An anoxic, Fe(II)-rich, U-poor ocean 3.46 billion years ago

The oxidation state of the atmosphere and oceans on the early Earth remains controversial. Although it is accepted by many workers that the Archean atmosphere and ocean were anoxic, hematite in the 3.46 billion-year-old (Ga) Marble Bar Chert (MBC) from Pilbara Craton, NW Australia has figured prominently in arguments that the Paleoarchean atmosphere and ocean was fully oxygenated. In this study, we report the Fe isotope compositions and U concentrations of the MBC, and show that the samples have extreme heavy Fe isotope enrichment, where δ⁵⁶Fe values range between +1.5‰ and +2.6‰, the highest δ⁵⁶Fe values for bulk samples yet reported. The high δ⁵⁶Fe values of the MBC require very low levels of oxidation and, in addition, point to a Paleoarchean ocean that had high aqueous Fe(II) contents. A dispersion/reaction model indicates that O₂ contents in the photic zone of the ocean were less than 10^-3μM, which suggests that the ocean was essentially anoxic. An independent test of anoxic conditions is provided by U-Th-Pb isotope systematics, which show that U contents in the Paleoarchean ocean were likely below 0.02ppb, two orders-of-magnitude lower than the modern ocean. Collectively, the Fe and U data indicate a reduced, Fe(II)-rich, U-poor environment in the Archean oceans at 3.46 billion years ago. Given the evidence for photosynthetic communities provided by broadly coeval stromatolites, these results suggests that an important photosynthetic pathway in the Paleoarchean oceans may have been anoxygenic photosynthetic Fe(II) oxidation.