Trapped gases point to local causes for a famous 2-billion-year-old carbon signal in Russia

The oxygen rise ran from roughly 2.5 to 2 billion years ago and was the largest chemical transformation recorded at Earth's surface, according to the Caltech release ^[1]. As oxygen built up, large amounts of microbial material were buried beneath the seafloor. That burial locked carbon into rock and left the isotopic signature now in dispute ^[2]. The usual way to read such a history is to measure the ratio of heavier to lighter carbon down a drill core and treat the sequence as a record of environmental change; the release compares it to reading a tree's growth rings ^[12].

Gas sealed inside the rock gives a second reading from the same cores. The samples chosen were rich in pyrobitumen, an insoluble organic carbon that forms when buried crude oil or kerogen is heated intensely at depth ^[11]. Rocks like that have their own history of oil generation. Thiagarajan called the Zaonega "one of the world's oldest known fossil oil fields" ^[8].

The team, she said, "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" ^[8]. The words "can be explained" matter. They establish that a local process is sufficient to produce the Karelian signal. On the evidence in the release, they do not establish that a global disturbance was absent, and Caltech's own framing is narrower: the study raises questions about whether the Russian evidence records a planetwide event ^[6]. The researchers reconstructed the sequence of changes preserved in Karelia's rocks after the first major rise in atmospheric oxygen ^[17].

Then there is the denominator. The global interpretation drew on drill cores from two basins, Karelia and the Francevillian Basin in Gabon ^[4]. This study reassesses one of them ^[20]. With Karelia given a sufficient local cause, I'd expect the case for a worldwide event to lean harder on the Gabon cores, which the supplied account does not re-examine. The release also does not give the number of samples measured or the isotope values themselves.

The project began with a dataset waiting for a method. Aivo Lepland of the Geological Survey of Norway, a co-author, arrived at Caltech on sabbatical with isotope measurements from Zaonega gases that had not been fully interpreted ^[15]. Thiagarajan and John Eiler had just finished work measuring isotope ratios in natural gases ^[16]. "This information is archived in the rocks, so, in order to study what happened, you have to study rocks," Lepland said ^[14]. He described the interval plainly: "Earth, in a way, went crazy during that time interval when oxygen appeared in the atmosphere" ^[13].