Anoxic event facts for kids

An anoxic event happens when parts of the ocean have very little or no oxygen. Imagine a part of the ocean where fish and other sea creatures can't breathe because there's not enough oxygen! These events are sometimes called oceanic anoxic events or deep ocean anoxic events.

Scientists have found signs of major anoxic events in the past, even though they haven't happened for millions of years. Evidence comes from dark, muddy layers (called black shales) found in rocks. These layers show that the oceans sometimes had very low oxygen. However, it was rare for these events to cause huge problems for life, except for one big event at the end of the Permian period, which led to a mass extinction.

Anoxic events might have caused times when many types of plants and animals died out. These "mass extinctions" were so important that geologists use them as markers to help figure out the age of rocks. Usually, these oceanic anoxic events lasted less than 500,000 years before the oceans got back to normal.

Today, there are a few places on Earth that show what anoxic events are like, but only in smaller areas. These are sometimes called 'dead zones'. You can find them off the East Coast of the United States in the Chesapeake Bay, in the Scandinavian strait Kattegat, the Black Sea, in the northern Adriatic, and off the coast of Louisiana.

What Causes Anoxic Events?

Scientists are not completely sure why anoxic events happened in the past. Here are some ideas:

• Warmer Climate: A warmer global climate could have led to a huge growth of tiny ocean plants (biomass). When these plants died, their decay used up a lot of oxygen.
• More Rainfall: Heavy rainfall might have washed a lot of plant and animal material from land into the oceans. This material would then decay, using up oxygen.
• Stopped Ocean Currents: If the deep ocean currents that move water between the poles and the equator stopped, the oxygen in the deep ocean wouldn't get replaced.
• Oxygen Used Up: Ocean oxygen might have been used up faster than it could be replaced.
• Poisonous Gas: A poisonous gas called hydrogen sulphide might have built up in the oceans, making them dangerous for most life.
• Hostile Oceans: The oceans might have become very difficult for most forms of life to survive in.

All of these things could have caused mass extinctions in the seas, changing how all animals that feed on sea creatures lived.

Past Examples of Anoxic Events

Scientists have found evidence of anoxic events from different times in Earth's history:

Jurassic Period

• Toarcian event: This happened about 183 million years ago (mya).

Lower Cretaceous Period

• Aptian event: This was a mid-Aptian extinction event, around 116 or 117 mya.

Upper Cretaceous Period

• Cenomanian–Turonian boundary event: During this time, dark, muddy layers (black shales) were laid down in ocean basins. This was about 91.5 mya (give or take 8.6 million years).

Cainozoic Era

• Palaeocene–Eocene Thermal Maximum (PETM): This event happened about 55.8 mya, when Earth's climate got much warmer.

Images for kids

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  Map of aquatic dead zones, January 2008. The red circles show the sizes and places of dead zones, with black dots meaning a dead zone of unknown size. The size and number of marine dead zones—areas where the deep water is so low in dissolved oxygen that sea creatures can't survive—have grown very quickly in the past 50 years. – NASA Earth Observatory

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  This world map of ocean currents shows how different parts of the world are connected by these moving waters.

See also

In Spanish: Evento anóxico para niños