BIFs are composed predominantly of ferric and ferrous minerals, including hematite (Fe 2O 3), magnetite (Fe 3O 4), and siderite (FeCO 3) 1. Thus, the first widespread development of marine red beds constrains the timing of deep-ocean oxygenation.īanded Iron Formations (BIFs) first appeared 3.85 billion years ago (Ga) in the Archean and were particularly prevalent around 2.6–2.4 Ga when they attained thicknesses of hundreds of metres 1, 2.

By contrast, banded iron formations formed when Fe-concentrations were much higher (> 50 μM). Geochemical analyses and thermodynamic modelling reveal that marine red beds formed when deep-ocean Fe-concentrations were > 4 nM. We find that marine red beds are a prominent feature of the sedimentary record since the middle Ediacaran (~580 million years ago). Here, to reconcile these observations, we track the evolution of oceanic Fe-concentrations by considering the temporal record of banded iron formations and marine red beds. The disappearance of banded iron formations at ~1.8 billion years ago was traditionally taken as evidence for the demise of ferruginous oceans, but recent geochemical studies show that ferruginous conditions persisted throughout the later Precambrian, and were even a feature of Phanerozoic ocean anoxic events.

Banded iron formations were a prevalent feature of marine sedimentation ~3.8–1.8 billion years ago and they provide key evidence for ferruginous oceans.

Oceans by Simon Holland