Abstract
The deep oceans were anoxic throughout the Precambrian until the first oxidation in the late Neoproterozoic; however, the precise timing and magnitude of this oxidation event are unknown. To examine the oxidation levels of the late Neoproterozoic-Early Paleozoic deep oceans, we investigated the chemical states of iron in deep-sea pelagic cherts of this age, based on 57Fe Mösbauer spectroscopy. The analyzed cherts occur as exotic blocks within accretionary complexes in Anglesey, Wales, UK (late Neoproterozoic), the Kurai area of the Gorny Altai Mountains, Russia (Ediacaran-Cambrian), Port au Port Bay, Newfoundland, Canada (Ordovician), and the Bayanhongor area of Mongolia (Devonian). The Mösbauer spectroscopy clarified that all the samples contain hematite as the main iron mineral, without pyrite. The common occurrence of hematite suggests that these deep-sea pelagic cherts were deposited under oxidizing conditions with a redox potential at least higher than that of the Fe(III)/Fe(II) transition (boundary between iron-hydroxide and dissolved Fe2+). The present results suggest that the late Neoproterozoic deep ocean was already ventilated up to the level required for the deposition of ferric iron.
