Mass extinctions may have been caused by evolution of tree roots

Geologists are finding 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 triggered by the evolution of tree roots. This is according to a research study conducted by scientists from Indiana University at Indianapolis Purdue University (IUPUI), along with colleagues from the UK.

Evidence for this new view of a remarkably volatile period in Earth’s prehistory was reported Nov. 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, Chancellor Professor of Earth Sciences in the IUPUI School of Science, and Matthew Smart, Ph.D. student in his laboratory at the time of the study.

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

The Devonian period, which occurred 419 to 358 million years ago, before life on earth evolved, is known for its mass extinction events, during which it is estimated that nearly 70 % of all 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 fuels vast “dead zones” in the Great Lakes and the Gulf of Mexico. , as excess nutrients from fertilizers and other agricultural runoff. trigger massive algal blooms that consume all the oxygen in the water.

The difference is that these past events were likely fueled by the roots of trees, which pulled nutrients from the earth during periods of growth, then dumped them abruptly into the Earth’s water during periods of decay.

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

Based on a chemical analysis of stone deposits from ancient lake beds – remnants of which persist across the globe, including the samples used in the study from sites in Greenland and off the North Coast – east of Scotland – researchers were able to confirm cycles of and lower levels of phosphorus, 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 formation – caused by root growth, with greater weathering indicating wet cycles with more roots and less weathering indicating dry cycles with fewer roots.

More importantly, the team found that dry cycles coincided with higher phosphorus levels, suggesting that dying roots were releasing their nutrients into the planet’s water during these times.

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

Given the phosphorus cycles that occur concurrently with the evolution of the first roots of trees — a feature of Archeopterisalso the first plant to grow leaves and reach a height of 30 feet – researchers were able to identify tree root decay as the prime suspect behind the Devonian extinction events.

Fortunately, Filippelli said, modern trees don’t cause similar destruction since nature has since developed systems to offset the impact of decaying wood. The depth of modern soil also retains more nutrients compared to the thin layer of soil that covered ancient Earth.

But the dynamics revealed in the study shed light on other, more recent threats to life in Earth’s oceans. The study authors note that others have argued that pollution from fertilizers, manure and other organic wastes, such as sewage, has put Earth’s oceans on the “verge of anoxia”. or a total lack of oxygen.

“This new information about the catastrophic results of natural events in the ancient world may serve as a warning about the consequences of similar conditions resulting from human activity today,” Fillipelli said.

Reference: “Enhanced Land Nutrient Release During Devonian Forest Emergence and Expansion: 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

The other authors of the article are William P. Gilhooly III from IUPUI and John Marshall and Jessica Whiteside from the University of Southampton, UK. Smart is currently an assistant professor of oceanography at the US Naval Academy. This study was partially funded by the National Science Foundation.