AbstractThe 13C/12C of dissolved inorganic carbon (δ13CDIC) carries valuable information on ocean biological C‐cycling, air‐sea CO2 exchange, and circulation. Paleo‐reconstructions of oceanic 13C from sediment cores provide key insights into past as changes in these three drivers. As a step toward full inclusion of 13C in the next generation of Earth system models, we implemented 13C‐cycling in a 1° 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 δ13CDIC over coarser resolution GFDL‐model implementations, capturing the Southern Ocean decline in surface δ13CDIC that propagates to the deep sea via deep water formation. Controls on δ13CDIC of the deep‐sea are quantified using both observations and model output. The biological control is estimated from the relationship between deep‐sea Pacific δ13CDIC and phosphate (PO4). The δ13CDIC:PO4 slope from observations is revised to a value of 1.01 ± 0.02‰ (μmol kg−1)−1, consistent with a carbon to phosphate ratio of organic matter (C:Porg) of 124 ± 10. Model output yields a lower δ13CDIC:PO4 than observed due to too low C:Porg. The ocean circulation impacts deep modern δ13CDIC in two ways, via the relative proportion of Southern Ocean and North Atlantic deep water masses, and via the preindustrial δ13CDIC of these water mass endmembers. The δ13CDIC 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 δ13CDIC relative to previous gas exchange parameterizations.