The Carbon Cycle

• Carbon is a chemical element, forming more compounds than any other element (scientists believe there are over 10 million carbon-based compounds in existence).
• Some examples of carbon compounds are carbon dioxide found in atmosphere, oceans and soil, calcium carbonate a solid compound in calcareous rocks, oceans and skeletons/shells of ocean creatures and bio-molecules which are complex carbon compounds in living things, like proteins, carbohydrates, fats/oils and DNA.
• Carbon is found in all life forms, as well as sedimentary rocks, diamond, graphite, coal, oil and gas.
• The interior of the Earth is mainly where carbon/CO₂ comes from. When Earth formed, it was stored in the mantle and has since escaped from constructive and destructive plate boundaries and volcanoes.
• Lots of CO₂ released from destructive margins formed from metamorphism of carbonate rocks subducting with ocean crust.
• Distribution of global carbon stores.
• Lots in Scotland, Northern Europe, Canada with around 500+ tonnes per hectare, this is because of presence of ice and vegetation.
• Band of heavy carbon storage around Equator – lots of tropical rainforest storing carbon.
• Very little in Northern Africa desert so not a lot of biosphere storage.
• Very little in Greenland and Antarctica so no vegetation storage.

Carbon Stores in the Carbon Cycle

• The United Nations climate change panel uses the units gigatonne of carbon dioxide (GtC) to measure amount of carbon dioxide in stores. One Gt is equivalent to 1 billion tonnes.
• Lithosphere which includes crust and uppermost mantle, constituting the hard-outer layer of the Earth. It is the largest store of carbon, with around 100,000 billion metric tonnes – equivalent to 100,000GtC.
• In the lithosphere, carbon is stored as organic and inorganic forms. Organic forms include litter, organic matter and humic (consist of humus) substances found in soils.
• Inorganic forms include fossil fuels like coal, natural gas, oil shale and limestone (plus other carbonate-based sedimentary deposits.
• Lithospheric carbon is distributed between the following stores: marine sediments/sedimentary rocks (up to 100 million GtC), soil organic matter (1500-1600 GtC), fossil fuel deposits (4100 GtC) and peat which is dead undecayed organic matter (250GtC).
• Hydrosphere – oceanic stores.
• The ocean is a key part of the carbon cycle. Measurements have been taken by the Global Ocean Data Analysis project (GLODAP) about the amounts of carbon in oceans. The total amount of oceanic carbon is between 37,000 and 40,000 GtC.
• Oceanic carbon stores are as follows: surface layer/euphotic zone where sunlight penetrates so photosynthesis can take place around 900GtC, intermediate and deep layer/twilight zone (37,100GtC), living organic matter such as fish, plankton, bacteria etc – 30GtC, dissolved organic matter (700GtC).
• Much of the oceanic carbon is there because CO₂ is absorbed into water. As well as that, the decay of dead organisms’ releases carbon dioxide into the water.
• Some oceanic carbon is locked in sedimentary rocks, which forms from the build-up carbon-rich sediment as carbon can be locked here for millions of years.
• This sedimentary layer of carbon storage may hold up to 100 million GtC.
• Biosphere is the total sum of all living matter. In this context, the carbon store is the terrestrial biosphere, which is estimated to contain 3170 GtC. The terrestrial biospheric storage of carbon is split into more minor stores:
• Living vegetation – globally, 19% of biospheric carbon is stored in plants, directly in the tissues of plants.
• It is estimated that half of carbon in forests is in high-latitude forests, and just over a third in low-latitude forests.
• Russia’s forests are put together the largest carbon forest reservoir with 25% of the world’s forest carbon, followed by the amazon at 20%.
• Plant litter is fresh, decomposed plant debris, like leaves, cones, needles, bark etc.
• Soil hummus originates from litter composition and is a thick brown/black substance that remains after litter is decomposed in all forest types, around 69% of the carbon is stored in the soil as opposed to 31% in the biomass in total, soils hold 2500GtC, with 1550GtC being organic and 950GtC being inorganic.
• Inorganic carbon soil is made up of carbon as well as carbonate materials like calcite, dolomite and gypsum.
• Peat is the accumulation of partially decayed vegetation/organic matter unique to natural areas which forms in wetland conditions where near permanent water saturation blocks oxygen from the ground, so anaerobic conditions result, meaning rates of plant litter decomposition are slowed down with peat storing over 250GtC.
• Animals only play a small storage role but are vital in the movement of carbon.

Atmosphere

• Anthropogenic carbon dioxide is that which is generated by human activity.
• Carbon dioxide is a trace gas in the atmosphere, and it is estimated that there is between 720 and 800 GtC in the atmosphere.
• CO₂makes up 0.04% of the atmospheric gases, which is 400pp, this value is the highest it has reached in the last 800,000 years, and probably for the past 20 million years.
• Carbon dioxide is a greenhouse gas, important in regulating the world’s temperature, but atmospheric levels of it have been rising significantly in recent years.
• Human activity has meant that we are burning more fossil fuels, and when this happens, carbon reacts with oxygen, making carbon dioxide, which is then released into the atmosphere.
• The Mauna Loa Observatory (MLO) in Hawaii has been monitoring atmospheric carbon levels since 1958, due to the location being in undisturbed air, remote and with minimal influences from vegetation and human activity.
• The average concentration in 1958 was 280ppm, and by 2015 it had increased to 400.3ppm.
• It has largely been because of anthropogenic sources (human), like combustion of fossil fuels and deforestation. At MLO, the daily average concentration is currently increasing at 2ppm/year, and this rate continues to increase.