State of the planet· The columns of time

Liquid water and anoxic air

4.4 Ga → 2.46 Ga

From 4.4 to 2.46 billion years ago, the Earth had oceans from the outset and an atmosphere without free oxygen, for nearly two billion years.

Regime characterised by the presence of oceans from the outset and by an atmosphere without free oxygen for nearly two billion years.

What the evidence shows

From 4.4 to 2.46 billion years before present, the atmosphere does not vary on the decisive point, which makes this the longest regime in the series. Oceans are present almost from the outset, and life very early. What is missing is free oxygen: the air is a mixture in which any present-day aerobic organism would suffocate, and it remains so for nearly half the history of the Earth. Micro-organisms had nonetheless been producing it for a long time, so that the absence proceeds not from a failure of production but from a sink: iron dissolved in seawater captured every molecule released, precipitated as oxide and settled to the floor before any reached the atmosphere.

Bounds and conventions

The early presence of oceans proceeds from a mineral grain and not from a coastal deposit: zircons formed before 4.3 billion years carry an oxygen isotope signature that only contact with cold liquid water produces, which establishes the existence of seas within the planet's first 250 million years. The constraint is strong on the presence of water and weak on everything else, a grain reporting neither the extent of the seas nor their position.