Toxic World Mass Extinction
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Devonian Life Sinks into Mass Extinction

The Reasons are Uncertain

Published in The EarthSphere Blog. Feature Image: Toxic Devonian World by WM House (©Archean Enterprises, LLC; ArcheanArt)


Prologue

The Forgotten Origins series has noted life’s success in the Devonian Period as extensive adaptive radiation of plant and animal species occurred on land and at sea. But success comes at a price.

Extinction

Despite life’s advances on land and at sea during the Devonian, the circle of time completed a loop. Between about 383 and 359 million years ago, 70 to 80 percent of the species on the planet disappeared. However, evidence defining the cause of this catastrophe is sparse. Perhaps life was too successful for its own good. After all, the Anthropocene extinction indicates human beings may be too smart for their own good.

It’s important to place this extinction event into the proper time frame. The Late Devonian is divided into two stages: the Frasnian followed by the Famennian. These two stages span 24 million years. 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 at the end of the Famennian. Large numbers of species disappeared from the geological records at these points in time.

The Late Devonian extinction didn’t occur as a single catastrophic blow to life. It was more like a slow-moving train wreck or death by a thousand cuts. So it seems doubtful an asteroid strike was the primary culprit. Impact craters like the Siljan Ring (Sweden), Alamo (USA, Nevada), and Woodleigh (Western Australia) all are roughly dated to the Late Devonian, but none of these craters are the size of the Chicxulub impact that caused the Late Cretaceous extinction. Also, the dating of these events is not precise enough to definitively tie them to any of the three extinction peaks.

Perhaps the 24-million-year interval encompassing the Late Devonian extinction represents a gradual development of severe chemical and environmental imbalances. Simply put, Devonian habitats became inhospitable to life.

Organic Shales

The term shale refers to fine-grained sedimentary rocks composed mainly of silt and clay-sized particles. Usually, these rocks originate from sediments deposited in waters away from currents and surface disturbances. The deposits then build layer upon layer over thousands to millions of years.

But there is a particular type of shale coveted by the oil exploration industry called black shale. Its dark color comes from the high levels of organic matter trapped in the rock matrix. When black shales are buried deep in the ground and temperatures rise to 110 to 120 degrees C, the organic matter begins converting to oil. Once it starts generating oil, it becomes a ‘source rock.’ Follow the source rock, and you find the oil.

The organic material in black shales is an indicator of ancient environmental conditions. Normally, organic materials like the remains of dead plants and animals fall to the ocean floor, where bacteria consume it. However, when the deep ocean waters are anoxic, there are no ocean bottom organisms to eat the organic matter reigning in from above, and it becomes incorporated into the sediments. The presence of black shales indicates persistent anoxia, and we know that the absence of free oxygen means the absence of most life.
 
Vast deposits of Devonian black shales cover much of the eastern United States, indicating widespread anoxia in the Late Devonian oceans. These shales point to severe ecological stress since most animal life still lived in shallow marine ecosystems and depended on oxygen for survival.

Late Devonian Anoxia

A variety of physical and environmental conditions cause ocean anoxia, and it is important to understand the potential sources of oxygen depletion. Today we have widespread anoxia in the Black Sea below about 100 meters. This anoxia results from density stratification between deep, cold, saline water and a thin shallow zone with fresher water. The density difference is high enough that the two don’t mix. Because ocean water requires contact with the atmosphere to oxygenate, no oxygen reaches the denser layer below the surface water. In this case, the physiographic conditions of the Black Sea basin led to its permanent anoxia.

But anoxia can also occur in bodies of water with no stratification. The process is biological and starts at the bottom of the food chain. Organic, nutrient-rich waters provide a paradise for algae, so algae populations explode when excess nutrients enter their ecosystem. As the algae die, bacteria consume the organic remains. But the bacteria have metabolic needs and require oxygen. So, when excessive dead organic matter appears, bacterial activity increases and slowly depletes the free oxygen.

The Devonian Period saw the rapid evolution of plants as they grew larger and developed complex root systems. During the Devonian, the plants and trees that ruled dry land helped break down rock into soil. The geological record indicates widespread forests developed during the Late Devonian. Under these conditions, the amount of organic material transported by streams and rivers to the oceans would have rapidly expanded. Also, increased chemical weathering due to forestation provided more nutrients to marine habitats.

Conditions were ripe in the Late Devonian for persistent anoxia in the extensive shallow oceans that were still the cradle of the animal kingdom. A perfect recipe for mass extinction.

(Next — Was there more bad news in the Late Devonian?)

ArcheanArt

Sources:

The Devonian extinction saw the oceans choke to death (By Chris Baraniuk, BBC) 

Oxic, Suboxic and Anoxic Conditions in the Black Sea (by James W. Murray, Keith Stewart Steven Kassakian, Marta Krynytzky, and Doug DiJulio ) 

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.