Cloud postulated that banded iron formations were a consequence of anoxic, iron-rich waters from the deep ocean welling up into a photic zone inhabited by cyanobacteria that had evolved the capacity to carry out oxygen-producing photosynthesis, but which had not yet evolved enzymes (such as superoxide dismutase) for living in an oxygenated environment. Such organisms would have been protected from their own oxygen waste through its rapid removal via the reservoir of reduced ferrous iron, Fe(II), in the early ocean. The oxygen released by photosynthesis oxidized the Fe(II) to ferric iron, Fe(III), which precipitated out of the sea water as insoluble iron oxides that settled to the ocean floor.