Era· The columns of time

Neoarchean

2.8 Ga → 2.5 Ga

In the Neoarchean, from 2.8 to 2.5 billion years ago, photosynthesis produced oxygen that the ocean absorbed as fast as it appeared.

Era of three hundred million years during which oxygen is produced by photosynthesis but consumed as fast as it appears, the ocean absorbing the entire output.

What the evidence shows

Oxygenic photosynthesis is the metabolic innovation with the widest consequences, and it belongs to the Archaean, to this era or the preceding one, between about 3.2 and 2.8 billion years. Its atmospheric effect is deferred by several hundred million years: iron dissolved in the oceans takes up the oxygen and precipitates, depositing the banded iron formations still worked today. A sink of that capacity absorbs the output for as long as it is not saturated. Free oxygen therefore does not accumulate in the atmosphere, and the era closes on unchanged air.

Bounds and conventions

A long interval separates the innovation from its consequence, and neither end of it is sharply dated. The phenomenon that characterises the era begins before it and ends after it, so that its limits mark no moment of the history they enclose.