Untersuchung der Variabilität des Erdmagnetfeldes in den letzten 30 Mio. Jahren: eine Fallstudie auf niedriger Breite (Äthiopien)
Zusammenfassung der Projektergebnisse
The geomagnetic field, generated by a dynamo process in the Earth’s outer core, is a dynamic object that can reverse its polarity but also produce during stable periods stochastic fluctuations in direction and intensity termed palaeosecular variation (PSV). The aim of this project was two-fold: (i) exploring the requirements to robustly quantify PSV from series of volcanic flows; (ii) better constrain PSV over the past 30 Myr through the analysis of Tertiary volcanic rocks from Ethiopia. Based on previous insights from numerical dynamo simulations, we showed that the relative variability ef in intensity can be practically estimated from relative palaeointensities obtained with the pseudo-Thellier method. In a more optimistic way than previous numerical results, we showed that the PSV proxies ef and the dispersion S of the virtual geomagnetic poles (VGP scatter) yield converged values to within 10% for series of 25 to 50 volcanic flows. Our thorough palaeomagnetic analysis of the Ethiopian traps indicates that the PSV proxy S was 50% higher during the Early Oligocene than during the Plio-Pleistocene. In parallel we also documented low absolute palaeointensity values (30±8 ZAm2 in terms of equivalent virtual dipole moment) during the Early Oligocene. Whether these two features are valid only at a local scale (our preferred interpretation) or can be generalised at a global scale remains an open question. From a geodynamic viewpoint, the magnetostratigraphy of the Belessa section (first field campaign results already published) confirms that the Ethiopian traps were emplaced at a minimum rate of 1 m/kyr, with a possible acceleration of the volume of volcanism through time. The magnetostratigraphy of the Lalibela section (second field campaign, results not published yet) will allow us to better explore the regional variability of the eruption rate. To better depict the global characteristics of the Earth’s magnetic field during the Cainozoic, we finally produced two new estimates of the PSV proxy ef (which has the advantage of being largely independent of site latitude in contrast to S): 35±5 % (N=20) for the Early Oligocene (Ethiopian traps); 44±5 % (N=47) for the Plio-Pleistocene (Boring Volcanic Field, USA). As predicted by numerical dynamo simulations and sometimes suggested from analyses of the VGP scatter, we found a correlation between PSV and reversal frequency. Further determinations of ef would be needed to validate (or invalidate) this correlation during the Phanerozoic eon.
Projektbezogene Publikationen (Auswahl)
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(2017), Variability of the 03 Ma palaeomagnetic field observed from the Boring Volcanic Field of the Pacific Northwest, Geophys. J. Int., 211(1), 69-79
Lhuillier, F., V. P. Shcherbakov, S. A. Gilder, and J. T. Hagstrum
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(2019), Impact of inner-core size on the dipole field behaviour of numerical dynamo simulations, Geophys. J. Int., 218(1), 179-189
Lhuillier, F., G. Hulot, Y. Gallet, and T. Schwaiger
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(2019), Palaeointensities of Oligocene and Miocene volcanic sections from Ethiopia: field behaviour during the Cainozoic, Geophys. J. Int., 216(2), 1482-1494
Lhuillier, F., V. P. Shcherbakov, V. V. Shcherbakova, S. Ostner, G. Hervé, and N. Petersen
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(2019), Palaeomagnetism and geochronology of Oligocene and Miocene volcanic sections from Ethiopia: geomagnetic variability in the Afro-Arabian region over the past 30 Ma, Geophys. J. Int., 216(2), 1446-1481
Lhuillier, F., and S. A. Gilder
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(2019), The secular variation of the Earth's magnetic field: investigations from palaeomagnetic data and dynamo simulations, Habilitation thesis Faculty of Geosciences University of Munich, Germany, 1-199 pp
Lhuillier, F.
