Carbonate compensation depth

The carbonate compensation depth (CCD) is the depth, in the oceans, at which the rate of supply of calcium carbonates matches the rate of solvation. That is, solvation 'compensates' supply. Below the CCD solvation is faster, so that carbonate particles dissolve and the carbonate shells (tests) of animals are not preserved. Carbonate particles cannot accumulate in the sediments where the sea floor is below this depth.

Calcite is the least soluble of these carbonates, so the CCD is normally the compensation depth for calcite. The aragonite compensation depth (ACD) is the compensation depth for aragonitic carbonates. Aragonite is more soluble than calcite, and the aragonite compensation depth is generally shallower than both the calcite compensation depth and the CCD.

Overview

As shown in the diagram, biogenic calcium carbonate (CaCO₃) tests are produced in the photic zone of the oceans (green circles). Upon death, those tests escaping dissolution near the surface settle, along with clay materials. In seawater, a dissolution boundary is formed as a result of temperature, pressure, and depth, and is known as the saturation horizon.^[3] Above this horizon, waters are supersaturated and CaCO₃ tests are largely preserved. Below it, waters are undersaturated, because of both the increasing solubility with depth and the release of CO₂ from organic matter decay, and CaCO₃ will dissolve. The sinking velocity of debris is rapid (broad pale arrows), so dissolution occurs primarily at the sediment surface.

At the carbonate compensation depth, the rate of dissolution exactly matches the rate of supply of CaCO₃ from above. At steady state this depth, the CCD, is similar to the snowline (the first depth where carbonate-poor sediments occur). The lysocline is the depth interval between the saturation and carbonate compensation depths.^[4][1]

Solubility of carbonate

Calcium carbonate is essentially insoluble in sea surface waters today. Shells of dead calcareous plankton sinking to deeper waters are practically unaltered until reaching the lysocline, the point about 3.5 km deep past which the solubility increases dramatically with depth and pressure. By the time the CCD is reached^{[clarification needed]} all calcium carbonate has dissolved according to this equation:

${\displaystyle {\ce {CaCO3 + CO2 + H2O <=> Ca^2+ (aq) + 2HCO_3^- (aq)}}}$

Calcareous plankton and

Variations in value of the CCD

Part of a series on the
Carbon cycle
By regions Terrestrial Marine Atmospheric Deep carbon Soil Permafrost Boreal forest Geochemistry
Carbon dioxide In the atmosphere Ocean acidification Removal Satellite measurements
Forms of carbon Total carbon (TC) Total organic carbon (TOC) Total inorganic carbon (TIC) Dissolved organic carbon (DOC) Dissolved inorganic carbon (DIC) Particulate organic carbon (POC) Particulate inorganic carbon (PIC) Primary production marine Black carbon Blue carbon Kerogen
Metabolic pathways Photosynthesis Chemosynthesis Calvin cycle Reverse Krebs cycle Carbon fixation C3 C4
Carbon respiration Ecosystem respiration Net ecosystem production Photorespiration Soil respiration
Carbon pumps Biological pump Martin curve Solubility pump Lipid pump Marine snow Microbial loop Viral shunt Jelly pump Whale pump Continental shelf pump
Carbon sequestration Carbon sink Soil carbon storage Marine sediment pelagic sediment
Methane Atmospheric methane Methanogenesis Methane emissions Arctic Wetland Aerobic production Clathrate gun hypothesis Carbon dioxide clathrate
Biogeochemical Marine cycles Nutrient cycle Carbonate–silicate cycle Carbonate compensation depth Great Calcite Belt Redfield ratio
Other Climate reconstruction proxies Carbon-to-nitrogen ratio Deep biosphere Deep Carbon Observatory Global Carbon Project Carbon capture and storage Carbon cycle re-balancing Territorialisation of carbon governance Total Carbon Column Observing Network C4MIP CO2SYS
Category
v t e

The exact value of the CCD depends on the solubility of calcium carbonate which is determined by temperature, pressure and the chemical composition of the water – in particular the amount of dissolved CO₂ in the water. Calcium carbonate is more soluble at lower temperatures and at higher pressures. It is also more soluble if the concentration of dissolved CO₂ is higher. Adding a reactant to the above chemical equation pushes the equilibrium towards the right producing more products: Ca²⁺ and HCO₃⁻, and consuming more reactants CO₂ and calcium carbonate according to Le Chatelier's principle.

At the present time the CCD in the

John Murray investigated and experimented on the dissolution of calcium carbonate and was first to identify the carbonate compensation depth in oceans.^[7]

Climate change impacts

Increasing

On the

• Carbonate pump
• Great Calcite Belt
• Lysocline
• Ocean acidification

References