41 000 years ago, the Earth’s magnetic field faded and practically disappeared, leaving our planet unprotected from the bombardment of cosmic rays.
Evidence for this event has been found in ocean sediment cores by a team from the Centre de Recherche et d’Enseignement de Géosciences de l’Environnement (CEREGE, CNRS/Aix-Marseille Université/IRD/Collège de France).
In the cores, the researchers measured variations in concentrations of beryllium-10, a radioactive isotope produced by the action of cosmic rays on oxygen and nitrogen atoms in the atmosphere.
The work, published in the Journal of Geophysical Research, is an important step towards developing a new method for studying the history of Earth’s magnetic field, which should shed light on why its strength has been declining over the past three thousand years.
The Earth’s magnetic field forms an efficient shield that deflects charged particles of cosmic origin headed for Earth. Far from being constant, the magnetic field has undergone many reversals, with the North magnetic pole shifting to the South geographic pole.
Such reversals are always accompanied by a disappearance of the magnetic field. The last such reversal took place 780 000 years ago. The magnetic field can also undergo excursions, periods when the field suddenly drops as if it was going to reverse, before recovering its normal polarity. The most recent event of this kind, known as the Laschamp excursion, took place 41 000 years ago.
Evidence for the event was uncovered by the researchers in sediment cores collected off the coasts of Portugal and Papua New Guinea. In the samples, they found an excess of beryllium-10, an isotope produced solely by collisions between particles of cosmic origin and atoms of nitrogen and oxygen.