The first land plants to develop penetrating root systems some 400 million years ago may very well have triggered a series of mass ocean extinctions.
The spread of plants to the mainland was a major moment on Earth that completely restructured 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 could have been just as profound.
During the Devonian period, spanning from 360 to 420 million years ago, the marine environment experienced numerous mass extinctions. A particularly destructive event towards the end of this period led to the extinction of up to nearly 60 percent of all species in the ocean.
Some scientists believe trees were the primary cause of these losses.
As plants moved away from water sources, they dug deeper and deeper for new sources of nutrients. Eventually their roots would have started drawing phosphorus from minerals trapped underground.
Once the tree rots, those nutrients in its biomass more easily dissolve into groundwater, which eventually ends up in the ocean.
As root systems became more complex and moved further inland in the Devonian, more and more phosphorus would have been dumped into the marine environment.
A new timeline of these nutrient pulses speaks to their destruction. The data are based on chemical analysis of rocks from ancient lake beds and coasts in Greenland and Scotland.
“Our analysis shows that tree root evolution likely flooded oceans with excess nutrients, leading to massive algal growth,” explains IUPUI Earth scientist Gabriel Filippelli.
“This rapid and destructive algal bloom would have deprived the oceans of most of the oxygen and triggered catastrophic mass extinctions.”
While scientists have previously suggested that tree roots play a role in the Devonian mass extinction, this study is one of the first to calculate the extent and timing of phosphorus release from land to water.
From site to site, the researchers found variations in the amount of phosphorus in the lake environment, but overall, most cases suggest there were large and rapid changes during the Devonian.
The fact that increases in ocean phosphorus levels during this period largely coincide with major extinction events suggests that the increased nutrient played a role in the crisis.
Peaks in phosphorus export did not necessarily coincide in time or magnitude at every site studied, but the authors say this is to be expected. Plant colonization of land was not “a single discontinuous event,” they explain, “but probably staggered geographically, peaking at different times in different parts of Euramerica and other parts of the Devonian soil.”
Phosphorus on land depleted at different rates depending on location, leading to marine extinction events lasting many millions of years. Although the exact processes behind nutrient uptake, plant growth and decay most likely vary, a general trend seems evident. During drier periods, researchers found that phosphorus supplies to lakes spiked, suggesting that when enough water isn’t available, tree roots could rot, leading to the release of their nutrients.
Today, trees are nowhere near as destructive to marine life as they were when they first appeared on the scene. The soil on land is now much deeper, allowing mineral-bound phosphorus to hide well out of reach of roots, allowing phosphorus-bearing organic molecules to more easily circulate through the ecosystem.
However, what is happening today shares worrying patterns with what happened hundreds of millions of years ago.
During the Devonian, atmospheric carbon dioxide and oxygen reached levels similar to those of recent years, but then the changes were largely due to the slow progression of plant life, as opposed to rapid changes caused by human activity.
Fertilizer and organic waste pollution does not require tree roots to enter the sea. It is pumped there by us and triggers low oxygen ‘dead zones’ in many key marine and lake environments.
“These new insights into the catastrophic consequences of natural phenomena in ancient times could serve as a warning of the consequences of similar conditions resulting from modern-day human activity,” says Fillipelli.
The study was published in GSA Newsletter.
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