The oxidation event: blessing or catastrophe ⋆ TheScientificRevelation

Bonjour folks. Welcome to another chapter of “The history of earth series” where we will travel back in time to witness some of the transforming events that shaped our earth today. In this chapter let’s dive into probably the first major extinction event in the earth’s history. The great oxidation event might be a blessing for us but it was a catastrophe for the abundant life that depended on carbon dioxide. It was a blessing for some and a curse for some. The first extinction event without any direct evidence. Now you might be thinking why are we talking about a hypothetical incidence with such a surety. Many scientists believe that it had happened for the sake of our current atmospheric composition and nutrient cycle.

The previous series was all about the genesis of the universe with a post on fundamental forces of nature, and the emergence of matter. In this, we will tackle the natural history part of the development of the earth.

As far as research goes the great oxidation event is the first great extinction on earth. We obviously don’t have fossil evidence for it so it is hypothetical. Scientist believes that it had happened and the microscopic life that depended on carbon and sulfur went extinct. There was no oxygen at the start of Archean eon and life was based on what was available that changed when oxygen came into the picture. It had not only changed the composition of the atmosphere but due to its highly reactive nature eat up fuels of most microscopic life.

The back story of oxidation

When archaeon eon started around 4 billion years ago the atmosphere was mostly of nitrogen but almost no oxygen. How do we know this? Well, we know it by the geological composition of rocks dates back to that time period. Scientists have found many rock compositions from archaeon eon that would have not existed if there would have been even a little oxygen in the atmosphere. Back then life was very small primitive bacterial and archaea cells that are still today. Because there was no oxygen these simple cell organisms were respiring and living in an oxygen-free environment. We don’t have evidence of these early lives for some reasons. They were singles celled simple life forms without any complexities so it would have been very difficult for them to fossilized.

On top of that, they found 4 billion years ago. It is a very long time for earth and its geology. Earth’s crust recycled and matter gets digested by the mantle. Plates move over each other creating subduction zones, the point where plates move inside the mantle. This way earth’s crust gets recycled and precious fossils inside can also get destroyed. The longer these fossils left in the earth’s crust there are higher the chances of them being burned down. That’s why it is very difficult to find fossils of older times.

The photosynthesis

By 3 billion years ago microbes were widespread all around the earth. Some examples of them are methanogens who produce methane as a by-product. They work in oxygen-deprived environments that often have high temperatures. Other bacteria like some who uses chemosynthesis for energy production. They use iron or nickel in the place of photons. These bacteria don’t need sunlight for photosynthesis because they take energy from these metals.

Back in archaeon eon cyanobacteria had first done oxygenic photosynthesis and produced oxygen as a by-product. There are pieces of evidence to support photosynthesis by cyanobacteria 2.7 billion years ago. There is one thing that needs to be cleared. The rise of oxygen was not like some gradual increase where cyanobacteria produced oxygen and then it filled the oceans followed by the atmosphere. To understand this just think about the distribution of cyanobacteria like it could more in some places and less in others. As well oxygen can only fill the atmosphere if it has oxidized every element and filled the oceans. The oxidation event was like infused with a series of peaks and dips. Oxygen levels had a very bumpy ride on way to reach us.

The science of biomarker

Although there is no direct evidence for fossils but scientists have found biomarkers. To explain it simply, there are many isotopes of carbon mainly C12, C13, C14 among them C12 is the lightest. On the other hand, C13 and C14 have one and two neutrons respectively. These extra neutrons make the carbon isotopes heavier. Plants prefer lighter carbon as it is easier to move around and they absorb it. So, the surrounding rocks absorb heavier isotopes. If we find more C13 and C14 isotopes in sedimentary layers we can deduce that the lightest one might have been taken by photosynthetic organisms (plants, bacteria). This indirect method came in handy in lots of situations for research.

For most of the time archaeon was a prokaryote without oxygen world. The transition between Proterozoic eon and archaeon eon is marked by the great oxidation event.

With every drop of water you drink, every breath you take, you are connected to the sea. No matter where on earth you live. Most of the oxygen in the atmosphere is generated by the sea.

Sylvia Earle

What caused the rise in oxygen levels?

The one-word answer for oxygen increment is obviously photosynthesis. As the population of cyanobacteria began to rise more oxygen was being poured into the atmosphere. It had diversified the cyanobacteria that were able to do photosynthesis more efficiently. Scientists are still unclear about the whole process of oxygen rise especially why it became overwhelming around the end of archaeon when oxidation was happening since far back. There are two explanations for this. Firstly yes, oxidation was happening for far longer and first, it reacted with chemicals on the earth’s surface and ocean. Oxidizing them and chemically changing the composition of the earth that became overwhelming. When it filled every niche it begun to rise in the atmosphere.

The escape route of hydrogen

The second one says that the hydrogen had left earth and escaped into space emptying the place for oxygen. water is made up of hydrogen and oxygen. when it vaporizes, water vapor goes into the atmosphere. As hydrogen is a lighter gas it escapes out to space Leaving oxygen behind. Every time hydrogen leaves earth becomes more oxidize. I have covered in my previous blog post The formation of the earth and the origin of life that the inner four planets are rocky planets. Due to powerful solar winds, hydrogen and other gases get swept away towards the periphery of the solar system. The gas giants are mostly made up of ice, hydrogen, and other gases that remain safe because solar winds have less effect on long distance. Due to this reason, hydrogen always escapes out to space. Learn more about this in my previous blog post.

The aftermath of oxygen increment

Due to this rise in oxygen, many microbes that depended on other gases died out leaving their remains behind. . The rising oxygen filled the atmosphere and when it reacted with UV radiations of the sun it formed the Ozone layer. These layers have been our first protection against the sun’s harmful radiation since then.

The emergence of complex life

There are some pieces of evidence that also suggest an increase in rock weathering. This had helped in the formation of soil and the nutrient leached out in water bodies. This nutrient had risen the productivity of oceans consequently increasing life growth. Now you had ozone’s protection and weathering nutrients to kick start a more complicated life. There was diversification in life forms as well as the appearance of eukaryotes.

Death and Ice age

The life that thrives without oxygen suffered due to the rise in oxygen. We know that these single-celled simple anaerobic bacteria were global. If their fuels and metabolism were getting severely affected due to oxygen growth there had to be a massive extinction. Some studies have also found the drop in nickel content during the volcanic eruption from archaeon eon. Nickel levels had dropped badly.

Many methanogen bacteria have nickel as an important component of their enzymes. As nickel levels dropped, methanogen bacteria’s population also dropped. Due to this Methane levels dropped drastically this time. This leads to a drop-dead decrease in temperature. Methane CH4 is a greenhouse gas that reacts strongly with oxygen and produces CO2. CO2 is a weaker greenhouse gas and it wasn’t able to hold the sun’s warmth. As more oxygen was being produced it converted methane into CO2 and there was nothing to fill in the methane gap so the temperature dropped more.

The mechanism of oxidation event

In recent times oxygen holds the second position in atmospheric composition with 21% after nitrogen 78%. I can’t even emphasize the fact that how much oxygen is important for life. Along with being the second most widespread gas in the atmosphere, oxygen is also the most abundant compound on the ocean, earth’s surface, and atmosphere. And the major source of oxygen on the earth is…… You guessed right, photosynthesis. Oxygen is abundant in the atmosphere and water bodies because oceanic life absorbs diffused oxygen from water. It dominates the earth’s crust in combination with silica that we call quartz.

Oxygen- a very Reactive gas

Oxygen is such an important gas its effect on the chemistry of the earth is massive. It takes part in redox reactions that means when it combines with other elements it takes the electron and thus getting a negative charge and that other elements attain a positive charge because it losses the electron. Basically, it oxidizes them. Oxygen accepts the electron and it oxidizes the element. At the time of Hadean and archaeon eon volcanic eruptions were extremely common. Earth was very hot and it cooled itself down by poring out through earth’s crust. At those times erupting volcanoes were also excreting minerals from inside like iron, silica, Nickel. When the water became abundant on the earth it combined with these minerals, especially iron.

Iron rust and clear ocean

In absence of oxygen iron rusted with a green hue and for millions of years earth’s ocean had a green tint. But when oxygen levels rose oceans were the first place where they spread. This oxygen had reacted with iron and produce the familiar red rust. Due to that for millions of years ocean was blood red in color. There was a lot of iron. Cyanobacteria were still pumping out oxygen so it had oxidized every bit of iron. Iron rust settled on the ocean floor and our oceans became the familiar blue. Many iron bed formations on land and oceans were of a time when oxygen level was on peak, sometime in between the transition of Proterozoic and archaeon eon. It is estimated that almost 90% of all the iron deposited between 2.6 and 1.9 billion years ago that same time oxidation was on peak.

The physics behind Huronian glaciation

The disappearance of GHGs

Oxygen reacted with methane converting it into carbon dioxide, a less efficient greenhouse gas. Methanogen bacteria was struggling to survive in an environment where oxygen is messing with its food as well as nickel is less available. In the absence of nickel, the temperature dropped and came the first and hardest ice age in the history of the earth. Huronian glaciation remained for millions of years changing the earth into an ice landmass famously known as The snowball earth. At the end of the oxidation event, we get numerous subsequent glaciation events. Scientists have found glacier deposits even at lower altitudes. Some estimate that glaciers have spread even to the equator with hard solid ice caps of miles covering the earth’s surface. In this situation where sunlight was unable to penetrate the ice, some argue that it was too harsh for photosynthetic bacteria.

The life under Snow

Many researchers think that it was more like a slush ball earth as the ice was not thick but kind of semi-liquid in the equator. These conditions were very harsh to live on. The very photosynthetic oxygen-producing bacteria which had created a graveyard for other bacteria suffered due to its excess production. Some studies have claimed that cyanobacteria had died out in bulk as the earth was covered under ice away from the warmth of the sun. The population of these oxygen-producing bacteria dropped and the already struggling other life forms suffered even more. This was a two-sided blast for the earth and its productivity. Scientists think that a series of volcanic eruptions had released greenhouse gases in the atmosphere that had increased temperature and ended the cold spell on earth.

A short summary

It was quite confusing for me to understand the great oxidation event. As a result to make it easier for me and my readers I have written a kind of summary about all things above and I hope it will help you all get a solid idea of The oxidation event.

The photosynthesis and iron

Note- This is photosynthesis by plants. cyanobacteria follow the same pattern.

The great oxidation event is route changing incidence in earth’s history. If it wouldn’t have happened, the earth now would have been a totally different place. The oxygen levels began to rise from the archaeon itself but its effect started appearing just after archaeon eon 2.5 billion years ago. Oxygen levels can only rise with the presence of photosynthesizers. For surety life was single-celled back then so most of the life was simple prokaryote cells based on iron and sulfur. It was surviving in oxygen nil places. Some mutation must have happened that have given rise to cyanobacteria, a probable candidate for the oxygen catastrophe.

In less concentration carbon dioxide was also present and cyanobacteria used it while releasing oxygen. Oxygen is a very reactive gas that had reacted with methane in the atmosphere and turned into more carbon dioxide. Because of this cyanobacteria again had CO2 for use. Methane was decreasing and so was the temperature. Back then the oceans were full of iron and probably green. In the absence of oxygen when iron rusted the rust was of green color. But when oxygen came into the picture iron reacted with it like crazy. We know something like this has happened because major iron beds and iron deposits have formed during this time.

Increased productivity and temperature drop

The iron rust was suspended in the oceans for a long time giving it a red hue. Oceans got their familiar blue when this rust settled onto the ocean floor. The oxygen had interacted with rocks and the earth’s surface oxidizing many chemicals. These chemicals released into oceans as nutrients and increased the productivity of the water ecosystem. On top of that volcanic activities were at their peak at that time that has released more carbon dioxide into the atmosphere. Both of these had promoted more cyanobacteria bloom that in turn had pumped more oxygen. Meanwhile, carbon dioxide replaced methane dropping temperature even more.

Cyanobacteria had released oxygen that rusted iron and filled oceans with it replacing other gases like hydrogen, iron methane, and sulfur. It was reacting with everyone creating by-products. As oxygen was using the fuels of non-oxygen based life forms, it was affecting them. When the ocean became outrun by the oxygen it escaped into the atmosphere and under UV conditions formed the ozone layer. The atmosphere was chocked full of oxygen and so as oceans. Cyanobacteria had taken the maximum amount of carbon dioxide, locking it as biomass.

In the end- Glaciation

There are two things I want to explain. In recent times also autotrophs absorb CO2 as biomass and cyanobacteria were also doing the same but there were no decomposers. The biomass locked up inside cyanobacteria sunk in the oceans without being decomposed. When decomposers eat the biomass they release back the carbon dioxide into the atmosphere and the carbon cycle runs smoothly. The CO2 was sucked out of the atmosphere and because decomposers evolved later it gets buried. This had removed carbon out of the cycle which has dire consequences. Secondly, less carbon dioxide also meant a lower temperature. Carbon dioxide had already replaced methane and its level was also dropping. Cyanobacteria had no/ less raw material so its population also decreased. This whole cycle of events bought Huronian glaciation, the longest ice age on earth.

Resources:

1. That time when oxygen almost killed everything by PBS Eon is the shortest and most informative video on this topic, covering every aspect of the Oxidation event.
2. Very informative workshop by European Geoscience Union explaining the great oxidation event with maximum details. It has simple engaging content. Give it a try.
3. A very detailed post on the great Oxidation event by BBC earth.
4. Another great youtube resource to check out How cyanobacteria took over the world.
5. The major information for this blog post had come from Podcast Common decent’s “The great oxidation event”.

I remember as a boy of 17 years of age, it was a fascinating thing for me: how we human being breathes out carbon dioxide into the air, the leaves of plants pick this carbon dioxide up, and the plants give up oxygen, which we can breathe in and keep our life going.

Percy Julian

oxygen turned out to be deadly for most of the life forms. So, for us maybe it is a blessing, at that time it was nothing less than poison for life. It is quite amazing to think that our beloved atmosphere was not like this from start. How little differences had bought big changes for life on this planet. There are many more fascinating stories to come by in the following chapters of “The history of Earth”. Stay tuned and do the Revelation.

Originally published at https://www.thescientificrevelation.com on January 1, 2021.