A Next Generation Ocean Carbon Isotope Model For Climate Studies I: Steady State Controls On Ocean C-13

The C-13/C-12 of dissolved inorganic carbon (delta C-13(DIC)) carries valuable information on ocean biological C-cycling, air-sea CO2 exchange, and circulation. Paleo-reconstructions of oceanic C-13 from sediment cores provide key insights into past as changes in these three drivers. As a step toward full inclusion of C-13 in the next generation of Earth system models, we implemented C-13-cycling in a 1 degrees lateral resolution ocean-ice-biogeochemistry Geophysical Fluid Dynamics Laboratory (GFDL) model driven by Common Ocean Reference Experiment perpetual year forcing. The model improved the mean of modern delta C-13(DIC) over coarser resolution GFDL-model implementations, capturing the Southern Ocean decline in surface delta C-13(DIC) that propagates to the deep sea via deep water formation. Controls on delta C-13(DIC) of the deep-sea are quantified using both observations and model output. The biological control is estimated from the relationship between deep-sea Pacific delta C-13(DIC) and phosphate (PO4). The delta C-13(DIC):PO4 slope from observations is revised to a value of 1.01 +/- 0.02 parts per thousand (mu mol kg(-1))(-1), consistent with a carbon to phosphate ratio of organic matter (C:P-org) of 124 +/- 10. Model output yields a lower delta C-13(DIC):PO4 than observed due to too low C:P-org. The ocean circulation impacts deep modern delta C-13(DIC) in two ways, via the relative proportion of Southern Ocean and North Atlantic deep water masses, and via the preindustrial delta C-13(DIC) of these water mass endmembers. The delta C-13(DIC) of the endmembers ventilating the deep sea are shown to be highly sensitive to the wind speed dependence of air-sea CO2 gas exchange. Reducing the coefficient for air-sea gas exchange following OMIP-CMIP6 protocols improves significantly surface delta C-13(DIC) relative to previous gas exchange parameterizations.