As oxygen levels first increased, vast amounts of microorganisms were buried on the ocean floor. Their burial trapped carbon in rocks and left behind unusual carbon isotope signals. For decades, many scientists have interpreted these traces as evidence that Earth’s carbon cycle became dramatically unbalanced on a global scale.

Ancient carbon signals from Russia and Gabon

Evidence for that interpretation comes from drill cores—long cylinders of solid rock recovered from deep underground—from ancient marine deposits in Karelia, Russia, and the French Villian Basin in Gabon. However, a new study led by researchers at the California Institute of Technology raises questions about whether the Russian evidence truly records a global event.

“One major debate has focused on anomalous carbon isotope signals that are often interpreted as evidence of global environmental change,” says Nivedita Thiagarajan, Ph.D. ’12, a senior research scientist at Caltech who works in the laboratory of John Eyler, the Robert P. Sharp Professor of Geology and Geochemistry and the Ted and Ginger Jenkins Chair in Geological and Planetary Sciences.

“We studied gas trapped in microscopic pockets within rocks of the Zaonega Formation in Russia’s Karelia, known as the world’s oldest fossil oil field. We found that the carbon isotope signal at this important location can be explained by local phenomena occurring in a sedimentary basin of several hundred square kilometers, rather than globally.”

Thiagarajan is the lead author of the recently published paper in Geology. The study describes how researchers reconstructed a series of changes preserved in Karelia’s rocks after the first major increase in atmospheric oxygen.

Carbon isotopes—heavier or lighter forms of carbon—can provide information about the origin of biological materials that accumulated billions of years ago. By measuring the relative abundance of these isotopes in drill cores, researchers can build a record of ancient environmental change, similar to reading tree rings.

The anomalous carbon signal found in both the Zaonega Formation and rocks in Gabon is known as the Shunga-Franceville phenomenon. Scientists have often cited it as evidence that Earth experienced a major global disruption of the carbon cycle about 2 billion years ago.

“In a sense, the Earth went crazy during that period when oxygen appeared in the atmosphere. What we are trying to assess is the causes and consequences of the oxygenation of the Earth,” explains Ivo Lepland, a researcher at the Norwegian Geological Survey in Trondheim and a co-author of the study. “This information is stored in the rocks, so we need to study the rocks to study what happened.”

Gas trapped in rock for 2 billion years

To examine the Shunga-Franceville phenomenon from another perspective, the researchers analyzed drill cores stored at the Norwegian Geological Survey. They focused on gas trapped inside microscopic fluid inclusions within samples of the pyrobitumen-rich Zaonega Formation.

The Zaonega Formation was once part of a marine sedimentary basin. Pyrobitumen is an insoluble form of organic carbon produced when buried crude oil or kerogen—a feedstock for natural gas—is exposed to intense heating deep beneath Earth’s surface.

The project began after Lepland arrived at Caltech for a sabbatical. He brought a new collection of isotope measurements from gas trapped in Zaonega’s rocks that had not yet been fully interpreted.

At the same time, Thiagarajan and Eyler had recently completed research measuring isotope ratios in natural gas. That work helped develop a broader framework for understanding how natural gas forms.

When the researchers combined their expertise and data, an alternative explanation for the ancient isotope signatures began to emerge.

Magma, methane and microorganisms

The researchers propose that a layer of magma penetrated marine sediments in the Zaonega Formation, which lay directly beneath the prehistoric ocean. Heat from the magma warmed the organic-rich deposits.

That heating produced hydrocarbons such as methane and propane. The gases then moved upward through the sediments, eventually reaching microorganisms living near the ocean floor. These microorganisms consumed the methane and produced biomass with light carbon isotope signatures, potentially explaining the anomalous signals preserved in the rocks.

Temperature evidence supports this scenario. The researchers identified a large temperature gradient, with temperatures reaching about 350 degrees Celsius near the magma intrusion and falling to about 72 degrees Celsius at an ancient submarine asphalt spill site approximately 300 meters higher.

“This set of geological and biological processes could be responsible for the anomalous carbon isotope signal recorded in the Zaonega Formation,” Thiagarajan says. “It was interesting to see that some of the same signatures observed in modern oil and gas reservoirs were present and preserved in samples from 2 billion years ago.”

The researchers stress that contributions from other processes cannot be completely excluded. Still, their results indicate that the carbon isotope anomalies preserved in Zaonega were primarily caused by events within the local sedimentary basin rather than by global disturbances.

“Since Zaonega is the reference location for the Shunga-Franceville event, our findings raise important questions about whether it should be considered a global event,” Thiagarajan says.

Testing the theory in Gabon

The next step is to determine whether the same explanation can account for similar isotope signals found in Gabon.

Researchers plan to analyze samples collected through the GOE-DEEP project, which is co-funded by the International Continental Scientific Drilling Program. The goal is to test whether local geological and biological processes like those identified in Russia also shaped Gabon’s rock record.

In the summer of 2025, Lepland spent four months in Gabon coordinating a drilling campaign. The newly recovered core arrived at the Norwegian Geological Survey in February and will be sampled later this year by an international research team representing 18 countries.

“By doing a similar study of the rocks in Gabon and comparing the two sites, we were able to really put things together,” Lepland said. “This is how science progresses.”

The study is titled “Paleoproterozoic thermogenic hydrocarbon production in the Zaonega Formation, Russia.” Additional authors are Florian Aichinger of Hydroisotop GmbH, a natural isotope analysis laboratory in Germany, and Anthony Prave of the University of St. Andrews in Scotland.