climate change during the early paleogene in the bogota basin colombia inferred from paleosol carbon isotope stratigraphy major oxides and environmental magnetism abstract paleosols in the bogota basin colombia record an increase in chemical weathering across the paleocene eocene p e transition rock magnetic properties and major element geochemistry exhibit an abrupt change in an interval identified as including the p e boundary as established by previously published biostratigraphy and a u pb date on volcanic zircons 56 2 1 6 ma during the stratigraphic interval that contains the p e transition magnetic susceptibility increases significantly with an order of magnitude higher concentration of magnetite maghemite and hematite the preponderance of pure stoichiometric magnetite in the paleosols of this interval indicates that the increase in magnetic susceptibility is due to changes in the rate of pedogenesis rather than an increase in the erosion of nearby volcanic rocks which would contribute titanomagnetite pedogenic structures preserved within the paleosols a lack of iron oxides as cement friability of the sandstones and previously published thermochronologic data are not consistent with burial diagenesis as an explanation for the origin of the magnetic mineral assemblage these enhanced pedogenesis and related chemical weathering is also confirmed by increases in the concentration of fe 2 o 3 and al 2 o 3 and loss on ignition values an increase in mean precipitation across this interval is inferred by the lack of carbonate nodules and a decrease in sio 2 in paleosols thus we hypothesize that there was an intensification of chemical weathering during the interval that contains the p e transition

discrimination of biogenic and detrital magnetite through a double verwey transition temperature magnetite occurs widely in natural environments in both inorganic and biogenic forms discrimination of the origin of magnetite has important implications from searching for past microbial activity to interpreting paleomagnetic and environmental magnetic records in a wide range of settings in this study we present rock magnetic and electron microscopic analyses of marine sediments from the continental margin of oman low temperature magnetic data reveal two distinct verwey transition tv temperatures that are associated with the presence of biogenic and inorganic magnetite this interpretation is consistent with room temperature magnetic properties and is confirmed by electron microscopic analyses our study justifies the use of two distinct tv temperatures as a diagnostic signature for discriminating inorganic and biogenic magnetite simple low temperature magnetic measurements therefore provide a tool to recognize rapidly the origin of magnetite within natural samples in addition our analyses reveal progressive down core dissolution of detrital and biogenic magnetite but with preservation of significant amounts of fine grained magnetite within sediments that have been subjected to severe diagenetic alteration we demonstrate that preservation of magnetite in such environments is due to protection of fine grained magnetite inclusions within silicate hosts our results therefore also provide new insights into diagenetic processes in marine sediments

a multi resolution approach to electromagnetic modelling we present a multi resolution approach for 3 d magnetotelluric forward modelling our approach is motivated by the fact that fine grid resolution is typically required at shallow levels to adequate

the paleomagnetism of single silicate crystals recording geomagnetic field strength during mixed polarity intervals superchrons and inner core growth 1 the basic features of the geomagnetic reversal chronology of the last 160 million years are well established the relationship between this history and other features of the field however has been elusive the determination of past field strength paleointensity is especially challenging commonly accepted results have come from analyses of bulk samples of lava historic lavas have been shown to faithfully record the past field strength when analyzed using the thellier double heating method data from older lavas however tend to show effects of in situ and laboratory induced alteration here we review an alternative approach single plagioclase crystals can contain minute magnetic inclusions 50 350 nm in size that are potential high fidelity field recorders thellier experiments using plagioclase feldspars from an historic lava on hawaii provide a benchmark for the method rock magnetic data from older lavas indicate that the feldspars are less susceptible to experimental alteration than bulk samples this resistance is likely related to the lack of clays in addition magnetic minerals are sheltered by the encasing silicate matrix from natural alteration that can otherwise transform the well defined thermoremanent magnetization into an irresolute chemical remanent magnetization if there is a relationship between geomagnetic reversal frequency and paleointensity it should be best expressed during superchrons intervals with few or no reversals thellier data sets based on single plagioclase crystals from lavas erupted during the cretaceous normal polarity superchron 83 120 million years ago suggest a strong 12 1022 am2 stable field consistent with an inverse relationship between reversal frequency and paleointensity superchrons may represent times when the pattern of core mantle boundary heat flux allows the geodynamo to operate at peak efficiency as suggested in some numerical models thellier data from single plagioclase crystals formed during times of moderate 10 reversals million years reversal occurrence suggest a weaker and more variable field these paleointensity data together with a consideration of paleomagnetic directions suggest that geomagnetic reversals field morphology secular variation and intensity are related the linkages over tens of millions of years imply a lower mantle control on the geodynamo on even longer timescales the magnetization held by plagioclase and other silicate crystals can be used to investigate the proterozoic and archean geomagnetic field during the onset of growth of the solid inner core data from plagioclase crystals separated from mafic dikes together with directional data from whole rocks indicate a dipole dominated field similar to that of the modern 2 5 2 7 billion years ago older archean rocks are of great interest for paleomagnetic and paleointensity investigations because they may record a time when the compositionally driven convection of the modern dynamo may not have been operating and a solid inner core did not play its current role in controlling the geometry of outer core flow most rocks of this age have been affected by low grade metamorphism investigations using single silicate grains provide arguably our best hope of seeing through secondary geologic events and reading the early history of the geodynamo absolute paleointensity measurements of the oldest rocks on the planet will require the further development of methods to investigate silicate crystals with exsolved magnetic minerals that address the uncertainties posed by thermocrystallization remanent magnetization anisotropy and slow cooling fortunately prior work in rock magnetism together with advances in analytical equipment and techniques provides a solid foundation from which these frontier issues can be approached

potbellies wasp waists and superparamagnetism in magnetic hysteresis because the response of a magnetic substance to an applied field depends strongly on the physical properties of the material much can be learned by monitoring that response through what is known as a magnetic hysteresis loop the measurements are rapid and quickly becoming part of the standard set of tools supporting paleomagnetic research yet the interpretation of hysteresis loops is not simple it has become apparent that although classic single domain pseudo single domain and multidomain loops described in textbooks occur in natural samples loops are frequently distorted having constricted middles wasp waisted loops or spreading middles and slouching shoulders potbellies such complicated loops are often interpreted in oversimplified ways leading to erroneous conclusions the physics of the problem have been understood for nearly half a century yet numerical simulations appropriate to geological materials are almost unavailable in this paper we discuss results of numerical simulations using the simplest of systems the single domain superparamagnetic sd sp system examination of the synthetic hysteresis loops leads to the following observations 1 wasp waisting and potbellies can easily be generated from populations of sd and sp grains 2 wasp waisting requires an sp contribution that saturates quickly resulting in a steep initial slope and potbellies require low initial slopes the sp contribution approaching saturation at higher fields the approach to saturation is dependent on volume hence the cube of grain diameter therefore there is a very strong dependence of hysteresis loop shape on the assumed threshold size 3 we were unable to generate potbellies using an sp sd threshold size as large as 30 nm and wasp waists cannot be generated using a threshold size as small as 8 nm the occurrence of both potbellies and wasp waists in natural samples is consistent with a room temperature threshold size of some 15 nm 5 4 simulations using a threshold size of 15 20 nm with populations dominated by sp grain sizes that is with a small number of sd grains produce synthetic hysteresis loops consistent with measured hysteresis loops and transmission electron microscopic observations from submarine basaltic glass 5 simulations and measurements using two populations with distinct coercivity spectra can also generate wasp waisted loops a relatively straightforward analysis of the resulting loops can distinguish the latter case from wasp waisting resulting from sp sd behavior

signature of magnetic enhancement in a loessic soil in nebraska united states of america our study of a loessic soil profile from east central nebraska shows that the a horizons of the modern soil are characterized by higher concentrations of fine grained b0 1 am magnetic minerals this pedogenic magnetic component leads to higher values of concentration dependent parameters such as magnetic susceptibility v isothermal remanent magnetization irm and anhysteretic remanent magnetization arm combined with increases in frequency dependent susceptibility vfd and arm irm ratios hysteresis properties are relatively insensitive towards the presence of this pedogenic magnetic component the magnetic properties of the soil profile are dominated by ferrimagnetic magnetite or maghemite analyses of bsoftq irm sirm and bhardq irm hirm however do show that approximately 80 90 of the remanence carrying magnetic component exists in the form of hematite or goethite and that the magnetically enhanced horizons are enriched in both ferri and antiferromagnetic minerals the pedogenic magnetic component is most likely caused by the conversion of paramagnetic iron bearing minerals to ferriand antiferromagnetic minerals soil compaction lessivage or decalcification cannot explain the observed magnetic soil properties magnetic analyses of loess paleosol sequences from the midwestern united states may yield valuable information about regional variability of paleoclimate based on the fine grained nature of the pedogenic magnetic component we expect grain size dependent proxies arm arm irm vfd to yield better paleoclimatic information than low field magnetic susceptibility used in previous analyses d 2004 elsevier b v all rights reserved

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

a paleotemperature record derived from dissolved noble gases in groundwater of the aquia aquifer maryland usa low 14c activities in groundwater of the confined part of the aquia aquifer in southeastern maryland suggest that most of this water infiltrated at least 30 000 years ago however radiocarbon contents of the dissolved inorganic carbon seem to be affected by isotopic exchange possibly with secondary calcite deposits in the formation leading to overestimated 14c ages whereas the geochemistry of the aquia aquifer complicates the application of the widely used 14c dating method the accumulation of radiogenic he seems to provide a viable alternative for establishing a chronology the quasi linear increase of he concentrations with flow distance observed in the aquia aquifer can be explained entirely by accumulation of in situ produced radiogenic he u and th concentrations in aquia sand were measured in order to determine the accumulation rate of 4he with sufficient confidence to establish a he time scale concentrations of dissolved atmospheric noble gases were used to derive mean annual ground temperatures at the time of infiltration these noble gas temperatures ngts clearly show the presence of water that infiltrated under much cooler conditions than at present ngts are correlated with chloride concentrations corroborating the hypothesis that chloride variations in this aquifer constitute a climate signal in contrast the stable isotope ratios 180 and d do not provide a clear record of past climatic changes in the aquia aquifer and the correlation between ngts and stable isotope ratios is weak the ngt record suggests that mean annual temperatures in this midlatitude coastal site during the last glacial maximum lgm were 9 0 0 6 c colder than during the holocene this difference is slightly lower than estimates derived from pollen data for this region but considerably larger than the rather uniform cooling of about 5 c indicated by noble gas studies in more southern locations of north america the larger cooling is ascribed to the influence of the laurentide ice sheet which at its maximum extension came as close as 250 km to our study site

rock magnetic properties of uncultured magnetotactic bacteria abstract understanding the magnetic properties of magnetite crystals fe3o4 produced by magnetotactic bacteria mtbs is of fundamental interest in fields of geosciences biomineralization fine particle magnetism and planetary sciences the database of bulk magnetic measurements on mtbs is however still too sparse to allow for generalizations due to difficulties in obtaining bacteria cells in sufficient quantities from natural environments and the fact that only a few cultivable strains are available here we report the first series of magnetic measurements on two air dried samples containing solely mtbs wild type cocci and magnetobacterium bavaricum which were directly isolated from carbonaceous lake sediments systematic rock magnetic studies show that 1 the magnetosomes in cells are dominated by single domain sd magnetite 2 the samples have delta ratios fc zfc higher than 2 3 the measured low temperature remanence cycling curves as well as the first order reversal curve forc diagrams are significantly different to those measured on synthetic sd magnetite samples and 4 the verwey transition temperature tv 100 k of mtb cells is distinctly lower than that from stoichiometric magnetite tv 120 125 k our results provide new insights on the magnetic properties of bacterial magnetite and advance the use of magnetic proxies for decoding the paleomagnetic signals of sediments containing bacterial magnetite

rock magnetic proxies of climate change in the loess palaeosol sequences of the western loess plateau of china summary results of the first detailed study of the climate proxy record in the loess palaeosol sequence at xining one of the few palaeoclimate sites in the currently arid western loess plateau of china illustrate the importance of making many types of rockmagnetic measurements other than susceptibility a multiparameter approach yielded confirmation that here as elsewhere in the loess plateau the susceptibility enhancement in palaeosols was caused primarily by ultrafine magnetite and maghaemite nevertheless magnetic enhancement was caused not exclusively by changes in relative grain size but also by variations in concentration and mineralogy of the magnetic fraction the effects of concentration variations were removed through normalization of susceptibility and anhysteretic remanence with saturation magnetization and saturation remanence respectively the resulting signal was ascribed more confidently to variation in magnetic grain size which in turn was interpreted as a better proxy of pedogenesis than simple susceptibility variations in magnetic mineralogy were also determined to constrain interpretations further the data were then used to discuss climate history at xining finally results from xining were compared with other western sites and contrasted with eastern sites in summary 1 data is presented from a new loess plateau site which also appears to yield a global climate signal 2 a demonstration is made of a more rock magnetically robust way to separate concentration composition and grain size controls on susceptibility and other magnetic parameters and 3 models are provided for inter regional comparisons of palaeoclimate proxy records

direct observation of ferrimagnetic ferroelastic domain interactions in magnetite below the verwey transition abstract the magnetic behaviour of magnetite at low temperatures is dominated by its transformation to a monoclinic crystal structure that is simultaneously ferrimagnetic ferroelastic and ferroelectric below 125 k the verwey transition here we use electron microscopy to reveal the relationship between ferrimagnetic and ferroelastic domain structures in monoclinic magnetite we present dynamic observations of magnetite during heating and cooling across the verwey transition revealing a diversity of unexpected interaction phenomena between crystallographic twins and magnetic domain walls this study provides a new understanding of the low temperature magnetic properties of magnetite that will affect a broad range of rock magnetic studies from the interpretation of magnetic remanence in terrestrial rocks and sediments to the search for biogenic magnetite in extraterrestrial materials

fracturing induced release of radiogenic 4he and 234u into groundwater during the last deglaciation an alternative source to crustal helium fluxes in periglacial aquifers external 4he sources have been invoked to explain 4he concentrations in groundwater greater than those expected from in situ u and th production in a fractured aquifer of ordovician age located in the st lawrence lowlands quebec canada 4he concentrations of up to 4 48 x 10 5 cm3 stp gh20 1 were measured such concentrations are ca 1000 times higher than would be expected from in situ production a concomitant increase in 4he concentration and 234u 238u activity ratio is shown suggesting a common release process in groundwater for 234u and 4he this process has tentatively been identified as glaciation induced rock fracturing following the laurentide ice sheet retreat the resulting increase in exposed grain surface facilitates 234u release by recoil and that of radiogenic 4he by diffusion using a model of helium diffusion from a spherical grain it is shown that rock fracturing facilitated the release of accumulated 4he at rates ranging from 4 2 x 10 10 cm3stp gh20 1 yr 1 to 1 06 x 10 8 cm3stp gh20 1 yr 1 these release rates are between 1 000 and 30 000 times higher than the local u and th steady state production rate of 3 5 1 4 x 10 13 cm3stp grock 1 yr 1 integration of 4he release rates over time yields a radiogenic 4he concentration of between 3 85 x 10 6 cm3stp gh20 1 and 7 12x 10 5 cm3stp gh20 1 in the range of concentrations measured in the st lawrence lowlands fractured aquifers results support the occurrence of a local radiogenic helium source to explain the excesses measured in groundwater without requiring a significant external he crustal flux

the different climatic response of pedogenic hematite and ferrimagnetic minerals evidence from particle sized modern soils over the chinese loess plateau abstract in recent years increasing interest in loess studies has focused on qualitative and quantitative paleoclimatic reconstruction using the imperfect antiferromagnetic mineral hematite however the linkage between the hematite formation and climatic variables remains controversial here we present the results of a comprehensive investigation of the magnetic properties and statistical analysis of a suite of clay and silt fractions of modern soil samples from 179 sites across the chinese loess plateau clp and adjacent regions our objective was to clarify the relationships between modern climatic variables and pedogenic hematite as well as pedogenic ferrimagnetic minerals first order reversal curve measurements were also conducted on representative particle sized subsamples from a n s transect to understand the differences in magnetic mineralogy between the two fractions our results show that pipette extraction separates the fine grained superparamagnetic sp and most of the single domain sd magnetic grains into the clay fraction and that the remaining silt fraction displays the magnetic properties of coarse pseudo single domain psd or a mixture of multidomain md psd and a few sd particles only the pedogenic clay fraction shows a strong correlation with climatic variables the application of redundancy analysis helps to distinguish the climate variables controlling the formation of ferrimagnetic minerals and hematite during pedogenesis on the clp pedogenic ferrimagnetic minerals are sensitive to mean annual precipitation while pedogenic hematite formation is preferentially dependent on mean annual temperature the confirmation of the temperature dependent nature of hematite on the clp provides a new possibility for quantitatively reconstructing the paleotemperature history of chinese loess paleosol sequences

changes in remanence coercivity and domain state at low temperature in magnetite abstract submicron magnetite crystals with mean sizes of 0 037 0 10 and 0 22 m undergo major changes in hysteresis properties and domain states in crossing the verwey transition t v 120 k the 0 037 m crystals are single domain sd both in the cubic phase at room temperature t 0 and in the monoclinic phase below t v the 0 10 and 0 22 m crystals have a mixture of sd and two domain 2d states at room temperature t 0 but mainly sd structures below t v in agreement with micromagnetic calculations coercive force h c increases on cooling through t v by a factor 3 5 in the submicron magnetites and 40 in a 1 3 mm single crystal because of the high crystalline anisotropy and magnetostriction of monoclinic magnetite as a result domain walls and sd moments are so effectively pinned below t v that all remanence variations in warming or cooling are reversible however between 100 k and t 0 remanence behavior is variable saturation remanence sirm produced in monoclinic magnetite at 5 k drops by 70 100 in warming across t v with minor recovery in cooling back through t v ultimate levels at 5 k of 23 37 for the submicron crystals and 3 for the 1 3 mm crystal in contrast sirm produced in the cubic phase at 300 k decreases 5 35 submicron or 95 1 3 mm during cooling from 300 to 120 k due to continuous re equilibration of domain walls but there is little further change in cooling through t v itself however the submicron magnetites lose a further 5 15 of their remanence when reheated through t v these irreversible changes in cycling across t v and the amounts of the changes have potential value in determining submicron magnetite grain sizes the irreversibility is mainly caused by 2d sd transformations on cooling through t v which preserve or enhance remanence while sd 2d transformations on warming through t v cause remanence to demagnetize

residence time indicators in groundwater the east midlands triassic sandstone aquifer the east midlands triassic sherwood sandstone aquifer which has been the subject of detailed radiometric age studies is used to investigate both inert and reactive constituents of groundwater as indicators of residence time detailed resampling of the aquifer in 1992 has provided a considerable body of new inorganic geochemistry data though without radiocarbon several inert indicators are defined including the isotopic ratios 18o 2h 36cl noble gas ratios as well as the halogen elements cl br f i and their element ratios these form a group of essentially unreactive tracers primarily reflecting changing rainfall inputs and palaeoclimatic conditions except at outcrop where human impacts are also seen clearly the concentrations of cl mainly from atmospheric sources remain below 25 mg l 1 cl over a distance of some 30 km from outcrop reactive indicators the result of time dependent water rock interactions include 13c mg ca sr ca na cl and show diagnostic trends along the flow lines however the concentrations of certain trace elements li rb cs mn and mo which are not limited by solubility constraints show linear trends along the present day flow gradient this water rock interaction is taking place in groundwaters with low total mineralisation and it can be demonstrated that reactions involving these elements and isotopes are occurring entirely within the aquifer since high salinity groundwaters are found below the sherwood sandstone there is good correlation between some of the elements and 14c activities in the 1977 data set and this has been used to derive a concentration age relationship for the 1992 set of data a chemical timescale for the aquifer is then established using a li and b a combination of five trace elements the age of the fresh groundwater is thus shown to be up to 100 ka bp indicating the likelihood of semi continuous recharge during the devensian glacial period preceding the glacial maximum when no recharge occurred 10 20 ka bp this approach may be of value in extending groundwater dating beyond the radiocarbon timescale as well as interpreting sites where no radiocarbon data are available the scatter from the mean age line indicates those groundwaters which are derived from either rapid or slower than average flow zones within the aquifer as well as age stratification this stratification is borne out by a depth profile of groundwater from a new borehole near to outcrop which is shown by a combination of chemical and isotopic tracers to contain holocene overlying late pleistocene water

3 dimensional magnetotelluric inversion including topography using deformed hexahedral edge finite elements and direct solvers parallelized on symmetric multiprocessor computers part ii direct data space inverse solution

the thermodynamic effect of nonhydrostatic stress on the verwey transition abstract experimental results on the change in the verwey transition temperature in magnetite under compression exhibit puzzling variability with slopes ranging from 6 to 16 k gpa our thermodynamic analysis of the verwey transition in magnetite explains much of this variability in terms of the transformation strain as magnetite changes in crystal structure from cubic to monoclinic because this strain involves a much larger change in shape than in volume the change in temperature of the phase boundary t v can be much more sensitive to nonhydrostatic stress than to hydrostatic pressure uniaxial compression and tension both are predicted to increase t v an effect that is opposite in sign and for favorable orientations up to six times greater per gigapascal than for pressure moreover because the monoclinic twin orientation with highest t v is thermodynamically favored our treatment also shows how any desired twin may be selected uniquely by applying stress of appropriate orientation thereby removing obstacles to understanding many kinds of low temperature phenomena in magnetite caused by the presence of multiple twins a similar analysis for the thermodynamic effects of strong magnetic fields is outlined predicting that t v will generally be lowered again the twin for which t v is highest i e is lowered the least will be the most stable twin selectivity per gigapascal of stress is up to two to three times stronger than that per tesla of magnetic field

modelling of helium transport in groundwater along a section in the pannonian basin underground water flow in sedimentary basins controls the distribution of dissolved salts and gases and their concentrations may therefore be used as indicators of the flow direction recent measurements of the 4he concentration in deep waters of the pannonian basin have great importance in this respect this paper presents an example of the simultaneous computation of water flow and helium distribution along a section crossing the great hungarian plain the model consists of three permeable layers the boundaries of the layers are prescribed using geologic sections constrained by ample borehole and seismic data the results of the finite difference calculations are fitted to the observed helium concentrations using a least squares algorithm that varies the model parameters the significance of the model is that it reconstructs the structure and flux of the groundwater flow and estimates the poorly known hydrogeological parameters of the flow regime such as hydraulic conductivities conductivity anisotropy and dispersion coefficients the statistical uncertainty of the estimated parameters is around half an order of magnitude an estimate of the regional average of the incoming helium flux is also obtained the total helium flux in the great hungarian plain at the surface is within the range observed in old stable continental areas of the earth

thermochemical remanent magnetization in precambrian rocks are we sure the geomagnetic field was weak 1 thellier paleointensity determinations from two dikes of the early proterozoic 2 46 ga matachewan dike swarm canada yield field values of 2 14 0 18 and 9 81 1 65 t corresponding values of virtual axial dipole moment are very low 0 54 0 05 1022 and 2 49 0 42 1022 am2 respectively when compared with the modern field the characteristic remanent magnetization chrm was isolated over a narrow range of high unblocking temperatures 520 580 c detailed rock magnetic analyses indicate that the chrm is carried by nearly stoichiometric pseudo single domain magnetite scanning electron microscopy sem reveals that the magnetite is in the form of fine intergrowths with ilmenite formed by oxyexsolution during cooling the high temperature oxidation defined in the sem images could have continued at temperatures below the curie point of magnetite in this case the chrm would be a thermochemical remanent magnetization tcrm rather than a thermal remanent magnetization trm estimates of the tcrm trm ratio show that the thellier data could underestimate the true field value by a factor of 4 without violating experimental selection criteria this uncertainty in trm fidelity translates into a potential range of field values that spans that defined by the modern field 8 1022 am2 and proposed low precambrian levels 2 1022 am2 therefore understanding further how tcrm is acquired and records the field represents a major challenge if these and many other similar rocks are to be used in thellier studies aimed at defining the strength of the precambrian field

magnetic hysteresis of pseudo single domain and multidomain magnetite below the verwey transition abstract hysteresis loops have been measured as a function of temperature between 10 k and room temperature for two size fractions of crushed magnetite 1 5 m pseudo single domain psd and 100 150 m multidomain md the material under study appears to be fairly close to stoichiometric magnetite with a verwey transition temperature of 115 k for the md sample and 118 k for the psd sample for both samples hysteresis properties below the verwey transition depend critically on whether or not the samples were cooled through the transition in the magnetic field for the psd sample an even more complex behavior has been observed temperature dependences of coercive force obtained after zero field cooling differ significantly for the demagnetized versus magnetized starting magnetic state

a review of single sample based models and other approaches for radiocarbon dating of dissolved inorganic carbon in groundwater abstract numerous methods have been proposed to estimate the pre nuclear detonation 14c content of dissolved inorganic carbon dic recharged to groundwater that has been corrected adjusted for geochemical processes in the absence of radioactive decay 14c0 a quantity that is essential for estimation of radiocarbon age of dic in groundwater the models approaches most commonly used are grouped as follows 1 single sample based models 2 a statistical approach based on the observed curved relationship between 14c and 13c data for the aquifer and 3 the geochemical mass balance approach that constructs adjustment models accounting for all the geochemical reactions known to occur along a groundwater flow path this review discusses first the geochemical processes behind each of the single sample based models followed by discussions of the statistical approach and the geochemical mass balance approach finally the applications advantages and limitations of the three groups of models approaches are discussed the single sample based models constitute the prevailing use of 14c data in hydrogeology and hydrological studies this is in part because the models are applied to an individual water sample to estimate the 14c age therefore the measurement data are easily available these models have been shown to provide realistic radiocarbon ages in many studies however they usually are limited to simple carbonate aquifers and selection of model may have significant effects on 14c0 often resulting in a wide range of estimates of 14c ages of the single sample based models four are recommended for the estimation of 14c0 of dic in groundwater pearson s model ingerson and pearson 1964 pearson and white 1967 han plummer s model han and plummer 2013 the iaea model gonfiantini 1972 salem et al 1980 and oeschger s model geyh 2000 these four models include all processes considered in single sample based models and can be used in different ranges of 13c values in contrast to the single sample based models the extended gonfiantini zuppi model gonfiantini and zuppi 2003 han et al 2014 is a statistical approach this approach can be used to estimate 14c ages when a curved relationship between the 14c and 13c values of the dic data is observed in addition to estimation of groundwater ages the relationship between 14c and 13c data can be used to interpret hydrogeological characteristics of the aquifer e g estimating apparent rates of geochemical reactions and revealing the complexity of the geochemical environment and identify samples that are not affected by the same set of reactions processes as the rest of the dataset the investigated water samples may have a wide range of ages and for waters with very low values of 14c the model based on statistics may give more reliable age estimates than those obtained from single sample based models in the extended gonfiantini zuppi model a representative system wide value of the initial 14c content is derived from the 14c and 13c data of dic and can differ from that used in single sample based models therefore the extended gonfiantini zuppi model usually avoids the effect of modern water components which might retain bomb pulse signatures the geochemical mass balance approach constructs an adjustment model that accounts for all the geochemical reactions known to occur along an aquifer flow path plummer et al 1983 wigley et al 1978 plummer et al 1994 plummer and glynn 2013 and includes in addition to dic dissolved organic carbon doc and methane ch4 if sufficient chemical mineralogical and isotopic data are available the geochemical mass balance method can yield the most accurate estimates of the adjusted radiocarbon age the main limitation of this approach is that complete information is necessary on chemical mineralogical and isotopic data and these data are often limited failure to recognize the limitations and underlying assumptions on which the various models and approaches are based can result in a wide range of estimates of 14c0 and limit the usefulness of radiocarbon as a dating tool for groundwater in each of the three generalized approaches single sample based models statistical approach and geochemical mass balance approach successful application depends on scrutiny of the isotopic 14c and 13c and chemical data to conceptualize the reactions and processes that affect the 14c content of dic in aquifers the recently developed graphical analysis method is shown to aid in determining which approach is most appropriate for the isotopic and chemical data from a groundwater system

observing thermomagnetic stability of nonideal magnetite particles good paleomagnetic recorders the thermomagnetic behavior of remanence induced magnetite fe3o4 particles in the pseudo single domain psd size range 0 1 10 m which dominate the magnetic signature of many rock lithologies is investigated using off axis electron holography construction of magnetic induction maps allowed for the visualization of the vortex domain state in an individual fe3o4 grain 200 nm in diameter as a function of temperature acquisition of a series of electron holograms at 100 c intervals during in situ heating up to 700 c demonstrates the vortex state of the fe3o4 grain in this instance remains thermally stable close to its unblocking temperature and exhibits a similar in plane remanent state upon cooling i e the particle is effectively behaving like a uniaxial single domain particle to temperatures near tc such particles are thought to be robust magnetic recorders it is suggested that evidence for psd behavior should therefore not preclude paleomagnetic investigation

paleomagnetic evidence for fossil biogenic magnetite in western crete abstract previous paleomagnetic research has shown that the concentration of fine grained magnetite preserved in miocene marine clays of western crete drops during two successive geomagnetic transitions although the mechanism responsible for this effect was not known magnetite precipitation by magnetotactic bacteria offers a straightforward explanation for this decrease reduced field strengths during a reversal will strain the competitive advantage of having intracellular magnetite and will force bacteria to precipitate correspondingly more magnetite if they are to maintain the same alignment in the geomagnetic field their ability to do this is limited by several magnetophysical constraints which should act to reduce the numbers of magnetite precipitating cells particularly in fields of less than about 12 of the present level this would lead to a decrease in the supply of bacterial magnitude to the sediments recovery of intact magnetotactic bacterial fossils may lead to a new method for estimating paleointensities from sedimentary rocks

magnetic properties of ilmenite hematite single crystals from the ecstall pluton near prince rupert british columbia paleomagnetic studies of the 91 ma ecstall pluton and other cretaceous plutons of british columbia imply large northward tectonic movements 2000 km may have occurred during the tectonic evolution of western north america however more recent studies have shown that the eastern edge of the ecstall pluton experienced considerable mineralogical changes as younger eocene plutons such as the 58 ma quottoon pluton were emplaced along its margins we investigated changes in the rock magnetic properties associated with this reheating event by examining isolated grains of intergrown ilmenite and hematite the primary paleomagnetic recorder in the ecstall pluton measurements of hysteresis properties low temperature remanence and room temperature isothermal remanent magnetization acquisition and observations from magnetic force microscopy and off axis electron holography indicate that samples fall into three groups the groups are defined by the presence of mineral microstructures that are related to distance from the quotoon plutonic complex the two groups closest to the quottoon pluton contain magnetite within hematite and ilmenite lamellae reheating of the ecstall pluton led to an increase in coercivity and magnetization as well as to development of mixed phase hysteresis these results indicate that shallow paleomagnetic directions from the western ecstall pluton are not affected by reheating and are therefore likely to record original field conditions at the time of pluton emplacement in the absence of structural deformation these shallow inclinations are consistent with large scale northward translation suggested by the baja british columbia hypothesis

development of a low temperature insert for the measurement of remanent magnetization direction using superconducting quantum interference device rock magnetometers data on the directional changes of a full magnetization vector during cycling to cryogenic temperatures can provide important insights into the low temperature magnetic properties of natural and synthetic materials these data also provide an empirical basis for the application of low temperature treatments in paleomagnetism for example the removal of viscous magnetization in magnetite bearing rocks however existing instruments only allow continuous measurement of magnetization along a single axis hampering experimental and theoretical advances in rock magnetism and the implementation of low temperature techniques into regular paleomagnetic practices here we describe development of a novel low temperature insert designed in collaboration with william s goree inc which allows measurement of directional behavior of a full magnetization vector during zero field low temperature cycling pilot experiments on well controlled polycrystalline samples of pseudo single domain psd and multidomain magnetite as well as on a natural sample containing psd magnetite indicate that the orientation of a saturation isothermal remanent magnetization sirm imparted at room temperature remains constant during low temperature cycling to 20 k this observation lends additional support to low temperature cycling as a cleaning technique in paleomagnetism the sirm imparted in an individual crystal of magnetite showed systematic albeit small changes upon both cooling and warming through the verwey temperature which may reflect switching between the easy magnetization directions however the switching effect may be significantly attenuated by crystallographic twinning in magnetite below the transition overall our results demonstrate the potential of the directional low temperature magnetometry for the advancement of our understanding of the properties of natural and synthetic materials

on the possibility of single domain pseudo single domain magnetic particles existing in the lower crust of mars source of the strong magnetic anomalies 1 this paper shows that single domain or pseudo single domain sd psd magnetite grains can exist in the upper 30 45 km of the martian crust the upper 50 km of the martian crust have likely formed by volcanic eruptions on a stagnant lithosphere a given lava flow cools very rapidly at the surface but is then gradually heated up through burial heating as it is overlain by other successive flows the thermal evolution models of a lava flow show that its temperature remains below the magnetic blocking temperatures of magnetite 480 580 c until it reaches 30 45 km depth the highly magnetic sd psd magnetite grains can easily explain the strong magnetic anomalies of mars less than 1 by volume of magnetite particles is sufficient

radioelements radiogenic helium and age relationships for groundwaters from the granites at stripa sweden abstract the solution of radioelements and radiogenic 4he by groundwaters in fractured rocks is dependent upon the radioelement distribution in the rock matrix and the extent of the rock water interface the 234 u 238 u activity ratio and the dissolved u rn and he contents of such groundwaters respond to changes in the flow regime with time although 234 u 238 u activity ratios change with groundwater residence time as a consequence of 234th recoil induced solution of 234u the activity ratio is strongly influenced by the u distribution within fractures by the extent of the rock water interface and by the amount of 238u in solution a model for the quantitative evaluation of these effects is presented groundwaters from depths up to 880 m in the stripa granite have variable dissolved uranium contents and 234 u 238 u activity ratios the uranium geochemistry is primarily determined by variations in flow path rather than by groundwater age dissolved radiogenic 4he in the groundwaters increases with their depth of origin and is dependent upon the u content of the granite and upon its fracture porosity it increases with groundwater residence time but movement of 4he by diffusion and transport processes make the actual groundwater age indeterminate

a tracer study of the floridan aquifer in southeastern georgia implications for groundwater flow and paleoclimate distributions of stable isotopes of water radiocarbon noble gases and chloride c1 in groundwater from the upper floridan aquifer in southeastern georgia suggest that down gradient of the gulf trough this aquifer contains waters representative of both regional and local groundwater flow systems in this area locally recharged waters tend to remain near the top of the aquifer and do not mix substantially with the regional groundwater flow system noble gas temperatures suggest that this region of georgia was 4 0 0 6 c cooler during the last glacial period lgp similar temperature changes have been reported for southern texas and northern new mexico suggesting that the southern united states cooled uniformly during the lgp stable isotopes of water became enriched down gradient from the recharge area these enrichments which have been observed before appear to result from local influx of shallow groundwater into the regional aquifer system rather than representing a climate change signal an inland gradient of the stable isotope composition 0 60 0 14 100 km in 18o was found in young holocene water after correcting for the change in the stable isotope composition of the ocean during the lgp water that was recharged during the lgp was found to be slightly depleted in stable isotopes relative to modern recharge 18o 0 6 0 4 assuming the modern inland gradient the change in 18o is consistent with a shift in the coastline which was caused by the lower sea level during the lgp

low temperature magnetic properties of magnetite using first order reversal curve analysis implications for the pseudo single domain state 1 low temperature magnetic hysteresis properties of nearly stoichiometric magnetite were studied using the first order reversal curve forc technique forc distributions were measured at 20 k after cooling in zero magnetic field or cooling in the presence of a strong 1 5 t magnetic field five polycrystalline samples ranged in mean grain size and shape representing mostly single domain sd pseudo single domain psd and multidomain md magnetic hysteresis behavior the forc distributions measured from psd magnetite after zero field cooling zfc manifest an unusual behavior characterized by two distinct maxima located almost symmetrically with respect to the zero interaction field hu 0 axis on a forc diagram in contrast the low temperature forc distributions for sd and md samples have a single maximum located at the hu 0 axis the appearance of two maxima on low temperature forc distributions is interpreted as a manifestation of a perminvar like behavior due to stabilization of the magnetic domain structure by irregularities of crystalline lattice induced by twinning in monoclinic magnetite the experimental data suggest that the effect of monoclinic twinning on remagnetization is only pronounced within a certain grain size range the double maxima zfc forc distribution appears to be a distinctive property of psd hysteresis behavior which cannot be reproduced by mixing sd and md magnetites

reactive transport of trace elements and isotopes in the eutaw coastal plain aquifer alabama 1 we integrate groundwater geochemistry mineralogy and numerical modeling techniques to study the reactive transport of heavy metals and isotopes in the eutaw coastal plain aquifer alabama geochemical data show that the elevated concentrations of fe mn and sr can be correlated with high ph and alkalinity these geochemical correlations suggest that that elevated metal concentrations may be derived from bacterial iron and manganese reduction geochemical modeling of bacterial fe iii and mn iv reduction shows that the biotransformation of iron and manganese minerals could control mobility and concentrations of fe and mn in coastal plain aquifers petrographic sem and edax studies of sediments show the formation of biogenic siderite rhodochrosite and pyrite in sediments associated with fe and mn rich groundwater rhodochrosite and siderite occur together as spheroids 1 0 mm in which rhodochrosite forms the center and siderite forms an outer rind which is consistent with the redox sequence of mineral precipitation predicted by the geochemical modeling the low 13c ratios of siderite and rhodochrosite 14 4 to 15 4 pdb and groundwater dic 20 6 to 14 1 pdb imply carbon of biogenic origin higher dic 13c levels are found to be correlated with elevated fe and mn concentrations and high ph values of groundwater this unexpected result implies novel carbon isotopic fractionation processes associated with bacterial fe iii reduction we used 36cl cl ratios of groundwater and isotope transport modeling to calculate the residence time of regional groundwater in the eutaw aquifer the calculation considering the natural decay only would yield 36cl levels that are significantly higher than field data suggesting that a significant mixing with cl rich older groundwater at depth is an important reason for substantial 36cl depletion along flow path

the relationship between he and cl as a criterion for the formation of groundwater composition abstract the analysis of the relationship between 1611 parallel determinations of helium and chloride concentrations in the groundwater of crystalline shields and sedimentary basins was performed the comparison showed that groundwaters have helium and chlorides concentrations in the range of 1 8 10 8 to 6 8 mol m3 and of 5 6 10 3 to 6488 mol m3 respectively and relatively stable he rcl ratios equal to 5 5 10 5 in sedimentary basins and 7 5 10 4 in crystalline shields such behavior of helium and chlorides shows that the formation of the groundwater composition is determined primarily by the processes of mixing fresh meteoric and saline sea waters with ancient brines from great depths during sedimentation sea water penetrates the subsurface and mixes with ancient brines on land fresh meteoric waters dilute the mixtures of seawater and ancient brines most of the radiogenic 4he and chlorides comes from the subsurface ancient brine the radiogenic 4he in the host rocks plays a secondary role therefore the concentration of 4he in saline groundwater reflects the size of the impurities of the ancient brine with a very high content of 4he rather than the residence time within the subsurface

experimental evaluation of magnetic interaction in pyrrhotite bearing samples pyrrhotite bearing metamorphic limestones have recently experienced an increasing relevance in paleomagnetic research simple univectorial remanences document the metamorphic uplift whereas more complex multicomponent ptrms may constrain its age for a successful application of the latter it is important to estimate the degree of magnetic interactions to ensure the additivity of individual ptrm segments we therefore have subjected the sized dispersed suite 5 250m of tte pyrrhotite to forc analysis and

electrical conductivity structure of north west fennoscandia from three dimensional inversion of magnetotelluric data abstract new magnetotelluric mt data in north west fennoscandia were acquired within the framework of the project magnetotellurics in the scandes masca the project focuses on the investigation of the crustal and upper mantle lithospheric structure in the transition zone from stable precambrian cratonic interior to passive continental margin beneath the caledonian orogen and the scandinavian mountains in western fennoscandia an array of 59 simultaneous long period and 220 broad band mt sites were occupied in the summers of 2011 to 2013 the 3 d inversion of the masca data was obtained using the modem 3 d code the full impedance and tipper data were used for the inversion the rocks of archaean and proterozoic basement towards east and the caledonian nappes towards west are modelled as resistive structures in the central and southern parts the whole crust is resistive and reflects the trans scandinavian igneous belt granitoids the middle to lower crust of the svecofennian province is conductive an uppermost crustal conductor is revealed in the skelleftea ore district the south end of the kittila greenstone belt is seen in the models as a strong upper to middle crustal conductor in the caledonides the highly conductive alum shales are observed along the caledonian thrust front a map of the crustal conductance for the north west fennoscandian shield is presented

low temperature behavior of single domain through multidomain magnetite abstract we have investigated the low temperature behavior of a suite of grown synthetic and natural magnetites that span single domain sd and multidomain md behavior synthetic samples had been grown in the laboratory either in an aqueous medium or in glass natural samples included sd magnetites occurring in plagioclase and truly md magnetites in the form of large octahedra in all experiments a sample was first given a saturation remanence at room temperature next moment was measured continuously during cooling and warming between 230 k and 60 k similar to results reported earlier by other workers magnetic memory is large in sd samples whereas truly md samples are almost completely demagnetized by cycling between room temperature and 60 k pseudo single domain samples exhibit behavior that is intermediate with respect to that of the sd and truly md states when data from this study are combined with data obtained by hartstra 10 from sized natural magnetites it is found that the percentage of total remanence that survives cycling between room temperature and 60 k decreases linearly with the logarithm of grain size and thus with increasing number of domains this relation suggests that memory can provide a reasonable estimate of grain size in those magnetite bearing rocks for which these samples provide good analogues remarkably some of the large natural octahedra provide a magnified view of md response to low temperatures and thus reveal two surprising and intriguing types of behavior first below approximately 180 k these octahedra demagnetize through a series of large barkhausen jumps second near 117 k these same octahedra exhibit a wild zone where magnetic moment executes large random excursions we interpret these two phenomena as direct evidence for the unpinning and irreversible displacement of domain walls in response to the drop in coercivity and possibly the broadening of domain walls as temperatures drop toward the isotropic point one implication of this behavior is that cooling to progressively lower temperatures could provide an effective method for stepwise removal of paleomagnetic components carried by md grains even without passage through the isotropic point of magnetite

magnetic hysteresis of 0 6 110 μm magnetites across the verwey transition we report saturation magnetization ms saturation remanence mrs coercive force hc and remanence coercivity hcr as a function of grain size d and temperature t for 0 6 135 m magnetites

low temperature cycling of isothermal and anhysteretic remanence microcoercivity and magnetic memory abstract this paper reports low temperature cycling ltc through the verwey transition of anhysteretic remanence arm partial arms and partially demagnetised saturation isothermal remanence sirm induced at room temperature in pseudo single domain and multidomain md magnetite the remanences were cooled in zero field to 50 k and then heated back to room temperature by inducing partial arms over different field ranges and by partially alternating field demagnetising sirms it was possible to isolate both low coercive force and high coercive force fractions of remanence on cooling through the verwey transition a sharp increase in the remanence was observed the relative size of the jump increased as the high coercive force fraction was increasingly isolated this behaviour is interpreted as being due to both an increase in the single domain multidomain threshold size on cooling through the verwey transition and to the reduction or elimination of closure domains in the low temperature phase in addition the memory ratio i e the fraction of remanence remaining after ltc divided by the initial remanence was found to be higher for the high coercive force fraction than the low coercive force fraction in our interpretation the high coercivity fraction behaviour is associated with reversible domain re organisation effects whilst the low coercive force fraction s behaviour is associated with irreversible domain re organisation and de nucleation processes due to the decrease in magnetocrystalline anisotropy on cooling to the verwey transition the high coercive force fraction is likely to be magnetoelastically controlled thus a rock displaying high coercive force behaviour is likely to carry a palaeomagnetically meaningful remanence with high unblocking temperatures in addition ltc analysis can be used to identify the domain state dominating the natural remanence in magnetite bearing rocks

low temperature magnetic properties of greigite fe3s4 1 we provide comprehensive low temperature magnetic results for greigite fe3s4 across the spectrum from superparamagnetic sp to multidomain md behavior it is well known that greigite has no low temperature magnetic transitions but we also document that it has strong domain state dependence of magnetic properties at low temperatures blocking of sp grains and increasing thermal stability with decreasing temperature is apparent in many magnetic measurements thermally stable single domain greigite undergoes little change in magnetic properties below room temperature for pseudo single domain psd md greigite hysteresis properties and first order reversal curve diagrams exhibit minor changes at low temperatures while remanence continuously demagnetizes because of progressive domain wall unpinning the low temperature demagnetization is grain size dependent for psd md greigite with coarser grains undergoing larger remanence loss ac susceptibility measurements indicate consistent blocking temperatures tb for all synthetic and natural greigite samples which are probably associated with surficial oxidation low temperature magnetic analysis provides much more information about magnetic mineralogy and domain state than room temperature measurements and enables discrimination of individual components within mixed magnetic mineral assemblages low temperature rock magnetometry is therefore a useful tool for studying magnetic mineralogy and granulometry of greigite bearing sediments

processes controlling 234u and 238u isotope fractionation and helium in the groundwater of the st lawrence lowlands quebec the potential role of natural rock fracturing abstract the goal of this study is to explain the origin of 234 u 238 u fractionation in groundwater from sedimentary aquifers of the st lawrence lowlands quebec canada and its relationship with 3 he 4 he ratios to gain insight regarding the evolution of groundwater in the region 234 u 238 u activity ratios or 234 u 238 u act were measured in 23 groundwater samples from shallow quaternary unconsolidated sediments and from the deeper fractured regional aquifer of the becancour river watershed the lowest 234 u 238 u act 1 14 0 01 was measured in ca hco 3 type freshwater from the quaternary shallower aquifer where bulk dissolution of the carbonate allows u to migrate into water with little 234 u 238 u isotopic fractionation the 234 u 238 u act increases to 6 07 0 14 in na hco 3 cl type groundwater preferential migration of 234 u into water by recoil is the underlying process responsible for this isotopic fractionation an inverse relationship between 234 u 238 u act and 3 he 4 he ratios has been observed this relationship reflects the mixing of newly recharged water with 234 u 238 u act close to the secular equilibrium and containing atmospheric tritiogenic helium and mildly mineralized older water 14 c ages of 6 6 kyrs with 234 u 238 u act of 6 07 and large amounts of radiogenic 4 he in excess of the steady state amount produced in situ the simultaneous fractionation of 234 u 238 u act and the addition of excess 4 he could be locally controlled by stress induced rock fracturing this process increases the surface area of the aquifer matrix exposed to pore water from which produced 4 he and 234 u can be released by recoil and diffusion this process would also facilitate the release of radiogenic helium at rates greater than those supported by steady state u th production in the rock consequently sources internal to the aquifers could cause the radiogenic 4 he excesses measured in groundwater

some additional parameters to estimate domain state from isothermal magnetization measurements abstract domain state and mixing analysis based on isothermal magnetization measurements is often restricted to the use of the day plot in order to make more information from simple rock magnetic measurements accessible for the quantitative analysis of domain state four additional parameters are proposed two of them a hysteresis shape parameter and a coercivity ratio can be directly calculated from standard measurements of the hysteresis loop and backfield curve the other two a transient energy dissipation ratio and the viscosity of isothermal remanent magnetization require additional measurements which however do not increase measurement time noticeably the theoretical background of the above parameters is discussed their ability to discern domain state is shown for a small set of synthetic magnetite and titanomagnetite samples and in a case study of a transect from the rim to the center of a dredged mid ocean ridge basalt

effect of the magnetic field applied during cooling on magnetic hysteresis in the low temperature phase of magnetite first order reversal curve forc analysis the strength of a magnetic field applied during cooling through the verwey transition can control the low temperature lt magnetic hysteresis properties of magnetite this effect is investigated using the first order reversal curve forc technique lt forc distributions were measured at 20 k from four polycrystalline samples after cooling from 300 k in zero magnetic field or in the presence of a magnetic field hfc from a 0 01 to 1 5 t range the samples represented nearly single domain sd pseudo single domain psd and multidomain md magnetic hysteresis behavior the lt forc distributions measured from psd magnetite after zero field cooling are characterized by two distinct maxima located almost symmetrically with respect to the zero interaction field hu 0 axis on a forc diagram application of an intermediate magnetic field 10 mt to 50 mt during cooling results in a shift of the peaks to near symmetry with respect to the hu hfc line the manifestation of the field controlled offset of forc distribution strongly depends on the direction of the field hfc the effect is best expressed when a forc diagram is measured parallel or antiparallel to the cooling field direction for hfc 100 mt the upper peak disappears and the lt forc distribution becomes single peaked and symmetrical with respect to the hu 0 axis in contrast no correlation between hfc and features of forc distributions is observed for sd and md magnetite these experimental results provide further support to a phenomenological model which suggests that magnetic hysteresis behavior of monoclinic magnetite is controlled by an intrinsic interplay between the magnetic and crystallographic twin domain structures

wasp waisted hysteresis loops mineral magnetic characteristics and discrimination of components in mixed magnetic systems rock magnetic studies of complex systems that contain mixtures of magnetic minerals or mixed grain size distributions have demonstrated the need for a better method of distinguishing between different magnetic components in geological materials hysteresis loops that are constricted in the middle section but are wider above and below the middle section are commonly observed in mixed magnetic assemblages such wasp waisted hysteresis loops have been widely documented particularly with respect to rare earth permanent magnets basaltic lava flows remagnetized paleozoic carbonate rocks and an increasingly wide range of other rocks our modelling combined with a review of previous work indicates that there are several conditions that give rise to as well as magnetic properties that are characteristic of wasp waisted hysteresis loops first at least two magnetic components with strongly contrasting coercivities must coexist this condition can arise from either mixtures of grain sizes of a single magnetic mineral or a combination of magnetic minerals with contrasting cocrcivities or a combination of these two situations second materials that give rise to wasp waisted hysteresis loops will have relatively high ratios of the coercivity of remanence to coercive force b cr b c because b0 is controlled by the soft low coercivity component whereas bcris controlled by the hard high coercivity component third values of b cr b c 10 usually only occur for strongly wasp waisted loops when the low coercivity component comprises an overwhelmingly large fraction of the total volume of magnetic grains fourth a given mixture of superparamagnetic and single domain sd grains is more likely to give rise to wasp waisted hysteresis loops than an equivalent mixture of sd and multidomain grains fifth our results provide empirical confirmation that the total magnetization of a material is the sum of the weighted contributions of each component in the absence of significant magnetic interaction between particles thus to contribute significantly to wasp waisted behavior a mineral magnetic component must give rise to a significant portion of the total magnetization of the rock as a result minerals with weak magnetic moments such as hematite need to occur in large concentrations to cause wasp waistedness in materials that also contain ferrimagnetic minerals we outline a method for determining the magnetic components that can give rise to wasp waisted hysteresis loops this method is based on high and low temperature magnetic measurements that are used to identify the dominant remanence bearing mineral s and on mineral magnetic techniques that are used to discriminate between different magnetic domain states the method is illustrated with several examples from archaeological geological and synthetic materials

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

alabama gulf coast groundwaters 4he and 14c as groundwater dating tools analyses of 4 he and 1 4 c in groundwaters from a miocene quartz aquifer on the alabama gulf coast show the usefulness of 4 he for dating these holocene groundwaters in this aquifer system of low alkalinities and low phs radiocarbon ages can be used without model correction the groundwaters studied ranged from 42 3 to 95 5 modern carbon and yielded uncorrected ages of 375 6790 yr radiogenic 4 he ages ranged from 50 7500 yr