We explore the impact of glacial–interglacial climate variations on the marine carbon-isotope record by means of the sophisticated marine biogeochemistry model RECOM. Different to most other marine carbon cycle models, RECOM does not rely on fixed Redfield ratios for organic soft tissue. Instead, the ratios of C:N and C:Chl in phytoplankton are calculated as a response to light, temperature and nutrient supply, which allows for assessing potential shifts in marine autotroph stochiometry. Our sensitivity studies feature a recently developed model version equipped with carbon isotopes (RECOM-ciso) forced with model output from fully coupled climate simulations.