one hundred million years of geomagnetic polarity history since 1968 absolute ages have been assigned to the late cretaceous cenozoic geomagnetic reversal time scale by fixing the ages of two or more calibration points in a composite marine magnetic anomaly profile and interpolating between or extrapolating beyond these points assuming constant spreading rates in each interval previously no more than 4 calibration points were used but it is now possible to specify 11 calibration points in addition to the 0 m y datum this improvement is based on magnetostratigraphic studies in italian pelagic limestones these studies closely tie the geomagnetic reversal sequence to the foraminiferal and coccolith zonations absolute ages of calibration points are provided by the best available dates on stage boundaries which are located from the biostratigraphic zonation the greatest changes from previous scales come in the late paleocene early eocene where the new ages are as much as 3 m y younger than in the 1977 scale of labrecque and others and as much as 1 8 m y younger than in the 1980 scale of ness and others

magnetic mineralogy of pelagic limestones summary due to their extremely small size and low concentration the carriers of magnetization in limestones are usually identified through indirect rock magnetic methods this paper describes a method for extracting tiny ferromagnetic grains from consolidated calcareous rocks and summarizes a scanning electron microscope sem investigation of magnetic phases from several palaeomagnetically important pelagic limestones details of surface characteristics and elemental composition are readily accessible with the sem and complement normal rock magnetic experiments in giving a more complete assessment of the major ferromagnetic phases their origin and the timing and acquisition of magnetization the results confirm many indications of the indirect methods and introduce several important new features in addition to detrital titanomagnetite diagenetic titanohaematite and iron sulphides there is a significant contribution by authigenic microcrystalline haematite detrital cosmic spherules chromian spinels and skeletal titanomaghaemite

comparison of magnetic hysteresis parameters of unremagnetized and remagnetized limestones for white magnetite bearing mesozoic pelagic limestones from italy which carry a primary magnetization the values of mrs ms and hcr hc generally lie in the pseudo single domain psd field of the day et al 1977 plot the logarithmic plot of mrs ms against hcr hc gives a straight line r 0 814 with slope and intercept close to the empirical mixing line of parry 1982 for single domain sd and multidomain md magnetite for one of the white pelagic limestone formations maiolica formation samples with hysteresis ratios closer to the md field display increased paramagnetic susceptibility and are from the upper part of the formation characterized by increased detrital clay we therefore associate the increased md magnetite with increased detrital influx for pinkish and reddish varieties of the italian pelagic limestones the presence of hematite is manifest by high saturation fields a wide range of hcr hc and wasp waisted hysteresis loops attributed to the mixing of magnetite and high coercivity authigenic hematite the hysteresis ratios for a collection of paleozoic and mesozoic remagnetized magnetite bearing limestones from britain nevada alaska and the appalachians lie mainly outside the psd field and appear to follow a power law trend following jackson et al 1993 the high values of hcr hc and the characteristically wasp waisted hysteresis loops can be interpreted in terms of a fine grained subspherical high coercivity sd magnetite mixed with a high proportion of superparamagnetic magnetite the slope and intercept of the power law relationship for mrs ms and hcr hc in the remagnetized limestones are distinct from those observed for the italian limestones and may provide a means of fingerprinting magnetite of primary as opposed to diagenetic origin

geochemical features of fe mn micronodules in deep sea sediments of the western north pacific ocean potential for co product metal extraction from rey rich mud abstract rare earth element and yttrium rey rich mud around minamitorishima island japan in the western north pacific ocean has received significant attention as a novel rey resource in rey rich muds industrially critical metals such as co ni and mo occur in relatively high concentrations we show that fe mn micronodules separated from rey rich muds are significantly enriched in these elements we measured co contents of almost 3000 ppm and ni contents that exceed 39 000 ppm through experiments we demonstrated that reductive chemical leaching can be used to extract major portions generally 80 of the metals associated with fe mn oxyhydr oxides from rey rich muds the fe mn micronodules were of an oxic diagenetic origin a simple mass balance calculation indicated that the contribution of the fe mn micronodules to the bulk fe mn oxyhydr oxides in the sediment can amount to 8 50 median 18 the other metals associated with fe mn oxyhydr oxides seem to be hosted in mn microparticles probably of a hydrogenetic origin we estimate that the most promising site for rey rich mud development contains 9 3 0 6 102 t 1 6 0 1 103 t and 2 8 0 2 102 t of leachable co ni and mo respectively in 1 km2 0 10 m below seafloor these amounts account for 0 66 0 04 0 061 0 004 and 0 097 0 007 of present annual global productions of co ni and mo respectively the feasibility of extracting these metals as co products during rey rich mud development depends on the cost efficiency of the recovery process which ultimately depends on their market price in the near future

the upper and lower nunivak sedimentary geomagnetic transitional records from southern sicily van hoof a a m van os b j h and langereis c g 1993 the upper and lower nunivak sedimentary geomagnetic transitional records from southern sicily phys earth planet inter 77 297 313 the detailed paleomagnetic records of the upper and lower nunivak polarity transitions have been determined from pliocene marine marls in southern sicily magnetites are the most important carrier of the remanence in both records the transitions are recorded in two components a low temperature and a high temperature component the two components do not represent the geomagnetic field because the changes in these components take place at lithological boundaries in addition the directional changes do not completely match directional changes of the same transitions recorded in calabria some 250 km away the character of the virtual geomagnetic poles paths is probably caused by smoothing of the stable directions before and after the transitions the directional changes as well as the smoothing mechanism can be explained by the diagenetic magnetite formation model van hoof et al 1992 in which shortly after burial the remanence carried by newly formed secondary magnetites is superposed on the initial remanence carried by primary magnetite

pliocene pleistocene evolution of the tyrrhenian arc paleomagnetic determination of uplift and rotational deformation abstract the jaramillo subchron and the brunhes matuyama transition have been identified in the pelagic pleistocene marls located on the calabrian side of the messina straits italy polarity of the characteristic magnetization has been deduced from an analysis of multicomponent magnetization this magnetostratigraphic age determination combined with the paleobathymetry of the quaternary marls implies uplift rates of fault bounded blocks of 0 7 1 0 m 1000 yr a positive fold test on three sites from the trubi formation 4 3 ma shows a clockwise rotation of 36 an antipodal test on pliocene pleistocene micritic limestones 0 9 2 0 ma also indicates a rotation of 14 with the same trend both results suggest a rotation rate of about 9 ma these results are in agreement with a tectonic mechanism involving simultaneous rotation and uplifts a discussion of the possible mechanisms indicates that a compression phase oriented ene wsw could have played a dominant role

regional patterns of magnetite authigenesis in the appalachian basin implications for the mechanism of late paleozoic remagnetization we report paleomagnetic and rock magnetic data from sedimentary carbonates of the appalachian basin that have implications for the mechanism of widespread late paleozoic remagnetization in this rock type sampling was accomplished mostly along two across basin transects one in devonian carbonates of new york state and the other in ordovician and mississippian carbonates of tennessee and alabama paleomagnetic investigations of the new york devonian carbonates along a transect from albany to buffalo indicate that these rocks were completely remagnetized in alleghenian time rock magnetic studies show that magnetite concentration varies in a regular fashion along the transect with a local maximum near syracuse and decreasing gradually to the east and west this pattern shows a striking correlation with the degree of diagenetic alteration of clay minerals in a devonian bentonite horizon which took place during alleghenian time we conclude that most of the magnetite present in these carbonates is of authigenic origin and that clay mineral alteration and magnetite authigenesis were coeval late paleozoic events that were controlled by the same diagenetic factors thermoviscous remagnetization processes cannot be ruled out in this setting in light of the observed degree of thermal maturity however our results suggest that chemical factors played a critical role in remagnetization since they allow us to infer that magnetite authigenesis and remagnetization are about the same age we propose a geochemical model for magnetite authigenesis wherein the iron is derived from detrital smectites during diagenetic illitization which is triggered by the introduction of potassium rich brines results from the tennessee transect from nashville to chattanooga show a very different pattern paleomagnetic studies indicate that late paleozoic remagnetization has affected ordovician carbonates of the nashville dome paleozoic carbonates in the overthrust belt near chattanooga also carry the late paleozoic remagnetization however mississippian carbonates between the dome and the overthrust belt are very weakly magnetized and show no evidence for late paleozoic remagnetization samples show only a present field magnetization or at one locality in northwestern alabama a dual polarity magnetization of probable mississippian age rock magnetic studies indicate that higher concentrations of magnetite are present in the remagnetized rocks of the nashville dome and the overthrust belt than in the nonremagnetized mississippian carbonates we therefore conclude that magnetite authigenesis and remagnetization are related events in this setting and that the remagnetization must be due to chemical processes the presence of a remagnetization shadow in the younger rocks now exposed between the fold thrust belt and the flexural arch is attributed to westward flow of magnetite forming aqueous fluids through deep sub carboniferous aquifers during alleghenian time upward migration of the fluids through the mississippian carbonates may have been prevented by the devono mississippian chattanooga shale an impermeable stratum that is present throughout the region

astronomical duration of polarity chron c31r lower maastrichtian cyclostratigraphy of odp site 762 indian ocean and the contessa highway section gubbio italy the duration of polarity chron c31r is estimated with a cyclostratigraphic approach two sites are investigated odp site 762 indian ocean and the contessa highway section gubbio italy cyclostratigraphic analysis is performed on greyscale variations site 762 and magnetic susceptibility variations contessa section both sites reveal an astronomical control of the sedimentation highlighted by the identification of all the orbital periodicities cyclostratigraphic signals are tuned on 405 ka eccentricity cycles extracted from the la04 astronomical solution in both sites cycle counting gives an estimate of the duration of polarity chron c31r of about 2 09 0 03 ma

a primary magnetization fingerprint from the cretaceous laytonville limestone further evidence for rapid oceanic plate velocities paleomagnetic data from the cretaceous 101 88 ma laytonville limestone of northern california are of geodynamic interest because they suggest a rate of absolute plate motion 15 cm yr that exceeds those of all present day plates such findings assume that a primary magnetization is preserved in these accreted pelagic limestone outcrops this assumption is supported by a trend in the paleomagnetic inclination data that matches in magnitude and sign the expected sense of motion predicted by comparing the mean paleolatitude value with the north american apparent polar wander path the pervasiveness of remagnetizations seen in shallow water carbonates of north america and elsewhere however raises questions as to whether the laytonville limestone is remagnetized regardless of tests indicating a primary magnetization the remagnetized carbonates have a distinctive wasp waisted magnetic hysteresis signature thought to reflect a bimodal distribution of magnetite grain sizes this signature can be quantified by the relationship between saturation remanence mr saturation magnetization ms coercivity hc and the coercivity of remanence hcr mr ms 0 89 hcr hc 06 this power law is compared with reference data from pacific deep sea carbonates deep sea drilling project sites 167 317 and 463 ocean drilling program site 806 and tethyan pelagic carbonates valle del mis section southern alps which together define a different curve mr ms 0 53 hcr hc 1 0 the laytonville limestone falls in two categories on the basis of unblocking temperature spectra resistance to alternating field demagnetization and behavior during the acquisition of isothermal remanent magnetization red laytonville limestone is thought to contain pigmentary hematite in addition to magnetite hysteresis curves from red laytonville limestone are wasp waisted but because such curves also typify magnetite hematite mixtures these data do not provide an unambiguous test for remagnetization some white laytonville limestone however appears to contain negligible amounts of hematite and can be directly compared with the remagnetized carbonates hysteresis parameters derived from white samples are indistinguishable from those of the reference pelagic limestones such samples have a stratigraphic significance occurring where paleolatitude data indicate an equatorial crossing these samples may have depleted pigmentary hematite due to increased magnetic dissolution as the limestone entered the equatorial belt of high productivity the lack of a remagnetization fingerprint in these white samples together with paleomagnetic lithologic and paleontologic data form a consistent data set supporting the primary nature of the laytonville limestone magnetization and rapid oceanic plate velocities a small plate size coupled with the unusually vigorous mantle plume volcanism of the cretaceous pacific basin could have combined to reduce the effectiveness of asthenospheric drag accounting for the rapid motion

miocene remagnetisation of carbonate platform and antalya complex units within the isparta angle sw turkey abstract a palaeomagnetic study has been carried out within the mesozoic and tertiary units of the relatively autochthonous carbonate platforms and the allochthonous deep sea volcanics and sediments of the antalya complex exposed around the isparta angle sw turkey the antalya complex is interpreted as a mosaic of carbonate platforms basinal sediments volcanic and ophiolitic rocks which formed within a southerly strand of the neotethyan ocean adjacent to gondwana the results indicate a widespread remagnetisation event negative fold tests show that the remanence at most sites is of secondary origin e g within the cirali lavas the magnetisation is carried by magnetite of presumed authigenic origin the remagnetisation event is believed to have occurred in the early middle miocene burdigalian langhian it was possibly triggered by the migration of orogenic fluids ahead of the advancing lycian nappes during their emplacement onto the carbonate platforms subsequent to remagnetisation a large segment of the isparta angle underwent an anticlockwise rotation of 30 this rotation is attributed to the overall convergence and bending of the hellenic arc and the final stages of emplacement of the lycian nappes during the late miocene in agreement with previous studies previously southerly palaeolatitudes were inferred from late triassic extrusives of the godene zone calbali dag unit the post folding magnetisation identified here within the cirali lavas of the godene zone to the south implies that these low palaeolatitudes result from the inappropriate application of structural tilt corrections the available data cannot be used to substantiate an origin for the antalya units south of the equator in the early mesozoic instead a position close to the northern margin of gondwana is indicated

paleoceanographic changes of the late pliensbachian early toarcian interval a possible link to the genesis of an oceanic anoxic event abstract secular records of the elemental and isotopic composition of belemnite calcite were studied in pliensbachian and toarcian sections from the yorkshire coast uk and southern germany to investigate oceanographic change during an interval prior to and including the toarcian oceanic anoxic event oae records from southern germany are correlated to the uk stratigraphy using strontium isotope stratigraphy the geochemical trends measured from belemnite calcite are consistent between the two sections and are interpreted in terms of temperature and salinity of the northwest european epi continental sea the data suggest that a dramatic environmental change coincided with the toarcian oae belemnite mg ca sr ca and na ca ratios increase by a factor of between 1 7 and 2 coincident with a 3 negative shift in 18o from the mid tenuicostatum zone until the lower falciferum zone of the uk ammonite biostratigraphy a period of 0 6 0 7 myr taken at face value the mg ca and 18o data argue for an abrupt warming of 6 7 c and substantial freshening during this interval global warming accompanied by an accelerated hydrological cycle and increased runoff is proposed to explain these changes prior to these events data from lower in the yorkshire section suggest a possible cooling accompanied by a shift to more saline waters during the period from the upper pliensbachian margaritatus zone to the toarcian lower tenuicostatum zone this earlier event may also have been important in causing density stratification in the northwest european epi continental sea

towards an astrochronological framework for the eastern paratethys mio pliocene sedimentary sequences of the focsani basin romania abstract a reliable geological timescale for the neogene sedimentary deposits of the romanian carpathian foredeep did not exist so far due to the lack of accurate isotopic datings and straightforward magnetostratigraphic correlations here we present a new chronology for the upper miocene to pliocene deposits of the westernmost part of the eastern paratethys based on high resolution magnetostratigraphic data from the focsani basin the thick and continuous river sections of putna 2 35 km and rimnicu s rat 7 32 km yield excellent magnetostratigraphic results and allow an unambiguous correlation to the polarity timescale for the interval between 7 and 2 5 ma the new age control permits the calculation of accumulation rates for each polarity zone which shows a significant increase at approximately 5 8 6 my the upper miocene to pliocene sedimentary rocks of the focsani basin display cyclic alternations of sandy units and finer intervals the calculated values for the average duration of the observed sedimentary cycles are very close to the average duration of precession 21 7 ky which indicates that the sedimentary cycles from the eastern carpathian foredeep deposits are astronomically forced we show that the usage of the unambiguous time control offered by magnetic polarity stratigraphy would give a more realistic picture of the processes that have taken place in this tectonically active region

integrated stratigraphy and palaeoenvironmental interpretation of the upper kimmeridgian to lower berriasian pelagic sequences of the velykyi kamianets section pieniny klippen belt ukraine abstract a continuous sedimentary and stratigraphic record of late kimmeridgian early berriasian age is presented from the pelagic succession from the eastern part of the pieniny klippen belt velykyi kamianets section ukraine the studied section 26 m thick was calibrated using biostratigraphy calpionellids calcareous dinocysts calcareous nannofossils magnetostratigraphy stable isotope stratigraphy 13c and 18o and gamma ray spectrometry magnetic polarity zones from m23r to m18n were identified the magnetostratigraphic calibration of calcareous dinocysts zonation around the kimmeridgian tithonian boundary has been performed for the first time the carpistomiosphaera borzai committosphaera pulla calcareous dinocyst zonal boundary is situated at the bottom of magnetozone m22n slightly lower than first occurrence of calcareous nannofossil conusphaera mexicana minor and higher than the standard kimmeridgian tithonian boundary m22ar m22an hybonoticeras beckeri hybonoticeras hybonotum ammonite zone boundary the position of the tithonian berriasian boundary in the magnetosubzone m19n2n crassicollaria calpionella calpionellid zonal boundary has been confirmed a decrease of terrigenous input and an increase of carbonate productivity correlate with increasing sedimentation rates from 0 7 to 1 1 m myr in the upper kimmeridgian m23r to m22r to 5 7 m myr in the uppermost tithonian and lower berriasian m19n to m18n transition between the saccocoma and calpionellid microfacies occurs in a narrow time interval mostly within polarity subchron m20n1r this event is traceable in similar stratigraphic position in several western tethyan sections and correlates perfectly with nannofossil calcification event of the early late tithonian

identifying goethite and hematite from rock magnetic measurements of soils and sediments we describe a sequence of experiments involving the acquisition of isothermal remanent magnetization irm in high 2 t fields followed by orthogonal demagnetization and the cooling and heating of remanent magnetization mr over the temperature range 196 c to 680 c the main aim of these experiments is to improve the basis for identifying the imperfect antiferromagnetic minerals contributing to the magnetic properties of natural soil and sediment samples which often contain complex mineral mixtures the procedures developed are first applied to a series of ore and synthetic samples of hematite and goethite each characterized by x ray diffraction xrd spectra and second to controlled mixtures of the two minerals the results confirm the mineral diagnostic nature of the experiments used they are subsequently used to identify goethite and hematite contributions to the rock magnetic properties of a variety of soils and recent sediments goethite can be unambiguously identified often accompanied by hematite in the majority of the samples studied despite indications from the synthetic mixtures that the ratio of goethite to hematite may have to exceed an order of magnitude before the goethite signatures are readily recognizable the rock magnetic tests are a much more sensitive basis for recognizing the presence of goethite in natural samples than are xrd analyses the results suggest that goethite s contribution to the rock magnetic properties of soil and sediment samples may often have been underestimated

rock magnetic properties of multicomponent natural remanent magnetization in alluvial red beds ne spain summary an earlier study of the cyclic miocene red bed sequence of la gloria spain by krijgsman et al indicated complicated behaviour of the natural remanent magnetization nrm in parts of the section particularly close to reversal boundaries we resampled part of the section with high resolution and used extensive rock magnetic analyses to characterize the magnetic remanence carriers below a conspicuous hydromorphous layer the magnetic contributions of haematite and magnetite are approximately equal while in the brown layers on top of the hydromorphous layer the contribution of magnetite is drastically reduced this is probably related to a change in hydrological conditions the nrm is characterized by 1 a low temperature 200 360 c overprint of unknown age 2 a medium temperature 360 580 c component interpreted as the characteristic remanent magnetization chrm and 3 a high temperature haematite component 600 680 c carrying the same directions as the medium temperature component there is no perceivable delay in nrm acquisition between the medium and the high temperature components they are both regarded as primary the behaviour of the nrm seems to be similar in the cyclic lithologies the acquisition of nrm thus seems to be independent of lithology in the cyclic part of the section the higher sampling resolution yielded the detection of a new polarity zone which probably represents a geomagnetic feature according to rock magnetic properties however the more detailed magnetostratigraphy of the resampled part of the section indicates that the earlier correlation to the geomagnetic polarity timescale is no longer tenable therefore the la gloria section should no longer be used as a magnetostratigraphic dating tool of mammal biochronology

magnetism of chinese loess deposits summary the age of chinese loess deposits has long been disputed biostratigraphical and earlier magnetostratigraphical investigations placed the entire loess formation within the pleistocene and ascertained a maximum loess age of about 1 2 myr a new collection of nearly 500 samples from a natural outcrop and a borehole section near lochuan lat 35 8 n long 109 2 e shaanxi province has been dated by magnetic stratigraphy thermal cleaning of the natural remanent magnetization nrm removes a strong secondary component of viscous origin along the present geomagnetic field which resides largely in magnetite the characteristic nrm component is due to haematite which is thought to be of chemical origin rhythmical intensity variations of nrm and initial susceptibility depend on the loess lithology and may reflect climatic changes during loess deposition the palaeomagnetic results are consistent between the two sections and yield a clearly defined magnetic polarity zonation the brunhes matuyama boundary and the jaramillo subchron have been positively identified in both outcrops at exactly the same stratigraphic level the olduvai subchron has been found in the borehole section which records the entire loess sequence most probably the formation of chinese loess began shortly after the matuyama gauss polarity transition therefore a late pliocene age of about 2 4 myr is assigned to the oldest loess sediments measured

a revised correlation of mesozoic polarity chrons and calpionellid zones investigation of four sections of tithonian to valanginian pelagic limestone have led to refinement of the correlation of calpionellid zones to the magnetic polarity time scale the correlations are self consistent but differ slightly from those previously published the discrepancy with the published correlation from the bosso section 1 has been resolved by re evaluation of the biostratigraphy of this sequence the revised correlation places the base of thechitinoidella zone in the lower part of polarity chron cm21n the base of zone a near the top of cm20n the a b boundary at the base of cm18 theb c boundary in the upper part of cm17 thec d boundary at the top of cm16 and thed e boundary at the top of cm14

paleomagnetic and structural evidence for neogene block rotations in the central apennines italy magnetic fabric and paleomagnetic analyses on sedimentary sequences in several structural units in the central apennines ranging from upper cretaceous pelagic carbonates to upper miocene turbidites provide evidence for at least two distinct phases of rotation the first counterclockwise ccw rotation is well recorded in eocene oligocene sediments and does not affect the lower middle miocene sequences the second phase is clearly recorded in lower middle and upper miocene sediments and shows both ccw and clockwise cw rotations in concordance with trends of the different thrust structures we detected three domains characterized by different rotation patterns the westernmost is the southern sabina area an arcuate thrust belt which underwent a cw rotation after the early miocene the second domain is the latium abruzzi carbonate platform where a ccw rotation occurred after the late miocene the third is the marsica area that underwent a cw rotation after the late miocene the magnetic fabric analyses confirm the existence of these different structural provinces and show a strong correlation between the present day strike of the anisotropy of magnetic susceptibility magnetic lineation and the rotations recorded in miocene sediments we attribute present day variations in orientation of the compressional structures in the central apennines to differential rotations about vertical axes due to strike slip faults and out of sequence thrust activity rather than to a change in the stress field orientation we also suggest that this deformation mechanism could be common in other collisional chains around the world

fast instantaneous oceanic plate velocities recorded by the cretaceous laytonville limestone paleomagnetic analysis and kinematic implications new paleomagnetic results from 101 89 ma laytonville limestone of the franciscan central belt melange of northern california confirm a southern hemisphere origin as first proposed by alvarez et al 1980 discovery and study of new laytonville limestone outcrops extend the paleomagnetic record and provide a definitive determination of age stratigraphic polarity and hemisphere of deposition paleontological determinations show that five sections are presently overturned and four are presently right side up as expected from a random distribution of nine blocks in a melange the exact accretion time is unknown but geologically reasonable estimates range between 70 and 50 ma leading to averaged estimates of poleward velocity between 30 and 14 cm yr the averaged values are obtained by dividing the total paleolatitude shift by the difference between magnetization and estimated accretion ages they reflect net poleward transport over a long time interval and are minimum estimates because they record only the poleward component of motion an instantaneous poleward velocity is recorded by the systematic inclination decrease of 160 paleomagnetic samples with respect to decreasing age over the 12 m y section conservative estimates place this minimum velocity at 15 cm yr the consistency between the values predicted by the average velocity calculations and those measured within the outcrops serves as an important check on the primary nature of the magnetization this minimum velocity is much higher than any present day plate velocity the subduction of old oceanic crust at the boundary of a relatively small plate is hypothesised as a driving mechanism to explain the measured velocities coupled with the minimum instantaneous poleward velocities this hypothesis suggests that the velocities of subducting plates may vary up to a factor of 5 similar plate geometries and resultant velocities have probably existed at other times in the past but their discovery may have been hindered by low preservation potential

magnetostratigraphy in pursuit of missing links since the last quadrennial report magneto stratigraphy has continued to flourish the primary issues have been 1 refinements to the geomagnetic reversal time scale grts including recalibrations correlation of biostratigraphic data to the crts and revision of the terminology 2 use of magnetostratigraphic data for estimating rates of such processes as sediment accumulation evolution and faunal migration and 3 magnetic mineralogy and the origin of remanence for convenience we have divided the publications into studies of continental and marine sediments and highlight the most exciting issues within each of these sub headings in the first section of this review we will concentrate on the development of the geomagnetic reversal time scale grts with particular emphasis on refinements from the last few years we then turn our attention to applications of magnetostratigraphy in marine and continental sequences

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

remagnetization by basinal fluids testing the hypothesis in the viola limestone southern oklahoma migration of orogenic or basinal fluids is a recently invoked mechanism to explain the widespread presence of late paleozoic secondary magnetizations in the rocks of north america paleomagnetic and geochemical results from the ordovician viola limestone in southern oklahoma are evaluated to assess the role of basinal fluids in leading to secondary magnetizations in the unit the viola limestone contains what we interpret to be a pervasive pennsylvanian synfolding magnetization residing in magnetite and a localized permian magnetization which resides in hematite and occurs in alteration zones around mineralized veins both secondary magnetizations are interpreted as chemical remanent magnetizations crm based on low burial temperatures and the presence of authigenic magnetic phases the relative proportion of the permian crm in hematite gradually decreases whereas the magnetite crm increases with distance from the veins fluid inclusion and sr isotope studies indicate that the vein mineralization calcite with mississippi valley type oxides and sulfides precipitated from basinal fluids which were warm saline and radiogenic the radiogenic 87sr 86sr values of the limestones in the alteration zones and the fact that there is more significant alteration closer to the veins suggests that the basinal fluids were also responsible for alteration in the limestones the coincidence of the geochemical and remagnetization trends suggests that the permian crm dates the migration of basinal fluids through the veins geochemical results from the viola limestone containing the pervasive crm indicate that it is relatively unaltered with no evidence for basinal fluids the lack of evidence for basinal fluids suggests that other mechanisms for the origin of the pervasive crm need to be tested the results of this study indicate that flow of basinal fluids was focused in veins and only locally altered the host limestone

new chronostratigraphy of the cretaceous paleogene boundary interval at bidart france abstract the cretaceous paleogene boundary section at bidart french basque coast was prevented for a long time to be considered as a reference section for the k t boundary due to a suspected hiatus comprising most of chron c29r in our new magneto and biostratigraphic study of this section we discovered that the underlying primary magnetic polarity is not revealed in the vector directions but in a small but consistent intensity excursion during laboratory demagnetization trends magnetite and maghemite are the main carriers of remanence with contribution of hematite to the magnetic mineralogy of the red brown lowermost danian and maastrichtian marly limestones magnetite is the main magnetic mineral in the white to pink danian limestones the nrm of marls and marly limestones is characterized by 1 a present day field overprint 2 a medium temperature 300 400 c reversed component interpreted as a relic of the characteristic remanent magnetization chrm carried by magnetite grains which are not completely oxidized into maghemite and 3 a high temperature up to 590 c normal component due to late diagenesis hematite growth the nrm of danian limestones is characterized by 1 a present day field overprint and 2 a chrm component of normal or reversed polarity the chrm directions allow a new correlation of the magnetic polarity sequence with the geomagnetic polarity time scale gpts chron c29r corresponds to the maastrichtian unit and extends up to 3 5 m above the k t boundary and chron c29n occurs at the top of the section there is no evidence for a hiatus at or below the k t boundary and the section does not extend up to chron c28r as previously suggested these new chronostratigraphic assignments are in good agreement with the previous biostratigraphic data and from the new biostratigraphic analyses based on calcareous nannofossils presented in this paper our study demonstrates that the section of bidart can be considered as a key section to study the k t boundary event s

contemporaneous thrusting and large scale rotations in the western sicilian fold and thrust belt in the western sicilian fold and thrust belt large clockwise rotations of allochthons occurred during late cenozoic contraction of part of the southern tethyan margin the magnitude of rotation decreases stepwise from over 120 in the upper sheets lying on the north coast of sicily to no appreciable rotation in the frontal portion of the belt flanking the southern coast of the island the allochthons are composed of imbricate thrust sheets derived primarily from individual basin and platform assemblages of the old tethyan margin paleomagnetic and structural data indicate that the rotation of the allochthons was accommodated by coherent torsional displacements on relatively low angle detachment surfaces timing relations for the imbrication history of the sicilian fold and thrust belt are derived from stratigraphic overlap and local involvement of sediments deposited in a series of foreland and piggyback basins the locus of deposition within successive foreland basins first migrated easterly then southerly during progressive deformation in the orogen imbrication began in the early miocene burdigalian langhian and continued at least through the early pleistocene and appears to be continuing today rotation is related to thrusting and accompanies a 70 change in the tectonic transport direction from easterly to southerly easterly striking right oblique transpressional faults and associated northeasterly trending folds postdate thrust sheet rotation in the interior of the thrust belt and were active contemporaneously with south directed thrusting in the foreland region pleistocene and possibly older late pliocene extension strongly modified the older thrust morphology along the tyrrhenian coast of northwestern sicily with the development of down to the north listric normal faults the extensional structures apparently are related to the opening and subsequent deformation of the tyrrhenian sea to the north

paleomagnetism and paleogeography of jurassic radiolarian cherts from the northern apennines of italy oriented samples of jurassic radiolarian chert were collected from the tuscan domain continental margin and the ligurid domain oceanic of the northern apennines for paleomagnetic study to determine the paleogeographic origins of these rocks the oceanic rocks are all thermochemically overprinted by a mostly reversed polarity component of magnetization b that was likely acquired during late miocene regional uplift of the northern apennines this component also dominates the lower brittle chert of the tuscan cherts but disappears upsection in the more clay rich and ductile siliceous marlstones in addition the tuscan cherts retain an inferred primary magnetization c isolated at temperatures between 560 and 660 c which passes a fold test and shows a polarity stratigraphy this component indicates a paleolatitude of 11 4 n and a counterclockwise vertical axis rotation of 29 9 with respect to the southern alps of italy of 49 8 with respect to africa and of 91 8 with respect to eurasia our results suggest that the tuscan domain was farther south than other deep water continental margin sections of adria and that transcurrent faulting might have played a significant role in the orogenic evolution of the northern apennines

environmental magnetism principles and applications 1 in environmental magnetism rock and mineral magnetic techniques are used to investigate the formation transportation deposition and postdepositional alterations of magnetic minerals under the influences of a wide range of environmental processes all materials respond in some way to an applied magnetic field and iron bearing minerals are sensitive to a range of environmental processes which makes magnetic measurements extremely useful for detecting signals associated with environmental processes environmental magnetism has grown considerably since the mid 1970s and now contributes to research in the geosciences and in branches of physics chemistry and biology and environmental science including research on climate change pollution iron biomineralization and depositional and diagenetic processes in sediments to name a few applications magnetic parameters are used to routinely scan sediments but interpretation is often difficult and requires understanding of the underlying physics and chemistry thorough examination of magnetic properties and of the environmental processes that give rise to the measured magnetic signal is needed to avoid ambiguities complexities and limitations to interpretations in this review we evaluate environmental magnetic parameters based on theory and empirical results we describe how ambiguities can be resolved by use of combined techniques and demonstrate the power of environmental magnetism in enabling quantitative environmental interpretations we also review recent developments that demonstrate the mutual benefit of environmental magnetism from close collaborations with biology chemistry and physics finally we discuss directions in which environmental magnetism is likely to develop in the future

goethite as a potential source of magnetic nanoparticles in sediments goethite is an important iron bearing mineral found in nearly all types of soils and sediments that typically occurs as particles of nanoscale crystallites here we show that poorly crystalline nanophase goethite can undergo dehydroxylation and alteration to produce strongly magnetic fe oxide nanoparticles after moderate reductive heating synthetic oriented aggregates of nanogoethite were converted to stoichiometric sub micron magnetite in contrast oxidative heating produced only nanocrystalline hematite we present a novel characterization of the products of nanogoethite alteration which represents an important pathway for the production of both superparamagnetic and stable single domain magnetic particles in a wide variety of sedimentary environments including soils and lacustrine and marine settings in which goethite is commonly present in nature the mechanism of magnetite formation observed in our experiments could contribute to the widespread phenomenon of magnetic enhancement in many soil types and could be responsible for secondary magnetizations often observed to accompany diagenesis in sedimentary rocks thus nanogoethite may be an important precursor of fine magnetic particles and magnetic remanence carriers in the environment particularly in soils and catchment areas affected by wildfire as well as in sediments subjected to deep burial diagenesis and low grade metamorphism

origin of red pelagic carbonates as an interplay of global climate and local basin factors insight from the lower devonian of the prague basin czech republic abstract red pelagic sediments are relatively common in the phanerozoic they are often interpreted as products of sea bottom oxidation during greenhouse climate showing a conspicuous alternation with black shales and thus carrying important palaeoceanographic information the lower devonian pragian carbonate strata of the prague basin czech republic praha formation contain a marked band of red pelagic carbonate up to 15 m thick which can be correlated for several tens of km we investigated seven sections 17 to 255 m thick of the prague basin using the methods of facies analysis outcrop gamma ray logging diffuse reflectance spectroscopy optical microscopy element geochemistry magneto mineralogy and electron microprobe analysis the aim was to find the mineral carriers of the red colour investigate the stratigraphic context of the red carbonates and evaluate the local and global prerequisites for their formation the red pigmentation represents enrichment by hematite with respect to goethite approximately 31 of the total reflectance falling in the red colour band represents a threshold for red coloration the red pigmentation is carried by submicronic hematite dispersed in argillaceous pelagic calcilutite and or inside skeletal allochems gamma ray log correlation indicates that the red carbonate band developed in stratigraphic levels with low sedimentation rates typically from 1 to 7 1 mm kyr which are comparable to the mesozoic rosso ammonitico facies the red beds and the whole praha formation pragian to early emsian are characterized by low toc values

nitrate reduction sulfate reduction and sedimentary iron isotope evolution during the cenomanian turonian oceanic anoxic event onset of the oae in italy have unusually low bulk d 57 fe values unlike those found in the black shale livello bonarelli deposited during the oceanic anoxic event itself these latter conform to the phanerozoic norm for organic rich sediments pyrite formation in the pre oae black shales has apparently taken place via dissimilatory iron reduction dir within the sediment a suboxic process that causes an approximately 2 fractionation between a lithogenic fe iii oxide source and fe ii aq in contrast bacterial sulfate reduction bsr at least partly in the water column characterized the oae itself and was accompanied by only minor iron isotope fractionation this change in the manner of pyrite formation is reflected in a decrease in the average pyrite framboid diameter from 10 to 7 mm the gradual albeit irregular increase in fe isotope values during the oae as recorded in the italian section is taken to demonstrate limited isotopic evolution of the dissolved iron pool consequent upon ongoing water column precipitation of pyrite under euxinic conditions given that evidence exists for both nitrate and sulfate reduction during the oae it is evident that redox conditions in the water column were highly variable in both time and space

paleomagnetic study of cretaceous rocks of peru south america evidence for rotation of the andes abstract paleomagnetic data for the cretaceous volcanic and sedimentary rocks in the andean region of peru are given reliable paleomagnetic field directions were obtained for three cretaceous albian to cenomanian formations from calcareous sediments in northern peru stable remanent magnetization directions were also derived from twelve cretaceous lava flows and dikes in coastal peru paleomagnetic data of the same age from the stable areas of south america such as brazil demonstrate that the paleomagnetic poles are nearly coincident with the present pole but peruvian paleomagnetic directions studied here showed several tens of degrees of counterclockwise declination shifts this suggests counterclockwise tectonic rotation of an extensive block which includes the whole of andean peru

rotational deformation during palaeogene thrusting and basin closure in eastern central greece palaeomagnetic evidence from mesozoic carbonates summary palaeomagnetic studies in greece have concentrated upon neogene sequences with mesozoic rocks of mainland greece receiving little attention the neogene data base is now sufficiently extensive however to allow meaningful interpretation of older units this paper presents new palaeomagnetic data from mesozoic and tertiary sequences of eastern central greece which document a pre neogene phase cf rotational deformation mesozoic carbonates have been sampled within the classic pelagonian zone in the argolis peninsula beotia and evvia pelagonian sequences record the development drowning and subsequent re establishment of a carbonate platform separating two strands of the neotethys ocean the pindos and vardar basins most platformal facies are tdo weakly magnetized for palaeomagnetic analysis however pelagic horizons consisting of pink condensed limestones have been deposited periodically in response to platform subsidence these middle late triassic jurassic and upper cretaceous sequences yield high quality palaeomagnetic data which reveal a consistent clockwise rotation of about 94 the constancy of rotation angle deduced from the mesozoic rocks suggests that the total tertiary rotation is uniform throughout the area studied this implies that observed variations in declination within the neogene sequences result from sampling of rocks of different ages deposited during a period of rapid overall rotation removal of the largest neogene rotation 45 from the mesozoic data set reveals a pre middle miocene post late cretaceous clockwise rotation of the southern pelagonian zone of 50 the major tectonic event during this time period was the closure of the pindos ocean during the eocene a large carbonate platform within the pindos basin now preserved in the parnassos zone presented a major obstacle to the advancing pelagonian nappes during sw directed thrusting present coordinates impingement of the nappes on to the parnassos platform resulted in pinning and subsequent clockwise rotation of thrust units around the south eastern margin of the platform the southernmost tip of the pelagonian zone southern argolis was detached along a pre existing line of weakness along the present migdhalitsa graben and experienced a different rotation history additional palaeomagnetic data from the pindos nappes in the eastern peloponnesos also provide evidence for significant rotation of thrust sheets during eocene basin closure the pindos sequences formed by pelagic sedimentation within the pindos ocean an overall anti clockwise rotation with respect to the underlying gavrovo tripolitza autochthon is implied by the data this probably reflects oblique emplacement of the pindos thrust sheets on to the adjacent platform variations in rotation angle probably result from variations in footwall topography

geologic significance of trace element abundances in the red scaglia limestones from gubbio central italy abstract contents of al cu fe mn pb tl and zn in 29 samples of pelagic limestones red scaglia have been determined by sequential chemical leaching cu mn pb and zn are mostly associated with the carbonate fraction while the bulk of fe is carried by the residual fraction and finally the bulk of al and the whole tl are contained in the residual mineral phases such as k bearing silicates the element distribution patterns in the limestones analyzed provide information about the materials sedimented and the redox conditions occurring during sedimentation and diagenesis in particular they suggest that most of the fe oxides carried by these rocks formed in the sea basin

cretaceous oceanic red beds as possible consequence of oceanic anoxic events article i nfo oceanic anoxic events oaes represent brief periods of burial of large amounts of organic carbon in the oceans during cretaceous time burial of organic carbon which preferentially sequestered isotopically lighter carbon during oaes resulted in positive 13 c excursions of 2 3 recognizable in global ocean these deposits are typically dark grey to black shales considered to be the result of interaction between extreme warm climate ocean circulation high bioproductivity and organic carbon preservation less interest has been given to periods between oaes when oxic deep sea deposits such as red marls and red shales were deposited during mid and late cretaceous the latter deposits are associated with very low content of organic carbon and oxic depositional environments which dominated western tethys in post turonian time up to the early eocene feedbacks among geochemical cycles in response to decreasing global temperatures increasing deep ocean circulation forced by high latitude deep water formation along the antarctic margin and widening and deepening of the interconnections between the oceanic basins may have been responsible for the major paleoceanographic change from deposition of organic carbon rich black shales during mid cretaceous to world wide deposition of cretaceous oceanic red beds corbs in the late cretaceous the presence of corbs sandwiched between mid cretaceous oaes may reflect major climate and paleoceanographic changes in a contrast to extremely warm climates during the oaes the corbs suggest cold periods and therefore oscillating climate shifts that have seldom been considered during modeling of cretaceous

continuous record of geomagnetic field intensity between 4 7 and 2 7 ma from downhole measurements abstract a continuous record of the geomagnetic field intensity from 4 7 to 2 7 ma has been obtained together with a precise magnetostratigraphy from downhole magnetic measurements at site 884 of odp leg 145 in the north pacific the record confirms the saw tooth pattern of geomagnetic field intensity proposed by valet et meynadier 10 reversals are characterized by a steep intensity decrease followed by a quick regeneration over each polarity interval rapid variations are superimposed over a progressive decrease in the mean intensity we find that the duration of each polarity interval is inversely proportional to the mean rate of decrease in the field intensity over this period and thus this duration seems to be pre determined

anisotropy of magnetic susceptibility in the scaglia rossa pelagic limestone abstract during folding of the scaglia rossa limestone in umbria italy deformation was mainly accommodated by pressure solution cleavage fossils between the cleavage planes appear visibly undeformed yet the limestone possesses a weak magnetic fabric the maximum and intermediate principal axes of the magnetic anisotropy ellipsoid define a distinct magnetic foliation plane within which a weak concentration of the maximum axes forms a magnetic lineation neither of these features is of sedimentary origin results from a slumped outcrop where bedding and a cleavage induced by overburden compaction have different attitudes show that the magnetic foliation is caused by the compaction comparisons with field derived structural data suggest that the magnetic lineation was produced tectonically during deformation of the apennine fold belt

rockmagnetic properties of fine grained natural low temperature haematite with reference to remanence acquisition mechanisms in red beds summary grain size dependence of some remanent magnetic properties of a natural low temperature haematite is reported the haematite was obtained from a gossan in montarnu france and upgraded by magnetic separation seven grain size fractions were prepared ranging from 10 down to 0 25 m by means of microprecision sieving and centrifuging the haematite has an average crystallite size of 60 nm and contains minor contents of si al and h2o its unit cell dimensions are very close to those of ideal haematite the magnetic properties of the haematite are strongly dependent on its thermal history for the original unheated haematite fractions the isothermal saturation remanence jrs decreases with grain size the remanent coercive force hcr remanent acquisition coercive force hcr and median destructive field of the saturation remanence h1 2i are maximal in the 1 0 0 5 m fraction this grain size range can be understood as the sd threshold size in analogy with ferrimagnetic minerals jrs hcr x xin is a useful grain size indicator lowtemperature cycling of jrs shows identical behaviour for all fractions the remanence decreases slightly and shows no sharp morin transition after annealing the haematite fractions at 700 c jrs is considerably reduced relative to the original haematite predominantly due to removal of the defect moment both hcr and hcr have increased and their maximal values are shifted to the 2 1 1 0 m fraction all fractions show a much sharper morin transition during low temperature cycling of jrs due to crystallite enlargement trm decreases with grain size the different rock magnetic properties before and after annealing might provide a basis for the discrimination in sediments between a haematite drm with a high temperature origin and a haematite crm which has a low temperature origin two natural red beds one with a crm and one with a drm associated with high temperature haematite broadly show the expected differences in coercivity and in annealing behaviour although a full account of all aspects of the observed behaviour can not yet be given

reversed polarity characteristic magnetizations in the albian contessa section umbrian apennines italy implications for the existence of a mid cretaceous mixed polarity interval paleomagnetic and paleontologic data from the valle della contessa umbrian apennines italy span a 14 m y gap in previous magnetostratigraphic sections and reveal several intervals of reversed characteristic remanent magnetization chrm in limestones and marls of the albian marne a fucoidi rock magnetic data indicate that the reversed chrm is carried by hematite having high unblocking temperatures 625 690 c hematite with these unblocking temperatures also carries a normal polarity chrm in other intervals a potential bias towards the identification of normal polarity is present since at every horizon measured a normal polarity component carried by magnetite and hematite is isolated at lower unblocking temperatures 300 600 c bulk magnetic properties vary throughout the measured section in the portion containing most of the reversed chrm intervals hematite is the dominant carrier of natural remanent magnetization while magnetite is the dominant carrier stratigraphically above and below these changes correspond approximately with lithology the zone of dominant hematite magnetization is marked by reddish bands pigmentary hematite which can be correlated throughout umbria and are thought to record changes in the oxidation state of the seafloor during deposition two models may account for the observed magnetization directions the reversed magnetizations may record a remagnetization which occurred at least 18 m y after deposition while the beds were still flat lying in reversed polarity chron 33r 83 79 ma or later if the reversed chrm represents a remagnetization the process is potentially of great importance since it can produce magnetization patterns which resemble polarity intervals alternatively the reversed magnetizations may have been acquired during intense seafloor oxidation episodes during the mid albian 107 104 ma and may record unrecognized intervals of reversed geomagnetic field polarity milankovitch like bedding cyclicity can be used to tune the sedimentation rate and obtain estimates of the duration of the potential reversed intervals using these estimates two of the potential reversed polarity intervals are of sufficient duration 100 kyr to be recognizable in both detailed stratigraphic sections and marine magnetic anomaly surveys a primary magnetization model predicts that several intervals of reversed magnetization should be found near the boundary of the biticinella breggiensis and ticinella primula foraminiferal zones and within the prediscosphaera cretacea nannofossil zone on a worldwide basis

age of the plio pleistocene boundary in the vrica section southern italy new palaeomagnetic data from the proposed plio pleistocene boundary stratotype section vrica section southern italy are presented here in order to improve the correlation of these sediments to the magnetic polarity time scale if the interpretation preferred here is correct the boundary falls above the olduvai normal subchron at approximately 1 6 myr

paleomagnetism of red cherts a case study in the inuyama area central japan this study is an attempt to isolate and identify the primary magnetization in red cherts red chert samples from a middle triassic sequence in the inuyama area 35 4 n 137 0 e central japan have a multicomponent magnetization which is delineated by thermal demagnetization two of the components have high coercivity and high blocking temperatures one is carried by hematite and the other by magnetite the former was concluded to be the primary magnetization for two reasons 1 it predates intraformational folding and 2 it records geomagnetic reversals the other component which accounts for the dominant part of natural remanent magnetization was probably acquired at a time long after the deposition two paleomagnetic results were drawn from the middle triassic red chert sequence 1 the mean inclination of the samples in the sequence was 1 4 6 8 therefore the paleolatitude amounts to 0 7 3 4 in contrast to the paleolatitude expected from the paleopole of eurasian continent which is about 70 and 2 the magnetostratigraphy of this sequence indicates that two geomagnetic epochs in the middle triassic are considerably longer more than several million years than the average of those in the cenozoic

timing of diagenetic haematite growth in red pelagic limestones from gubbio italy abstract magnetite haematite and to a minor extent maghaemite are recognised in the cretaceous and paleocene red pelagic limestones at gubbio the magnetite is detrital or biological whereas the haematite grew during diagenesis from a goethitic precursor thermal and af demagnetization of samples collected from close to reversal boundaries indicate that the various magnetization components do not record the polarity reversal at exactly the same stratigraphic level in the few tens of centimetersbelow a recorded geomagnetic reversal defined by the magnetite magnetization some of the haematite grains are magnetized in the post reversal field the blocking temperature spectra of this haematite fraction with post reversal magnetization are found to shift toward higher temperatures as the reversal boundary is approached the blocking temperature spectra reflect the grain size spectra of the haematite which we interpret as arising by the continual nucleation of grains down to a certain burial depth where the conditions are no longer conducive to further haematite growth the depth below reversal boundaries to which haematite with post reversal magnetization can occur is estimated to be about 60 cm after compaction and is equivalent to a time of about 105 years for these particular sediments a detailed study of the magnetization components at reversal boundaries indicates that the first diagenetic growth of haematite through the single domain critical volume occurs prior to the mechanical fixation of the detrital or biological magnetite subsequently the diagenetic haematite grains do not rotate in response to the ambient geomagnetic field polarity as easily as the magnetite because of their occurrence as pigmentary coatings on larger non magnetic grains

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

paleogene magnetic stratigraphy in umbrian pelagic carbonate rocks the contessa sections gubbio the umbrian sequence of pelagic carbonate rocks provides an opportunity for precise correlation between paleogene biostratigraphy and geomagnetic polarity history the red to pink paleocene to middle eocene scaglia rossa limestone the varicolored upper eocene scaglia variegata limestone and the gray green oligocene scaglia cinerea marlstone form a 250 m thick continuous exposure in the contessa valley near gubbio italy magnetostratigraphic lithostratigraphic and biostratigraphic investigations in three sections covering the entire paleogene have confirmed and dated the geomagnetic reversal sequence for most of this period the ferromagnetic mineral in the scaglia cinerea is magnetite and af demagnetization to peak fields of 20 mt is sufficient to define the characteristic remanent magnetization the scaglia variegata and pink scaglia rossa samples contain an additional hematite component which is very pronounced in dark red paleocene scaglia rossa samples changes of magnetic mineralogy take place in these limestones during heating especially above 500 c however thermal demagnetization is effective in isolating the characteristic remanence vectors which form almost antipodal clusters of directions representing normal and reversed polarities the directions are rotated counterclockwise partly due to post oligocene tectonism paleontological zonations of planktic foraminifera and calcareous nannofossils were hampered by poor preservation but the major epoch and stage 29 as in the gubbio bottaccione section paleocene eocene just above anomaly 25 eocene oligocene between anomalies 13 and 15 oligocene miocene just below anomaly 6c these correlations require slight modifications to previous conclusions on paleogene sea floor spreading rates

magnetostratigraphy of the hettangian langmoos section adnet austria evidence for time delayed phases of magnetization summary study of the magnetostratigraphy of the earliest jurassic mainly hettangian section of langmoos near adnet in austria northern calcareous alps provides a sequence of 10 magnetic polarity intervals these intervals are often evident in the natural remanent magnetization however rather than a true magnetostratigraphic sequence palaeomagnetic analyses reveal evidence for successive phases of magnetization within the section evidence includes 1 changes in lithology associated with changes in magnetic polarity 2 different combinations of components of both polarities and 3 largely non antipodal directions considering the palaeomagnetic directions we suggest that the apparent magnetostratigraphy is the result of two or three phases of magnetization which occurred at different stages of the tectonic emplacement of the northern calcareous alps

magnetostratigraphy and radio isotope dating of upper miocene lower pliocene sedimentary successions of the black sea basin taman peninsula russia abstract we present a new chronology for the upper miocene to pliocene deposits of the black sea basin based on high resolution magnetostratigraphic data coupled with 40 ar 39 ar dating from the 475 m long zheleznyi rog section on the taman peninsula russia this section comprises the stratigraphic interval of the khersonian to kimmerian regional stages of the eastern paratethys our magnetostratigraphic record is based on biogenic and early diagenetic greigite components and yields a magnetostratigraphic sequence of 8 polarity zones a volcaniclastic ash layer at the upper part of the khersonian is radio isotopically dated at 8 69 0 18 ma the age of the khersonian meotian transition arrives at 8 6 or 8 2 ma which is significantly younger than earlier estimates the new age has important consequences for the hipparion datum in the eastern paratethys and implies that all vallesian fossil sites are younger than 11 2 ma the meotian stage has a total duration of 2 2 to maximum 2 6 myr and is marked by multiple erosional events the end of the paratethys sea is dated at 5 5 ma when the black sea becomes isolated from the caspian sea the dacian basin and the mediterranean

integrated stratigraphy of the smirra core umbria marche basin apennines italy a new early paleogene reference section and implications for the geologic time scale pelagic sections of the umbria marche basin in the northern apennines italy have provided key geological archives for studying critical intervals of early paleogene time in addition to classical sections the smirra coring project provides a new record of relatively undisturbed sediments 120 m from 4 overlapping holes of the upper scaglia formation paleocene middle eocene here we present a new high resolution integrated magneto bio stratigraphy of the 93 m composite section drilled in smirra holes 1 and 2 the correlation of the magnetostratigraphy to published geomagnetic polarity timescales gptss ck95 gpts2004 gpts2012 as constrained by calcareous nannofossil biostratigraphy reveals that the cores range from chron c26r to c21n thanetian lutetian 60 to 46 ma sedimentation rates range from 0 6 to 1 cm kyr and are comparable with those from coeval sections in the umbria marche basin they point to significant discrepancies between existing time scales which mainly involve the duration of chron c23 and subchrons within c23n and c24n application of the average spectral misfit method on the magnetic susceptibility record reveals a strong signature of orbital forcing and allows an independent estimate of the sedimentation rates this analysis offers additional insights into solving time scale issues which mainly concern the duration of chron c23n our results imply a significantly longer duration for c23n 2n which would accommodate the presence of an extra third 405 kyr eccentricity related cycle in c23n the smirra record thus provides new constraints on the early paleogene time scale a key record for further cyclostratigraphic and astrochronological research and a reference framework for paleo climatic and oceanographic studies

new paleomagnetic results from the upper cretaceous red marls of the pieniny klippen belt western carpathians evidence for general ccw rotation and implications for the origin of the structural arc formation abstract the pieniny klippen belt pkb is a narrow arcuate structure separating the central and outer western carpathians formed during several cenozoic deformational stages the primary aim of this study was to obtain paleomagnetic constraints for the mechanism of formation of the arc we investigate albian santonian red pelagic marls from 14 localities distributed along a strike length of ca 400 km ams measurements reveal a pattern characteristic of weakly deformed sedimentary rocks and magnetic lineations do not correlate with the general strike of the pkb paleomagnetic analysis revealed well defined hematite based ancient magnetization components at 13 localities which are dated using fold and inclination tests a within locality fold test is negative for two localities exhibiting large ccw rotations of similar magnitude situated at the two ends of the pkb remanences of pre folding age were documented for 11 localities with an overall mean paleomagnetic direction of d 311 i 53 and 95 11 the indicated general ccw rotation most probably took place during the miocene together with western central and outer carpathians paleolatitudes for the pkb indicate a considerable separation from the southern margin of stable europe leaving space for coordinated rotation a paleomagnetic oroclinal test involving all localities with primary magnetizations was negative when localities with monoclinal steep dips are omitted due to possible declination bias the overall mean paleomagentic direction does not change significantly but correlation is observed between the general trend of the pkb and the paleomagnetic declinations thus we conclude that the present shape of the arc can be partly due to oroclinal bending this must have happened before oligocene since paleomagnetic declinations for neighboring paleogene basins in the central and outer western carpathians reveal a uniform ccw rotation of ca 50 magnitude irrespective of the position of the localities in relation to the carpathian arc

palaeomagnetic evidence from pelagic limestones for clockwise rotation of the ionian zone western greece abstract on the basis of a pilot study of all the major carbonate lithologies in the ionian zone of western greece the upper part of the upper senonian to upper eocene microbreccious limestones formation was selected as the most suitable facies for a detailed regional palaeomagnetic study the 51 sites sampled are situated on flanks of anticlines or monoclines which post date the acquisition of the characteristic remanent magnetization the regional folding occurred during post late eocene emplacement of nappe sheets which had undergone decollement on triassic evaporitic horizons the palaeomagnetic directions after correction for deviations of local fold axis orientation have a regional significance after this correction the mean palaeodeclination is about 35 e relative to the geographic pole with respect to the early tertiary african pole position these palaeodeclinations indicate an apparent 38 clockwise rotation of the nappe sheets the palaeoinclinations support a palaeogeographic model locating the carbonate sediments of the ionian zone on the northeastern margin of the adriatic promontory of the african plate the total rotation implied by the palaeodeclination data is possibly due to a combination of two different mechanisms 1 1 rotation of the nappe sheets during overthrust and emplacement during the main mid miocene compressional phase of deformation 2 2 rotation of the ionian zone basement and nappes associated with extension in the aegean basin during the last 13 m y

peri equatorial paleolatitudes for jurassic radiolarian cherts of greece abstract radiolarian rich sediments dominated pelagic deposition over large portions of the tethys ocean during middle to late jurassic time as shown by extensive bedded chert sequences found in both continental margin and ophiolite units of the mediterranean region which paleoceanographic mechanisms and paleotectonic setting favored radiolarian deposition during the jurassic and the nature of a tethys wide change from biosiliceous to biocalcareous mainly nannofossil deposition at the beginning of cretaceous time have remained open questions previous paleomagnetic analyses of jurassic red radiolarian cherts in the italian apennines indicate that radiolarian deposition occurred at low peri equatorial latitudes similar to modern day deposition of radiolarian rich sediments within equatorial zones of high biologic productivity to test this result for other sectors of the mediterranean region we undertook paleomagnetic study of mesozoic mostly middle to upper jurassic red radiolarian cherts within the aegean region on the peloponnesus and in continental greece sampled units are from the sub pelagonian zone on the argolis peninsula the pindos olonos zone on the koroni peninsula near karpenissi in central greece and the ionian zone in the varathi area of northwestern greece thermal demagnetization of samples from all sections removed low temperature viscous and moderate temperature overprint magnetizations that fail the available fold tests at argolis and koroni however the cherts carry a third high temperature magnetization that generally exhibits a polarity stratigraphy and passes the available fold tests we interpret the high temperature component to be the primary magnetization acquired during chert deposition and early diagenesis at kandhia and koliaky argolis the primary declinations and previous results indicate clockwise vertical axis rotations of 40 relative to stable europe due to ambiguities in hemispheric origin n or s and thus paleomagnetic polarity the observed declinations could indicate either clockwise cw or counterclockwise ccw vertical axis rotations thus at adriani koroni the primary declinations indicate either cw or ccw rotations of 95 or 84 depending on paleomagnetic polarity and age the primary inclinations for all peloponnesus sites indicate peri equatorial paleolatitudes similar to those found for coeval radiolarian cherts exposed in other mediterranean orogenic belts our new paleomagnetic data support the interpretation that mesozoic radiolarites within the tethys ocean were originally deposited along peri equatorial belts of divergence and high biologic productivity

silicate weathering machine at work rock magnetic data from the late paleocene early eocene cicogna section italy we describe a scenario of climate forcing on sedimentation recorded in the late paleocene early eocene cicogna marine section from the belluno basin ne italy previously published magneto biostratigraphic data revealed that the 81 m cicogna section extends from chron c25r to chron c23r spanning the np7 np8 np12 nannofossil zones 52 2 56 6 ma using previously published rock magnetic data augmented by data from this study we describe and thoroughly discuss a pronounced increase of hematite relative to maghemite or magnetite between 54 9 and 54 6 ma immediately above the paleocene eocene boundary followed by a second long term increasing trend from 54 ma up to 52 2 ma in the early eocene this hematite is essentially of detrital origin insofar as it is associated with a strong shallow bias of paleomagnetic inclinations and is interpreted to have formed on land by the weathering of fe bearing silicates and other primary minerals we speculate that the warm and humid climate typical of the paleocene eocene thermal maximum petm 54 9 ma as well as of the warming trend leading to the early eocene climatic optimum eeco 52 50 ma enhanced continental weathering of silicate rocks with the consequent production transport and sedimentation of detrital hematite grains this hypothesis is confirmed by a statistical correlation between the rock magnetic properties and global climate as revealed by a standard benthic oxygen isotope record from the literature our temporal coupling between oxidation state of sedimentary magnetic phases and global climate is therefore consistent with the existence in the paleocene eocene of the silicate weathering negative feedback mechanism for the long term stabilization of the earth s surface temperature

biostratigraphic and magnetostratigraphic intercalibration of latest cretaceous and paleocene depositional sequences from the deep water basque basin western pyrenees spain abstract latest cretaceous late maastrichtian and paleocene depositional sequences from the deep water basque basin have been calibrated with published 1 2 and newly acquired magnetostratigraphic data sequences mostly consist of hemipelagic marls and limestones and their relative ages are well constrained with planktic foraminifera they accumulated during a phase of tectonic tranquillity and reduced clastic input into the basin and therefore they can be attributed to eustatic sea level changes with reasonable confidence on the basis of the planktic foraminifera zonation a good match has been observed previously between the depositional sequences of the basque basin and specific sea level cycles of the 1988 version of the exxon global cycle chart gcc 3 here a new attempt at correlation using their respective magnetostratigraphic data has failed to confirm such a match the disagreements observed may indicate that for the studied interval 1 the current planktic foraminifera biostratigraphy lacks the necessary level of resolution to ensure synchrony between sequences of different basins and or 2 the magnetostratigraphy of the gcc needs to be revised whatever the case the new findings lend support for some criticism on the use of the gcc for interregional correlations

a paleomagnetic and geochemical record of the upper cochiti reversal and two subsequent precessional cycles from southern sicily italy abstract a detailed paleomagnetic and geochemical study of the upper cochiti n r reversal recorded in marine marls on southern sicily shows two consecutive and very rapid transitions r n and n r that coincide with distinct lithological boundaries a fe migration model is presented in which magnetite is formed under different diagenetic conditions during sulphate reduction originally formed primary magnetite is preserved this primary magnetite has recorded the pre transitional normal polarity after burial migration of ferrous iron occurs and subsequent oxidation produces secondary magnetite which records the post transitional reversed polarity and produces the two apparent transitions in addition low temperature lt and high temperature ht components record the reversal in a slightly different way the ht component being acquired with a delay it is probable that both components reside in magnetite but have a different origin and grain size distribution giving different blocking temperature spectra the actual upper cochiti reversal occurs at a level with an estimated astronomically calibrated age of 4 165 ma

paleomagnetism and rock magnetism of the mississippian leadville carbonate formation and implications for the age of sub regional dolomitization the mississippian leadville formation of central colorado is dolomitized near the sawatch uplift and hosts numerous pb zn ag mineral deposits paleomagnetic and rock magnetic data from several dolomite types away from centers of tertiary intrusive activity imply a late paleozoic late mississippian age for the dolomitization e g d 148 i 11 k 86 95 8 6 n 3 localities 31 samples in dolomitized units the magnetization is of diagenetic origin and was probably acquired during or soon after dolomitization tertiary thermochemical activity has overprinted dolomites close to intrusive centers data indicate that limestones removed from igneous activity were also remagnetized at the same time

upper cretaceous and palaeogene magnetic stratigraphy and biostratigraphy from the venetian southern alps abstract four sections of upper cretaceous eocene pelagic limestones from the belluno basin of the venetian alps italy yield well defined magnetic stratigraphies the correlation of magnetozones with nannofossil and foraminiferal zonations generally agrees well with that obtained from the type section at gubbio 1 the magnetostratigraphy supplements the palaeontology as a means of stratigraphic correlation variations in sedimentation rate and in the duration of hiatuses are clearly documented for the four different locations in the basin