The first land plants to develop penetrating root systems, about 400 million years agoit very well may have triggered a series of mass extinctions in the ocean.
The expansion of the plants to the mainland was great time on earth, completely restructuring the terrestrial biosphere. According to researchers from Indiana University-Purdue University of 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 from 360 million to 420 million years ago, the marine environment experienced numerous mass extinction events. One particularly destructive event towards the end of this period resulted in the extinction of almost 60 percent of all genera in the ocean.
Some scientists believe that the trees were the root cause of these losses.
As plant life moved away from water sources, they dug deeper and deeper in search of new sources of nutrients. At some point, its roots would have begun extracting phosphorus from minerals locked underground.
Once the tree decays, those nutrients within its biomass more easily dissolve into groundwater, which eventually ends up in the sea.
In the Devonian, as root systems became more complex and moved inland, more and more phosphorus would have been dumped into the marine environment.
A new timeline of these nutrient pulses tells of their destruction. The data is based on chemical analysis of stones from ancient lake beds and shorelines in Greenland and Scotland.
“Our analysis shows that the evolution of tree roots likely flooded the oceans with excess nutrients, causing massive algae growth.” Explain IUPUI earth scientist Gabriel Filippelli.
“These rapid and destructive algal blooms would have depleted most of the oxygen in the oceans, triggering catastrophic mass extinction events.”
While scientists have suspected tree roots to play a role in Devonian mass extinctions before, this study is one of the first to calculate the magnitude and timing of phosphorus delivery from land to water.
From one site to another, the researchers found differences in the amount of phosphorus present in the lake environment, but in general, most cases suggest that there were large and rapid changes during the Devonian.
The fact that rising ocean phosphorus levels largely align with major extinction events during this time suggests that the elevated nutrient 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. The colonization of the earth by plants was not a “single point event”, explain“but probably floundering geographically, peaking at different times in different parts of Euramerica and other parts of Devonian Earth.”
Phosphorus on land was depleted at different rates depending on location, leading to marine extinction events that lasted for many millions of years. Although the precise processes behind nutrient uptake, plant growth, and decomposition likely varied, a general trend seems apparent. During drier periods, the researchers found that phosphorous delivery to lakes spiked upward, suggesting that tree roots could rot if not enough water is available, leading to the release of their nutrients.
Today, trees are not as destructive to marine life as they were when they first appeared on the scene. The soil on earth is now much deeper, allowing mineral-bound phosphorus to hide well beyond the reach of roots to allow organic phosphorus-containing molecules to circulate more easily through the ecosystem.
That said, what’s 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 advance of plant life, as opposed to the rapid changes brought about by by human activity.
Pollution by fertilizers and organic waste does not require that the roots of the trees reach the sea. It is pumped there by us, and is triggering ‘dead zones’ of low oxygen level in many important marine and lake environments.
“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.” He says Filipelli.
The study was published in GSA Newsletter.