anhysteretic remanent magnetization of fine magnetic particles 1 various magnetic parameters are in common use for estimating the grain size of magnetic particles among these the ratio of the intensity of anhysteretic remanent magnetization arm to that of isothermal remanent magnetization as well as their alternating field af demagnetization curves are used as an indicator of the domain state of the particles several models have been proposed to describe physically the acquisition of arm in a biased af field jaep 1969 first developed a semiquantitative theory based entirely on the thermal fluctuation analysis developed by neel 1949 1954 1955 significant discrepancies were found between his model and experimental results on magnetite a new general theory of arm based on the work of jaep is presented here with particular regard to the influence of various parameters like grain size coercivity and mineralogy on arm intensity an analytical expression for arm intensity in the special case of very fine particles was derived from this theory and a good agreement with experimental results and data from the literature was found a new estimation of the atomic reorganization time was obtained from arm measurements on a sample of the yucca mountain tuff which has well known mineralogy and grain size distribution the results are in agreement with the value proposed by mcnab et al 1968 for magnetite some authors considered magnetic interactions as the key to understand the arm in fine particles and this is certainly true for strongly interacting samples in this case arm would be useless for the characterization of magnetic grains however many sediments have a very low concentration of well distributed magnetic grains for these samples the explanation of an arm in terms of intrinsic properties of the grains as qualitatively proposed by other authors is more suitable

magnetic properties in an ash flow tuff with continuous grain size variation a natural reference for magnetic particle granulometry the tiva canyon tuff contains dispersed nanoscale fe ti oxide grains with a narrow magnetic grain size distribution making it an ideal material in which to identify and study grain size sensitive magnetic behavior in rocks a detailed magnetic characterization was performed on samples from the basal 5 m of the tuff the magnetic materials in this basal section consist primarily of low impurity magnetite in the form of elongated submicron grains exsolved from volcanic glass magnetic properties studied include bulk magnetic susceptibility frequency dependent and temperature dependent magnetic susceptibility anhysteretic remanence acquisition and hysteresis properties the combined data constitute a distinct magnetic signature at each stratigraphic level in the section corresponding to different grain size distributions the inferred magnetic domain state changes progressively upward from superparamagnetic grains near the base to particles with pseudo single domain or metastable single domain characteristics near the top of the sampled section direct observations of magnetic grain size confirm that distinct transitions in room temperature magnetic susceptibility and remanence probably denote the limits of stable single domain behavior in the section these results provide a unique example of grain size dependent magnetic properties in noninteracting particle assemblages over three decades of grain size including close approximations of ideal stoner wohlfarth assemblages and may be considered a useful reference for future rock magnetic studies involving grain size sensitive properties

thermoremanence anhysteretic remanence and susceptibility of submicron magnetites nonlinear field dependence and variation with grain size we have measured initial susceptibility 0 and the dependence of anhysteretic remanent magnetization arm and thermoremanent magnetization trm on applied field for seven samples of magnetite with mean grain sizes from 40 to 540 nm trm acquisition is nonlinear in geomagnetically relevant weak fields contrary to the usual assumption made in paleointensity determination over the 40 540 nm range 0 varies with particle shape but only weakly with particle size d and can be used to correct for varying magnetite concentrations in sediment cores arm is strongly size dependent over the same range and is the best parameter for monitoring grain size variations in natural samples arm and trm have similar variations as d 1 for d 1 m suggesting a common source for this pseudo single domain psd dependence on grain size however trm is 10 20 times more intense than arm in grains around 0 2 m in size the trm microstate seems to be a two domain structure whereas the arm microstate may be a vortex structure which has never before been convincingly demonstrated by magnetic measurements independent evidence comes from theoretical fits to trm and arm field dependence data in moderate and strong fields trm in 215 540 nm magnetites is explained by two domain theory but arm is not in smaller grains d 0 1 m a psd theory predicts that trm is carried by single domain sd moments of entire grains but arm resides in moments 1 per grain 20 40 times weaker these remanence levels match those of metastable sd and vortex states respectively we therefore propose that magnetite grains in the 0 1 0 5 m size range can remain in metastable sd or two domain states following acquisition of trm but revert to a vortex ground state when field cycled for example in arm acquisition or alternating field demagnetization

ocean circulation beneath filchner ronne ice shelf from three dimensional model results a high resolution three dimensional ocean circulation model is applied to the cavity beneath filchner ronne ice shelf fris the model predicts predominantly barotropic currents which form a series of cyclonic gyres in the deep basins and anticyclonic circulations around the islands the surface circulation can be such that the water moves in the direction of decreasing or increasing ice thicknesses in the former case leading to freezing while melting at the ice shelf base results in the latter case the pattern of melting and freezing is consistent with known distributions of marine ice and melting areas beneath fris an anticyclonic circulation around the korff and henry ice rises with melting west of korff ice rise and freezing on the eastern side and north of henry ice rise is the main source for an ice pumping mechanism that produces the observed large marine ice body in the central filchner ronne ice shelf the estimates for net melting for realistic conditions at the open ocean boundary are 40 50 km 3 yr 1 indicating that ice shelf ocean interaction is an important contribution to the mass balance of the ice shelf

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

anomalous magnetic fabrics of dikes in the stable single domain superparamagnetic threshold this research has been partly funded by project cgl2008 04264 bte and by grant prx14 00303 to carles soriano

theoretical considerations on the anhysteretic remanent magnetization of interacting particles with uniaxial anisotropy 1 the anhysteretic remanent magnetization arm is widely used in rock magnetism and paleomagnetism because of its sensitivity to the domain state of magnetic particles and the close analogy to natural remanent magnetizations on the other hand the arm shares with other weak field magnetizations the property of being extremely sensitive to magnetostatic interactions therefore it is desirable to model the effects of interactions on natural assemblages of magnetic particles direct micromagnetic calculations of the arm acquisition process are not practicable therefore an analytical approach is used calculate the arm susceptibility of a system of interacting single domain sd particles the model is based on a statistical description of the interaction field the equations obtained have been used to evaluate the dependence of the arm on the packing fraction of the magnetic particles the effect of interactions on the anisotropy of arm aarm was evaluated as well the aarm of densely packed particles is complex and depends critically on the microcoercivity a physical interpretation of the aarm of highly interacting particles is therefore difficult

magnetic grain size variations through an ash flow sheet influence on magnetic properties and implications for cooling history rock magnetic studies of tuffs are essential to the interpretation of paleomagnetic data derived from such rocks provide a basis for interpretation of aeromagnetic data over volcanic terranes and yield insights into the depositional and cooling histories of ash flow sheets a rhyolitic ash flow sheet the miocene aged tiva canyon member of the paintbrush tuff contains both titanomagnetite phenocrysts present in the magma prior to eruption and cubic fe oxide microcrystals that grew after emplacement systematic variations in the quantity and magnetic grain size of the microcrystals produce large variations in magnetic properties through a section of the ash flow sheet penetrated in a borehole on the nevada test site natural remanent magnetization varies from less than 1 10 4 to more than 8 10 4 a m3 kg 1 and in phase magnetic susceptibility varies from less than 1 10 6 to more than 10 10 6 m3 kg 1 the microcrystals which include both magnetite and maghemite have curie points and maximum unblocking temperatures between 580 c and 640 c rock magnetic data including in phase and quadrature magnetic susceptibilities as well as hysteresis parameters demonstrate that these microcrystals are of superparamagnetic and single domain sizes titanomagnetite phenocrysts are the dominant remanence carriers in the central 50 m of the section whereas microcrystals are important contributors to remanent magnetization and magnetic susceptibility in two 15 m thick zones at the top and bottom within these zones the size of microcrystals decreases both toward the quenched margins and toward the interior of the sheet the decrease in microcrystal size toward the interior of the sheet is interpreted to indicate the presence of a cooling break possibly represented by a concentration of pumice laboratory heating produces changes in magnetic properties which vary complexly from sample to sample such changes include 1 decrease in saturation magnetization 2 both increase and decrease in in phase and quadrature magnetic susceptibilities and 3 shifts in coercivity spectra the complex sample to sample variation in response to heating is attributed to differences in initial grain size distributions

a comparison of different magnetic methods for determining the relative grain size of magnetite in natural materials some results from lake sediments abstract a new rapid method for identifying relative grain size variations in magnetic involves the parameter anhysteretic susceptibility arm i e specific arm obtained in a 1 oe steady field which is particularly sensitive to the single domain sd and small pseudo single domain psd grains of the finer magnetite fraction a second parameter low field susceptibility is relatively more sensitive to the coarser magnetite fraction larger psd and smaller multidomain md grains we can then obtain a measure of the ratio of coarse to fine grain magnetite for large numbers of samples by plotting arversus a simple idealized model based on sized magnetite samples is proposed to explain the use of the armversus plot for detecting relative grain size changes in the magnetic content of natural materials the sediments of three lakes that contain magnetite or a similar magnetic carrier and have a wide range of values of arm and are used to test the model the model is used to interpret the magnetic variations observed and the interpretations are supported by high field hysteresis measurements of the same sediments the combination of the high field hysteresis method of day et al 1 and the arm vs method is a powerful technique allowing the rapid identification of both the relative grain size and domain state for large numbers of samples containing magnetite the armvs method should be used as an intial means of identifying distinct groups of samples the high field hysteresis method should then be applied to a few representative samples from each group to confirm the initial interpretation

modeling the effects of frazil ice crystals on the dynamics and thermodynamics of ice shelf water plumes the seawater that comes into contact with the base of a floating ice shelf is modified as a result of the phase changes that occur melting is prevalent in the deepest parts of the subice cavity and this drives a buoyant flow of ice shelf water isw along the sloping ice shelf base the ascent of the isw toward the surface of the ocean causes supercooling because the freezing point rises with the falling pressure and this induces a change from melting to freezing assuming that seed crystals exist the isw now fulfills the condition for the rapid growth of disc shaped frazil ice crystals which may subsequently settle upward out of suspension under the action of gravity a simple numerical model of these processes has been developed based on the theory of inclined plumes the isw is treated as a turbulent particle laden gravity current ascending a reactive boundary and containing a suspended crystal load which evolves in response to the supercooling of the water and the inverted sedimentation of the crystals the frazil ice has two important effects on the behavior of the isw plume because the generation of crystals through out the plume provides such a large area over which phase changes can occur the conversion of supercooling into ice happens much more readily than is possible through turbulent transfer of heat and mass at the ice shelf base a suspended crystal load reduces the bulk density of the isw so that a growing suspension causes the plume to accelerate while deposition of crystals onto the ice shelf effects a deceleration there are positive feedbacks in that an acceleration of the plume induces more rapid crystal growth while a deceleration allows suspended crystals to settle out more easily

paleomagnetic constraints on the geometry and timing of deformation at yucca mountain nevada paleomagnetic data from miocene ash flow sheets indicate that southern yucca mountain has undergone about 30 of clockwise vertical axis rotation since emplacement of the tiva canyon member of the paintbrush tuff at about 13 ma declinations of remanent magnetization from 32 sites in the reversely magnetized tiva canyon member display a systematic southward increase over the 25 km north south extent of yucca mountain probable errors in estimating structural attitudes are a major source of uncertainty in determining the amount of rotation between any two sites analysis of tilt corrected directions of remanent magnetization from the tiva canyon sites indicates that structural attitudes determined from outcrop patterns on geologic maps are generally more accurate than field measurements of attitudes of compaction foliation or contacts attitudes determined from map patterns apparently minimize effects of 1 initial dips 2 poorly defined compaction foliation and 3 tilting that occurred after compaction but before blocking of remanence however such attitudes cannot accurately represent tilting related to unmapped local structures rotations implied by data from three older ash flow sheets the topopah spring member of the paintbrush tuff seven sites and the prow pass four sites and bullfrog three sites members of the crater flat tuff are generally consistent with the amount and sense of rotation documented by data from the tiva canyon member uncertainties in the declinations of tilt corrected site mean directions from these older units preclude evaluating whether older units have undergone more rotation than the tiva canyon member

episodic caldera volcanism in the miocene southwestern nevada volcanic field revised stratigraphic framework 40ar 39ar geochronology and implications for magmatism and extension the middle miocene southwestern nevada volcanic field swnvf is a classic example of a silicic multicaldera volcanic field in the great basin more than six major calderas formed between 15 and 7 5 ma the central swnvf caldera cluster consists of the overlapping silent canyon caldera complex the claim canyon caldera and the timber mountain caldera complex active from 14 to 11 5 ma and centered on topographic timber mountain locations of calderas older than the claim canyon caldera source of the tiva canyon tuff are uncertain except where verified by drilling younger peralkaline calderas black mountain and stonewall mountain formed northwest of the central swnvf caldera cluster we summarize major revisions of the swnvf stratigraphy that provide for correlation of lava flows and small volume tuffs with the widespread outflow sheets of the swnvf new laser fusion 40 ar 39 ar isotopic ages are used to refine and revise the timing of eruptive activity in the swnvf the use of high sensitivity mass spectrometry allowed analysis of submilligram sized samples with analytical uncertainties of 0 3 1 permitting resolution of age differences as small as 0 07 ma these results confirm the revised stratigraphic succession and document a pattern of episodic volcanism in the swnvf major caldera episodes belted range crater flat paintbrush timber mountain and thirsty canyon groups erupted widespread ash flow sheets within 100 300 k y time spans and pre and post caldera lavas erupted within 100 300 k y of the associated ash flows peak volcanism in the swnvf occurred during eruption of the paintbrush and timber mountain groups when over 4500 km 3 of metaluminous magma was erupted in two episodes within 1 35 m y separated by a 750 k y magmatic gap peralkaline and metaluminous magmatism in the swnvf overlapped in time and space the peralkaline tub spring and grouse canyon tuffs erupted early and the peralkaline thirsty canyon group tuffs and stonewall flat tuff erupted late in the history of the swnvf flanking the central volumetrically dominant peak of metaluminous volcanism magma chemistry transitional between peralkaline and metaluminous magmas is indicated by petrographic and chemical data particularly in the overlapping grouse canyon and area 20 calderas of the silent canyon caldera complex volcanism in the swnvf coincided with the miocene peak of extensional deformation in adjoining parts of the great basin although regional extension was concurrent with volcanism it was at a minimum in the central area of the swnvf where synvolcanic faulting was dominated by intra caldera deformation significant stratal tilting and paleomagnetically determined dextral shear affected the southwestern margin of the swnvf between the paint brush and timber mountain caldera episodes larger magnitude detachment faulting in the bullfrog hills southwest of the central swnvf caldera cluster followed the climactic timber mountain caldera episode postvolcanic normal faulting was substantial to the north east and south of the central swnvf caldera cluster but the central area of peak volcanic activity remained relatively unextended in postvolcanic time volcanism and extension in the swnvf area were broadly concurrent but swnvf area were broadly concurrent but in detail they were episodic in time and not coincident in space

melting of ice shelves and the mass balance of antarctica we calculate the present ice budget for antarctica from measurements of accumulation minus iceberg calving run off and in situ melting beneath the floating ice shelves the resulting negative mass balance of 469 gt year 1 differs substantially from other recent estimates but some components are subject to high temporal variability and budget uncertainties of 20 50 annual accumulation from an earlier review is adjusted to include the antarctic peninsula for a total of 2144 gt year 1 an iceberg production rate of 2016 gt year 1 is obtained from the volume of large icebergs calculated from satellite images since 1978 and from the results of an international iceberg census project ice shelf melting of 544 gt year 1 is derived from physical and geochemical observations of meltwater outflow glaciological field studies and modeling of the sub ice ocean circulation the highest melt rates occur near ice fronts and deep within sub ice cavities run off from the ice sheet surface and from beneath the grounded ice is taken to be 53 gt year 1 less than half of the negative mass balance need come from the grounded ice to account for the unattributed 0 45 mm year 1 in the ipcc best estimate of the recent global sea level rise

characterizing the superparamagnetic grain distribution f v hk by thermal fluctuation tomography 1 in 1965 d j dunlop showed that the joint distribution of particle volumes and microcoercivities f v hk0 can be determined for magnetically monomineralic thermally stable single domain ssd ensembles by taking advantage of the joint temperature and field dependence of relaxation time we have developed a procedure that follows dunlop s strategy to obtain f v hk0 for ensembles containing both superparamagnetic and ssd grains based on backfield remanence curves measured over a range of temperatures each point on the derivative curves represents the integrated contribution from grains that lie along a corresponding blocking contour on the neel plot a suitable set of such line integral samples can be used to reconstruct the f v hk0 distribution using the methods of tomographic imaging samples of the basal tiva canyon tuff have narrow size distributions of elongate ti poor titanomagnetite tomographic inversion of the low temperature backfield spectra yield sharply peaked f v hk0 distributions from which we calculate modal grain dimensions in good agreement with those observed by transmission electron microscopy analysis of synthetic samples containing bimodal populations clearly distinguishes the two modes because our simplified forward calculations incompletely account for the effects of orientation distribution the width of the coercivity distribution at each temperature is underestimated and consequently the inverse calculations yield grain distributions that are overly broad frequency and temperature dependent susceptibilities calculated for the inverted f v hk0 distributions accord fairly well with measured susceptibilities for the weakly interacting tiva canyon samples less well for a moderately interacting paleosol specimen and poorly for a strongly interacting ferrofluid

environmental magnetism principles and applications 1 in environmental magnetism rock and mineral magnetic techniques are used to investigate the formation transportation deposition and postdepositional alterations of magnetic minerals under the influences of a wide range of environmental processes all materials respond in some way to an applied magnetic field and iron bearing minerals are sensitive to a range of environmental processes which makes magnetic measurements extremely useful for detecting signals associated with environmental processes environmental magnetism has grown considerably since the mid 1970s and now contributes to research in the geosciences and in branches of physics chemistry and biology and environmental science including research on climate change pollution iron biomineralization and depositional and diagenetic processes in sediments to name a few applications magnetic parameters are used to routinely scan sediments but interpretation is often difficult and requires understanding of the underlying physics and chemistry thorough examination of magnetic properties and of the environmental processes that give rise to the measured magnetic signal is needed to avoid ambiguities complexities and limitations to interpretations in this review we evaluate environmental magnetic parameters based on theory and empirical results we describe how ambiguities can be resolved by use of combined techniques and demonstrate the power of environmental magnetism in enabling quantitative environmental interpretations we also review recent developments that demonstrate the mutual benefit of environmental magnetism from close collaborations with biology chemistry and physics finally we discuss directions in which environmental magnetism is likely to develop in the future

seasonal circulation under the eastern ross ice shelf antarctica an annual cycle of shelf water temperatures and salinities measured at depth near the eastern ross ice shelf front is used to force a two dimensional thermohaline circulation model adapted to different subice paths in the vicinity of roosevelt island these paths were assumed to have constant water column thicknesses of 160 200 and 240 m and lengths of 460 800 km additional simulations with the longer cavity included a 80 m thick interior water column in order to approximate conditions closer to the grounding line model results were compared with other long term measurements that showed outflow from beneath the ice shelf shelf water flowing into the cavity west of roosevelt island appears to follow a cyclonic route around the island the ice shelf base loses mass at a rate of 18 27 cm yr 1 with seasonal forcing increasing the spatial and temporal variability of circulation and property distributions in the larger cavities shallow cavities reduce the influence of shelf water variability with increasing length introducing a transient shelf water temperature rise of 0 01 c yr 1 for 100 years increases the melt rate by 4 5 times however this increase is smaller if salinity also decreases over the same period of time as might be expected from the added meltwater component

ocean interactions with the base of amery ice shelf antarctica using a two dimensional ocean thermohaline circulation model we varied the cavity shape beneath amery ice shelf in an attempt to reproduce the 150 m thick marine ice layer observed at the gi ice core site most simulations caused melting rates which decrease the ice thickness by as much as 400 m between grounding line and g1 but produce only minor accumulation at the ice core site and closer to the ice front changes in the seafloor and ice topographies revealed a high sensitivity of the basal mass balance to water column thickness near the grounding line to submarine sills and to discontinuities in ice thickness model results showed temperature salinity gradients similar to observations from beneath other ice shelves where ice is melting into seawater modeled outflow characteristics at the ice front are in general agreement with oceanographic data from prydz bay a freshwater flux across the grounding line derived from melting beneath the grounded ice sheet would have to be anomalously large to produce the basal marine ice layer and account for the ice shelf water outflow we concur with morgan s inference that the g 1 core may have been taken in a basal crevasse filled with marine ice this ice is formed from water cooled by ocean ice shelf interactions along the interior ice shelf base

goethite as a potential source of magnetic nanoparticles in sediments goethite is an important iron bearing mineral found in nearly all types of soils and sediments that typically occurs as particles of nanoscale crystallites here we show that poorly crystalline nanophase goethite can undergo dehydroxylation and alteration to produce strongly magnetic fe oxide nanoparticles after moderate reductive heating synthetic oriented aggregates of nanogoethite were converted to stoichiometric sub micron magnetite in contrast oxidative heating produced only nanocrystalline hematite we present a novel characterization of the products of nanogoethite alteration which represents an important pathway for the production of both superparamagnetic and stable single domain magnetic particles in a wide variety of sedimentary environments including soils and lacustrine and marine settings in which goethite is commonly present in nature the mechanism of magnetite formation observed in our experiments could contribute to the widespread phenomenon of magnetic enhancement in many soil types and could be responsible for secondary magnetizations often observed to accompany diagenesis in sedimentary rocks thus nanogoethite may be an important precursor of fine magnetic particles and magnetic remanence carriers in the environment particularly in soils and catchment areas affected by wildfire as well as in sediments subjected to deep burial diagenesis and low grade metamorphism

magnetic comparison of abiogenic and biogenic alteration products of lepidocrocite abstract lepidocrocite is a potentially important fe bearing precursor phase for the production of nanoscale fe oxide particles in the environment we present a detailed magnetic characterization of various alteration products of lepidocrocite resulting from thermal dehydroxylation reactions and bacterially induced bioreduction and remineralization accompanied by characterization with x ray diffraction xrd and transmission electron microscopy dehydroxylation during annealing at moderate temperatures produces a topotactic transformation from lepidocrocite to maghemite when heated in an oxidizing atmosphere or to magnetite when heated in a reducing atmosphere the abiotic fe oxide products form an oriented framework of strongly interacting superparamagnetic crystallites and are characterized by a distinctive porous nanostructure observed by electron microscopy lepidocrocite bioreduction by the iron reducing bacterium shewanella putrefaciens atcc 8071 produces nanoscale particles of a strongly magnetic phase this fe ii bearing mineral produced by bioreduction is highly crystalline and euhedral in shape with a broad grain size distribution and is indicated by magnetic and xrd measurements to be a cation excess magnetite we highlight the distinguishing microscopic characteristics of magnetite from both abiotic and bacterially induced mineralization that should allow them to be identified in natural settings moreover both mechanisms of alteration represent potential pathways for the direct formation of strongly magnetic fine grained fe oxide particles in sedimentary environments

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

sensitivity of the amery ice shelf antarctica to changes in the climate of the southern ocean abstract coupled ocean atmospheric general circulation models indicate that warming of up to 3 c may occur over the next century in waters immediately to the north of the amery ice shelf the impact of this warming on the ocean cavity under the amery ice shelf and the mass exchange at the interface between the ocean cavity and the ice shelf is investigated using a three dimensional ocean model warming of between 0 25 and 3 0 c is applied along the ice front in a series of model runs rather than in a single transient run changes in salinity are also considered for larger amounts of warming the model results show that the circulation in the ocean cavity changes as warming increases particularly in the gyres that dominate the horizontal circulation the changes in the heat flux from the warming increase the melt rates from the base of the amery ice shelf from the present day mean melt rate and net mass loss estimates of 0 28 m yr 1 and 14 2 gt yr 1 respectively by approximately 0 55 m yr 1 c 1 and 2

modeling thermodynamic ice ocean interactions at the base of an ice shelf abstract models of ocean circulation beneath ice shelves are driven primarily by the heat and freshwater fluxes that are associated with phase changes at the ice ocean boundary their behavior is therefore closely linked to the mathematical description of the interaction between ice and ocean that is included in the code an hierarchy of formulations that could be used to describe this interaction is presented the main difference between them is the treatment of turbulent transfer within the oceanic boundary layer the computed response to various levels of thermal driving and turbulent agitation in the mixed layer is discussed as is the effect of various treatments of the conductive heat flux into the ice shelf the performance of the different formulations that have been used in models of sub ice shelf circulation is assessed in comparison with observations of the turbulent heat flux beneath sea ice formulations that include an explicit parameterization of the oceanic boundary layer give results that

geomagnetic paleointensity in historical pyroclastic density currents testing the effects of emplacement temperature and postemplacement alteration thellier type paleointensity experiments were conducted on welded ash matrix or pumice from the 1912 novarupta nv and 1980 mt st helens msh pyroclastic density currents pdcs with the intention of evaluating their suitability for geomagnetic paleointensity studies pdcs are common worldwide but can have complicated thermal and alteration histories we attempt to address the role that emplacement temperature and postemplacement hydrothermal alteration may play in nonideal paleointensity behavior of pdcs results demonstrate two types of nonideal behavior unstable remanence in multidomain md titanomagnetite and nonideal behavior linked to fumarolic and vapor phase alteration emplacement temperature indirectly influences msh results by controlling the fraction of homogenous md versus oxyexsolved pseudo single domain titanomagnetite nv samples are more directly influenced by vapor phase alteration the majority of nv samples show distinct two slope behavior in the natural remanent magnetization partial thermal remanent magnetization plots we interpret this to arise from a thermo chemical remanent magnetization associated with vapor phase alteration and samples with high water content 0 75 loss on ignition generate paleointensities that deviate most strongly from the true value we find that pdcs can be productively used for paleointensity but that as with all paleointensity studies care should be taken in identifying potential postemplacement alteration below the curie temperature and that large welded flows may be more alteration prone one advantage in using pdcs is that they typically have greater areal spatial exposure than a basalt flow allowing for more extensive sampling and better assessment of errors and uncertainty

the effects of ocean warming on melting and ocean circulation under the amery ice shelf east antarctica using a three dimensional ocean model specially adapted to the ocean cavity under the amery ice shelf we investigated the present ocean circulation and pattern of ice shelf basal melting and freezing the differences which would result from temperature changes in the seas adjacent to the amery ice shelf and the ramifications of these changes for the mass balance of the ice shelf under present conditions we estimate the net loss from the amery ice shelf from excess basal melting over freezing at approximately 7 8 gt a 1 this comprises a gross loss of 11 4 gt a 1 at a mean rate of 0 42 m a 1 which is partially offset by freezing on of 3 6 gt a 1 at a mean rate of 0 19 m a 1 when the adjacent seas were assumed to warm by 1 c we found the net melt increased to 31 6 gt a 1 comprising 34 6 gt a 1 of gross melt and 3 0 gt a 1 of freezing

circulation beneath the filchner ice shelf antarctica and its sensitivity to changes in the oceanic environment a case study we investigate the sensitivity of the ocean circulation in the filchner trough to changes in the large scale oceanic environment and its impact on the mass balance of the filchner ice shelf antarctica three experiments with a three dimen sional ocean model describe i the current situation ii a scenario with increased ocean temperatures and iii a scenario with reduced sea ice formation rates on the adjacent continental shelf in the final discussion brief results of a combined scenario with increased ocean temperatures and reduced sea ice formation are presented the changes from the current situation affect the circulation in the filchnert rough and melting and freezing processes beneath the ice shelf the latter affect the amount and propert ies ofice shelf water isw a component of antarctic bottom water net basal melt rates provide an overall measure for the changes while the control run yields 0 35 m a i net melting averaged over the filchner ice shelf area the warming scenario results in a more than twofold increase in ice shelf mass loss reduced production of high salinity shelf water due to smaller sea ice formation rates in the second scenario leads on the other hand to a decrease in basal mass loss because the deep cavity is less well ventilated than in the con trol run isw is cooled and the ice shelf is stabilized under this scenario which is arguably the more likely development in the southern weddell sea

magnetic anisotropy in rhyolitic ignimbrite snake river plain implications for using remanent magnetism of volcanic rocks for correlation paleomagnetic studies and geological reconstructions some of the field and laboratory expenses for this work were covered by the natural environment research council nerc grant ne g005372 1 the salary and school fees for the first author were covered in part by a national science foundation nsf grant ear 443549 81179 awarded to x zhao

thermohaline circulation and interaction between ice shelf cavities and the adjacent open ocean the circulation system in an ice shelf cavity is driven by buoyancy fluxes due to melting and freezing of ice and horizontal pressure gradients at the interface between the cavity and the open ocean hence the inflow and outflow pattern and the hydrography in the open ocean influence the general hydrographic condition in the cavity which at least provides the potential for melting and freezing processes applying a three dimensional ocean general circulation model to an idealized ice shelf cavity geometry coupled with an open ocean at a topographic ice shelf barrier we found an important parameter controlling the interaction between these two systems idealized studies for different ice shelf and sea bottom topographies and forcing mechanisms for the open ocean show that the ice shelf edge represents a natural barrier for barotropic interaction because of the sudden decrease in water column thickness since the water column thickness and the coriolis force determine the characteristics for geostrophic flow separated circulation systems arise for the open ocean and the ice shelf cavity only in areas where constant water column thickness and from the oceanographic point of view constant f h contours can be observed across the barrier an increased barotropic current can surmount the ice edge and ventilate the water mass beneath the ice shelf this is only the case at lateral sloping sidewalls or at deep depressions which can be found for example in the southern weddell sea in all other cases the circulation in the ice shelf cavity is closed and almost unaffected by the hydrography outside the barrier

ocean circulation and ice ocean interaction beneath the amery ice shelf antarctica simulations of the ocean dynamics in the cavity under the amery ice shelf antarctica were carried out using a three dimensional numerical ocean model two different boundary conditions were used to describe the open ocean barotropic exchange at the ice front the simulations show that the circulation in the ocean cavity is predominantly barotropic and is generally steered by the cavity topography the circulation is driven by the density gradient in the cavity which is strongly influenced by the heat and salt fluxes from melting and freezing processes at the ice ocean interface and by the horizontal exchange of heat and salt across the open ocean boundary at the ice front the interaction at the ice ocean interface allows the basal component of the mass loss of the amery ice shelf to be estimated in the two simulations the computed losses were 5 8 gt yr 1 and 18 0 gt yr 1 values consistent with observations the bulk of the melting occurred near the southern grounding line of the ice shelf although substantial melting also occurred in areas where heat transport by horizontal circulation was large accretion was restricted to areas where water from upstream melting became supercooled as it ascended the ice shelf base

paleochemistry of lakes agassiz and manitoba based on ostracodes the ionic composition and salinity of lake manitoba and its late glacial precursor lake agassiz changed significantly over the past 11 000 years the paleochemical record reported here is based on modern analog environments of ostracodes identified in a new 14 5 m core from southern lake manitoba the ionic composition of lake manitoba today is dominated by na cl and hc03 with much less ca2 mg2 and k evaporative concentration of modern lake manitoba water would lead to greater salinity and the near depletion of ca2 due to continued precipitation of calcite during periods of highest salinity in the holocene however lake manitoba supported limnocythere staplini today this species inhabits waters in which ca2 hco3 including springs associated with groundwater in paleozoic bedrock discharging into lake winnipegosis and eventually after much dilution into lake manitoba further complicating the holocene record are intervals containing limnocythere friabilis that suggest periodic

magnetism and magnetic mineralogy of ash flow tuffs from yucca mountain nevada the magnetic susceptibility and remanent magnetization of an ash flow sheet are profoundly influenced by cooling history after emplacement spatial variations in these properties therefore provide a potentially useful tool for geological studies maxima and minima in measured along profiles at yucca mountain nevada identify persistent magnetic marker horizons within vitric portions of the tiva canyon and underlying topopah spring members of the paintbrush tuff the correlatable variations of with vertical position in these units exist over the 8 km of strike examined the observed stratigraphic changes in magnetic properties reflect variations in amounts and mineralogy of fe ti oxide phenocrysts and the presence shape size and mineralogy of magnetic fe oxide microcrystals that precipitated at high temperature after emplacement of each sheet vitrophyre near the top of the topopah spring member has a distinctive maximum in due to titanomagnetite phenocrysts susceptibility in the lower portion of the overlying tiva canyon member reaches maximum values of about 1 3 10 2 si near the base of the member beneath the columnarly jointed vitrophyre due to the presence of abundant superparamagnetic cubic fe oxide these elongate microcrystals are several tens of nm long and are typically deformed and tapered up section in the tiva canyon member the susceptibility decreases going to a minimum within the columnar vitrophyre at this stratigraphic level the precipitated fe oxides are larger and carry intense nrm of up to 30 a m is only 3 10 3 the whisker like microcrystals of fe oxide which typically nucleated on what appears to be chain silicate grew as needles up to 25 m in length the size variations of the precipitated fe oxides which were established using transmission electron microscopy tem and petrographic observation are consistent both with variations in magnetic susceptibility measured at the outcrop and with variations in the intensity of remanent magnetization several interpretations of the shape anisotropy of the precipitated feoxide are possible including growth by a dislocation mechanism additionally the observed elongation of precipitated microcrystals is consistent with theoretical predictions for growth in a uniaxial stress field susceptibility variations as established at the outcrop as well as in the borehole offer a potentially useful tool for stratigraphic correlation of ash flow sheets

the superparamagnetism of yucca mountain tuff yucca mountain tuff contains small titanomagnetite grains with narrow size distributions in the superparamagnetic range schlinger et al 1988 magnetic measurements on three samples comprising hysteresis loops at low and ambient temperatures acquisition and demagnetization of isothermal remanent magnetization thermal demagnetization of the saturation remanence and of a low temperature thermoremanence and frequency and temperature dependent susceptibilities allow evaluation of the magnetic properties in terms of neel s 1949 single domain theory precise grain volume distributions have been obtained by applying the blocking volume concept to thermal demagnetization results in contrast an attempt to derive mean particle volumes by fitting a langevin curve to the room temperature magnetization curves fails probably because the precondition for the langevin function kv kt 1 is not met it is only for the sample with the smallest grains and in weak fields 20 mt that a langevin fit provides a reasonable volume estimate there is good agreement between the experimental results and the calculated frequency and temperature dependence of susceptibility thus verifying that neel s theory is sufficient for the magnetic description of single domain assemblages spanning the superparamagnetic stable single domain boundary however some deviations between modeled and measured susceptibilities exist and the physical causes may include size dependent anisotropy nonuniform magnetizations and also an uncertain preexponential time constant 0 while 0 10 11 s gives the best fit for the sample with the largest grains 0 10 9 s is more reasonable for the others thus 0 may indeed be size and temperature dependent as predicted by brown 1959 the commonly cited parameter fd frequency dependence of susceptibility reaches 30 at room temperature rt for one sample with a blocking temperatures just below rt while fd 0 at rt for a superparamagnetic sample with smaller grains these results thus exemplify that fd is not limited to 15 as a number of studies suggest and that fd 0 must not be taken to imply the absence of superparamagnetic grains

a rapid method for magnetic granulometry with applications to environmental studies we have developed a rapid method of characterizing the fine scale variation sub micron to hundreds of microns m in the grain size of magnetite in long sediment cores this method is a new rock magnetic tool and can be readily applied to the following disciplines 1 environmental studies 2 hydrology 3 petrology and 4 geomagnetism the method consists of a comparative plot of the depth variation of two magnetic parameters the first parameter low field less than 0 1 millitesla mt or 1 oe susceptibility in a steady field is plotted on the abscissa the second parameter anhysteretic remanent magnetization arm imparted by a smoothly decreasing af of peak value 100 mt 1 000 oe in the presence of a weak steady field 0 01 0 1 mt or 0 1 1 oe is plotted on the ordinate for a sediment column in which the magnetic mineral is magnetite a plot of the two magnetic parameters shows transitions in the average magnetic grain size with depth as clearly discernible changes in the slope of a regression line fit to the individual points arm

deglacial environments in eastern prydz bay east antarctica abstract a high resolution continuous record of diatom census counts and diatom specific biomarkers in sediment core nbp0101 jpc24 allows assessment of oceanographic and environmental conditions in eastern prydz bay during the deglaciation 11 100 9000 cal yr bp at decadal timescale our study improves previous snapshots investigations based on resin embedded thin sections and presents a new proxy that compliments the diatom census counts our results suggest that the ice sheet retreat over the core site is dated at 11 100 cal yr bp setting the onset of local deglaciation and subsequent open marine conditions the glacial retreat in prydz bay is due to global warming initiated at 18 cal ka bp and the regional development of the prydz bay cyclonic gyre our results further demonstrate that the deglaciation in eastern prydz bay can be separated in four phases the first between 11 100 and 10 900 cal yr bp when the ice shelf was proximal and sea ice was almost perennial the second and the third phases between 10 900 10 400 cal yr bp and 10 400 9900 cal yr bp respectively when the ice shelf retreated and seasonal sea ice cycle consequently developed promoting warmer water to pump into the bay within the gyre which in turn forced the ice shelf recession and the yearly sea ice cycle establishment and the fourth between 9900 and 9000 cal yr bp when holocene condition were set with a recurrent seasonal sea ice cycle and a well established prydz bay gyre

ferromagnetic resonance characterization of greigite fe3s4 monoclinic pyrrhotite fe7s8 and non interacting titanomagnetite fe3 xtixo4 ferromagnetic resonance fmr spectroscopy has become an increasingly useful tool for studying the magnetic properties of natural samples magnetite fe3o4 is the only magnetic mineral that has been well characterized using fmr this limits the wider use of fmr in rock magnetism and paleomagnetism in this study we applied fmr analysis to a range of magnetic minerals including greigite fe3s4 monoclinic pyrrhotite fe7s8 magnetically non interacting titanomagnetite fe3 xtixo4 and synthetic magnetite chains to constrain interpretation of fmr analysis of natural samples and to explore applications of fmr spectroscopy we measured the fmr signatures of a wide range of well characterized samples at the x and q bands fmr spectra were also simulated numerically to compare with experimental results the effects of magnetic anisotropy mineralogy domain state and magnetostatic interactions on the fmr spectra are discussed for all studied minerals our experimental and theoretical analyses of magnetically non interacting tuff samples and magnetically interacting chains enable quantitative assessment of contributions of magnetostatic interactions and magnetic anisotropy to the fmr spectra our results also indicate that intact magnetosomes are a unique system with distinct fmr signatures while fmr analysis is useful for characterizing magnetic properties of natural samples care is needed when making interpretations because of overlaps in a range of fmr signatures of different magnetic minerals with different magnetic properties our analyses will help to constrain such interpretations in rock magnetic studies

the dynamic response of the greenland and antarctic ice sheets to multiple century climatic warming new calculations were performed to investigate the combined response of the greenland and antarctic ice sheets to a range of climatic warming scenarios over the next millennium use was made of fully dynamic 3d thermomechanic ice sheet models which were coupled to a two dimensional climate model the experiments were initialized with simulations over the last two glacial cycles to estimate the present evolution and were subsequently forced with temperature scenarios resulting from greenhouse emission scenarios which assume equivalent co2 increases of two four and eight times the present 1990 a d value by the year 2130 a d and a stabilization after that the calculations brought to light that during the next century short term effect the background evolution trend would dominate the response of the antarctic ice sheet but would be negligible for the greenland ice sheet on that timescale the greenland and antarctic ice sheets would roughly balance one another for the middle scenario similar to the ipcc96 is92a scenario with respective contributions to the worldwide sea level stand on the order of about 610 cm on the longer term however both ice sheets would contribute positively to the worldwide sea level stand and the most important effect would come from melting on the greenland ice sheet sensitivity experiments highlighted the role of ice dynamics and the height massbalance feedback on the results it was found that ice dynamics cannot be neglected for the greenland ice sheet not even on a century timescale but becomes only important for antarctica on the longer term the latter is related to an increased outflow of ice into the ice shelves and to the grounding line retreat of the west antarctic ice sheet which are both found to be sensitive to basal melting below ice shelves and the effective viscosity of the ice shelves stretching parameters to their limits yielded a combined maximum rate of sea level rise of 85 cm century21 of which 60 cm would originate from the greenland ice sheet alone

isopycnal mixing in ocean circulation models abstract a subgrid scale form for mesoscale eddy mixing on isopycnal surfaces is proposed for use in non eddy resolving ocean circulation models the mixing is applied in isopycnal coordinates to isopycnal layer thickness or inverse density gradient as well as to passive scalars temperature and salinity the transformation of these mixing forms to physical coordinates is also presented

two types of chemical remanent magnetization during the oxidation of magnetite abstract the chemical remanent magnetization crm m crm resulting from oxidation of equidimensional magnetite to a mixture of 90 hematite and 10 cation deficient magnetite is jointly controlled by the initial anhysteretic remanent magnetization m arm of the parent magnetite and the field h crm applied during oxidation m crm lies in the plane defined by h crm and m arm which were perpendicular in our experiments in an intermediate orientation determined by the relative strengths of h crm 0 05 0 1 and 0 2 mt and m arm 0 002 0 12 saturation remanence the crms are univectorial remanences that remain fixed in these stable intermediate directions throughout the course of stepwise alternating field and thermal demagnetization similar experiments in which acicular single domain cation deficient magnetite was oxidized to maghemite gave quite different results m crm was always parallel to m arm its direction was unaffected by field strengths as high as h crm 1 5 mt the essential factors controlling crm direction seem to be the number and crystallographic structure of oxidation products for single phase oxidation without change in crystal lattice crm remains parallel to the initial remanence of the parent mineral for multiphase oxidation to a mixture of rhombohedral and spinel phases both initial remanence and the field acting during oxidation influence the crm direction which therefore has an intermediate orientation that parallels neither m arm nor h crm

a preliminary magnetic study of soil samples from west central minnesota abstract various rock magnetic techniques were applied to characterize magnetically the samples of a soil profile taken from west central minnesota there is a marked change in magnetic properties as a function of depth in the core x ray analysis and curie temperature measurements carried out on the magnetic fractions indicate that magnetite is the dominant iron oxide in both the top soil and the subsoil the intensity of anhysteretic remanent magnetization arm decreases sharply as the depth increases in contrast the stability of arm was found to be higher for the subsoil the surface soil sample was capable of acquiring a significant amount of viscous remanent magnetization vrm the vrm acquisition coefficient s a of the subsoil s a 3 18 10 6 emu g 1 3 18 10 6 a m 2 kg 1 was about ten times weaker than that of the top soil sample s a 3 868 10 7emu g 1 3 868 10 7 a m 2 kg 1 the magnetic domain state indicator the ratio of coercivity of remanence to coercive force h cr h c was 1 5 and 3 85 for the top soil and subsoil respectively it appears that the observed variations in magnetic properties down the present soil core is due only to a difference in grain size we conclude that the magnetic grains in surface soil samples were more single domain sd like whereas the magnetite grains in the subsoil samples were more likely in pseudo single domain psd or small multidomain md range the observed lower stability for the surface soil samples is attributed to the presence of superparamagnetic grains whose presence was confirmed by transmission electron micrographs

comment on new late pennsylvanian paleomagnetic results from parana basin southern brazil is the recent giant gaussian process model valid for the kiaman superchron by brandt et al

magnetism of a red soil core derived from basalt northern hainan island china volcanic ash versus pedogenesis red soil volcanic ash versus pedogenesis similar to loess paleosol sequences in northwestern china terrestrial sedimentary sequences red soils in southern china also provide sensitive quaternary records of subtropical tropical paleoclimate and paleoenvironment compared with red clay sequences originated from eolian dust red soils derived from bedrock have received little attention in this study a long core of red soil derived from weathered basalt in northern hainan island china was systematically investigated by using detailed magnetic measurements and rare earth element analyses the results show that an extremely strong magnetic zone with a maximum magnetic susceptibility 10 10 5 m3 kg 1 is interbedded in the middle of the core profile this layer contains a significant amount of superparamagnetic magnetite maghemite particles that primarily originated from volcanic ash with secondary contributions from pedogenesis the former has an average grain size of 19 nm with a normal distribution of volume and the latter has a much wider grain size distribution the presence of volcanic ash within the red soil indicates that these quaternary basalts were not formed by continuous volcanic eruptions moreover the magnetic enhancement patterns differ between the upper and lower zones the upper zone is more magnetically enhanced and experienced higher precipitation and temperature than the lower zone discrimination of superparamagnetic particles originating from pedogenic processes and volcanic ash thus provides a sound theoretical base for accurate interpretation of magnetism in red soils in this region

a one dimensional model of ice shelf ocean interaction large scale oceanic circulation beneath antarctic ice shelves is driven by the thenmohaline differences which result from mass and energy exchange at the ice ocean interface dense saline waters are drawn underneath the ice shelves and emerge cooled and diluted as plumes of ice shelf water a simple one dimensional model of this process has been developed in which the ice shelf water plume is treated as a turbulent gravity current initiated at the inland margin by a flow of fresh meltwater emerging from beneath the grounded ice subsequent evolution of the plume as it ascends along an ice shelf base of specified geometry can be simulated the model has been applied to a flow line on ronne ice shelf antarctica to explain the observed distribution and rate of basal melting and freezing calculations indicate that the present mean melt rate of 0 6 m yr 1 would increase to 2 6 m yr 1 if the underlying water were to warm by 0 6 c this would not only lead to significant thinning of the ice shelf but could also cause a profound change in ocean circulation on the open continental shelf

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