Decaying tree roots may have caused mass extinctions

A new study published in the Geological Society of America Bulletin has found that the evolution of tree roots likely triggered a series of mass extinction events in our planet’s oceans during the Devonian Period (419 to 358 million years ago), before the emergence of life on land.

“Our analysis shows that the evolution of tree roots likely flooded past oceans with excess nutrients, causing massive algae growth,” said study co-author Gabriel Filippelli, a professor of Earth Sciences at Indiana University. “These rapid and destructive algae blooms would have depleted most of the oceans’ oxygen, triggering catastrophic mass extinction events.”

By performing a chemical analysis of stone deposits from ancient lake beds in Greenland and off the northeast coast of Scotland, the researchers managed to confirm previously identified fluctuations in levels of phosphorous, an important chemical element found in all life on Earth. 

The analysis also revealed wet and dry cycles based upon signs of “weathering” (soil formation caused by root growth), with greater weathering indicating wet cycles with more roots, and less weathering dry cycles with less roots. According to the experts, these dry cycles coincided with higher levels of phosphorous, suggesting that dying tree roots released their nutrients into the oceans during that period. 

Considering that these phosphorous cycles occurred at the same time as the evolution of the first tree roots (an important feature of Archaeopteris, the first plants to grow leaves and reach heights of up to 30 feet), the scientists argued that the decay of tree roots is the main cause of the Devonian mass extinctions. In a process known as “eutrophication,” decaying roots pulled nutrients from the land during times of growth, then suddenly dumped them into the waters during periods of decay, leading to the extinction of over 70 percent of marine life.

This process is remarkably similar to contemporary – yet smaller-scale – phenomena fueling the so-called “dead zones” in the Great Lakes and the Gulf of Mexico, where excess nutrients from fertilizers and other agricultural runoff trigger massive algal blooms that consume most of the water’s oxygen.

“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,” Fillipelli concluded.

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By Andrei Ionescu, Earth.com Staff Writer

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