Permian Black Shales
Daily Earth Science Geosphere Repost

Permian black shales

Some 252 million years ago, the earth was in crisis. The Permian period was ending, and the Triassic was beginning. About 96 percent of all marine species on the planet went extinct during this transition. The late Permian extinction is also known as the “Great Dying.” Clues to the causes of this catastrophe lie in the fossil and rock records of the Permian black shales. Most of the marine species that passed into the void of extinction died of suffocation. 

Oxygen is a sine qua non for most of life. Without oxygen, life on land and in the ocean perishes. There are two key terms describing oxygen levels in marine environments: hypoxia and anoxia. Hypoxia denotes low levels of oxygen, and anoxia references the complete absence of free oxygen. When widespread anoxic conditions prevailed in ancient oceans, they then produced a specific geological signature in the rock record – black shales.    

Black shales

Black shales are fine-grained sedimentary rocks containing a high percentage of organic matter. The organic matter is mixed in with clay and silt particles. Generally 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 what makes them distinct from regular shales is the organic matter.

Normally, organic material like the remains of dead plants and animals falls to the ocean floor where bacteria and other animals 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 in the sediments. This high organic content gives the resulting shales a dark, blackish color.

The oil industry explores for black shales buried deep in the earth because as temperatures rise, the organic material in the shales transforms into oil and gas deposits. A worldwide abundance of black shale at a specific time in the geological record indicates widespread anoxia in the world’s oceans.

End of the Permian

The late Permian was the time of Pangea where all of the earth’s continents had amalgamated into a single, super landmass. It was also the time when the Siberian Traps formed from a massive outpouring of flood basalts. These flood basalts covered an area estimated at seven million square kilometers (2.7 million square miles). So, for two million years, liquid rock poured from the earth in flood after flood. When the devastation finally stopped, one million cubic miles of rock had been pushed onto the earth’s surface. However, for life on earth, it was not the basalt that posed a threat; it was the degassing of the mantle plume that led the assault.

Carbon dioxide and other gases from the volcanic activity inundated the atmosphere, turning earth into a giant greenhouse. Global temperatures skyrocketed, and the ocean’s surface temperatures then rose some 10 degrees Celsius (20 degrees Fahrenheit). Fundamental physics and chemistry dictate that warm water holds less oxygen than cold water; thus, oxygen levels are temperature dependent. So, marine oxygen levels at the end of the Permian dropped by almost 80 percent. The resulting hypoxia and anoxia overwhelmed life, and 96 percent of all marine species disappeared. Anoxic ocean bottoms paved the way for massive deposits of Permian black shales.

 Breathable oceans

Life on land and in the oceans depends on oxygen. The degree to which a species’ metabolic needs balance with available oxygen defines a “breathability” factor. This factor is the “Metabolic Index (Φ), a species-specific measure of the environment’s capacity to meet temperature-dependent organismal oxygen demand.” 

Computer modeling indicates that breathability in the Permian oceans dropped so low that only four percent of life in the oceans survived. This same effect is also at work today. Global warming leads to ocean warming and a loss of oxygen in the surface waters. Then hypoxia stresses species low in the food chain, and their population falls. The knock-on effect is starvation for species higher in the food chain who depended on those fish. If you want to understand global warming run amuck, then study the late Permian black shales.


ArcheanWeb:

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

A hot earth analog (Source: ArcheanWeb) – https://archeanweb.com/2020/01/27/a-hot-earth-analog/  Also:

Heat, oxygen deprivation, and extinction (Source: ArcheanWeb) – https://archeanweb.com/2020/03/05/heat-oxygen-deprivation-and-extinction/ Also:

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


Sources:

CLIMATE CHANGE WAS BEHIND EARTH’S LARGEST EXTINCTION (BY HANNAH HICKEY; Futurity) – https://www.futurity.org/permian-mass-extinction-1927882/  Also:

Temperature-dependent hypoxia explains biogeography and severity of end-Permian marine mass extinction (By Justin L. Penn, Curtis Deutsch, Jonathan L. Payne, Erik A. Sperling; Science) – https://science.sciencemag.org/content/362/6419/eaat1327  Also:

Feature Image: Black Shale (Modified – By James St. John – Black shale (Sunbury Shale, Lower Mississippian; Tener Mountain roadcut, southern Ohio, USA) 5, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=84649232

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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