Marine micro-algae bind carbon dioxide, CO₂. During their decay, snowflake-like aggregates form that sink, remineralize and transport organically bound CO₂ to depth; this is referred to as the biological carbon pump. In our model study, we elucidate how variable aggregate composition impacts the global pattern of vertical carbon fluxes. Our mechanistic model approach advances the representation of the global biological carbon pump and promotes a more realistic projection under climate change.