ATMOSPHERE × EARTH
The Contested Whiff of Oxygen
~2.5 billion years ago (disputed)
A thin band of black shale, a dark rock formed from mud, found in Western Australia, seems to record free oxygen about 2.5 billion years ago, fifty million years before this gas took hold in the atmosphere, a disputed reading.
The whiff of oxygen is a pulse of free oxygen, meaning oxygen as a gas, that appears to be recorded in a thin band of black shale in Western Australia, fifty million years before oxygen took hold in the atmosphere. The evidence is two metals, molybdenum and rhenium. Oxygen releases both metals from rock, a breakdown called oxidative weathering, and they then move towards the sea. Researchers read an enrichment of the two metals in the sediment as evidence of oxygen upstream. This enrichment may be original, or circulating fluids may have introduced it much later, a possibility known as epigenetic contamination. The question remains argued in print.
Why it matters
If the whiff is real, the source of oxygen that would later transform the world was already operating, and its oxygen was removed as fast as it formed. If the whiff is not real, the shale is a warning about method: a fluid that entered the rock late can leave an enrichment that looks ancient.
Dating & uncertaintyDisputed
Anbar and colleagues (2007) interpreted the molybdenum and rhenium enrichments of the Mount McRae Shale as a pulse of oxygen fifty million years before the Great Oxidation Event, the lasting rise of oxygen in the atmosphere. In a re-examination published in 2022, Slotznick and colleagues attribute the enrichments to late fluids and recent weathering. Anbar and colleagues replied in 2023 that this argument is defective. The exchange is unresolved.
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