Around 2.5 to 2 billion years ago, Earth underwent the largest and most consequential chemical transformation ever recorded at its surface. Oxygen began accumulating in the planet’s atmosphere for the very first time, setting in motion a sweeping cascade of environmental and chemical changes that eventually helped make possible the rise of complex, multicellular organisms—including the plants and animals that populate the Earth today—about half a billion years ago.
As atmospheric oxygen levels increased during this distant epoch, enormous amounts of microscopic microbial material were buried deep beneath the ancient seafloor. That massive burial trapped vast quantities of carbon in rocks and left behind an unusual isotopic signature that has puzzled geologists and geochemists for generations. For decades, many scientists have interpreted that distinct chemical signature as definitive evidence that Earth’s global carbon cycle became dramatically unbalanced on a planetary scale during the rise of atmospheric oxygen.
A Famous Signal From Early Earth
Evidence supporting that interpretation has historically come from drill cores—long, cylindrical samples of solid rock pulled from deep underground—recovered from ancient marine sediments in Karelia, Russia, and the Francevillian Basin in Gabon. However, a new study led by researchers at the California Institute of Technology (Caltech) is now raising significant questions about whether the physical evidence gathered in Russia genuinely records a massive, planetwide event, or if scientists have been misinterpreting local geology all along.
"One major debate centers on an unusual carbon-isotope signal that has often been interpreted as evidence of a worldwide environmental change," says Nivedita Thiagarajan, a senior scientific researcher at Caltech who works in the laboratory of John Eiler, the Robert P. Sharp Professor of Geology and Geochemistry and the Ted and Ginger Jenkins Leadership Chair of the Division of Geological and Planetary Sciences.
"We studied gases trapped in microscopic pockets within rocks from the Zaonega Formation in Karelia, Russia, one of the world’s oldest known fossil oil fields, and found that the carbon-isotope signal at this key site can be explained by local phenomena that occurred in a several-hundred-square-kilometer sedimentary basin rather than across the entire globe," Thiagarajan explains.
Thiagarajan serves as the lead author of a recent study published in the scientific journal Geology. The research paper details how the scientific team successfully reconstructed the sequence of geological and biological changes preserved in Karelia’s ancient rocks following the initial major increase in atmospheric oxygen.
Carbon isotopes—which exist in heavier or lighter atomic forms of the element—provide critical clues about the origins and nature of biological material that accumulated billions of years ago. By meticulously measuring the relative amounts of these isotopes preserved within drill cores, researchers can build a detailed historical record of ancient environmental change, much like reading the concentric growth rings of a tree to understand past climates.
An unusual carbon signal discovered in both the Zaonega Formation in Russia and equivalent rocks from the Francevillian Basin in Gabon is widely known in the geological community as the Shunga-Francevillian event. For years, scientists have frequently pointed to this signal as prime evidence that Earth experienced a major global disruption of its carbon cycle approximately 2 billion years ago.
"Earth, in a way, went crazy during that time interval when oxygen appeared in the atmosphere. What we are trying to assess are the causes and consequences of Earth oxygenation," explains Aivo Lepland, a researcher at the Geological Survey of Norway (NGU) in Trondheim and a co-author on the new study. "This information is archived in the rocks, so, in order to study what happened, you have to study rocks."
Gases Trapped for 2 Billion Years
To examine the Shunga-Francevillian event from an entirely different perspective, the international research team analyzed preserved drill cores stored at the Geological Survey of Norway. Their scientific investigation focused specifically on gases sealed tightly inside microscopic fluid inclusions within rock samples from the Zaonega Formation that are exceptionally rich in pyrobitumen.
The Zaonega Formation was once part of a sprawling marine sedimentary basin millions of years ago. Pyrobitumen itself is an insoluble form of organic carbon that develops naturally when buried crude oil or kerogen—a primary source material for natural gas—is subjected to intense heating deep beneath the Earth’s surface over long spans of geologic time.
The collaborative project took shape after Lepland arrived at Caltech to spend a sabbatical. He brought with him a fresh collection of isotope measurements derived from gases trapped within Zaonega rocks, data that had not yet been fully interpreted or understood by his peers.
At the exact same time, Thiagarajan and Eiler had recently finished unrelated research measuring precise isotope ratios in modern natural gases. That separate body of work had helped them develop a broader, more sophisticated theoretical framework for understanding the chemical mechanisms of how natural gas forms beneath the surface.
When the researchers combined their two complementary sets of expertise and analytical data, a completely different explanation for the ancient isotope signatures began to emerge from the numbers.
Magma, Methane, and Microbes
The research team now proposes that a sheet of molten magma forced its way upward through thick layers of marine sediment at the Zaonega Formation, which at the time lay hidden beneath the waters of a prehistoric ocean. The intense heat radiating from the invading magma warmed surrounding sediments that were densely packed with organic material.
That thermal heating generated large volumes of hydrocarbons, including methane and propane. These newly formed gases then migrated upward through the overlying sediment layers and eventually reached communities of specialized microbes living near the seafloor that consumed methane as part of their metabolic processes.
Those microbes subsequently produced biological mass carrying a characteristically light carbon isotope signature, which could potentially explain the unusual chemical signal preserved in the rocks today.
The physical temperature evidence gathered by the team strongly supports this proposed scenario. The researchers identified a large thermal gradient across the geological formations, with temperatures reaching approximately 350 degrees Celsius right alongside the ancient magma intrusion, and falling off to about 72 degrees Celsius at an ancient seafloor asphalt spill located roughly 300 meters higher up in the sedimentary column.
"This chain of geological and biological processes can account for the unusual carbon-isotope signal recorded at the Zaonega Formation," Thiagarajan notes. "It was interesting to see that some of the same signatures that we observe in modern oil and gas basins are also there and preserved in 2-billion-year-old samples."
While the researchers emphasize that they cannot completely rule out minor contributions from other global or regional processes, their empirical results clearly indicate that the carbon isotope anomaly preserved at Zaonega was driven primarily by localized events operating within a single sedimentary basin, rather than by a massive, catastrophic disturbance of the global carbon cycle.
"Because Zaonega is a reference site for the Shunga-Francevillian event, our findings raise important questions about whether it should be considered a worldwide event," Thiagarajan warns.
Testing the Theory in Gabon
With the Russian data pointing toward a localized phenomenon, the logical next step for the research team will be to determine whether the same underlying explanation can also account for the similar carbon isotope signal found thousands of miles away in Gabon.
To answer this question, the researchers plan to analyze new samples collected through the GOE-DEEP project, an international scientific endeavor co-funded by the International Continental Scientific Drilling Program. The primary goal of this ongoing initiative is to test whether local geological and biological processes, similar to those identified in Russia, also shaped the ancient rock record in Gabon.
In the summer of 2025, Lepland spent four months on the ground in Gabon coordinating a major drilling campaign. The newly recovered rock cores arrived safely at the Geological Survey of Norway in February and are currently scheduled to be sampled later this year by an extensive international research team representing scientists from 18 different countries.
"Now we can really put things together by doing a similar type of study on the Gabonese rocks to compare the two sites," Lepland says. "This is how science moves forward."
The research paper published in Geology is titled "Paleoproterozoic thermogenic hydrocarbon generation, Zaonega Formation, Russia." Additional co-authors on the scientific study include Florian Eichinger of Hydroisotop GmbH, a specialized natural isotope analysis laboratory located in Germany, and Anthony Prave of the University of St. Andrews in Scotland.
