Marine biogeochemical cycles

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 are

brackish water of coastal estuaries. These biogeochemical cycles are the pathways chemical substances and elements

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 are biogeochemical cycles for the elements calcium, carbon, hydrogen, mercury, nitrogen, oxygen, phosphorus, selenium, and sulfur; molecular cycles for water and silica; macroscopic cycles such as the rock cycle; as well as human-induced cycles for synthetic compounds such as polychlorinated 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 particularly marine microorganisms are crucial for the functioning of many of these cycles. The forces driving biogeochemical cycles include metabolic processes within organisms, geological processes involving the Earth's mantle, as well as chemical 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.

Overview

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]

The water cycle

Main article: water cycle

Further information: Gulf Stream and thermohaline circulation

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 as solid, liquid, and gas 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 a

river system to the Gulf of Mexico. Runoff also plays a part in the carbon cycle, again through the transport of eroded rock and soil.^[11]

Ocean salinity

See also: Temperature–salinity diagram

Ocean salinity is derived mainly from the weathering of rocks and the transport of dissolved salts from the land, with lesser contributions from hydrothermal 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 in

  practical 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]

Sea spray

marine microorganisms, and all the substances and elements contained in their bodies, can be swept high into the atmosphere. There they become aeroplankton

and can travel the globe before falling back to Earth.

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 in sea 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 form

air bubbles, which are entrained by the wind stress during the whitecap 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]

Ocean circulation

See also: continental shelf pump

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, inducing Ekman transport

Two mechanisms which result in upwelling. In each case, if the wind direction were reversed it would induce downwelling.^[21]

Ventilation of the deep ocean

Antarctic bottom water

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

Global conveyor belt

Solar radiation affects the oceans: warm water from the Equator tends to circulate toward the

western 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 causes

Atlantic 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 the North 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]

Average reservoir residence times [32]
Reservoir	Average residence time
Antarctica	20,000 years
Oceans	3,200 years
Glaciers	20 to 100 years
Seasonal snow cover	2 to 6 months
Soil moisture	1 to 2 months
Groundwater: shallow	100 to 200 years
Groundwater: deep	10,000 years
Lakes (see lake retention time)	50 to 100 years
Rivers	2 to 6 months
Atmosphere	9 days

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]

Cycling of key elements

Some key elements involved in marine biogeochemical cycles
Element	Diagram	Description
Carbon		The acid-base chemistry in the oceans.
Oxygen		The molecular oxygen (O2) since it is the common product or reactant of many biogeochemical redox reactions within the cycle.[37] Processes within the oxygen cycle are considered to be biological or geological and are evaluated as either a source (O2 production) or sink (O2 consumption).[36][37]
Hydrogen		The hydrogen cycle consists of hydrogen exchanges between biotic (living) and abiotic (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), and ammonia (NH3). Many organic compounds also contain H atoms, such as hydrocarbons and organic matter. Given the ubiquity of hydrogen atoms in inorganic and organic chemical compounds, the hydrogen cycle is focused on molecular hydrogen (H2).
Nitrogen		The ecologists because nitrogen availability can affect the rate of key ecosystem processes, including primary production and decomposition. Human activities such as fossil fuel combustion, use of artificial nitrogen fertilizers, and release of nitrogen in wastewater have dramatically altered 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		The geologic time.[45] Humans have caused major changes to the global phosphorus cycle through shipping of phosphorus minerals, and use of phosphorus fertilizer , and also the shipping of food from farms to cities, where it is lost as effluent.
Sulphur		The essential element, being a constituent of many proteins and cofactors, and sulfur compounds can be used as oxidants or reductants in microbial respiration.[46] The global sulfur 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 major oxidizing agent.[47]
Iron		The microorganisms, especially iron-oxidizing bacteria. The abiotic processes include the rusting 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.[52][53]
Calcium		The calcium cycle is a transfer of calcium between dissolved and solid phases. There is a continuous supply of calcium ions into waterways from rocks, organisms, and soils.[54][55] Calcium ions are consumed and removed from aqueous environments as they react to form insoluble structures such as calcium carbonate and calcium silicate,[54][56] which can deposit to form sediments or the exoskeletons of organisms.[57] Calcium ions can also be utilized biologically, as calcium is essential to biological functions such as the production of bones and teeth or cellular function.[58][59] The calcium cycle is a common thread between terrestrial, marine, geological, and biological processes.[60] The marine calcium cycle is affected by changing atmospheric carbon dioxide due to ocean acidification.[57]
Silicon		The silica between the Earth's systems. Opal silica (SiO2), also called silicon dioxide, is a chemical compound of silicon. Silicon is a bioessential element and is one of the most abundant elements on Earth.[61][62] The silica cycle has significant overlap with the carbon cycle (see the carbonate–silicate cycle) and plays an important role in the sequestration of carbon through continental weathering, biogenic export and burial as oozes on geologic timescales.[63]

Box models

See also:

Climate box models

Box models are widely used to model biogeochemical systems.^[65] 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.^[64] 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.^[64]

Measurement units

Global biogeochemical box models usually measure:
            — reservoir masses in petagrams (Pg)
            — flow fluxes in petagrams per year (Pg yr⁻¹)
           Diagrams in this article mostly use these units
________________________________________________
 one

gigatonne = one billion (10⁹) tonnes

The

turnover 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.^[64]

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.[64] More complex multibox 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 the

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.^[66]

Dissolved and particulate matter

Further information:

Particulate organic carbon

fulvic acid, and humin) and non-humic material.^[71]

Biological pumps

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.^[74]

Further information: Biological pump

The

mollusks (carbonate pump).^[76]

The biological pump can be divided into three distinct phases,

coccolithophores and foraminifera) combine calcium (Ca) and dissolved carbonates (carbonic acid and bicarbonate

) 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 regenerative nutrient 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 in primary 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, M.D. (2017) "Bridging the gap between knowing and modeling viruses in marine systems—An upcoming frontier". Frontiers in Marine Science, 3: 284.
  doi:10.3389/fmars.2016.00284
• Beckett, S.J. and Weitz, J.S. (2017) "Disentangling niche competition from grazing mortality in phytoplankton dilution experiments". PLOS ONE, 12(5): e0177517.
  doi:10.1371/journal.pone.0177517
  .

Role of microorganisms

See also: marine microorganisms, microbial loop, viral shunt, and marine microbial symbiosis

DOM, POM and the viral shunt

dissolved organic matter (DOM) and particulate organic matter

(POM) through the marine food web

Carbon, oxygen and hydrogen cycles

Further information:

ocean carbon cycle, blue carbon, oxygen cycle, and hydrogen cycle

The

marine carbon cycle is composed of processes that exchange carbon between various pools within the ocean as well as between the atmosphere, Earth interior, and the seafloor. The carbon 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 both inorganic carbon (carbon not associated with a living thing, such as carbon dioxide) and organic

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^[79]

Oxygen cycle

Three main processes (or pumps) that make up the marine carbon cycle bring atmospheric

acid-base chemistry

in the oceans.

Forms of carbon [80]
Carbon form	Chemical formula	State	Main reservoir
carbon dioxide	CO2	gas	atmosphere
carbonic acid	H2CO3	liquid	ocean
bicarbonate ion	HCO3−	liquid (dissolved ion)	ocean
organic compounds	Examples: C6H12O6 (glucose) CH4 (methane)	solid gas	organic sediments (fossil fuels )
other carbon compounds	Examples: CaCO3 (calcium carbonate) CaMg(CO3)2 (calcium magnesium carbonate)	solid	shells sedimentary rock

Nitrogen and phosphorus cycles

land runoff cause excessive growth of microorganisms, which depletes oxygen and kills fauna. Worldwide, large dead zones are found in coastal areas with high human population density.^[1]

particulate organic material

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.^[81] Nitrogen enters the ocean through precipitation, runoff, or as N₂ from the atmosphere. Nitrogen cannot be utilized by phytoplankton as N₂ so it must undergo nitrogen fixation which is performed predominantly by cyanobacteria.^[82] Without supplies of fixed nitrogen entering the marine cycle, the fixed nitrogen would be used up in about 2000 years.^[83] Phytoplankton need nitrogen in biologically available forms for the initial synthesis of organic matter. Ammonia and urea are released into the water by excretion from plankton. Nitrogen sources are removed from the euphotic 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 to nitrite and nitrate but they are inhibited by light so this must occur below the euphotic zone.^[82] Ammonification or mineralization is performed by bacteria to convert organic nitrogen to ammonia. Nitrification can then occur to convert the ammonium to nitrite and nitrate.^[84] Nitrate can be returned to the euphotic zone by vertical mixing and upwelling where it can be taken up by phytoplankton to continue the cycle. N₂ can be returned to the atmosphere through denitrification.

Ammonium is thought to be the preferred source of fixed nitrogen for phytoplankton because its assimilation does not involve a redox 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 most Prochlorococcus and some Synechococcus that can only take up nitrogen as ammonium.^[83]

Marine nitrogen cycle

Marine phosphorus cycle

Phosphorus is an essential nutrient for plants and animals. Phosphorus is a

A

There is considerable overlap between the terms for the biogeochemical cycle and nutrient cycle. Some textbooks integrate the two and seem to treat them as synonymous terms.^[88] 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.^[89] Nutrient cycling occurs in ecosystems that participate in the "larger biogeochemical cycles of the earth through a system of inputs and outputs."^[89]: 425

Dissolved nutrients

Nutrients dissolved in seawater are essential for the survival of marine life. Nitrogen and phosphorus are particularly important. They are regarded as

The process of cycling nutrients in the sea starts with biological 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 as marine 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 the primary 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..."^[90]

Ocean nutrient 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 by continental divides. The grey areas are endorheic 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."^[90]

Marine sulfur cycle

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).^[91]

• cable bacteria

The sulfur cycle in marine environments has been well-studied via the tool of sulfur isotope systematics expressed as δ³⁴S. The modern global oceans have sulfur storage of 1.3 × 10²¹ g,^[92] mainly occurring as sulfate with the δ³⁴S value of +21‰.^[93] The overall input flux is 1.0 × 10¹⁴ g/year with the sulfur isotope composition of ~3‰.^[93] 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‰).^[94] 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.^[93] The residence time of sulfur in modern global oceans is 13,000,000 years.^[95]

In modern oceans, Hydrogenovibrio crunogenus, Halothiobacillus, and Beggiatoa are primary sulfur oxidizing bacteria,^[96][97] and form chemosynthetic symbioses with animal hosts.^[98] 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 form gypsum, which can form widespread deposits on near mid-ocean spreading centers.^[99]

Hydrothermal vents emit hydrogen sulfide that support the carbon fixation of chemolithotrophic bacteria that oxidize hydrogen sulfide with oxygen to produce elemental sulfur or sulfate.^[96]

Iron cycle and dust

Iron in the ocean cycles between plankton, aggregated particulates (non-bioavailable iron), and dissolved (bioavailable iron), and becomes sediments through burial.[100]^[105][106] Hydrothermal vents release ferrous iron to the ocean^[107] in addition to oceanic iron inputs from land sources. Iron reaches the atmosphere through volcanism,^[108] aeolian wind,^[109] and some via combustion by humans. In the Anthropocene, iron is removed from mines in the crust and a portion re-deposited in waste repositories.^[103][106]