on the reliability of mesozoic dipole low new absolute paleointensity results from parana flood basalts brazil 1 thellier paleointensity experiments were carried out on early cretaceous parana flood basalts forty two samples from 11 lava flows yielded apparently reliable absolute intensity determinations the mean paleointensity values per flow obtained in this study are ranging from 19 4 4 0 to 46 7 7 0 t and the corresponding virtual dipole moments are ranging from 4 0 0 6 to 10 5 1 1 10 22 am 2 these yield a mean value of 7 2 2 3 x 10 22 am 2 which is about 92 of the present geomagnetic axial dipole our results in conjunction with tarduno et al s 2001 data suggest that the paleostrength during early cretaceous period might be comparable or even much higher than recent field intensity and not anomalousely low as previously suggested thus the mesozoic dipole low mdl may probably be considered as unreliable

the paleomagnetism of single silicate crystals recording geomagnetic field strength during mixed polarity intervals superchrons and inner core growth 1 the basic features of the geomagnetic reversal chronology of the last 160 million years are well established the relationship between this history and other features of the field however has been elusive the determination of past field strength paleointensity is especially challenging commonly accepted results have come from analyses of bulk samples of lava historic lavas have been shown to faithfully record the past field strength when analyzed using the thellier double heating method data from older lavas however tend to show effects of in situ and laboratory induced alteration here we review an alternative approach single plagioclase crystals can contain minute magnetic inclusions 50 350 nm in size that are potential high fidelity field recorders thellier experiments using plagioclase feldspars from an historic lava on hawaii provide a benchmark for the method rock magnetic data from older lavas indicate that the feldspars are less susceptible to experimental alteration than bulk samples this resistance is likely related to the lack of clays in addition magnetic minerals are sheltered by the encasing silicate matrix from natural alteration that can otherwise transform the well defined thermoremanent magnetization into an irresolute chemical remanent magnetization if there is a relationship between geomagnetic reversal frequency and paleointensity it should be best expressed during superchrons intervals with few or no reversals thellier data sets based on single plagioclase crystals from lavas erupted during the cretaceous normal polarity superchron 83 120 million years ago suggest a strong 12 1022 am2 stable field consistent with an inverse relationship between reversal frequency and paleointensity superchrons may represent times when the pattern of core mantle boundary heat flux allows the geodynamo to operate at peak efficiency as suggested in some numerical models thellier data from single plagioclase crystals formed during times of moderate 10 reversals million years reversal occurrence suggest a weaker and more variable field these paleointensity data together with a consideration of paleomagnetic directions suggest that geomagnetic reversals field morphology secular variation and intensity are related the linkages over tens of millions of years imply a lower mantle control on the geodynamo on even longer timescales the magnetization held by plagioclase and other silicate crystals can be used to investigate the proterozoic and archean geomagnetic field during the onset of growth of the solid inner core data from plagioclase crystals separated from mafic dikes together with directional data from whole rocks indicate a dipole dominated field similar to that of the modern 2 5 2 7 billion years ago older archean rocks are of great interest for paleomagnetic and paleointensity investigations because they may record a time when the compositionally driven convection of the modern dynamo may not have been operating and a solid inner core did not play its current role in controlling the geometry of outer core flow most rocks of this age have been affected by low grade metamorphism investigations using single silicate grains provide arguably our best hope of seeing through secondary geologic events and reading the early history of the geodynamo absolute paleointensity measurements of the oldest rocks on the planet will require the further development of methods to investigate silicate crystals with exsolved magnetic minerals that address the uncertainties posed by thermocrystallization remanent magnetization anisotropy and slow cooling fortunately prior work in rock magnetism together with advances in analytical equipment and techniques provides a solid foundation from which these frontier issues can be approached

palaeomagnetism and 40ar 39ar age from a cretaceous volcanic sequence inner mongolia china implications for the field variation during the cretaceous normal superchron abstract an integrated palaeomagnetic and 40ar 39ar dating study was carried out on an early cretaceous volcanic lava sequence from the suhongtu section inner mongolia to determine the field behavior within the cretaceous normal superchron cns 40ar 39ar ages were obtained from 12 lava flows indicating that the studied lava was formed around 114 1 0 3 ma for the lower interval and 110 6 0 1 ma for the upper interval rock magnetic experiments and electron microprobe analyses indicate that the primary fe ti oxides are the main magnetic carriers all lava flows carry normal palaeomagnetic directions which can be grouped into 31 units by an f test with a fisher mean characteristic remanent magnetization chrm of d i 12 8 58 6 95 2 3 the corresponding palaeomagnetic pole is located at 80 3 n and 200 3 e a95 3 2 k 64 4 which is indistinguishable at the 95 confidence level from the eurasia pole derived from the apparent polar wander path for the early cretaceous using a modified thellier palaeointensity method with stringent acceptance criteria we obtained two time series of palaeointensity records from 15 independent palaeomagnetic units total 136 samples the virtual dipole moment vdm values varied from 2 53 1022 am2 to 9 92 1022 am2 the mean vdms for the upper and lower intervals are 5 38 2 06 1022 am2 and 4 61 2 67 1022 am2 respectively the observed time series of palaeointensity together with the previously available data for the cns suggest that magnetic field strength during the cns fluctuated significantly with time

palaeo ice stream onsets examples from the north eastern laurentide ice sheet in this paper we report on observations of glacial landscapes at the head of geomorphologically interpreted palaeo ice streams i e palaeo ice stream onsets our work is based on the results of sy

paleointensity determinations from the etendeka province namibia support a low magnetic field strength leading up to the cretaceous normal superchron paleointensity estimates provide much needed information on field generation within earth s core and upon the convective processes at work within the mantle we present new paleointensity estimates from the early cretaceous etendeka large igneous province in namibia 135ma which add to the sparse southern hemisphere data set the early cretaceous marks an important change in the earth s magnetic field from a state of rapid polarity reversals to one of long term stability associated with the onset of the cretaceous normal superchron at 121 ma paleointensity determinations using the izzi protocol were carried out on a total of 172 specimens from 14 sites encompassing the exposed stratigraphy of the etendeka province numerous checks of data reliability were considered before results were accepted including partial thermoremanent magnetization ptrm checks and ptrm tail checks hysteresis properties thermomagnetic analyses observations under reflected light and changes to room temperature susceptibility during the experiments following these checks a total of 64 individual samples from five sites were considered to provide reliable paleointensity determinations these results were combined to provide site mean data with an overall average virtual dipole moment vdm for the study of 2 5 1 0 1022 am2 this value equates to approximately 30 of present earth s field and when considered alongside existing studies suggests that earth s field strength was low in the time leading up to the cretaceous normal superchron

mantle variation within the canadian shield travel times from the portable broadband archean proterozoic transect 1989 we report travel times from the archean proterozoic transect 1989 this type of data set recorded by a transect of portable broadband instruments allows us to make inferences about mantle structure in the region between the wyoming craton and the superior province of the canadian shield with station separations of 50 to 100 km and frequencies up to 5 hz the resolution of lateral changes is increased by nearly an order of magnitude over previous studies to a scale that allows us to study the relation between velocity variation in the continental upper mantle surface geology tectonic features and age provinces travel times of direct and later phases are obtained from waveform matching the values are corrected for crustal contribution and inverted for the vertical path upper mantle delay tum under each station as well as the azimuthal dependence of this quantity the prominent feature in the upper mantle delays tum is the variation by at least 1 5 s for s much of which occurs over a narrow zone of just a few hundred kilometers width this suggests a major lateral upper mantle transition which does not coincide with the surface geological edge of the canadian shield but is located within the shield this same transition is also observed in shear wave splitting delay times surprisingly however the p delays do not exhibit a corresponding variation we address this apparent contradiction and show how it may be explained in conjunction with anisotropy in the subcontinental lithosphere a simplified thermal model of the lithospheric transition zone in which temperature controls the degree of crystallographic alignment and thus seismic anisotropy predicts this phenomenon

linear and nonlinear thellier paleointensity behavior of natural minerals 1 we tested single domain sd and multidomain md theories of the acquisition and thermal demagnetization of thermoremanent magnetization trm and partial trm ptrm in thellier paleointensity determination using as samples plagioclase and mafic minerals extracted from matachewan diabase dikes susceptibility hysteresis and coercivity data indicate that the plagioclase contains mainly sd grains of magnetite while the mafic minerals clinopyroxene biotite and actinolite contain mainly md magnetite the extracted minerals were used in simulated thellier experiments with laboratory trm as a proxy for natural remanent magnetization nrm the plagioclase behaved ideally as predicted by neel sd theory giving a linear nrm ptrm arai plot with positive ptrm checks the mafic minerals behaved nonideally with strongly curved arai plots sagging below the ideal sd line in accord with observations for md magnetites and with predictions that in heating to moderate temperatures t more trm is lost than can be regained as ptrm causing convex down arai plots in experiments with ptrm parallel to nrm all ptrm tail checks were zero but ptrm checks increasingly deviated from the arai curve toward the sd line as t increased however in repeat thellier experiments ptrms at all t agreed with the original ptrms the negative ptrm checks were therefore not due to physicochemical alteration but rather to departure from initial states with different magnetic histories than those of the original ptrms the inability of ptrm checks to reliably distinguish between md behavior and irreversible alteration could lead to overestimates of paleointensity from straight line fits to the low t unaltered part of the arai curve or underestimates from fits to high t data alternating field cleaning to 15 mt before each thellier step made the arai plot only marginally less curved low temperature demagnetization ltd zero field cycling through the 120 k verwey transition hardly changed the curvature of the arai plot for ptrm parallel to nrm for ptrm perpendicular to nrm ltd led to an inflected arai curve straddling the ideal sd line data of either form would not give reliable paleointensity values no presently available pretreatment renders the paleointensity data of md grains usable sd and nearly sd grains which behave ideally in the thellier experiment remain the only useful recorders of absolute paleointensity

the variation of intensity of earth s magnetic field with time abstract a new and larger database is established to assess the variation of paleointensity with time it is shown that submarine basaltic glass probably contains a grain growth chemical remanent magnetization in magnetite intensity estimates from these samples are used to establish a lower bound estimate on intensities statistical tests on various subsets of the database are used to establish a more reliable database for finding long term trends of the paleointensity we can obtain a reasonable estimate for intensity versus time for the cenozoic but not for the mesozoic or late paleozoic the distributions of data as a function of time also provide valuable information in particular the two well documented superchrons exhibit different intensity properties which suggests that there is not a simple correlation between reversal rate and intensity

numerical simulations of two layer flow past topography part i the leeside hydraulic jump abstractlaboratory observations of the leeside hydraulic jump indicate it consists of a statistically stationary turbulent motion in an overturning wave from the point of view of the shallow water equations swe the hydraulic jump is a discontinuity in fluid layer depth and velocity at which kinetic energy is dissipated to provide a deeper understanding of the leeside hydraulic jump three dimensional numerical solutions of the navier stokes equations nse are carried out alongside swe solutions for nearly identical physical initial value problems starting from a constant height layer flowing over a two dimensional obstacle at constant speed it is demonstrated that the swe solutions form a leeside discontinuity owing to the collision of upstream moving characteristic curves launched from the obstacle consistent with the swe solution the nse solution indicates the leeside hydraulic jump begins as a steepening of the initially horizontal density interface subsequently the nse solution indicates o

paleointensity on volcanic glass of varying hydration states abstract we have characterised the magnetic properties of variably hydrated volcanic glasses that were collected from rhyolitic deposits at blahnukur torfajokull iceland the glasses span the range from fresh obsidians to highly fractured perlites that contain 2 wt water lava hydration plays a key role in the formation of perlite and hence these rocks are ideal to study hydration effects on remanence carriers and reliability of the paleomagnetic record the total volatile content of the different samples was determined as a proxy for the degree of perlitisation hydration it was found that coercivity of remanence saturation magnetisation and saturation of remanence decrease with increasing hydration i e that magnetic remanence carriers get lost and that magnetic stability is reduced additionally thermal demagnetisation of a three component isothermal remanence revealed that mainly the high coercive material is destroyed within the more strongly hydrated samples while lower coercive material seems to be less affected grain sizes of all but one samples are in the pseudo single domain range the one exception shows multi domain characteristics it was impossible to unambiguously identify the remanence carriers but titanomagnetites are most likely responsible for the lower coercivity component while hemoilmenites possibly represent the higher one a modified thellier method was used to determine paleointensity values as most of the samples are hydrated it is not astonishing that the overall paleointensity data is not of very high quality however it is important to note that there are hydrated samples with well defined arai diagrams although seemingly of high quality these paleointensity values are incorrect as there is a trend towards lower paleointensity values observed with increasing perlitisation we attempted to test for magnetic anisotropy and cooling rate dependency but this was hampered by alteration during the experiments however we argue that both anisotropy and cooling rate dependency are unlikely to be responsible for the observed trend in paleointensity thus even well defined paleointensity values can be erroneous when obtained from hydrated glass this emphasises the need for unaltered samples and additional attention during paleointensity determinations

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

palaeomagnetism of flood basalts in the pilbara craton western australia late archaean continental drift and the oldest known reversal of the geomagnetic field 1 a late archaean circa 2775 2715 ma succession of terrestrial continental flood basalts mafic tuffs felsic volcanic rocks and clastic sedimentary rocks in the nullagine synclinorium and meentheena centrocline of the east pilbara basin western australia has been sampled for a palaeomagnetic study over 500 oriented mostly basalt drill cores were collected from the supracrustal succession and associated dykes thermal and alternating field demagnetization revealed two distinct components positive fold conglomerate and reversal tests confirm that the primary natural remanent magnetization nrm is still preserved the secondary component is interpreted as the record of remagnetization during a major thermal event possibly in the early proterozoic analysis of the primary nrm directions results in a magnetostratigraphy and an apparent polar wander path apwp for the 60 myr interval covered by the sampled succession assuming a geocentric axial dipole during this time interval the apwp shows that the pilbara craton was drifting during the late archaean and that drift rates probably varied significantly in particular a mean 27 2 shift in palaeolatitude is recorded across an unconformity that represents a relatively short time period and that marks a significant change in basalt geochemistry this study suggests that continents moved horizontally during the late archaean and that the rates of movement were significantly faster than in the phanerozoic in addition a reversed polarity interval with a positive reversal test is recorded we argue that it documents the oldest known geomagnetic reversals of the geomagnetic field

lower mantle convection and geomagnetism the statistical properties of recent marine magnetic anomaly time scales covering the period from 165 ma to the present are analyzed via the methods presented in the companion paper mcfadden this issue the data no longer support the hypothesis that during the last 86 ma there were two periods separated by a discontinuity at about 45 ma when the properties were stationary instead there is strong evidence that the rate of reversals decreased in an approximately linear manner with time from 165 ma until it reached zero resulting in the cretaceous normal polarity interval at about 86 ma ago the process of reversals began again and the rate of reversals increased linearly with time there is a slight indication that the rate of reversals reached a maximum about 10 ma ago and is again decreasing analysis of the reversal sequences shows there is no good evidence for the concept of differences in the relative stabilities of the two polarity states nor need such a concept be invoked to explain long intervals without reversals such as the cretaceous normal polarity interval quite clearly there has been some form of thermal perturbation at the base of the mantle with a characteristic time of the order of 108 years our interpretations require convection deep within the mantle and suggest that the temperature at the base of the mantle attains a maximum during long magnetic quiet intervals

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

high geomagnetic intensity during the mid cretaceous from thellier analyses of single plagioclase crystals recent numerical simulations have yielded the most efficient geodynamo having the largest dipole intensity when reversal frequency is low reliable paleointensity data are limited but heretofore have suggested that reversal frequency and paleointensity are decoupled we report data from 56 thellier thellier experiments on plagioclase crystals separated from basalts of the rajmahal traps 113 to 116 million years old of india that formed during the cretaceous normal polarity superchron these data suggest a time averaged paleomagnetic dipole moment of 12 5 1 4 1022 amperes per square meter three times greater than mean cenozoic and early cretaceous late jurassic dipole moments when geomagnetic reversals were frequent this result supports a correlation between intervals of low reversal frequency and high geomagnetic field strength

the cretaceous superchron geodynamo observations near the tangent cylinder abstract if relationships exist between the frequency of geomagnetic reversals and the morphology secular variation and intensity of earth s magnetic field they should be best expressed during superchrons intervals tens of millions of years long lacking reversals here we report paleomagnetic and paleointensity data from lavas of the cretaceous normal polarity superchron that formed at high latitudes near the tangent cylinder that surrounds the solid inner core the time averaged field recorded by these lavas is remarkably strong and stable when combined with global results available from lower latitudes these data define a time averaged field that is overwhelmingly dominated by the axial dipole octupole components are insignificant these observations suggest that the basic features of the geomagnetic field are intrinsically related superchrons may reflect times when the nature of core mantle boundary heat flux allows the geodynamo to operate at peak efficiency

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

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

the role of the earth s mantle in controlling the frequency of geomagnetic reversals a series of computer simulations of the earth s dynamo illustrates how the thermal structure of the lowermost mantle might affect convection and magnetic field generation in the fluid core eight different patterns of heat flux from the core to the mantle are imposed over the core mantle boundary spontaneous magnetic dipole reversals and excursions occur in seven of these cases although sometimes the field only reverses in the outer part of the core and then quickly reverses back the results suggest correlations among the frequency of reversals the duration over which the reversals occur the magnetic field intensity and the secular variation the case with uniform heat flux at the core mantle boundary appears most earth like this result suggests that variations in heat flux at the core mantle boundary of the earth are smaller than previously thought possibly because seismic velocity anomalies in the lowermost mantle might have more of a compositional rather than thermal origin or because of enhanced heat flux in the mantle s zones of ultra low seismic velocity

a new quality check for absolute palaeointensities of the earth magnetic field absolute palaeointensity determinations of recent lavas from mt etna have been carried out using the koenigsberger thellier thellier ktt method but with two different data presentations the first which is the standard method compares the natural remanent magnetization nrm demagnetized and the thermoremanent magnetization trm acquired in successive temperature intervals cumulatively the second compares the nrm and trm in the successive intervals in an incremental manner the latter method appears to distinguish aberrant behaviour better than the classical method such aberrant behaviour can be related to changes in hysteresis ratio parameters saturation remanent magnetization to saturation magnetization and remanent coercive force to coercive force which are reflected by displacements in the day et al 1977 plot these changes in hysteresis parameters are caused by irreversible changes in domain state or transformations of magnetic phases brought about by the thermal cycling the changes in hysteresis parameters serve as a useful quality factor for absolute palaeointensity determinations

thermochemical remanent magnetization in precambrian rocks are we sure the geomagnetic field was weak 1 thellier paleointensity determinations from two dikes of the early proterozoic 2 46 ga matachewan dike swarm canada yield field values of 2 14 0 18 and 9 81 1 65 t corresponding values of virtual axial dipole moment are very low 0 54 0 05 1022 and 2 49 0 42 1022 am2 respectively when compared with the modern field the characteristic remanent magnetization chrm was isolated over a narrow range of high unblocking temperatures 520 580 c detailed rock magnetic analyses indicate that the chrm is carried by nearly stoichiometric pseudo single domain magnetite scanning electron microscopy sem reveals that the magnetite is in the form of fine intergrowths with ilmenite formed by oxyexsolution during cooling the high temperature oxidation defined in the sem images could have continued at temperatures below the curie point of magnetite in this case the chrm would be a thermochemical remanent magnetization tcrm rather than a thermal remanent magnetization trm estimates of the tcrm trm ratio show that the thellier data could underestimate the true field value by a factor of 4 without violating experimental selection criteria this uncertainty in trm fidelity translates into a potential range of field values that spans that defined by the modern field 8 1022 am2 and proposed low precambrian levels 2 1022 am2 therefore understanding further how tcrm is acquired and records the field represents a major challenge if these and many other similar rocks are to be used in thellier studies aimed at defining the strength of the precambrian field

analysis of an updated paleointensity database q pi pint for 65 200 ma implications for the long term history of dipole moment through the mesozoic

superchron cycles driven by variable core heat flow 1 a numerical dynamo with time variable core heat flow and inner core growth rate links the thermal state of the earth s core to the long term trends in the geomagnetic polarity time scale gpts increasing core heat flow over 100 myr drives the numerical dynamo from a superchron state to reversing behavior whereas a similar decrease in core heat flow produces a decrease in reversal frequency driving the dynamo back into a superchron the polarity sequence including more than 400 reversals and two superchrons compares favorably with the gpts as does the dynamo frequency spectrum we find that core heat flow is positively correlated with polarity reversal frequency dipole variability and the relative strength of the non dipole field our results imply that changes in geomagnetic reversal frequency can be attributed to long period fluctuations in core heat flow and suggest heat flow minima during superchrons and a maximum near the present day

thellier paleointensity theory and experiments for multidomain grains 1 we extend theories of thermoremanent magnetization trm and partial trm ptrm in multidomain md grains to model thermal demagnetization and ptrm acquisition steps in thellier paleointensity determination because of the interleaving of zero field and in field heating cooling steps to increasing temperatures the initial state for any step is complex and theoretical modeling is more intricate than for ptrm production or thermal demagnetization separately at low to moderate temperature t trm lost exceeds ptrm regained causing convex down arai plots sagging below the ideal single domain sd line at moderate to high t ptrm acquisition outweighs trm loss as t approaches tcurie ptrm gain exactly equals trm loss and the arai plot becomes ideal when ptrms are produced perpendicular to the original trm and measured directly rather than by differencing field on and field off results there is less deviation from ideality at low to moderate t our theory agrees semiquantitatively with results for parallel and perpendicular ptrms for large md magnetites 135 m smaller md magnetites 6 and 20 m have less curved arai plots and the smallest magnetites 0 6 and 1 m have almost linear plots positive ptrm checks demonstrate that curved arai plots of md grains are reproducible unlike curved plots for rocks that alter physicochemically in the thellier experiment while negative ptrm tail checks indicate undemagnetized ptrm residuals low temperature demagnetization improves linearity only slightly practical applications of this work include using the predicted threshold t below which no net ptrm is produced in a parallel thellier experiment to screen data used for paleointensity fits straight line fits through low and medium t points in arai plots of md 135 m grains overestimated the paleofield by as much as 100 and for small pseudo single domain psd 0 6 and 1 m grains overestimated by about 25 however by using linear segments of medium to high t data with f values 0 5 it may be possible to obtain reasonable paleointensity estimates even for larger psd 6 and 20 m and md grains middle to high t fits for 0 6 m grains gave paleointensities within 4 of the correct value utilizing essentially the entire data set f 0 9 perpendicular data always gave superior linear fits orienting samples with their natural remanent magnetizations perpendicular to the laboratory field is therefore recommended for rocks containing psd and md grains however double heatings are preferable to single heatings because they allow ptrm tail checks to be carried out

geomagnetic secular variation in the cretaceous normal superchron and in the jurassic abstract it is now widely thought that geomagnetic polarity reversals occur spontaneously as a result of normal dynamo action rather than being externally triggered if this is the case then it may well be that periods of time in which the geomagnetic reversal frequency was dramatically different were characterised by different styles of secular variation two such periods were the cretaceous normal superchron cns 84 125 ma when the field was dominantly of a single polarity for 40 myr and the jurassic period 145 200 ma when reversals occurred at an average rate of as much as 4 6 myr 1 here we analyse a database of new and published palaeomagnetic directions from lavas emplaced during these periods in order to obtain first order descriptions of the palaeosecular variation psv during these times we then compare these records with one another and with that produced for the period 0 5 ma with average reversal frequency of 4 0 myr 1 our results are more equivocal than those obtained in a previous similar study mcfadden p l merrill r t mcelhinny m w lee s h 1991 reversals of the earths magnetic field and temporal variations of the dynamo families journal of geophysical research solid earth and planets 96 3923 3933 we demonstrate that this is probably a result of the previous study being affected by an artefact of their correction for within site scatter the usefulness of our jurassic record is severely limited by the restricted palaeolatitudinal span of the available data however our record for the cns is sufficient to allow us to conclude that it was likely that secular variation then was different from that in the 0 5 ma period this supports the hypothesis of a link between psv and reversal frequency and therefore endorses psv analysis as a first order tool for determining geomagnetic stability in the past

possible links between long term geomagnetic variations and whole mantle convection processes the earth s internal magnetic field varies on timescales of months to billions of years the field is generated by convection in the liquid outer core which in turn is influenced by the heat flowing from the core into the base of the overlying mantle much of the magnetic field s variation is thought to be stochastic but over very long timescales this variability may be related to changes in heat flow associated with mantle convection processes over the past 500 myr correlations between palaeomagnetic behaviour and surface processes were particularly striking during the middle to late mesozoic era beginning about 180 myr ago simulations of the geodynamo suggest that transitions from periods of rapid polarity reversals to periods of prolonged stability such as occurred between the middle jurassic and middle cretaceous periods may have been triggered by a decrease in core mantle boundary heat flow either globally or in equatorial regions this decrease in heat flow could have been linked to reduced mantle plume head production at the core mantle boundary an episode of true polar wander or a combination of the two

magnetic hysteresis monitoring of cretaceous submarine basaltic glass during thellier paleointensity experiments evidence for alteration and attendant low field bias on the basis of studies of holocene samples submarine basaltic glass sbg is thought to be an ideal paleointensity recorder because it contains unaltered single domain magnetic inclusions that yield thellier paleointensity data of exceptional quality to be useful as a recorder of the long term geomagnetic field older sbg must retain these optimal properties here we examine this issue through rock magnetic and transmission electron microscope tem analyses of cretaceous sbg recovered at ocean drilling program site 1203 northwestern pacific ocean these sbg samples have very low natural remanent magnetization intensities nrm 2 g and tem analyses indicate a correspondingly low concentration of crystalline inclusions thellier experiments on samples with the strongest nrm intensity 5 10 11 am 2 show a rapid acquisition of thermoremanent magnetization trm with respect to nrm demagnetization taken at face value this behavior implies magnetization in a very weak 17 t ambient field but monitoring of magnetic hysteresis properties during the thellier experiments on subsamples of the sbg samples used for paleointensity determinations indicates systematic variations in values over the same temperature range where the rapid trm acquisition is observed a similar change in properties during heating is observed on monitor sbg specimens using low temperature data with progressive heatings the verwey transition becomes more distinct we suggest that these experimental data record the partial melting and neocrystallization of magnetic grains in sbg during the thermal treatments required by the thellier method resulting in paleointensity values biased to low values we further propose that this process is pronounced in cretaceous and jurassic sbg relative to holocene sbg because devitrification on geologic time scales i e tens of millions of years lowers the transition temperature at which the neocrystallization can commence magnetic hysteresis monitoring may provide a straightforward means of detecting the formation of new magnetic inclusions in sbg during thellier experiments

low geomagnetic field intensity during the kiaman superchron thellier and microwave results from the great whin sill intrusive complex northern united kingdom 1 coe modified thellier cmt and microwave mw paleointensity experiments were conducted on samples from the multi phase late carboniferous ca 301 ma to early permian ca 270 ma great whin sill complex statistically comparable results were obtained with both techniques from high quality data forty three samples 25 with cmt and 18 with mw yielded mean paleointensity values of 12 0 5 8 t and 16 5 8 2 t for the early and late intrusive phases respectively corresponding to vdms of 1 6 0 8 1022 and 2 1 1 1 1022 am2 when combined with previous data the results suggest a generally low dipole moment during the krs reducing from 75 of the present value at 301 ma to 20 27 of the present value later in the superchron these data imply either a decoupling between processes controlling reversal rate and field strength or a more complex intriguing time lag relationship more absolute palaeointensity results are needed to further evaluate these alternatives

detecting multidomain magnetic grains in thellier palaeointensity experiments the thellier technique is the most commonly used method for absolute palaeointensity studies it is based on thellier s laws stating that partial thermoremanent magnetisations ptrms are independent and additive which however only are true for non interacting single domain sd grains for multidomain md grains individual ptrms are not independent and thellier palaeointensity experiments on rocks with a significant proportion of md grains may therefore lead to erroneous palaeofield estimates by introducing a ptrm tail check in the coe version of the thellier technique j geophys res 72 1967 3247 we present a simple method to detect and discard such rocks the usefulness of the ptrm tail check is exemplified by thellier palaeointensity experiments performed on two palaeomagnetic sample collections with different magnetomineralogy miocene baked sediments from french massif central are shown to be ideal for thellier studies with the remanent magnetisation residing in thermally stable sd grains for palaeocene eocene basaltic lavas from faeroe islands 64 of otherwise acceptable palaeointensity estimates are discarded due to the failure of ptrm tail checks indicating the presence of significant concentrations of md grains 2001 elsevier science b v all rights reserved

dipole strength and variation of the time averaged reversing and nonreversing geodynamo based on thellier analyses of single plagioclase crystals 1 single plagioclase crystals separated from lavas can contain minute magnetic inclusions that faithfully record the geomagnetic field while the silicate host limits natural and experimentally induced alteration here we use paleointensity analyses of plagioclase crystals to examine three characteristic geodynamo regimes spanning the last 200 myr an interval of moderate reversal frequency during the paleogene the nonreversing field of the cretaceous normal polarity superchron and a period of very high reversal occurrence during the late jurassic an inverse relationship between reversal rate and intensity is supported furthermore the strength of the time averaged reversing field is more variable than that of the nonreversing field these observations are consistent with an active lower mantle shaping core mantle boundary heat flux and efficiency of the geodynamo on timescales of tens of millions of years

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

geomagnetic palaeointensities during the cretaceous normal superchron measured using submarine basaltic glass high quality palaeointensity data have been obtained from thellier thellier experiments on recent and cretaceous submarine basaltic glasses whereas the recent samples faithfully yield today s geomagnetic intensity at the site the palaeointensities for the beginning and end of the cretaceous normal superchron are only 45 and 25 respectively of today s value the data thus extend the mesozoic dipole low into the cretaceous superchron and confirm that submarine basaltic glass is an excellent material for palaeointensity studies

revisiting a domain state independent method of palaeointensity determination abstract the use of a domain state independent method the wilson method of acquiring absolute palaeointensities is tested using three sets of historical lavas from mexico italy and iceland the wilson method works by plotting continuous thermal demagnetisation curve data for the original natural remanent magnetisation against those for a laboratory induced thermoremanence by comparing the predicted intensities with the known fields a new empirically derived rejection criterion is proposed that accounts for chemical alteration and improves the accuracy of the palaeointensity estimates the wilson palaeointensity estimates are compared with palaeointensity determinations made on sister samples using standard thellier type heating generally the wilson palaeointensity estimates compare favourably for samples that yielded technically correct thellier type determinations however at one locality the wilson method did not yield any estimates due to severe chemical alteration in contrast it was still possible to determine palaeointensity estimates from the lower temperature thellier data to assess the wilson palaeointensity protocol s ability to return a correct estimate for a true multidomain system a synthetic multidomain magnetite sample was imparted with a thermoremanent magnetisation the wilson protocol returned the correct palaeointensity within error in contrast the thellier type protocol failed as the wilson method is significantly faster than thellier type methods it is suggested that the wilson method should be tried as a first approach in palaeointensity studies or for samples dominated by multidomain material

do superchrons occur without any palaeomagnetic warning abstract the mainly dipolar geomagnetic field generated by the geodynamo within the earth s liquid core has reversed its polarity many times in the past this succession of intervals of alternate polarity defines the geomagnetic polarity time scale gpts usually interpreted as resulting from a gamma renewal process the rate of which would be controlled by the boundary conditions imposed by the mantle on the core in this interpretation boundary conditions would have occasionally evolved towards being unfavourable to reversals leading the reversal rate of the process to progressively decrease and reach zero at least twice causing the onset of the so called kiaman and cretaceous very long polarity intervals superchrons here we reconsider this causal link analysing the latest and best constrained gpts thanks to the continuous record provided by marine magnetic anomalies describing the past 160 myr we found that contrary to earlier claims no long term behaviour over the 40 myr preceding the cretaceous superchron can be seen in the reversal rate that could explain its onset at 120 ma more generally it turns out that hardly any special behaviour can be identified in the gpts which could have announced this superchron only the occurrence of the longest of all pre superchron intervals cm1n 3 myr before the onset of the superchron could be identified as a precursor such a behaviour could possibly be the consequence of medium term on the 10 myr time scale changes in the boundary conditions imposed by the mantle on the core but we note that the sole analysis of the gpts does not allow this to be tested in fact it appears that a single stationary process could also explain the entire pre superchron gpts with the only possible exception of cm1n this suggests that the occurrence of superchrons does not necessarily require changes in boundary conditions and could simply attest for a sudden non linear transition between a reversing and a non reversing state of the geodynamo

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

recent investigations of the 0 5 ma geomagnetic field recorded by lava flows we present a synthesis of 0 5 ma paleomagnetic directional data collected from 17 different locations under the collaborative time averaged geomagnetic field initiative tafi when combined with regional compilations from the northwest united states the southwest united states japan new zealand hawaii mexico south pacific and the indian ocean a data set of over 2000 sites with high quality stable polarity and declination and inclination measurements is obtained this is a more than sevenfold increase over similar quality data in the existing paleosecular variation of recent lavas psvrl data set and has greatly improved spatial sampling the new data set spans 78 s to 53 n and has sufficient temporal and spatial sampling to allow characterization of latitudinal variations in the time averaged field taf and paleosecular variation psv for the brunhes and matuyama chrons and for the 0 5 ma interval combined the brunhes and matuyama chrons exhibit different taf geometries notably smaller departures from a geocentric axial dipole field during the brunhes consistent with higher dipole strength observed from paleointensity data geographical variations in psv are also different for the brunhes and matuyama given the high quality of our data set polarity asymmetries in psv and the taf cannot be attributed to viscous overprints but suggest different underlying field behavior perhaps related to the influence of long lived core mantle boundary conditions on core flow psv as measured by dispersion of virtual geomagnetic poles shows less latitudinal variation than predicted by current statistical psv models or by previous data sets in particular the brunhes data reported here are compatible with a wide range of models from those that predict constant dispersion as a function of latitude to those that predict an increase in dispersion with latitude discriminating among such models could be helped by increased numbers of low latitude data and new high northern latitude sites tests with other data sets and with simulations indicate that some of the latitudinal signature previously observed in vgp dispersion can be attributed to the inclusion of low quality insufficiently cleaned data with too few samples per site our matuyama data show a stronger dependence of dispersion on latitude than the brunhes data the taf is examined using the variation of inclination anomaly with latitude best fit two parameter models have axial quadrupole contributions of 2 4 of the axial dipole term and axial octupole contributions of 1 5 approximately 2 of the octupole signature is likely the result of bias incurred by averaging unit vectors