ATMOSPHERE × EARTH
The Proof of an Oxygen-Free Atmosphere
~3.5 to 2.4 billion years ago
The mass-independent fractionation of sulfur is a sorting of sulfur isotopes, found in rocks older than 2.4 billion years, that only an atmosphere without oxygen allows.
The mass-independent fractionation of sulfur is a sorting of sulfur isotopes that ordinary chemical reactions cannot produce. Isotopes are atoms of the same element that differ in mass. Ordinary reactions sort isotopes by weight, in proportion to the mass difference, and this sorting is called fractionation. In rocks of the Archaean, an early eon of Earth history, the sorting of sulfur isotopes does not follow mass. One process is known to produce this result: ultraviolet light striking sulfur dioxide, a gas that contains sulfur, high in the atmosphere. The process requires air with almost no oxygen, for two reasons. Oxygen would form an ozone layer that blocks the ultraviolet. Oxygen would also alter the sulfur on its way down. The mass-independent fractionation disappears from rocks formed after 2.4 billion years ago.
Why it matters
Scientists base the statement that the early atmosphere contained no oxygen on the mass-independent fractionation of sulfur. The measurement also gives a figure: oxygen was at less than a hundred-thousandth of its present level.
Dating & uncertainty
The mass-independent fractionation of sulfur disappears from rocks between 2.4 and 2.3 billion years ago. This disappearance dates the oxygenation of the atmosphere more precisely than any other proxy, or indirect indicator. Researchers have yet to determine which photochemical reaction, a reaction driven by light, produces the fractionation and which molecule transports it to the sea floor.
Earth history as one day
05:29:5218 h 30 min before midnight
if Earth’s 4.54 billion years were compressed into a single day