Swamp kauri witnesses to a major climate shift

The stump of an ancient kauri tree from Ngāwhā. Photo / Nelson Parker

A new international study using ancient swamp kauri from Northland shows a temporary breakdown of Earth's magnetic field 42,000 years ago sparked major climate shifts, leading to 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.

Study authors, from UNSW Sydney, the South Australian Museum, NIWA and the University of Waikato, dubbed this episode 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 published last week in 'Science.'

"For the first time ever, we have been able to precisely date the timing and environmental impacts of the last magnetic pole switch," UNSW Science professor and co-lead author of the study Chris Turney said.

"The findings were made possible with ancient New Zealand kauri trees, which have been preserved in sediments for over 40,000 years. Using the ancient trees we could measure, and date, the spike in atmospheric radiocarbon (the radioactive isotope or type of carbon) levels caused by the collapse of Earth's magnetic field."

Cross-sections from several swamp kauri in NIWA's archive were analysed by Principal Scientist 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," Lorrey said.

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 high-precision liquid scintillation counting (HPLSC) at the University of Waikato.

Professor Alan Hogg, director of the University of Waikato Radiocarbon Dating Laboratory, said the technique provided the highest possible accuracy for samples of that age. The kauri radiocarbon data were charted alongside annual tree ring growth data, which served as an accurate internal timestamp.

While scientists already knew the magnetic poles temporarily flipped around 41,000-42,000 years ago (known as the Laschamps Excursion), they didn't know exactly how it impacted life on Earth, if at all. But the researchers were able to detail how Earth's atmosphere changed over that time via the kauri tree ring radiocarbon data and other data aligned to it.

"The kauri trees are helping us tie together records of environmental change in caves, ice cores and peat bogs around the world," co-lead Professor Alan Cooper, from the South Australian Museum, said.

The researchers 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 suggests 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.

"It's the most surprising and important discovery I've ever been involved in," Cooper said.

The findings come two years after a particularly important ancient kauri tree was uncovered at Ngāwhā. The massive tree, with a trunk spanning more than 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, from the University of New South Wales.