Wednesday 24 August 2005

PB2
1550-1710 hours

320
Plankton mineralisation in the Atlantic Ocean
Sanders, Richard¹, Poulton, Alex¹, Holligan, Patrick¹, Adey, Tim¹, Stinchcombe, Mark¹, Brown, Louise¹, Chamberlain, Katie²
1 National Oceanography Centre, Southampton, UK
2 Plymouth Marine Laboratory, Plymouth, UK
Author email: rics@noc.soton.ac.uk
The flux of organic carbon from the seasurface to the deep ocean 'the biological carbon pump' is mediated principally by sinking particles with an organic component. Recent work suggests that organic matter most efficiently reaches the seabed when associated with the relatively dense biogenic minerals opal and calcite which may act as 'ballast'. However the mechanisms underpinning these correlations are unclear with some authors suggesting that sinking organic matter scavenges non sinking mineral phases as it falls. We have made the first simultaneous measurements of photosynthesis and of calcite and silicate formation in the open ocean along a transect from 50N to 50S in the Atlantic. Bulk estimates of mineralisation coupled to organic carbon/ mineralisation ratios suggest that mineralising phytoplankton are responsible for 15% of total carbon fixation in the Atlantic with calcifiers being 2-3 times more significant than opal producers in this process. Thus one quarter of the organic carbon produced is pre-packaged with the ballast material which may be responsible for efficiently transferring organic carbon to the sea floor. However this association is not enough to ensure efficient export of carbon fixed by mineralising plankton as the mineral phases are massively decoupled from organic carbon during sinking with semi quantitative preservation of the minerals and in excess of 90% dissolution of organic carbon above 2000m. In summary the magnitude of organic carbon fixation by mineralising marine phytoplankton is such that an insignificant fraction of this organic carbon is quantitatively transported to the abyssal ocean. Further the semiquantitative transfer of minerals and the almost total mineralisation of organic carbon in the upper 2000m suggests that sinking mineral phases drive carbon export and not the reverse.

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