By Richard I. Gibson
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Transcript: Redbeds are sedimentary rocks such as sandstone, shale, and siltstone that contain enough oxidized iron, usually in the form of the mineral hematite, Fe2O3, to give the rock a reddish color. The fact that the iron is oxidized indicates that the sediments were exposed to oxygen-rich air or shallow, oxygenated water when they were deposited, and this in turn indicates that they probably formed on land or very shallow water such as lakes and ephemeral streams. Redbeds are associated with deserts in places.
In the book, I said that the oldest redbeds date to 2.6 billion years ago, but that’s incorrect. More recent work and better dating puts the oldest redbeds, the Jatulian rocks of Finland, at about 2.3 billion years old.
The age of the oldest redbeds can be used to infer the success of photosynthesis. By 2.3 billion years ago, there was enough free oxygen in the atmosphere to begin to react with iron in sediments, creating hematite and turning the resulting rocks red.
Flaming Cliffs of Mongolia, photo by Zoharby, GNU Free documentation license