Redox conditions of the deep ocean in the late Neoproterozoic-Early Paleozoic: ⁵⁷Fe Mössbauer spectroscopic study on deep-sea pelagic chert

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 Fe Mössbauer 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 Mössbauer 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 /Fe transition boundary between iron-hydroxide and dissolved Fe. The present results suggest that the late Neoproterozoic deep ocean was already ventilated up to the level required for the deposition of ferric iron.

57Fe Mössbauer spectroscopic analysis of deep-sea pelagic chert: Effect of secondary alteration with respect to paleo-redox evaluation

“…In a series of the 57 Fe Mössbauer studies (Kubo et al, 1996;Matsuo et al, 2003;Sato et al, 2009), we set the redox potential of the Fe(III)/Fe(II) transition (Berner, 1981) for the threshold in distinguishing the ''red state'' and ''black to dark-gray state'' of chert. Here, we use ''oxidizing'' or ''reducing'' for decribing redox potentials higher or lower than that of the Fe(III)/Fe(II) transition, respectively.…”

Memories of Pre-Jurassic Lost Oceans: How To Retrieve Them From Extant Lands

“…The second issue to note is the occurrence of red bedded chert at Llanddwyn that ubiquitously bears fine-grained hematite (Sato et al 2009). According to the Fe-mineral identification by Mössbauer spectroscopy (Matsuo et al 2003;Sato et al 2011), most of the Phanerozoic deep-sea chert beds share the primary brick-red colour, except for the cases of secondary alteration and peculiar anoxic horizons.…”

“…By utilizing rare earth element (REE) composition of accreted deepsea chert and associated banded iron formation (BIF) in Pilbara, Kato et al (2006) preliminarily reported that the onset of deep-sea oxygenation, in the limited context of iron oxide formation, took place sometime in the Neoarchean. Sato et al (2009) detected the occurrence of hematite-bearing non-metamorphosed red bedded chert of Ediacaran age from Anglesey in UK and the Gorny Altai Mtn. in Siberia.…”