Incorporating the stable carbon isotope 13C in the ocean biogeochemical component of the Max Planck Institute Earth System Model

The stable carbon isotopic composition (δ13 C) is an important variable to study the ocean carbon cycle across different timescales. We include a new representation of the stable carbon isotope 13 C into the HAMburg Ocean Carbon Cycle model (HAMOCC), the ocean biogeochemical component of the Max Planck Institute Earth System Model (MPI-ESM). 13 C is explicitly resolved for all oceanic carbon pools considered. We account for fractionation during air–sea gas exchange and for biological fractionation ϵp associated with photosynthetic carbon fixation during phytoplankton growth. We examine two ϵp parameterisations of different complexity: ϵ p Popp varies with surface dissolved CO 2 concentration ( Popp et al. , 1989 ) , while ϵ p Laws additionally depends on local phytoplankton growth rates ( Laws et al. , 1995 ) . When compared to observations of δ13 C of dissolved inorganic carbon (DIC), both parameterisations yield similar performance. However, with regard to δ13 C in particulate organic carbon (POC) ϵ p Popp shows a considerably improved performance compared to ϵ p Laws . This is because ϵ p Laws produces too strong a preference for 12 C, resulting in δ13 C POC that is too low in our model. The model also well reproduces the global oceanic anthropogenic CO 2 sink and the oceanic 13 C Suess effect, i.e. the intrusion and distribution of the isotopically light anthropogenic CO 2 in the ocean. The satisfactory model performance of the present-day oceanic δ13 C distribution using ϵ p Popp and of the anthropogenic CO 2 uptake allows us to further investigate the potential sources of uncertainty of the Eide et al. ( 2017 a ) approach for estimating the oceanic 13 C Suess effect. Eide et al. ( 2017 a ) derived the first global oceanic 13 C Suess effect estimate based on observations. They have noted a potential underestimation, but their approach does not provide any insight about the cause. By applying the Eide et al. ( 2017 a ) approach to the model data we are able to investigate in detail potential sources of underestimation of the 13 C Suess effect. Based on our model we find underestimations of the 13 C Suess effect at 200 m by 0.24 ‰ in the Indian Ocean, 0.21 ‰ in the North Pacific, 0.26 ‰ in the South Pacific, 0.1 ‰ in the North Atlantic and 0.14 ‰ in the South Atlantic. We attribute the major sources of underestimation to two assumptions in the Eide et al. ( 2017 a ) approach: the spatially uniform preformed component of δ13 C DIC in year 1940 and the neglect of processes that are not directly linked to the oceanic uptake and transport of chlorofluorocarbon-12 (CFC-12) such as the decrease in δ13 C POC over the industrial period. The new 13 C module in the ocean biogeochemical component of MPI-ESM shows satisfying performance. It is a useful tool to study the ocean carbon sink under the anthropogenic influences, and it will be applied to investigating variations of ocean carbon cycle in the past.