Topic 6 – The carbon cycle and energy security
Cycles, stores and fluxes:
Carbon the ‘building block of life’. Is present in the stores of:
- Atmosphere (as carbon dioxide / methane)
- Hydrosphere (dissolved CO2)
- Lithosphere (carbonate in limestone and fossil fuels)
- Biosphere (living in dead organisms)
Carbon moves from one sphere to another by linked processes known as the
biogeochemical carbon cycle.
- One of the most important drivers of the carbon cycle is the water cycle.
- Single carbon stores of the layer cycle can often have several fluxes, adding
and removing carbon at the same time.
Anthropogenic → Processes and actions associated with human activity.
,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.
There are two main components of the carbon cycle:
The geological carbon cycle
Slow part of the cycle is centered on the huge carbon stores in rocks and sediments,
with reservoir turnover rates of at least 100,000 years.
Reservoir turnover → The rate at which carbon enters and leaves stores is measured
by the mass of carbon in any store divided by the exchange flux.
The biological or physical carbon cycle
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,
soil and freshwater.
Sequestering → The natural storage of carbon by physical or biological processes
such as photosynthesis.
Geological origins
Most of the Earth’s carbon is geological, resulting from the formation of sedimentary
carbonate rocks in the oceans, and biologically derived carbon in rocks. Slow
geological processes release carbon into the atmosphere through chemical
weathering of rocks and volcanic outgassing at ocean ridges / subduction zones.
,Carbon in limestone and shale:
One of the Earth’s largest carbon stores is the Himalayas, this carbon is actively
weathered, eroded and transported back to the oceans.
In the oceans today, 80 per cent of carbon-containing rock is from shell-building
organisms and plankton. These are precipitated on to the ocean floor, form layers,
are cemented together and lithified into limestone. Over millions of years heat and
pressure compress the mud and carbon, forming sedimentary rock such as shale.
Geological processes:
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.
Chemical weathering
The geological part of the carbon cycle interacts with the rock cycle.
This process can be broken down into five phases:
1. Chemical weathering – in the atmosphere, water reacts with atmospheric
CO2 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.
3. Deposition – and burial turns the calcite sediment into limestone-
4. Subduction – of the sea floor under continental margins by tectonic
spreading.
a. Some of this carbon rises back up to the surface within heated magma,
then is degassed as CO2 and returned into the atmosphere.
b. Tectonic uplift cab also exposes previously buried limestone, as in the
Himalayans and Alps.
Volcanic outgassing
Pockets of CO2 exist in the Earth’s crust. Disturbance by volcanic eruptions or
earthquake activity may lead to outgassing.
Outgassing occurs at:
- Active or passive volcanic zones associated with tectonic plate boundaries.
(subduction zones and spreading ridges)
- Direct emissions from fractures in the Earth’s crust.
Two other locations contribute CO2 to the atmosphere:
- Isolated magma hotspots
- Tectonic collision zone volcanoes
A negative feedback mechanism regulates the natural geological carbon cycle:
