The Devonian Period, which occurred 419 million to 358 million years ago, is known for mass extinction events, during which it’s estimated that nearly 70 percent of all life on Earth perished.
According to a new study led by IU researchers Gabriel Filippelli and Matthew Smart, the evolution of tree roots may have triggered a series of these mass extinctions that rocked the Earth’s oceans during the Devonian Period.
Filippelli says their analysis shows that the evolution of tree roots likely flooded past oceans with excess nutrients, causing massive algae growth. He says these rapid and destructive algae blooms would have depleted most of the oceans’ oxygen, triggering catastrophic mass extinction events.
The process outlined in the study is remarkably similar to modern, albeit smaller-scale, phenomenon currently fueling broad “dead zones” in the Great Lakes and the Gulf of Mexico, where excess nutrients from fertilizers and other agricultural runoff trigger massive algae blooms that consume all of the water’s oxygen.
The difference is that the ancient events were likely fueled by tree roots, which pulled nutrients from the land during times of growth, then abruptly dumped them into the Earth’s water during times of decay.
Filippelli says the theory is based upon a combination of new and existing evidence.
Based upon a chemical analysis of rock deposits from ancient lake beds, the researchers were able to confirm previously identified cycles of higher and lower levels of phosphorus, a chemical element found in all life on Earth. They were also able to identify wet and dry cycles based upon signs of “weathering” — or soil formation — caused by root growth, with greater weathering indicating wet cycles with more roots and less weathering indicating dry cycles with fewer roots.
Most significantly, the team found that the dry cycles coincided with higher levels of phosphorus, suggesting that dying roots released their nutrients into the planet’s water during these times.
In light of the phosphorus cycles occurring at the same time as the evolution of the first tree roots, the researchers were able to pinpoint the action of tree roots releasing phosphorus from rocks as the prime suspect behind the Devonian Periods extinction events.
Fortunately, Filippelli says, modern trees don’t wreak similar destruction because nature has since evolved systems to balance out the impact of rotting wood. The depth of modern soil also retains more nutrients compared to the thin layer of dirt that covered the ancient Earth.
But the dynamics revealed in the study shed light on other newer threats to life in Earth’s oceans. The study’s authors note that others have made the argument that pollution from fertilizers, manure and other organic wastes, such as sewage, have placed the Earth’s oceans on the “edge of anoxia,” or a complete lack of oxygen.
Fllippelli says these new insights into the catastrophic results of natural events in the ancient world may serve as a warning about the consequences of similar conditions arising from human activity today.