The oxygen we need to live makes up about 21% of our atmosphere, but it wasn’t always so.
For its first 2⅟₂ billion years, Earth’s atmosphere had no oxygen. Then, with the advent of organisms capable of photosynthesis, there was suddenly this new gas to contend with. For many it was poisonous – oxygen is just so reactive – but for those who could use it, it enabled them to increase their metabolisms.
Oxygen levels remained low, just a few percent, until around 460 million years ago when it began a slow climb. But when did it reach anything close to the modern level of 21%? There are many computer models of ancient atmospheric oxygen concentrations, but actual evidence is sparse. An interesting bit of recent research on samples from two sites supplies some of that evidence.
Charcoal tested in ancient rocks
The first is a place called Rumney, in Wales. The samples come from a 1,041-foot borehole drilled by the British Geological Survey in 1978 (showing once again that rocks collected years ago can still be useful for modern research). The rocks, whose sediments were deposited in a near-shore ocean setting, are about 429 million years old, from the middle of the Silurian Period. The rocks contain many pieces of charcoal – burned plant material.
The second site is called Winnica, in the Kielce region of Poland, and the rocks date to around 424 million years ago. They also contain charcoal.
Both sites were much farther south than they are today, about 30 degrees south of the equator, and not that far from what would become North America.
More oxygen, hotter fires
What do those rocks have to do with oxygen concentrations? Experiments done under controlled conditions, such as in enclosed spaces where gas proportions can be controlled, show that a sustained burn, such as of a forest (although not in this case, since there were no trees yet) or a ground fire, requires an oxygen level of at least 16%.
And those fires were hot. Based on the reflectance or “shininess” of the charcoal, temperatures of 930 to 1,650 Fahrenheit are indicated. Such hot and extensive fires indicate oxygen levels were probably much higher than 16%, probably close to or even exceeding our present 21%.
There is some possibly older charcoal from a site in Passage Creek, Virginia, in rocks that are about 441 million years ago. In the subsequent period, the Devonian, there is a mysterious “charcoal gap” early in the period, but that ends later in the period when the first forests appeared and began pumping out oxygen (and getting burned by fires).
More oxygen, bigger bugs
In the following period, the Carboniferous, oxygen levels may have reached 31%. It has been suggested the large amount of available oxygen allowed the appearance of giant arthropods such as Meganeura, a relative of dragonflies with a 28-inch wingspan, or Arthropleura, a millipede six feet long.
The amount of oxygen in Earth’s atmosphere doesn’t affect organisms alone. It also affects how fast rock weathers, how quickly certain minerals form, and the frequency and intensity of fires.
Dale Gnidovec is curator of the Orton Geological Museum at Ohio State University.