10be signature of the matuyama brunhes transition from the heqing paleolake basin abstract we present a new 10be record spanning the matuyama brunhes m b reversal from the heqing paleolake basin in southern china it provides a robust proxy for past geomagnetic variations that can be compared with paleomagnetic records the m b transition is identified as a pronounced maximum of authigenic 10be 9be ratio between 768 6 and 778 5 ka that is consistent with all other available 10be proxy records from marine ice and loess archives however it is offset by approximately 0 60 cm depth from the magnetic signature of the polarity transition recorded in the same sediments we attribute this offset to the lock in effect of the remanence acquisition process which is similar to the phenomenon that exists in marine sediments we suggest that after eliminating the climatic hydrological signal the heqing sediments may be used as an archive for atmospheric 10be production rate changes the combination of magnetic remanence measurements and the cosmogenic 10be allows for a more precise geochronology of geomagnetic polarity reversals

geomagnetic cosmogenic and climatic changes across the last geomagnetic reversal from equatorial indian ocean sediments abstract high resolution records of beryllium 10be production and relative paleointensity have been obtained across the matuyama brunhes m b reversal from the equatorial indian ocean maldives area both magnetic and geochemical analyses were performed from the same discrete samples to avoid any artificial depth offset the authigenic 10be concentrations were normalized with respect to 9be in order to correct for potential environmental effects while the relative paleointensity was derived from the remanent magnetization intensity after accounting for changes in magnetic concentration within the sediment the relative paleointensity and the 10be 9be records are both characterized by large deviations which culminate in the middle of the reversal in contrast to most previous studies and despite relative high deposition rate 4 7 cm ka we observed a perfect synchronism between the 10be 9be peak the lowest value of relative paleointensity and the switch in direction which indicates that bioturbation and post depositional processes did not affect the magnetic record this leaves no ambiguity for the stratigraphic position of the reversal located within marine isotopic stage 19 as revealed by the planktonic 18o record from the same core the magnetic data depict a two phase process with a precursory event preceding the rapid polarity switch while only the second phase is present in the 10be record similarly to other low latitude records from the indonesian area using an orbitally tuned age model we obtain an age of 772 ka 5 ka for the middle of the transition while the precursory event occurred almost 20 ka before we believe that the bimodal distribution emerging from the compilations of the ages of the m b reversal results from the succession of these two events microtektites from the australasian impact were found at 0 6 m below the transition 790 ka 5 ka b p and confirm that this large event occurred 12 ka prior to the polarity transition the distribution of tektite abundance was used to deconvolve the 10be 9be signal the results confirm that the beryllium changes are concentrated during the transitional period thus likely in presence of a multipolar geomagnetic field or in the vicinity of a geomagnetic pole that favored the penetration of cosmic rays and consequently increased the 10be production the absence of 10be during the precursor indicates that the present site and the indonesian ones were far away from a geomagnetic pole and that interlatitudinal atmospheric mixing was limited the geomagnetic pole positions above the indonesian sites during the precursor would thus be incompatible with the corresponding inclined dipolar field during this period and suggest the dominance of low degree harmonics

late ediacaran magnetostratigraphy of baltica evidence for magnetic field hyperactivity abstract long term changes in characteristics of the geomagnetic field may signal evolutionary changes within the earth s interior among these long term variations the magnetic field reversal rate is probably the best known and most frequently used parameter to evaluate concomitant changes in the earth s core although the reversal rate is well known for the last 150 ma and reasonably constrained back to 300 ma knowledge of the early paleozoic and earlier times is limited hence the haunting question is whether the reversal pattern for the last 300 myr is a good representation of the field behavior over a much longer interval a paleomagnetic study of upper ediacaran sediments from the zigan fm baltica in the western south urals revealed a high temperature dual polarity component that is of primary origin levashova et al 2013 in this paper we present the results of a magnetostratigraphic study from the same sections that can be summed up as follows 1 30 magnetozones are found in the ca 110 meters thick composite section of the zigan fm 2 a similar reversal pattern was reported from upper ediacaran sediments from the white sea coast north russia and sw siberia but has never been found in clastic rocks of any other age 3 the reversal pattern in three remote ediacaran sections could not result from remagnetization or extremely slow sedimentation 4 the reversal rate in the late ediacaran exceeded 20 reversals per million years 5 the field appears to behave in a bipolar fashion during the period of hyperactivity

paleointensity variation across the matuyama brunhes polarity transition observations from lavas at punaruu valley tahiti 1 we have conducted a paleointensity study of the matuyama brunhes m b polarity transition recorded in 34 successive lava flows of punaruu valley on tahiti a reversed polarity is obtained from the lower part of the record major directional changes are derived from the middle part of the record and a normal polarity is recorded in the upper part of the record these paleomagnetic directions and five 40ar 39ar ages yielding a weighted mean of 771 8 1 ka indicate that 30 lava flows recorded the geomagnetic field across the m b transition the 215 specimens from 32 flows were subjected to the double heating technique of the shaw method combined with low temperature demagnetization ltd dht shaw method yielding 73 successful results from 18 flows for the reversed polarity period just prior to the major directional changes paleointensity shows an oscillation like variation between 3 and 38 t corresponding to virtual dipole moments vdms between 0 9 1022 and 9 6 1022 am2 for the major directional changes a weak paleointensity of 5 t is obtained which gives a vdm of 1 0 1022 am2 for the normal polarity period paleointensities are 14 21 t giving vdms of 3 5 5 2 1022 am2 for the reversed polarity period just prior to the major directional changes a linear relationship with a correlation coefficient of 0 96 is recognized on the diagram of vdm versus virtual geomagnetic pole latitude this linear relationship may be a precursory feature of the geodynamo at the onset of the m b transition

directions and intensities of the earth s magnetic field during a reversal results from the permo triassic siberian trap basalts russia abstract an extensive palaeomagnetic study was carried out on an approximately 250 ma old reversed to normal transition of the earth s magnetic field emf recorded in 86 volcanic lava flows of the siberian trap basalts north siberia russia in addition to the investigation of the directional behaviour of the field 700 specimens a total of 298 specimens was subjected to thellier type palaeointensity determinations adding several modifications to the original thellier experiment such as tests for md tails and the additivity of partial thermoremanent magnetisation yields highly reliable palaeointensity estimates of the late permian early triassic emf transitional directions of the emf were obtained from 20 flows during the reversal a clustering of the virtual geomagnetic poles vgps is observed 15 flows palaeointensity estimates suggest that this feature is not an artifact due to rapid flow emplacement since the directional cluster is associated with a well defined increase in palaeointensity from 6 to 13 t subsequently the next vgps move towards the pole position of normal polarity departing in a sudden movement from normal polarity the vgps form a second directional cluster comprising the results of 14 flows this feature is interpreted as a post transitional excursion but lacks the characteristic intensity variation recorded during the first transitional cluster the rest of the section 41 flows is of normal polarity the characteristic features of this reversal low intensities and directional clustering during the reversal and an excursion shortly after the reversal were also observed in records of polarity transitions of younger age this suggests that the underlying reversal processes were similar the mean virtual dipole moment calculated for the stable normal part of the studied section yields a rather low value of 2 2 0 9 1022 am2 these findings confirm that the mesozoic dipole low extends at least down to the late permian early triassic the geomagnetic vgp dispersion was calculated for the stable normal part of the section and yields values similar to those for the last 5 ma considering that the intensity of the emf was considerably higher in this time interval than in the early triassic a direct relationship between intensity and secular variation seems unlikely

a reversal of the earth s magnetic field recorded in mid miocene lava flows of gran canaria paleointensities 1 an extensive paleointensity study was carried out on an approximately 14 1 myr old reverse to normal transition of the geomagnetic field a total of 188 samples from a volcanic sequence on gran canaria canary islands were subjected to thellier type paleointensity determinations the paleodirectional results of this sequence are reported by leonhardt et al 2002 a modified thellier technique which facilitates the recognition of md tails and the formation of new magnetic remanences with higher blocking temperatures than the actual heating step was used on the majority of the samples we obtained reliable paleointensity results for 35 of the 87 sampled lava flows in general the obtained field intensities are lower than expected for the mid miocene this observation is very likely related to a long term reduction of the field close to transitions very low paleointensities with values 5 t were obtained during an excursion preceding the actual transition and also close to significant changes of the local field directions these are interpreted as nondipolar components becoming dominant for short periods and provoking a rapid change of field directions during the transition 15 successive lava flows recorded similar local field directions corresponding to a cluster of virtual geomagnetic poles close to south america chronologically within this cluster the paleointensity increases from about 9 to 28 t followed by a decrease back to approximately 9 t this variation of paleointensity between lava flows with similar direction indicates independent records of the geomagnetic field and gives strong evidence for a stable transitional state that lasted a significant period

eruptive history of the karoo lava flows and their impact on early jurassic environmental change this paper reports new paleomagnetic and geochronologic data from a 1500 m thick composite section belonging to the drakensberg group the thickest remnant of the karoo lavas in northern lesotho flow by flow analysis of paleomagnetic directions reveals twenty one magnetic directional groups dg corresponding to single eruptive events see and sixteen individual lava flows the new age determinations of lava flows range from 180 1 1 4 to 182 8 2 6 ma these data combined with previous results allow us to propose that the main part of the drakensberg group and the karoo intrusive complex dated around 181 183 ma may have been erupted over a period as short as 250 kyr and may have coincided with the two main phases of extinction in the early toarcian this scenario agrees well with the discontinuous rhythm of environmental and biotic perturbations in the late pliensbachian toarcian interval

formation of lava tubes and extensive flow field during the 1991 1993 eruption of mount etna detailed mapping during the 1991 1993 eruption of mount etna has shown that there is a relationship between tumuli ephemeral vents lava tubes and their parent lava flows during this eruption many tubes formed in stationary inflated a a lava flows ephemeral vents at the fronts of these stationary flows and above lava tubes fed secondary lava flows many of which subsequently developed new tubes the resulting complex network of tubes ephemeral vents and secondary flows was responsible for most of the widening thickening and lengthening of the 1991 1993 etna lava flow field the supply of relatively uncooled lava via tubes to distal parts of this flow field allowed lava to flow 3 km farther from the vent than the longest channel fed lava flow our observations suggest that lava tubes play a more important role in the formation of extensive a a flow fields on etna than has previously been recognized

lava flow identification and aging by means of lidar intensity mount etna case 1 an application of light detection and ranging lidar intensity for the identification and mapping of different lava flows from the mount etna italy active volcano is described in september 2004 an airborne lidar survey was flown over summit sectors of mount etna the information derived from lidar intensity values was used to compare the lava flows with respect to their age of emplacement analyzed lava flows vary in age between those dating prior to a d 1610 and those active during the survey 2004 2005 eruptions the target emitter distance as well as surface roughness and texture at the lidar footprint scale is the main parameter controlling the intensity response of lava flows variations in the roughness and texture of surfaces at a meter scale result from two main processes initial lava cooling and subsequent surface weathering both lead to variations in the original surface roughness of the flow in summary 1 initially from the time of emplacement the lidar intensity of lava flow surfaces decreases and 2 about 6 years after emplacement the lidar intensity of lava surfaces starts to increase with the age of flows lidar capability in terms of geometric accuracy of 1 m in plan position and less than 1 m in elevation and spectral lidar intensity depends on surface reflection at 1 064 m information can thus be effectively used to map lava flows and to define a relative chronology of lava emplacement

toothpaste lava characteristics and origin of a lava structural type transitional between pahoehoe and aa toothpaste lava an important basalt structural type which illustrates the transition from pahoehoe to aa is particularly well displayed on the 1960 kapoho lava of kilauea volcano its transitional features stem from a viscosity higher than that of pahoehoe and a rate of flow slower than that of aa viscosity can be quantified by the limited settling of olivine phenocrysts and rate of flow by field observations related to the low angle slope on which the lava flowed much can be learned about the viscosity rheologic condition and flow velocity of lavas long after solidification by analyses of their structural characteristics and it is possible to make at least a semiquantitative assessment of the numerical values of these parameters

late quaternary geomagnetic secular variation from historical and 14c dated lava flows on hawaii a paleomagnetic record of geomagnetic paleosecular variation psv is constructed for the last 4400 years based on 191 sites in historical and 14c dated lava flows from mauna loa kilauea and hualalai volcanoes on the island of hawaii the features of this new record are similar to those recorded by sediments from lake waiau near the summit of mauna kea volcano but overall mean inclinations for the lava flows 31 to 33 depending on window size are nearer the expected dipole field value 35 than is that for the sediments 27 divergence of the inclination records with increasing age suggests that the lake waiau values at depths below 2 m have been affected by compaction related inclination shallowing although magnetic terrain effects cannot be ruled out the rate of psv indicated by the record presented here is highly variable 20 century and a pronounced shift in inclination from 25 to 40 occurred between 1030 and 975 years b p paleomagnetic directions from undated materials can be correlated with our calibrated curve but the resolution is largely dependent on the psv rate and data densities for both the reference and unknown directions the upper part of the puna basalt 18 lava flows previously sampled for paleomagnetism along the northern wall of kilauea s caldera uwekahuna bluff was likely deposited sometime between 1030 and 750 years b p but the lowest two flows beneath the uwekahuna ash 2100 years b p are correlated with an age of 3034 years b p paleomagnetic data for 54 lava flows of the ka u basalt exposed in the northwest wall of mauna loa s summit caldera mokuaweoweo indicate that they probably accumulated over a relatively short time interval 200 years and are assigned to a 1000 to 1199 year b p time window the mean of ages within this window is 1030 years b p but mapping and other 14c dates indicate that these summit overflows are probably closer to 1200 years b p in age

a paleomagnetic and rock magnetic study of the manicouagan impact structure implications for crater formation and geodynamo effects we report rock magnetic and paleomagnetic data from the similar to 214ma manicouagan canada impact crater based on 25 widely distributed sites of impact melt and basement rocks collected at the surface as well as from boreholes drilled to depths 1 5km titanomagnetite and titanohematite carry the magnetic remanence in impact melts above 320m elevation and in most basement rocks impact melts below 320m contain solely titanomagnetite magnetic susceptibility and saturation magnetization proxies for titanomagnetite concentration increase more than tenfold toward the base of the thickest impact melt that underwent fractional crystallization the titanomagnetite enriched zone partially contributes to a 2000nt magnetic anomaly in the crater s center stepwise demagnetization reveals a single normal polarity magnetization component in all samples regardless of the magnetic phases present coeval lock in remanence times for titanomagnetite and titanohematite indicate that the titanohematite formed 570 degrees c during oxi exsolution the average paleomagnetic direction and intensity coincide well with 214ma reference values we find no evidence for an aberration of the geomagnetic field over the several thousands of years it took to cool a 481m thick portion of the impact melt body hence the energy released by the manicouagan impact that created one of the 10 largest known craters on earth provoked no measurable disturbance of the geodynamo magnetic anisotropy of clast free impact melts define magnetic lineation directions that are in places radially oriented with respect to the crater s center centrifugal flow of the melt within the evolving transient crater probably generated the fabric

the lobes of lava flows on earth and olympus mons mars the lobate distal margins of lava flows provide a useful source of morphological information on the rheology of the lava if the lobes are assumed to represent the arrest of free flowing isothermal bingham fluids on a slope the widths of lobes are a more useful practical index than lobe thicknesses because they are about an order of magnitude larger and can be more accurately measured from aerial photographs and other remote images lobes do not suffer from the changes in morphology that channels undergo during the course of eruptions a terrestrial data set of flow lobe and ancillary measurements from lavas throughout the range alkali olivine basalt to rhyolite shows some features that are predicted by the isothermal bingham fluid model these are correlation of width and thickness over more than two orders of magnitude and essentially no correlation of aspect ratio with slope there is a positive correlation of lobe width with silica content of the lava from a data set of measurements on lava flow lobes from the martian volcano olympus mons the mean value of aspect ratio 0 07 was found to be significantly less than that for the terrestrial data set 0 19 higher general levels of effusion rate on olympus mons are probably the factor responsible after normalisation lobe widths on olympus mons are found to be largely equivalent to those expected for terrestrial flows with andesitic basaltic silica contents

a 3d model of crustal magnetization at the pinacate volcanic field nw sonora mexico abstract the pinacate volcanic field pvf is located near the western border of the southern basin and range province in the state of sonora nw mexico and within the gulf of california extensional province this volcanic field contains the shield volcano santa clara which mainly consists of basaltic to trachytic volcanic rocks and reaches an altitude of 1200 m the pvf disrupts a series of discontinuous ranges of low topographic relief aligned in a nw direction which consist mainly of proterozoic metamorphic rocks and proterozoic through paleogene granitoids the pvf covers an area of approximately 60 by 55 km and includes more than 400 well preserved cinder cones and vents and eight maar craters it was active from about 1 7 ma until about 13 ka we have used the ages and magnetic polarities of the volcanic rocks along with mapped magnetic anomalies and their inverse modeling to determine that the pinacate volcanic field was formed during two volcanic episodes the oldest one built the santa clara shield volcano of basaltic and trachytic composition and occurred during the geomagnetic matuyama chron of reverse polarity which also includes the normal polarity jaramillo and olduvai subchrons thus imprinting both normal and reverse magnetization in the volcanic products the younger pinacate series of basaltic composition represents monogenetic volcanic activity that extends all around the pvf and occurred during the subsequent geomagnetic brunhes chron of normal polarity magnetic anomalies toward the north of the santa clara volcano are the most intense in the pvf and their inverse modeling indicates the presence of a large subsurface body magnetized in the present direction of the geomagnetic field this suggests that the magma chambers at depth cooled below the curie temperature during the brunhes chron

timing of the matuyama brunhes geomagnetic reversal decoupled thermal maximum and sea level highstand during marine isotope stage 19 recent high resolution magnetic and microfossil data from a long core penetrating the osaka group japan are astronomically tuned and reassessed to reveal detailed stratigraphic features across the matuyama brunhes mb polarity transition during marine isotope stage mis 19 the sediments have a uniform and high average accumulation rate of 63 cm ky astronomical tuning based on six calibration points including a clear mis 19 2 sea level lowstand dates the main mb boundary mbb to 777 6 ka the sea level peak of the mis 19 3 highstand to 780 7 ka and the climatic thermal maximum tm to 776 4 ka the mb transition began with a short lived normal polarity episode that occurred before the highstand at mis 19 3 and terminated with multiple rapid reversals that occurred between highstand mis 19 3 and lowstand 19 2 the tm just postdates the termination of the mb transition and occurs about 4000 years after the sea level peak the delayed tm is also observed at gesher benot ya aqov israel and on the mediterranean coast and possibly at lake baikal in high resolution records from chinese loess paleosols and deep sea sediments the rapid reversal zone of the mbb has a very similar climatostratigraphic character these magneto climatostratigraphic features in mis 19 are useful for correlating between terrestrial and marine sediments the delay in warming relative to the sea level highstand seems to have been due to a cloud albedo effect induced by an increase in galactic cosmic ray flux during an extremely low magnetic field intensity interval

absolute paleointensity and reversal records from the waianae sequence oahu hawaii usa abstract paleointensity experiments were performed on samples from four parallel sections of the waianae volcanics of oahu hawaii that document 0 4 myr of geomagnetic field variations the records of the three successive gilbert gauss lower and upper mammoth reversals confirm that large oscillations of directions precede or follow the reversals which are similar to waveforms generated by paleosecular variation with their amplitude being considerably amplified by the decrease of the dipole determinations of absolute paleointensity were attempted on 546 samples we only selected data which were obtained from segments covering a large part of the remanent magnetization more than 70 on the average and without concave up arai diagrams this procedure limited the success rate to 14 but provided consistent and reliable paleointensities in addition to other time intervals the results document the field variations surrounding the lower mammoth transition a weak field period dominated before the reversal then the transition was initiated by a transit from normal to reverse polarity followed by a short restoration of the field intensity in reverse polarity a second episode of a very weak field was accompanied by a return to positive inclinations before definitely reaching the reverse polarity a strong and apparent rapid recovery of the dipole following the completion of the reversal culminated at a value of 16 1022 am2 similar to field intensities reported for the other detailed volcanic records of reversals studied so far the asymmetry between the pre and the post reversal phases indicates the importance of field regeneration to initiate a new stable polarity interval

can downwelling at the top of the earth s core be detected in the geomagnetic secular variation abstract it has been argued based on recent seismic and mineral physics studies that the top of earth s liquid outer core is stably stratified here i analyze persistent geomagnetic secular variation features on the core mantle boundary to examine whether a kinematic signature of core fluid upwelling downwelling can be detected i focus on regions of intense high latitude geomagnetic flux patches that may be maintained by fluid downwelling in order to identify persistent patterns the radial field and its secular variation are stacked in the flux patch moving reference frame these stacked images are compared with forward solutions to the radial induction equation based on idealized field flow models clear advective secular variation signature below north america indicates that these intense flux patches may exhibit significant mobility stretching signature in the form of persistent positive secular variation correlated with the intense flux patch below the southern indian ocean may be considered as regional scale geomagnetic evidence for whole core convection although pure toroidal flow cannot be outruled

the distal segment of etna s 2001 basaltic lava flow etna s 2001 basaltic lava flow provided a good example of the distal flow segment between the flow front and stable channel across which the flow evolves from channel contained to dispersed this zone was mapped with meter precision using lidar data collected during 2004 and 2005 these data supported by field mapping show that the flow front comprised eight lobes each 10 to 20 m high the flow front appears to have advanced not as a single unit but as a series of lobes moving forward one lobe at a time primary lobes were centered on the channel axis and marginal lobes were off axis the lobes advanced as breakouts of low yield strength lava from the flow core of the stalled flow front marginal lobes were abandoned and contributed to marginal levees flanking the transitional channel for etna s 2001 flow the transitional channel is 140 m wide 700 m long and fed a 240 m long zone of dispersed flow the change from stable to transitional channel occurred at a major reduction in slope above this the stable channel is 5 2 km long 55 to 105 m wide and bounded by 15 to 25 m high levees and the stable channel is located over a previous channel in a final stage of activity lava ponding at the break in slope that marks the terminus of the stable channel put pressure on the eastern levee causing it to fail liberated lava then fed a final break out to the east similar flow front features occur at other volcanoes indicating that similar processes are characteristic of dispersed flow zones

constraining the age of the last geomagnetic reversal from geochemical and magnetic analyses of atlantic indian and pacific ocean sediments we studied four marine sediment records of the matuyama brunhes geomagnetic reversal from four sites located in the indian atlantic and western pacific oceans the results combine paleomagnetic cosmogenic nuclide beryllium be 10 and oxygen isotope analyses that were performed on the same samples in order to avoid any stratigraphic bias the three records from the equatorial indian ocean and north atlantic ocean did not reveal any offset between the authigenic be 10 be 9 ratio be ratio peak and the interval of low relative paleointensity rpi that characterizes the reversal the lower and upper limits of transitional directions are also concomitant with the increased atmospheric be 10 production that accompanied the geomagnetic dipole intensity decrease in contrast the record from western equatorial pacific ocean sediments was found 18 cm below the be ratio changes as a result of late magnetization acquisition at all four sites maximum be 10 production occurred at the same period soon after the maximum of marine isotope stage 19 mis 19 and therefore indicates its synchronous worldwide character such features are effectively observed from the be ratio signals and their relationship with the magnetic transition interval which further confirms the synchronous character of the transition taking all dating uncertainties 1 6 ka between sites and 5 ka for the age model into consideration our records suggest a mean reversal age of 772 4 6 6 ka the age of the transition in the atlantic ocean record is closer to 774 ka but this difference is within the limit of significance

tertiary extension and tilting in the queen charlotte islands evidence from dyke swarms and their paleomagnetism the queen charlotte islands form the western margin of the tertiary queen charlotte basin which is situated on the western margin of the north american plate they contain seven major dyke swarms of late eocene to miocene age a period when the relative motions of the pacific and the north american plates in this region were dominantly dextral strike slip transform margin with intervals of highly oblique divergence and convergence within each swarm dykes have a systematic trend however trends vary from swarm to swarm indicating that the stress field varied a total of 678 cores 1352 specimens were collected from 129 dykes in six swarms over a distance of about 200 km magnetic stability is variable one hundred and one dykes yielded records of the paleofield data are also reported from an oligocene pluton 5 sites 27 cores 52 specimens and miocene lavas 8 sites 52 cores 101 specimens both normal and reversed magnetizations occur but irrespective of sign the mean directions of remanen

magnetic stratigraphy constraints on the matuyama brunhes boundary recorded in a loess section at the southern margin of chinese loess plateau although the matuyama brunhes boundary mbb in the chinese loess plateau clp is very important in reliably correlating quaternary loess with other sediments in the world particularly with marine and polar ice cores its exact stratigraphic position remains controversial previous investigations usually placed the mbb between paleosol unit s8 and loess unit l8 in various locations to better understand the spatial differences in the mbb position a high resolution paleomagnetic study was conducted in a loess section of the lantian basin at the southern margin of the clp the results show that the mbb is situated in the middle of the relatively weak paleosol unit s7 consistent with a recent report on the mbb based on a be 10 study from the xifeng and luochuan loess sections of the central clp however the regional anomalously low magnetic susceptibility in paleosols s7 and s8 indicates that it is more reliable to determine the paleoclimate boundaries between loess and paleosol horizons of this segment with median grain size then the mbb in the yushan section can be correlated with the bottom of paleosol s7 corresponding to the older part of interglacial marine isotope stage 19 this result temporally reconciles the striking discrepancy of the position of the mbb recorded in between loess and other typical sedimentary sequences and further confirms that the stratigraphic position of the mbb could spatially vary to a certain extent due to regional sedimentary or paleoclimatic conditions in the marginal areas of the clp in the yushan section the high frequency variations of paleomagnetic directions during a long period of similar to 31 ka before the mbb however could not be attributed to a genuine response to the true geomagnetic behaviour moreover the climate offset defined by the magnetic susceptibility and median grain size of the section can be preliminarily attributed to the regional geology and paleoenvironment background a multiproxy based stratigraphic division is considered very necessary when paleomagnetic and climatic boundaries are defined exactly in a specific area of the southern clp

a model for the opening of ephemeral vents in a stationary lava flow abstract a model is proposed for the opening of ephemeral vents in the solid front of a stationary lava flow the front is considered to be a viscoelastic shell which deforms due to the pressure exerted on it by the inner fluid lava the vent opens when the normal stress in the front overcomes the tensile strength of solid lava cooling of the front is considered and the increase in crust thickness as the isothermal surface at the solidus temperature deepens into the lava body is calculated the thermal effect is negligible if the timescale for the opening of ephemeral vents is on the order of days as is often observed considering stationary flow lengths of a few hundred meters this timescale corresponds to an average viscosity of the crust of 10 12 pa s assuming reasonable values of the crust strength and initial thickness observation of ephemeral vents on mount etna is found to be consistent with the model predictions

the july august 2001 eruption of mt etna sicily the july august 2001 eruption of mt etna stimulated widespread public and media interest caused significant damage to tourist facilities and for several days threatened the town of nicolosi on the s flank of the volcano seven eruptive fissures were active five on the s flank between 3 050 and 2 100 m altitude and two on the ne flank between 3 080 and 2 600 m elevation all produced lava flows over various periods during the eruption the most voluminous of which reached a length of 6 9 km mineralogically the 2001 lavas fall into two distinct groups indicating that magma was supplied through two different and largely independent pathways one extending laterally from the central conduit system through radial fissures the other being a vertically ascending eccentric dike furthermore one of the eccentric vents at 2 570 m elevation was the site of vigorous phreatomagmatic activity as the dike cut through a shallow aquifer during both the initial and closing stages of the eruption for 6 days the magma column feeding this vent was more or less effectively sealed from the aquifer permitting powerful explosive and effusive magmatic activity while the eruption was characterized by a highly dynamic evolution complex interactions between some of the eruptive fissures and changing eruptive styles its total volume 25 106 m3 of lava and 5 10 106 m3 of pyroclastics was relatively small in comparison with other recent eruptions of etna effusion rates were calculated on a daily basis and reached peaks of 14 16 m3 s 1 while the average effusion rate at all fissures was about 11 m3 s 1 which is not exceptionally high the eruption showed a number of peculiar features but none of these except the contemporaneous lateral and eccentric activity represented a significant deviation from etna s eruptive behavior in the long term however the 2001 eruption could be but the first in a series of flank eruptions some of which might be more voluminous and hazardous placed in a long term context the eruption confirms a distinct trend initiated during the past 50 years toward higher production rates and more frequent eruptions which might bring etna back to similar levels of activity as during the early to mid seventeenth century

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

simulations of a time varying non dipole field during geomagnetic reversals and excursions abstract following brown et al 2007 reversals and excursions have been simulated by decreasing the axial dipole field to zero while leaving up a time varying non dipolar field similar to the cal7k model korte m constable c g 2005 continuous geomagnetic models for the past 7 millennia ii cals7k geochem geophys geosyst 6 2 doi 10 1029 2004gc000801 we also envisaged the situation in which the total field changes sign we first investigated the relationship between the non axial dipole structure of the historical and present field snapshots and the virtual geomagnetic poles vgps the accumulation of vgps in the vicinity of large flux concentrations in south atlantic and australia is constrained by the equatorial dipole only a few sites directly above the anomalies are associated with vgps in their vicinity therefore it seems delicate to associate recurring clusters of vgps during reversals with magnetic flux concentrations at the cmb the individual vgps resulting from simulations of reversals show all kinds of trajectories some being very simple with longitudinal confinement while others are complex with alternances of slow and fast changes as well as loopings of vgps preference for a specific longitude frequently happens in individual records but this is not systematic a direct consequence is that records of different reversals from a global array of sites are effectively dominated by preferred longitudinal bands thus a time varying non dipolar transitional field can generate anisotropy of the vgp distribution smearing of the signal by sedimentary records considerably amplifies the longitudinal confinement of the vgps overall these simulations account for most characteristics reported in detailed reversal records and thus strongly support the concept of a non dipolar transitional field we also simulated field excursions by assuming either a vanishing dipole followed by its return to initial polarity or by a short episode with the opposite polarity preceding its restoration the model accounts for the different characteristics inherent to the volcanic and sedimentary records of the recent excursions the simple structure of sedimentary vgp paths dominated by a large loop between northern and southern latitudes partly results from smearing of the signal therefore these records do not fully describe the complexity of the excursional field and remain exclusively sensitive to very low degree harmonics most detailed studies of excursions exhibit reversed directions which cannot exist without a very short dipolar episode of opposite polarity

matuyama brunhes reversal and kamikatsura event on maui paleomagnetic directions 40 ar 39 ar ages and implications abstract eighty nine basaltic lava flows from the northwest wall of haleakala caldera preserve a concatenated paleomagnetic record of portions of the matuyama brunhes m b reversal and the preceding kamikatsura event as well as secular variation of the full polarity reversed and normal geomagnetic field they provide the most detailed volcanic record to date of the m b transition the 24 flows in the transition zone show for the first time transitional virtual geomagnetic poles vgps that move from reverse to normal along the americas concluding with an oscillation in the pacific ocean to a cluster of vgps east of new zealand and back finally to stable polarity in the north polar region all but one of the 16 kamikatsura vgps cluster in central south america the full polarity flows with 40ar 39ar ages spanning a total of 680 kyr pass a reversal test and give an average vgp insignificantly different from the rotation axis with standard deviation consistent with that for other 0 5 ma lava flows of similar latitude precise 40ar 39ar dating consisting of 31 incremental heating experiments on 12 transitional flows yields weighted mean ages of 775 6 1 9 and 900 3 4 7 ka for the m b and kamikatsura transitional flows respectively this matuyama brunhes age is 16 kyr younger than ages for m b flows from the canary islands tahiti and chile that were dated using exactly the same techniques and standards suggesting that this polarity transition may have taken considerably longer to complete and been more complex than is generally believed for reversals

evidence from lava flows for complex polarity transitions the new composite steens mountain reversal record summary geomagnetic polarity transitions may be significantly more complex than are currently depicted in many sedimentary and lava flow records by splicing together paleomagnetic results from earlier studies at steens mountain with those from three newly studied sections of oregon plateau flood basalts at catlow peak and poker jim ridge 70 90 km to the southeast and west respectively we provide support for this interpretation with the most detailed account of a magnetic field reversal yet observed in volcanic rocks forty five new distinguishable transitional t directions together with 30 earlier ones reveal a much more complex and detailed record of the 16 7 ma reversed r to normal n polarity transition that marks the end of chron c5cr compared to the earlier r t n t n reversal record the new record can be described as r t n t n t r t n the composite record confirms earlier features adds new west and up directions and an entire large n t r t segment to the path and fills in directions on the path between earlier directional jumps persistent virtual geomagnetic pole vgp clusters and separate vgps have a preference for previously described longitudinal bands from transition study compilations which suggests the presence of features at the core mantle boundary that influence the flow of core fluid and distribution of magnetic flux overall the record is consistent with the generalization that vgp paths vary greatly from reversal to reversal and depend on the location of the observer rates of secular variation confirm that the flows comprising these sections were erupted rapidly with maximum rates estimated to be 85 120 m ka 1 at catlow and 130 195 m ka 1 at poker jim south paleomagnetic poles from other studies are combined with 32 non transitional poles found here to give a clockwise rotation of the oregon plateau of 11 4 5 6 with respect to the younger columbia river basalt group flows to the north and 14 5 4 6 with respect to cratonic north america 95 per cent confidence interval

the matuyama brunhes boundary interval 500 900 ka in north atlantic drift sediments summary ocean drilling program odp sites 984 and 983 located on north atlantic sediment drifts provide high resolution records across the matuyama brunhes boundary at odp site 984 bjorn drift iceland basin the mean sedimentation rate in the 500 900 ka interval is 12 cm kyr 1 based on an age model derived by matching the planktonic and benthic oxygen isotope records to an ice volume model the matuyama brunhes polarity transition at site 984 as defined by virtual geomagnetic polar vgp latitudes 50 has an apparent duration of 7 kyr with a mid point at 773 5 ka compared with 772 5 ka at neighbouring site 983 outside the polarity transition at both sites 984 and 983 excursions in vgp latitudes to values 20 at 540 590 and 670 ka correspond to troughs in the palaeointensity record new u channel palaeomagnetic data across the matuyama brunhes boundary for working and archive halves of core sections from three holes at both sites 984 and 983 augment data published by channell lehman 1997 and are compared with back to back 1 cm 3 discrete samples clusters of vgps in the south atlantic and northeast asia in both u channel and discrete sample records imply that polarity transition fields have characteristics similar to the modern non axial dipole nad field

a combined paleomagnetic dating investigation of the upper jaramillo transition from a volcanic section at tenerife canary islands abstract a coupled paleomagnetic dating investigation has been conducted on a sequence of 25 successive lava flows emplaced during the upper transition of the jaramillo subchron in tenerife canary islands this sequence is located along the western wall of the guimar collapse scar in the south central part of the island nine flows distributed throughout this sequence were dated using unspiked k ar and 40 ar 39 ar methods they bracket the section between 1009 22 ka and 971 21 ka 2 a first group of 8 flows at the bottom of the sequence is characterized by normal polarity with paleointensity values of the order of present day field intensity in the canary islands the virtual geomagnetic poles vgp of these 8 flows describe a short loop at high latitudes seven overlying flows are transitional in directions and dated between 991 14 ka and 1002 11 ka consistently with published ages of the upper jaramillo reversal this second group of flows is characterized by low paleointensity values around 8 12 t that are less than 30 of the present dipole value in tenerife the vgps of the first two transitional flows lie over northeastern pacific whereas the five following transitional flows have all negative inclinations and their vgps lie initially over east antarctica then describe a northward loop almost reaching new zealand the final group of ten flows yield intensities varying between 20 and 35 t and vgps close to the southern pole with two of them describing a small amplitude second loop to southeastern pacific assuming a constant extrusion rate as a very first approximation the distribution of the obtained ages suggests a duration of 7 6 5 6 ka for the transitional interval the obtained transitional positions of vgps are consistent with the path reported for the same reversal from north atlantic sediments but are different from the only other volcanic record from tahiti the intensity low characterizing the transitional interval remains the best tie point centered at 996 7 ka 2 relative to 28 02 ma fc sanidine

a precursor to the matuyama brunhes transition field instability as recorded in pelagic sediments abstract the period some 20 25 kyr just prior to the most recent generally recognized geomagnetic field polarity transition the matuyama to brunhes reversal appears to be marked by significant geomagnetic variability manifested as pronounced oscillations in intensity we compare several previously published paleomagnetic records with new high resolution paleomagnetic data obtained from five pelagic marine sites north atlantic dsdp hole 609b equatorial atlantic odp hole 665a and western equatorial pacific odp holes 803b 804c and 805b using standard rock magnetic normalization for all of the samples as well as a thellier thellier method on the sediments of hole 804c we consistently find a decrease in paleointensity dip of 1 approximately 15 kyr prior to the matuyama to brunhes transition in the five new records as well as in the previously published records despite sedimentation accumulation rates sar that range from 11 cm kyr to 1 cm kyr these sequences yield paleointensity curves that are broadly similar in form even at the lowest sars the intensity of the pre reversal low dip1 appears to be of the same magnitude as that of the transition itself dip2 of 1 in some of the records a directional excursion to nearly full normal polarity accompanies dip1 and remains after alternating field af and or thermal demagnetization whereas in other records the directional changes vanish with demagnetization and appear to be caused by overprinting a viscous remanent magnetization vrm contribution to nrm was identified in two of the records and until removed by af or thermal demagnetization was found to blur the double dip nature of the paleointensity profiles into an apparent single dip and also resulted in an apparent but erroneous sawtooth like post transitional sudden increase in paleointensity after appropriate normalization the magnitude of the post transitional recovery was much reduced the magnetic directions of three of the new records after cleaning and adjusting the stable declinations to either 0 normal or 180 reverse map to vgp positions lying in the pacific the directional variations however are far less consistent than the intensity variations the confirmed global existence of this dip so closely preceding a major reversal invites questions about its relation to the reversal itself the apparent normal character of this interval can also present problems for magnetostratigraphical interpretations based on coarse or incomplete sampling by mimicking the brunhes matuyama reversal some 15 kyr earlier in the section than its true location

detailed paleomagnetic study of two volcanic polarity transitions recorded in eastern iceland abstract two sequences of 45 and 49 individual lava flows respectively have been sampled in eastern iceland the two sections range in age from 12 09 to 10 21 ma as reported by watkins and walker watkins n walker g p l 1977 magnetostratigraphy of eastern iceland am j sci 277 513 584 and are labelled as profiles c and d stepwise alternating field af and thermal demagnetizations accompanied by investigations of rock magnetic properties indicate that the magnetization is primarily carried by titanomagnetite demagnetization experiments have identified eight and six transitional lavas respectively for these profiles the transitional virtual geomagnetic poles vgps of the r n profile c reversal are located mainly between patagonia and antarctica with a tendency of the virtual poles to move towards west coast of south america subsequently traveling to the northern hemisphere through several discrete steps located in the middle part of south america then to west africa and on to central asia before the poles settle into normal polarity the second and younger vgp path corresponding to profile d is a path characterized by a reverse to normal to reverse r n r motion of the virtual poles this path is characterized by poles located in west antarctica patagonia and the western and eastern part of south america the passage from the southern to the northern hemisphere is also through a discrete sets of steps along the southwestern pacific followed by a rapid motion to the northern siberian region continuing to the western equatorial part of south america before moving on to the central region of asia followed by a motion to the western pacific prior to the final move to the eastern part of antarctica the data from eastern iceland based on highly reliable transitional results as indicated by the demagnetizations and rock magnetic experiments derived from relatively spaced sites and different ages seem to indicate that the eastern icelandic high latitude 66 n profiles studied have several persistent transitional paleofield features that are uniquely observed at those sites particularly when compared to other volcanic records located at lower latitudes

a probabilistic assessment of the next geomagnetic reversal author s buffett b davis w abstract 2018 american geophysical union all rights reserved deterministic forecasts for the next geomagnetic reversal are not feasible due to large uncertainties in the present day state of the earth s core a more practical approach relies on probabilistic assessments using paleomagnetic observations to characterize the amplitude of fluctuations in the geomagnetic dipole we use paleomagnetic observations for the past 2 myr to construct a stochastic model for the axial dipole field and apply well established methods to evaluate the probability of the next geomagnetic reversal as a function of time for a present day axial dipole moment of 7 6 1022 a m2 the probability of the dipole entering a reversed state is less than 2 after 20 kyr this probability rises to 11 after 50 kyr an imminent geomagnetic reversal is not supported by paleomagnetic observations the current rate of decline in the dipole moment is unusual but within the natural variability predicted by the stochastic model

complexity in matuyama brunhes polarity transitions from north atlantic iodp odp deep sea sites abstract integrated ocean drilling program iodp expedition 303 to the north atlantic provided 16 records of the matuyama brunhes polarity transition mbt based on u channel and discrete samples from holes drilled at three sites sites u1304 u1305 and u1306 that have mean brunhes sedimentation rates of 16 18 cm kyr the mbt occurs during the transition from marine isotope stage mis 19c to mis 18e with mid point at 773 ka and a transition duration of 8 kyr combining the new mbt records including one new record for the top jaramillo with previously published north atlantic mbt records odp sites 983 984 and 1063 yields a total of more than 20 high sedimentation rate polarity transition records the mbt yields a repetitive pattern of transitional field states as virtual geomagnetic poles vgps move from high southern latitudes to loop over the pacific group in ne asia and transit into the mid latitude south atlantic before reaching high latitudes in the northern hemisphere the vgps for the top jaramillo transition feature a loop over the pacific then a ne asia group before transit over the indian ocean to high southerly latitudes the north atlantic mbt records described here contrast with longitudinally constrained vgp paths for the mbt indicating that relatively low sedimentation rate 4 cm kyr records of the mbt are heavily smoothed by the remanence acquisition process and do not adequately represent the mbt field the vgps at the mbt and top jaramillo as measured in the north atlantic have similarities with excursion iceland basin vgp paths and were apparently guided by maxima in downward vertical flux similar to those seen in the modern non dipole nd field implying longevity in nd features through time

analysis of the 2001 lava flow eruption of mt etna from three dimensional mapping 1 the 2001 etna eruption was characterized by a complex temporal evolution with the opening of seven eruptive fissures each feeding different lava flows this work describes a method adopted to obtain the three dimensional geometry of the whole lava flow field and for the reconstruction based on topographic data of the temporal evolution of the largest lava flow emitted from a vent located at 2100 m a s l preeruption and posteruption digital elevation models dem were extracted from vector contour maps comparison of the two dems and analysis of posteruption orthophotos allowed us to estimate flow area thickness and bulk volume additionally the two dimensional temporal evolution of the 2100 flow was precisely reconstructed by means of maps compiled during the eruption these data together with estimates of flow thickness allowed us to evaluate emitted lava volumes and in turn the average volumetric flow rates the analysis performed in this paper provided a total lava bulk volume of 40 1 106 m3 for the whole lava flow field most of which emitted from the 2100 vent 21 4 106 m3 the derived effusion rate trend shows an initial period of waxing flow followed by a longer period of waning flow this is in agreement not only with the few available effusion rate measurements performed during the eruption but also with the theoretical model of wadge 1981 for the temporal variation in discharge during the tapping of a pressurized source

reconstructing the holocene geomagnetic field abstract knowledge of the holocene evolution of earth s magnetic field is important for understanding geodynamo processes in the core is necessary for studying long term solar terrestrial relationships and can provide useful age constraints for archeologicaland stratigraphic applications continuous time varying global field models based on archeo and paleomagnetic data are useful tools in this regard we use a comprehensive data compilation and recently refined modelling strategies to produce cals10k 1b the first time varying spherical harmonic geomagnetic field model spanning 10 ky the model is an average obtained from bootstrap sampling to take account of uncertainties in magnetic components and ages in the data and hence has version number 1b instead of 1 this model shows less spatial and temporal resolution than earlier versions for 0 3 ka and particularly aims to provide a robust representation of the large scale field at the core mantle boundary cmb we discuss the geomagnetic dipole evolution and changes in holocene magnetic field morphology at the cmb as shown by the new reconstruction the results are compatible with earlier models cals3k 3 and cals3k 4 for 0 3 ka but reveal some clear deficiencies in the 0 7 ka cals7k 2 model prior to 3 ka cals10k 1b is able to resolve mobile and structurally evolving high latitude radial field flux lobes at the cmb in both hemispheres as well as persistent non zonal structure in the 10 ky average contributions to the average field from time varying structures in the equatorial indonesian australian region are particularly striking

morphology of basaltic lava channels during the mt etna september 2004 eruption from airborne laser altimeter data 1 the morphological features of active lava channels acquired over mt etna by means of airborne laser altimeter data are presented the data were acquired on 16th september 2004 using an optech altm 3033 laser altimeter the flight height was about 2000 m above ground during the flight the lava erupted from the eastern part of the mt etna summit area flowed into the valle del bove the measured morphologic features are channel width wc flow width wf levee height hl flow thickness tf and channel depth dc the main lava flow shows a well developed channel with levees the average measured values for the morphologic features of this flow are wc 16 m wf 52 m hl 7 m tf 5 m and dc 2 m the wc parameter shows linear increase from about 7 m near the vent up to about 25 m close to the front 1600 m in length

palaeosecular variation recorded by 9 ka to 2 5 ma old lavas from martinique island new evidence for the la palma aborted reversal 617 ka ago

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

modelling the palaeo evolution of the geodynamo although it is known that the geodynamo has been operating for at least 3 2 ga it remains difficult to infer the intensity dipolarity and stability occurrence of reversals of the precam brian magnetic field of the earth in order to assist the interpretation of palaeomagnetic data we produce models for the long term evolution of the geodynamo by combining core ther modynamics with a systematic scaling analysis of numerical dynamo simulations we update earlier dynamo scaling results by exploring a parameter space which has been extended in order to account for core aspect ratios and buoyancy source distributions relevant to earth in the precambrian our analysis highlights the central role of the convective power which is an output of core thermodynamics and the main input of our updated scalings as the thermal evolution of the earth s core is not well known two end member models of heat flow evolution at the core mantle boundary cmb are used respectively terminating at present heat flows of 11 tw high power scenario and 3 tw low power scenario the resulting models predict that until the appearance of the inner core a thermal dynamo driven only by secular cooling and without any need for radioactive heating can produce a dipole moment of strength comparable to that of the present field thus precluding an interpretation of the oldest palaeomagnetic records as evidence of the inner core presence the observed lack of strong long term trends in palaeointensity data throughout the earth s history can be rationalized by the weakness of palaeointensity variations predicted by our models relatively to the data scatter specifically the most significant internal magnetic field increase which we predict is associated to the sudden power increase resulting from inner core nucleation but the dynamo becomes deeper seated in the core thus largely cancelling the increase at the core and earth surface and diminishing the prospect of observing this event in palaeointensity data our models additionally suggest that the geodynamo has lied close to the transition to polarity reversals throughout its history in the precambrian we predict a dynamo with similar dipolarity and less frequent reversals than at present times due to conditions of generally lower convective forcing quantifying the typical cmb heat flow variation needed for the geodynamo to cross the transition from a reversing to a non reversing state we find that it is unlikely that such a variation may have caused superchrons in the last 0 5 ga without shutting down dynamo action altogether

correlation of paleointensity variation records in the brunhes matuyama polarity transition interval abstract paleomagnetic records of the 0 78 ma brunhes matuyama b m polarity reversal interval described from high resolution 8 11 cm kyr deep sea sediments from western equatorial pacific odp sites 767b and 769a show two successive and marked decreases in paleointensity of about 15 kyr wavelength each and a shift from full reversed to fuli normal polarity directions in only 2 kyr associated with the uppermost paleointensity decrease in contrast transition records from deep sea sediments with lower sedimentation rates tend to show simpler paleointensity variation with more complex patterns of directional change for example the b m transition record described from deep sea sediment core v16 58 from the southern indian ocean we suggest that the twin 10 be peaks reported in v16 58 correspond to increased production of this cosmogenic isotope associated with the double paleointensity dips recorded in odp 767b 769a the low resolution v16 58 paleomagnetic record can be accounted for by modeling of the geomagnetic field history in odp 767b at a nominal recording rate of 1 cm kyr and using an exponential lock in process filter with a half fixing depth of 16 cm the double paleointensity dips become a single broad magnetization low and the change in paleomagnetic directions is offset by about 16 cm below the actual level of the polarity transition similar to what we infer for v16 58 the low resultant magnetizations will be especially prone to overprinting which if in the present field will tend to give an americas vgp path the main features of the 10 be profile will be unaffected by the remanence lock in process and consequently retain their usefulness for correlation as a proxy of geomagnetic dipole intensity variation

the changing morphology of an open lava channel on mt etna an open channel lava flow on mt etna sicily was observed during may 30 31 2001 data collected using a forward looking infrared flir thermal camera and a minolta land cyclops 300 thermal infrared thermometer showed that the bulk volume flux of lava flowing in the channel varied greatly over time cyclic changes in the channel s volumetric flow rate occurred over several hours with cycle durations of 113 190 min and discharges peaking at 0 7 m3 s 1 and waning to 0 1 m3 s 1 each cycle was characterized by a relatively short high volume flux phase during which a pulse of lava with a well defined flow front would propagate down channel followed by a period of waning flow during which volume flux lowered pulses involved lava moving at relatively high velocities up to 0 29 m s 1 and were related to some change in the flow conditions occurring up channel possibly at the vent they implied either a change in the dense rock effusion rate at the source vent and or cyclic variation in the vesicle content of the lava changing its bulk volume flux pulses would generally overspill the channel to emplace p hoehoe overflows during periods of waning flow velocities fell to 0 05 m s 1 blockages forming during such phases caused lava to back up occasionally backup resulted in overflows of slow moving a that would advance a few tens of meters down the levee flank compound levees were thus a symptom of unsteady flow where overflow levees were emplaced as relatively fast moving p hoehoe sheets during pulses and as slow moving a units during backup small localized fluctuations in channel volume flux also occurred on timescales of minutes volumes of lava backed up behind blockages that formed at constrictions in the channel blockage collapse and or enhanced flow under around the blockage would then feed short lived wave like down channel surges real fluctuations in channel volume flux due to pulses and surges can lead to significant errors in effusion rate calculations

when and why sediments fail to record the geomagnetic field during polarity reversals we present four new records of the matuyama brunhes m b reversal from sediments of the equatorial indian ocean west equatorial pacific and north atlantic oceans with deposition rates between 2 cm kyr and 4 5 cm kyr the magnetic measurements were performed using 8 cc cubic samples and provided well defined reverse and normal polarity directions prior and after the last reversal in three records stepwise demagnetization of the transitional samples revealed a succession of scattered directions instead of a well defined characteristic component of magnetization there is no relationship with changes in magnetic mineralogy magnetic concentration and magnetic grain sizes this behavior could be caused by weakly magnetized sediment however the transitional samples of two cores have almost three orders of magnitude stronger magnetizations than the non transitional samples that yielded unambiguous primary directions in the other two cores moreover a similar proportion of magnetic grains was aligned in all records therefore the large amount of magnetic grains oriented by the weak transitional field did not contribute to improve the definition of the characteristic component we infer that the weakness of the field might not be only responsible assuming that the transitional period is dominated by a multipolar field it is likely that the rapidly moving non dipole components generated different directions that were recorded over the 2 cm stratigraphic thickness of each sample these components are carried by grains with similar magnetic properties yielding scattered directions during demagnetization in contrast the strongly magnetized sediments of the fourth core from the west equatorial pacific ocean were exempt of problems during demagnetization the declinations rotate smoothly between the two polarities while the inclinations remain close to zero this scenario results from post depositional realignment that integrated various amounts of pre and post transitional magnetic directions within each sample the present results point out the impossibility of extracting reliable information on geomagnetic reversals from low medium deposition rate sediments with the current sampling techniques among other features they put in question the relationship between reversal duration and site latitude

directional change during a miocene r n geomagnetic polarity reversal recorded by mafic lava flows sheep creek range north central nevada usa recurring transitional field directions during three miocene geomagnetic reversals provide evidence that lateral inhomogeneity of the lower mantle affects flow in the outer core we compare new paleomagnetic results from a composite sequence of 15 2 ma lava flows in north central nevada sheep creek range 40 7n 243 2e erupted during a polarity reversal to published data from steens mountain 250 km to the northwest in oregon and the newberry mountains 650 km to the south in california that document reversals occurring millions of years and many polarity switches earlier alternating field demagnetization followed by thermal demagnetization in half the samples clearly isolated the primary thermoremanent magnetization of sheep creek range flows we correlated results from our three sampled sections to produce a composite record that begins with a single virtual geomagnetic pole vgp at low latitude in the atlantic followed by two vgps situated near latitude 30n in ne africa after jumping to 83n one vgp the pole moves to equatorial south america one vgp back to ne africa three vgps to high southern latitudes two vgps back to equatorial south america three vgps and finally to high northern latitudes nine vgps the repeated visits of the transitional vgp to positions in south america and near ne africa as well as the similar behavior recorded at steens mountain and the newberry mountains suggest that lower mantle or core mantle boundary features localize core flow structures thereby imparting a discernible regional structure on the transitional geomagnetic field that persists for millions of years

nonrandom geomagnetic reversal times and geodynamo evolution abstract sherman s test applied to the geomagnetic polarity time scale gpts reveals that geomagnetic reversals in the phanerozoic deviate substantially from random times for 954 phanerozoic reversals exceeds the value expected for uniformly distributed random times by many standard deviations due to three constant polarity superchrons and clustering of reversals in the cenozoic c sequence reversals are nearly periodic in several portions of the mesozoic m sequence and during these times falls below random by several standard deviations according to some chronologies polarity reversals in a convection driven numerical dynamo with fixed control parameters have an overall value that is slightly lower than uniformly random due to weak periodicity whereas in a numerical dynamo with time variable control parameters the combination of superchrons and reversal clusters dominates yielding a large value that is comparable to the gpts sherman s test applied to shorter phanerozoic reversal sequences reveals two geodynamo time scales hundreds of millions of years represented by superchrons and reversal clusters that we attribute to time dependent core mantle thermal interaction plus unexplained variations lasting tens of millions of years characterized by alternation between random and nearly periodic reversals

the van zijl jurassic geomagnetic reversal revisited we have collected two new detailed records of what could be the second oldest well documented reversal the van zijl jurassic 180 ma reversal recorded in the thick basalt sequences of the karoo large igneous province in lesotho and south africa sections yielded 10 and 8 independent transitional paleomagnetic directions respectively over two continuous 130 m and 160 m sequences the corresponding vgp reversing paths share a number of consistent features and are integrated with a third record at bushmen s pass from prevot et al 2003 to yield a single reversal path with 1 a large directional jump with no transitional poles between 45 s and 45 n in the first main phase of the reversal 2 a large elongated loop multiple hairpin to the se with poles going down to 20 latitudes in the second phase of the reversal and 3 a rebound before finally settling in normal polarity we have measured relative paleo intensities on a set of carefully selected samples intensity was low prior to the directional changes and recovery to the following normal polarity was progressive transitional intensity is 80 to 90 lower than full polarity values there is no significant increase in intensity at the time of transitional directional clusters our results are consistent with a weak non dipolar reversing field with faster and larger directional secular variation as the equator is crossed by vgps they tend to vindicate the notion of a short duration of the directional reversal though one cannot draw global conclusions from a record limited to essentially a single location

high resolution 10be and paleomagnetic recording of the last polarity reversal in the chiba composite section age and dynamics of the matuyama brunhes transition we present new magnetic direction and relative paleointensity and beryllium isotope 10 be 9 be and 10 be 9 be ratio results covering the last geomagnetic reversal i e the matuyama brunhes transition mbt from the chiba composite section cbcs east central japan the very high sedimentation rates 90 cm ka of the studied site a candidate site for the global boundary stratotype section and point gssp of the lower middle pleistocene boundary allow the acquisition of a dataset of exceptional resolution coupled cosmogenic 10 be and magnetic results measured on the same samples demonstrate that the magnetization acquisition conserved the timing of rapid geomagnetic features allowing accurate paleomagnetic interpretations a polarity switch ps capturing most of the angular deviation occurring between 771 9 and 773 9 ka was followed by a directional instability rebound ic1 until 768 5 ka this sequence with 5 4 ka duration was characterized by a weak dipole field as low as 2 3 0 3 10 22 am 2 four rapid episodes of directional instabilities preceding and following the ps ic1 phase completed the reversal sequence the asymmetry observed between the long term dipole decay and sharp recovery and rapid oscillations 1 ka demonstrate the complex transitional field behavior during dipole moment low before reestablishment of the full polarity state our observations reinforce the fact that most reversal records do not integrate the full field behavior associated with the geodynamo action although this poses problems for understanding the underlying physical processes that produce reversals it does not hamper stratigraphic correlations among most geologic records

viscous newtonian laminar flow in a rectangular channel application to etna lava flows we introduce a 3d model for near vent channelized lava flows we assume the lava to be an isothermal newtonian liquid flowing in a rectangular channel down a constant slope the flow velocity is calculated with an analytical steady state solution of the navier stokes equation the surface velocity and the flow rate are calculated as functions of the flow thickness for different flow widths and the results are compared with those of a 2d model for typical etna lava flow parameters the influence of levees on the flow dynamics is significant when the flow width is less than 25 m the model predicts the volume flow rate corresponding to the surface velocity taking into account that both depend on flow thickness the effusion rate is a critical parameter to evaluate lava flow hazard we propose a model to calculate the effusion rate given the lava flow width the topograhic slope the lava density the surface flow velocity and either the lava viscosity or the flow thickness

authigenic 10be 9be ratio signature of the matuyama brunhes boundary in the montalbano jonico marine succession geomagnetic dipole moment gdm lows associated with polarity reversals or geomagnetic excursions induce significant modulation of the cosmogenic nuclide beryllium 10 10be production hence the reconstruction of atmospheric 10be production rates from natural archives such as marine sedimentary sequences or ice cores constitutes a complementary approach independent from paleomagnetic measurements to decipher past gdm fluctuations this is particularly important in the montalbano jonico succession south italy since it is candidate to host the global stratotype section and point of the middle pleistocene stage but where the magnetostratigraphic positioning of the matuyama brunhes boundary mbb has not been available up to now this study presents 1 original authigenic 10be cosmogenic nuclide and 9be stable isotope results and 2 new high resolution benthic oxygen isotope record covering termination ix and marine isotope stage mis 19 a robust chronological framework is established on the basis of i our oxygen isotope stratigraphy using the strong analogies between mis 1 and mis 19c in terms of orbital forcing and co2level and ii one precise 40ar 39ar date obtained in the tephra layer v4 the authigenic 10be 9be ratio record marks the atmospheric 10be overproduction linked to the dipole low accompanying the mbb transition with a characteristic twofold increase of the 10be production at the end of mis 19c and early mis 19b this signature is similar to those described in both marine and ice core records the detailed chronostratigraphy constrained by a radiometrically dated tephra layer 773 9 1 3ka within the mbb interval makesit possible to discuss the structure and to assess the timing of the 10be production changes and thus the mbb geomagnetic variations with an unprecedented accuracy for a marine archive sedimentation rates 80cm ka these new cosmogenic nuclide production signatures provide the only missing constraint required for retaining the montalbano jonico succession as a global scale correlation reference section for the early middle pleistocene boundary

multidecadally resolved polarity oscillations during a geomagnetic excursion polarity reversals of the geomagnetic field have occurred through billions of years of earth history and were first revealed in the early 20th century almost a century later details of transitional field behavior during geomagnetic reversals and excursions remain poorly known here we present a multidecadally resolved geomagnetic excursion record from a radioisotopically dated chinese stalagmite at 107 91 thousand years before present with age precision of several decades the duration of geomagnetic directional oscillations ranged from several centuries at 106 103 thousand years before present to millennia at 98 92 thousand years before present with one abrupt reversal transition occurring in one to two centuries when the field was weakest these features indicate prolonged geodynamo instability repeated asymmetrical interhemispheric polarity drifts associated with weak dipole fields likely originated in earth s deep interior if such rapid polarity changes occurred in future they could severely affect satellites and human society

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

petrologic evidence of a complex plumbing system feeding the july august 2001 eruption of mt etna sicily italy after the major 1991 1993 eruption mt etna resumed flank activity in july 2001 through a complex system of eruptive fissures cutting the ne and the s flanks of the volcano and feeding effusive activity fire fountains strombolian and minor phreatomagmatic explosions throughout the eruption magmas with different petrography and composition were erupted the vents higher than 2 600 m a s l hereafter upper vents uv erupted porphyritic plagioclase rich trachybasalt typical of present day summit and flank activity differently the vents located at 2 550 and 2 100 m a s l hereafter lower vents lv produced slightly more primitive trachybasalt dominated by large clinopyroxene olivine and uncommon minerals for etna such as amphibole apatite and orthopyroxene and containing siliceous and cognate xenoliths petrologic investigations carried out on samples collected throughout the eruption provided insights into one of the most intriguing aspects of the 2001 activity namely the coeval occurrence of distinct magmas we interpret this evidence as the result of a complex plumbing system it consists in two separate magma storage systems a shallow one feeding the activity of the uv and a deeper and more complex storage related to the activity of lv in this deep storage zone which is thermally and compositionally zoned the favourable conditions allow the crystallization of amphibole and the occurrence of cognate xenoliths representing wall cumulates throughout 2001 eruption uv and lv magmas remain clearly distinct and ascended following different paths ruling out the occurrence of mixing processes between them furthermore integrating the 2001 eruption in the framework of summit activity occurring since 1995 we propose that the 2001 magma feeding the vents lower than 2 600 m a s l is a precursor of a refilling event which reached its peak during the 2002 2003 etna flank eruption

increased production of cosmogenic 10 be recorded in oceanic sediment sequences information on the age duration and amplitude of the geomagnetic dipole moment minimum over the matuyama brunhes transition new high resolution authigenic be 10 be 9 ratio be ratio records covering the last geomagnetic reversal i e the matuyama brushes transition mbt have been obtained and set on a time scale using benthic delta o 18 cibicides wuellerstorfi records the geographic distribution of the four studied sites allows global comparison between the north atlantic indian and pacific oceans all be ratio records contain a twofold increase triggered by the geomagnetic dipole moment gdm collapse associated with the mbt the stratigraphic position of the be ratio spike relative to marine isotope stages allows establishment of a robust astrochronological framework for the mbt anchoring its age between 778 and 766 ka average mid peaks at 772 ka which is consistent with all other available be 10 proxy records from marine ice and loess archives the global be 10 atmospheric production doubling represents an increase of more than 300 atoms m 2 s 1 that is compatible with the increased magnitude of atmospheric be 10 production obtained by simulations between the present gdm and a null gdm the minimum be 10 derived gdm average computed for the 776 771 ka interval is 1 7 0 4 x 10 22 am 2 in agreement with model simulations and absolute paleointensities of transitional lava flows

a model for the shape of lava flow fronts 1 the shape of the front of a lava flow is controlled by the forces acting on the lava and in turn controls the flow dynamics we present an analysis of the shape of the front of an advancing lava flow considering both a newtonian fluid and a bingham fluid which is the most commonly used non newtonian rheological model for lava we assume than the flow front is moving at constant velocity on a sloping plane the flow is considered as having an infinite extent perpendicularly to the flow direction so that the problem is two dimensional the forces producing the flow advance are the gravity force and the pressure gradient due to the curvature of the flow surface the differential equation for the shape of the front is solved analytically both in the case of a newtonian fluid and in the case of the bingham fluid since the fluid is considered as homogeneous and isothermal the calculated shape applies to the fluid core of the flow front the presence of a solid crust at the top of the flow producing variable amounts of solid debris at the flow snout alters the observable shape of the front and is taken into account in order to compare the model front shapes to those observed in the field

millennial scale instability in the geomagnetic field prior to the matuyama brunhes reversal changes in the earth s magnetic field have global significance that reach from the outer core extending out to the uppermost atmosphere paleomagnetic records derived from sedimentary and volcanic sequences provide important insights into the geodynamo processes that govern the largest geomagnetic changes polarity reversals but dating uncertainties have hindered progress in this understanding here we report a paleomagnetic record from multiple lava flows on tahiti that bracket the matuyama brunhes m b polarity reversal 771 thousand years ago our high precision 40 ar 39 ar ages constrain several rapid and short lived changes in field orientation up to 33 000 years prior to the m b reversal these changes are similar to ones identified in other less well dated lava flows in maui chile and la palma that occurred during an extended period of reduced field strength recorded in sediments we use a simple stochastic model to show that these rapid polarity changes are highly attenuated in sediment records with low sedimentation rates this prolonged 33 000 year period of reduced field strength and increased geomagnetic instability supports models that show frequent centennial to millennial scale polarity changes in the presence of a strongly weakened dipole field

deciphering records of geomagnetic reversals polarity reversals of the geomagnetic field are a major feature of the earth s dynamo questions remain regarding the dynamical processes that give rise to reversals and the properties of the geomagnetic field during a polarity transition a large number of paleomagnetic reversal records have been acquired during the past 50 years in order to better constrain the structure and geometry of the transitional field in addition over the past two decades numerical dynamo simulations have also provided insights into the reversal mechanism yet despite the large paleomagnetic database controversial interpretations of records of the transitional field persist they result from two characteristics inherent to all reversals both of which are detrimental to an ambiguous analysis on the one hand the reversal process is rapid and requires adequate temporal resolution on the other hand weak field intensities during a reversal can affect the fidelity of magnetic recording in sedimentary records this paper is aimed at reviewing critically the main reversal features derived from paleomagnetic records and at analyzing some of these features in light of numerical simulations we discuss in detail the fidelity of the signal extracted from paleomagnetic records and pay special attention to their resolution with respect to the timing and mechanisms involved in the magnetization process records from marine sediments dominate the database they give rise to transitional field models that often lead to overinterpret the data consequently we attempt to separate robust results and their subsequent interpretations from those that do not stand on a strong observational footing finally we discuss new avenues that should favor progress to better characterize and understand transitional field behavior

exploring the influence of the non dipole field on magnetic records for field reversals and excursions summary we have used the model cals7k 2 to explore the possible influence of the time varying non dipole components of the geomagnetic field during field reversals and excursions our findings suggest that non dipole components could add significant structure to the field during the reversal and excursion processes globally the main polarity reversal is variable in duration and rapid reversals on subdecadal timescales are seen for a small number of locations the model generates variable reversal paths however there is a longitudinal preference both spatially and more weakly temporally directional reversal features are not globally synchronous some polarity changes finish before they start elsewhere global intensity variations however appear more coherent we also find support for the idea that field intensity changes occur some time before and after the major directional changes of the reversal large excursions appear naturally when the axial dipole has been reduced to 20 per cent for the whole time period however they are not globally synchronous or uniform

a self replication model for long channelized lava flows on the mars plains 1 a model is presented for channelized lava flows emplaced by a self replicating levee building process over long distances on the plains of mars such flows may exhibit morphologic evidence of stagnation overspills and upstream breakouts however these processes do not inhibit the formation and persistence of a prominent central channel that can often be traced for more than 100 km the two central assumptions of the self replication model are 1 the flow advances at the average upstream velocity of the molten core and 2 the fraction of the lava that travels faster than the average upstream velocity forms stationary margins in the advancing distal zone to preserve the self replication process for an exemplary 300 km long flow north of pavonis mons the model indicates that 8 m of crust must have formed during emplacement as determined from the channel and levee dimensions when combined with independent thermal dynamic estimates for the crustal growth rate relatively narrow constraints are obtained for the flow rate 2250 m3 s 1 emplacement duration 600 d and the lava viscosity of the molten interior 106 pa s minor transient overspills and breakouts increase the emplacement time by only a factor of 2 the primary difference between the prodigious channelized martian flows and their smaller terrestrial counterparts is that high volumetric flow rates must have persisted for many hundreds of days on mars in contrast to a few hours or days on earth

the c27n c26r geomagnetic polarity reversal recorded in the west greenland flood basalt province how complex is the transitional field 1 we present a paleomagnetic record of the c27n c26r 60 9 ma geomagnetic polarity reversal gpr sampled in two separate lava sequences in the vaigat formation west greenland flood basalt province the two profiles distanced 16 km apart consist of 45 and 35 sampled lava flows of which 31 and 23 respectively record transitional field directions a prominent feature of each profile is the presence of successive flows with similar paleomagnetic directions which based on geological geochemical and paleomagnetic evidences are interpreted to represent flow fields i e lavas erupted in a short period of time rather than periods when the geomagnetic field stood still multimodel photogrammetry shows that the typical flow length of vaigat formation picritic lavas is 2 km which explains why flow to flow correlation of paleomagnetic directions between the two profiles 16 km apart is not possible the two profiles therefore record different and independent time series of the same gpr allowing us to cast light on some of the questions that have arisen from single records particularly noteworthy is the fact that the two high resolution paleomagnetic records share no general trends clearly indicating that the c27n c26r transitional field was more complex and faster changing than can be resolved by either record in addition we obtained stable polarity paleomagnetic data from both pretransitional c27n and posttransitional c26r yielding a paleomagnetic pole latitude 64 8 n longitude 141 5 e n 14 k 19 8 a95 9 2 that passes reversal test

the duration of a yellowstone super eruption cycle and implications for the age of the olduvai subchron abstract small volume rhyolitic eruptions preceding and following a caldera forming eruption can provide insights into the tempo of eruption cycles and timing of magmatic recharge in this contribution high precision 40 ar 39 ar eruption ages were obtained on the three effusive eruptions bracketing the huckleberry ridge tuff which comprise yellowstone s first volcanic cycle these dates are supplemented with detailed paleomagnetic and rock magnetic analyses to resolve discrepancies with previous reported stratigraphy the huckleberry ridge tuff 2 08 ma was preceded by an eruption at 2 14 ma and followed by eruptions at 1 98 and 1 95 ma all of which occurred during four distinct periods of geomagnetic instability within the matuyama chron the first volcanic cycle of yellowstone has now been constrained to within a 200 kyr timespan or half of the previously proposed duration and similar to the duration of volcanic activity for caldera forming systems in the jemez volcanic field the maximum duration for magmatic recharge for the first yellowstone volcanic cycle is no greater than 100 kyr and likely closer to 40 kyr furthermore the combined 40 ar 39 ar eruption ages and paleomagnetic results provide polarity anchors for the pre olduvai excursion and olduvai subchron which are often used as tie points in studies of early pleistocene hominin evolution

time dependent mapping of the magnetic field at the core mantle boundary we consider the problem of constructing a time dependent map of the magnetic field at the core mantle boundary we use almost all the available data from the last 300 years to produce two maps one for the period 1690 1840 and the other for 1840 1990 we represent the spatial dependency of the field using spherical harmonics the time dependency using a cubic b spline basis and seek the smoothest solutions compatible with the observations particular attention must be paid to the effects of the crustal field in the data we argue that for observations from permanent magnetic observatories the most efficient strategy is to use first differences of annual means for satellite data the most efficient strategy is simply to limit the number of data used so as to minimize any tendency to map the crustal field into the core field the resulting model fits the observatory data better than any previous model yet has less power in the secular variation than previous models demonstrating that very simple models fit the data the resulting time dependent field map exhibits much of the same structure in the field and its secular variation identified in earlier studies

a high resolution record of the matuyama brunhes transition from the mediterranean region the valle di manche section calabria southern italy abstract high resolution palaeomagnetic and rock magnetic investigations on the valle di manche section crotone basin calabria southern italy provide a detailed record of the matuyama brunhes m b reversal that to our best knowledge is the only available record of the last geomagnetic reversal for the mediterranean on land marine stratigraphy the m b transition can be pinpointed precisely as it develops within a 3 cm thick interval located just above a prominent tephra layer the pitagora ash where the sedimentation rates are about 27 cm kyr demagnetization analyses indicate a stable palaeomagnetic behaviour throughout the section for both normal and reversed polarity directions with demagnetization vectors aligned toward the origin of zijderveld diagrams after the removal of a small viscous low coercivity remanence component in the lower part of the studied interval some samples acquired a spurious gyromagnetic remanent magnetization grm during af demagnetization in high fields rock magnetic analyses confirm that magnetite is the main magnetic carrier for all measured specimens which also have an abundant paramagnetic fraction only the lower part of the record well below the m b boundary is characterized by a downward increasing presence of iron sulphides greigite according to our chronology which is based on a robust cross validated age model the final reverse to normal directional change of the m b transition occurred at ca 786 9 5 ka error includes uncertainty in orbital tuning and was very rapid of the order of 100 years or less

paleomagnetism of the jurassic transantarctic mountains revisited evidence for large dispersion of apparent polar wander within less than 3myr the transantarctic mountains predominantly consist of jurassic continental flood basalts kirkpatrick and sills ferrar emplaced during the earliest phase of the break up of pangea published ages based on a variety of geochronological methods all agree rather well and cluster around 180 ma suggesting emplacement during a rather short time interval of not more than 3 myr paleomagnetic studies mostly carried out between the 60s and 90s of the last century however yield two well defined but significantly different groups of paleomagnetic pole positions plotting either in intermediate latitudes a or at latitudes exceeding 60 degrees s b pole positions belonging to group a are generally interpreted to be of primary origin whereas the significance of group b poles remains unclear here we report new data from the kirkpatrick basalts of northern victoria land antarctica where 157 oriented paleomagnetic samples were taken covering about 800 m of stratigraphy and 23 volcanic flows in gair mesa 73 4666 degrees s 162 8666 degrees e after removal of a steep magnetic overprint with rather low coercivities straight linear segments of exclusively normal polarity trending toward the origin of projection are identified in 151 samples from 22 sites maximum unblocking temperatures do not exceed 580 degrees c and maximum coercivities not 60 mt the resulting mean virtual geomagnetic pole vgp for the gair mesa plots at 66 4 degrees s 227 7 degrees e alpha 95 of 6 2 degrees k 25 7 comparable to group b poles reflecting light microscopy reveals that the magnetic inventory of all samples analyzed is dominated by magnetite showing shrinking cracks and broad ilmenite lamellae the latter being diagnostic for high temperature oxidation analysis of the distribution of the 22 pole positions obtained here shows that secular variation has successfully been averaged s b 15 9 degrees suggesting that the rocks studied here have recorded the time averaged geomagnetic field during early mid jurassic times based on these new results we postulate that both clusters a and b of pole positions reflect primary magnetizations and that the kirkpatrick basalts of gair mesa have been emplaced in a rather short normal polarity time interval of 3 myr if our reasoning is correct the apparent polar wander path for the latest early jurassic might be more complex than previously thought whether this complexity is evidence for massif true polar wander remains speculative c 2015 international association for gondwana research published by elsevier b v all rights reserved

dynamical similarity of geomagnetic field reversals volcanic records of the reversals of the geomagnetic field can be well matched under the assumption of a common reversal duration and imply that the reversal process comprises three phases a precursor a fast polarity switch and a rebound the properties of which have remained unchanged for about 180 million years the geomagnetic variations that prevail during magnetic field reversals convey important information on the nature of the geodynamo that drives earth s magnetic field in this paper jean pierre valet and co authors show that the ten most detailed volcanic records of geomagnetic reversals have a common duration as well as common dynamical characteristics they propose that the reversing process has remained the same for at least the past 180 million years characterized by three phases a precursor event a 180o polarity switch and then a rebound no consensus has been reached so far on the properties of the geomagnetic field during reversals or on the main features that might reveal its dynamics a main characteristic of the reversing field is a large decrease in the axial dipole and the dominant role of non dipole components1 2 3 other features strongly depend on whether they are derived from sedimentary or volcanic records only thermal remanent magnetization of lava flows can capture faithful records of a rapidly varying non dipole field but because of episodic volcanic activity sequences of overlying flows yield incomplete records here we show that the ten most detailed volcanic records of reversals can be matched in a very satisfactory way under the assumption of a common duration revealing common dynamical characteristics we infer that the reversal process has remained unchanged with the same time constants and durations at least since 180 million years ago we propose that the reversing field is characterized by three successive phases a precursory event a 180 polarity switch and a rebound the first and third phases reflect the emergence of the non dipole field with large amplitude secular variation they are rarely both recorded at the same site owing to the rapidly changing field geometry and last for less than 2 500 years the actual transit between the two polarities does not last longer than 1 000 years and might therefore result from mechanisms other than those governing normal secular variation such changes are too brief to be accurately recorded by most sediments

longitudinal confinement of geomagnetic reversal paths as a possible sedimentary artefact the positions of the virtual geomagnetic pole vgp during a large number of reversals of the earth s magnetic field seem to show a remarkable confinement to longitudes over the americas or to antipodal longitudes1 2 although it has been argued that this confinement is statistically insufficiently constrained3 it has also been pointed out4 that the same bands of longitude appear in other geophysical observations such as the pattern of fluid motion in the outer core and regions of higher seismic velocities in the lower mantle suggesting a causal relationship here we show that longitudinal confinement of vgps can arise from the smoothing of non antipodal stable directions just before and after a geomagnetic reversal because of the filtering effect of the remanence acquisition process in sediments the origin of this non antipodality is still uncertain and must remain speculative until more reversal records become available that include sufficiently long pre and post transitional intervals