By Nell Greenfieldboyce
An ancient, well-preserved tree that was alive the last time the Earth’s magnetic poles flipped has helped scientists pin down more precise timing of that event, which occurred about 42,000 years ago.
This new information has led them to link the flipping of the poles to key moments in the prehistoric record, like the sudden appearance of cave art and the mysterious extinction of large mammals and the Neanderthals. They argue that the weakening of the Earth’s magnetic field would have briefly transformed the world by altering its climate and allowing far more ultraviolet light to pour in.
Their provocative analysis, in the journal Science, is sure to get researchers talking. Until now, scientists have mostly assumed that magnetic field reversals didn’t matter much for life on Earth — although some geologists have noted that die-offs of large mammals seemed to occur in periods when the Earth’s magnetic field was weak.
The Earth is a giant magnet because its core is solid iron, and swirling around it is an ocean of molten metal. This churning creates a huge magnetic field, one that wraps around the planet and protects it from charged cosmic rays coming in from outer space.
Sometimes, for reasons scientists do not fully understand, the magnetic field becomes unstable and its north and south poles can flip. The last major reversal, though it was short-lived, happened around 42,000 years ago.
Giant kauri trees can live for thousands of years and can end up well preserved in bogs. “The trees themselves are quite unique,” says Cooper. “They’re a time capsule in a way that you don’t really get anywhere else in the world.”
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. More of these rays come in when the magnetic field is weak, so levels of this carbon go up.
The trees, with their calendar-like set of rings, took in this kind of carbon and laid it down as wood. That let the researchers see exactly when levels rose and peaked and then fell again. One tree in particular had a 1,700-year record that spanned the period of the greatest changes.
By creating a precise timeline, the research team was able to compare the magnetic field’s weakening to other well-established timelines in the archaeological and climate records.