Fossil fuel formation as a carbon store

Fossil fuels are carbon stores: they contain carbon that was taken from the environment by ancient organisms and later buried underground. As fossil fuel deposits formed, some carbon was locked away rather than remaining in the atmosphere as carbon dioxide, $$\ce{CO2}$$. This contributed to a fall in atmospheric carbon dioxide.

Plants and some microorganisms took in carbon dioxide during photosynthesis. Other organisms obtained carbon by feeding on them. When these organisms died, some of their remains were buried before they had fully decomposed. Over millions of years, heat and pressure changed the buried organic material into carbon-rich fuels.

Different remains formed different fossil fuels:

• Coal formed mainly from ancient plant material.
• Crude oil and natural gas formed mainly from microorganisms and other marine life buried in sediments.

In each case, carbon that had been part of living organisms became stored in underground deposits for long periods.

Limestone formation as a carbon store

Carbon dioxide can leave the atmosphere and become locked away in sedimentary rock. Limestone is a carbon store because it can hold this carbon for very long periods.

Carbon dioxide from the atmosphere dissolves in the oceans. Marine organisms use carbon from the dissolved substances to make shells and skeletons containing carbon compounds. When the organisms die, their remains can accumulate on the sea floor.

As more material collects, the remains can become buried. Over long periods, they form sedimentary rock such as limestone. The carbon in the rock was once part of atmospheric carbon dioxide, but is now stored in limestone deposits. This transfer helps to reduce the amount of carbon dioxide in the atmosphere.

Photosynthesis and atmospheric carbon dioxide

As algae and plants spread on Earth, they took in carbon dioxide from the atmosphere. They used it with water and energy from light to make sugars through photosynthesis. The sugars could then be used to make other carbon-containing substances in their biomass.

The carbon in this biomass had previously been part of carbon dioxide in the air. As photosynthetic organisms grew, carbon was therefore transferred from the atmosphere into living material, increasing the amount held in the biosphere. When organisms died, some of this carbon became part of dead organic matter.

Over long periods, the growth of algae and plants reduced the percentage of carbon dioxide in the atmosphere while increasing the amount of carbon held in living material and dead organic matter.

Changes in Earth's atmospheric composition

Earth’s early atmosphere is thought to have formed largely from gases released by volcanoes. It contained a high proportion of carbon dioxide, along with water vapour and some nitrogen, but little or no oxygen.

As Earth cooled, water vapour condensed to form oceans. Some carbon dioxide from the atmosphere dissolved in the ocean water, reducing the amount in the air.

Changes over long periods

The proportion of atmospheric carbon dioxide fell further as photosynthetic organisms removed it from the air. Carbon also became stored in sedimentary rocks and fossil fuels. For example, carbon in ocean water became part of the materials that formed some sedimentary rocks, while carbon in dead organisms became stored in fossil fuels over long periods.

Algae, and later plants, released oxygen through photosynthesis. Over time, this caused the proportion of oxygen in the atmosphere to increase.

As the proportions of gases changed, nitrogen became the largest component by percentage. The present atmosphere is approximately 78% nitrogen and 21% oxygen, with a much smaller proportion of carbon dioxide.