The OISO cruises, initiated in 1998, aim at monitoring and understanding the evolution of CO2 and pH in the South West Indian Ocean and the corresponding Southern Ocean sector. The observations collected onboard the Marion Dufresne once or twice a year are crucial to :

1. contribute to the annual update of the global carbon budget by regularly feeding the international databases, notably SOCAT and GLODAP ;
2. better understand the variability of oceanic CO2 and pH in the South Indian and Southern Oceans in order to evaluate the anthropogenic perturbation, and thus reduce the uncertainty on the evolution of the oceanic sink of CO2 and ocean acidification in the future (validation of model simulations).
3. collect in situ observations that allow the validation of autonomous observations (satellite data, Argo and BGC Argo floats, biologer) ;
4. document environnemental changes in relation to climate variability and evaluate the impact on marine organisms (coll., BIAF, MNHN, CEBC).

The objective of the OISO-32 cruise was to re-occupy the transects and stations between Reunion Island and the French Subantarctic Islands (Crozet, Kerguelen and Amsterdam) and in the Antarctic zone (down to 56.5°S) that were frequently sampled over the last 3 decades (during the INDIGO, MINERVE, OISO, KEOPS, SOCLIM cruises). Underway surface measurements were conducted during the entire cruise using the ship pumping system, and a rosette-CTD was deployed at 17 fixed stations located in the different biogeochemical zones.

Another objective of the OISO-32 cruise was to evaluate the impact of Patagonian dust on the biological pump of CO₂ in the future Southern Ocean (SO-dust project, M. Boyé). Indeed, in this region phytoplankton growth is limited by the very low iron inputs to the surface waters, and Patagonian dust is one of the major source of iron available for phytoplankton. Projections show an intensification of these depositions in the future in the Polar Front zone due to enhanced winds and an increase in the number and magnitude of storms, which could modify the global climate by stimulating the biological pump of CO₂ through delivering of iron (and other limiting micro-nutriments), as it was suggested for the Last Glacial Maximum. Projections also show ocean acidification and warming, but the cumulative impact of these anthropogenic changes on phytoplankton and the biological pump is not yet known. The SO-dust project aims at diagnosing the net effect of these multi-faceted changes on phytoplankton communities, by evaluating their individual and interactive effects. To do so, deck incubation of phytoplankton communities were performed following a reduced factorial experimental strategy (4 scenarios : actual conditions, future conditions and 2 intermediate conditions). A large volume of water (100 L) was also collected a the southernmost station (O11) to be used for monospecific incubations that will be performed at the laboratory after the cruise.