View source for Deep carbon cycle

{{short description|Movement of carbon through Earth's mantle and core}}
[[File:Reservoirs and fluxes of carbon in deep Earth.png|thumb|upright=1.8| {{center| Deep earth carbon}}]]

The '''deep carbon cycle''' (or '''slow carbon cycle''') is [[geochemical cycle]] (movement) of [[carbon]] through the Earth's [[mantle (geology)|mantle]] and [[Structure of the Earth#Core|core]].
It forms part of the [[carbon cycle]] and is intimately connected to the movement of carbon in the Earth's surface and atmosphere. By returning carbon to the deep Earth, it plays a critical role in maintaining the terrestrial conditions necessary for life to exist. Without it, carbon would accumulate in the atmosphere, reaching extremely high concentrations over long periods of time.<ref>{{Cite web|url=https://deepcarbon.net/feature/deep-carbon-cycle-and-our-habitable-planet|title=The Deep Carbon Cycle and our Habitable Planet|website=Deep Carbon Observatory|date=3 December 2015|access-date=2019-02-19|archive-date=2020-07-27|archive-url=https://web.archive.org/web/20200727084309/https://deepcarbon.net/feature/deep-carbon-cycle-and-our-habitable-planet|url-status=dead}}</ref>

Because the deep Earth is inaccessible to drilling, not much is conclusively known about the role of carbon in it. Nonetheless, several pieces of evidence—many of which come from laboratory simulations of deep Earth conditions—have indicated mechanisms for the element's movement down into the lower mantle, as well as the forms that carbon takes at the extreme temperatures and pressures of this layer. Furthermore, techniques like [[seismology]] have led to greater understanding of the potential presence of carbon in the Earth's core. Studies of the composition of basaltic [[magma]] and the flux of carbon dioxide out of volcanoes reveals that the amount of carbon in the [[Mantle (geology)|mantle]] is greater than that on the Earth's surface by a factor of one thousand.<ref name=":02">{{cite journal|last1=Wilson|first1=Mark|year=2003|title=Where do Carbon Atoms Reside within Earth's Mantle?|journal=Physics Today|volume=56|issue=10|pages=21–22|bibcode=2003PhT....56j..21W|doi=10.1063/1.1628990}}</ref>

== Quantity of carbon ==
{{Carbon cycle|By regions}}

There are about 44,000 gigatonnes of carbon in the atmosphere and oceans. A gigatonne is one billion [[tonne|metric tonnes]], equivalent to the mass of water in over 400,000 Olympic-size swimming pools.<ref>{{cite news |last1=Collins |first1=Terry |last2=Pratt |first2=Katie |title=Scientists Quantify Global Volcanic CO2 Venting; Estimate Total Carbon on Earth |url=https://deepcarbon.net/scientists-quantify-global-volcanic-co2-venting-estimate-total-carbon-earth |accessdate=17 December 2019 |work=Deep Carbon Observatory |date=1 October 2019 |language=en |archive-date=3 October 2019 |archive-url=https://web.archive.org/web/20191003010926/https://deepcarbon.net/scientists-quantify-global-volcanic-co2-venting-estimate-total-carbon-earth |url-status=dead }}</ref> Large as this quantity is, it only amounts to a small fraction of one percent of Earth's carbon. Over 90% may reside in the core, most of the rest being in the [[crust (geology)|crust]] and mantle.<ref name=Suarez2019>{{cite journal |last1=Suarez |first1=Celina A. |last2=Edmonds |first2=Marie |last3=Jones |first3=Adrian P. |title=Earth Catastrophes and their Impact on the Carbon Cycle |journal=Elements |date=1 October 2019 |volume=15 |issue=5 |pages=301–306 |doi=10.2138/gselements.15.5.301|doi-access=free }}</ref>

In the [[photosphere]] of the Sun, carbon is the [[Abundance_of_the_chemical_elements#Solar_system|fourth most abundant element]]. The Earth likely started with a similar ratio but lost a lot of it to evaporation as it [[Accretion (astrophysics)|accreted]]. Even accounting for evaporation, however, the [[silicate]]s making up the crust and mantle of the Earth have a carbon concentration that is five to ten times less than in [[CI chondrite]]s, a form of meteor that is believed to represent the composition of the [[presolar nebula|solar nebula before the planets formed]]. Some of this carbon may have ended up in the core. Depending on the model, carbon is predicted to contribute between 0.2 and 1 percent by weight in the core. Even at the lower concentration, this would account for half Earth's carbon.<ref name=Li2019>{{cite book |last1=Li |first1=Jie |last2=Mokkherjee |first2=Mainak |last3=Morard |first3=Guillaume |chapter=Carbon versus Other Light Elements in Earth’s Core |pages=40–65 |doi=10.1017/9781108677950.011 |editor-last1=Orcutt |editor-first1=Beth N. |editor-last2=Daniel |editor-first2=Isabelle |editor-last3=Dasgupta |editor-first3=Rajdeep |title=Deep carbon : past to present |date=2019 |publisher=Cambridge University Press |isbn=9781108677950|s2cid=210787128 }}</ref>

Estimates of the carbon content in the [[Upper mantle (Earth)|upper mantle]] come from measurements of the chemistry of [[mid-ocean ridge]] [[basalts]] (MORBs). These must be corrected for degassing of carbon and other elements. Since the Earth formed, the upper mantle has lost 40–90% of its carbon by evaporation and transport to the core in iron compounds. The most rigorous estimate gives a carbon content of 30 [[Parts-per notation|parts per million]] (ppm). The lower mantle is expected to be much less depleted – about 350 ppm.<ref name=Lee2019/>

== Lower mantle ==
Carbon principally enters the mantle in the form of [[carbonate]]-rich sediments on [[Plate tectonics|tectonic plates]] of ocean crust, which pull the carbon into the mantle upon undergoing [[subduction]]. Not much is known about carbon circulation in the mantle, especially in the deep Earth, but many studies have attempted to augment our understanding of the element's movement and forms within said region. For instance, a 2011 study demonstrated that carbon cycling extends all the way to the lower mantle. The study analysed rare, super-deep [[diamond]]s at a site in [[Juína|Juina, Brazil]], determining that the bulk composition of some of the diamonds' inclusions matched the expected result of basalt melting and [[Crystallization|crystallisation]] under lower mantle temperatures and pressures.<ref>{{Cite press release|last1=American Association for the Advancement of Science |url=https://www.sciencedaily.com/releases/2011/09/110915141227.htm |title=Carbon cycle reaches Earth's lower mantle: Evidence of carbon cycle found in 'superdeep' diamonds From Brazil |publisher=ScienceDaily |date=15 September 2011 |access-date=2019-02-06}}</ref> Thus, the investigation's findings indicate that pieces of basaltic oceanic lithosphere act as the principal transport mechanism for carbon to Earth's deep interior. These subducted carbonates can interact with lower mantle [[silicate]]s and metals, eventually forming super-deep diamonds like the one found.<ref>{{cite journal|last1=Stagno|first1=V.|last2=Frost|first2=D. J.|author2-link=Daniel Frost (earth scientist)|last3=McCammon|first3=C. A.|author-link3=Catherine McCammon|last4=Mohseni|first4=H.|last5=Fei|first5=Y.|date=5 February 2015|title=The oxygen fugacity at which graphite or diamond forms from carbonate-bearing melts in eclogitic rocks|journal=Contributions to Mineralogy and Petrology|volume=169|issue=2|pages=16|bibcode=2015CoMP..169...16S|doi=10.1007/s00410-015-1111-1|s2cid=129243867}}</ref>

{| class="wikitable floatright" 
|+ Carbon reservoirs in the mantle, crust and surface.<ref name=Lee2019>{{cite book |last1=Lee |first1=C-T. A. |last2=Jiang |first2=H. |last3=Dasgupta |first3=R. |last4=Torres |first4=M. |chapter=A Framework for Understanding Whole-Earth Carbon Cycling |pages=313–357 |doi=10.1017/9781108677950.011 |editor-last1=Orcutt |editor-first1=Beth N. |editor-last2=Daniel |editor-first2=Isabelle |editor-last3=Dasgupta |editor-first3=Rajdeep |title=Deep carbon : past to present |date=2019 |publisher=Cambridge University Press |isbn=9781108677950|s2cid=210787128 }}</ref>
|-
! Reservoir !! [[gigatonne]] C
|-
| Above surface || <math>(43-45) \times 10^3</math>
|-
| Continental crust and lithosphere || <math>(0.9-3.1) \times 10^8</math>
|-
| Oceanic crust and lithosphere || <math>1.4 \times 10^8</math>
|-
| Upper mantle || <math><3.2 \times 10^7</math>
|-
| Lower mantle || <math><1.0 \times 10^9</math>
|}

Carbonates descending to the lower mantle form other compounds besides diamonds. In 2011, carbonates were subjected to an environment similar to that of 1800&nbsp;km deep into the Earth, well within the lower mantle. Doing so resulted in the formations of [[magnesite]], [[siderite]], and numerous varieties of [[graphite]].<ref name="Fiquet 5184–5187">{{Cite journal|last1=Fiquet|first1=Guillaume|last2=Guyot|first2=François|last3=Perrillat|first3=Jean-Philippe|last4=Auzende|first4=Anne-Line|last5=Antonangeli|first5=Daniele|last6=Corgne|first6=Alexandre|last7=Gloter|first7=Alexandre|last8=Boulard|first8=Eglantine|date=2011-03-29|title=New host for carbon in the deep Earth |journal=Proceedings of the National Academy of Sciences|volume=108|issue=13|pages=5184–5187|doi=10.1073/pnas.1016934108 |pmid=21402927|pmc=3069163|bibcode=2011PNAS..108.5184B|doi-access=free}}</ref> Other experiments—as well as [[Petrology|petrologic]] observations—support this claim, finding that magnesite is actually the most stable carbonate phase in the majority of the mantle. This is largely a result of its higher melting temperature.<ref>{{Cite journal|last1=Dorfman|first1=Susannah M.|last2=Badro|first2=James|last3=Nabiei|first3=Farhang|last4=Prakapenka|first4=Vitali B.|last5=Cantoni|first5=Marco|last6=Gillet|first6=Philippe|date=2018-05-01|title=Carbonate stability in the reduced lower mantle |journal=Earth and Planetary Science Letters|volume=489|pages=84–91|doi=10.1016/j.epsl.2018.02.035 |bibcode=2018E&PSL.489...84D|osti=1426861|s2cid=134119145 |url=https://hal.archives-ouvertes.fr/hal-02363561/file/document-1.pdf}}</ref> Consequently, scientists have concluded that carbonates undergo [[Reduction (chemistry)|reduction]] as they descend into the mantle before being stabilised at depth by low oxygen [[fugacity]] environments. Magnesium, iron, and other metallic compounds act as buffers throughout the process.<ref>{{Cite journal|last1=Kelley|first1=Katherine A.|last2=Cottrell|first2=Elizabeth|date=2013-06-14|title=Redox Heterogeneity in Mid-Ocean Ridge Basalts as a Function of Mantle Source |journal=Science|volume=340|issue=6138|pages=1314–1317|doi=10.1126/science.1233299 |pmid=23641060|bibcode=2013Sci...340.1314C|s2cid=39125834|doi-access=free}}</ref> The presence of reduced, elemental forms of carbon like graphite would indicate that carbon compounds are reduced as they descend into the mantle. 

[[File:Carbon Outgassing (Dasgupta 2011).png|thumb|upright=1.8|left| {{center|Carbon outgassing processes<ref>{{Cite conference |url=http://www.deep-earth.org/postAGU2011/Dasgupta-cider-agu2011.ppt |title=The Influence of Magma Ocean Processes on the Present-day Inventory of Deep Earth Carbon |last=Dasgupta |first=Rajdeep |date=December 10, 2011|conference=Post-AGU 2011 CIDER Workshop |archive-url=https://web.archive.org/web/20160424031155/http://www.deep-earth.org/postAGU2011/Dasgupta-cider-agu2011.ppt |archive-date=24 April 2016 |access-date=20 March 2019}}</ref>}}]]

Nonetheless, [[Polymorphism (materials science)|polymorphism]] alters carbonate compounds' stability at different depths within the Earth. To illustrate, laboratory simulations and [[density functional theory]] calculations suggest that [[Tetrahedral molecular geometry|tetrahedrally-coordinated]] carbonates are most stable at depths approaching the [[core–mantle boundary]].<ref>{{cite book |doi=10.1016/B978-0-12-811301-1.00002-2 |chapter=Carbon-Bearing Magmas in the Earth's Deep Interior |title=Magmas Under Pressure |pages=43–82 |year=2018 |last1=Litasov |first1=Konstantin D. |last2=Shatskiy |first2=Anton |isbn=978-0-12-811301-1 }}</ref><ref name="Fiquet 5184–5187"/> A 2015 study indicates that the lower mantle's high pressures cause carbon bonds to transition from sp<sub>2</sub> to sp<sub>3</sub> [[Orbital hybridisation|hybridised orbitals]], resulting in carbon tetrahedrally bonding to oxygen.<ref>{{Cite journal|last1=Mao|first1=Wendy L.|author-link=Wendy Mao|last2=Liu|first2=Zhenxian|last3=Galli|first3=Giulia|last4=Pan|first4=Ding|last5=Boulard|first5=Eglantine|date=2015-02-18|title=Tetrahedrally coordinated carbonates in Earth's lower mantle|journal=Nature Communications|volume=6|pages=6311|arxiv=1503.03538|bibcode=2015NatCo...6.6311B|doi=10.1038/ncomms7311|pmid=25692448|s2cid=205335268}}</ref> CO<sub>3</sub> trigonal groups cannot form polymerisable networks, while tetrahedral CO<sub>4</sub> can, signifying an increase in carbon's [[coordination number]], and therefore drastic changes in carbonate compounds' properties in the lower mantle. As an example, preliminary theoretical studies suggest that high pressures cause carbonate melt viscosity to increase; the melts' lower mobility as a result of the property changes described is evidence for large deposits of carbon deep into the mantle.<ref>{{Cite journal|last1=Carmody|first1=Laura|last2=Genge|first2=Matthew|last3=Jones|first3=Adrian P.|date=2013-01-01|title=Carbonate Melts and Carbonatites |journal=Reviews in Mineralogy and Geochemistry|volume=75|issue=1|pages=289–322|doi=10.2138/rmg.2013.75.10 |bibcode=2013RvMG...75..289J}}</ref>

Accordingly, carbon can remain in the lower mantle for long periods of time, but large concentrations of carbon frequently find their way back to the lithosphere. This process, called carbon outgassing, is the result of carbonated mantle undergoing decompression melting, as well as [[mantle plume]]s carrying carbon compounds up towards the crust.<ref>{{Cite journal|last1=Dasgupta|first1=Rajdeep|last2=Hirschmann|first2=Marc M.|date=2010-09-15|title=The deep carbon cycle and melting in Earth's interior |journal=Earth and Planetary Science Letters|volume=298|issue=1|pages=1–13|doi=10.1016/j.epsl.2010.06.039 |bibcode=2010E&PSL.298....1D}}</ref> Carbon is oxidised upon its ascent towards volcanic hotspots, where it is then released as CO<sub>2</sub>. This occurs so that the carbon atom matches the oxidation state of the basalts erupting in such areas.<ref>{{cite journal |last1=Frost |first1=Daniel J. |last2=McCammon |first2=Catherine A. |title=The Redox State of Earth's Mantle |journal=Annual Review of Earth and Planetary Sciences |date=May 2008 |volume=36 |issue=1 |pages=389–420 |doi=10.1146/annurev.earth.36.031207.124322|bibcode=2008AREPS..36..389F }}</ref>

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== Core ==
{{biogeochemical cycle sidebar|carbon}}
Although the presence of carbon in the Earth's core is well-constrained, recent studies suggest large inventories of carbon could be stored in this region. [[S-wave|Shear (S) waves]] moving through the inner core travel at about fifty percent of the velocity expected for most iron-rich alloys.<ref name=DCOcore>{{Cite web|url=https://deepcarbon.net/feature/does-earths-core-host-deep-carbon-reservoir|title=Does Earth's Core Host a Deep Carbon Reservoir?|website=Deep Carbon Observatory|date=14 April 2015|access-date=2019-03-09|archive-date=2020-07-27|archive-url=https://web.archive.org/web/20200727092747/https://deepcarbon.net/feature/does-earths-core-host-deep-carbon-reservoir|url-status=dead}}</ref> Considering the core's composition is widely believed to be an alloy of crystalline iron with a small amount of nickel, this seismographic anomaly points to another substance's existence within the region. One theory postulates that such a phenomenon is the result of various light elements, including carbon, in the core.<ref name=DCOcore/> In fact, studies have utilised [[diamond anvil cell]]s to replicate the conditions in the Earth's core, the results of which indicate that [[Cementite|iron carbide]] (Fe<sub>7</sub>C<sub>3</sub>) matches the inner core's sound and density velocities considering its temperature and pressure profile. Hence, the iron carbide model could serve as evidence that the core holds as much as 67% of the Earth's carbon.<ref>{{Cite journal|last1=Li|first1=Jie|last2=Chow|first2=Paul|last3=Xiao|first3=Yuming|last4=Alp|first4=E. Ercan|last5=Bi|first5=Wenli|last6=Zhao|first6=Jiyong|last7=Hu|first7=Michael Y.|last8=Liu|first8=Jiachao|last9=Zhang|first9=Dongzhou|date=2014-12-16|title=Hidden carbon in Earth's inner core revealed by shear softening in dense Fe<sub>7</sub>C<sub>3</sub> |journal=Proceedings of the National Academy of Sciences|volume=111|issue=50|pages=17755–17758|doi=10.1073/pnas.1411154111 |pmid=25453077|pmc=4273394|bibcode=2014PNAS..11117755C|doi-access=free}}</ref> Furthermore, another study found that carbon dissolved in iron and formed a stable phase with the same Fe<sub>7</sub>C<sub>3</sub> composition—albeit with a different structure than the one previously mentioned.<ref>{{Cite journal|last1=Hanfland|first1=M.|last2=Chumakov|first2=A.|last3=Rüffer|first3=R.|last4=Prakapenka|first4=V.|last5=Dubrovinskaia|first5=N.|last6=Cerantola|first6=V.|last7=Sinmyo|first7=R.|last8=Miyajima|first8=N.|last9=Nakajima|first9=Y.|date=March 2015|title=High Poisson's ratio of Earth's inner core explained by carbon alloying |journal=Nature Geoscience|volume=8|issue=3|pages=220–223|doi=10.1038/ngeo2370 |bibcode=2015NatGe...8..220P}}</ref> Hence, although the amount of carbon potentially stored in the Earth's core is not known, recent research indicates that the presence of iron carbides could be consistent with geophysical observations.

<gallery mode=packed style=float:left heights=160px>
File:Flux of crustal material in the mantle.jpg| Movement of oceanic plates—which carry carbon compounds—through the mantle
File:Two models.jpg| {{center| Two models of the carbon content in Earth}}
</gallery>

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<gallery mode=packed style=float:left heights=190px>
File:Speeds of seismic waves.svg| Analysis of shear wave velocities has played an integral role in the development of knowledge about carbon's existence in the core
File:Carbon tetrahedral oxygen.png|Diagram of carbon tetrahedrally bonded to oxygen
</gallery>

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==Fluxes==
[[File:Carbon fluxes to, from, and within the Earth’s exogenic and endogenic systems.jpg|thumb|upright=3.5|left| {{center|'''Major fluxes of carbon to, from, and within the Earth’s exogenic and endogenic systems'''<br />Values give the maximum and minimum fluxes since 200 million years ago. The two major boundaries highlighted are<br />the [[Mohorovičić discontinuity]] (crust-mantle boundary; Moho) and the [[lithosphere-asthenosphere boundary]] (LAB).<ref name=Wong2019>{{cite journal |doi = 10.3389/feart.2019.00263|title = Deep Carbon Cycling over the Past 200 Million Years: A Review of Fluxes in Different Tectonic Settings|year = 2019|last1 = Wong|first1 = Kevin|last2 = Mason|first2 = Emily|last3 = Brune|first3 = Sascha|last4 = East|first4 = Madison|last5 = Edmonds|first5 = Marie|last6 = Zahirovic|first6 = Sabin|journal = Frontiers in Earth Science|volume = 7|page = 263|bibcode = 2019FrEaS...7..263W|s2cid = 204027259|doi-access = free}}</ref>}}]]

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== See also ==
*[[Deep Carbon Observatory]]
*[[Geochemistry of carbon]]

== References ==
{{reflist}}

== Further reading ==
{{Refbegin}}
*{{cite book |editor-last1=Hazen |editor-first1=Robert M. |editor-last2=Jones |editor-first2=Adrian P. |editor-last3=Baross |editor-first3=John A. |title=Carbon in Earth |series=Reviews in Mineralogy and Geochemistry |volume=75 |date=2013 |publisher=Mineralogical Society of America |isbn=978-0-939950-90-4 |url=http://www.minsocam.org/msa/rim/rim75.html |accessdate=13 December 2019}}
*{{cite book |last1=Hazen |first1=Robert M. |title=Symphony in C : carbon and the evolution of (almost) everything |date=2019 |publisher=W. W. Norton |isbn=9780393609448}}
*{{cite book |editor-last1=Orcutt |editor-first1=B |editor-last2=Dasgupta |editor-first2=R |title=Deep carbon: Past to present |date=2019 |publisher=Cambridge University Press |isbn=9781108677950 |doi=10.1017/9781108677950|hdl=10023/18736 |s2cid=241804383 }}
{{Refend}}

{{biogeochemical cycle}}

[[Category:Carbon cycle]]
[[Category:Geochemistry]]
[[Category:Earth]]
[[Category:Plate tectonics]]