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Marine Carbon Cycle

The oceanic 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. Carbon is an element that is essential to all living things; the human body is made up of approximately 18% carbon.[1] The carbon cycle circulates carbon around the planet, ensuring that carbon is available globally at short time-scales. 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 incorporated into a living thing).

The oceans play a key role in both storage and flux of carbon. Credit: NASA Earth Observatory.

The marine carbon cycle brings atmospheric carbon dioxide (CO2) into the ocean interior where three main processes move that carbon through the oceans. These three processes, or pumps, are: (1) the solubility pump, (2) the carbonate pump, and (3) the biological pump. These three pumps are interdependent and together transport carbon throughout the oceans. The total active pool of carbon at the Earth's surface is roughly 40,000 gigatons C (GtC, a gigaton is one billion tons, or the weight of approximately 6 million blue whales), and about 95% (~38,000 GtC) is stored in the ocean, mostly as dissolved inorganic carbon.[2][3]

Earth’s plants and algae (primary producers) are responsible for the largest annual carbon fluxes. Although the amount of carbon stored in marine biota (~3 GtC) is very small compared with terrestrial vegetation (~610 GtC), the amount of carbon exchanged (the flux) by these groups is nearly equal.[2] Marine and terrestrial organisms link the carbon and oxygen cycles through processes such as photosynthesis.[2] The marine carbon cycle is also tied to the nitrogen and phosphorus cycles by a constant stoichiometric ratio C:N:P of 106:16:1[4], meaning that an organism needs to take up nitrogen and phosphorus when carbon incorporates new organic matter. Likewise, organic matter decomposed by bacteria releases phosphorus and nitrogen.

The marine carbon cycle is also a main regulator of acid-base chemistry in the oceans. Some of the carbon species in the ocean, such as bicarbonate, are major contributors to alkalinity, a natural ocean buffer that prevents drastic changes in acidity (or pH). The marine carbon cycle also affects the reaction and dissolution rates of some chemical compounds, regulates the amount of carbon dioxide in the atmosphere and Earth's temperature.[5]

The human impacts on the marine carbon cycle are significant. Before the Industrial Revolution, the ocean was a net source of CO2 to the atmosphere whereas now the majority of the carbon that enters the ocean comes from atmospheric carbon dioxide (CO2).[6] The burning of fossil fuels and production of cement have changed the balance of carbon dioxide between the atmosphere and oceans.[2] Climate change, a result of excess CO2 in the atmosphere, has increased the temperature of the ocean and atmosphere (global warming).[7] The slowed rate of global warming occurring from 2000-2010[8] may be attributed to an observed increase in upper ocean heat content.[9][10]

Marine Carbon

Carbon compounds can be distinguished as either organic or inorganic depending on their composition. Organic carbon is the key component of organic matter - proteins, lipids, carbohydrates, and nucleic acids. Inorganic carbon is found primarily in simple compounds such as carbon dioxide, carbonic acid, bicarbonate, and carbonate (CO2, H2CO3, HCO3-, CO32- respectively).

Carbon is separated into four distinct pools based on whether it is organic/inorganic and whether it is dissolved/particulate. The processes associated with each arrow describe the transformation associated with the transfer of carbon from one reservoir to another.

Marine carbon is further separated into particulate and dissolved pools. These pools are distinguished by physical separation - dissolved carbon passes through a 0.2 μm filter.

Inorganic carbon

There are two main types of inorganic carbon that are found in the oceans. Dissolved inorganic carbon (DIC) is made up of bicarbonate, carbonate and carbon dioxide. DIC can be converted to particulate inorganic carbon (PIC) through calcification of CaCO3 (ie. shell formation). DIC can also be converted to particulate organic carbon (POC) through photosynthesis and chemoautotrophy (ie. primary production). DIC increases exponentially with depth as particles sink. Free oxygen decreases as DIC increases because the ratio of oxygen to carbon is much higher in inorganic carbon.

Particulate inorganic carbon (PIC) is the other form of inorganic carbon found in the ocean. Most PIC is the CaCO3 that makes up shells of various marine organisms. Fish also make calcium carbonate during osmoregulation.

Organic carbon

Like inorganic carbon, there are two main forms of organic carbon found in the ocean (dissolved and particulate). Dissolved organic carbon (DOC) can be thought of as any organic molecule that is smaller than 0.2 µm and therefore cannot be trapped on a filter. DOC can be converted into particulate organic carbon through heterotrophy and it can also be converted back to dissolved inorganic carbon (DIC) through respiration.

POC is comprised of organisms (dead or alive), their fecal matter, and detritus. POC can be converted to DOC through disaggregation of molecules and by exudation by phytoplankton. POC can also be converted to DIC through respiration. POC decreases exponentially with depth.

Marine Carbon Pumps

Solubility Pump

Full article: Solubility Pump

The oceans store the largest pool of reactive carbon on the planet due to the conversion of carbon dioxide to dissolved inorganic carbon - the solubility pump.[5] Aqueous CO2, carbonic acid, bicarbonate, and carbonate comprise dissolved inorganic carbon (DIC), the largest source of reactive carbon (~37,400 GtC)[5] in the world. The amount of dissolved inorganic carbon in the ocean is significantly higher in the mid to deep ocean (below 300 m depth).[11] The below chemical equations show the reactions that CO2 undergoes after it enters the ocean and transforms into its aqueous form.

First, carbon dioxide reacts with water to form carbonic acid.

Sea surface dissolved inorganic carbon concentration in the 1990s (from the GLODAP climatology)

(1)

Carbonic acid rapidly dissociates into free hydrogen ion (technically, hydronium) and bicarbonate.

(2)

The free hydrogen ion meets carbonate, already present in the water from the dissolution of CaCO3, and reacts to form more bicarbonate ion.

(3)

The dissolved species in the equations above, mostly bicarbonate, make up the carbonate alkalinity system, the dominant contributor to seawater alkalinity.[12]

Carbonate Pump

The carbonate pump, sometimes called the carbonate counter pump, starts with marine organisms producing particulate inorganic carbon (PIC) in the form of calcium carbonate (calcite or aragonite, CaCO3). This CaCO3 is what forms hard body parts like shells, or external formations such as coral reefs[5]. The formation of these shells increases atmospheric CO2 due to the production of CaCO3[12] in the following reaction:

(4) [13]

Coccolithophores, a nearly ubiquitous group of phytoplankton that produce shells of calcium carbonate, are the dominant contributors to the carbonate pump.[5] Due to their abundance coccolithophores have serious implications for the carbonate chemistry in the surface waters they inhabit as well as the ocean below.[14] The air-sea CO2 flux induced by a marine biological community can be determined by the rain ratio - the proportion of carbon from calcium carbonate compared to that from organic carbon in particulate matter sinking to the ocean floor, CCaCO3/COrganic.[13]

Biological Pump

Full article: Biological Pump

Organic carbon is exported from the upper ocean in a flux commonly termed the biological pump. The biological pump converts DIC into dissolved organic matter (DOM), which makes up a pool of tens to hundreds of thousands of carbon-rich organic molecules.[15] These molecules can be classified, based on how easily organisms can break them down for food, as labile, semilabile, or refractory. Photosynthesis by phytoplankton is the primary source for labile and semilabile molecules, and is the indirect source for most refractory molecules.[16][17] Labile molecules are present at low concentrations (in the picomolar range) and have half-lives of only minutes.[18] They are consumed by microbes within hours or days of production and reside in the surface oceans,[17] where they contribute a majority of the labile carbon flux.[19] Semilabile molecules, much more difficult to consume, are able to reach depths of hundreds of meters below the surface before being metabolized.[20] Refractory DOM largely comprises highly conjugated molecules like Polycyclic aromatic hydrocarbons or lignin.[16] Refractory DOM can reach depths greater than 1000 m and circulates through the oceans over thousands of years.[21][17][22] Over the course of a year, approximately 20 gigatons of photosynthetically-fixed labile and semilabile carbon is taken up by heterotrophs, whereas fewer than 0.2 gigatons of refractory carbon is consumed.[17] Marine dissolved organic matter (DOM) can store as much carbon as the current atmospheric CO2 supply,[22] but industrial processes are altering the balance of this cycle.[23] Life plays a key role in the global carbon cycle through respiration and photosynthesis (equations 5 and 6, respectively).[2] Dissolved inorganic carbon is biologically converted into organic matter by photosynthesis and other forms of autotrophy.[5] Organic carbon is used for energy (via respiration) by the organism that produced it or an organism higher in the food chain. Respiration turns organic carbon back into inorganic carbon.

(5)

carbohydrate + oxygen → carbon dioxide + water + heat

(6)

carbon dioxide + water + light energy → carbohydrate + oxygen

Inputs of Marine Carbon

Inputs to the marine carbon cycle are numerous, but the primary contributions, on a net basis, come from the atmosphere and rivers.[2] Hydrothermal vents supply carbon equal to the amount they consume.[5]

Atmosphere

Dissociation of carbon dioxide following Henry's Law

Before the Industrial Revolution, the ocean used to be a source of CO2 to the atmosphere; now it has become a sink for the excess CO2 in the atmosphere. Carbon dioxide is absorbed from the atmosphere at the ocean's surface at a known exchange rate. This exchange rate generally follows Henry's law (S = kP), where the solubility (S) of the carbon dioxide gas is proportional to the amount of gas in the atmosphere, or its partial pressure.[2] The amount of carbon dioxide that can be taken up by the oceans is related to the temperature of the water, where colder regions have higher uptake. The North Atlantic and Nordic oceans have the highest carbon uptake per unit area in the world.[24] With rising sea surface temperatures, the capacity of the oceans to take in carbon dioxide decreases. [25][12] A common way in oceanography to express how much carbon dioxide is being taken up by the oceans is the Revelle factor.[25][12] The Revelle Factor is a ratio of the change of carbon dioxide to the change in dissolved inorganic carbon. The Revelle Factor is a way to characterize the thermodynamic efficiency of the ocean to convert CO2 into bicarbonate. A high Revelle Factor means that more carbon dioxide has been taken up by the ocean, and has not been converted into bicarbonate. This excess CO2 decreases the pH of the ocean.

Rivers

Rivers can also transport carbon to the ocean through weathering of carbonate and aluminosilicate rocks on land, or by the decomposition of life (e.g. plant and soil material). Rivers contribute roughly equal amounts (~0.4 GtC/yr) of DIC and DOC to the oceans.[2] When it rains, an influx of carbon from land enters the rivers. It is estimated that approximately 0.8 GtC (DIC + DOC) is transported annually from the rivers to the ocean[2]. The rivers that flow into Chesapeake Bay (Susquehanna, Potomac, and James rivers) input approximately 0.004 Gt (6.5 x 1010 moles) DIC per year, or approximately 0.1 % of annual river influx.[26] The total carbon transport of rivers represents approximately 0.02 % of the total carbon in the atmosphere. Though it seems small, over long time scales (1000 to 10,000 years) the carbon that enters rivers (and therefore does not enter the atmosphere) serves as a stabilizing feedback for greenhouse warming.[27]

Outputs of Marine Carbon

The key outputs of the marine carbon system are organic matter and calcium carbonate preservation as well as reverse weathering.[2] A loss of CO2 to the atmosphere and hydrothermal processes does occur, but not at a net loss.[5]

Organic Matter Preservation

Marine sediments are important since they provide a thorough record of life on Earth and an important source of fossil fuel.[28] They are also the largest outlet for carbon from the oceanic system, due to sedimentation.[28] Oceanic carbon can exit the system in the form of dead phytoplankton that sink and are buried in the seafloor without being fully decomposed or dissolved. Ocean floor surface sediments account for 1.75x1015 kg of carbon in the global carbon cycle [29] At most, 4% of the particulate organic carbon from the euphotic zone, where light-powered primary production occurs, is buried in marine sediments.[28]

90% of organic carbon burial occurs in deposits of deltas and continental shelves and upper slopes.[30] Lignin and pollen are inherently resistant to degradation, and some studies show that inorganic matrices may also protect organic matter.[31] Preservation rates of organic matter depend on other interdependent variables that vary nonlinearly in time and space.[32] Although organic matter breakdown occurs rapidly in the presence of oxygen, microbes utilizing a variety of chemical species (via redox gradients) can degrade organic matter in anoxic sediments.[32] The burial depth at which degradation halts depends upon the sedimentation rate, the relative abundance of organic matter in the sediment, the type of organic matter being buried, and innumerable other variables.[32]

While decomposition of organic matter can occur in anoxic sediments when bacteria use oxidants other than oxygen (nitrate, sulfate, Fe3+), decomposition tends to end short of complete mineralization.[33] This occurs because of preferential decomposition of labile molecules over refractile molecules.[33]

The accumulation rate of organic carbon was 50% larger during the glacial maximum compared to interglacials. [34]

Calcium Carbonate Preservation

The precipitation and burial of calcium carbonate in the ocean removes carbon (POC) from the ocean and ultimately forms limestone.[5] The precipitation of calcium carbonate is important as it results in a loss of alkalinity as well as a release of CO2 (Equation 4), and therefore a change in the rate of preservation of calcium carbonate can alter the partial pressure of CO2 in Earth's atmosphere.[5] CaCO3 is supersatured in the ocean surface and undersaturated at depth[12], meaning the shells will dissolve as they sink, and deep ocean sediments have very little calcium carbonate.[5]

On time scales greater than 500,000 years Earth's climate is moderated by the flux of carbon in and out of the lithosphere.[35] Rocks formed in the ocean are brought to the surface and weathered or subducted into the mantle, the carbon outgassed by volcanoes.[2]

Reverse Weathering

Reverse weathering involves the chemical weathering process that breaks down rocks on land operating in reverse in marine sediments. The chemical reaction includes the consumption of bicarbonate in the formation of aluminosilicate rock.[36] Note that bicarbonate is produced in the weathering of both silicate and carbonate rocks and is likewise consumed in their formation.

Human Impacts on the Marine Carbon Cycle

Oceans can take up 15 - 40% of anthropogenic CO2,[37][38] and so far roughly 40% of the carbon from fossil fuel combustion has been taken up into the oceans.[39] Because the Revelle factor increases with increasing CO2, a smaller fraction of the anthropogenic flux will be taken up by the ocean in the future.[40] Current annual increase in atmospheric CO2 is approximately 4 gigatons of carbon.[41] This induces climate change that drives carbon concentration and carbon-climate feedback processes that modifies ocean circulation and the physical and chemical properties of seawater, which alters CO2 uptake.[42][43]

Ocean Acidification

Full article: Ocean Acidification

The pH of the oceans is declining due to uptake of atmospheric CO2.[44]  The rise in dissolved carbon dioxide reduces the availability of the carbonate ion, vital for many organisms that rely on CaCO3.[45] Carbonate ions preferentially bind to hydrogen ions to form bicarbonate[12], thus a reduction in carbonate ion availability increases the amount of unbound hydrogen ions (Equations 1-3). pH is a measurement of hydrogen ions, where a low pH means there are more unbound hydrogen ions. pH is therefore an indicator of carbonate speciation (the format of carbon present) in the oceans and can be used to assess how healthy the ocean is[45].

The list of organisms that may struggle due to ocean acidification include coccolithophores and foraminifera (the base of the marine food chain in many areas), human food sources such as oysters and mussels, and perhaps the most conspicuous, a structure built by microorganisms – the coral reefs.[45] Most surface water will remain supersaturated with respect to CaCO3 (both calcite and aragonite) for some time on current emissions trajectories[45], but the organisms that require carbonate will likely be replaced in many areas.[45] Coral reefs are under pressure from overfishing, nitrate pollution, and warming waters; ocean acidification will add additional stress on these important structures[45].

Iron Fertilization

Full article: Iron Fertilization

Iron fertilization is a facet of geoengineering, which purposefully manipulates the Earth’s climate system, typically in aspects of the carbon cycle or radiative forcing. Of current geoengineering interest is the possibility of accelerating the biological pump to increase export of carbon from the surface ocean. This increased export could theoretically remove excess carbon dioxide from the atmosphere for storage in the deep ocean. Ongoing investigations regarding artificial fertilization exist.[46] Due to the scale of the ocean and the fast response times of heterotrophic communities to increases in primary production, it is difficult to determine whether limiting-nutrient fertilization results in an increase in carbon export.[46]

Dams and Reservoirs

There are over 16 million dams in the world[47] that alter carbon transport from rivers to oceans.[48] Using data from the Global Reservoirs and Dams database, which contains approximately 7000 reservoirs that hold 77 % of the total volume of water held back by dams (8000 km3), it is estimated that the delivery of carbon to the ocean has decreased by 13 % since 1970 and is projected to reach 19 % by 2030.[49] The excess carbon contained in the reservoirs may emit an additional ~.184 Gt of carbon to the atmosphere per year[50] and an additional ~0.2 GtC will be buried in sediment.[49] Prior to 2000, the Mississippi, the Niger, and the Ganges River basins account for 25 – 31 % of all reservoir carbon burial.[49] After 2000, the Paraná (home to 70 dams) and the Zambezi (home to the largest reservoir) River basins exceeded the burial by the Mississippi.[49] Other large contributors to carbon burial caused by damming occur on the Danube, the Amazon, the Yangtze, the Mekong, the Yenisei, and the Tocantins Rivers.[49]

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