Abstract Offshore cultivated seaweed (or macroalgae) used as feedstock for biobased products is a rapidly developing research field for an innovative new industry. A model system including seaweed cultivation, biorefining and usage phases of the products is assessed on the basis of real experimental studies. The aim is to provide a dynamic model of the biogenic carbon cycle with a view to carbon neutrality of future macroalgae-based biorefinery systems. The model takes a holistic view of the system, including all processes directly and indirectly connected to the biorefinery in a cradle to cradle perspective. In the biorefinery, the biomass is converted to ethanol and the solid protein residue is isolated and used as an ingredient for fish feed. The aqueous extract enriched in minerals and organic nutrients is used as liquid fertilizer. Annual cultivation and processing of 1 ton of seaweed (dry weight) evaluated over a time horizon of 100 years results in a net reduction of 9.3 tons of atmospheric carbon (34 ton CO2). From one cultivation cycle, i.e. 1 ton of seaweed (dry weight), a net reduction of 0.035 tons of atmospheric carbon (0.13 tons of CO2), assessed 100 years later, is achieved. The main processes providing climate mitigation are carbon assimilation by growing seaweed and carbon retention in soil. The model can be used to more accurately quantify climate services provided by green industries, thus strengthening Life Cycle Assessment as a decision-support tool for sustainable management of offshore cultivated seaweed. The model is flexible since it can be adapted to different international case studies by entering local parameter values.