Dissolution of calcium carbonate particles proceeds when the concentration of carbonate ions in seawater decreases (pH drops) and pressure increases. The figure below illustrates how biogenic calcium carbonate particles produced in the ocean surface layer settle out and are deposited on the seafloor. It is divided into three layers. 

Calcium carbonate supersaturation (Ω > 1):Bathymetric zones from surface to bottom (Atlantic Ocean) and mid-depth (Pacific Ocean)

 Calcium carbonate particles do not dissolve anywhere near the ocean surface. This is because the seawater is supersaturated (Ω > 1) for calcium carbonate particles. Calcium carbonate saturation is often expressed as omega (Ω). (Oversaturation is not so oversaturated that it precipitates on its own.) If the water on the seafloor is oversaturated (Ω), calcium carbonate particles will accumulate on the seafloor, which will actually result in a whitish sediment layer.

Calcium carbonate saturation (Ω = 1):Bottom layer in the Atlantic Ocean and some depths in the mesopelagic layer in the Pacific Ocean

 In the ocean, saturation with respect to calcium carbonate decreases with depth. At a certain point, a depth appears where saturation (Ω = 1) for calcium carbonate is reached. That depth is called the calcium carbonate saturation depth.

Calcium carbonate unsaturated (Ω < 1):Bottom layer to seafloor surface in the Atlantic Ocean and mesopelagic layer to seafloor surface in the Pacific Ocean

 At depths deeper than the saturation depth, it becomes undersaturated (Ω < 1) and dissolution of the calcium carbonate particles proceeds.

Saturation depth (Ω = 1) to Seafloor surface in the depth band of CCD

 Since calcium carbonate is undersaturated (Ω < 1), calcium carbonate particles dissolve. Even if they reach saturation depth, their dissolution rate is quite slow. If the rate at which they fall from the surface layer exceeds the rate at which they dissolve, the accumulation of calcium carbonate particles on the seafloor surface will proceed slowly.

Calcium carbonate Compensation Depth: CCD

 The depth at which the rate of sedimentation from the surface is balanced by the rate of dissolution is called the Calcium carbonate Compensation Depth (CCD). At depths deeper than this, no calcium carbonate particles remain on the seafloor.

Here, the conditional equation for the dissolution of calcium carbonate particles in seawater is written out.

CaCO3 (s)  Ca2+ + CO32- ;Solubility equilibrium constant Ksp

Ksp = Ca2+】×【CO32-

The supersaturated state is, Ca2+】×【CO32-/ Ksp  >  1

The saturated (equilibrium) state is, Ca2+】×【CO32-/ Ksp  =  1

Undersaturated state is, Ca2+】×【CO32-/ Ksp  <  1.

 Since Ca2+ ions in seawater are abundant, Ca2+ can be considered unchanged. Therefore, as CO32- in seawater becomes smaller, it becomes undersaturated with respect to calcium carbonate and CaCO3 particles dissolve.

Conditions for small CO32- in seawater
 So what are the conditions under which CO32- in seawater becomes smaller? As we have already learned, as the pH of seawater decreases, the dissociation equilibrium of carbonic acid substances shifts, and the ratio of CO32- to DIC becomes smaller: at pH 8, CO32-/DIC = 9%, but at pH 7, CO32-/DIC = less than 1%.

Last modified: Tuesday, 3 October 2023, 9:48 AM