Everything about Solubility Pump totally explained
In
oceanic 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 :
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).
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 (CO
2 (aq)),
carbonic acid (H
2CO
3),
bicarbonate (HCO
3-) and
carbonate (CO
32-), and they interact with water as follows :
CO2 (aq) + H2O H2CO3 HCO3- + H+ CO32- + 2 H+
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 (for example
Na+,
K+,
Mg2+,
Ca2+) and negative (for example CO
32- itself,
Cl-,
SO42-,
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, T
CO2 or C
T),
Total alkalinity (T
ALK or A
T),
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 CO
2, the
dissolution/
precipitation of
CaCO3, or biological activity such as
photosynthesis/
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
Land-use changes, the
combustion of
fossil fuels, and the production of
cement have led to a flux of CO
2 to the atmosphere. Presently, about one third (approximately 2
gigatons of carbon per year) of anthropogenic emissions of CO
2 are believed to be entering the ocean. The solubility pump is the primary mechanism driving this flux, with the consequence that anthropogenic CO
2 is reaching the ocean interior via high latitude sites of deep water formation (particularly the North Atlantic). Ultimately, most of the CO
2 emitted by human activities will dissolve in the ocean, 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) predicted that the rate of CO
2 uptake will begin to saturate (reach the maximum rate) at 5
gigatons of carbon per year by 2100. This was partially due to
non-linearities in the seawater carbonate system, but also due to
climate change. Ocean warming decreases the solubility of CO
2 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) may act to decrease transport of dissolved CO
2 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
2 from the atmosphere acts to decrease climate change, it causes
ocean acidification which is believed will have negative consequences for marine ecosystems.
Further Information
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