Between 2.5 and 2 billion years ago, Earth underwent the most profound and sweeping chemical transformation ever recorded on its surface. During this pivotal era in deep time, oxygen began accumulating in the atmosphere for the very first time, setting into motion a complex cascade of environmental and geological changes. This atmospheric shift ultimately helped make possible the rise of complex, multicellular organisms, including the plants and animals that would populate the planet roughly half a billion years ago.
As atmospheric oxygen levels rose, enormous quantities of microbial organic material were buried beneath the seafloor. This massive burial sequestered carbon deep within the Earth’s crust, locking it away in rocks and leaving behind a distinct and unusual isotopic signature. For decades, many geologists and geochemists have interpreted this chemical signature as definitive evidence that Earth’s global carbon cycle experienced a dramatic, planetwide imbalance during the Paleoproterozoic era.
A Famous Signal From Early Earth
Evidence supporting the hypothesis of a global disruption has traditionally come from drill cores—long, cylindrical samples of solid rock extracted from deep underground. These cores were recovered from ancient marine sedimentary deposits in Karelia, Russia, and the Francevillian Basin in Gabon. However, a new study led by researchers at the California Institute of Technology (Caltech) is raising critical questions about whether the Russian evidence truly records a sweeping, planetwide event, or if scientists have been misinterpreting localized geological phenomena for global shifts.
"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 Ted and Ginger Jenkins Leadership Chair of the Division of Geological and Planetary Sciences.
Thiagarajan, who earned her PhD at Caltech in 2012, points out that the clues were locked away in microscopic details. "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," she explains. "And we 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 is the lead author of a recent paper published in the scientific journal Geology. The study details how the research team successfully reconstructed the chronological sequence of environmental and chemical changes preserved within Karelia’s ancient rocks following the initial major increase of oxygen in Earth’s early atmosphere.
To understand the significance of these findings, scientists look to carbon isotopes—which are heavier or lighter atomic forms of the element carbon. These isotopes provide vital information regarding the biological origins and environmental conditions of organic material that accumulated billions of years ago. By measuring the relative amounts of these isotopes preserved in drill cores, often referred to as carbon-isotope signals, researchers can construct a detailed historical record of ancient environmental changes. In many ways, this process is similar to reading the annual growth rings of an ancient tree to understand past climates.
The unusual carbon signal discovered in both the Russian Zaonega Formation and rocks from Gabon is widely known in the geological community as the Shunga-Francevillian event. For generations, scientists have pointed to this specific anomaly as prime evidence that Earth experienced a massive 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," explains Aivo Lepland, a researcher at the Geological Survey of Norway (NGU) in Trondheim and a co-author on the new study. "What we are trying to assess are the causes and consequences of Earth oxygenation. 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 research team analyzed drill cores that had been carefully stored and archived at NGU. Their investigation focused specifically on gases sealed tightly inside microscopic fluid inclusions within rock samples taken from the Zaonega Formation, which are exceptionally rich in pyrobitumen.
The Zaonega Formation was not always a rocky expanse; it was once part of an ancient marine sedimentary basin. Pyrobitumen itself is an insoluble form of organic carbon that develops when buried crude oil, or kerogen—which serves as the natural source material for crude oil and natural gas—is subjected to intense heating deep beneath the Earth’s surface over long spans of geological time.
The collaborative project began to take shape after Lepland arrived at Caltech to spend a sabbatical. He brought with him a fresh collection of isotope measurements derived from gases trapped inside Zaonega rocks, data that had not yet been fully interpreted or understood within the context of existing global models.
At roughly the same time, Thiagarajan and Eiler had recently concluded a separate body of research focused on measuring isotope ratios in modern and ancient natural gases. That independent work had helped the Caltech researchers develop a broader, more sophisticated theoretical framework for understanding the chemical pathways through which natural gas forms deep underground.
When the researchers combined their two complementary sets of expertise and analytical data, a radically different explanation for the ancient isotope signatures began to emerge. Instead of pointing outward toward a planetary crisis, the data pointed inward toward localized subterranean mechanics.
Magma, Methane, and Microbes
Based on their combined analysis, the research team proposes a dynamic local scenario. A sheet of molten magma forced its way upward, intruding through distinct layers of marine sediment at the Zaonega Formation, which at the time lay submerged beneath a prehistoric ocean. The immense heat radiating from this intruding magma warmed surrounding sediments that were densely packed with organic material.
That thermal heating process generated a variety of hydrocarbons, including significant amounts of methane and propane gases. These newly formed hydrocarbons then migrated upward through the overlying sediment column, eventually reaching near-surface environments where populations of microbes living near the ancient seafloor consumed the methane.
These methane-consuming microbes subsequently produced biomass carrying a characteristically light carbon isotope signature. According to the researchers, this biological activity can readily explain the unusual isotopic signal preserved in the rocks today, removing the need to invoke a global environmental catastrophe.
Physical evidence gathered from the region strongly supports this thermal scenario. The researchers identified a large and distinct thermal gradient across the rock layers. Temperatures reached approximately 350 degrees Celsius right alongside the magma intrusion, but dropped off significantly to about 72 degrees Celsius at an ancient seafloor asphalt spill located roughly 300 meters higher in the stratigraphic column.
"This chain of geological and biological processes can account for the unusual carbon-isotope signal recorded at the Zaonega Formation," Thiagarajan says. "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 environmental processes operating at the time, their analytical results clearly indicate that the carbon isotope anomaly preserved at Zaonega was driven primarily by events confined within the local sedimentary basin rather than by a sweeping global disturbance.
"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 notes.
Testing the Theory in Gabon
With the Russian site re-evaluated, the logical next step for the scientific team will be to determine whether this same local geological mechanism can also account for the similar isotope signal discovered thousands of miles away in Gabon.
To test this hypothesis, the researchers plan to analyze new rock 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 initiative is to test whether local geological and biological processes, similar to those identified in the Russian samples, also shaped the ancient rock record in Gabon.
In the summer of 2025, Lepland spent four months stationed in Gabon to oversee and coordinate the complex drilling campaign. The newly recovered drill cores successfully arrived at the NGU facilities in Norway in February and are 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, highlighting the collaborative nature of the ongoing work. "This is how science moves forward."
The research paper published in Geology is formally titled "Paleoproterozoic thermogenic hydrocarbon generation, Zaonega Formation, Russia." In addition to Thiagarajan, Eiler, and Lepland, additional co-authors on the study include Florian Eichinger of Hydroisotop GmbH, a specialized natural isotope analysis laboratory based in Germany, and Anthony Prave of the University of St. Andrews in Scotland.
