<P> Oceans contain the greatest quantity of actively cycled carbon in this world and are second only to the lithosphere in the amount of carbon they store . The oceans' surface layer holds large amounts of dissolved inorganic carbon that is exchanged rapidly with the atmosphere . The deep layer's concentration of dissolved inorganic carbon (DIC) is about 15% higher than that of the surface layer . DIC is stored in the deep layer for much longer periods of time . Thermohaline circulation exchanges carbon between these two layers . </P> <P> Carbon enters the ocean mainly through the dissolution of atmospheric carbon dioxide, which is converted into carbonate . It can also enter the oceans through rivers as dissolved organic carbon . It is converted by organisms into organic carbon through photosynthesis and can either be exchanged throughout the food chain or precipitated into the ocean's deeper, more carbon rich layers as dead soft tissue or in shells as calcium carbonate . It circulates in this layer for long periods of time before either being deposited as sediment or, eventually, returned to the surface waters through thermohaline circulation . </P> <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 . Because the rate of CO dissolution in the ocean is dependent on the weathering of rocks and this process takes place slower than current rates of human greenhouse gas emissions, ocean CO uptake will decrease in the future . CO absorption also 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>

Where is the store of oxygen in the carbon cycle