What are banded iron formations and what do they tell us about the evolution of Earth's atmosphere over geologic time?

Banded iron formations are sedimentary rocks consisting of alternating layers of iron-rich minerals and chert. They indicate periods of high oxygen levels in Earth's past, as the iron in these formations could only have been deposited in an oxygenated environment. The presence of banded iron formations suggests that significant amounts of atmospheric oxygen were produced by photosynthetic organisms, leading to the oxidation of iron in seawater.