Photosynthetic prokaryotic organisms that produced O as a waste product lived long before the first build-up of free oxygen in the atmosphere, perhaps as early as 3.5 billion years ago. The oxygen they produced would have been rapidly removed from the oceans by weathering of reducing minerals, most notably iron. This rusting led to the deposition of iron oxide on the ocean floor, forming banded iron formations. Thus, the oceans rusted and turned red. Oxygen only began to persist in the atmosphere in small quantities about 50 million years before the start of the Great Oxygenation Event.