Out in the open ocean, the store of living carbon as plants is tiny—less than in a desert on land. The floating cells of phytoplankton have lifetimes of only a few days before they sink and die or are eaten, so biomass cannot build up near the top of the ocean. The material that rains down from the surface into the deep ocean slowly rots and disperses into the water as it sinks, in a journey that may take a month. Often it clumps together as it sinks into what is aptly named "marine snow''. What reaches the sea floor thousands of meters below tends to be the most inert, indigestible material that bacteria and animals find difficulty making use of. It forms a loose gelatinous material that coats the sediment surface. No-one is quite sure how much carbon is held in the oceans as this fluff on the ocean floor, or in the form of organic molecules dissolved in the seawater, but it might rival the amount stored in soils on land.
There is another much vaster store of carbon in the ocean water, which forms an integral part of the carbon cycle. This is inorganic carbon in the form of C02 dissolved in the water. Rather than just existing like most gases would in the form of molecules floating around in solution, C02 actually chemically reacts with water to form an acid, known as carbonic acid, with the chemical formula H2C03.
It forms by this reaction:
C02 also reacts with carbonates to form bicarbonate, dissolved as ions in the ocean water. So, for example, if C02 reacts with calcium carbonate, an insoluble substance on the sea floor:
C02 + H20 + CaC03(solid) ^ Ca(HC03)2(dissolved) ^ Ca
(Note that in both cases the arrows point two ways, because the reaction is easily reversible. Both carbonic acid and bicarbonate can easily break down to yield C02 again if conditions shift.)
0ceanographers have chosen to call the dissolved bicarbonate and carbonate forms of carbon "alkalinity", although the term does not have much to do with pH and almost seems designed to confuse any newcomer to the subject! The oceans essentially control the C02 level of the atmosphere by storing most of the world's C02 in the form of this dissolved alkalinity. If the amount of C02 in the atmosphere suddenly goes up, the oceans will gradually dissolve most of it when it reacts with carbonate in the ocean and on the sea floor, forming bicarbonate so that only about one-eighth of the original amount is left in the atmosphere, like the end of an iceberg poking above the waterline when most of it is below. If, on the other hand, the C02 level in the atmosphere decreases, bicarbonate and carbonic acid break up to yield C02 and the oceans release carbon, pushing the atmosphere's C02 content back up again (Figure 7.2a, b). So, the oceans with their huge capacity to store and release
H2C03 C02 CO,
Figure 7.2. A huge amount of C02 is stored in the form of both bicarbonate and dissolved C02 in the ocean. (a) If the C02 concentration in the atmosphere becomes low, C02 will leave these reservoirs to top up the atmosphere, often depositing calcium carbonate on the sea floor as the bicarbonate breaks up to yield C02. (b) If C02 concentration in the atmosphere increases, this ocean reservoir will tend to soak up more C02 until most of it has been taken out of the atmosphere—often dissolving more carbonate from the sea bed to make the bicarbonate.
carbon act as a very effective buffer against any changes in C02 caused by living organisms, by volcanoes or by anything else. When land plants act to alter the C02 level in the atmosphere, they are always working against this massive buffer which rather limits how much they can change the composition of the atmosphere on the timescale of thousands of years. An increase or decrease in carbon storage in vegetation or soils may produce temporary changes lasting a few decades, but those changes will tend to be evened out by oceans taking up or releasing carbon over centuries and millennia. 0nly if the land plants work relentlessly over millions of years will they finally be able to overcome the effect of this big ocean reservoir and cause major changes to the C02 content of the atmosphere.
Was this article helpful?