Fossil eggshells from giant birds that lived more than 15 million years ago have given scientists a new way to explore how Earth’s plants responded during a much warmer climate. By analyzing a rare form of oxygen preserved inside the shells, the team found evidence suggesting that plant activity may have been significantly lower than it is today.
The middle Miocene, between about 17 million and 15 million years ago, is one of the closest natural comparisons scientists have for a warmer Earth. Atmospheric carbon dioxide levels were slightly higher than today, global temperatures were warmer, sea levels were elevated, and much of the planet’s water had not yet become locked in the large polar ice sheets.
As reported in Geochimica et Cosmochimica Acta, fossil eggshells can preserve chemical signatures that reach much deeper into Earth’s history, offering a new way to investigate how the planet’s biosphere functioned millions of years ago.
Fossil Eggshells Hold A Rare Atmospheric Signature
The study focused on eggshells laid by extinct giant birds that once lived in what is now the Namib Desert, which stretches about 2,000 kilometers across Angola, Namibia, and South Africa. Inside the shells, researchers measured oxygen-17, a rare stable isotope that can reveal information about ancient atmospheric processes.
Oxygen-17 is transferred into carbon dioxide through reactions involving ozone and sunlight in the upper atmosphere. During photosynthesis, plants absorb this carbon dioxide, altering atmospheric oxygen-17 levels. Birds then preserve this isotopic signature in their eggshells through the air they breathe, the food they eat, and the water they drink.
Measuring oxygen-17 in fossils has long been a technical challenge because the isotope is so scarce. The researchers developed a laser-based technique that requires ten times less sample material than previous methods, making it possible to analyze these ancient fossils while preserving much more of each specimen.
A New Way To Measure Ancient Plant Productivity
The team used oxygen-17 to estimate primary productivity, the rate at which plants absorb atmospheric carbon dioxide through photosynthesis. Determining primary productivity directly for ecosystems that vanished millions of years ago is impossible. Even today’s satellites cannot measure every plant or reconstruct biological activity from the distant past. Scientists therefore rely on indirect indicators preserved in fossils and rocks.
The study explains that oxygen-17 offers one such indicator. When plant growth is more active, more carbon dioxide, and with it oxygen-17, is removed from the atmosphere. Animals absorb that oxygen through normal biological processes, and its isotopic composition becomes locked inside hard tissues such as eggshells and teeth.
This approach extends well beyond the time covered by ice cores, allowing researchers to investigate periods of Earth’s history that were previously difficult to study using atmospheric evidence.
A Slowdown In Miocene Plant Activity
The prototype instrument was assembled during the first lockdown in 2020, after which the team spent three years analyzingdozens of fossil eggshell samples. The team found that around 15 million years ago, Earth’s biosphere appeared to be operating more slowly than it does today. Their measurements suggest that plants may have been about 40% less active at absorbing carbon dioxide than modern vegetation.
Researchers describe the findings as an early step. The models relating oxygen-17 in fossils to the global carbon cycle are still being refined, and independent laboratories will need to reproduce the results before they can be fully confirmed.
Even so, the work demonstrates that fossil eggshells can preserve detailed chemical records of Earth’s ancient atmosphere. By unlocking those signals, researchers have introduced a new tool for studying how plants and the carbon cycle functioned during one of the planet’s best-known greenhouse climates.
