The first land plants to develop penetrating root systems, about 400 million years ago, may have triggered a series of mass extinctions in the ocean.
The expansion of plants on the still land was a huge moment on Earth, completely restructuring 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 consequences for our oceans may have been just as profound.
During the Devonian period, which spanned from 360 million to 420 million years ago, the marine environment experienced many mass extinctions. A particularly devastating event at the end of this period led to the extinction of nearly 60 percent of all species in the ocean.
Some scholars believe that trees were the root cause of these losses.
As plant life moved away from water sources, they dug ever deeper for new sources of nutrients. At some point, their roots had begun to draw phosphorous from minerals trapped underground.
Once a tree decomposes, those nutrients in its biomass dissolve more easily into groundwater, which eventually ends up in the sea.
In the Devonian period, as root systems became more complex and moved inland, more and more phosphorus would have been dumped into the marine environment.
The new timeline of these trophic impulses indicates their destruction. The data is based on chemical analysis of stones from ancient lake beds and coastlines in Greenland and Scotland.
“Our analysis shows that the evolution of tree roots flooded past oceans with excess nutrients, causing massive algae growth,” explains Gabriele Filippelli, IUPUI Earth Scientist.
“The rapid and destructive algal blooms would have depleted most of the oxygen in the oceans, leading to the catastrophic mass extinction.”
While scientists have suspected tree roots to play a role in the Devonian mass extinction before, this study is one of the first to calculate the magnitude and timing of land-to-water phosphorus delivery.
From site to site, the researchers found differences in the amount of phosphorus present in the lake environment, but in general, most cases point to large and rapid changes occurring during the Devonian period.
The fact that rising phosphorus levels in the ocean align so closely with major extinction events during this time suggests that elevated nutrients played a role in the crisis.
The phosphorus export peaks did not necessarily match in time or magnitude at each site studied, but the authors say this is to be expected. Plant colonization of the land, they explained, was not “a single, discontinuous event,” “but likely overlapped geographically, peaking at different times in different parts of European America and other parts of the Devonian land.”
Phosphorus has been depleted on Earth at varying rates by location, leading to marine extinctions that have lasted for millions of years. Although the exact processes behind nutrient uptake, plant growth, and decay were likely diverse, the general trend appears clear. During droughts, the researchers found that the delivery of phosphorus to lakes shoots upwards, suggesting that tree roots may decay if not enough water is available, releasing their nutrients.
Today, the trees aren’t nearly as destructive to marine life as they were when they first arrived on the scene. Soils on Earth are now much deeper, allowing mineral-bound phosphorus to burrow out of reach of roots to allow organic molecules containing phosphorus to circulate more easily through the ecosystem.
However, what is happening today shares disturbing patterns with what happened hundreds of millions of years ago.
During the Devonian period, atmospheric carbon dioxide and oxygen reached levels similar to those in recent years, but at that time the changes were, in large part, due to the slow progression of plant life, as opposed to rapid changes through human activity.
Pollution from fertilizers and organic waste does not require tree roots to reach the sea. It gets pumped there by us, and leads to “dead zones” of low oxygen in many important marine and lake environments.
“These new insights into the cataclysmic consequences of natural events in the ancient world may serve as a warning about the consequences of similar conditions arising from human activity today,” says Filippelli.
The study has been published in GSA Bulletin.
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