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Jennifer Bannister / Uni Otago
A 23-million-year-old leaf: veins, holes pierced by insects and scar tissue are still visible
Fossil leaves from the remains of a 23-million-year-old forest suggest that some plants may adapt to grow faster as CO2 levels rise, says a study.
Scientists have recovered the very well-preserved leaves from an old lake on the South Island of New Zealand.
They have enabled scientists to link for the first time the high temperatures of the period with high levels of atmospheric CO2.
The results were published in the journal Climate of the Past.
In their scientific paper, the team shows that some plants produce carbon dioxide cords more efficiently for photosynthesis – the biological process that uses sunlight to produce food for the plant.
They say their findings could provide clues as to how the dynamics of plant life could shift as current CO2 levels rise to those of the distant past.
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What can we learn from these ancient leaves?
The team drilled 100 meters to near the bottom of the now dry lake bed, lying in the crater of a long-extinct volcano. The crater is located about a kilometer across.
Here biological material is fossil, including the remains of plants, algae, spiders, beetles, flies, fungi and other living things from a warm period known as the early Miocene Epoch.
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Daphne Lee / Uni Otago
The castle is located on a farm near the southern New Zealand city of Dunedin
Average global temperatures are thought to be between 3C and 7C higher than today, and ice largely disappeared from the pulse.
There has been debate among scientists about levels of CO2 over the period, which is one reason why this study is so interesting.
“The great thing is that these sheets are basically mummified, so we have their original chemical compositions, and can see all their fine features under a microscope,” said lead author Tammo Reichgelt, of the University of Connecticut at Storrs, US.
He says that they are so perfectly preserved that microscopic veins and stomata – the pores through which leaves can take up air and release water during photosynthesis – are visible.
The scientists analyzed the various chemical forms of carbon – like carbon isotopes – within leaves of half a dozen trees found at various levels in the burrow.
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Jennifer Bannister / Uni Otago
Low oxygen levels at the bottom of the lake helped maintain the leaves
This helped them to estimate the carbon content of the atmosphere over time.
They concluded that it was around 450 parts per million (ppm).
Previous studies – mainly use of marine organisms – have suggested that it was significantly lower, around 300 ppm.
This is similar to those in pre-industrial times, and not enough to justify the much higher temperatures of the early Miocene.
Human emissions have now shifted CO2 levels to around 415 ppm.
They are expected to reach 450 ppm in the next few decades – the same level that was experienced by those forests in New Zealand 23 million years ago.
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William D’Andrea
Scientists are researching the site for fossils
The researchers also analyzed the geometry of the leaf stomata and other anatomical features, and compared them with those of modern leaves.
They showed that the trees were unusually efficient at sucking carbon through the stomata, without leaking much water through the same route – a major challenge for all plants.
This allowed the trees to grow in marginal areas that would otherwise have been too forested.
The researchers said this higher efficiency was likely reflected in forests across the northern temperate latitudes, where more of the earth’s mass is located.
What does this tell us today?
As CO2 levels increase, many plants increase their rate of photosynthesis because they can remove carbon from the air more efficiently, saving water while doing so.
Data from Nasa satellites show a “global greening” effect mainly due to increasing levels of CO2 released by human activities in recent decades.
It is estimated that up to half of the planet’s vegetated lands have grown in leaf volume on trees and plants since about 1980.
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Tammo Reichgelt
Part of a mummified leaf, highly enlarged. Among the features on display are individual epidermal cells and oral stomata, openings through which the leaf takes carbon dioxide and releases water
The effect is expected to continue as CO2 levels increase.
But the authors of the new report say we should not assume that this is necessarily good news.
Increased CO2 absorption will not come close to compensating for what people are going through in the air.
And because much of today’s plant life evolved into a temperate, low CO2 world, some natural and agricultural ecosystems could be seriously disrupted by higher CO2 levels, along with the increasing temperatures and shifts in precipitation they bring.
Not all plants can benefit, and among those who do, the results may differ depending on temperature and availability of water as nutrients.
There is evidence that when some important crops rapidly photosynthesize, they absorb relatively less calcium, iron, zinc and other minerals that are essential for human nutrition.
“How it plays out is anyone’s guess,” said Dr Reichgelt. “It’s another layer of stress for plants. It can be great for some, and terrible for others.”
How are the leaves so well preserved?
The castle is located on a farm near the southern New Zealand city of Dunedin.
In the ancient crater lake, successive layers of sediment were built up from the surrounding environment over tens of millions of years.
The lake was deep and had low oxygen levels at the bottom, which means that all the prehistoric leaves that remained there were relatively well preserved, despite being 23 million years old.
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Jennifer Bannister / Uni Otago
Circular structures visible on this specimen are the reaction tissue of the leaf to some form of insect food or parasitism
These include countless leaves from a sub-tropical evergreen forest.
The castle has a layered structure with black-gray organic matter alternating with bands of white-ish silica-rich material laid by algae that bloom every spring.
The feature has only been recognized within about the last 15 years; scientists called it Foulden Maar.
It is the only known castle of its kind in the Southern Hemisphere, and has been far better preserved than the few similar ones known from the north.
What was it like working with such old material?
Tammo Reichgelt said he felt a great responsibility and “a strange kind of reverence” working with fossils of this quality that have left them restless for so long.
He described the excavation of the fossil material from a pit dug in deposits in the crater.
It was exposed to the elements, “very windy, sunny and saturated with rain”.
That made the job very challenging.
“The biggest blade I ever found was on a wet day and the brittle rock crawled into my hand with the blade on it,” he told the BBC.
“There was no saving. When this kind of thing happens, your stomach sinks and you feel like you just destroyed a pharaoh’s tomb.”
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