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

a high resolution paleointensity stack of the past 14 to 68 ka from black sea sediments abstract detailed paleo and mineral magnetic analyses of a sediment composite record from the southeastern black sea yielded a high resolution well dated paleointensity record though hampered by some larger hiatuses in some cores and contaminated by diagenetically formed greigite the paleomagnetic composite record obtained from the preserved primary detrital magnetite phase reflects a highly dynamic geomagnetic field during the last glacial period relative variations of paleointensity inferred from the sediments magnetisations were converted into a record of the virtual axial dipole moment vadm lowest vadm values are linked with the laschamp 0 50 10 22 a m 2 at 41 0 ka the norwegian greenland sea 1 5 10 22 a m 2 at 64 5 ka and the mono lake 3 0 10 22 a m 2 at 34 5 ka geomagnetic excursions the fully reversed field during the laschamp excursion exhibits a vadm of 2 0 10 22 a m 2 which is more than 25 of the present day axial dipole moment 7 628 10 22 a m 2 rates of change calculated from the black sea vadm record also give some information on how to assess the global decay of the present day geomagnetic field which is significantly enhanced in the area of the south atlantic anomaly comparison with provided c 14 and 10 be records confirm partly in the very detail the non linear anti correlation of geomagnetic field intensity and the production of cosmogenic radionuclides in the earth s upper atmosphere however discrepancies in the timing of lows and highs in the compiled records points out that the combination of different data sets from different archives remains a challenge

precise ages of the reunion event and huckleberry ridge excursion episodic clustering of geomagnetic instabilities and the dynamics of flow within the outer core the reunion event is one of the earliest recognized periods of normal polarity within the reversed matuyama chron named for the site at which it was first discovered on reunion island it has since purportedly been found globally in both volcanic rocks and sediments and thus has become a key chronostratigraphic marker however geochronologic results from several locations thought to have recorded this event have caused considerable confusion regarding not only its age and duration but also the number of reunion events new 40ar 39ar ages from eight reunion island lavas in three distinct sections are indistinguishable from one another thereby placing the event at 2 200 0 007 0 010 ma2 200 0 007 0 010 ma 2 2 analytical total uncertainty note this format is used throughout the paper the paleomagnetic behavior recorded at two of the island sites shows that the opposite normal polarity was reached and sustained for a period during which several lava flows were erupted whether this can be classified as a very short subchron bounded by a rapid set of back to back reversals or as a special case of a geomagnetic excursion is unclear hence we choose to continue labeling the dynamo activity recorded by these reunion island lavas as an event this event preceded a 38 kyr 38 kyr period of normal polarity that we name the feni subchron after its locality of discovery at odp site 981 the feni subchron was succeeded by the huckleberry ridge excursion for which 40ar 39ar sanidine and u pb zircon ages of 2 077 0 001 0 003 ma2 077 0 001 0 003 ma and 2 084 0 012 0 013 ma2 084 0 012 0 013 ma respectively from member b of the huckleberry ridge tuff in idaho are in agreement these findings suggest that the full normal polarity recorded on reunion island is a singular brief period of unstable field behavior at the onset of a 125 kyr 125 kyr bundling of dynamo instabilities from 2 20 to 2 07 ma disturbances to the axial dipole component of earth s magnetic field during this period and by analogy similar periods of temporally clustered excursions during the early and late portions of the brunhes chron may reflect disruptions to convective flow arising from parcels of material introduced into the outer core from either the inner core or core mantle boundaries a proposition that might be tested by future numerical dynamo simulations

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

late pleistocene chronology of sediments from the yermak plateau and uncertainty in dating based on geomagnetic excursions the yermak plateau is one of several regions in the arctic ocean where paleomagnetism yields controversial results despite low sedimentation rates late pleistocene paleomagnetic excursions have b

geomagnetic and mineral magnetic characterization of the anthropocene geomagnetic and mineral magnetic data provide geological indices that are both independent of human impact i e geomagnetic and respond to human induced environmental impact i e mineral magnetic we provide the first discussion of such magnetic events for help in defining the anthropocene within the holocene a potential geomagnetic marker for the anthropocene is the low dipole latitude at c 2700 cal a bp which is associated with distinct palaeosecular variation features in northerly mid to high latitude sites mineral magnetic records from lake and marine sediments identify major deforestation and soil delivery events from catchment systems in many parts of the world during the last 4000 years in europe clusters of these events occur around both 2600 cal a bp and ad 1100 the former coinciding with a low in geomagnetic field dipole latitude and peak intensity mineral magnetic records in peats and lake sediments can reflect particulate pollution from fossil fuel burning the expansion of major coal burning began c ad 1800 in western europe and eastern north america but around ad 1900 this expanded due to more widely distributed coal use and this event is the most clear mineral magnetic marker for the base of the anthropocene

loess stratigraphy using palaeomagnetism application to the poiana ciresului archaeological site romania abstract palaeomagnetic investigations were carried out at the archaeological site of poiana ciresului romania to check whether the loess records a reliable palaeomagnetic signal and to compare this signal to reference data the sediments are composed of loess and loess like sediments pedogenic alterations and a pronounced cultural layer occur in the section the archaeology bearing sedimentary geoarchive spans the time interval from ca 20 to ca 55 ka and was dated mainly by 14c and luminescence techniques the record of geomagnetic palaeosecular variation from this european loess site is discussed the presented palaeomagnetic record bears analogies to data from lac du bouchet france and is a reliable high resolution record mean sedimentation rates are ca 20 cm ky of palaeosecular variation the results imply that european loess geoarchives may record palaeosecular variation features even if loess is pedogenically altered the palaeosecular variation pattern of loess is here employed as a stratigraphic tool

geomagnetic field variations during the last 400 kyr in the western equatorial pacific paleointensity inclination correlation revisited 1 a paleomagnetic study was conducted on four piston cores newly obtained from the west caroline basin in the western equatorial pacific in order to investigate variations in paleointensity and inclination during the last 400 kyr an inclination intensity correlation was previously reported in this region using giant piston cores but the quality of the paleomagnetic data of the younger end the last ca 300 kyr was needed to be checked because the upper part of the giant piston cores could suffer from perturbation by oversampling age control is based on the oxygen isotope ratios for one core and inter core correlation using relative paleointensity for other cores stacked curves of paleointensity and inclination were constructed from the four cores it was confirmed that variations on the order of 104 to 105 years occur in inclination as well as paleointensity a cross correlation analysis showed that significant in phase correlation occurs between intensity and inclination for periods longer than about 25 kyr and power spectra of both paleointensity and inclination variations have peaks at 100 kyr periods the regional paleointensity stack with higher resolution than the sint 800 stack guyodo and valet 1999 should be useful for paleointensity assisted chronostratigraphy

basin wide magnetostratigraphic framework for the bighorn basin wyoming new paleomagnetic data from six different sections in the bighorn basin are combined with previously published results to construct a basin wide magnetostratigraphic framework geomagnetic polarity reversals between chrons c26r c26n c25r c25n c24r and c24n have been identifi ed among multiple stratigraphic sections in different parts of the basin using the new magnetostratigraphic framework paleontological paleobotanical and isotopic information from these varied locations in the basin can now be correlated and compared to similar records from elsewhere in the world these new data resolve previous uncertainty concerning the timing of an important episode of faunal turnover known as biohorizon b which is slightly below the chron c24r c24n boundary close to the position of the elmo isotope excursion in marine records backstripping analysis using these new magnetostratigraphic data helps defi ne the time transgressive onset of basin formation and shows the different subsidence histories of the northern and southern parts of the basin

the distinction between geomagnetic excursions and reversals summary two recent studies of the geomagnetic eld in the last 1 myr have found 14 excursions large changes in direction lasting 5 10 kyr each six of which are established as global phenomena by correlation between diierent sites the older picture of the geomagnetic eld enjoying long periods of stable polarity may not therefore be correct instead the eld appears to suier many dramatic changes in direction and concomitant reduction in intensity for 10 20 per cent of the time during excursions the eld may reverse in the liquid outer core which has timescales of 500 yr or less but not in the solid inner core where the eld must change by diiusion with a timescale of 3 kyr this timescale is consistent with the remarkably uniform duration of well dated excursions the disparity of dynamical timescales between the inner and outer cores a factor of 10 is consistent with the 10 excursions between full reversals

sawtooth variation in magnetic intensity profiles and delayed acquisition of magnetization in deep sea cores abstract recent records of normalised intensity of natural remanent magnetization nrm obtained from deep sea sediments document an asymmetrical saw tooth pattern associated with geomagnetic reversals this observation may reflect a fundamental aspect of the geodynamo time evolution or alternatively delay in acquisition of the nrm it is demonstrated here that a simple model in which a large fraction of the magnetic grains acquire nrm at deposition time while remaining grains reorientate or acquire magnetization after deposition is able to produce a saw tooth profile strikingly similar to that observed interestingly this mechanism of nrm acquisition does not damp out high frequency oscillations of the field such as those observed in the deep sea records exhibiting the saw tooth profile it provides an accurate record of field directions except for periods of very low field intensity

testing the geocentric axial dipole hypothesis using regional paleomagnetic intensity records from 0 to 300 ka absolute and relative geomagnetic paleointensity records reveal variations in geomagnetic dipole strength either via averaging time series of virtual axial dipole moments or through formal inversion strategies like the penalized maximum likelihood pml method used for the padm2m paleomagnetic axial dipole moment for 0 2 ma model however departures from the most basic geocentric axial dipole gad structure are obvious on centennial to millennial time scales and paleomagnetic records from igneous rocks suggest small deviations persist on million year time scales spatial variations in heat flow at the core mantle boundary inferred from large low shear velocity provinces llsvps are widely suspected to influence both the average geomagnetic field and its regional secular variation long term departures from a gad configuration should be visible from regional differences in paleointensity reconstructions we use a pml method to construct time varying models of regional axial dipole moment radms from a combined set of absolute and relative paleointensity data and compare results from the last 300 kyr radms are created from sediment records selected from specific latitude and longitude bands we also test whether grouping records lying above each of the 2 major llsvps centered on africa and the pacific produce radms that are distinct from those above regions lacking anomalous seismic structure systematic differences appear in the various regional results in the most recent part of the record regional differences are broadly similar to the holocene cals10k 1b time varying geomagnetic field model spanning 0 10 ka however lack of southern hemisphere records prevents direct confirmation of the hemispheric asymmetry present in cals10k 1b in both average virtual axial dipole moment and its variability as expected the 300 kyr radms exhibit greater overall temporal field variability than is seen over 0 10 ka average radm is higher in the pacific and in equatorial regions than in the atlantic and in mid high latitude northern hemisphere regions higher average radms are associated with lower overall field variability and less pronounced excursional signatures notably the lower variability in the pacific sector seen here defined by either longitude band or llsvp location suggests that the modern low paleosecular variation there extends over at least the past few hundred thousand years radms identified with llsvps show systematic deviations from the non llsvp group of records with distinct characteristics for the african and pacific provinces the african llsvp generates more pronounced radm minima associated with geomagnetic excursions and in general paleointensity decreases associated with excursions occur first in the atlantic longitude sector and over the african llsvp

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

mean mixed depth of sediments the wherefore and the why the activities and consequently the bioturbational effects of deposit feeding organisms are largely restricted to a narrow surficial zone of marine sediments with a worldwide environmentally invariant mean of 9 8 cm with a standard deviation of 4 5 cm currently available theories of infaunal behavior cannot predict quantitatively this observation a new simple model that accounts for the feedback between resource food abundance its reactivity and the intensity of bioturbation leads to a quantitative estimate of 9 7 cm this model constitutes a fundamental advance in our understanding bioturbation

paleosecular variations of the geomagnetic field during the holocene from eastern asia abstract high resolution paleomagnetic secular variation psv records bear great information of dynamics processes of the earth s geomagnetic field and can be further used for inter profile correlation and for dating sediments however effects of changes in the depositional environment on psv records have not been fully determined this study constructed holocene psv records for the gravity piston core zsqd34 obtained from the northern south china sea rock magnetic and transmission electron microscope tem results indicate that single sd and psd domain magnetites are the main carrier of the natural remanent magnetization comparable to the records derived from the freshwater lakes and the modeling results we observed that direction curves from these two environments of contrasting salinity content are rather consistent the direction curves are independent of the constructed salinity however the gradual increasing trend of relative intensity since about 5 kyr might be related to the decreasing sea surface salinity furthermore on the centennial and millennial time scale the relative intensity and salinity show some positive relation suggesting a potential contribution of salinity to the paleomagnetic relative intensity recording processes

multiple 10be records revealing the history of cosmic ray variations across the iceland basin excursion abstract cosmogenic 10be is a proxy of cosmic ray flux and its natural records provide vital information about the past intensity variability of the geomagnetic field and solar activity 10be records also serve as powerful tools for global synchronization among a variety of paleoarchives and for elucidating sedimentary processes on natural remanent magnetization acquisition however high resolution multi decadal to multi centennial records of 10be are scarce especially those older than several tens of thousands of years here we present multiple high resolution 10be records of the iceland basin geomagnetic excursion interval ca 170 200 kyr ago obtained from sediment cores authigenic 10be 9be ratio and an ice core atmospheric 10be flux comparing sedimentary 10be records with relative paleointensity from the same cores we found differences in the magnetic lock in depth even between adjacent cores the 10be proxy records from the sediment and ice cores exhibit common characteristics an asymmetric large scale variation a 7 kyr quasi plateau around the maximum with a characteristic mid term depression and multi millennial fluctuations in cosmic ray flux during this interval minimal synchronized and stacked 10be records show that maximum cosmic ray flux occurred 188 5 190 0 kyr ago and was double the present flux a wavelet analysis of the stacked curve reveals dominant 4 kyr and secondary 8 kyr periodicities both of which can be interpreted as intrinsic geomagnetic cycles the wavelet spectrum of the high resolution ice core record shows a periodicity of 1 7 kyr and somewhat intermingled multi centennial cycles around the maxima of 10be which likely represent solar cycles in this period high resolution 10be records from multiple paleoarchives provide both a robust proxy record of cosmic ray flux and a valuable tool for detailed global synchronization based on cosmic ray variations

rapid directional changes associated with a 6 5 kyr long blake geomagnetic excursion at the blake bahama outer ridge geomagnetic excursions are recognised as intrinsic features of the earth s magnetic field high resolution records of field behaviour captured in marine sedimentary cores present an opportunity to determine the temporal and geometric character of the field during geomagnetic excursions and provide constraints on the mechanisms producing field variability we present here the highest resolution record yet published of the blake geomagnetic excursion 125 ka measured in three cores from ocean drilling program odp site 1062 on the blake bahama outer ridge the blake excursion has a controversial structure and timing but these cores have a sufficiently high sedimentation rate 10 cm ka to allow detailed reconstruction of the field behaviour at this site during the excursion palaeomagnetic measurements of the cores reveal rapid transitions 230th excess measurements to assess variations in the sedimentation rates through the sections of interest this provides an age and duration for the blake excursion with greater accuracy and with constrained uncertainty we date the directional excursion as falling between 129 and 122 ka with a duration for the deviation of 6 5 1 3 kyr the long duration of this interval and the fully reversed field suggest the existence of a pseudo stable reversed dipole field component during the excursion and challenge the idea that excursions are always of short duration

post depositional remanent magnetization lock in depth in precisely dated varved sediments assessed by archaeomagnetic field models accurate and precise chronologies are needed to evaluate the existence and effect of a post depositional remanent magnetization lock in process on sedimentary palaeomagnetic records here we present lock in modelling results of two palaeomagnetic records from varved lake sediments lakes kalksjon and gyltigesjon in sweden by using model predictions based on archaeomagnetic data we used the c 14 wiggle match dating technique to improve the precision of the kalksjon varve chronology in the period between 3000 and 2000 cal bp which is characterized by pronounced palaeomagnetic secular variation this method allowed us to infer an age model with uncertainties of 20 years 95 4 probability range furthermore we compared the palaeomagnetic record of kalksjon to gyltigesjon which has a corresponding 14c wiggle matched chronology the ages of palaeomagnetic features derived from the wiggle matched varve chronologies are older than those predicted by the archaeomagnetic models lock in modelling was performed with different filter functions to explain the temporal offset and the amplitude of the lake sediment palaeomagnetic data the analyses suggest that a linear lock in function with lock in depths the depth below which no more natural magnetic remanence is acquired that range between 30 and 80 cm in kalksjon and 50 and 160 cm in gyltigesjon are most appropriate to explain the data these relatively deep lock in depths in sediments without a bioturbated mixed zone can be attributed to the relatively high organic contents and low density of the lake sediments which contribute to a thick unconsolidated upper zone of the sediment sequence in which re alignment of magnetic particles can take place c 2014 elsevier b v all rights reserved

geomagnetic secular variation as recorded in british lake sediments and its application to archaeomagnetic studies abstract lake sediments can play an important role in understanding and reconstructing temporal characteristics of the geomagnetic field as they potentially offer near continuous high resolution archives of magnetic information extending throughout the holocene to date lake sediment geomagnetic data has typically been excluded from british archaeomagnetic secular variation curves svc due to uncertainties with the acquisition of magnetisation by lake sediments this paper presents the argument that with regards to british datasets the real problem lies with poor chronological control and sets out to illustrate that with british archaeomagnetic data some progress has been made the results indicate that it is not currently possible to resolve secular variation on a time scale of 100 years from published british lake sediment data but more success has been made with data from archaeological sediments this level of detail has been considered necessary for the incorporation of lake sediment data into the british archaeomagnetic dataset as the ability to resolve short term geomagnetic changes is critical for the integration of any dataset into the british svc as the latter is predominantly employed to date archaeological architecture and artefacts requires that it has the ability to resolve changes over timescales relevant to human lifetimes using currently available data this retrospective critique confirms that in archaeological sediments depositional and thermoremanent magnetisation can record the same direction over the same time interval

the role of magnetostatic interactions in sediment suspensions summary the processes that influence a detrital remanent magnetization as well as the physical microscale factors that control formation of a stable post depositional remanent magnetization are still not fully understood previous laboratory studies and statistical numerical approaches have shown the possibility that sediment suspensions can display complex magnetization phenomena such behaviour has been attributed to the effect of magnetostatic interactions in the suspension which could provide one explanation for spurious magnetizations observed in marine sediment cores in laboratory experiments we investigated magnetization decay as a function of time in sediment suspensions produced with varying lithologies and particle concentrations a companion model takes into account the physics of magnetic particle particle interactions brownian motion and hydrodynamic forces to investigate numerically the magnetization behaviour of sediment suspensions when combined the experiments and the numerical models reveal a weak effect of magnetostatic interactions in the natural sediment suspensions which is expressed as an increase in the magnetization decay rate in addition a calculation of effective particle size based on the response of each suspension to brownian motion indicates that the majority of the sedimentary magnetic particles are attached to larger clay particles

the iceland basin excursion age duration and excursion field geometry the iceland basin geomagnetic excursion coincided with the marine isotope stage mis 6 7 boundary the age and duration of the excursion at seven north atlantic sites with sufficient isotope data are estimated by matching marine isotope stage mis 7a 7c to a calibrated template two criteria for defining the excursion virtual geomagnetic pole vgp latitudes 0 and 40 n yield excursion durations of 1 4 and 2 5 kyr respectively the midpoints of the excursion are in the 189 192 ka range with a mean of 190 2 ka although component magnetization directions are generally well defined rapid changes in field direction during a time of low field intensity are not adequately recorded during the excursion vgps transit southward over africa and the south atlantic reach high southern latitudes at the culmination of the excursion with partial recovery in relative paleointensity rpi and then track northward through the western pacific the high southern latitude vgps and the recovery in rpi imply that the earth s main axial dipole reversed polarity during the excursion if only for 1 kyr implying that excursions can be manifested globally and are important in millennial scale stratigraphy vgp clustering in the south atlantic and nw pacific roughly coincide with maxima in the vertical downward component of the modern nondipole nd field determined at the earth s surface which implies that the nd field became dominant as the geocentric dipole field weakened during the excursion and also that the nd field configuration is long lived on multimillennial timescales

magnetic record of deglaciation using forc pca sortable silt grain size and magnetic excursion at 26 ka from the rockall trough ne atlantic research supported by us nsf grants 0850413 and 1014506 and the european research council under the european union s seventh framework programme fp 2007 2013 erc grant agreement no 320750 the uk nerc and bgs funded the recovery of core md04 2822

synthetic and sedimentary records of geomagnetic excursions the geomagnetic excursion recorded in the sediments of lac st front massif central france is characterized by shallow and negative inclinations followed by a younger steep inclination interval vlag et al 1996 in the corresponding interval of the nearby lac du bouchet only steep inclinations are found sedimentary records of the mono lake excursion show similar inclination patterns complete records of this excursion show a succession of a shallow by a steep inclination interval while incomplete records only show only steep inclinations coe and liddicoat 1994 due to a non instantaneous acquisition of the remanence sedimentary records reflect only a smoothed geomagnetic signal it will be shown that smoothing of a small low intensity reversed interval embedded in a non antipodal normal field interval may result in records of a shallow inclination interval followed by a steep inclination interval while further smoothing results in only steep inclinations realignment of magnetic grains by the stronger normal field can also produce such records and may explain why such an unusual large lock in depth is required by the conventional smoothing model whatever the mechanism the similarities between these synthetic records and the excursional records of lac st front lac du bouchet and mono lake suggest that the latter are more or less affected by vector addition of two non antipodal directions

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

time variations in geomagnetic intensity 1 after many years spent by paleomagnetists studying the directional behavior of the earth s magnetic field at all possible timescales detailed measurements of field intensity are now needed to document the variations of the entire vector and to analyze the time evolution of the field components a significant step has been achieved by combining intensity records derived from archeological materials and from lava flows in order to extract the global field changes over the past 12 kyr a second significant step was due to the emergence of coherent records of relative paleointensity using the remanent magnetization of sediments to retrace the evolution of the dipole field a third step was the juxtaposition of these signals with those derived from cosmogenic isotopes contemporaneous with the acquisition of records new techniques have been developed to constrain the geomagnetic origin of the signals much activity has also been devoted to improving the quality of determinations of absolute paleointensity from volcanic rocks with new materials proper selection of samples and investigations of complex changes in magnetization during laboratory experiments altogether these developments brought us from a situation where the field changes were restricted to the past 40 kyr to the emergence of a coherent picture of the changes in the geomagnetic dipole moment for at least the past 1 myr on longer timescales the field variability and its average behavior is relatively well documented for the past 400 myr section 3 gives a summary of most methods and techniques that are presently used to track the field intensity changes in the past in each case current limits and potential promises are discussed the section 4 describes the field variations measured so far over various timescales covered by the archeomagnetic and the paleomagnetic records preference has always been given to composite records and databases in order to extract and discuss major and global geomagnetic features special attention has been devoted to discussing the degree of confidence to be put in the data by considering the integration of multiple data sets involving different techniques and or materials

a magnetostratigraphic reassessment of correlation between chinese loess and marine oxygen isotope records over the last 1 1 ma abstract we present a magnetic polarity based paleoclimatic correlation between chinese loess and marine oxygen isotope stage mis records over the last 1 1 ma this revised land ocean comparison indicates that inconsistent occurrence of the matuyama brunhes boundary which lies in mis 19 in marine sediments and occurs either at the base lower part of loess l8 or the uppermost part of soil s8 in chinese loess is due to regional and or local climate variability across the chinese loess plateau and does not need to be attributed to lock in effects of loess remanence acquisition associated with pedogenesis this result implies that the geomagnetic polarity boundaries in chinese loess in general are reliable chronostratigraphic time markers therefore astronomically tuned timescales for chinese loess that presume a downward shift of polarity boundaries due to a time lag in remanence acquisition may have overestimated ages of certain loess paleosol units

palaeosecular variation over the past 5 myr based on a new generalized database summary a generalized database of global palaeomagnetic data from 3719 lava flows and thin dykes of age 0 5 ma has been constructed for use with a relational database management system the database includes all data whose virtual geomagnetic poles vgp lie within 45 of the spin axis and can be used for studies of palaeosecular variation and for geomagnetic field modelling because many of these data were collected and processed more than 15 20 years ago each result has been characterized according to the demagnetization procedures carried out analysis of these data in terms of the latitude variation of the angular dispersion of vgps palaeosecular variation from lavas strongly suggests that careful data selection is required and that many of the older studies may need to be redone using more modern methods differences between the angular dispersions for separate normal and reverse polarity data sets confirm that many older studies have not been adequately cleaned magnetically therefore the use of the database for geomagnetic field modelling should be carried out with some caution using a vgp cut off angle that varies with latitude the best data set consists of 2636 results that show a smooth increase of vgp angular dispersion with latitude model g for palaeosecular variation which is based on modelling of the antisymmetric dipole and symmetric quadrupole dynamo families provides a good fit to these results

chemical magnetization in chinese loess the relationship between the intensity of characteristic remanent magnetization chrm and pedogenesis is examined in the luochuan section the results show a consistent link between the intensity values of remanent magnetization after every thermal demagnetization step and magnetic susceptiblity and other indexes of pedogenesis moreover the intensity of chrm is also much higher in paleosols than in loess these results suggest that chemical remanent magnetization crm is the main mode of nrm acquisition in paleosols and is linked to pedogenesis pedogenic processes overprint the magnetic signal to a certain depth in the underlying loess so the interpretation of the high resolution paleomagnetic records from paleosols requires a degree of caution

a magnetization process of sediments laboratory experiments on post depositional remanent magnetization summary palaeomagnetic studies require a theory of magnetization mechanism of sediments and a method of estimating magnetic field intensity from their remanences this paper establishes a physical basis for the generation of the remanence in deep sea and lake sediments experimentally redeposition experiments have been carried out under centrifugal force in weak magnetic fields the centrifuging method produces post depositional remanent magnetization post drm in the compacted sediment and its remanence and susceptibility are compatible with those of natural sediments and reconstituted materials of other redepositional experiments three properties of the post drm have been deduced from the experiments 1 the efficiency of acquisition of post drm decreases with increase in density during the compaction process 2 the total post drm is equal to the sum of the partial post drm addition law and 3 time is not a substantial factor for alignment of the magnetic particles these results lead to the conclusions that the magnetic particles do not rotate steadily but in a series of steps and that the density change is the crucial factor giving rise to the post drm a mathematical formula representing the remanence record in sediments is proposed on the basis of the experimental results and the model the principal equation is expressed as an integral of the product of three parameters over time when sediments have been compacted the field intensity variation characteristic function of the sediment and the time derivative of the density change

detrital remanent magnetization viscosity theory of the lock in zone post depositional remanent magnetization pdrm becomes permanent within a lock in zone whose apparent time delay depth and time span thickness are related to the characteristic time constants of compaction and magnetic grain rotation two simple models of the pdrm lock in zone are described using yaskawa s idea of effective sedimentary viscosity and mooney s empirical relation for the viscosity of a concentrated suspension of spheres the impulse response of the lock in zone determines the fidelity of the paleomagnetic recording

improvements in long core measurement techniques applications in palaeomagnetism and palaeoceanography we report on new developments in the long core measurement techniques using u channels in connection with new small access cryogenic magnetometers with these new instruments u channel samples allow us to measure a wide range of magnetic parameters along entire core length with a spatial resolution and accuracy virtually equivalent to that obtained using discrete cubic samples an additional advantage is that the sediment disturbance when sampling is significantly less when using u channels we present comparisons of results from both continuous and discrete measurements and examine the effectiveness of minimal deconvolution using various techniques examples of continuous measurements of low field susceptibility nrm arm irm hrc from north atlantic deep sea cores show that it is possible to generate logs of combination of parameters related to rock magnetic characteristics of the sediments useful in palaeoceanography in particular the continuous method allows us to rapidly detect core regions with fast changing magnetic parameters useful for correlation between cores and also for understanding changes in palaeo environmental conditions the fast scan also allows us to detect regions of the core where the rock magnetic parameters are suitable or not suitable for palaeomagnetic studies related to changes in the geomagnetic field such as the relative changes in palaeointensity

astronomical calibration of the matuyama brunhes boundary consequences for magnetic remanence acquisition in marine carbonates and the asian loess sequences we have compiled 19 records from marine carbonate cores in which the matuyama brunhes boundary mbb has been reasonably well constrained within the astronomically forced stratigraphic framework using oxygen isotopes by correlation of the 18o data to a timescale based on astronomical forcing we estimate astronomical ages for each of the mbb horizons in all but one record the mbb occurs within stage 19 most magnetostratigraphic sections in asian loess place the mbb within a loess interval since loess deposition is presumed to be associated with glacial intervals loess horizons should correspond to even numbered oxygen isotope stages a glacial age for the mbb is at odds with the results presented here which firmly place the mbb within interglacial stage 19 inconsistency among the many loess sections and between the loess and the marine records suggests that the magnetic interpretation of loess sections may be more complicated than hitherto supposed the mean of the stage 19 age estimates for the mbb is 777 9 1 8 n 18 inclusion of the single stage 20 age results in a mean of 778 8 2 5 n 19 the astronomical age estimate of the mbb compares favorably with an unweighted mean of 778 2 3 5 n 10 from a compilation of 40ar 39ar results of transitional lava flows combining the two independent data sets yields a grand mean of 778 0 1 7 n 28 the new compilation shows virtually no trend in placement of the mbb within isotope stage 19 as a function of sediment accumulation rate we interpret this to mean that the average depth of remanence acquisition is within a few centimeters of the sediment water interface separating the cores into two geographic regions an indo pacific caribbean ipc group and an atlantic group results in a significant difference in the position of the mid point of the reversal with respect to the astronomical time scale the data presented here suggest a difference of several thousand years between the two regions this observation could be caused by systematic differences between the two regions in sedimentation rate within the interval of interest systematic differences in remanence acquisition or by genuine differences in the timing of the directional changes between the two regions

a lock in model for the complex matuyama brunhes boundary record of the loess palaeosol sequence at lingtai central chinese loess plateau summary in most marine sedimentary records the matuyama brunhes boundary mbb has been found in interglacial oxygen isotope stage 19 in the magnetostratigraphic records of most chinese loess palaeosol profiles the mbb is located in loess layer l8 which was deposited during a glacial period the mbb at lingtai central chinese loess plateau also occurs in l8 and is characterized by multiple polarity flips the natural remanent magnetization is mainly carried by two coexisting components the higher coercivity harder component dominates in loess layers and is thought to be of detrital origin the lower coercivity softer component prevails in palaeosols and was most probably formed in situ by bio chemical processes a lock in model for the lingtai mbb record has been developed by extending the lithologically controlled pdrm model of bleil von dobeneck 1999 it assumes two lock in zones the nrm of the magnetically harder component is physically locked by consolidation shortly after loess deposition whereas the softer component is formed at greater depth by pedogenesis and acquires a chemical remanent magnetization of younger age at polarity boundaries grains carrying reversed and normal directions may therefore occur together within a single horizon the model uses arm coercivity spectra to estimate the relative contributions of the two components it is able to explain the observed rapid multiple polarity flips and low magnetization intensities as well as the stratigraphic shift of the lingtai mbb with respect to the marine records

is burial velocity a master parameter for bioturbation abstract the intensity of biological mixing in a marine sediment is widely believed to be related to the flux of labile organic matter at the same time this flux has been shown to be strongly correlated to the burial velocity one would then expect a significant correlation between biological mixing coefficient d b and burial velocity a nonlinear regression of existing data with a power law eqns 2 and 3 of the text produces a statistically significant correlation the statistical significance of the regression in no way implies causation and in fact it accounts for only about 22 of the total variance in the dataset thus reducing its physical significance a number of additional processes contribute to the large observed variance including timing of mixing events temporal and spatial heterogeneity differences in temporal scales sampling artifacts and specific particle associations with due care the d b relationship may prove useful in defining the likely range of d b values in a particular environment a similar analysis of the tracer identified surface mixed depth shows that it is essentially independent of burial velocity with a worldwide mean of 9 8 4 5 cm depth of mixing is probably more closely related to the energetic costs of deep burrowing

the importance of independent chronology in integrating records of past climate change for the 60 8 ka intimate time interval this paper provides a brief overview of the most common dating techniques applied in palaeoclimate and palaeoenvironmental studies including four radiometric and isotopic dating methods radiocarbon 230th disequilibrium luminescence cosmogenic nuclides and two incremental methods based on layer counting ice layer varves for each method concise background information about the fundamental principles and methodological approaches is provided we concentrate on the time interval of focus for the intimate integrating ice core marine and terrestrial records community 60 8 ka this dating guide addresses palaeoclimatologists who aim at interpretation of their often regional and local proxy time series in a wider spatial context and therefore have to rely on correlation with proxy records obtained from different archives from various regions for this reason we especially emphasise scientific approaches for harmonising chronologies for sophisticated and robust proxy data integration in this respect up to date age modelling techniques are presented as well as tools for linking records by age equivalence including tephrochronology cosmogenic 10be and palaeomagnetic variations finally to avoid inadequate documentation of chronologies and assure reliable correlation of proxy time series this paper provides recommendations for minimum standards of uncertainty and age datum reporting

geomagnetic paleointensity and directional secular variation at ocean drilling program odp site 984 bjorn drift since 500 ka comparisons with odp site 983 gardar drift normalized remanence relative geomagnetic paleointensity records for the last 500 kyr can be matched between two iceland basin ocean drilling program odp sites 984 and 983 and correlated with other high resolution records directional secular variation is however not easily correlated between sites due to inadequate recording at these mean sedimentation rates 12 15 cm kyr of the characteristic high frequency variability of the directional record both sites record the iceland basin event at 186 189 ka in which the characteristic magnetization component rotates through 180 and back coincident with a paleointensity low that lasted about 3 kyr other brunhes chron geomagnetic excursions appear to be manifest as intervals of higher amplitude secular variation during lows in paleointensity there is a tendency for paleointensity lows to correlate with peak interglacials in the oxygen isotope records however this does not translate into a correlation between paleointensity and percent carbonate or between paleointensity and magnetic concentration or grain size parameters

timing and lock in effect of the laschamp geomagnetic excursion in chinese loess 1 the laschamp geomagnetic excursion lge at 41 ka is a critical age control for hemispherical comparison of paleoclimate records from various kinds of geological archives e g ice core marine and lake sediments the timing of the lge in chinese loess however remains poorly constrained due to the lack of a reliable chronology and the complex acquisition processes of natural remanent magnetization here we systematically compare the paleomagnetic results of three optically stimulated luminescence dated loess sequences on the chinese loess plateau our results indicate that the timing of the lge in the northwestern loess plateau is slightly older than the absolute radiometric age determination and the timing inferred from greenland ice core 10be flux and marine i e relative paleointensity and authogenic 10be 9be stack records but younger than the counterparts in the central loess plateau we attribute the different timing of the lge in the three loess sections to a progressive southeastward increase in the lock in depth caused by the combined effects of postdepositional processes e g surface mixing chemical weathering and lock in delay on the remanence acquisition process we conclude that caution is needed to use the lge in chinese loess as a reliable tie event for high resolution chronological correlation to marine and ice core records only if the potential lock in effect can be precisely determined

an ice core perspective on the age of the matuyama brunhes boundary abstract two intervals of enhanced 10be flux thought to be associated with periods of low dipole intensity and identified as the matuyama brunhes transition and a precursor event have been observed in the bottom section of the epica dome c ice core the peaks span 764 776 ka and 788 798 ka on the new edc3 chronology with a stated absolute age uncertainty of 6 ka 2 this chronology uses orbital tuning of atmospheric oxygen 18 18oatm to correct for anomalies in ice flow in the bottom 500 m of the core an additional 28 18oatm data points have been measured to improve resolution and verify the accuracy of the tuning and the stated timescale uncertainty both the dating of the increased 10be and that relative to climatic records are compared to paleointensity records found in orbitally tuned marine sediments the mid point of the 10be peak associated with the m b is approximately 10 ka younger than the age determined radioisotopically from lavas with transitional orientations taking into account recent revisions to the 40ar 39ar dating standard and improved precision climatic constraints on the edc3 agescale make an error of this magnitude in the ice chronology implausible this age difference however is consistent with recent modeling suggesting that directional changes are spatially asynchronous and may precede the dipole intensity minimum in some locations although formally less precise than the published age from astrochronologically dated marine sediments ice core ages are potentially more accurate because they are not subject to lock in depth uncertainties

repeating waveform initiated by a 180 190 ka geomagnetic excursion in western north america implications for field behavior during polarity transitions and subsequent secular variation new paleomagnetic lithologic and stratigraphic data are presented from the sediments of lake chewaucan in the summer lake basin oregon the new data place better age constraints on the sediments and improve the accuracy of the previously published paleomagnetic record from this locality a complex yet distinct waveform is observed in all three components of the paleomagnetic vector the waveform begins as the 180 190 ka pringle falls long valley summer lake ii geomagnetic excursion and continues for two cycles after the excursion until the record is interrupted by an unconformity that we correlate to the oxygen isotope stage 6 5e boundary the waveform s directional morphology in virtual geomagnetic pole vgp space is defined by two clockwise loops followed by a distinctive counterclockwise clockwise counterclockwise looping sequence the vgp paths of the two cycles after the excursion are rotated 180 about earth s spin axis with respect to the vgp paths of the excursion cycle the waveform also consists of a relative paleointensity variation which repeats during the two cycles after the excursion the average paleointensity of the postexcursion waveform repetitions is high relative to the extremely low values that occur during the excursion this observation indicates that excursion initiated secular variations can occur after the field fully recovers from the low intensities which commonly typify excursions because of the similarities noted previously between this excursion and full polarity transitions trie et al 1991 our new observations constrain models for a wide range of field behavior including polarity transitions excursions and secular variation

measurement of the degree of smoothing in sediment paleomagnetic secular variation records an example from late quaternary deep sea sediments of the bermuda rise western north atlantic ocean abstract replicate paleomagnetic secular variation psv records for the last 15 000 yr have been recovered from two deep sea sediment gravity cores on the bermuda rise western north atlantic ocean the records have been correlated using several different parameters and dated by correlation to a nearby piston core which has a high resolution 14 c chronology the psv records are systematically subdued with respect to the expected local magnetic field variability during the holocene when sedimentation rates were about 10 cm kyr during the late pleistocene when sedimentation rates were a factor of 2 3 higher the psv variability was significantly larger we attribute this to smoothing of the psv records by a depositional post depositional remanence drm pdrm acquisition process with a 10 20 cm lock in interval the degree of psv smoothing was evaluated by comparing the vgp angular dispersion and vector spectral content of the bermuda rise cores with high resolution psv records from north america and great britain these results suggest a 50 reduction in the drm pdrm recording of vector variability during the holocene on the bermuda rise with most of the psv reduction occurring for features of less than 2000 yr in duration we can reproduce the subdued psv pattern with a mathematical model that simulates the site drm pdrm remanence acquisition process finally the replicate gravity cores document that psv records may be reproducible on a local scale even though they only retain a low pass filtered record of true magnetic field variability

two geomagnetic excursions during the brunhes chron recorded in chinese loess palaeosol sediments summary detailed palaeomagnetic integrated with rock magnetic studies have been carried out on a loess palaeosol sequence in baoji shaanxi province southern chinese loess plateau for most samples stepwise thermal demagnetization revealed two well defined magnetization components a low temperature component ltc which was isolated between 100 and 200 c is consistent with the present geomagnetic field direction a high temperature component htc which was isolated between 250 and 620 680 c shows normal reversed or transitional polarities our new magnetostatigraphy revealed two distinct geomagnetic excursions recorded in loess unit of l5 and palaeosol unit of s7 respectively and the matuyama brunhes m b polarity boundary in loess unit of l8 rock magnetic experiments demonstrated that the specimens from the excursion zones have the same magnetic properties as those from the brunhes normal or matuyama reversed polarity zones measurements of anisotropy of magnetic susceptibility ams showed that the sediments have primary sedimentary fabrics based on the palaeoclimatological and magnetostratigraphic age models the middle brunhes excursion in loess l5 is dated at 413 433 ka and the early brunhes excursion is estimated to occur 23 33 ka after the m b reversal comparing with previously reported geomagnetic excursions in the brunhes chron the middle brunhes excursion l5 is likely global for the early brunhes excursion s7 we need further studies to examine its global occurrence

holocene periodicity in north atlantic climate and deep ocean flow south of iceland climate fluctuations during the past millennium are relatively well documented1 on a longer timescale there is growing evidence of millennial scale variability of holocene climate at periodicities of 2 500 and 950 years possibly caused by changes in solar flux 2 3 and 1 500 years maybe related to an internal oscillation of the climate system 4 5 6 but the involvement of deep water masses in these holocene climate changes has yet to be established here we use sediment grain size data from the iceland basin to reconstruct past changes in the speed of deep water flow the study site is under the influence of iceland scotland overflow water isow the flow of which is an important component of the thermohaline circulation that modulates european climate flow changes coincide with some known climate events the little ice age and the mediaeval warm period and extend over the entire holocene epoch with aquasi periodicity of 1 500 years the grain size data indicate afaster isow flow when the climate of northern europe is warmer however a second mode of operation is observed in the early holocene when warm climate intervals are associated with slower isow flow at that time the melting remnant of land based glacial age ice may have provided a sufficient source of fresh water to the ocean to reduce isow flow south of iceland

integrated stratigraphic reconstruction for the last 80 kyr in a deep sector of the sardinia channel western mediterranean abstract a quantitative analysis of planktonic foraminifera coupled with petrophysical and paleomagnetic measurements and 14 c ams calibrations was carried out on a deep core recovered in the sardinia channel western mediterranean sea during the ciesm sub2 survey providing an integrated stratigraphic time framework over the last 80 kyr significant changes in the quantitative distribution of planktonic foraminifera allowed the identification of several eco bioevents useful to accurately mark the boundaries of the eco biozones widely recognized in the western mediterranean records and used for large scale correlations namely 10 eco biozones were identified based on the relative abundance of selected climate sensitive planktonic foraminiferal species sixteen codified eco bioevents were correlated with alboran sea planktonic foraminiferal data and several climatic global events sapropel s1 younger dryas greenland isotope interstadial 1 greenland isotope stadial 2 heinrich event h1 h6 were recognized the eco bioevents together with the 14 c ams calibrations allowed us to define an accurate age model spanning between 2 and 83 kyr the reliability of the age model was confirmed by comparing the colour reflectance 550 nm data of the studied record with the astronomically tuned record from the ionian sea odp site 964 a mean sedimentation rate of about 7 cm kyr included three turbidite event beds that were chronologically constrained within the relative low stand and regressive sea level phases of mis 4 and 3 the deep sea sedimentary record includes a distinct tephra occurring at the base of the core that dates 78 ka cal bp the paleomagnetic data provide a well defined record of the characteristic remanent magnetization that may be used to reconstruct the geomagnetic paleosecular variation for the mediterranean back to 83 kyr

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

dynamics of the earth magnetic field in the 10 75 kyr period comprising the laschamp and mono lake excursions new results from the french chaine des puys in a global perspective abstract we report here on a new paleomagnetic directions and intensities and coupled k ar and 40 ar 39 ar analysis of 35 different flows emplaced in the chaine des puys during the 75 to 10 kyr interval which contains the mono lake and laschamp excursions there is a remarkable agreement between the new set of absolute volcanic intensities and published sedimentary glopis 75 and cosmogenic 10 be and 36 cl records the laschamp and mono lake excursions are clearly revealed by a very significant intensity drop at 41 2 1 6 ka and 34 2 1 2 ka respectively the duration of the laschamp excursion is 1500 yr and about 640 yr when the drop of paleointensity or the directional change are considered respectively the intensity drop at the mono lake is twice as short in the 7 ka interval separating the two excursions the field intensity recovers to almost non transitional values the rate of decrease of the field intensity during these excursions attains 18 nt yr for the laschamp and even greater value 33 nt yr for the mono lake this figure is for the laschamp excursion similar to the present field intensity decrease in the last two centuries so that one may wonder whether such a high rate of change may be characteristic of an impending geomagnetic event reversal or excursion we suggest that the name auckland excursion should be used for the present day called mono lake

paleomagnetic full vector record of four consecutive mid miocene geomagnetic reversals abstract seventy mid miocene lava flows from flood basalt piles near neskaupstadur east iceland were sampled which provide a quasi continuous record of geomagnetic field variations samples were collected along the profile b of watkins and walker watkins n walker g p l 1977 magnetostratigraphy of eastern iceland am j sci 277 513 584 which was extended about 250 m farther down in a neighboring stream bed published radiometric age determinations harrison c mcdougall i watkins n 1979 a geomagnetic field reversal time scale back to 13 0 million years before present earth planet sci lett 42 143 152 range from 12 2 to 12 8 ma for the sampled sequence four reversals were recorded in this profile with 18 transitional lavas found within or between 17 normal and 30 reversed polarity flows the large amount of transitional lavas and the large virtual geomagnetic pole dispersion for stable field directions are noteworthy as such features are commonly observed in icelandic lavas and manifest in a far sidedness of the average vgp the reason for this characteristic which could be related to an anomaly beneath iceland a global field phenomenon local tectonics and or non horizontal flow emplacement is scrutinized non horizontal flow emplacement is likely in volcanic environments particularly if the sampled lavas are located on the paleoslopes of a central volcano from the difference of the observed paleomagnetic mean directions to the expected directions assuming a geocentric axial dipole gad a paleoslope which would explain the observed difference was calculated numerically the obtained dip and dip direction point consistently to a possible volcanic extrusion center of the lavas the determined paleodip however proved to be significantly too high compared to the usual slope of a central volcano suggesting further reasons for deviations from the gad other datasets of this age from europe also show enhanced vgp dispersion suggesting further contributions of geomagnetic origin for this observation basically all reversal paths move across the pacific transitions were identified as belonging to c5an 1r c5ar 3r based on the astronomically tuned neogene timescale lourens l hilgen f j laskar j shackleton n j wilson d 2004 a geological time scale cambridge university press we selected 122 samples for paleointensity measurements using a modified thellier method including tests for alteration and multidomain bias 85 of the measured samples yielded data of sufficient quality to calculate paleointensities for 26 lava flows the average paleointensity for stable field directions was 23 3 t whereas the intensity drops to a minimum of 5 8 t during field transitions the stable field intensities represent only about half of the present day field the saw tooth pattern of intensities which is characterized by a sharp increase of intensity directly after a reversal and then followed by a gradual decrease towards the next reversal was not found in this study

cosmogenic signature of geomagnetic reversals and excursions from the reunion event to the matuyama brunhes transition 0 7 2 14 ma interval long term variations of the geomagnetic dipole moment gdm during periods of stable polarity and in transitional states reversals and excursions provide key information for understanding the geodynamo regime following several studies dealing with the brunhes chron and the matuyama brunhes transition this study presents a new authigenic 10 be 9 be ratio be ratio record obtained from the md97 2143 core western pacific ocean this new be ratio series yields a record of gdm variations covering the early brunhes and mid to late matuyama time period i e 700 2140 ka independently from the relative paleointensity rpi record obtained from the same core that can be compared with available rpi records and stacks stratigraphic offsets measured between the be ratio peaks and the corresponding rpi minima reach 2 to 14 cm they can be assigned to post detrital remanent magnetization pdrm effects leading to magnetization locking in delays varying from 2 to 12 ka in the studied core 10 be overproduction episodes triggered by geomagnetic dipole moment lows gdl linked to polarity reversals and excursions confirm the global control exerted by the gdm on cosmogenic radionuclides production a dipole moment reconstruction derived from the beryllium 10 bedimo was compiled and calibrated using absolute paleointensity data this independent record complements the available paleomagnetic rpi records permitting 1 to overcome the pdrm lock in offsets induced below the mixing layer 2 to confront and increase the robustness and precision of gdm reconstructions and 3 to better constrain the chronology of geomagnetic field instabilities during the mid to late matuyama chron our new 10 be derived inventory is fully compatible with the gdl series linked to geomagnetic polarity reversals and events matuyama brunhes transition jaramillo and olduvai subchron boundaries cobb mountain reunion and it strengthens the occurrence of several excursions kamikatsura santa rosa punaruu bjorn gilsa gardar that were until now reported from only sparse locations

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

late quaternary sediments from deep sea sediment drifts on the antarctic peninsula pacific margin climatic control on provenance of minerals 1 we present results of detailed paleomagnetic investigations on deep sea cores from sediment drifts located along the pacific continental margin of the antarctic peninsula high resolution magnetic measurements on u channel samples provide detailed age models for three cores collected from drift 7 which document an age of 122 ka for the oldest sediments recovered near the drift crest at site sed 07 and a high sedimentation rate 11 cm kyr at site sed 12 located close to the alexander channel system low and high temperature magnetic measurements in conjunction with microscopic and mineralogic observations from drifts 4 5 and 7 indicate that pseudosingle domain detrital titanomagnetite partially oxidized and with limited ti substitution is the dominant magnetic mineral in the drift sediments the titanomagnetite occurs in two magnetic forms 1 a low coercivity form similar to laboratory synthesized titanomagnetite and 2 a high coercivity form bcr 60 mt these two forms vary in amount and stratigraphic distribution across the drifts we did not find evidence for diagenetic magnetic iron sulfides as has been previously suggested for these drift deposits the observed change of magnetic mineralogy in sediments deposited during heinrich events on drift 7 appears to be related to warming periods which temporarily modified the normal glacial transport pathways of glaciogenic detritus to and along the continental rise and thus resulted in deposition of sediments with a different provenance understanding this sediment provenance delivery signature at a wider spatial scale should provide information about ice sheet dynamics in west antarctica over the last 100 kyr

a stochastic model for palaeomagnetic field variations summary regeneration of the earth s magnetic field by convection in the liquid core produces a broad spectrum of time variation relative palaeointensity measurements in marine sediments provide a detailed record over the past 2 myr but an explicit reconstruction of the underlying dynamics is not feasible a more practical alternative is to construct a stochastic model from estimates of the virtual axial dipole moment the deterministic part of the model drift term describes time averaged behaviour whereas the random part diffusion term characterizes complex interactions over convective timescales we recover estimates of the drift and diffusion terms from the sint2000 model of valet et al and the padm2m model of ziegler et al the results are used in numerical solutions of the fokker planck equation to predict statistical properties of the palaeomagnetic field including the average rates of magnetic reversals and excursions a physical interpretation of the stochastic model suggests that the timescale for adjustments in the axial dipole moment is set by the dipole decay time d w e obtain d 29kyr from the stochastic models which falls within the expected range for the earth s core we also predict the amplitude of convective fluctuations in the core and establish a physical connection to the rates of magnetic reversals and excursions chrons lasting longer than 10 myr are unlikely under present day conditions however long chrons become more likely if the diffusion term is reduced by a factor of 2 such a change is accomplished by reducing the velocity fluctuations in the core by a factor of 2 which could be attributed to a shift in the spatial pattern of heat flux from the core or a reduction in the total core heat flow

bioturbation and holocene sediment accumulation fluxes in the north east atlantic ocean benthic boundary layer experiment sites bioturbation and holocene sediment accumulation are quantified in the three experimental areas of the benthic boundary layer benbo programme by means of the natural radionuclides 14c and 210pb and the artificial radionuclides 137cs and 241am the holocene accumulation rates determined by the radiocarbon method are 4 4 and 6 5 cm kyr 1 at sites b rockall plateau 1100 m water depth and c feni drift 1925 m water depth respectively accumulation at site a situated between feni drift and porcupine bank at 3570 m water depth was interrupted by an erosional event in the mid holocene which removed 0 25 m or more of the uppermost sediment present at that time the estimated accumulation rate since that event is 2 1 cm kyr 1 different estimates of surficial bioturbation mixing depths at site b are returned by the 210pbexcess and 14c methods with the former indicating 10 cm and the latter unusually deep at 17 cm at site c 210pbexcess and the fallout radionuclides 137cs and 241am are present in two distinctly separated depth zones with the deepest mixing down to 15 cm similar to the 14c mixed layer depth this is ascribed to deep burrowing by sipunculid or echiuran worms at site c and similar deep mixing is inferred to be necessary at site b to produce the differences in mixed layer depths derived from the longer and shorter lived radionuclide profiles although the deep burrowing episodes must be rare 1 event per 102 yr the greater accumulation rate at site c compared with site b is produced by an enhanced flux of current driven clay and silt sized material this fine material both dilutes the caco3 content of the site c sediment and is responsible for the higher corg content observed at site c compared with site b unlike the site b and c sediments which are fine grained the coarse 63 125 m size fraction is the most abundant size class in the late holocene sediments at site a suggesting that the sediments at this location are winnowed

a reassessment of post depositional remanent magnetism preliminary experiments with natural sediments abstract realignment of magnetic grains below the sediment water interface is thought to impart a post depositional remanent magnetization pdrm however there is little convincing evidence in the published literature that pdrm is the dominant mechanism by which sediments become magnetized we report here preliminary results from two kinds of laboratory experiments designed to investigate whether post depositional reorientation of magnetic particles is likely to occur in nature in the first experiment we monitored changes in the magnetization of natural sediments in response to changing laboratory fields our results are inconsistent with post depositional reorientation of magnetic particles in a second experiment we put live worms in a multi core tube with the original sediment water interface intact remagnetization was only observed in samples taken from a mound of fecal pellets formed at the surface these pellets had been suspended by a worm and redeposited in the laboratory field the other samples which were not resuspended but nonetheless bioturbated by the worms showed no change in magnetization our preliminary results do not support the hypothesis that post depositional reorientation occurs in natural undisturbed sediments below the sediment water interface

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

high resolution paleomagnetic secular variations and relative paleointensity since the late pleistocene in southern south america abstract paleomagnetic inclination declination and relative paleointensity were reconstructed from the sediments of laguna potrok aike in the framework of the international continental scientific drilling program icdp potrok aike maar lake sediment archive drilling project pasado here we present the u channel based full vector paleomagnetic field reconstruction since 51 2 ka cal bp the relative paleointensity proxy rpi was built by normalising the natural remanent magnetisation with the anhysteretic remanent magnetisation using the average ratio at 4 demagnetisation steps part of the chrm interval nrm arm 10 40 mt a grain size influence on the rpi was removed using a correction based on the linear relationship between the rpi and the median destructive field of the natural remanent magnetisation mdf nrm the new record is compared with other lacustrine and marine records and stacks from the mid to high latitudes of the southern hemisphere revealing consistent millennial scale variability the identification of the laschamp and possibly the mono lake geomagnetic excursions and a direction swing possibly associated to the hilina pali excursion at 20 ka cal bp nonetheless a global scale comparison with other high resolution records located on the opposite side of the earth and with various dipole field references hint at a different behaviour of the geomagnetic field around southern south america at 46 ka cal bp

magnetostratigraphy of the qiliting section se china and its implication for geochronology of the red soil sequences in southern china summary the red soil sequences in southern and southeastern china provide important terrestrial archives of palaeoclimate and palaeoenvironment and are important sources of palaeolithic settlements and therefore early human occupation here we present detailed palaeomagnetic investigation coupled with mineral magnetic and petrographic analyses of a red soil sequence at qiliting southeastern china the sediments contain both strongly magnetic minerals such as maghemite and magnetite and weakly magnetic minerals such as haematite and possibly goethite maghemite and haematite were identified as the main carriers of the natural remnant magnetisations only haematite dominates the characteristic remnant magnetisations palaeomagnetic findings suggest that the qiliting sedimentary sequence recorded the early brunhes chron and the late matuyama chron including the jaramillo subchron our finding coupled with previously published magnetostratigraphy establishes that the red soil sequences in southern china began to accumulate no later than the jaramillo age i e 1 0 ma it is further suggested that the capabilities of recording magnetic polarity stratigraphy in chinese red soil sequences are variable between areas because of different climate regimes

magnetic paleointensity stratigraphy and high resolution quaternary geochronology successes and future challenges abstract magnetic paleointensity stratigraphy is used to detect variations in the strength of earth s ancient magnetic field paleointensity studies have demonstrated that a dominantly dipolar geomagnetic signal can be recorded in a globally coherent manner in different types of sediments and in non sedimentary archives including ice core records and marine magnetic anomaly profiles the dominantly dipolar nature of geomagnetic paleointensity variations provides a global geophysical signal that has come to be widely used to date quaternary sediments despite the many successful applications of paleointensity assisted chronology the mechanisms by which sediments become magnetized remain poorly understood and there is no satisfactory theoretical foundation for paleointensity estimation in this paper we outline past successes of sedimentary paleointensity analysis as well as remaining challenges that need to be addressed to place such work on a more secure theoretical and empirical foundation we illustrate how common concepts for explaining sedimentary remanence acquisition can give rise to centennial to millennial offsets between paleomagnetic and other signals which is a key limitation for using paleointensity signals for geochronology our approach is intended to help non specialists to better understand the legitimate uses and limitations of paleointensity stratigraphy while pointing to outstanding problems that require concerted specialist efforts to resolve

the crust and upper mantle discontinuity structure beneath alaska inferred from receiver functions in this study three receiver function stacking methods are used to study the detailed crust and upper mantle structure beneath south central alaska we used teleseismic waveform data recorded by 36 stations in the broadband experiment across the alaska range beaar and 4 permanent stations in alaska h stacking method using p to s converted wave and its multiply reflected waves between the earth s surface and the moho discontinuity is adopted to estimate the crustal thickness h and average crustal vp vs ratio in this region the receiver function results for 24 stations show that the crustal thickness under alaska ranges from 26 0 to 42 6 km with an average value of 33 8 km and the vp vs ratio varies from 1 66 to 1 94 with an average value of 1 81 which corresponds to an average poisson s ratio of 0 277 with a range from 0 216 to 0 320 high poisson s ratios under some stations are possibly caused by partial melting in the crust and the uppermost mantle common converted point ccp stacking results of receiver functions along three lines show clear moho and slab images under this subduction zone the depths of the slab from our ccp stacking images are consistent with those estimated from the wadati benioff zone wbz in the area between two stations dh2 147 8 w 63 3 n and dh3 147 1 w 63 0 n a moho depth offset of about 10 km is found by both the h and ccp stacking techniques common depth point cdp stacking of receiver functions shows not only the 410 520 and 660 km discontinuities but also significant variations 30 to 15 km in the transition zone thickness under the southwest and southeast parts of the study region the transition zone becomes thinner by 20 30 km indicating that the temperature there is 150 200 k higher than that of the normal mantle 2004 elsevier b v all rights reserved

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

paleoclimatic context of geomagnetic dipole lows and excursions in the brunhes clue for an orbital influence on the geodynamo the hypothesis of an influence of the astronomical precession on the geodynamo energy budget was recently reappraised by theoreticians paleomagnetic indications of such an influence remain controversial because reconstructions of paleointensity variations from sediments are suspected to be contaminated by lithological paleoclimatically induced influences three sets of complementary indicators are however available 1 records of the direction of magnetization in sediments 2 records of magnetic anomalies of the deep sea floor basalts and 3 records of production variations of cosmogenic isotopes from sediment and ice cores these records confirm the genuine geomagnetic origin of paleointensity lows and their narrow link with excursions or short reversals recorded in various materials and often dated by radiometric methods the analysis of these time series and their comparison with 18o records of the paleoclimate suggest that such globally recorded geomagnetic dipole lows have preferentially occurred in the context of interglacial or transitional paleoclimates at the time of low or decreasing obliquity the dominant periods extracted with the complex continuous wavelet transform technique range from 40 to 125 ka further suggesting a link with orbital parameters these results encourage future efforts of research to improve the precision the resolution and the dating of the time series of geomagnetic dipole low in order to better decipher orbital signatures and understand their origin an important implication of this topic is that the next geomagnetic dipole low should be related with the present interglacial

effects of variable sedimentation rates and age errors on the resolution of sedimentary paleointensity records 1 synthetic u channel records of relative paleointensity have been generated with a numerical model simulating the recording process in sediments variable quality age control and variable sedimentation rates over a time interval of 500 ky simulation results indicate that paleointensity records with mean sedimentation rates up to 15 cm ky can reflect geomagnetic dipole intensity fluctuations with some amplitude differences between individual records of nongeomagnetic origin this study confirms that relative paleointensity records have great potential as a stratigraphic tool and that a stratigraphic precision of a few thousand years can be achieved with records characterized by high sedimentation rates spectral analyses suggest that caution should be used when interpreting the power spectra of individual records and that stacked records should be favored stacked u channel records with mean sedimentation rates of 1 cm ky do not provide reliable spectral information on the dipole intensity for wavelengths shorter than 25 50 ky and their utility is limited to long term trends in paleointensity for higher sedimentation rates the range of spectral information depends on the stack resolution sedimentation rates and the age model the best results are predictably obtained with high sedimentation rates and excellent age control in these cases the power spectra are reliable for wavelengths as short as 4 ky

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

the effects of aliasing and lock in processes on palaeosecular variation records from sediments studies of sedimentary records of palaeointensity variation report periods as long as 50 kyr archaeointensity data show geomagnetic periods of 2 kyr with large ampli tudes sampling of the sedimentary records can be as coarse as 8 kyr so the apparent long periods could be caused by aliasing the sedimentary lock in process could smooth the record and remove short periods thereby preventing aliasing from occurring we examine possible effects of aliasing by creating a 100 kyr long synthetic sequence of palaeointensity variation with a similar spectrum to that of archaeomagnetic data from the last 12 kyr and resampling at longer intervals with no lock in smoothing aliasing produces spurious energy in the spectra at long periods when smoothing by the sedimentation process is applied the amplitudes of the aliased peaks are reduced but still cause significant spurious long period energy in the spectra for some sedi mentation rates we restrict our analysis to palaeointensity data but similar problems may also exist for coarsely sampled directional data to avoid aliasing we recommend a maximum sampling interval of 2 kyr

magnetic properties of pelagic marine carbonates abstract pelagic carbonates are deposited far from continents usually at water depths of 3000 6000 m at rates below 10 cm kyr and are a globally important sediment type recent advances with recognition of widespread preservation of biogenic magnetite the inorganic remains of magnetotactic bacteria have fundamentally changed our understanding of the magnetic properties of pelagic carbonates we review evidence for the magnetic minerals typically preserved in pelagic carbonates the effects of magnetic mineral diagenesis on paleomagnetic and environmental magnetic records of pelagic carbonates and what magnetic properties can tell us about the open ocean environments in which pelagic carbonates are deposited we also discuss briefly late diagenetic remagnetisations recorded by some carbonates despite recent advances in our knowledge of these phenomena much remains undiscovered we are only at early stages of understanding how biogenic magnetite gives rise to paleomagnetic signals in sediments and whether it carries a poorly understood biogeochemical remanent magnetisation recently developed techniques have potential for testing how different magnetotactic bacterial species which produce different magnetite morphologies respond to changing nutrient and oxygenation conditions future work needs to test whether it is possible to develop proxies for ancient nutrient conditions from well calibrated modern magnetotactic bacterial occurrences a tantalizing link between giant magnetofossils and paleogene hyperthermal events needs to be tested much remains to be learned about the relationship between climate and the organisms that biomineralised these large and novel magnetite morphologies rather than being a well worn subject that has been studied for over 60 years the magnetic properties of pelagic carbonates hold many secrets that await discovery

geomagnetic palaeointensities and astrochronological ages for the matuyama brunhes boundary and the boundaries of the jaramillo subchron palaeomagnetic and oxygen isotope records from odp site 983 we have measured relative geomagnetic palaeointensity proxies palaeomagnetic directions and 18 o for the 700 1100 ka interval from odp site 983 gardar drift north atlantic where mean sedimentation rates are ca 13 cm kyr 1 the age model was generated by matching the benthic 18 o data to the ice volume model and confirmed by tuning the precessional components of both signals for the matuyama brunhes boundary mbb and the boundaries of the jaramillo subchronozone the duration of the polarity reversal process defined by virtual geomagnetic polar latitudes of less than 45 is ca 5 kyr whereas the generally accepted astrochronological estimates for the boundaries of the jaramillo subchronozone lie within the polarity transitions as recorded at site 983 the astrochronological age for the matuyama brunhes polarity transition 780 ka is ca 5 kyr older than the onset of this transition at site 983 775 ka the polarity reversals lie within palaeointensity lows with abrupt recovery of palaeointensity post reversal there is no progressive sawtooth decrease in palaeointensity within the jaramillo subchronozone or between the top of the jaramillo and the mbb but rather within polarity chrons several short intervals of low palaeointensity which sometimes coincide with high amplitude secular variation orbital 100 and 41 kyr periods are present in the palaeointensity record as they are not obviously attributable to climate lithology in these records they may be a feature of the geomagnetic field itself

magnetotaxis and acquisition of detrital remanent magnetization by magnetotactic bacteria in natural sediment first experimental results and theory 1 the widespread occurrence of magnetotactic bacteria mtb in several types of marine and freshwater sediment and the role of fossil magnetosomes magnetofossils as main remanent magnetization carriers therein has important paleomagnetic and paleoenvironmental implications despite numerous studies on mtb biology and on magnetofossil preservation in geological records no detailed information is yet available on how magnetotaxis i e the ability to navigate along magnetic field lines is performed in sedimentary environments and on how magnetofossils possibly record the earth magnetic field we provide for the first time experimental evidence for these processes mtb living in sediment are poorly aligned with the geomagnetic field contrary to what is observed in water this can explain the seemingly excessive magnetic moment of most mtb the observed alignment is sufficient for supporting magnetotaxis across the typical thickness of chemical gradients experiments with magnetofossil rich sediment suggest that a natural remanent magnetization nrm is acquired by magnetofossils in the so called benthic mixed layer where natural mtb populations usually occur the acquired nrm is proportional to the applied field at least up to 160 t and its intensity is compatible with values observed in nature for same sediment types therefore if fossil magnetosome chains are not subjected to further alteration by early diagenetic processes they can provide useful relative paleointensities we propose a preliminary model to explain early stages of magnetofossil nrm acquisition as the result of a dynamic equilibrium between magnetic torques and randomizing forces due to sediment mixing

paleomagnetism of quaternary sediments from lomonosov ridge and yermak plateau implications for age models in the arctic ocean abstract inclination patterns of natural remanent magnetization nrm in quaternary sediment cores from the arctic ocean have been widely used for stratigraphic correlation and the construction of age models however shallow and negative nrm inclinations in sediments deposited during the brunhes chron in the arctic ocean appear to have a partly diagenetic origin rock magnetic and mineralogical studies demonstrate the presence of titanomagnetite and titanomaghemite thermal demagnetization of the nrm indicates that shallow and negative inclination components are largely unblocked below 300 c consistent with a titanomaghemite remanence carrier following earlier studies on the mendeleev alpha ridge shallow and negative nrm inclination intervals in cores from the lomonosov ridge and yermak plateau are attributed to partial self reversed chemical remanent magnetization crm carried by titanomaghemite formed during seafloor oxidation of host detrital titanomagnetite grains distortion of paleomagnetic records due to seafloor maghemitization appears to be especially important in the perennially ice covered western mendeleev alpha ridge and central arctic ocean lomonosov ridge and to a lesser extent near the ice edge yermak plateau on the yermak plateau magnetic grain size parameters mimic the global benthic oxygen isotope record back to at least marine isotope stage 6 implying that magnetic grain size is sensitive to glacial interglacial changes in bottom current velocity and or detrital provenance

seven thousand year duration for a geomagnetic excursion constrained by 230thxs 1 the duration of geomagnetic excursions may provide important information about the geodynamo it has proven difficult however to assess the exact duration of both excursions and reversals we combine high resolution paleomagnetic records from marine sediments with measurements of the 230thxs flux to the seafloor in order to better constrain the duration of the iceland basin excursion ibe we find the duration of the ibe to be 7 8 kyr implying that it is considerably longer than previous estimates for the laschamp excursion 2 kyr and more similar in duration to that of full polarity reversals there may not be therefore a characteristic timescale for transitional field configurations that distinguishes excursions from reversals

magnetic granulometric and geochemical characterizations of loess sections in the eastern arid central asia implication for paleoenvironmental interpretations abstract loess deposits in the arid central asia aca contain sensitive messages concerning the evolution of the westerlies and the interior aridification of asia in this study we conducted a combined study of geophysical and geochemical characterization on loess sections in the eastern aca to discuss possible paleoclimate development since the late pleistocene era results show that the aca loess chiefly exhibited similar geophysical and geochemical compositions with loess deposits in other regions like the chinese loess plateau there are various accumulation rates that would have been responsible for the depositional difference within the aca loess profiles in addition there are three magnetic responses to the bulk grains and geochemical proxies in this study at the bottom segment of the section magnetic enhancement was associated with coarse grain sizes and water leaching magnetic enhancement responded to stronger pedogenesis and moderate paleo wind intensity in the uppermost parts moreover there is a transitional segment in the middle part of the section characterized by inordinately detrital inputs and weak pedogenic contributions thus it may be better to evaluate the magnetic climatology of the aca loess through different depositional stages furthermore this study generally reveals that the paleoclimate pattern in the aca was probably characterized by relatively low moisture during warm intervals and vice versa in the middle and bottom parts of the profiles but relatively enhanced moisture was indicated during the holocene period further studies are still necessary to obtain reliable proxies and chronology for a detailed paleoclimate interpretation of the aca loess

palaeoenvironmental change and glacier fluctuations in the high tian shan mountains during the last millennium based on sediments from lake ala kol kyrgyzstan abstract a suite of limnological proxies measured in clastic sediments of the glacier fed lake ala kol ne kyrgyzstan document palaeoenvironmental change and glacier fluctuations in the central tian shan over the last millennium rock magnetic and geochemical data provide supporting evidence for three glacier expansion episodes occurring in 1200 1400 ce 1500 1850 ce and 1900 1950 ce and glacier retreat periods during 1000 1200 ce and the intervening periods the maximal glacier extent as inferred from high lf si inc coh ca inc coh and low arm30 arm is recorded between 1500 and 1850 ce and corresponds to cold and wet conditions during the little ice age in the high central tian shan periods of glacier waxing and waning recorded at lake ala kol present a close correspondence to ice accumulation rate in the northern tibetan plateau suggesting that precipitation changes are the main driver of glacier fluctuations in central tian shan during the last millennium although temperature may have exerted a more complex influence on centennial timescales

magnetic mineral dissolution in pleistocene fluvio lacustrine sediments nihewan basin north china abstract the nihewan fluvio lacustrine sequence north china has recorded late pliocene pleistocene climatic and environmental changes and contains valuable information on early human evolution in high latitude east asia we carried out a combined mineral magnetic and geochemical investigation on a sequence from the xiantai section eastern nihewan basin results suggest that large amplitude magnetic property variations between so called high magnetic and low magnetic units mainly result from preservation dissolution cycles of detrital magnetic minerals in alternating oxic and anoxic depositional environments in our preservation dissolution model the high and low magnetic units represent glacial and interglacial deposits respectively this contributes to a better understanding of the link between magnetic properties and climate in the xiantai fluvio lacustrine sequence based upon this relationship early humans may have occupied the xiantai and xiaochangliang sites during an interglacial period during both interglacial and glacial periods at the donggutuo site and during a glacial period at the maliang site our work provides a paleoenvironmental context for early human adaptation and occupation in mainland east asia during the early pleistocene

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

archeomagnetic secular variation from korea implication for the occurrence of global archeomagnetic jerks abstract an archeomagnetic paleosecular variation psv was first defined in korea using baked materials collected from 26 kilns or hearths with ages ranging from 1100 bc to ad 1790 variations of geomagnetic declination and inclination from the korean peninsula are distinctively different from the prediction of a global model cals3k 3 or cals7k 2 for the past 3500 yr in particular a distinctive offset in magnitude and phase is noticeable between the observations and predictions at 745 bc ad 300 and ad 1400 1700 a bi plot of magnetic declination versus inclination displays three cusps at the corresponding time intervals these time intervals are nearly identical to or at least overlap with three of the four potential archeomagnetic jerks suggested by gallet et al 2003 from the european archeomagnetic data a comparison of the psv curves for neighboring countries regions revealed that european archeomagnetic jerks at 800 bc ad 200 ad 800 and ad 1400 were all preserved in east asia suggesting that the archeomagnetic jerks were global or at least northern hemispheric features

a detailed paleomagnetic record between 2 1 and 2 75 ma at iodp site u1314 in the north atlantic geomagnetic excursions and the gauss matuyama transition 1 this study investigated the detailed geomagnetic field variation between 2 1 and 2 75 ma from a sediment core iodp site u1314 with high sedimentation rate 10 cm kyr and good age control characteristic remanent magnetization directions were well resolved by stepwise alternating field demagnetization as a proxy of relative paleointensity natural remanent magnetization nrm normalized by anhysteretic remanent magnetization arm was used after testing that the influence of magnetic interaction in arm is negligible as a result the following features of the geomagnetic field in the studied period have been revealed during the transition of the gauss matuyama g m reversal and the reunion subchron the paleointensity decreased to the value lower than 20 of the average intensity in the whole studied interval in addition to these lows eight paleointensity lows were found associated with large directional changes that satisfy the definition of a geomagnetic excursion four of these have ages close to ages reported for geomagnetic excursions in prior studies whereas the other four excursions have not previously been observed in our results we confirm that the g m transition occurred in marine isotope stage 103 even if we consider the shift in depth due to the lock in process of magnetic particles the temporal variation in paleointensity showed asymmetric behavior associated with the g m transition with a gradual decrease prior to the transition and a rapid recovery after the transition