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

摘要

The isotopes of carbon ( δsup13/supC ) and nitrogen ( δsup15/supN ) 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 δsup13/supC and δsup15/supN in the ocean component of an Earth system model. We evaluate our simulated δsup13/supC and δsup15/supN against contemporary measurements, place the model's performance alongside other isotope-enabled models and document the response of δsup13/supC and δsup15/supN 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 sup°/sup latitude × 2.8 sup°/sup 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 δsup13/supC and δsup15/supN measurements, making it a useful tool for palaeoceanographic research. Changes to ecosystem function involve varying phytoplankton stoichiometry, varying CaCOsub3/sub production based on calcite saturation state and varying Nsub2/sub fixation via iron limitation. We find that large changes in CaCOsub3/sub production have little effect on δsup13/supC and δsup15/supN , while changes in Nsub2/sub 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, Nsub2/sub fixation and nutrient utilisation influence δsup15/supN . Hence, there is significant scope for isotope-enabled models to provide more robust interpretations of the proxy records.