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

magnetic properties of hydrothermally synthesized greigite fe3s4 i rock magnetic parameters at room temperature summary greigite of variable purity has been synthesized hydrothermally at 140 c by injecting a mohr s salt solution into a sodium sulphide solution containing dissolved zerovalent sulphur ageing for 30 960 min and rapidly quenching oxygen was rigorously excluded during the synthesis scanning electron microscope sem observations yield a grainsize range up to 400nm the variable purity was deduced from a varying specific saturation magnetization 3 29 am2 kg i the mjm or ors os ratio varies between 0 3 and 0 4 which is indicative of fine grained greigite concurring with sem observations hysteresis measurements at low temperatures indicate the presence of grains that are superparamagnetic at room temperature coercivity parameters b 13 43 mt bo cr 37 73 mt are the highest obtained to date for synthetic greigite and compare well with those measured for natural greigite ranges for other rock magnetic parameters are discussed greigite hysteresis parameters overlap those of magnetite to a large degree a diagnostic interparametric ratio for greigite is its high o xin ratio the range of the present greigite is 40 75 kam

on the statistical analysis of the rock magnetic s ratio summary the s ratio is a rock magnetic parameter employed to provide a relative measure of the contributions of low and high coercivity material to a sample s saturation isothermal remanent magnetization sirm the current form of expressing the s ratio involves reporting a single value that represents the relative contribution of the low coercivity material such as magnetite to the overall sirm expressing the s ratio in this form is somewhat misleading suggesting that the derived values are real numbers held in a euclidian sample space in fact s ratio data are constrained to a 1 d unit simplex meaning that many of the descriptive statistics reported for collections of data for example means and standard deviations are in error because the employed statistical methods are designed for unconstrained data only a simple example of this problem will be given followed by a demonstration of how meaningful statistics for the s ratio can be obtained using the additive log ratio transform of aitchison

magnetic fabric and structural setting of plio pleistocene clayey units in an extensional regime the tyrrhenian margin of central italy abstract results are presented of structural analysis and anisotropy of magnetic susceptibility ams analysis of plio pleistocene clayey units from the extensional tyrrhenian margin of central italy data have been collected in different tectonic settings mainly strike slip and transverse extensional structures at the sites with major evidence of plio pleistocene tectonics there appears a close relationship between magnetic lineations and stretching directions stretching processes that were able to modify the original magnetic fabric pre dated significant normal fault related tilting and therefore were active at the very first stage of extensional deformation the relationships between ams and structural trends in an extensional regime seem distinctively different from those reported in compressional regimes this kind of analysis may be useful in the geodynamic interpretation of ambiguous structures

oblique magnetic fabric in siderite bearing pelitic rocks of the upper carboniferous culm basin sw england an indicator for palaeo fluid migration

origin and consequences of cyclic ice rafting in the northeast atlantic ocean during the past 130 000 years deep sea sediment cores recovered from the northeast atlantic ocean were examined in order to elucidate the influence of the earth s orbital parameters on major ice rafting analyses of coarse grained ice rafted debris and planktonic foraminifers revealed a strong reaction to the precession signal since 130 000 yr b p dropstone layers have been deposited each half period of a precessional cycle 11 000 1000 yr ice rafting occurs during times of winter minimum summer maximum insolation and summer minimum winter maximum insolation in the first case high summer insolation forces meltwater discharge from the ice sheets into the polar seas which subsequently enhances formation of sea ice during the winter in the second case growth of continental ice enhances iceberg production which also leads to a salinity reduction of surface seawater both situations result in a southward penetration of polar water thus the marine record of dropstones documents ice rafting not only during weichselian stades but also during cold events within interstades the regularity of ice rafting yields a useful framework to calibrate and elucidate climatic changes not only in the region of the north atlantic ocean but also in remote areas such as the pacific ocean and the antarctic

investigating the reasons for the failure of palaeointensity experiments a study on historical lava flows from mt etna italy summary a refined palaeointensity experiment accompanied by rock magnetic studies has been carried out on six lava flows from 1910 and 1928 from mt etna the purpose of the study was to try to understand why these very young basaltic flows are generally unable to provide a correct estimate of the magnitude of the ambient magnetic field during flow cooling susceptibility versus temperature curves and ore microscopic studies show that 3 types of magnetic minerals phases h m and l are present in these flows some samples containing a single largely dominant magnetic phase while others contain a mixture of several phases phase h i s a thermally stable near magnetite phase resulting from titanomagnetite oxyexsolution phase l is a thermally stable titanomagnetite with a curie temperature of approximately 200 c phase m is a titanomagnetite phase of curie temperature between 450 490 c which is unstable at temperatures above 400 c in addition to the usual reliability checks of the thellier method nrm trm linearity ptrm checks our palaeointensity experiments included additional heating allowing determination of the md or psd sd character of each ptrm and determination of crm or transdomain remanences possibly acquired during heating from the 28 samples studied 20 provide a linear nrm trm plot over about 1 4 or more of total nrm however only six of them all containing near magnetite as a single phase display positive ptrm checks nevertheless these six samples yield a mean palaeointensity of about 52 t which exceeds the real field palaeomagnitude 42 t by some 25 per cent the reasons for this almost total failure of palaeointensity experiments are diverse for samples with a dominant l phase ptrms present a behaviour typical of large md grains with as much as 1 3 of remanence with unblocking temperatures exceeding the blocking range no crm is acquired yet a remanence does develop during heating in a field followed by cooling in zero field we suggest that this remanence is a transdomain remanence resulting from domain rearrangements these two observations are in conflict with some of the basic requirements of the thellier method phase m seems chemically stable up to 400 c but ptrm changes start at lower temperatures this thermal instability and the development of a significant transdomain remanence seem to be the causes of the failure of palaeointensity experiments the reason for the rather large and quite unexpected error in the average palaeointensity provided by the samples containing only a near magnetite phase h may lie in the fact that the low to medium temperature ptrms which represent a significant fraction of the total trm seem to be carried by small md particles in conclusion several modifications of the thellier method are proposed

improvements in procedure and paleointensity selection criteria picrit 03 for thellier and thellier determinations application to hawaiian basaltic long cores abstract in this paper we first describe the paleointensity furnaces and the procedures specifically designed to minimize oxidation of the samples during thellier and thellier paleointensity experiments routinely used at gif we also propose some improvements to the set of criteria commonly used to select the results of the thellier experiments in order to further increase the reliability of paleointensity determination finally to be included in databases the new set of paleointensity criteria picrit 03 paleointensity criteria in 2003 is discussed and the results obtained from two long hawaiian basaltic cores the scientific observation hole 4 soh 4 and the hawaiian scientific drilling project 1 hsdp1 are used to illustrate its effectiveness some of the results are re evaluated and a new composite record of absolute paleointensity and inclination spanning 420 kyr is presented the inclination values obtained from the two cores combined support the hypothesis of persistent shallow inclinations at hawaii in intensity the upper 100 kyr are covered by the data from soh 4 with 309 reliable determinations out of 766 examined obtained from 143 units which belong to the kilauea volcano at the top of this core the 341 reliable data covering the 100 420 kyr interval are from 149 units of core hsdp1 belonging to the mauna kea volcano the interval 320 to 420 kyr is particularly detailed because of high extrusion rate of the volcano the use of more stringent criteria decreases the number of determinations considered as reliable in previous publications but it improves their accuracy and robustness

long term variations in palaeointensity we compile a dataset of reliable palaeointensity estimates based both on published work and on new data from basaltic glass the basaltic glass data more than double the number of reliable thellie

characterization of the magnetic fabric of rocks abstract an inextensive system of magnetic susceptibility anisotropy factors is suggested which is sufficient in the majority of practical applications for characterizing the magnetic fabric of rocks the system involves three newly devised factors the corrected anisotropy degree the shape factor and the difference shape factor the first of these characterizes the quantity of anisotropy somewhat better than the commonly used anisotropy degree the shape factor characterizes the quality of anisotropy i e the shape of the susceptibility ellipsoid in a convenient way its definition is based on a certain analogy between the anisotropy ellipsoid and the strain ellipsoid the difference shape factor can replace the shape factor when only the differences between the principal susceptibilities are given while the mean susceptibility is unknown

the crystallographic fabric and texture of siderite in concretions implications for siderite nucleation and growth processes the crystallographic fabric of siderite in siderite concretions has been determined for upper carboniferous westphalian a non marine concretions and lower jurassic pliensbachian marine concretions compositional zoning indicates that individual siderite crystals grew over a period of changing pore water chemistry consistent with the concretions being initially a diffuse patch of cement which grew progressively the siderite crystallographic fabric was analysed using the anisotropy of magnetic susceptibility which is carried by paramagnetic siderite the siderite concretions from marine and non marine formations exhibit differences in fabric style although both display increases in the degree of preferred siderite c axis orientation towards the concretion margins the westphalian non marine siderites show a preferred orientation of siderite c axes in the bedding plane whereas the pliensbachian marine siderites have a preferred orientation of c axes perpendicular to the bedding in addition a single marine concretion shows evidence of earlier formed inclined girdle type fabrics which are intergrown with later formed vertical c axis siderite fabrics the marine and non marine fabrics are both apparently controlled by substrate processes at the site of nucleation which was probably clay mineral surfaces siderite nucleation processes on the substrate were most probably controlled by the bio chemistry of the pore waters which altered the morphology and crystallographic orientation of the forming carbonate the preferred crystallographic orientation of siderite results from the orientation of the nucleation substrate fabric changes across the concretions partially mimic the progressive compaction induced alignment of the clay substrates while the concretion grew during burial

are the iron carbonate minerals ankerite and ferroan dolomite like siderite important in paleomagnetism magnetic mossbauer effect and thermal properties have been evaluated for specimens of the mineral ankerite and are contrasted with similar measurements reported for the mineral siderite to determine if ankerite and ferroan dolomite like siderite may be important in paleomagnetism as a producer of secondary spurious remanent moments as the result of oxidation in air our data indicate that ankerite and siderite both have comparable thermal stabilities and that ankerite does break down to form iron oxides at temperatures as low as 250 c during thermal demagnetization we conclude from our data that any iron carbonate siderite ankerite or ferroan dolomite may readily oxidize in air to maghemite or hematite thus producing a spurious remanent moment rm that can dominate the magnetic properties of the specimen thermal demagnetization of ankerite or siderite at lower temperatures below 250 c does not appear to produce such a spurious rm as might be expected the quantity of secondary iron oxides produced by oxidation is limited by the iron content in the carbonate crystal lattice oxidized siderite produces significantly greater amounts of iron oxides than equivalent percentages of ankerite which in turn produces more iron oxides than equal amounts of ferroan dolomite in a given sample

time variations in geomagnetic intensity 1 after many years spent by paleomagnetists studying the directional behavior of the earth s magnetic field at all possible timescales detailed measurements of field intensity are now needed to document the variations of the entire vector and to analyze the time evolution of the field components a significant step has been achieved by combining intensity records derived from archeological materials and from lava flows in order to extract the global field changes over the past 12 kyr a second significant step was due to the emergence of coherent records of relative paleointensity using the remanent magnetization of sediments to retrace the evolution of the dipole field a third step was the juxtaposition of these signals with those derived from cosmogenic isotopes contemporaneous with the acquisition of records new techniques have been developed to constrain the geomagnetic origin of the signals much activity has also been devoted to improving the quality of determinations of absolute paleointensity from volcanic rocks with new materials proper selection of samples and investigations of complex changes in magnetization during laboratory experiments altogether these developments brought us from a situation where the field changes were restricted to the past 40 kyr to the emergence of a coherent picture of the changes in the geomagnetic dipole moment for at least the past 1 myr on longer timescales the field variability and its average behavior is relatively well documented for the past 400 myr section 3 gives a summary of most methods and techniques that are presently used to track the field intensity changes in the past in each case current limits and potential promises are discussed the section 4 describes the field variations measured so far over various timescales covered by the archeomagnetic and the paleomagnetic records preference has always been given to composite records and databases in order to extract and discuss major and global geomagnetic features special attention has been devoted to discussing the degree of confidence to be put in the data by considering the integration of multiple data sets involving different techniques and or materials

tectonic versus mineralogical contribution to the magnetic fabrics of epimetamorphic slaty rocks an example from the ardennes massif france belgium abstract the discrimination of the respective contribution of strain and mineralogy of the bulk anisotropy of low field magnetic susceptibility ams of rocks is addressed by a study of the ardennes variscan thrust belt unmetamorphosed to low medium grade rocks in the study area define a succession of magnetic parageneses hematite maghemite hematite maghemite magnetite magnetite pyrrhotite a direct control of the magnetic mineralogy on the magnitude of ams axes is shown in addition in all cases the interpretation of the directions of ams axes as kinematic indicators also applies to these rocks we show a progressive change in shearing direction from the south of the rocroi massif where the nappe transport is n directed in the north the vergence changes to northwestward

partial anhysteretic remanence and its anisotropy applications and grainsize dependence the distribution of remanence coercivities in a rock sample can be obtained by differentiating alternating field af demagnetization curves of anhysteretic remanent magnetization arm however significant errors can be introduced by grain interactions and by the noise inherent in differentiation we have found that 1 this error can be significantly reduced by measuring partial arm s parm s and 2 parm patterns show a strong empirical correlation with grain size parm s are imparted by exposing a sample to a small direct magnetic field in the presence of a large decreasing af the direct field is switched on over a specified range of af s thus magnetizing a fraction of the magnetite grains in the corresponding coercivity range we have further found in both synthetic and natural sediments that measurement of the anisotropy of parm enables us to detect the preferred orientations of distinct subpopulations of magnetite particles related to the size and shape of the grains

long term trends in paleointensity the contribution of dsdp odp submarine basaltic glass collections abstract the deep sea drilling project and the ocean drilling program have been collecting fresh appearing submarine basaltic glass from the world s oceans for over three decades this glass has proved nearly ideal for estimating paleointensity variations of the earth s magnetic field we compile here data for 726 paleointensity experiments from six publications on paleointensity using dsdp odp glass we also include new data for an additional 225 specimens these were obtained through the so called izzi paleointensity experiment of tauxe l staudigel h 2004 strength of the geomagnetic field in the cretaceous normal superchron new data from submarine basaltic glass of the troodos ophiolite geochem geophys geosyst 5 2 q02h06 doi 10 1029 2003gc000635 whereby infield zerofield steps are alternated with the zerofield infield steps to enhance quality assessment of the resulting data the entire collection of data from 951 experiments was prepared for uploading to the magic data base http earthref org including original measurements interpretations and useful metadata excellent results were obtained throughout the depth 1400 mbsf and age 0 160 ma range sampled dsdp odp glass data are compared with published paleointensity data meeting minimal acceptance criteria from the time interval 1 160 ma paleolatitudes were estimated for all cooling units in a self consistent manner for use in calculating virtual axial dipole moments we conclude 1 there is about a 20 difference in mean values between the sbg and the lava flow data 48 36 and 57 29 zam2 respectively the difference is caused by the fact that there are more higher values in the lava flow data than in the sbg data set rather than a difference in the minimum values lava flows cooling over a periods of days to months can account for the discrepancy 2 the positive relationship between polarity interval length and average paleofield intensity first hypothesized by cox a v 1968 lengths of geomagnetic polarity intervals j geophys res 73 3247 3260 is supported by data compiled here the brunhes data for which we have only a minimum estimate for polarity interval length are consistent with a long polarity interval suggesting that instead of racing toward reversal hulot g eymin c langlais b mandea m olsen n 2002 small scale structure of the geodynamo inferred from oersted and magsat satellite data nature 416 620 623 we could instead be in the midst of a long stable polarity interval 3 because the average value appears to be a function of polarity interval length it is probably not useful to calculate a mean value nonetheless it appears that most of the time apart from the brunhes and the longest polarity intervals the average dipole moment is substantially less than the present day value as suggested by juarez t tauxe l gee j s pick t 1998 the intensity of the earth s magnetic field over the past 160 million years nature 394 878 881

are there connections between the earth s magnetic field and climate understanding climate change is an active topic of research much of the observed increase in global surface temperature over the past 150 years occurred prior to the 1940s and after the 1980s the main causes invoked are solar variability changes in atmospheric greenhouse gas content or sulfur due to natural or anthropogenic action or internal variability of the coupled ocean atmosphere system magnetism has seldom been invoked and evidence for connections between climate and magnetic field variations have received little attention we review evidence for correlations which could suggest such causal or non causal connections at various time scales recent secular variation 10 100 yr historical and archeomagnetic change 100 5000 yr and excursions and reversals 10 3 10 6 yr and attempt to suggest mechanisms evidence for correlations which invoke milankovic forcing in the core either directly or through changes in ice distribution and moments of inertia of the earth is still tenuous correlation between decadal changes in amplitude of geomagnetic variations of external origin solar irradiance and global temperature is stronger it suggests that solar irradiance could have been a major forcing function of climate until the mid 1980s when anomalous warming becomes apparent the most intriguing feature may be the recently proposed archeomagnetic jerks i e fairly abrupt 100 yr long geomagneticfieldvariationsfoundatirregularintervalsoverthepastfewmillennia usingthearcheologicalrecordfromeuropetothe middle east these seem to correlate with significant climatic events in the eastern north atlantic region a proposed mechanism involves variations in the geometry of the geomagnetic field f i tilt ofthe dipole to lower latitudes resulting in enhanced cosmic ray induced nucleation of clouds no forcing factor be it changes in co2 concentration in the atmosphere or changes in cosmic ray flux modulatedbysolaractivityandgeomagnetism orpossiblyotherfactors canatpresentbeneglectedorshowntobetheoverwhelming single driver of climate change in past centuries intensive data acquisition is required to further probe indications that the earth s and sun s magnetic fields may have significant bearing on climate change at certain time scales 2006 elsevier b v all rights reserved

magnetic mineralogy changes in the pleistocene marine sequence of montalto di castro central italy and influences on the magnetic anisotropy summary the magnetic mineralogy of a pleistocene sedimentary sequence drilled near montalto di castro latium italy shows an extreme variability in a 40 m thick stratigraphic interval sedimentary fades in this interval also show rapid and strtng changes in the granulometry and colours of the deposits reflecting deposition in a coastal environment under a variety of energy and redox conditions rock magnetism analysed of the finegrained sediments from this stratigraphic interval indicate the presence in variable proportions of several magnetic minerals a new semi quantitative best fit method has been specifically developed for the estimation of the relative percentage contribution of the various magnetic minerals to the isothermal remanent magnetization the variability in the magnetic mineralogy throughout the whole sequence produces remarkable changes in the anisotropy of magnetic susceptibility of the deposits the interval with the major sedimentological and magnetic mineralogy changes is characterized by a fairly isotropic magnetic fabric unexpected for sediments this anomalous magnetic fabric is probably due to the balance between the inverse magnetic fabric produced by antiferromagnetic goethite and the normal magnetic fabric due to the paramagnetic matrix and other magnetic minerals

magnetic properties of the high himalayan leucogranites structural implications the magnetic properties of the high himalayan leucogranites have been investigated on 527 specimens in three plutons everest makalu 6 sites and manaslu 40 sites in nepal and gangotri 43 sites in india susceptibility varies between 2 and 100 10 6 si with an anisotropy ratio up to 1 16 high field and low temperature magnetic measurements together with comparison with weight percent iron demonstrate that anisotropy of magnetic susceptibility is carried by paramagnetic biotite and tourmaline the latter produces an inverse fabric i e with the minimum axis parallel to mineral lineation the magnetic fabric demonstrates complex patterns of stretching lineations during magmatic emplacement and its usefulness in semi quantitatively estimating petrofabric intensity is demonstrated for the biotite bearing facies natural remanent magnetization was measurable at only two sites in everest makalu where there are well defined reverse directions carried by titanomagnetite and pyrrhotite comparison of these preliminary results with predicted directions for stable india suggests northward tilting of about 10 and a small clockwise rotation of this massif

magnetic fabric variations in mesozoic black shales northern siberia russia possible paleomagnetic implications abstract a 28 m long section situated on the coast of the arctic ocean russia 74 n 113 e was extensively sampled primarily for the purpose of magnetostratigraphic investigations across the jurassic cretaceous boundary the section consists predominantly of marine black shales with abundant siderite concretions and several distinct siderite cemented layers low field magnetic susceptibility k ranges from 8 10 5 to 2 10 3 si and is predominantly controlled by the paramagnetic minerals i e iron bearing chlorites micas and siderite the siderite bearing samples possess the highest magnetic susceptibility usually one order of magnitude higher than the neighboring rock the intensity of the natural remanent magnetization m0 varies between 1 10 5 and 6 10 3 a m several samples possessing extremely high values of m0 were found there is no apparent correlation between the high k and high m0 values on the contrary the samples with relatively high m0 values possess average magnetic susceptibility and vice versa according to the low field anisotropy of magnetic susceptibility ams three different groups of samples can be distinguished in the siderite bearing samples i an inverse magnetic fabric is observed i e the maximum and minimum principal susceptibility directions are interchanged and the magnetic fabric has a distinctly prolate shape triaxial fabric samples ii showing an intermediate magnetic fabric are always characterized by high m0 values it seems probable that the magnetic fabric is controlled by the preferred orientation of paramagnetic phyllosilicates e g chlorite and mica and by some ferromagnetic mineral with anomalous orientation in relation to the bedding plane oblate fabric samples iii are characterized by a bedding controlled magnetic fabric and by moderate magnetic susceptibility and m0 values the magnetic fabric is controlled by the preferred orientation of phyllosilicate minerals and to a minor extent by a ferrimagnetic fraction most probably detrital magnetite considering the magnetic fabric together with paleomagnetic component analyses the siderite bearing and the high nrm samples about 15 of samples were excluded from further magnetostratigraphic research

biochemical vs detrital mechanism of remanence acquisition in marine carbonates a lesson from the k t boundary interval abstract an apparently complete carbonate rich cretaceous tertiary boundary interval in odp section 119 738c 20r 5 from the southern kerguelen plateau provides a unique insight into processes of magnetization acquisition in marine carbonates the boundary interval is characterized by a 1 m thick clay rich zone distinct depositional lamina are preserved within the basal 15 cm of this zone the upper part is bioturbated previous studies have demonstrated that the bulk of the detrital fraction in the laminated and bioturbated carbonates has the same local source and hence the two intervals likely had similar initial detrital assemblages magnetic properties of these rocks however differ significantly the laminated sediments have a higher content of non silicate bound iron yet approximately an order of magnitude lower intensity of the natural remanent magnetization compared to the bioturbated rocks our detailed rock magnetic study indicates that in the bioturbated interval the dominant iron bearing phase is single domain magnetite likely of biogenic origin in the laminated interval apart from a small ferromagnetic fraction with multi domain like behavior non silicate bound iron is mainly sequestered in paramagnetic phases likely poorly crystalline oxyhydroxides it appears that a shut down of biological productivity after the k t event allowed preservation of the initial detrital early authigenic iron phases that are dominated by reactive iron oxyhydroxides with recovery of the normal biological activity as evidenced by resumption of bioturbation the oxyhydroxides had been replaced with biogenic magnetite thus produced biochemical magnetization led to a several fold increase in the remanence our results suggest that in areas where bioavailable iron constitutes a significant part of the detrital input such as in pelagic marine environments distant from clastic sources the biochemical remanent magnetization may be the dominant process of magnetization acquisition

cross basin variations in magnetic susceptibility influenced by changing sea level paleogeography and paleoclimate upper devonian western canada sedimentary basin abstract upper devonian rocks in western canada were deposited as carbonate platform and surrounding slope and basinal rocks during a second order sea level cycle characterized by long term climatic warming that ended with cooling associated with the frasnian famennian mass extinction event in this contribution we assess the hypothesis that high resolution magnetic susceptibility ms data records cross basin variations that are intimately tied to sea level and associated climatic change the depositional environment and the paleogeographic location of siliciclastic sediment sources four main trends are apparent in the ms data 1 the magnitude of the ms signature increases from west to east across western canada 2 platform facies generally have low ms signatures compared with off platform facies 3 the overall ms signature increases and then decreases throughout the stratigraphic profile largely in response to changes in depositional setting i e item 2 4 ms is generally high during third and fourth order sea level lowstand and early transgression and lower during late transgression and highstand during the late middle givetian to early late frasnian devonian siliciclastic sediments silt and fine sand size sourced from the west alberta arch and peace river arch contributed to relatively high ms signatures in facies deposited proximal to these source areas other important detrital sediment sources were the ellesmerian orogenic belt in northern canada and the laurussian continental landmass to the east these latter sources supplied fine grained sediments that were dispersed basinwide and resulted in a nearly order of magnitude increase in ms values to the east this trend and fine grain size implies that some detrital sediment was delivered from the east as eolian dust from the old red continent by the early frasnian the west alberta arch was submerged and by middle to late frasnian time the peace river arch was largely covered during the late middle frasnian a pronounced rise in ms signatures can be tied directly to deposition of the clay and locally silt rich mount hawk formation during the late middle to late frasnian carbonate ramp facies prograded from east to west resulting in seaward migration of the shoreline and increased siliciclastic input to the ramp slope that influenced the pronounced increase in ms signatures to the east by the late frasnian cross basin ms profiles record uniformly low susceptibilities indicating that high ms siliciclastic detritus was either bypassed seaward was swamped by carbonate input or was dominated by diamagnetic quartz individual ms events can be accurately correlated across much of the basin at a resolution that is higher than single conodont zones despite the local influences on variability of the signature variations in the ms signature can thus be explained partly by depositional environment platforms with low ms basins with higher ms and partly by third and fourth order sea level and associated climatic changes fourth order changes in sea level are routinely interpreted to result from variations in paleoclimate in the milankovitch band and recent work indicates that third order sea level changes may be linked to longer term modulation of milankovitch band orbital variations the increase in ms during prolonged frasnian warming thus appears to be directly linked to paleoclimatic change and comparison of the ms signature with more direct measures like oxygen isotopes would serve as a test of its utility as a paleoclimate proxy

thermochemical remanence acquisition in single domain particle ensembles a case for possible overestimation of the geomagnetic paleointensity 1 single domain sd particle ensembles are the best studied systems in rock magnetism yet even for this simple system a complete theoretical treatment of the acquisition of thermochemical remanent magnetization tcrm is missing on the basis of the sd theory of chemical remanent magnetization it is generally taken for granted that tcrm acquisition must be less efficient than acquisition of a thermoremanent magnetization trm and that consequently the paleofield intensity is inevitably underestimated when a tcrm is interpreted as a trm here it is shown that this conclusion holds only when a continuous chemical change e g grain growth takes place during remanence blocking in cases where an initial trm at a later time and at lower temperature is modified by chemical processes the final tcrm can result in considerable paleofield overestimation when interpreted as a trm this is demonstrated for two different scenarios first partial grain dissolution after trm acquisition and second low temperature oxidation these mechanisms are not unlikely to occur in nature and numerical experiments indicate that both would lead to perfectly straight arai plots which makes such sd tcrms indistinguishable from true trms in the thellier experiment

toward an optimal geomagnetic field intensity determination technique 1 paleointensity determinations based on double heating techniques in field zero field cooling zero field in field cooling and two in field steps with opposite laboratory fields are generally considered to be functionally interchangeable producing equally reliable paleointensity estimates to investigate this premise we have developed a simple mathematical model we find that both the zero field first and in field first methods have a strong angular dependence on the laboratory field parallel orthogonal and anti parallel while the two in field steps method is independent of the direction of the laboratory produced field contrary to common practice each method yields quite different outcomes if the condition of reciprocity of blocking and unblocking temperatures is not met even with marginal 10 tails of partial thermoremanence our calculations suggest that the zero field first method with the laboratory produced field anti parallel to the natural remanence nrm is the most robust paleointensity determination technique when the intensity of the lab induced field is smaller than ancient field however the zero field first method with the laboratory field parallel to the nrm is the optimum approach when the intensity of the lab induced field is larger than the ancient field by far the best approach however is to alternatethe infield zerofield iz steps with zerofield infield zi steps

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

exploring the potential of acquisition curves of the anhysteretic remanent magnetization as a tool to detect subtle magnetic alteration induced by heating recently many new methods and improved protocols to determine the absolute paleointensity of lavas reliably have been proposed here we study eight recent flows from three different volcanic edifices mt etna la palma and hawaii with the so called multispecimen parallel differential ptrm msp method including the recently proposed domain state correction msp dsc fabian and leonhardt 2010 surprisingly apart from approximately correct paleointensity values we observe major underestimates of the paleofield these deviations are possibly related to alteration that is not revealed by rock magnetic analysis we explore the potential of high resolution acquisition curves of the anhysteretic remanent magnetization arm to detect subtle alteration in the samples it appears that assessing changes in the arm acquisition properties before and after heating to the desired msp temperature discriminates between underestimates and approximately correct estimations of the paleofield in the outcomes of the msp dsc protocol by combining observations from the domain state corrected msp protocol and arm acquisition experiments before and after heating an extended msp protocol is suggested which makes it possible to assess the best set temperature for the msp dsc protocol and to label msp results as being approximately correct or an underestimate of the paleofield

iaga paleointensity database distribution and quality of the data set abstract the aim of this paper is to provide details about the design of the paleointensity database and focus on the distribution of the available data set version 2003 of the paleointensity database comprises 3128 data from 215 references only absolute paleointensity determinations from igneous rocks and baked contacts are considered regardless which method of paleointensity determination was used the entries in the database correspond to cooling units all polarities as well as all units are considered normal reverse or transitional the earth s magnetic field is the only surface trace of the working of the geodynamo and is therefore a very important tool for our understanding of the earth s deep interior especially its variation with geologic time however to study the long term variation of the paleomagnetic field non dipole effects have to be removed in recent years the production rate of paleointensity data has strongly increased and the most reliable methods were more frequently used however the present data set still has an important temporal bias with 35 of the rocks having ages less than 1 ma as well as a strong geographic bias towards the northern hemisphere especially north america and europe for most time intervals secular variation cannot be averaged out neither temporally nor spatially therefore interpretation of the averaged field is very limited and is avoided here for the future a harmonization or a combination of all iaga databases would be desirable furthermore the input of raw data at the specimen level would be useful in order to allow reinterpretation of data with more developed and sophisticated methods based on our increasing understanding of rock magnetism finally new reliable data are very much needed

the origin of tectonic lineation in extensional basins combined neutron texture and magnetic analyses on undeformed clays abstract in extensional sedimentary basins fine grained sediments that appear undeformed at the outcrop scale can carry a magnetic fabric consistent with the regional deformation pattern the origin of the magnetic lineation which is often found in extensional basins is not yet well understood in clays from extensional basins in southern italy the magnetic lineation is tectonically controlled and oriented perpendicular to the main normal faults a combined analysis of magnetic and mineral fabrics was made to gain insight into the processes that lead to a lineation in extensional settings low field high field and low temperature susceptibility measurements were used to distinguish the ferrimagnetic and paramagnetic contributions to the magnetic susceptibility and its anisotropy the magnetic anisotropy of the sediments is predominantly carried by paramagnetic phyllosilicates neutron texture analysis was used to evaluate the spatial distribution of chlorite basal planes results demonstrate that the orientation of the magnetic lineation is related to the spatial distribution of chlorite lying parallel to the common axis of differently oriented basal planes a quantitative correlation between the magnetic and rock fabric was made comparing the low and high field magnetic anisotropy ams hfa to the theoretical anisotropy calculated from the chlorite preferred orientation a good linear correlation is found between the degree of theoretical anisotropy and the ams and hfa results show that the integrated approach of magnetic and mineral fabric investigations represents a valid alternative tool for detecting grain scale and regional deformation patterns in weakly deformed extensional basins where macroscopic evidence of deformation is often not visible

inverse magnetic fabric in carbonate bearing rocks abstract the observation of a magnetic susceptibility ellipsoid whose maximum axis corresponds to the minimum axis of petrofabric pole of bedding or schistosity is referred to as an inverse magnetic fabric the investigation of the magnetic properties of some ferroan carbonate monocrystals and paramagnetic limestones demonstrates that c axis preferred orientation of paramagnetic carbonates results in a maximum susceptibility parallel to the flattening direction inverse magnetic fabrics due to magnetite are also encountered in weakly deformed limestones a mineralogical model based on the property of single domain grains to have a zero susceptibility parallel to their long axis is proposed however more complex cases are also encountered

rock magnetic properties related to thermal treatment of siderite behavior and interpretation detailed analyses of rock magnetic experiments were conducted on the oxidation products of high purity natural crystalline siderite that were thermally treated in air atmosphere susceptibilities increase sharply between 400 and 530 c indicative of some new ferrimagnetic mineral phase generation both a drop between 540 and 590 c on the heating cycle and a dramatic increase from 590 c to 520 c on the cooling cycle occurred and are well consistent with the characteristic of magnetite a distinct hopkinson type susceptibility peak indicates that hematite is the terminal product if siderite is heated to 700 c over and over it has been revealed in detail that the original inverse magnetic susceptibility fabric contributed by the crystalline anisotropy of siderite in siderite bearing specimens is changed to a normal magnetic fabric during incremental heating over 410 490 c this is a result of dominant contributions from the distribution anisotropy of newly transformed ferromagnetic minerals a strong chemical viscous remanent magnetization could be produced during siderite oxidation in an external field rock magnetic experimental results show that magnetite maghemite and hematite are the transformation products of high temperature oxidation of siderite in air maghemite was not completely inverted to hematite even at temperature as high as 690 c during incremental thermal treatments the mineral transformation processes were confirmed by conventional optical microscopic observation x ray diffractometry and mossbauer spectroscopic analyses these results indicate that the rock magnetic methods used here are reliable and highly sensitive in detecting very small magnetic phase changes in rocks we conclude that these temperaturedependent variations of magnetic properties can be used as criteria for identification of siderite in rocks and sediments furthermore it is clear that great care should be exercised in thermal demagnetization of siderite bearing rocks in paleomagnetic magnetic anisotropy and rock magnetic studies

siderite formation in anoxic deep sea sediments a synergetic bacteria controlled process with improtant implications in paleomagnetism recent work on magnetic properties of limestones has demonstrated that the mineral siderite can be very important in paleomagnetism for two reasons first oxidation of siderite produces secondary daughter magnetic minerals magnetite maghemite and hematite either before during or after sampling these daughter products can completely change the magnetic properties of limestone samples and if unrecognized may be one of the primary reasons why many paleomagnetic studies of limestones especially paleozoic limestones are unsuccessful second siderite in weakly magnetized rocks may indicate the potential for successful paleomagnetic results because the presence of siderite indicates that the primary magnetic carriers are still intact appropriate demagnetization methods should yield successful results we conclude that microenvironmental conditions in anoxic marine sediments may permit the formation of siderite from iron ii produced during bacterial dissimilatory iron reduction

palaeomagnetic and rock magnetic studies of cretaceous rocks in the gongju basin korea implication of clockwise rotation palaeomagnetic and rock magnetic studies have been carried out for cretaceous non marine sedimentary rocks gongju group and volcanic rocks in the gongju basin located along the northern boundary of the ogcheon belt korea k ar age dating for the volcanic rocks was also performed it is found that the gongju group was remagnetised during the tilting of the strata with the characteristic remanent magnetisation chrm direction of d i 23 9 50 6 k 95 5 9 5 3 9 at 30 per cent untilting of the strata with a maximum value of precision parameter k while the volcanic rocks are revealed to acquire primary remanence with the direction of d i 204 2 43 8 k 36 6 9 5 8 6 after the tilt correction the k ar ages of the volcanic rocks range from 81 8 2 4 to 73 5 2 2 ma corresponding to the campanian stage of the late cretaceous electron microscope observations of samples from the gongju group show authigenic iron oxide minerals of various sizes distributed along the cleavage of chlorite and in the pore spaces indicating that the strata acquired the chemical remanent magnetisation due to the formation of secondary magnetic minerals under the influence of fluids the palaeomagnetic pole positions are at lat long 69 6 n 224 3 e dp 3 5 dm 5 2 calculated for the 30 per cent tilt corrected direction of the gongju group and at lat long 67 2 n 235 3 e a 9 5 8 9 for the volcanic rocks based on the results of this study it is interpreted that the volcanic rocks acquired the primary magnetisation almost at the same time as the remagnetisation of the gongju group in the late cretaceous comparisons of cretaceous palaeomagnetic poles from the korean peninsula with those from eurasia implies that the korean peninsula underwent clockwise rotation of 21 2 5 3 for the middle early cretaceous 12 6 5 4 for the late early cretaceous and 7 1 9 8 for the late cretaceous with respect to eurasia due to the sinistral motion of the tan lu fault

asynchronous deposition of ice rafted layers in the nordic seas and north atlantic ocean instabilities in ice stream flow within the north american laurentide ice sheet leading to the periodic release of armadas of icebergs into the north atlantic ocean over the past 60 000 years have produced extensive layers of coarse grained iceberg rafted debris heinrich layers in north atlantic sediments1 2 correlation of these layers with iceberg discharge events from the ice sheets on greenland iceland and scandinavia suggested in previous studies for some heinrich layers and in some areas3 4 5 would imply that ice sheet instability had been synchronous across the north atlantic presumably in response to a common environmental cause here we show a lack of widespread systematic correlations both between ice rafted debris layers in different sediment cores from the nordic seas and between the nordic layers and the north atlantic heinrich layers this suggests that the full glacial nordic ice sheets did not exhibit unstable behaviour coincident with iceberg discharge from the vast hudson bay drainage basin of the laurentide ice sheet6 7 off the hudson strait significant ice sheet discharge of melt water is indicated by size sorted sandy and muddy turbidite sediments different from the poorly sorted debris flows which dominate sedimentation on the margins of the nordic seas8 9 10 together these results suggest that the dynamics of quaternary ice sheets surrounding the nordic seas were different from the outlet glacier draining the hudson bay basin and they provide evidence against a common circum north atlantic mechanism driving the discharge of icebergs

the effects of secondary mineral formation on coe type paleointensity determinations theory and simulation thellier type experiments are the most widely applied approaches for determining the absolute paleointensities of earth s magnetic field one major problem however is that specimens are prone to thermal alteration due to the intensive thermal treatment during experiments linear arai plots with acceptable partial thermal remanent magnetization ptrm checks have been considered as evidence for the absence of or negligible effects of thermal alteration and as reliable indicators of high quality paleointensity estimates however by simulating the coe variant of the thellier method on assemblages of single domain sd magnetite particles it is demonstrated that new magnetic minerals which form during thermal treatments can result in linear concave up or concave down arai plots depending on the magnetic properties of both the primary and secondary magnetic phases among this range of behavior pseudoideal arai plots which are linear with acceptable ptrm check statistics would lead to paleointensity underestimates it is further demonstrated that ptrm checks are proportional to the degree of underestimate with a magnetic granulometry dependency for sd particles due to the complexity of this dependency ptrm check statistics are only comparable when specimens have similar magnetic properties this suggests that a universal threshold for ptrm check statistics is not likely to be effective since the criteria of linearity and low ptrm check statistics are insufficient to guarantee the fidelity of the estimates auxiliary rock magnetic methods such as temperature dependent hysteresis parameters and anhysteretic remanent magnetization are highly recommended to identify the presence of alteration

the magnetic fabric of the wolfcamp shale midland basin west texas understanding petrofabric variability hydrocarbon associations and iron enrichment rock fabrics have long been recognized as a key variable influencing unconventional reservoir properties and yet petrofabric analysis is rarely incorporated in reservoir characterization in a quantitative and scalable manner this work introduces anisotropy of magnetic susceptibility ams coupled with handheld x ray fluorescence to define the origin of petrofabrics in the wolfcamp shale and examine their relationship with total organic carbon toc and oil saturation indices osi standard paleomagnetic plugs were drilled from three vertical cores spanning the entire wolfcamp shale in the midland basin west texas low temperature magnetic experiments suggest paramagnetic minerals contribute between 93 4 and 98 6 to the low field bulk magnetic susceptibility klf across all lithotypes scanning electron microscopy revealed frequent occurrences of paramagnetic minerals such as chlorite illite ferroan dolomite marcasite and pyrite the fe rich clays such as chlorite carry the ams signal in mudrocks with a minor contribution from ferroan dolomite conversely ferroan dolomite carries the ams signal in most carbonate facies with a minor influence from fe rich clays the degree of magnetic anisotropy pj and shape factor t in mudrocks are compositionally dependent and respond to changes in both clay and framework grain concentration for carbonate facies pj and t vary widely because of changes in depositional facies and the concentration of ferroan dolomite the toc correlates well with pj and strongly oblate magnetic fabrics this relationship likely stems from the adsorption of organic matter om to clays during deposition and may indicate that organoclay composites were responsible for delivering om to the midland basin elevated osi are fabric selective favoring low pj and t fabrics suggesting that such fabrics facilitate hydrocarbon migration more so than high pj and t fabrics fuzzy c means clustering provides a framework to describe ams and geochemical transitions at the core scale and could serve as a tool to map barriers and pathways to hydrocarbon migration finally ti nb and klf data point to a mafic provenance lithotype and suggest layered intrusive complexes in the central basin uplift were exposed and eroded during deposition of the wolfcamp shale mafic source rocks probably increased supply of micronutrients such as dissolved iron and may have enhanced productivity in the midland basin

is this magnetic fabric normal a review and case studies in volcanic formations abstract when studying weakly anisotropic rocks non standard relationships between magnetic and expected petrofabric axes are sometimes observed in particular the so called inverse or intermediate fabrics a review of possible explanations for these effects is presented and leads to the conclusion that a majority of abnormal magnetic fabrics often encountered in magnetite rich volcanic rocks are due to abnormal preferred orientations or distributions of magnetite grains through case studies in dikes from skye island yemen and the iberian peninsula and magma and ash flows from the ethiopian traps and southern corsica the various possible origins of such fabrics are examined it is possible to retrieve reliable magma flow directions from these sites provided proper data selection is performed leading to non unique interpretations

testing the izzi protocol of geomagnetic field intensity determination 1 a new paleointensity determination protocol the izzi method was recently proposed the izzi technique combines the aitken in field zero field iz and coe zero field in field zi methods the izzi protocol of paleointensity method was experimentally tested showing a strong angular dependence resulting from the undemagnetized portions of partial thermoremanent magnetization ptrm tails the izzi method is better than the conventional techniques aitken coe and thellier in three respects 1 it can easily detect the angular dependence 2 it provides a quantitative estimate for the consistency of the outcome between iz and zi step and 3 it is quicker because the extra ptrm tail check step is unnecessary

eruptive timing and 200 year episodicity at 92 w on the hot spot influenced galapagos spreading center derived from geomagnetic paleointensity eruptive timing in mid ocean ridge systems is relatively poorly constrained despite being an important variable in our understanding of many mid ocean ridge processes including volcanic construction magma recharge flux and storage and the stability of hydrothermal systems and biological communities only a handful of absolute eruption chronologies exist yet they are essential in understanding how eruptive timing varies with important controlling variables to construct an eruptive history at one location on the galapagos spreading center we present age determinations derived from geomagnetic paleointensity to aid interpretation of the paleointensity data we also present results from on bottom magnetic anomaly measurements and forward modeling of topographic induced magnetic anomalies anomalies may lead to a 1 2 t bias in flow mean paleointensities which does not significantly affect the overall interpretation paleointensity results for the three youngest sampled units are indistinguishable consistent with the flows being emplaced in relatively rapid succession comparisons with models of geomagnetic field behavior suggest these flows were erupted sometime in the past 100 200 years the fourth sampled unit has a significantly higher paleointensity consistent with an age of roughly 400 years the possible bias in paleointensity data allows for ages as young as 50 years for the youngest three flows and 200 400 years for the oldest flow this age distribution demonstrates an episodicity in the emplacement of the largest flows at this location with a 200 300 year period of relative quiescence between emplacement of the oldest unit and the three youngest units

evidence from 40ar 39ar ages for a churchill province source of ice rafted amphiboles in heinrich layer 2 40ar 39 ar ages of most single ice rafted amphiboles from heinrich layer 2 h2 from a core in the labrador sea a core in the eastern north atlantic and a core in the western north atlantic range from 1600 to 2000 ma this range is identical to that for k ar ages from the churchill province of the canadian shield that outcrops at hudson strait and forms the basement of the northern part of hudson bay the ambient glacial sediment includes some younger and older grains derived from paleozoic mesoproterozoic and archean sources but still the majority of the amphiboles have ages in the 1600 2000 ma interval the ca k ratios of these 1600 2000 ma old amphiboles however have a bimodal distribution in contrast with the uniformity of the ca k ratios of h2 amphiboles this indicates that 1600 2000 ma old amphiboles of the ambient sediment were derived from an additional early proterozoic source besides churchill province in h2 churchill derived grains constitute 20 40 of the ice rafted debris ird the fraction in the ambient glacial sediment is 65 80 results presented here are consistent with the hypothesis that heinrich events were produced by a sudden intensification of the iceberg discharge through hudson strait that mixed in the north atlantic with icebergs that continued to calve from other ice sheets the shift from mixed sources in the background sediment to a large dominance of churchill province grains in h2 indicates that even if calving of other ice sheets intensified during the heinrich episode the increase in the iceberg discharge via hudson strait from the hudson bay drainage basin of the laurentide ice sheet was by far the largest

magnetic properties and metastability of greigite smythite mineralization in brown coal basins of the krusne hory piedmont bohemia finely dispersed forms of greigite or greigite smythite mineralization were found in layers a hundred and more metres thick in the miocene strata of the krusne hory erzgebirge piedmont brown coal basins under laboratory thermal treatment a pronounced instability of greigite or greigite smythite mineralization was revealed by the magnetic parameters conditioned by mineralogical metastability thermal treatment in oxidation conditions caused the most pronounced changes in magnetic parameters in a temperature range of 320 380 c in which a laboratory process of self reversal of remanence was observed the products obtained during thermal treatment were identified with the aid of x ray diffractographs co radiation fe filter when they were heated to 250 c no substantial changes were found while at 300 c the intensities of greigite became weaker and pyrite and marcasite originated to its detriment hexagonal pyrrhotite was generated in addition to pyrite and marcasite a total decomposition of bisulphides took place at temperatures above 400 c accompanied by the formation of various modifications of fe2o3 until finally at higher temperatures only fe2o3 was formed laboratory tests suggested self reversal of remanence in relation to the formation of pyrrhotite so far greigite or greigite smythite mineralization has been proven to exist in the bohemian massif solely in connection with miocene rocks containing fossil micro organic matter products of thermal alteration pyrite marcasite pyrrhotite and fe2o3 or fe3o4 depending on redox conditions may be expected in rocks of similar genesis but partly or completely carbonified as a result of a process of pyrolysis of the micro organic matter

magnetic fabric methods and applications fabric is a ubiquitous and significant feature of geological materials the processes involved in the formation and deformation of rocks and sediments leave their mark on the orientations of the constituent mineral grains petrofabrics thus provide essential keys to understanding the history of geological materials magnetic anisotropy is directly related to petrofabric and has become one of the most rapid sensitive and widely used tools for its characterization the relationship between magnetic fabric and petrofabric is complex and depends on various factors including the composition concentration and grain size of mineral grains ongoing research in geological applications is paralleled by studies of the fundamental mineral magnetic phenomena involved the papers in this book represent the current state of investigations in magnetic anisotropy studies as a discipline that integrates geological interpretations mineral fabric development technical advances and rock magnetic properties

effect of progressive deformation on remanent magnetization of permian redbeds from the alpes maritimes france abstract the relationships among the anisotropy of magnetic susceptibility the finite strain and the characteristic remanent magnetization chrm have been investigated in deformed red sediments from the alpes maritimes in southeast france the magnetic properties of these rocks are due to hematite the finite strain is known from field studies of the shapes and orientations of green reduction spots and from optical microscopy and x ray texture goniometry studies of chlorite and illite the strain data permit a classification of the magnetic results according to four main stages of progressive deformation in the compaction stage magnetic anisotropy is flattened in the bedding due to compactional loading the chrm directions approximate to a fisherian distribution about a mean value close to the known permian direction for the region as deformation proceeds pencil structure first develops and the anisotropy is characterized by a prolate pattern with maximum axis parallel to the subhorizontal azimuth of the pencil structure and fold axes with increasing deformation the magnetic susceptibility ellipsoid becomes oblate and finally triaxial at these stages of high deformation the anisotropy patterns show that the hematite grain orientations are strongly influenced by the cleavage the remanence directions in deformed rocks do not correspond to a fisher distribution and eventually deviate strongly from permian directions these results show that paleomagnetic vectors are affected by the distortional strains which accomodate shape change in the rock simple tilt correction without compensation for the strain would yield incorrect paleopole positions an attempt has been made to compensate for the effects of deformation by regarding the remanent vector as a passive marker march 1932 equations for strain removal were applied to the remanence in two ways 1 a using strain data directly obtained in the field for each site and 2 b by converting the magnetic susceptibility anisotropy of each sample to a corresponding finite strain value correcting for the finite strain by both methods improves the within site grouping of paleomagnetic vectors in strongly deformed rocks strain removal assuming the march model results in overcorrection of the remanent magnetization data in low strain regions distortion may produce mechanical rotation of the hematite particles but recrystallization may alter the deformation mechanism in regions of high strain the limits of applicability of the passive marker treatment could not be tested adequately in the deformed red sediments of the alpes maritimes

improving geomagnetic field reconstructions for 0 3 ka global geomagnetic field reconstructions on millennial time scales can be based on comprehensive paleomagnetic data compilations but especially for older data these still suffer from limitations in data quality and age controls as well as poor temporal and spatial coverage here we present updated global models for the time interval 0 3 ka where additions to the data basis mainly impact the south east asian alaskan and siberian regions we summarize recent progress in millennial scale modelling documenting the cumulative results from incremental modifications to the standard algorithms used to produce regularized time varying spherical harmonic models spanning 1000 bc to 1990 ad from 1590 to 1990 ad gauss coefficients from the historical gufm1 model supplement the paleomagnetic information in addition to absolute paleointensities calibrated relative paleointensity data from sediments are now routinely included iterative data rejection and recalibration of relative intensity records from sediments ensure stable results bootstrap experiments to generate uncertainty estimates for the model take account of uncertainties in both age and magnetic elements and additionally assess the impact of sampling in both time and space based on averaged results from bootstrap experiments taking account of data and age uncertainties we distinguish more conservative model estimates cals3k nb representing robust field structure at the core mantle boundary from relatively high resolution models cals3k n for model versions n 3 and 4 we assess the impact of newly available data and modifications to the modelling method by comparing the previous cals3k 3 the new cals3k 4 and the conservative new model cals3k 4b we conclude that with presently available data it is not feasible to produce a model that is equally suitable for relatively high resolution field predictions at earth s surface and robust reconstruction of field evolution avoiding spurious structure at the core mantle boundary cmb we presently consider cals3k 4 the best high resolution model and recommend the more conservative lower resolution version for studies of field evolution at the cmb

comparison of carbonate c and o stable isotope records across the jurassic cretaceous boundary in the tethyan and boreal realms abstract carbon and oxygen stable isotope records were compared for jurassic cretaceous j k boundary sections located in the tethyan realm brodno western slovakia and puerto escano southern spain bulk limestones and the boreal realm nordvik peninsula northern siberia belemnites since a detailed biostratigraphic correlation of these tethyan and boreal sections is impossible due to different faunal assemblages correlation of the isotope records was based on paleomagnetic data this novel approach can improve our understanding of the synchroneity of individual isotope excursions in sections where detailed biostratigraphic correlation is impossible no significant excursions in either the carbon or oxygen isotope records to be used for future boreal tethyan correlations were found around the j k boundary the upper tithonian and lower berriasian magnetozones m20n to m18n in the studied sections at the nordvik section where a much longer section middle oxfordian basal boreal berriasian was documented the transition from the middle oxfordian to the kimmeridgian and further to the volgian is characterized by a decrease in belemnite 18 o values from 18 o values up to 1 6 vs v pdb in the oxfordian to values between 0 3 and 0 8 in the late volgian and earliest boreal berriasian this trend which has previously been reported from the russian platform and tethyan realm sections corresponds either to gradual warming or a decrease in seawater 18 o supposing that the oxygen isotope compositions of seawater in the arctic boreal and tethyan realms were similar then the differences between oxygen isotope datasets for these records indicate differences in temperature the boreal tethyan temperature difference of 7 9 c in the middle and late oxfordian decreases towards the j k boundary indicating a significant decrease in latitudinal climatic gradients during the late jurassic two positive carbon isotope excursions recorded for the middle oxfordian and upper kimmeridgian in the nordvik section can be correlated with a similar excursion described earlier for the russian platform minor influence of biofractionation at the carbon isotopes and the influence of migration of belemnites to deeper slightly cooler water at the oxygen isotopes cannot be excluded for the obtained belemnite data

reliable paleointensity determinations from late cretaceous volcanic rocks in korea with constraint of thermochemical alteration abstract paleointensity determinations were carried out from late cretaceous 77 ma volcanic rocks in korea using a thellier type izzi experimental protocol with systematic partial thermal remanent magnetization ptrm checks various data selection criteria were used to estimate reliable paleointensities we set stringent threshold values for each parameter to ensure that there was 1 a linear relationship between natural remanent magnetization nrm lost and trm gained 2 negligible thermal alteration of magnetic minerals and 3 uni vectorial decay of nrm towards the origin from the 336 samples 88 were rejected because of an insufficient extrapolated nrm fraction in the best fit line fvds

diagenetic sensitivity of paleoenvironmental proxies a rock magnetic study of australian continental margin sediments a rock magnetic study of the upper 40 m of the ocean drilling program site 133 820a recovered at the outer edge of the northeastern australian continental margin shows that downcore variations in magnetic parameters are diagenetically driven and correlate with the changes in global sea level we identified intervals enriched in single domain sd magnetite in the studied section unlike previous studies that postulated a detrital source we show that the bulk of the sd fraction is biologically produced and is likely to be authigenic the abundance of sd magnetite thus cannot be used as an indicator for provenance or sediment transport mechanisms the biogenic magnetite was preserved during the high sedimentation rate periods likely due to a short residence time in the corrosive zone of active iron reduction the presence of the biogenic magnetite thus can be used as an indicator for a low degree of reductive dissolution more advanced dissolution during the periods of slow sedimentation coincident with sea level highstands resulted in significant changes in the composition of the detrital assemblage particularly in relative abundances of different mineral phases the overall stability of the detrital magnetic minerals toward dissolution varies in the studied section as hematite magnetite goethite due to the higher stability of hematite reductive diagenesis in general will lead to the lowering of the goethite hematite g h ratio which can be mistaken for an increased aridity in the sedimentary source areas in the conventional interpretation for this proxy the sensitivity of the g h proxy to diagenesis should be taken into account in paleoenvironmental studies

environmental magnetism past present and future environmental magnetism involves the application of rock and mineral magnetic techniques to situations in which the transport deposition or transformation of magnetic grains is influenced by environmental processes in the atmosphere hydrosphere and lithosphere the first explicit description of environmental magnetism as a distinct field was in 1980 since that time environmental magnetism has become a broad field that is finding application in an ever increasing array of scientific disciplines in this review of the present state of environmental magnetic studies we divide the field into three broad but arbitrary categories the first involves the use of mineral magnetic assemblages in the geological record to study physical processes in depositional environments this category includes the correlation of sediment cores using magnetic susceptibility measurements studies of geomagnetic field behavior the analysis of depositional and postdepositional mechanical processes that affect sediments and the examination of magnetic parameters that might represent proxies for paleoclimatic variation the second category encompasses studies of the processes responsible for variations in the magnetic minerals brought into a sedimentary environment these provenance investigations include studies of changes in catchment derived sediment in lakes fluctuations in contributions from terrigenous aeolian and glaciogenic components in deep sea sediments and the origin of atmospheric particulates the final category addresses in situ changes and transformations of magnetic minerals in sedimentary environments including pedogenesis authigenetic diagenetic formation of ferrimagnetic phases dissolution of magnetic minerals due to reductive diagenesis and contributions of biomagnetism to sedimentary magnetism because environmental magnetism can address problems in so many disciplines and because many of these problems may be inaccessible to other techniques it is likely that the scope of environmental magnetism will continue to expand rapidly environmental magnetism is capable of providing important data for studies of global environmental change climatic processes and the impact of humans on the environment all of which are major research initiatives in the international scientific community these factors suggest that environmental magnetism has a bright and diverse future

anomalous magnetic properties in rocks containing the mineral siderite paleomagnetic implications sampling and magnetic measurement of a 1 m bed in a new road cut of the upper cretaceous austin chalk northeastern texas has yielded anomalous magnetic results initial measurement of the anisotropy of magnetic susceptibility ams indicated unusually high anisotropies and low bulk susceptibilities characteristic of a magnetocrystalline anisotropy which might be expected for siderite feco3 natural remanent moments nrm for these samples were low 1 10 4 a m2 and directions were typical for samples which had acquired a normal geomagnetic field overprint at the site periodic remeasurement of the nrm yielded an increase in moment for some samples and periodic reversals of rm in a direction parallel or antiparallel to the ambient field in the laboratory isothermal remanent moments of these samples saturates at low induction values 200 mt ams remeasurement over a period of weeks to months revealed a general decrease in the anisotropy magnitudes an increase in susceptibility and a change in principal axis orientations these data were compared with heating oxidation experiments on austin chalk siderite ore and clastic sediments with siderite cements all of these results are consistent with the x ray diffraction and mossbauer data which indicate the presence of siderite in the austin chalk samples the mossbauer spectra of the samples obtained at room temperature and 78 k indicate the presence of approximately 80 pyrite 10 siderite and 10 of a clay component possibly chlorite when only the iron bearing mineral components are considered after sampling exposure to the air and subsequent oxidation in the laboratory the siderite in austin chalk samples appears to have altered to fe2o3 maghemite or fe3o4 magnetite increasing the magnetic moment and changing the nrm and ams directions in the samples the continuing changes appear to reflect a conversion from the less stable fe2o3 to fe2o3 hematite or oxidation of fe3o4 to fe2o3 we infer from our study that oxidation of siderite bearing limestone e g during the formation of unconformities or due to low magnitude tectonic events may produce a very stable secondary magnetic moment in the rock such changes may be very important when interpreting paleomagnetic data from limestones

rock magnetism of late neogene and pleistocene deep sea sediments relationship to sediment source diagenetic processes and sediment lithology we use bivariate scatter plots to illustrate variations in selected rock magnetic properties low field susceptibility anhysteretic and isothermal remanence of late neogene and pleistocene deep sea sediments from 16 sites in the arctic ocean north atlantic equatorial atlantic and north pacific ocean and the arabian sea our intention is to examine the ability of the rock magnetic properties to differentiate the sediments according to factors such as lithology geographical area and the dominant mode of terrigenous sedimentation which at these sites is via ice rafting via bottom currents or via eolian processes overall correlations between sediment magnetic properties and gross lithology is poor and factors such as the source and transport path of terrigenous sediment and detrital magnetic minerals together with the action of reductive diagenetic processes are the major controls on the magnetic properties on the bivariate scatter plots sites with major ice rafted contributions tend to have high sedimentary ferrimagnetic mineral concentrations relatively coarse ferrimagnetic grain sizes and scattered sample point distributions in contrast sites where we infer significant bottom current supply of terrigenous material have tightly grouped sample point distributions carbonate sediments in which the terrigenous component is supplied by eolian processes tend to have a broad range of magnetic mineral concentration caused by glacial interglacial fluctuations in carbonate accumulation and eolian activity sediments containing significant volcanogenic material have high concentrations of relatively coarse grained ferrimagnetic material reductive diagenesis is a significant determinant of sediment magnetic properties in high productivity areas and has the effect of preferentially removing the fine grained ferrimagnetic fraction causing a coarsening of the ferrimagnetic grain size distribution and a rise in the antiferromagnetic ferrimagnetic ratio

ams detection of inverse fabrics without aarm in ophiolite dikes inverse fabric contributions from single domain sd magnetite complicate interpretation of magmatic flow from anisotropy of magnetic susceptibility ams in the s troodos ophiolite dike swarm flow directions and magma chamber dimensions may only be inferred correctly if magnetic fabrics correspond to the orientation distribution od of minerals magnetite shape fabrics were isolated by anisotropy of anhysteretic remanence aarm but ams ambiguously merges magnetite and silicate fabrics however with the retrospective benefit of aarm in this area we recommend avoiding sd complications by excluding magnetite rich samples for which the bulk low field susceptibility 27 500 si magma flow was steeply upwards to the sw proximal to an axial magma chamber

thick dyke emplacement and internal flow a structural and magnetic fabric study of the deep seated dolerite dyke of foum zguid southern morocco 1 knowledge on forced magma injection and magma flow in dykes is crucial for the understanding of how magmas migrate through the crust to the earth s surface because many questions still persist we used the long thick and deep seated foum zguid dyke morocco to investigate dyke emplacement and internal flow by means of magnetic methods structural analysis petrography and scanning electron microscopy we also investigated how the host rocks accommodated the intrusion regarding internal flow 1 important variations of the rock magnetic properties and magnetic fabric occur with distance from dyke wall 2 anisotropy of anhysteretic remanent magnetization reveals that anisotropy of magnetic susceptibility ams results mainly from the superposition of subfabrics with distinct coercivities and that the imbrication between magnetic foliation and dyke plane is more reliable to deduce flow than the orientation of the ams maximum principal axis and 3 a dominant upward flow near the margins can be inferred the magnetic fabric closest to the dyke wall likely records magma flow best due to fast cooling whereas in the core the magnetic properties have been affected by high temperature exsolution and metasomatic effects due to slow cooling regarding dyke emplacement this study shows that the thick forceful intrusion induced deformation by homogeneous flattening and or folding of the host sedimentary strata dewatering related to heat as recorded by thick quartz veins bordering the dyke in some localities may have also helped accommodating dyke intrusion the spatial arrangement of quartz veins and their geometrical relationship with the dyke indicate a preintrusive to synintrusive sinistral component of strike slip

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

inverse magnetic fabric in deformed limestones of the morcles nappe switzerland anisotropy of magnetic susceptibility ams was measured at 15 sites in deformed helvetic limestones of the morcles nappe in western switzerland inverse magnetic fabrics characterized by axes of maximum susceptibility subperpendicular to bedding or cleavage were observed at four sites in the root zone of the nappe acquisition and thermal demagnetization of isothermal remanent magnetization show that the ferromagnetic mineralogy is dominated by pyrrhotite with various proportions of magnetite hematite and goethite the matrix minerals were studied in thin section with a staining technique to distinguish different carbonates by x ray texture goniometry and by microprobe analysis observations of ams at room and liquid nitrogen temperatures and of the temperature dependence of bulk susceptibility lead to the conclusion that the inverse fabrics result from a paramagnetic component due to fe2 ions in the calcite matrix

preserved magnetic fabrics vs annealed microstructures in the syntectonic recrystallised george granite south africa the saldanian basement of the cape fold belt of south africa outcrops in the kaaimans inlier with granite plutons intruded in low grade pelitic and quartzitic metasediments around 535 ma new field data support a ubiquitous saldanian top to the north thrust kinematics coeval with granite emplacement with no substantial cape tectonic overprint the granites and their contact aureoles display both synkinematic and post kinematic fabrics this and the high strain zone commonly observed all along the contact between the kaaimans inlier and the cape fold belt suggest a structural decoupling between the basement and its cover microstructures in the kaaimans inlier and in the george pluton establish a post kinematic pervasive and thermal overprint of saldanian age granites and country rocks record a medium temperature high strain deformation phase followed by a strong low temperature static recrystallisation two sets of andalusite porphyroblasts occur systematically in the contact aureoles of the studied plutons and cannot be explained by successive magmatic pulses the granites studied by the anisotropy of magnetic susceptibility ams technique are paramagnetic 20 km 300 si biotite is mostly at the origin of the bulk rock susceptibility although minor contributions of tourmaline or ferromagnetic phases may occur the contribution of biotite alone to the bulk magnetic susceptibility is supported by two quantitative models based respectively on whole rock compositions curie weiss law and on intrinsic mineral susceptibilities the magnetic foliations and lineations are homogeneous throughout the george pluton and are consistent with field structures the ams results mainly from the magneto crystalline anisotropy of biotite and from its lattice preferred orientation lpo in the rock the magnetic fabric reveals the biotite subfabrics that had been acquired before static recrystallisation and which was not modified by the subsequent thermal metamorphic event the magnetic fabric therefore preserves the emplacement related deformation fabric

strength of the geomagnetic field in the cretaceous normal superchron new data from submarine basaltic glass of the troodos ophiolite 1 we present here new paleointensity data from 39 sampling sites collected from the quenched margins of pillow lavas and dikes exposed within the troodos ophiolite similar to 92 ma formed during the cretaceous normal superchron cns a period of approximately 40 million years when the geomagnetic field reversed extremely infrequently if at all monte carlo simulations suggest that a minimum of 25 estimates are necessary for a reasonably robust estimate for the average field strength our data suggest a dipole strength equivalent to the present field or nearly twice the post cns average the mean and standard deviation of the dipole moment 81 43 zam 2 z 10 21 from the 57 data points compiled here agree remarkably well with those predicted from the long paleointensity record derived from dsdp site 522 the new data set for the cns suggests a picture of a strong and stable field during the period of time when it stopped reversing moreover the similarity of the cns data with the present geomagnetic field suggests that it is presently in a state of unusual polarity stability

single domain particles in rocks and magnetic fabric analysis magnetic susceptibility anisotropy is commonly used as a guide for determining stress and flow axes in rocks and for assessing the suitability of anisotropic rocks for palaeomagnetic purposes we have recently demonstrated however in laboratory produced samples that as theoretically expected susceptibility anisotropy is dependent on particle size multidomain particles of magnetite or similar strongly magnetic particles with shape anisotropy exhibit a maximum susceptibility parallel to their easy remanent magnetization axes whereas uniaxial single domain particles exhibit a maximum susceptibility perpendicular to their easy magnetization axes hence susceptibility anisotropy does not yield an unambiguous determination of particle alignment in rocks independent of domain state by demonstrating uniaxial single domain anisotropy characteristics for the first time in a rock specimen we show here that susceptibility measurements by themselves may not correctly identify the degree or type of alignment of ferro or ferrimagnetic particles in rocks in particular apparently foliated rocks can actually be lineated and vice versa moreover rocks with low anisotropy of susceptibility can have a much higher anisotropy of remanence remanence anisotropy methods are shown to give unambiguous determinations of the degree and type of alignment

subduction driven shortening and differential exhumation in a cadomian accretionary wedge the tepla barrandian unit bohemian massif abstract the tepla barrandian unit tbu of central europe s bohemian massif exposes perhaps the best preserved fragment of an accretionary wedge in the avalonian cadomian belt which developed along the northern active margin of gondwana during late neoproterozoic in the central tbu three ne sw trending lithotectonic units domains 1 3 separated by antithetic brittle faults differ in lithology style and intensity of deformation magnetic fabric ams and degree of cadomian regional metamorphism the flysch like domain 1 to the nw is the most outboard trenchward unit which has never been significantly buried and experienced only weak deformation and folding the central melange like domain 2 is characterized by heterogenous intense deformation developed under lower greenschist facies conditions and was thrust nw over domain 1 along a se dipping fault to the se the most inboard arcward domain 3 is lithologically monotonous dominated by graywackes and slates was buried to depths corresponding up to the lower greenschist facies conditions where it was overprinted by a pervasive se dipping cleavage and then was exhumed along a major nw dipping normal fault we interpret these domains to represent allochtonous tectonic slices that were differentially buried and then exhumed from various depths within the accretionary wedge during cadomian subduction the nw directed thrusting of domain 2 over domain 1 may have been caused by accretion at the wedge front whereas the se dipping cleavage and se side up exhumation of domain 3 may record inclined pervasive shortening during tectonic underplating and subsequent horizontal extension of the rear of the wedge the boundary faults were later reactivated during cambro ordovician extension and variscan compression compared to related terranes of the cadomian belt the tbu lacks exposed continental basement evidence for regional strike slip shearing and extensive backarc magmatism and lp ht metamorphism which could be interpreted to reflect flat slab cadomian subduction this in turn suggests that cadomian accretionary wedges developed in a manner identical to those of modern settings elevating the tbu to a key position for understanding the style kinematics and timing of accretionary processes along the avalonian cadomian belt

thermal alteration of the magnetic mineralogy in ferruginous rocks the northampton ironstone contains the paramagnetic minerals siderite and berthierine and a trace of the ferrimagnetic mineral magnetite the magnetic fabric of this sedimentary rock as defined by the anisotropy of magnetic susceptibility is controlled by bedding compaction but is inverse i e the maximum susceptibility axes are normal to the bedding plane this inverse fabric is attributed to the dominating presence of siderite the rocks were incrementally heated to 600 c and the magnetic fabric was measured at room temperature and at near liquid nitrogen temperature after each heating step cooling the samples in liquid nitrogen enhances the paramagnetic contribution to the magnetic fabric after heating the samples above 250 c the room temperature magnetic fabric became normal with the minimum susceptibility axes perpendicular to the bedding plane the low temperature magnetic fabric remains inverse until the samples are heated to about 500 c after heating to higher temperatures a normal fabric was observed at both room temperature and low temperature rock magnetic evidence shows that magnetite is created during the heating the start of mineralogical changes in the rock was detected by electron spin resonance spectroscopy powder x ray diffraction and acquisition of isothermal remanence berthierine starts to break down at approximately 250 c and is totally oxidized by 500 c the creation of magnetite in the heating experiments corresponds with this mineralogical change the change in the room temperature magnetic fabric is associated with the decomposition of berthierine part of which alters to magnetite the new magnetite phase has a normal magnetic fabric which overprints the inverse magnetic fabric of siderite

palaeointensity study of the hawaiian 1960 lava implications for possible causes of erroneously high intensities summary it is known that the hawaiian 1960 lava sometimes yields higher palaeointensities than the igrf 36 2 t in order to clarify the causes we have performed a comprehensive investigation of palaeointensity measurements on 19 cores from the lava according to various rock magnetic analyses they are classified into three groups with degrees of the deuteric oxidation in coe s version of the thellier method a significantly larger mean palaeointensity of 49 0 9 6 t n 17 was observed similar to previous studies these palaeointensities showed a dependence on the oxidation indices which can be explained by thermochemical remanent magnetization tcrm acquisition during the natural cooling stage of the lava although it is generally difficult to distinguish the tcrm from thermoremanent magnetization trm a lowtemperature demagnetization ltd treatment on nrm and trm in the thellier experiment seems to allow us to detect the influence of the tcrm on nrm another possible tool for tcrm detection is the double heating technique dht of the shaw method combined with ltd ltd dht shaw method it gave a better average of 39 4 7 9 t n 9 though the success rate was much lower most of the inappropriate results were screened by a combination of the double heating test the arm correction and the ltd treatment if two outliers are further excluded the average is improved to be 35 7 3 3 t n 7 which agrees well with the expected intensity the results of both methods emphasize the importance of taking and measuring samples from the parts of various oxidation degrees in a lava flow

palaeomagnetic field intensity variations suggest mesoproterozoic inner core nucleation the earth s inner core grows by the freezing of liquid iron at its surface the point in history at which this process initiated marks a step change in the thermal evolution of the planet recent computational and experimental studies have presented radically differing estimates of the thermal conductivity of the earth s core resulting in estimates of the timing of inner core nucleation ranging from less than half a billion to nearly two billion years ago recent inner core nucleation high thermal conductivity requires high outer core temperatures in the early earth that complicate models of thermal evolution the nucleation of the core leads to a different convective regime and potentially different magnetic field structures that produce an observable signal in the palaeomagnetic record and allow the date of inner core nucleation to be estimated directly previous studies searching for this signature have been hampered by the paucity of palaeomagnetic intensity measurements by the lack of an effective means of assessing their reliability and by shorter timescale geomagnetic variations here we examine results from an expanded precambrian database of palaeomagnetic intensity measurements selected using a new set of reliability criteria our analysis provides intensity based support for the dominant dipolarity of the time averaged precambrian field a crucial requirement for palaeomagnetic reconstructions of continents we also present firm evidence for the existence of very long term variations in geomagnetic strength the most prominent and robust transition in the record is an increase in both average field strength and variability that is observed to occur between a billion and 1 5 billion years ago this observation is most readily explained by the nucleation of the inner core occurring during this interval the timing would tend to favour a modest value of core thermal conductivity and supports a simple thermal evolution model for the earth

inverse to normal magnetic fabric transition in an upper miocene marly sequence from tuscany italy the magnetic susceptibility fabric of an upper miocene marly sequence exposed along the morra creek tuscany italy changes from inverse maximum susceptibility axis sub perpendicular to bedding to normal from the bottom to the top of the section the change is gradual with the presence of anomalous magnetic susceptibility fabrics at some levels the anhysteretic remanence anisotropy is always normal throughout the sequence the changes in the magnetic fabric seem to be controlled by sedimentological variations with the inverse susceptibility fabric related to the abundance of iron bearing carbonates in the lower part of the section in the upper part of the section subtle changes in the detrital inputs in the basin and particularly a larger contribution of clayey and ferrimagnetic particles turned the susceptibility fabric to a normal type