Ancient rock yields clues to early oxygen “oases” on Earth

New Scientist : Early Earth had no oxygen. However, at one point, oxygen-rich pockets of seawater may have started to form due to bacteria living in shallow, nutrient-rich marine areas. The oxygen then reacted with dissolved iron to form rock, which sank to the ocean’s bottom. The removal of the iron from the seawater allowed calcium carbonate to build up and form limestone. That geologic progression can be seen in rock samples from Steep Rock Lake in Ontario, Canada. They contain a mixture of iron minerals and limestone and layers of microbes, which are some 2.8 billion years old. Although such oxygen oases persisted for only about 5 million years, they forced early life forms to adapt to the chemically reactive gas before it would eventually spread around the world. Thus that period was a critical moment in the evolution of life on Earth, according to Robert Riding of the University of Tennessee and colleagues, whose study was published in the journal Precambrian Research.