rock magnetism of the early pliocene trubi formation at eraclea minoa sicily the most important magnetic mineral in the finegrained marls of the early pliocene trubi formation near eraclea minoa is magnetite some goethite is also present the magnetite is of high purity and of primary origin hysteresis and low temperature measurements show that most magnetite grains are single domain the presence of superpara magnetic grains is suspected multi domain grains are rare it is stewn that af demagnetization does not yield correct nrm directions

field dependent postdepositional grain realignment in the pdrm process experimental evidence and implications the field dependence of postdepositional remanent magnetization pdrm has been investigated by two component pdrm acquisition experiments in deviating magnetic fields of different strengths the experiments have been performed on samples consisting of crushed natural magnetite grains in pseudo single domain psd states embedded 0 1 by weight in a slowly curing epoxy araldite the increasing viscosity with time of the epoxy is thought to mimic consolidation processes in sediments which gradually inhibits magnetic grain realignment resulting pdrm directions are partly intermediate between final and initial field directions which are unresolvable by alternating field demagnetization to 60 mt the results suggest that partial pdrm acquired in initial low fields are modified by subsequent exposure to stronger fields this may be due to alignment of grains not affected by the initial field or by remobilization of grains previously aligned in the weaker initial field the suggested field dependent pdrm smoothing may prevent improvement of paleomagnetic records by deconvolution procedures and may imply that longitudinally confined paleomagnetic records in sediments of transitional polar paths during reversals may partly be artifacts of the pdrm recording process

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

periodicities of caco3cycles in the pliocene of sicily discrepancies with the quasi periods of the earth s orbital cycles a high resolution caco3 record in combination with a detailed magnetostratigraphy is presented for the rhythmically bedded marly sequence of the pliocene trubi formation on sicily italy magnetostratigraphic age calibration and subsequent spectral analysis of the variatrions in caco3 content yield a remarkably consistent discrepancy between the resulting periodicities and the corresponding quasi periods of the earth s orbital cycles this discrepancy can only be explained by assuming that ages assigned to polarity reversal boundaries in the geomagnetic polarity time scale lack sufficient precision a new polarity time scale is presented for the major part of the gilbert and gauss chron using the radiometrically dated gilbert gauss chron boundary at 3 40 ma as an age reference point and extrapolating an average quasi period of 21 7 kyr of the precession cycle as the periodicity of small scale colour annex caco3 cycles

the upper and lower thvera sedimentary geomagnetic reversal records from southern sicily detailed paleomagnetic records of the upper and lower thvera polarity transitions have been determined from pliocenc marine marls in southern sicily the dominant magnetic mineral is fine grained magnetite the two transitions have vgp paths following a great circle passing south america and the west coast of north america there are strong indications that the vgp paths result from smoothing of the non antipodal stable directions before and after the transitions the upper thvera transitional record is preceded by two excursions and followed by a third one the first excursion is a sedimentary artefact caused by post depositional migration of magnetic minerals and the third one by is caused by weathering of the sediment the upper thvera transition from sicily is compared with the record from calabria about 250 km away the two records show similarities as well as differences both transitions have identical vgp paths but in calabria the transition is recorded lower in the sediment the first and third excursions have different characters and the second one is not present apparently the registration of transitions of the earth s magnetic field in sediments is strongly influenced by smoothing and diagenetic processes after deposition

chronostratigraphy of uplifted quaternary hemipelagic deposits from the dodecanese island of rhodes greece an integrated magneto biostratigraphic study based on calcareous nannofossils and foraminifers together with the radiometric dating of a volcaniclastic layer found in several outcrops was carried out on the hemipelagic deposits of the lindos bay formation lbf at six localities on the island of rhodes greece our highly refined chronostratigraphic framework indicates that the lower and upper lithostratigraphic boundaries of the lbf are diachronous associated with the 40ar 39ar age determination of 1 85 0 08 ma for the volcaniclastic layer our data show that among the investigated outcrops the lindos bay type locality section provides the longest record 1 1 ma of the lbf hemipelagic deposition occurred continuously from the late gelasian 1 9 ma to the late calabrian 0 8 ma i e from chrons c2n olduvai to c1r 1r matuyama and from nannofossil zones cnpl7 to cnpl10 this long record together with the hemipelagic nature of the deposits make the lindos bay type locality section a unique element in the eastern mediterranean region allowing future comparisons with other early quaternary deep sea sections available in the central and western mediterranean regions

late devonian carbonate magnetostratigraphy from the oscar and horse spring ranges lennard shelf canning basin western australia a b s t r a c t the late devonian was a time of major evolutionary change encompassing the fifth largest mass extinction the frasnian famennian event in order to establish a chronological framework for global correlation before during and following the frasnian famennian mass extinction we carried out a coupled magnetostratigraphic and biostratigraphic study of two stratigraphic sections in the upper devonian carbonate reef complexes of the lennard shelf in the canning basin western australia magnetostratigraphy from these rocks provides the first high resolution definition of the late devonian magnetic polarity timescale a 581 m reference section and an 82 m overlapping section through the marginal slope facies napier formation of the oscar range as well as a 117 m section at horse spring virgin hills formation were sampled at decimeter to meter scale for magnetostratigraphy conodont biostratigraphy was used to correlate both sections and link magnetostratigraphic polarity zones to a globally established biostratigraphy as table characteristic remanent magnetization chrm with dual polarities ne shallowly upward and sw shallowly downward is recovered from 60 of all samples with magnetite inferred to be the chief magnetic carrier from thermal demagnetization characteristics these directions define a geomagnetic pole at 49 5 s 285 8 e and 95 2 4 n 501 placing the canning basin at 9 9 s during the late devonian consistent with carbonate reef development at this time ac onservative interpretation of the magnetostratigraphy shows the recovery of multiple reversals from both sections not including possible cryptochrons and short duration magnetozones field tests for primary remanence include positive reversal tests and matching magnetozones from an overlapping section in the oscar range as trong correlation was found between magnetic polarity stratigraphies of the oscar range and horse spring sections and we correlate 12 magnetostratigraphic packages the relative stratigraphic thicknesses of the isochronous sediments from these two sections indicate that carbonate accumulation was 4 5 faster in the middle slope deposits at oscar range than in the more distal lower slope horse spring deposits for the middle frasnian through famennian the magnetic field during the late devonian underwent a relatively high reversal frequency with good potential for regional and global correlation and should prove useful in deciphering a high resolution chronostratigraphy across the lennard shelf to enable higher confidence examination of reef development across a major biotic crisis

studies of the arctic ice cover and upper ocean with a coupled ice ocean model an ice ocean model has been developed by coupling the hibler ice model to a three dimensional ocean model that consists of a turbulent mixed layer model and an inverse geostrophic model the coupled model has a horizontal grid spacing of 127 km and has 17 vertical levels extending to the ocean bottom the model is forced with 12 hourly general circulation model derived atmospheric fluxes for the year 1986 for which good quality ice edge analyses and buoy tracks were available for comparisons the results are presented for the fifth year of a repetitive simulation with the 1986 fluxes at which time the system has reached a statistical equilibrium the seasonal and geographical variations of the ice cover and the upper few hundred meters of the ocean have been examined including the heat and salt budgets the computed heat fluxes and mixed layer depths mlds fall in the observed seasonal ranges with winter heat fluxes ranging from 15 w m 2 in the central arctic to about 500 w m 2 in the barents sea area and summer fluxes from about 5 w m 2 under the ice to about 30 w m 2 in the various marginal ice zone miz edge areas the corresponding winter mlds are found to be about 25 75 m in the central arctic to a deep 800 m in the greenland sea typical summer mlds are between 5 and 30 m in all seasons the miz was found to be the center of flux activity for both heat and salt with the processes of advection diffusion atmospheric forcing and vertical oceanic fluxes having their largest values here and of comparable magnitudes values of the heat flux components in the miz exceed those found under the ice by an order of magnitude or more and those in the open water region by a factor of 2 for salt the situation is similar except in the summer june through september when a lot of salt flux activity takes place under the ice comparisons are made with naval polar oceanography center npoc analyses for ice concentration and ice edge and with observed arctic buoy tracks in the same 1986 time period the computed ice edge positions have comparable accuracy to previous three dimensional coupled ice ocean studies with too much ice growth during the winter in the barents sea and too little ice east of greenland the ice thickness distributions however are much better with a monotonic increase of the ice thickness from the siberian coast east toward the canadian archipelago with maximum winter values of about 5 6 m

inconsistent magnetic polarities between greigite and pyrrhotite magnetite bearing marine sediments from the tsailiao chi section southwestern taiwan abstract to establish a magnetobiostratigraphy for a 620 m thick middle to late pleistocene mudstone sequence in the lower gutingkeng formation of the tsailiao chi tlc section in southwestern taiwan we conducted paleomagnetic and mineral magnetic measurements together with sediment granulometry and calcareous nannofossil identification paleomagnetic samples from 65 sites revealed two types of thermal demagnetization 25 400 c behavior 1 single component stable characteristic remanence in magnetite and pyrrhotite dominated samples type s and 2 abrupt changes in polarity when samples with significant greigite concentrations were heated above 320 340 c type c the characteristic polarities derived from type s samples and from magnetite dominated type c samples obtained above 340 c are consistent with those determined from nannofossil biostratigraphy this implies that the nrm carried by magnetite and pyrrhotite is reliable the essentially antiparallel remanence components in type c samples below 340 c are attributed to greigite the almost antiparallel direction could have resulted from delayed formation of greigite but in this case the different direction of this component must have resulted from variable remanence lock in times alternatively the opposite polarities may result from self reversal which warrants further investigation pyrrhotite and greigite may have both formed authigenically but there is no clear explanation for the observed differences in direction

east asia winter monsoon variations on a millennial time scale before the last glacial interglacial cycle grain size and magnetic susceptibility measurements on samples from a typical loess palaeosol sequence on the central chinese loess plateau are used to reconstruct the pleistocene east asian monsoon climate the coarse grained fraction i e the weight percentage 30 m of the bulk grain size distribution is used as a sensitive proxy index of the east asia winter monsoon strength on the basis of an absolute time scale time series variations of this proxy show that winter monsoon strengths varied on millennial time scales during the periods 145 165 240 280 320 350 390 440 600 640 860 890 900 930 and 1330 1400 kyr bp the wavelength of these climatic oscillations varied between 1 89 and 4 0 kyr as is shown by spectral analysis using the multitaper method although numerical simulation experiments show that high frequencies also can arise from measurement errors in the grain size analysis the frequencies prove to be sufficiently stable when the spectral analysis is repeated with a different number of tapers for the time being we do not correlate these climatic oscillations with palaeoclimatic records in the north atlantic deep sea sediments because both time scales need to be further improved our data however certainly demonstrate that millennial scale east asian winter monsoon variations in the last 1 4 million years can be detected from terrestrial loess records copyright 1999 john wiley sons ltd

a negative test of orbital control of geomagnetic reversals and excursions a 41 kyr periodic component has been reported in some sedimentary paleointensity records allowing speculation that there may be some component of orbital control of geomagnetic field generation such as by obliquity modulation however no discernable tendency is found for astronomically dated geomagnetic reversals in the plio pleistocene 0 to 5 3 ma or excursions in the brunhes 0 to 0 78 ma to occur at a consistent amplitude or phase of obliquity cyclicity nor of orbital eccentricity an implication is that paleointensity lows which are characteristically associated with these features are not distributed in a systematic way relative to obliquity and eccentricity supporting the idea that orbital forcing does not power the geodynamo

polarity transitions and excursions of the geomagnetic field the nature of geomagnetic field behavior during polarity transitions is one of the most hotly debated issues in modern geophysics dynamo action in the earth s fluid outer core is known to be responsible for generating the earth s main magnetic field however the mechanism for polarity transition is still unknown polarity transitions are a fundamentally important property of the geomagnetic field but because they have not taken place in historic time it is necessary to turn to the paleomagnetic record to understand the mechanism by which they occur

introduction to the special section on physical and biogeochemical processes responsible for the magnetization of sediments sediment palcomagnetism has made important contributions to magnetostratigraphy tectonic reconstructions and studies of geomagnetic field behavior interpretations of the sedimentary palcomagnetic record depend critically on when and how the natural remanent magnetization nrm is acquired and what postdepositional modifications have occurred this special section of the journal of geophysical research is a collection of 16 papers that address recent advances in our understanding of the physical chemical and biological processes which affect the acquisition and alteration of the nrm in sediments the issue is an outgrowth of a special symposium on magnetization processes in sediments that was convened at the 1987 spring agu meeting in baltimore maryland

rapid changes and near stationarity of the geomagnetic field during a polarity reversal one of the most important suggestions from many of the new reports of sedimentary geomagnetic records is that the time dependence of the changes of the geomagnetic field during a reversal might be more complex than early records indicated indeed alternating periods of rapid and slow field changes are apparent in some sedimentary records1 4 in volcanic records very fast directional changes recorded by individual lava flows during their progressive cooling have been reported5 6 but a major problem when inferring the temporal behaviour of the transitional field is that the extrusion rate of the successive lava flows is much more variable and more difficult to evaluate than the rate of accumulation of the sedimentary sequences here we report the results of a detailed study of a geomagnetic reversal recorded in middle miocene marine clays with temporal resolution almost comparable with that of the volcanic records and in our opinion a more reliable chronological control the record displays large fluctuations in the rate of directional change and different episodes of collapse and recovery of the field intensity a very fast n s directional change may be entirely controlled by a frozen flux dominated reversal mechanism

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

a critical re evaluation of the miocene pliocene boundary as defined in the mediterranean abstract a critical re evaluation of the miocene pliocene m p boundary as defined informally in the mediterranean is presented we show that ages reported earlier for this boundary ranging from 4 81 to 4 93 ma do not imply any diachroneity of the basal zanclean lithohorizon which reflects the pliocene flooding of the mediterranean and defines the m p boundary these ages most likely represent an artefact resulting from intraformational variations in sedimentation rate and inconsistent interpretation of coluor variations in terms of sedimentary cycles the best estimate for the age of this boundary arrives at 4 86 ma if the conventional geomagnetic polarity time scale gpts of berggren et al 1 is used and at 5 16 ma if the new gpts of cande and kent 2 is used an age of 5 32 ma is obtained if the astronomically calibrated time scale of hilgen 3 is adhered to because of its presumably global paleoclimatological significance the m p boundary as currently defined in the mediterranean is preferred to a redefinition of this boundary at the gilbert chron 5 boundary in a continous open marine sequence outside the mediterranean in addition this redefinition would lead to unnecessary confusion in global chronostratigraphy

multicomponent magnetization in paleomagnetic records of reversals from continental sediments in bolivia abstract a paleomagnetic study of geomagnetic reversals has been attempted from three continental sections of the altiplano in bolivia the first section viscachani shows a unique polarity interval with a magnetization carried by magnetite in the two other sections achocalla and irpavi the magnetization is characterized by a complex magnetic mineralogy dominated by magnetic sulphides mostly pyrrhotite and magnetite thermal demagnetization performed under vacuum was found to be much more efficient than in air in isolating the directions of remanence carried by a low temperature lt component due to sulphides and a high temperature ht component due to magnetite the directions of magnetization carried by the two minerals have been compared with particular attention being paid to records of geomagnetic reversals at achocalla the same polarity pattern is revealed by both components however the transitional directions of the lt component are located about 50 cm below the abrupt polarity change recorded by magnetite such observations are very similar to those from a recent study of pliocene marine marls in sicily 1 these characteristics are compatible with the results of a simulation which assumed that a very early acquisition of the remanence was carried by the lt component and that a post depositional reorientation of the magnetite occurred in the last section irpavi the observed lt and ht components have opposite polarities although the magnetic parameters indicate the presence of sulphides we cannot rule out the hypothesis that the lt component has a secondary origin due to coarse magnetite grains if we assume that the hypothesis of sulphides is correct this observation emphasizes the difficulties in establishing the origin of sulphide minerals

the upper kaena sedimentary geomagnetic reversal record from southern sicily a detailed paleomagnetic and rock magnetic record of the upper kaena polarity reversal was determined from late pliocene marine marls in southern sicily two separate characteristic magnetization components were distinguished a low temperature lt component between 200 and 50 c and a high temperature ht component between 500 and 580 610 c the two components record the reversal in a slightly different way but both were acquired in an early stage following deposition most probably by authigenic biogenic formation of magnetic minerals the actual r n reversal via e and horizontal inclinations seems to take 3 3 kyr and is followed by a stable normal interval 3 3 kyr and subsequently by a significant excursion 50 60 in declination 3 6 kyr relative intensities were lower during a time span of 5 6 kyr and show a minimum of 10 of the nontransitional values but during the excursion no intensity changes are apparent the vgps of the ht component are confined to a longitude between india and australia those of the lt component oscillate between this same longitude and one passing over north and south america we conclude that most if riot all observed features are largely determined by the remanence acquisition process which is complicated by authigenic formation of magnetic minerals and cyclically fluctuating paleoredox conditions this has not only resulted in considerable smoothing but also in delayed nrm acquisition in specific sedimentary intervals we believe that the observed excursion is a sedimentary artifact and not related to geomagnetic field behavior during transitions we emphasize that many reported geomagnetic features may in fact be due to the sedimentary remanence acquisition process

the gilbert gauss sedimentary geomagnetic reversal record from southern sicily a detailed paleomagnetic record of the gilbert gauss reversal boundary in deep sea marls from southern sicily shows that the remanence characterising the reversal is carried by a high temperature magnetite component a low temperature component is intermediate between the high temperature component and a present day overprint suggesting that it is obscured by weathering the actual reversal record is clearly affected by filtering caused by early diagenetic acquisition of a chemical remanent magnetisation crm intermediate vgps show a general confinement to a longitude over the americas more specifically they are restricted to two clusters a less distinct one off se south america and a very prominent one near ne north america the remarkable coincidence with two hoffmann clusters hoffmann 1992 suggests that even a crm acquisition filter cannot entirely obscure long lived transitional features

new late paleozoic paleopoles from the donbas foldbelt ukraine implications for the pangea a vs b controversy article i nfo the carboniferous to early permian apparent polar wander apw path for eurasia is not well constrained because of the paucity of reliable paleomagnetic poles this is at least partly responsible for the pangea a vs b controversy in the early permian is the overlap between the northern and southern continents during the early permian caused by a lack of reliable paleomagnetic data pangea a or must a large displacement along a mega shear zone be invoked pangea b here we present results from six paleomagnetic sampling sites ranging in age from the early carboniferous to the early permian from sedimentary rocks in the donbas foldbelt ukraine to improve the carboniferous early permian apw path for eurasia and to contribute to solving the pangea a vs b controversy six time intervals were sampled in the donbas foldbelt eastern ukraine which was filled with sediments and volcanic units during the late devonian to permian syn and post rift subsidence phases we present results from sediments that were corrected for inclination shallowing with the elongation inclination e i method we conclude that there is a general northward movement of the donbas foldbelt the resulting paleolatitudes are slightly but generally significantly higher than expected from existing apw paths the late carboniferous to early permian data provide three new reliable paleopoles for eurasia the early permian pole does not necessarily require a pangea b reconstruction it results in higher paleolatitudes for laurussia in the early permian and removes the overlap between gondwana and pangea we also reconstructed the position of laurussia based on carboniferous laurentian poles recently corrected for inclination shallowing which clearly favours a pangea b configuration it seems that the pangea a vs b debate is as lively as before the three early carboniferous paleopoles give reliable paleolatitudes but declinations significantly deviate from the expected directions we argue that the southernmost part of the donbas foldbelt underwent a counterclockwise rotation related to mesozoic compressional events that are recognised in paleostress analyses

preliminary results of a study of four successive sedimentary geomagnetic reversal records from the mediterranean upper thvera lower and upper sidufjall and lower nunivak abstract the results of a study of four successive early pliocene geomagnetic reversal records are presented the polarity transitions have been recorded in the trubi formation of calabria s italy which is composed of alternating carbonate poor and carbonate rich marly layers with moderately high 5 cm ka 1 accumulation rates the multicomponent nrm contains a well defined primary component in which two dominant magnetic phases can be recognized a magnetite and a low temperature lt i e removed at moderate temperatures phase most probably the lt phase is pyrrhotite the combined presence of pyrrhotite and magnetite in the sediment restricts the validity of a xarm x test for paleointensity studies the magnetite magnetizations reveal a smoother record relative to the records of the lt phase magnetizations between the two phases no significant delay in the moment of remanence acquisition is observed the durations of the major directional changes for the reversals are estimated to be in the range 0 3 3 0 ka the vgp path of the upper thvera record follows a great circle 90 w of the site meridian indicating a clearly non zonal transitional field whereas the vgp paths of the other reversal records are close to the site meridian indicating a mainly zonal transitional field the lower sidufjall and lower nunivak records are nearly identical and have a zonal harmonic content similar to records reported for the matuyama brunhes polarity transition

counter clockwise rotations in the southern apennines during the pleistocene paleomagnetic evidence from the matera area sediments from late pliocene to early pleistocene age have been sampled at six different localities sections 38 sites in the matera area which is in the southern part of peninsular italy basilicata three of the localities are from the apenninic belt the other three from the bradano unit both of them west of the earlier sampled apulian unit stepwise thermal demagnetisation revealed that usually two important components contribute to the natural remanent magnetisation nrm of these clay sediments a low temperature lt component removed between 100 and 300 320 c and a high temperature ht component removed between 300 320 and 430 450 c the ht component is a characteristic remanent magnetisation chrm for the sediments and has a clear primary pre folding origin in fresh unweathered sediments the lt component is identical in direction to the ht component but there is evidence that moderate weathering replaced a high intensity primary lt component with a low intensity secondary lt magnetisation the ht magnetisation is affected by this weathering process in a decrease of its intensity a counter clockwise deflection in declination is observed for the sites sampled in the apenninic belt the ht directions reveal a 23 counter clockwise rotation relative to the magnetic north dm 336 6 and im 57 2 sites in the bradano unit have no tectonic rotation dm 359 1 and im 56 3 earlier sampled sites from the apulian unit also showed no tectonic rotation dm 359 4 and im 57 0 paleomagnetic data support the hypothesis that the southern apennines rotated in the middle late pleistocene after the major phase of extension in the southern part of the tyrrhenian basin and contemporaneous with a regional compressional phase

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

misleading positions of geomagnetic reversal boundaries in eurasian loess and implications for correlation between continental and marine sedimentary sequences abstract the matuyama brunhes boundary mbb where the earth s magnetic field changed to its present orientation is one of the most frequently used time markers in continental quaternary stratigraphy although magnetic signals have been successfully recovered from loess deposits and an apparently coherent magnetostratigraphy seems to be in place a close examination of the published loess records from northern mid latitudes reveals significant uncertainty and variability for the position of the mbb in asia and part of central europe the mbb has been found in loess a glacial deposit this is in contrast to the well established palaeomagnetic records in deep sea sediments where the mbb is located in the interglacial oxygen isotope stage 19 this discrepancy has caused serious confusion in stratigraphic correlation between loess and marine sequences here we examine the causes of the discrepancy between the two sedimentary environments in their records of the mbb our approach includes a comparison of the mbb in loess and other continental records and an evaluation of the stratigraphic significance of the microtektites found around the mbb based on the occurrence of the mbb in an interglacial stage in lake and continental margin sequences and the correlative stratigraphic positions of the microtektites in marine sediments and loess we demonstrate that the measured positions of the mbb in loess are erroneous and misleading and its true occurrence was during the time of formation of the palaeosol corresponding to oxygen isotope stage 19 the most plausible cause for the apparent offset i e the mbb being found in the underlying loess instead is that the acquisition of remanent magnetisation in loess occurs at a certain depth substantially below the land surface the degree of such displacement is dependent on the lithology and structure of the deposits formed prior to the reversal and the time delay is estimated to be in the order of 103 104 years we show that other geomagnetic boundaries such as those of jaramillo and cobb mountain in loess have been similarly displaced though to a varying extent we suggest that the remanence acquisition in loess is closely related to the post depositional evolution in the structure of loessic materials which is in part influenced by regional climate conditions our conclusions on the displaced remanence acquisition have implications for loess chronology magnetostratigraphy as well as the geomagnetic interpretation of detailed magnetic signatures from loess

magnetite diagenesis in marine sediments from the oregon continental margin the magnetochemistry of sediments from the oregon continental margin is examined to determine the effects of iron sulfur diagenesis on the paleomagnetic record magnetic mineral dissolution and transformation into iron sulfides are a common feature in these suboxic to anoxic lutites these processes are evidenced in rapid decreases in natural remanent magnetization intensities and stabilities systematic changes in other rock magnetic properties and increases in solid phase sulfur concentrations with depth hysteresis measurements are used to evaluate downcore changes in magnetite concentration and grain size magnetite abundances decrease downcore from initial values of about 0 1 and nominal grain diameters lie within a narrow pseudosingle domain range of 0 08 to 0 6 m a first order surface area reaction model da dt ka is proposed to explain the magnetite dissolution mechanism where a is the total magnetite surface area and k is the rate constant the solution of this equation predicts that the surface area and concentration decrease exponentially and the concentration in addition depends on grain size application of this model in two cores where grain size varies with depth successfully explains the downcore profiles of both concentration and surface area despite extensive magnetite destruction magnetic directions in such sediments appear to reliably record long wavelength trends of the geomagnetic field

evidence for a change in the periodicity of tropical climate cycles at 2 4 myr from whole core magnetic susceptibility measurements the earth s orbit about the sun is influenced by gravitational interactions with the moon and other planets the resulting orbital perturbations give rise to cyclical variations in orbital eccentricity obliquity and precession with periods of 100 41 19 and 23 kyr respectively1 these variations are climatically important because they affect the seasonal and latitudinal distribution of solar radiation magnetic susceptibility measurements in deep sea sediment cores can be used as a sensitive indicator of temporal variations in the concentration of terrigenous material supplied to the sea bed here we use this approach to study the evolution of cycles of terrigenous sedimentation at ocean drilling program sites in the arabian sea and the eastern tropical atlantic we show from spectral analyses of the two susceptibility time series spanning the past 3 2 3 5 myr that the records are driven strongly by orbital forcing before 2 4 myr before present bp both records are strongly influenced by the 23 and 19 kyr periodicities but after 2 4 myr they are both significantly influenced by the 41 kyr periodicity this shift coincides with the beginning of major northern hemisphere glaciation which indicates for the indian ocean site that the supply of terrigenous sediment carried by monsoonal winds has responded to the rapid increase in ice cover at this time we cannot however exclude possible effects of changes in bottom water circulation on the susceptibility record of the atlantic site

effects of diagenesis on magnetic mineralogy in a jurassic claystone limestone succession from the paris basin we performed detailed rock magnetic and geochemical analyses on a previously published early jurassic magnetostratigraphic section the results improve our understanding of acquisition and preservation processes of magnetization in a series of alternating claystones and limestones the main carrier of magnetization is magnetite anhysteretic remanent magnetization arm varies by a factor of 40 and the arm variations are linked to magnetite grain size comparison of magnetic and geochemical data shows that when carbonate content is high 30 and 13 c0 magnetite is characterized by small grain sizes whereas when carbonate content is low 20 magnetite is coarse grained it appears that the oxidation of organic matter by sulfate reduction controls both 13 c and magnetite grain size h 2 s produced during sulfate reduction causes partial dissolution of magnetite grains with the finest magnetite grains those that best record the magnetic signal being dissolved first despite this partial dissolution both the direction and polarity of the original remanent magnetization are preserved

the last two geomagnetic polarity reversals recorded in high deposition rate sediment drifts high resolution records of geomagnetic polarity transitions are rare owing to the difficulty of preserving in the geological record details of a process with a duration of only a few thousand years lavas can record the instantaneous field on cooling but lava records are themselves discontinuous and therefore polarity transition fields are seldom captured sedimentary records are more continuous but typical deep sea sedimentation rates of a few cm kyr 1are too low to record transition fields adequately here we report transition records from north atlantic drift deposits with high sedimentation rates 12 cm kyr 1 at the ocean drilling program sites 983 and 984 virtual geomagnetic polar vgp paths are similar within and between sites for the last two geomagnetic reversals attesting to a memory effect spanning back to back reversals the complex vgp paths feature two vgp clusters one located in northeastern asia and the other in the south atlantic ocean off south america and provide a link between the longitudinally constrained transitional vgps often recorded in sediments1 2 which represent highly smoothed renditions of transition fields and the scattered transitional vgps3 and vgp clusters4 observed in lava records

a detailed palaeomagnetic record for the last interglacial period abstract a palaeomagnetic investigation of a late quaternary loess palaeosol sequence in central china has revealed two episodes of anomalous geomagnetic field behaviour in the lower and upper parts of soil complex s1 which spans the last interglacial interval comparison of the magnetic susceptibility record with the deep sea oxygen isotope stratigraphy provides a chronology which has been used to estimate the age and duration of these episodes the age of the younger episode has been estimated at 89 75 ka comparable with the age of a geomagnetic event recognised from sediments in the norwegian greenland sea and arctic ocean the older episode is associated with a broad low in relative palaeointensity which has been assigned to the time interval 130 110 ka this is consistent with existing age estimates of the so called blake event in contrast to a published record of the blake event from the loess at xining which shows three short episodes of almost full reversed polarity in the current record the blake event is represented by a loop in the vgp path which passes through shallow southern latitudes although both episodes occur within the same part of soil complex s1 the difference between the two records implies a greater degree of smoothing of the geomagnetic signal in the huanxian record this is probably a reflection of the lower accumulation rate at huanxian and may also indicate a greater time constant for the remanence aquisition at this site

reversal records in marine marls and delayed acquisition of remanent magnetization records of geomagnetic reversals frozen into the magnetic components of sediments provide a means to study the time dependent behaviour of the geomagnetic field during polarity transitions although sedimentary records have the advantage of being readily available and continuous the process by which they acquire a remanent magnetization is still not fully understood1 magnetites which lose their magnetization at relatively high temperatures 580 c are generally considered to carry the primary remanence of the sediment that is to record the palaeomagnetic direction here we describe two reversed to normal transitional records from marine marls in which this high temperature component shows a delayed remanence acquisition relative to a lower temperature component in these samples therefore the high temperature component does not reflect geo magnetic changes during the reversal at least for marine marls detailed palaeomagnetic rock magnetic and geochemical studies are apparently necessary to judge the validity of reversal records

fidelity and time resolution of the magnetostratigraphic record in siwalik sediments high resolution study of a complete polarity transition and evidence for cryptochrons in a miocene fluviatile section summary the surai khola section in southwest nepal a 5000 m continuously exposed record of fluvial sedimentation since middle miocene was revisited for high resolution magnetostratigraphy in sequences with expected cryptochrons and reversals of the geomagnetic field polarity intervals with durations of a few tens of thousands of years are recorded as zones of stable palaeomagnetic directions polarity transitions are recorded as zones with complex demagnetization behaviour of specimens in the sedimentary column almost antiparallel palaeoremanence directions residing in diverent haematite phases in the same specimens could generally not be separated properly by thermal demagnetization divering demagnetization paths for neighbouring specimens during a reversal suggest that measured transitional directions are not true geomagnetic field directions but rather are generated by the superposition of variable amounts of at least two almost antiparallel components of magnetization accompanying studies of recent river sand deposits demonstrate that these sediments acquire a true depositional remanent magnetization drm with considerable inclination errors and scattered directions for individual specimens

correlation of paleointensity variation records in the brunhes matuyama polarity transition interval abstract paleomagnetic records of the 0 78 ma brunhes matuyama b m polarity reversal interval described from high resolution 8 11 cm kyr deep sea sediments from western equatorial pacific odp sites 767b and 769a show two successive and marked decreases in paleointensity of about 15 kyr wavelength each and a shift from full reversed to fuli normal polarity directions in only 2 kyr associated with the uppermost paleointensity decrease in contrast transition records from deep sea sediments with lower sedimentation rates tend to show simpler paleointensity variation with more complex patterns of directional change for example the b m transition record described from deep sea sediment core v16 58 from the southern indian ocean we suggest that the twin 10 be peaks reported in v16 58 correspond to increased production of this cosmogenic isotope associated with the double paleointensity dips recorded in odp 767b 769a the low resolution v16 58 paleomagnetic record can be accounted for by modeling of the geomagnetic field history in odp 767b at a nominal recording rate of 1 cm kyr and using an exponential lock in process filter with a half fixing depth of 16 cm the double paleointensity dips become a single broad magnetization low and the change in paleomagnetic directions is offset by about 16 cm below the actual level of the polarity transition similar to what we infer for v16 58 the low resultant magnetizations will be especially prone to overprinting which if in the present field will tend to give an americas vgp path the main features of the 10 be profile will be unaffected by the remanence lock in process and consequently retain their usefulness for correlation as a proxy of geomagnetic dipole intensity variation

how the geomagnetic field vector reverses polarity a highly detailed record of both the direction and intensity of the earth s magnetic field as it reverses has been obtained from a miocene volcanic sequence the transitional field is low in intensity and is typically non axisymmetric geomagnetic impulses corresponding to astonishingly high rates of change of the field sometimes occur suggesting that liquid velocity within the earth s core increases during geomagnetic reversals

astronomical dating of the xiantai donggutuo and maliang paleolithic sites in the nihewan basin north china and implications for early human evolution in east asia abstract magnetostratigraphic studies have established a first order chronological framework for the paleolithic sites in the nihewan basin north china which enabled tracking early human evolution in east asia however to fully understand how well early humans were adapted to climate change a truly precise dating of the paleolithic sites is required here we established a high resolution astronomical timescale for the xiantai and donggutuo fluvio lacustrine successions at the eastern margin of the nihewan basin employing low field magnetic susceptibility as a climatic indicator aiming to further refine the ages of the xiantai donggutuo and maliang paleolithic sites starting from an initial age model constrained by geomagnetic reversals larger scale cycles were firstly tuned to orbital obliquity using an automatic orbital tuning method this first order tuning was followed by simultaneously tuning to both obliquity and precession the finally tuned records can be correlated almost cycle by cycle with the quartz grain size record of the chinese loess sequence and the marine 18o record the astronomically estimated age of the xiantai paleolithic site is ca 1 48 ma corresponding to paleosol layer s20 of the chinese loess sequences or marine oxygen isotope stage mis 49 an interglacial period the astronomical estimate for the donggutuo paleolithic site ranges from 1 06 ma to 1 12 ma corresponding to paleosol loess layers s11 s12 or mis 31 33 spanning both interglacial and glacial periods the astronomically estimated age of the maliang paleolithic site is 0 79 ma corresponding to loess layer l8 or mis 20 a glacial period this astronomical finding further implies that early humans may have permanently occupied china as far north as 40on since at least 1 1 ma and before this time the occupation may be intermittent

sedimentary cycles and volcanic ash beds in the lower pliocene lacustrine succession of ptolemais nw greece discrepancy between 40ar 39ar and astronomical ages a high resolution cyclostratigraphy for the rhythmically bedded lignite marl sequences of the lower pliocene ptolemais formation is combined with 40 ar 39 ar dating results of intercalated volcanic ash beds detailed field reconnaissance in three open pit lignite mines reveals three end member sediment types lignites composed primarily of organic material grey marls a mixture of carbonate and organic material and beige marls almost exclusively composed of carbonate these lithologies are arranged in two basic types of sedimentary cycles lignite grey marl and lignite beige marl cycles a cyclostratigraphic composite section comprising 56 lignite marl cycles is constructed which combines the consistent cycle patterns from three parallel sections the concordant positions of 20 volcanic ash beds in these sections confirm the cyclostratigraphic correlations and indicate that the lignite marl cycles result from regional basin wide forcing rather than lateral facies migrations 40 ar 39 ar ages on sanidine and biotite separates from nine volcanic ash beds were obtained by multiple total fusion and incremental heating experiments the 40 ar 39 ar ages range between 5 00 0 05 and 4 04 0 04 ma and are in general consistent with the stratigraphic order a least square linear regression using the measured 40 ar 39 ar ages gives an average duration of 21 8 0 8 kyr per lignite marl cycle evidently the lignite marl cycles in the ptolemais formation are linked to the precessional variation in the earth s orbit through its influence on mediterranean climate for the first time 40 ar 39 ar dating results totally independent from any other dating and or tuning technique confirm the astronomical theory of climate change the 40 ar 39 ar ages of the volcanic ash beds show a constant200 kyr 4 5 age discrepancy with the astronomical ages of the same ash beds this inconsistency remains difficult to explain the discrepancy is unlikely to have resulted from erroneous astronomical ages through incorrectness in the astronomical tuning inaccuracies of the magnetostratigraphic data or the orbital time series used and or errors in the apts the 40 ar 39 ar dating results neither give clear indications for a possible source of error from the excellent data set it is evident that neither loss of radiogenic 40 ar nor an underestimation of the contribution of ca and k derived ar isotopes could have caused the discrepancy moreover the discrepancy is also beyond the errors in the systematic variables like the decay constants of 40 k or the ages for the neutron fluence monitors 1999 elsevier science b v all rights reserved

magnetostratigraphic and cyclostratigraphic calibration of a prospective palaeocene eocene stratotype at zumaia basque basin northern spain detailed magnetostratigraphic work at the palaeocene eocene boundary interval in the zumaia section has led to a precise delineation of the position of polarity chron c25n from 35 0 m or 35 7 m to 25 2 m below the base of the carbon isotopic excursion that marks the palaeocene eocene boundary this chronostratigraphic constraint suggests that the lithological carbonate marl couplets making up most of the studied section are the expression of orbital precession cycles and confirms the expanded and continuous nature of the section with mean sediment accumulation rates of 2 5 cm kyr 1 the new data render zumaia a unique land based marine section with available magnetostratigraphic data coupled to cyclostratigraphy the revised chronostratigraphic framework presented supports previous cyclostratigraphic calibration of the latest palaeocene geomagnetic timescale it should facilitate better assessment of the different chemobiostratigraphic events that span this critical interval as a consequence of past climate and oceanographic change consequently the zumaia section is proposed as a candidate global stratotype section and point gssp for the palaeocene eocene boundary

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

biogenic magnetite and the magnetization of sediments biogenic magnetites are produced through the reduction of ferric iron by both biologically induced extracellular and biologically controlled intracellular processes with few exceptions all are ultra fine grained single domain magnetite biogenic magnetites formed by magnetotactic bacteria biologically controlled have been shown to contribute significantly to the natural remanent magnetization of carbonates and limestones hemipelagic and deep sea marine sediments the input into sediments of ultra fine grained magnetite produced by dissimilatory iron reducing bacteria biologically induced has yet to be firmly established but may be even more significant whether either type of authigenic biomagnetite is preserved is determined by postdepositional factors including oxidation reduction by substitution and dissolution unconsolidated and lithified sediments can be screened for putative biogenic magnetite by rock magnetic techniques it is not generally appreciated that magnetotactic bacteria and their magnetofossils can be identified by the unusual and in some cases unique morphologies size range and composition of the magnetite crystals magnetite produced by dissimilatory iron reducing bacteria have a distinctive morphology and size range but it is currently controversial as to whether these can be distinguished from certain chemically precipitated magnetites the presence of dissimilatory iron reducing bacteria however can be detected using microbiological techniques and sediment geochemistry biogenic magnetites are trace fossils and potentially useful environmental indicators and are considered to have significant input to the magnetization of most sediments both modern and ancient

the age of the miocene pliocene boundary in the capo rossello area sicily detailed correlations of magnetostratigraphy biostratigraphy and lithostratigraphy reveal that the basal pliocene is equally complete in the eraclea minoa and capo rossello sections sicily and the singa section calabria and that in accordance with the model of the pliocene flooding event in the mediterranean the deposition of the pelagic marls of the trubi formation started synchronously on sicily and in adjacent calabria in addition the data obtained from the trubi in the eraclea minoa section allows the age of the miocene pliocene boundary to be adjusted slightly from 4 83 4 84 1 to 4 86 ma because downward extrapolation of both sedimentation rate and average duration of small scale sedimentary cycles in the trubi yields this age for the boundary in this section linearly interpolated ages for the top of the sphaeroidinellopsis acme and the first substantial increase in globorotalia margaritae the fod of this species is non existent in the mediterranean pliocene at eraclea minoa arrive at 4 74 and 4 63 ma respectively because of the detailed magnetostratigraphy and the very accurate dating of the miocene pliocene boundary it is preferable to select the eraclea minoa section as the boundary stratotype rather than the capo rossello section finally this age of 4 86 ma for the miocene pliocene boundary suggests that the beginning of the pliocene is connected with the termination of a series of latest miocene glaciations and that the re establishment of open marine conditions in the mediterranean might be of glacio eustatic origin

a reassessment of post depositional remanent magnetism preliminary experiments with natural sediments abstract realignment of magnetic grains below the sediment water interface is thought to impart a post depositional remanent magnetization pdrm however there is little convincing evidence in the published literature that pdrm is the dominant mechanism by which sediments become magnetized we report here preliminary results from two kinds of laboratory experiments designed to investigate whether post depositional reorientation of magnetic particles is likely to occur in nature in the first experiment we monitored changes in the magnetization of natural sediments in response to changing laboratory fields our results are inconsistent with post depositional reorientation of magnetic particles in a second experiment we put live worms in a multi core tube with the original sediment water interface intact remagnetization was only observed in samples taken from a mound of fecal pellets formed at the surface these pellets had been suspended by a worm and redeposited in the laboratory field the other samples which were not resuspended but nonetheless bioturbated by the worms showed no change in magnetization our preliminary results do not support the hypothesis that post depositional reorientation occurs in natural undisturbed sediments below the sediment water interface

the rotation of iberia during the aptian and the opening of the bay of biscay abstract the cretaceous rotation of the iberian plate in a geodynamical active setting between africa and europe is contemporaneous with the opening of the northern atlantic ocean and provides fundamental temporal and kinematical constraints on the evolution of the bay of biscay and the pyrenees here we report new paleomagnetic data from 50 sites 1109 drilled cores from marine sediments of the cretaceous organya basin in the southern pyrenees to constrain the timing and magnitude of the rotation of iberia berriasian to barremian directions in exclusively limestone lithology are remagnetised in agreement with previous organya basin studies the aptian to cenomanian marls and limestones in the organya basin however provide primary magnetisations asserted by rock magnetic experiments and positive fold tests our results constrain the rotation of iberia to be finished just before the albian paleomagnetic data cannot well constrain the onset of the rotation but sea floor anomaly studies restrict the onset of the rotation to anomaly m0 or the barremian aptian boundary this implies that the entire rotation of iberia happened in one single phase and is confined to the aptian this is in contrast to earlier studies that suggested a two phase rotation of iberia but we argue that the large errors in published ages are not in disagreement with our results in summary we constrain the 35 rotation of iberia to the aptian period which implies a rotation rate of 2 7 5 1 myr depending on the controversial duration of the aptian

magnetostratigraphy based astronomical tuning of the early pliocene lacustrine sediments of ptolemais nw greece and bed to bed correlation with the marine record continental deposits from the early pliocene lacustrine ptolemais basin in nw greece display rhythmical alternations of lignite and marl beds three parallel sections from this area are studied using magnetostratigraphy and cyclostratigraphy the presence of the greater part of the gilbert chron enables the recognition of astronomical periodicities in the succession especially the precessional influence is evident as it determines the lithological cycles the continental ptolemais composite section is correlated to the most recent astronomical time scale and thus to the marine reference section the rossello composite from sicily c g langereis f j hilgen the rossello composite a mediterranean and global reference section for the early to early late pliocene earth planet sci lett 104 1991 211 225 on a bed to bed scale it is concluded that lignite corresponds to an insolation minimum beige layer in the rossello composite and marl to an insolation maximum grey layer in the rossello composite this implies a precipitation increase during insolation maxima in early pliocene continental greece 1998 elsevier science b v all rights reserved

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

diagenetic formation of ferrimagnetic iron sulphide minerals in rapidly deposited marine sediments south island new zealand abstract detailed magnetostratigraphic studies of late neogene siliciclastic sediments of the awatere group south island new zealand 41 45 s 174 05 e have revealed a wide range of palaeomagnetic behaviour examination of rock magnetic properties was undertaken using conventional palaeomagnetic techniques and thermomagnetic x ray diffraction and electron microprobe analyses these analyses indicate that the ferrimagnetic iron sulphide minerals greigite and pyrrhotite are responsible for a stable and intense magnetic remanence in fine grained sediments whereas titanomagnetite is the only remanence bearing mineral identified in coarser grained sediments which are less strongly and less stably magnetised than the fine grained sediments detrital titanomagnetite grains are likely to have undergone dissolution during early diagenesis as a result of iron sulphide formation which occurs commonly in rapidly deposited anoxic sediments that support active sulphate reduction and h 2 s formation preservation of greigite and pyrrhotite is inferred to result from the arrest of the pyritisation process probably due to the low permeability of the fine grained sediments and consumption of available h 2 s before full reaction to pyrite occurred relative palaeomagnetic instability and weak remanence intensities in coarser grained sediments is likely to be due to low titanomagnetite concentrations resulting from titanomagnetite dissolution

a 210 000 year record of barium variability in the deep northwest atlantic ocean the deep ocean is the storehouse for most of the carbon and nutrients in the ocean atmosphere system together with its out crops at high latitudes the deep ocean is of great importance in driving glacial interglacial changes in the carbon dioxide content of the atmosphere1 3 the chemical and physical structure of the deep glacial oceans can be reconstructed by means of palaeochemical tracers for example the cadmium and carbon isotope contents of benthic foraminifera have been used to reconstruct the phosphate or labile nutrient compositions of deep waters4 14 recently it was proposed15 that barium in benthic foraminifera could be used to reconstruct the distribution of refractory deep regenerated chemical properties in the water masses of the glacial oceans a reconstruction of barium concentra tions in the oceans at the time of the last glacial maximum revealed the greatest changes in the north atlantic where barium concentrations in deep water were up to 50 higher than at present16 here we present a record of the benthic ba ca ratio in the deep northwest atlantic stretching back to interglacial stage 7 210 kyr ago encompassing two full glacial cycles a comparison of the barium record with that of the other nutrient like tracers lends confidence in the general agreement between these tracers while showing some spectral variability unique to barium thus underscoring its utility as an additional palaeoceanographic tracer

the rossello composite a mediterranean and global reference section for the early to early late pliocene a high resolution magnetostratigraphy planktonic foraminiferal biostratigraphy and cyclostratigraphy is presented for the pliocene trubi marls in the punta di maiata section on sicily the integrated stratigraphy of the rossello composite section of hilgen 1 newslett stratigr 17 1987 is thereby completed this composite section provides an unprecedented high quality reference section for the early to early late pliocene containing a continuous sequence ranging from below the thvera subchron into the matuyama chron 4 86 2 45 ma the punta di maiata section extends from the sidufjall subchron of the gilbert into the gauss chron 4 50 3 30 ma linear interpolation between paleomagnetic datum planes in this section yields first order age estimates of 3 72 0 01 and 3 59 0 01 ma for the last common occurrence lco and the actual last occurrence lo of globorotalia margaritae in addition this age of 3 59 ma provides an accurate age for the zanclean piacenzian z p boundary provided the lo of g margaritae is maintained as a criterion to define this boundary in the mediterranean irrespective however of the criterion used the punta di maiata and punta piccola subsections of the rossello composite are at present the most suitable sections to be designated as stratotypes for the z p boundary the global significance of the rossello composite is further strongly enhanced by the establishment of an astronomically calibrated geomagnetic polarity time scale based on the correlation of the trubi sedimentary cycles with the astronomical record

two types of chemical remanent magnetization during the oxidation of magnetite abstract the chemical remanent magnetization crm m crm resulting from oxidation of equidimensional magnetite to a mixture of 90 hematite and 10 cation deficient magnetite is jointly controlled by the initial anhysteretic remanent magnetization m arm of the parent magnetite and the field h crm applied during oxidation m crm lies in the plane defined by h crm and m arm which were perpendicular in our experiments in an intermediate orientation determined by the relative strengths of h crm 0 05 0 1 and 0 2 mt and m arm 0 002 0 12 saturation remanence the crms are univectorial remanences that remain fixed in these stable intermediate directions throughout the course of stepwise alternating field and thermal demagnetization similar experiments in which acicular single domain cation deficient magnetite was oxidized to maghemite gave quite different results m crm was always parallel to m arm its direction was unaffected by field strengths as high as h crm 1 5 mt the essential factors controlling crm direction seem to be the number and crystallographic structure of oxidation products for single phase oxidation without change in crystal lattice crm remains parallel to the initial remanence of the parent mineral for multiphase oxidation to a mixture of rhombohedral and spinel phases both initial remanence and the field acting during oxidation influence the crm direction which therefore has an intermediate orientation that parallels neither m arm nor h crm

an attempt at reconstructing the geomagnetic field at the core mantle boundary during the upper olduvai polarity transition 1 66 myear abstract the palaeomagnetic directions of five published records obtained from sedimentary sections from various parts of the world were used to calculate maps of the radial field at the core mantle boundary cmb during the upper olduvai polarity reversal dated at 1 66 myear the five records were synchronized assuming that major changes in field direction occurred simultaneously at the five sites at the beginning and the end of the polarity transition low order coefficients of the field were determined using the hypothesis that the magnetic energy remained constant during the transition maps of the radial field at the cmb were calculated assuming a perfectly insulating mantle the results of this mapping exercise show a non axisymmetric field at the cmb in the first and last stages of the transition with strong flux moving in the longitudinal bands evidenced as preferred ones for transitional virtual geomagnetic poles vgp paths maps calculated at mid reversal do not show a simple geometry more palaeomagnetic records and more refined modelling techniques are desirable however the results are consistent with the idea that the transitional geodynamo is controlled by the thermal structure of the lower mantle

rock magnetic evidence for the dissolution and authigenic growth of magnetic minerals within anoxic marine sediments of the california continental borderland the rock magnetic properties of marine sediments from the california continental borderland san pedro santa catalina and san nicolas basins have been studied in order to quantitatively assess the effects of sediment diagenesis on magnetic minerals previous studies have noted that the natural remanent magnetization in these sediments primarily carried by detrital magnetite decays to 10 or less of its surface value soon after deposition this decrease is caused by magnetite dissolution related to sediment diagenesis and is unrelated to paleoclimatic variations or changes in the regional influx of detrital magnetic material detailed rock magnetic measurements show that shifts to softer remanent coercivity and differences in the rate and degree of magnetic intensity loss with depth can be related to the dissolution process the shift to softer remanent coercivity is related to a coarsening of the magnetic mineral grain sizes with depth due to preferential dissolution of the finest grained magnetic material the intensity decreases which are linearly proportional to magnetite concentration decreases indicate that dissolution occurs with rate constants ranging from 0 3 to 1 6 kyr 1 the rate constants sulfide concentrations and magnetite grain size estimates from the borderland are consistant with previous studies of magnetite dissolution our results demonstrate the importance of both sulfide and magnetite surface area in the dissolution process anomalous peaks in viscous remanence within the sediments suggest the authigenic growth of greigite and its subsequent transformation to pyrite

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

overprinting of natural magnetic remanence in lake sediments by a subsequent high intensity field there has been considerable debate recently on how well sedimentary rocks record transitional field directions during a polarity reversal1 5 this has not been an easy question to answer because the input geomagnetic signal for a given sedimentary record is not known independently and because the process by which remanent magnetization is acquired by sediments is difficult to duplicate in the laboratory the geomagnetic excursion recorded by sediments outcropping around the shores of mono lake california6 8 offers an unusually good opportunity to examine this question we report here a study of the mono lake excursion mle at a new locality on the southeast shore where although the records satisfy usually applied standards of palaeomagnetic quality it is clear from comparison with records from other localities that the field directions during a time of low field intensity have not been faithfully preserved the distinctive directions of natural remanence in the discrepant part of the record show that the southeast shore sediments were overprinted by the ensuing higher intensity field acting on the still unconsolidated sediment apparently by realigning some magnetic grains that had previously been locked in

diagenesis and remanence acquisition in the lower pliocene trubi marls at punta di maiata southern sicily palaeomagnetic and rock magnetic observations three new profiles lcm pmd and cmd through the lower cochiti polarity zone enable comparison of the effects of weathering and diagenesis on the magnetic properties of the early pliocene trubi marls these marls have been well dated by means of astronomical calibration and are of particular interest in terms of polarity zonation and regional tectonics the freshest exposures lcm profile carry a remanence associated with pyrrhotite which is particularly enhanced in some grey layers this is the first time that occurence of pyrrhotite as the mam remanence carrier in the trubi marls is documented sd magnetite was previously recognised as the fundamental ferromagnetic phase the mean chrm direction from the lcm record does not show the expected 30 35 clockwise rotation found in other studies and does not record the normal cochiti interval either i e is reverse throughout the sampled interval suggesting a relatively late origin for the nrm standard rock magnetic techniques are used to investigate the nature of the magnetization in the three laterally equivalent profiles at punta di maiata the actual data set seems to be better explained by a complex diagenetic history involving a multistage redox history that invokes a late anoxia event the previously postulated iron migration model van hoof et al 1993 was put forward for the suboxic situation and does not take into account the presence of magnetic sulfides which are clearly demonstrated in the zone under study the extended diagenetic model inferred here proposes a pathway that accounts for most of the new observations the salient feature is the postulated existence of a diagenetic phase that has altered the original nrm in some areas dissolution of magnetite resulting in a later stage acquisition of chemical remanence that does not date to the timing of deposition of the sediments the extension and the detailed mechanisms by which such a process has taken part deserve further research nonetheless the present investigation accounts for previously unrecognised features in the trubi marls from sicily and outlines how intricate the nature of the mechanisms contributing to the blocking of remanence in sediments can be

analysis of nrm directions from the rossello composite implications for tectonic rotations of the caltanissetta basin sicily abstract the paleomagnetic results of the pliocene marine marls from the rossello composite section 30 in the caltanissetta basin of sicily have been analysed to determine the amount and timing of tectonic rotations the analysis shows that there is a consistent inclination error for the nrm component which is removed between 500 and 600 c which may be caused by compaction of the sediment differences in inclination error throughout the composite section are related to differing carbonate content a higher carbonate content implying less compaction and smaller inclination error furthermore consistent differences are found between directions of normal and reversed polarities as well as between directions of the lower and upper part of the composite section these differences are to a large extent related to differences in carbonate content porosity and influence of weathering the caltanissetta basin shows a total clockwise rotation of 34 occurring since the early pliocene the magnetostratigraphy of the rossello composite section shows that from the miocene pliocene boundary 5 2 ma up to the gilbert gauss boundary 3 6 ma there were no tectonic rotations of the caltanissetta basin further we speculate that 10 of this total clockwise rotation occurred during approximately kaena times ca 3 05 ma and that at least 24 of this clockwise rotation is younger than the gauss matuyama boundary 2 6 ma

sedimentary records of reversal transitions magnetization smoothing artefact or geomagnetic field behaviour sedimentary records of reversals for the past 12 million years exhibit preferred longitudes of their vgp paths this observation may be related to long term aspects of core mantle dynamics however it has been suggested that sedimentary vgp paths could be an artefact due to smoothing which mixes non antipodal pre and post transitional directions we show here that smoothing over unrealistically long time scales is required to generate intermediate directions of magnetization which are purely mixtures of pre and post transitionals directions in sediments whose remanence is primarily depositional drm or post depositional remanence pdrm sedimentary records of transitions showing vgp paths confined to the planes of immediately pre and post transitional vgps have however been observed these can be explained by non axisymmetric bias in the non reversing field gubbins and bloxham 1987 constable 1992 which plays a role in reversals therefore provided that magnetization directions are properly cleaned to give single component directions the preferential longitudes of reversal vgp paths will be due to field behaviour and not to an artefact of the magnetization process if however mixtures of primary magnetization acquired at the time of the formation of the rock and secondary magnetization acquired later are measured then artefacts will be generated

astronomical dating of a tectonic rotation on sicily and consequences for the timing and extent of a middle pliocene deformation phase a detailed palaeomagnetic study of long and continuous middle pliocene sections from the caltanisetta basin on sicily reveals a differential clockwise rotation occurring around 3 21 ma the rotation appears to be a rapid event 80 000 100 000 years which suggests that the responsible tectonic processes also occur rapidly its timing corresponds closely to the transition from the trubi to the narbone formation at 3 19 ma this transition marks a major change in sedimentary environment on sicily and in calabria and it is coeval for instance with the onset of sapropel formation in the eastern mediterranean apparently it marks a synchronous and central eastern mediterranean wide event data from the oldest sediments overlying the tyrrhenian basement odp leg 107 suggest an acceleration in opening of the tyrrhenian sea during the middle pliocene we speculate that this acceleration is related to a transpressional event in the sicilian fold and thrust belt and extension which formed troughs in the foreland the strait of sicily thrust imbrication accompanying the transpressional event on sicily induced the middle pliocene clockwise rotation and resulted in shallowing of the caltanisetta basin causing the change in sedimentation regime characterised by the trubi narbone transition following this middle pliocene tectonic phase no rotation took place in the southern apennines calabria and sicily until the middle pleistocene 1 0 0 7 ma 1998 elsevier science b v all rights reserved

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

structural synthesis of s staffordshire uk implications for the variscan evolution of the pennine basin the presence of strata of westphalian a to possibly stephanian age in south staffordshire located at the southern margin of the pennine basin provides a rare control on the timing of late carboniferous deformation in central england during westphalian a c times sedimentation in south staffordshire was controlled by thermal subsidence with a superimposed influence of synsedimentary normal and reverse faulting alkaline basic igneous activity occurred during westphalian c times synchronous with uplift of the southern margin of the pennine basin uplift culminated during westphalian c d times in a phase of variscan inversion along basement lineaments and the development of a regional unconformity renewed subsidence and deposition of a relatively thick sedimentary sequence of westphalian d to possibly stephanian age was followed by a final phase of variscan deformation comprising possibly two pulses of uplift tilting and erosion observations presented for south staffordshire in conjunction with studies from other parts of the pennine basin indicate a complex interrelationship between rifting thermal subsidence and variscan basin inversion pulses of n s extension and e w to nw se compression are interpreted as the product of regional strike slip deformation superimposed upon a regime of thermal subsidence alkaline igneous activity is considered to be the product of mantle partial melting in response to limited decompression due to transtensional displacements on crustal thickness faults

longitudinal confinement of geomagnetic reversal paths as a possible sedimentary artefact the positions of the virtual geomagnetic pole vgp during a large number of reversals of the earth s magnetic field seem to show a remarkable confinement to longitudes over the americas or to antipodal longitudes1 2 although it has been argued that this confinement is statistically insufficiently constrained3 it has also been pointed out4 that the same bands of longitude appear in other geophysical observations such as the pattern of fluid motion in the outer core and regions of higher seismic velocities in the lower mantle suggesting a causal relationship here we show that longitudinal confinement of vgps can arise from the smoothing of non antipodal stable directions just before and after a geomagnetic reversal because of the filtering effect of the remanence acquisition process in sediments the origin of this non antipodality is still uncertain and must remain speculative until more reversal records become available that include sufficiently long pre and post transitional intervals

geomagnetic field stability reversal events and excursions abstract it has been suggested that there are several reverse subchrons in the brunhes and this suggestion is gaining some acceptance however such a suggestion has fundanmental implications for the reversal process because it implies that the reverse polarity state has been substantially less stable than the normal polarity state during the brunhes chron analysis of the latest polarity reversal timescale confirms that there is no reason to reject the hypothesis of a common stability for the two polarity states the cretaceous superchron appears to be a time when the reversal rate slowed to the point where the reversal process actually halted so does not indicate any difference in the stability of the two polarity states after examining the data we found no convincing evidence for even a single reversal event during the brunhes and we conclude that the only reversal excursions that are sufficiently well documented for the brunhes are the laschamp and the blake further we note that the question of whether of large excursions represent aborted reversals is unresolved so excursion data should not at this stage be grouped with transition polarity data finally we note that most of the time the nearly axially symmetric normal and reverse polarity states appear to be remarkably stable

relative geomagnetic field intensity and reversals from upper miocene sections in crete abstract two late miocene sections at potamida and kotsiana in western crete have been resampled at about 10 cm intervals 288 and 130 horizons respectively the two sections record the same two reversals bounding a normal polarity zone just below the tortonian messinian boundary mineral magnetic analysis identifies pseudo single domain magnetite as the carrier of the main fraction of magnetization in both sections normalizations with arm sirm and on different specimens of the same cores yield very similar results after checking that it is not correlated to bulk magnetic parameters the arm normalized record was used as a proxy for geomagnetic field intensity changes in the lower part of the reverse polarity zone sampled at potamida the field intensity appears to increase slightly then to decrease progressively to low values prior to the r n polarity transition there is no evidence for a fast recovery of field intensity across the two transitions the intensity remains very low after the r n reversal and only progressively builds to a maximum in the middle of the following normal chron it then progressively decreases until the n r transition no sudden recovery of the field intensity is observed across this reversal either these results are thus inconsistent with the saw tooth record obtained by valet and meynadier for the last 4 million years either the behaviour of the field is different for different intervals of time or the hypothesis of sedimentary artefacts in the original saw tooth record should be considered