Search

21 pages; International audience; Magnetotactic bacteria (MTB) intracellularly precipitate magnetite (Fe3O4) crystals that can be preserved in the geological record. When MTB die, the so-called magnetofossils constitute valuable proxies for paleoenvironmental reconstructions and are suspected to represent some of the oldest traces of biomineralization on Earth. Yet, the biogenicity of putative magnetofossils found in ancient terrestrial and extra-terrestrial samples is still largely debated and their significance for past climate still holds uncertainties. Here we studied a sedimentary sequence from the Indian Ocean (ODP Hole 711A) recording the Middle Eocene Climatic Optimum (MECO) through which a magnetofossil-rich interval was deposited. We investigated for the first time the potential of Fe isotopes as a biosignature in magnetofossils and thoroughly describe MECO related paleoenvironmental disruptions based on major and trace element concentrations. Bulk sediment Fe isotopes showed limited variations, with δ56Fe around −0.13 ± 0.04‰ (n = 24), linked to detrital iron rather than MTB activity. Hence, a sequential chemical extraction protocol was applied to determine the specific composition of magnetite. We discuss analytical biases related to this protocol (i.e. partial phyllosilicate and Mn-oxide leaching) and apply corrections to the data. Outside the magnetofossil-rich interval, Fe isotope compositions of oxides (mainly biotic and/or abiotic magnetites and possibly Fe coprecipitated with Mn-oxides) display a small range averaging −0.54 ± 0.05‰, and are interpreted as reflecting dominantly hydrothermal contribution, a conclusion also supported by prominent Eu anomaly. In contrast, the magnetofossil-rich interval shows larger δ56Fe variability in oxides, from −0.12 to −0.94‰, decreasing upwards in the stratigraphic section. This interval likely records enhanced Fe supply from atmospheric fallout, increase in biological productivity (illustrated by increased Ba accumulation rate) and subsequent development of ferruginous conditions in the sediment porewater. Covariations of Fe isotope compositions and Mn/Fe ratios can be explained by a vertical migration of a redox front and associated diagenetic modifications. Precipitation of barite (BaSO4) in the sediments after organic matter decay probably favored the preservation of magnetofossils by decreasing SO42- concentration in porewaters and subsequent H2S production, which usually dissolve magnetite in the sulfidic zone. Finally, we model the evolution of porewater fluid and estimate Fe isotope fractionation between magnetofossils and fluid to Δ56Femag-Fe(II)aq = 0.1–0.3‰, a value significantly different from abiotic magnetite fractionation (~1.5‰). Contrasting with recent results on MTB laboratory culture, no mass independent fractionation of Fe isotopes was observed in the present study. Nevertheless, the diverse geochemical proxies presented here provide important constraints on paleoclimate and magnetofossil biogenicity evaluation.

Magnetotactic bacteria (MTB) play an important role in biogeochemical cycles of iron and remanent magnetization acquisition of sediments. It is generally considered that magnetite-producing MTB prefer chemical conditions in the oxic-anoxic transition zone (OATZ). We studied the distribution of fossil MTB magnetite (magnetofossils) and microbes phylogenetically related to MTB within deep-sea surface sediments across the OATZ. Transmission electron microscope (TEM) observations and rock-magnetic proxies show that magnetofossils commonly occur both within the OATZ and above it, even at the top of surface oxidized layers, and that the proportion of magnetofossils with a teardrop morphology increases within the OATZ. Consistent results were obtained from the pyrosequencing of 16S rRNA gene fragments. Operational taxonomic units (OTUs) related to Nitrospirae and OP3 MTB, which produce teardrop-shape magnetosomes, were detected only within the OATZ, whereas OTUs related to alphaproteobacterial MTB, which produce equant or elongated magnetite particles, were distributed above the OATZ. These results suggest that magnetofossil morphology can be used to detect past OATZs, and that some MTB lineages can occur ubiquitously in marine sediments rather than being limited within the OATZ. Our results imply that the timing of remanent magnetization acquisition may depend on the inhabiting MTB lineages.

Reductive dissolution of magnetites is known to occur below the Fe-redox boundary in sediment columns. This study aims to document the detailed processes of biogenic magnetite dissolution. A sediment core taken from the Japan Sea was used for this purpose, in which reductive dissolution of magnetic minerals are known to start at about 1.3 m in depth and mostly complete within an interval of about 0.3 m. Using first-order reversal curve diagrams, preferential dissolution of biogenic magnetites within this interval is estimated from the observation that a narrow peak extending along the coercivity axis (the central ridge), which is indicative of biogenic magnetite, diminishes downcore. Transmission electron microscopy shows that the sediments contain the three morpho-types of magnetofossils: octahedron, hexagonal prism, and bullet shaped. With the progress of reductive dissolution, the proportion of bullet-shaped magnetofossils decreases, whereas that of hexagonal prisms increases. For hexagonal prisms, {111} caps are often etched while {110} side faces are almost intact. These observations can be explained by the differences in resistivity against dissolution among crystal planes of magnetite. A previous study reported that the dissolution rate of (111) planes is higher than that of (110) planes. Hexagonal prisms elongate in the [111] direction and are wrapped with {110} side faces, whereas octahedral and bullet-shaped magnetofossils have larger proportions of surface areas with {111} faces. Magnetosome morphology may reflect preference of inhabiting magnetotactic bacterial lineage for chemical conditions in sediments. One should, however, be cautious for possible alteration of original morphological composition during reductive diagenesis when magnetofossil morphology is used as a paleoenvironmental proxy.

金沢大学理工研究域地球社会基盤学系, Magnetic detection and classification of magnetofossils have been proposed as potential tools for paleoenvironmental studies. Magnetosomes in bacterial species living in different environmental conditions exhibit different grain morphologies and chain configurations, which determine their magnetic properties. Recently, abundant magnetofossils have been reported from unfossiliferous pelagic red clay. However, little is known about their geometry and magnetic properties. Here we report very low coercivity biogenic magnetite in red clay from Ocean Drilling Program (ODP) Site 777 in the northern Mariana Basin. Analyzed sediment showed non-interacting, stable single-domain-like magnetic behaviors. Acquisition of isothermal remanence was decomposed into five components, and a component with mean coercivity below 10mT accounted for around 25 % of the remanence in some samples. Based on comparisons with semi-quantitative transmission electron microscopy observations of magnetic extracts, this component appears to be carried by octahedral grains with size and shape very similar to biogenic magnetite in red clay from other sites. Micromagnetic calculations indicated that isolated maghemite octahedra may be responsible for the observed low coercivity component. Based on these results, we conclude that the low coercivity component represents non-chained biogenic magnetite that has been oxidized to maghemite. The crystal morphology, geological setting, and lithology do not suggest unusual environmental conditions for ODP Site 777, so the relative amount of chained versus non-chained grains may represent subtle environmental differences. We suggest that non-chained magnetofossils may be widespread in deep-sea sediments. Some methods could overlook the presence of non-chained magnetite, affecting the identification and quantification of magnetofossils. © 2021. American Geophysical Union. All Rights Reserved.

Sedimentary records of the Early Cenozoic indicate a series of events with climatic and carbon cycle variability known as hyperthermals. A ~350-kyr-long event of environmental disruption during the Paleocene, not described before and here named Selandian–Thanetian Transition Event (STTE), has been recognized and well constrained in the western Tethys Contessa Road section (Gubbio, Italy) through high-resolution biostratigraphic, geochemical, and rock-magnetic data. The STTE exhibits peculiar stressed ecological responses among calcareous nannofossils and foraminifera, which highlight marked environmental perturbation affecting the biosphere. The environmental instability is not confined within the photic zone but extends to the seafloor leading to little more trophic conditions of the sea surface waters with an enhanced, but of short measure, nutrient availability on the seafloor conditions and marked rise of lysocline. Magnetic Susceptibly (MS) is dominantly controlled by the balance between carbonate productivity and detrital supply, as evidenced by the strong correlation between MS and CaCO3 (%) (r2 = −0.72). However, we also document two components in the isothermal remanent magnetization (IRM) and first-order reversal curves (FORC) diagrams that prove the occurrence of biogenic magnetite throughout the STTE. Systematic variations in bio-geochemical and magnetic parameters show the relative abundance of carbonate production (or inversely dissolution of carbonate) versus detrital supply during the STTE, which induced higher populations of magnetotactic bacteria through increased terrigenous input and, therefore, increased nutrient supply. Noteworthy, the uppermost part of the STTE includes the equivalent of the suspected hyperthermal, short-lived Early Late Paleocene Event (ELPE). The ELPE event shows an episode of increase in magnetic properties of the sediments, including an increase in magnetofossil concentration, as indicated by IRM components and FORC diagrams. The comparison of biotic and abiotic records throughout the STTE at Contessa Road section with available data across the ELPE from former investigated ocean and land-based sites provides lines of evidence that this latter event might be indeed only the terminal part of a long-lasting environmental change than hitherto supposed.

We build micromagnetic models to investigate the magnetic properties of biogenic magnetite – a common type of magnetic mineral that is responsible for recording a wide range of biological, geophysical and geological processes on earth. The geometry of modelled particles is based on realistic size and shape distributions from nanoscale observations. Systematic changes in microstructures of biogenic magnetite ensembles are built and their magnetic properties are calculated, which enables a quantitative and separate assessment of the effect of crystal morphology and chain structures. Although the same particle size and shape distributions are used in all calculations, simulation results document large variations in magnetic properties, i.e., wide distributions of coercivity ( B c = ∼ 10 –60 mT), coercivity of remanence ( B cr = ∼ 14 –81 mT), dispersion parameter ( DP = ∼ 0.1 –0.5), and skewness values ( S = ∼ 0.7 –1.1) due to variable degree of anisotropy and magnetostatic interactions. Previously, the commonly observed “biogenic soft” and “biogenic hard” components on biogenic magnetite-bearing samples were often interpreted to reflect crystal morphologies, and that the small DP values of coercivity distributions were an indication of narrow particle size distributions. Our simulations suggest that these speculations are not always the case and that magnetosome microstructures likely exert a dominant control over their magnetic properties. Our modelling results provide a new theoretical perspective on the magnetic properties of biogenic magnetite, which is important for understanding magnetic proxy signals from magnetofossils in a wide range of environmental and geological settings, and for the search for biogenic magnetite in terrestrial rocks and in extra-terrestrial materials.

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

Redox variations govern a multitude of key geochemical and microbiological processes within lacustrine and marine systems, yet the interpretation of these geological archives can be limited because redox-sensitive microorganisms leave behind sparse fossil evidence. Here, we assess a biologically controlled magnetic proxy through investigation of a well-constrained sedimentary record covering a perturbation of redox-conditions driven by a complete trophic cycle in Lake Constance. Ferromagnetic resonance spectroscopy of sediments reveals strong uniaxial anisotropy, indicative of single-domain magnetite particles in intact or fragmentary chain arrangements, which are an unambiguous trait of magnetotactic bacteria (MTB) and their magnetofossil remains. We show that biogenic magnetite formed intra-cellularly in MTB faithfully records changing redox-conditions at or close to the sediment water-interface. Biogenic magnetite within sedimentary records points to the proliferation of MTB parallel to a decline in water column dissolved oxygen and the formation of sulfidic surface sediments in Lake Constance associated with an episode of eutrophication (1955–1991). We conclude that magnetofossils may serve as a sensitive geological proxy to reconstruct dynamic redox-changes along the sediment-water interface and bottom waters.

The oriented chains of nanoscale Fe-oxide particles produced by magnetotactic bacteria are a striking example of biomineralization. Several distinguishing features of magnetite particles that comprise bacterial magnetosomes have been proposed to collectively constitute a biosignature of magnetotactic bacteria (Thomas-Keprta et al., 2001). These features include high crystallinity, chemical purity, a single-domain magnetic structure, well-defined crystal morphology, and arrangement of particles in chain structures. Here, we show that magnetite derived from the inorganic breakdown of nanocrystalline goethite exhibits magnetic properties and morphologies remarkably similar to those of biogenic magnetite from magnetosomes. During heating in reducing conditions, oriented nanogoethite aggregates undergo dehydroxylation and transform into stoichiometric magnetite. We demonstrate that highly crystalline single-domain magnetite with euhedral grain morphologies produced abiogenically from goethite meets several of the biogenicity criteria commonly used for the identification of magnetofossils. Furthermore, the suboxic conditions necessary for magnetofossil preservation in sediments are conducive to the reductive alteration of nanogoethite, as well as the preservation of detrital magnetite originally formed from goethite. The findings of this study have potential implications for the identification of biogenic magnetite, particularly in older sediments where diagenesis commonly disrupts the chain structure of magnetosomes. Our results indicate that isolated magnetofossils cannot be positively distinguished from inorganic magnetite on the basis of their magnetic properties and morphology, and that intact chain structures remain the only reliable distinguishing feature of fossil magnetosomes., This work was supported by the “Agence nationale de la recherche” (France) under project 2010-BLAN-604-01. Dennis Kent and Joshua Feinberg are thanked for their constructive reviews. This is IPGP contribution 3826., Final draft post-review

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.

Magnetotactic bacteria (MTB) produce chains of intracellular magnetite and/or greigite crystals and respond to an ambient magnetic field. MTB are considered to be microaerophilic to anaerobic organisms that live at and below the oxic-anoxic transition zone of aquatic environments. On the basis of rock magnetic analyses, including first-order reversal curve diagrams and isothermal remanent magnetization component analyses, along with transmission electron microscopy, we demonstrate that bacterial magnetites (magnetofossils) dominate magnetic mineral assemblages throughout a 76 m thickness of red clay at Integrated Ocean Drilling Program Site U1365 in the South Pacific Gyre, as well as in subsurface red clay of the North Pacific Gyre, where the sediment column contains abundant dissolved oxygen and no oxic-anoxic transition zone exists. This implies that MTB inhabit red clay; this conflicts with widespread interpretations of MTB ecology, namely that they are microaerophilic, requiring low levels of oxygen to grow and produce magnetite, and that magnetotaxis is used to help them find optimal positions in a strong vertical chemical gradient. Most magnetofossils in the red clay have cubo-octahedral morphology. This supports the notion that magnetofossil morphology can be a paleoenvironmental indicator; the proportion of elongated magnetofossils increases in less oxic environments. Our results also have implications for red-clay paleomagnetism in that magnetofossils may cause much-delayed remanence acquisition if MTB can live at decimeter depths within red clay.

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.

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.

Magnetotactic bacteria contain chains of magnetically interacting crystals (magnetosome crystals), which they use for navigation (magnetotaxis). To improve magnetotaxis efficiency, the magnetosome crystals (usually magnetite or greigite in composition) should be magnetically stable single-domain (SSD) particles. Smaller single-domain particles become magnetically unstable owing to thermal fluctuations and are termed superparamagnetic (SP). Previous calculations for the SSD/SP threshold size or blocking volume did not include the contribution of magnetic interactions. In this study, the blocking volume has been calculated as a function of grain elongation and separation for chains of identical magnetite grains. The inclusion of magnetic interactions was found to decrease the blocking volume, thereby increasing the range of SSD behaviour. Combining the results with previously published calculations for the SSD to multidomain threshold size in chains of magnetite reveals that interactions significantly increase the SSD range. We argue that chains of interacting magnetosome crystals found in magnetotactic bacteria have used this effect to improve magnetotaxis.

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

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.

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.

International audience; There are longstanding and ongoing controversies about the abiotic or biological origin of nanocrystals of magnetite. On Earth, magne-totactic bacteria perform biomineralization of intracellular magne-tite nanoparticles under a controlled pathway. These bacteria are ubiquitous in modern natural environments. However, their identification in ancient geological material remains challenging. Together with physical and mineralogical properties, the chemical composition of magnetite was proposed as a promising tracer for bacterial magnetofossil identification, but this had never been explored quantitatively and systematically for many trace elements. Here, we determine the incorporation of 34 trace elements in magnetite in both cases of abiotic aqueous precipitation and of production by the magnetotactic bacterium Magnetospirillum magneticum strain AMB-1. We show that, in biomagnetite, most elements are at least 100 times less concentrated than in abiotic magnetite and we provide a quantitative pattern of this depletion. Furthermore, we propose a previously unidentified method based on strontium and calcium incorporation to identify magnetite produced by magnetotactic bacteria in the geological record. magnetotactic bacteria | magnetite | biomineralization | trace element incorporation

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 δFC / δZFC 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.

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.

Natural sediments are a complex mixture of magnetic minerals with different origins and different geochemical history, each of which is called a magnetic component. Magnetic components practically never occur in isolated form, and their characterization using bulk magnetic measurements relies on the individuation of the systematic variation of some parameters within a large group of samples. These variations can be interpreted either as a mixing trend or as the result of natural processes, which affect the physical and chemical properties of the magnetic particles. An alternative approach is offered by the analysis of magnetization curves using model functions, which are supposed to represent the magnetic properties of individual components. The success of this approach relies on (1) the choice of model functions that can reproduce the natural properties of a component with sufficient accuracy by varying a minimum number of parameters and (2) on very precise and accurate measurements, which are necessary to overcome the extreme sensitivity of the method to noise. In this paper, the analysis of remanent magnetization curves proposed by Egli (2003) is applied to a large set of representative sediments from the most variable environments and to a set of artificial magnetite samples. Despite the variety of materials and natural processes involved in the formation of these sediments, seven groups of magnetic components with well-defined and consistent properties could be identified. It has been found that both lacustrine and marine sediments contain two magnetically distinct groups of magnetosomes, which react differently to changes of the redox potential. The effects of some natural processes, such as weathering, reductive dissolution and transport could be observed on the individual components.

The characterization of individual magnetic components in sediments and sedimentary rocks is difficult, as these natural materials are often a complex mixture of magnetic mineral sources. The analysis of magnetization curves with model functions is the only practicable method of unmixing the magnetic components and characterizing their magnetic properties, if a priori information is not available. Unfortunately, such analyses rely on time consuming measurements and on the choice of appropriate model functions. The simplification of the unmixing problem is closely related to the number of parameters required to fully characterize a magnetic component, and the significance of these parameters in rock magnetic terms. A systematic analysis of synthetic and natural samples shows that a combination of four parameters, here called magnetic fingerprints, is sufficient for this purpose. The fingerprints of individual magnetic components isolated from a wide range of natural sediments and sedimentary rocks form well-defined groups with specific properties. These groups reflect common processes of formation, transport and dissolution of magnetic particles. A clear distinction can be made between two sorts of biogenic magnetite, atmospheric dust, urban pollution and ultrafine magnetite produced in soils and lacustrine/marine sediments. � 2004 Elsevier Ltd. All rights reserved.

The analysis of magnetization curves with model functions has been proposed independently in several works as a method to unmix and characterize magnetic minerals assemblages in sediments and sedimentary rocks. Unfortunately, a successful result of such analysis relies on time-consuming measurements and on the choice of appropriate model functions. However, once the magnetic properties of individual magnetic components have been determined on a small set of selected samples, a simpler and faster analysis of similar samples is possible. The fast analysis of a large number of samples allows investigation of the effect of natural processes on the properties of single magnetic components. The simplification of the unmixing problem proposed in this paper is based on an iterative linearization procedure, which considers the variability of magnetic components. Any simplification of the unmixing problem is limited by a minimum number of parameters, which are required to fully characterize a magnetic component. It has been shown that a combination of four parameters, so-called magnetic fingerprints, is sufficient for a complete characterization of the remanent magnetization of a component. The usefulness of magnetic fingerprints in tracking natural processes is demonstrated exemplarily for lake sediments. The response of Baldeggersee (Switzerland) to environmental changes has been investigated, with special regard to the role of bacterial magnetite in the iron cycle and its possible use as a sensitive paleoredox indicator. The relation between the magnetic properties of lake sediments on one hand, and climatic, tectonic- and human-driven environmental changes on the other, is strongly non-linear. Therefore, a classic correlation between so-called magnetic proxies and environmental signals should be considered with care.

Of the two nanocrystal (magnetosome) compositions biosynthesized by magnetotactic bacteria (MTB), the magnetic properties of magnetite magnetosomes have been extensively studied using widely available cultures, while those of greigite magnetosomes remain poorly known. Here we have collected uncultivated magnetite- and greigite-producing MTB to determine their magnetic coercivity distribution and ferromagnetic resonance (FMR) spectra and to assess the MTB-associated iron flux. We find that compared with magnetite-producing MTB cultures, FMR spectra of uncultivated MTB are characterized by a wider empirical parameter range, thus complicating the use of FMR for fossilized magnetosome (magnetofossil) detection. Furthermore, in stark contrast to putative Neogene greigite magnetofossil records, the coercivity distributions for greigite-producing MTB are fundamentally left-skewed with a lower median. Lastly, a comparison between the MTB-associated iron flux in the investigated estuary and the pyritic-Fe flux in the Black Sea suggests MTB play an important, but heretofore overlooked role in euxinic marine system iron cycle.

We studied intracellular magnetite particles produced by several morphological types of magnetotactic bacteria including the spirillar (helical) freshwater species, Magnetospirillum magnetotacticum, and four incompletely characterized marine strains: MV-1, a curved rod-shaped bacterium; MC-1 and MC-2, two coccoid (spherical) microorganisms; and MV-4, a spirillum. Particle morphologies, size distributions, and structural features were examined using conventional and high-resolution transmission electron microscopy. The various strains produce crystals with characteristic shapes. All habits can be derived from various combinations of the isometric {111}, {110}, and {100} forms. We compared the size and shape distributions of crystals from magnetotactic bacteria with those of synthetic magnetite grains of similar size and found the biogenic and synthetic distributions to be statistically distinguishable. In particular, the size distributions of the bacterial magnetite crystals are narrower and have a distribution asymmetry that is the opposite of the nonbiogenic sample. The only deviation from ideal structure in the bacterial magnetite seems to be the occurrence of spinel-law twins. Sparse multiple twins were also observed. Because the synthetic magnetite crystals contain twins similar to those in bacteria, in the absence of characteristic chains of crystals, only the size and shape distributions seem to be useful for distinguishing bacterial from nonbiogenic magnetite.

Core CHAT 1K from 3556 m depth east of New Zealand shows marked changes in magnetic susceptibility, correlating very well with δ18O of foraminifera over the last 150 ka. Susceptibility is high during interglacial stages 1 and 5, and substages 5a to 5e are well marked. The susceptibility is shown to be enhanced during interglacials and is neither an artefact of glacial carbonate dilution nor due to an interglacial increase of terrigenous material. Mineral magnetic property measurements (ARM, SIRM, S ratio) also show the same well-defined glacial–interglacial variability, interpreted as an interglacial increase of single domain magnetite grains less than 0.1 μm in size. Examination by TEM of magnetic extracts reveals abundant cubic to octahedral grains of 0.02–0.2 μm size, often arranged in long chains characteristic of bacterial magnetite. This material is much more abundant in interglacial than glacial samples. It is suggested that during interglacials, with lower productivity (and organic carbon input to the bed), the oxic zone in the sediments which can be colonised by magnetotactic bacteria is thicker and therefore is colonised for longer by magnetite producers. This yields a climate-controlled stratigraphy which leads that of infaunal benthic foraminifera by no more than a few centimetres.

Fresh fracture surfaces of the martian meteorite ALH84001 contain abundant polycyclic aromatic hydrocarbons (PAHs). These fresh fracture surfaces also display carbonate globules. Contamination studies suggest that the PAHs are indigenous to the meteorite. High-resolution scanning and transmission electron microscopy study of surface textures and internal structures of selected carbonate globules show that the globules contain fine-grained, secondary phases of single-domain magnetite and iron sulfides. The carbonate globules are similar in texture and size to some terrestrial bacterially induced carbonate precipitates. Although inorganic formation is possible, formation of the globules by biogenic processes could explain many of the observed features, including the PAHs. The PAHs, the carbonate globules, and their associated secondary mineral phases and textures could thus be fossil remains of a past martian biota.

[1] Single-domain magnetite particles produced by magnetotactic bacteria (MTB) and aligned in chains, called magnetosomes, are potentially important recorders of paleomagnetic, paleoenvironmental and paleolife signals. Rock magnetic properties related to the anisotropy of magnetosome chains have been widely used to identify fossilized magnetosomes (magnetofossils) preserved in geological materials. However, ambiguities exist when linking magnetic properties to the chain structure because of the complexity of chain integrity and magnetostatic interactions among magnetofossils that results from chain collapse during post-depositional diagenesis. In this paper, magnetic properties of three sets of samples containing extracted magnetosomes of the culturedMagnetospirillum magneticumstrain AMB-1 were analyzed to determine how chain integrity and particle concentration influence magnetic properties. Intact MTB and well-dispersed magnetosome chains are characterized by strong magnetic anisotropy and weak magnetostatic interactions, but progressive chain breakup and particle clumping significantly increase the degree of magnetostatic interaction. This results in a change of the magnetic signature toward properties typical of interacting, single-domain particles, i.e., a decrease of the ratio of anhysteretic remanent magnetization to the saturation isothermal remanent magnetization, decreasing in the crossing point of the Wohlfarth-Cisowski test and in the delta ratio between losses of field and zero-field cooled remanent magnetization across the Verwey transition, as well as vertical broadening of the first-order reversal curve distribution. We propose a new diagram that summarizes the Verwey transition properties, with diagnostic limits for intact and collapsed chains of magnetosomes. This diagram can be used, in conjunction with other parameters, to identify unoxidized magnetofossils in sediments and rocks.

We describe pulsed-magnetic-field remanence measurements of individual, killed, undisrupted cells of three different types of magnetotactic bacteria. The measurement technique involved the observation of aligned, individual magnetotactic bacteria with a light microscope as they were subjected to magnetic pulses of increasing amplitude. We show that for MM cells, the hysteresis loop is square, with the coercive field variable from cell to cell. This is consistent with just two magnetization states for the single chain of magnetite particles. For MR and MMP cells, the hysteresis loops are not square, indicating that there are several different magnetization states, and that individual cells can be demagnetized. The coercive fields in the MR and MMP cells are less variable than for the MM cells.

Magnetotactic bacteria biomineralize magnetic minerals with precisely controlled size, morphology, and stoichiometry. These cosmopolitan bacteria are widely observed in aquatic environments. If preserved after burial, the inorganic remains of magnetotactic bacteria act as magnetofossils that record ancient geomagnetic field variations. They also have potential to provide paleoenvironmental information. In contrast to conventional magnetofossils, giant magnetofossils (most likely produced by eukaryotic organisms) have only been reported once before from Paleocene-Eocene Thermal Maximum (PETM; 55.8 Ma) sediments on the New Jersey coastal plain. Here, using transmission electron microscopic observations, we present evidence for abundant giant magnetofossils, including previously reported elongated prisms and spindles, and new giant bullet-shaped magnetite crystals, in the Southern Ocean near Antarctica, not only during the PETM, but also shortly before and after the PETM. Moreover, we have discovered giant bullet-shaped magnetite crystals from the equatorial Indian Ocean during the Mid-Eocene Climatic Optimum (∼40 Ma). Our results indicate a more widespread geographic, environmental, and temporal distribution of giant magnetofossils in the geological record with a link to “hyperthermal” events. Enhanced global weathering during hyperthermals, and expanded suboxic diagenetic environments, probably provided more bioavailable iron that enabled biomineralization of giant magnetofossils. Our micromagnetic modelling indicates the presence of magnetic multi-domain (i.e., not ideal for navigation) and single domain (i.e., ideal for navigation) structures in the giant magnetite particles depending on their size, morphology and spatial arrangement. Different giant magnetite crystal morphologies appear to have had different biological functions, including magnetotaxis and other non-navigational purposes. Our observations suggest that hyperthermals provided ideal conditions for giant magnetofossils, and that these organisms were globally distributed. Much more work is needed to understand the interplay between magnetofossil morphology, climate, nutrient availability, and environmental variability, School of Ocean and Earth Science, National Oceanography Centre, University of Southampton, Reino Unido, Research School of Earth Sciences, The Australian National University, Australia, Grant Institute of Earth Science, University of Edinburgh, Reino Unido, Unidad de Zaragoza, Instituto Geológico y Minero de España, España, Instituto Oceanográfico da Universidade de São Paulo, Brasil, Department of Earth Science and Engineering, Imperial College London, Reino Unido

[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 1–15μm 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.

Sediments from the Tasman Basin in the southwest Pacific Ocean show a cyclic pattern of variation in mineral magnetic properties, including susceptibility and remanence, over the Brunhes Chron. The magnetic pattern is dominated by fluctuations in the concentration of a single domain ferrimagnetic component identified as bacterial magnetofossils by TEM investigation of the magnetic fraction. The intervals of low bacterial magnetite concentration, occurring in glacial periods, are accompanied by a change of the bacterial magnetosome morphology assemblage interpreted as recording a change in bacterial species assemblage. There is no evidence of iron or sulphate reducing conditions during low bacterial magnetite intervals. Concentrations of bacterial magnetite are low when oxygen concentrations in surface sediment pore waters dropped, either as the result of variation in bottom water oxygen concentration or sedimentation rate. The variations in the magnetic signal are therefore interpreted as resulting primarily from bacterial palaeoecological control rather than dissolution or alteration. The bacteria in this deep sea environment are apparently aerophilic and grow near the sediment surface within the oxic zone, implying that magnetic remanence is also acquired near the surface but up to tens of centimetres below where the oxic zone is very thick.

Magnetite (Fe$\_{3}$O$\_{4}$) crystals produced by two strains of cultured vibrioid magnetotactic bacteria were studied by high-resolution transmission electron microscopy (HRTEM). Both magnetotactic strains were characterized by single chains of magnetite crystals aligned along the long axes of the cells. The strains, designated as MV-2 and MV-4, produced crystals that differed markedly in size and morphology. Crystals present in MV-4 cells were generally larger than those in MV-2 cells and displayed significantly smaller aspect ratios. Crystallographic analysis of the magnetosomes of MV-2 revealed an elongated hexagonal habit based on a prism of {110} faces capped by {111} faces. The axis of elongation was parallel to the direction. This morphology closely resembles the crystal shape of magnetosomes in a previously described vibrioid species MV-1. In contrast, magnetosomes of MV-4 possessed a cubo-octahedral morphology which was modified by a small elongation along the direction. Although this morphology has not previously been observed in magnetotactic bacteria, it appears to be intermediate between the regular cubo-octahedral shape of magnetosomes in the cultured species Aquaspirillum magnetotacticum and the extensively elongated cubo-octahedral crystals of a previously studied uncultured ovoid-shaped magnetotactic bacterium. The results support the proposal that the crystal morphologies of bacterial magnetite are strain specific.

We report the discovery of exceptionally large biogenic magnetite crystals in clay-rich sediments spanning the Paleocene–Eocene Thermal Maximum (PETM) in a borehole at Ancora, NJ. Aside from previously described abundant bacterial magnetofossils, electron microscopy reveals novel spearhead-like and spindle-like magnetite up to 4 μm long and hexaoctahedral prisms up to 1.4 μm long. Similar to magnetite produced by magnetotactic bacteria, these single-crystal particles exhibit chemical composition, lattice perfection, and oxygen isotopes consistent with an aquatic origin. Electron holography indicates single-domain magnetization despite their large crystal size. We suggest that the development of a thick suboxic zone with high iron bioavailability—a product of dramatic changes in weathering and sedimentation patterns driven by severe global warming—drove diversification of magnetite-forming organisms, likely including eukaryotes.

Intracellular crystals of magnetite synthesized by cells of the magnetotactic vibroid organism, MV-1, and extracellular crystals of magnetite produced by the non-magnetotactic dissimilatory iron-reducing bacterium strain GS-15, were examined using high-resolution transmission electron microscopy, electron diffraction and57Fe Mo¨ssbauer spectroscopy. The magnetotactic bacterium contained a single chain of approximately 10 crystals aligned along the long axis of the cell. The crystals were essentially pure stoichiometric magnetite. When viewed along the crystal long axis the particles had a hexagonal cross-section whereas side-on they appeared as rectangules or truncated rectangles of average dimension, 53 × 35 nm. These findings are explained in terms of a three-dimensional morphology comprising a hexagonal prism of 110 faces which are capped and truncated by 111 end faces. Electron diffraction and lattice imaging studies indicated that the particles were structurally well-defined single crystals. In contrast, magnetite particles produced by the strain, GS-15 were irregular in shape and had smaller mean dimensions (14 nm). Single crystals were imaged but these were not of high structural perfection. These results highlight the influence of intracellular control on the crystallochemical specificity of bacterial magnetites. The characterization of these crystals is important in aiding the identification of biogenic magnetic materials in paleomagnetism and in studies of sediment magnetization.

[1] The Paleocene-Eocene Thermal Maximum, which occurred � 55.5 Ma, was caused by a massive release of carbon, as indicated by an � 3% negative carbon isotope excursion recorded in the marine, atmospheric, and terrestrial reservoirs. One suggested source for the carbon, a cometary impactor, is based on the sudden appearance and high concentration of single-domain (SD) magnetite in Paleocene-Eocene (P-E) boundary cores from the North Atlantic continental margin. We evaluate the potential sources of SD magnetite at the P-E boundary by presenting new magnetic hysteresis, low-temperature magnetic remanence, and transmission electron microscopy data from the North Atlantic coastal ocean. Our results show a similar increase in SD material but demonstrate that the magnetic material has a biogenic origin. These findings indicate that the high concentrations of SD magnetite immediately above the P-E boundary are the result of unusual accumulations and/or preservation of magnetotactic bacteria. Such bacteria typically occupy the oxic-anoxic transition zone near the sediment-water interface or in the water column. The high abundances of SD magnetite in sediments from across the shelf may be an artifact of nonsteady state redox conditions and exceptional preservation of SD magnetite. It may also indicate that the oxic-anoxic redox boundary shifted into the water column. The latter explanation implies transient eutrophy of the coastal ocean in this region, most likely due to seasonally enhanced runoff, and increased stratification and nutrient loading.

Previous workers identified a magnetically anomalous clay layer deposited on the northern United States Atlantic Coastal Plain during the Paleocene-Eocene Thermal Maximum (PETM). The finding inspired the highly controversial hypothesis that a cometary impact triggered the PETM. Here we present ferromagnetic resonance (FMR), isothermal and anhysteretic remanent magnetization, first order reversal curve, and transmission electron microscopy analyses of late Paleocene and early Eocene sediments in drillcore from Ancora, New Jersey. A novel paleogeographic analysis applying a recent paleomagnetic pole from the Faeroe Islands indicates that New Jersey during the initial Eocene had a ~6-9 degrees lower paleolatitude (~27.3 degrees for Ancora) and a more zonal shoreline trace than in conventional reconstructions. Our investigations of the PETM clay from Ancora reveal abundant magnetite nanoparticles bearing signature traits of crystals produced by magnetotactic bacteria. This result, the first identification of ancient biogenic magnetite using FMR, argues that the anomalous magnetic properties of the PETM sediments are not produced by an impact. They instead reflect environmental changes along the eastern margin of North America during the PETM that led to enhanced production and/or preservation of magnetofossils.

Carbonate muds deposited on continental shelves are abundant and well-preserved throughout the geologic record because shelf strata are difficult to subduct and peritidal carbonate units often form thick, rheologically strong units that resist penetrative deformation. Much of what we know about pre-Mesozoic ocean chemistry, carbon cycling, and global change is derived from isotope and trace element geochemistry of platform carbonates. Paleomagnetic data from the same sediments would be invaluable, placing records of paleolatitude, paleogeography, and perturbations to the geomagnetic field in the context and relative chronology of chemostratigraphy. To investigate the depositional and early diagenetic processes that contribute to magneitzation in carbonates, we surveyed over 500 core and surface samples of peritidal, often microbially bound carbonate muds spanning the last not, vert, similar 1000 yr and deposited on top of Pleistocene aeolianites in the Triple Goose Creek region of northwest Andros Island, Bahamas. Sedimentological, geochemical, magnetic and ferromagnetic resonance properties divide the sediment columns into three biogeochemical zones. In the upper sediments, the dominant magnetic mineral is magnetite, produced by magnetotactic bacteria and dissimiliatory microbial iron metabolism. At lower depths, above or near mean tide level, microbial iron reduction dissolves most of the magnetic particles in the sediment. In some cores, magnetic iron sulfides precipitate in a bottom zone of sulfate reduction, likely coupled to the oxidation of decaying mangrove roots. The remanent magnetization preserved in all oriented samples appears indistinguishable from the modern local geomagnetic field, which reflects the post-depositional origin of magnetic particles in the lower zone of the parasequence. While we cannot comment on the effects of late-stage diagenesis or metamorphism on remanence in carbonates, we postulate that early-cemented, thin-laminated parasequence tops in ancient peritidal carbonates are mostly likely to preserve syn-depositional paleomagnetic directions and magnetofossil stratigraphies.

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.

Distinct magnetic properties of marine sediments that record the Palaeocene–Eocene thermal maximum (PETM) have been suggested to be due to a bacterial magnetofossil signal that is linked to enhanced weathering conditions during the PETM. We document the dominance of bacterial magnetite in deep-sea sediments from southern Kerguelen Plateau (Ocean Drilling Program Hole 738C, southern Ocean) not only during the PETM, but also before and after the thermal event. This occurrence of magnetofossils throughout the PETM indicates that the occurrence of bacterial magnetosomes is not due to a preservation effect. Instead, we suggest that it is due to sustained mild iron-reducing conditions that dissolved the most labile aeolian-derived iron, which favoured continued magnetotactic bacterial activity without being strong enough to dissolve the less reactive magnetite and haematite. Enhanced aeolian haematite abundances at the beginning of the PETM indicate drier conditions on the neighbouring Antarctic continent at those times. Our results provide evidence that iron fertilisation by aeolian dust was the main limiting factor that conditioned proliferation of magnetotactic bacteria in the deep sea at the southern Kerguelen Plateau, with the exception of two short periods of rapidly changing palaeoenvironmental conditions at the onset and termination of the PETM. Increased iron supply from aeolian dust, that enhanced oceanic primary productivity and subsequent delivery of organic carbon to the seafloor, along with mild iron-reducing diagenetic conditions, seem to have been necessary to provide the iron needed for magnetite biomineralization by magnetotactic bacteria to drive their marked increase in abundance in the studied PETM record from southern Kerguelen Plateau. Our analyses of a deep-sea PETM record from Hole 1051B at Blake Nose (Atlantic Ocean) failed to identify magnetofossils despite evidence for the occurrence of magnetite and haematite of probable aeolian origin. Contrasting magnetic properties at these PETM sections indicate that further work is needed to understand the palaeoenvironmental and diagenetic factors whose interactions lead to production and preservation of magnetofossils in deep-sea sediments, Unidad de Zaragoza, Instituto Geológico y Minero de España, España, Research School of Earth Sciences, The Australian National University, Canberra, Australia, School of Ocean and Earth Science, National Oceanography Centre, University of Southampton, Reino Unido, Palaeomagnetic Laboratory ‘Fort Hoofddijk’, Institute of Earth Sciences, Universiteit Utrecht, Paises Bajos, Department of Geological Sciences, San Diego State University, Estados Unidos, Scripps Institution of Oceanography, University of California, Estados Unidos, Department of Earth and Planetary Sciences, University of California, Estados Unidos

Electron microscopic studies on magnetosomes in magnetotactic bacteria have revealed much information on their composition, structure, and even the formation of their mineral phase. The mineral phases of the magnetosomes are of two general types: iron oxides and iron sulfides. Iron oxide-type magnetosomes contain particles of the ferrimagnetic mineral magnetite (Fe3O4) while the iron sulfide-type contain ferrimagnetic greigite (Fe3S4), greigite and non-magnetic pyrite (FeS2), or possibly ferrimagnetic pyrrhotite (Fe7S8). Regardless of their composition, the crystalline particles in magnetosomes have a narrow size range: approximately 35 to 120 nm. Magnetite crystals in this size range are single-magnetic-domains and confer a permanent magnetic dipole moment to the cell. The single-domain size range for greigite is not known but is probably similar to that for magnetite. The morphology of the particles in the bacterial magnetosomes appears to be species-specific. Morphologies of magnetite crystals in different species of magnetotactic bacteria include cubo-octahedra, parallelepipedal (truncated hexahedral or octahedral prisms), and tooth- or bullet-shaped (anisotropic). Morphologies of greigite particles include cubo-octahedra and rectangular prismatic. The greigite-pyrite particles are generally pleomorphic with no consistent crystalline morphology. A membrane has been shown to surround the particles in some organisms and may be involved in the formation of the crystalline phase while also providing physical constraints on the size and the shape of the crystal. These results clearly indicate that the biomineralization process involved in the bacterial magnetosome, a good example of a self-assembled structure on a nanometer scale, is highly controlled by the organism.

Magnetotactic bacteria produce chains of magnetite crystals within a cell. Bacterial magnetites have characteristic morphologies and sizes that are strictly biologically controlled. We examined morphologies of fossil bacterial magnetites (magnetofossils) preserved in Pacific deep-sea sediments and their relations to organic carbon fluxes. Isotropic crystals dominate magnetofossils in sediments in relatively oxidized conditions, and anisotropic crystals predominate in more reduced conditions. Our finding has important implications for biomineralization processes and demonstrates the potential of magnetofossil morphology as a paleoenvironmental indicator.

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.

Concentrations of magnetite were determined, with depth, in sediments of varying H2S content taken from Long Island Sound (FOAM, NWC, and Sachem sites) and from a site in the rapidly depositing subaqueous portion of the Mississippi Delta. For the three Long Island Sound sites, dissolution of magnetite during burial is clearly demonstrated, and for all four sites the rate of magnetite dissolution is proportional to the concentration of dissolved pore water sulfide. By combining magnetite depth distributions with sedimentation rate data and measurements of dissolved sulfide, we find that magnetite dissolution is well described by the following rate law: dCmagdt = -1.1× 10−5C0.5BCmagAmag where Cmag is the concentration of magnetite (grams per gram of sediment), Cs is the concentration of dissolved sulfide (mM), and Amag is the specific surface area of magnetite in the sediment (cm2 per gram of magnetite.) This equation means that, for a typical range of magnetite surface areas and dissolved sulfide concentrations, the “half-life” of magnetite in anoxic marine sediments ranges from about 50 to 1000 years. SEM observations of magnetite grains from the different sediment sites reveal that magnetite dissolution may be accompanied by extensive replacement by pyrite. This only occurs if magnetite is in contact with high concentrations of H2S (>1 mM) for relatively long periods of time (several hundred years). With low concentrations of H2S (

Magnetite (Fe_3O_4) is one of the most stable carriers of natural remanent magnetization (NRM) in sedimentary rocks, and paleomagnetic studies of magnetite-bearing sediments, such as deep-sea cores and pelagic limestones, have provided a detailed calibration between the biostratigraphic and magnetic polarity time scales. Despite this important role, there is as yet a very poor understanding of how ultrafine-grained (< 0.1 µm) magnetite is formed, transported, and preserved in marine environments. A major conceptual advance in our understanding of these processes is the recent discovery that biogenic magnetite, formed by magnetotactic bacteria and/or other magnetite-precipitating organisms, is responsible for much of the stable magnetic remanence in many marine sediments and sedimentary rocks. Since these magnetite particles are of biogenic origin, they are termed properly magnetofossils (Kirschvink & Chang 1984).

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.

MAGNETOFOSSILS1, the fossil remains of bacterial magneto-somes2, are found in various deep-sea sediments, and have been linked to the preservation of stable natural remanent magnetization in many of them1,3–5. They have also been extracted and identified from lithified carbonates of Jurassic5 and Precambrian6 age, showing that magnetotactic bacteria have been a sedimentary source of fine-grained magnetite for much of geological time. Some clay-rich deep-sea sediments, however, do not record a stable remanent magnetization, for unknown reasons. Here we report results from a high-resolution transmission-electron-microscope study on samples of this type collected by the Deep Sea Drilling Project (DSDP); the material contains a complex mixture of single-domain magnetic minerals. Well-preserved magnetofossils show the same crystal structures as those found in magnetosomes from recent bacteria7–10, whereas others in these same preparations display a wide range of dissolution, corrosion and aggregation effects. Rock magnetic measurements are consistent with the presence of these alterations in many DSDP sediments, including those that preserve reliable palaeomagnetic directions. As we were unable to fina authigenic magnetic minerals in our sample, and as the magnetic fraction is dominated by well preserved magnetofossils, we suggest that the poor preservation of the magnetization is a result of diagenetic interactions between the magnetofossils and the clay minerals in the matrix.

Magnetite precipitation and tooth formation in chitons (Polyplacophora) proceeds through a biochemically-controlled reduction of the mineral ferrihydrite. Resulting crystals of single-domain magnetite are closely packed against one another and are typically near 0.1 μm in diameter. The natural magnetization of these teeth is characterized by abnormally low stability to alternating field demagnetization (m.d.f. near 12 mT) but has no appreciable decay due to low-temperature cycling. Chitons may be responsible for natural magnetizations on the order of 10 −6 G in marine sediments, whereas mud bacteria could produce remanence near 10 −8 G in both marine and freshwater sediments.

This paper reports on the magnetic properties of magnetosomes in the freshwater magnetotactic bacterium Aquaspirillum magnetotacticum . The magnetosomes are well crystallized particles of magnetite with dimensions of 40 to 50 nm, which are arranged within the cells in a single linear chain and are within the single-magnetic-domain (SD) size range for magnetite. A variety of magnetic properties have been measured for two samples of dispersions of freeze-dried cells consisting of (1) whole cells (M-1) and (2) magnetosomes chains separated from cells (M-2). An important result is that the acquisition and demagnetization of various type of remanent magnetizations are markedly different for the two samples and suggest that remanence is substantially affected by magnetostatic interactions. Interactions are likely to be much more important in M-2 because the extracted magnetosome chains are no longer separated from one another by the cell membrane and cytoplasm. Other experimental data for whole cells agree with predictions based on the chain of spheres model for magnetization reversal. This model is consistent with the unique linear arrangement of equidimensional particles in A. magnetotacticum . The magnetic properties of bacterial and synthetic magnetites are compared and the paleomagnetic implications are discussed.