A Next Generation Ocean Carbon Isotope Model for Climate Studies I: Steady State Controls on Ocean 13 C

The 13 C/ 12 C of dissolved inorganic carbon ( δ 13 C DIC ) carries valuable information on ocean biological C‐cycling, air‐sea CO 2 exchange, and circulation. Paleo‐reconstructions of oceanic 13 C from sediment cores provide key insights into past as changes in these three drivers. As a step toward full inclusion of 13 C in the next generation of Earth system models, we implemented 13 C‐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 δ 13 C DIC over coarser resolution GFDL‐model implementations, capturing the Southern Ocean decline in surface δ 13 C DIC that propagates to the deep sea via deep water formation. Controls on δ 13 C 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 δ 13 C DIC and phosphate (PO 4 ). The δ 13 C DIC :PO 4 slope from observations is revised to a value of 1.01 ± 0.02‰ ( μ 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 δ 13 C DIC :PO 4 than observed due to too low C:P org . The ocean circulation impacts deep modern δ 13 C DIC in two ways, via the relative proportion of Southern Ocean and North Atlantic deep water masses, and via the preindustrial δ 13 C DIC of these water mass endmembers. The δ 13 C DIC of the endmembers ventilating the deep sea are shown to be highly sensitive to the wind speed dependence of air‐sea CO 2 gas exchange. Reducing the coefficient for air‐sea gas exchange following OMIP‐CMIP6 protocols improves significantly surface δ 13 C DIC relative to previous gas exchange parameterizations.