The Abundance of Atmospheric CO₂ in Ocean Exoplanets: A Novel CO₂ Deposition Mechanism

We consider super-Earth sized planets which have a water mass fraction large enough to form an external mantle composed of high-pressure water-ice polymorphs and also lack a substantial H/He atmosphere. We consider such planets in their habitable zone, so that their outermost condensed mantle is a global, deep, liquid ocean. For these ocean planets, we investigate potential internal reservoirs of CO₂, the amount of CO₂ dissolved in the ocean for the various saturation conditions encountered, and the ocean-atmosphere exchange flux of CO₂. We find that, in a steady state, the abundance of CO₂ in the atmosphere has two possible states. When wind-driven circulation is the dominant CO₂ exchange mechanism, an atmosphere of tens of bars of CO₂ results, where the exact value depends on the subtropical ocean surface temperature and the deep ocean temperature. When sea-ice formation, acting on these planets as a CO₂ deposition mechanism, is the dominant exchange mechanism, an atmosphere of a few bars of CO₂ is established. The exact value depends on the subpolar surface temperature. Our results suggest the possibility of a negative feedback mechanism, unique to water planets, where a reduction in the subpolar temperature drives more CO₂ into the atmosphere to increase the greenhouse effect.