the carbon cycle

carbon stored in billion tonnes 

• atmosphere - 600
• oceans - 38,700
• sedimentary rocks - 60,000 - 100,000,000
• sea floor sediments - 6000
• fossil fuels - 4130
• land plants - 560
• soils - 2300 

carbonate rocks and deep ocean sediments are biggest stores of carbon. next largest is dissolved CO2 in oceans 

• carbon stored in rocks, sea floor sediment and fossil fuels is locked away for millions of years 
• total amount of carbon circulated by slow cycle is between 10 and 100 million tonnes a year
• CO2 diffuses from atmosphere into oceans where marine organisms make their shells and skeletons by fixing together dissolved carbon with calcium to form calcium carbonate 
• when organisms die, remains sink to ocean floor where they accumulate over millions of years, heat and pressure convert them into sediementary rocks 
• residence time of carbon in rocks is 150 million years 
• chemical weathering processes are the result of precipiation with high levels of CO2 from atmosphere containing weak acid 
• acid attacks rocks, releasing CO2 in atmosphere as well as dissolving it in streams, rivers and oceans 
• on land decomposed organic material may be buried beneath younger sediments - forms carbonaceous rocks such as coal, oil and natural gas. the fossil fuels act as carbon sinks for millions of years 

• carbon circulates most quickly between atmosphere, oceans and living organisms (biosphere) and soil 
• transfers are between 10 and 1000 times faster than slow carbon cycle 
• land plants and phytoplankton are key components to fast cycle - photosynthesis absorbs CO2 and releases oxygen 
• decomposition of plants and animals releases carbon to atmosphere again 
• carbon exchange also occurs between atmosphere and oceans - atmospheric CO2 dissolves in ocean surface waters, oceans ventilate CO2 back into atmosphere 
  • individual carbon atoms are stored by natural sequestrian in oceans for an average of 350 years 

• precipitation: atmospheric carbon dissolves in rainwater, forms weak carbonic acid.  rising CO2 in atmosphere from human activity (anthropogenic emissions) have increased rainfall acidity. this has also increased acidity of ocean surface waters 
• photosynthesis: carbon flowing from atmosphere to plants via photosynthesis averages 120 gigatonnes a year, carbon, sunlight and water are converted to glucose and oxygen
• weathering: break dwon of rocks on the earth's surface by chemical, biological and physical processes. most weathering involves rainwater containing dissolved CO2 breaking down limestone and chalk - process is called carbonation. this releases carbon from limestones to streams, atmosphere, rivers and ocean. chemical weathering transfers approx 0.3 billion tonnes of CO2 to atmosphere and oceans every year. freeze-thaw (physical) and chelation (biological) breaks down rocks  
• respiration: carbohydrates are converted to carbon when animals respire, reverse of photosynthesis. 
• decomposition: decomposers are organisms such as bacteria and fungi which dreak down dead organic matter, extract energy and release CO2 into atmosphere and nutrients into soil. decomposition rates depend on climate - faster rates occur in warm, humid environments.
• combustion: organic material burns and releases CO2 as well as other natural gases. can occur naturally or due to human activity. combustion of fossil fuels bad; lots of CO2 released

1. physical (inorganic pump) 

• mixing of surface and deep ocean waters by vertical currents, creating more even distribution of carbon. CO2 enters oceans from atmosphere by diffusion
• surface currents transport water (and therefore dissolved CO2) polewards where it becomes colder, more dense and sinks 
• this is called downwelling; only really occurs in cold regions of sea. individual carbon molecules remains in depths for centuries. deep ocean currents eventually transport carbon rich waters to areas of upwelling, where carbon-rich water diffuses back into atmosphere

2. biological (organic) pump

• carbon exchanged via marine animals - nearly half of all carbon fixation by photosynthesis takes place in oceans . 50 gigatonnes of CO2 is drawn from atmosphere by pump anually
• phytoplankton floats near surface and combines with sunlight, water and CO2 to produce organic material
• phytoplankton either accumulates in sediments on ocean floor or is decomposed and releases CO2 
• mollucs and crustaceans extract carbonate and calcium from water to construct shells and skeletons - ends up at bottom of sea and forms chalk or limestone

vegetation

• land plants, especially rainforest trees, contain huge stores of carbon 
• most of carbon, extracted from atmospheric CO2 through photosynthesis, stays locked away for decades