Virtually every steel beam in a skyscraper, every bridge, and every modern automobile exists thanks to a gigantic geochemical reaction that occurred over two billion years ago. On Archean Earth, the oceans were not blue, but rather a greenish hue due to enormous quantities of dissolved ferrous iron (Fe²⁺) in waters entirely devoid of free oxygen.
The planet's balance changed radically with the proliferation of the first photosynthetic cyanobacteria. These microorganisms began releasing oxygen as a byproduct of their metabolism, but that gas did not immediately escape into the atmosphere. Instead, it reacted instantaneously with the dissolved iron in seawater, triggering global-scale chemical precipitation: a steady "rain" of iron oxides (magnetite and hematite) that fell to the seafloor for hundreds of millions of years.
This process forged banded iron formations (BIFs), famous for their mesmerizing alternating layers of dark metallic oxides and silica-rich sheets or reddish jasper:
- The great planetary sink: Oceans functioned as a colossal chemical buffer that absorbed oxygen before it could accumulate in the air, delaying the atmospheric Great Oxidation until the sea's soluble iron was almost completely depleted.
- The foundation of modern industry: More than 60 % of the world's iron ore reserves mined today, from Australia's Pilbara deposits to the Lake Superior basin, come exclusively from these primordial marine sediments.
When we look at a modern steel structure, we are viewing the petrified remnant of the day the first life forms rusted an entire ocean.
Source: Geological Society of America / Nature Geoscience
