Banded iron formations

node · earth · Global; principally Western Australia and southern Africa

Laminated chemical sediments alternating iron oxide and silica layers, deposited on a scale nothing on the modern Earth approaches — the Hamersley Basin of Western Australia and the Transvaal Basin of South Africa each contain sequences hundreds of metres thick and hundreds of kilometres across, and they are the source of most of the world's iron ore. They require an ocean that could hold large quantities of dissolved ferrous iron, which is only possible when the deep ocean is anoxic, plus a periodic supply of oxidant to precipitate it. That combination is a signature of a transitional world rather than an oxygenated one. Deposition peaked in the late Archaean and earliest Proterozoic and then largely stopped after about 1.85 billion years ago, with a brief and separate return associated with the Neoproterozoic glaciations. The conventional reading is that the iron ran out because the oceans finally oxidised; a competing reading emphasises deep-ocean sulfide instead. The banding itself — the reason they are called banded — has no agreed explanation.