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map of anthropogenic carbon

Figure 1. Biogeochemical sensors on RAPID moorings.   MORE

BCP diagram

Figure 2. The biological carbon pump. MORE

Research Activities

ABC Fluxes is studying the flow of chemical tracers across 26.5°N:
- How do the fluxes of carbon dioxide, oxygen and nutrients vary with time?
- What causes this variability?
- How do the variations affect the ocean's capacity to take up and store carbon?

Our calculations are based on transport estimates from the RAPID array, which has been used to calculate the overturning circulation at 26.5°N since April 2004.[1] These measurements have four components (fig.1):


Basic transport estimates quantify the flow of water across the 26N latitude in the Atlantic. It is normally given in Sverdrup (Sv); 1 Sv = 106 m3 s-1 (1 million cubic metres per second). The water flow may be used to calculate the flow of anything transported around the globe by the ocean circulation; for example heat, salt, carbon, oxygen and nutrients.


Cunningham, S.A., T. Kanzow, D. Rayner, M.O. Baringer, W.E. Johns, J. Marotzke, H.R. Longworth, E.M. Grant, J. J.-M. Hirschi, L. M. Beal, C.S. Meinen, and H.L. Bryden (2007): Temporal Variability of the Atlantic Meridional Overturning Circulation at 26.5°N. Science, 317 (5840), 935-938. doi: 10.1126/science.1141304

  1. Florida Strait transport is measured using a submerged undersea telephone cable.[2]
  2. The Western Boundary Wedge (WBW) is measured using moorings immediately to the east of the Bahamas.
  3. Interior transport is calculated based on the density difference between end-point moorings.
  4. Ekman transport is estimated from satellite data and reanalyses of wind fields.

Baringer, M. O. and Larsen, J. C. (2001): Sixteen years of Florida Current Transport at 27° N. Geophysical Research Letters,, 28(16), 3179-3182. doi: 10.1029/2001GL013246

Ekman spiral

Ekman transport is the wind-driven portion of the circulation in the ocean surface layer. Due to the Coriolis effect surface currents flow at an angle of 45° to the wind at a speed of about about 2% of the wind speed. The current speed decreases with depth until there is no wind-driven flow, while the Coriolis effect continues to change the current direction. This is known as the Ekman spiral (see figure), and the layer of water from the surface to point of 0 flow is known as the Ekman layer.

If all flow over the Ekman layer is integrated, the net transporn is at 90° to the right of the surface wind in the northern hemisphere (left in the southern hemisphere).

Left: The Ekman spiral: (1) Wind, (2) force from above, (3) effective direction of the current flow, (4) Coriolis effect.
Credit: Timer. Source: Wikimedia Commons


Reanalysis is a systematic approach to produce data sets for climate monitoring and research, using numerical models that ingest (assimilate) all available observations every 6 to 12 hours over the period being analyzed. For atmospheric reanalyses, the observations may come from radiosondes, satellites, buoys, aircraft and ships. Typical output are time series of parameters such as air temperature, surface wind fields, atmospheric pressure etc. plotted on global or regional latitude/longitude grids.

Reanalysis wind fields give time series of the speed and direction of surface winds at different locations. They may, for example, be used to calculate the speed and direction of wind-driven surface currents.

The role of the subtropical gyre in carbon uptake

The North Atlantic Subtropical Gyre plays an important role in the uptake and storage of carbon from the atmosphere, mainly via the biological carbon pump (figure 2). Although biological production in the gyre is low, its large size means that the overall carbon uptake is significant. Three processes are important here:


Trichodesmium bloom MORE

  • Vertical transport of nutrients from depth to the sunlit zone.
  • Horizontal transport of nutrients and plankton by ocean currents.
  • Nitrogen fixation by phytoplankton such as Trichodesmium.

ABC has added biogeochemical sensors on the RAPID array moorings to get a better idea of how oxygen, nutrients and carbon dioxide vary - vertically and across the North Atlantic gyre at 26°N.

For more information about the sensors deployed on the array see New ABC observations

NOC rapid Southampton University Exeter University PML Met Office NOAA NERC
26N sensors

Figure 1. Biogeochemical sensors on the RAPID array at 26N.

Sub regions of the 26.5°N section across the Atlantic: Florida Strait, Western Boundary Wedge (WBW), Interior and Ekman (surface layer of both the Interior and the WBW). The black lines show the RAPID moorings.

Biogeochemical sensors deployed as part of the ABC Fluxes project are indicated on the moorings at WB1, WBH2, MAR1 and EB1.
Oxygen sensors will be on WB1 at 50m, 400m and 800m dept, at WBH2 at 1500m 2000m and 3500m, and at each of WB1, MAR1 and EB1 at 50m 400m 800m 1500m 2000m and 3500m.
Remote Access Samplers (RAS) will be deployed at WB1, MAR1 and EB1 at 50m and at WBH2 at 1500m.
pH and pCO2 sensor pairs will be deployed at 50m on WB1, MAR1 and EB1.

The plot shows anthropogenic carbon (Cant) in μmol kg-1, calculated using the delta C method with hydrographic data from the first RAPID cruise in 2004.


P.J. Brown, D.C.E. Bakker, U. Schuster, A.J. Watson (2010): Anthropogenic carbon accumulation in the subtropical North Atlantic. Journal of Geophysical Research - Oceans 115 (C4), C04016. doi: 10.1029/2008JC005043

Diagram of the biological carbon pump

Figure 2. 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 creating 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.

trichodesmium colony

Colony of the cynobacterium Trichodesmium

Nitrogen fixation

Nitrogen makes up 75% of the atmosphere and is an essential nutrient for plants, but most organisms cannot use its gaseous form (N2). Some marine blue-green algae (cyanobacteria) can absorb nitrogen from the atmosphere, turning it into ammonium (NH4) and nitrate (NO3) - forms readily available to plants.

Trichodesmium is one of the main nitrogen fixers. To fix the nitrogen, the bacteria use an enzyme called nitrogenase, which contains iron. The amount of iron available may be a limiting factor in the production of the enzyme. The major iron input to the tropical and subtropical Atlantic is dust blown from the Sahara Desert.

trichodesmium bloom

Bloom of Trichodesmium, also known as 'sea sawdust'.

Trichodesmium can form large blooms in nutrient-poor waters - usually when it has been calm for some time, and the surface temperature is over 20°C. The blooms release nitrogen and other nutrients, which then become available to other marine phytoplankton. As a result the blooms are important to the oceanic ecosystem and may contribute to an increase in overall plant productivity and carbon uptake.

Climate change is projected to increase stratification and reduce the depth of the wind-mixed surface layer. Both of these factors favour Trichodesmium blooms and may therefore increase their occurrence in the future.[1,2]


C.M. Moore, M.M. Mills, E.P. Achterberg, R.J. Geider, J. LaRoche, M.I. Lucas, E.L. McDonagh, Xi Pan, A.J. Poulton, M.J.A. Rijkenberg, D.J. Suggett, S.J. Ussher and E.M.S. Woodward (2009): Large-scale distribution of Atlantic nitrogen fixation controlled by iron availability. Nature Geoscience, 2, 867-871. doi:10.1038/ngeo667

B. Bergman, G. Sandh, S. Lin, H. Larsson and E.J. Carpenter (2013): Trichodesmium - a widespread marine cyanobacterium with unusual nitrogen fixation properties. FEMS Microbiology Reviews, 37, 1–17. doi:10.1111/j.1574-6976.2012.00352.x