GEOCHEMISTRY Banded iron formations (BIFs) are enigmatic, yet stunning, sedimentary rocks deposited mainly in the Late Archean to Early Proterozoic (2.7 to 1.9 billion years ago). They consist of micrometer- to centimeter-wide alternating layers of iron-rich and silica-rich material. The laminae were probably deposited in a quiet, marine, near-shore environment at a time when the ocean was nearly saturated in amorphous silica. The source of iron for the iron-rich layers may have been hydrothermal activity of the mid-ocean ridge or plume volcanism. The puzzling aspect is how ferrous iron from hydrothermal sources was oxidized into ferric iron in an oxygen-poor, early terrestrial setting. Konhauser et al . show quantitatively that iron-oxidizing bacteria, primarily chemolithoautotrophs, at cell densities much lower than what is found in modern environments could account for all of the ferric iron found in Precambrian BIFs. — LR Geology 30 , 1079 (2002).