A calcite sea is a sea in which low-magnesium

calcite Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scratc ...

is the primary inorganic marine calcium carbonate precipitate. An aragonite sea is the alternate seawater chemistry in which

aragonite Aragonite is a carbonate mineral, one of the three most common naturally occurring crystal forms of calcium carbonate, (the other forms being the minerals calcite and vaterite). It is formed by biological and physical processes, including pr ...

and high-magnesium calcite are the primary inorganic carbonate precipitates. The Early

Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and '' ...

and the Middle to Late

Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretace ...

oceans were predominantly calcite seas, whereas the Middle Paleozoic through the Early Mesozoic and the

Cenozoic The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configurat ...

(including today) are characterized by aragonite seas. The most significant geological and biological effects of calcite sea conditions include rapid and widespread formation of carbonate hardgrounds, calcitic

ooids Ooids are small (commonly ≤2 mm in diameter), spheroidal, "coated" (layered) sedimentary grains, usually composed of calcium carbonate, but sometimes made up of iron- or phosphate-based minerals. Ooids usually form on the sea floor, mo ...

, calcite cements, and the contemporaneous dissolution of aragonite shells in shallow warm seas. Hardgrounds were very common, for example, in the calcite seas of the

Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. The ...

and Jurassic, but virtually absent from the aragonite seas of the

Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleoz ...

Fossils A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in ...

invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...

organisms found in calcite sea deposits are usually dominated by either thick calcite shells and skeletons, were infaunal and/or had thick periostraca, or had an inner shell of aragonite and an outer shell of calcite. This was apparently because aragonite dissolved quickly on the seafloor and had to be either avoided or protected as a biomineral. Calcite seas were coincident with times of rapid

seafloor spreading Seafloor spreading or Seafloor spread is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. History of study Earlier theories by Alfred Wegener an ...

and global greenhouse climate conditions. Seafloor spreading centers cycle seawater through

hydrothermal vent A hydrothermal vent is a fissure on the seabed from which geothermally heated water discharges. They are commonly found near volcanically active places, areas where tectonic plates are moving apart at mid-ocean ridges, ocean basins, and hotspot ...

s, reducing the ratio of magnesium to calcium in the seawater through

metamorphism Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of chem ...

of calcium-rich minerals in basalt to magnesium-rich clays. This reduction in the Mg/Ca ratio favors the precipitation of calcite over aragonite. Increased seafloor spreading also means increased

volcanism Volcanism, vulcanism or volcanicity is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics, and volcanic gases erupt through a break in the surface called a ...

and elevated levels of carbon dioxide in the atmosphere and oceans. This may also have an effect on which polymorph of calcium carbonate is precipitated. Further, high calcium concentrations of seawater favor the burial of CaCO₃, thereby removing

alkalinity Alkalinity (from ar, القلوي, al-qaly, lit=ashes of the saltwort) is the capacity of water to resist acidification. It should not be confused with basicity, which is an absolute measurement on the pH scale. Alkalinity is the strength o ...

from the ocean, lowering seawater pH and reducing its acid/base buffering. Image:CalciteSeasTable.jpg, Table showing the conditions for calcite and aragonite seas Image:MgCaRatioChanges.jpg,

Tectonic Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents k ...

mechanism for changing Mg/Ca ratios in seawater File:OoidSurface01.jpg, Grainstone with calcitic

and sparry calcite cement; Carmel Formation, Middle Jurassic, of southern Utah Image:EncrustedBivalveMold.jpg, An encrusted

bivalve Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, biva ...

external mold showing contemporaneous dissolution of the original aragonite shell and calcitic cementation of the mold File:Encrusted Ordovician nautiloid InternalExternal.jpg, An encrusted

nautiloid Nautiloids are a group of marine cephalopods (Mollusca) which originated in the Late Cambrian and are represented today by the living ''Nautilus'' and ''Allonautilus''. Fossil nautiloids are diverse and speciose, with over 2,500 recorded species. ...

internal mold showing contemporaneous dissolution of the original aragonite shell and calcitic cementation Image:Paleosabella4CS.jpg, The boring ''Palaeosabella'' in an

shell. The borings penetrated an inner aragonitic shell layer which dissolved away. Image:Petroxestes_borings_Ordovician.jpg, ''

Petroxestes ''Petroxestes'' is a shallow, elongate boring (a type of trace fossil) originally found excavated in carbonate skeletons and hardgrounds of the Upper Ordovician of North America (Wilson and Palmer, 1988, 2006). These Ordovician borings were l ...

'' borings in an Upper

hardground Carbonate hardgrounds are surfaces of synsedimentarily cemented carbonate layers that have been exposed on the seafloor (Wilson and Palmer, 1992). A hardground is essentially, then, a lithified seafloor. Ancient hardgrounds are found in limestone ...

, southern Ohio

References

{{Reflist Limestone Marine geology Historical geology