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

mind the gap towards a biogenic magnetite palaeoenvironmental proxy through an extensive finite element micromagnetic simulation abstract magnetotactic bacteria mtb preserved in sediments magnetofossils are highly sensitive to environmental and climatic forcing and can therefore be used as an easy to measure palaeo environment proxy it is known that magnetofossils occur with various morphologies each with its own particular magnetic signature we present the first systematic extensive micromagnetic study relating magnetosome chain morphology to their hysteresis parameters we modelled more than 450 different intact magnetosome chains in the finite element micromagnetic modelling software merrill relating the morphology magnetosome size elongation intra chain spacing and chain length to the magnetic parameters of coercivity coercivity of remanence and saturation magnetization and analyzing domain states and switching modes it is shown that magnetic properties are far dominated by the intra chain spacing however analysis of a large set of published tem images of different mtb strains shows a strong correlation between intra chain spacing and grain size this correlation can in principle be used to infer mtb morphology purely from magnetic measurements providing a framework to use biogenic magnetite as a sensitive palaeo environment proxy additionally it is found that mtb carefully control intra chain spacing during magnetosome synthesis to maximize saturation remanence as well as magnetic stability

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

an appalachian amazon magnetofossil evidence for the development of a tropical river like system in the mid atlantic united states during the paleocene eocene thermal maximum on the mid atlantic coastal plain of the united states paleocene sands and silts are replaced during the paleocene eocene thermal maximum petm by the kaolinite rich marlboro clay the clay preserves abundant magnetite produced by magnetotactic bacteria and novel presumptively eukaryotic iron biomineralizing microorganisms using ferromagnetic resonance spectroscopy and electron microscopy we map the magnetofossil distribution in the context of stratigraphy and carbon isotope data and identify three magnetic facies in the clay one characterized by a mix of detrital particles and magnetofossils a second with a higher magnetofossil to detrital ratio and a third with only transient magnetofossils the distribution of these facies suggests that suboxic conditions promoting magnetofossil production and preservation occurred throughout inner middle neritic sediments of the salisbury embayment but extended only transiently to outer neritic sediments and the flanks of the embayment such a distribution is consistent with the development of a system resembling a modern tropical river dominated shelf

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

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

magnetic properties of a soil from southeastern brazil in the presence of magnetic biomineralization by social insects abstract it is known that the migratory ant pachycondyla marginata and its prey the termite neocapritermes opacus synthesize magnetic minerals which may contribute to the magnetic characteristics of soil a comparative study of the magnetic properties of the minerals found in those species as well as their nests and surrounding soil aimed at investigating the contribution of the biogenic minerals to the magnetization of the soil partially oxidized magnetite t v 120 k was found in the pachycondyla marginata as shown by the verwey transition in the zfc fc and rtsirm curves low temperature remanence measurements of the neocapritermes opacus indicated the presence of maghemite and or titanomaghemite while partially oxidized pseudo single domain magnetites and or titanomaghemite were recognized in the soil and insect nests based on the similarity in the magnetic characteristic of the samples it is concluded that there is significant influence of biogenic mineralization upon the pedogenic contribution

properties of intracellular magnetite crystals produced by desulfovibrio magneticus strain rs 1 desuffiovibrio magneticus strain rs 1 is an anaerobic sulfate reducing bacterium cells form intracellular nanocrystals of magnetite but are only weakly magnetotactic in order to understand the unusual magnetic response of this strain we studied magnetite crystals within cells grown with fumarate and sulfate many cells grown under either condition did not form magnetic crystals while others contained only i to 18 small similar to 40 nm magnetite containing magnetosomes bulk magnetic measurements of whole cells showed a superparamagnetic like behavior indicating that many of the magnetite crystals are too small to have a permanent magnetic moment at ambient temperature the temperature of the verwey transition is lower similar to 86 k than of magnetite from other magnetotactic strains likely indicating partial oxidation of magnetite into maghemite as a result of the small size and small number of magnetite magnetosomes the magnetic moments of most cells grown anaerobically with fumarate or sulfate are insufficient for magnetotaxis in addition to intracellular magnetite in some cultures another iron oxide hematite formed on the surfaces of cells the hematite grains are embedded in an extracellular polymeric material indicating that the crystals likely resulted from a biologically induced mineralization process although the hematite particles appear to consist of aggregations of many small 5 to 10 nm grains the grains have a consensus orientation and thus the whole particle diffracts as a single crystal the aligned arrangement of nanoparticles within larger clusters may reflect either a templated nucleation of hematite crystallites in an extracellular organic matrix or result from a self assembling process during the crystallization of hematite from ferric gels or ferrihydrite

compositionally zoned cl rich amphiboles from north dabie shan china monitor of high pressure metamorphic fluid rock interaction processes abstract amphiboles containing up to 4 2 wt cl are found in felsic granulites from yanzihe within the north dabie area of the dabie sulu ultrahigh and high pressure metamorphic belt in eastern china most amphibole grains show considerable zonations with cl contents ranging from 0 to 4 2 wt based on their textural features amphiboles can be divided into four generations 1 amphibole occurring as inclusions in orthopyroxene am in with cl contents around 3 5 wt 2 amphibole forming cores of grains in the matrix am i with cl contents between 3 0 and 4 2 wt 3 amphibole with cl contents of 0 2 to 2 5 wt am ii occurring as hydrothermally altered parts of the original amphibole 4 cl free amphibole am iii usually developed at the outermost rim of the grain major and rare earth elements show significant variations for am i am ii and am iii different generations of amphiboles are related to different metamorphic stages of the granulite in yanzihe and provide a monitor for fluid rock interactions and p t evolution during the high pressure metamorphism of dabie shan pressure and temperature estimates suggest that am in was formed during prograde metamorphism of 10 kbar and 700 800 c am i was formed under peak metamorphic conditions 20 kbar 800 960 c whereas am ii and am iii were formed during retrograde metamorphic stages 560 770 c and 5 7 kbar and 520 670 c and

use of magnetic hysteresis properties and electron spin resonance spectroscopy for the identification of volcanic ash a preliminary study summary this initial study investigates the possible use of hysteresis parameters and electron spin resonance esr spectroscopy to identify and correlate volcanic ash esr and hysteresis properties are sensitive to characteristics such as the chemical composition mineralogy and grain size and shape these characteristics are determined by the tectonic setting of the volcano and by the magmatic and eruptive history of the volcanic ash hysteresis properties and esr spectra therefore should be distinct for each ash eruption and may help to identify the eruptive source of the ash and to correlate ash from unknown sources we conducted esr spectroscopy at room temperature and magnetic hysteresis measurements on 19 samples of a single ash the 1974 october 14 eruption of the fuego volcano guatemala and on single samples of ash obtained from eight diverent volcanoes the fuego ash samples were obtained at increasing distances from the volcano for the single fuego ash esr spectra and hysteresis parameters become increasingly similar as the distance from the volcano increases at distances greater than 30 km esr spectra and hysteresis properties are uniform the variability of magnetic and esr properties with distance from fuego is due to the preferential fall out of phenocrysts closer to the volcano at large distances the ash is more uniform containing more glass and microcrystals all eight ash samples from the diverent volcanoes can be distinguished from the distal fuego 1974 october 14 ash using esr spectra and hysteresis parameters these results suggest that esr and hysteresis measurements have a potential to be used as tools to identify distal ash when used in conjunction with geochemical mineralogical and or other types of data

ferromagnetic resonance and low temperature magnetic tests for biogenic magnetite magnetite is both a common inorganic rock forming mineral and a biogenic product formed by a diversity of organisms magnetotactic bacteria produce intracellular magnetites of high purity and crystallinity magnetosomes arranged in linear chains of crystals magnetosomes and their fossils magnetofossils have been identified using transmission electron microscopy tem in sediments dating back to 510 570 ma and possibly in 4 ga carbonates in martian meteorite alh84001 we present the results from two rock magnetic analyses the low temperature moskowitz test and ferromagnetic resonance fmr applied to dozens of samples of magnetite and other materials the magnetites in these samples are of diverse composition size shape and origin biologically induced extracellular biologically controlled magnetosomes and chiton teeth magnetofossil synthetic and natural inorganic we confirm that the moskowitz test is a distinctive indicator for magnetotactic bacteria and provide the first direct experimental evidence that this is accomplished via sensitivity to the magnetosome chain structure we also demonstrate that the fmr spectra of four different strains of magnetotactic bacteria and a magnetofossil bearing carbonate have a form distinct from all other samples measured in this study we suggest that this signature also results from the magnetosomes unique arrangement in chains because fmr can rapidly identify samples with large fractions of intact isolated magnetosome chains it could be a powerful tool for identifying magnetofossils in sediments

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

formation of single domain magnetite by a thermophilic bacterium magnetite is a common product of bacterial iron reduction and may serve as a potential physical indicator of biological activity in geological settings here the authors report the formation of single domain magnetite under laboratory conditions by a thermophilic fermentative bacterial strain tor 39 that was isolated from the deep subsurface time course analyses were performed at 65 c to study the effect of bacterial activity on solution chemistry and magnetite formation during the growth of tor 39 run products were examined by transmission electron microscopy magnetite particles formed exclusively outside of bacterial cells and exhibited octahedral shapes having relatively equal length and width 15 difference tiny magnetite particles 12 nm nucleated between 10 and 11 h of incubation and increased to average lengths of 55 4 26 8 nm after 24 h of incubation between 24 h and 22 d of incubation magnetite particles maintained average lengths of 56 2 24 8 nm based on size constraints greater than 85 of the particles observed fell within the magnetic single domain little to no magnetite was detected in abiotic controls at 65 or 95 c or in tor 39 cultures whose activity was suppressed unlike mesophilic iron reducing bacteria e g gs 15 tor 39 produced crystals having shapes and sizesmore similar to some particles produced intracellularly by magnetotactic bacteria thus the single domain magnetite produced by thermophiles such as tor 39 may represent a heretofore unrecognized biological contribution to natural remanent magnetization in sedimentary basins and other geothermal environments less

a strong angular dependence of magnetic properties of magnetosome chains implications for rock magnetism and paleomagnetism 1 single domain magnetite particles produced by magnetotactic bacteria magnetosomes and aligned in chains are of great interest in the biosciences and geosciences here we investigated angular variation of magnetic properties of aligned magnetospirillum magneticum amb 1 cells each of which contains one single fragmental chain of magnetosomes with measurements at increasing angles from the chain direction we observed that i the hysteresis loop gradually changes from nearly rectangular to a ramp like shape e g bc and remanence decrease ii the acquisition and demagnetization curves of irm shift toward higher fields e g bcr increases and iii the forc diagram shifts toward higher coercivity fields e g bc forc increases for low temperature results compared to unoriented samples the samples containing aligned chains have a much lower remanence loss of field cooled fc and zero field cooled zfc remanence upon warming through the verwey transition higher ratio fc zfc for the measurement parallel to the chain direction and lower ratio larger fc and zfc values for the perpendicular measurement micromagnetic simulations confirm the experimental observations and reveal that the magnetization reversal of magnetosome chain appears to be noncoherent at low angles and coherent at high angles the simulations also demonstrate that the angular dependence of magnetic properties is related to the dispersion degree of individual chains indicating that effects of anisotropy need to be accounted for when using rock magnetism to identify magnetosomes or magnetofossils once they have been preserved in aligned chains additionally this study experimentally demonstrates an empirical correspondence of the parameter bc forc to bcr rather than bc at least for magnetite chains with strong shape anisotropy this suggests forc analysis is a good discriminant of magnetofossils in sediments and rocks

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

low temperature magnetic properties of pelagic sediments ocean drilling program site 805c tracers of maghemitization and magnetic mineral reduction magnetic properties of pelagic sediments from the western equatorial pacific ocean drilling program odp site 805c have been measured at low temperatures to investigate changes in magnetic mineralogy and the distribution of bacterial magnetite with depth a saturation isothermal remanent magnetization mrs was given to the samples by applying a strong magnetic field 2 5 t at 20 k after cooling both in the absence zero field cooling zfc and presence of a 2 5 t magnetic field field cooling fc the thermal demagnetization of both mrszfc and mrsfc was measured from 20 to 300 k no pronounced verwey transition was observed in the data from samples shallower than the iron redox boundary below the boundary the verwey transition is visible for all samples studied we interpret this behavior as reflecting low temperature oxidation maghemitization of primary magnetite and the subsequent dissolution of maghemite coatings below the iron redox boundary the low temperature oxidation may occur both at the water sediment interface and deeper in the suboxic zone of sediments as a result of bacteria mediated processes including the reduction of nitrate compounds mrszfc and mrsfc thermal demagnetization curves of all samples diverge for the entire temperature range measured the divergence may be caused by the low temperature stabilization of spontaneous magnetization vectors along with exchange interaction in the magnetite maghemite system together maghemitization and magnetic reduction diagenesis appear to play a dominant role in controlling the low temperature properties of the sediment studied as a result low temperature magnetic studies may provide more information on differences in reduction diagenesis than on the distribution of bacterial magnetite in natural sediments the changes in effective oxidation state below the iron redox boundary detected by low temperature methods may be useful in the study of some paleoenvironmental processes

structure and morphology of magnetite anaerobically produced by a marine magnetotactic bacterium and a dissimilatory iron reducing bacterium intracellular crystals of magnetite synthesized by cells of the magnetotactic vibroid organism mv 1 and extracellular crystals of magnetite produced by the non magnetotactic dissimilatory iron reducing bacterium strain gs 15 were examined using high resolution transmission electron microscopy electron diffraction and57fe mo ssbauer spectroscopy the magnetotactic bacterium contained a single chain of approximately 10 crystals aligned along the long axis of the cell the crystals were essentially pure stoichiometric magnetite when viewed along the crystal long axis the particles had a hexagonal cross section whereas side on they appeared as rectangules or truncated rectangles of average dimension 53 35 nm these findings are explained in terms of a three dimensional morphology comprising a hexagonal prism of 110 faces which are capped and truncated by 111 end faces electron diffraction and lattice imaging studies indicated that the particles were structurally well defined single crystals in contrast magnetite particles produced by the strain gs 15 were irregular in shape and had smaller mean dimensions 14 nm single crystals were imaged but these were not of high structural perfection these results highlight the influence of intracellular control on the crystallochemical specificity of bacterial magnetites the characterization of these crystals is important in aiding the identification of biogenic magnetic materials in paleomagnetism and in studies of sediment magnetization

geomagnetic field inclinations for the past 400 kyr from the 1 km core of the hawaii scientific drilling project a volcanic record of geomagnetic field inclination for the past 400 kyr at hilo hawaii has been obtained from the 941 5 m of core recovered by the hawaii scientific drilling project the analysis of 195 lava flows reveals six instances of near zero inclination and two instances of fully negative inclination reverse polarity within an otherwise normal polarity core in particular flow unit 23 178 m depth records a horizontal inclination and may be associated with the laschamp event flow units 40 and 42 260 m depth record negative inclinations and are close in age to the blake event and flow unit 55 320 m depth records a negative inclination with a relative declination change of 75 with respect to the overlying flow and is probably the jamaica biwa i pringle falls event the five instances of shallow inclination found below 400 m depth appear to have resulted from long term secular variation as they are part of inclination swings between 0 and 60 with a periodicity of 10 50 kyr in contrast the inclination shifts at 178 m and 320 m depths significantly deviate from long term trends suggesting the existence of at least two independent processes producing time variations of the geomagnetic field the secular variation has a mean of 30 9 95 2 27 which is significantly shallower than the expected dipole mean of 36 the dispersion 12 5 agrees with global paleosecular variation data for 0 5 ma and secular variation models

physical interpretation of hysteresis loops micromagnetic modeling of fine particle magnetite 1 hysteresis measurements have become an important part of characterizing magnetic behavior of rocks in paleomagnetic studies theoretical interpretation is often difficult owing to the complexity of mineral magnetism and published data sets demonstrate remanence and coercivity behavior that is currently unexplained in the last decade numerical micromagnetic modeling has been used to simulate magnetic particles such simulations reveal the existence of nonuniform remanent states between single and multidomain known as the flower and vortex configurations these suggest plausible explanations for many hysteresis measurements yet fall short of explaining high saturation remanence high coercivity data such as those commonly observed in fine grained submarine basalts in this paper we review the theoretical and experimental progress to date in understanding hysteresis of geological materials we extend numerical simulations to a greater variety of shapes and sizes including random assemblages of particles and shapes more complex than simple rods and cubes our simulations provide plausible explanations for a wide range of hysteresis behavior

early dolomitization of platform carbonates and the preservation of magnetic polarity results from a combination of techniques are presented to evaluate the nature of magnetization in shallow water platform carbonates which have undergone recrystallization during early calcification and dolomitization magnetic grain separates coercivity spectra modified lowrie fuller tests magnetization efficiency and magnetostratigraphic constraints indicate that the ultrafine grained magnetite is preserved during early burial geochemical regimes inversion from aragonite high magnesium calcite to low magnesium calcite and even pervasive dolomitization these single domain crystals are thought to occur as interacting multigrain clusters some of which may exceed 1 m in diameter these large clusters may help prohibit magnetic reorientation during diagenesis furthermore during both fabric preserving and fabric destructive dolomitization the ultrafine scale replacement process restricts reorientation of the clusters thus preserving depositional or early postdeposition magnetic orientation this early dolomitization matrix stabilization may even help protect and extend the subsurface lifespan of the original polarity

iron oxide formation in the active oxidation front above sapropel s1 in the eastern mediterranean sea as derived from low temperature magnetism low temperature magnetic properties of eastern mediterranean sediments from a box core have been investigated this box core contains the present day oxic suboxic boundary that is situated at the top of the relic of the youngest sapropel sl the upper half of sapropel s1 has been oxidized and fe oxides have precipitated in the oxidized sapropel zero field cooling zfc and field cooling fc saturation remanent magnetization m r induced in a field of 2 5 t at 20 k after cooling from 300 k with or without the 2 5 t field was measured during warming to 300 k an m r imparted at room temperature rtsirm was cycled to 20 k as well the difference between zfc and fc curves around the verwey transition as estimated by the parameter f c z f c suggests the presence of magnetosomes in the oxidized part of the sapropel we propose a new parameter d m 1 5 0 3 0 0 k r f c m 1 5 0 1 3 0 0 k r z f c m 3 0 0 k r f c where m 1 5 0 3 0 0 k r z f c represents the average remanence between 150 and 300 k to estimate the difference between zfc and fc curves above the verwey transition this is interpreted as being indicative of the number of defects and the extent of partial maghemitization at the oxic suboxic boundary where fe oxides actively precipitate the magnetite appears to be least maghemitized as inferred from low values for d further upward in the oxidized sapropel in older precipitates the magnetite is slightly more maghemitized just below the sapropel maghemitization is most pronounced in addition the initial slope of zfc and fc curves indicates that small grains or grain coatings that are superparamagnetic at room temperature sp are enriched at the oxic suboxic boundary higher in the oxidized sapropel the relative contribution of sp grains decreases presumably because they age to larger grains

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

electron spin resonance as a high sensitivity technique for environmental magnetism determination of contamination in carbonate sediments this study investigates the application of high sensitivity electron spin resonance esr to environmental magnetism in conjunction with the more conventional techniques of magnetic susceptibility vibrating sample magnetometry vsm and chemical compositional analysis using these techniques we have studied carbonate sediment samples from discovery bay jamaica which has been impacted to varying degrees by a bauxite loading facility the carbonate sediment samples contain magnetic minerals ranging from moderate to low concentrations the esr spectra for all sites essentially contain three components first a six line spectra centred around g 2 resulting from mn2 ions within a carbonate matrix second a g 4 3 signal from isolated fe3 ions incorporated as impurities within minerals such as gibbsite kaolinite or quartz third a ferrimagnetic resonance with a maxima at 230 mt resulting from the ferrimagnetic minerals present within the bauxite contamination depending upon the location of the sites within the embayment these signals vary in their relative amplitude in a systematic manner related to the degree of bauxite input analysis of the esr spectral components reveals linear relationships between the amplitude of the mn2 and ferrimagnetic signals and total mn and fe concentrations to assist in determining the origin of the esr signals coral and bauxite reference samples were employed coral representative of the matrix of the sediment was taken remote from the bauxite loading facility whilst pure bauxite was collected from nearby mining facilities we find esr to be a very sensitive technique particularly appropriate to magnetic analysis of ferri and para magnetic components within environmental samples otherwise dominated by diamagnetic carbonate minerals when employing typical sample masses of 200 mg the practical detection limit of esr to ferri and para magnetic minerals within a diamagnetic carbonate matrix is of the order of 1 ppm and 1 ppb respectively approximately 102 and 105 times the sensitivity achievable employing the vsm in our laboratory

biological control of paleomagnetic remanence acquisition in carbonate framework rocks of the tahiti coral reef abstract integrated ocean drilling program iodp expedition 310 recovered 632 meters of post last glacial max post lgm and late pleistocene coral reef framework rocks 37 holes at 22 sites from the current fore reef slopes surrounding the island of tahiti the cores were collected primarily to constrain the timing and amplitude of rising sea levels associated with the end of the last global glaciation 10 18 000 ybp to identify the climatic and environmental changes during that period and to evaluate the reef response to those combined changes the cores which were collected in three regions faaa tiarei and maraa are remarkable in that more than 60 of the total core volume is associated with microbialites rather than macroscopic coral or coralline algae assemblages we present here paleomagnetic data from 316 oriented paleomagnetic samples collected from the post lgm framework rocks of the maraa region 12 holes at 6 sites 96 of our samples were recovered from microbialites that have filled the primary cavities of the coral coralgal framework all samples appear to accurately record geomagnetic field variability at tahiti average inclination 30 6 alpha 95 2 9 site axial dipole inclination 32 6 and yet the samples are composed of 99 precipitated calcium carbonate our rock magnetic data indicate that the nrm resides almost entirely in detrital titanomagnetite grains derived from the tahiti volcanic edifice we present magnetic and microscopic evidence that the natural remanent magnetization in the carbonate rocks was locked in through mediation of organic biofilms mostly bacterial that held the grains in place until they were trapped by carbonate precipitation

a 13 200 year history of century to millennial scale paleoenvironmental change magnetically recorded in the palmer deep western antarctic peninsula abstract a 13 200 yr record of magnetic parameters from the palmer deep western antarctic peninsula records a sequence of five distinct shifts in glacimarine sedimentation coupled with century scale variations in paleoproductivity the five major shifts are manifested as abrupt order of magnitude changes in low field magnetic susceptibility accompanied by changes in magnetic particle size and mineralogy the late holocene 3 4 0 ka the early holocene 11 5 9 ka and the last glacial maximum prior to 13 2 ka are intervals of strong low field magnetic susceptibility and are characterized by multi domain md magnetite md magnetite is associated with zones of abundant gravel grains and is interpreted here as an indicator of material transported as ice rafted debris deglaciation 13 2 11 5 ka and the middle holocene 9 3 4 ka were times of enhanced productivity the middle holocene marks the onset of century scale productivity cycles seen in high field magnetic susceptibility which is responding to variations in biogenic silica deglaciation and the middle holocene interval contain pseudo single domain magnetite and titanomagnetite respectively these observations are surprising given the abundance of coarse md magnetite available in local source rocks the magnetic mineral assemblage in the deglaciation interval however can be explained by density sorting in meltwater plumes during the middle holocene the magnetic mineral assemblage suggests the reduction or cessation of locally derived terrigenous sediment and by inference the reduction or cessation of iceberg generation the early holocene middle holocene shift in terrigenous sedimentation may be responding in part to sea level which controls the position of the grounding line the palmer deep late holocene interval coincides with the neoglacial period a time of glacier re advances worldwide

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

elongated prismatic magnetite crystals in alh84001 carbonate globules potential martian magnetofossils using transmission electron microscopy tem we have analyzed magnetite fe 3o 4 crystals acid extracted from carbonate globules in martian meteorite alh84001 we studied 594 magnetites from alh84001 and grouped them into three populations on the basis of morphology 389 were irregularly shaped 164 were elongated prisms and 41 were whisker like as a possible terrestrial analog for the alh84001 elongated prisms we compared these magnetites with those produced by the terrestrial magnetotactic bacteria strain mv 1 by tem again we examined 206 magnetites recovered from strain mv 1 cells natural darwinian selection in terrestrial magnetotactic bacteria appears to have resulted in the formation of intracellular magnetite crystals having the physical and chemical properties that optimize their magnetic moment in this study we describe six properties of magnetite produced by biologically controlled mechanisms e g magnetotactic bacteria properties that collectively are not observed in any known population of inorganic magnetites these criteria can be used to distinguish one of the modes of origin for magnetites from samples with complex or unknown histories of the alh84001 magnetites that we have examined the elongated prismatic magnetite particles 27 of the total are indistinguishable from the mv 1 magnetites in five of these six characteristics observed for biogenically controlled mineralization of magnetite crystals

a biogenic origin for anomalous fine grained magnetic material at the paleocene eocene boundary at wilson lake new jersey 1 the paleocene eocene thermal maximum which occurred 55 5 ma was caused by a massive release of carbon as indicated by an 3 negative carbon isotope excursion recorded in the marine atmospheric and terrestrial reservoirs one suggested source for the carbon a cometary impactor is based on the sudden appearance and high concentration of single domain sd magnetite in paleocene eocene p e boundary cores from the north atlantic continental margin we evaluate the potential sources of sd magnetite at the p e boundary by presenting new magnetic hysteresis low temperature magnetic remanence and transmission electron microscopy data from the north atlantic coastal ocean our results show a similar increase in sd material but demonstrate that the magnetic material has a biogenic origin these findings indicate that the high concentrations of sd magnetite immediately above the p e boundary are the result of unusual accumulations and or preservation of magnetotactic bacteria such bacteria typically occupy the oxic anoxic transition zone near the sediment water interface or in the water column the high abundances of sd magnetite in sediments from across the shelf may be an artifact of nonsteady state redox conditions and exceptional preservation of sd magnetite it may also indicate that the oxic anoxic redox boundary shifted into the water column the latter explanation implies transient eutrophy of the coastal ocean in this region most likely due to seasonally enhanced runoff and increased stratification and nutrient loading

characterizing magnetism of individual magnetosomes by x ray magnetic circular dichroism in a scanning transmission x ray microscope article i nfo soft x ray scanning transmission x ray microscopy stxm was used to measure the fe l2 3 x ray magnetic circular dichroism xmcd signal from individual 30 nm diameter magnetosomes in a magnetotactic bacterium specifically the marine vibrio strain mv 1 the fe l2 3 spectra recorded with circularly polarized x rays from an elliptically polarizing undulator were very similar in shape and magnitude to those of fully saturated magnetite fe3o4 as previously determined by other techniques our results show that the magnetic moments of individual magnetosomes arranged in linear chains in intact cells of strain mv 1 are all oriented in the same direction the magnitude of the xmcd signal averaged over a chain of 9 magnetosomes in one cell is similar to that of single crystal magnetite the spectral shape is slightly different and indicates that magnetosomes have excess fe ii

low temperature remanence in stable single domain magnetite 1 a numerical model is developed in order to simulate the loss of low temperature remanence while warming through the verwey transition we concentrate on the amount of remanence lost fc and zfc during field cooled and zero field cooled low temperature demagnetization curves for stable single domain ssd particles of 1 inorganic magnetite and 2 biogenic magnetite in the form of magnetosome chains the model predicts that delta ratios fc zfc for inorganic ssd magnetite increase from 1 to 1 6 with increasing aspect ratio and that magnetosome chains have elevated delta ratios of at least 2 0 disaggregated magnetosomes are predicted to behave similarly to inorganic acicular ssd magnetite the numerical results agree with experimental results

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

the identification and biogeochemical interpretation of fossil magnetotactic bacteria magnetotactic bacteria which most commonly live within the oxic anoxic transition zone oatz of aquatic environments produce intracellular crystals of magnetic minerals specifically magnetite or greigite the crystals cause the bacteria to orient themselves passively with respect to the geomagnetic field and thereby facilitate the bacteria s search for optimal conditions within the sharp chemical gradients of the oatz the bacteria may also gain energy from the redox cycling of their crystals because magnetotactic bacteria benefit from their magnetic moments natural selection has promoted the development of traits that increase the efficiency with which the intracellular crystals impart magnetic moments to cells these traits also allow crystals produced by magnetotactic bacteria called magnetofossils when preserved in sediments to be distinguished from abiogenic particles and particles produced as extracellular byproducts of bacterial metabolism magnetofossils are recognizable based on their narrow size and shape distributions distinctive morphologies with blunt crystal edges chain arrangement chemical purity and crystallographic perfection this article presents a scheme for rating magnetofossil robustness based on these traits the magnetofossil record extends robustly to the cretaceous and with lesser certainty to the late archean because magnetotactic bacteria predominantly live in the oatz the abundance and character of their fossils can reflect environmental changes that alter the chemical stratification of sediments and the water column the magnetofossil record therefore provides an underutilized archive of paleoenvironmental information several studies have demonstrated a relationship between magnetofossil abundance and glacial interglacial cycles likely mediated by changes in pore water oxygen levels more speculatively a better developed magnetofossil record might provide constraints on the long term evolution of marine redox stratification more work in modern and ancient settings is necessary to explicate the mechanisms linking the abundance and character of magnetofossils to ancient biogeochemistry

magnetofossils the magnetization of sediments and the evolution of magnetite biomineralization magnetite fe 3o 4 is one of the most stable carriers of natural remanent magnetization nrm in sedimentary rocks and paleomagnetic studies of magnetite bearing sediments such as deep sea cores and pelagic limestones have provided a detailed calibration between the biostratigraphic and magnetic polarity time scales despite this important role there is as yet a very poor understanding of how ultrafine grained 0 1 m magnetite is formed transported and preserved in marine environments a major conceptual advance in our understanding of these processes is the recent discovery that biogenic magnetite formed by magnetotactic bacteria and or other magnetite precipitating organisms is responsible for much of the stable magnetic remanence in many marine sediments and sedimentary rocks since these magnetite particles are of biogenic origin they are termed properly magnetofossils kirschvink chang 1984

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

a multi parameter rock magnetic record of the last glacial interglacial paleoclimate from south central illinois usa pittsburg basin a small kettle lake in south central illinois was formed after the retreat of the illinoian ice sheet 130 ka b p and the sedimentary record includes the last interglacial glacial cycle curie temperature measurements xrd analyses and thermal demagnetization of a low temperature j indicate that the magnetic fraction is characterized by rs oxidized magnetite and magnetic iron sulfides the grain size of the magnetic fraction was estimated from armrj ratios rs frequency dependent susceptibility and hysteresis parameters these measurements show that glacial sediments contain coarse grained presumably detrital material while interglacial samples have an additional fine grained magnetic component which cannot be explained by a single catchment or climatic process early interglacial samples show magnetic properties characteristic of authigenic biogenically produced magnetite while late interglacial samples are probably influenced by pedogenesis around the basin q 1997 elsevier science b v

review of the low temperature magnetic properties of magnetite from a rock magnetic perspective the low temperature magnetic properties of magnetite are reviewed and implications for rock magnetism considered the behaviour of fundamental properties of magnetite at low temperatures near the verwey transition tv are documented and attention is given to various verwey transition theories the low temperature behaviour of the magnetic energies that control domain structure is reviewed in detail for the first time in rock magnetic literature the low temperature anomaly in spontaneous magnetization ms is documented and the differences between the saturation magnetization and ms near the verwey transition are discussed it is argued that the low temperature behaviour of the magnetocrystalline anisotropy and in particular the anomaly at tv is most likely to affect multidomain remanence during low temperature cycling for multidomain crystals it is calculated that the large increase in magnetocrystalline anisotropy intensity and reduction in symmetry on cooling through tv is likely to reduce the stability of closure domains

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

iron sulfides from magnetotactic bacteria structure composition and phase transitions using transmission electron microscopy we studied the structures and compositions of fe sulfides within cells of magnetotactic bacteria that were collected from natural habitats ferrimagnetic greigite fe3s4 occurred in all types of sulfide producing magnetotactic bacteria examined mackinawite tetragonal fes and tentatively sphalerite type cubic fes were also identified in contrast to earlier reports we did not find pyrite fes 2 or pyrrhotite fe12xs mackinawite converted to greigite over time within the bacteria that were deposited on electron microscope grids and stored in air orientation relationships between the two minerals indicate that the cubic close packed s substructure remains unchanged during the transformation only the fe atoms rearrange neither mackinawite nor cubic fes are magnetic and yet they are aligned in chains such that when converted to magnetic greigite the probable easy axis of magnetization 100 is parallel to the chain direction the resulting chains of greigite are ultimately responsible for the magnetic dipole moment of the cell both greigite and mackinawite magnetosomes can contain cu depending on the sampling locality because bacterial mackinawite and cubic fes are unstable over time only greigite crystals are potentially useful as geological biomarkers

chains clumps and strings magnetofossil taphonomy with ferromagnetic resonance spectroscopy magnetotactic bacteria produce intracellular crystals of magnetite or greigite the properties of which have been shaped by evolution to maximize the magnetic moment per atom of iron intracellular bacterial magnetite therefore possesses traits amenable to detection by physical techniques typically narrow size and shape distributions single domain size and arrangement in linear chains and often crystal elongation past strategies for searching for bacterial magnetofossils using physical techniques have focused on identifying samples containing significant amounts of single domain magnetite or with narrow coercivity distributions searching for additional of traits would however increase the likelihood that candidate magnetofossils are truly of biological origin ferromagnetic resonance spectroscopy fmr is in theory capable of detecting the distinctive magnetic anisotropy produced by chain arrangement and crystal elongation here we present analyses of intact and lysed magnetotactic bacteria dilutions of synthetic magnetite and sedimentary samples of modern carbonates from the great bahama bank oligocene miocene deep sea muds from the south atlantic and pleistocene lacustrine deposits from mono basin california we demonstrate that fmr can distinguish between intact bacterial magnetite chains collapsed chains and linear strings of magnetite formed by physical processes we also show that sediments in which the magnetization is likely carried by bacterial magnetite have fmr spectra resembling those of intact or altered bacterial magnetite chains