More than 360 million years ago, during the Devonian period, life was thriving. The first trees appeared on land when fish and invertebrates began to live in the sea. By the end of the Devonian period, however, more than half of all species on Earth had disappeared in a series of mass extinctions. New research shows how arboreal evolution contributed to these extinction events.
As land plants diversified, “more complex root systems began to grow, allowing them to reach greater distances. [down] To catch water,” says Matthew Smart, a marine biologist at the United States Naval Academy. Bulletin of the Geological Society of AmericaA type of tree called primitive evergreen Archeopteryx, even concentrated in virgin forests. Deep tree roots draw important minerals, such as phosphorus, from the bedrock, eventually decaying and forming mineral-laden soil. An influx of water has caused a toxic algae bloom, depriving the water of oxygen.
Researchers tracked this deadly pattern in five prehistoric lakebeds in Scotland and Greenland. They measured a gradual depletion of phosphorus in sedimentary layers during the middle to late Devonian. This is punctuated by sharp spikes in minerals with corresponding evidence of oxygen depletion.
Anne-Christine Da Silva, a sedimentologist at the University of Liège, said: Belgium not involved in new research. “This paper may provide an explanation.”
Planetary scientist Maya Elric of the University of New Mexico, who was not involved in the study, said the oxygen starvation event coincided with the extinction, but it’s unclear what role tree roots played. Phosphorus levels declined overall gradually as trees evolved, but this decline may have already occurred, she says.
Other hypotheses link the Devonian extinction to massive volcanic eruptions, meteor impacts, or disturbances in ocean currents. Smart’s team will then test the results using computer models to see if terrestrial plants may have caused ocean-wide oxygen depletion and corresponding extinction. This analysis is also useful in predicting the impact of modern algal blooms.
Poisonous flowers are on the rise in places such as the Gulf of Mexico, where agricultural fertilizers run off into the water. This leads to “dead zones” with no dissolved oxygen. “Those who don’t learn from history are doomed to repeat it,” says Smart. “In this case, we’re studying his 400-million-year-old history, and we can still learn the same lesson.”
