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The oxygen dance: the amount of oxygen in the atmosphere rose and fell in significant fluctuations until the appearance of land plants

The discovery also has value in a completely different field - the search for life on planets outside the solar system

the oxygen cycle. Image:
the oxygen cycle. Image:

By Alex Krause, Research Fellow in Earth Systems Modelling, UCL andBenjamin J. W. Mills, Associate Professor of Biogeochemical Modelling, University of Leeds

Are we alone in the universe? This is a question that has intrigued humans for centuries and inspired countless studies and works of fiction. But are we getting closer to discovering such a life? now, When the James Webb Space Telescope (JWST) worker, we may have made one giant leap that will allow us to answer this one day.

One of the four main goals of the JWST is to study planets outside our solar system, and to determine what gases their atmospheres are made of. now, the research Our new on changes in oxygen levels on Earth over geological ages offered clues as to what to really look for.

To try and understand how, when and why life might develop on other planets, it makes sense to look at the only planet we know of today that has life: Earth. Understanding the complicated evolutionary history of our planet may provide the key to finding other planets capable of supporting life.

Life and oxygen

We know that animals need oxygen to survive, although some, such as sponges, require less than others. However, while today oxygen is readily available, making up 21% of the atmosphere, we also Know that this has not been true for most of Earth's history.

If we were to travel deep into our past, to about 450 million years ago, we would have to carry a supply of oxygen cylinders with us. But what we are less sure about is the absolute amount of oxygen in the atmosphere and oceans over time and whether the increase in oxygen levels drove the evolution of animals, or vice versa. These questions have actually sparked many debates and decades of research.

Current theory holds that oxygen levels rose in three stages. The first, called "The Great Oxidation Event", occurred about 2.4 billion years ago, and transformed the Earth from a planet without oxygen in the atmosphere and oceans to a planet where oxygen is a permanent feature. The third occurred about 420 million years ago and is called "Event הOxidation in the Paleozoic", that since then the amount of oxygen in the atmosphere has increased until today. But in between, about 800 million years ago, lies the second stage:The Neoproterozoic oxidation event” or NOE.

Initially, information extracted from sedimentary rocks formed on the ocean floor suggested that × § ¤ The oxygen has already risen to the levels measured today.

However, more data collected since then has revealed An intriguing oxygen history More. It is important to note that the NOE occurred momentarily before the evidence of the first animals, which appeared about 600 million years ago.

Modeling oxygen levels

We set out to study and reconstruct atmospheric oxygen levels during the NOE to see under what conditions the first animals appeared. To do this, we built a computer model of the Earth, incorporating knowledge of the various processes that can add oxygen to the atmosphere or remove it.

We studied carbon-bearing rocks, deposited around the world, to calculate ancient photosynthesis rates. Photosynthesis is the process by which plants and bacteria use sunlight, water and carbon dioxide to create oxygen and energy in the form of sugars - and this is the main source of oxygen on Earth.

Carbon exists naturally in isotopes Many - atoms with a different number of neutrons in their nucleus. Different isotopes therefore have slightly different sizes and masses.

We examined isotopes of carbon called carbon-12 and carbon-13, which do not undergo radioactive decay. Plants prefer to use carbon-12 - the lightest isotope - during photosynthesis, and at the end of the process (with the death of the plants) what remains in the sea water and then in the rocks formed on the ocean floor - is carbon.

When we analyze these rocks, millions or even billions of years later, if we find more carbon-13 than carbon-12 we can estimate that the process of photosynthesis, and therefore also the production of oxygen, took place. We then modeled volcanic activity, which can release gases that react with oxygen, and remove it from the atmosphere.

This approach might sound a little strange, and you might ask why we didn't have a direct way to measure oxygen levels. This is because most geological evidence from this period is not preserved, and these carbon isotope ratios are one of the few well-defined data sets we have for this period.

What we found is that instead of a simple jump in oxygen levels during the Neoproterozoic, the amount of oxygen in the atmosphere changed significantly, and very rapidly on the scale of geological timescales. While 750 million years ago, oxygen made up 12% of the atmosphere, in just tens of millions of years it dropped to about 0.3% – a tiny fraction – before rising again a few million years later.

Our research shows that atmospheric oxygen likely continued these fluctuations between high and low levels until plants gained a foothold on land about 450 million years ago.

Looking for a foreign life

These findings are intriguing for several reasons. We have often thought that the relative stability that Earth has experienced for most of the past 4.5 billion years is necessary for life to flourish. After all, when major events have occurred, such as asteroid impacts, it hasn't ended well for some of Earth's inhabitants (sorry, dinosaurs).

But if the first animals did evolve amid highly variable oxygen levels, this suggests that some dynamic changes may be required to foster ecological innovation.

Our results suggest that periods of low atmospheric oxygen levels could have been important for the development of more complex life by driving the extinction of certain simple organisms and allowing survivors to reproduce and evolve into a variety of species when oxygen levels rose again. Therefore, a close examination of planets with atmospheres with a low oxygen level outside the solar system should not be ruled out.

Of course, this is a very mundane and even animal-centered view. Alien life may be completely different from life on Earth. For example, they could well exist in places like Titan - one of Saturn's moons - which has a sea of ​​liquid methane and ethane. But as a starting point in our search for extraterrestrial life, understanding the history of atmospheric oxygen on Earth is a useful guide.

For an article in The Conversation

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