Limestone is a sedimentary rock predominantly composed of calcium carbonate (\(\text{CaCO}_3\)), typically in the form of the minerals calcite or aragonite. Carbonate rocks collectively make up approximately 20 to 25% of all sedimentary rock sequences globally, representing one of the planet’s largest carbon reservoirs. Understanding the source of this \(\text{CaCO}_3\) is fundamental to grasping the long-term carbon cycle and the history of marine environments.
Biological Origin of Calcium Carbonate
The primary source for the \(\text{CaCO}_3\) found in most limestone deposits is the skeletal material produced by marine organisms, a process known as biomineralization. Organisms actively extract dissolved calcium ions (\(\text{Ca}^{2+}\)) and bicarbonate ions (\(\text{HCO}_3^-\)) from seawater to construct their hard parts, such as shells and external skeletons. This biological mechanism is far more productive than purely chemical precipitation in modern oceans.
Biomineralization occurs in a specialized, controlled space either inside or outside the organism’s cells. Organisms regulate the chemistry within this space, often by actively removing protons (\(\text{H}^+\)) to raise the \(\text{pH}\) level. This increase in alkalinity makes the precipitation of \(\text{CaCO}_3\) kinetically favorable, allowing the organism to build its protective structure. Upon death, these hard parts sink to the seafloor, forming carbonate sediments.
Microscopic plankton are prolific producers of this sediment, notably coccolithophores and foraminifera. Coccolithophores are single-celled phytoplankton that cover themselves in minute calcite plates called coccoliths. These organisms can account for 30 to 60% of modern open-ocean \(\text{CaCO}_3\) production.
Planktonic foraminifera, which are single-celled protists, also produce calcium carbonate shells. These shells and coccoliths, along with fragments from larger invertebrates, create calcareous ooze. Larger organisms, such as corals and mollusks, contribute significant \(\text{CaCO}_3\) volume, especially in shallow, tropical environments.
Environments Where Limestone Accumulates
The accumulation and preservation of biogenic \(\text{CaCO}_3\) require specific marine geological settings. Shallow marine environments, such as continental shelves and warm tropical seas, are highly conducive to limestone formation. High biological productivity from corals, calcareous algae, and mollusks rapidly generates skeletal debris here. The calm, warm, and shallow water minimizes dissolution, allowing thick layers of shell fragments and carbonate mud to build up quickly.
Deep marine environments also host limestone accumulation, though only under certain conditions dictated by ocean chemistry. As the biogenic material sinks, it is subject to dissolution, a process intensified by colder temperatures, greater pressure, and higher concentrations of dissolved carbon dioxide at depth. The Carbonate Compensation Depth (CCD) represents the depth at which the rate of \(\text{CaCO}_3\) supply exactly matches the rate of its dissolution.
Below the CCD, typically 4,000 to 5,000 meters in modern oceans, calcium carbonate dissolves completely before it can be buried, preventing the formation of limestone. Deep-sea limestones, often called chalks or pelagic oozes, are only preserved on the seafloor that remains above the CCD. Variations in water chemistry mean the CCD can be shallower in the Pacific Ocean and deeper in the Atlantic, influencing where deep-sea limestone can form.
Transforming Sediment into Solid Rock
The final stage in the limestone rock cycle is lithification, a process that converts the loose, water-rich sediment into solid rock over geological timescales. This transformation, also known as diagenesis, involves two primary mechanisms: compaction and cementation. As layers of new sediment accumulate, the weight of the overlying material causes compaction.
This pressure squeezes out the interstitial water from the pore spaces and forces the sediment grains closer together. Following compaction, cementation occurs as circulating groundwater, saturated with dissolved \(\text{CaCO}_3\), precipitates the mineral in the remaining pore spaces. This newly formed mineral cement acts as a glue, binding the skeletal fragments and carbonate mud together to create the solid limestone we observe in the geological record.