Calcite sea

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The alternation of calcite and aragonite seas through geologic time CalciteAragonite.jpg
The alternation of calcite and aragonite seas through geologic time

A calcite sea is a sea in which low-magnesium calcite is the primary inorganic marine calcium carbonate precipitate. An aragonite sea is the alternate seawater chemistry in which aragonite and high-magnesium calcite are the primary inorganic carbonate precipitates. The Early Paleozoic and the Middle to Late Mesozoic oceans were predominantly calcite seas, whereas the Middle Paleozoic through the Early Mesozoic and the Cenozoic (including today) are characterized by aragonite seas. [1] [2] [3] [4] [5] [6]

Jurassic hardground with encrusting oysters and borings CarmelHdgd.jpg
Jurassic hardground with encrusting oysters and borings

The most significant geological and biological effects of calcite sea conditions include rapid and widespread formation of carbonate hardgrounds, [7] [8] [9] calcitic ooids, [1] [10] calcite cements, [2] and the contemporaneous dissolution of aragonite shells in shallow warm seas. [6] [11] Hardgrounds were very common, for example, in the calcite seas of the Ordovician and Jurassic, but virtually absent from the aragonite seas of the Permian. [7]

Fossils of invertebrate organisms found in calcite sea deposits are usually dominated by either thick calcite shells and skeletons, [12] [13] [14] [15] were infaunal and/or had thick periostraca, [16] or had an inner shell of aragonite and an outer shell of calcite. [17] This was apparently because aragonite dissolved quickly on the seafloor and had to be either avoided or protected as a biomineral. [6]

Calcite seas were coincident with times of rapid seafloor spreading and global greenhouse climate conditions. [14] Seafloor spreading centers cycle seawater through hydrothermal vents, reducing the ratio of magnesium to calcium in the seawater through metamorphism of calcium-rich minerals in basalt to magnesium-rich clays. [2] [5] This reduction in the Mg/Ca ratio favors the precipitation of calcite over aragonite. Increased seafloor spreading also means increased volcanism 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. [5] Further, high calcium concentrations of seawater favor the burial of CaCO3, thereby removing alkalinity from the ocean, lowering seawater pH and reducing its acid/base buffering. [18]

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<span class="mw-page-title-main">Limestone</span> Type of sedimentary rock

Limestone is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of CaCO3. Limestone forms when these minerals precipitate out of water containing dissolved calcium. This can take place through both biological and nonbiological processes, though biological processes, such as the accumulation of corals and shells in the sea, have likely been more important for the last 540 million years. Limestone often contains fossils which provide scientists with information on ancient environments and on the evolution of life.

<span class="mw-page-title-main">Ordovician</span> Second period of the Paleozoic Era 485–444 million years ago

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 485.4 Ma to the start of the Silurian Period 443.8 Ma.

<span class="mw-page-title-main">Calcite</span> Calcium carbonate mineral

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 scratch hardness comparison. Large calcite crystals are used in optical equipment, and limestone composed mostly of calcite has numerous uses.

<span class="mw-page-title-main">Exoskeleton</span> External skeleton of an organism

An exoskeleton is a skeleton that is on the exterior of an animal to both support the body shape and protect the internal organs, in contrast to an internal endoskeleton which is enclosed underneath other soft tissues. Some large, hard and non-flexible protective exoskeletons are known as shell or armour.

<span class="mw-page-title-main">Lysocline</span> Depth in the ocean below which the rate of dissolution of calcite increases dramatically

The lysocline is the depth in the ocean dependent upon the carbonate compensation depth (CCD), usually around 5 km, below which the rate of dissolution of calcite increases dramatically because of a pressure effect. While the lysocline is the upper bound of this transition zone of calcite saturation, the CCD is the lower bound of this zone.

<span class="mw-page-title-main">Aragonite</span> Calcium carbonate mineral

Aragonite is a carbonate mineral and one of the three most common naturally occurring crystal forms of calcium carbonate, the others being calcite and vaterite. It is formed by biological and physical processes, including precipitation from marine and freshwater environments.

<span class="mw-page-title-main">Ooid</span> Small sedimentary grain that forms on shallow tropical seabeds

Ooids are small, 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, most commonly in shallow tropical seas. After being buried under additional sediment, these ooid grains can be cemented together to form a sedimentary rock called an oolite. Oolites usually consist of calcium carbonate; these belong to the limestone rock family. Pisoids are similar to ooids, but are larger than 2 mm in diameter, often considerably larger, as with the pisoids in the hot springs at Carlsbad in the Czech Republic.

<span class="mw-page-title-main">Dolomite (rock)</span> Sedimentary carbonate rock that contains a high percentage of the mineral dolomite

Dolomite (also known as dolomite rock, dolostone or dolomitic rock) is a sedimentary carbonate rock that contains a high percentage of the mineral dolomite, CaMg(CO3)2. It occurs widely, often in association with limestone and evaporites, though it is less abundant than limestone and rare in Cenozoic rock beds (beds less than about 66 million years in age). The first geologist to distinguish dolomite from limestone was Déodat Gratet de Dolomieu; a French mineralogist and geologist whom it is named after. He recognized and described the distinct characteristics of dolomite in the late 18th century, differentiating it from limestone.

<span class="mw-page-title-main">Bioerosion</span> Erosion of hard substrates by living organisms

Bioerosion describes the breakdown of hard ocean substrates – and less often terrestrial substrates – by living organisms. Marine bioerosion can be caused by mollusks, polychaete worms, phoronids, sponges, crustaceans, echinoids, and fish; it can occur on coastlines, on coral reefs, and on ships; its mechanisms include biotic boring, drilling, rasping, and scraping. On dry land, bioerosion is typically performed by pioneer plants or plant-like organisms such as lichen, and mostly chemical or mechanical in nature.

<span class="mw-page-title-main">Mid-ocean ridge</span> Basaltic underwater mountain system formed by plate tectonic spreading

A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about 2,600 meters (8,500 ft) and rises about 2,000 meters (6,600 ft) above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a divergent plate boundary. The rate of seafloor spreading determines the morphology of the crest of the mid-ocean ridge and its width in an ocean basin.

<span class="mw-page-title-main">Carbonate rock</span> Class of sedimentary rock

Carbonate rocks are a class of sedimentary rocks composed primarily of carbonate minerals. The two major types are limestone, which is composed of calcite or aragonite (different crystal forms of CaCO3), and dolomite rock (also known as dolostone), which is composed of mineral dolomite (CaMg(CO3)2). They are usually classified based on texture and grain size. Importantly, carbonate rocks can exist as metamorphic and igneous rocks, too. When recrystallized carbonate rocks are metamorphosed, marble is created. Rare igneous carbonate rocks even exist as intrusive carbonatites and, even rarer, there exists volcanic carbonate lava.

The carbonate compensation depth (CCD) is the depth, in the oceans, at which the rate of supply of calcium carbonates matches the rate of solvation. That is, solvation 'compensates' supply. Below the CCD solvation is faster, so that carbonate particles dissolve and the carbonate shells (tests) of animals are not preserved. Carbonate particles cannot accumulate in the sediments where the sea floor is below this depth.

<span class="mw-page-title-main">Carbonate hardgrounds</span>

Carbonate hardgrounds are surfaces of synsedimentarily cemented carbonate layers that have been exposed on the seafloor. A hardground is essentially, then, a lithified seafloor. Ancient hardgrounds are found in limestone sequences and distinguished from later-lithified sediments by evidence of exposure to normal marine waters. This evidence can consist of encrusting marine organisms, borings of organisms produced through bioerosion, early marine calcite cements, or extensive surfaces mineralized by iron oxides or calcium phosphates. Modern hardgrounds are usually detected by sounding in shallow water or through remote sensing techniques like side-scan sonar.

<span class="mw-page-title-main">Aragonite sea</span> Chemical conditions of the sea favouring aragonite deposition

An aragonite sea contains aragonite and high-magnesium calcite as the primary inorganic calcium carbonate precipitates. The chemical conditions of the seawater must be notably high in magnesium content relative to calcium for an aragonite sea to form. This is in contrast to a calcite sea in which seawater low in magnesium content relative to calcium favors the formation of low-magnesium calcite as the primary inorganic marine calcium carbonate precipitate.

Marine chemistry, also known as ocean chemistry or chemical oceanography, is the study of chemical content in marine environments as influenced by plate tectonics and seafloor spreading, turbidity, currents, sediments, pH levels, atmospheric constituents, metamorphic activity, and ecology. Marine life has adapted to the chemistries unique to Earth's oceans, and marine ecosystems are sensitive to changes in ocean chemistry.

<span class="mw-page-title-main">Shallow water marine environment</span>

Shallow water marine environment refers to the area between the shore and deeper water, such as a reef wall or a shelf break. This environment is characterized by oceanic, geological and biological conditions, as described below. The water in this environment is shallow and clear, allowing the formation of different sedimentary structures, carbonate rocks, coral reefs, and allowing certain organisms to survive and become fossils.

<span class="mw-page-title-main">Shell growth in estuaries</span>

Shell growth in estuaries is an aspect of marine biology that has attracted a number of scientific research studies. Many groups of marine organisms produce calcified exoskeletons, commonly known as shells, hard calcium carbonate structures which the organisms rely on for various specialized structural and defensive purposes. The rate at which these shells form is greatly influenced by physical and chemical characteristics of the water in which these organisms live. Estuaries are dynamic habitats which expose their inhabitants to a wide array of rapidly changing physical conditions, exaggerating the differences in physical and chemical properties of the water.

<span class="mw-page-title-main">Lawrence Alexander Hardie</span> American geologist (1933–2013)

Lawrence Alexander Hardie was an American geologist, sedimentologist, and geochemist.

<span class="mw-page-title-main">Marine biogenic calcification</span> Shell formation mechanism

Marine biogenic calcification is the production of calcium carbonate by organisms in the global ocean.

<span class="mw-page-title-main">Particulate inorganic carbon</span>

Particulate inorganic carbon (PIC) can be contrasted with dissolved inorganic carbon (DIC), the other form of inorganic carbon found in the ocean. These distinctions are important in chemical oceanography. Particulate inorganic carbon is sometimes called suspended inorganic carbon. In operational terms, it is defined as the inorganic carbon in particulate form that is too large to pass through the filter used to separate dissolved inorganic carbon.

References

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