ATMOSPHERE × EARTH

The Proof That the Air Had No Oxygen

~3.5 to 2.4 billion years ago

Archean · Paleoarchean

Sulfur in ancient rocks carries isotope ratios that no ordinary chemistry explains. Only one thing produces them: ultraviolet light striking sulfur dioxide, in air with almost no oxygen and no ozone to stop the light.

Why it matters

Behind the sentence "the air had no oxygen" stands this one measurement, and it puts a number on the claim: less than a hundred-thousandth of today's level.

Dating & uncertainty

The anomaly disappears from the record between 2.4 and 2.3 billion years ago, which dates the oxygenation of the atmosphere more sharply than any other proxy. Which photochemical reaction produces the anomaly, and which molecule carries it to the sea floor, is still being worked out.

Sources

1. Pavlov & Kasting (2002), Astrobiology · Mass-independent fractionation of sulfur isotopes in Archean sediments: strong evidence for an anoxic Archean atmosphere
2. Halevy (2013), PNAS · Production, preservation, and biological processing of mass-independent sulfur isotope fractionation in the Archean surface environment
3. Ono (2017), Annual Review of Earth and Planetary Sciences · Photochemistry of sulfur dioxide and the origin of mass-independent isotope fractionation in Earth's atmosphere

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