| |
The Earth\'s atmosphere is made up of 79% nitrogen, 21% oxygen and a small but growing trace of carbon dioxide. Less well known but much more interesting is the fact that the present-day composition is totally different from that of the primitive Earth, which was hostile to life. How and why has the atmosphere changed so much?
The earliest atmosphere probably consisted of gases like hydrogen, helium, methane, ammonia and carbon dioxide (CO2)—gases which occur in the atmospheres of the other planets. Most of these gases were lost and replaced by an atmosphere of gases emitted by volcanoes. Water vapour, CO2, nitrogen, sulphur oxides, chlorine and fluorine would have been the main constituents emitted. Water vapour would be rapidly lost as rain and the more chemically active gases combined with other substances. Hence, the atmosphere for most of the Earth\'s history consisted largely of CO2 with small amounts of nitrogen. The formation of the modern atmosphere required the removal of CO2 and the liberation of oxygen as well as a large increase in the amount of nitrogen.
A clue to the beginning of the change can be found in Precambrian rocks. Some of the earliest of sedimentary rocks are the banded ironstones, which could only have been deposited in the absence of oxygen. Later in the Precambrian era, iron began to be deposited as ‘red beds’, which imply the existence of a small amount of oxygen. Between these sediments, from about three billion years ago, are found deposits of stromatolites, formed by fossilization of beds of blue-green bacteria. The significance of blue-green bacteria, which continue to occur in the coastal waters of Australia, is that they do not require oxygen for respiration but liberate it through photosynthesis. They were responsible for the first free oxygen, but this did not raise the atmospheric concentration very far as it would have combined with trace gases and with materials liberated by weathering. Only after this resistance had been overcome could other processes accelerate the liberation of oxygen and the removal of CO2.
The first green plants were the phytoplankton, which lived in the surface waters of the oceans. Like other green plants, they liberated oxygen through photosynthesis but then reused it in respiration. However, because they lived in the deep ocean they caused a small net addition to oxygen levels because some of the carbon compounds incorporated in their cells sank to the bottom of the oceans and were locked into sediments which do not decompose because of the cold, dark and oxygen-free conditions.
As land plants grew larger and more elaborate, they provided another sink for carbon and another source of free oxygen. From 350 million years ago, especially in the period now called the Carboniferous, some land plants have been preserved from decomposition by being trapped in marshy conditions, and were eventually transformed into coal or petroleum. During this period, the oxygen content of the atmosphere reached 1%.
More recently, especially in the Cretaceous period from 145 million years ago, appeared the largest of the sinks for carbon. Small marine animals drew on dissolved CO2 in the sea to build shells of calcium carbonate. When the animals died the shells, called coccoliths, sank to the sea floor and were transformed into chalk. The immense amounts of chalk—visible in rocks like those of the white cliffs of Dover—show that this was a major way of taking CO2 away from the atmosphere.
The effectiveness of these processes is shown by the amounts of carbon now estimated to be locked into the various ‘reservoirs’. If the amount remaining in the atmosphere is taken as one unit, there are 5 units in land vegetation and soil, over 14,000 units dissolved in the oceans or in ocean sediments and over 71,000 units in the rocks, mostly in the form of calcium carbonate. So natural processes have the capacity to remove huge quantities of CO2, though probably too slowly to avert the prospect of global warming as a result of human activity, especially the burning of fossil fuels.
Plants and bacteria were also involved in the build up in levels of nitrogen, though the chemically unreactive nature of nitrogen also contributed. While other volcanic gases were removed from the atmosphere, nitrogen was more inert and thus more likely to remain. Some nitrogen was also derived from ammonia liberated by volcanoes. Today, nitrogen passes through soils and plants, but amounts fixed permanently into plant or animal tissues and preserved from decomposition are very small indeed. There is some loss of nitrate in solution, but this is taken up by algae and recycled. Overall, the losses no more than counteract the continuing emissions from volcanoes and consequently nitrogen has become the largest component of the atmosphere.
The development of the atmosphere to its current form has involved interlocking physical and biological processes, including the evolution of progressively more complex forms of life. The way these processes have produced an atmosphere ideal for complex forms of life has driven some interpreters to look for divine intervention and others to accept the concept of Gaia. PS |
|
