The Story of O2

Banded Iron Formation

There's a really nice article in this week's Science about the history of gaseous oxygen in the earth's atmosphere. Everyone agreed that the earth started out without any gaseous diatomic oxygen. There was some debate about when free oxygen first appeared. Minerals older than 2.4 billion years or so showed no signs of oxidized iron (rust). But some researchers believed there was evidence of atmospheric oxygen even earlier. But now a detailed analysis of atmospheric sulfur chemistry supports the later date for oxygen. So there seems to have been a "Great Oxidation Event" around 2.4 billion years ago in which banded iron formations were laid down around the world. Photosynthetic algae appear at around 2.7 billion years ago and perhaps at that point they were producing enough oxygen to create the iron formations.

In the same issue of Science, there's also an article with evidence that the earth held on to its gaseous hydrogen for longer than previously thought, which would help explain some of the history of early life on the planet.