ATMOSPHERE × CLIMATE

The Sky Becomes a Filter

~430 million years ago

Phanerozoic · Paleozoic · Silurian

Oxygen split by sunlight makes ozone, and ozone absorbs the ultraviolet that breaks DNA. The oxygen came from the first land plants, and by an unexpected route: their tissue is far poorer in phosphorus than anything in the sea, so a given supply of phosphorus washed off the rocks built twenty times more carbon, and carbon buried unrotted is what leaves oxygen behind. By 420 to 400 million years ago there was enough of it to leave charcoal in the rocks, which means fire.

Why it matters

Life had been ashore in patches for billions of years, but always under a sky that let the hard ultraviolet through. A thick ozone layer is what let it stand up in the open. It also brought fire, and fire holds the oxygen steady: too much and the forests burn, which is the feedback that has regulated the air ever since.

Dating & uncertainty

Oxygen reached something like modern levels between about 435 and 392 Ma, and charcoal in the rocks shows it passed 15 percent by 420 to 400 Ma. Chemistry-climate models find the ozone column was thinner than earlier estimates for a given oxygen level, so the shield came later and more gradually than the textbook version.

Sources

1. Lenton et al. (2016), PNAS · Earliest land plants created modern levels of atmospheric oxygen
2. Cooke et al. (2022), Royal Society Open Science · A revised lower estimate of ozone columns during Earth's oxygenated history
3. Tostevin & Mills (2020), Interface Focus · Reconciling proxy records and models of Earth's oxygenation during the Neoproterozoic and Palaeozoic

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