Burrowing Animals May Have Stabilized Earth's Oxygen 540 Million Years Ago

Could burrowing animals be key to Earth's oxygen? Scientists have found that their activity significantly influenced Earth's phosphorus cycle and, as a result, the amount of oxygen in Earth's atmosphere.

About 540 million years ago, the first burrowing animals evolved. These worms first appeared in the Earth's oceans, mixing up sediments in a process known as bioturbation. Marine organic carbon burial is a source of oxygen to the atmosphere, and its rate is proportional to the amount of phosphate in the oceans. This means that anything that decreases the size of the ocean phosphate reservoir also decreases oxygen.

In order to better understand the influence of these creatures on the Earth's oxygen, the scientists employed computer model that linked data from the fossil record to well established connections between the phosphorus and oxygen cycles. More specifically, they focused on the burial of phosphorus in the organic matter in ocean sediments.

"It has long been appreciated that organic phosphorus burial is greater from the kind of well oxygenated, well-mixed sediments that animals inhabit, than from poorly mixed, low oxygen 'laminated' sediments," said Richard Boyle, one of the researchers, in a news release. "The key argument we make in this paper is that this difference is directly attributable to bioturbation."

The researchers found that over 540 million years ago, burrowing animals significantly increased the extent to which oxygenated waters came into contact with ocean sediments. This contact caused the bacteria in the sediments to store phosphate in their cells. This, in turn, caused an increase in phosphorus burial which led to decreases in marine phosphate concentrations, productivity, organic carbon burial and oxygen.

"We already think this cycle was key to helping stabilize atmospheric oxygen during the Pherozoic (the last 542 million years)-and that oxygen stability is a good thing for the evolution of plants and animals," said Tim Lenton, co-author of the new study. "What is new in this study is it attributes the oxygen stabilization to biology-the presence or absence of animals stirring up the ocean sediments."

The findings are published in the journal Nature Geoscience.