Geomagnetic field reversals: Morphology and Consequences

During the last century the global geomagnetic field lost more than 10% of its strength. Reasons and consequences, however, remain elusive. One hypothesis proposes that the geomagnetic field is about to change its polarity in near future. Geomagnetic field reversals occurred hundreds of times during the Earths history, and on an average every 200 000 years during the last 5 Myr. However, the current polarity lasts already for 780 000 years. Our main research objective is to recover and report reliable new records of recent reversals, providing information on the geomagnetic fields full vector (direction and intensity) variation in time. Such records require geological archives which 1) acquire a stable remanent magnetization during their formation (paleomagnetism), 2) store this information unchanged throughout the geological past, and 3) allow the reconstruction of both, direction and intensity of the field. These requirements are ideally met by volcanic rocks. Because in addition a very good temporal resolution is desirable, continuous, effusive volcanism, typically found during shield building states of certain volcanoes, is preferred. A detailed and well dated sequence of volcanic rocks can contain an almost continuous consistent record of geomagnetic field variation across the transitional state of a reversal. Here, three volcanic sequences are projected: 1) The Austrian volcanic region around the shield volcano Gleichenberg in Styria, as well as the hot-spot related volcanic islands 2) St. Helena in the southern hemisphere and 3) Cape Verde (Northern hemisphere). All regions contain Pleistocene to Miocene sequences with comparable magmatic histories and very high extrusion rates. The outcrop situation is perfect, whilst no studies on full vector data have been performed before. Subsequently, a second goal will be the global interpretation of the recovered reversal recordings. Direct data-based knowledge is essential to understand the geodynamo processes within the Earths interior. A detailed reconstruction of the global geomagnetic field evolution is hampered by poor spatial and temporal distribution of high quality paleomagnetic records. However, the database of such records is continuously growing by efforts as projected in our main objective above. Here, also new ideas and approaches towards an improvement of our previous paleomagnetic inversion technique for reconstruction of global a geomagnetic model (Leonhardt and Fabian, 2007) will be developed. The application of these techniques to other geomagnetic field reversals, and a comparison of evolution histories of different events, is essential for finding regularities in the real geodynamo process. Improving global geomagnetic models of reversals (and excursions) will be of essential interest to other disciplines, since solar-magnetospheric interactions as well as radionuclide production in the atmosphere are dependent on the morphology and strength of the geomagnetic field in its extreme states.

Investigating the occurence, morphology and consequences of geomagnetic field reversals is the primary goal of this project. Within the framwork of an FWF proposal (P30523-N29) we aimed on identification and sampling transitional geomagnetic field information at three sites: in Styria (Austria), São Nicolau (Cabo Verde) and St. Helena. The first target for the paleomagnetic search of intermediate field directions were the Pleistocene and Pliocene volcanic rocks from South-East-Austria (Styria). 28 sites were sampled on eight Pliocene volcanoes of which four gave paleo-directions and intensities in the range expected from secular variation. The remaining 4 volcanoes have low paleo-inclinations and intensities. A transitional geomagnetic field configuration was recorded during the short eruptive activity of these volcanoes. Two new 39Ar-40Ar ages allow the correlation of the Styrian transitional directions with cryptochron C2r.2r-1 of the geomagnetic polarity time scale. As a cryptochron has a short duration of less than 10 to 30 ka, at least 4 of the investigated 8 Styrian volcanoes have been formed in a much shorter time interval than previously suggested. Results are published here: https://doi.org/10.1186/s40623-021-01518-w. The second sampling campaign was conducted on island São Nicolau (Cape Verde) in October 2018. The island is built up by basalts of predominantly late Miocene to Pleistocene age. We sampled three promising sections containing field reversal, but did font find transitional directions. A publication on secular variation is in preparation. Finally, a third sampling campaign on St. Helena was undertaken. We sampled in detail and expanded a section already investigated by our collaborators in Liverpool in which a field transition is recorded. Additionally, one other section with mixed polarity was sampled. Most of the obtained paleointensities are considerably lower than the present day or the dipole field intensity. During the polarity transition a drop in the field strength is apparently not present. Although we could discover transitional geomagnetic field directions in two of the projected three volcanic provinces modelling of the geomagnetic field during a field transition is hampered. Examples of such global models can be found here: https://cobs.zamg.ac.at/data/index.php/en/models-and-databases.