19 Feb 2021

Ancient swamp kauri give clues on atmospheric changes to Earth from 42,000 years ago

4:00 pm on 19 February 2021

Ancient swamp kauri tree rings have shed light on crucial changes to the Earth's atmosphere, which may have led to mass extinction of megafauna and the beginning of cave art.

The northern lights stream across the arctic sky near Yellowknife, Northwest Territories in Canada.

The Adams Event would have caused frequent and dazzling light shows in the sky - similar to the Northern and Southern Lights. Photo: 123RF

Scientists from NIWA, the University of New South Wales Sydney (UNSW), the South Australian Museum, and the University of Waikato, have been working together on the study.

They looked at what was happening just before the event known as the Laschamp Excursion - the last time when the magnetic poles switched (north became south, south became north). This period lasted about 800 years, before the poles then flipped back.

Research up until this point has been particularly focused on the Laschamp Excursion.

It has looked at what that did to the Earth, and found that at the time, the magnetic field was about 28 percent of present-day strength.

But the new research showed that the most tumultuous period was in the build up, as the poles migrated across the face of the earth.

Using the radiocarbon data from the kauri trees, the researchers assessed what was happening to the geomagnetic field as the poles were migrating.

"We've known for a long time that the Earth's magnetic field polarity changed, about 41,000 years ago, we can see that in our natural archives," explained Andrew Lorrey, principal climate and environmental applications scientist at NIWA.

Dr Andrew Lorrey, Principal Scientist - Climate and Environmental Applications.

Dr Andrew Lorrey Photo: NIWA / Rebekah Parsons-King

"The interesting component here is that the strength of the magnetic field and how that waned through this period, that's not something that's been very well defined in the past," Dr Lorrey said.

This sequence has been dubbed the Adams Event, after Douglas Adams, the science fiction writer who wrote in The Hitchhikers Guide to the Galaxy that 42 was the answer to life, the universe, and everything.

Radiocarbon data ascertained from the trees showed at this time, the geomagnetic field would have been between zero and 6 percent of what it is today, he said.

"We essentially had no magnetic field at all - our cosmic radiation shield was totally gone," said professor Chris Turney, from UNSW Science.

This would have caused frequent and dazzling light shows in the sky - similar to the Northern and Southern Lights - taking place all around the world. Electrical storms would also have been more prevalent.

Mass extinction event and the beginning of cave art

"The magnetic field protects us, from a lot of what happens with the sun, and also bombardment by cosmic rays," Dr Lorrey said.

"What we found was the magnetic field was greatly diminished prior to the full polarity flip, and that's when we think some climatic activity and response happened on Earth, about 42,000 years ago."

Ancient rock art, depicting animals and humans, is seen at the Chiribiquete National Park in Colombia on 2 July, 2018.

The study hypothesises that such an event led to the arrival of cave art. Photo: Diego Camilo Carranza Jimenez / Anadolu Agency / AFP

Evidence from around the world shows that during this period there was a mass extinction of megafauna.

"Unfiltered radiation from space ripped apart air particles in Earth's atmosphere, separating electrons and emitting light - a process called ionisation," Prof Turney said.

"The ionised air 'fried' the ozone layer, triggering a ripple of climate change across the globe."

The paper hypothesised that such an event led to the arrival of cave art, as early humans sought shelter from the extreme weather patterns.

It also suggested it could explain other evolutionary mysteries, such as the extinction of Neanderthals.

Kauri a 'valuable' resource for research

The discovery was only possible because of the unique circumstances offered to scientists by ancient swamp kauri.

The trunk of an ancient kauri tree from Ngāwhā, Northland.

The trunk of an ancient kauri tree from Ngāwhā, Northland. Photo: Supplied by NIWA / Nelson Parker

In the northern hemisphere, the peat bogs which preserved ancient trees were scraped away by the last ice age.

"Northland on the other hand, during the global last glacial maximum, was not glaciated," Dr Lorrey said. "So our peats in many places survived intact, and we have this wonderful treasure trove of older bog sediments, which have been preserved with wood in them.

"Nowhere else in the world do we have the volume, and time coverage of wood, and especially wood that allows us to make very long tree ring records.

"At any given clip, we can extract swamp kauri trees, and it's quite common to find 600 to 1000 rings on those trees, so they're very valuable for climate research and radio carbon research."

Meanwhile, he said the facilities at the University of Waikato were equally important.

It allowed the scientists to conduct high precision liquid scintillation to extract sequential blocks of wood consisting of 40 annual rings, from four ancient kauri logs.

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