EQ1 - how does the carbon cycle operate to maintain planetary health
1.1 - most global carbon is locked in terrestrial stores as part of the long term geological cycle
1.1.1 - the biogeochemical carbon cycle consists of carbon stores of different sizes (terrestrial, oceans
and atmosphere) with annual fluxes between stores of varying size (measured in Pg/Gt) rates and on
different timescales
carbon cycle
1) stores
- measured in petagrams (Pg) or gigatonnes (Gt)
1. atmosphere → CO2, CH4
2. hydrosphere → dissolved CO2
3. lithosphere → carbonates in limestone and fossil fuels like coal, oil and gas
4. biosphere → living and dead organisms
2) fluxes → movement or transfer of carbon between stores
3) human activity (anthropogenic)
- part of the carbon cycle
- amount added by humans are tiny in compared with the natural flows exchanged every year
- planetary health is at risk as more carbon enters the atmosphere
180 Gt has been added to the atmosphere as a result of burning fossil fuels - this is enough to alter the
concentration of greenhouse gases in the atmosphere, triggering climate change
forms of carbon
- inorganic → found in rocks as bicarbonates and carbonate
- organic → found in plant material
- gaseous → found as CO2, CH4 and CO
terrestrial carbon store
- igneous and metamorphic rocks do not contain much carbon
- sedimentary rocks do have high concentrations, eg limestone contains about 42% calcium carbonate
by weight
- geological processes have trapped carbon in the form of coal, oil (85% carbon) and natural gas
- the terrestrial store is the largest
atmospheric carbon
- volcanic activity, respiration, wildfires and outgassing emit carbon dioxide into the atmospheric store
- very small compared with other stores
- however, small changes in concentration affect global temperature
- between 2012-2017 the average CO2 concentration increased by 3%, mostly due to human
emissions
ocean carbon store
- carbon dioxide is dissolved by oceans from the atmosphere, but it only makes up a small proportion of
the seawater mass
- 93% of oceanic CO2 is stored in undersea algae, plants and coral
- most CO2 is stored in intermediate and deep water, with only about 2.5% in surface water
slow carbon cycle - closed cycle (without humans)
, 1) release of CO2 into the atmosphere by volcanoes
2) reacts with water vapour in atmosphere to create acid (carbonic) rain
CO2 +H20 = H2CO3
3) acid rain reacts with rocks which contain calcium carbonate, producing calcium bicarbonate
- carbonation → acid rain reacting with carbonate in a rock
4) runoff from weathering and erosion of these rocks is then carried by rivers and into the ocean
5) organisms use that carbon to create shells
6) when animals die, shells layer up at the bottom of the ocean and form limestone
7) subduction of carbonate rocks → melts into magma, and is then used by volcanoes
1.1.2 - most of the earth’s carbon is geological, resulting from the formation of sedimentary carbonate
rocks (limestone) in the oceans and biologically derived carbon in shale, coal and other rocks
geological processes form sedimentary carbonate rocks and release carbon to the atmosphere
sedimentary carbonate rocks
- formed from calcareous ooze, shells and skeletons that collect at the bottom of oceans
1. marine creatures such as corals and phytoplankton absorb carbon from seawater
2. when their remains collect on the seabed the calcium carbonate is compacted by the weight of
new layers above them, and cemented together to form an organic limestone rock
- calcareous oozes are not found where the ocean is too deep, because the pressure causes calcium
carbonate to dissolve
biologically derived carbon rocks
- formed from the remains of living organisms deposited in layers within sedimentary rocks (such as
shale)
- organic carbon from tropical coastal swamps is buried to produce coal
- the hardness depends on the amount of pressure and heat when squashed during rock
formation
- anaerobic reactions may convert organic carbon into a liquid (oil) that moves within rock layers until
trapped
- by product of coal and oil formation is natural gas, which is also trapped within layers of sedimentary
rocks
1.1.3 - chemical weathering removes carbon from silicate rocks, the carbon ends up in the ocean as
carbonate rock, carbon is released via outgassing at ocean ridges, hotspot volcanoes and subduction
zones
volcanic out-gassing
- tectonic forces may bring limestone rocks into contact with extreme heat, causing chemical changes
and releasing CO2 into the atmosphere
- volcanic activity at plate boundaries or intraplate hot spots may release CO2 into the atmosphere
- eg, nearly 12% of all the volcanic gas emissions in Hawaii
- geothermal areas also release CO2 through hot springs or oversaturated pools
- degassing occurs because CO2 is not dissolved easily and so is released early in eruptions
chemical weathering of rocks
- limestone rocks are easily weathered by rain because it becomes a weak carbonic acid as it falls
through the air and absorbs some carbon dioxide
- this acid then dissolves the calcium carbonate of the rock, and dissolved carbon is then carried by
water for deposition on the seabed or released as a gas
, 1.2 - biological processes sequester carbon on land and in on the oceans on shorter timescales
1.2.1 - phytoplankton sequester atmospheric carbon during photosynthesis in surface ocean waters
carbonate shells/ tests move into the deep ocean water through the carbonate pump and action of the
thermohaline circulation
biological carbon pump
- phytoplankton → mostly microscopic single celled plants found in the warmer surface waters of oceans
and seas
- phytoplankton consume carbon dioxide from the atmosphere during the process of photosynthesis,
storing it in their bodies as a carbohydrate
- when they die, the phytoplankton transfer carbon to deeper ocean areas (forming calcium carbonate
when they sink to the seabed) or shallower layers if eaten by zooplankton (which then transfer carbon
to deeper waters when they die)
marine carbonate pump
- marine food web extends from phytoplankton to zooplankton, and then to other organisms such as
corals, oysters and crabs which not only consume carbon but absorb it to make their shells and
skeletons
- while some carbon dioxide is returned to the atmosphere through respiration, these marine creatures
collectively use enough carbon to create room for the sea to absorb more from the atmosphere
- in a relatively short time carbon is taken from the atmosphere into the ocean and then chemically
transferred to the seabed to become fixed in sedimentary rocks (this is the marine carbonate pump)
- cold, denser seawater sinks to deep ocean areas where slow-moving currents enable CO2 gas to be
stored
thermohaline circulation
1) warm, salty ocean currents transfer heat energy from tropical areas towards the poles and cold, less
salty currents transfer colder water towards the equator
- the temperature and salinity differences cause the large scale circulation of seawater
2) warmer waters tend to travel near the surface, while large volumes of cold water tend to move at
depths below 3km
- this makes a large-scale global circulation of interlinked surface warm currents and deep cold currents
moving seawater around the world between the oceans
- in this way, the Pacific, Indian, Southern and Atlantic Oceans are linked together by the thermohaline
circulation
3) part of the thermohaline circulation helps to transfer CO2 from equatorial ocean source areas (eg the
Atlantic between Brazil and West Africa) to polar ocean sink areas (such as the Nordic Sea between
Iceland and Norway, and the Weddell Sea near the Antarctic Peninsula)
- upwelling and downwelling currents move dissolved CO2 in what has been called a physical carbon
pump
1.2.2 - terrestrial primary producers sequester carbon during photosynthesis, some of this carbon is
returned to the atmosphere during respiration by consumer organisms
autotrophs - plants
heterotrophs - animals
sequester - use and store
anaerobic decomposition - decomposition without oxygen
humus - decomposed plant material
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