Devonian Extinction
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Mystery of the Late Devonian extinction

Between about 385 and 359 million years ago, life on earth was under attack, and 70 to 80 percent of the species on the planet disappeared. However, evidence defining the cause of this catastrophe is sparse.  Also, controversy exists over whether there was one event or many events leading to extinction and how long it took. Some researchers propose multiple events over about 20 million years. The answer is uncertain, but there are some clues to the mystery of the Late Devonian extinction.

The Late Devonian is divided into two stages: the Frasnian followed by the Famennian. Paleontologists point to three periods in the Late Devonian where major changes occur in the fossil records: the beginning and end of the Frasnian and the end of the Famennian. Large numbers of species disappeared from the geological records at these points in time

Black Shales

Black shales provide one piece of evidence about Devonian environmental conditions. Layers of black shales deposited at the end of the Frasnian and the end of the Famennian provide a clue. The significance of these shales is that they provide evidence of widespread anoxia (lack of oxygen) in the late Devonian seas. Without oxygen, both fish and bottom-dwelling creatures perished. 

This observation raises a second question. What happened to the oxygen? During the Devonian, animals had only recently crawled out of the ocean, and the vascular plants that evolved during the preceding Silurian period dominated dry land. The Devonian saw the rapid evolution of plants as they grew larger and developed complex root systems. So, during the Devonian, the plants and trees that ruled dry land were helping to break down rock into soil. Additionally, the geological record indicates that vast forests rapidly expanded in the Late Devonian. Therefore, the combination of organic remains from dead plants and the nutrients from rapidly weathering soils washed into streams, rivers, and finally the oceans. 

Organic, nutrient-rich waters provided a paradise for algae, and so algae populations exploded. As the algae died, bacteria played their role by consuming the organic remains. But the bacteria had metabolic needs and required oxygen. Slowly they sucked the free oxygen from the oceans creating anoxic conditions that were ripe for the formation of black shales. Perhaps the success of plants on land caused the downfall and extinction of so many species. 

Asteroids

Some researchers propose an asteroid impact as the cause of extinction, much like the demise of the dinosaurs at the end of the Cretaceous. Late Devonian impact craters like the Siljan Ring (Sweden), Alamo (USA, Nevada), and Woodleigh (Western Australia) all roughly tie to extinction events. Still, the precise dating and direct evidence to make them the primary culprits do not exist. These impacts were not the size of the Chicxulub impact that caused the Late Cretaceous extinction, and thus their role in the Devonian is uncertain.

Ozone

Recent research at the University of Southampton identifies another possible cause – a weakened ozone layer. The researchers studied the cell walls of spores and pollen from the Late Devonian and discovered a high incidence of genetic damage in these plant cells about 359 million years ago. 

The cause for the ozone problem is postulated as high continental temperatures that allow more water transport into the upper atmosphere. The water vapor carried chemical compounds, including chlorine, which is produced naturally by some plants and algae. When the chlorine reached the ozone layer, it broke down the ozone, thus allowing damaging ultraviolet radiation to the surface of the earth. Essentially the mechanism was a weakened ozone layer caused by climate warming. The researchers note that many plant species went extinct at this time, creating a collapse of the Late Devonian forest ecosystems.

Cascading events

Mass Extinctions are caused by rapidly changing environmental conditions. The specific cause of the Devonian extinction is a mystery, but there are various bits of evidence that hint at possible causes. It is also possible that the answer is all of the above. Perhaps the Devonian extinction was the result of a series of cascading natural events that collectively toppled the ancient world’s ecosystems. 

One scenario might be: Global collapse of the Devonian forest ecosystems from ozone depletion created a massive influx of organic material into the oceans. This flood of organic material and nutrients drove algae blooms and then ocean anoxia. Or, maybe the forests were already stressed from the effects of an asteroid strike, and the coup de grâce was ozone depletion.

We don’t know the answer, but we do know that all extinction events seem to be associated with some form of rapid and dramatic climate change. The current Anthropocene extinction is no exception. The only real difference is that past extinctions occurred in response to natural causes. However, the current high rate of species extinction is directly related to the activity of a single species: Homo sapiens.


ArcheanWeb:


Permian black shales (Source: ArcheanWeb) – https://archeanweb.com/2020/05/27/permian-black-shales/  Also:

Mass Extinction Events: Life’s Struggle for Survival (Source: ArcheanWeb) – https://archeanweb.com/2019/12/01/mass-extinction-events-lifes-struggle-for-survival/  Also:Paragraph

Cascading natural events: The perfect storm (Source: ArcheanWeb) – https://archeanweb.com/2020/06/02/cascading-natural-events-the-perfect-storm/  Also:


Sources:

The Devonian extinction saw the oceans choke to death (By Chris Baraniuk, BBC) – http://www.bbc.com/earth/story/20150624-the-day-the-oceans-died  Also:

Prehistoric Climate Change Damaged the Ozone Layer and Led to a Mass Extinction (By  John Marshall; National Interest) – https://nationalinterest.org/blog/reboot/prehistoric-climate-change-damaged-ozone-layer-and-led-mass-extinction-159476  Also:

Feature Image: Psammolepis Alata (Reworked) –  By Vladislav Egorov (3D model), Jaagup Metsalu (render) – Estonian Museum of Natural History, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=82586973

William House
William is an earth scientist and writer with an interest in providing the science "backstory" for breaking environmental, earth science, and climate change news.

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