Oceanic carbon-13 and nitrogen-15 isotopes simulated by CSIRO Mk3L-COAL version 1.0

The isotopes of carbon (delta13C) and nitrogen (delta15N) 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 delta13C and delta15N in the ocean component of an Earth system model.

We evaluate our simulated delta13C and delta15N against contemporary measurements, place the model's performance alongside other isotope enabled models, and document the response of delta13C and delta15N 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 x 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 delta13C and delta15N 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 delta13C and delta15N, 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 delta15N. Hence, there is significant scope for isotope enabled models to provide more robust interpretations of the proxy records.

These simulations were run by Dr Pearse J Buchanan of the ARC Centre of Excellence for Climate Systems Sciences, as part of his PhD in quantitative marine science.

A full list of the variables available is available in the readme file.

A full description of the CSIRO Mk3L v1.2 climate system model can be found in both:

    Phipps, S. J., et al. (2012) and Phipps, S. J., et al. (2011)

Chronological descriptions of the developing biogeochemical ocean model within the CSIRO Mk3L-COAL v1.0 Earth System Model

that was used can be found in:   Buchanan, P. J., et al. (2018),  Buchanan, P. J., et al. (2019) and Matear, R. J., and Lenton, A. (2014). 

These simulations were run by Dr Pearse J Buchanan of the ARC Centre of Excellence for Climate Systems Sciences, as part of his PhD in quantitative marine science.