Science & Tech

Ancient Deepsea Shells Reveal 66 Million Years Of Carbon Dioxide Levels

Co-authored by two Texas A&M scientists, a new analysis using ancient marine organisms provides important past climate data and context to modern carbon emissions.
By Leslie Lee, Texas A&M University College of Geosciences June 14, 2021

black and white image of a microscopic foraminifera shell
Chemical fingerprints of past carbon dioxide levels are preserved in microscopic fossil shells such as this foraminifera.

Foraminifera Project

In a new study, scientists have estimated carbon dioxide levels from the past 66 million years using two methods analyzing tiny organisms found in sediment cores from the deep seafloor, and found a consistent picture of the evolution of the ocean-atmosphere carbon dioxide levels.

Co-authored by Yige Zhang, assistant professor in the Department of Oceanography at Texas A&M University, and oceanography graduate student Xiaoqing Liu, the research was conducted by an international team and recently published in the Annual Review of Earth and Planetary Sciences.

This new environmental reconstruction research shows that 21st century carbon emissions have the potential to return atmospheric carbon dioxide (CO2) to levels not seen since the much warmer climates of Earth’s distant past.

The study found CO2 levels of more 1500 parts per million (ppm) in the atmosphere associated with extreme global warmth 50 million years ago, a climate so warm that no ice existed on the poles, the researchers said.

“For instance, at CO2 levels of around 1500 ppm, last seen about 50 million years ago, it was so warm that we find fossilized alligators in the Arctic,” said James Rae, lead author of the paper and reader in the School of Earth and Environmental Sciences at the University of St. Andrews.

For the last one million years, CO2 levels slowly oscillated between 180 ppm and 300 ppm, as Earth’s climate transitioned in and out of the ice ages, with the dawn of agriculture and human settlement only establishing when CO2 and climate stabilized at the end of the last ice age, around 10,000 years ago, Rae said.

“In the last 100 years, fossil fuel burning has driven CO2 from 280 up to 414 ppm,” he said.

Understanding Ancient Atmosphere Using Tiny Ocean Organisms

“Societally, we all care about carbon dioxide in the atmosphere, and in paleoclimatology we have these methods to look at CO2 variations in Earth’s history,” Zhang said. “And in particular, these two-ocean marine-based methods are the two most common and arguably the most accurate. We used those two state-of-the-art methods, analyzed how they compare with each other, and looked at what history they inform us about for the last 66 million years.”

The researchers used two proxy methods to reconstruct carbon dioxide levels during the last 66 million years, also known as the Cenozoic Era: one method using carbon isotopes in alkenones, and another method using boron isotopes in planktic foraminifera – single-celled organisms with calcite shells, common in marine cores.

Zhang collaborated with researchers from the University of St. Andrews and the University of South Hampton, both in the U.K., and the Swiss Federal Institute of Technology in Zürich.

“These data allow us to see how CO2 has changed through Earth’s history, and the impact this has had on climate,” Rae said. “Over the last 66 million years we find that CO2 and global climate go hand in hand, with higher CO2 levels associated with dramatically different climates.”

chart graphic
At CO2 levels of around 1500 ppm, last seen about 50 million years ago, it was so warm that we find fossilized alligators in the Arctic, researchers say.

Courtesy of James Rae


Zhang helped oversee the alkenones method team, along with Heather Stoll, professor at ETH Zürich, and Liu assisted in the research. Rae worked on the boron isotope methods, along with Gavin Foster of the University of Southampton and Ross Whiteford of the University of St. Andrews.

“The alkenone method begins with biomarkers that were produced by algae millions of years ago, and these algae, like algae today, were photosynthetic – just like plants or trees on land,” Zhang said. “When these algae photosynthesize, they fractionate carbon isotopes. That process is determined by a number of factors, and one of them is the amount of CO2 available to them in seawater.”

These biomarkers leave a timeline of carbon levels, he said.

Samples were taken from cores of mud from the deep sea floor, where microscopic fossils and ancient molecules accumulate, preserving the history of CO2 and climate. By firing these ancient atoms through super sensitive instruments, scientists can detect the chemical fingerprints of past changes in CO2.

“The shells and molecules we measure capture the chemistry of seawater at the time they grow,” Rae said. “Over the last 20 years scientists have figured out how to use these chemical fingerprints to reconstruct how much CO2 was in seawater and the atmosphere above it.”

These methods have been the focus of the researchers’ various efforts for years, and together they aim for this paper to contribute to wider efforts to combine a broader range of paleo-CO2 proxies. Recent refinements to these methods now allow more accurate and precise reconstructions of the ancient atmosphere.

“This refined method produces CO2 estimates that are in line with the ice core-derived CO2 records over the last one million years,” Zhang said. “This gives us more confidence in extending the reconstructions to the entire Cenozoic era.

The last time CO2 was as high as it is today was three million years ago, Rae said, and it drove enough warming to melt all of the ice from Greenland and West Antarctica and raise sea level by around 60 feet.

“It takes time for global warming to catch up with CO2 levels, but the geological record shows us where we’re headed if we don’t use cleaner ways to get our energy,” Rae said.

Media contacts:

  • Robyn Blackmon, College of Geosciences,
  • Leslie Lee, College of Geosciences,

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