How do iron-rich banded rocks reveal oxygen in ancient oceans?

Banded iron formations are distinctive sedimentary rocks made of alternating layers rich in iron minerals and silica. They mostly formed during the Precambrian, especially between about 2.5 and 1.8 billion years ago, when oxygen released by early photosynthetic microbes began reacting with dissolved iron in seawater. As oxygen increased, the iron precipitated and accumulated on the seafloor. These rocks are important evidence for the Great Oxidation Event, one of the biggest changes in Earth's atmosphere and oceans.