bacterial magnetite produced in water column dominates lake sediment mineral magnetism lake ely usa summary environmental magnetic studies of annually laminated sediments from lake ely northeastern pennsylvania usa indicate that bacterial magnetite is the dominant magnetic mineral in the lake sediment in previous studies of lake ely sediment the dark organic rich layers in the annual laminae were interpreted to have high intensity saturation isothermal remanent magnetizations sirms while the light coloured silt rich layers have low intensity sirms to test the hypothesis that the magnetic grains in the sediments were an authigenic product of magnetotactic bacteria rather than detrital magnetic grains eroded from the watershed we analysed samples from the water column the lake sediment and a sediment trap installed near the lake bottom direct microscopic observation of the water column samples showed the presence of magnetotactic bacteria in and below the oxic anoxic transition zone oatz to characterize the magnetic minerals rock magnetic parameters were measured for material from the water column the sediment trap and the dark and light coloured lake sediments low temperature magnetic measurements tested for the presence of magnetosomes in separated dark and light coloured layer samples numeric unmixing of the low temperature results showed that biogenic magnetites were present in the lake sediment and contributed more significantly to the sirm in the dark organic rich layers than in the light coloured inorganic silt rich layers observations under the transmission electron microscope tem of magnetic extracts also show the abundance of magnetosomes in the lake sediment the presence of live magnetotactic bacteria in the water column and the predominance of bacterial magnetites in filtered particulate matter sediment traps and recent lake sediment all suggest that bacterial magnetites are the main magnetic minerals in lake ely sediment this finding suggests that changes in environmental factors that control the productivity of magnetic bacteria in the lake likely contribute to the variability of magnetic mineral concentrations observed in the lake sediments

environmental conditions for the presence of magnetofossils in the last glacial maximum inferred from magnetic parameters of sediments from the ulleung basin east sea abstract magnetic characterization of sediments from the ulleung basin in the east sea japan sea was carried out to decipher changes in environmental conditions over the last 39 000 years magnetic susceptibility intensities of natural remanent magnetization nrm anhysteretic remanent magnetization arm partial arm parm and saturation isothermal remanent magnetization sirm were measured for selected samples irm acquisition hysteresis and first order reversal curve forc measurements were made the ratios of rock magnetic parameters such as arm sirm arm and s ratio were determined to identify magnetic grain size and mineralogical variations along with electron microscope observations of magnetic mineral extracts down core variations in rock magnetic parameters for the 5 21 m long sediment core reveal distinct intervals with different grain size concentration and composition of magnetic minerals the uppermost portion and the high magnetic intensity zone hiz in marine isotope stage mis 2 are characterized by higher concentrations of fine grained magnetite whereas the mis 1 and 3 intervals contain lower concentrations of coarser grained magnetic minerals with increasing influence of hematite and or iron sulphide minerals the hiz interval is inferred to contain high occurrences of fine grained bacterial magnetosomes based on geochemical proxies east sea conditions for the hiz interval coincident with the last glacial maximum lgm can be summarized as having low total organic carbon toc enhanced eolian contributions from terrestrial organic particles and iron bearing minerals under the influence of an intensified east asian winter monsoon along with iron reducing but not sulphidic conditions that favoured growth and preservation of magnetotactic bacteria the delicate balance between reductive dissolution of magnetofossils under sulphidic conditions and production and preservation of magnetotactic bacteria in overlying iron reducing sediments for the hiz interval is responsible for the resultant abundance variations of magnetosomes in the studied sediments

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

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

characterization of hematite α fe2o3 goethite α feooh greigite fe3s4 and pyrrhotite fe7s8 using first order reversal curve diagrams 1 first order reversal curve forc diagrams have become a standard tool in rock magnetism yet magnetite is the only magnetic mineral that is well characterized using forc diagrams we present forc diagrams for predominantly single domain sd synthetic aluminous hematite a fe2 xalxo3 and goethite a feal ooh and natural greigite fe3s4 and pyrrhotite fe7s8 to constrain interpretation of forc diagrams from natural samples hematite and goethite have low spontaneous magnetizations and negligible magnetic interactions while greigite and pyrrhotite have higher spontaneous magnetizations and can have strong magnetic interactions the coercivity of hematite systematically increases with al content only for samples produced using the same synthesis method but it is variable for samples produced with different methods even for similar al content this precludes use of magnetic coercivity alone to quantify the al content of natural hematites goethite has much higher coercivity than hematite for all measured samples sd and superparamagnetic sp behavior is common in natural greigite samples with peak coercivities ranging from

reductive diagenesis magnetite dissolution greigite growth and paleomagnetic smoothing in marine sediments a new view abstract in many anoxic sedimentary environments the onset of sulfate reduction and pyritization of detrital iron bearing minerals leads to a precipitous decline in magnetic mineral concentration during early diagenesis the usefulness of the surviving paleomagnetic record in such environments is usually argued to depend on how much of the primary detrital magnetic assemblage survives diagenetic dissolution detailed rock magnetic and electron microscope analyses of rapidly deposited 7 cm kyr latest pleistocene holocene sediments from the continental margins of oman 22 22 4 n 60 08 0 e and northern california 38 24 8 n 123 58 2 w demonstrate that pyritization during early diagenesis also leads to the progressive down core growth of the ferrimagnetic iron sulfide greigite greigite growth begins with nucleation of large concentrations of superparamagnetic sp nanoparticles at the inferred position of the sulfate methane transition which can explain the apparently paradoxical suggestion that diagenetically reduced sediments contain enhanced concentrations of sp particles looping of hysteresis parameters on a day plot records the dissolution of single domain sd titano magnetite and the formation of sp greigite which then slowly and progressively grows through its sd blocking volume and acquires a stable paleomagnetic signal this looping trend is also evident in data from several published records oregon margin korea strait japan sea niger fan argentine margin and the ontong java plateau indicating that these processes may be widespread in reducing environments our observations have profound implications for paleomagnetic records from sulfate reducing environments the paleomagnetic signal recorded by greigite is offset from the age of the surrounding sediments by 10 s of kyr and ongoing growth of greigite at depth results in smoothing of the recorded signal over intervals of 10 s to 100 s of kyr we therefore expect the presence of greigite to compromise paleomagnetic records in a wide range of settings that have undergone reductive diagenesis

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

experimental observation of magnetosome chain collapse in magnetotactic bacteria sedimentological paleomagnetic and evolutionary implications magnetotactic bacteria precipitate intracellular crystals of single domain magnetite fe3o4 and or greigite fe3s4 which have often been implicated in carrying the natural remanent magnetization nrm of freshwater and marine sediments in vivo the magnetic crystals are usually aligned in chains such that their moments add together generating net cellular moments high enough to rotate the cells passively to align with the geomagnetic field a magnetostatic biophysical analysis demonstrates that this arrangement is out of dynamic equilibrium and would collapse spontaneously without a support mechanism past rock magnetic analyses of shallow water marine carbonates suggest that partial collapse does occur during diagenesis and dolomitization to calibrate this effect we induced magnetosome chain collapse in magnetospirillum magnetotacticum strain ms 1 by progressive sonification and treatment with detergents and monitored the changes with rock magnetic analysis and tem although it has been speculated that the cell wall and associated membrane structures act to prevent magnetosome chain collapse our data indicate that magnetosome linearity persists long after cells are disrupted this is consistent with prior observations that in some magnetotcocci the magnetosome chains pass through the cell interior precluding continuous contact with the cell wall and implying additional support structures exist in some species using tem tomographic reconstructions prepared with a magnetic technique that prevents chain collapse we examined the three dimensional ultrastructure of magnetosomes without the problem of post mortem magnetosome motion this method revealed the presence of an intracellular organic sheath beyond that of actin like filaments reported recently that follows the chain of magnetosomes which we postulate evolved to hold the crystals in place and enhances their ability to preserve nrm in sediments as the genomes of two magnetotactic bacteria contain several apparent homologues of known eukaryotic cytoskeletal proteins natural selection for magnetotaxis may have played a role in the evolution of precursors to the eukaryotic cytoskeleton the presence of this sheath is also consistent with the observation of electron translucent material associated with putative magnetofossil chains in alh84001

direct estimates of pedogenic magnetite as a tool to reconstruct past climates from buried soils 1 variations in magnetic properties of buried soils can be used to reconstruct past climatic conditions during paleosol formation most methods however are based on comparisons between the magnetically enriched upper soil horizons and the magnetically unaltered parent material in thin loess paleosol sequences such a comparison can be problematic because all horizons soil and underlying loess may be affected to varying degrees by pedogenesis we propose two direct estimates of pedogenic magnetite based on the analysis of anhysteretic remanent magnetization ratios carm isothermal remanent magnetization and coercivity distributions these estimates are independent of any information regarding the parent material and are possible if pedogenic minerals have similar magnetic properties throughout the study region this condition seems to be met throughout the midwestern united states and a few loessic soils elsewhere the remanence carrying part of pedogenic magnetite is composed of single domain particles with consistent well constrained magnetic properties these particles are extremely well dispersed in the soil matrix as indicated by the absence of noticeable magnetostatic interaction effects our analyses of over 70 modern loessic soil profiles demonstrate that the abundance of pedogenic magnetite correlates well with modern climate and that the method is suited for reconstruction of past climates from paleosols

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

mineral magnetic study of lacustrine sediments from lake pumoyum co southern tibet over the last 19 ka and paleoenvironmental significance abstract high resolution proxy records from tibetan plateau are essential to understand the past climatic and environmental changes in this work we conducted systematic environmental magnetic studies on lacustrine sediments from lake pumoyum co southern tibet spanning from the last deglaciation 19 ka to present we correlated the magnetic proxies in the lake core with environmental changes the magnetic proxies from the 3 units were studied both unit 3 19 13 2 ka and unit 2 13 2 8 4 ka contain evidences for an anoxic or sub anoxic environment fine grained greigite dominates the bulk magnetic properties of unit 3 while both magnetite and greigite coexist in unit 2 overall this reveals a transition from an anoxic environment to an oxic environment in contrast unit 1 8 4 ka present spans the holocene when the lake environment was oxic there is an increasing trend in the concentration of magnetic minerals which reflects an increasing transport of magnetic minerals from the catchment into the lake due to the increased post glacial melt water flow although different mechanisms affect the magnetic assemblage and associated magnetic properties in the different units our results suggest that the magnetic proxies of the local lake environment on southern tibet are sensitive to environmental changes

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

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

identification of paleomagnetic remanence carriers in ca 3 47 ga dacite from the duffer formation the pilbara craton abstract the ca 3 47 ga duffer formation has been considered to carry one of the oldest paleomagnetic records yet the lack of rock magnetic data limits the interpretation of the nature of the remanence we conducted a rock magnetic and paleomagnetic investigation on columnar dacite of the duffer formation the main magnetic minerals are phenocrysts of titanomagnetite and magnetite and secondary hematite in groundmass detailed thermal demagnetization revealed more complex natural remanence than previously estimated consisting of four components with typical unblocking temperature of 200 350 200 500 590 and 690 c combined with alternating field demagnetization and rock magnetic data they are attributed to titanomagnetite coarse grained magnetite fine grained magnetite and hematite respectively the comparison of unblocking temperature and coercivity suggests that the previously proposed secondary component is carried by fine grained magnetite as well as hematite while the putative primary component is carried by coarse grained magnetite and titanomagnetite microscopic observations showed that coarse grained magnetite and titanomagnetite are primary crystals although this does not necessarily indicate they preserve primary remanence the remanence directions of all components revealed higher scatter than the previous studies suggesting the need for caution in interpretation the low unblocking temperature of titanomagnetite suggests that if their remanence is truly primary the rocks must have kept below 250 c for 3 47 billion years

classification seasonality and persistence of low frequency atmospheric circulation patterns abstract orthogonally rotated principle component analysis rpca of northern hemisphere 1 month mean 700 mb heights is used to identify and describe the seasonality and persistence of the major modes of interannual variability the analysis is detailed and comprehensive in that 1 a high resolution approximately equal area 358 point grid is used for the virtually maximum possible 35 year period of record 2 a positive bias in the nmc data base in the early 1950s in the subtropics is largely eliminated for the first time and 3 homogeneous separate analyses of each month of the year are carried out detailing the mouth to month changes in the dominant circulation patterns winter results are similar to those of other recent rpca and teleconnection studies except that some less obvious patterns are identified and further detail of the better known patterns is provided two north south dipole patterns are found over the pacific ocean west pacific oscillation and east pacific pattern and over the atla

determining the porosity exponent m and lithology factor a for sandstones and their control by overburden pressure a case study from the gulf of suez egypt the porosity exponent m represents an important parameter in archie s equation it is routinely used in well logging interpretation and formation evaluation it reflects some petrophysical rock properties such as the petrofabrics pore fabrics and overburden pressure obp data measured at ambient conditions for a total of 55 sandstone core samples were obtained from 8 wells varying in depositional facies geologic age degree of consolidation and cementation material and then measured again at obp similar to the actual reservoir conditions to check the effect of obp on archie s parameters and to introduce a model of high reliability other published data sets were also used to confirm the study results based on the best fit regression analysis several empirical relationships were introduced to relate the porosity exponent to pore volume lithology factor and permeability the effect of obp on the porosity exponent was also investigated in relation to the petrophysical parameters given the above a new statistical model is proposed to relate the porosity exponent to the permeability of the studied sandstone samples in the gulf of suez egypt the effect of confining pressure on the studied porosity exponent m and the lithology factor a is significant when porosity is less than or equal to 10 from the present study categorization of the studied samples into flow units and rock types using the hydraulic flow unit and the discrete rock types increased the reliability of the obtained relationships and increased the applicability of the obtained equations in addition the value of the lithology factor a fluctuates around 1 0 92 1 04 regardless of the applied processing method whereas the value of the porosity exponent m fluctuates around 1 83 the obtained relationships between the formation resistivity factor and porosity have a very high reliability 0 949 correlation coefficient 0 969

fossil bacterial magnetite in deep sea sediments from the south atlantic ocean the magnetostratigraphy of deep sea sediment cores has proven to be a powerful and high resolution method of dating and comparing sedimentary records in contrast to this widespread application relatively few attempts have been made to analyse and identify the mineralogy grain size and origin of the carrier of the remanent magnetization1 2 it is widely assumed that dominant ferrimagnetic minerals in deep sea sediments are of lithogenic origin that is that they derive from oceanic or continental detritus volcanic ashfalls micrometeorites hydrothermal precipitation or chemical mineralization processes a very different view was taken by kirschvink and lowenstam3 who suggested that fossil remnants of magnetite mineralizing organisms in particular magnetotactic bacteria might play an important part in the magnetization of sediments deposited in certain detritus poor aquatic systems we have used rock magnetic diagnostic methods to characterize the magnetic phases in deep sea sediments from the south atlantic these phases were then extracted and studied with the electron microscope our results indicate that fossil bacterial magnetite is the main carrier of the natural remanent magnetization in sediments ranging in age from quarternary to eocene different types of bacterial magnetosomes were detected most of them occurring in chains of identical particles and nearly all of them falling into the theoretically determined4 size range of single domain magnetitite

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

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

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

biogenic magnetite as a primary remanence carrier in limestone deposits abstract studies on the microbial communities and magnetic phases of samples collected from carbonate oozes at sugarloaf key fl u s a and calcareous laminated sediments from laguna figueroa baja california mexico have revealed the existence of magnetotactic bacteria and ultrafine grained single domain magnetite in both environments magnetotactic bacteria were identified by light and electron microscopy the single domain magnetite was detected by coercivity spectra analysis with a squid magnetometer and examined under the transmission electron microscope the similarity in terms of size and shape between the single domain magnetite found in these sediments and the magnetite observed in the bacterial magnetosome from enriched cultures indicates the ultrafine grained magnetite in these two marine environments was biologically formed these results combined with the common occurrences of ultrafine grained magnetite in limestone deposits detected rock magnetically suggest biogenic magnetite may be present and contribute to the magnetic remanence in these rocks several cambrian limestone samples separately collected from siberia china and kazakhstan were examined for the presence of bacterial magnetite samples from the lower cambrian sinskian formation at siberia platform were found to contain both a large amount of apparently bacterial magnetite particles and a very stable primary magnetic component post cambrian diagenesis does not seem to affect the microgranulometry of these apparently bacterial magnetite crystals or the magnetic remanence carried by them assessing the potential role of biogenic magnetite as a primary remanence carrier in other phanerozoic limestone deposits ought to be further pursued

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

magnetite diagenesis in marine sediments from the oregon continental margin the magnetochemistry of sediments from the oregon continental margin is examined to determine the effects of iron sulfur diagenesis on the paleomagnetic record magnetic mineral dissolution and transformation into iron sulfides are a common feature in these suboxic to anoxic lutites these processes are evidenced in rapid decreases in natural remanent magnetization intensities and stabilities systematic changes in other rock magnetic properties and increases in solid phase sulfur concentrations with depth hysteresis measurements are used to evaluate downcore changes in magnetite concentration and grain size magnetite abundances decrease downcore from initial values of about 0 1 and nominal grain diameters lie within a narrow pseudosingle domain range of 0 08 to 0 6 m a first order surface area reaction model da dt ka is proposed to explain the magnetite dissolution mechanism where a is the total magnetite surface area and k is the rate constant the solution of this equation predicts that the surface area and concentration decrease exponentially and the concentration in addition depends on grain size application of this model in two cores where grain size varies with depth successfully explains the downcore profiles of both concentration and surface area despite extensive magnetite destruction magnetic directions in such sediments appear to reliably record long wavelength trends of the geomagnetic field

bacterial magnetite and the magnetic properties of sediments in a swedish lake a comparison of the magnetic properties of lake sediments and catchment materials from pajep njakajaure north sweden demonstrates that high concentrations of single domain sd magnetite in the upper 70 cm of gyttja in the deepest part of the basin are not of detrital origin tem and light microscope examination revealed the existence of bacterial cells and magnetosomes in the upper sediments which are responsible for the presence of the sd magnetite most of the fossil magnetosomes appear to have been dissolved through reductive processes at lower depths although some are preserved in the lower sediments and carry a post depositional remanent magnetization pdrm the results highlight the complex effects of post depositional processes on mineral magnetic assemblages and natural remanent magnetization aquisition and demonstrate how mineral magnetic measurements may be used to identify magnetic grain size changes brought about by sediment diagenesis

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

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

late miocene early pleistocene paleoclimate history of the chinese loess plateau revealed by remanence unmixing previous studies show that the quaternary loess sequence and the late miocene pliocene red clay sequence on the chinese loess plateau clp share similar magnetic properties despite their being deposited under different climate conditions to solve this paradox we apply a novel remanence unmixing technique and demonstrate that loess and red clay on the central clp contain a similar low coercivity pedogenic component but their high coercivity components differ dramatically reflecting different oxidation and temperature conditions we infer that temperatures on the chinese loess plateau cooled from the late miocene to the quaternary in a manner similar to sea surface temperature records for the same time interval this coherency between marine and terrestrial records argues for a co2 forcing on long term paleoclimatic variations key points backfield remanence data of chinese loess and red clay consist of two components loess and red clay have similar pedogenic but different detrital components the loess plateau experienced a cooling trend from the late miocene to the quaternary

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

bacterial magnetite in swedish varved lake sediments a potential bio marker of environmental change abstract detailed mineral magnetic measurements were carried out on three varved lake sediment sequences in sweden which extend to ca 7000 bc the comparison of the magnetic properties of the organic rich varved lake sediments with their respective catchment materials indicates that the magnetic properties of the sediments are dominated by relatively high concentrations of single domain magnetite magnetosomes produced by magnetotactic bacteria the dimictic nature of the lakes which help form and preserve the varves due to the weakly oxic or suboxic environment also appear to be suitable for a high degree of magnetosome preservation the concentration of the magnetosomes in two of the sequences as determined by mass specific magnetic measurements exhibits a positive linear relationship with the total organic carbon content and suggests that magnetic remanence measurements reflect lake productivity via processes of organic matter accumulation and decomposition further research should focus on the potential to use the fossil magnetosomes as proxy climate environmental change indicators and as recorders of geomagnetic field variations and behaviour

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

magnetostatic interactions in deep sea sediments inferred from first order reversal curve diagrams implications for relative paleointensity normalization 1 understanding magnetostatic interactions in sediments is important for paleomagnetism and rock magnetism magnetostatic interactions are known to affect significantly the efficiency of anhysteretic remanent magnetization arm acquisition and hence the ratio of arm susceptibility arm to saturation isothermal remanent magnetization sirm a magnetic grain size proxy widely used for environmental applications can be controlled by magnetostatic interactions relative paleointensity estimations may also be influenced by magnetostatic interactions because arm is often used as a normalizer to correct for the difference in magnetizability of sediments in this study characterization of magnetic grains in deep sea sediments from the north pacific ocean was conducted using first order reversal curve forc diagrams and irm acquisition curves the forc diagrams indicate that the magnetic grains consist mainly of a noninteracting single domain sd component and an interacting sd component and additionally of a multidomain md component the relative abundances of these components were semiquantitatively estimated by curve fitting of cross sections along a line parallel to the axis of local interaction fields hu and through a peak in the coercivity hc three components were fitted assuming a gaussian distribution of interaction fields hu the irm acquisition curves can be described by two dominant components assuming a log gaussian distribution the mean coercivities of the two components are 40 and 100 mt respectively it is estimated that the former component is carried by biogenic magnetite and roughly corresponds to the noninteracting sd component derived from the forc diagrams and that the latter component is carried by eolian maghemite corresponding to the interacting sd component the arm sirm ratio decreases with increasing concentration of the interacting sd component this implies that the arm sirm ratio is significantly affected by magnetostatic interactions and that it does not necessarily reflect magnetic grain size the effect of magnetostatic interactions on natural remanent magnetization acquisition in sediments is not well understood but if it is similarly sensitive to the sirm normalization by arm in relative paleointensity estimations would overcompensate for the concentration of magnetic grains and cause the significant coherence between the normalized intensity and the normalizer that has been reported in the literature

radioisotopic age constraints for glacial terminations ix and vii from aggradational sections of the tiber river delta in rome italy abstract buried sedimentary aggradational sections deposited between 800 ka and 600 ka in the tiber river coastal alluvial plain have been studied using borecores from around rome 40ar 39ar ages on sanidine and or leucite from intercalated tephra layers and paleomagnetic investigation of clay sections provide geochronologic constraints on the timing of aggradation of two alluvial sections demonstrating that they were deposited in response to glacio eustatic sea level rise caused by glacial terminations ix and vii 40ar 39ar age data indicate ages of 802 8 ka 1 full errors and 649 3 ka 1 full errors for glacial terminations ix and vii respectively providing a rare test beyond the range of u series dating for corals and speleothems 500 ka of the astronomically calibrated timescale developed for oxygen isotope records from deep sea cores

biogenic magnetite from surface holocene carbonate sediments great bahama bank magnetic separates and rock magnetic tests from holocene carbonate surface sediments of the bahamas indicate that fine grained magnetite maghemite is common at or near the sediment water interface and forms a major portion of the magnetic fraction magnetite crystals range from 400 to 1000 a in diameter as observed under the transmission electron microscope tem and often occur as elongate chains or multi grain clusters crystals are very similar in dimension and grain habit to those of known biogenic origin mainly bacteria all measured grains are within the single domain stability field for magnetite based on crystal dimensions coercivity distributions are fairly broad 5 100 mt or greater likely the result of partial oxidation of the magnetic minerals no large multidomain grains were observed in the tem study individual grains occur in several shapes in the microscope image mainly hexagonal and cuboidal prismatic widespread occurrence of single domain magnetite in this isolated carbonate setting suggests that biogenic precipitation is a major contributor of early in situ depositional magnetization a contribution of magnetite from authigenic precipitation during burial has not been documented in these near surface environments

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

depth distribution of magnetofossils in near surface sediments from the blake bahama outer ridge western north atlantic ocean determined by low temperature magnetism 1 fe oxide and fe sulfide trace minerals in sediments and sedimentary rocks provide proxy records of biogeochemical processes record past variations in the geomagnetic field and can serve as proxies for climatic variations an important class of these fe oxides is produced by bacteria magnetic particles produced by magnetotactic bacteria have been proposed as a primary recorder of the geomagnetic field in many terrestrial marine sediments and have also been suggested to represent fossil evidence of life on the planet mars to better understand their distribution and preservation in the sediment column and their relationship to other biochemical processes we present rock magnetic data that document the occurrence and abundance of fossil biogenic magnetite magnetofossils in marine sediments from the blake bahama outer ridge magnetic hysteresis and low temperature magnetism both indicate that the occurrence of magnetofossils is closely linked to the depth of the modern fe redox boundary within the sediment column and that a fraction of the magnetic minerals in the sediment column above the fe redox boundary are in the form of intact and relatively unaltered chains of nanophase magnetite crystals below the fe redox boundary the abundance of these magnetofossils is markedly decreased the important conclusions of this work are to demonstrate that nondestructive rock magnetic methods can be used to successfully document the occurrence and relative abundance of magnetofossils in geologic materials

early diagenetic greigite as a recorder of the palaeomagnetic signal in miocene pliocene sedimentary rocks of the carpathian foredeep romania summary during the miocene pliocene the carpathian region represented the westernmost part of the so called eastern paratethys a palaeobioprovince that covered central and eastern europe as well as parts of southwest asia previous palaeomagnetic investigations provide a highresolution magnetochronology for the sedimentary sequences of the carpathian foredeep and indicate a marked transition in magnetic carriers from iron oxides to iron sulphides in chron c3r here we demonstrate using detailed rock magnetic investigations and scanning electron microscope sem analyses that the major magnetic iron sulphide mineral is greigite fe3s4 thermomagnetic runs indicate an irreversible decrease in magnetization with increasing temperature up to 400 c and sem observations indicate octahedral grain morphologies and fe s ratios that are indicative of greigite hysteresis loops have rectangular shapes which are typical of single domain behaviour with coercivity and coercivity of remanence at values of bc 35 45 mt and bcr 52 67 mt respectively first order reversal curve diagrams have contours that close around single domain peaks with bc values of 45 90 mt isothermal remanent magnetization component analysis reveals a small dispersion for the greigite component dispersion parameter dp 0 15 log units indicating a narrow grain size distribution a positive fold test inclination shallowing and two positive reversal tests are additional arguments for the syn depositional formation of this greigite we thus argue that most of the greigite was formed under early diagenetic conditions that is within 1000 yr of deposition of the sediment in this setting and that it thus can be considered as a reliable recorder of the palaeomagnetic signal in this situation the appearance of greigite during chron c3r between 6 0 and 5 5 ma in the carpathian foredeep is most likely related to regional tectonic and or climatic events that reshaped the basin configuration and changed the palaeoenvironmental conditions

effects of internal mineral structures on the magnetic remanence of silicate hosted titanomagnetite inclusions an electron holography study 1 titanomagnetite inclusions in pyroxene and plagioclase are carriers of stable magnetic remanence in some slowly cooled rocks such as gabbros anorthosites granulites and diorites needle shaped inclusions with average dimensions of 1 1 25 m form epitaxially by exsolution from their host silicate close examination of clinopyroxene hosted inclusions reveals an internal microstructure which consists of magnetite fe3o4 prisms and ulvospinel fe2tio4 lamellae that formed as a result of phase unmixing during initial cooling this internal structure exerts a profound influence on the magnetic remanence properties of each inclusion primarily by transforming it from a multidomain grain into an assemblage of magnetostatically interacting single domain prisms here we use off axis electron holography to image the magnetization states of individual prisms and the magnetostatic interactions between them we show that the inclusions exhibit both single domain and collective magnetic states that depend primarily on the shape anisotropies of individual magnetite prisms magnetostatic interactions between closely spaced prism stacks and the shape anisotropy of the needle itself prisms that are separated by thick ulvospinel lamellae show uniformly magnetized and or vortex states in contrast closely spaced magnetite prisms behave as multipart vortices or as long composite columns whose strong net magnetization may not be related directly to the orientation and shape of either the needle or the constituent prisms the overall remanence direction recorded by clinopyroxene crystals containing finely exsolved inclusions is a reflection of both the inclusions elongation directions and the prism arrangements within them

maghemite to magnetite reduction across the fe redox boundary in a sediment core from the ontong java plateau influence on relative palaeointensity estimation and environmental magnetic application summary during reduction diagenesis production of dissolved fe2 by reduction of ferric oxide starts at the fe redox boundary the associated magnetic property changes may influence palaeomagnetic and environmental magnetic records of marine sediments however this has not been evaluated thoroughly in this study using a gravity core from the ontong java plateau we document in detail rock magnetic changes across the fe redox boundary and investigate their influence on relative palaeointensity estimation and on a magnetic proxy for the proportion of terrigenous biogenic magnetic minerals the magnetic mineral assemblage above the fe redox boundary is characterized by a component with a mean coercivity of 100 mt in isothermal remanent magnetization irm component analyses and low s ratios s 0 1t low temperature irm measurements and mossbauer spectroscopy indicate that the degree of maghemitization is higher above the fe redox boundary from these observations we conclude that reduction of maghemite to magnetite occurs at the fe redox boundary and we infer that a maghemite skin on magnetic grains is lost across the boundary relative palaeointensity variations obtained by normalizing nrm intensity with sirm agree well with regional and global palaeointensity stacks which suggests that relative palaeointensity estimation is not significantly affected by the fe redox boundary temporal variations of the ratio of anhysteretic remanent magnetization susceptibility and saturation irm arm sirm coincide with the regional pattern across the ontong java plateau it is also possible to estimate variations in the proportion of terrigenous to biogenic components using the arm sirm ratio across the fe redox boundary

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

remanence acquisition efficiency in biogenic and detrital magnetite and recording of geomagnetic paleointensity relative paleointensity rpi variations of earth s magnetic field are widely used to understand geomagnetic field behavior and to develop age models for sedimentary sequences rpi estimation is based on a series of assumptions one key assumption that is rarely considered is that all magnetic particles in the sediment acquired a magnetization in an identical manner in this paper we test this assumption for sediments from the eastern equatorial pacific ocean that record well documented global rpi variations over the last 780 kyr the magnetization is carried by two stable single domain magnetic components which we identify as magnetite magnetofossils and titanomagnetite nanoparticle inclusions within larger silicate particles by analyzing signals carried by the two components separately we determine for the first time that magnetic nanoparticle inclusions can cause their host particles to record reliable but inefficient sedimentary paleomagnetic signals the magnetization carried by biogenic magnetite is acquired more efficiently than that carried by the nanoparticle inclusions variations in the concentration of both components are modulated climatically so that they record nearly identical rpi signals in many sediment types there is no correlation between the concentrations of different magnetic components so that variable remanence acquisition efficiency will complicate rpi recording our work demonstrates that detailed assessment of paleomagnetic recording by each constituent magnetic component needs to become a routine part of sedimentary rpi analysis

a simple inorganic process for formation of carbonates magnetite and sulfides in martian meteorite alh84001 we show experimental evidence that the zoned mg fe ca carbonates magnetite and fe sulfides in martian meteorite alh84001 may have formed by simple inorganic processes chemically zoned carbonate globules and fe sulfides were rapidly precipitated under low temperature 150 c hydrothermal and non equilibrium conditions from multiple fluxes of ca mg fe co2 s h2o solutions that have different compositions chemically pure single domain defect free magnetite crystals were formed by subsequent decomposition of previously precipitated fe rich carbonates by brief heating to 470 c the sequence of hydrothermal precipitation of carbonates from flowing co2 rich waters followed by a transient thermal event provides an inorganic explanation for the formation of the carbonate globules magnetite and fe sulfides in alh84001 in separate experiments kinetically controlled 13c enrichment was observed in synthetic carbonates that is similar in magnitude to the 13c enrichment in alh84001 carbonates

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

comparison of hysteresis thermomagnetic and low temperature magnetic properties of particle size fractions from loess and palaeosol samples in central asia and the chinese loess plateau

ultrafine grained magnetite in deep sea sediments possible bacterial magnetofossils a new extraction technique now permits ultrafine magnetite crystals to be separated from a variety of deep sea sediments morphologic characterization of these particles with transmission electron microscopy reveals the presence of several distinct crystal types some of which closely resemble those formed by the magnetotactic bacteria the apparently biogenic magnetite particles are of single domain size and dominate the population in calcareous deep sea sediments bacterially precipitated magnetite may therefore be a major source of the stable magnetic remanence in some marine sediments these objects possibly constitute the smallest mineral fossils yet recovered from the sedimentary record

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

analysis of the field dependence of remanent magnetization curves 1 a new method to calculate and analyze coercivity distributions of measured acquisition demagnetization curves of remanent magnetization is presented the acquisition demagnetization curves are linearized by rescaling both the field and the magnetization axes an appropriate filtering of the linearized curves efficiently removes measurement errors prior to evaluating the coercivity distributions the filtered coercivity distributions are modeled using a set of generalized probability density functions in order to estimate the contributions of different magnetic components an error estimation is calculated for these functions with analytical and numerical methods in order to evaluate whether the model is significantly different from the measured data three sediment samples from baldeggersee switzerland and three samples of urban atmospheric particulate matter pm have been analyzed using this method it is found that the coercivity distributions of some of the magnetic components show significant and consistent deviations from a logarithmic gaussian function large deviations are found also in the coercivity distributions of theoretical af demagnetization curves of single domain and multidomain particles constraints in the shape of model functions affect the identification and quantification of magnetic components from remanent magnetization curves and should be avoided as far as possible the generalized probability density function presented in this paper is suitable for appropriate modeling of gaussian and a large number of non gaussian coercivity distributions

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