Anoxic event

An anoxic event was a time when large parts of Earth's oceans had very little dissolved oxygen (O₂). This made the water toxic and hard for sea creatures to live in.

These events happened many times in the past. They often happened at the same time as big die-offs of plants and animals, called mass extinctions. Some scientists think that when ocean water moved less, the climate got warmer, and there was more of a certain gas in the air, it made these events more likely. Others believe that increased volcanism, which released a gas called CO₂, helped start these events.

Today, human actions like putting nutrients from farms and sewage into the water create smaller areas with little oxygen, called dead zones. Experts say it would take thousands of years for the whole ocean to become like this again because of modern climate change.

Background

The idea of oceanic anoxic events was first suggested in 1976 by Seymour Schlanger and geologist Hugh Jenkyns. They noticed this after findings from the Deep Sea Drilling Project in the Pacific Ocean. They found black, carbon-rich rocks in Cretaceous sediments on submarine volcanic plateaus like the Shatsky Rise and Manihiki Plateau. These rocks were the same age as similar ones found in the Atlantic Ocean and in Europe, especially in the Apennines in Italy. This showed that there were unusual, oxygen-poor conditions in the world’s oceans during certain times in geological time.

Modern studies of these organic-rich rocks show fine layers that were not disturbed by sea creatures, suggesting oxygen-poor conditions on the seafloor. There might have also been a poisonous layer of hydrogen sulfide, H₂S. Recent studies found special molecules from certain bacteria that needed light and hydrogen sulfide, showing that oxygen-poor conditions reached high into the upper water.

This understanding is new, put together over the last thirty years. These oxygen-poor events are connected to large oil reserves found in bands of black shale in the geologic record.

Euxinia

Anoxic events with euxinic (oxygen-poor and containing sulfur) conditions have been linked to big bursts of volcanic activity. Volcanoes put more CO₂ into the air, raising global temperatures. This sped up the hydrological cycle, putting extra nutrients into the oceans, which helped tiny plants grow a lot. In some areas where water layers stayed separate, these conditions could have led to euxinia. With low oxygen or euxinic conditions, phosphate stayed in the water instead of settling in mud, allowing more tiny plants to keep growing.

Mechanism

During warm times in Earth's past, like the Jurassic and Cretaceous, the oceans had less oxygen. This made it easier for areas with no oxygen to form. Scientists have some ideas about why these events happened quickly.

One idea is that lots of plant-like material in the water didn’t break down because there wasn’t enough oxygen. This happened in some ocean shapes, but it doesn’t explain why it happened in many places.

Another idea is that more tiny plants and bacteria grew because there were more nutrients like nitrate and phosphate in the water. This would have used up even more oxygen.

These events might also have been caused by huge amounts of carbon dioxide from volcanoes. This would have warmed the planet and caused more nutrients to flow into the oceans. This led to more plant growth and used up oxygen. When the extra carbon was buried, the climate cooled again, ending the event.

The timing of these events matches with large volcanic areas, suggesting a connection.

Main article: Oceanic anoxic event

Main articles: Anoxic event, Oceanic anoxic event

Further information: Oceanic anoxic events

Occurrence

Oceanic anoxic events happened when Earth's oceans lost oxygen. These events mostly happened during very warm times, when there was a lot of carbon dioxide in the air and temperatures were higher than today. One idea is that huge amounts of natural gas (methane) suddenly escaped from deep places in the Earth, like frozen deposits under the ocean floor. Methane is a powerful greenhouse gas, which can warm the planet a lot.

We know about these events from warm times like the Cretaceous and Jurassic Periods, but they may have also happened in earlier times like the late Triassic, Permian, Devonian, Ordovician, and Cambrian. These events usually didn't last very long, often less than a million years, before things went back to normal.

Consequences

Oceanic anoxic events had many important effects. Scientists think these events caused many sea creatures to disappear, especially during certain times long ago.

These events also helped create much of the world's oil and natural gas. When the oceans lacked oxygen, more plant and animal material was saved in the mud. Over millions of years, this saved material turned into oil and gas.

Some scientists think that during these events, a harmful gas called hydrogen sulfide may have risen from the ocean into the air. This gas could have hurt plants and animals on land and in the water.

Anoxic events in Earth's history

Cretaceous

Main article: Cretaceous

Long ago, the oceans were very different from today. During a time called the Cretaceous period, special conditions made large parts of the ocean lack oxygen, the air we breathe. Scientists have found evidence of two big events when the oceans lost oxygen. One happened around 120 million years ago, and another around 93 million years ago.

Jurassic

Main article: Jurassic § Toarcian Oceanic Anoxic Event

There was a big event when the oceans lost oxygen during the Jurassic period, about 183 million years ago. We know about this event from rocks found all around the world.

Paleozoic

Main article: Paleozoic

Big changes in the oceans happened many times during the Paleozoic era. One major change happened at the end of the Permian period. Another happened between the Ordovician and Silurian periods, when the oceans lost oxygen many times.

Archean and Proterozoic

It is thought that for most of Earth's early history, the oceans did not have much oxygen. During the Archean time, the oceans lacked a special chemical called sulfate. Later, during the Proterozoic, these conditions became more common.

Several times when the oceans lost oxygen are known from the late Neoproterozoic.