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

Abstract

The stable carbon isotopic composition (δ13C) is an important variable to study the ocean carbon cycle across different timescales. We include a new representation of the stable carbon isotope 13C into the HAMburg Ocean Carbon Cycle model (HAMOCC), the ocean biogeochemical component of the Max Planck Institute Earth System Model (MPI-ESM). 13C is explicitly resolved for all oceanic carbon pools considered. We account for fractionation during air-sea gas exchange and for biological fractionation μp associated with photosynthetic carbon fixation during phytoplankton growth. We examine two μp parameterisations of different complexity: μpPopp varies with surface dissolved CO2 concentration , while μpLaws additionally depends on local phytoplankton growth rates . When compared to observations of δ13C of dissolved inorganic carbon (DIC), both parameterisations yield similar performance. However, with regard to δ13C in particulate organic carbon (POC) μpPopp shows a considerably improved performance compared to μpLaws. This is because μpLaws produces too strong a preference for 12C, resulting in δ13CPOC that is too low in our model. The model also well reproduces the global oceanic anthropogenic CO2 sink and the oceanic 13C Suess effect, i.e. the intrusion and distribution of the isotopically light anthropogenic CO2 in the ocean. The satisfactory model performance of the present-day oceanic δ13C distribution using μpPopp and of the anthropogenic CO2 uptake allows us to further investigate the potential sources of uncertainty of the approach for estimating the oceanic 13C Suess effect. derived the first global oceanic 13C 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 approach to the model data we are able to investigate in detail potential sources of underestimation of the 13C Suess effect. Based on our model we find underestimations of the 13C Suess effect at 200ĝ€¯m by 0.24ĝ€¯‰ in the Indian Ocean, 0.21ĝ€¯‰ in the North Pacific, 0.26ĝ€¯‰ in the South Pacific, 0.1ĝ€¯‰ in the North Atlantic and 0.14ĝ€¯‰ in the South Atlantic. We attribute the major sources of underestimation to two assumptions in the approach: the spatially uniform preformed component of δ13CDIC 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 δ13CPOC over the industrial period. The new 13C 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.