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

sedimentation at manop site h eastern equatorial pacific over the past 400 000 years climatically induced redox variations and their effects on transition metal cycling gravity cores recovered from manganese nodule project site h 6 33 n 92 49 w show marked downcore variations in the abundance of calcium carbonate organic carbon opal manganese and other components deposited over the past 400 000 years variations in the downcore abundance of organic carbon which ranges from 0 2 to 1 0 can be used to hindcast redox conditions in the surface sediments over this time the results indicate that the depth to the manganese redox boundary varied from about 5 to 25 cm below the seafloor during four major cycles downcore variations in solid phase mn ni and cu can be produced by such changes in redox conditions a model which predicts that solid phase mn can be trapped and buried when the mn redox boundary migrates rapidly upward is consistent with the observed organic carbon and mn records and supports the reconstructed redox variations the history of redox variations at site h can be explained by changes with time in surface water productivity major productivity variations at the site occur over 100 kyr cycles with relatively higher productivity occurring during glacial stages thus quaternary climate changes influence surface water productivity redox conditions in sediments and the cycling of transition metals

estimating superparamagnetism in marine sediments with the time dependency of coercivity of remanence superparamagnetic grains ultrafine 50 nm particles that do not retain a magnetic remanence at room temperature can form through several biogeochemical pathways in marine sediments knowing the distribution of such particles therefore provides a means of better understanding the past diagenetic history of sediments and evaluating their potential record of environmental change herein we employ magnetic hysteresis measurements in a new way to determine the abundance of superparamagnetic sp grains we start with a phenomenological model that describes a dependency of coercivity of remanence hcr with measurement time as a function of superparamagnetic grain content calculated time dependencies of hcr agree with experimental data suggesting that the model can be used to derive quantitative sp estimates when applied to pelagic sediments of the western equatorial pacific ocean ocean drilling program site 805c the time dependencies of hcr reveal a 25 30 sp increase starting just above the modern iron redox boundary irb next we compare our approach with a previous method that detects sp grains through the thermal demagnetization of saturation remanence acquired at very low temperature 20 k we find this method fails to record the sp changes detected using the time dependency of hcr and suggest that the cause is a sensitivity of the low temperature data to small differences in magnetic mineralogy that accompany diagenesis specifically differential maghemitization causes varying suppression of the remanence changes associated with the verwey transition at 120 k the removal of maghemite shells on magnetic grains below the irb results in greater remanence changes at the verwey transition and a corresponding sp underestimate using the low temperature method in addition some iron oxyhydroxides paramagnetic at room temperature can become ferromagnetic at very low temperatures resulting in an sp overestimate in oxic sediments above the irb the increase of sp content in the zone of magnetic mineral reduction observed with the time dependency of hcr may be related to the breakdown of larger magnetic grains and the activity of dissimilatory iron reducing bacteria superparamagnetism measured by the time dependency of hcr may be useful for tracking the history of these biogeochemical processes in marine sediments

middle eocene to late oligocene antarctic glaciation deglaciation and southern ocean productivity during the eocene oligocene transition earth cooled significantly from a greenhouse to an icehouse climate nannofossil assemblages from southern ocean sites enable evaluation of paleoceanographic changes and hence of the oceanic response to antarctic ice sheet evolution during the eocene and oligocene a combination of environmental factors such as sea surface temperature and nutrient availability is recorded by the nannofossil assemblages of and can be interpreted as responses to the following changes a cooling trend started in the middle eocene was interrupted by warming during the middle eocene climatic optimum and by short cooling episodes the cooling episode at 39 6 ma preceded a shift toward an interval that was dominated by oligotrophic nannofossil assemblages from 39 1 to 36 2 ma we suggest that oligotrophic conditions were associated with increased water mass stratification low nutrient contents and high efficiency of the oceanic biological pump that in turn promoted sequestration of carbon from surface waters which favored cooling after 36 2 ma we document a large synchronous surface water productivity turnover with a dominant eutrophic nannofossil assemblage that was accompanied by a pronounced increase in magnetotactic bacterial abundance this turnover reflects a response of coccolithophorids to changed nutrient inputs that was likely related to partial deglaciation of a transient antarctic ice sheet and or to iron delivery to the sea surface eutrophic conditions were maintained throughout the oligocene which was characterized by a nannofossil assemblage shift toward cool conditions at the eocene oligocene transition finally a warm nannofossil assemblage in the late oligocene indicates a warming phase

biogeochemical remanent magnetization in pelagic sediments of the western equatorial pacific ocean paleomagnetic data from pelagic sediments are an important source of information on the history of the geomagnetic field however fundamental uncertainties about how these sediments acquire their magnetizations remain here we address whether biogenic magnetite formed at depth within the sediments can contribute to remanent magnetizations in sediments of the western equatorial pacific ocean odp site 805 biogenic magnetite found near the fe redox boundary carries a natural remanent magnetization because the production of biogenic magnetite is tied to the geochemical zonation of the sediments we call this magnetization a biogeochemical remanence these data indicate that some pelagic sediments can continue to acquire remanence significantly after the post depositional lock in of detrital magnetic grains

magnetic hysteresis in young mid ocean ridge basalts dominant cubic anisotropy magnetic hysteresis data from young mid ocean ridge basalts include samples with saturation remanence to saturation magnetization mrs ms ratios greater than 0 5 the theoretical limit for an assemblage of single domain grains with uniaxial anisotropy under the usual assumption of dominant uniaxial anisotropy the narrow single domain grain size distribution implied by these high mrs ms values is difficult to reconcile with petrographic and remanence data that suggest the presence of larger multidomain grains dominant cubic anisotropy provides a plausible explanation for the high mrs ms ratios and if generally valid requires reinterpretation of granulometric and domain state inferences made from hysteresis data

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

controlled biomineralization of magnetic minerals by magnetotactic bacteria magnetotactic bacteria produce two general types of minerals as the mineral phases of their magnetosomes iron oxides and iron sulfides the iron oxides include only ferrimagnetic magnetite fe3o4 and the iron sulfides ferrimagnetic greigite fe3s4 and non magnetic pyrite fes2 regardless of composition the magnetosome particles have a very narrow size range 35 120 nm and are single magnetic domains these intracellular particles confer a permanent magnetic dipole moment to the cell resulting in the cell s magnetotactic response i e a motile biomagnetic compass the morphology of the magnetosome particles is species and or strain specific thus the organism exerts significant control over the biomineralization process es involved in magnetosome formation however there is some accumulating evidence that suggests that in certain organisms under specific situations local environmental conditions appear to affect the magnetosome mineral phase in at least three different ways these include 1 variations in the stoichiometry of the metal and non metal components of the magnetosome mineral phase 2 the replacement of iron with other transition metal ions in the metal component of the magnetosome mineral phase and 3 the replacement of oxygen with sulfur or vice versa in the non metal component of the magnetosome mineral phase examples of each type of mineral change or variation are described and discussed

astronomical calibration of the matuyama brunhes boundary consequences for magnetic remanence acquisition in marine carbonates and the asian loess sequences we have compiled 19 records from marine carbonate cores in which the matuyama brunhes boundary mbb has been reasonably well constrained within the astronomically forced stratigraphic framework using oxygen isotopes by correlation of the 18o data to a timescale based on astronomical forcing we estimate astronomical ages for each of the mbb horizons in all but one record the mbb occurs within stage 19 most magnetostratigraphic sections in asian loess place the mbb within a loess interval since loess deposition is presumed to be associated with glacial intervals loess horizons should correspond to even numbered oxygen isotope stages a glacial age for the mbb is at odds with the results presented here which firmly place the mbb within interglacial stage 19 inconsistency among the many loess sections and between the loess and the marine records suggests that the magnetic interpretation of loess sections may be more complicated than hitherto supposed the mean of the stage 19 age estimates for the mbb is 777 9 1 8 n 18 inclusion of the single stage 20 age results in a mean of 778 8 2 5 n 19 the astronomical age estimate of the mbb compares favorably with an unweighted mean of 778 2 3 5 n 10 from a compilation of 40ar 39ar results of transitional lava flows combining the two independent data sets yields a grand mean of 778 0 1 7 n 28 the new compilation shows virtually no trend in placement of the mbb within isotope stage 19 as a function of sediment accumulation rate we interpret this to mean that the average depth of remanence acquisition is within a few centimeters of the sediment water interface separating the cores into two geographic regions an indo pacific caribbean ipc group and an atlantic group results in a significant difference in the position of the mid point of the reversal with respect to the astronomical time scale the data presented here suggest a difference of several thousand years between the two regions this observation could be caused by systematic differences between the two regions in sedimentation rate within the interval of interest systematic differences in remanence acquisition or by genuine differences in the timing of the directional changes between the two regions

stress field in the ryukyu kyushu wadati benioff zone by inversion of earthquake focal mechanisms abstract the study addresses the space distribution of the stress field in the kyushu ryukyu wadati benioff zone based on homogeneous data of earthquake focal mechanisms and the inverse technique by gephart and forsyth j geophys res 89 1984 9305 the used data set consists of 148 harvard cmt solutions and 22 earthquake focal mechanisms listed in previous studies the stress field parameters are determined for 0 40 41 100 and h 100 km depth ranges the top 100 km layer of the wadati benioff zone wbz is characterized by strike normal maximum compression 1 and steeper than the slab minimum compression 3 the last indicating for unbalanced slab pull force the tokara channel divides the subduction into two parts of different stress regime at depth greater than 100 km to the south of the channel the slab is under slab parallel 1 and slab normal 3 while its northern part beneath kyushu is under slab parallel extension and slab normal compression the results of recent studies on the regional velocity structure and geochemistry of the volcanic lava indicate that the most plausible reason for the observed stress field difference below 100 km in the northern and rest part of the arc is the presence of hot low viscosity upper mantle west of kyushu the results of this study indicate that the forces involved in the contemporary subduction dynamics in the ryukyu kyushu wadati benioff zone are related to the convergence between the philippine sea plate and the eurasian plate the trench suction force slab pull the slab anchor force and in the southern central part of the arc mantle resistance

non steady state magnetic mineral reduction chemical lock in and delayed remanence acquisition in pelagic sediments abstract magnetic hysteresis data collected from pelagic sediment sites in the western equatorial pacific ocean define a progressive deepening of modern redox boundaries with decreased organic carbon input paleomagnetic data from the site with the highest organic carbon shows marked changes in the natural remanent magnetization at the modern fe redox boundary suggesting a type of chemical lock in process in which some magnetic grains form in situ above the boundary and freely orient in the field the depth of chemical lock in suggests paleomagnetic data will be smoothed on time scales ranging from less than 40 kyr to more than 425 kyr the higher values are compatible with models calling for a delayed remanence acquisition to account for the saw tooth pattern observed in some sedimentary paleointensity records

environmental magnetism of pleistocene sediments in the north pacific and ontong java plateau temporal variations of detrital and biogenic components 1 an environmental magnetic study was conducted to investigate temporal variations of the sources of magnetic minerals in the pacific ocean using sediment cores of pleistocene age the proportion of interacting i to noninteracting n i single domain components was estimated from first order reversal curve diagrams and the proportion of middle m to low l coercivity components was determined from decomposition of isothermal remanent magnetization irm acquisition curves the i and m components are interpreted to be carried by terrigenous maghemite while the n i and l components represent biogenic magnetite an inverse correlation between the ratio of anhysteretic remanent magnetization susceptibility arm to saturation irm sirm and the i n i ratio was confirmed for sediments across wide expanses in the pacific ocean this implies that arm sirm can be used to estimate the relative abundances of biogenic and terrigenous components in sediments in the north pacific the relative abundance of the i and m components increases in glacial periods variations of arm sirm and s ratio s 0 1t resemble each other and decrease in the same time intervals these variations reflect increases of terrigenous input in glacial periods as eolian dust on the ontong java plateau in the western equatorial pacific on the other hand minima in arm sirm and s 0 1t occur at glacial to interglacial transitions and the relative abundance of the i and m components increases in these periods terrigenous material in this region is considered to be transported mainly from the new guinea and solomon islands by the equatorial undercurrent the higher proportion of the terrigenous component at glacial to interglacial transitions is coeval with carbonate preservation maxima reported from the ontong java plateau suggesting a linkage between the two

early diagenesis in an organic rich turbidite and pelagic clay sequence from the cape verde abyssal plain ne atlantic magnetic and geochemical signals abstract processes of early diagenesis in sediments from the cape verde abyssal plain are investigated using the complimentary techniques of environmental magnetism and solid phase geochemistry the sequence captured by a 2 m long box core comprises an organic rich turbidite derived from the nw african continental margin emplaced ca 330 ka bp overlain by brown pelagic clay diagenesis in the pelagic clay mainly involves slow in situ oxidation of the iron oxide phases present which mossbauer effect data suggest are detrital haematite and magnetite possibly maghaemitised in the turbidite diagenesis has proceeded in two stages the first involving suboxic reductive processes following depletion of porewater o2 and the second involving oxidative processes associated with the slow descent of an oxidation front through the bed due to diffusion of o2 from the overlying seawater carbonate free magnetic parameters interparametric quotients and remanence acquisition demagnetisation and coercivity measurements of the core reveal that significant depletion of detrital ferrimagnetic iron fe2 fe3 oxide grains like titano magnetite has occurred within the turbidite during suboxic diagenesis and confirm previous suggestions that reductive dissolution of such components is a grain size selective process however the higher oxidation state fe3 canted antiferromagnetic iron oxide grains like haematite and goethite are much less susceptible to reductive diagenesis remobilisation of transition metal ions from below the oxidation front in the turbidite has led to authigenic precipitation of ferrimagnetic fe mn micronodules in a distinct layer immediately overlying the front and above this a further interval of mn enriched diagenetic laminae however within the suboxic organic rich zone of the turbidite well below the oxidation front there is clear evidence for the presence of a significant concentration of fine single domain ferrimagnetic iron oxide grains which are most probably associated with a population of live magnetotactic bacteria between this zone and the oxidation front there is a transitional interval in which the number of live magnetotactic bacteria declines rapidly and fossil magnetosomes in the sediment are being reductively dissolved progressively up sequence these results are broadly similar though differing in some respects to magnetic and geochemical studies of hemipelagic and high productivity pelagic sediments in which mn and fe redox boundaries have been recognised and further illustrate the value of environmental magnetic parameters for characterising early diagenetic processes in sediments of this kind

emergence and evolution of the south atlantic anomaly revealed by the new paleomagnetic reconstruction shawq2k abstract the south atlantic anomaly saa is one of the most outstanding features of the present geomagnetic field thus a good knowledge of the saa is clue for a better understanding of the dynamical behavior of the geomagnetic field to achieve this goal paleomagnetic data are crucial since they provide the unique way to investigate past changes in the earth s magnetic field here we present a new global geomagnetic field reconstruction the shawq2k model which is based on a critical revision of the global archeomagnetic and volcanic dataset the new model provides an improved description of the geomagnetic field during the last 2 millennia and yields surprising outcomes about the emergence and development of the saa it shows that the reversed flux patch observed at the core mantle boundary and linked to the saa emerged in the southern hemisphere from at least 950 ad this patch moved westward from the equator to southern latitudes being clearly isolated after 1400 ad in addition since 1550 ad a second reversed flux patch moving northeastward is observed in the north atlantic the new data now available for the southern hemisphere coming from africa and south america together with the use of an appropriated weighting scheme in the modeling process have improved our understanding of past geomagnetic field behavior and showed new evolutionary aspects of the saa

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

temporal trends of magnetic dissolution in the pelagic realm gauging paleoproductivity abstract magnetic hysteresis data from the western equatorial pacific suggest that the preservation of magnetic remanence in pelagic carbonates is controlled by paleoproductivity hysteresis excursions 100 k y intervals are linked here to greater magnetic dissolution associated with increased glacial organic carbon supply changes in the redox boundary coupled to glacial terminations result in anomalous magnetic features which are preserved in the sedimentary record these high frequency variations are superimposed on a longer term signal of magnetic dissolution which may reflect decreased productivity since the miocene

early sediment diagenesis on the blake bahama outer ridge north atlantic ocean and its effects on sediment magnetism sediment magnetic and geochemical studies of a suite of deep sea sediment cores from the blake bahama outer ridge bbor north atlantic ocean have identified two current redox boundaries in surficial holocene carbonate rich sediments over much of the bbor the upper mn 4 mn 2 redox boundary is associated with a spike in the concentration of solid phase mn as mno2 the lower fe 3 fe 2 redox boundary is associated with a spike in the concentration of solid phase fe as goethite feooh over much of the bbor high sediment magnetic intensities occur in surficial carbonate rich sediments associated with these redox boundaries and lower intensities occur in deeper late pleistocene carbonate poor sediments this relationship is opposite to that expected if sediment magnetism simply reflects the clastic noncarbonate sediment fraction the surficial high sediment magnetic intensities are due primarily to the following two factors 1 magnetic mineral authigenesis associated with early diagenesis and 2 the presence of abundant 0 1 m magnetite crystals interpreted to be bacterial magnetosomes magnetosomes are almost absent in the late pleistocene low carbonate sediments owing most likely to local pleistocene environmental conditions high clastic flux low organic flux which did not favor their growth the sediment natural remanent magnetization is strongly correlated with the sediment clastic fraction and is relatively unaffected by early diagenesis and the presence of abundant bacterial magnetite if this is typical bacterial magnetite may be more abundant in nature but less important to sediment paleomagnetic records than previously thought

the fidelity of paleomagnetic records carried by magnetosome chains article history received 30 may 2013 received in revised form 19 september 2013 accepted 20 september 2013 available online xxxx editor j lynch stieglitz magnetotactic bacteria mtb which use aligned chains of magnetosomes magnetic crystals as a navi gation tool are found in a wide range of modern day marine river and lacustrine environments and their fossilized remains are being increasing recognized in geological records despite an increasing realization that biogenically derived magnetic particles may play a key role in sedimentary magnetizations little is known about the influence that they may have on the fidelity of paleomagnetic recordings using cultured magnetospirillum magneticum strain amb 1 we have conducted simple 2d i e zero inclination deposi tion experiments to assess the efficiency with which magnetosome chains align along magnetic field lines and the implications that this has for paleomagnetic records our results indicate that the natural rema nent magnetization nrm acquired by deposited mtb is near linear with applied field 0 120 t but that nrm acquisition does not perfectly follow the assumed linear trend and over a six fold increase in applied field only a factor 5 5 increase in nrm intensity is observed i e 9 lower than the assumed trend both anhysteretic remanent magnetization arm and saturation isothermal remanent magnetiza tion sirm normalized relative paleointensities rpis can successfully recover field strength variations under identical situations when the mtb concentration of the initial solution is varied up to a factor 8 nrm carried by mtb responds in the expected fashion i e a doubling of bacteria produces a doubling of nrm intensity both arm and sirm however do not respond to as expected a doubling of con centration corresponds to an increase of 1 76 in sirm intensity and an increase of only 1 63 in arm intensity both are influenced by magnetostatic interactions that arise from the close packing of bacterial cells or subchain interactions within a single bacterial cell first order reversal curve forc diagrams however are too insensitive to indicate the presence of paleomagnetically relevant magnetic interactions remanence based methods i e the arm ratio are more appropriate to detect such interactions

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

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

depth of post depositional remanence acquisition in deep sea sediments a case study of the brunhes matuyama reversal and oxygen isotopic stage 19 1 although post depositional magnetization pdrm in deep sea sediments appears to be acquired during the earliest stages of sediment compaction the natural variability of the pdrm lock in depth in deep sea sediments is poorly understood and as yet unquantified here we consider variations in the relative stratigraphic positions of oxygen isotopic interglacial stage 19 1 and the brunhes matuyama b m chronozone reversal for eight deep sea sediment cores results from a similar study of the displacement between a widespread microtektite layer and the b m boundary are also included 1 the pdrm lock in depth and the temporal relationships between the b m and stage 19 1 datums can be determined from the offsets between the paleomagnetic and the isotopic and microtektite stratigraphies for cores with sedimentation rates greater than 1 cm kyr a the depth offset between the paleomagnetic and isotopic datums is a linear function of sedimentation rate a simple model r 2 0 77 demonstrates that 1 pdrm acquisition occurs 16 cm below the sediment surface and 2 the b m reversal occurs 6 kyr 4 2 kyr after the stage 19 1 datum and the stage 19 1 datum occurs 9 kyr 4 3 kyr after the deposition of the australasian microtektite strewnfield this example argues against simple geophysical models linking geomagnetic field reversals to climate change or impact events the b m boundary is anomalously deep 30 50 cm in very low accumulation rate sediments 1 cm kyr 1 and this may reflect the unusual physical properties of these sediments a review of the geotechnical literature suggests that very low accumulation rate sediments have sufficient time to develop enhanced interparticle rigidity structural strength which inhibits early compaction and hence pdrm acquisition

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

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

superparamagnetism and reduction diagenesis in pelagic sediments enhancement or depletion superparamagnetic grains ultra fine d 0 05 m particles unable to retain a remanence at room temperature have come under increased scrutiny as indicators of the diagenetic and authigenic history of sedimentary rocks in marine sediments fine grained magnetite with large surface area to volume is thought to be preferentially removed during magnetic mineral reduction on the basis of new magnetic hysteresis low temperature and geochemical studies of pelagic sediments from the western equatorial pacific ocean it is proposed that reduction processes increase rather than deplete the ultra fine magnetic grain population at the modern fe redox boundary enhanced superparamagnetism coincides with a coarsening of remanence carrying grains superparamagnetism also tracks proposed temporal changes in magnetite reduction caused by climatically driven fluctuations in organic carbon corg supply together with hysteresis characteristics changes in superparamagnetism may help the identification of paleointensity artifacts resulting from non steady state magnetic mineral reduction

magnetic properties and geochemistry of the active oxidation front and the youngest sapropel in the eastern mediterranean sea summary magnetic properties irm arm xin s ratio at 0 3 t room temperature rt hysteresis and thermomagnetic curves and geochemical data fe s mn al ti organic c were studied in two eastern mediterranean boxcores abc26 and bc19 at a resolution of 3 5 mm the boxcores contain sapropel s1 9 6 kyr bp at a few decimetres below seafloor the magnetic fraction consists predominantly of single domain sd to pseudo singledomain psd magnetite in the entire cores the original input of magnetic grains comes from two sources aeolian dust both cores and volcanic ash from the minoan eruption of santorini core bc19 only non steady state diagenesis has changed the magnetic mineralogy considerably in these alternating organic rich organic poor sediments during deposition of sapropel s1 reductive diagenesis and pyritization in and just below the sapropel caused lower magnetic intensities coarser magnetic grain sizes and partial maghemitization in thermomagnetic curves two types of pyrite can be identified one oxidizes below 450 uc and the other above 450 uc the higher oxidation temperature is predominantly found below the sapropel this may be related to the microtexture of pyrite which is euhedral below sapropels and mainly framboidal within sapropels since the end of sapropel deposition a downward moving oxidation front has oxidized the upper half c 5 cm of the sapropel the oxidized part of the sapropel is enriched in diagenetically formed fe oxides with relatively high coercivity and arm the maximum coercivity is found in a distinct layer between the present day mn and fe redox boundaries at the top of the unoxidized sapropel the freshly precipitated fe oxides in this centimetre thick layer contain a mixture of superparamagnetic sp grains and high coercivity sd magnetite higher in the oxidized zone the freshly precipitated fe oxides have aged into generally slightly lower coercivity sd grains with relatively high arm in addition to the diagenetic formation of fe oxides at the top of the sapropel formation of a ferrimagnetic fe monosulphide may have occurred within the sapropel during later stages of diagenesis which may have enhanced the arm signal in the organic rich interval in particular

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

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

the first precambrian gold deposit in north xinjiang nw china geochronology metallogenic character and ore genesis of the dajingou gold deposit abstract the dajingou gold deposit is one of the typical gold deposits in the quruqtagh metamorphic terrane north of the tarim craton the deposit mainly comprises gold bearing quartz veins hosted in precambrian metamorphic rocks and structurally controlled by the subsidiary faults and ductile shear zone to determine the age of gold mineralization in the dajingou deposit a single sericite 40ar 39ar age of 829 4 4 8 ma mswd 0 57 and hydrothermal zircon weighted average u pb ages 812 15 ma mswd 3 2 from auriferous quartz veins were obtained in this study indicating that mineralization occurred in the mid neoproterozoic ca 0 83 ga corresponding to regional metamorphism of the tarim craton termed the tarim orogeny the dajingou deposit is the first precambrian gold deposit identified in north xinjiang nw china the 18o and d of quartz ranges from 14 7 to 15 9 and 104 to 75 respectively the calculated 18oh2o of the fluids ranges from 0 9 to 5 0 this suggests that the ore forming fluids in the dajingou gold deposit were derived from dehydration of metamorphic rocks during regional metamorphism with a subsequent input of meteoric water based on its similarities with typical orogenic gold deposits the dajingou deposit is proposed to be an orogenic gold deposit formed during the mid neoproterozoic tarim orogeny in the tarim craton

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

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