Marine biogeochemical cycles

The dominant feature of the planet viewed from space is water – oceans of liquid water flood most of the surface while water vapour swirls in atmospheric clouds and the poles are capped with ice. Taken as a whole, the oceans form a single marine system where liquid water – the "universal solvent" – dissolves nutrients and substances containing elements such as oxygen, carbon, nitrogen and phosphorus. These substances are endlessly cycled and recycled, chemically combined and then broken down again, dissolved and then precipitated or evaporated, imported from and exported back to the land and the atmosphere and the ocean floor. Powered both by the biological activity of marine organisms and by the natural forces of the Sun and tides and movements within the Earth's crust, these are the marine biogeochemical cycles.

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Marine biogeochemical cycles arebiogeochemical cycles that occur withinmarine environments, that is, in thesaltwater of seas or oceans or thebrackish water of coastalestuaries. These biogeochemical cycles are the pathwayschemical substances andelements move through within the marine environment. In addition, substances and elements can be imported into or exported from the marine environment. These imports and exports can occur as exchanges with the atmosphere above, the ocean floor below, or as runoff from the land.

There arebiogeochemical cycles for the elementscalcium,carbon,hydrogen,mercury,nitrogen,oxygen,phosphorus,selenium, andsulfur; molecular cycles forwater andsilica; macroscopic cycles such as therock cycle; as well as human-induced cycles for synthetic compounds such aspolychlorinated biphenyl (PCB). In some cycles there are reservoirs where a substance can be stored for a long time. The cycling of these elements is interconnected.

Marine organisms, and particularlymarine microorganisms are crucial for the functioning of many of these cycles. The forces driving biogeochemical cycles includemetabolic processes within organisms, geological processes involving the Earth's mantle, as well aschemical reactions among the substances themselves, which is why these are called biogeochemical cycles. While chemical substances can be broken down and recombined, the chemical elements themselves can be neither created nor destroyed by these forces, so apart from some losses to and gains from outer space, elements are recycled or stored (sequestered) somewhere on or within the planet.

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.^[1]

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.^[2]

Water is the medium of the oceans, the medium which carries all the substances and elements involved in the marine biogeochemical cycles. Water as found in nature almost always includes dissolved substances, so water has been described as the "universal solvent" for its ability to dissolve so many substances.^[3][4] This ability allows it to be the "solvent of life"^[5] Water is also the only common substance that exists assolid, liquid, andgas in normal terrestrial conditions.^[6] Since liquid water flows, ocean waters cycle and flow in currents around the world. Since water easily changes phase, it can be carried into the atmosphere as water vapour or frozen as an iceberg. It can then precipitate or melt to become liquid water again. All marine life is immersed in water, the matrix and womb of life itself.^[7] Water can be broken down into its constituent hydrogen and oxygen by metabolic or abiotic processes, and later recombined to become water again.

While the water cycle is itself abiogeochemical cycle, flow of water over and beneath the Earth is a key component of the cycling of other biogeochemicals.^[8] Runoff is responsible for almost all of the transport oferodedsediment andphosphorus from land towaterbodies.^[9] Culturaleutrophication of lakes is primarily due to phosphorus, applied in excess toagricultural fields infertilizers, and then transported overland and down rivers. Both runoff and groundwater flow play significant roles in transporting nitrogen from the land to waterbodies.^[10] Thedead zone at the outlet of theMississippi River is a consequence ofnitrates from fertilizer being carried off agricultural fields and funnelled down theriver system to theGulf of Mexico. Runoff also plays a part in thecarbon cycle, again through the transport of eroded rock and soil.^[11]

Ocean salinity is derived mainly from the weathering of rocks and the transport of dissolved salts from the land, with lesser contributions fromhydrothermal vents in the seafloor.^[12] Evaporation of ocean water and formation of sea ice further increase the salinity of the ocean. However these processes which increase salinity are continually counterbalanced by processes that decrease salinity, such as the continuous input of fresh water from rivers, precipitation of rain and snow, and the melting of ice.^[13] The two most prevalent ions in seawater are chloride and sodium. Together, they make up around 85 per cent of all dissolved ions in the ocean. Magnesium and sulfate ions make up most of the rest. Salinity varies with temperature, evaporation, and precipitation. It is generally low at the equator and poles, and high at mid-latitudes.^[12]

• 
  Annual mean sea surface salinity, measured in 2009 inpractical salinity units ^[14]

• 
  Vertical differences in sea salinity between the surface and a depth of 300 metres. Salinity increases with depth in red regions and decreases in blue regions.^[15]

A stream of airborne microorganisms circles the planet above weather systems but below commercial air lanes.^[16] Some peripatetic microorganisms are swept up from terrestrial dust storms, but most originate from marine microorganisms insea spray. In 2018, scientists reported that hundreds of millions of viruses and tens of millions of bacteria are deposited daily on every square meter around the planet.^[17][18] This is another example of water facilitating the transport of organic material over great distances, in this case in the form of live microorganisms.

Dissolved salt does not evaporate back into the atmosphere like water, but it does formsea salt aerosols insea spray. Manyphysical processes over ocean surface generate sea salt aerosols. One common cause is the bursting ofair bubbles, which are entrained by the wind stress during thewhitecap formation. Another is tearing of drops from wave tops.^[19] The total sea salt flux from the ocean to the atmosphere is about 3300 Tg (3.3 billion tonnes) per year.^[20]

Upwelling

Upwelling caused by an offshore wind in friction with the ocean surface

Upwelling can be caused if an alongshore wind moves towards the equator, inducingEkman transport

Two mechanisms which result inupwelling. In each case, if the wind direction were reversed it would inducedownwelling.^[21]

Ventilation of the deep ocean

Antarctic bottom water

Antarctic Circumpolar Current, with branches connecting to the global conveyor belt

Solar radiation affects the oceans: warm water from the Equator tends to circulate toward thepoles, while cold polar water heads towards the Equator. The surface currents are initially dictated by surface wind conditions. Thetrade winds blow westward in the tropics,^[22] and thewesterlies blow eastward at mid-latitudes.^[23] This wind pattern applies astress to the subtropical ocean surface with negativecurl across theNorthern Hemisphere,^[24] and the reverse across theSouthern Hemisphere. The resultingSverdrup transport is equatorward.^[25] Because of conservation ofpotential vorticity caused by the poleward-moving winds on thesubtropical ridge's western periphery and the increased relative vorticity of poleward moving water, transport is balanced by a narrow, accelerating poleward current, which flows along the western boundary of the ocean basin, outweighing the effects of friction with the cold western boundary current which originates from high latitudes.^[26] The overall process, known aswestern intensification, causes currents on the western boundary of an ocean basin to be stronger than those on the eastern boundary.^[27]

As it travels poleward, warm water transported by strong warm water current undergoes evaporative cooling. The cooling is wind driven: wind moving over water cools the water and also causesevaporation, leaving a saltier brine. In this process, the water becomes saltier and denser. and decreases in temperature. Once sea ice forms, salts are left out of the ice, a process known as brine exclusion.^[28] These two processes produce water that is denser and colder. The water across the northernAtlantic Ocean becomes so dense that it begins to sink down through less salty and less dense water. This downdraft of heavy, cold and dense water becomes a part of theNorth Atlantic Deep Water, a southgoing stream.^[29]

Winds drive ocean currents in the upper 100 meters of the ocean's surface. However, ocean currents also flow thousands of meters below the surface. These deep-ocean currents are driven by differences in the water's density, which is controlled by temperature (thermo) and salinity (haline). This process is known as thermohaline circulation. In the Earth's polar regions ocean water gets very cold, forming sea ice. As a consequence the surrounding seawater gets saltier, because when sea ice forms, the salt is left behind. As the seawater gets saltier, its density increases, and it starts to sink. Surface water is pulled in to replace the sinking water, which in turn eventually becomes cold and salty enough to sink. This initiates the deep-ocean currents driving the global conveyor belt.^[30]

Thermohaline circulation drives a global-scale system of currents called the "global conveyor belt." The conveyor belt begins on the surface of the ocean near the pole in the North Atlantic. Here, the water is chilled by Arctic temperatures. It also gets saltier because when sea ice forms, the salt does not freeze and is left behind in the surrounding water. The cold water is now more dense, due to the added salts, and sinks toward the ocean bottom. Surface water moves in to replace the sinking water, thus creating a current. This deep water moves south, between the continents, past the equator, and down to the ends of Africa and South America. The current travels around the edge of Antarctica, where the water cools and sinks again, as it does in the North Atlantic. Thus, the conveyor belt gets "recharged." As it moves around Antarctica, two sections split off the conveyor and turn northward. One section moves into the Indian Ocean, the other into the Pacific Ocean. These two sections that split off warm up and become less dense as they travel northward toward the equator, so that they rise to the surface (upwelling). They then loop back southward and westward to the South Atlantic, eventually returning to the North Atlantic, where the cycle begins again. The conveyor belt moves at much slower speeds (a few centimeters per second) than wind-driven or tidal currents (tens to hundreds of centimeters per second). It is estimated that any given cubic meter of water takes about 1,000 years to complete the journey along the global conveyor belt. In addition, the conveyor moves an immense volume of water—more than 100 times the flow of the Amazon River (Ross, 1995). The conveyor belt is also a vital component of the global ocean nutrient and carbon dioxide cycles. Warm surface waters are depleted of nutrients and carbon dioxide, but they are enriched again as they travel through the conveyor belt as deep or bottom layers. The base of the world's food chain depends on the cool, nutrient-rich waters that support the growth of algae and seaweed.^[31]

The global average residence time of a water molecule in the ocean is about 3,200 years. By comparison the average residence time in the atmosphere is about nine days. If it is frozen in the Antarctic or drawn into deep groundwater it can be sequestered for ten thousand years.^[32][33]

Some key elements involved in marine biogeochemical cycles
Element	Diagram	Description
Carbon		Themarine carbon cycle involves processes that exchangecarbon between various pools within the ocean as well as between the atmosphere, Earth interior, and theseafloor. Thecarbon cycle is a result of many interacting forces across multiple time and space scales that circulates carbon around the planet, ensuring that carbon is available globally. The marine carbon cycle is a central to the global carbon cycle and contains bothinorganic carbon (carbon not associated with a living thing, such as carbon dioxide) andorganic carbon (carbon that is, or has been, incorporated into a living thing). Part of the marine carbon cycle transforms carbon between non-living and living matter. Three main processes (or pumps) that make up the marine carbon cycle bring atmosphericcarbon dioxide (CO2) into the ocean interior and distribute it through the oceans. These three pumps are: (1) the solubility pump, (2) the carbonate pump, and (3) the biological pump. The total active pool of carbon at the Earth's surface for durations of less than 10,000 years is roughly 40,000 gigatons C (Gt C, a gigaton is one billion tons, or the weight of approximately 6 millionblue whales), and about 95% (~38,000 Gt C) is stored in the ocean, mostly as dissolved inorganic carbon.[34][35] The speciation of dissolved inorganic carbon in the marine carbon cycle is a primary controller ofacid-base chemistry in the oceans.
Oxygen		Theoxygen cycle involves biogeochemical transitions ofoxygenatoms between differentoxidation states inions,oxides, andmolecules throughredox reactions within and between thespheres/reservoirs of the planet Earth.[36] The word oxygen in the literature typically refers tomolecular oxygen (O2) since it is the commonproduct orreactant of many biogeochemical redox reactions within the cycle.[37] Processes within the oxygen cycle are considered to bebiological orgeological and are evaluated as either asource (O2 production) or sink (O2 consumption).[36][37]
Hydrogen		Thehydrogen cycle consists ofhydrogen exchanges betweenbiotic (living) andabiotic (non-living) sources and sinks of hydrogen-containing compounds. Hydrogen (H) is the most abundant element in the universe.[38] On Earth, common H-containing inorganic molecules include water (H2O), hydrogen gas (H2),methane (CH4),hydrogen sulfide (H2S), andammonia (NH3). Many organic compounds also contain H atoms, such ashydrocarbons andorganic matter. Given the ubiquity of hydrogen atoms in inorganic and organic chemical compounds, the hydrogen cycle is focused on molecular hydrogen (H2).
Nitrogen		Thenitrogen cycle is the process by whichnitrogen is converted into multiple chemical forms as it circulates amongatmosphere,terrestrial, andmarine ecosystems. The conversion of nitrogen can be carried out through both biological and physical processes. Important processes in the nitrogen cycle includefixation,ammonification,nitrification, anddenitrification. 78% of theEarth's atmosphere is molecular nitrogen (N2),[39] making it the largest source of nitrogen. However, atmospheric nitrogen has limited availability for biological use, leading to ascarcity of usable nitrogen in many types ofecosystems. The nitrogen cycle is of particular interest toecologists because nitrogen availability can affect the rate of key ecosystem processes, includingprimary production anddecomposition. Human activities such as fossil fuel combustion, use of artificial nitrogen fertilizers, and release of nitrogen in wastewater have dramaticallyaltered the global nitrogen cycle.[40][41][42] Human modification of the global nitrogen cycle can negatively affect the natural environment system and also human health.[43][44]
Phosphorus		Thephosphorus cycle is the movement ofphosphorus through thelithosphere,hydrosphere, andbiosphere. Unlike many other biogeochemical cycles, theatmosphere does not play a significant role in the movement of phosphorus, because phosphorus and phosphorus-based compounds are usually solids at the typical ranges of temperature and pressure found on Earth. The production ofphosphine gas occurs in only specialized, local conditions. Therefore, the phosphorus cycle should be viewed from whole Earth system and then specifically focused on the cycle in terrestrial and aquatic systems. Locally, transformations of phosphorus are chemical, biological and microbiological: the major long-term transfers in the global cycle, however, are driven bytectonic movements ingeologic time.[34] Humans have caused major changes to the global phosphorus cycle through shipping of phosphorus minerals, and use of phosphorusfertilizer, and also the shipping of food from farms to cities, where it is lost as effluent.
Sulphur		Thesulfur cycle is the collection of processes by whichsulfur moves between rocks, waterways and living systems. Such biogeochemical cycles are important ingeology because they affect many minerals. Biochemical cycles are also important for life because sulfur is anessential element, being a constituent of manyproteins andcofactors, and sulfur compounds can be used as oxidants or reductants in microbial respiration.[45] The globalsulfur cycle involves the transformations of sulfur species through different oxidation states, which play an important role in both geological and biological processes. Earth's main sulfur sink is the oceans SO42−, where it is the majoroxidizing agent.[46]
Iron		Theiron cycle (Fe) is the biogeochemical cycle ofiron through theatmosphere,hydrosphere,biosphere andlithosphere. While Fe is highly abundant in the Earth's crust,[47] it is less common in oxygenated surface waters. Iron is a key micronutrient inprimary productivity,[48] and a limiting nutrient in the Southern ocean, eastern equatorial Pacific, and the subarctic Pacific referred to asHigh-Nutrient, Low-Chlorophyll (HNLC) regions of the ocean.[49] Iron exists in a range ofoxidation states from -2 to +7; however, on Earth it is predominantly in its +2 or +3 redox state and is a primary redox-active metal on Earth.[50] The cycling of iron between its +2 and +3 oxidation states is referred to as the iron cycle. This process can be entirelyabiotic or facilitated bymicroorganisms, especiallyiron-oxidizing bacteria. The abiotic processes include therusting of iron-bearing metals, where Fe2+ is abiotically oxidized to Fe3+ in the presence of oxygen, and the reduction of Fe3+ to Fe2+ by iron-sulfide minerals. The biological cycling of Fe2+ is done by iron oxidizing and reducing microbes.[51][52]
Calcium		Thecalcium cycle is a transfer of calcium betweendissolved andsolid phases. There is a continuous supply ofcalcium ions into waterways fromrocks,organisms, andsoils.[53][54] Calcium ions are consumed and removed from aqueous environments as they react to form insoluble structures such ascalcium carbonate and calcium silicate,[53][55] which can deposit to form sediments or theexoskeletons of organisms.[56] Calcium ions can also be utilizedbiologically, as calcium is essential to biological functions such as the production ofbones andteeth or cellular function.[57][58] The calcium cycle is a common thread between terrestrial, marine, geological, and biological processes.[59] The marine calcium cycle is affected by changingatmospheric carbon dioxide due toocean acidification.[56]
Silicon		Thesilica cycle involves the transport ofsilica between the Earth's systems.Opal silica (SiO2), also calledsilicon dioxide, is a chemical compound ofsilicon. Silicon is a bioessential element and is one of the most abundant elements on Earth.[60][61] The silica cycle has significant overlap with thecarbon cycle (see thecarbonate–silicate cycle) and plays an important role in the sequestration of carbon through continentalweathering, biogenic export and burial asoozes on geologic timescales.[62]

Box models are widely used to model biogeochemical systems.^[64] Box models are simplified versions of complex systems, reducing them to boxes (or storagereservoirs) for chemical materials, linked by materialfluxes (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.^[63] 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 materialM under consideration, as defined by chemical, physical or biological properties. The sourceQ is the flux of material into the reservoir, and the sinkS 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 asteady state ifQ =S, that is, if the sources balance the sinks and there is no change over time.^[63]

Theturnover time (also called the renewal time or exit age) 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.^[63] 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.^[63] More complexmultibox models are usually solved using numerical techniques.

The diagram above shows a simplified budget of ocean carbon flows. It is composed of three simple interconnected box models, one for theeuphotic zone, one for theocean interior or dark ocean, and one forocean sediments. In the euphotic zone, netphytoplankton 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 asparticles (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.^[65]

Importance of Antarctic krill in biogeochemical cycles

Processes in the biological pump

Numbers given are carbon fluxes (Gt C yr−1) in white boxes
and carbon masses (Gt C) in dark boxes

Phytoplankton convert CO₂, which has dissolved from the atmosphere into the surface oceans into particulate organic carbon (POC) during primary production. Phytoplankton are then consumed by krill and small zooplankton grazers, which in turn are preyed upon by higher trophic levels. Any unconsumed phytoplankton form aggregates, and along with zooplankton faecal pellets, sink rapidly and are exported out of the mixed layer. Krill, zooplankton and microbes intercept phytoplankton in the surface ocean and sinking detrital particles at depth, consuming and respiring this POC to CO₂ (dissolved inorganic carbon, DIC), such that only a small proportion of surface-produced carbon sinks to the deep ocean (i.e., depths > 1000 m). As krill and smaller zooplankton feed, they also physically fragment particles into small, slower- or non-sinking pieces (via sloppy feeding, coprorhexy if fragmenting faeces), retarding POC export. This releases dissolved organic carbon (DOC) either directly from cells or indirectly via bacterial solubilisation (yellow circle around DOC). Bacteria can then remineralise the DOC to DIC (CO₂, microbial gardening). Diel vertically migrating krill, smaller zooplankton and fish can actively transport carbon to depth by consuming POC in the surface layer at night, and metabolising it at their daytime, mesopelagic residence depths. Depending on species life history, active transport may occur on a seasonal basis as well.^[72]

Thebiological pump, in its simplest form, is the ocean's biologically driven sequestration ofcarbon from the atmosphere to the ocean interior andseafloor sediments.^[73] It is the part of theoceanic carbon cycle responsible for the cycling oforganic matter formed mainly byphytoplankton duringphotosynthesis (soft-tissue pump), as well as the cycling ofcalcium carbonate (CaCO₃) formed into shells by certain organisms such asplankton andmollusks (carbonate pump).^[74]

The biological pump can be divided into three distinct phases,^[75] the first of which is the production of fixed carbon by planktonicphototrophs in theeuphotic (sunlit) surface region of the ocean. In these surface waters,phytoplankton usecarbon dioxide (CO₂),nitrogen (N),phosphorus (P), and other trace elements (barium,iron,zinc, etc.) during photosynthesis to makecarbohydrates,lipids, andproteins. Some plankton, (e.g.coccolithophores andforaminifera) combine calcium (Ca) and dissolved carbonates (carbonic acid andbicarbonate) to form a calcium carbonate (CaCO₃) protective coating.

Biological pump

The oceanic whale pump where whales cycle nutrients through the water column

Once this carbon is fixed into soft or hard tissue, the organisms either stay in the euphotic zone to be recycled as part of the regenerativenutrient cycle or once they die, continue to the second phase of the biological pump and begin to sink to the ocean floor. The sinking particles will often form aggregates as they sink, greatly increasing the sinking rate. It is this aggregation that gives particles a better chance of escaping predation and decomposition in the water column and eventually make it to the sea floor.

The fixed carbon that is either decomposed by bacteria on the way down or once on the sea floor then enters the final phase of the pump and is remineralized to be used again inprimary production. The particles that escape these processes entirely are sequestered in the sediment and may remain there for millions of years. It is this sequestered carbon that is responsible for ultimately lowering atmospheric CO₂.

External videos
Marine oxygen and carbon dioxide cycles

• Brum JR, Morris JJ, Décima M and Stukel MR (2014) "Mortality in the oceans: Causes and consequences".Eco-DAS IX Symposium Proceedings, Chapter 2, pages 16–48. Association for the Sciences of Limnology and Oceanography.ISBN 978-0-9845591-3-8.
• Mateus, Marcos D. (6 January 2017)."Bridging the Gap between Knowing and Modeling Viruses in Marine Systems—An Upcoming Frontier".Frontiers in Marine Science.3: 284.Bibcode:2017FrMaS...3..284M.doi:10.3389/fmars.2016.00284.
• Beckett, Stephen J.; Weitz, Joshua S. (15 May 2017)."Disentangling niche competition from grazing mortality in phytoplankton dilution experiments".PLOS ONE.12 (5) e0177517.Bibcode:2017PLoSO..1277517B.doi:10.1371/journal.pone.0177517.PMC 5432111.PMID 28505212.

DOM, POM and the viral shunt

Connections between the different compartments of the living (bacteria/viruses and phyto−/zooplankton) and the nonliving (DOM/POM and inorganic matter) environment^[76]

Theviral shunt pathway facilitates the flow ofdissolved organic matter (DOM) andparticulate organic matter (POM) through the marine food web

Themarine carbon cycle is composed of processes that exchangecarbon between various pools within the ocean as well as between the atmosphere, Earth interior, and theseafloor. Thecarbon cycle is a result of many interacting forces across multiple time and space scales that circulates carbon around the planet, ensuring that carbon is available globally. The Oceanic carbon cycle is a central process to the global carbon cycle and contains bothinorganic carbon (carbon not associated with a living thing, such as carbon dioxide) andorganic carbon (carbon that is, or has been, incorporated into a living thing). Part of the marine carbon cycle transforms carbon between non-living and living matter.

Marine carbon cycle^[77]

Oxygen cycle

Three main processes (or pumps) that make up the marine carbon cycle bring atmosphericcarbon dioxide (CO₂) into the ocean interior and distribute it through the oceans. These three pumps are: (1) the solubility pump, (2) the carbonate pump, and (3) the biological pump. The total active pool of carbon at the Earth's surface for durations of less than 10,000 years is roughly 40,000 gigatons C (Gt C, a gigaton is one billion tons, or the weight of approximately 6 millionblue whales), and about 95% (~38,000 Gt C) is stored in the ocean, mostly as dissolved inorganic carbon.^[34][35] Thespeciation of dissolved inorganic carbon in the marine carbon cycle is a primary controller ofacid-base chemistry in the oceans.

The nitrogen cycle is as important in the ocean as it is on land. While the overall cycle is similar in both cases, there are different players and modes of transfer for nitrogen in the ocean.^[79] Nitrogen enters the ocean through precipitation, runoff, or as N₂ from the atmosphere. Nitrogen cannot be utilized byphytoplankton as N₂ so it must undergonitrogen fixation which is performed predominantly bycyanobacteria.^[80] Without supplies of fixed nitrogen entering the marine cycle, the fixed nitrogen would be used up in about 2000 years.^[81] Phytoplankton need nitrogen in biologically available forms for the initial synthesis of organic matter.Ammonia andurea are released into the water by excretion from plankton. Nitrogen sources are removed from theeuphotic zone by the downward movement of the organic matter. This can occur from sinking of phytoplankton, vertical mixing, or sinking of waste of vertical migrators. The sinking results in ammonia being introduced at lower depths below the euphotic zone. Bacteria are able to convert ammonia tonitrite andnitrate but they are inhibited by light so this must occur below the euphotic zone.^[80] Ammonification ormineralization is performed by bacteria to convert organic nitrogen to ammonia.Nitrification can then occur to convert the ammonium to nitrite and nitrate.^[82] Nitrate can be returned to the euphotic zone by vertical mixing andupwelling where it can be taken up by phytoplankton to continue the cycle. N₂ can be returned to the atmosphere throughdenitrification.

Ammonium is thought to be the preferred source of fixed nitrogen for phytoplankton because its assimilation does not involve aredox reaction and therefore requires little energy. Nitrate requires a redox reaction for assimilation but is more abundant so most phytoplankton have adapted to have the enzymes necessary to undertake this reduction (nitrate reductase). There are a few notable and well-known exceptions that include mostProchlorococcus and someSynechococcus that can only take up nitrogen as ammonium.^[81]

Marine nitrogen cycle

Marinephosphorus cycle

Phosphorus is an essential nutrient for plants and animals. Phosphorus is alimiting nutrient for aquatic organisms. Phosphorus forms parts of important life-sustaining molecules that are very common in the biosphere. Phosphorus does enter the atmosphere in very small amounts when the dust is dissolved in rainwater and seaspray but remains mostly on land and in rock and soil minerals. Eighty per cent of the mined phosphorus is used to make fertilizers. Phosphates from fertilizers, sewage and detergents can cause pollution in lakes and streams. Over-enrichment of phosphate in both fresh and inshore marine waters can lead to massivealgae blooms which, when they die and decay leads toeutrophication of freshwaters only. Recent research suggests that the predominant pollutant responsible for algal blooms in saltwater estuaries and coastal marine habitats is nitrogen.^[83]

Phosphorus occurs most abundantly in nature as part of theorthophosphate ion (PO₄)³⁻, consisting of a P atom and 4 oxygen atoms. On land most phosphorus is found in rocks and minerals. Phosphorus-rich deposits have generally formed in the ocean or from guano, and over time, geologic processes bring ocean sediments to land.Weathering of rocks and minerals release phosphorus in a soluble form where it is taken up by plants, and it is transformed into organic compounds. The plants may then be consumed byherbivores and the phosphorus is either incorporated into their tissues or excreted. After death, the animal or plant decays, and phosphorus is returned to the soil where a large part of the phosphorus is transformed into insoluble compounds.Runoff may carry a small part of the phosphorus back to theocean.^[84]

Anutrient cycle is the movement and exchange oforganic andinorganic matter back into theproduction of matter. The process is regulated by the pathways available inmarine food webs, which ultimately decompose organic matter back into inorganic nutrients. Nutrient cycles occur within ecosystems. Energy flow always follows a unidirectional and noncyclic path, whereas the movement ofmineral nutrients is cyclic. Mineral cycles include thecarbon cycle,oxygen cycle,nitrogen cycle,phosphorus cycle andsulfur cycle among others that continually recycle along with other mineral nutrients intoproductive ecological nutrition.

There is considerable overlap between the terms for thebiogeochemical cycle and nutrient cycle. Some textbooks integrate the two and seem to treat them as synonymous terms.^[86] However, the terms often appear independently. Nutrient cycle is more often used in direct reference to the idea of an intra-system cycle, where an ecosystem functions as a unit. From a practical point, it does not make sense to assess a terrestrial ecosystem by considering the full column of air above it as well as the great depths of Earth below it. While an ecosystem often has no clear boundary, as a working model it is practical to consider the functional community where the bulk of matter and energy transfer occurs.^[87] Nutrient cycling occurs in ecosystems that participate in the "larger biogeochemical cycles of the earth through a system of inputs and outputs."^[87]: 425

Nutrients dissolved in seawater are essential for the survival of marine life. Nitrogen and phosphorus are particularly important. They are regarded aslimiting nutrients in many marine environments, because primary producers, like algae and marine plants, cannot grow without them. They are critical for stimulatingprimary production byphytoplankton. Other important nutrients are silicon, iron, and zinc.^[88]

The process of cycling nutrients in the sea starts withbiological pumping, when nutrients are extracted from surface waters by phytoplankton to become part of their organic makeup. Phytoplankton are either eaten by other organisms, or eventually die and drift down asmarine snow. There they decay and return to the dissolved state, but at greater ocean depths. The fertility of the oceans depends on the abundance of the nutrients, and is measured by theprimary production, which is the rate of fixation of carbon per unit of water per unit time. "Primary production is often mapped by satellites using the distribution of chlorophyll, which is a pigment produced by plants that absorbs energy during photosynthesis. The distribution of chlorophyll is shown in the figure above. You can see the highest abundance close to the coastlines where nutrients from the land are fed in by rivers. The other location where chlorophyll levels are high is in upwelling zones where nutrients are brought to the surface ocean from depth by the upwelling process..."^[88]

Oceannutrient cycle

Ocean nutrient flux

Land runoff drains nutrients and pollutants to the ocean

The drainage basins of the principal oceans and seas of the world are marked bycontinental divides. The grey areas areendorheic basins that do not drain to the ocean.

"Another critical element for the health of the oceans is the dissolved oxygen content. Oxygen in the surface ocean is continuously added across the air-sea interface as well as by photosynthesis; it is used up in respiration by marine organisms and during the decay or oxidation of organic material that rains down in the ocean and is deposited on the ocean bottom. Most organisms require oxygen, thus its depletion has adverse effects for marine populations. Temperature also affects oxygen levels as warm waters can hold less dissolved oxygen than cold waters. This relationship will have major implications for future oceans, as we will see... The final seawater property we will consider is the content of dissolved CO₂. CO₂ is nearly opposite to oxygen in many chemical and biological processes; it is used up by plankton during photosynthesis and replenished during respiration as well as during the oxidation of organic matter. As we will see later, CO₂ content has importance for the study of deep-water aging."^[88]

Sulfate reduction in the seabed is strongly focused toward near-surface sediments with high depositional rates along the ocean margins. The benthic marine sulfur cycle is therefore sensitive to anthropogenic influence, such as ocean warming and increased nutrient loading of coastal seas. This stimulates photosynthetic productivity and results in enhanced export of organic matter to the seafloor, often combined with low oxygen concentration in the bottom water (Rabalais et al., 2014; Breitburg et al., 2018). The biogeochemical zonation is thereby compressed toward the sediment surface, and the balance of organic matter mineralization is shifted from oxic and suboxic processes toward sulfate reduction and methanogenesis (Middelburg and Levin, 2009).^[89]

• cable bacteria

The sulfur cycle in marine environments has been well-studied via the tool ofsulfur isotope systematics expressed as δ³⁴S. The modern global oceans have sulfur storage of 1.3 × 10²¹ g,^[90] mainly occurring as sulfate with the δ³⁴S value of +21‰.^[91] The overall input flux is 1.0 × 10¹⁴ g/year with the sulfur isotope composition of ~3‰.^[91] Riverine sulfate derived from the terrestrial weathering of sulfide minerals (δ³⁴S = +6‰) is the primary input of sulfur to the oceans. Other sources are metamorphic and volcanic degassing and hydrothermal activity (δ³⁴S = 0‰), which release reduced sulfur species (e.g., H₂S and S⁰). There are two major outputs of sulfur from the oceans. The first sink is the burial of sulfate either as marine evaporites (e.g., gypsum) or carbonate-associated sulfate (CAS), which accounts for 6 × 10¹³ g/year (δ³⁴S = +21‰). The second sulfur sink is pyrite burial in shelf sediments or deep seafloor sediments (4 × 10¹³ g/year; δ³⁴S = -20‰).^[92] The total marine sulfur output flux is 1.0 × 10¹⁴ g/year which matches the input fluxes, implying the modern marine sulfur budget is at steady state.^[91] The residence time of sulfur in modern global oceans is 13,000,000 years.^[93]

In modern oceans,Hydrogenovibrio crunogenus,Halothiobacillus, andBeggiatoa are primary sulfur oxidizing bacteria,^[94][95] and form chemosynthetic symbioses with animal hosts.^[96] The host provides metabolic substrates (e.g., CO₂, O₂, H₂O) to the symbiont while the symbiont generates organic carbon for sustaining the metabolic activities of the host. The produced sulfate usually combines with the leached calcium ions to formgypsum, which can form widespread deposits on near mid-ocean spreading centers.^[97]

Hydrothermal vents emit hydrogen sulfide that support the carbon fixation ofchemolithotrophic bacteria that oxidize hydrogen sulfide with oxygen to produce elemental sulfur or sulfate.^[94]

Theiron cycle (Fe) is the biogeochemical cycle ofiron through theatmosphere,hydrosphere,biosphere andlithosphere. While Fe is highly abundant in the Earth's crust,^[102] it is less common in oxygenated surface waters. Iron is a key micronutrient inprimary productivity,^[48] and alimiting nutrient in the Southern ocean, eastern equatorial Pacific, and the subarctic Pacific referred to asHigh-Nutrient, Low-Chlorophyll (HNLC) regions of the ocean.^[49]

Iron in the ocean cycles between plankton, aggregated particulates (non-bioavailable iron), and dissolved (bioavailable iron), and becomes sediments through burial.^{[98][103][104]}Hydrothermal vents release ferrous iron to the ocean^[105] in addition to oceanic iron inputs from land sources. Iron reaches the atmosphere through volcanism,^[106]aeolian wind,^[107] and some via combustion by humans. In theAnthropocene, iron is removed from mines in the crust and a portion re-deposited in waste repositories.^[101][104]

global dust

Map of dust in 2017

Global oceanic distribution of dustdeposition

Iron is an essential micronutrient for almost every life form. It is a key component of hemoglobin, important to nitrogen fixation as part of theNitrogenase enzyme family, and as part of the iron-sulfur core offerredoxin it facilitates electron transport in chloroplasts, eukaryotic mitochondria, and bacteria. Due to the high reactivity of Fe²⁺ with oxygen and low solubility of Fe³⁺, iron is a limiting nutrient in most regions of the world.

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Carbon cycle
By regions Terrestrial Marine Atmospheric Deep carbon Soil Permafrost Boreal forest Geochemistry
Carbon dioxide In the atmosphere Ocean acidification Removal Satellite measurements
Forms of carbon Total carbon (TC) Total organic carbon (TOC) Total inorganic carbon (TIC) Dissolved organic carbon (DOC) Dissolved inorganic carbon (DIC) Particulate organic carbon (POC) Particulate inorganic carbon (PIC) Primary production marine Black carbon Blue carbon Kerogen
Metabolic pathways Photosynthesis Chemosynthesis Calvin cycle Reverse Krebs cycle Carbon fixation C3 C4
Carbon respiration Ecosystem respiration Net ecosystem production Photorespiration Soil respiration
Carbon pumps Biological pump Martin curve Solubility pump Lipid pump Marine snow Microbial loop Microbial carbon pump Viral shunt Jelly pump Whale pump Continental shelf pump
Carbon sequestration Carbon sink Soil carbon storage Marine sediment pelagic sediment
Methane Atmospheric methane Methanogenesis Methane emissions Arctic Wetland Aerobic production Clathrate gun hypothesis Carbon dioxide clathrate
Biogeochemical Marine cycles Nutrient cycle Carbonate–silicate cycle Carbonate compensation depth Great Calcite Belt Redfield ratio
Other Climate reconstruction proxies Carbon-to-nitrogen ratio Deep biosphere Deep Carbon Observatory Global Carbon Project Carbon capture and storage Carbon cycle re-balancing Territorialisation of carbon governance Total Carbon Column Observing Network C4MIP CO2SYS
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Thecalcium cycle is a transfer of calcium betweendissolved andsolid phases. There is a continuous supply ofcalcium ions into waterways fromrocks,organisms, andsoils.^[53][110] Calcium ions are consumed and removed from aqueous environments as they react to form insoluble structures such ascalcium carbonate and calcium silicate,^[53][111] which can deposit to form sediments or theexoskeletons of organisms.^[56] Calcium ions can also be utilizedbiologically, as calcium is essential to biological functions such as the production ofbones andteeth or cellular function.^[57][58] The calcium cycle is a common thread between terrestrial, marine, geological, and biological processes.^[59] Calcium moves through these different media as it cycles throughout the Earth. The marine calcium cycle is affected by changingatmospheric carbon dioxide due toocean acidification.^[56]

Biogenic calcium carbonate is formed when marine organisms, such ascoccolithophores,corals,pteropods, and othermollusks transform calcium ions andbicarbonate into shells andexoskeletons ofcalcite oraragonite, both forms of calcium carbonate.^[56] This is the dominant sink for dissolved calcium in the ocean.^[59] Dead organisms sink to the bottom of the ocean, depositing layers of shell which over time cement to formlimestone. This is the origin of both marine and terrestrial limestone.^[56]

Calcium precipitates into calcium carbonate according to the following equation:

Ca²⁺ + 2HCO₃⁻ → CO₂+ H₂O + CaCO₃^[110]

The relationship between dissolved calcium and calcium carbonate is affected greatly by the levels of carbon dioxide (CO₂) in the atmosphere.

Increased carbon dioxide leads to morebicarbonate in the ocean according to the following equation:

CO₂ + CO₃²⁻ + H₂O → 2HCO₃^−[112]

With its close relation to thecarbon cycle and the effects of greenhouse gasses, both calcium and carbon cycles are predicted to change in the coming years.^[115] Tracking calcium isotopes enables the prediction of environmental changes, with many sources suggesting increasing temperatures in both the atmosphere and marine environment. As a result, this will drastically alter the breakdown of rock, the pH of oceans and waterways and thus calcium sedimentation, hosting an array of implications on the calcium cycle.

Due to the complex interactions of calcium with many facets of life, the effects of altered environmental conditions are unlikely to be known until they occur. Predictions can however be tentatively made, based upon evidence-based research. Increasing carbon dioxide levels and decreasing ocean pH will alter calcium solubility, preventing corals and shelled organisms from developing their calcium-based exoskeletons, thus making them vulnerable or unable to survive.^[116][117]

Most biological production ofbiogenic silica in the ocean is driven bydiatoms, with further contributions fromradiolarians. These microorganisms extract dissolvedsilicic acid from surface waters during growth, and return this by recycling throughout thewater column after they die. Inputs of silicon to the ocean from above arrive via rivers andaeolian dust, while those from below include seafloor sediment recycling, weathering, andhydrothermal activity.^[118]

"Biological activity is a dominant force shaping the chemical structure and evolution of the earth surface environment. The presence of an oxygenated atmosphere-hydrosphere surrounding an otherwise highly reducing solid earth is the most striking consequence of the rise of life on earth. Biological evolution and the functioning of ecosystems, in turn, are to a large degree conditioned by geophysical and geological processes. Understanding the interactions between organisms and their abiotic environment, and the resulting coupled evolution of the biosphere and geosphere is a central theme of research in biogeology. Biogeochemists contribute to this understanding by studying the transformations and transport of chemical substrates and products of biological activity in the environment."^[119]

"Since the Cambrian explosion, mineralized body parts have been secreted in large quantities by biota. Because calcium carbonate, silica and calcium phosphate are the main mineral phases constituting these hard parts, biomineralization plays an important role in the global biogeochemical cycles of carbon, calcium, silicon and phosphorus"^[119]

Deep cycling involves the exchange of materials with themantle. Thedeep water cycle involves exchange of water with the mantle, with water carried down bysubducting oceanic plates and returning through volcanic activity, distinct from thewater cycle process that occurs above and on the surface of Earth. Some of the water makes it all the way to thelower mantle and may even reach theouter core.

Part of a series on
Biogeochemical cycles
Water cycle Water cycle deep water cycle
Carbon cycle Global atmospheric terrestrial oceanic Sequestration carbon sink deep carbon cycle soil carbon mycorrhizal fungi Boreal forests
Nutrient cycle Hydrogen cycle Nitrogen cycle human impact nitrification nitrogen and lichens fixation assimilation Oxygen cycle Phosphorus cycle assimilation Sulfur cycle assimilation
Rock cycle Calcium cycle Silica cycle Carbonate–silicate cycle
Marine cycle Marine biogeochemical cycles Biological pump microbial loop viral shunt Calcareous ooze Siliceous ooze
Methane cycle Atmospheric methane Methane clathrate clathrate gun hypothesis Arctic methane emissions
Other cycles aluminum arsenic boron bromine cadmium chlorine chromium copper fluorine gold iodine iron lead lithium manganese mercury ozone–oxygen selenium vanadium zinc
Related topics Biogeochemistry geochemical cycle chemical cycling environmental chemistry Biosequestration Deep biosphere Ocean acidification acid rain Biogeochemical planetary boundaries
Research groups DAAC GEOTRACES IMBER NOBM SOLAS
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In the conventional view of the water cycle (also known as thehydrologic cycle), water moves between reservoirs in theatmosphere and Earth's surface or near-surface (including theocean,rivers andlakes,glaciers andpolar ice caps, thebiosphere andgroundwater). However, in addition to the surface cycle, water also plays an important role in geological processes reaching down into thecrust andmantle. Water content inmagma determines how explosive a volcanic eruption is; hot water is the main conduit for economically important minerals to concentrate inhydrothermal mineral deposits; and water plays an important role in the formation and migration ofpetroleum.^[120] Petroleum is afossil fuel derived from ancientfossilizedorganic materials, such aszooplankton andalgae.^[121][122]

Water is not just present as a separate phase in the ground. Seawater percolates into oceanic crust andhydrates igneous rocks such asolivine andpyroxene, transforming them into hydrous minerals such asserpentines,talc andbrucite.^[123] In this form, water is carried down into the mantle. In theupper mantle, heat and pressure dehydrates these minerals, releasing much of it to the overlyingmantle wedge, triggering the melting of rock that rises to formvolcanic arcs.^[124] However, some of the "nominally anhydrous minerals" that are stable deeper in the mantle can store small concentrations of water in the form ofhydroxyl (OH⁻),^[125] and because they occupy large volumes of the Earth, they are capable of storing at least as much as the world's oceans.^[120]

The conventional view of the ocean's origin is that it was filled by outgassing from the mantle in the earlyArchean and the mantle has remained dehydrated ever since.^[127] However, subduction carries water down at a rate that would empty the ocean in 1–2 billion years. Despite this, changes in theglobal sea level over the past 3–4 billion years have only been a few hundred metres, much smaller than the average ocean depth of 4 kilometres. Thus, the fluxes of water into and out of the mantle are expected to be roughly balanced, and the water content of the mantle steady. Water carried into the mantle eventually returns to the surface in eruptions atmid-ocean ridges andhotspots.^[128]: 646 Estimates of the amount of water in the mantle range from1⁄4 to 4 times the water in the ocean.^{[128]: 630–634}

Thedeep carbon cycle is the movement ofcarbon through the Earth'smantle andcore.It forms part of thecarbon 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.^[129]

Aquaticphytoplankton andzooplankton that died and sedimented in large quantities underanoxic conditions millions of years ago began forming petroleum and natural gas as a result ofanaerobic decomposition (by contrast,terrestrial plants tended to formcoal and methane). Overgeological time thisorganicmatter, mixed withmud, became buried under further heavy layers of inorganic sediment. The resulting hightemperature andpressure caused the organic matter to chemicallyalter, first into a waxy material known askerogen, which is found inoil shales, and then with more heat into liquid and gaseous hydrocarbons in a process known ascatagenesis.Such organisms and their resulting fossil fuels typically have an age of millions of years, and sometimes more than 650 million years,^[130] the energy released in combustion is still photosynthetic in origin.^[131]

Such as trace minerals, micronutrients, human-induced cycles for synthetic compounds such aspolychlorinated biphenyl (PCB).

• 
  Lead cycle
• 
  Mercury cycle

• Nordin, B. E. C., ed. (1988).Calcium in Human Biology. ILSI Human Nutrition Reviews.doi:10.1007/978-1-4471-1437-6.ISBN 978-1-4471-1439-0.
• Schlesinger, William H.; Bernhardt, Emily S. (2020).Biogeochemistry: An Analysis of Global Change.doi:10.1016/C2017-0-00311-7.ISBN 978-0-12-814608-8.

• James, Rachael (2005).Marine Biogeochemical Cycles. Butterworth-Heinemann.ISBN 978-0-7506-6793-7.

• Biogeochemical cycle
• Geochemistry
• Biogeography
• Marine organisms
• Oceanography

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