Biological processes sequestering carbon
This section outlines the processes involved in the fast carbon cycle, linking the atmosphere, ocean and ecosystems.
Oceanic sequestration
- The oceans are the Earth’s largest carbon store, being 50 times greater than that of the atmosphere; 93 per cent of CO, is stored in undersea algae, plants and coral, with the remainder in a dissolved form.
- Small changes in oceanic carbon cycling can have significant global impacts.
- The COz gas exchange flux between oceans and atmosphere operates on a timescale of several hundred years.
- There is also a significant input of both organic carbon and carbonate ions from continental river run-off.
- Only a small proportion of this carbon is eventually buried in ocean sediments, but these are important long-term carbon stores with fluxes operating over millennia, unlike most terrestrial systems.
the ocean fact
The average depth of the ocean is about 3688 m, with the deepest part in the Mariana Trench (10,994 m) in the western Pacific Ocean.
Far less is known about this store than about terrestrial stores.
Carbon cycle pumps
The specification requires you to understand two key processes in depth when looking at the carbon cycle pump: the biological pump, and the actions of the linked carbonate pump involving thermohaline circulation. This circulation is part of a third important process called the physical pump. These pumps flux surface ocean COz to the deep ocean, as illustrated in Figure 4.5 and summarised in Table 4.4 (page 84).
Carbon cycle pumps
The processes operating in oceans to circulate and store carbon. There are three sorts: biological, carbonate and physical.
Thermohaline circulation
The global system of surface and deep water ocean currents is driven by temperature (thermo) and salinity (haline) differences between areas of oceans.
Biological pump
p1
- This is the organic sequestration of CO2 to oceans by phytoplankton.
- These microscopic, usually single-celled, marine plants float near the ocean surface to access sunlight to photosynthesise.
- They are the base of the marine food web.
- Although minute, their huge numbers make up half of the planet’s biomass.
- Phytoplankton have rapid growth rates, called net primary productivity (NPP), especially in the shallow waters of continental shelves, where rivers carry nutrients far out to sea, and in nutrient upwelling locations.
Biological pump
p2
- The Arctic and Southern Oceans are very productive areas.
- Carbon is then passed up the food chain by consumer fish and zooplankton, which in turn release CO, back into the water and atmosphere.
- Most is recycled in surface waters.
- Only 0.1% reaches the sea floor after the dead phytoplankton sink, where they either decompose or are turned into sediment.
- Decomposition is faster than on land because of the lack of woody plant structures.
- Phytoplankton sequester over 2 billion metric tonnes of COz annually to the deep ocean.
Carbonate pump
- This relies on inorganic carbon sedimentation.
- Marine organisms may utilise calcium carbonate (CaCOz) to make hard outer shells and inner skeletons, such as some plankton species, coral, oysters and lobsters.
- When organisms die and sink, many shells dissolve before reaching the sea floor sediments.
- This carbon becomes part of deep ocean currents.
- Shells that do not dissolve build up slowly on the sea floor, forming limestone sediments such as those in the ‘White Cliffs’ of Dover.
Physical pump
- This is based on the oceanic circulation of water including upwelling, downwelling and the thermohaline current
- COz in the oceans is mixed much more slowly than in the atmosphere, so there are large spatial differences in COz concentration.
- The colder the water, the more potential for CO, to be absorbed.
- CO, concentration is 10% higher in the deep ocean than at the surface, and polar oceans store more CO, than tropical oceans.
- Warm tropical waters release CO, to the atmosphere, whereas colder high-latitude oceans take in CO from the atmosphere.
- More than twice as much CO, can dissolve into cold polar waters than in warm equatorial waters.
- As major ocean currents such as the North Atlantic Drift (Gulf Stream) move waters from the tropics to the poles, the water cools and can absorb more atmospheric CO2.
- High latitude and Arctic zones with deep oceans have cooler water, which sinks because of its higher density, taking COz accumulated at the surface downwards.
Thermohaline circulation
- Thermohaline circulation is a vital component of the global ocean nutrient and carbon dioxide cycles.
- Ocean currents circulate carbon, with water flows equivalent to over 100 times that of the Amazon River.
- It takes 1000 years for any given cubic metre of water to travel around the system.
- Warm surface waters are depleted of nutrients and COz, but they are enriched again as they travel through the conveyor belt as deep or bottom layers.
- The foundation of the planet’s food chain depends on the cool, nutrient-rich waters that support algae and seaweed growth.
- The circulation also helps shift carbon in the carbonate pump cycle from upper to deeper waters.
- The balance of total carbon uptake (92 PgC) and carbon loss (90 PgC) from the ocean is therefore dependent on organic and inorganic processes acting at both surface and deep ocean locations.
- Until the start of the twenty-first century oceans were able to sequester increased COz emissions, but evidence now suggests a slowing of this storage.
- Increased oceanic acidification, due to increased COz, reduces the capacity for extra COz storage.
steps in Thermohaline circulation
1 The main current begins in polar oceans where the water gets very cold; sea ice forms; surrounding seawater gets saltier, increases in density and sinks.
2 The current is recharged as it passes Antarctica by extra cold salty, dense water.
3 Division of the main current:
northward into the Indian Ocean and into the western Pacific.
4 The two branches warm and rise as they travel northward, then loop back southward and westward.
5 The now-warmed surface waters continue circulating around the globe. On their eventual return to the North Atlantic they cool and the cycle begins again.
Terrestrial sequestration
This part of the carbon cycle, based on organic carbon, has the shortest temporal (time) scale - only seconds, minutes or years
- Primary producers - plants - take carbon out of the atmosphere through photosynthesis and release COz back into the atmosphere through respiration.
- When consumer animals eat plants, carbon from the plant becomes part of its fats and proteins.
- Micro-organisms and detritus feeders such as beetles feed on waste material from animals, and this becomes part of these micro-organisms.
- After plant and animal death, tissues such as leaves decay faster than more resistant structures, such as wood. Decomposition is fastest in tropical climates with high rainfall, temperatures and oxygen levels; it is very slow in cold, dry conditions or where there is a shortage of oxygen. In Arctic biomes, ecosystems are ‘locked down’ by extreme cold for substantial time periods.
Globally, the most productive biomes are
tropical forests, savannah and grassland, which together account for half of global NPP. Figure 4.11 on page 91 shows typical values for different biomes of productivity and carbon storage capacity. Storage is mainly in plants and soils, with smaller amounts in animals, and micro-organisms (bacteria and fungi). The largest store is in trees, which can live tens, hundreds and even thousands of years.
Carbon fluxes vary:
- diurnally - during the day the fluxes are positive, from the atmosphere to the ecosystem; at night the fux is negative, with loss from the ecosystem to the atmosphere
- seasonally - in the northern hemisphere winter, when few land plants are growing and many are decaying, atmospheric COz concentrations rise; during the spring, when plants begin growing again, concentrations drop.
Carbon sinks can become
carbon sources with anthropogenic influence, for example by forest burning.
Tropical rainforests
- These are one of the largest organic stores of carbon on Earth.
- The Amazon rainforest alone, covering 5.3 million sq km, sequesters 17% of all terrestrial carbon, more than any other land-based biome.
- Some species, such as Brazil nut trees, dominate this process.
- 1% of the Amazon’s 16,000 tree species store 50 per cent of its carbon, removing COz from the atmosphere for centuries.
Wetlands and peatlands
- Wetlands that contain peat, an organic sediment, are important carbon stores.
- Many peatlands formed during the Holocene have been a store for thousands of years; with climate change and overuse, however, they are becoming net carbon sources.
Biological carbon
Soils store 20-30 per cent of global carbon, sequestering about twice the quantity of carbon as the atmosphere and three times that of terrestrial vegetation. However, whether it sequesters or actually emits CO2 depends on local conditions.
There are two sources of carbon in soils.
There are two sources of carbon in soils.
Arid and semi-arid soils, and those developed on limestone, contain inorganic carbon. However, the most important store is from organic sources through plant photosynthesis and subsequent decomposition both above and below ground.
Living organisms represent about five per cent of the total soil organic matter. They have seasonal as well as daily patterns, and not all are active at the same time.
Since all parts of plants are made of carbon, any
- loss to the ground means a transfer or flux from the plant to the soil.
- Litterfall and branch litter includes whole plants, leaves and branches shed during any year.
- Roots may be shed as well.
- Carbon is stored as dead organic matter in soils for years, decades or even centuries in colder climates or wetland environments, before being broken down by soil microbes and released back to the atmosphere.
- After death there are thousands of compounds in the soil to be decomposed.
- The most long-term process is the formation of humus.
- This is seen easily in soils as it has a dark, rich colour.
- Humus soils are 60 per cent carbon and are important for sequestration as well as for water storage.
- In general, carbon cycling and formation is most active in topsoil horizons.
- Stabilised carbon, with longer turnover times, is located in deeper soil layers.
- In permafrost regions, over 61 per cent of carbon is stored deeper than 30 cm.
- An additional long-term carbon store in many soils is pyrogenic carbonaceous matter, formed from biomass burned and carbonised during wildfires.
- This resists microbial decomposition and can remain in soils for long periods.
The capacity of soil to store organic carbon is determined by:
- Climate
- Soil type
- Management and use of soils
- Climate
dictates plant growth and microbial and detritivore activity. Rapid decomposition occurs at higher temperatures or under waterlogged conditions. Places with high rainfall have an increased potential carbon storage than the same soil type in lower rainfall places. Arid soils store only 30 tonnes per hectare compared with 800 tonnes per hectare in cold regions.
- Soil type:
clay-rich soils have a higher carbon content than sandy soils. Clay protects carbon from decomposition.
