Yes, most people assume that wood and plant matter mostly comes from the Earth, and this is of course FALSE. The main component of plant matter is CARBON and comes mostly from the CARBON dioxide in the Atmosphere. This is why plants (in the oceans firstly as you pointed out and in forests secondly) PRODUCE the oxygen we breathe and are considered "the lungs of the Earth". CO2 minus the O2 that goes into the atmosphere gets you, obviously, pure carbon.

Oh that evil CO2 that the Earth after billions of years hasn't learned to manage herself! Let's intervene quickly before we suffocate!!!