Washington: Scientists have found that the rise in oxygen levels is linked to a rapid increase in the burial of sediment containing large amounts of carbon-rich organic matter – a process that leads to the formation of fossil fuels. Oxygen enables the chemical reactions that animals use to get energy from stored carbohydrates in food.
So it may be no coincidence that animals appeared and evolved during the “Cambrian explosion,” which coincided with a spike in atmospheric oxygen roughly 500 million years ago, researchers said.
It was during the Cambrian explosion that most of the current animal designs appeared. In green plants, photosynthesis separates carbon dioxide into molecular oxygen and carbon.
However, photosynthesis had already been around for at least 2.5 billion years. Researchers wondered what accounted for the sudden spike in oxygen during the Cambrian. The key is to recognise that sediment storage blocks the oxidation of carbon, said Shanan Peters, a professor at the University of Wisconsin-Madison in the US.
Without burial, this oxidation reaction causes dead plant material on Earth’s surface to burn. That causes the carbon it contains, which originated in the atmosphere, to bond with oxygen to form carbon dioxide. For oxygen to build up in our atmosphere, plant organic matter must be protected from oxidation.
That is exactly what happens when organic matter – the raw material of coal, oil and natural gas – is buried through geologic processes, researchers said.
To make this case, researchers mined a unique data set called Macrostrat, an accumulation of geologic information on North America. The parallel graphs of oxygen in the atmosphere and sediment burial, based on the formation of sedimentary rock, indicate a relationship between oxygen and sediment.
Both graphs show a smaller peak at 2.3 billion years ago and a larger one about 500 million years ago. “It’s a correlation, but our argument is that there are mechanistic connections between geology and the history of atmospheric oxygen,” said postdoctoral fellow Jon Husson.
“When you store sediment, it contains organic matter that was formed by photosynthesis, which converted carbon dioxide into biomass and released oxygen into the atmosphere,” Husson said. “Burial removes the carbon from Earth’s surface, preventing it from bonding molecular oxygen pulled from the atmosphere,” he added.
Some of the surges in sediment burial that researchers identified coincided with the formation of vast fields of fossil fuel that are still mined today, including the oil-rich Permian Basin in Texas and the Pennsylvania coal fields of Appalachia.
“Burying the sediments that became fossil fuels was the key to advanced animal life on Earth,” Peters said. The study was published in the journal Earth and Planetary Science Letters.