All Natural, All Nuclear
The Oklo uranium mine in Gabon contains well known evidence of natural nuclear reactors, but how widespread were they? A team of researchers has proposed a scenario to account for the disappearance of a radioactive mineral from the geological record. Part of their hypothesis is that a surge of oxygen billions of years ago caused the creation of millions of tiny nuclear reactors. If true, this primordial nuclear age could have played a role in the evolution of early life forms.
Appearing in the Geological Society of America's GSA Today, Laurence A. Coogan and Jay T. Cullen, both from the School of Earth and Ocean Sciences, University of Victoria, have put forth a radical idea about natural nuclear reactors and the evolution of early life on Earth. In their article, entitled “Did natural reactors form as a consequence of the emergence of oxygenic photosynthesis during the Archean?,” they conjecture that once oxygen producing photosynthesis evolved, it produced local oxygen enrichment in surface water. This helped carry uranium into solution, which was redeposited at the margins of oxygen oases. These uranium deposits, they claim, would have the potential to form natural reactors due to the high concentration of 235U during the Archean Eon (3.8 – 2.5 billion years ago). How this happened is shown in the figure below, taken from the article.
Cartoon showing a possible mechanism by which oxygenic photosynthesis could lead to formation of natural fission reactors. Uraninite weathered out of igneous and metamorphic rocks is transported to isolated basins and deposited in shallow water environments, providing a ready source of U as soon as the waters become oxidizing. Photolytically produced H2O2 rains out of the atmosphere and oxidizes the uppermost water column, reducing the concentration of electron donors required by anoxygenic photosynthesizers such as H2S and Fe2+. This provides the selective pressure required for the emergence of oxygenic photosynthesis due to the abundance of H2O as an alternative electron donor.
At the time all of this was taking place, Earth's atmosphere was very oxygen poor compared with modern levels. An oxygenated atmosphere is generally considered prerequisite for the evolution of complex life. On Earth, atmospheric oxygen is produced through photosynthesis. It is widely, although not unanimously, accepted that oxygen levels in Earth’s atmosphere were very low throughout the first ~2 billion years of Earth’s history. According to the article, “[e]vidence from paleosols for soil development under reducing conditions and the occurrence of clastic sediments containing minerals that are highly soluble under oxic conditions, such as pyrite and uraninite, suggest low atmospheric oxygen before ca. 2.3 Ga.”
At about the same time, a volcanically produced mineral known as uraninite began to disappear. It is known that oxygenated water dissolves uraninite and, because most of Earth's early oxygen was present in the seas, Coogan and Cullen think the two events are linked. According to them, when the uraninite dissolved, grains of radioactive uranium-235 (235U) broke free and were eventually deposited on banks and shorelines. When enough 235U accumulated—a mass about the size of a basketball—nuclear fission occurred. Coogan and Cullen calculated that enough 235U existed at the time to have started millions of these reactors.
The ancient oceans were a happening place 2.3 billion years ago.
In 1956, while at the University of Arkansas, Dr. Paul Kuroda described the conditions under which a natural nuclear reactor could occur and there is at least one location where natural fission is known to have occurred. That is in the well known Oklo region of Gabon. When the Oklo reactors were discovered, the conditions found there were very similar to Kuroda's predictions. Concentrations of uranium in the Oklo geological formations show chemical evidence that 17 ancient reactors once operated there for more than a million years during the Proterozoic Eon (~1.8 Ga). James Lovelock, in The Ages of Gaia, put it this way:
A bizarre consequence of the appearance of oxygen was the advent the world's first nuclear reactors. Nuclear power from its inception has rarely been described publicly except in hyperbole. The impression has been given that to design and construct a nuclear reactor is a feat unique to physical science and engineering creativity. It is chastening to find that, in the Proterozoic, an unassertive community of modest bacteria built a set of nuclear reactors that ran for millions of years.
The newly proposed millions of ancient reactors would have emitted neutrons, irradiating anything near by, and it is difficult to determine the impact of near-surface natural reactors on the Archean biosphere. There is a ubiquitous bacterial strain, Deinococcus radiodurans, which is naturally resistant to otherwise lethal doses of radiation. So far, scientists have been at a loss as to how that resistance evolved. Coogan and Cullen suggest, “Investigation of the evolution of radiation tolerance in some bacteria (e.g., Deinococcus radiodurans and members of the cyanobacteria), for which there is no other obvious terrestrial selective pressure, may prove fruitful.”
The nuclear reactor hypothesis is “plausible,” says geophysicist Norman Sleep of Stanford University, commenting in Science. But if the reactors were widespread, scientists should see more variation in Earth's current ratio of 235U to 238U, the two radioactive isotopes that make up uraninite. Aside from measurements taken at Oklo, this ratio is consistent everywhere on Earth, Sleep says. For more information regarding the Oklo site see page 347 in Chapter 18 of The Resilient Earth.
Site of the Oklo natural nuclear reactors in Gabon. Photo US DOE.
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, in Science (see “Did Ancient Earth Go Nuclear?”). 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.”
The widespread ancient nuclear reactor hypothesis remains controversial, and the link between such reactors and the evolution of life on our planet even more so. Still, it is interesting to note that nuclear energy, the favorite boogeyman of eco-activists everywhere prior to the advent of the global warming hysteria, has proven to be just another natural phenomenon. Most rational scientists know this, which is why the AAAS Pew poll found that 70% of scientists favor the expanded use of nuclear energy.
Still, atavistic eco-activists go into meltdown at the mere mention of building new nuclear power plants. But the world's energy needs continue to rise and, whether you believe that CO2 emissions will turn Earth into a living hell or just that being an energy independent nation is a good thing, something must be done. Instead of raising forests of twirling wind turbines, which slaughter birds, bats and the occasional skydiver, or slathering every available surface with costly and intermittent solar cells, I say we go nuclear—what could be more natural?
Be safe, enjoy the interglacial and stay skeptical.