Atmospheric

Part 6 - Atmospheric History

The Earth has changed dramatically over the life of the planet, from being almost entirely frozen at times, to being much warmer than it is now. Between 56 and 34 million years ago the Arctic Ocean was not permanently frozen. Fossils tell us that the yearly average temperature near the poles was as much as 25°C (45°F) warmer than it is today but the tropics were not much hotter because Earth's warmth was more uniformly distributed.

Earth's atmosphere has also changed radically. It now consists mainly of two gases, nitrogen (N)(78%) and oxygen (O2)(21%). However, 4.5 billion years ago, the Earth's atmosphere was quite different; being mainly methane (CH4), ammonia (NH3) and water vapour (H2O). There was no free oxygen (O2). 

The earliest traces of life are about 3.8 billion years old. There are no fossils but a thin vein of graphite, (a form of carbon) found in Greenland's ancient surface rocks suggests it is the remains of microbial life. Carbon has two stable isotopes C-12 and C-13 (which has one extra neutron in the nucleus) and the graphite is unusually enriched with the lighter C-12 which is preferred by all living organisms.

While most of life on Earth depends on the energy provided by solar radiation, a number of living organisms exist because of the heat from the Earth's core. These ancient organisms evolved in the seas, probably using energy from the Earth's core which heated the water and provided dissolved chemicals needed by the organisms; And these required no oxygen.

By 3.5 billion years ago, the atmosphere was predominantly nitrogen (N) and carbon dioxide (CO2). The sun's radiation had broken down the ammonia (NH3) (to release the nitrogen; a stable, unreactive gas) and the methane (CH4) to release the carbon, which subsequently combined with oxygen taken from metal oxides to produce carbon dioxide (CO2); a stable, unreactive gas. Three billion years ago, the atmosphere consisted of nitrogen, hydrogen sulfide, methane, and 0.4 to 8 % carbon dioxide (ten to 200 times more than today's atmosphere); There was still no free gaseous oxygen; this was all locked in chemical combinations with hydrogen (water is H2O) and many other elements.

So what happened to all the carbon dioxide (CO2) and where did the oxygen come from? The answer is microbes.

About 3.5 billion years ago single celled Cyanobacteria were probably the first microbes to extract oxygen by photosynthesis. They evolved in shallow seas and began using solar radiation to split carbon from atmospheric carbon dioxide (CO2) and to extract hydrogen from water (H2O). They used the hydrogen to create complex hydro-carbons and more cyanobacteria. In the process, they released a gas toxic to most anaerobic life forms . . . oxygen.

This did not immediately cause free oxygen to accumulate in the atmosphere as, when the organisms died, most of the freed hydrogen quickly combined with any free oxygen. Also oxygen was rapidly absorbed (burned) by other elements like methane gas (CH4), which produced water (H2O) and carbon dioxide (CO2). Oxygen was also rapidly absorbed by other elements including iron to form iron oxide (rust) which precipitated on the sea beds, for millions of years, forming bands of rust in sedimentary rocks.

However, three billion years ago, the sun was 30% less bright than it is today but the remaining methane and carbon dioxide in the Earth's atmosphere trapped enough of the sun's heat to prevent Earth from freezing over.