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

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

10be evidence for delayed acquisition of remanent magnetization in marine sediments implication for a new age for the matuyama brunhes boundary abstract fluxes of the meteoric cosmogenic radionuclide 10 be vary with changes in the incoming cosmic rays modulated by geomagnetic field intensity variations the variability in the 10 be flux can be used to synchronize ice cores as well as marine sediments by comparison with the relative paleointensity variations of the geomagnetic field however lock in of the paleomagnetic signal at some depth below the sediment water interface in marine sediments through acquisition of a post depositional remanent magnetization pdrm adds uncertainty to synchronization despite the long history of such studies the magnitude of the pdrm lock in depth remains controversial in this article we present clear evidence for a downward offset of the paleointensity minimum relative to the 10 be flux anomaly at the matuyama brunhes m b geomagnetic polarity boundary which we interpret to result from a 15 cm pdrm lock in depth this lock in depth indicates that up to several tens of thousands years of age offset probably occurs when a paleomagnetic record is used for dating marine sediments and the age of the m b boundary should be revised to ca 10 kyr younger which is consistent with a younger ice core derived age of 770 6 ka 2 this cosmogenic age tuning strategy will contribute to refining paleomagnetic based age models for marine sediments and identifying of lead lag relationships for global abrupt environmental changes

paleomagnetic reconstruction of the global geomagnetic field evolution during the matuyama brunhes transition iterative bayesian inversion and independent verification abstract the earth s magnetic field changed its polarity from the last reversed into today s normal state approximately 780 000 years ago while before and after this so called matuyama brunhes reversal the earth magnetic field was essentially an axial dipole the details of its transitional structure are still largely unknown here a bayesian inversion method is developed to reconstruct the spherical harmonic expansion of this transitional field from paleomagnetic data this is achieved by minimizing the total variational power at the core mantle boundary during the transition under paleomagnetic constraints the validity of the inversion technique is proved in two ways first by inverting synthetic data sets from a modeled reversal here it is possible to reliably reconstruct the gauss coefficients even from noisy records second by iteratively combining four geographically distributed high quality paleomagnetic records of the matuyama brunhes reversal into a single geometric reversal scenario without assuming an a priori common age model the obtained spatio temporal reversal scenario successfully predicts most independent matuyama brunhes transitional records therefore the obtained global reconstruction based on paleomagnetic data invites to compare the inferred transitional field structure with results from numerical geodynamo models regarding the morphology of the transitional field it is found that radial magnetic flux patches form at the equator and move polewards during the transition our model indicates an increase of non dipolar energy prior to the last reversal and a non dipolar dominance during the transition thus the character and information of surface geomagnetic field records is strongly site dependent the reconstruction also offers new answers to the question of existence of preferred longitudinal bands during the transition and to the problem of reversal duration different types of directional variations of the surface geomagnetic field continuous or abrupt are found during the transition two preferred longitudinal bands along the americas and east asia are not predicted for uniformly distributed sampling locations on the globe similar to geodynamo models with cmb heatflux derived from present day lower mantle heterogeneities a preference of transitional vgps for the pacific hemisphere is found the paleomagnetic duration of reversals shows not only a latitudinal but also a longitudinal variation even the paleomagnetically determined age of the reversal varies significantly between different sites on the globe the described bayesian inversion technique can easily be applied to other high quality full vector reversal records also its extension to inversion of secular variation and excursion data is straightforward

magnetic properties of sedimentary greigite fe3s4 an update greigite fe3s4 is an authigenic ferrimagnetic mineral that grows as a precursor to pyrite during early diagenetic sedimentary sulfate reduction it can also grow at any time when dissolved iron and sulfide are available during diagenesis greigite is important in paleomagnetic environmental biological biogeochemical tectonic and industrial processes much recent progress has been made in understanding its magnetic properties greigite is an inverse spinel and a collinear ferrimagnet with antiferromagnetic coupling between iron in octahedral and tetrahedral sites the crystallographic c axis is the easy axis of magnetization with magnetic properties dominated by magnetocrystalline anisotropy robust empirical estimates of the saturation magnetization anisotropy constant and exchange constant for greigite have been obtained recently for the first time and the first robust estimate of the low field magnetic susceptibility is reported here the curie temperature of greigite remains unknown but must exceed 350 c greigite lacks a low temperature magnetic transition on the basis of preliminary micromagnetic modeling the size range for stable single domain behavior is 17 200 nm for cubic crystals and 17 500 nm for octahedral crystals gradual variation in magnetic properties is observed through the pseudo single domain size range we systematically document the known magnetic properties of greigite at high ambient and low temperatures and with alternating and direct fields and illustrate how grain size variations affect magnetic properties recognition of this range of magnetic properties will aid identification and constrain interpretation of magnetic signals carried by greigite which is increasingly proving to be environmentally important and responsible for complex paleomagnetic records including widespread remagnetizations

reconciling astrochronological and 40ar 39ar ages for the matuyama brunhes boundary and late matuyama chron when five matuyama brunhes m b boundary records from the north atlantic are placed on isotope age models produced by correlation of the 18o record directly or indirectly to an ice volume model the m b boundary lies consistently at the young end of marine isotope stage 19 with a mean age for the midpoint of the reversal of 773 1 ka standard deviation 0 4 kyr 7 kyr younger than the presently accepted astrochronological age for this polarity reversal 780 781 ka two recently proposed revisions of the age of the 40ar 39ar fish canyon sanidine fcs standard to 28 201 0 046 ma and 28 305 0 036 ma would adjust 40ar 39ar ages applicable to the m b boundary and other reversals and excursions back to 1 2 ma to ages older than the new astrochronological ages by 8 24 kyr the variables used to construct the ice volume models cannot account for the discrepancy the fcs standard age that best fits the astrochronological ages is 27 93 ma which is within the uncertainty associated with the commonly used value of 28 02 0 16 ma but younger than the recently proposed fcs ages the edc2 and edc3 age models in the dome c antarctic ice core yield ages of 771 7 ka and 766 4 ka respectively for the 10be flux peak that denotes the paleointensity minimum at the reversal boundary implying that the edc2 rather than edc3 age model is consistent with the observations from marine sediments at least close to the m b boundary

a comparison of two sequential geomagnetic polarity transitions upper olduvai and lower jaramillo from the southern hemisphere abstract two normal to reverse n to r geomagnetic polarity transitions were obtained from a southern hemisphere deep sea sediment core 35 91 s 59 97 e the upper jaramillo reversal was continuously sampled by taking 0 5 cm thick samples across 55 cm of section and the upper olduvai reversal was sampled in a similar manner across 60 cm both records are characterized by full normal and reverse polarity directions which are in very good agreement with those predicted by axial dipole fields for the core site latitude each record also exhibits a zone yielding intermediate directions because of a large increase in both the within level and the between level scatter in the upper jaramillo transition zone it is not considered to represent an accurate record of the geomagnetic field the upper olduvai transition however appears to be a more coherent record it is characterized by directions that shallow early in the reversal but then rapidly steepen to nearly vertical upward directions the declinations do not change until the inclinations have passed through the vertical the total directional change occurs within a broad nrm intensity minimum the virtual geomagnetic pole vgp path calculated for this record is not longitudinally constrained but instead exhibits a westward progression through the reversal when considered together with the lower jaramillo transition previously reported from this core these records constitute a set of sequential southern hemisphere transitions a number of striking similarities exist in the two records such as steep upward directions and a shallowing which occurs between the near vertical and the full polarity directions the results of zonal harmonic modeling of these records are consistent with the idea of a standing field which persisted across both the upper olduvai and lower jaramillo reversals

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

snapshot of the matuyama brunhes reversal process recorded in 40ar 39ar dated lavas from guadeloupe west indies 1 we obtain new 40ar 39ar ages for three lavas that record part of the matuyama brunhes geomagnetic field reversal process on guadeloupe these lavas record a reversed transitional reversed magnetostratigraphy and yield a weighted mean isochron age of 785 2 5 2 ka 2 analytical uncertainty relative to an age for the fish canyon sanidine standard of 28 201 ma this age is of greater accuracy than ages obtained previously by k ar dating these lavas may record directional fluctuations occurring between the matuyama brunhes precursor and the final directional reversal such variations have been observed in some globally distributed coupled paleomagnetic and oxygen isotope marine sedimentary records of the matuyama brunhes reversal previous paleointensity estimates from these lavas indicate reduced but fluctuating field strength during this time

geomagnetic field evolution during the laschamp excursion abstract since the last geomagnetic reversal 780 000 years ago the earth s magnetic field repeatedly dropped dramatically in intensity this has often been associated with large variations in local field direction but without a persistent global polarity flip the structure and dynamics of geomagnetic excursions and especially the difference between excursions and polarity reversals have remained elusive so far for the best documented excursion the laschamp event at 41 000 years bp we have reconstructed the evolution of the global field morphology by using a bayesian inversion of several high resolution palaeomagnetic records we have obtained an excursion scenario in which inverse magnetic flux patches at the core mantle boundary emerge near the equator and then move poleward contrary to the situation during the last reversal leonhardt r fabian k 2007 paleomagnetic reconstruction of the global geomagnetic field evolution during the matuyama brunhes transition iterative bayesian inversion and independent verification earth planet sci lett 253 172 195 these flux patches do not cross the hydrodynamic boundary of the inner core tangent cylinder while the last geomagnetic reversal began with a substantial increase in the strength of the non dipolar field components prior to the laschamp excursion both dipolar and non dipolar field decay at the same rate this result suggests that the nature of an upcoming geomagnetic field instability can be predicted several hundred years in advance even though during the laschamp excursion the dipolar field at the earth s surface was dominant the reconstructed dynamic non dipolar components lead to considerable deviations among predicted records at different locations the inverse model also explains why at some locations no directional change during the laschamp excursion is observed

geomagnetic excursions and paleointensities in the matuyama chron at ocean drilling program sites 983 and 984 iceland basin 1 we report natural remanent magnetization nrm directions and geomagnetic paleointensity proxies for part of the matuyama chron 0 9 2 2 ma interval from two sites located on sediment drifts in the iceland basin at ocean drilling program sites 983 and 984 mean sedimentation rates in the late matuyama chron are 15 9 and 11 5 cm kyr 1 respectively for the older part of the record 1 2 ma oxygen isotope data are too sparse to provide the sole basis for age model construction the resemblance of the volume susceptibility record and a reference 18o record led us to match the two records to derive the age models this match based on site 983 984 susceptibility is consistent with available site 983 984 benthic 18o data paleointensity proxies were derived from the slope of the nrm versus anhysteretic remanent magnetization plot for alternating field demagnetization in the 30 60 mt peak field range paleointensity lows correspond to polarity reversals at the limits of the jaramillo olduvai cobb mountain and reunion subchrons and to seven excursions in nrm component directions magnetic excursions defined here by virtual geomagnetic polar latitudes crossing the virtual geomagnetic equator are observed at 932 1048 1115 1190 1215 cobb mountain subchron 1255 1472 1480 1567 1575 gilsa subchron and 1977 ka the results indicate that geomagnetic directional excursions associated with paleointensity minima are a characteristic of the matuyama chron and probably of polarity chrons in general

tectonic and geochronological implications of variably timed magnetizations carried by authigenic greigite in marine sediments from new zealand detailed scanning electron microscope observations coupled with elemental microanalysis confirm the presence of a chemical remanent magnetization carried by authigenic greigite fe 3 s 4 in uplifted neogene marine sediments from the hikurangi margin of new zealand normal polarity samples from the studied section have declinations that are deflected 60 clockwise of reversed polarity samples indicating the presence of two distinct magnetizations separated by several million years of tectonic rotation about a vertical axis however although multiple generations of iron sulfide growth are observed petrographically we see no clear differences in the relative timing of greigite formation between samples carrying these two magnetizations not only can the diagenetic growth of greigite in fine grained marine sediments occur long after deposition obscuring tectonic and magnetostratigraphic information but such remagnetizations are also difficult to distinguish from a more primary signal in the absence of constraints from field tests our observations emphasize that considerable care is necessary when interpreting paleomagnetic data from greigite bearing sediments

apparent magnetic polarity reversals due to remagnetization resulting from late diagenetic growth of greigite from siderite summary a mixed polarity zone representing alternations between remagnetized and non remagnetized strata has been documented within the lower few metres of the crp 1 core ross sea antarctica detailed rock magnetic investigation of this interval indicates that the normal polarity remagnetization is carried by magnetostatically interacting single domain particles of a ferrimagnetic iron sulphide mineral while the reversed polarity magnetization of non remagnetized strata is carried by magnetite with a broad range of grain sizes and negligible magnetostatic interactions scanning electron microscope observations of polished sections indicate that the ferrimagnetic iron sulphide mineral is greigite fe3s4 based on microtextural relationships it is not possible to determine the relative timing of formation for much of the greigite however a significant proportion of the greigite has grown on the surface of authigenic siderite feco3 grains that occur as microconcretions and as cement surrounding detrital matrix grains in such cases microtextural relationships indicate that siderite post dates early diagenetic pyrite and that greigite post dates the siderite siderite usually forms in environments with abundant dissolved iron and carbonate but without dissolved pore water h2s this set of geochemical conditions occurs in methanic settings below the sulphate reduction zone in which early diagenetic pyrite forms we interpret the observed remagnetization of the lower part of the crp 1 core as due to a late diagenetic pore water migration event where abundant iron on the surface of siderite grains reacted with fluids containing limited dissolved sulphide thereby causing precipitation of greigite the distribution of siderite and associated greigite in the lower part of the crp 1 core is patchy which accounts for the apparent alternation of polarities this study is part of a growing catalogue of remagnetizations involving greigite which suggests that occurrences of greigite should be treated with caution in palaeomagnetic and environmental magnetic studies

magnetite dissolution diachronous greigite formation and secondary magnetizations from pyrite oxidation unravelling complex magnetizations in neogene marine sediments from new zealand detailed rock magnetic and electron microscope analyses indicate that the magnetic signature of neogene marine sediments from the east coast of new zealand is dominated by the authigenic iron sulphide greigite the greigite is present as a mixed population of stable single domain and superparamagnetic grains which is consistent with authigenic growth from solution this growth can result from pyritization reactions soon after deposition which also leads to dissolution of most detrital magnetite however where constrained by field tests our data suggest that remanence acquisition can occur 1 myr after deposition and can vary in timing at the outcrop scale strong viscous overprints result from oxidation of the iron sulphides probably during percolation of oxic ground water this process can sometimes destroy any ancient remanent magnetization this complex magnetic behaviour particularly the presence of late forming magnetizations carried by greigite means that the remanence in new zealand cenozoic sediments and in similar sediments elsewhere cannot be assumed to be primary without confirmation by field tests the reversals test should be employed with caution in such sediments as patchy remagnetizations can lead to false polarity stratigraphies

temperature dependent residual shear strength characteristics of smectite bearing landslide soils this paper presents experimental investigations regarding the effect of temperature on the residual strength of landslide soils at slow to moderate shearing velocities we performed ring shear tests on 23 soil samples at temperatures of 6 29 c the test results show that the shear strength of smectite rich soils decreased when temperatures were relatively low these positive temperature effects strength losses at lower temperatures observed for smectite bearing soils are typical under relatively slow shearing rates in contrast under relatively high shearing rates strength was gained as temperature decreased as rheological properties of smectite suspensions are sensitive to environmental factors such as temperature ph and dissolved ions we inferred that temperature dependent residual strengths of smectitic soils are also attributed to their specific rheological properties visual and sem observations of ca bentonite suggest that slickensided shear surfaces at slow shearing rates are very shiny and smooth whereas those at moderate shearing rates are not glossy and are slightly turbulent indicating that platy smectite particles are strongly orientated at slow velocities the positive temperature effect is probably due to temperature dependent micro friction that is mobilized in the parallel directions of the sheet structure of hydrous smectite particles on the contrary the influence of micro viscous resistance which appears in the vertical directions of the lamination is assumed to increase at faster velocities our results imply that if slip surface soils contain high fractions of smectite decreases in ground temperature can lead to lowered shear resistance of the slip surface and trigger slow landslide movement

natural remanent magnetization acquisition in bioturbated sediment general theory and implications for relative paleointensity reconstructions we present a general theory for the acquisition of natural remanent magnetizations nrm in sediment under the influence of a magnetic torques b randomizing torques and c torques resulting from interaction forces dynamic equilibrium between a and b in the water column and at the sediment water interface generates a detrital remanent magnetization drm while much stronger randomizing torques may be provided by bioturbation inside the mixed layer these generate a so called mixed remanent magnetization mrm which is stabilized by mechanical interaction forces during the time required to cross the surface mixed layer drm is lost and mrm is acquired at a rate that depends on bioturbation intensity both processes are governed by a mrm lock in function the final nrm intensity is controlled mainly by a single parameter that is defined as the product of rotational diffusion and mixed layer thickness divided by sedimentation rate this parameter defines three regimes 1 slow mixing 10 with mrm acquisition and full drm randomization and 3 intermediate mixing because the acquisition efficiency of drm is larger than that of mrm nrm intensity is particularly sensitive to in case of mixed regimes generating variable nrm acquisition efficiencies this model explains 1 lock in delays that can be matched with empirical reconstructions from paleomagnetic records 2 the existence of small lock in depths that lead to drm preservation 3 specific nrm acquisition efficiencies of magnetofossil rich sediments and 4 some relative paleointensity artifacts

untangling inconsistent magnetic polarity records through an integrated rock magnetic analysis a case study on neogene sections in east timor inconsistent polarity patterns in sediments are a common problem in magnetostratigraphic and paleomagnetic research multiple magnetic mineral generations result in such remanence haystacks here we test whether end member modeling of isothermal remanent magnetization acquisition curves as a basis for an integrated rock magnetic and microscopic analysis is capable of isolating original magnetic polarity patterns uppermost miocene pliocene deep marine siliciclastics and limestones in east timor originally sampled to constrain the uplift history of the young timor orogeny serve as case study an apparently straightforward polarity record was obtained that however proved impossible to reconcile with the associated biostratigraphy our analysis distinguished two magnetic end members for each section which result from various greigite suites and a detrital magnetite suite the latter yields largely viscous remanence signals and is deemed unsuited the greigite suites are late diagenetic in the cailaco river section and early diagenetic thus reliable in the viqueque type section by selecting reliable sample levels based on a quality index a revised polarity pattern of the latter section is obtained consistent with the biostratigraphy and unequivocally correlatable to the geomagnetic polarity time scale although the cailaco river section lacks a reliable magnetostratigraphy it does record a significant postremagnetization tectonic rotation our results shows that the application of well designed rock magnetic research based on the end member model and integrated with microscopy and paleomagnetic data can unravel complex and seemingly inconsistent polarity patterns we recommend this approach to assess the veracity of the polarity of strata with complex magnetic mineralogy

toward an understanding of the multicomponent magnetization of uplifted neogene marine sediments in new zealand new zealand contains many sequences of tectonically uplifted miocene pliocene marine sediments which have been central to the development of a biostratigraphic framework for temperate southern latitudes chronological control is almost invariably dependent on magnetostratigraphy interpretation of the natural remanent magnetization nrm of these sediments is however notoriously difficult because of the weak magnetization secondary components that resist demagnetization and thermal instability of the clay minerals a new paleomagnetic study of wanganui basin sediments has yielded unusually well defined demagnetization data from which systematic characterization explanation and interpretation of the multicomponent nature of the nrm have been possible in general the magnetization consists of three components 1 a low 250 c t b component that is considered to be diagenetic in origin this interpretation of the intermediate and high t b components is supported by rock magnetic data which show that specimens carrying strong high t b components have a distinctly different spectrum of ferrimagnetic grains specifically a population with much higher coercivities than is seen in specimens which lack a high t b component it is argued that these high coercivity high t b grains are secondary in origin leading to the conclusion that the intermediate t b component is the primary detrital magnetization consistency of the paleomagnetic data within and between sites also supports this conclusion it is suggested that failure to recognize these three components has in some cases in the past led to misidentification of the primary magnetization using this hypothesis of the multi component magnetization the turakina river data of mcguire 1989 have been reinterpreted yielding a high resolution magnetostratigraphy from the upper gilbert chron 3n circa 4 ma to the lower matuyama chron 2r the correlation of this magnetostratigraphy with the geomagnetic polarity timescale is supported by biostratigraphic constraints

reactivation of intrabasement structures during rifting a case study from offshore southern norway abstract pre existing structures within crystalline basement may exert a significant influence over the evolution of rifts however the exact manner in which these structures reactivate and thus their degree of influence over the overlying rift is poorly understood using borehole constrained 2d and 3d seismic reflection data from offshore southern norway we identify and constrain the three dimensional geometry of a series of enigmatic intrabasement reflections through 1d waveform modelling and 3d mapping of these reflection packages we correlate them to the onshore caledonian thrust belt and devonian shear zones based on the seismic stratigraphic architecture of the post basement succession we identify several phases of reactivation of the intrabasement structures associated with multiple tectonic events reactivation preferentially occurs along relatively thick c 1 km relatively steeply dipping c 30 structures with three main styles of interactions observed between them and overlying faults i faults exploiting intrabasement weaknesses represented by intra shear zone mylonites ii faults that initiate within the hangingwall of the shear zones inheriting their orientation and merging with said structure at depth or iii faults that initiate independently from and cross cut intrabasement structures we demonstrate that large scale discrete shear zones act as a long lived structural template for fault initiation during multiple phases of rifting

relative paleointensity rpi and oxygen isotope stratigraphy at iodp site u1308 north atlantic rpi stack for 1 2 2 2 ma narpi 2200 and age of the olduvai subchron 2015 elsevier ltd integrated ocean drilling program iodp site u1308 49 53 n 24 14 w water depth 3871 m provides a record of relative paleointensity rpi and benthic stable isotope stratigraphy back to 3 2 ma the record since 1 5 ma was published previously and here we present the interval from 1 5 ma to 3 2 ma early pleistocene late pliocene the benthic oxygen isotope record in this interval is correlated to marine isotope stage mis 51 to km2 with an apparent hiatus that removed part of the interval spanning mis 104 g2 2 6 2 65 ma at the gauss matuyama boundary the mean sedimentation rate for the 1 5 3 2 ma period is 8 5 cm kyr the age model was built by correlation of the benthic oxygen isotope record to a global stack lr04 apart from the expected polarity reversals three magnetic excursions are recorded punaruu in mis 31 32 at 1092 ka gilsa in mis 54 55 at 1584 ka and a newly recognized excursion labeled porcupine after the nearby porcupine abyssal plain in mis g6 g7 at 2737 ka the ages of polarity reversals at site u1308 on the lr04 time scale are consistent with the current geomagnetic polarity timescale gpts with the exception of the base of the olduvai subchron that occurred in mis 73 corresponding to 1925 ka on the lr04 time scale 25 kyr younger than in the current gpts the rpi record at site u1308 is calibrated using the oxygen isotope age model and combined with four other north atlantic records to obtain a north atlantic rpi stack for 1 2 2 2 ma narpi 2200 that is compared with published rpi stacks epapis sint 2000 and padm2m for 2 2 3 2 ma the site u1308 rpi record is compared with a rpi record from north atlantic iodp site u1314 and with the pacific epapis stack the mean sedimentation rates of the north atlantic sites in narpi 2200 are greater by about an order of magnitude than most of the records incorporated in other stacks the comparison of epapis and narpi 2200 yields an apparent lag for narpi 2200 relative to epapis attributed the epapis age model constructed by correlation of magnetic concentration parameters a proxy for carbonate percentage to a calibrated oxygen isotope record the long rpi record from site u1308 yields a very similar mean value for the brunhes and matuyama virtual axial dipole moments 7 05 1022 am2 implying no polarity bias in the strength of the main geomagnetic dipole in contrast to interpretations from sint 2000 and padm2m the results strengthen the case that rpi can be used to improve global stratigraphic correlation for sites with mean sedimentation rates up to several decimeters kyr

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

diagenetic formation of greigite and pyrrhotite in gas hydrate marine sedimentary systems mineral magnetic results and electron microscope observations from gas hydrate bearing marine sediments cored at southern hydrate ridge during ocean drilling program leg 204 sites 1244 to 1252 cascadia margin offshore oregon demonstrate that authigenic greigite and pyrrhotite formed as a byproduct of microbially mediated diagenetic reactions in the sulphate the anaerobic oxidation of methane aom and the methanic gas hydrate zones geochemical conditions favourable for formation and preservation of greigite and pyrrhotite appear to be a limited source of sulphide whether it derives from microbially driven sulphate reduction in the sulphate zone in the aom zone or in deep sediments undergoing aom so that pyritization reactions are not driven to completion our results indicate that rock magnetic identification of greigite and pyrrhotite should be useful for detecting ancient gas hydrate systems in the marine sedimentary record because it can enable rapid screening of ancient sediments for potential horizons where methane and disseminated gas hydrates might have occurred formation of authigenic greigite and pyrrhotite at different depths within the gas hydrate stability zone also implies that the magnetization of the host sediments will have been acquired at variable times which is likely to compromise paleomagnetic results from greigite and pyrrhotite bearing marine sediments

detailed paleomagnetic records during the brunhes matuyama geomagnetic reversal and a direct determination of depth lag for magnetization in marine sediments abstract detailed paleomagnetic records were studied from homoclinal bathyal siltstone deposited during the brunhes matuyama geomagnetic reversal outcropping in the boso peninsula central japan the records are based on three continuous sediment cores from three sections within a distance of 14 km the stratigraphic horizons were correlated using a volcanic ash bed which occurs about 2 m above the brunhes matuyama reversal the sedimentation rate according to the oxygen isotope stratigraphy for the three sections decreases eastward and is 3 70 2 20 and 1 83 m ka 1 the depth lag of magnetization in the sediments was calculated directly as 42 cm based on the differences in the relative position of the key ash layer in the correlated virtual geomagnetic pole vgp paths for the three sections vgp movement during the geomagnetic reversal is characterized by short intervals of very rapid motion between periods of relative stability the last swing of the vgp from the southern hemisphere to the northern hemisphere in the brunhes matuyama geomagnetic reversal took only 38 yr intensities of the remanent magnetization normalized by anhysteretic remanent magnetization indicate that the intensity of the geomagnetic field decreased to one fifth for about 5 4 ka including the geomagnetic reversal the movement of the vgp longitude is 0 36 yr 1 westward and this westward drift can be related to the rotation of the non dipole field

magnetic characteristics of synthetic pseudo single domain and multi domain greigite fe3s4 1 we report the magnetic properties of pure synthetic pseudo single domain psd and multi domain md greigite and the grain size dependence of the magnetic properties of greigite for the first time the dominantly psd like and md like behavior are demonstrated by hysteresis first order reversal curve diagrams low temperature cycling ltc of room temperature saturation isothermal remanent magnetization sirm and low temperature sirm warming curves variations in a range of magnetic properties clearly correlate with grain size characteristic psd md behavior is preserved at low temperatures which coupled with the small decrease in remanence during warming rule out the presence of substantial superparamagnetic behavior in the studied samples ltc sirm measurements indicate a continuous demagnetization of remanence during cooling knowledge of this expanded range of magnetic properties of greigite should be widely useful in environmental magnetic and paleomagnetic studies

upper and lower jaramillo polarity transitions recorded in iodp expedition 303 north atlantic sediments implications for transitional field geometry sediments collected during the integrated ocean drilling program iodp expedition 303 in the north atlantic provide records of polarity transitions and geomagnetic excursions at a high resolution here we investigate polarity transitions at the upper and lower boundaries of the jaramillo subchronozone at sites u1305 u1304 and u1306 the sediments carry strong natural remanent magnetizations nrm with median destructive fields consistent with magnetite as the dominant magnetic carrier both polarity transitions are characterized by low values of relative paleointensity the u1305 record of the lower jaramillo reversal exhibits a marked cluster of virtual geomagnetic poles vgps over southern south america and a secondary accumulation in the region of ne asia north pacific the main south american vgp cluster is also visible at site u1304 which documents a less complex pattern possibly because of a higher degree of smoothing records of the upper jaramillo polarity transition document a vgp loop over the americas followed by north to south motion including a secondary vgp accumulation near india the similarity between several records of the upper jaramillo transition obtained at various sites in the north atlantic is a strong indication that geomagnetic field changes have been faithfully captured results suggest that a transverse possibly dipolar field component has fluctuated during these polarity reversals and that these fluctuations combined with a reduced axial dipole component yielded the observed field at the earth s surface during the polarity transitions the lower jaramillo transition also exhibits vgp clusters in the vicinity of south america and eastern asia but these clusters are shifted relative to the upper jaramillo vgp clusters implying a memory exerted by the upper mantle on polarity transition geometry

paleomagnetic record at odp site 980 feni drift rockall for the past 1 2 myrs 1 a detailed record of variations in the direction and intensity of the geomagnetic field over the past 1 2 myrs has been obtained from an 120 m thick sedimentary section cored at odp site 980 on the feni drift north atlantic the record has high resolution due to high mean sedimentation rates 11 3 cm kyr for the brunhes chron and 5 5 cm kyr for the matuyama chron paleomagnetic measurements every 1 cm along u channel samples and high quality isotopic age control the iceland basin event is manifest by virtual geomagnetic poles crossing the equator at 190 ka the base of the recovered section lies immediately below the cobb mountain subchronozone that occurs within marine isotope stage 35 normalized remanence data can be correlated to paleointensity records from odp sites 983 and 984 700 km to the nw from site 980 and with lower resolution paleointensity data from the pacific ocean differences between the site 980 paleointensity record and the pacific records are attributed to variable sedimentation rates variable quality age control and inadequacies in the normalization procedure used to derive the paleointensity proxies

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

age calibrated relative paleointensity for the last 1 5 myr at iodp site u1308 north atlantic integrated ocean drilling program iodp site u1308 central north atlantic records paleomagnetic directional and relative paleointensity rpi variations for the last 1 5 myr in 110 m of the sediment sequence at a mean sedimentation rate of 7 3 cm kyr a detailed benthic oxygen isotope record was combined with rpi to produce an integrated high resolution magneto isotopic stratigraphy for site u1308 apart from the well known polarity reversals in this interval the punaruu excursion is recorded at 1092 ka and the cobb mountain subchron in the 1182 1208 ka interval the paleointensity proxies are determined as slopes of nrm versus arm and nrm versus armaq arm acquisition with linear correlation coefficients to monitor the quality of the linear fit the rpi record for site u1308 is compared with the three other paleointensity records one from the western equatorial pacific and two from the north atlantic that cover the same time interval and have accompanying oxygen isotope records the match protocol of lisiecki and lisiecki 2002 is used to optimize the correlation of paleointensity records beginning with the original published age models for each record the match routine is used to optimize the rpi correlations to site u1308 with checks to ensure compatibility with oxygen isotope records squared wavelet coherence wtc indicates significant improvement in rpi and oxygen isotope correlations after matching each rpi record to site u1308 particularly for periods 10 kyr the level of coherence for the atlantic rpi records and the lower resolution pacific record implies synchronous global variability at scales 10 kyr that can be attributed to the axial dipole geomagnetic field

identification and environmental interpretation of diagenetic and biogenic greigite in sediments a lesson from the messinian black sea greigite fe3s4 is a widespread authigenic magnetic mineral in anoxic sediments and is also commonly biosynthesized by magnetotactic bacteria in aqueous environments while the presence of fossilized bacterial magnetite fe3o4 has now been widely demonstrated the preservation of greigite magnetofossils in the geological record is only poorly constrained here we investigate mio pliocene sediments of the former black sea to test whether we can detect greigite magnetofossils and to unravel potential environmental controls on greigite formation our magnetic analyses and transmission electron microscope tem observations indicate the presence of both diagenetic and bacterial greigite and suggest a potentially widespread preservation of greigite magnetofossils in ancient sediments which has important implications for assessing the reliability of paleomagnetic records carried by greigite tem based chemical and structural analyses also indicate the common presence of nickel substituted diagenetic iron sulfide crystals with a ferrimagnetic greigite structure in addition our cyclostratigraphic framework allows correlation of magnetic properties of messinian black sea sediments taman peninsula russia to global climate records diagenetic greigite enhancements appear to be climatically controlled with greigite mainly occurring in warm wet periods diagenetic greigite formation can be explained by variations in terrigenous inputs and dissolved pore water sulfate concentrations in different sedimentary environments our analysis demonstrates the usefulness of greigite for studying long term climate variability in anoxic environments key points we provide evidence for the presence of biogenic greigite in ancient sediments diagenetic greigite enhancements are climatically controlled greigite is a paleoenvironmental indicator in anoxic environments

millennial to submillennial scale features of the matuyama brunhes geomagnetic polarity transition from osaka bay southwestern japan 1 a high resolution magnetic record of the matuyama brunhes mb reversal transition was obtained from homogeneous marine clay in a 1700 m core from osaka bay the transition stretches over a core length of 6 4 m and yields four short reversal episodes two predate the main mb boundary mbb and two postdate it we made diatom analyses of sediments to estimate relative sea level changes and constructed an age model by correlation with the astronomically calibrated marine oxygen isotope curve the age model shows the short reversal episodes with durations of 1 kyr occurred at 4 5 kyr intervals within stage 19 and the mbb occurred about 5 kyr after the sea level highstand corresponding to isotopic event 19 3 it also shows the paleointensity minimum of 15 20 of the postreversal mean occurred at about 6 kyr before the mbb and another minimum at about 1 kyr after four virtual reverse to normal polarity boundaries found in this study suggest different ages for the mbb could be observed in lava flow studies some millennial to submillennial scale transition features found in this study are compatible with the previous numerical simulations of geomagnetic field reversal

alder creek sanidine acs 2 a quaternary 40ar 39ar dating standard tied to the cobb mountain geomagnetic event accurate 40ar 39ar dating of quaternary and holocene samples requires a standard neutron fluence monitor of appropriate age this standard should be isotopically homogenous at the scale of analysis and should be well intercalibrated with other standards several such standards have been proposed including sanidine from the 1 19 ma alder creek rhyolite acs turrin et al 1994 turrin b d donnely nolan j m hearn b c jr 1994 40ar 39ar ages from the rhyolite of alder creek california age of the cobb mountain normal polarity subchron revisited geology 22 251 254 this sanidine is satisfactorily homogeneous well intercalibrated with fish canyon sanidine fcs renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 and is linked to the astronomically calibrated magnetic polarity time scale since the alder creek rhyolite lava is the type occurrence of the cobb mountain event cm we collected 408 kg of this rock from which we anticipate being able to extract more than 8 kg of 425 m sanidine crystals analyses of sanidine phenocryts from the acs 2 from three separate irradiations and two distinct grain sizes yield ages ranging from 1 190 0 004 to 1 194 0 003 ma1 with a weighted mean age of 1 193 0 001 ma mswd 0 74 n 225 based on fcs 28 02 ma renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 the value of the intercalibration factor renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 is rfcsacs 2 0 04229 0 00012 n 8 identical with that of the original acs measurements 0 04229 0 00012 renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 the age of acs 2 is in agreement with the recent astronomical calibration of the cm event 1 215 to 1 190 ma channell j e t mazaud a sullivan p turner s raymo m e 2002 geomagnetic excursions and paleointensities in the matuyama chron at ocean drilling program sites 983 and 984 iceland basin j geophys res 107 6 10 1029 2001jb000491 the apparent absence of xenocrysts and excess argon suggests that multigrain analyses are feasible and therefore the amount of material can be tailored to experimental requirements aliquots from various grain sizes will be available upon request zircon is also potentially available which may be of interest to u th he researchers