The oceans are able to buffer rising CO2 levels both physically and biologically.

But it's unlikely the saturation point can be reached easily. It's not really a closed system. Remember that the White Cliffs of Dover, the marble backbone of Italy and the amazingly vast limestone stores of the American midwest consist of CaCO3 that was once atmospheric CO2 fixed by plankton and essentially removed from the system.

Considering that the atmospheric CO2 level during the Carboniferous Period was in excess of 1500ppm, that the planet didn't evaporate, that Life not only didn't die off, but indeed flourished, suggests that the saturation point is not easily, and not likely to be reached