ATMOSPHERE × CLIMATE
The Ozone Layer Forms
~430 million years ago
Phanerozoic · Paleozoic · Silurian
The ozone layer, which absorbs the ultraviolet wavelengths that damage DNA, formed about 430 million years ago from oxygen released by the first land plants.
Ozone forms when sunlight splits oxygen molecules and the fragments recombine in threes. It absorbs the ultraviolet wavelengths that damage DNA. The oxygen came from the first land plants, by an indirect route. Phosphorus is a nutrient that is washed off the rocks. Plant tissue contains far less phosphorus per unit of carbon than marine life does, a difference expressed as the carbon-to-phosphorus ratio. The same phosphorus therefore built about twenty times more carbon in plants than in marine life. Carbon that is buried without decaying leaves oxygen behind. By 420 to 400 million years ago there was enough oxygen for fire. Charcoal, also called fusain, appears in the rocks, and fire will not propagate below about sixteen per cent oxygen.
Why it matters
Life had occupied terrestrial environments in patches for billions of years, but always under a sky that let short-wavelength ultraviolet through. The rise of oxygen had two effects. The ozone formed from the oxygen made it possible to occupy environments exposed to the open sky. The oxygen itself made fire possible. Fire then stabilised the concentration of oxygen: when oxygen is in excess, forests burn. This is a feedback, an effect that acts back on its cause, and it has regulated the atmosphere since.
Dating & uncertainty
Redox proxies, chemical indicators in rocks of the oxygen once present in the water, show that the deep oceans were oxygenated between about 435 and 392 million years ago. Fossil charcoal establishes that atmospheric oxygen passed 15 per cent between 420 and 400 million years ago. The ozone column is the total ozone above a point on the ground. For a given oxygen concentration, the models of Cooke and colleagues (2022), which couple chemistry and climate, obtain a thinner ozone column than earlier estimates allowed. The ozone shield therefore formed later and more gradually than the usual account indicates.
Earth history as one day
21:43:362 h 16 min before midnight
if Earth’s 4.54 billion years were compressed into a single day