Particles of iron in sediments orient themselves in accordance with the local magnetic field of the earth. As the sediment consolidates and lithifies over the course of several decades, the particles of iron continue to lie in the direction of the original magnetic field. They thus preserve data on the state of the magnetic field when the sediment was being deposited. These data are used world-wide to date geological strata. The researchers developed a method which provides information on the carriers of the magnetic signal in sediments. Carriers include the iron oxides magnetite and hematite.
Using this method, the researchers were able to demonstrate that a short-lived change in the earth's magnetic field really did take place about ten million years ago. Hitherto, researchers had been unable to exclude the possibility that subsequent chemical or physical processes had altered the magnetic signal in the sediment.
The Utrecht University researchers made another remarkable discovery using the new method. In an organically rich layer in the eastern Mediterranean, they found that bacteria had formed magnetic material. With the new method, they were able to distinguish between the bacterial material and the original magnetic signal.
Studies of the earth's past magnetic field are important for our understanding of the 'geodynamo'. Researchers use the term 'geodynamo' to refer to the idea that the geomagnetic field is generated in the centre of the earth. The hypothesis is that the electricity that runs through the molten iron of the earth's outer core causes the magnetic field. The earth's magnetic field now points south, meaning that a compass needle points north. Some 800,000 years ago, a compass needle would have pointed south, having previously pointed north. Changes in the direction of the earth's magnetic field are referred to as reversals.
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