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Here we constrain the speciation of carbon that may outgas in ocean exoplanets. Ocean exoplanets likely have at least a few percent by mass of water, which is sufficient to build a high-pressure ice layer between a rocky interior and the outer hydrosphere. We study the possible formation of a filled ice in the ternary system H _2 O–CO _2 –CH _4 . The incorporation of CH _4 and CO _2 in filled ice would be an important mechanism for transporting carbon across a high-pressure ice mantle into the atmosphere. The CH _4 –CO _2 pair is also important as a potential biosignature. We find that a filled ice in the system H _2 O–CO _2 –CH _4 is possible though enriched in CH _4 . CO _2 cannot account for more than 15% by mole of the carbon content of the filled ice. Such a filled ice is less dense than an overlying ocean and would therefore discharge into the ocean, depressurize, and outgas its carbon content into the atmosphere. A high-pressure, water-rich mantle in ocean worlds may therefore support the transport of carbon from the interior into the atmosphere. More than 75% by mole of this carbon would be reduced. As long as CH _4 exists/is produced in the interior and the ice mantle convects, thus transporting chemical species outward, a flux of carbon enriched in CH _4 would outgas. If this persists over geological time it would negate atmospheric sinks for CH _4 , and explain low concentrations of atmospheric CO _2 . If the contrary is correct than the interior of the planet may be oxidizing.