  We consider super-Earth sized planets which have a water mass fraction that
is large enough to form an external mantle composed of high pressure water ice
polymorphs and that 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 steady state the abundance of CO2 in the
atmosphere has two possible states. When the 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.
