Geochemistry

Oxidation Before Oxygen

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Science  19 Jul 2013:
Vol. 341, Issue 6143, pp. 217
DOI: 10.1126/science.341.6143.217-a
CREDIT: J. E. JOHNSON ET AL., PROC. NATL. ACAD. SCI. U.S.A. 110, 28 (24 JUNE 2013) © 2013 NATIONAL ACADEMY OF SCIENCES

The emergence of oxygen-producing photosynthesis had a profound effect on Earth's surface environment. It eventually oxidized the oceans and atmosphere, paving the way for aerobic life. Determining the timing of photosynthesis and the subsequent transformation of biogeochemical cycles relies on analyzing clues in ancient buried sediments. Johnson et al. analyzed the mineralogy and isotopic signatures of carbon and sulfur in 2.4-billion-year-old drill cores from South Africa. Synchrotron-based x-ray absorption spectroscopy revealed that abundant Mn oxides were hosted in carbonate deposits, which were probably formed via oxidation of soluble Mn(II). Several lines of geochemical evidence based on redox-sensitive proxies, however, preclude oxygen as the primary oxidant acting on Mn. Moreover, because the rise of oxygen from oxygenic cyanobacteria would not occur for another 200 million years after the sediments were deposited, a primitive (or transitional) form of photosynthesis may have been responsible for forming the Mn oxides. In this scenario, the water-oxidizing complex in the early photosynthesizing enzyme machinery utilized Mn(II) as an electron donor.

Proc. Natl. Acad. Sci. U.S.A. 110, 11238 (2013).

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