James Lovelock’s Gaia hypothesis and natural uranium reactors
As promised, I return to the subject of James Lovelock’s Gaia hypothesis which considers the Earth as an intimately linked system of physical, chemical and biological processes, interacting in a self-regulating way to maintain the conditions necessary for life.
Reservations about Gaia
Clearly Gaia could not have existed before lifeforms had got established on Earth so one wonders precisely at what stage during the evolution of life Gaia came into being. This is reminiscent of the debate about when during the course of evolution did man acquire his immortal soul.
Whatever reservations one might have about Gaia, it must be acknowledged that the hypothesis has stimulated useful research and discussion. There are in fact many examples in the history of science where doubtful, or even barmy, hypotheses have stimulated excitement and useful outcomes – the alchemists were doomed not to achieve their primary objective, but their experiments provided the basis for modern chemistry, and Joseph Priestley’s belief in phlogiston did not prevent him from discovering oxygen.
Lovelock’s involvement with atmospheric studies goes back a long way. In the 1950s he developed super-sensitive instruments for detecting trace impurities in the air and these were used to reveal the ubiquity of DDT in the atmosphere. It was such studies that prompted Rachel Carson to write Silent Spring.
In his most recent book, The Revenge of Gaia, Lovelock expresses deep concern over global warming and his conviction that nuclear power is the only realistic antidote. In view of Lovelock’s enthusiasm for nuclear power and his interest in life processes, I am surprised he hasn’t given greater emphasis in his writings to the role of primitive lifeforms in the creation, some 1.8 billion years ago, of a number of natural uranium reactors at what is now a uranium mine at Oklo in the Gabon in West Africa.
Natural uranium reactors in West Africa
Here is a brief account of that amazing phenomenon. At present natural uranium consists of 0.7% of the fissile isotope U235 with the remainder being essentially the fertile isotope U238. However, U235 decays radioactively more than six times faster than U238 so that 1.8 billion years ago its concentration in natural uranium would have been about three per cent. At this U235 level ordinary ground water could, in principle, act as a moderator and a nuclear chain reaction be initiated, but only within a deposit where the uranium mineral content was above about 30%, which is fifty times the normal deposit level. In fact, such rich ore deposits were achieved at Oklo
At times earlier than 1.8 billion years ago there was hardly any oxygen in the atmosphere so the uranium ore over a complete water-shed at Oklo was in the reduced insoluble state. Then the arrival of photosynthetic blue-green algae raised the oxygen level of the atmosphere and converted the uranium ore to its more soluble oxidised form.
The ore at the bottom of streams dissolved and was carried down-stream but at certain locations anaerobic conditions were again encountered due to the accumulation of rotting organic detritus and the process was reversed. The consequent precipitation led to ore concentrations which permitted the fission process. There were in fact six sites at Oklo where natural reactors operated for more than half a million years, and all due to the action of the most primitive of lifeforms.
What has all this to do with da Vinci?, I can hear you ask. It was in fact Leonardo da Vinci who first proposed that the whole earth was a macrocosm of the human body. Maybe someday, someone will write a sequel to The Da Vinci Code, with more import and verisimilitude.