- (C) The primary source of CO 2 is outgassing from Earth’s interior at mid-
ocean ridges, hot-spot volcanoes, and volcanic arcs. Much of the CO 2
released at subduction zones is derived from the metamorphism of carbonate
rocks subducting with the ocean crust. Much of the overall outgassing of
CO 2 , especially at mid-ocean ridges and hot-spot volcanoes, was stored in
the mantle when Earth formed. Some of the outgassed carbon remains as
CO 2 in the atmosphere, some is dissolved in the oceans, some carbon is held
as biomass in living or dead and decaying organisms, and some is bound in
carbonate rocks.
