Biological carbon pump

The biological carbon pump is a major term in the global carbon cycle, transferring approximately 5-15 GT C yr-1 from the surface ocean to the oceans interior (Henson et al., 2011). It is of comparable magnitude to the annual increase in CO2 in the atmosphere driven by anthropogenic remobilisation of fossil fuel reserves and without it we believe that atmospheric CO2 would be order 200ppm higher (Parekh et al., 2006). Small changes in its functioning and or strength could radically affect ocean atmosphere partitioning of CO2.

The biological carbon pump (BCP) consists of the photosynthetic fixation of carbon in the upper ocean followed by the downward flux of some of this material due to gravitational settling. Our recent compilation of thorium-derived estimates of carbon flux suggests that the major uncertainty in our estimate of globally integrated export flux is derived from the very high variability in e ratio at very low temperatures. We believe that this is related to the intense seasonality and weak seasonal cycle in surface temperature in high latitude systems which makes them a high priority area for future research into BCP strength.

The processes that cause this settling to occur are not well understood with excess density imparted through the ballasting by biominerals such as opal, calcite and celestite (Martin et al., 2011, Sanders et al., 2010) and the compaction and packaging of material by grazing being important factors. As the material sinks through the upper ocean the flux decreases and material is mineralised (Steinberg L+O). The rate at which this occurs is highly variable, with the shape of the mineralisation curve regulating at some level the timescale over which the CO2 is stored in the interior.

Important factors which regulate the shape of the mineralisation curve are though to include temperature, oxygen and ecosystem structure (John Dunne, Princeton). We are currently using the neutrally buoyant trap PELAGRA to estimate the shape of this curve in diverse oceanic provinces. We are also attempting to budget the processes responsible for consuming carbon in the interior following suggestions that we do not even understand this most basic aspect of the problem – in some locations integrated interior respiration calculated from various proxies exceeds the supply of organic carbon by significant margins.

Ultimately our work is focussed on improving the representation of the BCP in numerical models, to this end we are compiling databases of BCP parameters and formulating new algorithms of mineralisation and export for inclusion in the latest generation of numerical models developed by our colleagues in the numerical modelling group.

The Biological Carbon Pump, illustrating the movement of carbon in and around the ocean system. Image adapted from US JGOFS.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

References

  • Parekh, P., S. Dutkiewicz, M. J. Follows, and T. Ito (2006), Atmospheric carbon dioxide in a less dusty world, Geophysical Research Letters, 33(3).
  • Henson, S. A., R. Sanders, E. Madsen, P. J. Morris, F. Le Moigne, and G. D. Quartly (2011), A reduced estimate of the strength of the ocean's biological carbon pump, Geophysical Research Letters, 38.


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