NZ’s Rosetta Stone: Ancient kauri reveals cataclysmic natural events that could repeat

An ancient kauri, a kind of Rosetta Stone, has revealed evidence of cataclysmic natural events including climate change and solar storms that could repeat in modern times. By Kate Evans.

In the car park of Ngāwhā marae, in Northland, a massive kauri trunk lies in pieces, the whale-sized log next to its upright root ball. The ends are sealed with lacquer to keep moisture out, and its scalloped bark is slowly peeling off to land in curled flakes in the gravel.

When I stand at the trunk’s cut face and stretch my arms out, I can’t reach all the way across. The wood is rough against my cheek, and I can smell the spicy, piney resin.

It feels and smells as if it could have been toppled a year ago. Yet when this tree last stood upright, Neanderthals and Denisovans still walked the Earth. Homo sapiens, still many millennia away from reaching New Zealand, had only recently colonised Europe and begun to make art.

In 2019, construction foreman Mark Magee was clearing a platform for a new geothermal power plant near Ngāwhā, using a 45-tonne digger, when it hit the obstinate, seemingly endless object 9 metres down. He called in more drivers to peel away the mudstone encasing it. Uncovered, complete with its Medusa-like rootball, it measured 25m long and 2.75m across and weighed 60 tonnes.

Although the kauri had clearly been buried for thousands of years, Magee was astonished to see recognisable leaves and cones stuck to its underside that were still green.

The power company Top Energy called in Dargaville sawmiller Nelson Parker to examine the find. As soon as his chainsaw bit into the bark, Parker knew from the dark-yellow colour of the sawdust, and from the smell, that the tree was very old and worth a lot of money.

He also knew that ancient swamp kauri are worth a lot to science. One this large would be of special interest to a group of scientists who study the information that the preserved trees have coded into their rings. After removing the roots, he cut a 10cm-thick slice from the base of the trunk and sent it to them for analysis. (Top Energy subsequently gifted the kauri to the marae.)

What Parker couldn’t know then was that this particular tree held the key to understanding an ancient environmental catastrophe, and how it may have shaped our collective past.

Analysis at the University of Waikato’s radiocarbon dating lab and further scientific modelling revealed that the Ngāwhā kauri lived about 42,000 years ago, its 1600-year lifespan coinciding with dramatic geomagnetic and solar events that triggered several centuries of climate change.

A paper resulting from an international scientific collaboration, just published in Science magazine, sheds light on this period and the lessons it may hold for the planet’s future.

The study opens up the possibility that, contrary to established wisdom, severe geomagnetic and solar events could affect global climate, though the scientists involved emphasise that these events don’t explain current warming.

“It gives you a view of a much more changeable planet, and a much more changeable climate system,” says one of the lead authors, Alan Cooper, an ancient-DNA researcher at the South Australian Museum.

Stories to tell

There are other ancient trees in the world, but none as old, as long-lived or as numerous as the buried kauri.

They fall into two age clusters: young trees that died anywhere between a few thousand and about 13,000 years ago, and ancient ones that were alive more than 25,000 years ago, before the last glacial maximum. In the northern hemisphere, because migrating ice sheets demolished everything in their path, few trees survived the glacial periods. Northland, however, remained ice-free.

Swamp kauri logs unearthed from paddocks, wetlands and building sites in Northland and Auckland have important stories to tell. Samples can help scientists answer questions about climate patterns such as El Niño and add detail to know­ledge about sea-level rise, the frequency of volcanic eruptions and the timing of past events, including species extinctions.

“The kauri are globally unique,” says Andrew Lorrey, a climate scientist from the National Institute of Water and Atmospheric Research (Niwa), who contributed to the Science paper. Lorrey came here from the US in 2002 to study swamp kauri. “There’s no other wood resource like it for this part of Earth’s history, full stop.”

Other natural climate archives, such as ice cores, lake sediments and stalactites and stalagmites, also allow scientists to peer into the past. But trees are the “gold standard”, Lorrey says, because they directly sample the atmosphere and make a record of it and other aspects of the environment in each annual growth ring of wood they lay down.

Unlike ice cores and lake sediments, tree rings don’t compress over time. Multiple trees growing at the same time can be cross-referenced, too, smoothing out any local or individual variation that may interfere with broad conclusions about the climate. (Imagine a single tree growing poorly for a few seasons because its roots were waterlogged or it was shaded by others.) Long-lived, well-preserved kauri are therefore a kind of “high-resolution time-capsule”, Lorrey says.

Another contributor to the study, Alan Hogg, is director of Waikato’s radiocarbon dating lab and a globally recognised expert in the process of determining the age of objects containing organic material by measuring their radioactive isotopes. Dating swamp kauri doesn’t just tell scientists how long ago the trees lived, he says. By combining that data with radiocarbon information from other natural archives, such as lake sediments or stalagmites, they can reconstruct more precise timelines of the deep past.

Tiny amounts of radiocarbon, or 14C – a radioactive carbon isotope – build up in plants as they absorb carbon dioxide from the atmosphere as a result of photosynthesis, Hogg explains. Animals, including humans, acquire 14C, too, by eating the plants.

From the moment the plant or animal dies, the radiocarbon in its cells begins to decay at a constant rate. “If you had a pile of 14C atoms in your hand, in about 5730 years, half of them would be gone. After about 60,000 years, they would be impossible to detect.”

Climate and extinction

After the Ngāwhā log was unearthed, fist-sized samples of it ended up in Hogg’s lab, where it was ground up, purified, dehydrated and its carbon transformed into a vial of pure benzene.

Further high-tech analysis, repeated several times, confirmed the kauri had a radiocarbon age of between 36,000 and 39,000 years. But the radiocarbon age is not the same as the tree’s calendar age, because although the rate of 14C decay remains constant, the amount of 14C in the atmosphere varies through time as a result of changes in solar radiation, the strength of the Earth’s magnetic field and, more recently, human activities such as setting off nuclear bombs. That means tree rings from two different years can end up with the same amount of 14C.

To correct this variance, scientists must assemble a reliable historical record of radiocarbon variation – a calibration curve – that translates radiocarbon ages into calendar dates. A group of international scientists, including Hogg, spent seven years working on the latest version of the calibration curve, released in 2020. The more precise curve will help to date ancient ruins and archaeological finds more accurately and clarify the frequency of volcanic eruptions and earthquakes.

It has already led to new insights. About 15,000 years ago, melting ice sheets caused sea levels to rise more than 16m and flood Southeast Asia’s Sunda Shelf, which stretches from the Malay Peninsula across Borneo to Bali. The new curve reveals that the flooding took place even faster than scientists thought, over just 160 years –a metre a decade. The lesson that sea levels can rise suddenly, rather than steadily tracking global temperatures, may have implications for us as the Antarctic ice sheets melt.

The new curve also indicates that modern humans and Neanderthals coexisted in Europe for only 4000 years, not 6000 as previously thought. Likewise, scientists now know that the oldest painting in France’s Chauvet Cave is about 450 years older than earlier estimates, dating back 36,500 years.

Each ancient kauri discovered and sampled has the potential to add more detail to the curve, increasing its accuracy further, and revealing more about climate change, extinctions and human prehistory. “What happened when is everything,” Lorrey says. “If we can pin down the timing, we can understand cause and effect – and that’s what kauri can help us figure out. It’s the cipher.”

Sun-flare dangers

Beyond their use in dating, the radiocarbon patterns in kauri rings may also have something to teach us about changes in Earth’s geomagnetic field. In 1859, the sun erupted in a massive solar flare and coronal mass ejection, sending a barrage of radiation towards Earth.

The resulting day-long geomagnetic storm – called the Carrington Event – took out the nascent telegraph system. The aurora was visible in Mexico, Cuba, Hawaii and Queensland. The flares were so bright in the northeastern United States that people could read the newspaper by their light, and gold miners in the Rocky Mountains began preparing breakfast in the middle of the night because they mistook the glow for dawn.

Radiocarbon research has pinpointed even larger solar storms further back in history. In 2012, Japanese researcher Fusa Miyake used Japanese tree rings to show that a much more powerful flare – now called a Miyake Event – occurred about 774AD, leaving a dramatic radiocarbon spike in the wood. The same year, the medieval Anglo-Saxon Chronicle – a history written in the ninth century – recorded an eerie “red crucifix” in the heavens.

An identical radiocarbon spike showed up in swamp kauri rings at the same time, according to a Swiss study that included contributions from Hogg and University of Auckland dendrochronologist Gretel Boswijk. Scientists identified similar Miyake Events in 994AD and 5480 BCE. High-resolution swamp kauri records could reveal yet more from tens of thousands of years ago.

Researchers’ interest in these events is not merely academic. Even a Carrington-sized geomagnetic storm, while not harmful to humans, may be powerful enough to knock out electrical and communications systems across the planet, including satellites, GPS, power grids and the internet.

“We need to know if something 100 times more powerful than a Carrington Event could happen,” says Hogg. “This is not scaremongering. These are huge events that could fry everything. We want to know, ‘What is the most damage that the sun can do?'”

A deadly boundary

Solar storms aren’t the only geomagnetic events to write themselves into kauri wood. About 42,000 years ago, the Earth’s magnetic field went wandering for a millennium – and briefly reversed entirely – in what’s called a geomagnetic excursion. Scientists named it the Laschamps Event, after the French village where they first detected the excursion’s magnetic signature in a lava flow. If one happened today, it could also cause serious problems for global communications systems, with financial losses estimated in the billions.

Given that satellite data has shown the Earth’s magnetic field is weakening and the magnetic North Pole is moving more rapidly than expected, some scientists speculate that an excursion is nigh. We don’t know enough about these events to be able to predict when they’re going to happen, and what effect they might have on life on Earth when they do, but the Ngāwhā kauri offers some stunning clues.

When Hogg adjusted the radiocarbon dates of the tree based on the calibration curve, he realised they spanned the onset of the Laschamps Event. The kauri had sprouted into a sapling about 42,715 years ago; 1600 years later, it crashed into the mud.

Hogg and Lorrey worked with Cooper and Chris Turney, a professor of climate change and earth science at the University of New South Wales, to match the information in the tree’s rings with other natural archives to build a precise sequence of events. They were among 33 scientists from 10 countries who contributed to the paper in Science. (This is the first paper New Zealand-born Cooper has led since he was fired from the University of Adelaide in December 2019 following bullying allegations.)

When the Ngāwhā kauri was a few centuries old, Earth’s magnetic field weakened dramatically, Cooper says. Coincidentally, the sun went into one of its periodic, century-long dormant periods, called a “grand solar minima”, in which it produces many fewer sunspots than usual and gives off less energy.

This, in turn, decreased the flow of solar wind around the planet, which normally provides another protective magnetic field. With little defence against cosmic and solar radiation, Earth’s atmosphere was bombarded for a few hundred years with ionising particles from space. Lightning likely raged, auroras shimmered even in temperate skies and weather patterns abruptly altered. “It would have been freaky as,” says Cooper. The Ngāwhā kauri lived through it all.

The Adams Event

Over the following centuries, ice sheets rapidly expanded across North America, while Australia shifted to a more arid climate, its inland lakes drying up. Cooper suggests those changes are what drove the extinction of numerous species of Australian megafauna – such as the giant, wombat-like diprotodon – more than 10,000 years after modern humans arrived on the continent. The implication is that humans alone weren’t responsible for the extinctions: they had only to stake out diminishing waterholes to finish off the ill-fated animals.

The study’s authors suggest the combination of the solar minima and the weakened magnetic field triggered climate changes and flow-on effects for humans and animals. “It explains a bunch of patterns about 42,000 years ago that had previously been mysteries,” says Cooper. The paper proposes calling this turbulent transition period at the start of Laschamps the “Adams Event”, after Douglas Adams. In the British author’s Hitchhiker’s Guide to the Galaxy novels, the answer to “Life, the Universe, and Every­thing” is 42.

“The Adams Event appears to represent a major climatic, environmental and archaeological boundary that has previously gone largely unrecognised,” the paper’s authors write. It is roughly when figurative art first appeared in caves all over the world, from Europe to Indonesia. Neanderthals went extinct just afterwards – about 40,900 years ago.

Cooper suggests the onset of the Laschamps Event may have something to do with those events, too – though some other scientists think this is where the paper gets too speculative.

But the possibility that severe geomagnetic and solar events may affect global climate could launch a whole new multidisciplinary field of research, Cooper says.

The kauri dug up in Ngāwhā was the key to it all – a kind of Rosetta Stone. “It’s very unusual to get the Earth’s geomagnetic field this low,” says Hogg, “and to get such a long-lived tree that grew right through it – we should be doing more with it. This is just the start.”

• A version of this story was first published in bioGraphic, a project of the California Academy of Sciences

Source: Read Full Article