This research focuses on the role of the Southern Ocean (SO) in the global biogeochemical cycling of elements such as C, N, Si, P, Fe and in climate regulation, notably through its capacity to absorb atmospheric CO2, a major greenhouse gas. This research is carried out by BELCANTO (BELgian research on Carbon uptake in the ANTarctic Ocean) an existing interdisciplinary network of belgian biologists, geochemists, and physical and ecological modelers. The overall objective of the network is to construct and validate a realistic 3D ice-ocean biogeochemical model for the area south of 30°S. This requires a thorough understanding of the factors regulating ocean-atmosphere interactions, oceanic circulation and biogeochemical processes involving biogenic matter. The research methodology involve and combine (i) new process-level studies under laboratory-controlled conditions; (ii) field work in key SO areas with direct measurements of core biogeochemical parameters along with an original use of multi-proxies and rate measurements; and (iii) numerical development and experimentation. The biogeochemical model plays a central role as integrator of new knowledge synthesised from experimental studies. As starting point, we use the SWAMCO-4 model implemented in the 3D ice-ocean model ORCA-LIM. Two important weaknesses were identified from previous model runs: the crude representation of iron in the water and sea-ice and the parameterization of microbial processes in the mesopelagic layer. Process-level studies are therefore conducted under laboratory conditions to improve our understanding of iron cycling focusing on the interaction between organic matter and bioavailable iron. In order to improve the model parameterization, the organic matter degradation in the mesopelagic layer is investigated in the field, based on combined measurements of mesopelagic particulate Ba and bacterial mineralization rates (oxygen consumption) at different depths in the twilight zone and making use of new sampling devices (e.g. IRS sediment traps) to sort particles according to their settling velocity and/or size class. The network know-how will also be further developed through the acquisition of new skills in molecular biology (e.g. Fe-bioreporters), the determination of plankton-group specific biochemical compounds with their stable isotopic composition and the use of 30Si isotopes to estimate silica production- dissolution. Additional laboratory experiment are conducted to assess and parameterize the micro-organism response to future expectation of increased temperature and acidification. Upgraded SWAMCO model will be tested for its capacity to reproduce seasonal and interannual variability. For this purpose field data collected in the Australian sector during BELCANTO I and II are integrated in a joint data base and completed with data gained during this project. Of particular importance will be the assessment of the model capability to simulate atmospheric CO2 uptake, export production from the surface layer, bacterial degradation in the mesopelagic zone and the stoichiometric signature of exported matter and subducted water. Field measurements such as air-sea CO2 fluxes and multi-proxies developed by BELCANTO I and II (the proxy-tool box: 234Th-deficit; 15N-New production; meso-Baxs, d29Si) will be used to validate the upgraded SWAMCO model. Numerical experimentation with upgraded 3D ORCA-LIM SWAMCO will include better initial fields, performance assessment in contrasted regions and seasons where data are available, model scenarios for better understanding of the SO system behaviour (relative contribution of physical and biological processes) and predicting its future evolution. Finally, properly validated, the model will (i) reduce uncertainty linked to the assessment of the role of SO as source/sink of atmospheric CO2 and estimate the related impact on biogeochemical cycles and (ii) improve our capability to predict the SO response to future increase of atmospheric CO2 and global warming.