Automicrite

Last updated September 24, 2024

Automicrite is autochthonous micrite, that is, a carbonate mud precipitated in situ (no transporting) and made up of fine-grained calcite or aragonite micron-sized crystals. It precipitates on the sea floor or within the sediment as an authigenic mud thanks to physicochemical, microbial, photosynthetic and biochemical processes. It has peculiar fabrics and uniform mineralogical and chemical composition.^[1]

Environments where automicrite is formed

Automicrite deposits are found in different environments, as lakes, tidal flats, or the aphotic zone of marine slopes and basins. However, the environment where automicrite is most common is in carbonate mud mounds, a type of carbonate platform. In this kind of carbonate platform, common in the geological past but nearly absent today, automicrite deposits made carbonate structures of many tens to hundreds of metres of relief called "mud mounds". Automicritic mud mounds seemed to form more easily in areas of low sedimentation rate of siliciclastic extrabasinal sediment.^[2]

Usually, the presence of mud in sedimentary rocks is an indicator of low-energy conditions in a depositional system, so in high-energy conditions it is nearly impossible to find mud. Instead, automicrite can precipitate internally in cavities and sediment pores below the sediment–water interface, also in sediments that are formed in high-energy condition. Automicrite can also crystallize in suspension within cavities and then deposit on the cavity bottom. Usually, this process forms peloids, a kind of carbonate grains make by automicrite.^[3]

Features

In present carbonate platforms, automicrite is composed by aragonite needle-shaped crystals which have dimension of 1- 5 μm. Instead, in old limestones micrite is composed by μm-size calcite crystals.^[3]

Formation

Knowledge of the automicrites generation processes allow to make paleo-environmental interpretations, so it can become good instrument for basin analysis. Carbonate mud or micrite may originate through several processes, including the abiotic precipitation from highly supersaturated seawater or precipitation induced by microbial activity.^[4]

1. Abiotic micrites precipitation

There are two kinds of “inorganic” micrite:^[3]

- Internal micrite which precipitates inside cavities and inter-granular pores of sediments. It is a pore-filling micrite which has peloidal or clotted textures, and when it is found in limestones it may prove that marine lithification occurred.

- Seafloor micrite which precipitates at the sediment–water interface.

The equilibrium relationship for ${\ce {CO2, CaCO3}}$ in water is:

${\ce {H2O + CO2 + CaCO3 (solid) <-> Ca2+ (aq) + 2HCO3- (aq)}}$

Through temperature increase, pressure decrease, or a decrease in pH, the loss of ${\ce {CO2}}$ makes the reaction shift to the left. While ${\ce {CaCO3}}$ should precipitate spontaneously from seawater, which is supersaturated with respect to calcium carbonate, calcite precipitation is inhibited by the presence of Mg in seawater and aragonite crystallization is inhibited by organophosphatic molecules. In modern environments, carbonate mud seems to form spontaneously in seawater in whitings.^[3]

Whitings are clouds of suspended carbonate crystals (aragonite and Mg-calcite) that make the sea white. This phenomenon is common in tropically environments such as the Great Bahama Bank. Some attribute the formation of whitings to resuspension of carbonate sediment from sea bottom by waves.^[4]

2. Precipitation induced by microbial activity

Precipitation of carbonate mud from seawater may be triggered by biological activity as photosynthesis. In fact, the metabolism of some organisms remove dissolved CO₂ from seawater and thus promotes the precipitation of carbonate. In present sedimentary environments, the production of carbonate mud by cyanobacteria is known to occur in peritidal environments (Bahamas, Florida Bay, Persian Gulf) and in highly saline lakes (Coorong, Australia; Lake Tanganyika, eastern Africa). In this process is relevant the rule of filamentous cyanobacteria, in fact the calcite crystals formed within the organic filaments of cyanobacteria are retained after the death of the organism and consequently end up in the formation of layered carbonate mud rocks, the stromatolites. Micritic rocks of the geological past, e.g., the stromatolites of the Late Triassic Hauptdolomit of the Alps, may have had a similar origin. In many Jurassic micritic and peloidal limestones, remains of benthic coccoid cyanobacterial mats have been found.^[1]

Also bacterial sulfate reduction may promote carbonate mud precipitation from seawater. Bacterial sulfate reduction may be represented by the simplified reaction:

2CH + SO₄²⁻ → HCO₃⁻ + HS⁻ + H₂O

This process has the potential to promote precipitation by producing hydrogen carbonate ions, which are one of the reactants in the precipitation of carbonate from seawater.

Related Research Articles

<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.

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">Calcium carbonate</span> Chemical compound

Calcium carbonate is a chemical compound with the chemical formula CaCO₃. It is a common substance found in rocks as the minerals calcite and aragonite, most notably in chalk and limestone, eggshells, gastropod shells, shellfish skeletons and pearls. Materials containing much calcium carbonate or resembling it are described as calcareous. Calcium carbonate is the active ingredient in agricultural lime and is produced when calcium ions in hard water react with carbonate ions to form limescale. It has medical use as a calcium supplement or as an antacid, but excessive consumption can be hazardous and cause hypercalcemia and digestive issues.

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.

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.

A speleothem is a geological formation by mineral deposits that accumulate over time in natural caves. Speleothems most commonly form in calcareous caves due to carbonate dissolution reactions. They can take a variety of forms, depending on their depositional history and environment. Their chemical composition, gradual growth, and preservation in caves make them useful paleoclimatic proxies.

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.

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). One of the first geologists 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">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.

Beachrock is a friable to well-cemented sedimentary rock that consists of a variable mixture of gravel-, sand-, and silt-sized sediment that is cemented with carbonate minerals and has formed along a shoreline. Depending on location, the sediment that is cemented to form beachrock can consist of a variable mixture of shells, coral fragments, rock fragments of different types, and other materials. It can contain scattered artifacts, pieces of wood, and coconuts. Beachrock typically forms within the intertidal zone within tropical or semitropical regions. However, Quaternary beachrock is also found as far north and south as 60° latitude.

<span class="mw-page-title-main">Calcite sea</span> Sea chemistry favouring low-magnesium calcite as the inorganic calcium carbonate precipitate

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 are characterized by aragonite seas.

<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.

<span class="mw-page-title-main">Cementation (geology)</span> Process of chemical precipitation bonding sedimentary grains

A brief, easy-to-understand description of cementation is that minerals bond grains of sediment together by growing around them. This process is called cementation and is a part of the rock cycle.

Micrite is a limestone constituent formed of calcareous particles ranging in diameter up to four μm formed by the recrystallization of lime mud.

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.

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.

A whiting event is a phenomenon that occurs when a suspended cloud of fine-grained calcium carbonate precipitates in water bodies, typically during summer months, as a result of photosynthetic microbiological activity or sediment disturbance. The phenomenon gets its name from the white, chalky color it imbues to the water. These events have been shown to occur in temperate waters as well as tropical ones, and they can span for hundreds of meters. They can also occur in both marine and freshwater environments. The origin of whiting events is debated among the scientific community, and it is unclear if there is a single, specific cause. Generally, they are thought to result from either bottom sediment re-suspension or by increased activity of certain microscopic life such as phytoplankton. Because whiting events affect aquatic chemistry, physical properties, and carbon cycling, studying the mechanisms behind them holds scientific relevance in various ways.

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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

1 2 Erik., Flügel (2010). Microfacies of carbonate rocks : analysis, interpretation and application. Munnecke, Axel. (2nd ed.). Heidelberg: Springer. ISBN 9783642037962. OCLC 663093942.
↑ Keim, Lorenz; Schlager, Wolfgang (1999). "Automicrite facies on steep slopes (Triassic, Dolomites, Italy)". Facies. 41 (1): 15–25. Bibcode:1999Faci...41...15K. doi:10.1007/bf02537457. ISSN 0172-9179. S2CID 129934640.
1 2 3 4 Sam., Boggs (2009). Petrology of sedimentary rocks (2nd ed.). Cambridge: Cambridge University Press. ISBN 9780511516429. OCLC 500960599.
1 2 Wolfgang, Schlager (2005). Carbonate Sedimentology and Sequence Stratigraphy. SEPM (Society for Sedimentary Geology). Tulsa, OK. ISBN 978-1565761162. OCLC 61364867.{{cite book}}: CS1 maint: location missing publisher (link)

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[:0-1] 1 2 Erik., Flügel (2010). Microfacies of carbonate rocks : analysis, interpretation and application. Munnecke, Axel. (2nd ed.). Heidelberg: Springer. ISBN 9783642037962. OCLC 663093942.

[2] Keim, Lorenz; Schlager, Wolfgang (1999). "Automicrite facies on steep slopes (Triassic, Dolomites, Italy)". Facies. 41 (1): 15–25. Bibcode:1999Faci...41...15K. doi:10.1007/bf02537457. ISSN 0172-9179. S2CID 129934640.

[:1-3] 1 2 3 4 Sam., Boggs (2009). Petrology of sedimentary rocks (2nd ed.). Cambridge: Cambridge University Press. ISBN 9780511516429. OCLC 500960599.

[:2-4] 1 2 Wolfgang, Schlager (2005). Carbonate Sedimentology and Sequence Stratigraphy. SEPM (Society for Sedimentary Geology). Tulsa, OK. ISBN 978-1565761162. OCLC 61364867.{{cite book}}: CS1 maint: location missing publisher (link)

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