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The evolution of photosynthesis was a critical evolutionary step in the history of life on Earth, and it has been proposed that anoxygenic photosynthesis preceded oxygenic photosynthesis, largely on the basis that the Archean Earth atmosphere and oceans are generally considered to have been anoxic. The first rise in atmospheric oxygen, termed the “Great Oxidation Event” (GOE), has been proposed to have occurred between ~2.45 and 2.2 Ga. The GOE model was developed based on geologic occurrences of detrital pyrite, siderite, and uraninite in sediments deposited before the GOE, as well as retention of Fe in paleosols after the GOE. Support for the GOE model comes from the disappearance of mass-independent fractionation of S isotope after the GOE. Recent geochemical studies, however, increasingly provide evidence for a more complex evolution of atmospheric O₂ levels prior to the GOE as compared to a simple a “step-function”.

In this study, we focus on the origin and environmental significance of hematite from the 3.46 Ga Marble Bar Chert (MBC) from the Pilbara craton, Western Australia. Other workers have interpreted hematite in the MBC to have precipitated from a fully-oxygenated Archean ocean at the time of deposition at 3.46 Ga, which in turn would suggest that oxygenic photosynthesis had evolved prior to that time. We combine Fe isotopes, which can constrain the extent of oxidation, with U-Th-Pb isotopes, which provide an estimate of seawater U contents and tests for post-depositional alteration. Our results indicate that hematite in the MBC was precipitated by a very small extent of oxidation from an Fe(II)-rich, U-poor ocean at 3.46 Ga. We conclude, therefore, that hematite in the 3.46 Ga MBC cannot be used to infer oxygenic photosynthesis prior to 3.5 Ga.

The MBC samples have very high δ⁵⁶Fe values, ranging between +1.53 ‰ and +2.63 ‰, that define the upper limit of measured δ⁵⁶Fe values for terrestrial samples. The likely source of aqueous Fe(II) that was oxidized to form hematite in the MBC was hydrothermal fluids, which should have had a δ⁵⁶Fe value of around 0 ‰, or slightly negative. Oxidation of aqueous Fe(II) in modern marine hydrothermal systems produces precipitates that have slightly negative δ⁵⁶Fe values, reflecting essentially quantitative oxidation. The very high δ⁵⁶Fe values of hematite in the MBC, therefore, do not support a fully oxidized Archean ocean. Dispersion/reaction modeling indicates that very low oxidation rates and low degrees of Fe(II) oxidation are needed to produce the highly positive δ⁵⁶Fe values. In particular, modeling strongly suggests that the photic zone of the 3.46 Ga ocean was reduced and buffered by excess Fe(II). In fact, modeling suggests that photic zone oxygen contents were less than 10-3 mmicromolar , less than 0.0003% of modern photic zone levels.