Solubility pump
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In oceanic biogeochemistry, the solubility pump is a physicochemical process that transports carbon as dissolved inorganic carbon (DIC) from the ocean's surface to its interior.[1] The solubility pump can be influenced by several environmental and biotic factors, which can enhance or reduce the strength of the solubility pump and the solubility of carbon dioxide in the ocean.
The solubility pump can act as a sink for carbon dioxide released into the atmosphere by anthropogenic activities. Though most of the carbon dioxide released via anthropogenic activities will dissolve in the ocean eventually, the exact rate that this will occur is unknown.[2] Increased warming of the ocean due to climate change, as well as slowed thermohaline circulation in the ocean[3], can reduce to amount of carbon dioxide taken up from the atmosphere into the ocean. Additionally, increased uptake of carbon dioxide in the ocean leads to ocean acidification, which can be problematic for marine life.[4]
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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 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 denser
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 might be expected from average surface concentrations. 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.
Other environmental factors also affect the solubility pump, such as sea ice formation. The formation of sea ice has dynamics that can both increase and decrease exchange between the air and the sea.[5]
There can also be regional differences in the effect of the solubility pump due to factors such a varying temperatures and differences in wind stress on the solubility pump.[6][7][8]
The exchange of carbon within the solubility pump can be monitored by various gasses. Nitrogen and noble gasses such as arsenic and krypton can be used to trace the amount of cycling of gasses, including carbon, between the air and the sea.[9] In addition, ocean chemistry in general has an effect on the amount of carbon dioxide in the atmosphere.[10] Marine biota can also alter the amount of δ13CO2 in the atmosphere.[11]
The solubility pump has a biological counterpart known as the biological pump. For an overview of both pumps, see Raven & Falkowski (1999).[1] While the solubility pump is effected by factors such as noble gas exchange and environmental conditions, the biological pump can be affected by factors such as the marine food web, and the biological pump can impact atmospheric carbon dioxide concentrations overall.[12][13]
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 dissolved inorganic carbon, or DIC). These are dissolved free carbon dioxide (CO2(aq)), carbonic acid (H2CO3), bicarbonate (HCO−3) and carbonate (CO2−3), and they interact with water as follows:
- CO2(aq) + H2O ⇌ H2CO3 ⇌ HCO−3 + H+ ⇌ CO2−3 + 2 H+
The balance of these carbonate species (which ultimately affects the solubility of carbon dioxide), is dependent on factors such as pH, as shown in a Bjerrum plot. In seawater this is regulated by the charge balance of a number of positive (e.g. Na+, K+, Mg2+, Ca2+) and negative (e.g. CO2−3 itself, Cl−, SO2−4, Br−) 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, TCO2 or CT), Total alkalinity (TALK or AT), pH, and pCO2. 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 air-sea flux of CO2, the dissolution or precipitation of CaCO3, or biological activity such as photosynthesis or respiration. Each of these has different effects on each of the four basic parameters, and together they exert strong influences on global cycles. The net and local charge of the oceans remains neutral during any chemical process.
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