MEPS 291:53-64 (2005)  -  doi:10.3354/meps291053

Magnitude of excess carbon sequestration into the deep ocean and the possible role of TEP

W. Koeve1,2,*

1Zentrum für marine Umweltwissenschaften (Marum), Fachbereich Geowissenschaften (FB5), Universität Bremen, Postfach 330440, 28334 Bremen, Germany
2Present address: Observatoire Midi Pyrénées (OMP), LEGOS, 14, av. Edouard-Belin, 31400 Toulouse, France

ABSTRACT: It has been suggested that TEP (transparent exopolymer particles) may provide a vehicle for non-Redfield excess carbon export into the deep sea. Here, the hypothesis that organic carbon produced in excess of the Redfield C:N ratio is sequestered in the deep ocean is tested by a model-based mass balance approach. The model looks for a value of the C:N ratio of export production which is consistent with current knowledge of the rapid change of organic matter fluxes over depth, the remineralisation C:N ratio in the interior of the ocean, and the observed C:N ratios of sinking particles in the deep sea and at the sea floor. It is estimated that the contribution of excess carbon export into the deep ocean is equivalent to 3 to 5.6% (medians, depending on model assumptions; overall range: –16 to 21%) of the conventional Redfield biological pump (C:N = 6.6). Elevated C:N ratios of sinking particles in the deep ocean of 9 to 23, and their increase with depth, can be explained by C:N ratios of export production being only slightly larger than the vertically integrated C:N ratio of remineralisation in the interior of the ocean. The basin scale effect of this preferential nitrogen remineralisation, within the seasonal thermocline, on carbon sequestration is 1 order of magnitude lower compared with Redfield equivalent remineralisation or CaCO3 sequestration. The often observed increase in the C:N ratio of sinking particles with depth does not require that the remineralisation C:N ratio increases with depth, but can also arise under conditions of constant C:N remineralisation ratios. It is concluded that only a small fraction of carbon overconsumption in the surface ocean is sequestered into the deep ocean. The majority appears to be remineralised in the upper twilight zone.

KEY WORDS: Carbon flux · Net community production · C:N ratio · Redfield · TEP

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