The first land plants to develop penetrating root systems about 400 million years ago may well have caused a series of mass extinctions in the ocean.
The expansion of plants into solid ground was a great moment on Earth, completely re-structuring the terrestrial biosphere. According to researchers from Indiana University – Purdue University Indianapolis (IUPUI) in the US and the University of Southampton in the UK, the impact on our oceans may have been equally dramatic.
During the Devonian, which spanned from 360 million to 420 million years ago, the marine environment experienced numerous mass extinctions. A particularly destructive event near the end of this period resulted in the extinction of nearly 60 percent of all ocean genera.
Some scientists think that trees were the cause of these losses.
As plant life moved away from water sources, they dug deeper and deeper for new sources of nutrients. At some point, their roots would have started to extract phosphorus from minerals trapped underground.
Once the tree decays, those nutrients in the biomass more easily dissolve in the groundwater, which eventually ends up in the sea.
As root systems in the Devonian became more complex and moved further inland, more and more phosphorus would have been dumped into the marine environment.
A new timeline of these nutrient pulses speaks of their destruction. The data is based on the chemical analysis of rocks from ancient lake beds and shorelines in Greenland and Scotland.
“Our analysis shows that tree root evolution is likely to have flooded oceans with excess nutrients, creating massive algal growth,” explained IUPUI earth scientist Gabriel Filippelli.
“This rapid and destructive algal bloom would have depleted most of the oxygen in the oceans, causing catastrophic mass extinctions.”
While scientists have previously suspected that tree roots play a role in Devonian mass extinctions, this study is one of the first to calculate the magnitude and timing of phosphorus release from land to water.
From site to site, researchers found differences in the amount of phosphorus present in the lake’s environment, but overall, most cases suggest that there were major and rapid changes during the Devonian.
The fact that rising ocean phosphorus levels largely correspond to major extinction events during this time suggests that the elevated nutrient played a role in the crisis.
Peaks of phosphorus export did not necessarily coincide in time or magnitude at each site studied, but the authors say this is to be expected. The colonization of the land by plants was not “a single intermittent event,” they explain, “but probably spread geographically, peaking at different times in different parts of Euramerica and other parts of the Devonian Earth.”
Land-based phosphorus was depleted at different rates depending on location, leading to marine extinction events lasting many millions of years. While the precise processes behind nutrient uptake, plant growth and decay more than likely varied, a general trend seems clear. During drier periods, researchers found that phosphorus releases to lakes skyrocketed, suggesting that tree roots can rot if not enough water is available, leading to the release of their nutrients.
Today, trees are not nearly as destructive to marine life as they were when they first arrived on the scene. Soil on land is now much deeper, allowing mineral-bound phosphorus to hide far beyond the reach of roots, making it easier for organic molecules containing phosphorus to cycle through the ecosystem.
That said, what’s happening today shares worrying patterns with what happened hundreds of millions of years ago.
During the Devonian, carbon dioxide and oxygen in the atmosphere reached similar levels to those of recent years, but at the time the changes were largely due to the slow advance of plant life, as opposed to rapid changes from human activity.
Fertilizer and organic waste pollution does not require tree roots to reach the sea. It is pumped through us there, causing low oxygen “dead zones” in many major marine and lake environments.
“These new insights into the catastrophic consequences of ancient natural events may serve as a warning about the consequences of similar conditions arising from human activities today,” says Fillipelli.
The study is published in GSA bulletin.