ATMOSPHERE × LIFE
The Billion-Year Oxygen Ceiling
~1.6 to 0.8 billion years ago
Proterozoic · Mesoproterozoic · Calymmian
Atmospheric oxygen stayed at a thousandth of today's level at most from 1.6 to 0.8 billion years ago, according to chromium isotopes measured in sediments of that age.
The oxygen ceiling is the upper limit under which atmospheric oxygen remained for most of a billion years. The estimate comes from chromium isotopes, forms of chromium whose atoms differ in mass. In sediments of that age, they place atmospheric oxygen at a thousandth of present levels at most. Chromium serves as a marker because its oxidation on land requires oxygen. The oxidised chromium has a fractionated signature, that is, altered proportions of its isotopes, which is then recorded in marine sediment. Little oxygen meant little ozone, so more ultraviolet reached the surface. Much of the deep sea remained anoxic, without oxygen. It may also have been short of trace metals, the metals that enzymes, the proteins that speed up a cell's reactions, require in minute amounts. Complex cells persisted throughout that time, but no animal appeared. This absence is usually attributed to the oxygen requirement of animal tissue, which must draw oxygen from its surroundings faster than diffusion alone, the passive spreading of a gas, will supply it.
Why it matters
The longest interruption in the history of life, the interval of the oxygen ceiling, is ordinarily attributed to the slowness of biological innovation. The chromium estimate makes the planet the cause: too little oxygen, too much ultraviolet, and an ocean held to be poor in essential trace metals. On this reading, the interruption ends when those limits lift.
Dating & uncertaintyDisputed
Chromium isotopes place the oxygen of the mid-Proterozoic, the middle part of the Proterozoic eon, at no more than a thousandth of the present level, which would leave too little oxygen for animals. Other proxies, measurable quantities from which a past condition is inferred, and other models allow several per cent of the present level. With the low value, animals appeared late because the planet prevented them. With the higher values, they appeared late because the biology required that time.
Earth history as one day
15:32:308 h 27 min before midnight
if Earth’s 4.54 billion years were compressed into a single day