further rock magnetic and chronostratigraphic results on reversal excursions during the last 50 ka as derived from northern high latitudes and discrepancies in precise ams 14c dating summary a total of five sediment cores from three sites the arctic ocean the fram strait and the greenland sea yielded evidence for geomagnetic reversal excursions and associated strong lows in relative palaeointensity during oxygen isotope stages 2 and 3 a general similarity of the obtained relative palaeointensity curves to reference data can be observed however in the very detail results from this high resolution study differ from published records in a way that the prominent laschamp excursion is clearly characterized by a significant field recovery when reaching the steepest negative inclinations whereas only the n r and r n transitions are associated with the lowest values two subsequent excursions also reach nearly reversed inclinations but without any field recovery at that state a total of 41 accelerator mass spectrometry ams 14 c ages appeared to allow a better age determination of these three directional excursions and related relative palaeointensity variations however although the three sites yielded more or less consistent chronological as well as palaeomagnetic results a comparison to another site ps2644 in the iceland sea revealed significant divergences in the ages of the geomagnetic field excursions of up to 4 ka even on basis of uncalibrated ams 14 c ages this shift to older 14 c ages cannot be explained by a time transgressive character of the excursions because the distance between the sites is small when compared with the size of and the distance to the geodynamo in the earth s outer core the most likely explanation is a difference of reservoir ages and or mixing with old 14 c depleted co2 from glacier ice expelled

why are geomagnetic excursions not always recorded in sediments constraints from post depositional remanent magnetization lock in modelling abstract many geomagnetic excursions have been documented during the brunhes chron however high resolution paleomagnetic studies of sediments often provide evidence of few if any excursions we have investigated the reasons for this observation by modelling the post depositional remanent magnetization pdrm lock in process a cubic lock in function produces more rapid lock in compared to linear and exponential functions proposed in the literature we therefore used a cubic function to model best case scenarios for the quality of the paleomagnetic record when a high frequency geomagnetic input signal is convolved with the sediment lock in function for a wide range of sedimentation rates even for a lock in depth of 10 cm where 95 of the pdrm is locked in 5 cm below the surface mixed layer an input signal containing abundant excursions with 1 kyr duration is smoothed so drastically at low sedimentation rates 1 3 cm kyr that no excursions are recorded excursions are further attenuated by increasing the lock in depth as well as by sampling at discrete stratigraphic intervals the pdrm process acts as a low pass filter for high frequency secular variation signals even at moderate sedimentation rates 10 cm kyr we have also modelled the effects of modulation of sedimentation rate by a climatic forcing function e g insolation even though the average sedimentation rate might be substantial pdrm recording becomes much less reliable in intervals where sedimentation rates are lowest in these cases some excursions are reliably recorded in detail while others may not be recorded at all our modelling suggests that in order to consistently detect the presence of geomagnetic excursions it is ideal to work with sediments that maintain minimum sedimentation rates above 10 cm kyr if the normal spectrum of geomagnetic field behaviour contains abundant excursions failure to document excursions in many apparently high resolution analyses probably results from pdrm lock in within relatively slowly deposited sediments or to unrecognized intervals of slow sedimentation in environments with higher average sedimentation rates

the co evolution of black sea level and composition through the last deglaciation and its paleoclimatic significance abstract the black sea was an inland lake during the last ice age and its sediments are an excellent potential source of information on eurasian climate change showing linkages between regionally and globally recognized millennial scale climate events of the last deglaciation here we detail changes from the last glacial maximum lgm through the transition to an anoxic marginal sea using isotopic strontium and oxygen and trace element sr ca ratios in carbonate shells which record changing input sources and hydrologic conditions in the basin and surrounding region sr isotope records show two prominent peaks between 18 and 16 ka bp cal reflecting anomalous sedimentation associated with meltwater from disintegrating eurasian ice sheets that brought black sea level to its spill point following a sharp drop in sr isotope ratios back toward glacial values two stages of inorganic calcite precipitation accompanied increasing oxygen isotope ratios and steady sr isotope ratios these calcite peaks are separated by an interval in which the geochemical proxies trend back toward glacial values the observed changes reflect negative water balance and lake level decline during relatively warm periods bolling allerod and preboreal and increasing river input less evaporation resulting in higher lake levels during the intervening cold period the younger dryas a final shift to marine values in sr and oxygen isotope ratios at 9 4 ka bp cal corresponds to connection with the global ocean and marks the onset of sedimentation on the black sea continental shelf this date for the marine incursion is earlier than previously suggested based on the appearance of euryhaline fauna and the onset of sapropel formation in the deep basin

paleomagnetic evidence for long term 104 year memory and periodic behavior in the earth s core dynamo process a new paleomagnetic secular variation psv record from the late quaternary wilson creek beds of mono lake california contains a distinctive periodic vector waveform that follows and is almost certainly related to the mono lake excursion a comparison of all published psv results from the wilson creek beds suggests that the magnetic field at mono lake went through an interval 36 000 28 000 ybp of very low amplitude psv followed by the mono lake excursion 28 000 27 000 ybp and four subsequent recurrences 27 000 12 500 ybp of the excursion waveform with relatively diminished amplitudes lower than the excursion amplitude but higher than psv amplitudes at mono lake since 12 500 ybp this pattern clearly suggests that the mono lake excursion is related to typical psv it also suggests that the core dynamo process responsible for psv is capable of near impulse onset persistence over a time scale of 15 000 years and quasi periodic behavior that is non wave length dispersive in time

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

the last 40 ka tephrostratigraphic record of lake ohrid albania and macedonia a very distal archive for ash dispersal from italian volcanoes abstract a 1075 cm long core lz1120 was recovered in the south eastern part of the lake ohrid republics of macedonia and albania and sampled for identification of tephra layers magnetic susceptibility investigations show rather high magnetic values throughout the core with peaks unrelated to the occurrence of tephra layers but instead to the relative abundance of detrital magnetic minerals in the sediment naked eye inspection of the core allowed us to identify of two tephra layers at 896 897 cm and 1070 1075 cm laboratory inspection of the grain size fraction 125 m allowed for the identification of a third cryptotephra at 310 315 cm major element analyses on glass shards of the tephra layers at 896 897 cm and 1070 1075 cm show a trachytic composition and indicate a correlation with the regionally dispersed y 3 and y 5 tephra layers dated at ca 30 and 39 cal ka bp the cryptotephra at 310 315 cm has a mugearitic benmoreitic composition and was correlated with the fl eruption of mt etna dated at 3370 70 cal yr bp these ages are in agreement with five 14 c ams measurements carried out on plant remains and macrofossils from the lake sediments at different depths along the core the recognition of distal tephra from italian volcanoes allows us to link the lake ohrid succession to other archives located in the mediterranean area and in eastern europe the benmoreitic mugearitic tephra layer at 310 315 cm is the first recognition in the balkan area of a distal ash deposit from a mid intensity explosive eruption of mt etna as far as 600 km from the source

evidence for an increase in cosmogenic 10be during a geomagnetic reversal reversals in the geomagnetic field which occur every few hundred thousand years represent a dramatic change in the earth s environment although there is no satisfactory theory for such reversals it is generally accepted that the dipole field intensity decreases to 20 of its normal value for a few thousand years during the change in direction1 because the galactic and solar cosmic rays which impinge on the earth s atmosphere are charged a significant fraction about half of them are deflected by the geomagnetic field2 at the time of a reversal this magnetic shielding is greatly reduced and it has been suggested that the increased flux of high energy particles could have effects on evolutionary3 or climatic4 processes for example the statistically significant coincidence in levels of some marine faunal extinctions and reversal boundaries in ocean sediments5 could be caused directly or indirectly by the decreased geomagnetic intensity during the reversal we report here evidence in marine sediments for an increase in cosmogenic 10be production in the earth s atmosphere during the brunhes matuyama reversal 730 000 yr ago in addition to confirming an increase in cosmogenic isotope production the results provide information on the magnitude and duration of the geomagnetic intensity decrease during such an event and the depth at which rmanent magnetism is acquired in marine sediments

reconstruction of global 10be production over the past 250ka from highly accumulating atlantic drift sediments in this study we present a reconstruction of the global 10be production rate over the past 250 000 years from three marine sediment cores located in high accumulation environments in the north northwest and south atlantic ocean odp sites 983 1063 and 1089 the 10be records are corrected for oceanic transport processes and principal component analysis pca is used to extract the common signal from the three records which we interpreted as variations of the global 10be production rate the reconstruction presented here may serve as i a record of past flux of galactic cosmic rays gcr ii a proxy for past geomagnetic dipole strength and iii as a global matching tool to synchronize marine archives with ice cores and terrestrial records

geomagnetic excursions knowns and unknowns geomagnetic excursions are short lived episodes when earth s magnetic field deviates into an intermediate polarity state understanding the origin frequency amplitude duration and field behavior associated with excursions is a forefront research area within solid earth geophysics recent advances in excursion research are summarized here and key further research is suggested to resolve major unanswered questions improving the global distribution of excursion records particularly from the southern hemisphere obtaining high resolution sedimentary excursion records with good age control from sites with sedimentation rates 10 cm kyr obtaining volcanic excursion records coupled with high precision geochronology and estimating excursion duration with high chronological precision will all facilitate hypothesis testing concerning the deep earth dynamics that generate geomagnetic excursions

late brunhes polarity excursions mono lake laschamp iceland basin and pringle falls recorded at odp site 919 irminger basin abstract component natural remanent magnetizations derived from u channel and 1 cm3 discrete samples from odp site 919 irminger basin indicate the existence of four intervals of negative inclinations in the upper brunhes chronozone according to the age model based on planktic oxygen isotope data these excursional intervals occur in sediments deposited during the following time intervals 32 34 ka 39 41 ka 180 188 ka and 205 225 ka these time intervals correspond to polarity excursions detected elsewhere known as mono lake laschamp iceland basin and pringle falls the isotope based age model is supported by the normalized remanence paleointensity record that can be correlated to other calibrated paleointensity records for the 0 500 ka interval such as that from odp site 983 for the intervals associated with the mono lake and laschamp excursions virtual geomagnetic poles vgps reach equatorial latitudes and mid southerly latitudes respectively for intervals associated with the iceland basin and pringle falls excursions repeated excursions of vgps to high southerly latitudes indicate rapid directional swings rather than a single short lived polarity reversal the directional instability associated with polarity excursions is not often recorded probably due to smoothing of the sedimentary record by the process of detrital remanence drm acquisition

insights into the age of the mono lake excursion and magmatic crystal residence time from u th he and 230 th dating of volcanic allanite we present new data for the age of the mono lake excursion at its type locality using the u th he system on allanite we dated wilson creek ash 15 lajoie 1968 to 38 7 1 2 ka 2 se the new age for this ash supports the hypothesis kent et al 2002 zimmerman et al 2006 that the mono lake excursion is coincident with and probably the same event as the laschamp geomagnetic excursion 40 4 2 ka an event that shares similar magnetic characteristics with the excursion identified at mono lake we also estimate an allanite magma residence time of slightly less than 30 ka based on 230 th 238 u disequilibrium and the u th he based eruption age

origin and age of the directions recorded during the laschamp event in the chaine des puys france abstract excursions of the geomagnetic field are likely even more frequent than reversals and thus must be seen as a major characteristic of the geodynamo the laschamp event discovered in lava flows of massif central france is the youngest and the most studied field excursion its geomagnetic origin has been controversial due to the existence of self reversal processes taking advantage of new dated sites the initial aim of this work was to find additional flows with intermediate or reverse directions but also to investigate the magnetic properties of normal polarity flows surrounding the event this study combines thermal and alternating field demagnetization of 272 samples from 21 units including 12 new localities and 12 k ar datings of main units no new site with intermediate or reverse polarity was found ten sites have a normal polarity and all sites studied at olby louchadiere and royat display intermediate but scattered directions magnetic mineralogy is characterized by primary titanomagnetite variable amounts of titanomaghemite and almost pure magnetite we confirmed that reverse polarity flows are affected by self reversals but we found that this is also the case for normal flows a direct consequence is that self reversals cannot be taken as responsible for the reverse directions but they likely contribute to generate dispersion however the overall coherency of the directions indicates that this process was limited and despite complex magnetization the geomagnetic origin of the laschamp in the chaine des puys is not questioned all volcanic pole positions vgps published so far show significant scatter but they remain consistent with each other interestingly they do not coincide with the longitudinal loops seen in the sedimentary records and are compatible with a dominant non dipolar field geometry new k ar datings provide a coherent chronology of the successive polarities the 37 ka old reverse directions of the olby flow are chronologically consistent with the 41 9 kyr old normal polarity flows preceding the event the royat flow is only marginally intermediate and indicates that the end of the event was not younger than 33 3 ka in the chaine des puys

laschamp and mono lake geomagnetic excursions recorded in new zealand eight basaltic lavas from the auckland volcanic field new zealand record three distinct sets of excursional geomagnetic field directions and low paleointensities however the timing and therefore paleomagnetic significance of these records have been poorly understood radiocarbon k ar and thermoluminescence dating constrain these lavas to have erupted during the last 75 ka a period during which as many as three excursions have been recorded differentially at several northern and fewer southern hemisphere sites forty 40ar 39ar incremental heating experiments conducted on groundmass from seven of these excursional lavas indicate that they erupted during at least two periods at 39 1 4 1 and 31 6 1 8 ka coincident with 40ar 39ar astrochronologic and 14c ages determined for the laschamp and mono lake excursions respectively samples from a lava flow associated with a third cluster of virtual geomagnetic poles vgps are complicated by low concentrations of radiogenic argon in the presence of excess argon and thus yield discordant age spectra and an imprecise age of 26 6 8 1 ka our findings indicate that the laschamp and mono lake excursions until recently identified unequivocally and isotopically dated only in the northern hemisphere were globally synchronized at 40 1 and 32 1 ka however the vgps of lavas that record the laschamp excursion in new zealand and france are inconsistent with a simple clockwise looping geometry inferred from vgp paths obtained in eight marine sediment cores spanning the laschamp excursion we suggest that the differences in vgps recorded at the various sites are significant and may point to non axial dipole components and lower mantle control on transitional fields during short lived excursions

late glacial to holocene climate and sedimentation history in the nw black sea abstract gravity cores from the continental slope in the northwestern black sea were studied using high resolution stable isotope grain size and xrf scanning data the measurements provide a 30 000 years ams 14c dated record of variations in the hydrological regime of the black sea and give insight into changing paleoenvironments in the surrounding areas stable climatic conditions during the last glacial maximum were followed by a series of meltwater pulses most likely originating from the scandinavian ice sheet between 18 000 and 15 500 yr bp 1 this meltwater input rose the level of the caspian sea to a point that caspian water could spill into the black sea via the manych depression north of the caucasian mountains high frequency oscillations in the xrf data during this period suggest a probable link to the arctic climate regime later during the bolling allerod and the early holocene prevailing high temperatures led to authigenic calcite precipitation through increased phytoplankton activity interrupted by the younger dryas and the 8200 yr bp cold event with dominant clastic sedimentation

geomagnetic field intensity and inclination records from hawaii and the reunion island geomagnetic implications abstract we report on new paleointensity and inclination records obtained in hawaii from 386 samples drilled in 137 subaerial flows of the hsdp2 long basaltic core that we have combined in a composite record with the other hawaiian results to produce a unique and accurate lava record of absolute geomagnetic field intensity and inclination at hawaii for the last 405 kyr these data are considered together with published results from la reunion island for about the same time period in order to compare them to model results in hawaii when recognized excursional periods are omitted the average vadm vdm value is about 8 2 8 4 1022 am2 and the inclination is on the average 29 8 i e about 6 shallower than the expected value at this location gad value in the reunion island we selected published results in which both inclination and intensities values are obtained using modern methods and strict criteria the average vadm vdm is 9 4 2 3 8 7 2 3 1022 am2 and the average inclination value when transitional data are ignored is 44 4 2 4 about 7 steeper than the gad value these results compared with model intensities predicted by dynamo solutions that incorporate lateral variations in core mantle boundary heat flow derived from seismic tomography are too high for both localities but they decrease as the amplitude of thermal boundary anomalies increases with strong boundary heat flow the model inclination anomaly at hawaii fits with the data but at la reunion model inclination anomalies have the wrong sign this comparison indicates that strong boundary heat flow anomalies give the best fit to the data and that future improvements will be obtained by proportionally increasing both the rayleigh number and the amplitude of heat flow variations

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

paleointensity of the earth s magnetic field during the laschamp excursion and its geomagnetic implications the reversed paleomagnetic direction of the laschamp and olby flows represents a specific feature of the geomagnetic field this is supported by paleomagnetic evidence showing that the same anomalous direction was recorded at several distinct sites including scoria of the laschamp volcano to examine this anomalous geomagnetic fluctuation we studied the paleointensity of the laschamp and olby flows using the thellier method twenty five samples were selected for the paleointensity experiments and from seven we obtained reliable results because the paleointensity results of the olby and laschamp flows as well as laschamp scoria are very similar they can be represented by a single mean paleointensity f 7 7 t considering that this low paleointensity is less than 1 6 of the present geomagnetic field and is more characteristic of transitional behavior our results suggest that the paleomagnetic directions of the laschamp and olby flows were not acquired during a stable reversed polarity interval a more likely explanation is that the laschamp excursion represents an unsuccessful or aborted reversal

dynamics of the laschamp geomagnetic excursion from black sea sediments abstract investigated sediment cores from the southeastern black sea provide a high resolution record from mid latitudes of the laschamp geomagnetic polarity excursion age constraints are provided by 16 ams 14c ages identification of the campanian ignimbrite tephra 39 28 0 11 ka and by detailed tuning of sedimentologic parameters of the black sea sediments to the oxygen isotope record from the greenland ngrip ice core according to the derived age model virtual geomagnetic pole vgp positions during the laschamp excursion persisted in antarctica for an estimated 440 yr making the laschamp excursion a short lived event with fully reversed polarity directions the reversed phase centred at 41 0 ka is associated with a significant field intensity recovery to 20 of the preceding strong field maximum at 50 ka recorded field reversals of the laschamp excursion lasting only an estimated 250 yr are characterized by low relative paleointensities 5 relative to 50 ka the central fully reversed phase of the laschamp excursion is bracketed by vgp excursions to the sargasso sea 41 9 ka and to the labrador sea 39 6 ka paleomagnetic results from the black sea are in excellent agreement with vgp data from the french type locality which facilitates the chronological ordering of the non superposed lavas that crop out at laschamp olby in addition vgps between 34 and 35 ka reach low northerly to equatorial latitudes during a clockwise loop inferred to be the mono lake excursion

on the age of the laschamp geomagnetic excursion abstract the laschamp geomagnetic excursion is a critical tie point found directly in deep sea sediment cores and revealed in polar ice as an abrupt change in the rate of cosmogenic nuclide flux despite the importance of this excursion to quantifying paleoclimate proxy records archived in sediment and ice and to providing an independent calibration of the radiocarbon calendar its timing remains poorly known previous k ar 40 ar 39 ar and u th isochron determinations from lava flows at the type locality in the massif central france vary widely are imprecise and suggest a mean age of about 46 2 2 5 ka 2 results of 6 new unspiked k ar and 13 40 ar 39 ar incremental heating experiments on subsamples from three sites on the laschamp and olby flows are concordant and give a weighted mean age of 40 4 1 1 ka 2 uncertainty including analytical sources only that is 10 younger than the previous estimates considering that the 40 k 40 ar decay constant is not known to a precision better than 2 4 the most probable radioisotopic age for the laschamp excursion is 40 4 2 0 ka 2 analytical plus decay constant uncertainties this new age for the laschamp excursion agrees precisely with that deduced from the napis 75 deep sea sediment paleointensity stack when calibrated against the gisp2 ice core chronology using the o isotopes in ice and the magnetic properties of the marine cores

chronology and correlation of quaternary magnetostratigraphy and nannofossil biostratigraphy in norwegian greenland sea sediments paleomagnetic analyses of a series of sediment cores from the norwegian greenland sea document the existence of seven short lived geomagnetic events during the last about 500 000 years calcareous nannofossil biostratigraphy correlated to oxygen isotope chronostratigraphy provides a time framework to infer age and duration of these periods of extreme divergence of the earth s magnetic paleofield from the regular normal polarity configuration of the brunhes chron the geomagnetic events typically extend over 5 to 10 kyr two events are observed within the oxygen isotope stage 2 4 interval a short event is recorded at the stage 4 5 boundary and another just below the stage 5 6 boundary additional three geomagnetic events are identified in oxygen isotope stage 6 at the stage 9 10 boundary and near the top of oxygen isotope stage 13 respectively

magnetic hysteresis properties of greigite fe3s4 and a new occurrence in holocene sediments from swedish lappland a new occurrence of ferrimagnetic greigite fe3s4 is reported in freshwater holocene sediments from gaskkamus gorsajavri in swedish lappland the presence of greigite was inferred from the mineral magnetic analysis of sediment cores the greigite bearing sediments possess a high saturation isothermal remanent magnetisation to susceptibility sirm ratio a characteristic shared by other natural sediments known to contain greigite the hysteresis properties measured using a vibrating sample magnetometer of natural greigite extracted from lake sediments show much higher saturation coercivity b0 c and mrsms ratio values approximately 50 mt and 0 5 respectively than synthetic samples of greigite approximately 12 mt and 0 2 the results suggest that synthetic greigite samples contain crystals ranging from small non remanence carrying superparamagnetic grains to large multi domain grains in contrast the natural samples probably consist predominantly of pseudo single domain and single domain crystals capable of carrying a chemical remanent magnetisation however the unstable nature of greigite under laboratory conditions creates difficulty in characterising its physical attributes

a summary of brunhes paleomagnetic field variability recorded in ocean drilling program cores the ocean drilling program odp has recovered many long sediment sequences from around the world that contain medium to high resolution paleomagnetic records of brunhes age these records have provided an important new global view of geomagnetic field variability during stable magnetic dipole polarity that could not be recovered by conventional terrestrial or deep sea piston coring the odp paleomagnetic records of directional variability can be routinely recovered and used regionally as a very highresolution relative chronostratigraphic tool 200 yr resolution odp paleomagnetic records of brunhes age relative paleointensity have dramatically improved our understanding of global geomagnetic field intensity these records document the global scale pattern of paleointensity and demonstrate its use as a high resolution relative chronostratigraphic tool that under ideal conditions could have a precision of 500 yr odp paleomagnetic records and associated oxygen isotope chronostratigraphies have also greatly improved our understanding of the number and ages of brunhes geomagnetic field excursions and their relationship to normal directional secular variation and paleointensity variability although significant progress has been made though odp paleomagnetic studies of brunhes aged sediments it is clear that more work is needed to further resolve and define the detailed global pattern of brunhes psv and its relationship to excursions and true polarity reversals

a stable chemical remanence in holocene sediments the ferrimagnetic iron sulphide greigite has been extracted from freshwater sediments from loch lomond and llyn geirionydd and identified by x ray diffraction and thermomagnetic analyses the authigenic greigite is probably the dominant carrier of a stable natural remanent magnetization in the freshwater sediments underlying marine sediments in loch lomond and in the lateglacial interstadial sediments from llyn geirionydd mineral magnetic analysis shows that greigite tends to have a higher sirm x ratio approximately 70 kam 1 than many natural magnetite assemblages this magnetic characteristic coupled with greigite s ability to oxidize easily and to change to a less magnetic mineral upon exposure to air may be used to indicate its presence in downcore magnetic profiles alternating field demagnetization cannot be used to distinguish chemical from detrital remanences in loch lomond or llyn geirionydd on account of the similar magnetic stabilities of greigite and magnetite the time lag between the acquisition of the primary detrital remanence and the secondary chemical remanence remains unknown and creates difficulties in understanding the record of paleomagnetic secular variation contained in these lake sediments

timing and climatic impact of greenland interstadials recorded in stalagmites from northern turkey a 50 kyr long exceptionally well dated and highly resolved stalagmite oxygen 18o and carbon 13c isotope record from sofular cave in northwestern turkey helps to further improve the dating of greenland interstadials gi 1 and 3 12 timing of most gi in the sofular record is consistent within 10 to 300 years with the iconic hulu cave record larger divergences 500 years between sofular and hulu are only observed for gi 4 and 7 the sofular record differs from the most recent ngrip chronology by up to several centuries whereas age offsets do not increase systematically with depth the sofular record also reveals a rapid and sensitive climate and ecosystem response in the eastern mediterranean to gi whereas a phase lag of 100 years between climate and full ecosystem response is evident finally results of spectral analyses of the sofular isotope records do not support a 1 470 year pacing of gi

high resolution record of northern hemisphere climate extending into the last interglacial period high resolution record of northern hemisphere climate extending into the last interglacial period

wood decay of xenoxylon yunnanensis feng sp nov from the middle jurassic of yunnan province china abstract a permineralised gymnosperm wood xenoxylon yunnanensis feng sp nov is described from the middle jurassic shaximiao formation in huize county yunnan province southwest china the pycnoxylic wood consists of thick walled tracheids and thin walled rays it is characterised by uniseriate contiguous compressed bordered pits on the radial walls of tracheids and one large simple pit in each cross field growth rings exhibit narrow latewood the mean sensitivity of analysis of 25 growth rings is 0 2 suggesting that a climate with abundant rainfall and weak seasonality could have prevailed in yunnan at least locally during the middle jurassic the outer portion of the wood is largely decayed and shows poorly preserved cells lacking details decayed tracheids displaying various features including removal of middle lamellae separations of secondary walls presence of thickened corners and appositions are recognised as cell reactions to fungal activity irregularly distributed micro cavities are formed by complete decomposition of tracheids and rays in the wood abundant fungal remains occurring in the wood generally consist of multiple branched hyphae these fungal hyphae are either confined to individual cell lumens or extend through adjacent cells although the probable reproductive structure and typical clamp connection are very rarely observed with few septate hyphae the affinities of these fungal hyphae are somewhat obscure nevertheless our study sheds light on the palaeoecology of middle jurassic terrestrial ecosystems of southwest china

palaeointensities of the auckland geomagnetic excursions by the ltd dht shaw method we report new palaeointensity results concerning the auckland geomagnetic excursions using the double heating technique of the shaw method with low temperature demagnetisation ltd dht shaw method the excursional palaeodirections recorded in six volcanoes of the auckland volcanic field new zealand have been classified into three groups north down nd west up wu and south up su directions in the present study five to six consistent palaeointensities have been obtained from each of five volcanoes recording the auckland geomagnetic excursions the wiri 27 ka crater hill and puketutu volcanoes nd group yielded mean palaeointensities of 10 6 1 2 1 11 8 2 8 and 11 1 0 4t respectively the hampton park volcano 55 ka wu group gave 9 5 1 2t while the mclennan hills volcano su group gave 2 5 0 5t it is notable that consistent palaeointensities have been obtained from the three different volcanoes which have almost the same palaeodirections nd group possibly supporting the reliability of the palaeointensity data these five palaeointensities for the auckland geomagnetic excursions correspond to virtual dipole moments vdms of 0 6 2 1 10 22 am 2 whereas three mean palaeointensities obtained from the auckland volcanoes having non excursional palaeodirections are 13 1 40 0 t giving stronger vdms of 2 1 6 9 10 22 am 2 these results suggest that the dipole component of the geomagnetic field reduced to about 2 10 22 am 2 or less during the auckland geomagnetic excursions

the campanian ignimbrite and codola tephra layers two temporal stratigraphic markers for the early upper palaeolithic in southern italy and eastern europe tephra layers from archaeological sites in southern italy and eastern europe stratigraphically associated with cultural levels containing early upper palaeolithic industry were analysed the results confirm the occurrence of the campanian ignimbrite tephra ci ca 40 cal ka bp at castelcivita cave southern italy temnata cave bulgaria and in the kostenki borshchevo area of the russian plain this tephra originated from the largest eruption of the phlegrean field caldera represents the widest volcanic deposit and one of the most important temporal stratigraphic markers of western eurasia at paglicci cave and lesser sites in the apulia region we recognise a chemically and texturally different tephra which lithologically chronologically and chemically matches the physical and chemical characteristics of the plinian eruption of codola a poorly known late pleistocene explosive event from the neapolitan volcanoes likely somma vesuvius for this latter we propose a preliminary age estimate of ca 33 cal ka bp and a correlation to the widespread c 10 marine tephra of the central mediterranean the stratigraphic position of both ci and codola tephra layers at castelcivita and paglicci help date the first and the last documented appearance of early upper palaeolithic industries of southern italy to ca 41 40 and 33 cal ka bp respectively or between two interstadial oscillations of the monticchio pollen record to which the ci and codola tephras are physically correlated corresponding to the greenland interstadials 10 9 and 5 in eastern europe the stratigraphic and chronometric data seem to indicate an earlier appearance of the early upper palaeolithic industries which would predate of two millennia at least the overlying ci tephra the tephrostratigraphic correlation indicates that in both regions the innovations connected with the so called early upper palaeolithic encompassing subsistence strategy and stone tool technology appeared and evolved during one of the most unstable climatic phases of the last glacial period on this basis the marked environmental unpredictability characterising this time span is seen as a potential ecological factor involved in the cultural changes observed

changes in the carbon cycle during the last deglaciation as indicated by the comparison of 10be and 14c records the variations in atmospheric radiocarbon c 14 concentration during the last 50 000 years can be attributed to changes in the c 14 production rate due to changes in solar activity the geomagnetic field and or interstellar galactic cosmic ray flux and to changes in the global carbon cycle the relative contributions of these processes is the subject of current debate although the discrepancies between the various reconstructions of the past atmospheric radiocarbon concentration increase with age the relatively good agreement over the last 25 000 years allows a quantitative discussion of the causes of the observed c 14 variations for this period using be 10 measurements from greenland summit ice cores we show that in addition to solar and geomagnetically induced production rate changes significant changes in the carbon cycle have to be considered to explain the measured c 14 concentrations there is evidence that these changes are connected to 1 global deglaciation and 2 climate changes in the north atlantic region on centennial to millennial time scales related to changes in the ocean circulation differences between be 10 and geomagnetic field records however suggest that uncertainties of about 20 still exist in determinations of past changes in the c 14 production rate c 2004 elsevier b v all rights reserved less

pleistocene water intrusions from the mediterranean and caspian seas into the black sea the hydrological balance of the black sea is governed by riverine input and by the exchange with the mediterranean sea a speleothem record from a cave in northern turkey that tracks the isotopic signature of black sea surface water suggests an open connection to the mediterranean sea in at least twelve periods in the past 670 000 years

the mono lake excursion recorded in phonolitic lavas from tenerife canary islands paleomagnetic analyses and coupled k ar and ar ar dating abstract we present a coupled paleomagnetic dating investigation conducted on three different lava flows from the island of tenerife canary islands spain erupted during the mono lake excursion mle paleomagnetic analyses consist in zero field demagnetizations af and or thermal and of thellier and thellier experiments using the picrit 03 set of criteria to select reliable intensity determinations one of the flows is characterized by a direction largely deviated from the one expected from an axial geocentric dipole gad field its paleointensity value is very low 7 8 t the two other sites are characterized by inclinations slightly shallower than the gad value and by low intensity values about 12 and 21 t present value 38 t the three k ar ages combined with two 40 ar 39 ar ages range from 32 0 to 33 2 ka and they are not statistically distinguishable from one another it therefore appears that these lavas have recorded the mle the only excursion in this time interval confirming its brief duration shorter than the minimum age uncertainties available the mean age is younger but within the uncertainties consistent with the age of the 10 be peak and of the marine intensity low when reported in the most recent ice age model these new results are the first ones with radiometric dating produced from the northern hemisphere combined with existing cosmogenic marine and volcanic paleomagnetic data these results are discussed in terms of dating and geometry of the earth magnetic field during the excursion

a new tool for determining ionospheric currents from magnetic satellite data an algorithm for determining ionospheric currents at high latitudes from magnetic measurements made by low altitude satellites is presented and illustrated using magsat data the ionospheric currents are assumed to flow in a height of 115 km perpendicular to the orbit plane of the satellite which corresponds to east west direction in the case of the dawn dusk magsat orbit the method allows for studying the time development of both the strength and the location of the auroral electrojets application to 13 days of magsat data in winter 1979 yields a correlation coefficient of r 0 88 between the ae index and the strength of the auroral electrojets their latitudinal location moves southward with increasing magnetospheric ring current as measured by the dst index

the greenland ice core chronology 2005 15 42 ka part 2 comparison to other records a new greenland ice core chronology gicc05 based on multi parameter counting of annual layers has been obtained for the last 42 ka here we compare the glacial part of the new time scale which is based entirely on records from the northgrip ice core to existing time scales and reference horizons covering the same period these include the grip and northgrip modelled time scales the meese sowers gisp2 counted time scale the shackleton fairbanks grip time scale sfcp04 based on 14c calibration of a marine core the hulu cave record three volcanic reference horizons and the laschamp geomagnetic excursion event occurring around greenland interstadial 10 gicc05 is generally in good long term agreement with the existing greenland ice core chronologies and with the hulu cave record but on shorter time scales there are significant discrepancies around the last glacial maximum there is a more than 1 ka age difference between gicc05 and sfcp04 and a more than 0 5 ka discrepancy in the same direction between gicc05 and the age of a recently identified tephra layer in the northgrip ice core both sfcp04 and the tephra age are based on 14c dated marine cores and fixed marine reservoir ages for the laschamp event gicc05 agrees with a recent independent dating within the uncertainties

self reversal explanation for the laschamp olby geomagnetic field excursion abstract the reversed natural remanent magnetisation nrm of the volcanic features at laschamp and olby chaine des puys auvergne france is commonly believed to document the youngest excursion of the geomagnetic field during the present brunhes epoch recent radiometric dating determines the age of these lavas to be 35000 45000 y continuous thermal demagnetisation indicates that under laboratory conditions the nrm of many olby samples undergoes complete or partial self reversal to a lesser extent this is also true for the laschamp material thus self reversal may be another possible explanation of the reversed nrm directions magnetic optical and microprobe analyses suggest that the self reversal mechanism is based upon magnetostatic interaction between differently oxidised titanomagnetite phases

deep sea sediment records of the laschamp geomagnetic field excursion 41 000 calendar years before present 1 we have recovered two new high resolution paleomagnetic records of the laschamp excursion 41 000 calendar years b p from deep sea sediments of the western north atlantic ocean the records document that the laschamp excursion was characterized locally by 1 declination changes of 120 2 inclination changes of more than 140 3 1200 year oscillations in both inclination and declination 4 near 90 out of phase relationships between inclinations and declinations that produced two clockwise loops in directions and virtual geomagnetic poles vgps followed by a counterclockwise loop 5 excursional vgps during both intervals of clockwise looping 6 magnetic field intensities less than 10 of normal that persisted for almost 2000 years 7 marked similarity in excursional directions over 5000 km spatial scale length and 8 secular variation rates comparable to historic field behavior but persisting in sign for hundreds of years all of these features with the exception of anomalously large directional amplitude are consistent with normal magnetic field secular variation comparison of our laschamp excursion paleomagnetic records with other late quaternary excursion records suggests that there is a group of excursions which we term class i which have strikingly similar patterns of field behavior and likely share a common cause as part of the overall core dynamo process three general models of secular variation are described that can qualitatively produce class i excursions on the basis of these observations we conclude that class i excursions epitomized by the laschamp excursion are more closely related to normal secular variation and are not necessarily a prelude to magnetic field reversal

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

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

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

cosmogenic nuclides during isotope stages 2 and 3 abstract changes in the concentration of cosmogenic nuclides measured in natural archives are related mainly to changes in the cosmic ray induced production rate in the atmosphere but also to the subsequent transport and deposition processes which depend on the geochemical properties of the nuclides a comparison of 10 be and 36 cl data from ice cores from greenland ice cores and 14 c data from tree rings and other archives provides the following information for the isotope stages 2 and 3 1 changes in the cosmogenic nuclide production rate on time scales longer than approximately 2 kyr agree well with paleomagnetic data derived from deep sea sediments in particular the geomagnetic excursions leading to weak fields around 40 kyr bp laschamp and 32 kyr bp mono lake can be clearly seen 2 production variations on shorter time scales contain information on solar variability as an example the 205 yr devries cycle already known from the analysis of 14 c tree ring data during the holocene is also present in the 10 be data of the time period 25 55 kyr bp 3 a comparison of 14 c calculated using 10 be data with the measured 14 c record points to a reduction of the global deep water formation during the younger dryas cold event 4 using information available on the production rates of cosmogenic nuclides in the past derived from paleomagnetic data the paleoprecipitation rate can be calculated this is crucial for establishing precise time scales

rock magnetism of late neogene and pleistocene deep sea sediments relationship to sediment source diagenetic processes and sediment lithology we use bivariate scatter plots to illustrate variations in selected rock magnetic properties low field susceptibility anhysteretic and isothermal remanence of late neogene and pleistocene deep sea sediments from 16 sites in the arctic ocean north atlantic equatorial atlantic and north pacific ocean and the arabian sea our intention is to examine the ability of the rock magnetic properties to differentiate the sediments according to factors such as lithology geographical area and the dominant mode of terrigenous sedimentation which at these sites is via ice rafting via bottom currents or via eolian processes overall correlations between sediment magnetic properties and gross lithology is poor and factors such as the source and transport path of terrigenous sediment and detrital magnetic minerals together with the action of reductive diagenetic processes are the major controls on the magnetic properties on the bivariate scatter plots sites with major ice rafted contributions tend to have high sedimentary ferrimagnetic mineral concentrations relatively coarse ferrimagnetic grain sizes and scattered sample point distributions in contrast sites where we infer significant bottom current supply of terrigenous material have tightly grouped sample point distributions carbonate sediments in which the terrigenous component is supplied by eolian processes tend to have a broad range of magnetic mineral concentration caused by glacial interglacial fluctuations in carbonate accumulation and eolian activity sediments containing significant volcanogenic material have high concentrations of relatively coarse grained ferrimagnetic material reductive diagenesis is a significant determinant of sediment magnetic properties in high productivity areas and has the effect of preferentially removing the fine grained ferrimagnetic fraction causing a coarsening of the ferrimagnetic grain size distribution and a rise in the antiferromagnetic ferrimagnetic ratio

diagenetic formation of ferrimagnetic iron sulphide minerals in rapidly deposited marine sediments south island new zealand abstract detailed magnetostratigraphic studies of late neogene siliciclastic sediments of the awatere group south island new zealand 41 45 s 174 05 e have revealed a wide range of palaeomagnetic behaviour examination of rock magnetic properties was undertaken using conventional palaeomagnetic techniques and thermomagnetic x ray diffraction and electron microprobe analyses these analyses indicate that the ferrimagnetic iron sulphide minerals greigite and pyrrhotite are responsible for a stable and intense magnetic remanence in fine grained sediments whereas titanomagnetite is the only remanence bearing mineral identified in coarser grained sediments which are less strongly and less stably magnetised than the fine grained sediments detrital titanomagnetite grains are likely to have undergone dissolution during early diagenesis as a result of iron sulphide formation which occurs commonly in rapidly deposited anoxic sediments that support active sulphate reduction and h 2 s formation preservation of greigite and pyrrhotite is inferred to result from the arrest of the pyritisation process probably due to the low permeability of the fine grained sediments and consumption of available h 2 s before full reaction to pyrite occurred relative palaeomagnetic instability and weak remanence intensities in coarser grained sediments is likely to be due to low titanomagnetite concentrations resulting from titanomagnetite dissolution

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

self reversal of remanent magnetization in basalts due to partially oxidized titanomagnetites summary a range of basaltic samples from olby france and vogelsberg germany displaying the phenomenon of partial and complete self reversal was studied in order to decipher the physical mechanism responsible for self reversed magnetic remanence in basalts microscopic observations and rock magnetic measurements show titanomagnetite to be the carrier of remanence due to low temperature oxidation the titanomagnetites are present in two magnetic phases forming close side by side phase assemblages the ore grains consist of a non oxidized magnetically soft titanomagnetite part with curie temperatures between 140 and 300 c and a magnetically hard low temperature oxidized part with curie temperatures between 410 and 590 c magnetic force microscopy observations present evidence that the oxidation process does not only influence the curie temperatures and thus the blocking temperature spectrum but also the domain configuration during acquisition of a thermoremanence the two phases are magnetically coupled leading to a remanent magnetization of the low curie temperature phase which is antiparallel to the applied external magnetic field computational modelling of the remanence acquisition process explains the coupling of the two phases by magnetostatic interaction as the observed partial oxidation of ore grains is not uncommon in subaerial basalts partial and complete self reversal is probably a frequently occurring phenomenon nevertheless it remains unnoticed in most cases as it cannot be detected by the standard stepwise thermal demagnetization technique despite the complex remanence properties our investigations indicate that the upper part of the blocking temperature spectrum still carries reliable palaeodirectional information

evidence for two intervals of enhanced 10 be deposition in antarctic ice during the last glacial period we have previously reported concentration profiles of cosmic ray produced cosmogenic 10be in deep ice cores from dome c and vostok antarctica1 2 in both these cores we found a concentration of 10be approximately two times larger in ice from the late glacial period than in the holocene ice we interpreted this as probably resulting from a lower precipitation rate on the antarctic plateau during glacial periods compared to interglacial periods in the vostok profile there was one sample corresponding to 60 000 yr bp which gave an unusually large 10be concentration not correlated with any obvious climatic event we suggested that this sample might be reflecting increased 10be production as for example during a period of reduced solar modulation2 we have now measured a much more detailed concentration profile for 10be in the vostok core the results confirm a 10be peak lasting 1 000 2 000 years at 60 000 yr bp and show another similar peak at 35 000 yr bp we have also observed the latter peak in the dome c core we discuss possible sources for these peaks and their potential as stratigraphic markers

the greenland ice core chronology 2005 15 42 ka part 1 constructing the time scale the greenland ice core chronology 2005 gicc05 is extended back to 42 ka b2k before 2000 ad i e to the end of greenland stadial 11 the chronology is based on independent multi parameter counting of annual layers using comprehensive high resolution measurements available from the north greenland ice core project ngrip these are measurements of visual stratigraphy conductivity of the solid ice electrolytical melt water conductivity and the concentration of na c a 2 s o 4 n o 3 n h 4 a n uncertainty estimate of the time scale is obtained from identification of uncertain annual layers which are counted as 0 570 5 years the sum of the uncertain annual layers the so called maximum counting error of the presented chronology ranges from 4 in the warm interstadial periods to 7 in the cold stadials the annual accumulation rates of the stadials and interstadials are on average one third and half of the present day values respectively and the onset of the greenland interstadials 2 3 and 8 based on 20 year averaged d 18 o

geomagnetic field behavior during the iceland basin and laschamp geomagnetic excursions a simple transitional field geometry we present four new records of the iceland basin excursion ibe and five new records of the laschamp excursion le obtained from rapidly deposited marine sediments in the north atlantic ocean the nordic seas the gulf of mexico the south china sea and the southern indian ocean marked minima in relative paleointensity correspond with the paleomagnetic directional changes associated with all of the excursion records the virtual geomagnetic pole vgp paths of the four ibe records are all similar the vgps move southward over europe and africa reaching the southern hemisphere three reach antarctica and then move to more eastern longitudes before returning northward over australia and east asia describing a large counterclockwise loop the same vgp pattern is observed in other published records the vgp paths observed for the le are similar to those of the ibe however they loop clockwise instead of counterclockwise despite the different sense of looping the marked similarity among the paths for the two excursions suggests that a similar relatively simple geometry dominated the transitional field during both the ibe and the le similar dynamo mechanisms must therefore have been active in the earth s core for both excursions the duration of the excursions is estimated at 2 000 years which supports the suggestion that a difference exists between the mechanisms for excursions and reversals however the coincidence of the longitudinal bands for vgps associated with excursions compared to some reversal paths could also indicate an inherent link between the mechanisms for reversals and excursions

40ar 39ar k ar and 230th 238u dating of the laschamp excursion a radioisotopic tie point for ice core and climate chronologies abstract a brief period of enhanced 10 be flux that straddles the interstadial warm period known as dansgaard oeschger event 10 in greenland and its counterpart in antarctica the antarctic isotope maximum 10 is but one consequence of the weakening of earth s magnetic field associated with the laschamp excursion this 10 be peak measured in the grip ice core is dated at 41 250 y b2k before year 2000 ad in the most recent gicc05 age model obtained from the northgrip core via multi parameter counting of annual layers uncertainty in the age of the 10 be peak is however no better than 1630 y at the 95 confidence level reflecting accumulated error in identifying annual layers the age of the laschamp excursion guillou h singer b s laj c kissel c scaillet s jicha b 2004 on the age of the laschamp geomagnetic excursion earth planet sci lett 227 331 343 is revised on the basis of new 40 ar 39 ar unspiked k ar and 238 u 230 th data from three lava flows in the massif central france together with the 40 ar 39 ar age of a transitionally magnetized lava flow at auckland new zealand combined these data yield an age of 40 700 950 y b2k where the uncertainty includes both analytical and systematic 40 k and 230 th decay constant errors taking the radioisotopic age as a calibration tie point suggests that the layer counting chronologies for the northgrip and gisp2 ice cores are more accurate and precise than previously thought at depths corresponding to the laschamp excursion

greigite detected as dominating remanence carrier in late pleistocene sediments lisan formation from lake kinneret sea of galilee israel summary a rock magnetic investigation of three sedimentary cores of lisan formation of late pleistocene age from lake kinneret sea of galilee northern israel demonstrates that the magnetization of these sediments is controlled by various degrees of a secondary chemical remanent magnetization crm carried by greigite fe3s4 this crm is superimposed on a primary detrital remanent magnetization drm that resides in ti magnetite this finding is independently confirmed by x ray diffraction xrd measurements performed on magnetic extracts of the sediments the domain state of the greigite is largely single domain behaviour sd thus dominating the magnetization therefore the magnetic record retrieve from of these sediments is not reflecting geomagnetic variations but rather chemical rock magnetic properties resulting from diagenetic processes the results of our study suggest that paleomagnetic record of greigite bearing sediments should be interpreted with caution because of the following reasons 1 geomagnetic secular variations can be biased due to large coercivity overlap between magnetite and greigite 2 alternating field af demagnetization can produce erroneous directions due to vector distortion by acquisition of a gyro remnant magnetization grm 3 estimation of relative paleointensity can be hampered by large scale variations in natural remnant magnetization nrm intensity caused by the acquisition of a secondary crm of unknown age and unknown extent superimposed on the drm the precipitation of greigite requires reducing conditions at the sediment water interface and or interstitial water associated with the presence of sulphur and iron oxides reducing conditions are typical of stratified lakes and other stratified water bodies such as the dead sea paleo lake lisan temporarily the eastern mediterranean sea the caspian sea the black sea and many other marine basins according to our findings paleomagnetic records from such environments carried dominantly by greigite therefore should be treated with care

geomagnetic excursions reflect an aborted polarity state abstract geomagnetic excursions represent short episodes of a few thousand years at most during which the field considerably exceeds its normal range of variability during a polarity state paleomagnetic records have now been obtained with extremely high temporal resolution which have improved our knowledge of these short events we have compiled the most detailed records of excursions that had occurred during the brunhes and matuyama chrons we show that virtual geomagnetic poles vgps of at least one record of each event are able to reach the opposite polarity in the next step we have computed different simulations of excursions during which the dipole progressively vanishes before growing back without reversing this scenario produces very few reversed directions which are only visible at some latitudes we infer that it is impossible to reach the ratio of reversed to intermediate vgps present in the paleomagnetic records if the excursions were not associated with a short period of reversed dipole field therefore excursions should be regarded as two successive reversals bracketing an aborted polarity interval we propose that the same underlying mechanisms prevail in both situations excursions or reversals and that below a certain strength the field reaches an unstable position which preludes either the achievement of a reversal or its return to the former polarity

magnetic properties of sedimentary greigite fe3s4 new data are presented here that document the previously poorly known magnetic characteristics of sedimentary greigite fe3s4 several diagnostic magnetic properties can be used in conjunction to rapidly screen sediments to detect the presence of greigite such discriminants are necessary because greigite displays a similar or higher range of coercivities compared to most ferrimagnetic minerals and the commonly used method of alternating field demagnetization cannot be used to distinguish between palaeomagnetic remanence components due to greigite and other ferrimagnetic minerals sedimentary greigite has high ratios of the saturation isothermal remanent magnetization to magnetic susceptibility mrs which are accompanied by hysteresis ratios of mrsms 0 5 and bcr bc 1 5 ms is the saturation magnetization bc is the coercive force and bcr is the coercivity of remanence greigite has no low temperature phase transition whereas the ferrimagnetic iron sulphide pyrrhotite undergoes a major phase transition at about 34 k and unoxidized magnetite displays the verwey transition at 118 k low temperature measurements may therefore prove valuable in distinguishing between magnetite pyrrhotite and greigite although the absence of a low temperature transition cannot be taken as definitive evidence for the presence of greigite greigite also displays characteristic high temperature behaviour with a major drop in magnetization between 270 and 350 c determination of the mrs ratio is an excellent means of rapidly screening sediments to identify stratigraphic intervals that may contain greigite subsequent low and high temperature analyses can then provide unambiguous identification of greigite and enable the evaluation of the presence of other magnetic minerals