Extremely thorough revision notes for A Level Edexcel Geography for the carbon cycle topic. I achieved an A* in Geography in the 2022 A Level exams with these notes.
The carbon cycle & energy
security
The carbon cycle
= a biochemical cycle whereby carbon is stored & moved between spheres through
flows/fluxes. It’s a closed system but made up of interlinked subsystems which are open & have
inputs/outputs. Carbon stores can add carbon to atmosphere (sources) & remove it (sinks).
2 main components of the cycle –
1. The (slow) geographical carbon cycle – carbon stores in rocks & sediments
2. The (fast) biological carbon cycle – carbon within vegetation, soils &
atmosphere
Carbon cycle processes
Þ Stores (also referred to as pools, stocks & reservoirs) - there are terrestrial
(land), oceanics (oceans/seas) & atmospheric (air around us) stores. They
function as sources (adding carbon to atmosphere) & sinks (removing carbon
from atmosphere)
Þ Fluxes - refer to movement/transfer of carbon between stores (fluxes create
cycles & feedbacks), providing the motion in the carbon cycle
Carbon fluxes between carbon stores of carbon cycle are measured in either
petagrams (Pg) or gigatonnes (Gt) of carbon per year. The major fluxes are
between the oceans & the atmosphere & between the land & atmosphere via the
biological processes of photosynthesis & respiration. These fluxes vary not only in
terms of flow but also on different timescales.
Longer time scale (10-500 years)
Shortest time scale (seconds to mins)
à carbon from dead plant material can be
à plants take carbon out the
incorporated into soils where it might reside
atmosphere through photosynthesis &
for years, decades or centuries before being
release it back into the atmosphere via
broken down by soil microbes & released
respiration
back into atmosphere
, There’re 3 forms of carbon in carbon cycle
• Inorganic – found in rocks as bicarbonates & carbonates
• Organic – found in plant material & living organisms
• Gaseous – found as CO2 & CH4 (methane)
If sources = sinks à carbon cycle is in equilibrium
Changes in the system result in negative or positive feedback. There’s generally a
balance between sources & sinks, sometimes it takes a while for equilibrium to be
reached (e.g., after volcanic eruption)
, The geological carbon cycle (slow)
à carbon stored in rocks & sediments is exchanged very slowly, it’s referred to as the
terrestrial carbon stores & stores in over the long term (100-200mil years). Largest
carbon store is geological – over 100mil Pg of carbon in lithosphere.
Most of Earth’s carbon is stored in rocks as fossil fuels (e.g., coal) & in sedimentary
carbonate rocks (e.g., limestone – formed in the ocean but also shale & clay which
is created biologically)
It moves through carbon cycle through chemical weathering of rocks & volcanic
outgassing –
Volcanic outgassing
à they release co2 during eruptions & also move geological carbon from mantle to
surface (constructive p boundary)
à they emit 0.15-0.25 Gt annually (very small)
à emission of co2 from Kilauea, Hawaii is responsible for around 35% of total
Carbon trapped in sedimentary rock is subducted into mantle at a destructive pb & is
released during volcanic eruption. It becomes igneous rock & releases small amounts of
carbon.
Chemical weathering of rocks
= water reacts with atmospheric co2 & carbonic acid forms
1. Transportation of calcium by rivers from land into the oceans - these combine to form
calcium carbonate & form minerals such as calcite.
2. Deposition & burial turns calcite sediment into limestone (process known as diagenesis –
conversion of sediment to sedimentary rock)
3. Subduction of the sea floor
4. Some of this carbon rises back to the surface within heated magma, then degassed as
co2 & returned to the atmosphere
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