complete dynamic modeling calibration for the thurwieser rock avalanche italian central alps abstract few rock avalanches have been witnessed and no exact real time description is available the 2 5 m m 3 punta thurwieser rock avalanche september 2004 italian central alps is a unique event for which many quantitative observations have been gathered this is the first case ever of a natural rock avalanche being recorded on video and for which an almost exact value of the total time duration from initial detachment to final deposition is available the landslide travelled over 2 9 km from its source partially in a glacial environment the initial failure and the time for the complete evacuation of the source area were studied and assessed by a series of photos the mean front velocities along the path have been estimated from the video the final geometry and the characteristics of the deposit have been obtained by gps measurements aerial photos field surveys and laboratory tests a quasi 3d continuum dynamic analysis of the landslide motion has been performed assuming both a voellmy and a frictional rheology in the model dan 3d for the first time the landslide front velocities have been used for back analyses and calibration of the rheological parameters together with the final shape and thickness of the deposit and its developing extension the dynamic flow resistance parameters initially calibrated with dan were adopted and validated in dan 3d showing good consistency they are consistent with the existing database for calibrated landslides of similar type and scale flow velocities on the path segment underlain by glacial deposits up to 55 65 m s 1 are more accurately predicted by adopting a frictional rheology up to 50 55 m s 1 with zero pore pressure and a 26 friction angle than by a voellmy rheology up to 40 45 m s 1 both observations and modelling show that glacial ice plays a substantial role in increasing the rock avalanche mobility

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

remagnetization of the paleogene tibetan himalayan carbonate rocks in the gamba area implications for reconstructing the lower plate in the india asia collision netherlands organization for scientific research nwo 825 15 016 institute for rock magnetism irm at the university of minnesota instruments and facilities program of nsf

paleolatitudes of the tibetan himalaya from primary and secondary magnetizations of jurassic to lower cretaceous sedimentary rocks the tibetan himalaya represents the northernmost continental unit of the indian plate that collided with asia in the cenozoic paleomagnetic studies on the tibetan himalaya can help constrain the dimension and paleogeography of greater india the indian plate lithosphere that subducted and underthrusted below asia after initial collision here we present a paleomagnetic investigation of a jurassic limestones and lower cretaceous volcaniclastic sandstones section of the tibetan himalaya the limestones yielded positive fold test showing a prefolding origin of the isolated remanent magnetizations detailed paleomagnetic analyses rock magnetic tests end member modeling of acquisition curves of isothermal remanent magnetization and petrographic investigation reveal that the magnetic carrier of the jurassic limestones is authigenic mag netite whereas the dominant magnetic carrier of the lower cretaceous volcaniclastic sandstones is detrital magnetite our observations lead us to conclude that the jurassic limestones record a prefolding remagnetiza tion whereas the lower cretaceous volcaniclastic sandstones retain a primary remanence the volcaniclastic sandstones yield an early cretaceous paleolatitude of 55 5 s 52 5 s 58 6 s for the tibetan himalaya suggesting it was part of the indian continent at that time the size of greater india during jurassic time cannot be estimated from these limestones instead a paleolatitude of the tibetan himalaya of 23 8 s 21 8 s 26 1 s during the remagnetization process is suggested it is likely that the remagnetization caused by the oxidation of early diagenetic pyrite to magnetite was induced during 103 83 or 77 67 ma the inferred paleolatitudes at these two time intervals imply very different tectonic consequences for the tibetan himalaya

rock magnetic criteria for the detection of biogenic magnetite abstract we report results on the magnetic properties of magnetites produced by magnetotactic and dissimilatory iron reducing bacteria magnetotactic bacterial mtb strains ms1 mv1 and mv2 and dissimilatory iron reducing bacterium strain gs 15 grown in pure cultures were used in this study our results suggest that a combination of room temperature coercivity analysis and low temperature remanence measurements provides a characteristic magnetic signature for intact chains of single domain sd particles of magnetite from mtbs the most useful magnetic property measurements include 1 acquisition and demagnetization of isothermal remanent magnetization irm using static pulse and alternating fields 2 acquisition of anhysteretic remanent magnetization arm and 3 thermal dependence of low temperature 20 k saturation irm after cooling in zero field zfc or in a 2 5 t field fc from 300 k however potentially the most diagnostic magnetic parameter for magnetosome chain identification in bulk sediment samples is related to the difference between low temperature zero field and field cooled sirms on warming through the verwey transition t 100 k intact chains of unoxidized magnetite magnetosomes have ratios of fc zfc greater than 2 where the parameter is a measure of the amount of remanence lost by warming through the verwey transition disruption of the chain structure or conversion of the magnetosomes to maghemite reduces the fc zfc ratio to around 1 similar to values observed for some inorganic magnetite maghemite greigite and gs 15 particles numerical simulations of fc zfc ratios for simple binary mixtures of magnetosome chains and inorganic magnetic fractions suggest that the fc zfc parameter can be a sensitive indicator of biogenic magnetite in the form of intact chains of magnetite magnetosomes and can be a useful magnetic technique for identifying them in whole sediment samples the strength of our approach lies in the comparative ease and rapidity with which magnetic measurements can be made compared to techniques such as electron microscopy

warm ocean is eroding west antarctic ice sheet 1 satellite radar measurements show that ice shelves in pine island bay have thinned by up to 5 5 m yr 1 over the past decade the pattern of shelf thinning mirrors that of their grounded tributaries the pine island thwaites and smith glaciers and ocean currents on average 0 5 c warmer than freezing appear to be the source the synchronised imbalance of the inland glaciers is the result of reduced lateral and basal tractions at their termini and the drawdown of grounded ice shows that antarctica is more sensitive to changing climates than was previously considered

characterizing magnetofossils from first order reversal curve forc central ridge signatures the central ridge structure of a first order reversal curve forc distribution is indicative of uniaxial noninteracting single domain magnetic particles which provides the opportunity to identify and characterize biogenic magnetic mineral remains magnetofossils in sediments recent studies have shown that magnetofossils are widespread in the geological record and that they carry useful environmental information and contribute to paleomagnetic recording which makes it essential to quantify how these biogenic components contribute to the magnetic properties of sediments we present results from six sedimentary sequences whose magnetic mineral assemblages contain a significant magnetofossil contribution using principal component analysis we find that the central ridge properties exhibit both intra sequnce and inter sequence variability that may be ascribed to external environmental factors while samples from individual sediment sequences tend to cluster together there is a continuum of inter sequence behavior that appears to be related to a variety of magnetofossil properties we demonstrate the complexity of biogenic magnetic components in sedimentary environments but also the power and potential of forc central ridges for understanding magnetic mixtures and unraveling environmental information

magnetization carriers and remagnetization of bedded chert abstract article history received 22 october 2010received in revised form 11 january 2011accepted 25 february 2011editor p demenocalkeywords bedded chertremanencebiogenic magnetitehematite bedded chert is the only sediment type representative of the paleozoic to early mesozoic pelagic marineenvironment becauseoftheirassociationwithophiolitesandislandarcrocks presenceofdatablemicrofossilsandpaleohorizontalreferenceprovidedbywell developedbeddingsurfaces beddedchertsequencesareoftentargeted for paleomagnetic tectonic studies however processes of magnetization acquisition in biosiliceoussediments and consequently the signi cance of their magnetic record are not well understood our rockmagnetic study of a triassic jurassic radiolarian chert sequence the mino terrane central japan shows thatthe ferrimagnetic assemblage of the gray chert units is of detrital origin while the red chert s assemblage isdominated by authigenic phases pigmentary hematite and biogenic magnetite which contribute to thenatural remanent magnetization the presence of magnetofossils places red oxic chert in the category ofprospective environmental archives magnetite producing magnetotactic bacteria were apparently able totolerate elevated concentrations of dissolved silica as well as a steep redox gradient in sedimentary pore waters during the deposition of red chert layers a strong uniaxial anisotropy due to chain alignment of thebiogenic magnetite grains likely contributes to the acquisition of anomalously stable partial thermoviscousmagnetization by chert even at low metamorphic temperatures 2011 elsevier b v all rights reserved

the chicxulub asteroid impact and mass extinction at the cretaceous paleogene boundary the cretaceous paleogene boundary similar to 65 5 million years ago marks one of the three largest mass extinctions in the past 500 million years the extinction event coincided with a large asteroid impact at chicxulub mexico and occurred within the time of deccan flood basalt volcanism in india here we synthesize records of the global stratigraphy across this boundary to assess the proposed causes of the mass extinction notably a single ejecta rich deposit compositionally linked to the chicxulub impact is globally distributed at the cretaceous paleogene boundary the temporal match between the ejecta layer and the onset of the extinctions and the agreement of ecological patterns in the fossil record with modeled environmental perturbations for example darkness and cooling lead us to conclude that the chicxulub impact triggered the mass extinction

ice avalanches some empirical information about their formation and reach evaluation d apres l etude d environ 100 avalanches de glace des zones de rupture et des distances d arret

precipitation of fe iii oxyhydroxide deposits from shallow water hydrothermal fluids in tutum bay ambitle island papua new guinea abstract previous research on sea floor fe iii oxyhydroxide deposits has focused primarily on deep sea hydrothermal systems found along volcanically active portions of the mid ocean ridges and on hydrogenetic deposits formed in deep basins and along continental shelves there is however not much known about their formation in shallow water settings associated with volcanic islands the hydrothermal system at ambitle island papua new guinea provides an excellent opportunity to study the formation of fe iii oxyhydroxides in a shallow water setting precipitation from the hydrothermal solution is caused by mixing with seawater based on a 87 sr 86 sr mixing model the calculated minimum and maximum seawater fractions are approximately 11 and 57 respectively thus precipitation of tutum bay fe iii oxyhydroxides takes place at a temperature range between approximately 60 and 93 c the chemical composition shows low mn contents fe mn 600 and elements that are usually enriched in fe iii oxyhydroxides such as co and v are below crustal abundance and well below their concentrations in island arc volcanics arsenic concentrations on the other hand are by two orders of magnitude higher than those in other marine deposits rare earth element ree concentrations reflect their concentration in the hydrothermal fluids rather than seawater the crystallinity of the deposits increases with age as protoferrihydrite is apparently altered to fe smectite and hematite and as bearing minerals are formed contact with seawater and therefore oxidizing conditions seems to be the factor increasing the crystallinity

detection of noninteracting single domain particles using first order reversal curve diagrams we present a highly sensitive and accurate method for quantitative detection and characterization of noninteracting or weakly interacting uniaxial single domain particles unisd in rocks and sediments the method is based on high resolution measurements of first order reversal curves forcs unisd particles have a unique forc signature that can be used to isolate their contribution among other magnetic components this signature has a narrow ridge along the hc axis of the forc diagram called the central ridge which is proportional to the switching field distribution of the particles therefore the central ridge is directly comparable with other magnetic measurements such as remanent magnetization curves with the advantage of being fully selective to sd particles rather than other magnetic components this selectivity is unmatched by other magnetic unmixing methods and offers useful applications ranging from characterization of sd particles for paleointensity studies to detecting magnetofossils and ultrafine authigenically precipitated minerals in sediments

remagnetization of carbonate rocks in southern tibet perspectives from rock magnetic and petrographic investigations netherlands organization for scientific research nwo with a rubicon grant 825 15 016 institute for rock magnetism irm at the university of minnesota instruments and facilities program of nsf erc starting grant 306810 nwo vidi 864 11 004

the effects of rockfall volume on runout distance the main goal of this paper is to clarify the effects of rockfall volume on its fluidization for this purpose outdoor rockfall experiments were carried out to analyze runout distances and individual movements of rockfall blocks and numerical simulations were conducted for these experiments to learn more about the mechanism of rockfall fluidization the rockfall experiments were conducted using an artificial slope on which granite slabs were surfaced and overlaid with cubiformed granite blocks the size and number of blocks were varied in this series of experiments further numerical simulations were carried out in which the coordinates of individual rockfall blocks were traced in three dimensional space from rockfall initiation to final deposition it became clear from the experiments and simulations that the runout distance had a positive correlation with the rockfall volume number of rockfall blocks and the runout distance of the gravity center of deposited rockfall mass had a negative correlation with the rockfall volume to clarify the mechanism of these two phenomena the positions of individual blocks from initial arrangements to final depositions were traced in the experimental and numerical simulations this revealed that the relative positions of each block along the slope direction were not changed during rockfall movement the reason for the block sequence preservation was that front facing blocks in the initial cube arrangement accelerated and rear facing blocks decelerated along the slope by internal collision between blocks further as the rockfall volume increased the opportunities for impact among the rockfall blocks increased with the front facing blocks pushed farther whereas the gravity center of the deposited rockfall mass had an inclination to travel shorter distances as rockfall blocks increased according to the increase of kinetic energy dispersed by the collision of blocks

biochemical vs detrital mechanism of remanence acquisition in marine carbonates a lesson from the k t boundary interval abstract an apparently complete carbonate rich cretaceous tertiary boundary interval in odp section 119 738c 20r 5 from the southern kerguelen plateau provides a unique insight into processes of magnetization acquisition in marine carbonates the boundary interval is characterized by a 1 m thick clay rich zone distinct depositional lamina are preserved within the basal 15 cm of this zone the upper part is bioturbated previous studies have demonstrated that the bulk of the detrital fraction in the laminated and bioturbated carbonates has the same local source and hence the two intervals likely had similar initial detrital assemblages magnetic properties of these rocks however differ significantly the laminated sediments have a higher content of non silicate bound iron yet approximately an order of magnitude lower intensity of the natural remanent magnetization compared to the bioturbated rocks our detailed rock magnetic study indicates that in the bioturbated interval the dominant iron bearing phase is single domain magnetite likely of biogenic origin in the laminated interval apart from a small ferromagnetic fraction with multi domain like behavior non silicate bound iron is mainly sequestered in paramagnetic phases likely poorly crystalline oxyhydroxides it appears that a shut down of biological productivity after the k t event allowed preservation of the initial detrital early authigenic iron phases that are dominated by reactive iron oxyhydroxides with recovery of the normal biological activity as evidenced by resumption of bioturbation the oxyhydroxides had been replaced with biogenic magnetite thus produced biochemical magnetization led to a several fold increase in the remanence our results suggest that in areas where bioavailable iron constitutes a significant part of the detrital input such as in pelagic marine environments distant from clastic sources the biochemical remanent magnetization may be the dominant process of magnetization acquisition

magnetic fabric variations in mesozoic black shales northern siberia russia possible paleomagnetic implications abstract a 28 m long section situated on the coast of the arctic ocean russia 74 n 113 e was extensively sampled primarily for the purpose of magnetostratigraphic investigations across the jurassic cretaceous boundary the section consists predominantly of marine black shales with abundant siderite concretions and several distinct siderite cemented layers low field magnetic susceptibility k ranges from 8 10 5 to 2 10 3 si and is predominantly controlled by the paramagnetic minerals i e iron bearing chlorites micas and siderite the siderite bearing samples possess the highest magnetic susceptibility usually one order of magnitude higher than the neighboring rock the intensity of the natural remanent magnetization m0 varies between 1 10 5 and 6 10 3 a m several samples possessing extremely high values of m0 were found there is no apparent correlation between the high k and high m0 values on the contrary the samples with relatively high m0 values possess average magnetic susceptibility and vice versa according to the low field anisotropy of magnetic susceptibility ams three different groups of samples can be distinguished in the siderite bearing samples i an inverse magnetic fabric is observed i e the maximum and minimum principal susceptibility directions are interchanged and the magnetic fabric has a distinctly prolate shape triaxial fabric samples ii showing an intermediate magnetic fabric are always characterized by high m0 values it seems probable that the magnetic fabric is controlled by the preferred orientation of paramagnetic phyllosilicates e g chlorite and mica and by some ferromagnetic mineral with anomalous orientation in relation to the bedding plane oblate fabric samples iii are characterized by a bedding controlled magnetic fabric and by moderate magnetic susceptibility and m0 values the magnetic fabric is controlled by the preferred orientation of phyllosilicate minerals and to a minor extent by a ferrimagnetic fraction most probably detrital magnetite considering the magnetic fabric together with paleomagnetic component analyses the siderite bearing and the high nrm samples about 15 of samples were excluded from further magnetostratigraphic research

magnetotactic bacterial response to antarctic dust supply during the palaeocene eocene thermal maximum distinct magnetic properties of marine sediments that record the palaeocene eocene thermal maximum petm have been suggested to be due to a bacterial magnetofossil signal that is linked to enhanced weathering conditions during the petm we document the dominance of bacterial magnetite in deep sea sediments from southern kerguelen plateau ocean drilling program hole 738c southern ocean not only during the petm but also before and after the thermal event this occurrence of magnetofossils throughout the petm indicates that the occurrence of bacterial magnetosomes is not due to a preservation effect instead we suggest that it is due to sustained mild iron reducing conditions that dissolved the most labile aeolian derived iron which favoured continued magnetotactic bacterial activity without being strong enough to dissolve the less reactive magnetite and haematite enhanced aeolian haematite abundances at the beginning of the petm indicate drier conditions on the neighbouring antarctic continent at those times our results provide evidence that iron fertilisation by aeolian dust was the main limiting factor that conditioned proliferation of magnetotactic bacteria in the deep sea at the southern kerguelen plateau with the exception of two short periods of rapidly changing palaeoenvironmental conditions at the onset and termination of the petm increased iron supply from aeolian dust that enhanced oceanic primary productivity and subsequent delivery of organic carbon to the seafloor along with mild iron reducing diagenetic conditions seem to have been necessary to provide the iron needed for magnetite biomineralization by magnetotactic bacteria to drive their marked increase in abundance in the studied petm record from southern kerguelen plateau our analyses of a deep sea petm record from hole 1051b at blake nose atlantic ocean failed to identify magnetofossils despite evidence for the occurrence of magnetite and haematite of probable aeolian origin contrasting magnetic properties at these petm sections indicate that further work is needed to understand the palaeoenvironmental and diagenetic factors whose interactions lead to production and preservation of magnetofossils in deep sea sediments

multiproxy analysis of a new terrestrial and a marine cretaceous paleogene k pg boundary site from new zealand an integrated study of palynology mossbauer spectroscopy mineralogy and osmium isotopes has led to the detection of the first k pg boundary clay layer in a southern hemisphere terrestrial setting the k pg boundary layer was independently identified at centimetre resolution by all the above mentioned methods at the marine k pg boundary site of mid waipara and the terrestrial site of compressor creek greymouth coal field new zealand mossbauer spectroscopy shows an anomaly of fe containing particles in both k pg boundary sections jarosite at mid waipara and goethite at compressor creek this anomaly coincides with a turnover in vegetation indicated by an interval dominated by fern spores and extinction of key pollen species in both sections in addition to the terrestrial floristic changes the mid waipara section reveals a turnover in the dinoflagellate assemblages and the appearance of global earliest danian index species geochemical data reveal relatively small iridium enrichments in the boundary layers of 321 pg g at mid waipara and 176 pg g at compressor creek unradiogenic os 187 os 188 values of the boundary clay reveal the presence of a significant extraterrestrial component we interpret the accumulation of fe nano phases at the boundary as originating from both the impactor and the crystalline basement target rock the goethite and jarosite are interpreted as secondary phases formed by weathering and diagenesis the primary phases were probably controlled by the initial composition of the vapor plume and condensation kinetics rather than condensation thermodynamics this investigation indicates that identification of fe in nano phases by mossbauer spectroscopy is an accurate and cost effective method for identifying impact event horizons and it efficiently complements widely used biostratigraphic and geochemical methods c 2010 elsevier ltd all rights reserved less

challenges in isolating primary remanent magnetization from tethyan carbonate rocks on the tibetan plateau insight from remagnetized upper triassic limestones in the eastern qiangtang block abstract carbonate rocks widely used for paleomagnetically quantifying the drift history of the gondwana derived continental blocks of the tibetan plateau and evolution of the paleo meso neo tethys oceans are prone to pervasive remagnetization identifying remagnetization is difficult because it is commonly undetectable through the classic paleomagnetic field tests here we apply comprehensive paleomagnetic rock magnetic and petrographic studies to upper triassic limestones in the eastern qiangtang block our results reveal that detrital biogenic magnetite which may carry the primary natural remanent magnetization nrm is rarely preserved in these rocks in contrast authigenic magnetite and hematite pseudomorphs after pyrite and monoclinic pyrrhotite record three episodes of remagnetization the earliest remagnetization was induced by oxidation of early diagenetic pyrite to magnetite probably related to the collision between the northeastern tibetan plateau and the qiangtang block after closure of the paleo tethys ocean in the late triassic the second remagnetization residing in hematite and minor goethite which is the further subsurface oxidation product of pyrite magnetite is possibly related to the development of the localized cenozoic basins soon after india asia collision in the paleocene the youngest remagnetization is a combination of thermoviscous and chemical remanent magnetization carried by authigenic magnetite and pyrrhotite respectively our analyses suggest that a high supply of organic carbon during carbonate deposition prevailing sulfate reducing conditions during early diagenesis and widespread orogenic fluid migration related to crustal shortening during later diagenesis have altered the primary remanence of the shallow water tethyan carbonate rocks of the tibetan plateau we emphasize that all paleomagnetic results from these rocks must be carefully examined for remagnetization before being used for paleogeographic reconstructions future paleomagnetic investigations of the carbonate rocks in orogenic belts should be accompanied by thorough rock magnetic and petrographic studies to determine the origin of the nrm

testing the reliability of detrital cave sediments as recorders of paleomagnetic secular variations seso cave system central pyrenees spain abstract a paleomagnetic study has been carried out on a waterlaid detrital sedimentary sequence of 240 cm thick within the seso cave system west central pyrenees in these sediments seven charcoal samples were dated using 14c ams ranging from 2080 to 650 cal yr bp 130 bc 1300 ad two levels of human occupation of the cave have been recognized by ceramics associated to the iberian period and to the roman period respectively the detrital sedimentary sequence is made of autochthonous piping detached material from the eocene marls host rock inside of the cavity and allochthonous stream transported sediments from the outside sediments the autochthonous material first 100 cm made of fine grain laminated sediments lutites and marls corresponds to pond facies the allochthonous material 190 240 cm is made of lutites and sands and corresponds to stream facies and both facies are mixed from 100 to 190 cm the increase in sedimentation rate towards the end of the sequence stream facies points to an intensification of the alluvial activity as a possible consequence of a more arid climate during the medieval climate anomaly for the paleomagnetic study 44 discrete cylindrical samples were taken along the detrital sequence the values of the natural remanent magnetization and magnetic susceptibility are significantly lower in the pond sediments than in the stream sediments the declination and inclination of the paleomagnetic characteristic component sister samples analyzed by both alternating field and temperature demagnetizing procedure of each depth point is compared to the spanish archeomagnetic catalog and available geomagnetic models arch3k 1 cals3k 4 cals10k 1b and scha dif 3k in order to determine the accuracy of these sediments recording the earth s magnetic field results suggest that these sediments poorly record the earth s magnetic field however paleomagnetic inclination shows similar results between both demagnetizing methods and the inclination is well recorded especially in the younger stream facies the lack of archeological remains with absolute dates from 925 to 1545 cal yr bp in the iberian paleomagnetic secular variation reference curve has prevented up to now the study of that time period therefore the inclination data from the seso cave deposit is the first record of the iberian paleomagnetic secular variation during most part of the medieval time and they are closer to the inclination values of one geomagnetic model cals10k 1b

new paleomagnetic results from middle jurassic limestones of the qiangtang terrane tibet constraints on the evolution of the bangong nujiang ocean

magnetotactic bacterial abundance in pelagic marine environments is limited by organic carbon flux and availability of dissolved iron article i nfo article history magnetotactic bacteria intracellularly biomineralize magnetite of an ideal grain size for recording palaeomagnetic signals however bacterial magnetite has only been reported in a few pre quaternary records because progressive burial into anoxic diagenetic environments causes its dissolution deep sea carbonate sequences provide optimal environments for preserving bacterial magnetite due to low rates of organic carbon burial and expanded pore water redox zonations such sequences often do not become anoxic for tens to hundreds of metres below the seafloor nevertheless the biogeochemical factors that control magnetotactic bacterial populations in such settings are not well known we document the preservation of bacterial magnetite which dominates the palaeomagnetic signal throughout eocene pelagic carbonates from the southern kerguelen plateau southern ocean we provide evidence that iron fertilization associated with increased aeolian dust flux resulted in surface water eutrophication in the late eocene that controlled bacterial magnetite abundance via export of organic carbon to the seafloor increased flux of aeolian iron bearing phases also delivered iron to the seafloor some of which became bioavailable through iron reduction our results suggest that magnetotactic bacterial populations in pelagic settings depend crucially on particulate iron and organic carbon delivery to the seafloor

magnetic mineral diagenesis abstract reduction oxidation redox reactions occur during burial because sediments contain reactive mixtures of oxidised and reduced components diagenetic chemical reactions represent the approach of all sedimentary components toward equilibrium and control the long term stability of sedimentary iron bearing minerals magnetic minerals are sensitive indicators of sedimentary redox conditions and of changes in these conditions through time with diagenetic effects ranging from subtle to pervasive despite the importance of magnetic mineral diagenesis in paleomagnetism rock magnetism and environmental magnetism and the usefulness of these subjects in the earth and environmental sciences there is no systematic single published treatment of magnetic mineral diagenesis this paper is an attempt to provide such a treatment for the full range of diagenetic environments magnetic mineral diagenesis during early burial is driven largely by chemical changes associated with organic matter degradation in a succession of environments that range from oxic to nitrogenous to manganiferous to ferruginous to sulphidic to methanic where the free energy yielded by different oxidants decreases progressively in each environment in oxic environments the most important diagenetic processes involve surface oxidation of detrital minerals and precipitation of fe3 bearing minerals from solution in ferruginous environments the most reactive detrital and authigenic iron oxides undergo dissolution often mediated by dissimilatory iron reducing bacteria which releases fe2 that becomes available for other reactions the fe2 in solution can diffuse upward where it is oxidised to form new authigenic iron oxyhydr oxide minerals or it can become bioavailable to enable magnetotactic bacteria to biomineralise magnetite generally at the base of the overlying nitrogenous zone alternatively dissimilatory iron reducing bacteria can produce extracellular magnetite within ferruginous environments in sulphidic environments iron bearing detrital mineral assemblages undergo more radical alteration hydrogen sulphide which is a byproduct of bacterial sulphate reduction or of anaerobic oxidation of methane reacts with the fe2 released from iron mineral dissolution or directly with solid iron oxyhydr oxide minerals to form iron sulphide minerals mackinawite greigite and pyrite authigenic growth of ferrimagnetic greigite has important implications for paleomagnetic recording secondary iron sulphide formation can also occur as a result of anaerobic oxidation of methane methane migration through sediments in association with biogenic or thermogenic methane production or in association with gas hydrate dissociation can disrupt the diagenetic steady state and give rise to greigite and monoclinic pyrrhotite formation that remagnetises sediments most of the above described diagenetic processes occur below 50 c with continuing burial above 50 c but at sub metamorphic temperatures magnetic minerals can undergo further thermally induced chemical changes that give rise to a wide range of mineralogical transformations that affect the magnetic record of the host sediment these changes include remagnetisations magnetic analysis can provide much valuable information concerning diagenesis in environmental processes the range of processes discussed in this paper should assist researchers in analysing sediment magnetic properties for which the assessment of diagenetic effects has become a necessary component

first order reversal curve forc diagrams for pseudo single domain magnetites at high temperature abstract the recently developed first order reversal curve forc technique for rapidly examining magnetic domain state has great potential for paleomagnetic and environmental magnetic investigations however there are still some gaps in the basic understanding of forc diagrams in particular the behavior of pseudo single domain psd grains and the contribution of magnetostatic interactions in this paper we address some of these problems we report the first forc diagrams measurements on narrowly sized and well characterized synthetic psd through multidomain md magnetite samples the forc diagrams evolve with grain size from single domain sd like to md like through the psd grain size range since each sample contains grains of essentially a single size individual psd grains evidently contain contributions from both sd like and md like magnetic moments in proportions that vary with grain size the evolving forc diagrams cannot be due to physical mixtures of sd and md grains of widely different sizes the forc diagrams were all asymmetric small psd samples have forc diagrams with a distinctive closed contour structure the distributions of the larger md grains display no peak and lie closer to the interaction field axis to assess the effect of magnetostatic interactions we measured forc diagrams between room temperature and the curie temperature on heating the forc distributions contract without changing shape until 500 c above this temperature the diagrams become more md like and in addition become more symmetric the temperature dependence of the interaction field parameter is proportional to that of the saturation magnetization in accordance with neel s interpretation of the preisach diagram the decrease in asymmetry with heating suggests that the origin of the asymmetry lies in magnetostatic interactions the magnetic hysteresis parameters as a function of temperature were determined from the forc curves as the grain size decreased the normalized coercive force was found to decrease more rapidly with temperature

k t boundary extinction geologically instantaneous or gradual event evidence from deep sea benthic foraminifera a stepped pattern of benthic foraminiferal decimation survival and recovery identified across the cretaceous tertiary k t boundary at caravaca southeast spain suggests that an exceptionally large influx of nutrients reached the sea floor suddenly at the time of the k t extinction event rapid reestablishment of pre k t boundary environmental conditions followed our data demonstrate a close coupling between the collapse of the ecologic system in the upper water column at caravaca and a correspondent decimation of bottom dwellers on the sea floor the dramatic environmental collapse of deep water benthic foraminiferal communities at the end of the cretaceous at caravaca suggests that the k t boundary event was geologically instantaneous and catastrophic

paleoenvironmental signature of the selandian thanetian transition event stte and early late paleocene event elpe in the contessa road section western neo tethys abstract sedimentary records of the early cenozoic indicate a series of events with climatic and carbon cycle variability known as hyperthermals a 350 kyr long event of environmental disruption during the paleocene not described before and here named selandian thanetian transition event stte has been recognized and well constrained in the western tethys contessa road section gubbio italy through high resolution biostratigraphic geochemical and rock magnetic data the stte exhibits peculiar stressed ecological responses among calcareous nannofossils and foraminifera which highlight marked environmental perturbation affecting the biosphere the environmental instability is not confined within the photic zone but extends to the seafloor leading to little more trophic conditions of the sea surface waters with an enhanced but of short measure nutrient availability on the seafloor conditions and marked rise of lysocline magnetic susceptibly ms is dominantly controlled by the balance between carbonate productivity and detrital supply as evidenced by the strong correlation between ms and caco3 r2 0 72 however we also document two components in the isothermal remanent magnetization irm and first order reversal curves forc diagrams that prove the occurrence of biogenic magnetite throughout the stte systematic variations in bio geochemical and magnetic parameters show the relative abundance of carbonate production or inversely dissolution of carbonate versus detrital supply during the stte which induced higher populations of magnetotactic bacteria through increased terrigenous input and therefore increased nutrient supply noteworthy the uppermost part of the stte includes the equivalent of the suspected hyperthermal short lived early late paleocene event elpe the elpe event shows an episode of increase in magnetic properties of the sediments including an increase in magnetofossil concentration as indicated by irm components and forc diagrams the comparison of biotic and abiotic records throughout the stte at contessa road section with available data across the elpe from former investigated ocean and land based sites provides lines of evidence that this latter event might be indeed only the terminal part of a long lasting environmental change than hitherto supposed

magnetic detection and characterization of biogenic magnetic minerals a comparison of ferromagnetic resonance and first order reversal curve diagrams australian research council grant number dp120103952 australia new zealand iodp consortium anzic grant number le0882854

chemical and physical characteristics of iron oxides in riverine and glacial meltwater sediments abstract the speciation and grain size distribution of iron have been examined in several riverine and glacial sediments in order to evaluate controls on the concentrations and modes of occurrence of iron oxides in weathering derived particulates three operationally defined fractions have been recognised relating to the reactivity of iron towards dissolved sulphide 1 highly reactive iron fe hr consisting of ferric oxides 2 poorly reactive iron fe pr consisting of magnetite and some sheet silicate iron and 3 unreactive residual silicate iron fe u the finer fractions of each riverine and glacial suite are enriched in fe hr and usually also have higher fe hr fet ratios however coarser fractions may also have elevated fe hr contents and relationships of fe hr to both surface area and al indicate that iron oxides dominantly occur in association with the surfaces of aluminosilicate minerals which are common in finer grained fractions but may also be relatively enriched in coarser fractions a similar range of fe hr surface loadings is evident in the riverine and glacial meltwater suites from 0 6 to 1 8 mg fe m 2 for the riverine sediments and 0 5 to 1 5 mg fe m 2 for the glacial meltwater sediments despite wide variations in catchment bedrock lithology and climatic conditions significantly lower iron oxide loadings 0 1 mg fe m 2 were found in subglacial sediments most fe hr is closely associated with organic c although the latter occurs as blebs or smears rather than continuous coatings e g ransom et al 1997 ransom b bennett r h baerwald r shea k 1997 tem study of in situ organic matter on continental margins occurrence and the monolayer hypothesis mar geol 138 1 9 electron microscope observations show that iron oxides may occur as nanoparticulate 10 20 nm spheres at the edges of sheet aluminosilicate minerals as crystalline oxides infilling surface pits in coarser grained aluminosilicate minerals and occasionally as discrete crystalline oxides the similar occurrences of iron oxides in both riverine and glacial environments indicates that the delivery of reactive iron oxides to the oceans during glacial interglacial cycles will be affected by the masses and grain size characteristics of sediments delivered by glacial and riverine fluxes rather than by differences in iron mineralogy these delivery rates will be modified by behaviour of the oxide nanoparticles during transport from the continents

enhanced primary productivity and magnetotactic bacterial production in response to middle eocene warming in the neo tethys ocean abstract earth s climate experienced a warming event known as the middle eocene climatic optimum meco at 40 ma which was an abrupt reversal of a long term eocene cooling trend this event is characterized in the deep southern atlantic pacific and indian oceans by a distinct negative 18 o excursion over 500 kyr we report results of high resolution paleontological geochemical and rock magnetic investigations of the neo tethyan monte cagnero mca section northeastern apennines italy which can be correlated on the basis of magneto and biostratigraphic results to the meco event recorded in deep sea sections in the mca section an interval with a relative increase in eutrophic nannofossil taxa and decreased abundances of oligotrophic taxa spans the culmination of the meco warming and its aftermath and coincides with a positive carbon isotope excursion and a peak in magnetite and hematite goethite concentration the magnetite peak reflects the appearance of putative magnetofossils while the hematite goethite apex is attributed to an enhanced detrital mineral contribution likely as aeolian dust transported from the continent adjacent to the neo tethys ocean during a drier more seasonal climate during the peak meco warming based on our new geochemical paleontological and magnetic records the meco warming peak and its immediate aftermath are interpreted as a period of high primary productivity sea surface iron fertilization is inferred to have stimulated high phytoplankton productivity increasing organic carbon export to the seafloor and promoting enhanced biomineralization of magnetotactic bacteria which are preserved as putative magnetofossils during the warmest periods of the meco event in the mca section together with previous studies our work reinforces the connection between hyperthermal climatic events and the occurrence or increased abundance of putative magnetofossils in the sedimentary record

evidence for the recovery of terrestrial ecosystems ahead of marine primary production following a biotic crisis at the cretaceous tertiary boundary the fossil record demonstrates that mass extinction across the cretaceous tertiary k t boundary is more severe in the marine than the terrestrial realm we hypothesize that terrestrial ecosystems were able to recover faster than their marine counterparts to test this hypothesis we measured sedimentary 13 c as a tracer for global carbon cycle changes and compared it with palaeovegetational changes reconstructed from palynomorphs and cuticles across the k t boundary at sugarite new mexico usa different patterns of perturbation and timescales of recovery of isotopic and palaeobotanical records indicate that the 13 c excursion reflects the longer recovery time of marine versus terrestrial ecosystems

the detection of bacterial magnetite in recent sediments of lake chiemsee southern germany the sediments of lake chiemsee located in the alpine foreland in southern germany host a variety of magnetotactic bacteria mtb which contain intracellular crystals of magnetite arranged in linear chains to detect bacterial magnetite in the carbonate dominated surface sediments and further quantify its contribution to the magnetic signal of the sediments we conducted detailed rock magnetic measurements as well as complimentary non magnetic analyses electron microscopy powder x ray diffraction and sediment pore water analysis our results demonstrate that biogenic single domain magnetite characterized by bullet and truncated hexagonal prismatic shapes is the dominant ferrimagnetic component in the topmost few centimetres of the sediment the changes of magnetic properties with depth are due to the occurrence of live mtb and the downward increasing dissolution of biogenic magnetite moreover the ratios of remanence loss on warming through the verwey transition after field cooling and zero field cooling of saturation isothermal remanence dfc dzfc were determined as 1 47 and 1 25 for freeze dried and air dried sediment samples containing mtb respectively these low ratios suggest that the bacterial magnetite chains were disrupted to a large extent and or that the bacterial magnetosomes might have undergone partial

searching for single domain magnetite in the pseudo single domain sedimentary haystack implications of biogenic magnetite preservation for sediment magnetism and relative paleointensity determinations 1 magnetic hysteresis measurements of sediments have resulted in widespread reporting of pseudo single domain like magnetic properties in contrast the ideal single domain sd properties that would be expected to be responsible for high quality paleomagnetic records are rare determining whether sd particles are rare or common in sediments requires application of techniques that enable discrimination among different magnetic components in a sediment we apply a range of such techniques and find that sd particles are much more common than has been reported in the literature and that magnetite magnetofossils the inorganic remains of magnetotactic bacteria are widely preserved at depth in a range of sediment types including biogenic pelagic carbonates lacustrine and marine clays and possibly even in glaci marine sediments thus instead of being rarely preserved in the geological record we find that magnetofossils are widespread this observation has important implications for our understanding of how sediments become magnetized and highlights the need to develop a more robust basis for understanding how biogenic magnetite contributes to the magnetization of sediments magnetofossils also have grain sizes that are substantially smaller than the 1 15 m size range for which there is reasonable empirical support for relative paleointensity studies the different magnetic response of coexisting fine biogenic and coarser lithogenic particles is likely to complicate relative paleointensity studies this issue needs much closer attention despite the fact that sediments have been subjected to paleomagnetic investigation for over 60 years much remains to be understood about how they become magnetized

aeolian input of bioavailable iron to the ocean 1 atmospheric deposition of mineral dust supplies much of the essential nutrient iron to the ocean presumably only the readily soluble fraction is available for biological uptake previous ocean models assumed this fraction was constant here the variable solubility of fe in aerosols and precipitation is parameterized with a two step mechanism the development of a sulfate coating followed by the dissolution of iron hydr oxide on the dust aerosols the predicted soluble fe fraction increases with transport time from the source region and with the corresponding decrease in dust concentration the soluble fraction is 1 percent near sources but often 10 40 percent farther away producing a significant increase in soluble fe deposition in remote ocean regions our results may require more rapid biological and physicochemical scavenging of fe than used in current ocean models we further suggest that increasing so2 emission alone could have caused significant fe fertilization in the modern northern hemisphere oceans

diagenetic sensitivity of paleoenvironmental proxies a rock magnetic study of australian continental margin sediments a rock magnetic study of the upper 40 m of the ocean drilling program site 133 820a recovered at the outer edge of the northeastern australian continental margin shows that downcore variations in magnetic parameters are diagenetically driven and correlate with the changes in global sea level we identified intervals enriched in single domain sd magnetite in the studied section unlike previous studies that postulated a detrital source we show that the bulk of the sd fraction is biologically produced and is likely to be authigenic the abundance of sd magnetite thus cannot be used as an indicator for provenance or sediment transport mechanisms the biogenic magnetite was preserved during the high sedimentation rate periods likely due to a short residence time in the corrosive zone of active iron reduction the presence of the biogenic magnetite thus can be used as an indicator for a low degree of reductive dissolution more advanced dissolution during the periods of slow sedimentation coincident with sea level highstands resulted in significant changes in the composition of the detrital assemblage particularly in relative abundances of different mineral phases the overall stability of the detrital magnetic minerals toward dissolution varies in the studied section as hematite magnetite goethite due to the higher stability of hematite reductive diagenesis in general will lead to the lowering of the goethite hematite g h ratio which can be mistaken for an increased aridity in the sedimentary source areas in the conventional interpretation for this proxy the sensitivity of the g h proxy to diagenesis should be taken into account in paleoenvironmental studies

detection of live magnetotactic bacteria in south atlantic deep sea sediments abstract since they were first descrbed in 1975 1 magnetotactic bacteria have been observed in a wide variety of environments including soils 2 rivers freshwater lakes and coastal marine sediments 3 4 and in one case at a water depth of about 600 m in a hemipelagic sediment 5 here we report the first systematic findings of magnetotactic bacteria in pelagic and hemipelagic sediments of the eastern south atlantic different morphologies were identified cocci spirilla vibrionic and rod shaped forms at water depths to about 3000 m on the african continental margin between the equator and 30 s and on the walvis ridge in a pelagic environment about 1400 km off the coast no actively swimming magnetotactic bacteria could be detected in deeper waters magnetotactic bacteria were observed only in the upper 10 cm of the sediments with the highest concentrations always found in a well defined narrow subsurface layer the depth of this layer is dependent on the input and turnover rates of organic matter in the sediment measurements of oxygen and nitrate concentrations 6 7 provide an assignment of depth distributions of magnetotactic bacteria to the stratification of terminal electron acceptors in the sediment the data show clearly that the majority of marine magnetotactic bacteria live in the anaerobic zone in contrast to former assumptions that they may inhabit microaerobic layers 8 11

variable remanence acquisition efficiency in sediments containing biogenic and detrital magnetites implications for relative paleointensity signal recording widespread geological preservation of biogenic magnetite makes it important to assess how such particles contribute to sedimentary paleomagnetic signals we studied a sediment core from the south china sea that passes the strict empirical criteria for magnetic uniformity used in relative paleointensity studies such assessments are based routinely on bulk magnetic parameters that often fail to enable identification of mixed magnetic mineral assemblages using techniques that enable component specific magnetic mineral identification we find that biogenic and detrital magnetites occur in approximately equal concentrations within the studied sediments we analyzed normalized remanence signals associated with the two magnetite components to assess whether co occurring biogenic and detrital magnetites record geomagnetic information in the same way and with the same efficiency paleomagnetic directions for the two components have no phase lag which suggests that the biogenic and detrital magnetites acquired their magnetizations at equivalent times however we find that the biogenic magnetite is generally 2 4 times more efficient as the detrital magnetite in contributing to the natural remanent magnetization nrm despite their approximately equal magnetic contributions variations in the concentration and efficiency of remanence acquisition of the two components suggest that a significant part of the nrm is controlled by nongeomagnetic factors that will affect relative paleointensity recording we recommend that methods suited to the detection of variable recording efficiency associated with biogenic and detrital magnetites should be used on a routine basis in relative paleointensity studies

combined use of magnetometry and spectroscopy for identifying magnetofossils in sediments identification of the mineral remains of magnetotactic bacteria mtb known as magnetofossils is of particular interest because their occurrence can be used for environmental and climatic reconstructions single domain magnetite particles which are biomineralized in the cell body of mtb have characteristic properties that can be used to detect their fossil remains acquisition of anhysteretic and isothermal remanent magnetization arm and irm first order reversal curve forc diagrams and ferromagnetic resonance fmr spectra were used to detect the magnetic mineral inventory in holocene lake sediments a comparative analysis in terms of the discriminatory power of these methods is presented the forc diagrams contain two distinct features a sharp horizontal ridge centered on the horizontal axis bc and a feature with symmetric spread along the vertical bb axis the coercivity spectra derived from the central ridge coincides with that derived from arm and irm acquisition curves and is compatible with the presence of noninteracting linear chains of single domain magnetite the second feature on forc diagrams is indicative of interacting particles in clusters in the fmr spectra from bulk sediment two populations are separated empirically based on the forc information an asymmetric signal is taken to describe the population which contains single domain particles in clusters empirical spectral separation of this contribution results in fmr spectra that are similar to those of intact mtb which strongly suggests that a fraction of linear magnetosome chains is present combination of fmr and forc results demonstrates the strong potential of these methods for identifying magnetofossils based on alignment and interaction patterns of magnetic particles

nanogoethite is the dominant reactive oxyhydroxide phase in lake and marine sediments iron oxides affect many elemental cycles in aquatic sediments via numerous redox reactions and their large sorption capacities for phosphate and trace elements the reactive ferric oxides and oxyhydroxides are usually quantified by operationally defined selective chemical extractions that are not mineral specific we have used cryogenic 57fe mossbauer spectroscopy to show that the reactive iron oxyhydroxide phase in a large variety of lacustrine and marine environments is nanophase goethite feooh rather than the assumed surface complex stabilized two line ferrihydrite and accompanying mixture of clay and oxyhydroxide fe bearing phases this result implies that the kinetic and stability parameters of the type of nanogoethite that we observe to be present in sediments should be first determined and then used in models of early diagenesis the identity and characteristics of the reactive phase will also set constraints on the mechanisms of its authigenesis

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 kolka karmadon rock ice slide of 20 september 2002 an extraordinary event of historical dimensions in north ossetia russian caucasus on 20 september 2002 an enormous rock ice slide and subsequent mud flow occurred on the northern slope of the kazbek massif northern ossetia russian caucasus it started on the north northeast wall of dzhimarai khokh 4780 ma s l and seriously affected the valley of genaldon karmadon immediate governmental actions available scientific information first reconstructions hazard assessments and monitoring activities as well as initial expert judgments recommendations are documented in order to enable more detailed analyses and modelling of the event by the wider scientific community among the most remarkable aspects related to this event are 1 the relation between the recent event and somewhat smaller but quite similar events that occurred earlier in historical times 1835 1902 2 the interactions between unstable local geological structures and complex geothermal and hydraulic conditions in the starting zone with permafrost cold to polythermal hanging glaciers and volcanic effects hot springs in close contact with each other 3 the erosion and incorporation of a debris covered valley glacier largely enhancing the sliding volume of rocks ice firn snow water and probably air to a total of about 100 106m3 and 4 the astonishingly high flow velocities up to 300 km h 1 and enormous length of travel path 18 km plus 15 km of debris mud flow this extraordinary case illustrates that large catastrophic events in high mountain regions typically involve a multitude of factors and require integrated consideration of complex chains of processes a task which must be undertaken by qualified groups of experts

challenging the sensitivity limits of paleomagnetism magnetostratigraphy of weakly magnetized guadalupian lopingian permian limestone from kyushu japan despite their utility for bio and chemostratigraphy many carbonate platform sequences have been difficult to analyze using paleomagnetic techniques due to their extraordinarily weak natural remanent magnetizations nrms however the physical processes of magnetization imply that stable nrms can be preserved that are many orders of magnitude below our present measurement abilities recent advances in reducing the noise level of superconducting magnetometer systems particularly the introduction of dc squid sensors and development of a low noise sample handling system using thin walled quartz glass vacuum tubes have solved many of these instrumentation problems increasing the effective sensitivity by a factor of nearly 50 over the previous techniques of squid moment magnetometry here we report the successful isolation of a two polarity characteristic remanent magnetization from middle late permian limestone formed in the atoll of a mid oceanic paleo seamount now preserved in the jurassic accretionary complex in japan which had proved difficult to analyze in past studies paleothermometric indicators including conodont alteration indices carbonate petrology and clumped isotope paleothermometry are consistent with peak burial temperatures close to 130 c consistent with rock magnetic indicators suggesting fine grained magnetite and hematite holds the nrm the magnetic polarity pattern is in broad agreement with previous global magnetostratigraphic summaries from the interval of the early middle permian kiaman reversed superchron and the permian triassic mixed interval and ties the tethyan panthalassan fusuline zones to it elevated levels of hematite associated with the positive 13 c carb of the kamura event argue for a brief spike in environmental oxygen the results also place the paleo seamount at a paleolatitude of 12 s in the middle of the panthalassan ocean and imply a n nw transport toward the asian margin of pangea during triassic and jurassic times in accordance with the predicted trajectory from its tectono sedimentary background these developments should expand the applicability of magnetostratigraphic techniques to many additional portions of the geological time scale

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

a stable southern margin of asia during the cretaceous paleomagnetic constraints on the lhasa qiangtang collision and the maximum width of the neo tethys

airborne remote sensing applications to coastal wave research airborne sensors provide effective coverage of a broad region and are suitable for large scale experiments in this paper two scanning sensors that use the direct ranging technique to measure surface wave displacement are described on a nasa p 3 aircraft the sensors can complete one run across a 100 km continental shelf in 17 min a case study is presented using radar measured two dimensional surface topography to derive wave damping due to bottom friction the results are in good agreement with an analytical model based on a quadratic formulation of bottom shear stress this study demonstrates that remote sensing measurements can be used for rapid characterization of surface waves on the continental shelf and in coastal regions examples illustrated in this paper include the derivation of wavenumber spectra and estimation of the dissipation rate of shoaling ocean swell

global plate motion frames toward a unified model 1 plate tectonics constitutes our primary framework for understanding how the earth works over geological timescales high resolution mapping of relative plate motions based on marine geophysical data has followed the discovery of geomagnetic reversals mid ocean ridges transform faults and seafloor spreading cementing the plate tectonic paradigm however so called absolute plate motions describing how the fragments of the outer shell of the earth have moved relative to a reference system such as the earth s mantle are still poorly understood accurate absolute plate motion models are essential surface boundary conditions for mantle convection models as well as for understanding past ocean circulation and climate as continent ocean distributions change with time a fundamental problem with deciphering absolute plate motions is that the earth s rotation axis and the averaged magnetic dipole axis are not necessarily fixed to the mantle reference system absolute plate motion models based on volcanic hot spot tracks are largely confined to the last 130 ma and ideally would require knowledge about the motions within the convecting mantle in contrast models based on paleomagnetic data reflect plate motion relative to the magnetic dipole axis for most of earth s history but cannot provide paleolongitudes because of the axial symmetry of the earth s magnetic dipole field we analyze four different reference frames paleomagnetic african fixed hot spot african moving hot spot and global moving hot spot discuss their uncertainties and develop a unifying approach for connecting a hot spot track system and a paleomagnetic absolute plate reference system into a hybrid model for the time period from the assembly of pangea 320 ma to the present for the last 100 ma we use a moving hot spot reference frame that takes mantle convection into account and we connect this to a pre 100 ma global paleomagnetic frame adjusted 5 in longitude to smooth the reference frame transition using plate driving force arguments and the mapping of reconstructed large igneous provinces to core mantle boundary topography we argue that continental paleolongitudes can be constrained with reasonable confidence

quantifying magnetite magnetofossil contributions to sedimentary magnetizations under suitable conditions magnetofossils the inorganic remains of magnetotactic bacteria can contribute to the natural remanent magnetization nrm of sediments in recent years magnetofossils have been shown to be preserved commonly in marine sediments which makes it essential to quantify their importance in palaeomagnetic recording in this study we examine a deep sea sediment core from offshore of northwestern western australia the magnetic mineral assemblage is dominated by continental detritus and magnetite magnetofossils by separating magnetofossil and detrital components based on their different demagnetization characteristics it is possible to quantify their respective contributions to the sedimentary nrm throughout the brunhes chron in the studied core the contribution of magnetofossils to the nrm is controlled by large scale climate changes with their relative importance increasing during glacial periods when detrital inputs were low our results demonstrate that magnetite magnetofossils can dominate sedimentary nrms in settings where they are preserved in significant abundances

room temperature magnetic properties of ferrihydrite a potential magnetic remanence carrier abstractroom temperature magnetic techniques are rapidly becoming an important tool for the discrimination and quantification ofminerals in a broad range of geological environmental and extraterrestrial materials the utility of these techniques is dependentupon the comprehensiveness and consistency of data describing the magnetic characteristics of individual mineral phases in thisregard there is a paucity of data pertaining to the magnetic properties of ferrihydrite this paper reports room temperaturemagnetic properties of ferrihydrite that are relevant to studies concerning geological and environmental materials under surficialconditions synthetic ferrihydrite both 2 and 6 line varieties exhibit a susceptibility of 2 7 10 6 m 3 kg 1 at 298 k significantly at 298 k synthetic ferrihydrite exhibits a saturation remanent magnetization r rs of 1 19 and 0 86 10 4 am 2 kg 1 for 2 and 6 line ferrihydrite respectively these observations suggest that ferrihydrite is a potential magnetic remanencecarrier under ambient environmental conditions measurements of mineral mixtures show that magnetic techniques candiscriminate between ferrihydrite and goethite at low concentrations natural samples of ferrihydrite exhibit magnetic propertiesthat are consistent with those obtained for synthetic ferrihydrite thus magnetic techniques using room temperature magneticproperties may find important applications in studies of the biogeochemical cycling of fe in dynamic settings d 2005 elsevier b v all rights reserved

palaeomagnetism of middle triassic red bedded cherts from southwest japan equatorial palaeolatitude of primary magnetization and widespread secondary magnetization summary middle triassic anisian red bedded cherts were collected from the ajiro area in kyushu island southwest japan for a palaeomagnetic study thermal demagnetization showed five distinct remanent magnetization components from the cherts the lowest temperature component below 200 c component a is a present day viscous overprint the second up to 420 c component b and third demagnetized up to 580 c component c components are secondary magnetizations of thermoviscous and chemical origin respectively both of which reside in magnetite the fourth removed component component d was demagnetized at 630 c and is a secondary magnetization carried by haematite components b and c are considered to be a result of widespread remagnetization in southwest japan during the cretaceous the basis of this consideration is that red chert remagnetizations from an area about 600 km away from the present study area are of the same origins with unblocking spectra similar to those observed in this study the last removed component during thermal demagnetization up to 695 c component e is a primary magnetization carried by haematite the directions of this component after correction for the inclination flattening yielded a mean palaeolatitude of 2 s at the time of deposition in the middle triassic the ajiro cherts accreted as part of an accretionary complex to the eastern margin of the south china block during the middle jurassic and were then translated northwards from the continental margin to their present position through a margin parallel translation process owing to the oblique convergence of the oceanic plate

unraveling driving factors for large rock ice avalanche mobility large rock ice avalanches have attracted attention from scientists for decades and some of these events have caused high numbers of fatalities a relation between rock slope instabilities in cold high mountain areas and climate change is currently becoming more evident and questions about possible consequences and hazard scenarios in densely populated high mountain regions leading beyond historical precedence are rising to improve hazard assessment of potential rock ice avalanches their mobility is a critical factor this contribution is an attempt to unravel driving factors for the mobility of large rock ice avalanches by synthesizing results from physical laboratory experiments and empirical data from 64 rock ice avalanches with volumes 1x106 m3 from glacierized high mountain regions around the world the influence of avalanche volume water and ice content low friction surfaces and topography on the apparent coefficient of friction as a measure of mobility is assessed in laboratory experiments granular ice in the moving mass was found to reduce bulk friction up to 20 while water led to a reduction around 50 for completely saturated material compared with dry flows evidence for the effects of water as a key driving factor to enhance mobility was also found in the empirical data while the influence of the ice content could not be confirmed to be of much relevance in nature besides liquefaction it was confirmed that mobility increases with volumes and that frictional surface characteristics such as flow paths over glaciers are also dominant variables determining mass movement mobility effects of the topography along the flow path as well as channeling are assumed to be other critical factors the results provide an empirical basis to roughly account for different path and flow characteristics of large rock ice avalanches and to find appropriate ranges for friction parameters for scenario modeling and hazard assessments copyright 2011 john wiley sons ltd

low temperature magnetic properties of pelagic carbonates oxidation of biogenic magnetite and identification of magnetosome chains 1 pelagic marine carbonates provide important records of past environmental change we carried out detailed low temperature magnetic measurements on biogenic magnetite bearing sediments from the southern ocean ocean drilling program odp holes 738b 738c 689d and 690c and on samples containing whole magnetotactic bacteria cells we document a range of low temperature magnetic properties including reversible humped low temperature cycling ltc curves different degrees of magnetite oxidation are considered to be responsible for the observed variable shapes of ltc curves a dipole spring mechanism in magnetosome chains is introduced to explain reversible ltc curves this dipole spring mechanism is proposed to result from the uniaxial anisotropy that originates from the chain arrangement of biogenic magnetite similar to published results for uniaxial stable single domain sd particles the dipole spring mechanism reversibly restores the remanence during warming in ltc measurements this supports a previous idea that remanence of magnetosome chains is completely reversible during ltc experiments we suggest that this magnetic fingerprint is a diagnostic indicator for intact magnetosome chains although the presence of isolated uniaxial stable sd particles and magnetically interacting particles can complicate this test magnetic measurements through the eocene section of odp hole 738b reveal an interval with distinct magnetic properties that we interpret to originate from less oxidized biogenic magnetite and enrichment of a biogenic hard component co occurrence of these two magnetic fingerprints during the late eocene in the southern ocean indicates less oxic conditions probably due to increased oceanic primary productivity and organic carbon burial

catastrophic landslide deposits in the karakoram himalaya in july 1986 three catastrophic landslides deposited about 20 x 106 cubic meters of debris on bualtar glacier in the karakoram himalaya a sudden acceleration and superficial breakup of the glacier provided an opportunity to examine the fresh deposits in depth beneath a surface layer of large boulders finer materials mainly sand and silt made up half of the total volume the fine materials were formed during the rock avalanche from mostly intact massive rock of the source zone velocity estimates suggest that this disaggregation occurred in less than 2 minutes coarse materials remained in bands of uniform lithology but the fine materials had diffused throughout the landslides a small amount of carbonate appears to have been calcined by frictional heating presumably at the base of the initial sliding masses these observations are relevant to understanding the mechanisms of catastrophic landslides other nearby rock avalanche deposits indicate that landslides are an important geomorphic process in the area and that they pose a continuing risk to human activity

contributions from glacially derived sediment to the global iron oxyhydr oxide cycle implications for iron delivery to the oceans estimates of glacial sediment delivery to the oceans have been derived from fluxes of meltwater runoff and iceberg calving and their sediment loads the combined total 2900 tg yr 1 of the suspended sediment load in meltwaters 1400 tg yr 1 and the sediment delivered by icebergs 1500 tg yr 1 are within the range of earlier estimates high resolution microscopic observations show that suspended sediments from glacial meltwaters supraglacial and proglacial sediments and sediments in basal ice from arctic alpine and antarctic locations all contain iron oxyhydr oxide nanoparticles which are poorly crystalline typically 5 nm in diameter and which occur as single grains or aggregates that may be isolated or attached to sediment grains nanoparticles with these characteristics are potentially bioavailable a global model comparing the sources and sinks of iron present as oxyhydr oxides indicates that sediment delivered by icebergs is a significant source of iron to the open oceans beyond the continental shelf iceberg delivery of sediment containing iron as oxyhydr oxides during the last glacial maximum may have been sufficient to fertilise the increase in oceanic productivity required to drawdown atmospheric co2 to the levels observed in ice cores

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

rock magnetism of remagnetized carbonate rocks another look authigenic formation of fine grained magnetite is responsible for widespread chemical remagnetization of many carbonate rocks authigenic magnetite grains dominantly in the super paramagnetic and stable single domain size range also give rise to distinctiverock magnetic prop erties now commonly used as a fingerprint of remagnetization we re examine the basis of this association in terms of magnetic mineralogy and particle size distribution in remagnetized carbon ates having these characteristic rock magnetic properties including wasp waisted hysteresis loops high ratios of anhysteretic remanence to saturation remanence and frequency dependent susceptibility new measurements on samples from the helderberg group allow us to quantify the proportions of superparamagnetic stable single domain and larger grains and to evaluate the mineralogical composition of the remanence carriers the dominant magnetic phase is magnetite like with sufficient impurity to completely suppress the verwey transition particle sizes are extremely fine approximately 75 of the total magnetite content is superparamagnetic at room temperature and almost all of the rest is stable single domain although it has been pro posed that the single domain magnetite in these remagnetized carbonates lacks shape anisotropy and is therefore controlled by cubic magnetocrystalline anisotropy we have found strong exper imental evidence that cubic anisotropy is not an important underlying factor in the rock magnetic signature of chemical remagnetization the usefulness of a palaeomagnetic remanence is directly related to the accuracy with which its age is known e g van der voo 1990 primary remanence acquiredduringorverysoonafterrockformation pro videsdirectinformationonpalaeofieldorientationand strengthatthattime thepossibilityofpartialorcom plete remagnetization at a later time complicates palaeomagneticinterpretation ashaslongbeenrecog nized e g graham 1949 every robust palaeomag netic study must therefore include some effort to constrain the ages of identified components of the natural remanent magnetization nrm relative dating with respect to sedimentary processes struc tural tilting or cross cutting relationships is the basis of the classical geometric palaeomagnetic tests fold test and conglomerate test graham 1949 baked contact test everitt clegg 1962 unconformity test kirschvink 1978 the special circumstances required for application of these tests are often not available however or positive test results may providerelativelylooseconstraintsallowingmagneti zation ages that significantly post date the age of the rock formation as a result other evidence although generally more indirect becomes essential in evaluating the age origin and significance of nrm components such evidence commonly includes petrographic observations isotopic andgeo chemical data and rock magnetic characterization by their very existence magnetic overprints provide evidence of some process or event of