Oceanic carbon cycle

Sea surface dissolved inorganic carbon concentration in the 1990s (from the GLODAP climatology)

The oceans contain around 36,000 gigatons of carbon, mostly in the form of bicarbonate ion (over 90%, with most of the remainder being carbonate).

Movement of carbon in the ocean

The oceanic biospheric carbon cycle
A schematic diagram of the carbon cycle in the ocean.[7]

CO2 is absorbed from the atmosphere at the ocean's surface and converted into dissolved inorganic carbon (DIC). It is then converted in gross primary production (GPP) by phytoplankton into organic carbon. About half of the GPP is autotrophically respirated and converted back into DIC. The rest stays in the form of net primary production (NPP). Some of the organic carbon sinks into the lower ocean levels as detritus or calcium carbonate in shells. Some soft tissue is converted into particulate organic carbon or dissolved organic carbon and, from these forms, into dissolved inorganic carbon. The rest sinks to the ocean floor. Shells out of calcium carbonate are also deposited on the ocean floor as sediment, whereas the carbon can dissolve and reach the lower ocean levels again. Thermohaline circulation can bring carbon in the deep ocean levels to the upper levels, where it can again be exchanged with the atmosphere. Units are in gigatons.[7]

In the oceans, dissolved carbonate can combine with dissolved calcium to precipitate solid calcium carbonate, CaCO3, mostly as the shells of microscopic organisms. When these organisms die, their shells sink and accumulate on the ocean floor. Over time these carbonate sediments form limestone which is the largest reservoir of carbon in the carbon cycle. The dissolved calcium in the oceans comes from the chemical weathering of calcium-silicate rocks, during which carbonic and other acids in groundwater react with calcium-bearing minerals liberating calcium ions to solution and leaving behind a residue of newly formed aluminium-rich clay minerals and insoluble minerals such as quartz.

Carbon exchange between the ocean and other systems

Main article: Carbon cycle

Atmosphere

Carbon is readily exchanged between the atmosphere and ocean. Each year, approximately 90 gigatons of carbon are exchanged between the atmosphere and the ocean in each direction, leading to a quick equilibration of surface ocean and atmospheric carbon levels.[4] In regions of oceanic upwelling, carbon is released to the atmosphere. Conversely, regions of downwelling transfer carbon (CO2) from the atmosphere to the ocean. Interactions with the atmosphere also influence the rate of carbon uptake from other systems. Extreme storms such as hurricanes and typhoons bury extremely large amounts of carbon, because they wash away so much sediment.[8]

Inorganic carbon, that is carbon compounds with no carbon-carbon or carbon-hydrogen bonds, is important in its reactions within water. This carbon exchange becomes important in controlling pH in the ocean and can also vary as a source or sink for carbon. When CO2 enters the ocean, it participates in a series of reactions which are locally in equilibrium:

Solution:

CO2 (atmospheric) ⇌ CO2 (dissolved)

Conversion to carbonic acid:

CO2 (dissolved) + H2O ⇌ H2CO3

First ionization:

H2CO3 ⇌ H+ + HCO3(bicarbonate ion)

Second ionization:

HCO3 ⇌ H+ + CO32−(carbonate ion)

This set of reactions, which of each has its own equilibrium coefficient, determines the form that inorganic carbon takes in the oceans.[9] The coefficients, which have been determined empirically for ocean water, are themselves functions of temperature, pressure, and the presence of other ions (especially borate). In the ocean the equilibria strongly favor bicarbonate. Since this ion is three steps removed from atmospheric CO2, the level of inorganic carbon storage in the ocean does not have a proportion of unity to the atmospheric partial pressure of CO2. The factor for the ocean is about ten: that is, for a 10% increase in atmospheric CO2, oceanic storage (in equilibrium) increases by about 1%, with the exact factor dependent on local conditions. This buffer factor is often called the Revelle factor, after Roger Revelle.

Anthropogenic influences

The ocean's buffer capacity is limited and depends on the relatively slow weathering of rocks, which takes place much slower than current CO2 emissions into the atmosphere.[4] Current emission rates exceed the supply of the necessary mineral cations for these reactions, meaning that on a time scale of millennia, the ocean's ability to absorb CO2 will decrease. A major contributing factor to this decrease in capacity is the ocean's falling pH levels. The absorption of carbon dioxide makes the ocean more acidic and given the projected rates of increases in atmospheric CO2, ocean surface water pH will decrease by about 0.2 by 2100. This will additionally slow the biological precipitation of calcium carbonates.[10][11]

References

  1. Sarmiento, J.L.; Gruber, N. (2006). Ocean Biogeochemical Dynamics. Princeton University Press, Princeton, New Jersey, USA.
  2. C.Michael Hogan. 2010. Virus. Encyclopedia of Earth. National Council for Science and the Environment. eds. S.Draggan and C.Cleveland
  3. ""Sinkers" provide missing piece in deep-sea puzzle" (PDF) (Press release). Monterey Bay Aquarium Research Institute (MBARI). 2005-06-09. Retrieved 2007-10-07.
  4. 1 2 3 Falkowski, P.; Scholes, R. J.; Boyle, E.; Canadell, J.; Canfield, D.; Elser, J.; Gruber, N.; Hibbard, K.; Högberg, P.; Linder, S.; MacKenzie, F. T.; Moore b, 3.; Pedersen, T.; Rosenthal, Y.; Seitzinger, S.; Smetacek, V.; Steffen, W. (2000). "The Global Carbon Cycle: A Test of Our Knowledge of Earth as a System". Science. 290 (5490): 291–296. doi:10.1126/science.290.5490.291. PMID 11030643.
  5. Eric T. Sundquist; Wallace S. Broecker (1985). The Carbon Cycle and Atmospheric CO2: Natural Variations, Archean to Present. American Geophysical Union. ISBN 978-0-87590-060-5. Retrieved 8 June 2012.
  6. Sarmiento, J. L.; Bender, M. (1994). "Carbon biogeochemistry and climate change". Photosynthesis Research. 39 (3): 209. doi:10.1007/BF00014585.
  7. 1 2 Prentice, I.C. (2001). "The carbon cycle and atmospheric carbon dioxide". Climate change 2001: the scientific basis: contribution of Working Group I to the Third Assessment Report of the Intergouvernmental Panel on Climate Change / Houghton, J.T. [edit.] Retrieved 31 May 2012.
  8. Typhoons Bury Tons of Carbon in the Oceans Newswise, Retrieved on July 27, 2008.
  9. Millero, Frank J. (2005). Chemical Oceanography (3 ed.). CRC Press. ISBN 0-8493-2280-4.
  10. Kleypas, J. A.; Buddemeier, R. W.; Archer, D.; Gattuso, J. P.; Langdon, C.; Opdyke, B. N. (1999). "Geochemical Consequences of Increased Atmospheric Carbon Dioxide on Coral Reefs". Science. 284 (5411): 118–120. doi:10.1126/science.284.5411.118. PMID 10102806.
  11. Langdon, C.; Takahashi, T.; Sweeney, C.; Chipman, D.; Goddard, J.; Marubini, F.; Aceves, H.; Barnett, H.; Atkinson, M. J. (2000). "Effect of calcium carbonate saturation state on the calcification rate of an experimental coral reef". Global Biogeochemical Cycles. 14 (2): 639. doi:10.1029/1999GB001195.
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