Ocean carbon and nitrogen isotopes in CSIRO Mk3L-COAL version 1.0: a tool for palaeoceanographic research

The isotopes of carbon (δ¹³C) and nitrogen (δ¹⁵N) are commonly used proxies for understanding the ocean. When used in tandem, they provide powerful insight into physical and biogeochemical processes. Here, we detail the implementation of δ¹³C and δ¹⁵N in the ocean component of an Earth system model. We evaluate our simulated δ¹³C and δ¹⁵N against contemporary measurements, place the model's performance alongside other isotope-enabled models and document the response of δ¹³C and δ¹⁵N to changes in ecosystem functioning. The model combines the Commonwealth Scientific and Industrial Research Organisation Mark 3L (CSIRO Mk3L) climate system model with the Carbon of the Ocean, Atmosphere and Land (COAL) biogeochemical model. The oceanic component of CSIRO Mk3L-COAL has a resolution of 1.6^∘ latitude × 2.8^∘ longitude and resolves multimillennial timescales, running at a rate of ∼400 years per day. We show that this coarse-resolution, computationally efficient model adequately reproduces water column and core-top δ¹³C and δ¹⁵N measurements, making it a useful tool for palaeoceanographic research. Changes to ecosystem function involve varying phytoplankton stoichiometry, varying CaCO₃ production based on calcite saturation state and varying N₂ fixation via iron limitation. We find that large changes in CaCO₃ production have little effect on δ¹³C and δ¹⁵N, while changes in N₂ fixation and phytoplankton stoichiometry have substantial and complex effects. Interpretations of palaeoceanographic records are therefore open to multiple lines of interpretation where multiple processes imprint on the isotopic signature, such as in the tropics, where denitrification, N₂ fixation and nutrient utilisation influence δ¹⁵N. Hence, there is significant scope for isotope-enabled models to provide more robust interpretations of the proxy records.