Banded Iron Formations (by Ale*)
The Banded Iron Formations (BIFs) are a series of stratigraphic rock units deposited at different times in different environments. They contain higher-than-usual amounts of iron (Fe). These deposits are all Precambrian in age, and the ones in the image in particular are Archean (older than 2.5 billion years ago, that is 2,500,000,000 years). The current explanation for the abundance of Fe in these layers, albeit not totally accepted by the scientific community, is that Fe existed in solution in Archean ocean waters in its reduced form. Sudden increase in the amount of available oxygen, caused by the beginning of photosynthetic life in the form of stromatolite-building cyanobacteria, would have caused the reduced Fe to oxidize to hematite (and possibly magnetite). There is evidence in the fossil and rock records of an increase in atmospheric oxygen right after the last deposited BIFs. BIFs were never to form again. At the very beginning, before any BIFs were ever deposited, the most common form of Fe on Earth’s surface was pyrite. Pyrite today would not stand a chance of preservation in subaerial environment because of the abundance of oxygen in the atmosphere.