Ancient kauri trees reveal a turning point in Earth’s history 42,000 years ago
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 are published today 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," says Chris Turney, a professor at UNSW Science and co-lead author of the study.
"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" Dr Lorrey says.
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 in New Zealand.
Professor Alan Hogg, Director of the University of Waikato Radiocarbon Dating Laboratory, says the technique provides the highest possible accuracy for samples of this age.
The kauri radiocarbon data was charted alongside annual tree ring growth data, which acts as an accurate, internal timestamp.
While scientists already knew the magnetic poles temporarily flipped around 41-42,000 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 this 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," says co-lead Professor Alan Cooper from the South Australian Museum.
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 Neandertals 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," says Prof. Cooper.
The perfect (cosmic) storm
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 41,000-42,000 years the north and south pole swapped places entirely.
"The Laschamps Excursion was the last time the magnetic poles flipped," says Prof. Turney. "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," says Prof. Turney. "We essentially had no magnetic field at all - our cosmic radiation shield was totally gone."
Into the caves
Dazzling light shows would have been frequent in the sky during the Adams Event.
Aurora borealis and aurora australis, also known as the northern and southern lights, are caused by solar winds hitting the Earth’s magnetosphere.
Usually confined to the polar northern and southern parts of the globe, the colourful sights may have been widespread during the breakdown of Earth’s magnetic field.
Ionised air - which is a great conductor for electricity - would have also increased the frequency of electrical storms.
"Unfiltered radiation from space ripped apart air particles in Earth’s atmosphere, separating electrons and emitting light - a process called ionisation," says Prof. Turney. "The ionised air ‘fried’ the Ozone layer, triggering a ripple of climate change across the globe."
The researchers theorise that the dramatic environmental changes may have caused early humans to seek more shelter. This could explain the sudden appearance of cave art around the world roughly 42,000 years ago.
"We think that the sharp increases in UV levels, particularly during solar flares, would suddenly make caves very valuable shelters," says Prof. Cooper. "The common cave art motif of red ochre handprints may signal it was being used as sunscreen, a technique still used today by some groups. "
A consequence of the weakened magnetic field and intensification of space weather, was a very strong increase in the amount of radiocarbon generated in the upper atmosphere and permanently recorded in tree-rings.
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," says UNSW’s Dr Jonathan Palmer, a specialist in dating tree rings (dendrochronology).
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," says Prof. Turney. "It was uncanny. "Douglas Adams was clearly on to something, after all."
This work was made possible by funding from the Australian Research Council, The Royal Society of New Zealand Marsden Fund, the University of Waikato, the National Institute of Water and Atmospheric Research with support from Ngāpuhi iwi and Top Energy, and many other international partners.
See the research article in Science: A global environmental crisis 42,000 years ago. [DOI: 10.1126/science.abb8677] published 19 Feb 2021.