Abstract
Pliocene volcanic rocks from south-east Austria were paleomagnetically investigated. Samples were taken from 28 sites located on eight different volcanoes. Rock magnetic investigations revealed that magnetic carriers are Ti-rich or Ti-poor titanomagnetites with mainly pseudo-single-domain characteristics. Characteristic remanent magnetization directions were obtained from alternating field as well as from thermal demagnetization. Four localities give reversed directions agreeing with the expected direction from secular variation. Another four localities of the Klöch–Königsberg volcanic complex (3) and the Neuhaus volcano (1) have reversed directions with shallow inclinations and declinations of about 240° while the locality Steinberg yields a positive inclination of about 30° and 200° declination. These aberrant directions cannot be explained by local or regional tectonic movements. All virtual geomagnetic pole positions are located on the southern hemisphere. Four virtual geomagnetic poles lie close to the geographic pole, while all others are concentrated in a narrow longitude sector offshore South America (310°–355°) with low virtual geomagnetic pole latitudes ranging from − 15° to − 70°. The hypothesis that a transitional geomagnetic field configuration was recorded during the short volcanic activity of these five localities is supported by 9 paleointensity results and 39Ar/40Ar dating. Virtual geomagnetic dipole moments range from 1.1 to 2.9·1022 Am2 for sites with low VGP latitudes below about 60° and from 3.0 to 9.3·1022 Am2 for sites with higher virtual geomagnetic pole latitudes. The new 39Ar/40Ar ages of 2.51 ± 0.27 Ma for Klöch and 2.39 ± 0.03 Ma for Steinberg allow for the correlation of the Styrian transitional directions with cryptochron C2r.2r-1 of the geomagnetic polarity time scale.Graphic abstract
Highlights
The Earth’s magnetic field is generated in the outer core by an interacting system of liquid and electrical currents, which form the so-called geodynamo
In addition to the stable field configuration and its secular variation, documenting transitional field configurations during reversals or excursions are of high interest in order to constrain geodynamo mechanisms and support modeling of the geomagnetic field evolution during such critical events (e.g., Leonhardt et al 2009)
Rock magnetism The thermomagnetic behavior of susceptibility was measured for at least one sample per site. It was studied more intensively for the sites selected for paleointensity determination using thermal cycling runs
Summary
The Earth’s magnetic field is generated in the outer core by an interacting system of liquid and electrical currents, which form the so-called geodynamo. For our understanding of the geodynamo knowledge of the temporal change of the geomagnetic field is crucial. Direct observations cover only a few hundred years (e.g., Arneitz et al.2017). Paleomagnetic records obtained from rocks provide information on the geological past (e.g., Gubbins and Herrero-Bervera 2007). Volcanic rocks give spot readings of the geomagnetic field and eventually provide a temporal succession if stratigraphy is known. In addition to the stable field configuration and its secular variation, documenting transitional field configurations during reversals or excursions are of high interest in order to constrain geodynamo mechanisms and support modeling of the geomagnetic field evolution during such critical events (e.g., Leonhardt et al 2009)
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