EARTH × LIFE

The Rust at the Bottom of the Sea

~3.5 to 1.8 billion years ago

Archean · Paleoarchean

For nearly two billion years the ocean held iron dissolved in it, as water holds salt, because there was no oxygen to rust it. Whenever oxygen appeared, the iron rusted out of the water and settled as a red layer; when the oxygen failed, silica settled instead. The result is a striped rock, hundreds of metres thick, that still supplies most of the world's iron ore.

Why it matters

Those stripes record the early ocean's chemistry layer by layer. They are also why steel is cheap: nearly every iron mine on the planet works one of these formations.

Dating & uncertainty

Deposition peaks around 2.5 billion years ago and stops almost entirely by 1.8 billion. What oxidised the iron is still argued: oxygen produced by cyanobacteria, or a form of photosynthesis that consumes iron directly and requires no oxygen at all. Both may have operated.

Sources

1. Bekker et al. (2010), Economic Geology · Iron formation: the sedimentary product of a complex interplay among mantle, tectonic, oceanic, and biospheric processes
2. Thompson et al. (2019), Science Advances · Photoferrotrophy, deposition of banded iron formations, and methane production in Archean oceans
3. Robbins et al. (2019), Nature Geoscience · Hydrogeological constraints on the formation of Palaeoproterozoic banded iron formations

Earth history as one day

05:29:5218 h 30 min before midnight

if Earth’s 4.54 billion years were compressed into a single day