9.11 Evolution of the Atmosphere
Syllabus
For 200 million years, the proportions of different gases in the atmosphere have been much the same as they are today: about four-fifths (approx 80%) Nitrogen; one-fifth (approx 20%) Oxygen; small proportions of various other gases, including Carbon Dioxide, water vapour and Noble Gases.
Theories about what was in the Earth’s early atmosphere and how the atmosphere was formed have changed and developed over time. Evidence for the early atmosphere is limited because of the time scale of 4.6 billion years.
One theory suggests that during the first billion years of the Earth’s existence there was intense volcanic activity that released gases that formed the early atmosphere and water vapour that condensed to form the oceans. At the start of this period the Earth’s atmosphere may have been like the atmospheres of Mars and Venus today, consisting of mainly Carbon Dioxide with little or no Oxygen gas.
Volcanoes also produced Nitrogen which gradually built up in the atmosphere and there may have been small proportions of Methane and Ammonia.
When the oceans formed Carbon Dioxide dissolved in the water and Carbonates were precipitated producing sediments, reducing the amount of Carbon Dioxide in the atmosphere. No knowledge of other theories is required.
Students should be able to, given appropriate information, interpret evidence and evaluate different theories about the Earth’s early atmosphere.
Algae and plants produced the Oxygen that is now in the atmosphere by photosynthesis, which can be represented by the equation: 6CO2 + 6H2O --> C6H12O6 + 6O2 Carbon Dioxide + Water --> Glucose + Oxygen
Algae first produced Oxygen about 2.7 billion years ago and soon after this Oxygen appeared in the atmosphere. Over the next billion years plants evolved and the percentage of Oxygen gradually increased to a level that enabled animals to evolve.
Algae & plants decreased the percentage of Carbon Dioxide in the atmosphere by photosynthesis.
Carbon Dioxide was also decreased by the formation of some sedimentary rocks & fossil fuels.
Students should be able to:
describe the main changes in the atmosphere over time and some of the likely causes of these changes
describe and explain the formation of deposits of limestone, coal, crude oil and natural gas.
What does this mean?
History can be dull if it's all about learning dates.
The History of the Atmosphere doesn't have any dates we can be certain of because the Earth is so old (4.6 billion years next Tuesday) and most of the events we think we really important in making today's atmosphere happened 2.7 billion years ago - long before people evolved.
And the atmosphere has been fairly stable for 200 million years - also long before there were people.
It's likely that the very early Earth had very little atmosphere - like some other planets close to the Sun (eg Mars, Venus)
It may have started with almost no atmosphere at all - like St Patrick's.
Volcanic Degassing
The theory is that volcanoes produced enough gas to create an atmosphere of some sort.
But it wouldn't be like our atmosphere because volcanoes don't produce Oxygen.
They do produce water vapour and Carbon Dioxide.
They sometimes produce Ammonia (NH3), Nitrogen and Methane (CH4).
Nothing would have been able to live in this atmosphere and the Earth was anyway too hot for life.
But eventually it cooled and the water vapour could condense, forming oceans.
Where did all the Carbon Dioxide go?
Carbon Dioxide is very soluble in water.
So when oceans formed a lot of it would simply have dissolved in the water.
And dissolving CO2 forms carbonate ions which combine with some dissolved metals ions to form insoluble metal carbonates that precipitate into sedimentary rocks.
Where did all the Oxygen come from?
Once there were oceans and a reasonable temperature simple plants could evolve - probably something like the algae we have today.
Plants photosynthesise.
This reduces the amount of Carbon Dioxide even more.
And it produces Oxygen!
When there was enough Oxygen it was possible for animals to evolve.
Billions of tons of living things turned into fossil fuels over millions of years.
Living things are largely Carbon, so this (almost) permanently removed some of the Carbon from the atmosphere.
In Geography you have probably learned that the shells of sea-creatures are made up of Calcium Carbonate and that these can turn into Limestone and Chalk.
The Carbonate comes from dissolved CO2 in the ocean, so turning it into rock has locked up even more of the early CO2 for millions of years.
The balance of photosynthesis (plants only) and respiration (all living things) keeps the balance of Oxygen and CO2 fairly constant even today.
Although combustion (burning) and decaying matter are also involved in the balance.
Today's Atmosphere
The amount of Water Vapour in the atmosphere varies with temperature, season, location, time of day, biome etc.
But you should learn that when air is dried it is almost the same everywhere in the world.
It is
Around 20% Oxygen
Around 80% Nitrogen
With the rest being Noble Gases (mostly Argon) with a tiny amount (0.03%) CO2.
Methane and Ammonia
We don't really know for sure how much Methane or Ammonia there ever was in the atmosphere.
We just assume it would have been there because they are among the gases that come from some volcanoes today
In the graph above we see the theory that any Hydrogen and Helium in the atmosphere would disappear quickly because these light gases can drift into space, and Hydrogen can react.
The theory also shows that when the water vapour condenses into oceans, the CO2 declines soon after.
We think that there would have been lots of Nitrogen right from the start because we have found this coming from volcanoes too.
We know that Ammonia is a very reactive gas, so we assume that it would have reacted with rocks and other substances quite quickly - which is why the % of Ammonia decreased quickly.
Methane also reacts quite easily so it also would not have existed in the atmosphere for long.
Videos
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Keywords
· Atmosphere – the mixture of gases in the air
· Noble Gases – Group 0 elements, unreactive
· Billion – a thousand millions = 1,000,000,000
· Volcanic Activity – Volcanic eruptions and release of gases from volcanoes
· Condense – Gas turning to liquid
· Sediments – solids that fall to sea floor eg mud, sand, Carbonates
· Carbonates – substances that contain a metal bonded to Carbon and Oxygen only
· Sedimentary Rocks – rocks formed at the bottom of the sea from sediments eg mudstone (from mud) sandstone (from sand, Limestone (from Carbonates)
· Algae – simple plants
· Photosynthesis – chemical reaction carried out by plants that uses water and CO2 to form sugars and Oxygen
· Respiration – chemical reaction carried out by all living things to release energy
· Acid Rain – rain with a pH less than 5.5
· Respiratory problems – difficulties breathing (eg Asthma or Bronchitis)
· Pollutant – substance in the environment that causes problems, usually released by people
· Particulates – tiny solid particles in the atmosphere (usually due to burning)