CROZEX

   
Why Southern Ocean?
 

2. Scientific rationale - Southern Ocean

Measurements of new production in the Southern Ocean allow us to estimate the magnitude of biological sequestration of CO2 into the deep ocean, via the sinking of particulate material (the biological pump). Surface water macro nutrient (N, P and Si) concentrations are high over much of the southern oceans, yet chlorophyll levels are low, probably due to coupled iron (Boyd et al., 2000) and light (Nelson and Smith, 1991) limitation (Sunda and Huntsman, 1997). However in some regions (around islands, over shallow topography, at fronts and in coastal regions) it appears that chlorophyll and primary productivity levels are high (de Baar et al., 1997, Blain et al., 2000, Whitehouse et al., 1996). It is likely that this is due to iron fertilization. Direct observations of enhanced iron concentrations have been made around Kerguelen (Blain et al., 2000), and the nitrogen assimilation characteristics of phytoplankton around South Georgia (Sanders et al., in prep) are consistent with high water column iron levels. Thus determining and explaining the spatial distribution of iron around Crozet is a key objective of CROZEX.

These high productivity regions may be disproportionately important for carbon export compared to low productivity open water areas for two reasons. Firstly they support high biomass levels. Secondly export rates from these blooms, which are often dominated by large diatoms (de Baar et al., 1997, Whitehouse et al., 1996), are potentially high. This latter suggestion is uncertain as these large diatoms may sink as empty frustules and thus not contribute to carbon export (Pondaven et al., 2000, Nelson et al., 2002)). Further, SOIREE demonstrated both increased productivity and a decrease in surface water fCO2 as a result of iron enrichment, but observed little export (Boyd et al., 2000). This, together with the persistence of the SOIREE bloom (Abraham et al., 2000), provides strong indirect evidence for efficient Fe (and therefore C?) recycling within the mixed layer rather than export following a Southern Ocean iron fertilized bloom. SOIREE however did not continue for long enough to confirm this hypothesis. More recent experiments (EISENEX, EIFEX) have seeded eddies and remained long enough to observe export. The length of the CROZEX cruise is dictated by the importance of catching the export phase and attempting to estimate the export flux in as many ways as possible. This information is essential if are to predict the likely impact on carbon export of sustained large-scale iron fertilization of the Southern Oceans, whether natural (possibly during the Last Glacial Maximum) or anthropogenic (deliberate injection).


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