Paleomagnetic reconstruction of the global geomagnetic field evolution during the Matuyama/Brunhes transition: Iterative Bayesian inversion and independent verification |
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| Institution: | 1. Department für Geo- und Umweltwissenschaften, Ludwig-Maximilians-Universität, 80333 München, Germany;2. Universität Bremen, FB Geowissenschaften, Postfach 330 440, 28334 Bremen, Germany;1. Department of Earth Sciences, University of Southern California, Los Angeles, United States;2. Woods Hole Oceanographic Institution, Woods Hole, MA, United States;3. Department of Ocean, Earth, and Atmospheric Sciences, Old Dominion University, Norfolk, VA, United States;1. Department of Earth and Planetary Science, University of California, Berkeley, CA 94720, USA;2. Division of Geological & Planetary Sciences, Caltech, Pasadena, CA 91125, USA;1. Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany;2. Institute of Geophysics, University of Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany;1. Department of Earth Sciences, University of Southern California, Los Angeles, USA;2. Large Lakes Observatory, University of Minnesota-Duluth, USA;1. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;2. Key Laboratory of the Earth''s Deep Interior, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;3. Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093-0220, USA;4. School of History and Culture, Shandong University, Jinan 250100, China;5. Zhejiang Provincial Institute of Cultural Relics and Archaeology, Hangzhou 310014, China;6. Hebei Province Institute of Cultural Relics, Shijiazhuang 050000, China |
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| Abstract: | The Earth's magnetic field changed its polarity from the last reversed into today's normal state approximately 780 000 years ago. While before and after this so called Matuyama/Brunhes reversal, the Earth magnetic field was essentially an axial dipole, the details of its transitional structure are still largely unknown. Here, a Bayesian inversion method is developed to reconstruct the spherical harmonic expansion of this transitional field from paleomagnetic data. This is achieved by minimizing the total variational power at the core–mantle boundary during the transition under paleomagnetic constraints. The validity of the inversion technique is proved in two ways. First by inverting synthetic data sets from a modeled reversal. Here it is possible to reliably reconstruct the Gauss coefficients even from noisy records. Second by iteratively combining four geographically distributed high quality paleomagnetic records of the Matuyama/Brunhes reversal into a single geometric reversal scenario without assuming an a priori common age model. The obtained spatio-temporal reversal scenario successfully predicts most independent Matuyama/Brunhes transitional records. Therefore, the obtained global reconstruction based on paleomagnetic data invites to compare the inferred transitional field structure with results from numerical geodynamo models regarding the morphology of the transitional field. It is found that radial magnetic flux patches form at the equator and move polewards during the transition. Our model indicates an increase of non-dipolar energy prior to the last reversal and a non-dipolar dominance during the transition. Thus, the character and information of surface geomagnetic field records is strongly site dependent. The reconstruction also offers new answers to the question of existence of preferred longitudinal bands during the transition and to the problem of reversal duration. Different types of directional variations of the surface geomagnetic field, continuous or abrupt, are found during the transition. Two preferred longitudinal bands along the Americas and East Asia are not predicted for uniformly distributed sampling locations on the globe. Similar to geodynamo models with CMB heatflux derived from present day lower mantle heterogeneities, a preference of transitional VGPs for the Pacific hemisphere is found. The paleomagnetic duration of reversals shows not only a latitudinal, but also a longitudinal variation. Even the paleomagnetically determined age of the reversal varies significantly between different sites on the globe. The described Bayesian inversion technique can easily be applied to other high quality full vector reversal records. Also its extension to inversion of secular variation and excursion data is straightforward. |
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