Scientists study iron chemistry and mineralogy in sedimentary rocks in order to gain insights into the redox (oxidation and reduction) conditions on the ancient Earth. Today, iron is not very soluble in seawater, but 2.3 billion years ago it may have been a different story. Some theories suggest that during this time, just after the rise of atmospheric oxygen, the planet’s oceans and lakes would have been rich in soluble iron and these conditions could have persisted for a billion years.

A team of researchers has examined iron-bearing minerals in 1.4 billion-year-old rocks from Montana and Idaho (in the lower Belt Group, Belt Supergroup) in order to glean details about the ancient environment. To read Earth’s history in the rocks, the team had to first identify which minerals were representative of the environment in which they were formed (primary minerals); and those that were altered after formation (secondary) and provide clues about the changes that the materials underwent over time. The team employed a novel method (microscale textural studies coupled with bulk rock magnetic techniques) to study the samples. Their results indicate that shallow waters in the Belt Basin supported oxic environments. In addition, they found no indication of ferrous iron in the water column. These findings are consistent with eukaryotic fossils found in the same strata.

The study, “Mid-Proterozoic ferruginous conditions reflect postdepositional processes,” was published in the journal Geophysical Research Letters. The work was supported by NASA Astrobiology through the Exobiology Program.