ATMOSPHERE × EARTH

A Contested Whiff of Oxygen

~2.5 billion years ago (disputed)

Archean · Neoarchean

One thin band of black shale in Western Australia appears to record oxygen in its waters fifty million years before the atmosphere filled with it. The trace is written in molybdenum and rhenium, metals that only travel when oxygen frees them from rock; whether the signal is original, or was written in much later by passing fluids, is still being argued in print.

Why it matters

If the whiff is real, the machinery that would change the world was already running, its product erased as fast as it formed. If it is not, this rock is a warning: a later fluid can forge an ancient signal.

Dating & uncertaintyDisputed

Molybdenum and rhenium enrichments in the Mount McRae Shale were read in 2007 as a pulse of oxygen 50 million years before the Great Oxidation Event. A 2022 reexamination attributes them to later fluids and recent weathering; the original authors answered in 2023 that the argument is flawed. The exchange is unresolved.

Sources

1. Anbar et al. (2007), Science · A whiff of oxygen before the Great Oxidation Event?
2. Slotznick et al. (2022), Science Advances · Reexamination of 2.5-Ga whiff of oxygen interval points to anoxic ocean before the GOE
3. Anbar et al. (2023), Science Advances · Technical comment on the reexamination of the 2.5-Ga whiff interval

Earth history as one day

10:46:4313 h 13 min before midnight

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