Seasonal variability of mesopelagic organic carbon remineralisation in the naturally iron-fertilized Kerguelen area (Southern Ocean)

We report on the distribution of excess, biogenic particulate barium-barite, a proxy for mesopelagic zone remineralization of organic matter, in the naturally iron-fertilized Kerguelen area which was visited during an early and late phase of the growth season (Oct.-Nov. 2011, KEOPS 2 and Fe.-Mar. 2005, KEOPS 1). An important question relates to the fate of the surface exported organic carbon: how much carbon escapes remineralization at mesopelagic depths to sink to deeper layers where longer term sequestration is likely. Results for mesopelagic organic carbon remineralization are compared with primary production regime and export production in order to estimate mesopelagic carbon transfer efficiency. Variability of transfer efficiency at different stages of the productive season and possible controls, including effects of iron-availability and community structure, are discussed and compared with earlier results under artificially induced (EIFEX 2004 cruise) and natural Fe-replete (SAZ-Sense 2007 cruise; mid-austral summer) conditions.