Did natural reactors form as a consequence of the emergence of oxygenic photosynthesis during the Archean?
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The advent of oxygenic photosynthesis changed Earth’s surface environment in numerous ways, perhaps most notably by making possible the evolution of large and complex life-forms. Current models suggest that organisms that can perform oxygenic photosynthesis first took hold in isolated marine and freshwater basins, producing local oxygen oases. Here we present calculations that suggest that uranium deposits could have formed at the margins of these basins due to the strong local reduction-oxidation gradients. Because of the high abundance of 235U at this time, these uranium deposits could have formed widespread, near-surface, critical natural fission reactors. These natural reactors would have represented point sources of heat, ionizing radiation, and free radicals. Additionally, they would have far-field effects through the production of mobile short- and long-lived radioactive daughter isotopes and toxic byproducts. It is possible that these fission products provided a negative feedback, helping to limit the proliferation of the cyanobacteria in the Archean environment. Secular decreases in the abundance of 235U in turn decreased the probability of such deposits forming critical fission reactors during the early Proterozoic.
Manuscript received 14 January 2009; accepted 30 July 2009.