Jurassic drop in ocean oxygen lasted a million years

May 12, 2017
Credit: CC0 Public Domain

Dramatic drops in oceanic oxygen, which cause mass extinctions of sea life, come to a natural end - but it takes about a million years.

The depletion of in the oceans is known as "anoxia", and scientists from the University of Exeter have been studying how periods of anoxia end.

They found that the drop in oxygen causes more organic carbon to be buried in sediment on the ocean floor, eventually leading to rising oxygen in the atmosphere which ultimately re-oxygenates the ocean.

Scientists believe the modern ocean is "on the edge of anoxia" - and the Exeter researchers say it is "critical" to limit carbon emissions to prevent this.

"Once you get into a major event like anoxia, it takes a long time for the Earth's system to rebalance," said lead researcher Sarah Baker, a geographer at the University of Exeter.

"This shows the vital importance of limiting disruption to the to regulate the Earth system and keep it within habitable bounds."

The researchers, who also include Professor Stephen Hesselbo from the Camborne School of Mines, studied the Toarcian Oceanic Anoxic Event, which took place 183 million years ago and was characterized by a major disturbance to the , depleted oxygen in Earth's oceans and mass extinction of marine life.

Numerical models predicted that increased burial of - due to less decomposition and more plant and marine productivity in the warmer, carbon-rich environment - should drive a rise in atmospheric oxygen, causing the end of an anoxic event after one million years.

To test the theory, the scientists examined fossil charcoal samples to see evidence of wildfires - as such fires would be more common in oxygen-rich times.

They found a period of increased wildfire activity started one million years after the onset of the anoxic event, and lasted for about 800,000 years.

"We argue that this major increase in fire activity was primarily driven by increased atmospheric oxygen," said Baker.

"Our study provides the first fossil-based evidence that such a change in could occur in a period of one million years."

The increase in fire activity may have also helped end ocean anoxia by burning and reducing the amount of plants on land.

This is because plants can help to erode rocks on the land that contain nutrients needed for marine life - therefore with fewer plants, fewer nutrients are available to be carried to the sea and used to support marine life in the oceans.

Less - that would use oxygen to breathe - would mean less oxygen being used in the oceans, and could therefore help the oceans to build up a higher oxygen content, ending anoxia.

It may therefore be essential to maintain the natural functioning of wildfire activity to help regulate the Earth system in the long-term, the researchers say.

The charcoal sediment tests were carried out at Mochras in Wales and Peniche, Portugal.

The paper, published in the journal Nature Communications, is entitled: "Charcoal evidence that rising terminated Early Jurassic anoxia."

Explore further: Ancient rocks reveal how Earth recovered from mass extinction

More information: "Charcoal evidence that rising atmospheric oxygen terminated Early Jurassic ocean anoxia" Nature Communications (2017). DOI: 10.1038/ncomms15018

Related Stories

Humble moss helped create our oxygen-rich atmosphere

August 15, 2016

The evolution of the first land plants including mosses may explain a long-standing mystery of how Earth's atmosphere became enriched with oxygen, according to an international study led by the University of Exeter.

Ocean toxicity hampered the rapid evolution of complex life

December 4, 2015

By examining rocks at the bottom of ancient oceans, an international group of researchers have revealed that arsenic concentrations in the oceans have varied greatly over time. But also that in the very early oceans, arsenic ...

Elementary new theory on mass extinctions that wiped out life

November 5, 2015

Throughout the past 600 million years there have been five major mass extinction events that devastated life on Earth. While some of these events are very well studied, such as the killer asteroid that wiped out the dinosaurs ...

Recommended for you

New hope for limiting warming to 1.5 C

September 18, 2017

Significant emission reductions are required if we are to achieve one of the key goals of the Paris Agreement, and limit the increase in global average temperatures to 1.5°C; a new Oxford University partnership warns.

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.