AO1 (3 marks)/AO2 (9 marks)

Marking instructions
Markers must apply the descriptors in line with the general marking guidance and the qualities outlined in the levels-based mark scheme below. Responses that demonstrate only AO1 without any AO2 should be awarded
marks as follows:

Level 1 AO1 performance: 1 mark
Level 2 AO1 performance: 2 marks
Level 3 AO1 performance: 3 marks.

Indicative content guidance
The indicative content below is not prescriptive and candidates are not required to include all of it. Other relevant material not suggested below must also be credited. Relevant points may include:

AO1

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.
Phytoplankton sequester atmospheric carbon during photosynthesis in surface ocean waters; carbonate shells move into the deep ocean water through the carbonate pump and action of the thermohaline circulation.
Terrestrial primary producers sequester carbon during photosynthesis; some of this carbon is returned to the atmosphere during respiration by consumer organisms.
The process of fossil fuel combustion has altered the balance of carbon pathways particularly from geological stores to atmospheric stores.

AO2

A primary role of the oceans is acting as a store of carbon. The total ocean store (consisting of all four components) has the largest non-geologic store of carbon at over 38,000 GtC making the ocean a vital component of carbon cycle and so maintaining planetary health.
Another key role of the oceans is its ability to absorb carbon dioxide into the surface ocean store (900 GtC). Crucially, as the oceans can absorb more carbon than it emits (a net gain of 1.2 Gtc yr-1), this means that it is considered a carbon sink and so highlights the importance of the oceans in regulating the carbon cycle.
Crucially, there are three key processes that occur in oceans that allows them to be a carbon sink with the atmosphere – a physical, solubility and biological cycle. The physical cycle is caused when dissolved carbon dioxide is taken from the surface ocean to the intermediate and deep oceans through downwelling currents. The solubility cycle is caused when the carbon dioxide absorbed by the oceans forms carbonic acid which in turn dissociates with hydrogen ions to form bicarbonates and then further reactions forms carbonates which are stored in the upper ocean. The biological cycle allows carbon dioxide to be sequestered in the ocean through photosynthesis by phytoplankton and other marine biota which converts the carbon dioxide into organic matter.
Another key role of the oceans is therefore by allowing these physical, solubility and biological cycles to increase the amount of carbon dioxide that can enter the oceans from the atmosphere thereby helping to regulate the carbon cycle.
Furthermore, another key role of the oceans is to act as a biological pump transporting carbon from the surface oceans to the intermediate and deep oceans (37100 GtC). This occurs when as these biological organisms die, their dead cells, shells and other parts sink into the mid and deep water.
In addition, decay of these organism also releases carbon dioxide into this intermediate and deep water (dissolved carbon 700 GtC) stores. Thus the oceans role is to move carbon from the surface oceans where it may vent back into the atmosphere and instead store it the mid and deep ocean store as well as the dissolved carbon store and so regulate the carbon cycle.
Some material sinks right to the bottom of the ocean and forms the sea bed sediment store (1750 GtC) where over time, through chemical and physical processes, the carbon is transformed to rocks such as limestone. The carbon sequestered in this store can remain for geological epochs and so regulates the carbon cycle.
Yet there is also the upwelling of carbon from intermediate and deep oceans to the surface oceans (90 GtC yr-1) through upwelling currents and turbulence created by surface winds allowing carbon previously stored in the intermediate and deep ocean store to return to the surface oceans and then back to the atmosphere (706.4 Gtc) highlighting that the role of oceans in regulating the carbon cycle is a two way relationship with carbon being moved both downwards and upwards through the oceans.
Furthermore, the rise in temperature caused by global warming is leading to ocean acidification and leading in some oceans to stratification occurring which reduces the ability of the oceans to absorb carbon dioxide reducing the role of the oceans as a carbon sink. Furthermore land use changes and the burning of fossil fuels is perhaps changing the atmospheric levels of carbon cycle beyond the ability of the oceans to counter act in maintaining a balanced carbon cycle.
Overall, the oceans play a vital role in regulating the carbon cycle by being the largest non-geologic store of carbon as well as being a carbon sink. This may be compared to the whole working of the carbon cycle. In addition through the biological pump they are able to sequester carbon from the atmosphere and store it in the intermediate and deep oceans and eventually some is stored as sea bed sediments. Yet the ability of the oceans to offset the impact of burning fossil fuels is small and if fossil fuel use continues to accelerate the oceans will have an increasingly smaller role to play in maintaining the carbon cycle.

Accept other assessments of the role of oceans in regulating the carbon cycle.

Carbon Cycle (Edexcel A Level Geography)

Topic Questions

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