculation effects on depositional remanence formation have been limited to spherical bodies with translational and rotational motion acting independently of each other in the case of non spherical flocs the translational and rotational motion are coupled and such bodies will describe a complex trajectory through the water column calculations will be presented that show the torque exerted on a non spherical floc by the surrounding water can be orders of magnitude greater than the magnetic torque non spherical flocs will therefore align less efficiently with the geomagnetic field and hydrodynamic effects may play an important role in controlling the magnitude of sedimentary remanence

a rapid method for magnetic granulometry with applications to environmental studies we have developed a rapid method of characterizing the fine scale variation sub micron to hundreds of microns m in the grain size of magnetite in long sediment cores this method is a new rock magnetic tool and can be readily applied to the following disciplines 1 environmental studies 2 hydrology 3 petrology and 4 geomagnetism the method consists of a comparative plot of the depth variation of two magnetic parameters the first parameter low field less than 0 1 millitesla mt or 1 oe susceptibility in a steady field is plotted on the abscissa the second parameter anhysteretic remanent magnetization arm imparted by a smoothly decreasing af of peak value 100 mt 1 000 oe in the presence of a weak steady field 0 01 0 1 mt or 0 1 1 oe is plotted on the ordinate for a sediment column in which the magnetic mineral is magnetite a plot of the two magnetic parameters shows transitions in the average magnetic grain size with depth as clearly discernible changes in the slope of a regression line fit to the individual points arm

widespread occurrence of silicate hosted magnetic mineral inclusions in marine sediments and their contribution to paleomagnetic recording magnetic mineral inclusions occur commonly within other larger mineral phases in igneous rocks and have been demonstrated to preserve important paleomagnetic signals while the usefulness of magnetic inclusions in igneous rocks have been explored extensively their presence in sediments has only been speculated upon the contribution of magnetic inclusions to the magnetization of sediments therefore has been elusive in this study we use transmission electron microscope tem and magnetic methods to demonstrate the widespread preservation of silicate hosted magnetic inclusions in marine sedimentary settings tem analysis reveals detailed information about the microstructure chemical composition grain size and spatial arrangement of nanoscale magnetic mineral inclusions within larger silicate particles our results confirm the expectation that silicate minerals can protect magnetic mineral inclusions from sulfate reducing diagenesis and increase significantly the preservation potential of iron oxides in inclusions magnetic inclusions should therefore be considered as a potentially important source of fine grained magnetic mineral assemblages and represent a missing link in a wide range of sedimentary paleomagnetic and environmental magnetic studies in addition we present depositional remanent magnetization drm modeling results to assess the paleomagnetic recording capability of magnetic inclusions our simulation demonstrates that deposition of larger silicate particles with magnetic inclusions will be controlled by gravitational and hydrodynamic forces rather than by geomagnetic torques thus even though these large silicates may contain ideal single domain particles they can not contribute meaningfully to paleomagnetic recording however smaller silicate grains e g silt and clay sized silicates with unidirectionally magnetized magnetic inclusions can potentially record a reliable drm

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

detrital remanence inclination errors and anhysteretic remanence anisotropy quantitative model and experimental results summary we suggest that inclination errors in detrital remanent magnetization drm may be recognized and corrected by measurement of the anisotropy of anhysteretic remanent magnetization arm arm anisotropy reflects directional variations in the remanence capacity of relatively fine grained magnetic particles in rocks or sediments generally the same particles that carry the stable component of drm relative vertical and horizontal drm magnitudes are controlled by this directional remanence capacity as well as by the alignment efficiency of the particle magnetic moments which in turn is governed by the relative intensities of the vertical and horizontal components of the ambient magnetic field thus the respective vertical and horizontal components of palaeointensity and therefore palaeofield inclination may be obtained by normalizing the measured drm components by parallel arm intensities synthetic sediments containing silica kaolin and sized magnetite powders have previously been found to acquire experimental drm with an inclination error that is a direct function of kaolin concentration further experiments have now shown that arm anisotropy in the synthetic sediments is also a direct function of kaolin content and thus a correlation exists between drm inclination errors and arm anisotropy multiplying the measured drm vector by the inverse of the arm anisotropy matrix yields an improved estimate of the actual laboratory depositional field in all cases for samples with acicular 0 5 m magnetite this procedure reduces a mean inclination error of over 5 to less than 0 5 samples with subequant magnetite of slightly larger grain size 0 75 m exhibit similar arm anisotropy 10 per cent but somewhat larger inclination errors 10 we interpret this in terms of stronger horizontal alignment but weaker particle anisotropies for the larger more equant magnetites

volcanic ash layers from the last glacial termination in the ngrip ice core the tephrochronological record of the 1400 1640 m depth 10 000 16 000 calendar ice core years before present of the ngrip ice core has been established by particle screening of selected samples ash was identified in 20 samples correlation with ice marine and terrestrial records from volcanic source regions in the northern hemisphere positively identifies the saksunarvatn ash and the vedde ash ash zone 1 major element chemistry of the remaining identified ash layers mainly points towards an icelandic origin this tephrochronological record provides new important marker horizons for correlating the timing of the climatic changes associated with the last glacial termination within the north atlantic region as well as outlining more details concerning the frequency and composition of volcanic eruptions occurring at this deglaciation copyright 2005 john wiley sons ltd

recent volcanic history of the veidivotn fissure swarm southern iceland an approach to volcanic risk assessment abstract the recent volcanic history of the southwestern part of the veidivotn fissure swarm southern iceland provides a basis for assessment of volcanic risk in an area of large hydropower potential local tephrastratigraphy and regional tephrochronology provide relative and absolute dating of individual eruptions as well as information on the volume and distribution of the products formed in each eruption three large eruptions took place in this area in 1480 a d 900 a d and 150 a d respectively each eruption produced approx 1 km3 dre of basaltic and minor amounts of silicic lava and tephra on fissures up to 42 km long no evidence is found of smaller eruptions during this period the estimated eruption frequency one eruption every 600 800 years implies that this part of the veidivotn fissure swarm is inactive for long periods between relatively large volcanic events a change in the mode of eruption from effusive to explosive took place during this period the hazards posed by this area include far reaching lava flows widespread heavy tephra fall with thicknesses in excess of 2 m at distances of 10 km and damming of a large glacial river with the consequent formation of unstable lakes a volcano tectonic model which explains the observed eruption frequency and provides a basis for a long term monitoring program is proposed eruptions on the veidivotn fissure swarm are interpreted as corollaries of rifting episodes initiated in the bardarbunga central volcano volcano tectonic episodes affect the fissure swarm at an average interval of 100 years minor episodes are limited to the central volcano and adjacents parts of the fissure swarm during the less frequent major episodes rifting and volcanic activity extends to the extreme southwestern part of the fissure swarm seismic monitoring of the bardarbunga central volcano could provide an early warning of renewed activity on the veidivotn fissure swarm a major rifting episode resulting in eruption on its southwestern part can be expected during the next 100 to 300 years

effects of internal mineral structures on the magnetic remanence of silicate hosted titanomagnetite inclusions an electron holography study 1 titanomagnetite inclusions in pyroxene and plagioclase are carriers of stable magnetic remanence in some slowly cooled rocks such as gabbros anorthosites granulites and diorites needle shaped inclusions with average dimensions of 1 1 25 m form epitaxially by exsolution from their host silicate close examination of clinopyroxene hosted inclusions reveals an internal microstructure which consists of magnetite fe3o4 prisms and ulvospinel fe2tio4 lamellae that formed as a result of phase unmixing during initial cooling this internal structure exerts a profound influence on the magnetic remanence properties of each inclusion primarily by transforming it from a multidomain grain into an assemblage of magnetostatically interacting single domain prisms here we use off axis electron holography to image the magnetization states of individual prisms and the magnetostatic interactions between them we show that the inclusions exhibit both single domain and collective magnetic states that depend primarily on the shape anisotropies of individual magnetite prisms magnetostatic interactions between closely spaced prism stacks and the shape anisotropy of the needle itself prisms that are separated by thick ulvospinel lamellae show uniformly magnetized and or vortex states in contrast closely spaced magnetite prisms behave as multipart vortices or as long composite columns whose strong net magnetization may not be related directly to the orientation and shape of either the needle or the constituent prisms the overall remanence direction recorded by clinopyroxene crystals containing finely exsolved inclusions is a reflection of both the inclusions elongation directions and the prism arrangements within them

dipole strength and variation of the time averaged reversing and nonreversing geodynamo based on thellier analyses of single plagioclase crystals 1 single plagioclase crystals separated from lavas can contain minute magnetic inclusions that faithfully record the geomagnetic field while the silicate host limits natural and experimentally induced alteration here we use paleointensity analyses of plagioclase crystals to examine three characteristic geodynamo regimes spanning the last 200 myr an interval of moderate reversal frequency during the paleogene the nonreversing field of the cretaceous normal polarity superchron and a period of very high reversal occurrence during the late jurassic an inverse relationship between reversal rate and intensity is supported furthermore the strength of the time averaged reversing field is more variable than that of the nonreversing field these observations are consistent with an active lower mantle shaping core mantle boundary heat flux and efficiency of the geodynamo on timescales of tens of millions of years

remanence acquisition efficiency in biogenic and detrital magnetite and recording of geomagnetic paleointensity relative paleointensity rpi variations of earth s magnetic field are widely used to understand geomagnetic field behavior and to develop age models for sedimentary sequences rpi estimation is based on a series of assumptions one key assumption that is rarely considered is that all magnetic particles in the sediment acquired a magnetization in an identical manner in this paper we test this assumption for sediments from the eastern equatorial pacific ocean that record well documented global rpi variations over the last 780 kyr the magnetization is carried by two stable single domain magnetic components which we identify as magnetite magnetofossils and titanomagnetite nanoparticle inclusions within larger silicate particles by analyzing signals carried by the two components separately we determine for the first time that magnetic nanoparticle inclusions can cause their host particles to record reliable but inefficient sedimentary paleomagnetic signals the magnetization carried by biogenic magnetite is acquired more efficiently than that carried by the nanoparticle inclusions variations in the concentration of both components are modulated climatically so that they record nearly identical rpi signals in many sediment types there is no correlation between the concentrations of different magnetic components so that variable remanence acquisition efficiency will complicate rpi recording our work demonstrates that detailed assessment of paleomagnetic recording by each constituent magnetic component needs to become a routine part of sedimentary rpi analysis

low frequency earthquakes at the torfajokull volcano south iceland abstract torfajokull is a large rhyolitic volcanic edifice with a 12 km diameter caldera and abundant high temperature geothermal activity it is located in the neovolcanic zone in south iceland at the junction of the eastern rift zone and a transform zone with the intraplate volcanic flank zone of south iceland the latest eruption at torfajokull occurred about 500 years ago torfajokull is a source of persistent small scale seismicity where two types of earthquakes occur high frequency events are concentrated in the western part of the caldera and low frequency events cluster in the south small low frequency earthquakes have been observed at torfajokull since the installation of a local analogue seismograph station in 1985 they typically occur in swarms up to 300 earthquakes per day have been observed the low frequency events have a frequency content of about 1 3 hz and are difficult to locate because of the emergent nature of their phases the 160 events located during the years 1994 2000 using the permanent icelandic seismic network cluster in the southern part of the torfajokull caldera a closer study of low frequency events was carried out between may and october 2002 with a dense network of twenty guralp 6td broadband seismometers in the torfajokull area no distinct swarm activity was observed during this period but small low frequency events occurred almost on a daily basis about 330 low frequency events were detected during the study period they are located in the southern part of the caldera between two small glaciers areas of intensive geothermal activity surround the cluster of low frequency events it is argued that these earthquakes are associated with active magma in the south part of the torfajokull caldera possibly a rising cryptodome

magnetic properties of pelagic marine carbonates abstract pelagic carbonates are deposited far from continents usually at water depths of 3000 6000 m at rates below 10 cm kyr and are a globally important sediment type recent advances with recognition of widespread preservation of biogenic magnetite the inorganic remains of magnetotactic bacteria have fundamentally changed our understanding of the magnetic properties of pelagic carbonates we review evidence for the magnetic minerals typically preserved in pelagic carbonates the effects of magnetic mineral diagenesis on paleomagnetic and environmental magnetic records of pelagic carbonates and what magnetic properties can tell us about the open ocean environments in which pelagic carbonates are deposited we also discuss briefly late diagenetic remagnetisations recorded by some carbonates despite recent advances in our knowledge of these phenomena much remains undiscovered we are only at early stages of understanding how biogenic magnetite gives rise to paleomagnetic signals in sediments and whether it carries a poorly understood biogeochemical remanent magnetisation recently developed techniques have potential for testing how different magnetotactic bacterial species which produce different magnetite morphologies respond to changing nutrient and oxygenation conditions future work needs to test whether it is possible to develop proxies for ancient nutrient conditions from well calibrated modern magnetotactic bacterial occurrences a tantalizing link between giant magnetofossils and paleogene hyperthermal events needs to be tested much remains to be learned about the relationship between climate and the organisms that biomineralised these large and novel magnetite morphologies rather than being a well worn subject that has been studied for over 60 years the magnetic properties of pelagic carbonates hold many secrets that await discovery

holocene volcanic activity at grimsvotn bardarbunga and kverkfjoll subglacial centres beneath vatnajokull iceland assessment of potential future eruptive behaviour of volcanoes relies strongly on detailed knowledge of their activity in the past such as eruption frequency magnitude and repose time the eruption history of three partly subglacial volcanic systems grimsvotn bardarbunga and kverkfjoll was studied by analysing tephra from soil profiles around the vatnajokull ice cap which extend back to 7 6 ka well known regional holocene marker tephra e g h3 h4 h5 were utilized to correlate profiles stratigraphic positions and geochemical compositions were used for fine scale correlation of basaltic tephra around vatnajokull ice cap 345 tephra layers were identified of which 70 originated from grimsvotn bardarbunga or kverkfjoll the eruption frequency of each volcanic system was estimated grimsvotn has been the most active with an average of 7 eruptions 100 years range 4 14 during prehistoric time before 870 ad bardarbunga has been the second most active with 5 eruptions 100 years range 1 8 and kverkfjoll has remained essentially calm with 0 3 eruptions 100 years but showing periodic activity with repose times of 1000 years all three volcanic systems experienced lulls in activity from 5 ka to 2 ka referred to as the mid holocene low this reduced eruption frequency appears to have resulted from a decrease in magma generation and delivery from the mantle plume rather than from changes in ice load glacier thickness in prehistoric time there was a time lag of 1000 3000 years between a peak of activity at volcanoes directly above the mantle plume versus at volcanoes located in the non rifting part of the eastern volcanic zone closer to the periphery of the island this time space relationship suggests that a significant future increase in volcanism can be expected there following increased levels of volcanism above the plume

geochemistry dispersal volumes and chronology of holocene silicic tephra layers from the katla volcanic system iceland at least 12 silicic tephra layers silk tephras erupted between ca 6600 and ca 1675 yr bp from the katla volcanic system have been identified in southern iceland in addition to providing significant new knowledge on the holocene volcanism of the katla system which typically produces basaltic tephra the silk tephras form distinct and precise isochronous marker horizons in a climatically sensitive location close to both the atmospheric and marine polar fronts with one exception the silk tephras have a narrow compositional range with sio2 between 63 and 67 geochemically they are indistinguishable from ocean transported pumice found on beaches in the north atlantic region although they differ significantly from the silicic component of the north atlantic ash zone one naazo volumes of airborne silk tephra range from 0 05 to 0 3 km3 we present new isopach maps of the six largest layers and demonstrate that they originate within the katla caldera the apparently stable magma system conditions that produced the silk tephras may have been established as a consequence of the eruption of the silicic component of naazo ca 10 3 ka and disrupted by another large scale event the tenth century ad eldgja eruption ca 1 ka despite the current long repose silicic activity of this type may occur again in the future presenting hitherto unknown hazards copyright 2001 john wiley sons ltd

the tephrochronology of iceland and the north atlantic region during the middle and late quaternary a review the tephrochronology of iceland and the north atlantic region is reviewed in order to construct a unified framework for the last 400 kyr bp nearly all of the tephra layers described are also characterised geochemically a number of new tephra layers are analysed for the first time for their geochemical signature and a number of pre holocene tephra layers have been given an informal denotation the tephrostratigraphy of ash zone ii is highlighted where possible the rhyolitic tephra layers found outside iceland have been correlated to known icelandic tephra layers or to the volcanic source area the application of tephra fallout in various depositional environments is described and discussed copyright 2000 john wiley sons ltd

on the origin of inclination shallowing in redeposited sediments we studied the detrital remanent magnetization of laboratory redeposited lake sediments previously shown to be excellent recorders of the paleomagnetic field the redeposited and original natural sediments have indistinguishable stabilities to alternating fields af demagnetization and remanence intensities indicating that their remanences reside in the same magnetic particles thermomagnetic behavior hysteresis parameters and relatively high af stabilities of the detrital and anhysteretic remanences are essentially identical throughout the sediment column suggesting uniform distributions of submicron magnetite grains the declination is accurately recorded but inclination shallowing averaging 18 o is observed for routinely hand stirred rapidly redeposited sediments however sonically disaggregated sediments settle more slowly with anomalous shallowing averaging 7 o decreasing upward in the redeposited column these results indicate that thorough disaggregation of the sediment reduces the magnitude of the inclination shallowing probably by more efficiently isolating the magnetic grains from the matrix thus diminishing the dominance of the gravitational over geomagnetic aligning torques during deposition the progressive decrease of the inclination shallowing upward in the sediment column might be due to a parallel upward increase in the fraction of isolated magnetic particles which undergo brownian motion and alignment by the geomagnetic field the dominance of gravitational over geomagnetic aligning torques during deposition might be a cause for inclination shallowing in some coarser sediments in contrast inclination shallowing would be minimized for slow deposition of isolated submicron magnetite grains during natural sedimentation

paleointensity results from the late archaean modipe gabbro of botswana 1 rock magnetic and thellier thellier coe paleointensity results are reported for a new collection of samples from the 2 78 ga modipe gabbro of botswana the magnetic properties are very favorable leading to an unusually high success rate and a well constrained result of 36 40 t 95 confidence interval we discuss the long standing problem of allowing for the enormous differences between the natural and laboratory cooling rates and apply a temperature dependent correction derived from first order reversal curve forc data whereas pure single domain sd corrections often lead to quite large decreases in paleointensity estimates sometimes exceeding 50 we find a modest increase of about 10 the earth s magnetic dipole moment in the late archaean is thereby estimated to have been 6 1022 am2

rock magnetism of tiny exsolved magnetite in plagioclase from a paleoarchean granitoid in the pilbara craton granitoids are widespread in precambrian terranes as well as the phanerozoic orogenic belts but they have garnered little attention in paleomagnetic studies because granitoids often contain abundant coarse grained magnetically unstable oxides in this study the first example of tiny needle shaped exsolved oxides in plagioclase in a paleoarchean granitoid is reported the magnetic properties of single plagioclase crystals with the exsolved oxide inclusions have been studied to determine their paleomagnetic recording fidelity demagnetization experiments and hysteresis parameters indicate that the oxide inclusions are near stoichiometric magnetite and magnetically very stable first order reversal curve forc diagrams reveal negligible magnetostatic interactions minimal interactions are also reflected by very efficient acquisition of anhysteretic remanent magnetization single plagioclase crystals exhibit strong magnetic remanence anisotropies which require corrections to their paleodirectional and paleointensity data nonetheless quantitative consideration of anisotropy tensors of the single plagioclase crystals indicates that the bias can be mitigated by properly averaging data from a few tens of single crystals from the nonlinear thermoremanence acquisition of the plagioclase crystals we estimate that the plagioclase crystals can reconstruct paleointensity up to 50 t local metamorphic condition suggests that those magnetite may carry remanence of 3 2 to 3 3 ga we suggest that exsolved magnetite in granitoids is potentially a suitable target for the study of the early history of the geomagnetic field and prompt detailed microscopic investigations as well as paleomagnetic tests to constrain the age of remanence