Multi-method palaeointensity data of the geomagnetic field during the past 500 kyrs from European volcanoes

Abstract

The Earth’s magnetic field is generated in the liquid outer core of our planet and acts as a shield against harmful radiation from space. Without it, life on Earth would not be possible. However, the geomagnetic field is not stable: its poles continually move around and sometimes even swap positions (geomagnetic reversals) and its intensity varies. This intensity has been decreasing for the past few hundred years, which according to some researchers may be a sign of an impending reversal. In order to gain a better understanding of the present-day field and how it’s generated, we need more information on how it behaved in the past. This information can be obtained using sedimentary and volcanic rocks, as well as archaeological artefacts, which store the field as it was on the moment of deposition (sediments) or cooling (lavas and archaeological artefacts). However, while it is fairly straightforward to obtain data on the past direction of the Earth’s magnetic field, it is much more difficult to obtain reliable data on its intensity. Most palaeointensity methods involve several heating steps, which often induce alteration in the rock samples. This alteration, as well as other characteristics of the used rock samples, causes many samples to fail commonly used reliability criteria. Success rates are often in the range of 10 to 20%. As a result, there is a lack of data on the past intensity of the geomagnetic field. In this thesis, a multi-method approach was used to determine reliable palaeointensity data. Because the four methods are all based on different principles, it is possible that if one method does not work, one or more of the others do, thus increasing the success rates. Furthermore, if more than one method works on the same site, consistency between their results provides an additional reliability criterion. For these palaeomagnetic directions and intensities to be truly useful, it is necessary to have precise and accurate ages for the volcanic rocks used in these experiments. These can be obtained from the radioactive decay of isotopes such as 14C or 40K. In this thesis, 40Ar/39Ar ages were obtained using the ThermoFisher Helix multi-collector mass spectrometer located at the Vrije Universiteit Amsterdam. Its high resolution enables a more accurate determination of the amount of 36Ar and therefore the amount of 40Ar of atmospheric rather than radiogenic origin. This thesis consists of four parts. Part I is an introduction to palaeomagnetism and 40Ar/39Ar dating. Part II focusses on different methods of palaeointensity determination, whereas Part III is about the ThermoFisher Helix MC mass spectrometer. Finally, in part IV the methods described in the previous chapters are applied to volcanic rocks from three regions in Europe. By using multiple palaeointensity methods, combined with strict selection criteria, it was possible to determine reliable palaeointensity data for 82% of these flows. The obtained values are somewhat low compared to the present-day field, but are in agreement with the average intensity since the last reversal, 780,000 years ago.