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The vast majority of the world’s iron is known as Banded Iron Formation (BIF). Archean banded-iron deposits are marine sedimentary rocks that are very rich in iron and today account for 90% of the iron mined in the world.

Figure 1.2. The figure shows changes in the abundance of elements over time, mainly sulfur (S) and iron (Fe). The color gradations indicate a transition from anoxic oceans, e.g. low in sulfur, before 2.4 billion years (light blue) to oceans rich in H2S between 1.8 billion and 800 million years (dark blue), and then to complete oxygenation of the oceans (green). Courtesy of Ariel Anbar (2008). For a color version of this figure, see www.iste.co.uk/chapouthier/life.zip

Figure 1.3. Banded Iron Formation (BIF). Courtesy of Pierre Thomas (2011). For a color version of this figure, see www.iste.co.uk/chapouthier/life.zip

The ocean and the Earth’s surface were without oxygen 4 to 2.5 billion years ago.

The weathering of minerals from iron-rich continents produced ferrous ions (Fe²⁺) that were soluble in water, and therefore particularly mobile, and which were able to spread into the oceans. Volcanic activity at hydrothermal springs may also have contributed to the presence of ferrous ions in solution.

Oxygenation of the oceans by the oxygenic photosynthesis of cyanobacteria, up to about 2.4 billion years ago, caused soluble ferrous iron (Fe²⁺) to disappear by oxidation into insoluble ferric iron (Fe³⁺), which precipitated as magnetite and hematite.

When most of the reduced forms of iron were oxidized in the Paleoproterozoic era, sedimentation of banded iron deposits became rare. As a result, the O2 content first increased in the oceans, then in the atmosphere, becoming toxic to anaerobic organisms. This was the Great Oxidation or “Oxygen Catastrophe”.

Given that sea iron precipitated in an insoluble form (Fe³⁺) in the Archean era, the sea water of that time contained iron in solution, in a soluble form (Fe²⁺). This proves that the sea of that time was reduced, as was the overlying atmosphere.

Another type of photosynthesis, which is rare and little known, could explain an abundant precipitation of iron oxide: photoferrotrophy, a process where iron provides electrons.

Photoferrotrophy is a photosynthesis (less energy efficient than conventional photosynthesis) that oxidizes the iron (Fe²⁺) of FeO into iron (Fe³⁺) of Fe2O3; it can be written in a very simplified way: 2 FeO + H₂O + photons → Fe₂O₃ + 2 H⁺ + 2 e^-.

The H+ ions and e^- electrons are then used by mechanisms, similar to those of classical photosynthesis, to synthesize carbohydrates from CO₂; this metabolism requires the presence of iron (Fe²⁺) in the environment and leads to the massive precipitation of hematite Fe₂O₃.

Life is determined by the environment and, as multiple environments coexist, the origins of life and biodiversity coincide and evolve together.