Banded Iron Formations (BIFs) are a visually striking group of sedimentary rocks that are iron rich and almost exclusively deposited in the Precambrian. Their existence points to a major marine iron cycle that does not operate today. Several theories have been proposed to explain how the BIFs formed. While they all involve the precipitation of ferric (Fe3+) iron hydroxides from the seawater via oxidation of dissolved ferrous (Fe2+) iron that was abundant when the oceans contained very low levels of free oxygen, they disagree as to how this oxidation occurred. In the podcast, Clark Johnson describes how oxidation could have occurred without the presence of abundant free oxygen in the oceans. Clark Johnson is a Professor Emeritus in the Department of Geoscience at the University of Wisconsin-Madison.