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Most geochemical proxies and models of atmospheric evolution suggest that the amount of free O-2 in Earth’s atmosphere stayed below 10(-5) present atmospheric level (PAL) until the Great Oxidation Event (GOE) that occurred between similar to 2.2 and 2.4 Ga, at which time free O-2 in the atmosphere increased to approximately 10(-1) to 10(-2) PAL. Although photosynthetically produced "O-2 oases" have been proposed for the photic zone of the oceans prior to the GOE, it has been difficult to constrain absolute O-2 concentrations and fluxes in such paleoenvironments. Here we constrain free O-2 levels in the photic zone of a Late Archean marine basin by the combined use of Fe and Mo isotope systematics of Ca-Mg carbonates and shales from the 2.68 to 2.50 Ga Campbellrand-Malmani carbonate platform of the Kaapvaal Craton in South Africa. Correlated Fe and Mo isotope compositions require a key role for Fe oxide precipitation via oxidation of aqueous Fe(II) by photosynthetically-derived O-2, followed by sorption of aqueous Mo to the newly formed Fe oxides. A dispersion/reaction model illustrates the effects of Fe oxide production and Mo sorption to Fe oxides, and suggests that a few to a few tens of mu M free O-2 was available in the photic zone of the Late Archean marine basin, consistent with some previous estimates. The coupling of Fe and Mo isotope systematics provides a unique view into the processes that occurred in the ancient shallow ocean after production of free O-2 began, but prior to oxygenation of the deep ocean, or significant accumulation of free O-2 in the atmosphere. These results require oxygenic photosynthesis to have evolved by at least 2.7 Ga and suggest that the Neoarchean ocean may have had a different oxygenation history than that of the atmosphere. The data also suggest that the extensive iron formation deposition that occurred during this time was unlikely to have been produced by anoxygenic photosynthetic Fe(II) oxidation. Finally, these data indicate that the ocean had significant amounts of O-2 at least 150 Myr prior to previously proposed "whiffs" of O-2 at the Archean to Proterozoic transition. (C) 2012 Elsevier Ltd. All rights reserved.