THE OXYGENATED DEEP OCEAN IN THE LATE NEOPROTEROZOIC–EARLY PALEOZOIC: 57FE MöSSBAUER SPECTROSCOPIC EVIDENCE FROM ANCIENT DEEP-SEA CHERTS

The deep oceans were generally regarded to have been anoxic throughout the Precambrian until the first oxidation in the late Neoproterozoic; however, the precise timing and magnitude of this oxidation event are poorly constrained. In order to clarify the oxidation levels of the deep oceans around the Precambrian-Phanerozoic transition, we investigated the chemical states of iron in deep-sea pelagic cherts of the late Neoproterozoic-Early Paleozoic age, based on ⁵⁷Fe Mössbauer spectroscopy. All the analyzed cherts occur as exotic blocks within ancient accretionary complexes in Anglesey, Wales (UK), the Kurai area of the Gorny Altai Mountains (Russia), Port au Port Bay, Newfoundland (Canada), and the Bayanhongor area (Mongolia). The Mössbauer analysis clarified that all the samples contain hematite, without pyrite, just like Late Paleozoic and Mesozoic red cherts in Japan. The common occurrence of hematite and absence of pyrite suggest that these deep-sea cherts were deposited under relatively oxidizing conditions with a redox potential at least higher than that of the Fe(OH)₃/Fe²⁺ transition (boundary between iron-hydroxide and dissolved ferrous iron). The present results clarified that the Precambrian deep ocean, at least in part, became oxygenated by the latest Neoproterozoic enough for stabilizing ferric iron in sediments.