Solubility pump
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[[Image:CO2 pump hg.png|thumb|Air-sea exchange of CO<sub>2</sub>]]
In [[ocean]]ic [[biogeochemistry]], the '''solubility pump''' is a physico-chemical process that transports [[carbon]] (as dissolved inorganic carbon) from the ocean's surface to its interior.
==Overview==
The solubility pump is driven by the coincidence of two processes in the ocean :
* The [[solubility]] of [[carbon dioxide]] is a strong inverse function of [[sea surface temperature|seawater temperature]] (i.e. solubility is greater in cooler water)
* The [[thermohaline circulation]] is driven by the formation of deep water at high latitudes where seawater is usually cooler and more dense
Since deep water (that is, seawater in the ocean's interior) is formed under the same surface conditions that promote carbon dioxide solubility, it contains a higher concentration of dissolved inorganic carbon than one might otherwise expect. Consequently, these two processes act together to pump carbon from the atmosphere into the ocean's interior.
One consequence of this is that when deep water upwells in warmer, equatorial latitudes, it strongly outgasses carbon dioxide to the atmosphere because of the reduced solubility of the gas.
The solubility pump has a biological counterpart known as the [[biological pump]]. For an overview of both pumps, see Raven & Falkowski (1999).<ref name=raven99>Raven, J. A. and P. G. Falkowski (1999). [http://www.blackwell-synergy.com/links/doi/10.1046/j.1365-3040.1999.00419.x Oceanic sinks for atmospheric CO<sub>2</sub>.] ''Plant Cell Environ.'' '''22''', 741-755.</ref>
==Carbon dioxide solubility==
[[Carbon dioxide]], like other gases, is soluble in water. However, unlike many other gases ([[oxygen]] for instance), it reacts with water and forms a balance of several ionic and non-ionic species (collectively known as [[Total inorganic carbon|dissolved inorganic carbon]], or DIC). These are dissolved free carbon dioxide (CO<sub>2</sub> <sub>(aq)</sub>), [[carbonic acid]] (H<sub>2</sub>CO<sub>3</sub>), [[bicarbonate]] (HCO<sub>3</sub><sup>-</sup>) and [[carbonate]] (CO<sub>3</sub><sup>2-</sup>), and they interact with water as follows :
<center> CO<sub>2</sub> <sub>(aq)</sub> + H<sub>2</sub>O <math>\leftrightarrow</math> H<sub>2</sub>CO<sub>3</sub> <math>\leftrightarrow</math> HCO<sub>3</sub><sup>-</sup> + H<sup>+</sup> <math>\leftrightarrow</math> CO<sub>3</sub><sup>2-</sup> + 2 H<sup>+</sup> </center>
The balance of these carbonate species (which ultimately affects the solubility of carbon dioxide), is dependent on factors such as [[pH]]. In [[seawater]] this is regulated by the charge balance of a number of positive (e.g. [[sodium|Na<sup>+</sup>]], [[potassium|K<sup>+</sup>]], [[magnesium|Mg<sup>2+</sup>]], [[calcium|Ca<sup>2+</sup>]]) and negative (e.g. CO<sub>3</sub><sup>2-</sup> itself, [[chlorine|Cl<sup>-</sup>]], [[sulfate|SO<sub>4</sub><sup>2-</sup>]], [[bromine|Br<sup>-</sup>]]) ions. Normally, the balance of these species leaves a net positive charge. With respect to the carbonate system, this excess positive charge shifts the balance of carbonate species towards negative ions to compensate. The result of which is a reduced concentration of the free carbon dioxide and carbonic acid species, which in turn leads to an oceanic uptake of carbon dioxide from the atmosphere to restore balance. Thus, the greater the positive charge imbalance, the greater the solubility of carbon dioxide. In carbonate chemistry terms, this imbalance is referred to as [[alkalinity]].
In terms of measurement, four basic parameters are of key importance: '''[[Total inorganic carbon]]''' (TIC, T<sub>CO2</sub> or C<sub>T</sub>) , '''Total [[alkalinity]]''' (T<sub>ALK</sub> or A<sub>T</sub>), '''pH''', and '''[[partial pressure|pCO<sub>2</sub>]]'''. Measuring any two of these parameters allows for the determination of a wide range of pH-dependent species (including the above mentioned species). This balance can be changed by a number of processes. For example, the [[flux|air-sea flux]] of CO<sub>2</sub>, the [[solvation|dissolution]]/[[precipitation (chemistry)|precipitation]] of [[Calcium Carbonate|CaCO<sub>3</sub>]], or biological activity such as [[photosynthesis]]/[[cellular respiration|respiration]]. Each of these has different effects on each of the four basic parameters, and together they exert strong influences on global cycles. It is important to note, the net and local charge of the oceans remains neutral during any chemical process.
==Anthropogenic changes==
[[Image:AYool GLODAP invt aCO2.png|thumb|right|200px|Vertical inventory of "present day" (1990s) anthropogenic CO<sub>2</sub>]]
Land-use changes, the [[combustion]] of [[fossil fuel]]s, and the production of [[cement]] have led to a flux of CO<sub>2</sub> to the atmosphere. Presently, about one third (approximately 2 [[gigaton]]s of carbon per year)<ref name=tak02>Takahashi, T., S. C. Sutherland, C. Sweeney, A. Poisson, N. Metzl, B. Tilbrook, N. Bates, R. Wanninkhof, R. A. Feely, C. Sabine, J. Olafsson and Y. C. Nojiri (2002) [http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6VGC-452W7KK-2&_user=10&_coverDate=12%2F31%2F2002&_rdoc=2&_fmt=summary&_orig=browse&_srch=doc-info(%23toc%236035%232002%23999509990%231%23FLA%23display%23Volume)&_cdi=6035&_sort=d&_docanchor=&view=c&_ct=19&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=1eb682d5552c8d58911ddf6e2012da02 Global sea-air CO<sub>2</sub> flux based on climatological surface ocean ''p''CO<sub>2</sub>, and seasonal biological and temperature effects.] ''Deep-Sea Res. Pt. II'' '''49''', 1601-1622.</ref><ref name=orr01>Orr, J. C., E. Maier-Reimer, U. Mikolajewicz, P. Monfray, J. L. Sarmiento, J. R. Toggweiler, N. K. Taylor, J. Palmer, N. Gruber, C. L. Sabine, C. Le Quéré, R. M. Key and J. Boutin (2001). Estimates of anthropogenic carbon uptake from four three-dimensional global ocean models. ''Global Biogeochem. Cycles'' '''15''', 43-60.</ref> of anthropogenic emissions of CO<sub>2</sub> are believed to be entering the ocean. The solubility pump is the primary mechanism driving this flux, with the consequence that anthropogenic CO<sub>2</sub> is reaching the ocean interior via high latitude sites of deep water formation (particularly the North Atlantic). Ultimately, most of the CO<sub>2</sub> emitted by human activities will dissolve in the ocean<ref name=arch05>Archer, D. (2005). Fate of fossil fuel CO<sub>2</sub> in geologic time. ''J. Geophys. Res.'', '''110''', doi:10.1029/2004JC002625.</ref>, however the rate at which the ocean will take it up in the future is less certain.
In a study of carbon cycle up to the end of the 21st century, Cox ''et al.'' (2000)<ref name=cox00>Cox, P. M., Betts, R. A., Jones, C. D., Spall, S. A. and Totterdell, I. J. (2000). [http://www.nature.com/nature/journal/v408/n6809/abs/408184a0.html Acceleration of global warming due to carbon-cycle feedbacks in a coupled climate model.] ''Nature'', '''408''', 184-187.</ref> predicted that the rate of CO<sub>2</sub> uptake will begin to saturate (reach the maximum rate) at 5 [[gigaton]]s of carbon per year by 2100. This was partially due to [[nonlinearity|non-linearities]] in the seawater carbonate system, but also due to [[climate change]]. Ocean warming decreases the solubility of CO<sub>2</sub> in seawater, slowing the ocean's response to emissions. Warming also acts to increase ocean stratification, isolating the surface ocean from deeper waters. Additionally, changes in the ocean's [[thermohaline circulation]] (specifically slowing)<ref name=bryd05>Bryden, H. L., Longworth, H. R. and Cunningham, S. A. (2005). [http://www.nature.com/nature/journal/v438/n7068/abs/nature04385.html Slowing of the Atlantic meridional overturning circulation at 25° N.] ''Nature'', '''438''', 655-657.</ref> may act to decrease transport of dissolved CO<sub>2</sub> into the deep ocean. However, the magnitude of these processes is still uncertain, preventing good long-term estimates of the fate of the solubility pump.
While ocean absorption of anthropogenic CO<sub>2</sub> from the atmosphere acts to decrease climate change, it causes [[ocean acidification]] which is believed will have negative consequences for marine ecosystems<ref name=orr05>Orr, J. C. ''et al.'' (2005). [http://www.ipsl.jussieu.fr/~jomce/acidification/paper/Orr_OnlineNature04095.pdf Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms.] ''Nature'' '''437''', 681-686.</ref>.
==See also==
* [[Biological pump]]
* [[Continental shelf pump]]
* [[Ocean acidification]]
==References==
{{reflist}}
[[Category:Aquatic ecology]]
[[Category:Carbon]]
[[Category:Chemical oceanography]]
[[Category:Geochemistry]]
[[Category:Oceanography]]