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

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

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

long lived transitional states of the geomagnetic field and the two dynamo families palaeomagnetic data from lavas produced at the hawaii and society islands hotspots suggest that one or more specific long lived field configurations recur during successive polarity reversals spanning perhaps several million years the apparent quasistationarity of these states and the relative placement of the two recording sites offer a rare opportunity to explore the fundamental geometrical properties of transitional fields

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

limestone marl couplets origin of the early pliocene trubi marls in calabria southern italy abstract a detailed micropaleontological and geochemical study of early pliocene limestone marl couplets from the capo spartivento section in calabria southern italy indicates that the rhythmic sedimentation pattern is related to changes in surface water productivity relative to the marls the limestone units are typically marked by higher abundances of the planktonic foraminifera g bulloides higher numbers of benthic foraminifera higher 13c values and lower 18o values these characteristics are interpreted as indicating that cool highly productive conditions existed during the deposition of the limestone units time series analysis indicates that a significant amount of the variance in the carbonate record is associated with periods of 17 and 23 ky similarly cross spectral analysis demonstrates that the precession signal and the carbonate record are strongly coherent over the 23 ky frequency band we propose a model in which intensified winds associated with precession maxima result in increased upwelling higher productivity and limestone deposition

a comparison of different magnetic methods for determining the relative grain size of magnetite in natural materials some results from lake sediments abstract a new rapid method for identifying relative grain size variations in magnetic involves the parameter anhysteretic susceptibility arm i e specific arm obtained in a 1 oe steady field which is particularly sensitive to the single domain sd and small pseudo single domain psd grains of the finer magnetite fraction a second parameter low field susceptibility is relatively more sensitive to the coarser magnetite fraction larger psd and smaller multidomain md grains we can then obtain a measure of the ratio of coarse to fine grain magnetite for large numbers of samples by plotting arversus a simple idealized model based on sized magnetite samples is proposed to explain the use of the armversus plot for detecting relative grain size changes in the magnetic content of natural materials the sediments of three lakes that contain magnetite or a similar magnetic carrier and have a wide range of values of arm and are used to test the model the model is used to interpret the magnetic variations observed and the interpretations are supported by high field hysteresis measurements of the same sediments the combination of the high field hysteresis method of day et al 1 and the arm vs method is a powerful technique allowing the rapid identification of both the relative grain size and domain state for large numbers of samples containing magnetite the armvs method should be used as an intial means of identifying distinct groups of samples the high field hysteresis method should then be applied to a few representative samples from each group to confirm the initial interpretation

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

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

compaction induced inclination shallowing of the post depositional remanent magnetization in a synthetic sediment summary a synthetic sediment comprised of kaolinite distilled water and either equidimensional or acicular magnetite was given a post depositional remanent magnetization pdrm by being stirred in a magnetic field this sediment was compacted under pressures which varied continuously from 0 to 0 14 mpa in a water tank consolidometer and to higher pressure steps 2 53 mpa in a standard soil consolidometer compaction took place in the same magnetic field in which the sample was given its pdrm the compaction caused shallowing of the sample s magnetic inclination this shallowing was found to be a function of the sample s initial magnetic inclination and the degree of sample compaction tan i 1 aav tan lo where ir is the remanent inclination after compaction av is the volume change i is the initial magnetic inclination and a is an empirically derived constant the data show a maximum inclination shallowing of 12 for an initial inclination of 54 in good agreement with the maximum inclination shallowing predicted by the above equation we propose an electrostatic mechanism to be the cause of the inclination shallowing in this model positively charged magnetite grains adhere to the surface of negatively charged clay grains with their long dimension parallel to the clay grain s surface as the clay grains become reorientated due to compaction the easy axes of magnetization are rotated away from the axis of compression alternating field demagnetization data reveal that our samples have shallower characteristic magnetizations than their post compaction nrms implying that the smaller higher coercivity grains are most affected by the compaction process these data support our model since a surface charge mechanism for inclination shallowing would predict that the smallest magnetic grains would be preferentially affected

placing bounds on lithospheric deformation in the central pacific ocean abstract lineations in the gravity field in the central pacific ocean have been explained by models of both small scale convection and lithospheric boudinage the boudinage model requires deformation of the plate and we have investigated bounds on the amount of deformation we calculate separations between a pair of fracture zones on the pacific plate which occur in an area where the lineations are well developed and compare these values to the separation of the corresponding fracture zone pair on the nazca plate where no lineations have been observed satellite and surface ship data was used to locate fracture zones and small circles were fitted to sections of picks which were chosen around their points of intersection with magnetic isochrons great circle distances from these small circles to stage poles were calculated and the difference between these distances gave the separations between the fracture zone pairs we found that between anomalies 13 and 7 the difference in fracture zone spacing on the pacific plate compared to the nazca plate appears to increase as the age of oceanic lithosphere decreases the maximum difference calculated was 97 km about 4 of the distance between the fracture zones and we attribute this difference to the re orientation of the ancient pacific farallon spreading centre between anomalies 7 and 5 the main lithospheric stretching models require the fracture zone separation to increase with age but this is not seen in our results

messinian astrochronology of the melilla basin stepwise restriction of the mediterranean atlantic connection through morocco the melilla basin ne morocco formed the easternmost part of the rifian corridor which was an important mediterranean atlantic gateway during the late miocene the sedimentary infill of the basin consists of a shallow marine precession related cyclic marl diatomite succession laterally grading into a marginal carbonate complex three bio sedimentary events have been recorded within the marl succession 1 onset of diatomite deposition 2 major change in foraminiferal assemblages and 3 transition to halimeda rich carbonates and porites coral reef build ups recent 40 ar 39 ar dating has provided a good age control for the melilla carbonate sequences but a high resolution astronomical time frame is necessary to solve the climatic signature of the basin sediments this study focuses on the shallow marine marl succession of the melilla basin integrated magneto cyclo and biostratigraphy allowed a detailed correlation to the astronomical target curve resulting in a high resolution time frame for the late miocene evolution of the basin comparison of the melilla data with previous results from other moroccan and mediterranean basins indicates that the input of atlantic waters through the rifian corridor became restricted after 6 84 ma and was minimized by 6 58 ma in the final period 6 58 5 96 ma towards the messinian salinity crisis the melilla basin can be considered as a marginal basin of the mediterranean the astronomical time frame for the marl sequences of the melilla basin moreover enables a direct comparison between the independent isotopic and astrochronological dating techniques as astronomical ages have also been assigned to intercalated volcanic tuffs for which 40 ar 39 ar ages were previously determined we conclude that the isotopic ages are systematically younger

extension of the astronomically calibrated polarity time scale to the miocene pliocene boundary the early pleistocene to late pliocene astronomically calibrated time scale of shackleton et al 1 and hilgen 2 is extended to the miocene pliocene boundary this is done by correlating the detailed record of caco3 cycles in the trubi and the lower part of the overlying narbone formation rossello composite section sicily to the astronomical record using 1 inferred phase relations between these caco3 cycles and the corresponding orbital cycles of precession and eccentricity and 2 calibration points provided by the previously established astronomical calibration of sapropelitic layers which occur in the topmost part of the caco3 record 2 this correlation allows all small scale caco3 cycles in the rossello composite section to be coded after the correlative precession peak and to be dated astronomically with an accuracy of 1 ky in combination with the available high resolution magnetostratigraphy it further allows the construction of an astronomically calibrated geomagnetic polarity time scale for the major part of the gilbert and gauss chrons this new time scale gives ages for the reversal boundaries of the kaena 3 04 3 11 ma and mammoth 3 22 3 33 ma the gilbert gauss chron boundary 3 58 ma and the reversal boundaries of the cochiti 4 18 4 29 ma nunivak 4 48 4 62 ma sidufjall 4 80 4 89 ma and thvera 4 98 5 23 ma these ages are significantly older than the ages given by conventional time scales age discrepancies gradually increase with increasing age the age of 5 32 ma for the miocene pliocene boundary as defined by the base of the trubi on sicily is almost 0 5 m y 9 5 older than the presently accepted age of 4 86 ma 3 these considerable discrepancies with the conventional time scale can either be explained by an consistent error in the decay constants used in k ar radiometric dating or more likely by diffusional loss of radiogenic argon

locating and characterizing a crack in concrete with diffuse ultrasound a four point bending test this paper describes an original imaging technique named locadiff that benefits from the diffuse effect of ultrasound waves in concrete to detect and locate mechanical changes associated with the opening of pre existing cracks and or to the development of diffuse damage at the tip of the crack after giving a brief overview of the theoretical model to describe the decorrelation of diffuse waveforms induced by a local change the article introduces the inversion procedure that produces the three dimensional maps of density of changes these maps are interpreted in terms of mechanical changes fracture opening and damage development in addition each fracture is characterized by its effective scattering cross section

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

role of tropical pacific ssts in global medieval hydroclimate a modeling study 1 the role of tropical pacific ssts in driving global medieval hydroclimate is assessed using fossil coral records from palmyra atoll tropical pacific sea surface temperature sst boundary conditions are derived for the period 1320 1462 a d these boundary conditions consist of la nina like mean state conditions in the tropical pacific with inter annual and decadal variability about that altered state the reconstructed ssts in the tropical pacific are used to force a 16 member ensemble of atmospheric general circulation model agcm simulations coupled to a one layer ocean model outside of the tropical pacific the agcm simulations of medieval climate are compared with modern climate simulations for the period 1856 2005 a d and are shown to reproduce many aspects of medieval hydroclimate found in paleo proxy records for much of the western hemisphere northern eurasia and the northern tropics these results suggest that many features of global medieval hydroclimate changes can be explained by changes in tropical pacific ssts though the potential role for other oceans is also discussed

a method to study alterations of magnetic minerals during thermal demagnetization applied to a fine grained marine marl trubi formation sicily summary the method presented is based on monitoring changes in the coercivity spectrum of an isothermal remanent magnetization irm during stepwise thermal demagnetization the procedure allows one to determine alteration temperatures and the coercivities and blocking temperatures of the magnetic minerals involved in the alterations it distinguishes between thermal decay due to unblocking of the remanence and demagnetization due to alterations of magnetic minerals as a result the initial magnetic mineralogy can also be determined the method is demonstrated with samples of fine grained marine marls from the pliocene trubi formation in sicily italy single domain sd magnetite is the dominant remanence carrier in this sediment a first alteration during heating to only 145 c is accompanied by a considerable reduction of coercivities higher than 0 1 t this alteration is thought to be due to the reduction of stress in superficially maghemitized sd magnetite grains between 390 and 480 c grains with magnetite like properties are produced most likely by oxidation of pyrite in the sediment at about 560 c the breakdown of iron bearing silicates also produces magnetic grains the magnetic grains formed during both alteration processes have low blocking temperatures demagnetization above 510 c is not only due to unblocking of the irm alteration of magnetite to haematite proves to be an important demagnetization mechanism

the origin of rhythmic bedding in the pliocene trubi formation of sicily southern italy abstract foraminiferal palynological stable isotope geochemical clay mineralogical and granulometric data have been gathered for the successive layers of four rhythmic alterations of relatively carbonate poor and carbonate rich beds in the early pliocene trubi formation of sicily southern italy the marly rhythmites consist of a grey layer and a white more indurated layer in which an additional beige less indurated layer is intercalated they are therefore more complex than the well known marl limestone couples elsewhere these rhythmites probably originate from the alternate predominance of increased dilution of river supplied terrigenous material grey and decreased carbonate production in the surface waters beige the grey layers are considered to correspond to periods of increased local run off and higher surface water temperatures and productivity the beige layers signal more arid and cooler climatic conditions lower surface water productivity and stronger or more frequent dust loaded southern winds the white layers apparently reflect intermediate climatic and environmental conditions these periodical changes probably brought about by astronomical precession appear to have occurred not only during the pliocene but also during the late miocene

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

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

complex behavior of the geomagnetic field during three successive polarity reversals 11 12 m y b p detailed paleomagnetic records of both the directional and relative field intensity changes occurring during three sequential polarity reversals have been obtained from marine clays of serravallian tortonian boundary age in the island of zackinthos greece the three reversals are characterized by a complicated geometrical and temporal structure periods of rapid changes of the geomagnetic field vector alternate with periods of near stationarity different episodes of collapse and recovery of the field intensity and the presence of rebounds have been documented overall the records obtained here are reminiscent of volcanic records and appear much more complex and quite different from those previously obtained from upper tortonian marine clays in crete in the same geographical area the bearing of these results on some of the published reversal models is discussed and it is also suggested that the greater complexity of the zackinthos records might be related to the fact that they are exactly coeval with the last reversal spurt documented by reversal frequency analyses

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

depth of post depositional remanence acquisition in deep sea sediments a case study of the brunhes matuyama reversal and oxygen isotopic stage 19 1 although post depositional magnetization pdrm in deep sea sediments appears to be acquired during the earliest stages of sediment compaction the natural variability of the pdrm lock in depth in deep sea sediments is poorly understood and as yet unquantified here we consider variations in the relative stratigraphic positions of oxygen isotopic interglacial stage 19 1 and the brunhes matuyama b m chronozone reversal for eight deep sea sediment cores results from a similar study of the displacement between a widespread microtektite layer and the b m boundary are also included 1 the pdrm lock in depth and the temporal relationships between the b m and stage 19 1 datums can be determined from the offsets between the paleomagnetic and the isotopic and microtektite stratigraphies for cores with sedimentation rates greater than 1 cm kyr a the depth offset between the paleomagnetic and isotopic datums is a linear function of sedimentation rate a simple model r 2 0 77 demonstrates that 1 pdrm acquisition occurs 16 cm below the sediment surface and 2 the b m reversal occurs 6 kyr 4 2 kyr after the stage 19 1 datum and the stage 19 1 datum occurs 9 kyr 4 3 kyr after the deposition of the australasian microtektite strewnfield this example argues against simple geophysical models linking geomagnetic field reversals to climate change or impact events the b m boundary is anomalously deep 30 50 cm in very low accumulation rate sediments 1 cm kyr 1 and this may reflect the unusual physical properties of these sediments a review of the geotechnical literature suggests that very low accumulation rate sediments have sufficient time to develop enhanced interparticle rigidity structural strength which inhibits early compaction and hence pdrm acquisition

the plumbing of old faithful geyser revealed by hydrothermal tremor 1 old faithful geyser in yellowstone national park usa has attracted numerous scientific investigations for over two centuries to better understand its geological structure the physics of its eruptions and the controls of its intermittency using data acquired with a seismic array in 1992 we track the sources of hydrothermal tremor produced by boiling and cavitation inside the geyser the location of seismic sources identifies a previously unknown lateral cavity at 15 m below the surface on the sw side of the vent and connected to the conduit this reservoir is activated at the beginning of each geyser eruption cycle and plays a major role in the oscillatory behavior of the water level in the conduit before each eruption

palaeomagnetic constraints on the geometry of the geomagnetic field during reversals palaeomagnetic records of the path of the pole during reversals of the earth s magnetic field provide a test of the hypothesis that dipolar or low order axisymmetric components of the field dominate during reversals multiple records of reversals during the past 12 myr show no simple or consistent geographical pattern although a more robust analysis of the transitional field awaits a greater number of well distributed sampling sites the present data are not inconsistent with the simplest models in which a field reminiscent of the non dipole component of the present day field becomes dominant

seismic interferometry using persistent noise sources for temporal subsurface monitoring in passive source seismology seismic interferometry typically refers to the cross correlation of ambient noise to construct an estimate of the green s function between sensors the presence of persistent natural and or anthropogenic sources can bias or prevent the retrieval of these estimated green s functions here we show how these strong persistent sources can be used to measure small changes in the medium between a source and either or both source sensor pairs the method relies on localizing the sources and using this information to identify and select cross correlation functions for each source of interest we illustrate this method by monitoring growth of a block cave at an underground mine using three nearly continuously operating ore crushers which dominate the wavefield this technique should work equally well in natural environments using sources such as volcanic tremor hydrothermal bubble cavitation and microseisms

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

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

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

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