Saturation state[edit]
The saturation state of seawater for a mineral (known as Ω) is a measure of the thermodynamic potential for the mineral to form or to dissolve, and is described by the following equation:
{Omega} = frac{left[ extrm{Ca}^{2+}
ight] left[ extrm{CO}_{3}^{2-}
ight]}{K_{sp}}
Here Ω is the product of the concentrations (or activities) of the reacting ions that form the mineral (Ca2+
and CO2−
3), divided by the product of the concentrations of those ions when the mineral is at equilibrium (K
sp), that is, when the mineral is neither forming nor dissolving.[43] In seawater, a natural horizontal boundary is formed as a result of temperature, pressure, and depth, and is known as the saturation horizon, or lysocline.[20] Above this saturation horizon, Ω has a value greater than 1, and CaCO
3 does not readily dissolve. Most calcifying organisms live in such waters.[20] Below this depth, Ω has a value less than 1, and CaCO
3 will dissolve. However, if its production rate is high enough to offset dissolution, CaCO
3 can still occur where Ω is less than 1. The carbonate compensation depth occurs at the depth in the ocean where production is exceeded by dissolution.[44]
The decrease in the concentration of CO32− decreases Ω, and hence makes CaCO
3 dissolution more likely.
Calcium carbonate occurs in two common polymorphs (crystalline forms): aragonite and calcite. Aragonite is much more soluble than calcite, so the aragonite saturation horizon is always nearer to the surface than the calcite saturation horizon.[20] This also means that those organisms that produce aragonite may be more vulnerable to changes in ocean acidity than those that produce calcite.[10] Increasing CO
2 levels and the resulting lower pH of seawater decreases the saturation state of CaCO
3 and raises the saturation horizons of both forms closer to the surface.[45] This decrease in saturation state is believed to be one of the main factors leading to decreased calcification in marine organisms, as the inorganic precipitation of CaCO
3 is directly proportional to its saturation state.[46]
Saturation state[edit]
The saturation state of seawater for a mineral (known as Ω) is a measure of the thermodynamic potential for the mineral to form or to dissolve, and is described by the following equation:
{Omega} = frac{left[ extrm{Ca}^{2+}
ight] left[ extrm{CO}_{3}^{2-}
ight]}{K_{sp}}
Here Ω is the product of the concentrations (or activities) of the reacting ions that form the mineral (Ca2+
and CO2−
3), divided by the product of the concentrations of those ions when the mineral is at equilibrium (K
sp), that is, when the mineral is neither forming nor dissolving.[43] In seawater, a natural horizontal boundary is formed as a result of temperature, pressure, and depth, and is known as the saturation horizon, or lysocline.[20] Above this saturation horizon, Ω has a value greater than 1, and CaCO
3 does not readily dissolve. Most calcifying organisms live in such waters.[20] Below this depth, Ω has a value less than 1, and CaCO
3 will dissolve. However, if its production rate is high enough to offset dissolution, CaCO
3 can still occur where Ω is less than 1. The carbonate compensation depth occurs at the depth in the ocean where production is exceeded by dissolution.[44]
The decrease in the concentration of CO32− decreases Ω, and hence makes CaCO
3 dissolution more likely.
Calcium carbonate occurs in two common polymorphs (crystalline forms): aragonite and calcite. Aragonite is much more soluble than calcite, so the aragonite saturation horizon is always nearer to the surface than the calcite saturation horizon.[20] This also means that those organisms that produce aragonite may be more vulnerable to changes in ocean acidity than those that produce calcite.[10] Increasing CO
2 levels and the resulting lower pH of seawater decreases the saturation state of CaCO
3 and raises the saturation horizons of both forms closer to the surface.[45] This decrease in saturation state is believed to be one of the main factors leading to decreased calcification in marine organisms, as the inorganic precipitation of CaCO
3 is directly proportional to its saturation state.[46]
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