Anomalies and Reversals of the Earth’s Magnetic Field

Abstract

The Earth’s magnetic field is an invisible shield that protects our planet from the solar wind, which is essential for the origin and evolution of life, as well as modern technology (Chapter 1). Thanks to satellites, the field can now be measured with remarkable precision. To forecast future behavior, however, we must reconstruct how the field behaved in the past. To study the historical behavior of the Earth’s magnetic field, we perform paleomagnetic measurements on rocks to recover the ancient magnetic field preserved within them. While paleodirections are usually straightforward to recover, paleointensities remain challenging. Chapters 2 and 3 present two new approaches to recover paleointensities from volcanic rocks using pseudo-Thellier measurements. The LAVA method in Chapter 3 performs especially well and recovers intensities for all sites within 5.6 μT, far outperforming traditional thermal methods. The past behavior of the Earth’s magnetic field is reconstructed by combining paleomagnetic data from around the world into geomagnetic models that reveal how processes deep inside the Earth shape the magnetic field through time. The South Atlantic Anomaly (SAA), a region of low magnetic intensity over South America, is arguably the most striking present-day feature. Current models indicate that the SAA has been drifting westward for centuries and that it originated east of Africa, but the scarcity of southern hemisphere data means its modeled evolution is still highly speculative. Chapters 4, 5, and 6 provide new paleomagnetic records from Chile and Fiji covering the last two millennia. The Chilean data indicate that the SAA only arrived at the start of the twentieth century, much later than predicted by models. The Fiji data reveal an intensity minimum of 12 μT about six hundred years ago, roughly 25 μT below model predictions and the lowest value found globally for the Holocene. Chapters 6 and 7 present geomagnetic models that include new data from Chile, Réunion, Bali, and Fiji. Chapter 6 demonstrates that the Fiji low is not part of the SAA, but a separate West Pacific Anomaly linked to a reversed flux patch (RFP) on the Core-Mantle Boundary (CMB) near the edge of the Pacific Large Low Shear Velocity Province (LLSVP). Chapter 7 shows that the SAA originated near southern Africa around 1,000 AD and migrated westward ever since, with RFPs consistently forming near the edge of the African LLSVP on the CMB. The appearance of RFPs along the edges of both LLSVPs strengthens the idea that processes in the lowermost mantle influence the geodynamo. Geomagnetic reversals are events in which the magnetic field switches polarity, yet they remain poorly understood because of their short duration. Chapter 8 develops a model of the most recent (Matuyama-Brunhes) reversal using a new method that synchronizes sedimentary records with a global paleointensity marker rather than classical age-depth modelling, yielding a shorter and more symmetric reversal than previously assumed. Chapter 9 offers a high-resolution volcanic record from Gran Canaria that captures the chaotic, multipolar behavior of a mid-Miocene reversal, including rapid inclination switches and strong non-dipole intensities.