paleomagnetism from deception island south shetlands archipelago antarctica new insights into the interpretation of the volcanic evolution using a geomagnetic model deception island shows the most recent exposed active volcanism in the northern boundary of the bransfield trough the succession of the volcanic sequence in the island is broadly divided into pre and post caldera collapse units although a well constrained chronological identification of the well defined successive volcanic episodes is still needed a new paleomagnetic investigation was carried out on 157 samples grouped in 20 sites from the volcanic deposits of deception island south shetlands archipelago antarctic peninsula region distributed in 1 volcanic breccia 3 sites and lavas 2 sites prior to the caldera collapse 2 lavas emplaced after the caldera collapse 10 sites and 3 dikes cutting pre and the lowermost post caldera collapse units 5 sites the information revealed by paleomagnetism provides new data about the evolution of the multi episodic volcanic edifice of this quaternary volcano suggesting that the present day position of the volcanic materials is close to their original emplacement position the new data have been combined with previous paleomagnetic results in order to tentatively propose an age when comparing the paleomagnetic data with a global geomagnetic model despite the uncertainties in the use of averaged paleomagnetic data per volcanic units the new data in combination with tephra occurrences noted elsewhere in the region suggest that the pre caldera units f1 and f2 erupted before 12 000 year bc the caldera collapse took place at about 8300 year bc and post caldera units s1 and s2 are younger than 2000 year bc

geomagnetic field intensity between 70 000 and 130 000 years b p from a volcanic sequence on la reunion indian ocean abstract a detailed paleointensity study was made of a sequence of 70 successive lava flows of the piton des neiges volcano on the island of la reunion indian ocean radiometric dating brackets the age of this sequence between 130 3 ka and 72 3 ka rock magnetic investigations show that titanomagnetites in the pseudo single domain range are the main magnetic carrier of the natural remanent magnetisation nrm over 350 samples were used for paleointensity determinations carried out with the thellier method in vacuum or in an argon atmosphere of these 89 samples yielded reliable results with within flow scatter often lower than 20 these results indicate that the geomagnetic field intensity has varied at la reunion between 13 and 65 t during the period of time explored the average value 42 t is higher than the present field at la reunion 35 t the results from the upper part of the section are consistent with previous results obtained for the 82 98 ka period also at la reunion 1 and document a broad low around 95 ka not associated with large directional changes on a larger geographic scale the paleointensity values from la reunion are significantly higher than those obtained from mount etna 2 precise comparison is however difficult because of the lack of detail in the etna results in the lower part of the section a marked intensity low coinciding with significant deviation from the dipole field direction is observed at 115 ka and could correspond to the end of the blake event

paleosecular variation record of geomagnetic full vector during late miocene from the nayarit area mexico abstract we have sampled a sequence of 45 late miocene consecutive lava flows in the tepic area nayarit state mexico the age of the volcanic units lies between 8 and 9 million years ma according to available radiometric data all samples were stepwise demagnetized partly with alternating field af partly thermally with very similar results most of the rocks exhibited well defined one component remanent magnetisation with high unblocking temperatures mostly above 525 c and high median destructive fields mdf 40 50 mt rock magnetic experiments combined with microscopy show that in most cases the main magnetic mineral is ti poor titanomagnetite associated with exsoluted ilmenite continuous susceptibility measurements with temperature and hysteresis experiments yield in most cases nearly reversible curves with curie points close to that of magnetite and pseudo single domain characteristics characteristic remanent magnetisations chrm isolated after the first steps of demagnetisation are all normal polarity according to the dispersion of virtual geomagnetic pole vgp directions paleosecular variation was abnormally lower than the one observed in general during miocene considering our paleomagnetic results together with available radiometric data it seems that the volcanic units have been emplaced during a very short time span of about 0 08 million years the mean paleomagnetic directions obtained from this study do not differ significantly from that expected for the middle miocene thirty one samples from eight individual flows yielded acceptable paleointensity estimates the site mean paleointensities range from 27 8 0 9 to 42 0 7 9 t the virtual dipole moments vdm range from 5 9 to 9 5 1022 am2 this corresponds to a mean value of 7 6 1 4 1022 am2 which is higher than the average vdm value for late miocene altogether our data suggest the existence of relatively high geomagnetic field strength undergoing low fluctuations these results support the theoretical suggestion about an inverse relationship between secular variation and local field strength as result of electromagnetic coupling between the solid inner core and liquid outer core with the inner core tending to stabilise core convection and hence the field when intensity is high some fluctuation of absolute intensity was detected within the same directional group dg indicating that the intensity of the geomagnetic field varies faster than its direction

paleomagnetism and 40ar 39ar chronology of lavas from meseta del lago buenos aires patagonia 1 as part of a larger project investigating paleosecular variation in south america 36 paleomagnetic and geochronology sites were sampled on meseta del lago buenos aires 47 s 289 e 200 km east of the active plate boundary in southern patagonia basaltic lavas sampled range from late miocene to late pleistocene yet most of the individual lavas are younger than 3 3 ma thirty eight isochron ages determined via 40ar 39ar incremental heating experiments frame a detailed stratigraphy the isochron ages range from 66 9 4 1 ka for scoria cones and a youthful lava flow in the rio pinturas valley to 10 12 1 35 ma for thick plateau forming lavas exposed along the se edge of the meseta the last 3 3 myr is characterized by seven episodes of volcanism at ca 3 2 3 0 ma 2 4 ma 1 7 ma 1 35 ma 1 0 ma 750 ka 430 330 ka and 110 ka the bulk of lavas forming the surface of the meseta erupted in the last 1 7 my all sites have stable magnetization and after step demagnetization using either thermal or alternating field techniques yield characteristic directions held by magnetite and or titanomagnetite ten sites have distinct transitional directions defined by pole latitudes 55 and associated ages indicate possible connections to known reversals within the matuyama chron including the onset of the jaramillo subchron 1 016 0 01 ma the cobb mountain subchron 1 25 0 03 ma the ontong java 1 event 1 37 0 03 ma and the termination of the olduvai subchron 1 72 0 02 ma remaining sites are divided into normal 14 sites and reversed polarity 12 sites at 95 confidence mean normal and reversed directions overlap the mean direction for 26 sites is i 63 0 d 3 4 95 5 4 which is indistinguishable from the expected geocentric axial dipole paleosecular variation measured by the dispersion of virtual geomagnetic poles about the rotation axis is higher than expected at 20 0 model g using data for the past 5 myr predicts a dispersion of 17 this discrepancy may be due to true dispersion of the field in southern south america or it may be an artifact of inadequate sampling

plio pleistocene time averaged field in southern patagonia recorded in lava flows 1 paleomagnetic directions were obtained from stepwise alternating field or thermal demagnetization of 53 lava flows from southern patagonia latitudes 49 5 52 1 s that include the pali aike volcanic field and the meseta viscachas plateau lavas in addition to previous miocene late quaternary ages of these flows 40ar 39ar dates spanning from 0 1 to 15 4 ma were obtained for 17 of the sites all except one of the magnetic polarities coincide with the expected polarities of the magnetic polarity timescale cande and kent 1995 for the obtained 40ar 39ar ages the mean direction from 33 sites eliminating sites 4 ma that pass a selection criteria of 95 5 is dec 358 7 inc 68 2 95 3 5 a value that coincides within the statistical uncertainty with the direction of the geocentric axial dipole for that area inc 68 1 likewise the mean virtual geomagnetic pole vgp coincides within the statistical uncertainty with the geographic north pole the secular variation described by the vgp angular standard deviation for these sites is 17 1 a value expected for that latitude according to model g of paleosecular variation mcfadden et al 1988 the characteristics of the data presented are optimum for time averaged field taf studies because of the good age control and good quality of the paleomagnetic data 1 primary components of magnetization were obtained using principal component analysis kirschvink 1980 from at least five points and maximum angular deviation 5 2 site means were calculated with fisher statistics using at least three samples and c 38 of the 53 flows had 95 5 no results five sites or high 95 values 5 were obtained primarily from sites affected by lightning

paleomagnetic secular variation of the honolulu volcanic series 33 700 ka o ahu hawaii abstract thirty years after the precursory study j geophys res 13 1972 2129 19 dated sites with ages between 33 and 700 ka from the honolulu volcanic series hvs southeastern part of the island of o ahu in hawaii have been revisited a minimum of 16 samples per site have been studied using alternating field and thermal demagnetization both techniques yielded similar results coeval flows could be identified by relying on their k ar ages similar rock magnetic parameters and identical directions reliable paleomagnetic directions have been obtained for all sites two site mean directions which deviate significantly from the geocentric axial dipole gad correspond to periods of low dipole intensity and may thus reflect a larger proportion of non dipole components the average value of the site mean declinations is 358 6 while the mean inclination of 34 5 is 3 5 lower than the value of the geocentric dipole at the site the north geographic pole is largely contained within the uncertainties surrounding the position of the mean virtual geomagnetic poles vgp latitude the deviation of 2 3 in latitude is further reduced by 1 if we consider the existence of a standing small quadrupole component that would represent 5 of the dipole the angular dispersion of the virtual geomagnetic poles 12 is only 1 7 smaller than expected at the site latitude and thus not a firm indication of low secular variation thus there is no significant difference with the global time averaged field for the same period other recent results from the same island for an older time period display similar characteristics but with reverse and normal polarities that are not perfectly antipodal it is suggested that this could result from changes in the mean dipole field intensity during these time intervals this points out to the importance of scrutinizing the entire field vector in any analysis of paleosecular changes these results do not bring any evidence for a persistent field anomaly under hawaii

the growth collapse and quiescence of teno volcano tenerife new constraints from paleomagnetic data tenerife basically consists of three miocene shield volcanoes the anaga the teno and central shield as well as the pliocene canadas volcano the temporal evolution and structural significance of each volcano with respect to the history of tenerife is still a matter of debate we present paleomagnetic results in order to enhance the view of the volcanic history of the teno volcano by means of magnetostratigraphy it is found that the initial subaerial phase shows reverse magnetizations throughout after two major sector collapses dominantly normally magnetized lavas extruded comparisons of observed magnetic polarities with the geomagnetic polarity timescale show that these volcanic activities occurred within 0 4 myr between 6 3 and 5 9 ma significantly younger flows 5 3 myr old according to their radiometric age revealed again normal polarity throughout the absence of inversely magnetized lavas in between the two normal periods indicates a volcanic hiatus or erosional phase the evolutionary sequence and the estimated high production rates for the initial building phase are similar as would be expected for a hotspot volcano the average geomagnetic field for 6 0 0 2 ma is close to an axial dipole field showing a slight far sided right handed effect the field strength determined by thellier type intensity determinations corresponds to a virtual axial dipole moment of 4 9 1022 a m2 this value is approximately half of the present day field strength but similar to values obtained for the mid miocene it also corresponds to the proposed tertiary low field level of the geomagnetic dipole moment

palaeomagnetic results from the indian heaven volcanic field south central washington summary palaeomagnetic data have been obtained from plio pleistocene rocks of the indian heaven volcanic field ihvf located in the high cascade province in southern washington alternating field demagnetization yielded well defined directions from 56 individual lava flows sites from 23 volcanic units all units sampled were normally magnetized suggesting that volcanic activity within the indian heaven region occurred primarily during the last 0 73 myr and not over the last 3 75 myr as suggested by k ar ages combining palaeomagnetically similar site mean directions from lava flows from the same volcanic unit resulted in n 35 the mean direction calculated from this data set is i 65 2 and d 0 2 095 3 3 k 56 4 while the palaeomagnetic pole calculated using virtual geomagnetic pole vgp data is located at 87 7 n 231 4 e or95 4 7 k 27 2 this pole is not significantly different from the pole determined from all data n 56 or the rotational axis and is slightly near sided with respect to the mean sampling site longitude the between site angular dispersion of the vgps about the mean for n 35 and n 56 although low is not statistically different at the 95 confidence level from the expected value predicted by model f of mcfadden mcelhinny 1984 our data together with other recent data from the western united states do not confirm the presence of a persistent standing non dipole field component in this region during the plio pleistocene as suggested by brown mertzman 1979 additionally these data suggest that vgp angular dispersion associated with the geomagnetic field has been decreasing over the last 5 myr

an assessment of the accuracy of admittance and coherence estimates using synthetic data previous work has shown that estimates of the admittance between topography and free air gravity anomalies are often biased by spectral leakage even after the application of multiple prolate spheroidal wavefunction data tapers despite this a number of authors who have used the free air admittance method to estimate the weighted average effective elastic thickness of the lithosphere te and to identify topography supported by mantle convection have not tested their methods using synthetic data with a known relationship between topography and gravity in this paper i perform a range of such tests using both synthetic surface data and synthetic line of sight los accelerations of satellites orbiting around an extra terrestrial planet it is found that spectral leakage can cause the estimated admittance and coherence to be significantly in error but only if the box in which they are estimated is too small the definition of small depends on the redness of the gravity spectrum there is minimal error in the whole box weighted average estimate of te if the admittance between surface gravity and topography is estimated within a box at least 3000 km wide when the synthetic uniform te is less than 20 km and the coherence is high the errors in te are mostly 5 km for all box sizes greater than 1000 km on the other hand when the true te is greater than 20 km and the box size is 1000 km the best fitting te is likely to be at least 5 10 km less than the true te however even when the coherence is high it is not possible to use elastic plate admittance models to distinguish between real and spurious small fractions of internal loading when the boxes are smaller than 2000 km in width noise in the gravity introduces error and uncertainty but no additional bias into the estimates of the admittance it does however bias estimates of te calculated using the coherence between bouguer gravity and topography the admittance at wavelengths between 1000 and 4000 km which is of interest when studying mantle convection is only well resolved when the box size is larger than 2000 km or the synthetic te is less than 15 km and the true long wavelength admittance is greater than 30 mgal km 1 when the box size is 1000 km it is not possible to distinguish between real and spurious convective support of long wavelength topography at the altitude of most orbiting satellites 200 400 km gravity is much redder than it is at ground level therefore it is necessary to use much larger boxes when using los accelerations to estimate the admittance than it is when using surface measurements however gradual changes in the altitude of the satellite or even occasional sudden changes make almost no difference to the estimate of the admittance finally three short investigations are undertaken concerning the effect on the admittance of the application of irregular windows to the data of lateral variations in te and of the infilling of flexural moats by low density sediment it is found that the bias due to the application of an irregular window is not significantly greater than the bias due to the application of a regular window of similar size that the best fitting te is strongly biased by the value associated with the highest amplitude topography and that sediment infill raises the admittance at wavelengths corresponding to the width of the moat

paleomagnetism and 40ar 39ar ages from volcanics extruded during the matuyama and brunhes chrons near mcmurdo sound antarctica maps of virtual geomagnetic poles derived from international geomagnetic reference field models show large lobes with significant departures from the spin axis these lobes persist in field models for the last few millenia the anomalous lobes are associated with observation sites at extreme southerly latitudes to determine whether these features persist for millions of years paleomagnetic vector data from the continent of antarctica are essential we present here new paleomagnetic vector data and ar 40 ar 39 ages from lava flows spanning the brunhes and matuyama chrons from the vicinity of mcmurdo sound antarctica oriented paleomagnetic samples were collected from 50 lava flows by e mankinen and a cox in the 1965 1966 austral summer season preliminary data based largely on the natural remanent magnetization nrm directions were published by mankinen and cox 1988 we have performed detailed paleomagnetic investigations of 37 sites with multiple fully oriented core samples to investigate the reliability of results from this unique sample collection of these only one site fails to meet our acceptance criteria for directional data seven sites are reversely magnetized the mean normal and reverse directions are antipodal the combined mean direction has d over bar 12 i over bar 86 alpha 4 kappa 37 and is indistinguishable from that expected from a gad field we obtained reproducible absolute paleointensity estimates from 15 lava flows with a mean dipole moment of 49 zam 2 and a standard deviation of 28 zam 2 ar 40 ar 39 age determinations were successfully carried out on samples from 18 of the flows our new isotopic ages and paleomagnetic polarities are consistent with the currently accepted geomagnetic reversal timescales

paleomagnetism of miocene volcanics on sao tome paleosecular variation at the equator and a comparison to its latitudinal dependence over the last 5 myr a collection was made in january 2009 of 10 oriented samples from each of 54 sites in lavas on sao tome island nominal location 0 3o n 6 5o e some sites were affected by lightning leaving a total of 42 sites for analysis of paleosecular variation overall magnetic properties were excellent highly stable magnetizations carried by pseudo single domain magnetite after principal component analysis of progressive alternating field demagnetization data for the samples twenty two sites had normal polarity magnetizations d 0 6o i 8 3o 95 4 3o 53 1 and 20 had reverse magnetizations d 176 0o i 4 2o 95 7 3o 20 8 the directions are within 5 of antiparallel yielding a positive reversal test the combined data set of 42 site mean virtual geomagnetic poles converted to common normal polarity yields a pole position at 86 0 n 211 5oe a95 3 1o ar ar and k ar dating reveals that these rocks are miocene in age 5 11 ma old enough to allow northerly plate motion to help explain the slightly far sided pole position the between site dispersion in virtual geomagnetic poles was estimated as the angular standard deviation sb and equaled 11 4 with 95 confidence interval between 9 9 and 13 4 this article is protected by copyright all rights reserved

paleomagnetic directions and k ar dating of 0 to 1 ma lava flows from la guadeloupe island french west indies implications for time averaged field models twenty six lava flows spanning the last million years were sampled in la guadeloupe french west indies because of the lack of continuous volcano stratigraphic sections in la guadeloupe dating is necessary in order to describe the temporal evolution of the geomagnetic field in this time interval new k ar ages ranging from 50 ka to 1 ma have been obtained on andesites using the cassignol gillot technique at the universite paris sud institut de physique du globe de paris orsay laboratory additional flows with k ar ages obtained using the same dating technique were also sampled for paleomagnetic investigations more than 200 samples were analyzed using both alternating field af and thermal stepwise demagnetization techniques duplicate samplings of two flows at three different sites demonstrate that within flow dispersion is negligible for the andesitic lava flows sampled in this study direct comparison with an earlier paleomagnetic study performed on the island indicates that for the three investigated flows modern demagnetization techniques yield much better defined paleomagnetic directions the matuyama brunhes transition was recorded in a three flow section and is dated at 781 18 ka in good agreement with other recent radiometric age determinations the mean paleomagnetic pole calculated from the 23 normal polarity flows is indistinguishable from geographic north which implies that no significant persistent axial quadrupole term can be identified at this site for the last million years this result contradicts earlier results and has important implications for models of the time averaged field taf an occam algorithm was used to construct a taf model from the global volcanic database from the last 5 ma substitution of the mean direction calculated from the earlier study for the lesser antilles by our new mean value reduces the quadrupole term by more than 30 this effect which was produced by changing data from a single site demonstrates that older paleomagnetic sites may need to be reinvestigated furthermore it also highlights the limitations of taf models that can be inferred from paleomagnetic databases in their present state

paleomagnetic directions from mid latitude sites in the southern hemisphere argentina contribution to time averaged field models back arc volcanism located to the east of the andean cordillera was sampled in the argentina provinces of mendoza and neuquen for paleomagnetic time average field and paleosecular investigations the activity ranges from 2 ma to very recent time with a large variety of products from basalts to highly differentiated lavas after removal of sites affected by lightning those with 95 higher than 10 and combining of nearby sites displaying close directions we present new paleomagnetic results from 31 flows units belonging to two volcanic massifs the payun matru and the cerro nevado previous and new k ar age determinations constrain the volcanic activity of these massifs from 300 to 0 ka and from 1 9 to 0 9 ma respectively most paleomagnetic samples have nrm intensities between about 1 and 20 a m and depict progressive removal of magnetization components in a consistent fashion during stepwise af or thermal demagnetization nineteen flows yielded a normal direction declination 354 8 inclination 53 0 95 6 8 and 12 flows a reverse direction declination 181 0 inclination 52 3 95 5 9 the combined data yielded a mean direction declination 357 3 inclination 52 8 95 4 6 which is not statistically different from the axial dipole field g10 expected at this latitude 36 s the angular dispersion of virtual geomagnetic poles calculated from flows with normal directions asd 16 5 compares well with the observed value from global datasets for this site latitude but flows with reverse directions display a surprisingly low dispersion asd 12 5 since most reverse directions were sampled from flows ranging between 1 9 and 0 9 ma this can be interpreted as an interval of low paleomagnetic secular variation additional data also with accurate time constraints are obviously needed to better support this observation finally no convincing evidence for a complex time average field significantly different from the axial dipole can be supported by this study for the last 2 myr

paleomagnetism of the southwestern u s a recorded by 0 5 ma igneous rocks 1 the issue of permanent nondipole contributions to the time averaged field lies at the very heart of paleomagnetism and the study of the ancient geomagnetic field in this paper we focus on paleomagnetic directional results from igneous rocks of the southwestern u s a in the age range 0 5 ma and investigate both the time averaged field and its variability about the mean value several decades of work in the southwestern united states have resulted in the publication of paleomagnetic data from over 800 individual paleomagnetic sites as part of a new investigation of the san francisco volcanics we collected paleomagnetic samples from 47 lava flows many of which have been previously dated the new data combined with published data are highly scattered contributions to the scatter were considered and we find that removal of data sets from tectonically active areas and judicious selection according to fisher s 1953 precision parameter results in an axially symmetric data distribution with normal and reverse modes that are indistinguishable from antipodal monte carlo simulations suggest that a minimum of 5 samples per site are needed to estimate the precision parameter sufficiently accurately to allow its use as a determinant of data quality numerical simulations from statistical paleosecular variation models indicate the need for several hundred paleomagnetic sites to get an accurate determination of the average field direction and are also used to investigate the directional bias that results from averaging unit vectors rather than using the full field vector average directions for the southwestern u s a show small deviations from a geocentric axial dipole field but these cannot be considered statistically significant virtual geomagnetic pole vgp dispersions are consistent with those from globally distributed observations analyzed by mcelhinny and mcfadden 1997 however a systematic investigation of the effect of imposing a cutoff on vgps with large deviations from the geographic axis indicates that while it may reduce bias in calculating the average direction such a procedure can result in severe underestimates of the variance in the geomagnetic field a more satisfactory solution would be to use an unbiased technique for joint estimation of the mean direction and variance of the field distribution

identification of groundwater flow paths in complex aquifer systems a methodology was developed and applied to the tindouf southwestern algeria and the annaba bouteldja aquifers northeastern algeria in order to understand better the hydrogeology of the complex aquifers despite the scarcity of the available data graphical representation of deuterium versus oxygen 18 and principal components analysis pca are statistical techniques used to combine various disciplinary data in order to identify chemical and isotopic groups which are in turn used to define groundwater flow paths the results of this study agree with the generally accepted hydrogeological conceptual model of the aquifers in addition we obtained new results using the pca method 1 a description of the complex flow system by grouping various qualitative and quantitative parameters 2 the definition and characterization of the main groundwater flow paths from their sources to the discharge zones these flow paths are defined by their water categories which are represented by salinity and origin of groundwater this approach is useful for analysing aquifers despite the lack of important database and may also be helpful for studying other complex groundwater basins copyright 2006 john wiley sons ltd

changes of the geomagnetic field vector obtained from lava sequences on the island of vulcano aeolian islands sicily abstract a combined geochronologic k ar and palaeomagnetic study has been conducted on five volcanic sections 70 flows on the island of vulcano the sections span the interval from 15 2 to 135 4 ka with large hiatuses rock magnetic investigations indicate that the natural remanent magnetisation nrm is carried by titanomagnetites in the four oldest sections with a contribution from high coercivity magnetic minerals in the youngest section palaeomagnetic directions obtained by thermal demagnetisation document a large magnetic excursion at around 110 ka with virtual geomagnetic poles vgps that lie over eastern siberia these directions may represent a volcanic record of the termination of the blake event only about 15 of the samples from 19 flows proved suitable for palaeointensity thellier and thellier determinations these measurements indicate that the geomagnetic field has varied at vulcano between 16 and 53 t in the explored time interval when combined with previous results obtained at mt etna for the period from 140 to 60 ka tric et al 1994 phys earth planet inter 85 113 129 these results document a broad low at around 115 ka the palaeointensity values from vulcano and etna are consistent with global values suggesting the presence of a dominant dipole field contribution these values on the other hand are significantly smaller than those recently obtained from la reunion in the southern hemisphere this suggests that large long lived non dipolar components of the geomagnetic field may be present at la reunion in the interval from 130 to 90 ka these observations are consistent with the fact that the vgp angular dispersion calculated from non excursional data is significantly smaller at vulcano than at la reunion

padm2m a penalized maximum likelihood model of the 0 2 ma palaeomagnetic axial dipole moment summary we present a new time varying model for palaeomagnetic axial dipole moment padm for the past 2 myr and compare it with earlier virtual axial dipole moment vadm reconstructions which have been based on stacking and averaging scaled relative palaeointensity records the padm is derived from both absolute and relative palaeointensity data and constructed using a new penalized maximum likelihood pml approach to recover a cubic b spline representation of axial dipole field variations on million year timescales the pml method is explicitly intended to reduce bias in estimating the true axial dipole moment that arises in average vadm reconstructions we apply the pml method to a set of 96 032 published data 1800 palaeointensities from igneous rocks 3300 archaeointensities and 86 relative palaeointensity time series of variable lengths and resolutions two models are discussed padm2mp is a trial model based on a subset of the nine longest available sedimentary records padm2m uses a comprehensive data set 76 records 81 446 data 10 records were eliminated and is our preferred model padm2m has a lower mean than existing vadm reconstructions but shows similarities in long period variability some differences in timing amplitude and resolution of certain features can be attributed to variations in age assignments others result from our more comprehensive data set and a reduction in bias attributable to pml modelling padm2m has an average axial dipole moment over 0 2 ma of 5 3 1022 am2 with a standard deviation of 1 5 1022 am2 the brunhes chron average 6 2 1022 am2 is higher than for earlier epochs of matuyama 4 8 1022 am2 as seen in some previous studies the power spectrum for our model agrees with previous estimates of the global palaeomagnetic power spectrum for frequencies up to about 102 myr 1 we see no distinctive evidence in the power spectrum for orbital forcing of geodynamo behaviour

revised and updated paleomagnetic results from costa rica 1 paleomagnetic results from globally distributed lava flows have been collected and analyzed under the time averaged field initiative tafi a multi institutional collaboration started in 1996 and designed to improve the geographic and temporal coverage of the 0 5 ma paleomagnetic database for studying both the time averaged field and its very long term secular variations paleomagnetic samples were collected from 35 volcanic units either lava flows or ignimbrites in costa rica in december 1998 and february 2000 from the cordilleras central and guanacaste the underlying canas liberia and bagaces formations and from volcano arenal age estimates range from approximately 40 ka to slightly over 6 ma although initial results from these sites were used in a global synthesis of tafi data by johnson et al 2008 a full description of methodology was not presented this paper documents the definitive collection of results comprising 28 paleomagnetic directions 24 normal 4 reversed with enhanced precision and new geological interpretations adding two paleointensity estimates and 19 correlated 40ar 39ar radiometric ages the average field direction is consistent with that of a geocentric axial dipole and dispersion of virtual geomagnetic poles 17 3 4 6 is in general agreement with predictions from several statistical paleosecular variation models paleointensity estimates from two sites give an average field strength of 26 3 t and a virtual axial dipole moment of 65 zam2 the definitive results provide a useful augmentation of the global database for the longer term goal of developing new statistical descriptions of paleomagnetic field behavior

paleomagnetic results from the newer volcanics of victoria contribution to the time averaged field initiative 1 the newer volcanics of victoria australia were sampled in 2001 and 2003 as part of the time averaged field initiative which is designed to provide a reliable data set for modeling the earth s magnetic field over the past 5 my the collection consisted of ten cores per site at 42 sites complete demagnetization was carried out on all samples using af or thermal demagnetization and line fitting techniques were employed to determine the resultant vector thirty two of these sites gave results that met the criterion that n 5 and 95 was less than 6 20 sites were reversely magnetized dec 173 4 inc 54 6 95 3 8 and 13 normally magnetized dec 2 1 inc 62 3 95 6 3 the data gave a global mean paleomagnetic pole lat 87 2 long 21 9 and a95 4 4 which is not significantly different from the pole of rotation for the present location of victoria and for its latitude 2 5 million years ago when most of the lavas were erupted

evidence for geomagnetic excursions recorded in brunhes and matuyama chron lavas from the trans mexican volcanic belt 1 this study presents paleomagnetic data from 59 independent lava flows from the trans mexican volcanic belt tmvb with ages from 6 4 ma to recent 52 being younger than 1 ma and 11 new 40ar 39ar age determinations most remanence carriers are ti poor titanomagnetite of pseudosingle domain magnetic structure nine lavas contain small amounts of titanomaghemite and four lavas additional titano hematite paleosecular variation of lava flows younger than 1 7 ma is consistent with latitude dependent model g and also in agreement with other pleistocene paleomagnetic data from the tmvb the directional record of brunhes and matuyama chrons lavas was correlated to the geomagnetic polarity timescale and there is evidence for at least four geomagnetic excursions one lava flow dated at 592 20 ka has a fully reversed paleodirection and most likely erupted during the big lost excursion another fully reversed flow dated at 671 12 ka gives new volcanic evidence for the delta stage 17 excursion this excursion is supported by a reversed intermediate direction of another flow from a different volcanic field but of very close age of 673 10 ka from the matuyama age lavas one flow with normal polarity magnetization dated at 949 37 ka could either be related to the kamikatsura or the santa rosa excursion and a normal polarity flow dated at 1628 56 ka could have been emplaced during the gilsa excursion the results presented here confirm in one case but disagree in four cases with results presented in two previous studies of the same lava flows and interpreted as geomagnetic excursions

geomagnetic intensity and inclination variations at hawaii for the past 98 kyr from core soh 4 big island a new study and a comparison with existing contemporary data abstract reliable thellier thellier paleointensity determinations were obtained from 321 out of 766 examined samples from the upper 200 units 675 m of core soh 4 drilled in the kilauea volcano the main trend of the soh 4 record is a marked decrease from values higher than present in the last 4 kyr to values about 40 smaller from about 20 kyr to the bottom of the sampled interval three main lows coinciding with shallow or negative inclinations observed in the record were identified with an excursion previously documented in hawaii and the mono lake and laschamps events for the last 15 kyr the soh 4 intensities are largely in agreement with the worldwide distribution of volcanic data in the 15 75 kyr interval the general lack of volcanic data makes comparison with the soh 4 data difficult the soh 4 record is similar to a composite record of paleointensity largely based on a recent north atlantic sedimentary paleointensity stack napis 75 the soh 4 record is also generally consistent with a record of geomagnetic field intensity obtained from a study of 36 cl in the grip ice core calculated with the assumption that the production rate of 36 cl is controlled only by the geomagnetic field one pronounced discrepancy between the different records is observed around 50 kyr bp when the sedimentary and cosmogenic based records document high intensities which are not observed in soh 4 these high intensities are also absent in a recent record based on the analyses of sea floor magnetic anomalies the histogram of vadms obtained from soh 4 is different from that obtained from the sedimentary or 36 cl based records and is more skewed than the distribution given by kono and tanaka for the last 2 5 million years

a paleomagnetic and paleointensity study on pleistocene and pliocene basaltic flows from the djavakheti highland southern georgia caucasus abstract new paleomagnetic rock magnetic and paleointensity results obtained on samples from 23 basaltic lava flows belonging to four different flow sequences mashavera kvemo orozmani zemo karabulaki and diliska of pleistocene and pliocene age from the eastern djavakheti highland in southern georgia are presented radiometric dating of these sequences yields ages between 1 8 and 2 18 ma for mashavera 2 07 and 2 58 ma for zemo karabulakhi and 2 12 and 3 27 for diliska no radiometric ages are available for the kvemo orozmani sequence which is considered to be coeval to the mashavera sequence rock magnetic experiments including measurement of thermomagnetic hysteresis and irm acquisition curves suggest low ti titanomagnetite as main carrier of remanence although a lower curie temperature component was also observed in several cases reversible and non reversible curves were recorded in thermomagnetic experiments paleomagnetic analysis generally indicated the presence of a single component mainly in the mashavera sequence but also two more or less superimposed components in some other cases in 21 sites a characteristic component could be determined and all except one were characterised by normal polarity directions flows from the mashavera sequence had a rather steep inclination 73 1 nevertheless a mean paleomagnetic direction of all four sequences is obtained d 8 5 i 60 8 n 4 95 11 7 k 62 7 which agrees with the plio quaternary directions obtained in previous studies in georgia the paleomagnetic pole obtained latitude 82 1 longitude 118 2 a 95 8 0 k 240 7 agrees with the pole values of both the 0 ma and the 5 ma windows of the synthetic eurasian polar wander path from besse and courtillot 2002 in order to analyse the behaviour of secular variation the scatter of paleosecular variation of virtual geomagnetic poles of both the mashavera flow and all 18 studied flows of pleistocene age was calculated it could be observed that both data sets seem to fit well the expected scatter at latitude 41 n paleointensity experiments were carried out with the coe modification of the thellier method twenty five out of 84 samples 30 provided reliable paleointensity results these successful results were mainly obtained in the mashavera sequence most flows yielded paleointensity results in the 30 45 t range in accordance with expected pliocene to present day intensities two flows however located near the top of the mashavera sequence yield high paleointensity values around 60 t anomalous paleointensity results in the upper lying mashavera flows together with the steep inclinations observed in that sequence could perhaps signal the near onset of the olduvai matuyama reversal

geomagnetic field for 0 3 ka 1 new data sets for global modeling 1 paleomagnetic and archeomagnetic records are used in both regional and global studies of earth s magnetic field we present a description and assessment of five newly compiled data sets also used in the companion paper by korte et al 2009 to produce a series of time varying spherical harmonic models of the geomagnetic field for the last 3000 years data are drawn from our compilation of lake sediment records and from the online database geomagia50v2 the five selections are available from the earthref digital archive at http earthref org cgi bin erda cgi n 944 data are grouped according to the source of material and we conducted separate assessments of reliability for archeomagnetic artifacts and lava flows the arch3k dat data set and for sediments sed3k dat the overall number of data is 55 greater than in previous compilations constrained data sets were selected using different criteria for each group winnowing of archeological data was based on uncertainties supplied by the original data providers the lake sediment data assessment relied on preassigned age uncertainties and one or more of the following comparisons with archeomagnetic data from the same region regional consistency among several lakes and consistency with global archeomagnetic models we discuss relative merits of a larger unconstrained data set or a smaller possibly more reliable one the constrained data sets eliminate a priori up to 35 of the available data in each case and rely on potentially subjective assessments of data quality given the limited data available our analyses indicate that iterative rejection of a small number 1 1 5 of outlying data during global field modeling is a preferable approach specific regional comparisons among the models and data support the conclusion that korte et al s outlier free cals3k 3 model based on all available measurements from sediments and archeological artifacts currently provides the best global representation of the 0 3 ka field the arch3k 1 model provides a better fit to the denser european archeomagnetic data and may be better in that region

the time averaged paleomagnetic field 0 5 ma persistent departures from the geocentric axial dipole field model of the time averaged paleomagnetic field over the past 5 myr have been analyzed using oceanic data from deep sea cores and continental data from igneous rocks and sediments the data set comprises the equivalent of 9490 spot readings of the field 5831 normal and 3659 reverse from 930 groups of data variations in declination anomalies between groups of data strongly suggest the existence of second order tectonic effects small rotations that make it difficult if not impossible to identify any nonzonal terms that might exist inclination anomalies have therefore been modeled in terms of low degree zonal harmonics the calculated inclination anomalies within 10 latitude bands are widely scattered for both oceanic and continental data and do not appear to have been drawn from distributions with common means we attribute this large scatter to previously undetected or uncorrected second order tectonic effects in the continental data or in the oceanic data to small departures from the vertical in deep sea cores if it is assumed that these effects are random between groups of data and without a systematic bias they can be averaged out to a large extent if there are sufficient numbers of groups within each latitude band this method also has the major advantage that it maximizes time averaging of the field when this is done an acceptable fit to the inclination anomaly data is found using a zonal harmonic model although the point estimates for the reverse polarity zonal quadrupole term are consistently larger than those for the normal polarity zonal quadrupole term the difference is not significant and is likely due to contamination effects this finding is true for a combination of all the data or for separate combinations of continental igneous rocks and ocean sediments this conclusion differs from all previous analyses the major differences occur in model estimates of the zonal octupole term the estimates varying widely depending on the particular data combination we show that false octupole terms can be introduced by several factors including inclination errors in sediments and lavas the use of unit vectors in the analyses and incomplete magnetic cleaning particularly of reversely magnetized rocks thus we cannot impute a geomagnetic significance to the estimates of a zonal octupole term we suggest that the best estimates for the zonal quadrupole g2 g20 g10 are obtained from the combined brunhes age igneous rocks and oceanic normal data 0 033 0 019 and for the combined matuyama age igneous rocks and oceanic reverse data 0 042 0 022 when these two sets are combined the overall best estimate of g2 is 0 038 0 012

age and magnetism of lavas in jokuldalur area eastern iceland gilsa event revisited abstract we present results of paleomagnetic measurements and k ar age determinations of 38 lava flows collected in five separate sections in the jokuldalur area of eastern iceland including one section previously studied by watkins et al watkins n d kristjansson l mcdougall i 1975 a detailed paleomagnetic survey of the type location for the gilsa geomagnetic polarity event earth planet sci lett 27 436 444 these sites are close to the type locality of the normal gilsa event in the matuyama chron first identified by mcdougall and wensink mcdougall i wensink h 1966 paleomagnetism and geochronology of the pliocene pleistocene lavas in iceland earth planet sci lett 1 232 236 using new experimental results as well as field observations we could correlate the sequence of lava flows in the jokuldalur area spanning the time interval between 1 8 and 0 5 ma the magnetic polarities and ages obtained in this study are quite consistent with the standard time scale for brunhes matuyama ages given by cande and kent cande s c kent d v 1995 revised calibration of the geomagnetic polarity timescale for the late cretaceous and cenozoic j geophys res 100 6093 6095 based on marine magnetic anomalies except that the gilsa event needs to be added existence of olduvai age lavas under gilsa was inferred but not certain our results are fairly in good agreement with the former studies by wensink wensink h 1964a secular variation of earth magnetism in plio pleistocene basalts of eastern iceland geol mijnbouw 43 403 413 wensink h 1964b paleomagnetic stratigraphy of younger basalts and intercalated plio pleistocene tillites in iceland geol rund 54 364 384 and watkins et al watkins n d kristjansson l mcdougall i 1975 a detailed paleomagnetic survey of the type location for the gilsa geomagnetic polarity event earth planet sci lett 27 436 444 but the assignment to the polarity zones is different because of the newly obtained k ar ages based on the present study we propose that the gilsa event is a short normal subchron in the matuyama chron distinct from and above the more well established oldvai subchron

anisotropic paleosecular variation models implications for geomagnetic field observables abstract we present a family of statistical models for paleosecular variation psv of the geomagnetic field that are compatible with paleodirectional and paleointensity variations in lava flows sampling the last 5 ma and explore what paleomagnetic observables might be used to discriminate among the various family members we distinguish statistical models with axial anisotropy which provide a suitable description for an earth with homogeneous boundary conditions at the core mantle interface from those with more general anisotropy corresponding to geographically heterogeneous boundary conditions the models revise and extend earlier ones which are themselves descendants of cp88 devised by constable and parker constable c g parker r l 1988 statistics of the geomagnetic secular variation for the past 5 m y j geophys res 93 11569 11581 in cp88 secular variation is described by statistical variability of each gauss coefficient in a spherical harmonic description of the geomagnetic field with each coefficient treated as a normally distributed random variable the gauss coefficients of the non dipole part of the field exhibit isotropic variability and the variances are derived from the present field spatial power spectrum the dipole terms have a special status in cp88 with a non zero mean for the axial dipole and lower variance than predicted from the spatial power spectrum all non dipole terms have zero mean except the axial quadrupole cp88 is untenable for two reasons it fails to predict the observed geographic dependence of directional variability in the magnetic field and it grossly underpredicts the variance in paleointensity data the new models incorporate large variance in the axial dipole and in the non axial quadrupole gauss coefficients g21 and h21 the resulting variance in paleomagnetic observables depends only on latitude zonal models unless the variance in h21 is different from that in g21 non zonal models non zonal longitudinal variations in psv such as the flux lobes seen in the historical magnetic field are simulated using the non zonal models both the zonal and non zonal models fit summary statistics of the present dataset we investigate the influence of persistent non zonal influences in psv on various paleomagnetic observables it is shown that virtual geomagnetic pole vgp dispersion is rather insensitive to longitudinal variations in structure of psv and that inclination dispersion has the potential to be more informative given the right site distribution there is also the possibility of using paleointensity and geographic variations in the frequency of occurrence of excursional directions to identify appropriate psv models

paleosecular variation and the average geomagnetic field at 20 latitude we assembled a new paleomagnetic directional data set from lava flows and thin dikes for four regions centered on 20 latitude hawaii mexico the south pacific and reunion we investigate geomagnetic field behavior over the past 5 myr and address whether geographical differences are recorded by our data set we include inclination data from other globally distributed sites with the 20 data to determine the best fitting time averaged field taf for a two parameter longitudinally symmetric zonal model values for our model parameters the axial quadrupole and octupole terms are 4 and 6 of the axial dipole respectively our estimate of the quadrupole term is compatible with most previous studies of deviations from a geocentric axial dipole gad field our estimated octupole term is larger than that from normal polarity continental and igneous rocks and oceanic sediments but consistent with that from reversed polarity continental and igneous rocks the variance reduction compared with a gad field is 12 and the remaining signal is attributed to paleosecular variation psv we examine psv at 20 using virtual geomagnetic pole vgp dispersion and comparisons of directional distributions with simulations from two statistical models regionally the hawaii and reunion data sets lack transitional magnetic directions and have similar inclination anomalies and vgp dispersion in the pacific hemisphere hawaii has a large inclination anomaly and the south pacific exhibits high psv the deviation of the taf from a gad contradicts earlier ideas of a pacific dipole window and the strong regional psv in the south pacific contrasts with the generally low secular variation found on short timescales the taf and psv at hawaii and reunion are distinct from values for the south pacific and mexico demonstrating the need for time averaged and paleosecular variation models that can describe nonzonal field structures investigations of zonal statistical psv models reveal that recent models are incompatible with the empirical 20 directional distributions and cannot fit the data by simply adjusting relative variance contributions to the psv the 20 latitude data set also suggests less psv and smaller persistent deviations from a geocentric axial dipole field during the brunhes

dependence of the duration of geomagnetic polarity reversals on site latitude an important constraint on the processes governing the geodynamo the flow in the outer core responsible for generating earth s magnetic field is the duration of geomagnetic polarity reversals that is how long it takes for earth s magnetic field to reverse1 it is generally accepted that earth s magnetic field strength drops to low levels during polarity reversals and the field direction progresses through a 180 change while the field is weak1 the time it takes for this process to happen however remains uncertain with estimates ranging from a few thousand up to 28 000 years here i present an analysis of the available sediment records of the four most recent polarity reversals these records yield an average estimate of about 7 000 years for the time it takes for the directional change to occur the variation about this mean duration is not random but instead varies with site latitude with shorter durations observed at low latitude sites and longer durations observed at mid to high latitude sites such variation of duration with site latitude is predicted by simple geometrical reversal models in which non dipole fields are allowed to persist while the axial dipole decays through zero and then builds in the opposite direction and provides a constraint on numerical dynamo models

40ar 39ar chronology and paleomagnetism of quaternary basaltic lavas from the persani mountains east carpathians abstract quaternary volcanism in the persani mountains forms an na alkali basaltic province inside the bend area of the carpathians in the southeastern part of europe previous k ar ages and paleomagnetic data reveal several transitional virtual geomagnetic poles which were tentatively associated with the cobb mountain subchron and a brunhes chron excursion we report a new paleomagnetic and rock magnetic study coupled with 40ar 39ar geochronology to better constrain the age of geomagnetic reversals or excursions that might be recorded and the timing of volcanism of the paleomagnetic directions obtained from sampled lava flows 4 are reversed polarity 19 are normal polarity and 16 have transitional polarity 40ar 39ar plateau ages determined from incremental heating experiments on groundmass indicate that two of the reversely magnetized lavas erupted at 1142 41 and 800 25 ka four of the normally magnetized lavas erupted at 1060 10 1062 24 684 21 and 683 28 ka and two transitionally magnetized lavas formed at 1221 11 and 799 21 ka both the new 40ar 39ar ages and the paleomagnetic data suggest at least five episodes of volcanic activity with the most active periods during the jaramillo and brunhes chrons this results shows that the last phases of alkalic and calc alkaline magmatism in the south east carpathians were contemporaneous the age of the older transitionally magnetized lava flow is within error of recent unspiked k ar and astrochronologic ages for the reversal that defines the onset of the cobb mountain normal polarity subchron the age of the younger transitional lava is similar to that of an excursion that preceded the matuyama brunhes polarity reversal and which has come to be known as the matuyama brunhes precursor omitting the excursion data the dispersion of the virtual geomagnetic poles around 19 is larger than the expected value around 45 n from the global compilation but closer to the value obtained only from the time averaged geomagnetic field initiative studies

holocene paleosecular variation from dated lava flows on maui hawaii ow inclination and low secular variation seem to be present for at least the past 5 myear in central pacific the period covering the past 10 kyear is crucial to the study of the field variability and to fill the gap between historical field measurements and long term paleomagnetic records we have conducted a paleomagnetic study of 13 sites of basaltic lava flows from maui island with 14c ages between 10 3 and 0 2 ka two other sites dated at 45 and 730 ka were also sampled eight to 10 samples from each site were demagnetized using thermal treatment and companion specimens from the same samples were demagnetized by alternating fields af thermomagnetic and hysteresis measurements indicated that magnetite 575 c in fine grains was the dominant magnetic carrier although in many cases we also observed a low temperature phase which is likely carried by titanomagnetite with low titanium content the existence of relatively high coercivities associated with these two mineralogical phases generated overlapping components which could not be properly isolated using af demagnetization successful results were obtained after thermal demagnetization for 13 sites with a mean inclination of 36 9 4 the mean inclination inc 36 3 of the eleven sites younger than 10 5 ka is also very close to the value 37 of the geocentric axial dipole gad at the site latitude but the angular dispersion of the virtual geomagnetic poles vgps about the spin axis 9 with n 13 and 6 7 with n 11 is significantly lower than the predictions of the models of paleosecular variation at this latitude the inclination variations for the past 10 kyear are in excellent agreement with the very detailed dataset which has previously been obtained from the big island of hawaii the mean inclination of all volcanic records n 132 is 1 8 lower than expected but this is likely caused by the lack of records between 5 and 7 ka b p we note also that the inclinations from lake waiau sediments big island are shallower than those of the volcanic records mauna kea hawaii and were thus probably affected by compaction we infer that there is no striking evidence for an inclination anomaly under hawaii during this period being aware of the need for additional records covering at least the 5 7 ka b p time interval the absence of a systematic deviation going beyond the dispersion of the inclinations obtained for the past millions of years neither pleads for a significant long term persistent anomaly however all studies report a low dispersion of the vgps which must reflect low secular variation

geomagnetic paleosecular variation recorded in plio pleistocene volcanic rocks from possession island crozet archipelago southern indian ocean possession island in the crozet archipelago consists of volcanic units erupted mainly between 5 and 0 5 ma a paleomagnetic sampling was carried out along several sections distributed near the northern eastern and southeastern coasts a total of 45 independent flows were sampled 320 samples for each flow a precisely defined characteristic remanence direction was usually isolated after a careful progressive cleaning in alternating fields however particularly complex remanence behavior is often observed the magnetostratigraphy of the lava pile is quite simple with reversed rocks in the lower part and normal units in the upper part of two sections a third section is of normal polarity throughout its whole thickness including three excursional directions we did not find any intermediate directions between the normal and reverse magnetozones thus we have no evidence for the recording of the matuyama brunhes transition expected from a previous study watkins 1972 the amplitude of paleosecular variation estimated from between flow dispersion from the field of an axial dipole is 11 8 with 95 confidence limits between 9 3 deg and 14 0 deg this value is consistent with the general anisotropic statistical model for paleosecular variation of constable and johnson 1999

paleomagnetic field properties at high southern latitude 1 statistical analyses of paleomagnetic data from lava flows are used to study geomagnetic field behavior on million year timescales previous paleomagnetic studies have lacked high latitude measurements necessary to investigate the persistence of geomagnetic anomalies observed in the recent and historical field and replicated in some numerical geodynamo simulations these simulations suggest that reduced convective flow inside the tangent cylinder may affect the magnetic field at high latitude whereas lower latitude observations are expressions of columnar helical flow outside the tangent cylinder this paper presents new paleointensity and paleodirectional data from 100 volcanic sites in the erebus volcanic province evp antarctica and 21 new age determinations by the 40ar 39ar incremental heating method the new evp data are combined with previously published paleomagnetic and geochronological results providing 133 sites 91 having radioisotopic dates modified thellier thellier paleointensity estimates are reported for 47 sites 37 have dates ages for the combined data set span 0 03 to 13 42 ma the 125 high quality evp directional data selected from the merged data set have a non fisherian distribution and a mean direction with an inclination anomaly of 3 but 95 confidence limits include the prediction from a geocentric axial dipole virtual geomagnetic pole vgp dispersions for brunhes matuyama and the combined 0 5 ma data set are consistently high compared with values from middle to low latitude regions regardless of the criterion used to determine transitional fields with vgp latitude cut off at 45 the dispersion 23 9 2 1 for the combined 0 5 ma evp data set is consistent with earlier high latitude data and paleosecular variation psv in model g but not with some more recent statistical psv models mean evp paleointensity of 31 5 2 4 t derived from 41 high quality sites is about half the current value at mcmurdo 63 t the result is essentially independent of data selection criteria high vgp dispersion and low intensity values support the global observation of anticorrelation between directional variability and field strength simulations of time varying dipole strength show that uneven temporal sampling may bias the mean evp intensity estimate but the possibility of persistently anomalous field behavior at high latitude cannot be excluded

paleosecular variation and gad studies of 0 2 ma flow sequences from the aleutian islands alaska 1 a study was undertaken in the late 1960s to investigate paleosecular variations of the geomagnetic field as recorded in volcanic rocks from the aleutian islands the early results were internally consistent but to be wholly credible by today s standards needed more detailed demagnetization complete thermal demagnetization protocols have been applied to the unmeasured archived samples from the six flow sequences used in the initial study and 40ar 39ar techniques have been used to improve the time resolution the flow sequences have ages ranging from about 50 ka to 2 ma and the number of sequential flows in the individual sequences varies from 8 to 21 after strict selection criteria were applied mad 5 95 5 for both demagnetization data from samples and samples within one flow the number of acceptable flows per flow sequence dropped to between 5 and 15 with the exception of a sequence showing transitional field behavior the between flow dispersion and the 95 values for the other sequences were notably low with respect to secular variation models and their mean directions were very close to the gad field since the time represented by the individual sequences is not well determined the low dispersion could represent very short eruption times in contrast the lack of dispersion with respect to the gad field can be taken to indicate good time averaging since the locations of the sampled flows are at roughly the same latitude about 50 n but are spread over about 10 of longitude the dispersion was calculated for both the locality means and the flow means these data represent the whole 2 myr and give a dispersion which is lower than current secular variation models predict a similar data set published for locations in western canada that are at roughly the same latitude and overlap in age with the aleutian sites gives dispersions that are close to the model predictions at face value this can be interpreted as indicating low secular variation for the aleutian sites

paleomagnetic results from the snake river plain contribution to the time averaged field global database 1 this study presents paleomagnetic results from the snake river plain srp in southern idaho as a contribution to the time averaged field global database paleomagnetic samples were measured from 26 sites 23 of which 13 normal 10 reverse yielded site mean directions meeting our criteria for acceptable paleomagnetic data flow ages on 21 sites range from 5 ka to 5 6 ma on the basis of 40ar 39ar dating methods the age and polarity for the 21 dated sites are consistent with the geomagnetic reversal time scale except for a single reversely magnetized site dated at 0 39 ma this is apparently the first documented excursion associated with a period of low paleointensity detected in both sedimentary and igneous records combining the new data from the srp with data published from the northwest united states between the latitudes of 40 and 50 n there are 183 sites in all that meet minimum acceptability criteria for legacy and new data the overall mean direction of 173 normally magnetized sites has a declination of 2 3 inclination of 61 4 a fisher concentration parameter of 58 and a radius of 95 confidence 95 of 1 4 reverse sites have a mean direction of 182 4 declination 58 6 inclination of 50 and 95 of 6 9 normal and reversed mean directions are antipodal and indistinguishable from a geocentric axial dipole field at the 95 confidence level virtual geomagnetic pole dispersion was found to be circularly symmetric while the directional data were elongate north south an updated and corrected database for the northwestern u s region has been contributed to the magnetics information consortium magic database at http earthref org

paleosecular variation and time averaged field recorded in late pliocene holocene lava flows from mexico 1 this paper presents results from 13 paleomagnetic sites from an area west of mexico city and 7 sites from an area of dispersed monogenetic volcanism in the state of san luis potosi accompanied by seven 40ar 39ar radiometric dates an analysis of secular variation and time averaged paleomagnetic field in the trans mexican volcanic belt tmvb using compiled data both newly obtained and from the literature is presented interpretation can best be constrained after excluding from the data set sites that appear to be tectonically affected the selected data include 187 sites of late pliocene holocene age the mean direction among these sites is dec 358 8 inc 31 6 95 2 0 k 29 this direction does not overlap the expected geocentric axial dipole gad but is consistent with a gad plus a 5 quadrupole the virtual geomagnetic pole scatter of this group of sites 12 7 with lower and upper 95 confidence limits of 11 9 and 14 1 is consistent with the value expected from model g 13 6

paleosecular variation of brunhes age lava flows from british columbia canada 1 brunhes age lava flows have been sampled for paleosecular variation studies from several volcanic fields of southern british columbia silverthrone garibaldi lake mt meager clearwater wells gray park and kelowna area a total of 52 lava flows were sampled and 7 to 10 samples were drilled at each site previous radiometric studies indicate that the ages of these lava flows range from 2 3 to 760 ka stepwise thermal demagnetization 14 to 21 steps was carried out for all the samples in each site and af demagnetization was performed on one sample per site forty five sites were selected based on rigorous criteria 95 5 to calculate a mean direction d 356 9 i 70 2 95 2 8 that is statistically indistinguishable from the direction of the dipole field at the area i 68 3 virtual geomagnetic poles vgps do not show the far sided effect and the angular standard deviation is 17 5 a value in agreement with the paleomagnetic field dispersion for that latitude these high quality results are expected to improve the time averaged field taf and secular variation models

paleomagnetism and 40ar 39ar ages from la palma in the canary islands 1 the structure of the time averaged geomagnetic field has been known forcenturies to be approximately dipolar significant departures of the time averagedfield from that of an axial geocentric dipole however have been reportedfor decades the data on which time averaged field models are based must beof the highest quality in order to document subtle long term features wepresent here new paleomagnetic data and 40ar 39ar ages for the island of la palma the canary islands paleomagnetic samples were obtained from 28 lava flows of these 21 met ourminimum acceptance criteria for use in time averaged field models the 40ar 39ar age determinationswere successfully carried out on samples from eight of the flows our isotopicages and paleomagnetic polarities are consistent with the currently acceptedgeomagnetic reversal timescales the reversed data in the updated databaseare antipodal to the normal data within the uncertainties and the time averageddirection is indistinguishable from that expected from a geocentric axialdipole

time averaged field at the equator results from ecuador seventy sites were collected from pliocene to recent lavas in ecuador between 0 19 and 1 1 s latitude lightning affected many of the sites sampled and the final data set consisted of 51 sites with 95 of 10 or less 21 flows yielded normal directions of magnetization declination 354 6 inclination 5 95 7 2 and 30 yielded reverse declination 183 5 inclination 5 6 95 5 these sites pass a reversal test at a high level the combined data give a direction of declination 359 9 inclination 5 4 and 95 4 2 which is just significantly different from the gad field but is in agreement with a gad field plus 5 quadrupole field mean vgps are not significantly different from the geographic poles the asd for the combined sites is 13 3 which is in agreement with model g paleointensity measurements yield values of vadm which range from 1 39 1022 to 22 6 1022 am2 the mean value for the reverse field is 3 2 1022 am2 while for the normal field it is 10 2 1022 am2 the 40ar 39ar dates on selected sites show that they are 2 6 ma and younger the age dates and magnetic polarity indicate that the chacana structure is brunhes matuyama in age

insights from geodynamo simulations into long term geomagnetic field behaviour abstract detailed knowledge of the long term spatial configuration and temporal variability of the geomagnetic field is lacking because of insufficient data for times prior to 10 ka we use realisations from suitable numerical simulations to investigate three important questions about stability of the geodynamo process is the present field representative of the past field does a time averaged field actually exist and supposing it exists how long is needed to define such a field numerical geodynamo simulations are initially selected to meet existing criteria for morphological similarity to the observed magnetic field a further criterion is introduced to evaluate similarity of long term temporal variations allowing for reasonable uncertainties in the observations observed and synthetic axial dipole moment frequency spectra for time series of order a million years in length should be fit by the same power law model this leads us to identify diffusion time as the appropriate time scaling for such comparisons in almost all simulations intervals considered to have good morphological agreement between synthetic and observed field are shorter than those of poor agreement the time needed to obtain a converged estimate of the time averaged field was found to be comparable to the length of the simulation even in non reversing models suggesting that periods of stable polarity spanning many magnetic diffusion times are needed to obtain robust estimates of the mean dipole field long term field variations are almost entirely attributable to the axial dipole nonzonal components converge to long term average values on relatively short timescales 15 20 kyr in all simulations the time averaged spatial power spectrum is characterised by a zigzag pattern as a function of spherical harmonic degree with relatively higher power in odd degrees than in even degrees we suggest that long term spatial characteristics of the observed field may emerge on averaging times that are within reach for the next generation of global time varying paleomagnetic field models

paleomagnetic study of the ages of lavas on the island of lanai i hawai i abstract a detailed paleomagnetic study of tholeiitic basaltic of eight sites drilled from kaumalapau harbor manele bay and maunalei gulch areas was conducted to investigate the volcanic evolution of the island of lana i stepwise alternating field af demagnetizations accompanied by investigations of rock magnetic properties such as low field susceptibility versus temperature k t and saturation isothermal remanent magnetization sirm indicate that the characteristic remanent magnetization chrm is primarily carried by ti poor magnetite demagnetization experiments have identified eight mean flow directions characterized by negative polarities our paleomagnetic results along with the radiometric age determinations of the lavas in question indicate that the tholeiitic flows that formed the lanai volcano were erupted very rapidly and only during the matuyama polarity chron 0 780 2 58 ma and not during the brunhes chron 0 0 78 ma as reported previously for the k ar dated lavas in the maunalei gulch

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