About 41,000 years ago, something remarkable happened: Earth’s magnetic field flipped and, for a temporary period, magnetic north was south and magnetic south was north.
Palaeomagnetists refer to this as a geomagnetic excursion. This event, which is different to a complete magnetic pole reversal, occurs irregularly through time and reflects the dynamics of Earth’s molten outer core.
The strength of Earth’s magnetic field would have almost vanished during the event, called the Laschamp excursion, which lasted a few thousand years.
Earth’s magnetic field acts as a shield against high-energy particles from the Sun and outside the solar system. Without it the planet would be bombarded by these charged particles.
We don’t know when the next geomagnetic excursion will happen. But if it happened today, it would be crippling.
Satellites and navigation apps would be rendered useless — and power distribution systems would be disrupted at a cost of between US$7 billion and US$48 billion each day in the United States alone.
Obviously, satellites and electric grids didn’t exist 41,000 years ago. But the Laschamp excursion — named after the lava flows in France where it was first recognised — still left its mark.
We recently detected its signature in Australia for the first time, in a 5.5 metre-long sediment core taken from the bottom of Lake Selina, Tasmania.
Within these grains lay 270,000 years of history, which we unpack in our paper published in the journal Quaternary Geochronology.