View source for Martin curve

{{Short description|Mathematical representation of particulate organic carbon export to ocean floor}}
[[File:POC.png|thumb|upright=1.7|right| {{center|Surface concentrations of particulate organic matter (POM)<br />as [[Moderate-Resolution Imaging Spectroradiometer|imaged by satellite]] in 2011}}]]

The '''Martin curve''' is a power law used by [[oceanographers]] to describe the export to the ocean floor of [[particulate organic carbon]] (POC). The curve is controlled with two parameters: the reference depth in the [[water column]], and a [[remineralisation]] parameter which is a measure of the rate at which the vertical [[flux]] of POC attenuates.<ref name=Olli2015>{{cite journal |doi = 10.1016/j.pocean.2015.05.016|title = Unraveling the uncertainty and error propagation in the vertical flux Martin curve|year = 2015|last1 = Olli|first1 = Kalle|journal = Progress in Oceanography|volume = 135|pages = 146–155|bibcode = 2015PrOce.135..146O}}</ref> It is named after the American oceanographer [[John Martin (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 {{CO2}} levels.

==Background==
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"The flux of sinking particles out of the surface ocean and its attenuation with depth is most often quantified in field observations by a fit to POC sinking flux data as introduced by Martin et al.:<ref name=Martin1987 />

: <math>F_{z}=F_{100} \left(\frac{z}{100}\right)^{-b}</math> {{space|20}} (1)

"This "Martin curve" was derived from a selected set of open ocean stations in the Pacific measuring POC flux where ''F<sub>z</sub>'' is the sinking flux of POC at depth ''z'' measured using [[sediment trap]]s and ''F<sub>100</sub>'' is the trap flux at a fixed 100-m reference depth. An exponent ''b'' of 0.86 captured the average attenuation of POC flux, with approximately a 90% decline in POC flux between 100 m and 1,000 m in this dataset."<ref name=Buesseler2020>{{cite journal |doi = 10.1073/pnas.1918114117|title = Metrics that matter for assessing the ocean biological carbon pump|year = 2020|last1 = Buesseler|first1 = Ken O.|last2 = Boyd|first2 = Philip W.|last3 = Black|first3 = Erin E.|last4 = Siegel|first4 = David A.|journal = Proceedings of the National Academy of Sciences|volume = 117|issue = 18|pages = 9679–9687|pmid = 32253312|pmc = 7211944}}</ref>

"As noted by Cael and Bisson,<ref name=Cael2018 /> numerous POC flux-vs.-depth parameterizations are possible that are statistically indistinguishable from the Martin curve, while having very different implications for flux attenuation and the mechanisms governing the BCP. Importantly, since the Martin curve is not mechanistically based, this relationship does not allow for projected alterations of BCP efficiencies as ecosystems change in response to altered climate or other forcings.<ref name=Boyd2015>{{cite journal |doi = 10.3389/fmars.2015.00077|doi-access = free|title = Toward quantifying the response of the oceans' biological pump to climate change|year = 2015|last1 = Boyd|first1 = Philip W.|journal = Frontiers in Marine Science|volume = 2}}</ref> At this time of their 1987 publication, Martin et al. emphasized that this equation was used as a statistical expedient and it was not based on a priori physics.<ref name=Martin1987 /> Critically, the Martin curve is based on a power law with a fixed shallow reference depth, and thus the flux curve pivots around this point. As a result, the interpolated fluxes are sensitive to the availability of sufficient vertically resolved data just below the fixed reference depth needed to quantify the b value, and such resolution is rare in most sediment trap studies".<ref>{{cite journal |doi = 10.1016/j.dsr.2006.06.003|title = On the variability of the exponent in the power law depth dependence of POC flux estimated from sediment traps|year = 2006|last1 = Primeau|first1 = François|journal = Deep Sea Research Part I: Oceanographic Research Papers|volume = 53|issue = 8|pages = 1335–1343|bibcode = 2006DSRI...53.1335P|url = https://escholarship.org/uc/item/6sj4874q}}</ref><ref name=Olli2015>{{cite journal |doi = 10.1016/j.pocean.2015.05.016|title = Unraveling the uncertainty and error propagation in the vertical flux Martin curve|year = 2015|last1 = Olli|first1 = Kalle|journal = Progress in Oceanography|volume = 135|pages = 146–155|bibcode = 2015PrOce.135..146O}}</ref><ref name=Buesseler2020 />
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[[File:Flux of POC with depth.png|thumb|upright=1.2|{{center|Flux of POC with depth}}]]
{{carbon cycle|pump}}

The dynamics of the [[particulate organic carbon]] (POC) pool in the ocean are central to the [[marine carbon cycle]]. POC is the link between surface [[Marine primary production|primary production]], the [[deep ocean]], and [[marine sediment]]s. The rate at which POC is degraded in the [[dark ocean]] can impact atmospheric CO<sub>2</sub> concentration.<ref name=Kharbush2020>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. {{doi|10.3389/fmars.2020.00518}}. [[File:CC-BY icon.svg|50px]] Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref>

The [[biological carbon pump]] (BCP) is a crucial mechanism by which atmospheric CO<sub>2</sub> is taken up by the ocean and transported to the ocean interior. Without the BCP, the pre-industrial atmospheric CO<sub>2</sub> concentration (~280 ppm) would have risen to ~460 ppm.<ref>{{cite journal |doi = 10.1002/2016GL069233|title = Geographical, seasonal, and depth variation in sinking particle speeds in the North Atlantic|year = 2016|last1 = Villa-Alfageme|first1 = M.|last2 = De Soto|first2 = F. C.|last3 = Ceballos|first3 = E.|last4 = Giering|first4 = S. L. C.|last5 = Le Moigne|first5 = F. A. C.|last6 = Henson|first6 = S.|last7 = Mas|first7 = J. L.|last8 = Sanders|first8 = R. J.|journal = Geophysical Research Letters|volume = 43|issue = 16|pages = 8609–8616|bibcode = 2016GeoRL..43.8609V| hdl=11441/98535 | s2cid=3728545 |hdl-access = free}}</ref> At present, the [[particulate organic carbon]] (POC) flux from the surface layer of the ocean to the ocean interior has been estimated to be 4–13 Pg-C year<sup>−1</sup>.<ref>{{cite journal |doi = 10.5194/bg-11-1177-2014|title = Dynamics of particulate organic carbon flux in a global ocean model|year = 2014|last1 = Lima|first1 = I. D.|last2 = Lam|first2 = P. J.|last3 = Doney|first3 = S. C.|journal = Biogeosciences|volume = 11|issue = 4|pages = 1177–1198|bibcode = 2014BGeo...11.1177L|hdl = 1912/6674|hdl-access = free | doi-access=free }}</ref> 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<sub>2</sub> 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]] of atmospheric CO<sub>2</sub> that would result in a negative feedback on global warming.<ref name=Honda2020>{{cite journal |doi = 10.3389/feart.2020.00366|title = Effective Vertical Transport of Particulate Organic Carbon in the Western North Pacific Subarctic Region|year = 2020|last1 = Honda|first1 = Makio C.|journal = Frontiers in Earth Science|volume = 8|page = 366|bibcode = 2020FrEaS...8..366H|doi-access = free}} [[File:CC-BY icon.svg|50px]] Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref> Different researchers have investigated the vertical attenuation of the POC flux since the 1980s.<ref name="Particulate organic carbon flux in">{{cite journal |doi = 10.1038/288260a0|title = Particulate organic carbon flux in the oceans—surface productivity and oxygen utilization|year = 1980|last1 = Suess|first1 = Erwin|journal = Nature|volume = 288|issue = 5788|pages = 260–263|bibcode = 1980Natur.288..260S|s2cid = 4275275}}</ref><ref>{{cite journal |doi = 10.1016/0198-0149(84)90068-2|title = Primary productivity and particle fluxes on a transect of the equator at 153°W in the Pacific Ocean|year = 1984|last1 = Betzer|first1 = Peter R.|last2 = Showers|first2 = William J.|last3 = Laws|first3 = Edward A.|last4 = Winn|first4 = Christopher D.|last5 = Ditullio|first5 = Giacomo R.|last6 = Kroopnick|first6 = Peter M.|journal = Deep Sea Research Part A. Oceanographic Research Papers|volume = 31|issue = 1|pages = 1–11|bibcode = 1984DSRA...31....1B}}</ref><ref>Berger, W. H., Fischer, K., Lai, C., and Wu, G. (1987). "Ocean carbon flux: global maps of primary production and export production,". In: Biogeochemical Cycling and Fluxes between the Deep Euphotic Zone and Other Oceanic Realms", Vol. 3, ed. C. Agegian (Silver Spring, MD: NOAA), pages 87–30.</ref><ref>{{cite journal |doi = 10.1038/325803a0|title = Primary production, new production and vertical flux in the eastern Pacific Ocean|year = 1987|last1 = Pace|first1 = Michael L.|last2 = Knauer|first2 = George A.|last3 = Karl|first3 = David M.|last4 = Martin|first4 = John H.|journal = Nature|volume = 325|issue = 6107|pages = 803–804|bibcode = 1987Natur.325..803P|s2cid = 4353764}}</ref><ref name=Honda2020 />

In 1987, Martin ''et al''. proposed the following power law function to describe the POC flux attenuation:<ref name=Martin1987>{{cite journal |doi = 10.1016/0198-0149(87)90086-0|title = VERTEX: Carbon cycling in the northeast Pacific|year = 1987|last1 = Martin|first1 = John H.|last2 = Knauer|first2 = George A.|last3 = Karl|first3 = David M.|last4 = Broenkow|first4 = William W.|journal = Deep Sea Research Part A. Oceanographic Research Papers|volume = 34|issue = 2|pages = 267–285|bibcode = 1987DSRA...34..267M}}</ref>

: <math>F_{z}=F_{100} \left(\frac{z}{100}\right)^{-b}</math> {{space|16}} (1)
<!-- : <math>POCF_{(z)}=POCF_{(100)}\times \left(\frac{z}{100}\right)^{-b}</math> -->

where ''z'' is water depth (m), and ''F''<sub>''z''</sub> and ''F''<sub>100</sub> are the POC fluxes at depths of ''z'' metres and 100 metres respectively. Although other functions, such as an [[exponential curve]], have also been proposed and validated,<ref name="digitalcommons.uri.edu">{{cite journal |doi = 10.1002/2016GB005535|title = Global evaluation of particulate organic carbon flux parameterizations and implications for atmospheric pCO2|year = 2017|last1 = Gloege|first1 = Lucas|last2 = McKinley|first2 = Galen A.|last3 = Mouw|first3 = Colleen B.|last4 = Ciochetto|first4 = Audrey B.|journal = Global Biogeochemical Cycles|volume = 31|issue = 7|pages = 1192–1215|bibcode = 2017GBioC..31.1192G| s2cid=133746237 |url = https://digitalcommons.uri.edu/gsofacpubs/639}}</ref><ref name="Cael2018"/> 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.<ref>Berelson, W. M. (2001). "The flux of particulate organic carbon into the ocean interior.: a comparison of four U.S. JGOFS regional studies". ''Oceanography'', '''14''': 59–64.</ref> Moreover, numerical simulations have shown that a change in the value of b would significantly change the atmospheric CO<sub>2</sub> concentration.<ref>{{cite journal |doi = 10.1029/96gb00634|title = The role of the vertical fluxes of particulate organic matter and calcite in the oceanic carbon cycle: Studies using an ocean biogeochemical general circulation model|year = 1996|last1 = Yamanaka|first1 = Yasuhiro|last2 = Tajika|first2 = Eiichi|journal = Global Biogeochemical Cycles|volume = 10|issue = 2|pages = 361–382|bibcode = 1996GBioC..10..361Y}}</ref><ref>{{cite journal |doi = 10.1038/ngeo612|title = The impact of remineralization depth on the air–sea carbon balance|year = 2009|last1 = Kwon|first1 = Eun Young|last2 = Primeau|first2 = François|last3 = Sarmiento|first3 = Jorge L.|journal = Nature Geoscience|volume = 2|issue = 9|pages = 630–635|bibcode = 2009NatGe...2..630K| s2cid=59058813 |url = https://escholarship.org/uc/item/30d5m3m9}}</ref><ref>{{cite journal |doi = 10.5194/bg-16-2923-2019|title = Sensitivity of atmospheric CO<sub>2</sub> to regional variability in particulate organic matter remineralization depths|year = 2019|last1 = Wilson|first1 = Jamie D.|last2 = Barker|first2 = Stephen|last3 = Edwards|first3 = Neil R.|last4 = Holden|first4 = Philip B.|last5 = Ridgwell|first5 = Andy|journal = Biogeosciences|volume = 16|issue = 14|pages = 2923–2936|bibcode = 2019BGeo...16.2923W|s2cid = 202987265|doi-access = free}}</ref><ref name=Honda2020 />

Subsequently, other researchers have derived alternative [[remineralization]] profiles from assumptions about particle degradability and sinking speed.<ref name="A new, mechanistic model for organi">{{cite journal |doi = 10.1016/S0967-0645(01)00101-1|title = A new, mechanistic model for organic carbon fluxes in the ocean based on the quantitative association of POC with ballast minerals|year = 2001|last1 = Armstrong|first1 = Robert A.|last2 = Lee|first2 = Cindy|last3 = Hedges|first3 = John I.|last4 = Honjo|first4 = Susumu|last5 = Wakeham|first5 = Stuart G.|journal = Deep-Sea Research Part II: Topical Studies in Oceanography|volume = 49|issue = 1–3|pages = 219–236|bibcode = 2001DSRII..49..219A}}</ref><ref name="New views on the degradation and di">{{cite journal |doi = 10.1016/0198-0149(90)90058-4|title = New views on the degradation and disposition of organic particles as collected by sediment traps in the open sea|year = 1990|last1 = Banse|first1 = Karl|journal = Deep Sea Research Part A. Oceanographic Research Papers|volume = 37|issue = 7|pages = 1177–1195|bibcode = 1990DSRA...37.1177B}}</ref><ref name="Cael2018"/><ref>{{cite journal |doi = 10.5194/bg-5-55-2008|title = On the treatment of particulate organic matter sinking in large-scale models of marine biogeochemical cycles|year = 2008|last1 = Kriest|first1 = I.|last2 = Oschlies|first2 = A.|journal = Biogeosciences|volume = 5|issue = 1|pages = 55–72|bibcode = 2008BGeo....5...55K| s2cid=3840184 |doi-access = free}}</ref><ref>{{cite journal |doi = 10.1029/2000GB001383|title = Regional variability in the vertical flux of particulate organic carbon in the ocean interior|year = 2002|last1 = Lutz|first1 = Michael|last2 = Dunbar|first2 = Robert|last3 = Caldeira|first3 = Ken|journal = Global Biogeochemical Cycles|volume = 16|issue = 3|pages = 11-1-11-18|bibcode = 2002GBioC..16.1037L|doi-access = free}}</ref><ref>{{cite journal |doi = 10.1016/0016-7037(89)90239-1|title = A simple rate model for organic matter decomposition in marine sediments|year = 1989|last1 = Middelburg|first1 = Jack J.|journal = Geochimica et Cosmochimica Acta|volume = 53|issue = 7|pages = 1577–1581|bibcode = 1989GeCoA..53.1577M}}</ref><ref>{{cite journal |doi = 10.5194/gmd-9-2589-2016|title = Evaluation of NorESM-OC (Versions 1 and 1.2), the ocean carbon-cycle stand-alone configuration of the Norwegian Earth System Model (NorESM1)|year = 2016|last1 = Schwinger|first1 = Jörg|last2 = Goris|first2 = Nadine|last3 = Tjiputra|first3 = Jerry F.|last4 = Kriest|first4 = Iris|last5 = Bentsen|first5 = Mats|last6 = Bethke|first6 = Ingo|last7 = Ilicak|first7 = Mehmet|last8 = Assmann|first8 = Karen M.|last9 = Heinze|first9 = Christoph|journal = Geoscientific Model Development|volume = 9|issue = 8|pages = 2589–2622|bibcode = 2016GMD.....9.2589S| s2cid=59034696 |doi-access = free}}</ref><ref name="Particulate organic carbon flux in"/> 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.<ref name=Martin1987 /><ref>{{cite journal |doi = 10.1016/j.ocemod.2011.05.001|title = Numerical effects on organic-matter sedimentation and remineralization in biogeochemical ocean models|year = 2011|last1 = Kriest|first1 = I.|last2 = Oschlies|first2 = A.|journal = Ocean Modelling|volume = 39|issue = 3–4|pages = 275–283|bibcode = 2011OcMod..39..275K}}</ref>

The Martin curve can be expressed in a slightly more general way as:<ref name=Lauderdale2021>{{cite journal |doi = 10.1029/2020GL091746|title = Impact of Remineralization Profile Shape on the Air-Sea Carbon Balance|year = 2021|last1 = Lauderdale|first1 = Jonathan Maitland|last2 = Cael|first2 = B. B.|journal = Geophysical Research Letters|volume = 48|issue = 7|pages = e2020GL091746|pmid = 34219838|pmc = 8243937|bibcode = 2021GeoRL..4891746L|hdl = 1721.1/130486|hdl-access = free}} [[File:CC-BY icon.svg|50px]] Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref>

: <math>f_p{(z)}=C_pz^{-b}</math>

where ''f''<sub>''p''</sub>(''z'') is the fraction of the flux of particulate organic matter from a productive layer near the surface{{hsp}}<ref>{{cite journal |doi = 10.1073/pnas.1918114117|title = Metrics that matter for assessing the ocean biological carbon pump|year = 2020|last1 = Buesseler|first1 = Ken O.|last2 = Boyd|first2 = Philip W.|last3 = Black|first3 = Erin E.|last4 = Siegel|first4 = David A.|journal = Proceedings of the National Academy of Sciences|volume = 117|issue = 18|pages = 9679–9687|pmid = 32253312|pmc = 7211944| bibcode=2020PNAS..117.9679B |doi-access = free}}</ref> sinking through the depth horizon ''z'' [m], ''C''<sub>''p''</sub> [''m''<sup>''b''</sup>] is a scaling coefficient, and ''b'' is a nondimensional exponent controlling how ''f<sub>p</sub>'' decreases with depth. The equation is often normalised to a reference depth ''z<sub>o</sub>'' but this parameter can be readily absorbed into ''C<sub>p</sub>''.<ref name=Lauderdale2021 />

==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 mineral|ballast]] to settle gravitationally in the ocean. Materials that may serve as ballast include [[biogenic opal]] (hereinafter "[[opal]]"), CaCO<sub>3</sub>, and [[aluminosilicate]]s. In 1993, Ittekkot hypothesized that the drastic decrease from ~280 to ~200 [[parts per million|ppm]] of atmospheric CO<sub>2</sub> that occurred during the last glacial maximum was caused by an increase of the input of [[aeolian dust]] (aluminosilicate ballast) to the ocean, which strengthened the BCP.<ref>{{cite journal |doi = 10.1016/0921-8181(93)90060-2|title = The abiotically driven biological pump in the ocean and short-term fluctuations in atmospheric CO2 contents|year = 1993|last1 = Ittekkot|first1 = Venugopalan|journal = Global and Planetary Change|volume = 8|issue = 1–2|pages = 17–25|bibcode = 1993GPC.....8...17I}}</ref> In 2002, Klaas and Archer , as well as Francois ''et al.'' who compiled and analyzed global [[sediment trap]] data, suggested that CaCO<sub>3</sub>, 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]]) is higher in subtropical and tropical areas where CaCO<sub>3</sub> is a major component of [[marine snow]].<ref>{{cite journal |doi = 10.1029/2001GB001765|title = Association of sinking organic matter with various types of mineral ballast in the deep sea: Implications for the rain ratio|year = 2002|last1 = Klaas|first1 = Christine|last2 = Archer|first2 = David E.|journal = Global Biogeochemical Cycles|volume = 16|issue = 4|pages = 63-1-63-14|bibcode = 2002GBioC..16.1116K|doi-access = free}}</ref><ref name=Francois2002>{{cite journal |doi = 10.1029/2001GB001722|title = Factors controlling the flux of organic carbon to the bathypelagic zone of the ocean|year = 2002|last1 = Francois|first1 = Roger|last2 = Honjo|first2 = Susumu|last3 = Krishfield|first3 = Richard|last4 = Manganini|first4 = Steve|journal = Global Biogeochemical Cycles|volume = 16|issue = 4|pages = 34-1-34-20|bibcode = 2002GBioC..16.1087F| s2cid=128876389 |doi-access = free}}</ref><ref name=Honda2020 />

Reported sinking velocities of CaCO<sub>3</sub>-rich particles are high.<ref>{{cite journal |doi = 10.5194/bg-6-85-2009|title = Sinking rates and ballast composition of particles in the Atlantic Ocean: Implications for the organic carbon fluxes to the deep ocean|year = 2009|last1 = Fischer|first1 = G.|last2 = Karakaş|first2 = G.|journal = Biogeosciences|volume = 6|issue = 1|pages = 85–102|bibcode = 2009BGeo....6...85F| s2cid=45117580 |doi-access = free}}</ref><ref>{{cite journal |doi = 10.1016/j.dsr.2018.11.004|title = Sinking dynamics of particulate matter in the subarctic and subtropical regions of the western North Pacific|year = 2019|last1 = Sukigara|first1 = Chiho|last2 = Mino|first2 = Yoshihisa|last3 = Kawakami|first3 = Hajime|last4 = Honda|first4 = Makio C.|last5 = Fujiki|first5 = Tetsuichi|last6 = Matsumoto|first6 = Kazuhiko|last7 = Wakita|first7 = Masahide|last8 = Saino|first8 = Toshiro|journal = Deep Sea Research Part I: Oceanographic Research Papers|volume = 144|pages = 17–27|bibcode = 2019DSRI..144...17S|s2cid = 133654627}}</ref> 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.<ref>{{cite journal |doi = 10.1029/2004GL020613|title = Simulating oceanic CaCO3export production in the greenhouse|year = 2004|last1 = Heinze|first1 = C.|journal = Geophysical Research Letters|volume = 31|issue = 16|bibcode = 2004GeoRL..3116308H|doi-access = free}}</ref> 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]] or [[mixed layer]]) to [[net primary productivity]]) in subtropical and tropical areas is low because high temperatures in the upper layer increase POC decomposition rates.<ref>{{cite journal |doi = 10.1029/1999gb001229|title = Temperature effects on export production in the open ocean|year = 2000|last1 = Laws|first1 = Edward A.|last2 = Falkowski|first2 = Paul G.|last3 = Smith|first3 = Walker O.|last4 = Ducklow|first4 = Hugh|last5 = McCarthy|first5 = James J.|journal = Global Biogeochemical Cycles|volume = 14|issue = 4|pages = 1231–1246|bibcode = 2000GBioC..14.1231L| s2cid=53998465 | url=https://scholarworks.wm.edu/vimsarticles/1198 |doi-access = free}}</ref> The result might be a higher transfer efficiency and a strong positive correlation between POC and CaCO<sub>3</sub> 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.<ref name="Francois2002"/><ref name=Henson2012 /><ref name=Honda2020 />

[[File:Effective Martin curve slope for POC.png|thumb|upright=2| {{center|'''Effective Martin curve slope for POC'''}} {{center|<small>(according to a global biogeochemical model called M<sup>4</sup>AGO{{hsp}}<ref>{{cite journal |doi = 10.5194/bg-17-1765-2020|title = Microstructure and composition of marine aggregates as co-determinants for vertical particulate organic carbon transfer in the global ocean|year = 2020|last1 = Maerz|first1 = Joeran|last2 = Six|first2 = Katharina D.|last3 = Stemmler|first3 = Irene|last4 = Ahmerkamp|first4 = Soeren|last5 = Ilyina|first5 = Tatiana|journal = Biogeosciences|volume = 17|issue = 7|pages = 1765–1803|bibcode = 2020BGeo...17.1765M|hdl = 21.11116/0000-0004-BD3E-3|s2cid = 216206324|hdl-access = free | doi-access=free }}</ref>)</small>}}]]

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]]s,<ref>{{cite journal |doi = 10.1029/97GB03366|title = The decoupling of production and particulate export in the surface ocean|year = 1998|last1 = Buesseler|first1 = Ken O.|journal = Global Biogeochemical Cycles|volume = 12|issue = 2|pages = 297–310|bibcode = 1998GBioC..12..297B|doi-access = }}</ref><ref>{{cite journal |doi = 10.1126/science.1137959|title = Revisiting Carbon Flux Through the Ocean's Twilight Zone|year = 2007|last1 = Buesseler|first1 = K. O.|last2 = Lamborg|first2 = C. H.|last3 = Boyd|first3 = P. W.|last4 = Lam|first4 = P. J.|last5 = Trull|first5 = T. W.|last6 = Bidigare|first6 = R. R.|last7 = Bishop|first7 = J. K. B.|last8 = Casciotti|first8 = K. L.|last9 = Dehairs|first9 = F.|last10 = Elskens|first10 = M.|last11 = Honda|first11 = M.|last12 = Karl|first12 = D. M.|last13 = Siegel|first13 = D. A.|last14 = Silver|first14 = M. W.|last15 = Steinberg|first15 = D. K.|last16 = Valdes|first16 = J.|last17 = Van Mooy|first17 = B.|last18 = Wilson|first18 = S.|journal = Science|volume = 316|issue = 5824|pages = 567–570|pmid = 17463282|bibcode = 2007Sci...316..567B|s2cid = 8423647}}</ref><ref>{{cite journal |doi = 10.1016/j.dsr2.2008.04.024|title = VERTIGO (VERtical Transport in the Global Ocean): A study of particle sources and flux attenuation in the North Pacific|year = 2008|last1 = Buesseler|first1 = K.O.|last2 = Trull|first2 = T.W.|last3 = Steinberg|first3 = D.K.|last4 = Silver|first4 = M.W.|last5 = Siegel|first5 = D.A.|last6 = Saitoh|first6 = S.-I.|last7 = Lamborg|first7 = C.H.|last8 = Lam|first8 = P.J.|last9 = Karl|first9 = D.M.|last10 = Jiao|first10 = N.Z.|last11 = Honda|first11 = M.C.|last12 = Elskens|first12 = M.|last13 = Dehairs|first13 = F.|last14 = Brown|first14 = S.L.|last15 = Boyd|first15 = P.W.|last16 = Bishop|first16 = J.K.B.|last17 = Bidigare|first17 = R.R.|journal = Deep-Sea Research Part II: Topical Studies in Oceanography|volume = 55|issue = 14–15|pages = 1522–1539|bibcode = 2008DSRII..55.1522B|url = https://digital.library.unt.edu/ark:/67531/metadc898605/}}</ref> Honda and Watanabe proposed in 2010 that [[opal]], rather than CaCO<sub>3</sub>, is crucial as [[ballast mineral|ballast]] for effective POC vertical transport in subarctic regions.<ref>{{cite journal |doi = 10.1029/2009GL041521|title = Importance of biogenic opal as ballast of particulate organic carbon (POC) transport and existence of mineral ballast-associated and residual POC in the Western Pacific Subarctic Gyre|year = 2010|last1 = Honda|first1 = Makio C.|last2 = Watanabe|first2 = Shuichi|journal = Geophysical Research Letters|volume = 37|issue = 2|pages = n/a|bibcode = 2010GeoRL..37.2605H|doi-access = }}</ref> Weber et al. reported in 2016 a strong negative correlation between transfer efficiency and the [[picoplankton]] fraction of plankton as well as higher transfer efficiencies in high-latitude areas, where large phytoplankton such as diatoms predominate.<ref name=Weber2016>{{cite journal |doi = 10.1073/pnas.1604414113|title = Deep ocean nutrients imply large latitudinal variation in particle transfer efficiency|year = 2016|last1 = Weber|first1 = Thomas|last2 = Cram|first2 = Jacob A.|last3 = Leung|first3 = Shirley W.|last4 = Devries|first4 = Timothy|last5 = Deutsch|first5 = Curtis|journal = Proceedings of the National Academy of Sciences|volume = 113|issue = 31|pages = 8606–8611|pmid = 27457946|pmc = 4978250|bibcode = 2016PNAS..113.8606W|doi-access = free}}</ref> They also calculated that the fraction of vertically transported CO<sub>2</sub> 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.<ref name=Weber2016 /><ref name=Honda2020 />

In contrast, Bach et al.conducted in 2019 a [[mesocosm]] 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.<ref name=Bach2019>{{cite journal |doi = 10.1029/2019GB006256|title = The Influence of Plankton Community Structure on Sinking Velocity and Remineralization Rate of Marine Aggregates|year = 2019|last1 = Bach|first1 = L. T.|last2 = Stange|first2 = P.|last3 = Taucher|first3 = J.|last4 = Achterberg|first4 = E. P.|last5 = Algueró-Muñiz|first5 = M.|last6 = Horn|first6 = H.|last7 = Esposito|first7 = M.|last8 = Riebesell|first8 = U.|journal = Global Biogeochemical Cycles|volume = 33|issue = 8|pages = 971–994|bibcode = 2019GBioC..33..971B|s2cid = 201329875|doi-access = free}}</ref> 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<sub>3</sub> export flux.<ref name=Henson2012>{{cite journal |doi = 10.1029/2011GB004099|title = Global patterns in efficiency of particulate organic carbon export and transfer to the deep ocean|year = 2012|last1 = Henson|first1 = Stephanie A.|last2 = Sanders|first2 = Richard|last3 = Madsen|first3 = Esben|journal = Global Biogeochemical Cycles|volume = 26|issue = 1|pages = n/a|bibcode = 2012GBioC..26.1028H|doi-access = free}}</ref><ref name=Honda2020 />

[[File:Pelagiczone.svg|thumb|upright=0.8| {{center|Zones in the [[water column]]}}]]

Key factors affecting the rate of biological decomposition of sinking POC in the water column are water temperature and the [[dissolved oxygen]] (DO) concentration: the lower the water temperature and the DO concentration, the slower the biological respiration rate and, consequently, the POC flux decomposition rate.<ref name=Marsay2015>{{cite journal |doi = 10.1073/pnas.1415311112|title = Attenuation of sinking particulate organic carbon flux through the mesopelagic ocean|year = 2015|last1 = Marsay|first1 = Chris M.|last2 = Sanders|first2 = Richard J.|last3 = Henson|first3 = Stephanie A.|last4 = Pabortsava|first4 = Katsiaryna|last5 = Achterberg|first5 = Eric P.|last6 = Lampitt|first6 = Richard S.|journal = Proceedings of the National Academy of Sciences|volume = 112|issue = 4|pages = 1089–1094|pmid = 25561526|pmc = 4313834|bibcode = 2015PNAS..112.1089M|doi-access = free}}</ref><ref name=DeVries2017>{{cite journal |doi = 10.1002/2016GB005551|title = The export and fate of organic matter in the ocean: New constraints from combining satellite and oceanographic tracer observations|year = 2017|last1 = Devries|first1 = Tim|last2 = Weber|first2 = Thomas|journal = Global Biogeochemical Cycles|volume = 31|issue = 3|pages = 535–555|bibcode = 2017GBioC..31..535D| s2cid=132192896 }}</ref><ref>{{cite journal |doi = 10.1002/2017GB005643|title = Temperature and oxygen dependence of the remineralization of organic matter|year = 2017|last1 = Laufkötter|first1 = C.|last2 = John|first2 = Jasmin G.|last3 = Stock|first3 = Charles A.|last4 = Dunne|first4 = John P.|journal = Global Biogeochemical Cycles|volume = 31|issue = 7|pages = 1038–1050|bibcode = 2017GBioC..31.1038L| s2cid=102953187 |url = https://boris.unibe.ch/153163/}}</ref><ref name="Pavia2019"/> 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).<ref name=Marsay2015 /> In addition, Bach ''et al.'' found POC decomposition rates are high (low) when diatoms and ''[[Synechococcus]]'' (harmful algae) are the dominant phytoplankton because of increased (decreased) [[zooplankton]] abundance and the consequent increase (decrease) in [[grazing pressure]].<ref name=Bach2019 /><ref name=Honda2020 />

Using radiochemical observations ([[Thorium-233|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 ([[hypoxia (environmental)|hypoxic]]) eastern Pacific equatorial zone than in other areas; that is, vertical attenuation of the POC flux was smaller in the hypoxic area.<ref name=Pavia2019>{{cite journal |doi = 10.1073/pnas.1901863116|title = Shallow particulate organic carbon regeneration in the South Pacific Ocean|year = 2019|last1 = Pavia|first1 = Frank J.|last2 = Anderson|first2 = Robert F.|last3 = Lam|first3 = Phoebe J.|last4 = Cael|first4 = B. B.|last5 = Vivancos|first5 = Sebastian M.|last6 = Fleisher|first6 = Martin Q.|last7 = Lu|first7 = Yanbin|last8 = Zhang|first8 = Pu|last9 = Cheng|first9 = Hai|last10 = Edwards|first10 = R. Lawrence|journal = Proceedings of the National Academy of Sciences|volume = 116|issue = 20|pages = 9753–9758|pmid = 31036647|pmc = 6525517|bibcode = 2019PNAS..116.9753P|doi-access = free}}</ref> 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.<ref name=Pavia2019 /> 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.<ref>{{cite journal |doi = 10.1002/2014GB004935|title = Effects of sinking velocities and microbial respiration rates on the attenuation of particulate carbon fluxes through the mesopelagic zone|year = 2015|last1 = McDonnell|first1 = A. M. P.|last2 = Boyd|first2 = P. W.|last3 = Buesseler|first3 = K. O.|journal = Global Biogeochemical Cycles|volume = 29|issue = 2|pages = 175–193|bibcode = 2015GBioC..29..175M|hdl = 1912/7333| s2cid=53453555 |hdl-access = free}}</ref> 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.<ref name=Henson2019>{{cite journal |doi = 10.1029/2018GB006158|title = Drivers of Carbon Export Efficiency in the Global Ocean|year = 2019|last1 = Henson|first1 = Stephanie|last2 = Le Moigne|first2 = Fred|last3 = Giering|first3 = Sarah|journal = Global Biogeochemical Cycles|volume = 33|issue = 7|pages = 891–903|pmid = 32063666|pmc = 7006809|bibcode = 2019GBioC..33..891H}}</ref> They attributed the low export ratio during the late bloom to grazing pressure by microzooplankton and bacteria.<ref name=Henson2019 /><ref name=Honda2020 />

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]] and a depth of 1000 m it is relatively high.<ref name=Marsay2015/> 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.<ref name=Marsay2015 /> Gloege et al. discussed in 2017 parameterization of the vertical attenuation of POC flux,<ref name="digitalcommons.uri.edu"/> and reported that vertical attenuation of the POC flux in the [[Mesopelagic zone|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{{hsp}}<ref name="New views on the degradation and di"/> and a ballast model.<ref name="A new, mechanistic model for organi"/><ref name=Honda2020 />

However, the exponential model tends to underestimate the POC flux in the [[midnight zone]] (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.<ref name=Cael2018>{{cite journal |doi = 10.3389/fmars.2018.00395|title = Particle Flux Parameterizations: Quantitative and Mechanistic Similarities and Differences|year = 2018|last1 = Cael|first1 = B. B.|last2 = Bisson|first2 = Kelsey|journal = Frontiers in Marine Science|volume = 5|hdl = 1912/23677|hdl-access = free|doi-access = free}}</ref> 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".<ref name=Martin1987 /> 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.<ref>{{cite journal |doi = 10.1073/pnas.1814559115|title = Episodic organic carbon fluxes from surface ocean to abyssal depths during long-term monitoring in NE Pacific|year = 2018|last1 = Smith|first1 = Kenneth L.|last2 = Ruhl|first2 = Henry A.|last3 = Huffard|first3 = Christine L.|last4 = Messié|first4 = Monique|last5 = Kahru|first5 = Mati|journal = Proceedings of the National Academy of Sciences|volume = 115|issue = 48|pages = 12235–12240|pmid = 30429327|pmc = 6275536|bibcode = 2018PNAS..11512235S|doi-access = free}}</ref> 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.<ref name=Henson2012/><ref name=Marsay2015/><ref name=DeVries2017 /> 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 CaCO<sub>3</sub> is a major component of deep-sea marine snow. Moreover, it is also possible that POC is more [[refractory]] in low-latitude areas than in high-latitude areas.<ref name=Henson2012 /><ref name=Marsay2015 /><ref name=DeVries2017 /><ref name=Honda2020 />

==Uncertainty in the biological pump==
{{further|Biological pump}}

The ocean's [[biological pump]] regulates atmospheric carbon dioxide levels and climate by transferring [[organic carbon]] produced at the surface by [[phytoplankton]] to the ocean interior via [[marine snow]], where the organic carbon is consumed and respired by [[marine microorganisms]]. 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 statistics|parametric]] uncertainty) or the underlying equations ([[structural]] uncertainty) that describe organic matter export. In 2021, Lauderdale evaluated structural uncertainty using an ocean [[biogeochemistry]] model by systematically substituting six alternative [[remineralisation]] profiles fit to a reference power-law curve. Structural uncertainty makes a substantial contribution, about one-third in atmospheric pCO<sub>2</sub> 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.<ref name=Lauderdale2021 /><ref>[https://scitechdaily.com/new-research-reveals-uncertainty-in-how-much-carbon-the-ocean-absorbs-over-time-climate-projections-could-be-off/ New Research Reveals Uncertainty in How Much Carbon the Ocean Absorbs Over Time – Climate Projections Could Be Off] ''SciTechDaily'', 8 May 2021.</ref>

Carbon and nutrients are consumed by phytoplankton in the surface ocean during [[Marine primary production|primary production]], 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 carbon|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 CO<sub>2</sub> levels by several hundred microatmospheres.<ref>{{cite journal |doi = 10.1357/0022240054663231|title = Preformed phosphate, soft tissue pump and atmospheric CO<SUB>2</SUB>|year = 2005|last1 = Ito|first1 = Takamitsu|last2 = Follows|first2 = Michael J.|journal = Journal of Marine Research|volume = 63|issue = 4|pages = 813–839}}</ref><ref>{{cite book |doi = 10.1029/GM032p0099|chapter = Ocean Carbon Pumps: Analysis of Relative Strengths and Efficiencies in Ocean-Driven Atmospheric CO2 Changes|title = The Carbon Cycle and Atmospheric CO2 : Natural Variations Archean to Present|series = Geophysical Monograph Series|year = 2013|last1 = Volk|first1 = Tyler|last2 = Hoffert|first2 = Martin I.|pages = 99–110|isbn = 9781118664322}}</ref> The biological pump resists simple mechanistic characterisation due to the complex suite of biological, chemical, and physical processes involved,<ref>{{cite journal |doi = 10.1038/s41586-019-1098-2|title = Multi-faceted particle pumps drive carbon sequestration in the ocean|year = 2019|last1 = Boyd|first1 = Philip W.|last2 = Claustre|first2 = Hervé|last3 = Levy|first3 = Marina|last4 = Siegel|first4 = David A.|last5 = Weber|first5 = Thomas|journal = Nature|volume = 568|issue = 7752|pages = 327–335|pmid = 30996317|bibcode = 2019Natur.568..327B|s2cid = 119513489|url = https://hal.archives-ouvertes.fr/hal-02117441/file/pdf%20merged%20of%20final%20submission%20317960_6_merged_1547169923.pdf}}</ref> so the fate of exported organic carbon is typically described using a depth-dependent profile to evaluate the degradation of sinking particulate matter.<ref name=Lauderdale2021 />

==See also==
* [[Particulate inorganic carbon]]

==References==
{{reflist}}

[[Category:Oceanography]]
[[Category:Carbon]]