The cause of the mass extinction may be the evolution of tree roots

According to new research, the evolution of tree roots may have caused a series of mass extinctions.

Geologists find parallels between ancient extinction events on a global scale and modern threats to Earth’s oceans.

A series of mass extinctions that rocked Earth’s oceans during the Devonian period more than 300 million years ago may have been caused by the evolution of tree roots. This is according to a research study led by scientists at Indiana University – Purdue University Indianapolis (IUPUI), along with colleagues in the United Kingdom.

Evidence for this new view of a remarkably volatile period in Earth’s previous history was reported on November 9 in the scientific journal Bulletin of the Geological Society of America. It is one of the oldest and most respected publications in the field of geology. The study was led by Gabriel Filippelli, Professor of Earth Sciences in the IUPUI School of Science, and Matthew Smart, Ph.D. Student in his lab while studying.

“Our analysis shows that tree root evolution flooded past oceans with excess nutrients, causing a massive algal growth,” said Filippelli. “This rapid and destructive algal bloom would have depleted most of the oxygen in the oceans, leading to the catastrophic mass extinction.”

Ymir Island in East Greenland

Scientists are collecting rock samples on Ymer Island in eastern Greenland, one of several sites whose analysis has provided insight into the chemical makeup of Devonian lake basins. Credit: John Marshall, University of Southampton

The Devonian period, which occurred 419 million to 358 million years ago, before life evolved on Earth, is known for mass extinction events, during which it has been estimated that nearly 70 percent of life on Earth perished.

The process described in the study – known scientifically as eutrophication – is remarkably similar to the modern phenomenon, albeit on a smaller scale, which currently feeds extensive “dead zones” in the Great Lakes and Gulf of Mexico, as excess nutrients from fertilizers and other agricultural runoff lead to massive algal blooms that It consumes all the oxygen of the water.

The difference is that these past events were most likely driven by tree roots, which pulled nutrients from the ground during times of growth, and then suddenly dumped them into the land’s water during times of decay.

Filippelli said the theory is based on a mixture of new and existing evidence.

Gabriel Filippli

Gabriel Filippelli. Credit: Liz Kay, Indiana University

Based on chemical analysis of stony sediments from ancient lakebeds — whose remains are still found worldwide, including samples used in the study from sites in Greenland and off the northeastern coast of Scotland — the researchers were able to confirm previously identified higher cycles and lower levels of Phosphorous, a chemical element present in all life on Earth.

They were also able to identify wet and dry cycles based on signs of “weathering” – or soil composition – caused by root growth, with increased weathering indicating wet cycles with more roots and reduced weathering indicating dry cycles with fewer roots.

Matthew Smart

Matthew Smart. Credit: Image courtesy Matthew Smart

Importantly, the team found that the drought cycles coincided with higher levels of phosphorous, suggesting that the dead roots released their nutrients into the planet’s waters during these times.

“It’s not easy to look back over 370 million years in the past,” Smart said. “But rocks have long memories, and there are still places on Earth where you can use chemistry as a microscope to unravel mysteries of the ancient world.”

In view of the phosphorous cycles that occur at the same time as the development of the roots of the first tree – a feature of Archaeopteriswhich is also the first plant to grow leaves up to 30 feet in height — researchers have been able to identify decomposition of tree roots as the prime suspect behind the Devonian extinction events.

Fortunately, modern trees do not cause similar devastation because nature has since developed systems to counterbalance the effect of rotting wood, Filippelli said. 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 authors note that others have advanced the argument that pollution from fertilizer, manure and other organic waste, such as sewage, has put Earth’s oceans on the “brink of hypoxia,” or total hypoxia.

“These new insights into the catastrophic consequences of natural events in the ancient world may serve as a warning about the consequences of similar conditions arising from human activity today,” Filippelli said.

Reference: “Enhanced release of terrestrial nutrients during the emergence and expansion of Devonian forests: evidence from laxative phosphorous and geochemical records” by Matthew S Smart, Gabriel Filippelli, William B. 2022, GSA Bulletin.
DOI: 10.1130 / B36384.1

Additional authors on the paper are William B. Gilhuly III of IUPUI and John Marshall and Jessica Whiteside of the University of Southampton, UK. Smart is currently an assistant professor of oceanography at the US Naval Academy. This study was supported in part by the National Science Foundation.

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