Mass extinctions may have been driven by the development of tree roots

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

A series of mass extinctions that rocked Earth’s oceans during the Devonian period over 300 million years ago may have been triggered by the development of tree roots. This is according to a research study led by researchers at Indiana University–Purdue University Indianapolis (IUPUI), together with colleagues in Great Britain.

Evidence for this new view of a remarkably volatile period in Earth’s prehistory was reported Nov. 9 in the journal Science Geological Society of America Bulletin. It is one of the oldest and most respected publications in the field of geology. The study was led by Gabriel Filippelli, Chancellor’s Professor of Earth Sciences in the School of Science at IUPUI, and Matthew Smart, a Ph.D. student in his lab at the time of the study.

“Our analysis shows that the development of tree roots likely flooded the ocean with excess nutrients, causing massive algal growth,” Filippelli said. “These rapid and destructive algal blooms would have depleted most of the oceans’ oxygen, triggering catastrophic mass extinction events.”

The Devonian period, which occurred 419 million to 358 million years ago, before the development of life on land, is known for mass extinction events, during which it is estimated that nearly 70 percent of all life on Earth disappeared.

The process described in the study — known scientifically as eutrophication — is remarkably similar to the modern, albeit smaller-scale, phenomenon that currently fuels wide “dead zones” in the Great Lakes and Gulf of Mexico, as excess nutrients from fertilizers and other runoff from agriculture trigger massive algal blooms that consume all the oxygen in the water.

The difference is that these earlier events were likely fueled by tree roots, which pulled nutrients from the ground during times of growth, then suddenly dumped them into Earth’s waters during times of decay.

The theory is based on a combination of new and existing evidence, Filippelli said.

Based on a chemical analysis of rock deposits from ancient lake beds – remnants of which remain around the world, including the samples used in the study from sites in Greenland and off the north-east coast of Scotland – 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 on 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 importantly, the team found that the dry cycles coincided with higher levels of phosphorus, suggesting that dying roots were releasing their nutrients into the planet’s waters during these times.

“It’s not easy to see over 370 million years into 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 unlock the mysteries of the ancient world.”

In light of the phosphorus cycles that occur at the same time as the development of the first tree roots — a characteristic of Archaeopterisalso the first plant to grow leaves and reach heights of 30 feet—scientists were able to point to the decay of tree roots as the prime suspect behind the Devonian extinction.

Fortunately, Filippelli said, modern trees do not cause similar destruction because nature has since developed systems to balance out the effects of rotting wood. The depth of modern soil also retains more nutrients compared to the thin layer of dirt that covered the ancient soil.

But the dynamics revealed in the study shed light on other more recent 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 waste, such as sewage, has placed Earth’s oceans on the “edge of anoxia,” or a complete lack of oxygen.

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

Reference: “Enhanced terrestrial nutrient release during the Devonian emergence and expansion of forests: Evidence from lacustrine phosphorus and geochemical records” by Matthew S. Smart, Gabriel Filippelli, William P. Gilhooly III, John EA Marshall and Jessica H. Whiteside, November 9 2022, GSA Bulletin.
DOI: 10.1130/B36384.1

Additional authors on the paper are William P. Gilhooly 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.