Calcite precipitation in lakes

---

Calcite precipitation in lakes

Calcite precipitation can be caused in lakes by increased photosynthetic activity, such as during algal blooms. An indicator of calcite precipitation in lakes can be a white colour. Calcite precipitation can enhance sedimentation of carbon; thus, these events can take carbon out of the short-term carbon cycle and store it in the long-term carbon cycle. Primary production causes calcite precipitation due to macroscale changes in pH and microscale processes.

Macroscale processes

Photosynthetic activity can decrease the amount of carbonic acid through carbon uptake and thereby alter the lake's pH. Dissolved inorganic carbon (DIC) uptake by primary producers can lower surface DIC. The decrease in H₃O+ (protons in water) will push the pH-dependent dissolved inorganic carbon equilibrium towards the right (see DIC equilibrium). Effectively, this increases the calcite saturation state (Ω_CaCO3). {CO₃^2-} and {Ca²⁺} are calculated from their concentrations and the ionic strength of major ion concentrations. An increase in the concentration of the reactants in the calcite equilibrium will increase the calcite saturation state. The increased availability of CO₃^2- will push the calcite equilibrium to precipitate CaCO₃. Depending on the change in pH, bicarbonate ions can be more important for calcite precipitation than carbonate ions. When precipitated CaCO₃ crystals sink into the Hypolimnion, they partially dissolve due to the lower calcite saturation state. The amount of calcium carbonate that gets sedimented depends on the trophic state index of the lake.

Dissolved inorganic carbon equilibrium:

CO_2(g) + H₂O ⇌ [CO_2(aq) + H₂O ⇌ H₂CO₃ + H₂O] ⇌ HCO^3- + H₃O⁺ + H₂O ⇌ CO₃^2- + 2H₃O⁺

Calcite equilibrium:

CO₃^2-_(aq) + Ca²⁺_(aq) ⇌ CaCO_3(s)

Bicarbonate-calcite equilibrium:

2HCO^3-_(aq) + Ca²⁺_(aq) ⇌ CaCO_3(s) + CO₂ + H₂O

Calcite saturation state:

Ω_CaCO3 = ({CO₃^2-} {Ca²⁺}) / K_S0

Ω_CaCO3 := calcite saturation state; {CO₃^2-} := carbonate-ion activity; {Ca²⁺} := calcium-ion activity; K_S0 := solubility constant of calcite

Microscale processes

Complementing the aforementioned macroscale process, algal removal of carbon dioxide changes alkalinity more extremely in close proximity to the cell. The cell wall catalyses the process by providing a crystallization site. The growth of the calcite crystals is limited by several factors. Firstly, the precipitation of calcium carbonate can reduce the calcite saturation state within the diffusive boundary layer. This happens in the case that calcite precipitation takes out Ca²⁺ and CO₃^2- faster than the diffusion rate that resupplies them. Secondly, growing crystals eventually sink out of the productive zone into zones with a lower calcite saturation state, which reduces the resupply of Ca²⁺ and CO₃^2-, inhibiting crystal growth.

This article "Calcite precipitation in lakes" is from Wikipedia. The list of its authors can be seen in its historical and/or the page Edithistory:Calcite precipitation in lakes. Articles copied from Draft Namespace on Wikipedia could be seen on the Draft Namespace of Wikipedia and not main one.

---