Burying fossil fuels helped keep oxygen in the atmosphere.

Sediments trapped biomass like this trilobite, leading to the creation of fossil fuels and increased oxygen in the atmosphere. Photo: Shutterstock

The creation of fossil fuels may have helped kick off the evolution of animal life on the planet.

Why? Animals need oxygen, not just to breathe but also to process energy from food. But large amounts of carbon in the form of plant material were left on the surface of the earth for billions of years. The plant material, left on the surface, burned and allowed the carbon in it to bond with oxygen and create carbon dioxide.

When these carbon-rich plant materials were buried due to geological processes that are not yet fully understood, that carbon was prevented from bonding with oxygen, and fossil fuels were created. The result was that there was a lot more oxygen in the atmosphere. Much more than there is today, in fact.

The researchers used a data set called Macrostrat, an accumulation of geologic information on North America, to graph oxygen in the atmosphere and sediment burial—based on the formation of sedimentary rock—and found that oxygen levels rose as sediments got buried.

Researcher Shanan Peters says, “Why is there oxygen in the atmosphere? The high school explanation is ‘photosynthesis.’ But we’ve known for a long time … that building up oxygen requires the formation of [sedimentary] rocks like black shale.”

The burial of sediment, and thus the creation of fossil fuels, happened during the Cambrian period. The burial led to the “Cambrian explosion”—a period when an incredible number of different creatures started to evolve—happened. Most animals as we know them today got their start in the Cambrian.

The researchers are still not sure why sediment storage accelerated so quickly in the Cambrian period. “There are many ideas to explain the different phases of oxygen concentration,” says researcher Jon Husson. “We suspect that deep-rooted changes in the movement of tectonic plates or conduction of heat or circulation in the mantle may be in play, but we don’t have an explanation at this point.”

Most people are well aware that burning those fuels has many effects on the world at large, but one that tends to get overlooked is that by burning carbon, we bond it with oxygen to create carbon dioxide. In addition to acting as a greenhouse gas, that carbon dioxide is also removing oxygen from the atmosphere.

Carbon needs to be buried in order to keep oxygen in the atmosphere. “The secret to having oxygen in the atmosphere is to remove a tiny portion of the present biomass and sequester it in sedimentary deposits. That’s what happened when fossil fuels were deposited,” says Husson.

Long story short: We need to keep fossil fuels in the ground in order to avoid further depletion of oxygen in the atmosphere.