Iron isotopic systematics and ferric iron carriers of Phanerozoic continental red beds and implications for atmospheric oxygenation

Abstract

Continental red beds (CRB) are characteristic for their predominant red colour due to the widespread presence of ferric oxides (hematite or goethite) that might suggest distinctly oxygenated atmospheric conditions during their formation. Therefore, elemental and isotopic systematics of iron, a redox-sensitive element, can provide critical constraints on Fe cycling and possibly also palaeoredox conditions. However, our knowledge of the iron isotopic systematics (δ⁵⁶Fe) of CRB remains very limited. This prevents to evaluate whether these rocks are useful to trace past atmospheric oxygenation. To fill the gap, we present an extensive dataset of Fe elemental and isotopic data, paralleled by Mössbauer and diffuse reflectance spectra, for classic Phanerozoic examples of CRB supplemented by the analyses of colour detrital grain coatings. The data reveal goethite as the principal phase within grey-green siliciclastic lithologies and detrital grain coatings. These samples show a positive correlation between Fe³⁺/Fe_T and δ⁵⁶Fe (fine-grained lithologies) suggesting percolation of late-stage diagenetic fluids at variable redox and pH conditions connected with hematite dissolution. In contrast, the red facies are characterized by the predominance of hematite, largely variable Fe_T contents and δ⁵⁶Fe values overlapping with those estimated for the upper continental crust. The overall lighter and more homogeneous δ⁵⁶Fe values in Phanerozoic red beds compared to their Palaeoproterozoic (∼2.2 Ga) counterparts are consistent with more oxygenated atmosphere during the Phanerozoic. Therefore, the appearance of CRB may serve as an important marker of atmospheric oxygenation in the past.