A Sino-Norwegian study of pollen over 1.7 million years shows how surprisingly little the species mix of plants and trees have changed east of the Tibetan Plateau. Researchers also saw how natural fluctuations were constantly repeated.
A Sino-Norwegian study of pollen over 1.7 million years shows how surprisingly little the species mix of plants and trees have changed east of the Tibetan Plateau. Researchers also saw how natural fluctuations were constantly repeated.

Chinese and Norwegian scientists have drilled an almost 600-meter deep hole into the Tibetan Plateau. This gives them a window into the past.

Among their findings is that ice age cycles suddenly became stronger just over 600,000 years ago. Researchers were also struck by how little plant life has changed over the past 1.7 million years.

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In the far east of the high Tibetan Plateau in China lies a place called Zoige.

For millions of years and until not many thousands of years ago, this area was covered by a large lake.

The existence of the lake — and its disappearance — has provided exciting opportunities for paleoecologists John Birks and Vivian Astrup Felde from Bergen.

Along with their Chinese counterparts, they have examined the contents of a nearly 600-metre deep drill core retrieved from the bottom of the now-dried-up lake. Scientists have worked layer by layer through ever-older sediments from the Earth's past.

These sediments are full of old pollen. Pollen not only provides a record of what plants and trees grew in the area in the past — but gives researchers insights into climate change and ice ages.

The study has now been published in the academic journal Science Advances.

A look at life in the past

Paleoecologists study life in the past.

Their work often consists of studying plant and tree pollen that has been buried in sediment layers in marshes or in lake bottoms.

The Zoige drill core on the Tibetan Plateau is the second oldest core containing remnants of ancient plant life that has ever been retrieved. It provides researchers a snapshot of changes in pollen quantities from more than 1.7 million years back in time.

Today, the landscape east of the Tibetan Plateau is covered with grass and scrub. During the ice ages, the landscape here was more or less forested.
Today, the landscape east of the Tibetan Plateau is covered with grass and scrub. During the ice ages, the landscape here was more or less forested.

More powerful ice ages 600,000 years ago

Over the last two million years, the Earth has experienced something very special: shifts between ice ages and interglacial periods.

The most recent ice ages have lasted around 100,000 years each. Warm interglacial periods between them have lasted about 10,000 years each.

But scientists report in this study that something happened to the climate —620,000 years ago.

  • Up until 620,000 years ago, scientists saw quite clearly that there have been cycles of 20,000 to 40,000 years between big climate shifts. But then these cycles became more intermittent and weaker.
  • After this period, 100,000-year ice age cycles became the dominant feature of major climate change.

A very special drill core

For many years, John Birks has been a central figure in paleoecology in Norway.

He has a large international network, so it’s not surprising that Chinese scientists and engineers wanted him to participate in this particular study, which almost literally sets the world record for digging into the past.

Along with his Chinese counterparts, John Birks (pictured) and Vivian Astrup Felde from the Bjerknes Centre for Climate Research and the Department of Biological Sciences at the University of Bergen have now published a study that describes the evolution of vegetation and climate on the Tibetan Plateau over the last 1.74 million years.
Along with his Chinese counterparts, John Birks (pictured) and Vivian Astrup Felde from the Bjerknes Centre for Climate Research and the Department of Biological Sciences at the University of Bergen have now published a study that describes the evolution of vegetation and climate on the Tibetan Plateau over the last 1.74 million years.

Birks was also involved in investigating a sediment core that was retrieved from the bottom of the crater lake in El'gygytgyn in northeast Siberia a few years ago. The core goes back a full 3.5 million years in time and thus holds a clear world record.

However, this latest sediment core from China may contain a lot more information.

By comparison, there is little material from Norway that is older than from the end of the last ice age, about 12,000 years ago. Also, there are no continuous ice cores from Antarctica that have any information that is similar to this one from China.

“It goes without saying that it is difficult to get everything up under complicated drilling like this. But the Chinese engineers managed to bring up as much as 96 percent of what was downhole. This is very good,” says Birks.

Plant life goes in cycles

Understanding the Earth’s past climate has become very important in this time of human-caused climate change. This allows us to more easily distinguish between natural and human-caused changes.

Birks, Felde and their Chinese counterparts are the first in the world to show that fluctuations between high and low humidity have also been important for plant life far back in time.

Scientists and lay people alike have thought that temperature fluctuations must have been the most important factor controlling plant life, as the Earth's orbit and tilt changed and the climate shifted between ice ages and interglacial periods.

“The samples we have collected clearly show us how plant life has gone through cycles the entire time,” Birks said. “But the changes have been the same over and over again.”

The figure shows the changes in forest and climate on the Tibetan Plateau 1.7 million years back in time. Note C, which shows the amount of pollen from trees. D at the bottom shows the average summer temperature. Also note how the peaks in the interglacial period have become much clearer over the last 600,000 years, both in terms of pollen and temperature. The same goes for the lowest points of the ice age. The dotted lines show how the average climate has become colder, spread over four periods.
The figure shows the changes in forest and climate on the Tibetan Plateau 1.7 million years back in time. Note C, which shows the amount of pollen from trees. D at the bottom shows the average summer temperature. Also note how the peaks in the interglacial period have become much clearer over the last 600,000 years, both in terms of pollen and temperature. The same goes for the lowest points of the ice age. The dotted lines show how the average climate has become colder, spread over four periods.

Not much change in plant species

Pollen from plants and trees that Birks and colleagues have studied from 1.7 million years back shows how the landscape of the eastern Tibetan Plateau has hovered on the verge of becoming a high mountain desert during dry periods.

At the same time, scientists see how the forest has almost completely taken hold over and covered the landscape when the climate was more humid.

One interesting facet of the pollen record was that there were no major changes in the kind of plants that grew here — for more than a million years.

“When the climate changes, the plant life doesn’t change much. No new species are established,” Birks said. “What is changing is the abundance of different species.”

The figure below shows exactly this.

Over 1.7 million years — and a number of ice ages and interglacial periods — scientists have found surprisingly few changes in the kinds of plants and trees that have grown on the Tibetan Plateau. But the absolute numbers of each species vary, of course. For example, notice the change in spruce trees (Picea) 600,000 years ago. You can also see the changes in pine trees (Pinus), birch trees (Betula), wormwood (Artemisia), grass (Poaceae) or buttercups (Ranunculaceae). At the far right you can see changes in the landscape type over the same 1.7 million years, from forest to steppe, meadow, scrub and desert.
Over 1.7 million years — and a number of ice ages and interglacial periods — scientists have found surprisingly few changes in the kinds of plants and trees that have grown on the Tibetan Plateau. But the absolute numbers of each species vary, of course. For example, notice the change in spruce trees (Picea) 600,000 years ago. You can also see the changes in pine trees (Pinus), birch trees (Betula), wormwood (Artemisia), grass (Poaceae) or buttercups (Ranunculaceae). At the far right you can see changes in the landscape type over the same 1.7 million years, from forest to steppe, meadow, scrub and desert.

Glacial Periods and Ice Ages

We are now living in a glacial period.

At the same time, we are probably living at a time between two ice ages — in an interglacial period.

Over the last two million years, there have probably been about 40 ice ages. But it is only possible to find evidence of the last two in Norway. Any evidence of older ice ages and interglacial periods have been obliterated by more recent ice ages.

The last glacial period on Earth was as long as 330-250 million years ago.

Scientists have assumed that the ice ages must have caused major changes in plant life communities. But this new research suggests perhaps this isn’t correct.


Source: Store Norske Leksikon (Great Norwegian Encyclopedia)

Translated by Nancy Bazilchuk

Reference:

Yan Zhao et al.: "Evolution of vegetation and climate variability on the Tibetan Plateau over the past 1.74 million years", Nature Science Advances, May 6, 2020. DOI: 10.1126/sciadv.aay6193

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Read the Norwegian version of this article at forskning.no