1,720,984 research outputs found
The Effect of Low-Temperature Oxidation on Large Multi-Domain Magnetite Decay of the virtual dipole moment during polarity transitions and geomagnetic excursions
No full textNatural samples of oxidized and unoxidized magnetite have been used to study the effect of low-temperature oxidation on magnetic domain state. All of the magnetites studied were large multi-domain (MD) grains, ranging in size from 53 µm to at least 250 µm. Hysteresis data typical of MD grains are obtained from unoxidized magnetite samples while data from partially oxidized samples are characteristic of pseudo-single domain (PSD) grains. Our results demonstrate that low-temperature oxidation can seriously affect the magnetic properties of magnetite by significantly increasing the PSD-MD threshold size. The presence of a suppressed Verwey transition at 118 K in the oxidized samples indicates that the oxidation may only be surficial and that there may exist a core of magnetite under the maghemitized surface shell. We suggest three possible mechanisms for the observed PSD-like behavior. First, the magnetite core could be reduced sufficiently in volume to make it a PSD grain, independent of the surficial maghemite. Second, internal stress in the composite grain due to lattice mismatches between the magnetite core and maghemite rim will increase the domain wall energy and make it more difficult to add walls for a given grain size. Coupled with a reduced volume of the magnetite core, this could give rise to PSD-like behavior. Third, the composite grain could be a mixture of SD maghemite and MD magnetite which gives rise to bulk PSD-like properties. Our results have potentially important implications for paleomagnetic records because they provide a mechanism whereby a stable chemical remanent magnetization can be acquired by MD grains in sediments and igneous rocks. <br/
Middle/Late Pleistocene relative palaeointensity of the geomagnetic field from lacustrine sediments, Lake Chewaucan, western United States
No full textDetailed palaeomagnetic and rock magnetic studies of a 15 m succession of Middle/Late Pleistocene lacustrine sediments from ancient Lake Chewaucan, southern Oregon, western United States, indicate that the remanence-bearing grains are sufficiently uniform to be applicable to relative palaeointensity studies. We have used ARM, SIRM and ? for normalization of the NRM. All three parameters give essentially identical results in their relative stratigraphic variations, which indicates that the normalizations efficiently remove the effects of variation in magnetic mineral concentration. Patterns in grain-size variation, as indicated by small-scale quasi-cyclic fluctuations in hysteresis parameters, may be due to environmental changes such as lake-level variation. However, these fluctuations are within the acceptable range of grain sizes for palaeointensity studies and cannot be correlated with any of the features of the normalized remanence record. We therefore conclude that the large-scale variations in the normalized remanence record are due to geomagnetic palaeointensity fluctuations. Parts of the normalized remanence record, where firm chronological constraints exist, may correlate with features of relative palaeointensity records from deep-sea sediments. Our results also confirm the observation that low geomagnetic field intensities dominate during geomagnetic excursions. Further studies of relative palaeointensity of the geomagnetic field may enable the development of an independent time-scale which would make possible the direct correlation of palaeoclimate records from deep-sea and continental environments.<br/
Characterizing interactions in fine magnetic particle systems using first order reversal curves
No full textWe demonstrate a powerful and practical method of characterizing interactions in fine magnetic particle systems utilizing a class of hysteresis curves known as first order reversal curves. This method is tested on samples of highly dispersed magnetic particles, where it leads to a more detailed understanding of interactions than has previously been possible. In a quantitative comparison between this method and the dM method, which is based on the Wohlfarth relation, our method provides a more precise measure of the strength of the interactions. Our method also has the advantage that it can be used to decouple the effects of the mean interaction field from the effects of local interaction field variance
Decay of the virtual dipole moment during polarity transitions and geomagnetic excursions Decay of the virtual dipole moment during polarity transitions and geomagnetic excursions
No full textWe have analyzed paleointensity data from five polarity transitions and geomagnetic excursions recorded by lava flows. We find a significant correlation between the logarithm of the virtual dipole moment and the angle between the virtual geomagnetic pole and the Earth's rotation axis. This correlation implies that any physical model for geomagnetic field behavior during transitions and excursions must address the question of how changes in direction are linked to changes in intensity. In addition, large changes in paleointensity may occur even when the virtual geomagnetic pole is still at high latitude so the concept of transitional direction must be redefined. <br/
Relative Geomagnetic Paleointensity across the Jaramillo Subchron and the Matuyama/Brunhes Boundary
No full textAnalysis of old paleomagnetic data and new mineral magnetic data from a sediment core from the central equatorial Pacific Ocean indicates that this core can be used for determination of relative geomagnetic paleointensity. These data indicate that during the Jaramillo subchron, there was a gradual decrease in paleointensity from the onset of the subchron to its termination. The data from the subchron also display high-amplitude, fine-scale features that may be coherent over the scale of an ocean basin. Furthermore, there was a rapid rise in paleointensity immediately after the termination of the Matuyama /Brunhes polarity transition. These observations support the hypothesis that an asymmetrical saw-tooth pattern in paleointensity may be a common characteristic of polarity transitions; however, a better understanding of the high-amplitude features is needed before definitive conclusions can be drawn about the overall behavior of the field. The coherence of fine-scale features across such large distances does not support the suggestion that the asymmetrical sawtooth pattern is an artifact of viscous remanent remagnetization. <br/
Variations of the geomagnetic field during the Holocene: Relative paleointensity and inclination record from the West Pacific (ODP Hole 1202B)
No full textEnvironmental magnetism: Past, present, and future
No full textEnvironmental magnetism involves the application of rock and mineral magnetic techniques to situations in which the transport, deposition, or transformation of magnetic grains is influenced by environmental processes in the atmosphere, hydrosphere, and lithosphere. The first explicit description of environmental magnetism as a distinct field was in 1980. Since that time environmental magnetism has become a broad field that is finding application in an ever-increasing array of scientific disciplines. In this review of the present state of environmental magnetic studies, we divide the field into three broad, but arbitrary, categories. The first involves the use of mineral magnetic assemblages in the geological record to study physical processes in depositional environments. This category includes the correlation of sediment cores using magnetic susceptibility measurements, studies of geomagnetic field behavior, the analysis of depositional and postdepositional mechanical processes that affect sediments, and the examination of magnetic parameters that might represent proxies for paleoclimatic variation. The second category encompasses studies of the processes responsible for variations in the magnetic minerals brought into a sedimentary environment. These provenance investigations include studies of changes in catchment-derived sediment in lakes, fluctuations in contributions from terrigenous, aeolian and glaciogenic components in deep-sea sediments, and the origin of atmospheric particulates. The final category addresses in situ changes and transformations of magnetic minerals in sedimentary environments, including pedogenesis, authigenetic/diagenetic formation of ferrimagnetic phases, dissolution of magnetic minerals due to reductive diagenesis, and contributions of biomagnetism to sedimentary magnetism. Because environmental magnetism can address problems in so many disciplines and because many of these problems may be inaccessible to other techniques, it is likely that the scope of environmental magnetism will continue to expand rapidly. Environmental magnetism is capable of providing important data for studies of global environmental change, climatic processes, and the impact of humans on the environment, all of which are major research initiatives in the international scientific community. These factors suggest that environmental magnetism has a bright and diverse future. <br/
Wasp-waisted hysteresis loops: Mineral magnetic characteristics and discrimination of components in mixed magnetic systems
No full textRock magnetic studies of complex systems that contain mixtures of magnetic minerals or mixed grain size distributions have demonstrated the need for a better method of distinguishing between different magnetic components in geological materials. Hysteresis loops that are constricted in the middle section, but are wider above and below the middle section, are commonly observed in mixed magnetic assemblages. Such “wasp-waisted” hysteresis loops have been widely documented, particularly with respect to rare earth permanent magnets, basaltic lava flows, remagnetized Paleozoic carbonate rocks, and an increasingly wide range of other rocks. Our modelling, combined with a review of previous work, indicates that there are several conditions that give rise to, as well as magnetic properties that are characteristic of, wasp-waisted hysteresis loops. First, at least two magnetic components with strongly contrasting coercivities must coexist. This condition can arise from either mixtures of grain sizes of a single magnetic mineral, or a combination of magnetic minerals with contrasting coercivities, or a combination of these two situations. Second, materials that give rise to wasp-waisted hysteresis loops will have relatively high ratios of the coercivity of remanence to coercive force (B cr /B c ) because B0 is controlled by the soft (low coercivity) component, whereas Bcris controlled by the hard (high coercivity) component. Third, values of B cr /B c ? 10 usually only occur for strongly wasp-waisted loops when the low coercivity component comprises an overwhelmingly large fraction of the total volume of magnetic grains. Fourth, a given mixture of superparamagnetic and single-domain (SD) grains is more likely to give rise to wasp-waisted hysteresis loops than an equivalent mixture of SD and multidomain grains. Fifth, our results provide empirical confirmation that the total magnetization of a material is the sum of the weighted contributions of each component, in the absence of significant magnetic interaction between particles. Thus to contribute significantly to wasp-waisted behavior, a mineral magnetic component must give rise to a significant portion of the total magnetization of the rock. As a result, minerals with weak magnetic moments such as hematite need to occur in large concentrations to cause wasp-waistedness in materials that also contain ferrimagnetic minerals. We outline a method for determining the magnetic components that can give rise to wasp-waisted hysteresis loops. This method is based on high- and low-temperature magnetic measurements that are used to identify the dominant remanence-bearing mineral/s and on mineral magnetic techniques that are used to discriminate between different magnetic domain states. The method is illustrated with several examples from archaeological, geological, and synthetic materials
Magnetic interactions and their effect on the low temperature behavior of fine magnetic particles
No full textEnvironmental magnetic record of Antarctic palaeoclimate from Eocene/Oligocene glaciomarine sediments, Victoria Land Basin
No full textThe onset of continent-wide glaciation in Antarctica is still poorly understood, despite being one of the most important palaeoclimatic events in the Cenozoic. The Eocene/ Oligocene boundary interval has recently been recognized as a critical time for Antarctic climatic evolution, and it may mark the preglacial-glacial transition. Magnetic susceptibility, intensity of natural and artificial remanences, hysteresis parameters and magnetic anisotropy of the lower half ( late Eocene/early Oligocene) of the CIROS-1 core (from McMurdo Sound, Antarctica) reveal alternating intervals of high and low magnetic mineral concentrations that do not correspond to lithostratigraphic units in the core. Pseudo-single-domain magnetite is the main magnetic mineral throughout the sequence, and sharp changes in magnetite concentration match changes in clay mineralogy beneath and at the Eocene/Oligocene boundary. The detrital magnetite originated from weathering of the Ferrar Group (which comprises basic extrusive and intrusive igneous rocks). Weathering processes and input of magnetite to the Victoria Land Basin were intense during periods when the Antarctic climate was warmer than today, but during intervals when the climate was relatively cool, chemical weathering of the Ferrar Group was suppressed and input of detrital magnetite to the Victoria Land Basin decreased. Our results also indicate that a cold and dry climate was not established in Antarctica until the Eocene/Oligocene boundary, with major ice sheet growth occurring at the early/late Oligocene boundary. Some earlier cold intervals are identified, which indicate that climate had begun to deteriorate by the middle/late Eocene boundary
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