Carbon stores and cycles

Carbon is integral to life on earth. It is found as carbon based molecules in various forms – as carbon dioxide and methane in the atmosphere, as organic matter in soils formed by the decomposition of organic material, sedimentary ocean bed layers, in carbon-rich rocks and in vegetation. The key carbon cycles operate at the terrestrial, atmospheric and oceanic level.

The cycling of carbon between the land and atmosphere is known as the fast carbon cycle. This is the movement of carbon from living things up into the atmosphere. With carbon dioxide, this takes place through the process of respiration from plants and animals and both CO2 and methane (CH4) are released through the decomposition of plants and animals. Ultimately this transfer of carbon into the atmosphere is directly affected by humans by land use change and industrial processes.

As well as terrestrial processes, the ocean carbon cycle is important to consider. Carbon is stored here as dissolved CO2 in the water and also in carbon compounds in marine organisms. The input to the ocean store is through absorption via a gas exchange with the atmosphere. Carbon is also transferred into the oceans through precipitation of naturally (and anthropogenic) acid rain. Much carbon is stored in a carbon sink on the floor of shallow oceans as accumulating sediments from a persistent ‘rain’ of dead and decaying marine organism remains and the excrement of plankton and other creatures in the upper sea layers.

The cycling of carbon between surface bedrock and atmospheric or ocean stores is known as the slow carbon cycle. The weathering of surface carbon-bearing rocks by acid rain (carbonic acid formed as a result of atmospheric moisture reacting with carbon dioxide) over millions of years leads to a terrestrial-ocean carbon transfer as rivers transport weathered rock into the oceans. Vast quantities of carbon are stored in sedimentary deposits on the ocean floor. Over millions of year, with tectonic plate movement, they are eventually subducted into the mantle at a destructive plate margin. The carbon content is then returned to the atmosphere through volcanic activity (usually as CO2), where it contributes to the formation of acid rain to start the cycle again.