Anoxic event

An anoxic event describes a time when large parts of Earth's oceans had very little dissolved oxygen (O₂). This created toxic waters that could not support many sea creatures. Although such events have not happened for millions of years, the geologic record shows they occurred many times in the past. These events often happened at the same time as big die-offs of plants and animals, called mass extinctions, and may have helped cause them.

Many scientists think that when the movement of ocean water slowed down, the climate became warmer, and there was more of a certain type of gas in the air, it made these anoxic events more likely. Some researchers believe that increased volcanism, which released a gas called CO₂, was a main reason these events started.

Today, human actions like putting nutrients from farms and sewage into the water create smaller areas with little oxygen, called dead zones. However, 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 periods in Earth's history, like the Jurassic and Cretaceous, the oceans had less oxygen, making it easier for areas without oxygen to form. Scientists have a few 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 certain ocean shapes, but it doesn’t explain why it happened in many places around the world.

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 warming the planet. This would have caused more nutrients to flow into the oceans, leading to more plant growth and using 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 occurred 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 places deep 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. For example, the Toarcian and Cenomanian-Turonian anoxic events happened at the same time as big die-offs of sea life.

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. Most of the world's oil comes from these ancient, oxygen-poor oceans.

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, possibly causing more mass extinctions. It might have also reached high into the sky and damaged the layer that protects Earth from the Sun's strong rays, making life even harder to survive.

Anoxic events in Earth's history

Cretaceous

Main article: Cretaceous

Long ago, during a time when there were no ice caps, the oceans were much higher, and the land looked very different, special conditions happened in the water. These conditions made large parts of the ocean lack oxygen, which is the air we breathe. This happened in a part of Earth's history called the Cretaceous period.

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. These events changed the chemistry of the oceans and affected many living things.

Jurassic

Main article: Jurassic § Toarcian Oceanic Anoxic Event

There was one big event when the oceans lost oxygen during the Jurassic period, about 183 million years ago. We know about this event from rocks found on land 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, linked to huge volcanic eruptions. 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, which stopped some bad conditions from happening. Later, during the Proterozoic, these bad conditions became more common.

Several times when the oceans lost oxygen are known from the late Neoproterozoic, including one that may have happened at the same time as a big change in life on Earth.