Calcium Carbonate Dynamics in Marine Environments

Calcium carbonate (CaCO₃) underpins a suite of fundamental processes in the ocean, from the construction of coral reef frameworks and shell formation in marine organisms to large-scale biogeochemical cycling that regulates atmospheric carbon dioxide. In surface waters, biogenic calcifiers such as coccolithophores, molluscs and reef-building corals precipitate CaCO₃, drawing down dissolved inorganic carbon and producing alkalinity. Beneath the mixed layer, the balance between carbonate production and dissolution is governed by chemical parameters including pH, carbonate ion concentration and temperature, which collectively determine the saturation state of different CaCO₃ polymorphs. As particles sink, they may dissolve above or below the lysocline, contributing alkalinity to deep waters and influencing global carbon storage. In sediments, early diagenetic reactions—driven by organic matter degradation—modulate porewater pH and total alkalinity, controlling the preservation or dissolution of carbonate deposits. Human influences such as ocean acidification, warming and ecological shifts are altering saturation horizons, compensation depths and rates of production versus dissolution, with cascading effects on ecosystem function, sediment budgets and the ocean’s long-term capacity to buffer CO₂ emissions.

Research from Nature Portfolio

Recent studies have elucidated the role of marine fishes in inorganic carbon cycling by quantifying carbonate excretion. Analysis across hundreds of reef fish species has demonstrated that body size and intestinal morphology are primary predictors of excreted CaCO₃ rates and mineralogy. Larger fishes with relatively longer intestines excrete proportionally less carbonate, while temperature influences the proportion of calcite, aragonite and more soluble phases such as high-Mg calcite. These findings refine our understanding of how shifts in fish community composition will impact sediment generation and alkalinity fluxes under changing ocean conditions. Another investigation has explored the interplay between aragonite and calcite dissolution at the seafloor. High-resolution reactive transport models reveal that aragonite dissolution can locally elevate alkalinity and suppress calcite dissolution within surface sediments, effectively ‘galvanising’ the preservation of calcite. This mechanism highlights a previously underappreciated feedback whereby the vulnerability of aragonite-producing organisms to acidification may indirectly affect calcite stability and the buffering capacity of deep-sea environments.

Calcium Carbonate Dynamics in Marine Environments publication trend

The graph below shows the total number of articles in calcium carbonate dynamics in marine environments across all publications each year (not limited to Nature Index journals).

Technical terms

Calcium carbonate (CaCO₃): A mineral composed of calcium and carbonate ions, forming polymorphs such as calcite and aragonite, fundamental to marine biomineralisation and sediment composition.

Saturation state (Ω): A dimensionless measure of the thermodynamic tendency for CaCO₃ to precipitate or dissolve; values above unity favour precipitation, below unity favour dissolution.

Carbonate compensation depth (CCD): The depth in the ocean at which carbonate dissolution balances supply, below which CaCO₃ does not accumulate in sediments.

Total alkalinity (TA): The sum of all titratable bases in seawater, primarily bicarbonate and carbonate ions, indicating the capacity of water to neutralise acids.

Early diagenesis: Chemical and biological transformations of sediments and porewaters occurring shortly after deposition, affecting mineral stability, pH and alkalinity.

References

1. Temperature, species identity and morphological traits predict carbonate excretion and mineralogy in tropical reef fishes. Nature Communications (2023).
2. Aragonite dissolution protects calcite at the seafloor. Nature Communications (2022).
3. Correction of Inexactitude in Surficial Sediment Calcium Carbonate Content Data of the Global Ocean. Ocean-Land-Atmosphere Research (2023).
4. Carbonate chemistry in sediment porewaters of the Rhône River delta driven by early diagenesis (northwestern Mediterranean). Biogeosciences (2016).