About 400 million years ago, Earth’s oceans were undergoing a series of biological crises known as the Devonian extinction. While the disappearance of nearly 70% of marine species at that time is known to be linked to a critical lack of oxygen in the oceans, the exact cause was still poorly understood. Scientists, however, have just established that the main culprit would be the rapid development of tree roots during this period.
the DevonianDevonian, which ranges from 419 to 358 million years ago, is one of the most tormented geological periods that the Earth has known. In just 61 million years, six biological crises have followed one another, leading to the extinction of many speciesspecies marines. Among them is one of five major mass extinctions.
If during this period, marine biodiversity is therefore severely affected, on land, however, things happen very differently. The Devonian is indeed a rather positive period for the development of trees and plants. Although diametrically opposed, could these two events be linked?
Oceans devoid of oxygen
We now know that the marine extinctions of the Devonian are associated with a drastic decrease in the level of oxygen in the oceans. These are events that are called anoxic. Over the entire period, the repetition of these events would have led to the disappearance of nearly 70% of the species, which were then mainly marine.
Several causes have been proposed to explain these repetitive drops in oxygen levels in the oceans: glaciationsglaciationsimpacts ofasteroidsasteroids, volcanic eruptionsvolcanic eruptions… or the establishment of vascular plants on the continents. In an article published in the journal Geological Society of America Bulletina team of scientists supports this last hypothesis with new results.
The development of land plants has indeed been accompanied by the establishment of numerous and complex root systems. However, by puncturing the nutrientsnutrients in depth, the roots would have induced a significant modification of the biogeochemistry of the soil, without common measure with what the Earth would have known previously.
The roots behind a massive release of nutrients into the oceans
The extremely rapid development of root systems, linked to the explosion of the plant kingdom at that time, would thus have led to an excessive pumping of nutrients from the soil. However, at the death of the plants, and in the absence of soil formation capable of retaining them, these nutrients would have found themselves leached towards the oceans, in extremely large quantities, favoring the massive development of certain algaealgae. By consuming a large part of the oxygen present in the water, these microorganismsmicroorganisms would therefore be the cause of anoxic events that led to repeated extinctions in the marine environment.
The scientists based themselves on the study of sedimentary rockssedimentary rocks deposited at the bottom of ancient Devonian lakes to identify cyclic variations in the level of phosphorusphosphorusa chemical elementchemical element which turns out to be a essential nutrientessential nutrient to life on earth. By comparing these cycles to those of the paleoclimate, the researchers found that the dry periods coincided with high levels of phosphorus in the sedimentsedimentsignifying a massive influx of this nutrient into ocean basins and lakelake. Dry periods are indeed associated with increased plant mortality, and therefore an excessive release of nutrients trapped in decaying roots.
Ocean anoxia, a current problem but with a different cause
One can ask the question: why is this link of cause and effect no longer observed today? Quite simply because over 400 million years, the Earth system has adapted to the presence of significant plant cover and new interactions have arisen, in particular thanks to burrowing organisms such as earthworms, which allow trap in the soil the nutrients released during the decomposition of the mattermatter organic.
This problem of eutrophication by the development of algae and the beginning ofanoxiaanoxia of the ocean environment, yet we encounter it now. Except that today, it is not plants that are the source of excess nutrients in the oceans, but our agricultural practices and soil degradation linked to human activity. The Devonian period should alert us to the dramatic consequences caused in the short and medium terms by these deteriorated environmental conditions.