Records of Paleomagnetic Field Variations
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Records of Paleomagnetic Field Variations Karl Fabian and Roman Leonhardt
3.1 Origin and Carriers of Paleomagnetic Signals The Earth’s internal magnetic field is the only direct active signal from Earth’s core, which can be detected nearly undistorted at Earth’s surface, and it has been perpetually recorded by magnetic remanence carriers in rocks throughout the Earth’s history. This rock magnetic record of the paleomagnetic field is an extremely valuable archive. On one hand, it is used for plate-tectonic reconstruction, and provides exact magnetostratigraphic information about motion and chronology of the crust. On the other hand, rock magnetic data constitute the only direct recording of the history of magnetic field generation in the deep interior of the Earth. Experimental determinations of the paleomagnetic field are now challenged to produce data sets that can be compared to numerical geodynamo models. Only when such data are sufficiently reliable and detailed, it can be decided which of the different imaginable processes within the Earth’s outer core most closely resembles reality. It is likely that during the next decades, the fundamental physical mechanisms, which are driving the enormous internal machinery of our planet, will be revealed by the combined effort of the community of geoscientists. The main physical database on which the success of this endeavor depends are rock magnetic determinations of paleofield direction and intensity. While the determination of the paleofield direction from the stepwise demagnetization of a rock usually can be accomplished without complications, the determination of paleofield intensity is extremely challenging. Unfortunately, even a global cover by directional data would allow only to infer the geometry of the paleofield. In order to assess the activity of the geodynamo, the knowledge of the past field intensity is indispensable. In consequence, the optimal determination of the paleointensity of the Earth’s magnetic field by means of rock magnetic measurements is a central experimental Karl Fabian Geological Survey of Norway Leiv Eiriksons vei 39 7491 Trondheim Norwegen Roman Leonhardt Department Angewandte Geowissenschaften und Geophysik Lehrstuhl Technische ¨ Okosystemanalyse Universit¨at Leoben Peter Tunner Straße 25-27 8700 Leoben Austria
K.-H. Glaßmeier et al. (eds.), Geomagnetic Field Variations, Advances in Geophysical and Environmental Mechanics and Mathematics, c Springer-Verlag Berlin Heidelberg 2009
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K. Fabian and R. Leonhardt
problem, which recedes the progress of retracing the history of the geodynamo, and therefore of revealing the physical processes in the Earth’s liquid core. Therefore, paleointensity determination lies at the heart of the two main themes of this chapter: (1) to understand the physical origin of the rock magnetic signal in sediments and lava flows, and (2) to relate it to the past variations of the Earth’s magnetic field. The mechanisms of remanence acquisition, and our current state of knowledge about them, are completely
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