structural:summarization
Formation

Limestone forms when calcite or aragonite precipitate out of water containing dissolved calcium, which can take place through both biological and nonbiological processes. The solubility of calcium carbonate (CaCO) is controlled largely by the amount of dissolved carbon dioxide (CO) in the water. This is summarized in the reaction:

Increases in temperature or decreases in pressure tend to reduce the amount of dissolved CO and precipitate CaCO. Reduction in salinity also reduces the solubility of CaCO, by several orders of magnitude for fresh water versus seawater.

Near-surface water of the earth's oceans are oversaturated with CaCO by a factor of more than six. The failure of CaCO to rapidly precipitate out of these waters is likely due to interference by dissolved magnesium ions with nucleation of calcite crystals, the necessary first step in precipitation. Precipitation of aragonite may be suppressed by the presence of naturally occurring organic phosphates in the water. Although ooids likely form through purely inorganic processes, the bulk of CaCO precipitation in the oceans is the result of biological activity. Much of this takes place on carbonate platforms.

The origin of carbonate mud, and the processes by which it is converted to micrite, continue to be a subject of research. Modern carbonate mud is composed mostly of aragonite needles around 5 μm (0.20 mils) in length. Needles of this shape and composition are produced by calcareous algae such as Penicillus, making this a plausible source of mud. Another possibility is direct precipitation from the water. A phenomenon known as whitings occurs in shallow waters, in which white streaks containing dispersed micrite appear on the surface of the water. It is uncertain whether this is freshly precipitated aragonite or simply material stirred up from the bottom, but there is some evidence that whitings are caused by biological precipitation of aragonite as part of a bloom of cyanobacteria or microalgae. However, stable isotope ratios in modern carbonate mud appear to be inconsistent with either of these mechanisms, and abrasion of carbonate grains in high-energy environments has been put forward as a third possibility.

Formation of limestone has likely been dominated by biological processes throughout the Phanerozoic, the last 540 million years of the Earth's history. Limestone may have been deposited by microorganisms in the Precambrian, prior to 540 million years ago, but inorganic processes were probably more important and likely took place in an ocean more highly oversaturated in calcium carbonate than the modern ocean.