the ocean gets its co2 or carbon from fish or marine life as they breath by exchanging carbon with oxygen found in the oceans water , from the carbon stored in dead marine life as it decomposes and from dissolved rock and the carbon cycle between the oceans and the atmosphere etc...
the ocean will absorb co2 from the atmosphere when the
co2 pressure of the atmosphere is higher than the co2 pressure
in the ocean.
therefore , the cutting down of 1/2 the trees would lower the oxygen ppm in the atmosphere and increase the co2 ppm in the atmosphere which would increase the co2 pressure of the atmosphere and trigger co2 absorbtion into the oceans... but the oceans have a limit as to the amount of co2 that they can absorb.
so this is yet another reason other than the lower oxygen ppm
to not cut down the trees as a means of reducing co2 levels
in the atmosphere.
