NZ's ancient kauri yields major scientific discovery

The trunk of an ancient kauri tree from Ngāwhā, Northland. Swamp kauri has revealed more about a mysterious event 42,000 years ago. Photo / Nelson Parker

Secrets hidden within ancient swamp kauri have revealed a temporary breakdown of Earth's magnetic field 42,000 years ago, prompting global environmental change and mass extinctions.

This dramatic turning point in Earth's history was triggered by a reversal of Earth's magnetic poles and changing solar winds.

"For the first time ever, we have been able to precisely date the timing and environmental impacts of the last magnetic pole switch," said Professor Chris Turney, a co-lead author of a new study just published in major journal Science.

The episode has now been dubbed the "Adams Transitional Geomagnetic Event", or "Adams Event" for short - a tribute to science fiction writer Douglas Adams, who wrote in The Hitchhikers Guide to the Galaxy that "42" was the answer to life, the universe, and everything.

The findings were made possible with ancient New Zealand kauri trees, which have been preserved in sediments for more than 40,000 years.

Using the ancient Northland trees, Turney, of the University of New South Wales, and his Kiwi and Australian colleagues were able measure, and date, a spike in atmospheric radiocarbon levels caused by the collapse of Earth's magnetic field.

Cross-sections from several swamp kauri in Niwa's archive were analysed by Dr Andrew Lorrey to determine their age.

"These kauri trees also lived during the time period leading into the Adams Event and provide a baseline of normal radiocarbon levels prior to the unprecedented rise associated with the Adams Event," said Lorrey, a principal scientist at Niwa.

Using radiocarbon dating, the team tracked the changes in radiocarbon levels during the magnetic pole reversal.

Sequential blocks of wood consisting of 40 annual rings were extracted from four ancient kauri logs and dated by a process called high-precision liquid scintillation counting.

Professor Alan Hogg, director of the University of Waikato Radiocarbon Dating Laboratory, said this technique provided the highest possible accuracy for samples of this age.

The kauri radiocarbon data was charted alongside annual tree ring growth data, which acted as an accurate, internal timestamp.

While scientists already knew the magnetic poles temporarily flipped around 41-42,000 years ago - something known as the Laschamps Excursion - they didn't know exactly how it impacted life on Earth - if at all.

As we know it today, the magnetic north pole doesn't have a fixed location.

It usually wobbles close to the North Pole - the northern-most point of Earth's axis - over time due to dynamic movements within the Earth's core, just like the magnetic south pole.

Sometimes, for reasons that aren't clear, the magnetic pole movements are drastic.

Around 41,000 to 42,000 years ago the north and south pole swapped places entirely.

"The Laschamps Excursion was the last time the magnetic poles flipped," Turney said.

"They swapped places for about 800 years before changing their minds and swapping back again."

Until now, scientific research has focused on changes that happened while the magnetic poles were reversed, when the magnetic field was weakened to about 28 per cent of its present-day strength.

But according to this new research, the most dramatic part was the lead-up to the reversal, when the poles were migrating across the Earth.

"Earth's magnetic field dropped to only 0-6 per cent strength during the Adams Event," Turney said.

"We essentially had no magnetic field at all - our cosmic radiation shield was totally gone."

In the study, the researchers were able to detail how Earth's atmosphere changed over the period via the kauri tree-ring radiocarbon data and other data aligned to it.

They used the newly created kauri radiocarbon timescale and other records from sites across the Pacific with global climate modelling to tie large shifts in major wind belts, tropical climate and glacier activity back to the Adams Event.

One of their first clues was that megafauna across mainland Australia and Tasmania went through simultaneous extinctions 42,000 years ago.

The paper suggested that the Adams Event could explain a lot of other evolutionary mysteries, like the extinction of Neanderthals and the sudden widespread appearance of figurative art in caves around the world.

These findings come two years after a particularly important ancient kauri tree was uncovered at Ngāwhā, Northland.

The massive tree - with a trunk spanning over two and a half metres - was alive during the Laschamps.

"Like other entombed kauri logs, the wood of the Ngāwhā tree is so well preserved that the bark is still attached," said Dr Jonathan Palmer, of the University of New South Wales.

This new ancient kauri timescale has helped to reveal what happened during a dramatic period in Earth's history. The team were able to reconstruct a chain of environmental and extinction events using climate modelling and the kauri records.

"The more we looked at the data, the more everything pointed to 42," Turney said.

"It was uncanny. Douglas Adams was clearly on to something, after all."