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BCP diagram

Figure 1. The biological carbon pump. MORE
 

moorings and profiling floats

Figure 2. Automated chemical sensors. MORE
 

different types of sediment trap

Figure 3. Sampling the downward particle flux. MORE

The biological carbon pump

The 'biological carbon pump' (BCP) contributes to the ocean's role in taking up and storing carbon dioxide from the atmosphere. Without the BCP the atmospheric concentration of CO2 would be much higher.
carbon cycle

What is the biological carbon pump?

Just like plants on land, the microscopic marine phytoplankton take up carbon dioxide [CO2] and water [H2O] from their surrounding and use energy from sunlight to turn it into glucose [C6H12] and oxygen [O2]. The glucose powers the metabolism of the plankton cells, and can be turned into other organic compounds. If enough nutrients are available the plankton will grow and multiply.

Phytoplankton are the 'grass of the sea' - at the bottom of the marine food chain. Respiration by animals, bacteria and plants 'remineralises' the organic carbon - turning it back into carbon dioxide and water.

When plants and animals die their remains sink into deeper water as detritus and decompose, releasing carbon dioxide and nutrients back into the water. This is why nutrients such as nitrate are scarce in surface water, but found in much higher concentrations in the deep ocean.

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Detritus is the remains of dead plant and animals or faecal pellets colonised by bacteria. The bacterial 'feed' on the dead remains, and change the organic carbon back into carbon dioxide, water and mineral nutrients.

The transformation of carbon dioxide and nutrients into organic carbon, its sinking into the in the deep ocean, and its decomposition at depth, is known as the biological carbon pump. It contributes to the ocean's uptake and storage of carbon dioxide and keeps atmospheric CO2 about 200 ppm lower than it would be if the ocean were without life.

Remineralisation depth is key to the ocean's carbon uptake

Most of the organic carbon produced by phytoplankton is remineralised in the surface ocean. From there the CO2 that is released through respiration can easily return to the atmosphere. Only organic carbon that makes it into the deep ocean below the thermocline contributes to the biological carbon pump.

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The permanent thermocline is a layer 1000-1200m below the surface where the temperature decreases rapidly with depth. It acts as a barrier to vertical mixing, keeping the deep ocean isolated from the surface ocean and atmosphere. Carbon that makes it through the thermocline is effectively stored away from the atmosphere for hundreds of years.

marine snow

Marine snow MORE

The depth at which organic carbon is remineralised therefore determines how much is stored in the interior. Thus understanding what controls remineralisation depth is key to understanding how effective the biological carbon pump is at regulating atmospheric carbon dioxide.

Remineralisation depth may depend on temperature, ecosystem structure and oxygen. Understanding how these interact to affect the efficiency of the biological carbon pump requires accurate measurements of how nutrients and carbon vary with depth, location and time.

Such measurements are now feasible, thanks to the development of modern, automated observing systems that can measure oxygen and nutrients or sample the downward particle flux at different depths at much shorter time intervals than is possible with dedicated research cruises.

NOC rapid Southampton University Exeter University PML Met Office NOAA NERC
Diagram of the biological carbon pump

Figure 1. The biological carbon pump. Credit: NOC/V.Byfield. PDF available on request.

The biological carbon pump (BCP)

Just like plants on land, the microscopic marine phytoplankton take up carbon dioxide [CO2] and water [H2O]from their surrounding and use energy from sunlight to turn it into glucose [C6H12] and oxygen [O2].

The glucose powers the metabolism of the plankton cell, and can be turned into other organic compounds. If enough nutrients are available the plankton will grow and multiply. Phytoplankton is the 'grass' of the sea - at the bottom of the marine food chain. Eventually the plankton and the animals that feed on them die and sink into deeper water, where they decompose.

The production of organic carbon through photosynthesis, the sinking of organic matter and its subsequent decomposition in the deep ocean is known as the 'biological carbon pump'. It contributes to the ocean's uptake and storage of carbon dioxide, and keeps atmospheric CO2 about 200 ppm lower than it would be if the ocean were without life.

rapid mooring, argo float cycle and

Left: RAPID and Argo use instruments that record and store data automatically. Right: Biogeochemical data from a biological Argo float.

Chemical sensors on moorings and floats

There are no a wide range of sensors capable of recording physical and biogeochemical parameters when mounted on moorings such as the RAPID array or drifting floats such as those in the Argo programme.

The RAPID mooring array currently measures temperature, pressure and current speeds. However biogeochemcial sensors will be added to the array so that it can also collect time series of nitrate, oxygen, and other biogeochemcial parameters that can be used to give better estimates of the remineralisation depth at 26°N.

The international Argo programme has over 3000 drifting floats that record temperature and salinity profiles from the surface to 2000m. Floats that can dive to depths of 4-5000m are now being developed and tested. Several floats now also carry sensors that deliver profiles of nitrate and oxygen, as well as phytoplankton chlorophyll and light available for photosynthesis (PAR - photosynthetically available radiation).

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Different ways to sample the downward flux of organic particles. Credit: NOC/V.Byfield.

Sampling the downward particle flux

To understand the ocean's role in the global carbon cycle, it is important to quantify the downward flux of organic particles. Traditionally this has been done by taking water samples at different depths during oceanographic research cruises. The large water sampler and the rosette of bottles surrounding the CTD in the figure are examples of this.

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CTD is short for conductivity, temperature and depth. From conductivity we can calculate salinity. A CTD thus records salinity, temperature and depth as it is lowered through the water. The CTD frame may also hold instruments to measure other paratmeters such as oxygen. Sample bottles surrounding the CTD are triggered to shut and trap water samples from different depths.

In recent years autonomous sediment traps are increasingly used to give time series of measurements. These consist of a trap to collect the sediment particles, and transfer these to individual sample bottles at set time intervals. The sample bottles are treated to stop the organic carbon from decomposing once it has been trapped.

There are different traps designs. Surface waters contain higher concentrations of suspended particles, so traps with relatively small collection areas are used. The particle concentration decreases with depth; so funnels with a large collection area necessary to sample marine snow in the deep ocean.

Moored or anchored traps rely on collecting particles suspended in the water as the currents flow past the trap. Drifting sediment traps catch falling material instead of letting it sweep past in the current. They can be configured to be neutrally buoyant at a given depth. Floats that are moored or drift at several depths can give a vertical profile of the particle concentration with depths. From this we can calculate how much of the organic material is remineralised at different depths in the water.

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Marine snow

Marine snow is particles of detritus that sink slowly through the water, a little like snow-flakes in air. This rain of particles decreases with depth, as the organic carbon is remineralised by the bacteria that colonise the surface of the detritus flakes. Here the marine snow is caught in the headlights of a submersible.