Why did the appearance of oxygen in Earth’s atmosphere change the transport of iron?

The appearance of oxygen in Earth’s atmosphere fundamentally altered the transport of iron by changing its chemical state and solubility.

In the early, anoxic oceans, iron existed primarily as ferrous iron ($F{e}^{2+}$), which is water-soluble and could be easily transported throughout the world's oceans. However, as oxygen levels rose, it reacted with this dissolved ferrous iron to produce ferric iron ($F{e}^{3+}$). Unlike its ferrous counterpart, ferric iron is highly insoluble.12

This chemical transition caused the iron to precipitate out of the water column and settle onto the ocean floor, forming banded-iron formations. Consequently, the introduction of oxygen effectively ended the widespread mobility of iron in the oceans, as the iron was "locked away" in sedimentary deposits rather than remaining in solution. This process continued for millions of years until the majority of the dissolved iron had been precipitated, eventually allowing oxygen to accumulate in the seawater and outgas into the atmosphere.132