The first land plants to develop penetrating root systems, around 400 million years ago, could very well have triggered a series of mass extinctions in the ocean.
The expansion of plants on dry land was a great moment on Earth, completely restructuring the terrestrial biosphere. According to researchers from Indiana University and Purdue University Indianapolis (IUPUI) in the US and the University of Southampton in the UK, the consequences for our oceans could have been just as profound. .
During the Devonian Period, which spanned 360 million to 420 million years ago, the marine environment experienced many mass extinction events. A particularly destructive event near the end of this period led to the extinction of nearly 60% of all genera in the ocean.
Some scientists believe that trees are the cause of these losses.
As plant life moved away from water sources, they dug deeper and deeper to find new sources of nutrients. At some point, their roots would have started extracting phosphorus from minerals locked underground.
Once the tree decomposes, these nutrients in its biomass dissolve more easily into groundwater, which eventually ends up in the sea.
During the Devonian, as root systems 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 impulses speaks of their destruction. The data is based on chemical analysis of stones from ancient lake beds and coasts in Greenland and Scotland.
“Our analysis shows that the evolution of tree roots likely flooded past oceans with excess nutrients, causing massive algal growth,” says Gabriel Filippelli, earth scientist at IUPUI.
“These rapid and destructive algal blooms would have depleted most of the oxygen in the oceans, triggering catastrophic mass extinction events.”
While scientists previously suspected tree roots played a role in Devonian mass extinctions, this study is one of the first to calculate the extent and timing of phosphorus input from land to water. .
From site to site, the researchers found differences in the amount of phosphorus present in the lake environment, but overall most cases suggest there were large and rapid changes. during the Devonian.
The fact that rising ocean phosphorus levels largely corresponded to major extinction events during this period suggests that increased nutrients played a role in the crisis.
The phosphorus export peaks did not necessarily coincide in time or magnitude at each site studied, but the authors say that is to be expected. Land colonization by plants was not a “single punctuated event”, they explain, “but probably staggered geographically, culminating at different times in different parts of Euramérica and other parts of Devonian Earth”.
Terrestrial phosphorus has been depleted at varying rates depending on location, leading to marine extinction events that spanned millions of years. Although the precise processes behind nutrient uptake, plant growth and decomposition more than likely vary, a general trend seems apparent. During drier periods, researchers found that phosphorus input to lakes increased, suggesting that tree roots could decay if there is not enough water available, causing the release of their nutrients.
Today, the trees are not as destructive to marine life as they were when they arrived on the scene. The soil on earth is now much deeper, allowing mineral-bound phosphorus to hide well beyond the reach of roots to let organic phosphorus-containing molecules flow through the ecosystem more easily.
That said, what is happening today shares disturbing patterns with what happened hundreds of millions of years ago.
During the Devonian, atmospheric carbon dioxide and oxygen reached levels similar to recent years, but at the time the changes were largely due to the slow advance of plant life, as opposed to rapid changes due to human activity.
Fertilizer and organic waste pollution does not need tree roots to reach the sea. It is pumped there by us and it triggers low oxygen “dead zones” in many important marine and lake environments. .
“This new knowledge 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,” says Fillipelli.
The study was published in GSA Bulletin.
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