Continental and sea ice iron sources fertilize the Southern Ocean in synergy

International audienceIron release from melting continental and sea ice is deemed important for phytoplankton, the growth of which is iron-limited in the Southern Ocean. Both sources are generally considered separately, yet their effects on the biological carbon pump could interact. Using a global ocean-sea-ice-biogeochemical model with a representation of both continental and sea ice iron sources, we find them to have an overall additive effect on phytoplankton activity, increasing carbon export by +13.9% of the Southern Ocean total, with continental ice contributing +4.5% and sea ice +8.0%. The +1.4% residual is due to a coupled fertilization effect: When the iron source from continental ice is activated, iron in sea ice increases by 16%, so does iron transport toward low production areas. Overall, this increases phytoplankton activity: Fertilization is more efficient where sea ice melts than at locations of initial iron release by continental ice