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Question 1

Carbon cycle:

Answer 1

The biogeochemical cycle by which carbon moves from one sphere to another. It acts as a closed system made up of linked subsystems that have inputs, throughputs and outputs.
Carbon stores function as sources (adding carbon to the atmosphere) and sinks (removing carbon from the atmosphere).

Question 2

Fluxes

Answer 2

Movements of organic compounds through an ecosystem.

Question 3

Terrestrial carbon stores

Answer 3

This section considers the role of land-based processes in the carbon cycle, focusing on slow movements and longer-term stores of carbon.

Question 4

Carbon is called the main ‘building block of life’. It is present in the stores of:

Answer 4

the atmosphere, as carbon dioxide (COz) and compounds such as methane (CH4)
the hydrosphere, as dissolved COz
the lithosphere, as carbonates in limestone and fossil fuels such as coal, oil and gas
the biosphere, in living and dead organisms.

Question 5

Carbon moves from

Answer 5

• one sphere to another by linked processes known as the biogeochemical carbon cycle.
• This includes every microbe, leaf, puddle, grain of rock, dead being and volcanic eruption.
• Complete decomposition of organic matter results in carbon returning to inorganic forms such as COz and carbonates contained in rock and seawater.
• Processes including photosynthesis and diffusion drive the flows or fluxes between the stores, operating at local and global scales.
• If sources equal the sinks, the carbon cycle is balanced, or in equilibrium, with no change in the size of the stores.
• Changes in the system may result in negative or positive feedback.

Question 6

diagram of the carbon cycle

Answer 6

• main stores and fluxes of the carbon cycle before (in black) and after (in red) major anthropogenic (human) influences.
• The numbers represent estimated carbon pool sizes in PgC (petagrams of carbon) and the magnitude of the different exchange fluxes in PgC yr 1 (petagrams of carbon per year), averaged between 2000 and 2009.
• One of the most important drivers of the carbon cycle is the water cycle for example, run-off and rivers transport eroded rock and soil into oceans.
• Single carbon stores of the larger cycle can often have several fluxes, adding and removing carbon at the same time.

Question 7

Key concept: System feedback

Answer 7

• Earth systems normally operate by negative (stabilising) feedbacks, maintaining a stable state by preventing the system moving beyond certain thresholds.
• Any change is cancelled out, maintaining equilibrium.
• Positive (amplifying) feedback loops occur when a small change in one component causes changes in other components.
• This shifts the system away from its previous state and toward a new one.

Question 8

Intergovernmental Panel on Climate Change (IPCC):

Answer 8

The leading international organisation for the scientific assessment of climate change.

Question 9

Anthropogenic

Answer 9

Processes and actions associated with human activity.

Question 10

Petagrams (Pg) or Gigatonnes (Gt)

Answer 10

The units used to measure carbon; one petagram (Pg), also known as a gigatonne (Gt), is equal to a trillion kilograms, or 1 billion tonnes.

Question 11

Reservoir turnover:

Answer 11

The rate at which carbon enters and leaves a store is measured by the mass of carbon in any store divided by the exchange flux.

Question 12

There are two main components of the carbon cycle.

Answer 12

The geological carbon cycle
The biological or physical carbon cycle

Question 13

The geological carbon cycle

Answer 13

• This slow part of the cycle is centred on the huge carbon stores in rocks and sediments, with reservoir turnover rates of at least 100,000 years.
• Organic matter that is buried in deep sediments, protected from decay, takes millions of years to turn into fossil fuels.
• Carbon is exchanged with the fast component through volcanic emissions of COz, chemical weathering, erosion and sediment formation on the sea floor.

Question 14

The biological or physical carbon cycle

Answer 14

• This fast component of the carbon cycle has relatively large exchange fluxes and ‘rapid’ reservoir turnovers of a few years up to millennia.
• Carbon is sequestered in, and flows between, the atmosphere, oceans, ocean sediments and on land in vegetation, soils and freshwater.
• Fluxes are measurements of the rate of flow of material between the stores.
• Because fluxes are a rate, the units are mass per unit time.
• At a global scale they are expressed as Pg per year: PgC yr 1, or a GtC yr 1.
• You will need to be able to construct proportional arrows to show these varying fluxes.

Question 15

Sequestering:

Answer 15

The natural storage of carbon by physical or biological processes such as photosynthesis.

Question 16

long term carbon stores

Answer 16

Question 17

short term carbon stores

Answer 17

Question 18

Processes:

Answer 18

The physical mechanisms that drive the flux of material between stores.

Question 19

Key concept: Geological fluxes

Answer 19

These are small on an annual basis, but without them the carbon stored in rocks would accumulate and remain there forever, eventually depleting the sources of CO2 that are vital to life forms.

Question 20

Geological origins

Answer 20

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 rocks like shale and coal. Slow geological processes release carbon into the atmosphere through chemical weathering of rocks, shown in Table 4.2, and volcanic outgassing at ocean ridges/subduction zones.
Limestone, shale and fossil fuels are important carbon stores.

Question 21

key processes in the geological carbon cycle

Answer 21

Question 22

Carbon in limestone and shale

Answer 22

• One of Earth’s largest carbon stores is the Himalayas, which started off as oceanic sediments rich in calcium carbonate.
• Folded up by mountain building, this carbon is being actively weathered, eroded and transported back to the oceans.
• In the oceans today, 80 per cent of carbon-containing rock is from shell-building (calcifying) organisms (corals) and plankton.
• These are precipitated on to the ocean floor, form layers, are cemented together and lithified (turned to rock) into limestone.
• The remaining twenty per cent of rocks contain organic carbon from organisms that have been embedded in layers of mud.
• Over millions of years heat and pressure compress the mud and carbon, forming sedimentary rock such as shale.

Question 23

Carbon fossil fuels

Answer 23

• Fossil fuels are so called because they were made up to 300 million years ago from the remains of organic material.
• Organisms, once dead, sank to the bottom of rivers and seas, were covered in silt and mud, and then started to decay anaerobically (without the presence of oxygen).
• This process operates over millennia.
• The deeper the deposit, the more heat and pressure is exerted on the deposits.
• When organic matter builds up faster than it can decay, layers of organic carbon become oil, coal or natural gas instead of shale.

Question 24

formation of Oil and natural gas

Answer 24

• Formed from the remains of tiny aquatic animals and plants
• Gas and oil occur in ‘pockets’ in rocks, migrating up through the crust until meeting caprocks
• Natural gas, such as methane, is made up of the fractions of oil molecules, so small they are in gas form not liquid, and usually found with crude oil
• Other hydrocarbon deposits include oil shales, tar sands and gas hydrates

Question 25

formation of Coal

Answer 25

Formed from the remains of trees, ferns and other plants There are four main types of coal:
* anthracite is the hardest coal; it has the most carbon and, hence, a higher energy content
* bituminous coals are next in hardness and carbon content
* soft coals such as lignite and brown coal are lower in carbon (25-35%) and energy potential; these are the major global source of energy supplies but emit more CO2 than hard coals
* peat is the stage before coal; it is an important carbon and energy source

Question 26

Geological processes

Answer 26

• slow geological processes are an important control on the carbon cycle.
• Through a series of chemical reactions and tectonic activity, carbon takes between 100 and 200 million years to move between rocks, soil, ocean and atmosphere.
• On average, 10^13 to 10^14 grams (10-100 million metric tonnes) of carbon move through this slow carbon cycle annually.
• This compares with the faster carbon cycles of ecosystems (10^16 to 10^17 grams annually) and anthropogenic cycles (10^15 grams annually).
• The specification focuses on two specific processes: the chemical weathering of rocks, and volcanic outgassing at ocean ridges and subduction zones

Question 27

Chemical weathering

Answer 27

The geological part of the carbon cycle interacts with the rock cycle, a series of constant processes through which Earth’s materials change from one form to another over millennia. These processes can be broken down into five phases:

Question 28

These processes can be broken down into five phases:

Answer 28

1. Chemical weathering: in the atmosphere, water reacts with atmospheric CO, and carbonic acid forms. Although only weakly acidic, once this water reaches the surface as rain, it reacts with some surface minerals, slowly dissolving them into their component ions.
2. Transportation of calcium ions by rivers from the land into oceans. These combine with bicarbonate ions to form calcium carbonate and precipitate out as minerals such as calcite (CaCO3).
3. Deposition and burial turns the calcite sediment into limestone.
4. Subduction of the sea floor under continental margins by tectonic spreading.
5. Some of this carbon rises back up to the surface within heated magma, then is ‘degassed’ as CO2 and returned to the atmosphere.
   Diamonds, the purest form of carbon, have recently been discovered to be formed up to 435 miles (700 km) deep, proving that carbon is cycled between Earth’s surface and the lower mantle. Tectonic uplift can also expose previously buried limestone, as in the Himalayas and Alps.

Question 29

Volcanic outgassing

Answer 29

Pockets of CO2 exist in the Earth’s crust. Disturbance by volcanic eruptions or earthquake activity may allow pulses or more diffuse fluxes into the atmosphere.
Outgassing occurs at:

Question 30

Outgassing occurs at:

Answer 30

• active or passive volcanic zones associated with tectonic plate boundaries, including subduction zones and spreading ridges
• places with no current volcanic activity, such as the hot springs and geysers in Yellowstone National Park, USA
• direct emissions from fractures in the Earth’s crust.

Question 31

volcanoes vs humans

Answer 31

Volcanoes currently emit 0.15-0.26 Gt CO2 annually. In comparison, humans emit about 35 Gt, mainly from fossil fuel use, so volcanic degassing is relatively insignificant.

Question 32

volcanoes examples

Answer 32

• About 70 surface volcanoes are currently active.
• An example of a major degassing as a pulse is the 1991 eruption of Mt Pinatubo in the Philippines, part of an island arc created by a subduction zone.
• Such eruptions not only return CO, to the atmosphere; the fresh silicate rock erupted starts the carbon cycle again
• Volcanically active mid-ocean ridges, such as in Iceland, are found on the growing edges of tectonic plates.
• Constructive plate boundaries zigzag 37,000 miles across the sea floor.
• Currently they seem to be in a fairly languid state, despite producing more lava annually than land volcanoes.
• Their magmas are basic and low in silicate.
• Hence, although they produce much lava, the CO, emitted is about the same as the smaller number of alkaline magma land volcanoes, about 88 million metric tonnes a year.
• CO, can drive explosive eruptions in normally effusive lava flows.
• Two other locations contribute CO, to the atmosphere: isolated magma hotspots such as Kilauea in Hawaii, and tectonic collision zone volcanoes such as Etna in Sicily.
• An interesting negative feedback mechanism regulates the natural geological carbon cycle

Question 33

diagram of the slow geological carbon cycle

Answer 33

Question 34

negative feedback in regulating the geological carbon cycle

Answer 34