Global changes in intensity of the Earth's magnetic field during the past 800kyr

Recent advances in palaeomagnetic and dating techniques have led to increasingly precise records of the relative intensity of the Earths past magnetic field at numerous field sites. The compilation and analysis of these records can provide important constraints on changes in global magnetic field intensity and therefore on the Earth's geodynamo itself. A previous compilation for the past 200 kyr integrated 17 marine records into a composite curve1, with the geomagnetic origin of the signal supported by an independent analysis of Be production made on different cores2. The persistence of long-term features in the Earth's magnetic intensity or the existence of long-term periodic changes cannot, however, be resolved in this relatively short time span. Here we present the integration of 33 records of relative palaeointensity into a composite curve spanning the past 800 kyr.We find that the intensity of the Earth's dipole field has experienced large-amplitude variations over this time period with pronounced intensity minima coinciding with known excursions in field direction, reflecting the emergence of non-dipole components. No stable periodicity was found in our composite record and therefore our data set does not support the hypothesis that the Earth's orbital parameters have a direct and strong influence on the geodynamo

X-ray magnetic circular dichroïsm provides strong evidence for tetrahedral iron in ferrihydrite

International audience[1] Ferrihydrite is an important iron oxyhydroxide for earth and environmental sciences, biology, and technology. Nevertheless, its mineral structure remains a matter of debate. The stumbling block is whether a significant amount of tetrahedrally coordinated iron is present. Here we present the first X-ray magnetic circular dichroïsm (XMCD) measurements performed on a well characterized synthetic sample of 6-line fer-rihydrite, at both K and L 2,3 energy edges of iron. XMCD results demonstrate unambiguously the presence of tetrahedrally coordinated Fe(III) in the mineral structure, in quantities compatible with the latest extended X-ray absorption fine structure (EXAFS) analyses suggesting a concentration of 20–30%. Moreover , we find an antiferromagnetic coupling between tetrahedral and octahedral sublattices, with the octa-hedral sublattice parallel to the external magnetic field. Components: 5100 words, 3 figures

Expedition 306 summary

The overall aim of the North Atlantic paleoceanography study of Integrated Ocean Drilling Program Expedition 306 is to place late Neogene–Quaternary climate proxies in the North Atlantic into a chronology based on a combination of geomagnetic paleointensity, stable isotope, and detrital layer stratigraphies, and in so doing generate integrated North Atlantic millennial-scale stratigraphies for the last few million years. To reach this aim, complete sedimentary sections were drilled by multiple advanced piston coring directly south of the central Atlantic “ice-rafted debris belt” and on the southern Gardar Drift. In addition to the North Atlantic paleoceanography study, a borehole observatory was successfully installed in a new ~180 m deep hole close to Ocean Drilling Program Site 642, consisting of a circulation obviation retrofit kit to seal the borehole from the overlying ocean, a thermistor string, and a data logger to document and monitor bottom water temperature variations through time

A New Tool for Separating the Magnetic Mineralogy of Complex Mineral Assemblages from Low Temperature Magnetic Behavior

Article 61 Citation: Lagroix F and Guyodo Y (2017)International audienceOne timeless challenge in rock magnetic studies, inclusive of paleomagnetism and environmental magnetism, is decomposing a sample's bulk magnetic behavior into its individual magnetic mineral components. We present a method permitting to decompose the magnetic behavior of a bulk sample experimentally and at low temperature avoiding any ambiguities in data interpretation due to heating-induced alteration. A single instrument is used to measure the temperature dependence of remanent magnetizations and to apply an isothermal demagnetization step at any temperature between 2 and 400 K. The experimental method is validated on synthetic mixtures of magnetite, hematite, goethite as well as on natural loess samples where the contributions of magnetite, goethite, hematite and maghemite are successfully isolated. The experimental protocol can be adapted to target other iron bearing minerals relevant to the rock or sediment under study. One limitation rests on the fact that the method is based on remanent magnetizations. Consequently, a quantitative decomposition of absolute concentration of individual components remains unachievable without assumptions. Nonetheless, semi-quantitative magnetic mineral concentrations were determined on synthetic and natural loess/paleosol samples in order to validate and test the method as a semi-quantitative tool in environmental magnetism studies

Global changes in intensity of the Earth's magnetic field during the past 800kyr

Recent advances in palaeomagnetic and dating techniques have led to increasingly precise records of the relative intensity of the Earths past magnetic field at numerous field sites. The compilation and analysis of these records can provide important constraints on changes in global magnetic field intensity and therefore on the Earth's geodynamo itself. A previous compilation for the past 200 kyr integrated 17 marine records into a composite curve1, with the geomagnetic origin of the signal supported by an independent analysis of Be production made on different cores2. The persistence of long-term features in the Earth's magnetic intensity or the existence of long-term periodic changes cannot, however, be resolved in this relatively short time span. Here we present the integration of 33 records of relative palaeointensity into a composite curve spanning the past 800 kyr.We find that the intensity of the Earth's dipole field has experienced large-amplitude variations over this time period with pronounced intensity minima coinciding with known excursions in field direction, reflecting the emergence of non-dipole components. No stable periodicity was found in our composite record and therefore our data set does not support the hypothesis that the Earth's orbital parameters have a direct and strong influence on the geodynamo

A comparison of relative paleointensity records of the Matuyama Chron for the period 0.75–1.25 Ma

International audienceA comparison of 15 records of relative geomagnetic paleointensity, spanning the time interval 0.75–1.25 Ma, has been performed after synchronizing the records at the paleointensity minima corresponding to geomagnetic reversals. Filtering and smoothing of the records indicated that, after synchronization, paleointensity features with wavelengths larger than about 20 ky can be matched from one record to another, although age offsets can still be observed in several records. Further synchronization of the records, using intensity minima, permitted construction of a composite stack of these records. This stack provides useful information about the long-term evolution of the geomagnetic field intensity over this period of time, and confirms the existence of a rapid recovery of the field intensity after each reversal. Analysis of the standard deviation and jackknife calculation suggest that improvement in the stack accuracy and resolution could still be achieved with better-constrained age models

The nature of a cryptochron from a paleomagnetic study of chron C4r.2r recorded in sediments off the Antarctic Peninsula

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