relative geomagnetic intensity of the field during the last 140 ka relative magnetic field palaeointensities have been obtained from three marine cores in the somali basin western indian ocean the correlation between the records based on changes in the low field susceptibility and the inclination is extremely detailed a time versus depth correlation has been established from the 18o record the remanence intensity was normalized with respect to the anhysteretic remanent magnetization with special attention to changes in grain sizes coercivities and mineralogy the synthetic record of relative palaeointensity has been extended to cover the last 140 ka the quasi cyclic pattern observed during the last 80 ka confirms the results previously obtained in the mediterranean sea and thereby establishes its dominantly dipolar character between 140 and 80 ka the field intensity exhibits a much longer cycle spectral analysis of the intensity records shows the presence of two dominant periodicities but the length of the signal is too short to document the presence of a stationary process

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

high resolution magnetostratigraphy of four sediment cores from the greenland sea ii rock magnetic and relative palaeointensity data summary high resolution magnetostratigraphic analysis of three sediment cores from the base of the volcanic seamount vesteris banken in the greenland basin and one from the jan mayen fracture zone revealed records of three pronounced geomagnetic events the mono lake excursion 28 27 ka the laschamp event 37 33 ka and the biwa i jamaica event 189 179 ka rock magnetic investigations measurements of magnetic susceptibility and arm and irm acquisition demagnetization experiments show that only fine grained titano magnetite 0 5 m is the dominant magnetic carrier mineral in the hemipelagic muds there is no evidence for high amounts of high coercivity minerals like haematite or goethite volcanic basaltic ash layers in sediments around vesteris banken seamount are characterized by extremely high values of magnetic susceptibility reaching 16 000 times 10 6 si as well as by low median destructive fields of the anhysteretic remanent magnetization mdfarm indicating magnetite of larger grain sizes 1 m besides this no significant variation in rock magnetic parameters across intervals of intermediate to reversed chrm inclinations are observed therefore these are interpreted as true records of geomagnetic field variations the recovered sediments characterized by only moderate fluctuations in concentration as well as grain size related rock magnetic parameters provided good conditions for the reconstruction of the relative palaeointensity of the geomagnetic field the recorded geomagnetic events are clearly linked to a drastic reduction in the relative palaeointensity of the field the lowermost values were obtained for the time of the polarity transition with a trend to higher relative intensities in the course of the polarity events event ages and the relative palaeointensity variations based on different methods derived from the greenland sea sediments investigated are in good agreement with data published in the literature

four dimensional analysis of fracture arrays at theaspohard rock laboratory se sweden abstract during excavation and construction ofaspohrl an underground laboratory for the study of spent nuclear fuel geological information is systematically recorded and filed in a data base called geotab statistical analyses have been performed on fracture information from geotab and stochastically chosen outcrops have been mapped in detail based on systematic recording of crosscutting relations a succession of minerals and characteristics of decreasing age have been outlined these include epidote quartz oxidized walls iron oxides chlorite and calcite the sequential fragmentation of the rock mass is described in terms of evolution of fracture arrays in time a correction procedure for sampling biases has been performed and fracture characteristics have been analysed it is concluded that fracture lengths follow a lognormal distribution with mean mode and standard deviation estimated to 2 84 m 1 52 m and 1 43 m respectively spacing has been evaluated for different fracture sets and varies between 0 32 m 1 to 8 45 m 1 it is concluded that the fracture array consist of six sets with different kinematics fracture zones underground are compared to a fracture zone at the surface

paleointensity in the quaternary west eifel volcanic field germany preliminary results abstract preliminary paleointensity results are presented from 36 sites with virtual geomagnetic pole vgp latitudes of about 30 90 normal polarity in the quaternary west eifel volcanic field a strong correlation between vgp latitude and the earth s virtual magnetic dipole moment vdm is observed with low intensities for low vgp latitudes indicating possibly an emplacement during an excursion or event of the brunhes epoch the age distribution of the west eifel volcanics is as yet poorly known also the mean vdm value for sites with high vgp latitudes is considerably lower than the present day dipole moment of the earth

improvements in procedure and paleointensity selection criteria picrit 03 for thellier and thellier determinations application to hawaiian basaltic long cores abstract in this paper we first describe the paleointensity furnaces and the procedures specifically designed to minimize oxidation of the samples during thellier and thellier paleointensity experiments routinely used at gif we also propose some improvements to the set of criteria commonly used to select the results of the thellier experiments in order to further increase the reliability of paleointensity determination finally to be included in databases the new set of paleointensity criteria picrit 03 paleointensity criteria in 2003 is discussed and the results obtained from two long hawaiian basaltic cores the scientific observation hole 4 soh 4 and the hawaiian scientific drilling project 1 hsdp1 are used to illustrate its effectiveness some of the results are re evaluated and a new composite record of absolute paleointensity and inclination spanning 420 kyr is presented the inclination values obtained from the two cores combined support the hypothesis of persistent shallow inclinations at hawaii in intensity the upper 100 kyr are covered by the data from soh 4 with 309 reliable determinations out of 766 examined obtained from 143 units which belong to the kilauea volcano at the top of this core the 341 reliable data covering the 100 420 kyr interval are from 149 units of core hsdp1 belonging to the mauna kea volcano the interval 320 to 420 kyr is particularly detailed because of high extrusion rate of the volcano the use of more stringent criteria decreases the number of determinations considered as reliable in previous publications but it improves their accuracy and robustness

climato hydrological fluctuations printed in long lacustrine records in lake hovsgol mongolia abstract a provisional age model is proposed for hdp 04 sedimentary records from lake hovsgol on the mongolian plateau of a middle latitude zone during the brunhes epoch the model is established on the basis of two procedures first age model linear age model with some dates c 14 238 u 234 u 230 th and paleomagnetic events and a second age model with auto orbital tuning for the first age model the age model suggests that the mis 1 to mis 7 interval is included in the upper part and the mis 11 to mis 19 is included in the lower part in addition to the missing part of about 100 ky the glacial interglacial cycle is most clearly presented by aridity water level change three milankovitch parameters precession obliquity and eccentricity are detected in most datasets grain size is occasionally large in glacial intervals and small in interglacial ones in the upper part above the hiatus possibly related to material mobility with water level change change in erosion area and wind availability a deep depression cold stage comparable to glacial periods corresponded to an insolation minimum is detected at about the mis 5d in records indicating that this region is highly sensitive to solar insolation another depression found during the mis 11 interval also corresponded to a small insolation minimum showing that the interval was not a simple prolonged interglacial period

esr dating of xenolithic quartz in volcanic rocks abstract selected quaternary volcanoes of the eifel volcanic field germany and of the eger rift west bohemia were dated using esr on quartz xenoliths in basanites typical sizes of the quarzites and sandstones were a few cm with the exception of one large xenolith with dimensions of 22 and 46 cm investigations on the optical and thermal properties of the al center in our samples show that these properties are determined by one or several unknown electron traps esr ages using the al center show good agreement with independent ages up to 500 ka but also show that the thermal stability can vary significantly from sample to sample age overestimation due to incomplete resetting of the esr signals at the time of eruption is found for two samples and weak or non existent e and ohc signals are discussed as possible indicators of complete zeroing

timing of quaternary geomagnetic reversals and excursions in volcanic and sedimentary archives abstract we review the ages of quaternary magnetic polarity reversals and magnetic excursions improvements in radioisotopic 40ar 39ar dating of reversals and excursions reflect the use of multi collector mass spectrometers along with ultra low backgrounds laser based incremental heating methods and adoption of 28 201 ma for the fish canyon sanidine fcs standard age kuiper et al 2008 these improvements have brought 40ar 39ar ages of reversals and excursions into close correspondence with astrochronological ages based on the correlation of 18o data to lr04 lisiecki and raymo 2005 and in a few cases correlation to ice core chronologies ages of two polarity reversals the matuyama brunhes boundary and base olduvai are at odds with ages given in popular geomagnetic polarity timescales sedimentary records of the laschamp and iceland basin excursion are numerous and widespread and attest to global manifestation and global synchronicity of these excursions records of other quaternary excursions are less numerous the blake excursion 115 ka has been recorded in north atlantic sediments chinese loess indian ocean volcanics and spanish speleothems the santa rosa excursion 932 ka has been recorded in north atlantic and phillipine basin sediments as well as in volcanic rocks from new mexico and galapagos the punaruu excursion 1115 ka has been observed in sediments from the north atlantic and california margin as well as in volcanic rocks on tahiti uniformity of individual excursion ages using astrochronological 40ar 39ar and u th methods argues for synchronous global manifestation of excursions strengthening the usefulness of magnetic excursions and reversals in quaternary stratigraphy

multiple brunhes chron excursions recorded in the west eifel germany volcanics support for long held mantle control over the non axial dipole field volcanic records of excursional geomagnetic field behavior in particular paleointensity estimates are fragmentary for the pleistocene the west eifel volcanic field is unique in that 12 of 66 measured lava flow sites record virtual geomagnetic poles vgps clustered between 34 45 n latitude and 30 50 e

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

mono lake excursion in mono basin california and at carson sink and pyramid lake nevada summary a restudy of the mono lake excursion in the mono basin california using two new localities 20 km apart shows that the palaeomagnetic record in the basin is the most complete and accurate for the entire excursion the excursion is also recorded in exposed sediment from pleistocene lake lahontan in northwestern nevada about 250 km north of the mono basin sampling of the lake lahontan sediment was done at pyramid lake and in the carson sink following the initial discovery of the mono lake excursion by denham cox 1971 liddicoat coe 1979 showed that the excursion consists of two parts split by a volcanic ash bed at pyramid lake and in the carson sink the older half of the excursion is not recorded but the younger half can be recognized two factors appear to have contributed to the partial recording of the excursion at pyramid lake and in the carson sink 1 the relative strength of the palaeomagnetic field low during the older half of the excursion and high during the younger half and 2 the slower sedimentation in lake lahontan estimated to be about half the rate of deposition for the lacustrine sediment in the mono basin tephrochronology and carbon 14 dates indicate an age of 28 000 yr for the middle of the mono lake excursion on the assumption that the sedimentation rate was a constant 25cm kyr 1 in the mono basin the span of the full excursion is about 2000 yr

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

asymmetrical saw tooth pattern of the geomagnetic field intensity from equatorial sediments in the pacific and indian oceans abstract a record of relative paleointensity from marine sediments in the equatorial indian ocean spanning the last 4 ma completes a previous dataset from ocean drilling program odp leg 138 sites in the equatorial pacific 1 the timescale was defined with a precision better than 20 kyr with different and independent methods most of the features present in the record from the equatorial pacific are confirmed the overall saw tooth pattern is observed across every field reversal and the short term fluctuations superimposed on the slow intensity decrease preceding the reversals appear to be similar at both sites the global character of these features reinforces their interpretation in terms of changes in dipole field intensity and provides new important constraints on models of the geodynamo a synthetic curve of field intensity changes during the last 4 ma is proposed as a first paleointensity timescale for future stratigraphic studies

late pleistocene eifel eruptions insights from clinopyroxene and glass geochemistry of tephra layers from eifel laminated sediment archive sediment cores

determination of geomagnetic palaeointensities from the quaternary west eifel volcanic field germany summary determination of geomagnetic palaeointensities has been attempted for 418 specimens from 51 volcanic units of the quaternary west eifel volcanic field this paper describes the measuring procedure using a modification of the thellier method the analysis of the derived data points the determination of linear segments and the quality of the individual palaeointensity values calculated from the corresponding slopes according to a series of acceptance and rejection criteria finally 312 reliable individual palaeointensity results have been obtained about 73 per cent of the accepted regression lines have correlation coefficients better than 0 990 and less than 10 per cent of the accepted thellier experiments have quality factors less than 1 0 the carriers of magnetization of the west eifel volcanic rocks are titanomagnetites with a broad range of titanium contents and oxidation states in a more or less unweathered mostly foiditic rock matrix accompanying rock magnetic experiments allowed a rough classification of the specimens corresponding to the oxidation state of the titanomagnetites specimens with highly oxidized titanomagnetites have been the most successful candidates for the thellier experiments however a prediction of the suitability of the specimens for the thellier experiments by sample selection according to this classification was not always appropriate instead for most volcanic units pilot specimens were first measured to determine the suitability of a specific sample type the temperature interval from which the evaluation of palaeointensity was possible and the best laboratory field intensity to keep the slope errors of the thellier experiments to a minimum palaeointensity determination was not possible for 14 volcanic units because only unsuitable specimens were available for each of the 37 remaining volcanic units at least five successful thellier experiments have been carried out leading to a final mean palaeointensity result these 37 values have relative standard errors of between 4 and 23 per cent mostly around 10 per cent specimens with differing rock magnetic properties from the same volcanic unit yielded consistent palaeointensity values therefore all west eifel palaeointensity results have to be regarded as reliable estimates of the earth s magnetic palaeofield strength mean palaeointensities range from 7 to 62 pt with an unexpectedly large number of palaeointensity values less than 30 pt about half of the low palaeointensity values are associated with intermediate virtual geomagnetic pole latitudes which may be due to a geomagnetic excursion

a link between geomagnetic reversals and events and glaciations abstract the apparent duration of geomagnetic polarity events in arctic ocean sediments is much longer than in sediments from lower latitudes in fact while the remanence of brunhes age sediment cores from the yermak plateau at 82 n is fully reversed for 30 of their lengths 1 the events often evade detection in many other continuously deposited sediments for example the laschamp event is absent in an otherwise high resolution record of secular variation from lac du bouchet 2 which is located near the laschamp volcanics where the event was first detected very short event durations of a few hundred years at the most have been suggested before 2 3 because sedimentation rates in the arctic ocean were increased during glaciations the exaggerated proportion of reverse polarities in sediments from high latitudes suggests a link between glaciation and field reversals this suggestion is supported by magnetostratigraphic results obtained from thick loess paleosol sequences in china 4 these demonstrate that all polarity boundaries separating chrons and subchrons since the gauss matuyama field reversal have been recorded in loess and thus during periods of cold climate although conflicting evidence exists for some boundaries furthermore the ages of 22 events and chron boundaries have been compared with the oxygen isotope record 5 thought to represent global ice volume all events and reversals younger than 2 6 ma may have occurred during periods of global cooling or during cold stages however some ages are still too poorly dated for a definite correlation climatic signals also exist in the two longest relative paleointensity records 6 7 but these are suspected to be caused by climatically driven variations in the rock magnetic parameters a mechanism for field reversals may be the acceleration of the earth s rotation caused by lowering of the sea level during glaciations the short duration of events also implies that the geomagnetic field can reverse an order of magnitude faster than commonly assumed

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

full vector geomagnetic field records from the east eifel germany abstract to create meaningful models of the geomagnetic field high quality directional and intensity input data are needed however while it is fairly straightforward to obtain directional data intensity data are much scarcer especially for periods before the holocene here we present data from twelve flows age range 200 to 470 ka in the east eifel volcanic field germany these sites had been previously studied and are resampled to further test the recently proposed multi method palaeointensity approach samples are first subjected to classic palaeomagnetic and rock magnetic analyses to optimise the subsequent palaeointensity experiments four different palaeointensity methods izzi thellier the multispecimen method calibrated pseudo thellier and microwave thellier are being used in the present study the latter should be considered as supportive because only one or two specimens per site could be processed palaeointensities obtained for ten sites pass our selection criteria two sites are successful with a single approach four sites with two approaches three more sites work with three approaches and one site with all four approaches site averaged intensity values typically range between 30 and 35 t no typically low palaeointensity values are found in line with paleodirectional results which are compatible with regular palaeosecular variation of the earth s magnetic field results from different methods are remarkably consistent and generally agree well with the values previously reported they appear to be below the average for the brunhes chron there are no indications for relatively higher palaeointensities for units younger than 300 ka however our young sites could be close in age and therefore may not represent the average intensity of the paleofield three of our sites are even considered coeval encouragingly these do yield the same palaeointensity within uncertainty bounds

a 640 kyr geomagnetic and palaeoclimatic record from lake baikal sediments summary magnetic remanence vectors for 1737 samples from two 100 m cores of lake baikal sediments are reported along with complete magnetic susceptibility profiles obtained from a passthrough system chronological control is established by means of two independent correlations first by matching susceptibility variations to the oceanic oxygen isotope record and second by matching the relative palaeointensity variations to the sint 800 global reference curve these both imply an average deposition rate of 15 cm kyr 1 and a basal age of 640 ka spectral analysis reveals the presence of milankovitch signals at 100 kyr eccentricity 41 kyr obliquity and 23 and 19 kyr precession stable remanence vectors are almost all of normal polarity the few exceptions comprise brief intervals of low and or negative inclinations which probably represent geomagnetic excursions however these are far less numerous than the high sedimentation rate would lead one to expect furthermore only four of them can be readily matched to the still poorly understood global pattern these are the laschamp the albuquerque the iceland basin and perhaps the west eifel excursions which occurred at 38 000 146 000 at 180 000 190 000 and at 480 000 495 000 yr ago respectively

geomagnetic field strength during late miocene first paleointensity results from baja california 1 thellier paleointensity experiments were carried out on 73 selected samples from the late miocene baja california volcanic flows for the first time the selected samples showed a stable remanent magnetization observed upon thermal treatment and reasonably reversible continuous susceptibility curves compatible with ti poor titanomagnetites judging from the ratios of hysteresis parameters it seems that the samples fall in the small pseudosingle domain grain size region thirty three samples from 10 individual flows yielded acceptable paleointensity estimates the natural remanent magnetization nrm fractions used for paleointensity determination range from 37 to 81 and the quality factor varies between 4 5 and 24 5 being normally 5 the mean paleointensity values per flow obtained in this study range from 18 1 3 1 to 45 6 4 3 t and the corresponding virtual dipole moments vdms range from 3 8 0 6 to 8 8 0 8 1022 a m2 these values correspond to mean value of 6 6 1 7 1022 a m2 which is 85 of the present geomagnetic axial dipole the average vdm value for late miocene according to all available selected data is 6 1 1 5 1022 a m2 which is higher than average paleofield for the period 5 160 ma 4 2 2 3 1022 a m2 after juarez and tauxe 2000 given the large dispersion and the very poor distribution of reliable absolute intensity data it is hard to draw any firm conclusions regarding the time evolution of the geomagnetic field more high quality determinations are strongly needed

paleomagnetic evidence from cape verde islands basalts for fully reversed excursions in the brunhes chron abstract in this study we present paleomagnetic data from two lava sequences on santo antao cape verde islands the tarrafal and agua nova profiles from which 63 and 43 lava flows were sampled respectively previous 40ar 39ar ages have constrained the two profiles to the brunhes chron which is in accordance with the normal polarity of the majority of the flows some individual lava flows as well as flow sequences with virtual geomagnetic poles deviating more than 45 from the geographic north pole are interpreted to represent geomagnetic excursions based on interpretation of the directional data three excursions are recorded in the tarrafal profile t i t ii and t iii and four in the agua nova profile ag i ag ii ag iii and ag iv both 40ar 39ar results and paleomagnetic directional data indicate that an excursion defined by eight flows in the tarrafal profile t i is the same as one recorded in six flows in the agua nova profile ag i this excursion which passes the reversal test with classification c is the first geomagnetic excursion in the brunhes chron to pass the reversal test the 40ar 39ar ages suggest that this excursion could represent chryptochron c1n 1 0 493 0 504 ma the only chryptochron or tiny wiggle identified in the marine magnetic anomaly record for the brunhes chron the remaining excursions may not yet be unambiguously correlated between the two profiles another excursion recorded by two flows in the top of the tarrafal profile t iii also displays completely reversed polarity

the blake geomagnetic event transition geometry dynamical characteristics and geomagnetic significance abstract two records of the blake geomagnetic event have been obtained from marine cores in the mediterranean the upper bound for duration of the event calculated from accurate oxygen isotope stratigraphy and or tephrochronology is 4000 years one record is very detailed with 70 transitional directions and allows some characteristics of the event to be precisely retrieved the directional changes do not occur at constant speed rather a stop and go behaviour is observed as in detailed records of reversals the virtual geomagnetic pole vgp path confined to two bands of longitude over the americas and antipodal to them is virtually identical to the only other path of this event obtained from lacustrine sediments in the western usa suggesting that a dipolar component may be present in the transitional field during the blake event the two bands of longitude coincide with those of many recent reversals suggesting that the physical processes leading to reversals or events are basically the same and that the same mechanism is statistically observed over long periods of time

the relationship between the magnitude and direction of the geomagnetic field during the late tertiary in eastern iceland summary during the late tertiary well over 1000 lavas were extruded in eastern iceland potentially they each record an instantaneous description of the palaeomagnetic field we have systematically attempted to determine the palaeofield magnitude from one specimen from each lava about one in every four studied has yielded a palaeomagnitude value to go with the palaeofield direction value previously obtained there is a slight preference for reversely magnetized specimens to yield results results from 106 lavas indicate an arithmetic mean value for the virtual dipole moment vdm of 8 2 0 5 1022 am2 the data suggest an asymmetry whereby the normal polarity field was maintained in magnitude over a wider range of directions than was the reversed polarity field possibly indicating its greater stability

geomagnetic field intensity at hawaii for the last 420 kyr from the hawaii scientific drilling project core big island hawaii four hundred twenty five new paleointensity thellier thellier determinations out of 545 analyzed samples have been obtained from core hsdp which penetrates about 1000 meters 208 flows of the mauna loa and mauna kea volcanic series encompassing the last 420 kyr rock magnetic investigations identify pseudo single domain magnetite as the main magnetic mineral inclinations are shallower than expected from a geocentric dipole field but are consistent with data from other geographical regions at the same latitude the inclination record reveals three episodes of negative inclination whose interpolated age correlates well with that of known geomagnetic events the paleointensity record from the mauna loa sequence is not very detailed and does not allow precise comparison with other data in the 0 50 kyr interval the record from the mauna kea sequence on the contrary is very detailed and documents relatively short lived episodes of low and high field strength from 15 to 60 t the average virtual dipole moment 8 7 3 0 10 22 a m 2 is not significantly different from the value reported by kono and tanaka 1995 for the last 2 5 myr a comparison with other data from hawaii and other geographical regions is described in detail there are no drastic changes in paleointensity with the inclination anomaly in agreement with previous results from hawaii but in contrast with most published results which however consider data from polarity transition spectral analysis of a particularly detailed portion of the record between 420 and 326 kyr documents significant periodicities at 36 8 5 and 4 ka in the inclination record but not in the intensity record suggesting that changes in time of the inclination are to a certain extent independent from those of the intensity

paleointensity of the geomagnetic field during the last 80 000 years high resolution records of the relative paleointensity of the geomagnetic field have been obtained from five marine cores three duplicate records were used to estimate the regional coherency of the data within a single area tyrrhrenian sea while the two others document the field variations in the eastern mediterranean and the southern indian ocean careful investigations of distinct rock magnetic parameters have established the downcore uniformity of the sediments in terms of magnetic mineralogy and grain sizes the time depth control was provided by oxygen isotopes and small scale variations in the deposition rates were constrained by means of tephrachronology the synthetic curve calculated from the mediterranean records provides a continuous record of the intensity variations during the last 80 000 years 80 kyr which correlates well with the sparse volcanic data available for the period 0 40 kyr the fact that identical behavior is seen in both data sets and that they also compare quite well with results from a core collected in the pacific ocean establishes the truly dipolar character of these variations the dipole field moment is characterized by large scale changes as shown by the existence of pronounced drops at 39 and 60 kyr alternating with periods of higher intensity the record suggests a periodic nature for these intensity variations however the period studied is not sufficiently long to state this conclusively these results demonstrate the potential of sediments for such studies and constitute a first step towards obtaining a global paleointensity record over a long period of time

the intensity of the time averaged geomagnetic field the last 5 myr the existing database for paleointensity estimates of the ancient geomagnetic field contains more than 1500 data points collected through decades of effort despite the huge amount of work put into obtaining these data there remains a strong bias in the age and global distribution of the data toward very young results from a few locations also few of the data meet strict criteria for reliability and most are of unknown quality in order to improve the age and spatial distribution of the paleointensity database we have carried out paleointensity experiments on submarine basaltic glasses from a number of dsdp sites of particular interest are the sites that provide paleointensity data spanning the time period 0 3 5 ma a time of relatively few high quality published data points our new data are concordant with contemporaneous data from the published literature that meet minimum acceptance criteria and the combined data set yields an average dipole moment of 5 49 2 36u10 22 am 2 this average value is comparable to the average paleofield for the period 5 160 ma 4 2 2 3u10 22 am 2 t juarez l tauxe j s gee and t pick 1998 nature 394 878 881 and is substantially less than the value of approximately 8u10 22 am 2 often quoted for the last 5 myr e g mcfadden

late pleistocene geomagnetic excursion in icelandic lavas confirmation of the laschamp excursion in 1980 kristjansson and gudmundsson 1 reported a late glacial geomagnetic excursion in three hills in the reykjanes peninsula iceland with shallow negative inclinations and westerly declinations they named it the skalamaelifell excursion more extensive field work has identified the same excursional paleomagnetic direction declination 258 inclination 15 at four additional outcrops in a 10 10 km area in the reykjanes peninsula the excursion lavas are olivine tholeiites with similar petrography and chemical compositions paleointensity determinations by the thellier method average 4 2 0 2 t for 8 samples more than an order of magnitude weaker than the present geomagnetic field in iceland together these results suggest extrusion of the excursion lavas in a very brief span of time probably less than a few hundred years kar dating of the excursion lavas gives a mean age for 19 determinations of 42 9 7 8ka 2 compilation of thirty kar ages of the laschamp and olby flows by three laboratories yield a new age for the laschamp excursion in france of 46 6 2 4ka 2 the age of the excursion in southwestern iceland is statistically indistinguishable from the laschamp excursion at the 95 confidence level and both have very low paleointensities therefore we suggest that the laschamp and olby flows in france and the skalamaelifell units of iceland recorded essentially the same geomagnetic excursion differences in the virtual paleomagnetic poles vgps of these excursions may be due to 1 the probable non dipole character of the geomagnetic field during the excursion 2 rapid geomagnetic secular variation and possible small age differences of the extrusive rocks in france and iceland and or 3 crustal magnetic anomalies which might dominate the local geomagnetic field during the excursion at either or both locations

absolute paleointensities recorded during the brunhes chron at la guadeloupe island abstract ten absolute paleointensity determinations within the brunhes chron have been obtained on andesitic lava flows from la guadeloupe island french west indies f w i the thellier and thellier method performed on 124 specimens under either ambient or argon atmosphere allows reliable determinations from high temperature steps a satisfactory within flow reproducibility has been observed and at least two samples for each flow have been used for calculation of the weighted mean paleointensity the average virtual axial dipole moment vadm 7 1 1 8 1022 a m2 for normal polarity flows from the brunhes chron recorded at la guadeloupe is compatible with the historical field the 10 vadm values obtained show a relatively good agreement with the deep sea composite record sint800 of relative paleointensity focusing on the 100 75 ka interval a 20 ka oscillation observed from a compilation of available volcanic data and present in high resolution sedimentary records has been tentatively related to the axial dipole moment variations finally a paleointensity decrease of a factor of 10 is observed for flows emitted during the matuyama brunhes m b transition as already observed in other records of this reversal because a large area of the globe around the caribbean islands was previously devoid of data the present dataset fills a gap towards construction of a global paleointensity database for the brunhes chron

evidence for a new geomagnetic reversal from lava flows in idaho discussion of short polarity reversals in the brunhes and late matuyama polarity chrons k ar ages and paleomagnetic data for basalt samples from a new core hole site e at the idaho national engineering laboratory inel indicate that the age of the reversed polarity event recorded in snake river plain lavas is older than 465 50 ka 1000 years before present reported previously by champion et al 1981 nine basalt flows eight with normal polarity and one with reversed polarity were recognized in the site e core hole the flows above and below the reversed flow have ages of 491 80 ka and 580 93 ka respectively the inclination of the paleomagnetic field direction of the reversed flow at site e agrees with the inclination of reversed flows elsewhere at inel which have an age of 565 14 ka these reversed flows were previously thought to be correlative with the emperor event we suggest that this polarity event is an older event which we name the big lost reversed polarity subchronozone and subchron a review of data documenting short reversal records from volcanic and sedimentary rocks shows that there is evidence for eight polarity subchrons in the brunhes and two besides the jaramillo in the late matuyama these 10 short subchrons begin to indicate the many short events that cox 1968 hypothesized must exist if polarity interval lengths have a poisson distribution these events are true subchrons not excursions and may or may not have low associated paleointensities although low field strengths might explain why the reversal process aborts the mean sustained polarity interval length since late matuyama chron time is 90 000 years the similarity of this number with the 105 year period of the earth s orbital eccentricity suggests anew that linkage between geomagnetic paleoclimatic and possible underlying earth orbital parameters should be evaluated

40ar 39ar dating links albuquerque volcanoes to the pringle falls excursion and the geomagnetic instability time scale abstract incremental heating of basaltic lava groundmass from the albuquerque volcanoes new mexico and plagioclase from ash d at pringle falls oregon yield 40ar 39ar isochron ages of 218 14 and 211 13 ka respectively sediments in which ash d was deposited and the eight lava flows of the albuquerque volcanoes display excursional paleomagnetic data with virtual geomagnetic poles vgps in the southern hemisphere and together with the statistically indistinguishable 40ar 39ar dates establish that both sites record the pringle falls excursion this excursion is also recorded by the 227 8 ka mamaku ignimbrite new zealand and by high deposition rate sediments at odp site 919 in the north atlantic ocean that are dated astrochronologically at 209 207 ka we propose that the names albuquerque and jamaica excursion be abandoned and that a radioisotopic age of 211 13 ka be adopted for the pringle falls excursion which is one of five globally expressed well documented excursions in marine sediment cores or dated by 40ar 39ar methods that took place from 220 to 30 ka together with at least five other well dated excursions between 730 and 520 ka some ten excursions define the geomagnetic instability time scale gits for the bruhnes chron eight of these excursions have been dated using 40ar 39ar methods if the temporal clustering of excursions in the gits is not a sampling artefact this suggests that the geodynamo is not intrinsically unstable but rather that instability in the flow pattern of the outer core fluid flow causes the main dipole field to be considerably weakened such that it becomes unstable with a 200 to 300 ka recurrence interval excursions are an important part of dynamo behavior over geologic time scales that in addition to reversals of the main dipole field need to be fully considered when assessing whether theoretical and numerical simulations of the dynamo produce earth like results

paleointensity of the earth s magnetic field and k ar dating of the louchadiere volcanic flow central france new evidence for the laschamp excursion we report paleointensity results of the earth s magnetic field from a late pleistocene lava flow louchadiere central france which recorded an intermediate geomagnetic field direction 5 sites mean declination 114 1 inclination 58 2 k 130 95 6 7 new k ar age determinations confirm that this flow is contemporaneous with the laschamp and olby flows and that this excursion occurred around 45ka ago using the thellier double heating method reliable paleointensities have been obtained for ten samples from three different sites providing an average field strength of 12 9 t 3 3 this low value and previous results of the laschamp excursion from france and iceland confirm that the earth s magnetic field was in an intermediate state during the laschamp excursion

odp site 1063 bermuda rise revisited oxygen isotopes excursions and paleointensity in the brunhes chron an age model for the brunhes chron of ocean drilling program odp site 1063 bermuda rise is constructed by tandem correlation of oxygen isotope and relative paleointensity data to calibrated reference templates four intervals in the brunhes chron where paleomagnetic inclinations are negative for both u channel samples and discrete samples are correlated to the following magnetic excursions with site 1063 ages in brackets laschamp 41 ka blake 116 ka iceland basin 190 ka pringle falls 239 ka these ages are consistent with current age estimates for three of these excursions but not for pringle falls which has an apparent age older than a recently published estimate by 28 kyr for each of these excursions termed category 1 excursions virtual geomagnetic poles vgps reach high southerly latitudes implying paired polarity reversals of the earth s main dipole field that apparently occurred in a brief time span 2 kyr in each case several times shorter than the apparent duration of regular polarity transitions in addition several intervals of low paleomagnetic inclination low and negative in one case are observed both in u channel and discrete samples at 318 ka mis 9 412 ka mis 11 and in the 500 600 ka interval mis 14 15 these category 2 excursions may constitute inadequately recorded category 1 excursions or high amplitude secular variation

combined paleointensity and 40ar 39ar age spectrum data from volcanic rocks of the west eifel field germany evidence for an early brunhes geomagnetic excursion results of virtual geomagnetic pole position paleointensity and isotopic age determinations have been derived from lavas of the quaternary west eifel volcanic field germany the characteristic remanent magnetization carm directions and corresponding positions of virtual geomagnetic poles vgp have mainly been determined by bohnel et al they are exclusively of normal polarity the vgps form a strongly non fisherian distribution characterized by about 30 pole positions with latitudes below 60 n which are mainly confined to the longitude sector from 20 w to 120 e eight independent poles are clustered at 40 n and 45 e geomagnetic paleointensity values from 37 volcanic units of the west eifel have been determined paleointensities range from 7 to 62 t with an unexpectedly large number of paleointensity values below 30 t about half of the low paleointensity values are associated with intermediate vgp latitudes isotopic age determinations from west eifel volcanic rocks have been published fuhrmann and lippolt additional isotopic ages are presented here the west eifel volcanic field is usually devided into an older part in the nw and a younger one in the se based on petrological models and geological evidence all available isotopic ages of the west eifel volcanic rocks which are exclusively from the northwestern part of the field lie in the interval between 460 and 660 kyr for seven of the clustered low latitude vgps it was possible to determine paleointensities and 40ar 39ar ages within their error limits six poles have identical virtual dipole moments vdm at 3 5 1022a m2 furthermore the individual isotopic age intervals of the 40ar 39ar data overlap the assigned age of 510 30 kyr is considered to be the age of a geomagnetic excursion for higher latitude vgp positions 45 n vdm values range from 2 to 10 1022a m2 the mean vdm value about 7 5 1022a m2 of the younger volcanoes ages estimated to be below 100 kyr is not remarkably different from the present dipole moment of the earth in contrast the older volcanoes with higher latitude vgp positions yielded a mean value of only 5 3 1022a m2 and a bimodal frequency distribution of the vdm values isotopic ages determined for two of these volcanic units are close to the inferred excursion age suggesting a decrease of the geomagnetic dipole moment during a time interval which surpassed the duration of the excursion

geomagnetic paleointensities from excursion sequences in lavas on oahu hawaii paleomagnetic data demonstrating three late tertiary excursions in the direction of the geomagnetic field recorded in sequences of basaltic lavas on the island of oahu hawaii were published by r r doell and g b dalrymple in 1973 we have determined geomagnetic paleointensities by the thelliers method for 14 lavas from the three sites during these experiments considerable difficulty was encountered because of the presence of titanomaghemite in many lavas and the contamination of natural remanent magnetization by lightning in many others moreover we often observed the production of spurious high temperature chemical remanent magnetization during the thellier experiments an analysis of this particularly troublesome problem is presented two of the sites showed low paleointensities associated with angular departures of the paleomagnetic field direction from that of a geocentric axial dipole which suggests that these excursions represent aborted reversals or fragments of reversals at the third site however the paleointensity did not become low as the field diverged this excursion may reflect the variation of a large nondipole source near hawaii

mid brunhes magnetic excursions in marine isotope stages 9 13 14 and 15 286 495 540 and 590 ka at north atlantic iodp sites u1302 3 u1305 and u1306

pinning down the brunhes matuyama and upper jaramillo boundaries a reconciliation of orbital and isotopic time scales until recently the temporal calibration of the plio pleistocene portion of the geomagnetic reversal time scale grts was based exclusively on kar isotopic ages of volcanic rocks the so called chronogram technique estimates the age that minimizes discrepancies between a given age assignment for a reversal boundary and the population of dates of known polarity the most widely quoted date 0 73 ma for the brunhes matuyama boundary was derived in such a manner 1 radioactive decay is not the only source of temporal information in the geologic record however variations in several of the orbital parameters of the earth e g precession obliquity eccentricity can be calculated for the last several million years and the climatic response approximated a recent analysis of the so called astronomical technique on oxygen isotopic data from deep sea sediments suggested an age of 0 78 ma for the brunhes matuyama boundary 2 some 50 000 years older than the chronogram estimate we reconcile the two estimates for the age of the last reversal by first presenting new magnetostratigraphic data tied to high quality40ar39ar dates our date for the upper jaramillo boundary is0 992 0 039 ma considerably older than the estimate of 0 91 ma in the standard chronogram based time scale 3 furthermore an age of0 746 0 009 ma was obtained for sediments immediately overlying the brunhes matuyama boundary these data are consistent with the astronomical estimates of 0 78 ma and 0 99 ma for the brunhes matuyama and upper jaramillo boundaries respectively finally we present a new method for estimating the uncertainties of ages calculated using the chronogram technique that employs a simple bootstrap resampling scheme the 95 confidence region for the chronogram estimate of the brunhes matuyama boundary using an updated compilation of date polarity pairs spans from 0 73 to 0 78 ma we find therefore that the discrepancies between the isotopic data set and the astronomical calibration of the brunhes matuyama boundary disappear when appropriate uncertainties are applied for the most recent compilation of isotopic data

a sixty thousand year paleomagnetic record from gulf of california sediments secular variation late quaternary excursions and geomagnetic implications a unique inclination record of geomagnetic secular variation for the past 60 000 years 60 ka has been obtained from continuously deposited sediments in the gulf of california recovered during deep sea drilling project dsdp leg 64 by hydraulic piston coring at site 480 the chronology of hole 480 was determined by 18o stratigraphy and varve counts indicating sedimentation rates approaching 1 m ka the paleomagnetic results of the upper 49 m show 1 the average inclination over this interval is identical to the geocentric axial dipole value at the sampling site 2 excursion directions occur between about 53 and 22 ka before present bp during this time the geomagnetic field was generally noisier than in the overlying and underlying sections with greater dispersion of the inclination 3 the laschamp excursion was apparently recorded at hole 480 between about 51 and 49 ka bp and the mono lake excursion between about 29 and 26 ka bp in addition a narrow 0 3 0 4 m zone near 23 ka bp has a very similar paleomagnetic signature as the excursion observed at summer lake oregon 1 and we suggest that the summer lake is distinct from and younger than the mono lake excursion by 3 5 ka and of considerably shorter duration lasting no more than a few hundred years 4 recurring inclination fluctuations were identified at site 480 characterized by end points with steep inclinations and shallow intermediate value s as compared with the geocentric axial dipole the inclination cycles are particularly apparent from 54 to 24 ka bp with a characteristic period of about 4 4 ka 5 the noisier inclination record between 54 and 24 ka bp might be related to the generally reduced dipole moment between about 20 and 50 ka and particularly low paleointensities for the laschamp and mono lake excursions

marine magnetic anomalies as recorders of geomagnetic intensity variations in addition to providing a robust record of past geomagnetic polarity reversals marine magnetic anomalies often show shorter wavelength variations which may provide information on geomagnetic intensity variations within intervals of constant polarity to evaluate this possible geomagnetic signal we compare sea surface profiles of the central anomaly with synthetic profiles based on brunhes age 0 0 78 ma paleointensity records derived from deep sea sediments the similarity of the synthetic profiles and observed profiles from the ultra fast spreading southern east pacific rise suggests that geomagnetic intensity variations play an important role in the magnetization of the oceanic crust this interpretation is further supported by systematic variations in the pattern of the central anomaly at slower spreading ridges which are entirely consistent with a progressively smoother record of the sediment derived paleointensity if the sedimentary records as calibrated to available absolute paleointensity data accurately record variations in dipole intensity over the brunhes it follows that much of the brunhes was characterized by geomagnetic intensities lower than either the mean dipole moment for the past 10 ka or the average for the period from 0 05 to 5 0 ma furthermore the sediment paleointensity records reflect the significant increase in geomagnetic intensity from a low of 2 1022 am2 near 40 ka to a peak value 11 1022 am2 at 3 ka that has been well documented from absolute paleointensity determinations we suggest that geomagnetic intensity variations may be the most important cause of the rapid changes in the source layer magnetization near the ridge crest and the resultant central anomaly magnetic high

mono lake or laschamp geomagnetic event recorded from lava flows in amsterdam island southeastern indian ocean summary we report a survey carried out on basalt flows from amsterdam island southeastern indian ocean in order to check the presence of intermediate directions interpreted as belonging to a geomagnetic field excursion within the brunhes epoch completing this palaeomagnetic record with palaeointensity determinations and radiometric dating because the palaeomagnetic sampling was done over a few hours during the resupply of the french scientific base martin du viviers by the marion dufresne vessel we could collect only 29 samples from four lava flows the directional results corroborate the findings by watkins nougier normal polarity is found for two units and an intermediate direction with associated virtual geomagnetic poles vgps close to the equator for the other two units a notable result is that these volcanic rocks are well suited for absolute palaeointensity determinations 50 per cent of the samples yields reliable intensity values with high quality factors an original element of this study is that we made use of the thermomagnetic criterion ptrm tail test of shcherbakova et al to help in the interpretation of the palaeointensity measurements doing this only the high temperature intervals beyond 400 c were retained to obtain the most reliable estimate of the strength of the ancient magnetic field however not applying the ptrm tail test does not change the flow mean values significantly because the samples we selected by conventional criteria for estimating the palaeointensity carry only a small proportion of their remanence below 400 c the normal units yield virtual dipole moments vdm of 6 2 and 7 7 10 22 am 2 and the excursional units yield values of 3 7 and 3 4 10 22 am 2 these results are quite consistent with the other thellier determinations from brunhes excursion records all characterized by a decrease of the vdm as the vgp latitude decreases 40 ar 39 ar isotopic age determinations provide an estimate of 26 15 and 18 9 kyr for the transitional lava flows which could correspond to the mono lake excursion however the large error bars associated with these ages do not exclude the hypothesis that this event is the laschamp

some aspects of the late tertiary geomagnetic field in iceland summary in 1972 78 the late n d watkins and others carried out ajoint field programme of geological mapping in the mio pliocene flood basalts of iceland including sampling for k ar dating and palaeomagnetic research the major part of the palaeomagnetic sampling is represented by 2462 lavas in five long composite sections through the lava pile this paper deals with various statistical properties of this data set it is concluded that geomagnetic reversals occur more frequently than is assumed in the current ocean floor polarity time scale there is no evidence for significant asymmetries between normal and reverse polarity states of the field neither as regards chron lengths secular variation or virtual dipole moment magnitude intensities of remanence in these lavas are shown to be well approximated in terms of a hyperbolic distribution the latitude distribution of virtual magnetic poles can be fitted with a bingham function having k 4 5 and low latitude poles do not occur preferentially in any particular longitude interval

the galapagos axial magnetic anomaly evidence for the emperor event within the brunhes and for a two layer magnetic source the axial brunhes magnetic anomaly over the galapagos spreading center shows consistent short wavelength anomalies that are not consistent with a uniformly magnetized layer we interpret this anomaly to confirm the presence of the emperor reversed event at 0 49 0 05 mybp with a duration about 0 01 m y further this anomaly is consistent with a two layer magnetic source with the lower layer having very broad transition zones offset away from the axis from those of the upper layer by about 10 km

the geomagnetic dipole moment variation between 250 and 800 ka bp reconstructed from the authigenic 10be 9be signature in west equatorial pacific sediments the authigenic 10 be 9 be ratio proxy of the cosmogenic radionuclide 10 be production rate in the atmosphere was measured in sediments deposited in the west equatorial pacific gulf of papua 10 25 s 146 15 e in order to reconstruct the geomagnetic dipole moment variations in the 250 800 ka time interval independently from paleomagnetic methods the pelagic clayey carbonate muds continuously deposited between marine isotope stages 8 and 20 were subsampled every 10 cm the 10 be production rate record reconstructed for the 250 800 ka interval is described and compared over the same time interval with global paleomagnetic stacks sint 2000 and piso 1500 the highlighted cosmogenic 10 be overproductions appear to be triggered by the series of geomagnetic dipole lows gdl that have occurred since the brunhes matuyama reversal calibration using absolute values of the geomagnetic dipole moment drawn from a paleomagnetic database produces a new record of the dipole moment variations over the 250 800 ka time interval the timing and amplitude of the gdl succession revealed by such records will contribute to improving knowledge of the geodynamo rhythms and rate of changes at millennial to million year scales

a possible lacustrine paleomagnetic record of the blake episode from pringle falls oregon u s a abstract we have studied a high resolution paleomagnetic record of a magnetic polarity episode recovered from a 20 m section of diatomaceous lacustrine sediments from pringle falls near la pine oregon 43 7 n 238 6 e a total of 474 samples 982 specimens were collected at about 5 cm intervals from the 20 m section the age of this record is at present constrained between 0 6 ma and 18 000 yr based upon a pollen date from the older part of the lakebed sequence and the age of the younger glacial deposits that overlie the lakebeds the characteristic magnetization of the samples was determined by alternating field af demagnetization to 15 mt the average normal polarity stable declination and inclination of the non magnetic polarity episode sampled section 61 95 2 6 is statistically equivalent to that expected at this site for a geocentric axial dipole 62 the record provides a virtual geomagnetic pole vgp path that identifies a well defined clockwise ne sw loop with a radius of 60 followed by a second younger feature that traces out a counter clockwise loop 15 in radius residing in the ne quadrant and characterized by low latitudes interpretation of the natural remanent magnetization anhysteretic remanent magnetization nrm arm ratios for the samples indicates that the field intensity may have fallen and then increased above the average dipole field intensity the low intensity interval ends before the onset of the polarity episode as defined by directions and may suggest that the episode is a result of an unstable regeneration of the field after a significant lowering of overall field intensity these two characteristic features are similar to the two features present in the blake geomagnetic polarity episode at 114 000 and 105 000 yr bp observed in different parts of the world

differential subsidence of the forearc wedge of the ryukyu nansei shoto arc caused by subduction of ridges on the philippine sea plate abstract the philippine sea plate psp carrying several ridges has been sudbucting under the ryukyu nansei shoto arc since middle miocene because no extensive accretionary prism has been growing along the ryukyu trench the arc provides an opportunity to examine effects of ridge subduction on structure of the forearc wedge and a clue to reconstruct ancient plate motion of the psp that is inferred to have changed between nw and nnw to examine this perspective we clarified structure of the ryukyu forearc wedge based on seismic profiles and bathymetric data and related them to ridge subduction an erosional unconformity between pre neogen and neogene rocks is widely recognized through the ryukyu arc and we divided the forearc wedges into zones i to iv from southwest to northeast by difference of depth of the erosional unconformity we correlated these four zones to the locations of ridge subduction that have been shifting ne or sw along the ryukyu trench zone i is underlain by the largely subsided unconformity and we attributed the structure to tearing of the eurasia plate due to subduction of the western margin of the psp including the luzon arc zone ii consists of a wide terrace on the shallow erosional unconformity and no ridge that was subducting in this zone is known zone iii is characterized by the seaward descending unconformity covered with a thick sequence of neogene sediments and is related to subduction of the nw se trending daito ridge and the e w trending amami plateau zone iv has the deeper unconformity and slope geometry and corresponds to subduction of the nnw sse trending kyushu palau ridge the structure of the ryukyu forearc is consistently related to subduction of ridges on the psp suggesting that the change of the plate motion of the psp can be precisely reconstructed by further detailed survey

paleosecular variation of field intensities and dipole moments abstract in order to use paleointensity data for paleosecular variation studies we first obtain analytical for the intensity of the geomagnetic field and the corresponding dipole moment in terms of gauss coefficients next statistical parameters for these quantities are derived under the assumption that gauss coefficients vary randomly with time and that the axial dipole component is much larger than all the other gauss coefficients using these expressions we construct a homogeneous background model for paleosecular variation in which gauss coefficients other than the axial dipole term are zero mean random variables with variances decaying exponentially with the degree of the harmonic we compare this model with the present field and the data from the last 10 ma collected in the paleointensity database although we have to make certain allowances because some of the paleointensity data may not be characteristic of the time averaged paleomagnetic field we obtain a broad agreement between the observation and a slightly modified form of the homogeneous background model from this match we conclude that the amplitude of the non dipole components may be about twice that predicted from the present field and that the contribution from the equatorial dipole is very small the present analysis suggests that a more rigorous selection criteria should be employed to secure the reliability of data and also that the data set should more adequately sample the time interval before we can obtain a more detailed knowledge of the structures of the ancient magnetic field

geomagnetic paleointensities derived from volcanic rocks of the quaternary east eifel volcanic field germany geomagnetic paleointensities have been determined for 18 rock units of basanitic thephritic or nephelinitic composition from the quaternary east eifel volcanic field germany in general the rocks are well suited for paleointensity determination and gave consistent results of paleointensity for each rock unit at least five individual values have been measured with the thellier method compared with the present day field of 48 t the unweighted mean paleointensity values range from 29 to 62 t with relative standard errors mostly below 10 paleointensity values are concentrated at 30 and 50 t with a gap at 40 t the distribution is bimodal rather than gaussian although all paleopoles lie close to the geographic pole this cannot be explained by the very discontinuous distribution of volcanic activity in time which took place mainly around 200 and 400 ka in each age group both low and high paleointensity values are found however a further temporal separation within each age group cannot be excluded furthermore the paleointensity values of the west eifel volcanic field show a similar bimodal distribution which indicates that this case seems to be connected with a geomagnetic excursion recorded in those rocks therefore it might be considered that the east eifel rocks recorded a characteristic feature of the earth s magnetic field namely a quasi cyclic variation of paleointensity on the other hand the now available global data set of 128 virtual dipole moment values from the brunhes chron and older than 30 ka shows an almost gaussian distribution with a mean value of 7 4 3 4 1022 am2 if 14 values with transitional pole positions are excluded a quasi cyclic variation of the dipole moment therefore does not seem to be a persistent global characteristic of the earth s magnetic field

geomagnetic excursion captured by multiple volcanoes in a monogenetic field 1 five monogenetic volcanoes within the quaternary auckland volcanic field are shown to have recorded a virtually identical but anomalous paleomagnetic direction mean inclination and declination of 61 7 and 351 0 respectively consistent with the capture of a geomagnetic excursion based on documented rates of change of paleomagnetic field direction during excursions this implies that the volcanoes may have all formed within a period of only 50 100 years or less these temporally linked volcanoes are widespread throughout the field and appear not to be structurally related however the general paradigm for the reawakening of monogenetic fields is that only a single new volcano or group of closely spaced vents is created typically at intervals of several hundred years or more therefore the results presented show that for any monogenetic field the impact of renewed eruptive activity may be significantly under estimated especially for potentially affected population centres and the siting of sensitive facilities