2.2. Numerical Ocean Biogeochemistry Model

Alternative remineralization profiles are substituted into global ocean simulations of a coarse resolution (2.8°, 15 vertical levels) configuration of the Massachusetts Institute of Technology general circulation model, MITgcm (Marshall et al., 1997), coupled to an idealized marine biogeochemistry model that considers the coupled cycles of dissolved inorganic carbon, alkalinity, phosphate, dissolved organic phosphorus, oxygen, and dissolved iron (Dutkiewicz et al., 2006; Parekh et al., 2005, 2006).

Two‐thirds of surface net community production (that depends on light, phosphate, and iron using Michaelis‐Menten kinetics) is channeled into dissolved organic matter that is largely remineralized in the surface ocean with a timescale of 6 months (Yamanaka & Tajika, 1997), while one‐third is exported to the ocean interior via sinking particulate organic matter subject to depth‐dependent remineralization rates. Elemental biological transformations are related using fixed stoichiometric ratios RC:N:P:Fe:O2=117:16:1:4.68×10−4:−170 (Anderson & Sarmiento, 1994) with a prescribed inorganic to organic rain ratio of 7% (Yamanaka & Tajika, 1996). The total atmosphere‐ocean carbon inventory is conserved as there is no riverine carbon input or sediment carbon burial, which may impact the model's transient behavior and steady state (Roth et al., 2014). Atmosphere‐ocean exchange of CO₂ captures the magnitude and variation of observed air‐sea fluxes (Lauderdale et al., 2016).

Our model includes tracers to separate the in situ concentrations of carbon into (i) a component subducted from the surface layer and transported conservatively by ocean circulation (the “preformed” carbon concentration, C _pre), and (ii) a component that integrates export and remineralization of sinking particles as a water mass transits the ocean interior (the “biological” carbon concentration, C _bio), which encompasses both soft tissue regeneration and carbonate dissolution, and connects more directly to the biological pump (Ito & Follows, 2005; Volk & Hoffert, 1985). We integrate simulations for 10,000 years toward steady state in atmosphere‐ocean carbon partitioning.