ATMOSPHERE × LIFE

Oxygen Before the Oxygenation

~2.7 to 2.5 billion years ago

Archean · Neoarchean

Long before the air changed, oxygen was gathering in the shallow seas above the continental shelves, made by microbes and consumed almost as fast. Five chemical witnesses, each read in a different element, point to the same thing, and in places the water held oxygen from the surface all the way down to the sea floor.

Why it matters

Oxygen was not switched on at 2.4 billion years. For three hundred million years it had been trickling out into the world, and something kept mopping it up.

Dating & uncertainty

Isotopes of nitrogen, iron, molybdenum and thallium point to oxygen in shallow water during the few hundred million years before the atmosphere changed; selenium enrichments may record oxidative weathering on land. Each proxy has its own caveats, and the size and duration of these patches are estimates, not measurements.

Sources

1. Ostrander et al. (2019), Nature Geoscience · Fully oxygenated water columns over continental shelves before the Great Oxidation Event
2. Koehler et al. (2018), PNAS · Transient surface ocean oxygenation recorded in the 2.66-Ga Jeerinah Formation, Australia
3. Czaja et al. (2012), Geochimica et Cosmochimica Acta · Evidence for free oxygen in the Neoarchean ocean based on coupled iron-molybdenum isotope fractionation

Earth history as one day

09:43:3614 h 16 min before midnight

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