invariant and changing transitional field configurations in a sequence of geomagnetic reversals in the past few years the increasing number of detailed records of geomagnetic transitions has provided new and important information about the behaviour of the earth s magnetic field during a reversal surprisingly however no attempt has been made to obtain information about the evolution of the transitional field characteristics over a long period of time we report here the results of a palaeomagnetic study of four different geomagnetic reversals three of which are successive recorded at different stratigraphical levels of two upper tortonian marine clay sections in western crete greece this sequence represents 1 5 myr of geomagnetic field history the results obtained suggest that the process of reversal from either starting polarity may remain invariant for a long period of time then change rather abruptly between successive reversals

stratigraphy and remanence acquisition of a palaeomagnetic reversal in alluvial siwalik rocks of pakistan a high resolution record of a palaeomagnetic reversal is documented in miocene alluvial rocks of pakistan we examined lateral variability of lithostratigraphy and palaeomagnetic stratigraphy through the same palaeomagnetic reversal in six correlated sections each section contains one or more palaeomagnetic sites with directions between fully reversed and fully normal the position of the reversal illustrates local relief in the study area and the time transgressive nature of certain stratigraphic units variability in the thickness of the transitional interval indicates contemporaneous variability in sediment accumulation rates different characteristics of palaeomagnetic remanence are associated with the depositional and postdepositional history of these sediments variability in patterns of remanence behaviour is the basis for inferences about post depositional processes we discuss two magnetic parameters that express coherency of palaeomagnetic samples the maximum angle of deviation and the circular standard deviation of particular interest are samples with incoherent palaeomagnetic signals the incoherency of samples is inversely correlated with the thickness of the transitional interval a low rate of sediment accumulation suggested by a thin transitional interval may facilitate a prolonged period of remagnetization through pedogenic or hydrological processes alternatively transition intervals defined by coherent magnetization may be thin as a result of pedogenically induced incoherency

long term polarity stability and strength of the geomagnetic dipole palaeomagnetically observed reversals so far provide the strongest evidence1 that reversals originate from processes of core magnetohydrodynamics mhd thus furnishing a crucial constraint for geomagnetic dynamo modelling the long term variations in reversal frequency favouring2 independent mechanisms for the dynamo and its reversals provide an added constraint here we present evidence for a correlation between polarity superchrons and palaeointensity data and also marine magnetic and mags at anomalies our results indicate that the dipole moment is correlated to the reversal frequency being appreciably greater during the fixed polarity superchrons this new information derived from the reversal record provides fresh insight into the workings of the geodynamo and also explains why the oceanic crust associated with the cretaceous quiet zone carries such a strong thermoremanence

palaeomagnetic excursions recorded in mid weichselian cave sediments from skjonghelleren valderoy w norway abstract anomalous single component palaeomagnetic directions in laminated clay silt sediments in the non karstic skjonghelleren cave w norway are interpreted to record two high resolution but incomplete records of geomagnetic field excursions a depositional non distorted origin of magnetization carried by ultra fine magnetite maghemite grains has been inferred from field evidence redeposition experiments and magnetic fabric properties the sediments deposited sub glacially between 56 ka to 12 ka determined by consistent isotope ages obtained from bones and travertine the youngest excursion

paleomagnetic record of two successive miocene geomagnetic reversals in western crete abstract two geomagnetic reversals r n followed by n r have been recorded in a sequence of miocene marine clays of tortonian messinian age in western crete greece the time span of each transition is found to be of the order of 5000 10 000 years the transitional vgp paths are largely constrained in longitude either near the site longitude r n or opposite to it n r a normalized magnetization intensity record has also been obtained and its variations during reversals are found to be in excellent agreement with the predictions of hoffman s phenomenological model

an attempt to constrain the age duration and eruptive history of the karoo flood basalt naude s nek section south africa we have carried out paleomagnetic sampling of a similar to 750 m sequence of the karoo large igneous province naude s nek pass south africa k ar dating cassignol gillot has been performed on four samples from the 650 m upper unit mean age 179 2 1 8 ma and a sample from the lower unit 184 8 2 6 ma a succession of two phases of volcanism is suggested the lower 25 flows 115 m thick have recorded a reversed polarity the next 23 flows 135 m thick are transitional and contribute a detailed record of the van zijl 1962 jurassic reversal the upper 38 flows 500 m thick have normal polarity directional groups dgs of lava flows with quasi identical remanence directions indicate eruption durations too short to have recorded geomagnetic secular variation and hence are interpreted as single eruptive events altogether 19 dgs and 10 sheet lobes yield a sequence of 29 distinct directions this could correspond to a total eruptive activity shorter than 3000 years less than one per mil of the total duration over which the section was emplaced we obtain a new paleomagnetic pole for south africa at similar to 180 ma lambda 75 2 degrees n phi 276 4 degrees e a 95 5 8 degrees n 19 which is consistent with earlier reports

first paleomagnetic results from mio pliocene series of the hellenic sedimentary arc abstract paleomagnetic measurements have been conducted on the mio pliocene series along the external sedimentary hellenic arc in order to detect the presence or absence of rotational deformations in different sections of the arc in the central eastern part of the arc crete and rhodes paleomagnetic directions are very closely aligned along the north south direction showing that this portion of the arc has not undergone significant rotation since at least 7 m y for crete and 4 m y for rhodes in the western and northwestern parts results from south peloponnesos zakinthos kephallinia and corfu indicate that this section of the arc has undergone a clockwise rotation detailed analysis of the data from the three ionian islands shows that their rotation began about 5 m y b p and has proceeded since then at roughly 5 m y giving a total amplitude of about 26 it is suggested that the beginning of this rotation is related to the lower pliocene compressive phase which most probably initiated the tectonic activity of the present day ionian margin

the character of the field during geomagnetic reversals it is well established that the geomagnetic field which is predominantly dipolar at the earth s surface has alternated between normal and reverse polarity through geologic time figure 1 evidence of this behavior is preserved in the ancient magnetism of rocks the remanence of radio metrically dated lava flows for example shows that the field has reversed at least 23 times over the past 5 million years figure 2 reversals are not periodic it is apparent in figure 2 that lengths of recent polarity intervals range from lo4 to lo6 years and that the intervals follow a very irregular pattern this behavior contrasts sharply with that of the sun s field which switches polarity every 1 1 years paleomagnetic observations discussed in more detail below show that a geomagnetic reversal takes thousands of years to complete what happens to the geomagnetic field during the transitional period does the entire field just rotate from one orientation to the other does the field disappear

the zonal harmonic model of polarity transitions a test using successive reversals a recently developed zonal model for the last geomagnetic field reversal which describes time and latitude dependent transitional behavior of intensity and inclination in terms of dominance of low order field harmonics was tested using a latitudinal and chronological succession of transition records the primary data were derived from a high resolution study of five matuyama to brunhes deep sea cores collected along a 40 meridional band in the north central pacific the transitions analyzed were the on sets and terminations of the olduvai and jaramillo events and the matuyama brunhes boundary supplementary data published by previous workers included a jaramillo onset record from the southern indian ocean and a transition recorded in the triassic chugwater formation of north america in a general sense the measured inclination and intensity records indicate the jaramillo transitions and the last reversal to be remarkably similar to each other and to differ from the olduvais crosscorrelation of the measured data sets with synthetic zonal harmonic records for the core latitudes indicates that the zonal harmonic model does predict the behavior of matuyama brunhes reversal in the northern hemisphere for the older reversals however the present model requires substantial modification this is particularly so in the case of the jaramillo onset although two inclination and intensity records of this transition from northern and southern hemisphere sediments strongly resemble each other the current model produces a synthetic record for one hemisphere that is incompatible with that hemisphere s measured data further to model zonally the olduvai reversals for which a wide latitudinal northern hemisphere sampling was available a substantially different ratio of low order multipole components from that of the standard model is required and the modeled solution was unsatisfactory at very low latitude a modified zonal harmonic approach which introduces a small sectoral component may offer a solution to the incompatibilities uncovered by the present study

secular variation as recorded in lake sediments a discussion of north american and european results type curves depicting secular variations in declination and inclination through holocene time have been constructed by stacking data from individual cores of lake sediment from east central north america and the u k there is a strong positive correlation in declination with zero phase shift froiti 0 to 4500 14 c years b p but from 4500 to 8500 14 c years b p the correlation is strongly negative which we interpret in terms of fluctuations in intensity of standing geomagnetic sources a source located under the atlantic ocean appears to have been dominant before ca 4500 14c years b p and then another similar source located either to the east or to the west of both observation areas gained dominance however the inclination type curves show optimum positive correlation for a phase shift of ca 400 years suggesting that the geomagnetic sources that caused them were drifting westward at ca 0 2 degrees a year there is no way of adjusting the timescales so as to obviate this apparent paradox secular variations of the total geomagnetic vector have been investigated by noting the sense clockwise anticlockwise of looping of the v g p paths though this cannot be interpreted uniquely in terms of westward or eastward drifting sources in the core as demonstrated by specific radial dipole models

the steens mountain oregon geomagnetic polarity transition 2 field intensity variations and discussion of reversal models we carried out an extensive paleointensity study of the 15 5 0 3 m y miocene reversed to normal polarity transition recorded in lava flows from steens mountain south central oregon one hundred eighty five samples from the collection whose paleodirectional study is reported by mankinen et al this issue were chosen for paleointensity investigations because of their low viscosity index high curie point and reversibility or near reversibility of the strong field magnetization curve versus temperature application of the thellier stepwise double heating method was very successful yielding 157 usable paleointensity estimates corresponding to 73 distinct lava flows after grouping successive lava flows that did not differ significantly in direction and intensity we obtained 51 distinguishable complete field vectors of which 10 are reversed 28 are transitional and 13 are normal the record is complex quite unlike that predicted by simple flooding or standing nondipole field models it begins with an estimated several thousand years of reversed polarity with an average intensity of 31 5 8 5 t about one third lower than the expected miocene intensity this difference is interpreted as a long term reduction of the dipole moment prior to the reversal when site directions and intensities are considered truly transitional directions and intensities appear almost at the same time at the beginning of the transition and they disappear simultaneously at the end of the reversal large deviations in declination occur during this approximately 4500 1000 year transition period that are compatible with roughly similar average magnitudes of zonal and nonzonal field components at the site the transitional intensity is generally low with an average of 10 9 4 9 t for directions more than 45 away from the dipole field and a minimum of about 5 t the root mean square of the three field components x y and z are of the same order of magnitude for the transitional field and the historical nondipole field at the site latitude however a field intensity increase to pretransitional values occurs when the field temporarily reaches normal directions which suggests that dipolar structure could have been briefly regenerated during the transition in an aborted attempt to reestablish a stationary field changes in the field vector are progressive but jerky with at least two and possibly three large swings at astonishingly high rates each of those transitional geomagnetic impulses occurs when the field intensity is low less than 10 t and is followed by an interval of directional stasis during which the magnitude of the field increases greatly for the best documented geomagnetic impulse the rapid directional change corresponds to a vectorial intensity change of 6700 2700 nt yr 1 which is about 15 50 times larger than the maximum rate of change of the nondipole field observed during the last centuries the occurrence of geomagnetic impulses seems to support reversal models assuming an increase in the level of turbulence within the liquid core during transitions the record closes with an estimated several thousand years of normal polarity with an average intensity of 46 7 20 1 t agreeing with the expected miocene value however the occurrence of rather large and apparently rapid intensity fluctuations accompanied by little change in direction suggests that the newly reestablished dipole was still somewhat unstable

intensity of the earth s magnetic field evidence for a mesozoic dipole low the variation of the strength of the earth s magnetic field throughout geological time is known only poorly large uncertainties spoil indirect paleointensity determinations based on magnetic anomaly amplitude or remanence of sedimentary rocks even when paleointensity data are obtained from direct thermomagnetic studies of magmatic units fundamental problems related to the actual nature of the primary remanence and to the effects of different cooling rates may result in erroneous determinations these uncertainties combined with well known experimental difficulties can explain the large scatter in paleointensity data reported in most previous review papers because experimental difficulties are more rigourously dealt with using the thellier method which also offers some opportunities to minimize some fundamental uncertainties we carried out a compilation of all triassic and younger paleointensity determinations made using this method this selected data set indicates the occurrence of large long term changes of the geomagnetic dipole strength with time the average dipole strength seems to have been approximately constant since late cretaceous time but a dipole strength only one third of the cenozoic value prevailed during most of mesozoic time following a period of larger strength ending possibly during early jurassic times the mesozoic dipole low may be correlated with a cessation of relative motion between the geomagnetic axis and the mantle the terrestrial dynamo seems therefore to exhibit different long term stable states which are probably controlled by the thermal structure in the lowermost mantle

research on groundwater radon as a fluid phase precursor to earthquakes groundwater radon monitoring work carried out in southern california by the university of southern california since 1974 is summarized here this effort began with a sampling network over a locked segment of the san andreas fault from tejon to cajon and was later expanded to cover part of the southern transverse mountain ranges groundwater samples were brought back weekly to the laboratory for high precision scintillation counting needs for more frequent sampling and less labor prompted the development of an economical and field worthy instrument known as the continuous radon monitor about 10 have been installed in the network since early 1980 the groundwater radon content was found to show anomalous increases mostly at a single station before a number of moderate and nearby earthquakes our work is hampered by a lack of large earthquakes that may have a regional impact on radon anomalies and by the complexity of the underground hydrological regime to circumvent this difficulty we have chosen to monitor only deep artesian wells or hot spring wells

paleomagnetic stratigraphy a correlation method abstract interdisciplinary applications of paleomagnetism have in general been restricted to the construction of chronostratigraphic time frames based on correlation with the magnetopolarity time scale for quaternary deposits beyond the range of the traditional 14c dating method paleomagnetic excursions may eventually prove useful as chronostratigraphic markers excursions are inferred from high amplitude paleomagnetic records of short duration and the reality extent and duration of most claimed excursions are still controversial these problems may be overcome by developing procedures to discriminate genuine records of excursions from distorted paleomagnetic signals one suggested approach is a more extensive application of magnetic fabric analysis in conjunction with systematic laboratory investigations of remanence acquisition processes in different sedimentary environments

geomagnetic reversals and excursions their paleomagnetic record and implications for the geodynamo since the earliest reports of paleomagnetic evidence in support of the existence of geomagnetic field reversals much attention has been focused on understanding the relevant core mechanism given the far reaching implications such an understanding would have on the whole of geomagnetism past investigations of this phenomenon have involved experimentalists and theoreticians alike in particular during the previous quadrennium 1975 1978 significant contributions were gained from each of various approaches namely experimental studies of paleomagnetic records of polarity transitions statistical studies involving the frequency of occurrence of reversals as well as the time averaged full polarity states and theoretical studies concerning the dynamo continued progress along many fronts has been made during the last four years see the recent review by hoffman 1982a such is the subject of this report

high resolution record of geomagnetic secular variation from late pleistocene lake lisan sediments paleo dead sea we measured geomagnetic secular variation in lake lisan sediments paleo dead sea more than 1500 oriented samples were collected from a 27 3 m section of alternating aragonite and detritus laminae in the dead sea basin ranging in age from 67 to 32 ka the natural remanent magnetization nrm is carried by titanomagnetite in the detrital laminae whereas the aragonite is diamagnetic the nrm is very stable and was acquired several hundred years after deposition the mean direction of 878 horizons is dd 005o id 45o 95d 1o d 22 we observed three modes of directional geomagnetic variation as a function of and by inference time very rapid inter sample changes slow variation in mean direction and inclination shallowing of about 1o m the overall rate of change in direction is 0 57 0 57o year not significantly different from zero for about 83 of the record the rate of change is less than 1o year and comparable to historical values high rates of change are observed more frequently in the lisan than in historical records and peak rates are up to ten times faster a smoothed curve resulting in a maximum rate of change of 0 66o year and a mean 0 10 0 10o year may be a more realistic representation of the field behavior no reverse nrms were observed but geomagnetic field excursions may be present where the vgps deviate by more than 40o from the geographic north at about 52 and 41 ka the latter may represent the laschamp event 1998 elsevier science b v all rights reserved

two different r n geomagnetic reversals with identical vgp paths recorded at the same site we have studied two reverse normal r n geomagnetic reversals recorded at different stratigraphical levels of a single section of tortonian marine clays in western crete we report here that the virtual geomagnetic pole vgp paths associated with these two transitions which are separated in time by a period of 1 myr are identical although the possibility of an accidental coincidence cannot be rejected this result suggests that the basic mechanisms in the geodynamo leading to a reversal might persist unchanged over a long period of time moreover the two vgp paths are largely constrained in longitude along a mean great circle located 80 w of the site so that the transitional field cannot be correctly described in terms of pure axial symmetry

multiple skeletal and dental pathologies in a late miocene mesotheriid mammalia notoungulata from the altiplano of bolivia palaeoecological inferences we report a partial adult skeleton of the notoungulate mesotheriid plesiotypotherium achirense showing multiple skeletal and dental pathologies from the late miocene locality of achiri bolivian altiplano the material shows bilateral absence of hypselodont permanent molars further associated with exostosis on various limb bones the main pathology relates with the loss of two first molars on both sides of the jaw perhaps due to long lasting periodontal diseases other craniomandibular afflictions likely related to the latter pathology are an over growth of two first upper molars on both sides and the anomalous development of masticatory muscle insertions m masseter pterygoideus medialis temporalis and temporalis pars profundis a hypothetical pathological sequence of the masticatory apparatus was reconstructed according to distinctive wear of ever growing crowns of two first upper molars and the unequal lower alveoli reabsorption additional pathologies are exostoses observed on several articular surfaces of limb bones from the scapula to distal phalanges they likely decreased the range of movements during locomotion of this individual given its multiple pathologies we might have expected this abnormal individual to be a potential easy prey however the long time survival of this animal suggests a low predatory pressure a hypothesis that is consistent with the virtual absence of carnivorous vertebrates in the achiri fossil assemblage and the composition of coeval mammalian guilds in the bolivian altiplano and or a herd behaviour for plesiotypotherium achirense as seen in many modern grazing herbivore ungulates

geomagnetic reversals from impacts on the earth the impact of a large extraterrestrial object on the earth can produce a geomagnetic reversal through the following mechanism dust from the impact crater and soot from fires trigger a climate change and the beginning of a little ice age the redistribution of water near the equator to ice at high latitudes alters the rotation rate of the crust and mantle of the earth if the sea level change is sufficiently large 10 meters and rapid in a few hundred years then the velocity shear in the liquid core disrupts the convective cells that drive the dynamo the new convective cells that subsequently form distort and tangle the previous field reducing the dipole component near to zero while increasing the energy in multipole components eventually a dipole is rebuilt by dynamo action and the event is seen either as a geomagnetic reversal or as an excursion sudden climate changes from other causes such as volcanic eruptions could also trigger reversals this mechanism may not be the sole cause of geomagnetic reversals but it can account for the rapid drop of the dipole component preceding a reversal the predominance of multipole components during a transition the associations of microtektites temperature drops and extinctions with reversals and the possible correlation between peaks in the geomagnetic reversal rate and the times of mass extinctions the model may also account for the long term changes in the average rate of reversals we make several testable predictions

paleomagnetic volcanic data and geometric regularity of reversals and excursions mostly on the basis of paleomagnetic sedimentary data it has been suggested that maps of virtual geomagnetic poles vgps corresponding to directions of the magnetic field at each site tend to fall along american and asian longitudes during reversals and excursions such geometric regularity in transitional fields may indicate that the core and mantle are dynamically coupled however studies of paleomagnetic lava data have thus far failed to show any pattern in transitional fields in this paper we examine a paleomagnetic lava database covering reversals and excursions which have occurred over the last 20 myr volcanic eruptions occur sporadically thus we normalize the data to account for the fact that reversal and excursional events at the various sites are recorded by different numbers of intermediate directions but we prefer not to use averaging methods of previous investigators who discarded or combined directions when they appeared to be similar we find that volcanic data give intermediate vgps which tend to fall along american and asian longitudes roughly consistent with the sedimentary data this result is not an apparent artifact arising from the nonuniform geographic distribution of volcanic sites provided the appropriate polarity is assigned to intermediate vgps we find that icelandic vgps tend to fall along asian longitudes other patterns in the data for example latitudinal clustering of vgps or distinguishing longitudinal preferences of excursions from reversals are not resolved however it appears that in general transitional fields are nondipolar

numerical models of the geodynamo and observational constraints 1 the past few years have seen the emergence of a large number of numerical simulations of the geodynamo in parallel both new and old geomagnetic archeomagnetic and paleomagnetic observations have been interpreted as actual geomagnetic features and used as constraints for dynamo models naturally model predictions should be tested against actual characteristics of the geomagnetic field despite huge differences sometimes in excess of a billion between the values of parameters used in the simulations and those estimated for the earth it is intriguing that many available simulations succeed in producing largely axial dipolar magnetic fields with weaker nondipolar structures in agreement with the first order characteristics of the geomagnetic field yet when considering finer characteristics there are significant differences and failures to actually produce a number of fundamental characteristic features in this presentation we first review numerical results obtained to date then we attempt to summarize which field characteristics derived from observational data sets can be considered robust on the basis of simple criteria used to evaluate the degree of confidence that can be placed in each datum we sort presumably characteristic geomagnetic features into three categories robust controversial and unlikely we conclude that numerical models should be illustrated with a number of key predictions averaged over at least 10 dipole diffusion times these predictions should be tested against the subset of robust observations only controversial observations should await additional confirmation

a miocene polarity transition r n from the agno batholith luzon a geomagnetic reversal record has been obtained from the agno batholith 16 15 n 120 39 e in luzon island the philippines this body is a biotite hornblende quartz diorite dated at between 14 5 and 15 0 m y by the k ar whole rock method hysteresis properties show that the magnetic remanence is dominated by multidomain low titanium titanomagnetite a stable component of magnetization is isolated upon both thermal and alternating field demagnetization normal and reversed polarity zones within the batholith give tightly grouped results which when compared to other miocene poles from the baguio area allow a tilt correction to be made the resulting reversed to normal reversal sequence has a far sided vgp path and a decrease in intensity of magnetization of approximately a factor of 5 with a longer duration than the directional change the vgp path is confined in a longitudinal band 120 from the site and so differs from the characteristic reversed to normal transition paths in that it is not near sided intermediate field geometry cannot therefore always be the same for individual reversals the agno reversal record is consistent with a quadrupole field and reversal initiated in the northern hemisphere an octupolar intermediate field would require simultaneous flooding from both polar latitude bands

analysis of the early jurassic geomagnetic data recorded at the breggia gorge ticino switzerland abstract geomagnetic directions recorded in a lower jurassic stratigraphic sequence that outcrops at the breggia gorge are analyzed a new method to identify directions suitable for calculating a time averaged dipolar geomagnetic direction has been applied to calculate a hypothetical early jurassic geocentric axial dipole the data were separated to identify normal reverse directions corresponding to periods of stable field and intermediate directions from an analysis of the data in the palaeogeographic coordinates of the breggia gorge at the time of the acquisition of the remanence it is suggested that polarity transitions in the early jurassic behaved in a similar manner to that described for late cenozoic reversals with the vgps preferentially following longitudinal paths the preferred paths coincide with a time averaged location of the jurassic subduction area of pangea this agreement could indicate a connection between the geometry of the reversals and the subduction zones an analysis to test whether both n r and intermediate directions have similar preferred distributions suggests that the underlying structure of the early jurassic geomagnetic field was similar to that observed for the late cenozoic the dispersion of directions with inclinations around 0 is higher than those belonging to n r polarity directions suggesting that there was an enhancement of fluctuations of the geomagnetic field during intermediate states in the early jurassic

paleosecular variation during sequential geomagnetic reversals from hawaii abstract paleomagnetic directions measured from 175 flows in four volcanic sequences on the island of o ahu hawaii cover about 0 3 m y of geomagnetic field changes and include detailed records of the successive gilbert gauss 3 57 ma and lower mammoth reversals 3 33 ma the dominant pattern of the directional changes is the presence of large inclination variations with increasing amplitude and steep values during the transitional periods the morphology of these recurring features during different transitions is reminiscent of the characteristics of the non dipole field inferred from studies of paleosecular variation in the pacific these observations are consistent with transitional mechanisms yielding the emergence of non dipole components with characteristics similar to those during periods of stable polarity the three different records of the lower mammoth reversal show that clusters of transitional pole positions are not reproducible between parallel sections and are thus linked to local eruptions and not to long lived states of the transitional field

a method for the display and analysis of transitional paleomagnetic data application of a simple coordinate transformation to data from polarity transition and excursion records renders a form of display having strong similarities yet distinct advantages over the use of virtual geomagnetic poles specifically the procedure consists of a single rotation about the east west line so as to align the z axis of the local cartesian system with the normal polarity axial dipole field direction associated with the recording site at the time of the event the method makes possible the redefinition of concepts relevant to transitional field studies in terms of vector directions a more meaningful approach than the use of virtual poles when nondipole field components are significant visually polar stereographic plots in d i space present paleodirectional movement as if one were literally peering down the dipole direction analysis of individual transition records in this rotated directional space makes easy the determination of 1 the angular distance of a given paleodirection from that of the axial dipole field 2 the number of strictly transitional directions contained within a given record and 3 the degree of near sidedness or far sidedness of each paleodirection in addition the comparison of records from distant sites for the purpose of harmonic analysis can be accomplished without the need for additional normalization such comparison plots in d i space display transitional data in a form strongly reminiscent of virtual geomagnetic poles plotted with respect to a common site longitude records of polarity transitions and excursions are presented which illustrate the utility of the proposed approach

a detailed record of normal reversed polarity transition obtained from a thick loess sequence at jiuzhoutai near lanzhou china summary a record of normal reversed polarity transition has been obtained from a 4 m thickness of loess exposed at a section near lanzhou china magnetostratigraphic studies suggest it may represent a reversal bounding the onset of a reversed polarity zone within the jaramillo normal subchron the natural remanent magnetization consists of two components a low coercivity i 20 mt low unblocking temperature i 300 c component of viscous origin and a high coercivity 20 mt high unblockingtemperature 25g700 c component carrying the characteristic remanence mineral magnetic analyses confirmed the presence of magnetite its low temperature oxidation products and haematite each contributing to the remanence properties grain size and concentration showed limited variations and there was little evidence for the presence of the ultrafine magnetic phase commonly associated with palaeosol formation pedogenic processes appeared negligible and their effects unimportant with detrital processes dominating the mineralogy and most probably the acquisition of the characteristic remanence the reversal record was characterized by the decay and recovery of the geocentric axial dipole term with large directional swings occurring during periods of reduced relative palaeofield intensity the virtual geomagnetic poles traced a complex path exhibiting no particular geographical confinement relative palaeofield intensity determinations were insensitive to the choice of normalization parameter and showed a distinctive asymmetry striking similarities were observed with the matuyama jaramillo reversal record obtained from the same section rolph 1993 and the steens mountain reversal record prcvot et al 1985 lending further support for the existence of unusually high post transitional field intensities

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

on the anisotropy of secular variation deduced from paleomagnetic volcanic data we seek to establish whether or not secular variation the rate at which the magnetic field is changing in time is a function of field direction is secular variation anisotropic from a large number of paleomagnetic lava data many of which record polarity transitions we find that vectors from stratigraphically adjacent flows are most least correlated during nontransitional transitional periods since volcanic activity is unrelated to and therefore uncorrelated with magnetic secular variation this relationship indicates that secular variation is enhanced during transitions perhaps by a factor of 2 thus secular variation is anisotropic being a function of the deviation of field direction from an axial dipole despite this spatial dependence we find little convincing evidence that transitional fields exhibit persistent recurring quasi stationary states which manifest themselves in terms of midlatitude virtual geomagnetic poles vgps as has sometimes been hypothesized on the other hand we identify tentatively some relative quiescence in secular variation during midtransition when vgps fall near the equator numerous subsets of the database are examined for consistency and statistical errors are estimated by bootstrap analyses statistical models of secular variation need to incorporate the information content in serial correlations like those analyzed here from stratigraphie sections if the lava data are to be fully exploited

chemical variations in fumarolic gases at vulcano island southern italy seasonal and volcanic effects abstract because of increasing in fumarolic activity at vulcano since september 1987 regular sampling and analysis is conducted on the well known f5 crater fumarole t 330 5 c significant chemical variations have been recorded over the period 1987 1989 variations of water vapour in the fumarolic fluid follow seasonal pattern and can be related to both external seasonal and internal volcanic influences suggesting a control by a shallow water aquifer significant variations of h 2 o and of some species in the anhydrous gas phase h 2 so 2 hcl hf n 2 he probably result from chemical or dynamic changes in the feeding system at depth several mechanism are considered for explaining the chemical trends results in relationship to the recent increase in activity a growing input of deep magmatic fluids is not supported by the low co content and the apparent constancy of the 3 he 4 he ratio apparent equilibrium temperatures close to the fumarolic outlet temperature 350 400 c were calculated suggesting isothermal expansion of the gas from a shallow equilibration zone comparison with the fumaroles at sea level porto di levante suggests that these latter may be fed a different hydrothermal systems variable interaction between both systems may account for some of the chemical variations observed at f5 fumarole

olduvai onset polarity transition two detailed paleomagnetic records from north central pacific sediments abstract paleomagnetic analyses of two azimuthally oriented deep sea sediment cores from the north central pacific ocean which contain a record of the onset of the olduvai subchron polarity transition show 1 intermediate directions of magnetization with transitional inclinations reaching values up to 70 2 up to three sequential 180 declination changes two of which imply rapid oscillations of the geomagnetic field before the main reversal and 3 a pronounced drop in intensity of magnetization within the transitional interval the data further indicate that the intensity decay and recovery cycle took longer than the directional changes of the reversal whereas the directional changes lasted about 5000 years the duration of the recorded intensity cycles suggest that the complete onset of the olduvai reversal may have lasted as much as 28 000 years anhysteretic remanent magnetization studies and thermomagnetic analyses indicate that uniform magnetic mineralogy characterizes the transition zones of the cores this further suggests that the j 10 arm 70 ratio may represent a relative paleointensity estimate and may thus indicate a decrease of the intensity of the geomagnetic field during the onset of the olduvai reversal virtual geomagnetic pole paths of the two records are near sided directions in projected d i space indicate that the olduvai onset transitional field geometry possessed a contribution from non axisymmetric terms in addition to the dominating zonal harmonics the analyses also suggest that the transitional field during the onset of the olduvai reversal was dominated by harmonics different from those of the matuyama brunhes boundary jaramillo transitions and upper olduvai reversal

a long term geomagnetic excursion from plio pleistocene sediments in java a paleomagnetic record from the plio pleistocene sediments in sangiran central java represents a long period change of the geomagnetic field and reveals an existence of large scale swing of declination between the olduvai and jaramillo events the field directions in reverse polarity show a gradual clockwise swing to almost west pointing directions followed by a rapid return to south pointing directions the geomagnetic behavior has been well duplicated from the plio pleistocene sediments in mojokerto 200 km west of sangiran at both sites the declinations have maximum deflection more than 70 from the axial dipole field direction and the inclinations remain low during the swing the full duration of the swing recorded over sections about 40 m thick at both sites was estimated about 130 000 years and a zone of most deviated fields 50 000 years assuming a constant sedimentation rate the phenomenon was regarded as an excursion neither geomagnetic excursion nor geomagnetic event corresponding to the entire duration of the excursion has been reported at present although only the final stage a duration of most deviated fields may be associated with the geomagnetic events reported from north america and europe an upward pointing eccentric radial dipole model rather than a downward pointing one seems plausible to explain the geomagnetic behavior during the excursion the former becomes one possible source for the inclination anomalies in the time averaged fields of the plio pleistocene times in the equatorial areas a short sequence of the upper olduvai transitional fields obtained from mojokerto exhibits a vgp path confined to a band of longitude 150 w 20 inconsistent with those from other sites on the globe this suggests that a dipolar component was not dominant globally in the upper olduvai transitional fields

palaeomagnetic excursions aborted reversals and transitional fields recordings of palaeomagnetic excursions have revealed apparent field behaviour ranging from rather erratic directional movements and loopings1 4 to single highly defined events5 7 excursions of the latter variety show what seems to be a rapid change in direction such that the path of the associated virtual geomagnetic poles vgps is well constrained in longitude such behaviour is not unlike that observed at the onset of some recorded polarity transitions7 9 indeed it has been suggested that excursions may occur during unsuccessful or aborted reversals2 5 7 10 we show here that available records of palaeomagnetic excursions together with our present understanding of field behaviour associated with geomagnetic reversals strongly support this hypothesis

the matuyama brunhes and upper jaramillo transitions recorded in a loess section at weinan north central china abstract the paleomagnetic records of the matuyama brunhes mb and upper jaramillo uj polarity reversals have been obtained from a loess section at weinan shaanxi province in china an investigation of the rock magnetic properties using stepwise irm acquisition hysteresis loops thermomagnetic analysis and x ray diffraction on magnetic extracts identifies pseudo single domain magnetite as the carrier of the main fraction of the remanence associated with maghemite and hematite the total duration of the directional change based upon independent loess accumulation rates is about 5000 yr for the mb reversal and about 3200 yr for the uj reversal these values are consistent with previous estimates obtained from marine or lacustrine sediments apart from a few intermediate points observed on north america mb and in the pacific uj the transitional virtual geomagnetic pole vgp paths are situated in two longitudinal bands over eastern asia and australia mb and the americas uj the bulk of the uj record and the part of the mb record over the americas are not consistent with the hypothesis of a record arising from smoothing of pre and post transitional directions 1 a reappraisal of the statistical approach of mcfadden et al 2 shows that the longitudinal distribution of the records of reversals is consistent with the hypothesis of genuine geographically preferred sectors for the paths independent of site longitude with a probability higher than 0 45 when the two weinan records are considered thus these results obtained from wind blown loess sustain the hypothesis of two preferential longitudinal bands for reversing transitional vgps suggested from the study of marine and lacustrine sediments

grain size dependence of initial susceptibility and saturation magnetization related parameters of four natural magnetites in the psd md range summary initial susceptibility coercive force and several types of remanent coercive forces were measured on a set of artificial rock specimens containing grain size fractions of a natural magnetite and titanomagnetite both of which are optically homogeneous and a natural titanomagnetite and titanomaghemite both of which show exsolution phenomena it was found subsequently that the optically homogeneous titanomagnetite contains some submicroscopic fe rich inclusions these inclusions are magnetically screened in coarse grains only for the lower grain sizes studied they have an appreciable influence on the data the size of the fractions varied from 5 to 250 m the parameters and several parameter ratios are plotted as a function of grain size several of these parameters can be used as mineralogical and grain size indicators the varying degree of exsolution of the minerals is reflected in the gradients of the plots from plots of initial susceptibility versus grain size it is concluded that the shape of the grains does not play a dominant role in determining the change of this parameter with grain size variation of the various parameters with grain size is explained in terms of a gradual transition from large psd grains to md grains without there being any sharp transition for titanomagnetite x 0 55 the transition takes place at grain sizes that are about 15 m larger than for magnetite

gauss matuyama polarity transition the gauss matuyama g m polarity transition has been identified in a 9 cm rotary drilled core from scaries valley california the lithology recovery and preservation of the sediments allow a detailed investigation of the transition the transition is estimated to have lasted approximately 2000 years about 2000 years earlier a short reversal also occurred that lasted 2000 years the relative field intensity decreased by at least 70 both during this short reversal and the g m transition the virtual geomagnetic poles path for the g m transition is restricted to a meridional band 20 wide in the atlantic ocean and closely matches the v g p path for the matuyama brunhes transition recorded at lake tecopa california on the other hand the pole path for the g m transition recorded in russia about 180 in longitude from searles valley is nearly antipodal at low latitudes to the learies valley pole path these data indicate that the transition field associated with the reversal was dominated by non dipole components

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

anomalously shallow palaeomagnetic inclinations and the question of the age of the canarian archipelago summary extensive geological geophysical and geochronological data available from the canary islands establish conclusively that formation of the oldest subaerial volcanic structure of the islands began during the miocene a mid cretaceous age for these volcanic formations has been postulated in previous works on the basis of palaeomagnetic determinations the results obtained in the present palaeomagnetic study of lanzarote include analysis of the record of variations of magnetic inclination in boreholes that penetrate the whole of the oldest volcanic series they show that the excessive age previously assigned to these formations is due to the utilization of volcanic units with abnormally low 15 magnetic inclinations lgis in lanzarote lavas exhibiting lgis appear interbedded in a volcanic series that overall shows a typical miocene inclination 45 the units stratigraphically beneath and above the lgi horizons give in fact directions consistent with the middle upper miocene field direction d 359 i 45 with k 29 a95 6 7 and a palaeopole of 87 n 178 e thereby confirming the miocene age of the oldest subaerial volcanics of lanzarote short excursions of the geomagnetic field seem likely to be the explanation for these lgis since other factors such as tectonic tilting post eruptive modification of the primary remanence or errors in sample orientation can be disregarded the detection in the canary archipelago of volcanic units with abnormally low magnetic inclinations seems to be related to the relatively continuous record of the geomagnetic field in rapidly growing volcanic edifices as seems to be indicated by the presence of a few usually one or two short polarity events in volcanic suites of several hundred metres thickness

paleomagnetism of a polarity transition in the lower triassic chug water formation wyoming a total of 156 oriented specimens taken at stratigraphic intervals of 3 to 20 cm have been collected from the red peak member of the chugwater formation at three localities approximately 25 km southeast of dubois wyoming the stratigraphic interval containing the polarity transition in the remanent magnetic field is about 1 0 m all specimens were thermally demagnetized at 500 550 620 and 660 c and show directions of magnetization of normal reversed and transitional character as well as a remarkable decrease of the remanent intensity in the transition zone the section sampled is divided into five intensity periods two periods recording the dipolar field one recording a dipolar field of decreasing intensity one recording a dipolar field of increasing intensity and a period of low intensity recording the field present during the 180 directional swing a virtual geomagnetic pole vgp path obtained for a n r polarity transition makes a fairly smooth transit with the path confined to a sector of longitude between 85 w and 110 e the reversal s onset is first indicated by a slow movement of the vgp across northern latitudes the vgp then lingers to two positions 60 n and 40 n 35 e to 70 e and 20 n and 40 n 335 e to 5 e suggesting brief periods of field stability or nondipolar field dominance the onset of the transition appears to be more rapid than the recovery the vgp path observed at a northern hemisphere site is located about 90 from the site the pole position derived from stable normal and reversed portions of this study 115 5 e 46 4 n 95 4 1 dp 2 2 dm 4 2 lies approximately in the center of the main group of poles reported for other triassic units

a detailed record of the lower jaramillo polarity transition from a southern hemisphere deep sea sediment core a detailed record of the lower jaramillo reversed to normal polarity transition was obtained from a southern hemisphere deep sea sediment core latitude 35 91 e longitude 59 97 e the record consists of over 850 samples taken across 140 cm of section the transition itself is recorded across approximately 70 cm and is represented by more than 475 specimens from about 160 levels giving intermediate directions the transition is identified by a nearly 180 shift from directions in good agreement with a reversed axial dipole field to those closely aligned with a normal axial dipole field for the core site latitude the inclinations shallow gradually early in the reversal and pass through very steep negative values 80 late in the transition the declinations show little appreciable variation until the inclinations have moved through the near vertical and then slowly approach values in agreement with a normal polarity field an intensity low accompanies the directional change during which the intensity drops to less than 15 of the maximum values observed in this sample interval the intensity fluctuation spans a wider interval than the directional change decreasing prior to any systematic change in the directions and then increasing to pre transition levels by the same depth at which the directions have stabilized the vgp path constructed for this reversal is longitudinally constrained to a certain extent between 140 and 230 for intermediate vgp latitudes and is roughly centered 120 from the site longitude this path is therefore a far sided vgp path in hoffman s 1977 terminology assuming a constant sedimentation rate 67m ma through the jaramillo subchron the duration of the transition is estimated to be 11 200 years to 4 500 years depending on the criteria for the directional change whereas the associated intensity variation occurred over 15 000 to 20 000 years considered together with records of the most recent reversal matuyama brunhes in light of current transitional field models this record strongly suggests that the lower jaramillo transitional field was dominated by different harmonics than the matuyam brunhes transitional field

high resolution sedimentary record of a geomagnetic reversal a detailed study of a geomagnetic reversal shows that during the transition the geomagnetic field undergoes both short and long term fluctuations the first are thought to be linked to the reversal process the second are present with the same time constants both before and during the reversal this suggests that the secular variation at least in its longer period terms is not affected by the reversal of the main field this feature has important consequences for the validity of the frozen flux approximation

continuous monitoring of high temperature fumaroles on an active lava dome volcan colima mexico evidence of mass flow variation in response to atmospheric forcing rapid multichannel monitoring of fumaroles on volcan colima mexico provides new insight into the time scales and magnitudes of fumarole temperature variation temperatures in five fumaroles all located along a single fracture cutting the summit lava dome of the volcano were monitored at 20 min intervals between may 1991 and may 1992 measurements were made using a programmable data logger deployed near the fumarole field and data were radiotelemetered to a nearby volcano observatory at regular intervals mean fumarole temperatures varied between 350 c and 550 c statistical analysis of these time series shows that significant diurnal variation occurs in each fumarole magnitudes of these daily fluctuations are generally between 25 c and 50 c although larger amplitude variations occur especially in cooler fumaroles simultaneous monitoring of atmospheric pressure at the fumaroles indicates that these variations in temperature are inversely correlated with barometric pressure these observations indicate that fumarole temperatures respond to atmospheric forcing a numerical model developed to explore the dependence of fumarole temperature on mass flow demonstrates that many aspects of observed temperature variation are accounted for by mass flow variation resulting from small changes in barometric pressure the relationship between mass flow and fumarole temperature is nonlinear the response of fumarole temperatures to a given change in mass flow is greatest in fumaroles with low mass flow and cool temperatures the nature of this dependence is little affected by fumarole geometry for the cases considered continuous measurement of fumarole temperatures may be an effective means of monitoring local mass flow on volcanoes at volcan colima average temperatures changed by less than 100 c during the 1 year sampling period during and immediately following effusive activity changes in degassing were abrupt and inconsistent along the length of the fracture following this period temperatures decreased gradually and there was a higher degree of correlation between fumaroles the method described here represents a substantial improvement over traditional fumarole monitoring techniques because subtle variation can be quickly identified using standard statistical techniques and the method provides regular information about thermal activity on a volcano minimizing the hazards normally associated with the collection of these data on a regular basis

olduvai termination detailed palaeomagnetic analysis of a north central pacific core summary a high resolution record of the termination of the olduvai normal to reverse subchron 1 77 myr polarity transition was obtained from a fully azimuthally oriented deep sea sediment piston core recovered at a northern mid latitude site 37 22 4 n 179o36 l w the n r olduvai reversal was continuously sampled at 3 to 5 mm intervals across 130 cm of section this continuous sampling was achieved by staggering the 1 cc plastic boxes incrementally so that offset segments corresponding to sampling windows of 3 to 5 mm were obtained the reversal is well defined and at least 10 truly intermediate directions have been recorded displaying a definite directional change of moo with a pronounced decrease in intensity of magnetization the decrease of magnetization and the directional change coincide closely the inclination record during the transition is characterized by a brief steepening trend early in the reversal passing through high positive values followed by a gradual shallowing trend this time passing through the horizontal from this point on the inclinations show a final settling about the reversed polarity mean this record is characterized by full normal and reverse polarity directions which are in good agreement f2 with those predicted by the geocentric axial dipole field model for the core site latitude the estimated duration of the directional change was about 10 ooo yr whereas that of the intensity decrease was between 12 ooo and 16 ooo yr anhysteretic remanent magnetization arm studies and thermomagnetic analyses indicate that a uniform magnetic mineralogy characterizes the transition zone of the core this further suggests that the j10 arm70 ratio may represent a relative palaeointensity estimate and may thus indicate a decrease of the intensity of the geomagnetic field during the reversal the virtual geomagnetic pole vgp path of the record is far sided directions in projected d 1 space indicate that the n r olduvai reversal transition field geometry had contributions from both non axisymmetric and axisymmetric terms the results based on this single record show that 1 the palaeofield at this locality during the olduvai transition was characterized by contributions from both zonal and non zonal terms but the contribution of non zonal quadrupolar terms was predominant this is in contrast to the observation by herrero bervera et al 1991 based on four deep sea records in which the vgps show general confinement within the longitude band of the americas and their antipode that the transitional field may be dominated by a dipolar component 2 the field intensity dropped to about 22 per cent of its initial or final recovered value during the transition it stayed at or close to this minimum for approximately half or less of the overall intensity change

late miocene magnetobiostratigraphy of crete six upper miocene marine clay sections on crete greece have been subjected to a detailed magnetobiostratigraphic analysis six correlatable polarity zones are recognized and these demonstrate the regional synchrony of planktonic foraminiferal biohorizons by way of correlation to the magnetic polarity time scale of lowrie and alvarez 1981 the cretan sequence is assigned to polarity chronozones 5 anomaly 3a and 6 the new chronology provides an age of 5 6 ma for the first occurrence datum fod of thegloborotalia conomiozea group in the mediterranean an age of 6 0 ma for the fod ofg menardii form 5 and an age of 6 6 ma for the last occurrence datum lod ofg menardii form 4 correlating the polarity record of the new zealand blind river section kennett and watkins 1974 with the magnetic polarity time scale provides an age of 6 0 ma for the evolutionary appearance ofgloborotalia conomiozea which is in complete agreement with the age of 6 1 0 1ma given by loutit and kennett 1979 the demonstrated diachrony of 0 4 ma between the new zealand fod ofg conomiozea and its mediterranean counterpart is explicable in view of the different nature of the two events the one in new zealand being evolutionary and the one in the mediterranean migrational the fod of theg conomiozea group in the tortonian messinian boundary stratotype section coincides with the level proposed by colalongo et al 1979 to mark the base of the messinian since the fod of theg conomiozea group in crete and in sicily are most probably time equivalent the age of the tortonian messinian boundary is fixed at 5 6 ma the youngest sediments incorporated in this study extend into the gilbert chronozone and antedate the main evaporitic phase consequently the messinian evaporitic body is younger than the base of the gilbert chronozone the age of which is fixed at 5 3 ma adopting an age of 5 0 ma for the miocene pliocene boundary would imply that evaporites and post evaporitic lago mare sediments were deposited in some 300 000 years and suggests that in the central parts of the mediterranean basins evaporites must have accumulated at rates of some 3 m per 1000 years