Some of the components dissolved in seawater, especially calcium (Ca2+) and hydrogen carbonate (HCO3-) ions , are taken up by many groups of marine organisms to produce shells or skeletons composed of calcium carbonate, mainly as calcite.
When these organisms die, their remains accumulate on the sea floor and, if in sufficient abundance, eventually form limestones. Fragments of calcite shells from animal groups such as bivalves, brachiopods, trilobites, echinoids and crinoids are often found in ancient limestones. Some marine plants produce calcium carbonate too: for example, the Cretaceous Chalk deposits of southeast England and elsewhere are made mainly of the calcite plates of tiny phytoplankton called coccolithophores, along with some foraminiferans.
An influx of clay and silt in seawater inhibits the growth of many carbonate producing organisms, so limestones tend to indicate deposition in clear water, away from land-derived supplies of sediment (i.e. silicate minerals and rock fragments). The great majority of limestones are of marine origin, and most are composed of calcite.
The majority of ancient and modern carbonate sediments formed in shallow seas of the tropical–subtropical belt, between about 30oN and 30oS of the Equator. Here, reef-building organisms such as corals are common. Corals forming reefs today, such as the reef explored in Rough Science 5, are restricted in their distribution, thriving only in warm, clear, shallow, tropical seas, and many reefs that developed in similar circumstances can be recognized in ancient deposits.
Among the many components of limestones, there is fine carbonate mud, which can form in various ways. Imagine you have found a bed of limestone formed almost completely of calcium carbonate mud, which is now a hard rock. What inference can you make about the strength of currents active when the bed was deposited, and about the proximity at that time to sources of land-derived sediment?
The limestone must have accumulated in very quiet, low-energy conditions where currents were not strong enough to remove the carbonate mud. There was probably no nearby source of land-derived sediment, because this would have introduced material such as clay minerals and quartz.
The limestone contains small fragments of broken and haphazardly arranged crinoid stems and plates, some of which exceed five milimetres in length. The currents required to break up and move these sizeable crinoid fragments were certainly stronger than those that deposited only fine carbonate muds.
Some limestones contain calcium carbonate directly precipitated from seawater. For example, many well-known British limestones used for building, such as Portland Stone and Cotswold Stone, contain near-spherical grains about one milimetre across (resembling fish roe), in addition to shell fragments. Similar round grains form today by the precipitation of calcium carbonate around some kind of nucleus (such as a tiny shell fragment); they grow as they are rolled around in warm, wave-agitated, currentswept, shallow seas such as those at present around the Bahamas.