What is an anoxic event?

What is an anoxic event? The history of the Earth is full of innumerable phenomena and mysterious, surprising, natural and supernatural phenomena. These include dozens of biological, chemical, geological, meteorological and physical phenomena. 
What is an anoxic event?
Scientists and geologists have developed and explained many of them for our understanding and knowledge. One of these unique phenomena is known as 'anoxic event' or 'oceanic anoxic event'.
Concept of anoxic events
During anoxic events, vast areas of the ocean are completely depleted of oxygen. 
Although humans have not experienced this phenomenon for millions of years, geological discoveries have established that our oceans have witnessed anoxic events several times. 
In 1968, the National Science Foundation and Regents of the University of California initiated an ocean drilling project to collect data to display the history of the Earth. 
He successfully examined the seabed and ocean plates to collect geological evidence and samples that supported many concepts, hypotheses and theories. Seymour Schlanger and Hugh Jenkyns were the first to propose this theory based on the findings of offshore drilling projects.
Even today, there are more than four hundred places worldwide where seawater contains anoxic characteristics on a very small scale. 
In geological terms, these patches of water are known as "dead zones." These dead zones can be found all over the world, from North and South America to Europe, China, New Zealand and Japan.
Causes of anoxic events
Geologists and scientists are still unsure about what triggered these events, but there are many possible theories for these strange events. Some geologists believe that these events were the result of global warming. 
An increase in temperature caused massive biomass growth that resulted in oxygen depletion. Few others suggest that irregular deep sea water circulation between the North Pole and the South Pole and the equator triggered this phenomenon. 
Another possibility points to the irregular supply of oceanic oxygen. Describe the events as a result of the rapid use of oxygen in water without adequate replacement. 
Some blamed the heavy rains as responsible for the events. They believe that these intense rains swept massive amounts of organic materials from the earth that eventually deposited in the oceans and triggered the appearance of anoxic events. 
Some geological evidence pointed to a large collection of poisonous hydrogen sulfide on the surface of the ocean and the atmosphere that caused anoxic and euxinia events.
Historical chronology of anoxic events
There are four main anoxic events observed by geologists based on deep water drilling findings. 
The oldest evidence we have for anoxic events dates back to 183 million years ago, during the Jurassic period. The second reliable source confirms an anoxic event 116 millions of years ago, during the Lower Cretaceous period. 
The third great test of the anoxic event suggests its occurrence 90 million years ago, during the Upper Cretaceous period. 
The last recorded anoxic event took place around 55 million years ago, during the Cainozoic period.
Consequences of anoxic events
Geologists directly link anoxic events with the mass extinction of many marine organisms and life forms. 
They also blame these events for the extinction of many plants and animals on land, as these events released hydrogen sulfide poison gas into the atmosphere. They claim that it even affected the ozone layer that protects the Earth from the harmful ultraviolet rays of the sun. 
On the other hand, anoxic events played an important role in the deposit of large underwater reserves of oil and gas and many other metal reserves, especially iron.

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