Geochemistry of carbon

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The geochemistry of carbon is the study of the transformations involving the element carbon within the systems of the Earth. To a large extent this study is organic geochemistry, but it also includes the very important carbon dioxide. Carbon is transformed by life, and moves between the major phases of the Earth, including the water bodies, atmosphere, and the rocky parts. Carbon is important in the formation of organic mineral deposits, such as coal, petroleum or natural gas. Most carbon is cycled through the atmosphere into living organisms and then respirated back into the atmosphere. However an important part of the carbon cycle involves the trapping of living matter into sediments. The carbon then becomes part of a sedimentary rock when lithification happens. Human technology or natural processes such as weathering, or underground life or water can return the carbon from sedimentary rocks to the atmosphere. From that point it can be transformed in the rock cycle into metamorphic rocks, or melted into igneous rocks. Carbon can return to the surface of the Earth by volcanoes or via uplift in tectonic processes. Carbon is returned to the atmosphere via volcanic gases. Carbon undergoes transformation in the mantle under pressure to diamond and other minerals, and also exists in the Earth's outer core in solution with iron, and may also be present in the inner core.

Carbon can form a huge variety stable compounds. It is an essential component of living matter. Living organisms can live in a limited range of conditions on the Earth that are limited by temperature and the existence of liquid water. The potential habitability of other planets or moons can also be assessed by the existence of liquid water.

Carbon makes up only 0.08% of the combination of the lithosphere, hydrosphere, and atmosphere. Yet it is the twelfth most common element there. In the rock of the lithosphere, carbon commonly occurs as carbonate minerals containing calcium or magnesium. It is also found as fossil fuels in coal and petroleum and gas. Native forms of carbon are much rarer, requiring pressure to form. Pure carbon exists as graphite or diamond.

The deeper parts of Earth such as the mantle are very hard to discover. Few samples are known, in the form of uplifted rocks, or xenoliths. Even fewer remain in the same state they were in where the pressure and temperature is much higher. Some diamonds retain inclusions held at pressures they were formed at, but the temperature is much lower at the surface. Iron meteorites may represent samples of the core of an asteroid, but it would have formed under different conditions to the Earth's core. Therefore, experimental studies are conducted in which minerals or substances are compressed and heated to determine what happens in similar conditions to the planetary interior.

The two common isotopes of carbon are stable. On Earth, carbon 12, ¹²C is by far the most common at 98.894%. Carbon 13 is much rarer averaging 1.106%. This percentage can vary slightly and its value is important in isotope geochemistry whereby the origin of the carbon is suggested.

Origins

Carbon cycle

Formation

Carbon can be produced in stars at least as massive as the Sun by fusion of three helium-4 nuclei: ⁴He + ⁴He + ⁴He --> ¹²C. This is the triple alpha process. In stars as massive as the Sun, carbon 12 is also converted to carbon 13 and then onto nitrogen 14 by fusion with protons. ¹²C + ¹H --> ¹³C + e⁺. ¹³C + ¹H --> ¹⁴N. In more massive stars, two carbon nuclei can fuse to magnesium, or a carbon and an oxygen to sulfur.

Astrochemistry

In molecular clouds, simple carbon molecules are formed, including carbon monoxide and dicarbon. Reactions with the trihydrogen cation of the simple carbon molecules yield carbon containing ions that readily react to form larger organic molecules. Carbon compounds that exist as ions, or isolated gas molecules in the interstellar medium, can condense onto dust grains. Carbonaceous dust grains consist mostly of carbon. Grains can stick together to form larger aggregates.

Earth formation

Meteorites and interplanetary dust shows the composition of solid material at the start of the Solar System, as they have not been modified since its formation. Carbonaceous chondrites are meteorites with around 5% carbon compounds. Their composition resembles the Sun's minus the very volatile elements like hydrogen and noble gases. The Earth is believed to have formed by the gravitational collapse of material like meteorites.

Important effects on Earth in the first Hadian Era include strong solar winds during the T-Tauri stage of the Sun. The Moon forming impact caused major changes to the surface. Juvenile volatiles outgased from the early molten surface of the Earth. These included carbon dioxide and carbon monoxide. The emissions probably did not include methane, but the Earth was probably free of molecular oxygen. The Late Heavy Bombardment was between 4.0 and 3.8 billion years ago (Ga). To start with, the Earth did not have a crust as it does today. Plate tectonics in its present form commenced about 2.5 Ga.

Early sedimentary rocks formed under water date to 3.8 Ga. Pillow lavas dating from 3.5 Ga prove the existence of oceans. Evidence of early life is given by fossils of stromatolites, and later by chemical tracers.

Organic matter continues to be added to the Earth from space via interplanetary dust, which also includes some interstellar particles. The amounts added to the Earth were around 60,000 tonnes per year about 4 Ga.

Isotope

Biological sequestration of carbon causes enrichment of carbon-12, so that substances that originate from living organisms have a higher carbon-12 content. Due to the kinetic isotope effect, chemical reactions can happen faster with lighter isotopes, so that photosynthesis fixes lighter carbon-12 faster than carbon-13. Also lighter isotopes diffuse across a biological membrane faster. Enrichment in carbon 13 is measured by delta ¹³C(o/oo) = [(¹³C/¹²C)sample/(¹³C/¹²C)standard - 1] * 1000. The common standard for carbon is Cretaceous Peedee formation belemnite.

Stereoisomers

Complex molecules, in particular those containing carbon can be in the form of stereoisomers. With abiotic processes they would be expected to be equally likely, but in carbonaceous chondrites this is not the case. The reasons for this are unknown.

Crust

The outer layer of the Earth, the crust along with its outer layers contain about 10²⁰ kg of carbon. This is enough for each square meter of the surface to have 200 tons of carbon.

Sedimentation

Carbon added to sedimentary rocks can take the form of carbonates, or organic carbon compounds. In order of source quantity the organic carbon comes from phytoplankton, plants, bacteria and zooplankton. However terrestrial sediments may be mostly from higher plants, and some oxygen deficient sediments from water may be mostly bacteria. Fungi and other animals make insignificant contributions. On the oceans the main contributor of organic matter to sediments is plankton, either dead fragments or faecal pellets termed marine snow. Bacteria degrade this matter in the water column, and the amount surviving to the ocean floor is inversely proportional to the depth. This is accompanied by biominerals consisting of silicates and carbonates. The particulate organic matter in sediments is about 20% of known molecules 80% of material that cannot be analysed. Detritivores consume some of the fallen organic materials. Aerobic bacteria and fungi also consume organic matter in the oxic surface parts of the sediment. Coarse-grained sediments are oxygenated to about half a meter, but fine grained clays may only have a couple of millimetres exposed to oxygen. The organic matter in the oxygenated zone will become completely mineralized if it stays there long enough.

Deeper in sediments where oxygen is exhausted, anaerobic biological processes continue at a slower rate. These include anaerobic mineralization making ammonium, phosphate and sulfide ions; fermentation making short chain alcohols, acids or methyl amines; acetogenesis making acetic acid; methanogenesis making methane, and sulfate, nitrite and nitrate reduction. Carbon dioxide and hydrogen are also outputs. Under freshwater, sulfate is usually very low, so methanogensis is more important. Yet other bacteria can convert methane, back into living matter, by oxidising with other substrates. Bacteria can reside at great depths in sediments. However sedimentary organic matter accumulates the indigestible components.

Deep bacteria may be lithotrophes, using hydrogen, and carbon dioxide as a carbon source.

In the oceans and other waters there is much dissolved organic materials. These are several thousand years old on average, and are called gelbstoff (yellow substance) particularly in fresh waters. Much of this is tannins. The nitrogen containing materials here appear to be amides, perhaps from peptidoglycans from bacteria. Microorganisms have trouble consuming the high molecular weight dissolved substances, but quickly consume small molecules.

From terrestrial sources black carbon produced by charring is an important component. Fungi are important decomposers in soil.

Macromolecules

Proteins are normally hydrolysed slowly even without enzymes or bacteria, with a half life of 460 years, but can be preserved if they are desiccated, pickled or frozen. Being enclosed in bone also helps preservation. Over time the amino acids tend to racemize, and those with more functional groups are lost earlier. Protein still will degrade on the timescale of a million years. DNA degrades rapidly, lasting only about four years in water. Cellulose and chitin have a half life in water at 25° of about 4.7 million years. Enzymes can accelerate this by a factor of 10¹⁷. About 10¹¹ tons of chiting are produced each year, but it is almost all degraded.

Lignin is only efficiently degraded by fungi, white rot, or brown rot. These require oxygen.

Lipids are hydrolysed to fatty acids over long time periods. Plant cuticle waxes are very difficult to degrade, and may survive over geological time periods.

Preservation

More organic matter is preserved in sediments if there is high primary production, or the sediment is fine-grained. The lack of oxygen helps preservation greatly, and that also is caused by a large supply of organic matter. Soil does not usually preserve organic matter, it would need to be acidified or water logged, as in the bog. Rapid burial ensures the material gets to an oxygen free depth, but also dilutes the organic matter. A low energy environment ensures the sediment is not stirred up and oxygenated. Salt marshes and mangroves meet some of these requirements, but unless the sea level is rising will not have a chance to accumulate much. Coral reefs are very productive, but are well oxygenated, and recycle everything before it is buried.

Sphagnum bog

In dead Sphagnum, sphagnan a polysaccharide with D-lyxo-5-hexosulouronic acid is a major remaining substance. It makes the bog very acidic, so that bacteria cannot grow. Not only that, the plant ensures there is no available nitrogen. Holocellulose also absorbs any digestive enzymes around. Together this leads to major accumulation of peat under sphagnum bogs.

Mantle

Earth's mantle is a significant reservoir of carbon. The mantle contains more carbon than the crust, oceans, biosphere, and atmosphere put together. The figure is estimated to be very roughly 10²² kg. Carbon concentration in the mantle is very variable, varying by more than a factor of 100 between different parts.

The form carbon takes depends on its oxidation state, which depends on the oxygen fugacity of the environment. Carbon dioxide and carbonate are found where the oxygen fugacity is high. Lower oxygen fugacity results in diamond formation, first in eclogite, then peridotite, and lastly in fluid water mixtures. At even lower oxygen fugacity, methane is stable in contact with water, and even lower, metallic iron and nickel form along with carbides. Iron carbides include Fe₃C and Fe₇C₃.

Minerals that contain carbon include calcite and its higher density polymorphs. Other significant carbon minerals include magnesium and iron carbonates. Dolomite is stable above 100 km depth. Below 100 km, dolomite reacts with orthopyroxine (found in peridotite) to yield magnesite (an iron magnesium carbonate). Below 200 km deep, carbon dioxide is reduced by ferrous iron (Fe²⁺), forming diamond, and ferric iron (Fe³⁺). Even deeper pressure induced disproportionation of iron minerals produces more ferric iron, and metallic iron. The metallic iron combines with carbon to form the mineral cohenite with formula Fe₃C. Cohenite also contains some nickel substituting for iron. This form or carbon is called "carbide". Diamond forms in the mantle below 150 km deep, but because it is so durable, it can survive in eruptions to the surface in kimberlites, lamproites, or ultramafic lamprophyres.

Xenoliths can come from the mantle, and different compositions come from different depths. Above 90 km (3.2 GPa) spinel peridotite occurs, below this garnet peridotite is found.

Inclusions trapped in diamond can reveal the material and conditions much deeper in the mantle. Large gem diamonds are usually formed in the transition zone part of the mantle, (410 to 660 km deep) and crystallise from a molten iron-nickel-carbon solution, that also contains sulfur and trace amounts of hydrogen, chromium, phosphorus and oxygen. Carbon atoms constitute about 12% of the melt (about 3% by mass). Inclusions of the crystallised metallic melt are sometimes included in diamonds. Diamond can be caused to precipitate from the liquid metal, by increasing pressure, or by adding sulfur.

Fluid inclusions in crystals from the mantle have contents that most often are liquid carbon dioxide, but which also include carbon oxysulfide, methane and carbon monoxide

Material is added by subduction from the crust. This includes the major carbon containing sediments such as limestone, or coal. Each year 2×10¹¹ kg of CO₂ is transferred from the crust to the mantle by subduction. (1700 tons of carbon per second).

Upwelling mantle material can add to the crust at mid oceanic ridges. Fluids can extract carbon from the mantle and erupt in volcanoes. At 330 km deep a liquid consisting of carbon dioxide and water can form. It is highly corrosive, and dissolves incompatible elements from the solid mantle. These elements include uranium, thorium, potassium, helium and argon. The fluids can then go on to cause metasomatism or extend to the surface in carbonatite eruptions. The total mid oceanic ridge, and hot spot volcanic emissions of carbon dioxide match the loss due to subduction: 2×10¹¹ kg of CO₂ per year.

In slowly convecting mantle rocks, diamond that slowly rises above 150 km will slowly turn into graphite or be oxidised to carbon dioxide or carbonate minerals.

Core

Earth's core is believed to be mostly an alloy of iron and nickel. The density indicates that it also contains a significant amount of lighter elements. Elements such as hydrogen would be stable in the Earth's core, however the conditions at the formation of the core would not be suitable for its inclusion. Carbon is a very likely constituent of the core. Preferential partitioning of the carbon isotope¹²C into the metallic core, during its formation, may explain why there seems to be more ¹³C on the surface and mantle of the Earth compared to other solar system bodies (−5‰ compared to -20‰). The difference can also help to predict the value of the carbon proportion of the core.

The outer core has a density around 11 cm⁻³, and a mass of 1.3×10²⁴kg. It contains roughly 10²² kg of carbon. Carbon dissolved in liquid iron affect the solution of other elements. Dissolved carbon changes lead from a siderophile to a lithophile. It has the opposite effect on tungsten and molybdenum, causing more tungsten or molybdenum to dissolve in the metallic phase. The measured amounts of these elements in the rocks compared to the Solar System can be explained by a 0.6% carbon composition of the core.

The inner core is about 1221 km in radius. It has a density of 13 g cm⁻³, and a total mass of 9×10²² kg and a surface area of 18,000,000 square kilometers. Experiments with mixtures under pressure and temperature attempt to reproduce the known properties of the inner and outer core. Carbides are among the first to precipitate from a molten metal mix, and so the inner core may be mostly iron carbides, Fe₇C₃ or Fe₃C. At atmospheric pressure (100 kPa) the iron-Fe₃C eutectic point is at 4.1% carbon. This percentage decreases as pressure increases to around 50 GPa. Above that pressure the percentage of carbon at the eutectic increases. The pressure on the inner core ranges from 330 GPa to 360 GPa at the centre of the Earth. The temperature at the inner core surface is about 6000 K. The material of the inner core must be stable at the pressure and temperature found there, and more dense than that of the outer core liquid. Extrapolations show that either Fe₃C or Fe₇C₃ match the requirements. Fe₇C₃ is 8.4% carbon, and Fe₃C is 6.7% carbon. The inner core is growing by about 1 mm per year, or adding about 18 cubic kilometres per year. This is about 18×10¹²kg of carbon added to the inner core every year. It contains about 8×10²¹ kg of carbon.

High pressure experimentation

In order to determine the fate of natural carbon containing substances deep in the Earth, experiments have been conducted to see what happens when high pressure, and or temperatures are applied. Such substances include carbon dioxide, carbon monoxide, graphite, methane, and other hydrocarbons such as benzene, carbon dioxide water mixtures and carbonate minerals such as calcite, magnesium carbonate, or ferrous carbonate. Under super high pressures carbon may take on a higher coordination number than the four found in sp³ compounds like diamond, or the three found in carbonates. Perhaps carbon can substitute into silicates, or form a silicon oxycarbide. Carbides may be possible.

Carbon

At 15 GPa graphite changes to a hard transparent form, that is not diamond. Diamond is very resistant to pressure, but at about 1 TPa (1000 GPa) transforms to a BC-8 form.

Carbides

Carbides are predicted to be more likely lower in the mantle as experiments have shown a much lower oxygen fugacity for high pressure iron silicates. Cohenite remains stable to over 187 GPa, but is predicted to have a denser orthorhombic Cmcm form in the inner core.

Carbon dioxide

Under 0.3 GPa pressure, carbon dioxide is stable at room temperature in the same form as dry ice. Over 0.5 GPa carbon dioxide forms a number of different solid forms containing molecules. At pressures over 40 GPa and high temperatures, carbon dioxide forms a covalent solid that contains CO₄ tetrahedra, and has the same structure as β-cristobalite. This is called phase V or CO₂-V. When CO₂-V is subjected to high temperatures, or higher pressures, experiments show it breaks down to form diamond and oxygen. In the mantle the geotherm would mean that carbon dioxide would be a liquid till a pressure of 33 GPa, then it would adopt the solid CO₂-V form till 43 GPa, and deeper than that would make diamond and fluid oxygen.

Carbonyls

High pressure carbon monoxide forms the high energy polycarbonyl covalent solid, however it is not expected to be present inside the Earth.

Hydrocarbons

Under 1.59 GPa pressure at 25 °C, methane converts to a cubic solid. The molecules are rotationally disordered. But over 5.25 GPa the molecules become locked into position and cannot spin. Other hydrocarbons under high pressure have hardly been studied.

Carbonates

Calcite changes to calcite-II and calcite-III at pressures of 1.5, and 2.2 GPa. Siderite undergoes a chemical change at 10 GPa at 1800K to form Fe₄O₅. Dolomite decomposes 7GPa and below 1000 °C to yield aragonite and magnesite. However, there are forms of iron containing dolomite stable at higher pressures and temperatures. Over 130 GPa aragonite undergoes a transformation to a SP³ tetrahedrally connected carbon, in a covalent network in a C222₁ structure. Magnesite can survive 80 GPa, but with more than 100 GPa (as at a depth of 1800 km it changes to forms with three-member rings of CO₄ tetrahedra (C₃O₉⁶⁻). If iron is present in this mineral, at these pressures it will convert to magnetite and diamond. Melted carbonates with SP³ carbon are predicted to be very viscous.

Some minerals that contain both silicate and carbonate exist, spurrite and tilleyite. But high-pressure forms have not been studied. There have been attempts to make silicon carbonate. Six coordinated silicates mixed with carbonate should not exist on Earth, but may exist on more massive planets.

Biosphere

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Biosphere

 

A false-color composite of global oceanic and terrestrial photoautotroph abundance, from September 2001 to August 2017. Provided by the SeaWiFS Project, NASA/Goddard Space Flight Center and ORBIMAGE.

The biosphere (from Greek βίος bíos "life" and σφαῖρα sphaira "sphere"), also known as the ecosphere (from Greek οἶκος oîkos "environment" and σφαῖρα), is the worldwide sum of all ecosystems. It can also be termed the zone of life on Earth. The biosphere is virtually a closed system with regard to matter, with minimal inputs and outputs. With regard to energy, it is an open system, with photosynthesis capturing solar energy at a rate of around 130 Terawatts per year. However it is a self-regulating system close to energetic equilibrium. By the most general biophysiological definition, the biosphere is the global ecological system integrating all living beings and their relationships, including their interaction with the elements of the lithosphere, cryosphere, hydrosphere, and atmosphere. The biosphere is postulated to have evolved, beginning with a process of biopoiesis (life created naturally from non-living matter, such as simple organic compounds) or biogenesis (life created from living matter), at least some 3.5 billion years ago.

In a general sense, biospheres are any closed, self-regulating systems containing ecosystems. This includes artificial biospheres such as Biosphere 2 and BIOS-3, and potentially ones on other planets or moons.

Origin and use of the term

A beach scene on Earth, simultaneously showing the lithosphere (ground), hydrosphere (ocean) and atmosphere (air)

The term "biosphere" was coined by geologist Eduard Suess in 1875, which he defined as the place on Earth's surface where life dwells.

While the concept has a geological origin, it is an indication of the effect of both Charles Darwin and Matthew F. Maury on the Earth sciences. The biosphere's ecological context comes from the 1920s (see Vladimir I. Vernadsky), preceding the 1935 introduction of the term "ecosystem" by Sir Arthur Tansley (see ecology history). Vernadsky defined ecology as the science of the biosphere. It is an interdisciplinary concept for integrating astronomy, geophysics, meteorology, biogeography, evolution, geology, geochemistry, hydrology and, generally speaking, all life and Earth sciences.

Narrow definition

Geochemists define the biosphere as being the total sum of living organisms (the "biomass" or "biota" as referred to by biologists and ecologists). In this sense, the biosphere is but one of four separate components of the geochemical model, the other three being geosphere, hydrosphere, and atmosphere. When these four component spheres are combined into one system, it is known as the Ecosphere. This term was coined during the 1960s and encompasses both biological and physical components of the planet.

The Second International Conference on Closed Life Systems defined biospherics as the science and technology of analogs and models of Earth's biosphere; i.e., artificial Earth-like biospheres. Others may include the creation of artificial non-Earth biospheres—for example, human-centered biospheres or a native Martian biosphere—as part of the topic of biospherics.

Earth's biosphere

Age

Stromatolite fossil estimated at 3.2–3.6 billion years old

The earliest evidence for life on Earth includes biogenic graphite found in 3.7 billion-year-old metasedimentary rocks from Western Greenland and microbial mat fossils found in 3.48 billion-year-old sandstone from Western Australia. More recently, in 2015, "remains of biotic life" were found in 4.1 billion-year-old rocks in Western Australia. In 2017, putative fossilized microorganisms (or microfossils) were announced to have been discovered in hydrothermal vent precipitates in the Nuvvuagittuq Belt of Quebec, Canada that were as old as 4.28 billion years, the oldest record of life on earth, suggesting "an almost instantaneous emergence of life" after ocean formation 4.4 billion years ago, and not long after the formation of the Earth 4.54 billion years ago. According to biologist Stephen Blair Hedges, "If life arose relatively quickly on Earth ... then it could be common in the universe."

Extent

Rüppell's vulture

 

Xenophyophore, a barophilic organism, from the Galapagos Rift.

Every part of the planet, from the polar ice caps to the equator, features life of some kind. Recent advances in microbiology have demonstrated that microbes live deep beneath the Earth's terrestrial surface, and that the total mass of microbial life in so-called "uninhabitable zones" may, in biomass, exceed all animal and plant life on the surface. The actual thickness of the biosphere on earth is difficult to measure. Birds typically fly at altitudes as high as 1,800 m (5,900 ft; 1.1 mi) and fish live as much as 8,372 m (27,467 ft; 5.202 mi) underwater in the Puerto Rico Trench.

There are more extreme examples for life on the planet: Rüppell's vulture has been found at altitudes of 11,300 m (37,100 ft; 7.0 mi); bar-headed geese migrate at altitudes of at least 8,300 m (27,200 ft; 5.2 mi); yaks live at elevations as high as 5,400 m (17,700 ft; 3.4 mi) above sea level; mountain goats live up to 3,050 m (10,010 ft; 1.90 mi). Herbivorous animals at these elevations depend on lichens, grasses, and herbs.

Life forms live in every part of the Earth's biosphere, including soil, hot springs, inside rocks at least 19 km (12 mi) deep underground, the deepest parts of the ocean, and at least 64 km (40 mi) high in the atmosphere. Microorganisms, under certain test conditions, have been observed to survive the vacuum of outer space. The total amount of soil and subsurface bacterial carbon is estimated as 5 × 10¹⁷ g, or the "weight of the United Kingdom". The mass of prokaryote microorganisms—which includes bacteria and archaea, but not the nucleated eukaryote microorganisms—may be as much as 0.8 trillion tons of carbon (of the total biosphere mass, estimated at between 1 and 4 trillion tons). Barophilic marine microbes have been found at more than a depth of 10,000 m (33,000 ft; 6.2 mi) in the Mariana Trench, the deepest spot in the Earth's oceans. In fact, single-celled life forms have been found in the deepest part of the Mariana Trench, by the Challenger Deep, at depths of 11,034 m (36,201 ft; 6.856 mi). Other researchers reported related studies that microorganisms thrive inside rocks up to 580 m (1,900 ft; 0.36 mi) below the sea floor under 2,590 m (8,500 ft; 1.61 mi) of ocean off the coast of the northwestern United States, as well as 2,400 m (7,900 ft; 1.5 mi) beneath the seabed off Japan. Culturable thermophilic microbes have been extracted from cores drilled more than 5,000 m (16,000 ft; 3.1 mi) into the Earth's crust in Sweden, from rocks between 65–75 °C (149–167 °F). Temperature increases with increasing depth into the Earth's crust. The rate at which the temperature increases depends on many factors, including type of crust (continental vs. oceanic), rock type, geographic location, etc. The greatest known temperature at which microbial life can exist is 122 °C (252 °F) (Methanopyrus kandleri Strain 116), and it is likely that the limit of life in the "deep biosphere" is defined by temperature rather than absolute depth. On 20 August 2014, scientists confirmed the existence of microorganisms living 800 m (2,600 ft; 0.50 mi) below the ice of Antarctica. According to one researcher, "You can find microbes everywhere – they're extremely adaptable to conditions, and survive wherever they are."

Our biosphere is divided into a number of biomes, inhabited by fairly similar flora and fauna. On land, biomes are separated primarily by latitude. Terrestrial biomes lying within the Arctic and Antarctic Circles are relatively barren of plant and animal life, while most of the more populous biomes lie near the equator.

Annual variation

Artificial biospheres

Biosphere 2 in Arizona.

Experimental biospheres, also called closed ecological systems, have been created to study ecosystems and the potential for supporting life outside the Earth. These include spacecraft and the following terrestrial laboratories:

• Biosphere 2 in Arizona, United States, 3.15 acres (13,000 m²).
• BIOS-1, BIOS-2 and BIOS-3 at the Institute of Biophysics in Krasnoyarsk, Siberia, in what was then the Soviet Union.
• Biosphere J (CEEF, Closed Ecology Experiment Facilities), an experiment in Japan.
• Micro-Ecological Life Support System Alternative (MELiSSA) at Universitat Autònoma de Barcelona

Extraterrestrial biospheres

No biospheres have been detected beyond the Earth; therefore, the existence of extraterrestrial biospheres remains hypothetical. The rare Earth hypothesis suggests they should be very rare, save ones composed of microbial life only. On the other hand, Earth analogs may be quite numerous, at least in the Milky Way galaxy, given the large number of planets. Three of the planets discovered orbiting TRAPPIST-1 could possibly contain biospheres. Given limited understanding of abiogenesis, it is currently unknown what percentage of these planets actually develop biospheres.

Based on observations by the Kepler Space Telescope team, it has been calculated that provided the probability of abiogenesis is higher than 1 to 1000, the closest alien biosphere should be within 100 light-years from the Earth.

It is also possible that artificial biospheres will be created in the future, for example with the terraforming of Mars.

 

Deep carbon cycle

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Deep_carbon_cycle 

 

Deep earth carbon

The deep carbon cycle is geochemical cycle (movement) of carbon through the Earth's mantle and core. It forms part of the carbon cycle and is intimately connected to the movement of carbon in the Earth's surface and atmosphere. By returning carbon to the deep Earth, it plays a critical role in maintaining the terrestrial conditions necessary for life to exist. Without it, carbon would accumulate in the atmosphere, reaching extremely high concentrations over long periods of time.

Because the deep Earth is inaccessible to drilling, not much is conclusively known about the role of carbon in it. Nonetheless, several pieces of evidence—many of which come from laboratory simulations of deep Earth conditions—have indicated mechanisms for the element's movement down into the lower mantle, as well as the forms that carbon takes at the extreme temperatures and pressures of this layer. Furthermore, techniques like seismology have led to greater understanding of the potential presence of carbon in the Earth's core. Studies of the composition of basaltic magma and the flux of carbon dioxide out of volcanoes reveals that the amount of carbon in the mantle is greater than that on the Earth's surface by a factor of one thousand.

Quantity of carbon

Carbon cycle

There are about 44,000 gigatonnes of carbon in the atmosphere and oceans. A gigatonne is one billion metric tonnes, equivalent to the mass of water in over 400,000 Olympic-size swimming pools. Large as this quantity is, it only amounts to a small fraction of one percent of Earth's carbon. Over 90% may reside in the core, most of the rest being in the crust and mantle.

In the photosphere of the Sun, carbon is the fourth most abundant element. The Earth likely started with a similar ratio but lost a lot of it to evaporation as it accreted. Even accounting for evaporation, however, the silicates making up the crust and mantle of the Earth have a carbon concentration that is five to ten times less than in CI chondrites, a form of meteor that is believed to represent the composition of the solar nebula before the planets formed. Some of this carbon may have ended up in the core. Depending on the model, carbon is predicted to contribute between 0.2 and 1 percent by weight in the core. Even at the lower concentration, this would account for half Earth's carbon.

Estimates of the carbon content in the upper mantle come from measurements of the chemistry of mid-ocean ridge basalts (MORBs). These must be corrected for degassing of carbon and other elements. Since the Earth formed, the upper mantle has lost 40–90% of its carbon by evaporation and transport to the core in iron compounds. The most rigorous estimate gives a carbon content of 30 parts per million (ppm). The lower mantle is expected to be much less depleted – about 350 ppm.

Lower mantle

Carbon principally enters the mantle in the form of carbonate-rich sediments on tectonic plates of ocean crust, which pull the carbon into the mantle upon undergoing subduction. Not much is known about carbon circulation in the mantle, especially in the deep Earth, but many studies have attempted to augment our understanding of the element's movement and forms within said region. For instance, a 2011 study demonstrated that carbon cycling extends all the way to the lower mantle. The study analysed rare, super-deep diamonds at a site in Juina, Brazil, determining that the bulk composition of some of the diamonds' inclusions matched the expected result of basalt melting and crystallisation under lower mantle temperatures and pressures. Thus, the investigation's findings indicate that pieces of basaltic oceanic lithosphere act as the principal transport mechanism for carbon to Earth's deep interior. These subducted carbonates can interact with lower mantle silicates and metals, eventually forming super-deep diamonds like the one found.

Carbon reservoirs in the mantle, crust and surface.

Reservoir	gigatonne C
Above surface	${\displaystyle (43-45)\times 10^{3}}$
Continental crust and lithosphere	${\displaystyle (0.9-3.1)\times 10^{8}}$
Oceanic crust and lithosphere	${\displaystyle 1.4\times 10^{8}}$
Upper mantle	${\displaystyle <3.2\times 10^{7}}$
Lower mantle	${\displaystyle <1.0\times 10^{9}}$

Carbonates descending to the lower mantle form other compounds besides diamonds. In 2011, carbonates were subjected to an environment similar to that of 1800 km deep into the Earth, well within the lower mantle. Doing so resulted in the formations of magnesite, siderite, and numerous varieties of graphite. Other experiments—as well as petrologic observations—support this claim, finding that magnesite is actually the most stable carbonate phase in the majority of the mantle. This is largely a result of its higher melting temperature. Consequently, scientists have concluded that carbonates undergo reduction as they descend into the mantle before being stabilised at depth by low oxygen fugacity environments. Magnesium, iron, and other metallic compounds act as buffers throughout the process. The presence of reduced, elemental forms of carbon like graphite would indicate that carbon compounds are reduced as they descend into the mantle.

Carbon outgassing processes

Nonetheless, polymorphism alters carbonate compounds' stability at different depths within the Earth. To illustrate, laboratory simulations and density functional theory calculations suggest that tetrahedrally-coordinated carbonates are most stable at depths approaching the core–mantle boundary. A 2015 study indicates that the lower mantle's high pressures cause carbon bonds to transition from sp₂ to sp₃ hybridised orbitals, resulting in carbon tetrahedrally bonding to oxygen. CO₃ trigonal groups cannot form polymerisable networks, while tetrahedral CO₄ can, signifying an increase in carbon's coordination number, and therefore drastic changes in carbonate compounds' properties in the lower mantle. As an example, preliminary theoretical studies suggest that high pressures cause carbonate melt viscosity to increase; the melts' lower mobility as a result of the property changes described is evidence for large deposits of carbon deep into the mantle.

Accordingly, carbon can remain in the lower mantle for long periods of time, but large concentrations of carbon frequently find their way back to the lithosphere. This process, called carbon outgassing, is the result of carbonated mantle undergoing decompression melting, as well as mantle plumes carrying carbon compounds up towards the crust. Carbon is oxidised upon its ascent towards volcanic hotspots, where it is then released as CO₂. This occurs so that the carbon atom matches the oxidation state of the basalts erupting in such areas.

Core

Part of a series on
Biogeochemical cycles

Although the presence of carbon in the Earth's core is well-constrained, recent studies suggest large inventories of carbon could be stored in this region. Shear (S) waves moving through the inner core travel at about fifty percent of the velocity expected for most iron-rich alloys. Considering the core's composition is widely believed to be an alloy of crystalline iron with a small amount of nickel, this seismographic anomaly points to another substance's existence within the region. One theory postulates that such a phenomenon is the result of various light elements, including carbon, in the core. In fact, studies have utilised diamond anvil cells to replicate the conditions in the Earth's core, the results of which indicate that iron carbide (Fe₇C₃) matches the inner core's sound and density velocities considering its temperature and pressure profile. Hence, the iron carbide model could serve as evidence that the core holds as much as 67% of the Earth's carbon. Furthermore, another study found that carbon dissolved in iron and formed a stable phase with the same Fe₇C₃ composition—albeit with a different structure than the one previously mentioned. Hence, although the amount of carbon potentially stored in the Earth's core is not known, recent research indicates that the presence of iron carbides could be consistent with geophysical observations.

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  Movement of oceanic plates—which carry carbon compounds—through the mantle

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  Two models of the carbon content in Earth

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  Analysis of shear wave velocities has played an integral role in the development of knowledge about carbon's existence in the core

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  Diagram of carbon tetrahedrally bonded to oxygen

Fluxes

Major fluxes of carbon to, from, and within the Earth’s exogenic and endogenic systems
Values give the maximum and minimum fluxes since 200 million years ago. The two major boundaries highlighted are the Mohorovičić discontinuity (crust-mantle boundary; Moho) and the lithosphere-asthenosphere boundary (LAB).

 

Biogeochemical cycle

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Biogeochemical_cycle 

A biogeochemical cycle is the pathway by which a chemical substance cycles (is turned over or moves through) the biotic and the abiotic compartments of Earth. The biotic compartment is the biosphere and the abiotic compartments are the atmosphere, hydrosphere and lithosphere. There are biogeochemical cycles for chemical elements, such as for calcium, carbon, hydrogen, mercury, nitrogen, oxygen, phosphorus, selenium, iron and sulfur, as well as molecular cycles, such as for water and silica. There are also macroscopic cycles, such as the rock cycle, and human-induced cycles for synthetic compounds such as polychlorinated biphenyls (PCBs). In some cycles there are reservoirs where a substance can remain or be sequestered for a long period of time.

Overview

Generalized biogeochemical cycle 

Energy flows directionally through ecosystems, entering as sunlight (or inorganic molecules for chemoautotrophs) and leaving as heat during the many transfers between trophic levels. However, the matter that makes up living organisms is conserved and recycled. The six most common elements associated with organic molecules—carbon, nitrogen, hydrogen, oxygen, phosphorus, and sulfur—take a variety of chemical forms and may exist for long periods in the atmosphere, on land, in water, or beneath the Earth's surface. Geologic processes, such as weathering, erosion, water drainage, and the subduction of the continental plates, all play a role in this recycling of materials. Because geology and chemistry have major roles in the study of this process, the recycling of inorganic matter between living organisms and their environment is called a biogeochemical cycle.

The six aforementioned elements are used by organisms in a variety of ways. Hydrogen and oxygen are found in water and organic molecules, both of which are essential to life. Carbon is found in all organic molecules, whereas nitrogen is an important component of nucleic acids and proteins. Phosphorus is used to make nucleic acids and the phospholipids that comprise biological membranes. Sulfur is critical to the three-dimensional shape of proteins. The cycling of these elements is interconnected. For example, the movement of water is critical for leaching sulfur and phosphorus into rivers which can then flow into oceans. Minerals cycle through the biosphere between the biotic and abiotic components and from one organism to another.

Ecological systems (ecosystems) have many biogeochemical cycles operating as a part of the system, for example, the water cycle, the carbon cycle, the nitrogen cycle, etc. All chemical elements occurring in organisms are part of biogeochemical cycles. In addition to being a part of living organisms, these chemical elements also cycle through abiotic factors of ecosystems such as water (hydrosphere), land (lithosphere), and/or the air (atmosphere).

The living factors of the planet can be referred to collectively as the biosphere. All the nutrients—such as carbon, nitrogen, oxygen, phosphorus, and sulfur—used in ecosystems by living organisms are a part of a closed system; therefore, these chemicals are recycled instead of being lost and replenished constantly such as in an open system.

The diagram on the right shows a generalised biogeochemical cycle. The major parts of the biosphere are connected by the flow of chemical elements and compounds. In many of these cycles, the biota plays an important role. Matter from the Earth's interior is released by volcanoes. The atmosphere exchanges some compounds and elements rapidly with the biota and oceans. Exchanges of materials between rocks, soils, and the oceans are generally slower by comparison.

The flow of energy in an ecosystem is an open system; the sun constantly gives the planet energy in the form of light while it is eventually used and lost in the form of heat throughout the trophic levels of a food web. Carbon is used to make carbohydrates, fats, and proteins, the major sources of food energy. These compounds are oxidized to release carbon dioxide, which can be captured by plants to make organic compounds. The chemical reaction is powered by the light energy of the sun.

Sunlight is required to combine carbon with hydrogen and oxygen into an energy source, but ecosystems in the deep sea, where no sunlight can penetrate, obtain energy from sulfur. Hydrogen sulfide near hydrothermal vents can be utilized by organisms such as the giant tube worm. In the sulfur cycle, sulfur can be forever recycled as a source of energy. Energy can be released through the oxidation and reduction of sulfur compounds (e.g., oxidizing elemental sulfur to sulfite and then to sulfate).

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  Examples of major biogeochemical processes
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  The oceanic whale pump showing how whales cycle nutrients through the ocean water column

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  The implications of shifts in the global carbon cycle due to human activity are concerning scientists.

Although the Earth constantly receives energy from the sun, its chemical composition is essentially fixed, as the additional matter is only occasionally added by meteorites. Because this chemical composition is not replenished like energy, all processes that depend on these chemicals must be recycled. These cycles include both the living biosphere and the nonliving lithosphere, atmosphere, and hydrosphere.

Biogeochemical cycles can be contrasted with geochemical cycles. The latter deals only with crustal and subcrustal reservoirs even though some process from both overlap.

Reservoirs

The chemicals are sometimes held for long periods of time in one place. This place is called a reservoir, which, for example, includes such things as coal deposits that are storing carbon for a long period of time. When chemicals are held for only short periods of time, they are being held in exchange pools. Examples of exchange pools include plants and animals.

Plants and animals utilize carbon to produce carbohydrates, fats, and proteins, which can then be used to build their internal structures or to obtain energy. Plants and animals temporarily use carbon in their systems and then release it back into the air or surrounding medium. Generally, reservoirs are abiotic factors whereas exchange pools are biotic factors. Carbon is held for a relatively short time in plants and animals in comparison to coal deposits. The amount of time that a chemical is held in one place is called its residence time or turnover time (also called the renewal time or exit age).

Box models

Basic one-box model

Box models are widely used to model biogeochemical systems. Box models are simplified versions of complex systems, reducing them to boxes (or storage reservoirs) for chemical materials, linked by material fluxes (flows). Simple box models have a small number of boxes with properties, such as volume, that do not change with time. The boxes are assumed to behave as if they were mixed homogeneously. These models are often used to derive analytical formulas describing the dynamics and steady-state abundance of the chemical species involved.

The diagram at the right shows a basic one-box model. The reservoir contains the amount of material M under consideration, as defined by chemical, physical or biological properties. The source Q is the flux of material into the reservoir, and the sink S is the flux of material out of the reservoir. The budget is the check and balance of the sources and sinks affecting material turnover in a reservoir. The reservoir is in a steady state if Q = S, that is, if the sources balance the sinks and there is no change over time.

The residence or turnover time is the average time material spends resident in the reservoir. If the reservoir is in a steady state, this is the same as the time it takes to fill or drain the reservoir. Thus, if τ is the turnover time, then τ = M/S. The equation describing the rate of change of content in a reservoir is

${\displaystyle {\frac {dM}{dt}}=Q-S=Q-{\frac {M}{\tau }}}$

When two or more reservoirs are connected, the material can be regarded as cycling between the reservoirs, and there can be predictable patterns to the cyclic flow. More complex multibox models are usually solved using numerical techniques.

Simple three box model
simplified budget of ocean carbon flows 

More complex model with many interacting boxes
export and burial rates of terrestrial organic carbon in the ocean 

Measurement units

Global biogeochemical box models usually measure:
            — reservoir masses in petagrams (Pg)
            — flow fluxes in petagrams per year (Pg yr⁻¹)
 ________________________________________________
 one petagram = 10¹⁵ grams = one gigatonne = one billion (10⁹) tonnes

The diagram on the left above shows a simplified budget of ocean carbon flows. It is composed of three simple interconnected box models, one for the euphotic zone, one for the ocean interior or dark ocean, and one for ocean sediments. In the euphotic zone, net phytoplankton production is about 50 Pg C each year. About 10 Pg is exported to the ocean interior while the other 40 Pg is respired. Organic carbon degradation occurs as particles (marine snow) settle through the ocean interior. Only 2 Pg eventually arrives at the seafloor, while the other 8 Pg is respired in the dark ocean. In sediments, the time scale available for degradation increases by orders of magnitude with the result that 90% of the organic carbon delivered is degraded and only 0.2 Pg C yr⁻¹ is eventually buried and transferred from the biosphere to the geosphere.

The diagram on the right above shows a more complex model with many interacting boxes. Reservoir masses here represents carbon stocks, measured in Pg C. Carbon exchange fluxes, measured in Pg C yr⁻¹, occur between the atmosphere and its two major sinks, the land and the ocean. The black numbers and arrows indicate the reservoir mass and exchange fluxes estimated for the year 1750, just before the Industrial Revolution. The red arrows (and associated numbers) indicate the annual flux changes due to anthropogenic activities, averaged over the 2000–2009 time period. They represent how the carbon cycle has changed since 1750. Red numbers in the reservoirs represent the cumulative changes in anthropogenic carbon since the start of the Industrial Period, 1750–2011.

Compartments

Biosphere

Role of marine organisms in biogeochemical cycling in the Southern Ocean 

 

Main article: Biosphere

Microorganisms drive much of the biogeochemical cycling in the earth system.

Atmosphere

Main article: Atmosphere

Hydrosphere

Main article: Hydrosphere

The global ocean covers more than 70% of the Earth's surface and is remarkably heterogeneous. Marine productive areas, and coastal ecosystems comprise a minor fraction of the ocean in terms of surface area, yet have an enormous impact on global biogeochemical cycles carried out by microbial communities, which represent 90% of the ocean's biomass. Work in recent years has largely focused on cycling of carbon and macronutrients such as nitrogen, phosphorus, and silicate: other important elements such as sulfur or trace elements have been less studied, reflecting associated technical and logistical issues. Increasingly, these marine areas, and the taxa that form their ecosystems, are subject to significant anthropogenic pressure, impacting marine life and recycling of energy and nutrients. A key example is that of cultural eutrophication, where agricultural runoff leads to nitrogen and phosphorus enrichment of coastal ecosystems, greatly increasing productivity resulting in algal blooms, deoxygenation of the water column and seabed, and increased greenhouse gas emissions, with direct local and global impacts on nitrogen and carbon cycles. However, the runoff of organic matter from the mainland to coastal ecosystems is just one of a series of pressing threats stressing microbial communities due to global change. Climate change has also resulted in changes in the cryosphere, as glaciers and permafrost melt, resulting in intensified marine stratification, while shifts of the redox-state in different biomes are rapidly reshaping microbial assemblages at an unprecedented rate.

Global change is, therefore, affecting key processes including primary productivity, CO₂ and N₂ fixation, organic matter respiration/remineralization, and the sinking and burial deposition of fixed CO₂. In addition to this, oceans are experiencing an acidification process, with a change of ~0.1 pH units between the pre-industrial period and today, affecting carbonate/bicarbonate buffer chemistry. In turn, acidification has been reported to impact planktonic communities, principally through effects on calcifying taxa. There is also evidence for shifts in the production of key intermediary volatile products, some of which have marked greenhouse effects (e.g., N₂O and CH₄, reviewed by Breitburg in 2018, due to the increase in global temperature, ocean stratification and deoxygenation, driving as much as 25 to 50% of nitrogen loss from the ocean to the atmosphere in the so-called oxygen minimum zones  or anoxic marine zones, driven by microbial processes. Other products, that are typically toxic for the marine nekton, including reduced sulfur species such as H₂S, have a negative impact for marine resources like fisheries and coastal aquaculture. While global change has accelerated, there has been a parallel increase in awareness of the complexity of marine ecosystems, and especially the fundamental role of microbes as drivers of ecosystem functioning.

Lithosphere

Main article: Lithosphere

Fast and slow cycles

There are fast and slow biogeochemical cycles. Fast cycle operate in the biosphere and slow cycles operate in rocks. Fast or biological cycles can complete within years, moving substances from atmosphere to biosphere, then back to the atmosphere. Slow or geological cycles can take millions of years to complete, moving substances through the Earth's crust between rocks, soil, ocean and atmosphere.

As an example, the fast carbon cycle is illustrated in the diagram below on the left. This cycle involves relatively short-term biogeochemical processes between the environment and living organisms in the biosphere. It includes movements of carbon between the atmosphere and terrestrial and marine ecosystems, as well as soils and seafloor sediments. The fast cycle includes annual cycles involving photosynthesis and decadal cycles involving vegetative growth and decomposition. The reactions of the fast carbon cycle to human activities will determine many of the more immediate impacts of climate change.

The fast cycle operates through the biosphere, including exchanges between land, atmosphere, and oceans. The yellow numbers are natural fluxes of carbon in billions of tons (gigatons) per year. Red are human contributions and white are stored carbon.

The slow cycle operates through rocks, including volcanic and tectonic activity

The slow cycle is illustrated in the diagram above on the right. It involves medium to long-term geochemical processes belonging to the rock cycle. The exchange between the ocean and atmosphere can take centuries, and the weathering of rocks can take millions of years. Carbon in the ocean precipitates to the ocean floor where it can form sedimentary rock and be subducted into the earth's mantle. Mountain building processes result in the return of this geologic carbon to the Earth's surface. There the rocks are weathered and carbon is returned to the atmosphere by degassing and to the ocean by rivers. Other geologic carbon returns to the ocean through the hydrothermal emission of calcium ions. In a given year between 10 and 100 million tonnes of carbon moves around this slow cycle. This includes volcanoes returning geologic carbon directly to the atmosphere in the form of carbon dioxide. However, this is less than one percent of the carbon dioxide put into the atmosphere by burning fossil fuels.

Deep cycles

Further information: Deep carbon cycle

The terrestrial subsurface is the largest reservoir of carbon on earth, containing 14–135 Pg of carbon  and 2–19% of all biomass. Microorganisms drive organic and inorganic compound transformations in this environment and thereby control biogeochemical cycles. Current knowledge of the microbial ecology of the subsurface is primarily based on 16S ribosomal RNA (rRNA) gene sequences. Recent estimates show that <8% of 16S rRNA sequences in public databases derive from subsurface organisms  and only a small fraction of those are represented by genomes or isolates. Thus, there is remarkably little reliable information about microbial metabolism in the subsurface. Further, little is known about how organisms in subsurface ecosystems are metabolically interconnected. Some cultivation-based studies of syntrophic consortia and small-scale metagenomic analyses of natural communities suggest that organisms are linked via metabolic handoffs: the transfer of redox reaction products of one organism to another. However, no complex environments have been dissected completely enough to resolve the metabolic interaction networks that underpin them. This restricts the ability of biogeochemical models to capture key aspects of the carbon and other nutrient cycles. New approaches such as genome-resolved metagenomics, an approach that can yield a comprehensive set of draft and even complete genomes for organisms without the requirement for laboratory isolation  have the potential to provide this critical level of understanding of biogeochemical processes.

Some examples

Some of the more well-known biogeochemical cycles are shown below:

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  Carbon cycle

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  Nitrogen cycle

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  Nutrient cycle

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  Oxygen cycle

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  Phosphorus cycle

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  Sulfur cycle

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  Rock cycle

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  Water cycle

Many biogeochemical cycles are currently being studied for the first time. Climate change and human impacts are drastically changing the speed, intensity, and balance of these relatively unknown cycles, which include:

• the mercury cycle, and
• the human-caused cycle of PCBs.

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  Chloroplasts conduct photosynthesis in plant cells and other eukaryotic organisms.

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  Kerogen cycle

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  Coal is a reservoir of carbon

Biogeochemical cycles always involve active equilibrium states: a balance in the cycling of the element between compartments. However, overall balance may involve compartments distributed on a global scale.

As biogeochemical cycles describe the movements of substances on the entire globe, the study of these is inherently multidisciplinary. The carbon cycle may be related to research in ecology and atmospheric sciences. Biochemical dynamics would also be related to the fields of geology and pedology.

Vladimir Vernadsky 1934
father of biogeochemistry 

The chemistry of the arena of life — that is Earth’s biogeochemistry — will be at the center of how well we do, and all biogeochemists should strive to articulate that message clearly and forcefully to the public and to leaders of society, who must know our message to do their job well.

— William H. Schlesinger 2004