Earliest land plants created modern levels of atmospheric oxygen

The progressive oxygenation of the Earth’s atmosphere was pivotal

to the evolution of life, but the puzzle of when and how

atmospheric oxygen (O2) first approached modern levels (~21%)

remains unresolved. Redox proxy data indicate the deep oceans

were oxygenated during 435-392 Ma, and the appearance of

fossil charcoal indicates O2>15-17% by 420-400 Ma. However,

existing models have failed to predict oxygenation at this time.

Here we show that the earliest plants, which colonized the land

surface from ~470 Ma onwards, were responsible for this mid-

Paleozoic oxygenation event, through greatly increasing global

organic carbon burial – the net long-term source of O2. We use

a trait-based ecophysiological model to predict that cryptogamic

vegetation cover could have achieved ~30% of today’s global

terrestrial net primary productivity by~445 Ma. Data from modern

bryophytes suggests this plentiful early plant material had a much

higher molar C:P ratio (~2000) than marine biomass (~100), such

that a given weathering flux of phosphorus could support more

organic carbon burial. Furthermore, recent experiments suggest

that early plants selectively increased the flux of phosphorus (relative

to alkalinity) weathered from rocks. Combining these effects

in a model of long-term biogeochemical cycling, we reproduce a

sustained +2‰ increase in the carbonate carbon isotope (δ13C)

record by ~445 Ma, and predict a corresponding rise in O2 to

present levels by 420-400 Ma, consistent with geochemical data.

This oxygen rise represents a permanent shift in regulatory regime

to one where fire-mediated negative feedbacks on organic carbon

burial stabilise high O2 levels.