A promising novel recorder of Earth’s ancient magnetic field: fossil micrometeorites as paleomagnetic archives

The paleomagnetic record from sedimentary rocks is often ambiguous, limiting the recovery of reliable paleomagnetic data, particularly when compared to paleomagnetic data obtained from igneous rocks. We explore the potential of a novel type of paleomagnetic recorder in sedimentary rocks: fossil micrometeorites. Micrometeorites (MM) are small cosmic particles (50 μm-2mm) which account for a significant portion of the approximately 40,000 tons of extraterrestrial material that arrives on Earth annually. Many MMs melt during atmospheric entry, at altitudes of 80-90 km and transform into cosmic spherules (CS). CS have long been recognised as magnetic and can be extracted from sedimentary rocks. During melting and formation of CS, MM oxidise and form new minerals, particularly wüstite and magnetite. Subsequently, the cosmic spherules quench within a few seconds. This rapid cooling through the Curie temperature leads to the formation of thermoremanent magnetisations within CS. Diagnostic features such as metal beads and vesicles inside a proportion of CS make it possible to determine their flight trajectory, which can be combined with the magnetisation to reconstruct the polarity of EMF. While modern CS from Antarctica have already been shown to be good recorders of the geomagnetic field¹, the deep-time paleomagnetic records of CS remain unexplored. We present the results of a paleomagnetic study of anthropogenic magnetite spherules that resemble iron-type (I-type) CS, as well as several Paleozoic I-type CS. We demonstrate that individual spherules are measurable using existing paleomagnetic methods and equipment, while computed tomography (NanoCT) scanning enabled reliable reconstruction of the fall trajectory of a Paleozoic CS. We show that fossil CS with sizes of 50-300 μm are a promising, novel class of paleomagnetic recorder, accessible with existing techniques and capable of opening a new window on the ancient geomagnetic field.

¹Suavet, C., Gattacceca, J., Rochette, P. & Folco, L. Constraining the terrestrial age of micrometeorites using their record of the Earth’s magnetic field polarity. Geology 39, 123–126 (2011).