Trapped gases in ancient rocks challenge global explanation for 2-billion-year-old carbon anomaly
Roughly 2.5–2 billion years ago, Earth's surface experienced the biggest chemical change in its history when oxygen became abundant in the atmosphere, eventually leading to the evolution of complex life forms such as plants and animals that emerged half a billion years ago.
During this first rise in oxygen, large quantities of microbial biomass were buried on the seafloor and locked carbon into rocks with an anomalous isotopic signature. Many researchers think this signature is a sign of the carbon cycle thrown off balance across the whole planet.
Chemical evidence supporting a global event has been found in drill cores—long cylinders of solid rock pulled from deep underground—extracted from ancient seabed successions in Karelia, Russia, and the Francevillian Basin in Gabon. Now, though, those geochemical clues have been called into question by new work led by researchers at Caltech, who say the Russian drill-core evidence could have another explanation.
"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 (Ph.D. '12), a senior scientific researcher at Caltech who works in the lab 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.
"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."
The signal under debate
Thiagarajan is the lead author on a recent paper published in the journal Geology that explains the team's approach to reconstructing the changes seen in the rock record at Karelia during the aftermath of the first major buildup of oxygen in the atmosphere.
Carbon isotopes—heavier or lighter forms of the element—left geochemical signals in the biomass that accumulated billions of years ago and yield clues about its source. Measurements of ratios of these carbon isotopes, or carbon-isotope signals, found in drill cores act as a timeline of past environmental changes on the planet, much like rings in a tree.
The carbon-isotope anomaly seen in core samples from the Zaonega Formation in Karelia and at another location in Gabon is known as the Shunga–Francevillian event and has been cited as evidence for a global carbon-cycle change roughly 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 study. "This information is archived in the rocks, so, in order to study what happened, you have to study rocks."
Tracing gases in ancient rocks
To explore the Shunga–Francevillian event from a different angle, the research team used drill cores housed at NGU to investigate the molecular and isotopic composition of gases trapped in fluid inclusions in pyrobitumen-rich samples of the Zaonega Formation, which is part of an ancient marine sedimentary basin. Pyrobitumen is an insoluble type of organic carbon that forms when trapped crude oil or kerogen—a source material for natural gas—is exposed to intense heat deep underground.
The collaboration began when Lepland came to Caltech for a sabbatical. Lepland brought along a new dataset of isotope signatures from trapped gases in Zaonega rocks that had yet to be interpreted. Meanwhile, Thiagarajan and Eiler had just completed work measuring isotope ratios in natural gases, which had led them to develop a broad theory to explain the mechanisms of natural-gas formation.
Combining their data and expertise, the group arrived at a surprising explanation for the trapped-gas isotope signatures. Their hypothesis suggests that a sheet of magma intruded through layers of marine sediments in the Zaonega Formation—then deep beneath a prehistoric ocean—and heated organic-rich sediments.
This produced hydrocarbon molecules like methane and propane that migrated upward and fed methane-consuming microbes near the seafloor, producing biomass with a light carbon isotope signature. The team's measurements revealed a broad temperature gradient ranging from 350°C (662°F) next to the magma intrusion to 72°C (162°F) at an ancient seafloor asphalt spill roughly 300 meters (980 feet) above it.
"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."
Questions now shift to Gabon
While the researchers say they cannot fully exclude contributions from other processes, their data support a predominantly local rather than global driver for the carbon-isotope anomaly recorded in the Zaonega Formation.
"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 says.
Next, the team plans to look at samples from Gabon collected via the GOE-DEEP project, co-funded by the International Continental Scientific Drilling Program, to explore whether the same local processes can explain the similar isotopic signals seen in the record there. In the summer of 2025, Lepland spent four months in Gabon coordinating the drilling campaign; the cores arrived at NGU in February and will be sampled by an international science team from 18 countries later this year.
"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."
Publication details
Nivedita Thiagarajan et al, Paleoproterozoic thermogenic hydrocarbon generation, Zaonega Formation, Russia, Geology (2026). DOI: 10.1130/g54338.1
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Citation: Trapped gases in ancient rocks challenge global explanation for 2-billion-year-old carbon anomaly (2026, August 10) retrieved 10 August 2026 from https://phys.org/news/2026-08-gases-ancient-global-explanation-billion.html
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