A surge of oxygen littered early Earth with millions of tiny nuclear reactors, blasting ancient life with radiation. That's the scenario a team of researchers has proposed to account for the disappearance of a radioactive mineral from the geological record. If true, this primordial nuclear age could have played a role in the evolution of early life forms.
Roughly 2.5 billion to 3 billion years ago, significant quantities of oxygen began entering Earth's atmosphere. Scientists credit new types of photosynthetic bacteria that produced the gas as a waste product. At about the same time, a volcanically produced mineral known as uraninite began to vanish. Because most of this early oxygen was present in Earth's seas (where ancient life resided), and given that oxygenated water dissolves uraninite, geologist Laurence Coogan and chemical oceanographer Jay Cullen of the University of Victoria in Canada propose this month in GSA Today that the two events were linked.
But their analysis goes further. When the uraninite dissolved, the team argues, grains of radioactive uranium-235 (235U) broke free and were eventually deposited on banks and shorelines. When enough 235U accumulates--about a basketball-sized volume--a fission reaction begins, releasing neutron radiation. Coogan and Cullen calculated that enough 235U existed at the time to have started millions of these reactors.
Coogan says there's at least one location where natural fission is known to have occurred, the Oklo region of Gabon. The concentrations of uranium in the geological formations there show chemical evidence that 17 ancient--and now dormant--reactors once burned. There's also a ubiquitous bacterial strain, Deinococcus radiodurans, which is naturally resistant to otherwise lethal doses of radiation. So far, scientists have not been able to determine how that resistance evolved.
The nuclear reactor hypothesis is "plausible," says geophysicist Norman Sleep of Stanford University in Stanford, California. But if the reactors were widespread, scientists would see more variance in Earth's current ratio of 235U to 238U, which is based on the half-lives of the two isotopes that make up uraninite. Yet, Oklo aside, this ratio is consistent everywhere on Earth, Sleep says.
Still, the paper is "not only fascinating reading, but it also generates ideas for testable hypotheses," says health physicist and radiological specialist P. Andrew Karam of the New York City Department of Health and Mental Hygiene. If it bears fruit, he adds, "the fact that ancient Earth may have hosted scores of natural nuclear reactors is certainly relevant to today's debates over nuclear energy, radioactive waste disposal, and the putative health effects of exposure to low levels of radiation."