Roughly 2.5 to 2 billion years ago, the 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 of 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 in support of 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 in the Francevillian Basin in Gabon. Now, though, those geochemical clues have been brought 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 (PhD '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."
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 in 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, Africa, 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."
To explore the Shunga–Francevillian event anomaly from a different angle, the research team used drill cores housed at NGU to investigate the molecular and isotopic composition of gases trapped as 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 were 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 at the Zaonega Formation—then deep under the waters of a prehistoric ocean—and heated organic-rich sediments. This produced hydrocarbon molecules like methane and propane that then migrated upward and fed methane-consuming microbes near the seafloor that produced biomass with a light carbon isotope signature. The team's measurements revealed a broad temperature gradient ranging from approximately 350 degrees Celsius next to the magma intrusion to 72 degrees Celsius at an ancient seafloor asphalt spill roughly 300 meters 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."
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," says Thiagarajan.
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."
The Geology paper is titled "Paleoproterozoic thermogenic hydrocarbon generation, Zaonega Formation, Russia." Additional authors on the study are Florian Eichinger of Hydroisotop GmbH, a natural isotope analysis laboratory in Germany, and Anthony Prave of the University of St. Andrews in Scotland.
