A chemical signature preserved in rocks for nearly 2 billion years has long been used to reconstruct one of the most consequential chapters in Earth’s early history. Scientists had interpreted the unusual carbon-isotope pattern as evidence of a major disruption to the planet’s carbon cycle, occurring around the time oxygen began accumulating in the atmosphere.
A new study from researchers at the California Institute of Technology (Caltech), however, suggests that at least one key record of this event may have had a much smaller origin.
The signal found in the Zaonega Formation in Karelia, Russia, may have been produced by a combination of magma, heat, hydrocarbons and methane-consuming microbes acting within a relatively confined marine basin, rather than by a planetwide environmental shift.
The findings, published in the journal Geology , do not settle the wider debate over what happened during Earth’s oxygenation.
But they do raise an important question: if a famous carbon signal can be created by local geological processes, how much of the ancient rock record can safely be treated as evidence of a global event?
A carbon signal that shaped the story of early Earth
During the time interval of 2.5 to 2 billion years ago, there was a dramatic change in the geochemical composition of the Earth's surface due to oxygen accumulation in the atmosphere.
This time interval is called the Great Oxidation Event, which was characterised by the chemical evolution of the ocean, atmosphere, and rocks, leading to the formation of an environment for the evolution of complex life much later.
Carbon isotopic composition is one of the parameters used by scientists to study this period of time since various biological and geological processes result in different ratios of carbon isotopes in rocks.
The appearance of such specific ratios in rocks in distant regions may suggest some changes that occurred globally.
The peculiar geochemical anomaly known as the Shunga-Francevillian event has gained much attention due to the similarity in carbon-isotope ratios in ancient rocks from the Zaonega Formation of Karelia and the Francevillian Basin of Gabon.
The new Caltech-led research takes a closer look at the Russian record and suggests that the interpretation may not be as straightforward as previously assumed.
Nivedita Thiagarajan, a senior scientific researcher at Caltech and lead author of the study, and her colleagues examined gases preserved in microscopic pockets inside rocks from the Zaonega Formation. Their results point toward a chain of geological and biological processes that unfolded within the ancient basin itself.
Ancient gases reveal what happened beneath the seafloor
The researchers examined fluid inclusions — tiny sealed pockets within minerals that can preserve gases and other materials from the distant past.
The rock samples were taken from those areas of the Zaonega Formation that are full of pyrobitumen, a carbon-rich substance formed when crude oil/kerogen is subjected to heat. These rocks have their origin in a marine basin formed about 2 billion years ago.
The geological setting provided an important clue.
Magma, according to the findings of the study, intruded into the sediments rich in organic material below the primordial ocean. The heat released during the intrusion may have led to the transformation of the organic material into hydrocarbons like methane and propane.
These gases would then migrate through the surrounding sediments.
The generation of methane was not the final step in the chain of reactions. The scientists suggest that methane got to microorganisms located near the seabed. The microorganisms oxidised methane and formed a residue with unique isotope composition.
That biological signature could then have become preserved in the surrounding rocks.
The data is derived from the temperature record of the formation.
The temperature near the magma intrusion is believed to have been around 350 degrees Celsius, but at a distance where an old seafloor asphalt seep is located 300 meters higher in the stratigraphic column, the temperature would have been 72 degrees Celsius.
These findings point towards a situation where the heat generated by the magma caused the formation of hydrocarbons, while the microbes used up some of the methane.
Why a local event could change the global Earth story
The difference between a local geological phenomenon and a worldwide environmental phenomenon is important since scientists frequently employ old rocks as parts of a much longer planetary history.
The fact that a carbon isotope anomaly was created in a local environment does not automatically mean that the whole world had gone through that particular change.
The Caltech team argues that the Zaonega signal can be explained largely by processes confined to a sedimentary basin spanning several hundred square kilometres. That would make the Russian record less direct evidence for a worldwide disruption of Earth's carbon cycle than previously thought.
Thiagarajan said the study cannot completely exclude contributions from other processes.
The researchers are therefore not arguing that every aspect of the Shunga-Francevillian event has been explained, nor that a global environmental change did not occur during the period.
Instead, their findings question whether the Zaonega Formation should automatically be treated as a reference record for a global event.
This difference is especially critical since the occurrence of oxygen in the atmosphere of Earth was a planetary shift itself. It is critical to understand the difference between signals that were caused by planetary shifts and those that came from the geology of a particular area.
The same carbon-isotope signal can be used in Gabon as a test.
Scientists will now test the Gabon rocks
The next stage of the research will be devoted to studying the ancient rocks found in the Francevillian Basin in Gabon.
The core samples have been taken within the framework of the GOE-DEEP project under the auspices of the International Continental Scientific Drilling Program. It is assumed that the study of gases trapped in the core samples taken in Gabon will show whether the same process took place there.
Drilling has been arranged in Gabon in 2025, and the new core samples arrived at the Geological Survey of Norway in February. The team of 18 different countries will start to sample the materials in 2026.
Comparing the Russian and Gabonese records could help determine whether their similar carbon signals share a common origin or arose independently from local geological conditions.
Aivo Lepland of the Geological Survey of Norway, one of the study's co-authors, said the comparison will allow researchers to examine the two sites using a similar approach.
The study, titled “Paleoproterozoic thermogenic hydrocarbon generation, Zaonega Formation, Russia,” was published in Geology . Other authors include Florian Eichinger of Hydroisotop GmbH in Germany and Anthony Prave of the University of St. Andrews in Scotland.
For scientists studying Earth's early oxygenation, the result is a reminder that an ancient rock does not necessarily preserve a simple snapshot of the whole planet. Sometimes, the chemical clue may have been written by what was happening just beneath one ancient seafloor.
FAQ
1. What is the 2-billion-year-old carbon clue?It is an unusual carbon-isotope signature known as the Shunga-Francevillian event. It has been associated with major changes in Earth's carbon cycle during the period when atmospheric oxygen began increasing.
2. What does the new study suggest caused the signal?
It is suggested that the magma heating up the sediment rich in organics below the ocean could generate hydrocarbons like methane. The organisms that consume methane could have led to the formation of the carbon isotopes seen in the rock samples. 3. Does the study disprove a global environmental change 2 billion years ago?
No. The researchers say their findings mainly challenge whether the Zaonega Formation in Russia can be used as straightforward evidence of a global carbon-cycle disturbance. Other evidence may still indicate that major planetary changes occurred during Earth's oxygenation.
4. Why are scientists studying rocks from Gabon next?
A similar carbon-isotope signal occurs in ancient rocks from Gabon. Researchers plan to examine newly recovered cores to determine whether local geological and biological processes can also explain the Gabonese record.
