Devonian Period occured almost 419 million to 358 million years ago, before the evolution of life on land, is known for mass extinction events. During this event, nearly 70 percent of all life on Earth perished.

A new study- by scientists at IUPUI in collaboration with colleagues in the United Kingdom- suggests that the evolution of tree roots may have triggered a series of mass extinctions that rocked the Earth’s oceans during the Devonian Period. The evolution of tree roots likely flooded past oceans with excess nutrients, causing massive algae growth.

Gabriel Filippelli, Chancellor’s Professor of Earth Sciences in the School of Science at IUPUI, said, “These rapid and destructive algae blooms would have depleted most of the oceans’ oxygen, triggering catastrophic mass extinction events.”

Scientists, in the study, highlighted the process called eutrophication. The process is mostly similar to the modern, albeit smaller-scale, a phenomenon currently fueling broad “dead zones” in the Great Lakes and the Gulf of Mexico, as excess nutrients from fertilizers and other agricultural runoff trigger massive algae blooms that consume all of the water’s oxygen.

Filippelli said, “The difference is that these past 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.”

“The theory is based upon a combination of new and existing evidence.”

Scientists performed chemical analyses of the stone deposit from ancient lake beds. They confirmed previously identified cycles of higher and lower phosphorus levels, a chemical element found in all life on Earth.

Additionally, they could distinguish between wet and dry cycles using the signals of “weathering,” or soil formation, which is brought on by root growth. Greater weathering indicated wet cycles with more roots, whereas less weathering indicated dry cycles with fewer roots.

Importantly, scientists found that the dry cycles coincided with higher phosphorous levels. It means dying roots released their nutrients into the planet’s water during these times.

Matthew Smart, a Ph.D. student in his lab at the time of the study, said, “It’s not easy to peer over 370 million years into the past. 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.”

The scientists were able to identify the decay of tree roots as the primary suspect behind the Devonian Periods extinction events due to the fact that the phosphorus cycles took place at the same time as the evolution of the first tree roots, a characteristic of Archaeopteris, which was also the first plant to grow leaves and reach heights of 30 feet.

Filippelli said, “Fortunately, 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.”

However, the study’s dynamics made it clear that there are other, more recent dangers to ocean life. The paper’s authors point out that other people have argued that sewage, manure, fertilizers, and other organic wastes have polluted the Earth’s oceans to the point where they are “on the edge of anoxia,” or completely devoid of oxygen.

Fillipelli said, “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.”

Journal Reference:

  1. Matthew S. Smart et al. Enhanced terrestrial nutrient release during the Devonian emergence and expansion of forests: Evidence from lacustrine phosphorus and geochemical records. Geological Society of America Bulletin. DOI: 10.1130/B36384.1