What trees caused the Devonian extinction?

Archaeopteris, considéré comme le premier arbre

The Devonian extinction occurred approximately 380 to 359 million years ago. Nearly 75% of species have been seen to disappear, mainly hitting marine life. On dry land, plants and animals seem well-preserved, but the poverty of the terrestrial fossil record makes accurate assessment difficult. The dynamics of this extinction itself raises questions, as scientists estimate that it lasted 20 million years in 3 episodes. As such, the causes of this mass extinction are still being debated, and a new theory has already been formulated by scientists at Indiana University.

The appearance of trees

The Devonian is also called the “Age of Fishes” and during this period there was an important diversification of this group and the appearance of the lineage that is the origin of modern vertebrates, as well as the first forms of life emerging from it. ‘Water. But precisely on dry land, a major transformation also took place: the rise of the first trees. with’Archaeopteris As a symbol, specialists consider the tree closest to the current forms. The arrival of trees profoundly changed the shape of the Earth and possibly the oceans, paleontologists claim in a study published in the journal Earth. Geological Society of America Bulletin. According to them, the rotting of the roots of the first trees created a massive flow of organic matter into the sea. This resulted in widespread algae blooms, which used up almost all of the ocean’s oxygen and led to a period of ocean extinction.

This process is called Eutrophication, similar to what is happening now on a smaller scale in some regions around the world. The reason is different: waste from agricultural activities (slurries, fertilizers, etc.) provides the organic matter that feeds the algae. Indeed, over 350 million years, nature has adapted and can compensate for the biological contribution associated with the decomposition of trees. A large part is now absorbed by soils which are much deeper than the Devonian and therefore do not terminate in water.

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The collection of samples from Ymer Island in East Greenland is one of several sites that have led to a better understanding of the chemical composition of Devonian lake beds. Credit: John Marshall, University of Southampton.

Wet and dry cycles

To support their theory, the researchers studied the rock deposits of ancient lakes, remnants of which survive around the world, particularly in Greenland. Scientists were particularly interested in the amount of phosphorus, a chemical element found in all life on Earth. Their analysis thus reveals a phosphorus cycle in the Devonian with dry periods corresponding to high levels of phosphorus, wet periods with dying roots of trees and wet periods with lower levels of this element and more roots growing in the soil. A result consistent with their hypothesis suggesting root erosion as the primary cause of the Devonian extinction.

However, other hypotheses such as volcanic or even The explosion of a star near the Sun Can also explain oceanic anoxia. This last explanation is mainly valid for the final event of the Devonian extinction, which marks the end of this period and the beginning of the Carboniferous, known as the “Hangenberg Crisis”.

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