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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 multi-millennial timescales, running at a rate of ∼400 years per day. We show that this coarse resolution, computationally
efficient model adequately reproduces water column and coretop 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.
Created: 2019-02-01
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- DOI : 10.25914/5C6643F64446C