<P> Oceanic absorption of CO is one of the most important forms of carbon sequestering limiting the human - caused rise of carbon dioxide in the atmosphere . However, this process is limited by a number of factors . CO absorption makes water more acidic, which affects ocean biosystems . The projected rate of increasing oceanic acidity could slow the biological precipitation of calcium carbonates, thus decreasing the ocean's capacity to absorb carbon dioxide . </P> <P> The geologic component of the carbon cycle operates slowly in comparison to the other parts of the global carbon cycle . It is one of the most important determinants of the amount of carbon in the atmosphere, and thus of global temperatures . </P> <P> Most of the earth's carbon is stored inertly in the earth's lithosphere . Much of the carbon stored in the earth's mantle was stored there when the earth formed . Some of it was deposited in the form of organic carbon from the biosphere . Of the carbon stored in the geosphere, about 80% is limestone and its derivatives, which form from the sedimentation of calcium carbonate stored in the shells of marine organisms . The remaining 20% is stored as kerogens formed through the sedimentation and burial of terrestrial organisms under high heat and pressure . Organic carbon stored in the geosphere can remain there for millions of years . </P> <P> Carbon can leave the geosphere in several ways . Carbon dioxide is released during the metamorphosis of carbonate rocks when they are subducted into the earth's mantle . This carbon dioxide can be released into the atmosphere and ocean through volcanoes and hotspots . It can also be removed by humans through the direct extraction of kerogens in the form of fossil fuels . After extraction, fossil fuels are burned to release energy, thus emitting the carbon they store into the atmosphere . </P>

Which of the following is the largest reservoir in the carbon cycle