Earth's Redox History

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Science  23 Dec 2011:
Vol. 334, Issue 6063, pp. 1654-1655
DOI: 10.1126/science.1216481

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Beginning with photosynthetic bacteria and continuing to green plants, organisms have been transferring electrons from inorganic donors and producing organic carbon for billions of years. The main oxidized products, accumulating in the Earth's crust along with organic material, have been ferric iron (Fe3+) and sulfate (SO42−), the former accounting for about 75% of the total (1). Molecular oxygen (O2), confined to the ocean and atmosphere, accounts for only 2 or 3% of the oxidized products. Moreover, O2 accumulated only if its production overbalanced consumption by reactions with organic carbon, sulfide (S2−), and ferrous iron (Fe2+), all delivered to the ocean by geological processes. On page 1694 of this issue, Kump et al. (2) provide new information that will strongly affect efforts to decipher Earth's redox history.