ATMOSPHERE × LIFE
Oxygen Before the Great Oxidation
~2.7 to 2.5 billion years ago
Oxygen oases are local accumulations of oxygen in the shallow seas above the continental shelves, the submerged edges of the continents, that formed between 2.7 and 2.5 billion years ago, before oxygen changed the atmosphere.
Oxygen oases, as they are sometimes called, are local and transient accumulations of oxygen in the sea, formed before the atmosphere changed. Microbes produced the oxygen in the shallow seas above the continental shelves. Reaction with rock and dissolved iron consumed it almost as fast. Five independent chemical indicators, each measured in a different element, show that oxygen was present. In places, the water contained oxygen from the surface down to the sea floor. Oxygen oases do not imply an oxygenated atmosphere. When oxygen is produced faster than it is consumed in one marine basin, it can remain there without reaching the air.
Why it matters
By 2.4 billion years ago, oxygen had already been diffusing into the environment for three hundred million years. Throughout that time, some mechanism was absorbing the oxygen as fast as it formed.
Dating & uncertainty
Researchers measured isotopes, the heavy and light forms of an element, of nitrogen, iron, molybdenum and thallium. These measurements indicate the presence of oxygen in shallow water during the few hundred million years preceding the change in the atmosphere. Enrichments in selenium may record oxidative weathering on land, the alteration of rock by oxygen. Each of these proxies, or indirect indicators, comes with its own qualifications. The extent and duration of the oxygen oases are estimates and not measurements.
Earth history as one day
09:43:3614 h 16 min before midnight
if Earth’s 4.54 billion years were compressed into a single day