![POC](https://upload.wikimedia.org/wikipedia/commons/e/e5/POC.png)
The Martin curve is a power law used by
oceanographers
Oceanography (), also known as oceanology and ocean science, is the scientific study of the oceans. It is an Earth science, which covers a wide range of topics, including ecosystem dynamics; ocean currents, waves, and geophysical fluid dynami ...
to describe the export to the ocean floor of
particulate organic carbon
Particulate organic matter (POM) is a fraction of total organic matter operationally defined as that which does not pass through a filter pore size that typically ranges in size from 0.053 to 2 millimeters.
Particulate organic carbon (POC) is ...
(POC). The curve is controlled with two parameters: the reference depth in the
water column
A water column is a conceptual column of water from the surface of a sea, river or lake to the bottom sediment.Munson, B.H., Axler, R., Hagley C., Host G., Merrick G., Richards C. (2004).Glossary. ''Water on the Web''. University of Minnesota-D ...
, and a
remineralisation
In biogeochemistry, remineralisation (or remineralization) refers to the breakdown or transformation of organic matter (those molecules derived from a biological source) into its simplest inorganic forms. These transformations form a crucial link ...
parameter which is a measure of the rate at which the vertical
flux of POC attenuates.
It is named after the American oceanographer
John Martin.
The Martin Curve has been used in the study of
ocean carbon cycling and has contributed to understanding the role of the ocean in regulating atmospheric levels.
Background
![Flux of POC with depth](https://upload.wikimedia.org/wikipedia/commons/2/28/Flux_of_POC_with_depth.png)
The dynamics of the
particulate organic carbon
Particulate organic matter (POM) is a fraction of total organic matter operationally defined as that which does not pass through a filter pore size that typically ranges in size from 0.053 to 2 millimeters.
Particulate organic carbon (POC) is ...
(POC) pool in the ocean are central to the
marine carbon cycle
The oceanic carbon cycle (or 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 Seabed, seafloor. The carbon cycle is a result of ma ...
. POC is the link between surface
primary production
In ecology, primary production is the synthesis of organic compounds from atmospheric or aqueous carbon dioxide. It principally occurs through the process of photosynthesis, which uses light as its source of energy, but it also occurs through ...
, the
deep ocean
The deep sea is broadly defined as the ocean depth where light begins to fade, at an approximate depth of 200 metres (656 feet) or the point of transition from continental shelves to continental slopes. Conditions within the deep sea are a combin ...
, and
marine sediments. The rate at which POC is degraded in the
dark ocean can impact atmospheric CO
2 concentration.
[Kharbush, J.J., Close, H.G., Van Mooy, B.A., Arnosti, C., Smittenberg, R.H., Le Moigne, F.A., Mollenhauer, G., Scholz-Böttcher, B., Obreht, I., Koch, B.P. and Becker, K. (2020) "Particulate Organic Carbon Deconstructed: Molecular and Chemical Composition of Particulate Organic Carbon in the Ocean". ''Frontiers in Marine Science'', 7: 518. . ]
Material was copied from this source, which is available under
Creative Commons Attribution 4.0 International License
The
biological carbon pump
The biological pump (or ocean carbon biological pump or marine biological carbon pump) is the ocean's biologically driven sequestration of carbon from the atmosphere and land runoff to the ocean interior and seafloor sediments.Sigman DM & GH ...
(BCP) is a crucial mechanism by which atmospheric CO
2 is taken up by the ocean and transported to the ocean interior. Without the BCP, the pre-industrial atmospheric CO
2 concentration (~280 ppm) would have risen to ~460 ppm. At present, the
particulate organic carbon
Particulate organic matter (POM) is a fraction of total organic matter operationally defined as that which does not pass through a filter pore size that typically ranges in size from 0.053 to 2 millimeters.
Particulate organic carbon (POC) is ...
(POC) flux from the surface layer of the ocean to the ocean interior has been estimated to be 4–13 Pg-C year
−1. To evaluate the efficiency of the BCP, it is necessary to quantify the vertical attenuation of the POC flux with depth because the deeper that POC is transported, the longer the CO
2 will be isolated from the atmosphere. Thus, an increase in the efficiency of the BCP has the potential to cause an increase of ocean
carbon sequestration
Carbon sequestration is the process of storing carbon in a carbon pool. Carbon dioxide () is naturally captured from the atmosphere through biological, chemical, and physical processes. These changes can be accelerated through changes in lan ...
of atmospheric CO
2 that would result in a negative feedback on global warming.
[ ]
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Creative Commons Attribution 4.0 International License
Different researchers have investigated the vertical attenuation of the POC flux since the 1980s.
In 1987, Martin ''et al''. proposed the following power law function to describe the POC flux attenuation:
:
(1)
where ''z'' is water depth (m), and ''F''
''z'' and ''F''
100 are the POC fluxes at depths of ''z'' metres and 100 metres respectively. Although other functions, such as an
exponential curve
Exponential growth is a process that increases quantity over time. It occurs when the instantaneous rate of change (that is, the derivative) of a quantity with respect to time is proportional to the quantity itself. Described as a function, a ...
, have also been proposed and validated,
this
power law function, commonly known as the "Martin curve", has been used very frequently in discussions of the BCP. The
exponent b in this equation has been used as an index of BCP efficiency: the larger the exponent b, the higher the vertical attenuation rate of the POC flux and the lower the BCP efficiency. Moreover, numerical simulations have shown that a change in the value of b would significantly change the atmospheric CO
2 concentration.
Subsequently, other researchers have derived alternative
remineralization
In biogeochemistry, remineralisation (or remineralization) refers to the breakdown or transformation of organic matter (those molecules derived from a biological source) into its simplest inorganic forms. These transformations form a crucial link ...
profiles from assumptions about particle degradability and sinking speed.
However, the Martin curve has become ubiquitous as the model that assumes slower-sinking and/or labile organic matter is preferentially depleted near the surface causing increasing sinking speed and/or remineralization timescale with depth.
The Martin curve can be expressed in a slightly more general way as:
[ ]
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Creative Commons Attribution 4.0 International License
:
where ''f''
''p''(''z'') is the fraction of the flux of particulate organic matter from a productive layer near the surface sinking through the depth horizon ''z''
''C''
''p'' ''b''">'m''''b''is a scaling coefficient, and ''b'' is a nondimensional exponent controlling how ''f
p'' decreases with depth. The equation is often normalised to a reference depth ''z
o'' but this parameter can be readily absorbed into ''C
p''.
Vertical attenuation rate
The vertical attenuation rate of the POC flux is very dependent on the sinking velocity and decomposition rate of POC in the water column. Because POC is labile and has little negative buoyancy, it must be aggregated with relatively heavy materials called
ballast
Ballast is material that is used to provide stability to a vehicle or structure. Ballast, other than cargo, may be placed in a vehicle, often a ship or the gondola of a balloon or airship, to provide stability. A compartment within a boat, ship ...
to settle gravitationally in the ocean. Materials that may serve as ballast include
biogenic opal (hereinafter "
opal
Opal is a hydrated amorphous form of silica (SiO2·''n''H2O); its water content may range from 3 to 21% by weight, but is usually between 6 and 10%. Due to its amorphous property, it is classified as a mineraloid, unlike crystalline form ...
"), CaCO
3, and
aluminosilicate
Aluminosilicate minerals ( IMA symbol: Als) are minerals composed of aluminium, silicon, and oxygen, plus countercations. They are a major component of kaolin and other clay minerals.
Andalusite, kyanite, and sillimanite are naturall ...
s. In 1993, Ittekkot hypothesized that the drastic decrease from ~280 to ~200
ppm of atmospheric CO
2 that occurred during the last glacial maximum was caused by an increase of the input of
aeolian dust
Dust is made of fine particles of solid matter. On Earth, it generally consists of particles in the atmosphere that come from various sources such as soil lifted by wind (an aeolian process), volcanic eruptions, and pollution. Dust in homes ...
(aluminosilicate ballast) to the ocean, which strengthened the BCP. In 2002, Klaas and Archer , as well as Francois ''et al.'' who compiled and analyzed global
sediment trap data, suggested that CaCO
3, which has the largest density among possible ballast minerals, is globally the most important and effective facilitator of vertical POC transport, because the transfer efficiency (the ratio of the POC flux in the deep sea to that at the bottom of the surface
mixed layer
The oceanic or limnological mixed layer is a layer in which active turbulence has homogenized some range of depths. The surface mixed layer is a layer where this turbulence is generated by winds, surface heat fluxes, or processes such as evaporat ...
) is higher in subtropical and tropical areas where CaCO
3 is a major component of
marine snow
In the deep ocean, marine snow (also known as "ocean dandruff") is a continuous shower of mostly organic detritus falling from the upper layers of the water column. It is a significant means of exporting energy from the light-rich photic zone to ...
.
Reported sinking velocities of CaCO
3-rich particles are high. Numerical simulations that take into account these findings have indicated that future ocean acidification will reduce the efficiency of the BCP by decreasing ocean calcification. In addition, the POC export ratio (the ratio of the POC flux from an upper layer (a fixed depth such as 100 metres, or the
euphotic zone
The photic zone, euphotic zone, epipelagic zone, or sunlight zone is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological proc ...
or
mixed layer
The oceanic or limnological mixed layer is a layer in which active turbulence has homogenized some range of depths. The surface mixed layer is a layer where this turbulence is generated by winds, surface heat fluxes, or processes such as evaporat ...
) to
net primary productivity) in subtropical and tropical areas is low because high temperatures in the upper layer increase POC decomposition rates. The result might be a higher transfer efficiency and a strong positive correlation between POC and CaCO
3 in these low-latitude areas: labile POC, which is fresher and easier for microbes to break down, decomposes in the upper layer, and relatively refractory POC is transported to the ocean interior in low-latitude areas.
![Effective Martin curve slope for POC](https://upload.wikimedia.org/wikipedia/commons/5/59/Effective_Martin_curve_slope_for_POC.png)
On the basis of observations that revealed a large increase of POC fluxes in high-latitude areas during diatom blooms and on the fact that diatoms are much bigger than
coccolithophore
Coccolithophores, or coccolithophorids, are single celled organisms which are part of the phytoplankton, the autotrophic (self-feeding) component of the plankton community. They form a group of about 200 species, and belong either to the king ...
s, Honda and Watanabe proposed in 2010 that
opal
Opal is a hydrated amorphous form of silica (SiO2·''n''H2O); its water content may range from 3 to 21% by weight, but is usually between 6 and 10%. Due to its amorphous property, it is classified as a mineraloid, unlike crystalline form ...
, rather than CaCO
3, is crucial as
ballast
Ballast is material that is used to provide stability to a vehicle or structure. Ballast, other than cargo, may be placed in a vehicle, often a ship or the gondola of a balloon or airship, to provide stability. A compartment within a boat, ship ...
for effective POC vertical transport in subarctic regions. Weber et al. reported in 2016 a strong negative correlation between transfer efficiency and the
picoplankton
Picoplankton is the fraction of plankton composed by cells between 0.2 and 2 μm that can be either prokaryotic and eukaryotic phototrophs and heterotrophs:
* photosynthetic
* heterotrophic
They are prevalent amongst microbial plankton communit ...
fraction of plankton as well as higher transfer efficiencies in high-latitude areas, where large phytoplankton such as diatoms predominate.
They also calculated that the fraction of vertically transported CO
2 that has been sequestered in the ocean interior for at least 100 years is higher in high-latitude (polar and subpolar) regions than in low-latitude regions.
In contrast, Bach et al.conducted in 2019 a
mesocosm thumb , Diagram of a small form closed system mesocosm.
A mesocosm (''meso-'' or 'medium' and ''-cosm'' 'world') is any outdoor experimental system that examines the natural environment under controlled conditions. In this way mesocosm studie ...
experiment to study how the plankton community structure affected sinking velocities and reported that during more productive periods the sinking velocity of aggregated particles was not necessarily higher, because the aggregated particles produced then were very fluffy; rather, the settling velocity was higher when the phytoplankton were dominated by small cells.
In 2012, Henson et al. revisited the global sediment trap data and reported the POC flux is negatively correlated with the opal export flux and uncorrelated with the CaCO
3 export flux.
![Pelagiczone](https://upload.wikimedia.org/wikipedia/commons/b/bc/Pelagiczone.svg)
Key factors affecting the rate of biological decomposition of sinking POC in the water column are water temperature and the
dissolved oxygen
Oxygen saturation (symbol SO2) is a relative measure of the concentration of oxygen that is dissolved or carried in a given medium as a proportion of the maximal concentration that can be dissolved in that medium at the given temperature. It ca ...
(DO) concentration: the lower the water temperature and the DO concentration, the slower the biological respiration rate and, consequently, the POC flux decomposition rate.
For example, in 2015 Marsay with other analysed POC flux data from neutrally buoyant sediment traps in the upper 500 m of the water column and found a significant positive correlation between the exponent b in equation (1) above and water temperature (i.e., the POC flux was attenuated more rapidly when the water was warmer).
In addition, Bach ''et al.'' found POC decomposition rates are high (low) when diatoms and ''
Synechococcus
''Synechococcus'' (from the Greek ''synechos'', in succession, and the Greek ''kokkos'', granule) is a unicellular cyanobacterium that is very widespread in the marine environment. Its size varies from 0.8 to 1.5 µm. The photosynthetic c ...
'' (harmful algae) are the dominant phytoplankton because of increased (decreased)
zooplankton
Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek word for ''animal''). Plankton are aquatic organisms that are unable to swim effectively against currents, and consequently drift or are carried along by ...
abundance and the consequent increase (decrease) in
grazing pressure
Grazing pressure is defined as the number of grazing animals of a specified class (age, species, physiological status like pregnant) per unit weight of herbage (herbage biomass). It is well established in general usage.
Definition
Grazing pre ...
.
Using radiochemical observations (
234Th-based POC flux observations), Pavia et al. found in 2019 that the exponent b of the Martin curve was significantly smaller in the low-oxygen (
hypoxic
Hypoxia means a lower than normal level of oxygen, and may refer to:
Reduced or insufficient oxygen
* Hypoxia (environmental), abnormally low oxygen content of the specific environment
* Hypoxia (medical), abnormally low level of oxygen in the t ...
) eastern Pacific equatorial zone than in other areas; that is, vertical attenuation of the POC flux was smaller in the hypoxic area.
They pointed out that a more hypoxic ocean in the future would lead to a lower attenuation of the POC flux and therefore increased BCP efficiency and could thereby be a negative feedback on global warming.
McDonnell et al. reported in 2015 that vertical transport of POC is more effective in the Antarctic, where the sinking velocity is higher and the biological respiration rate is lower than in the subtropical Atlantic. Henson et al. also reported in 2019 a high export ratio during the early bloom period, when primary productivity is low, and a low export ratio during the late bloom period, when primary productivity is high.
They attributed the low export ratio during the late bloom to grazing pressure by microzooplankton and bacteria.
Despite these many investigations of the BCP, the factors governing the vertical attenuation of POC flux are still under debate. Observations in subarctic regions have shown that the transfer efficiency between depths of 1000 and 2000 m is relatively low and that between the bottom of the
euphotic zone
The photic zone, euphotic zone, epipelagic zone, or sunlight zone is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological proc ...
and a depth of 1000 m it is relatively high.
[ Marsay et al. therefore proposed in 2015 that the Martin curve does not appropriately express the vertical attenuation of POC flux in all regions and that a different equation should instead be developed for each region.] Gloege et al. discussed in 2017 parameterization of the vertical attenuation of POC flux, and reported that vertical attenuation of the POC flux in the twilight zone (from the base of the euphotic zone to 1000 m) can be parameterised well not only by a power law model (Martin curve) but also by an exponential model and a ballast model.
However, the exponential model tends to underestimate the POC flux in the midnight zone
The bathypelagic zone or bathyal zone (from Greek βαθύς (bathýs), deep) is the part of the Pelagic zone, open ocean that extends from a depth of below the Sea level, ocean surface. It lies between the mesopelagic above, and the abyssopelagi ...
(depths greater than 1000 metres). Cael and Bisson reported in 2018 that the exponential model (power law model) tends to underestimate the POC flux in the upper layer, and overestimate it in the deep layer. However, the abilities of both models to describe POC fluxes were comparable statistically when they were applied to the POC flux dataset from the eastern Pacific that was used to propose the "Martin curve". In a long-term study in the northeastern Pacific, Smith et al. observed in 2018 a sudden increase of the POC flux accompanied by an unusually high transfer efficiency; they have suggested that because the Martin curve cannot express such a sudden increase, it may sometimes underestimate BCP strength. In addition, contrary to previous findings, some studies have reported a significantly higher transfer efficiency, especially to the deep sea, in subtropical regions than in subarctic regions. This pattern may be attributable to small temperature and DO concentration differences in the deep sea between high-latitude and low-latitude regions, as well as to a higher sinking velocity in subtropical regions, where CaCO3 is a major component of deep-sea marine snow. Moreover, it is also possible that POC is more refractory
In materials science, a refractory material or refractory is a material that is resistant to decomposition by heat, pressure, or chemical attack, and retains strength and form at high temperatures. Refractories are polycrystalline, polyphase, ...
in low-latitude areas than in high-latitude areas.
Uncertainty in the biological pump
The ocean's biological pump
The biological pump (or ocean carbon biological pump or marine biological carbon pump) is the ocean's biologically driven sequestration of carbon from the atmosphere and land runoff to the ocean interior and seafloor sediments.Sigman DM & GH ...
regulates atmospheric carbon dioxide levels and climate by transferring organic carbon
Total organic carbon (TOC) is the amount of carbon found in an organic compound and is often used as a non-specific indicator of water quality or cleanliness of pharmaceutical manufacturing equipment. TOC may also refer to the amount of organic c ...
produced at the surface by phytoplankton to the ocean interior via marine snow
In the deep ocean, marine snow (also known as "ocean dandruff") is a continuous shower of mostly organic detritus falling from the upper layers of the water column. It is a significant means of exporting energy from the light-rich photic zone to ...
, where the organic carbon is consumed and respired by marine microorganisms
Marine microorganisms are defined by their habitat as microorganisms living in a marine environment, that is, in the saltwater of a sea or ocean or the brackish water of a coastal estuary. A microorganism (or microbe) is any microscopic livin ...
. This surface to deep transport is usually described by a power law relationship of sinking particle concentration with depth. Uncertainty in biological pump strength can be related to different variable values ( parametric uncertainty) or the underlying equations (structural
A structure is an arrangement and organization of interrelated elements in a material object or system, or the object or system so organized. Material structures include man-made objects such as buildings and machines and natural objects such ...
uncertainty) that describe organic matter export. In 2021, Lauderdale evaluated structural uncertainty using an ocean biogeochemistry
Biogeochemistry is the scientific discipline that involves the study of the chemical, physical, geological, and biological processes and reactions that govern the composition of the natural environment (including the biosphere, the cryosphere, ...
model by systematically substituting six alternative remineralisation
In biogeochemistry, remineralisation (or remineralization) refers to the breakdown or transformation of organic matter (those molecules derived from a biological source) into its simplest inorganic forms. These transformations form a crucial link ...
profiles fit to a reference power-law curve. Structural uncertainty makes a substantial contribution, about one-third in atmospheric pCO2 terms, to the total uncertainty of the biological pump, highlighting the importance of improving biological pump characterisation from observations and its mechanistic inclusion in climate models.
Carbon and nutrients are consumed by phytoplankton in the surface ocean during primary production
In ecology, primary production is the synthesis of organic compounds from atmospheric or aqueous carbon dioxide. It principally occurs through the process of photosynthesis, which uses light as its source of energy, but it also occurs through ...
, leading to a downward flux of organic matter. This "marine snow" is transformed, respired, and degraded by heterotrophic organisms in deeper waters, ultimately releasing those constituents back into dissolved inorganic form. Oceanic overturning and turbulent mixing return resource-rich deep waters back to the sunlit surface layer, sustaining global ocean productivity. The biological pump maintains this vertical gradient in nutrients through uptake, vertical transport, and remineralisation of organic matter, storing carbon in the deep ocean that is isolated from the atmosphere on centennial and millennial timescales, lowering atmospheric CO2 levels by several hundred microatmospheres. The biological pump resists simple mechanistic characterisation due to the complex suite of biological, chemical, and physical processes involved, so the fate of exported organic carbon is typically described using a depth-dependent profile to evaluate the degradation of sinking particulate matter.
See also
* Particulate inorganic carbon
Particulate inorganic carbon (PIC) can be contrasted with dissolved inorganic carbon (DIC), the other form of inorganic carbon found in the ocean. These distinctions are important in chemical oceanography. Particulate inorganic carbon is someti ...
References
{{reflist
Oceanography
Carbon