Stable carbon isotopes have now been implemented in REcoM (Hauck et al., 2016), the marine ecosystem and biogeochemistry model applied at AWI. In an ocean-only setup imple- mented in the MITgcm 3D-OGCM we here show how changing boundary conditions influence the simulated δ13C fields. Different to most other marine biogeochemistry models, RECOM does not rely on fixed stoichiometric ratios of phytoplankton organic matter. Instead, the composition of phytoplankton organic matter is calculated as a response to light, tempera- ture and nutrient supply, which allows for assessing potential stoichiometric shifts between the past and present. We study different parametrisations of biogenic carbon-isotopic fractiona- tion of marine phytoplankton during photosynthesis (Laws versus Rau) and their influence on model–data comparisons for the Last Glacial Maximum and the Holocene. Furthermore, we perform simulations, in which the climatic boundary and initial condi- tions (SST, wind, precipitation, runoff, salinity) and / or the dust fluxes are prescribed for preindustrial or LGM conditions based on previous studies (Zhang et al., 2014; Völker & Köh- ler, 2013; Albani et al., 2016). This gives us four simulations, from which we will analyse how especially dust via iron fertilization of the marine biology versus mainly physical (ocean overturning) changes will influence simulated δ13C fields. In doing so we will quantify how relevant the silicic acid leakage hypothesis and ocean overturning changes are for glacial δ13C.