Caltech finds 2-billion-year carbon signal was local, not global

PASADENA, Calif. For decades the standard account of early Earth treated the Shunga-Francevillian event as a planetary milestone. Roughly 2 billion years ago a distinct carbon-isotope anomaly appears in rock cores pulled from ancient seafloor sediments in Karelia, Russia, and in the Francevillian Basin in Gabon. Geologists read that signature as evidence that Earth's carbon cycle had been thrown off balance across the entire planet during the rise of atmospheric oxygen.

A study led by researchers at the California Institute of Technology now argues that the Russian half of that evidence does not record a global event at all. The team analyzed gases trapped inside microscopic pockets of rock and concluded that the anomaly was produced locally by magma heating organic-rich sediment, which generated methane that fed communities of methane-consuming microbes on the ancient seafloor.

"One major debate centers on an unusual carbon-isotope signal that has often been interpreted as evidence of a worldwide environmental change," said Nivedita Thiagarajan (PhD '12), a senior scientific researcher at Caltech and lead author of the study published in the journal Geology. "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."

What the Signal Was Supposed to Prove | Oxygen and Buried Carbon

Between about 2.5 billion and 2 billion years ago, Earth's surface underwent the largest chemical transformation in its history. Oxygen began accumulating in the atmosphere in an episode known as the Great Oxidation Event, a change that eventually made possible the evolution of complex organisms such as plants and animals roughly half a billion years ago.

As oxygen levels rose for the first time, enormous quantities of microbial material were buried beneath the seafloor. That burial trapped carbon in rock and left behind an isotopic signature. Because living organisms preferentially process the lighter of carbon's two stable isotopes, a spike in buried biomass depresses the ratio of ¹³C to ¹²C preserved in sediment. Read across many sites at once, those ratios function like tree rings, giving geologists a timeline of ancient environmental change.

The anomaly shared by the Zaonega Formation in Karelia and the Francevillian Basin in Gabon became a cornerstone of that timeline, cited as proof that carbon burial had gone globally out of balance.

Magma, Methane, and Microbes | Reconstructing a Local System

Thiagarajan works in the lab of John Eiler, the Robert P. Sharp Professor of Geology and Geochemistry at Caltech. Aivo Lepland, a researcher at the Geological Survey of Norway in Trondheim and a co-author, arrived at Caltech for a sabbatical carrying an uninterpreted dataset of isotope measurements from gases trapped in Zaonega rocks. Thiagarajan and Eiler had at the same time finished measuring isotope ratios in natural gases, work that produced a broader theory of how natural gas forms.

Combining the two datasets changed the reading. The team examined the molecular and isotopic composition of gases sealed inside fluid inclusions in pyrobitumen-rich samples. Pyrobitumen is an insoluble form of organic carbon that develops when buried crude oil or kerogen is subjected to intense heat deep underground.

The proposed sequence runs as follows. A sheet of magma forced its way through layers of marine sediment at Zaonega, which then lay beneath a prehistoric ocean. Heat from the intrusion cooked organic-rich sediment into hydrocarbon molecules including methane and propane. Those gases migrated upward through the sediment and reached microbes living near the seafloor that consume methane. The microbes produced biomass carrying a light carbon-isotope signature, which was then hardened into the local rock record.

Temperature data supports the scenario. The researchers identified a steep thermal gradient running from roughly 350 degrees Celsius beside the magma intrusion down to about 72 degrees Celsius at an ancient seafloor asphalt spill approximately 300 meters higher. That gradient is the fingerprint of a local hydrothermal system, not of ocean chemistry shifted planet-wide.

"This chain of geological and biological processes can account for the unusual carbon-isotope signal recorded at the Zaonega Formation," Thiagarajan said. "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."

The Flip Side | What This Finding Does Not Disprove

The result is a correction to how one measurement is interpreted, not a reversal of the oxygen story. The researchers state that atmospheric oxygen did rise during the Great Oxidation Event, and they say plainly that they cannot fully exclude contributions from other processes at Zaonega. What their data support is a predominantly local rather than global driver for the anomaly recorded at that single site.

The distinction matters because Zaonega is a reference site. "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 said. If one basin's petroleum chemistry was mistaken for a planetary signal, then the calibration used to measure the scale of ancient carbon cycling needs rechecking at every site that leaned on it.

That is the same reasoning behind our coverage of the study that relocated Earth's water to the inner solar system, and the pattern our ocean deoxygenation reporting tracks in the modern ocean. Established consensus and the reassessment of how that consensus was measured often arrive in the same dataset.

The Gabon Test | Where the Research Goes Next

The open question is whether the same local explanation accounts for the matching signal in Gabon. The team plans to analyze cores recovered through the GOE-DEEP project, co-funded by the International Continental Scientific Drilling Program. Lepland spent four months in Gabon during the summer of 2025 coordinating the drilling campaign. The cores arrived at the Geological Survey of Norway in February and are scheduled for sampling later this year by a research team representing 18 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 said. "This is how science moves forward."

The outcome will decide how much of a textbook chapter gets rewritten. If Gabon also resolves to local basin chemistry, the Shunga-Francevillian event shrinks from a planetary benchmark to a regional archive of the same petroleum-generating processes that geologists read in modern oil fields. The finding also deepens the link between geology and biology that our reporting on chemosynthetic vent ecosystems documented, where microbes still turn geochemical energy into biomass at the seafloor. HoneyNewspaper's science desk, environment desk, and ethics desk will continue tracking how much confidence the record of deep time can carry.