Microbially assisted recording of the Earth's magnetic field in sediment

Sediments continuously record variations of the Earth's magnetic field and thus provide an important archive for studying the geodynamo. The recording process occurs as magnetic grains partially align with the geomagnetic field during and after sediment deposition, generating a depositional remanent magnetization (DRM) or post-DRM (PDRM). (P) DRM acquisition mechanisms have been investigated for over 50 years, yet many aspects remain unclear. A key issue concerns the controversial role of bioturbation, that is, the mechanical disturbance of sediment by benthic organisms, during PDRM acquisition. A recent theory on bioturbation-driven PDRM appears to solve many inconsistencies between laboratory experiments and palaeomagnetic records, yet it lacks experimental proof. Here we fill this gap by documenting the important role of bioturbation-induced rotational diffusion for (P) DRM acquisition, including the control exerted on the recorded inclination and intensity, as determined by the equilibrium between aligning and perturbing torques acting on magnetic particles

Microbially assisted recording of the Earth's magnetic field in sediment

Sediments continuously record variations of the Earth's magnetic field and thus provide an important archive for studying the geodynamo. The recording process occurs as magnetic grains partially align with the geomagnetic field during and after sediment deposition, generating a depositional remanent magnetization (DRM) or post-DRM (PDRM). (P) DRM acquisition mechanisms have been investigated for over 50 years, yet many aspects remain unclear. A key issue concerns the controversial role of bioturbation, that is, the mechanical disturbance of sediment by benthic organisms, during PDRM acquisition. A recent theory on bioturbation-driven PDRM appears to solve many inconsistencies between laboratory experiments and palaeomagnetic records, yet it lacks experimental proof. Here we fill this gap by documenting the important role of bioturbation-induced rotational diffusion for (P) DRM acquisition, including the control exerted on the recorded inclination and intensity, as determined by the equilibrium between aligning and perturbing torques acting on magnetic particles

Paleomagnetic evidence for rapid vertical-axis rotation in the Peruvian Cordillera ca. 8 Ma

Paleomagnetic results from 31 Neogene sites in the Peruvian Andes yield primary magnetizations, as demonstrated by positive fold and reversal tests. Strata dated as 18–9 Ma record a significant counterclockwise rotation (−11° ± 5°), whereas unconformably overlying younger strata (7–6 Ma) are not rotated. The age of rotation thus is between 9 and 7 Ma, a period that coincides with the widespread Quechua 2 deformation phase. Moreover, eight independent studies on 107–9 Ma rocks from Peru between 9°S and 15°S reveal similar and significant rotations (−15° ± 6°). This suggests that the region rotated during a 2 m.y. period of deformation ca. 8 Ma, when the Andes underwent rapid uplift and important deformation commenced in the Subandean zone

Anatomy of a pressure-induced, ferromagnetic-to-paramagnetic transition in pyrrhotite: Implications for the formation pressure of diamonds

Meteorites and diamonds encounter high pressures during their formation or subsequent evolution. These materials commonly contain magnetic inclusions of pyrrhotite. Because magnetic properties are sensitive to strain, pyrrhotite can potentially record the shock or formation pressures of its host. Moreover, pyrrhotite undergoes a pressure-induced phase transition between 1.6 and 6.2 GPa, but the magnetic signature of this transition is poorly known. Here we report room temperature magnetic measurements on multidomain and single-domain pyrrhotite under nonhydrostatic pressure. Magnetic remanence in single-domain pyrrhotite is largely insensitive to pressure until 2 GPa, whereas the remanence of multidomain pyrrhotite increases 50\% over that of initial conditions by 2 GPa, and then decreases until only 33\% of the original remanence remains by 4.5 GPa. In contrast, magnetic coercivity increases with increasing pressure to 4.5 GPa. Below ∼1.5 GPa, multidomain pyrrhotite obeys Néel theory with a positive correlation between coercivity and remanence; above ∼1.5 GPa, it behaves single domain-like yet distinctly different from uncompressed single-domain pyrrhotite. The ratio of magnetic coercivity and remanence follows a logarithmic law with respect to pressure, which can potentially be used as a geobarometer. Owing to the greater thermal expansion of pyrrhotite with respect to diamond, pyrrhotite inclusions in diamonds experience a confining pressure at Earth’s surface. Applying our experimentally derived magnetic geobarometer to pyrrhotite-bearing diamonds from Botswana and the Central African Republic suggests the pressures of the pyrrhotite inclusions in the diamonds range from 1.3 to 2.1 GPa. These overpressures constrain the mantle source pressures from 5.4 to 9.5 GPa, depending on which bulk modulus and thermal expansion coefficients of the two phases are used

Distribution of magnetic remanence carriers in the human brain

That the human brain contains magnetite is well established;however, its spatial distribution in the brain has remained unknown. We present room temperature, remanent magnetization measurements on 822 specimens from seven dissected whole human brains in order to systematically map concentrations of magnetic remanence carriers. Median saturation remanent magnetizations from the cerebellum were approximately twice as high as those from the cerebral cortex in all seven cases (statistically significantly distinct, p = 0.016). Brain stems were over two times higher in magnetization on average than the cerebral cortex. The ventral (lowermost) horizontal layer of the cerebral cortex was consistently more magnetic than the average cerebral cortex in each of the seven studied cases. Although exceptions existed, the reproducible magnetization patterns lead us to conclude that magnetite is preferentially partitioned in the human brain, specifically in the cerebellum and brain stem

Evidence for coeval Late Triassic terrestrial impacts from the Rochechouart (France) meteorite crater

High temperature impact melt breccias from the Rochechouart (France) meteorite crater record magnetization component with antipodal, normal and reverse polarities. The corresponding paleomagnetic pole for this component lies between the 220 Ma and 210 Ma reference poles on the Eurasian apparent polar wander path, consistent with the 214 $\pm$ 8 Ma 40Ar/39Ar age of the crater. Late Triassic tectonic reconstructions of the Eurasian and North American plates place this pole within 95% confidence limits of the paleomagnetic pole from the Manicouagan (Canada) meteorite impact crater, which is dated at 214 $\pm$ 1 Ma. Together, these observations reinforce the hypothesis of a Late Triassic, multiple meteorite impact event on Earth

Greigite (Fe3S4) Formation in Artificial Sediments via Solid‐State Transformation of Lepidocrocite

Greigite (Fe3S4) is a ferrimagnetic iron‐sulfide mineral that forms in sediments during diagenesis. Greigite growth can occur diachronously within a stratigraphic profile, complicating or overprinting environmental and paleomagnetic records. An important objective for paleo‐ and rock‐magnetic studies is to identify the presence of greigite and to discern its formation conditions. Greigite detection remains, however, challenging and its magnetic properties obscure due to the lack of pure, stable material of well‐defined grain size. To overcome these limitations, we report a new method to selectively transform lepidocrocite to greigite via the intermediate phase mackinawite (FeS). In‐situ magnetic characterization was performed on discrete samples with different sediment substrates. Susceptibility and chemical remanent magnetization increased proportionally over time, defining two distinct greigite growth regimes. Temperature dependent and constant initial growth rates indicate a solid‐state FeS to greigite transformation with an activation energy of 78–90 kJ/mol. Low and room temperature magnetic remanence and coercivity ratios match with calculated mixing curves for superparamagnetic (SP) and single domain (SD) greigite and suggest ∼25% and ∼50% SD proportions at 300 and 100 K, respectively. The mixing trend coincides with empirical data reported for natural greigite‐bearing sediments, suggesting a common SP endmember size of 5–10 nm that is likely inherited from mackinawite crystallites. The average particle size of 20–50 nm determined by X‐ray powder diffraction and electron microscopy accords with theoretical predictions of the SP/SD threshold size in greigite. The method constitutes a novel approach to synthesize greigite and to investigate its formation in sediments.Plain Language Summary: Sediments provide continuous records of Earth's ancient magnetic field, which lend insights into the workings of the geodynamo and help to establish the geologic time scale through global magnetostratigraphic correlation. Greigite is a magnetic iron sulfide mineral that commonly forms after deposition, thereby remagnetizing the sediment and complicating interpretation of the magnetic record. Understanding greigite formation and detecting its presence is fundamental for obtaining reliable records of the paleomagnetic field, yet knowledge of how greigite grows and how its magnetic properties evolve during growth remains limited. This article outlines a novel approach to form greigite in sediments and to monitor its growth kinetics, grain size and magnetic remanence acquisition. The magnetic properties of the synthetic sediments resemble those of natural greigite‐bearing sediments and match well with theoretical calculations, which can help quantify grain sizes in sedimentary greigite. The reported method and our results contribute to a better understanding of greigite formation and chemical magnetic remanence acquisition in sediments.Key Points: We present a new method to grow greigite in aqueous sediments and create a chemical remanent magnetization under controlled conditions. Greigite grain sizes of 20–50 nm span the superparamagnetic to single domain threshold, consistent with theoretical predictions. Our experimental hysteresis data coincide with calculated mixing curves allowing better quantification of greigite particle sizes in nature.Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659https://doi.org/10.5281/zenodo.652165

Competition of magnetic and hydrodynamic forces on ellipsoidal particles under shear: Influence of the Earth's magnetic field on particle alignment in viscous media

International audience[1] We present a model that describes the rotation of ellipsoidal magnetic particles in a viscous fluid under the influence of hydrodynamic and magnetic forces, with an aim to better understand how sediments acquire their remanent magnetizations. Analyses of the governing equations elucidate how magnetic particles will rotate for different values of leading parameters including particle shape, remanent and induced magnetic intensity, magnetic field intensity and direction, strain rate, shear direction, and viscosity. Numerical solution of the governing equations makes it possible to visualize the rotation path and the magnetic direction of a particle through time. Thus the model can discern the timescales and trajectories of magnetic particles rotating due to torque of the magnetic field couple while simultaneously entrained in a velocity gradient. For example, in a layer of viscosity 10 4 Pa s, prolate magnetite starting at any initial orientation and subjected to simple shear with a strain rate of 3.17 Â 10 À8 s À1 needs 4 months to rotate within 3° of the Earth's field direction. Under the same conditions, hydrodynamic forces will govern the orientation of oblate hematite whose moment will be perpetually randomly oriented with respect to the magnetic field direction. When applied to laboratory experiments, the viscous model successfully matches the observed data, particularly after accounting for mechanical interaction and flocculation effects. Magnetic anisotropies calculated from multiparticle systems of hematite yield typical sedimentary fabrics with relatively low percentages of anisotropy (<5%) and maximum principal axes that lie in the sedimentation plane. Citation: Jezek, J., and S. A. Gilder (2006), Competition of magnetic and hydrodynamic forces on ellipsoidal particles under shear: Influence of the Earth's magnetic field on particle alignment in viscous media

Greigite (Fe3S4) Formation in Artificial Sediments via Solid‐State Transformation of Lepidocrocite

Abstract Greigite (Fe3S4) is a ferrimagnetic iron‐sulfide mineral that forms in sediments during diagenesis. Greigite growth can occur diachronously within a stratigraphic profile, complicating or overprinting environmental and paleomagnetic records. An important objective for paleo‐ and rock‐magnetic studies is to identify the presence of greigite and to discern its formation conditions. Greigite detection remains, however, challenging and its magnetic properties obscure due to the lack of pure, stable material of well‐defined grain size. To overcome these limitations, we report a new method to selectively transform lepidocrocite to greigite via the intermediate phase mackinawite (FeS). In‐situ magnetic characterization was performed on discrete samples with different sediment substrates. Susceptibility and chemical remanent magnetization increased proportionally over time, defining two distinct greigite growth regimes. Temperature dependent and constant initial growth rates indicate a solid‐state FeS to greigite transformation with an activation energy of 78–90 kJ/mol. Low and room temperature magnetic remanence and coercivity ratios match with calculated mixing curves for superparamagnetic (SP) and single domain (SD) greigite and suggest ∼25% and ∼50% SD proportions at 300 and 100 K, respectively. The mixing trend coincides with empirical data reported for natural greigite‐bearing sediments, suggesting a common SP endmember size of 5–10 nm that is likely inherited from mackinawite crystallites. The average particle size of 20–50 nm determined by X‐ray powder diffraction and electron microscopy accords with theoretical predictions of the SP/SD threshold size in greigite. The method constitutes a novel approach to synthesize greigite and to investigate its formation in sediments