Although global cyanobacteria release massive amounts of oxygen “waste,” no surplus builds up in Earth's atmosphere. Oxygen gas reacts immediately with hydrogen, carbon and iron to form oxides such as water, calcium carbonate (limestone) and iron ore (hematite and magnetite).
For 600 million years, sediments of alternately higher and lower concentrations of iron oxide settle out on the ocean floors. Some of these sediments metamorphose into massive banded-iron formations, or BIFs, which are the principal source of iron mined by humans two thousand million years in the future.
Scientists theorize that fluctuations in these iron oxide deposits were caused by some combination of seasonal upwelling of ironrich waters from the depths of the ocean, seasonal variations in photosynthetic activity, periodic volcanic eruptions, and seasonal variations in the oxygen production of cyanobacteria.
One interpretation is that the layers may be read as growth rings, indicating increases in oxygen “exhaust” by cyanobacteria in warmer seasons and lower exhaust in cooler seasons.