That let the researchers see exactly when levels rose and peaked and then fell again. The trees, with their calendar-like set of rings, took in this kind of carbon and laid it down as wood.

More of these rays come in when the magnetic field is weak, so levels of this carbon go up. Inside trees that lived during the last magnetic flip, the researchers and their colleagues looked for a form of carbon created when cosmic rays hit the upper atmosphere. 'They're a time capsule in a way that you don't really get anywhere else in the world.' 'The trees themselves are quite unique,' says Cooper. Giant kauri trees can live for thousands of years and can end up well preserved in bogs. He and colleague Chris Turney, an earth scientist at the University of New South Wales, found a new way to study the exact timing of all this, using unusual trees in New Zealand. These changes were accompanied by a weakening in the magnetic field, he says, to as low as about 6% of its strength today. He explains that it 'then zoomed down through the Pacific really fast to Antarctica and hung out there for about 400 years and then shot back up through the Indian Ocean to the North Pole again.'

'Even though it was short, the North Pole did wander across North America, right out towards New York, actually, and then back again across to Oregon,' says Alan Cooper, an evolutionary biologist with Blue Sky Genetics and the South Australian Museum. Iron molecules embedded in sediments around the world also captured a record of this magnetic wobble, which unfolded over about a thousand years. Volcanic activity back then, during the flip, produced this distinctive iron signature as the molten lava cooled and locked the iron into place. This reversal is called the Laschamp excursion, after lava flows in France that contain bits of iron that are basically pointed the wrong way.

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