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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. Rajdeep Dasgupta claims that "outgassing from the mantle is controlled by decompression melting of carbonated mantle", as well as carbonates rising due to mantle plumes.<ref>{{Cite journal|last=Dasgupta|first=Rajdeep|last2=Hirschmann|first2=Marc M.|date=2010-09-15|title=The deep carbon cycle and melting in Earth's interior|url=http://www.sciencedirect.com/science/article/pii/S0012821X10004140|journal=Earth and Planetary Science Letters|volume=298|issue=1|pages=1–13|doi=10.1016/j.epsl.2010.06.039|issn=0012-821X}}</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 web|url=https://login.stanford.idm.oclc.org/login?url=https://www.annualreviews.org/doi/full/10.1146/annurev.earth.36.031207.124322|title=Shibboleth Authentication Request|website=login.stanford.idm.oclc.org|doi=10.1146/annurev.earth.36.031207.124322|access-date=2019-02-07}}</ref><br />
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. Rajdeep Dasgupta claims that "outgassing from the mantle is controlled by decompression melting of carbonated mantle", as well as carbonates rising due to mantle plumes.<ref>{{Cite journal|last=Dasgupta|first=Rajdeep|last2=Hirschmann|first2=Marc M.|date=2010-09-15|title=The deep carbon cycle and melting in Earth's interior|url=http://www.sciencedirect.com/science/article/pii/S0012821X10004140|journal=Earth and Planetary Science Letters|volume=298|issue=1|pages=1–13|doi=10.1016/j.epsl.2010.06.039|issn=0012-821X}}</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 web|url=https://login.stanford.idm.oclc.org/login?url=https://www.annualreviews.org/doi/full/10.1146/annurev.earth.36.031207.124322|title=Shibboleth Authentication Request|website=login.stanford.idm.oclc.org|doi=10.1146/annurev.earth.36.031207.124322|access-date=2019-02-07}}</ref><br />
[[File:Speeds of seismic waves.PNG|thumb|Analysis of shear wave velocities has played an integral role in the development of knowledge about carbon's existence in the core]]

=== Carbon in the Core ===
=== Carbon in the Core ===
Although the presence of carbon in the earth's deep interior is not known, recent studies suggest the core possessing large inventories of the element. For instance, [[S-wave|shear (S) waves]] moving through the core travel at about fifty percent of the speed expected for most iron alloys. One theory postulates that such a phenomena is the result of various light elements, including carbon, in the core.<ref>{{Cite web|url=https://deepcarbon.net/feature/does-earths-core-host-deep-carbon-reservoir|title=Does Earth's Core Host a Deep Carbon Reservoir? {{!}} Deep Carbon Observatory|website=deepcarbon.net|access-date=2019-02-05}}</ref> In fact, the results from a study run by scientists at [[Deep Carbon Observatory]] 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. [[University of Michigan]] professor Jackie Li—who was on the investigation team—states: "Should it hold up to various tests, the model would imply that as much as two-thirds of the planet's carbon is hidden in its center sphere, making it the largest reservoir of carbon on Earth."<ref>{{Cite journal|last=Li|first=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 Fe7C3|url=https://www.pnas.org/content/111/50/17755|journal=Proceedings of the National Academy of Sciences|language=en|volume=111|issue=50|pages=17755–17758|doi=10.1073/pnas.1411154111|issn=0027-8424|pmid=25453077}}</ref> Furthermore, Clemens Prescher of the [[University of Bayreuth]] and his colleagues utilised a diamond anvil cell to replicate the conditions in the earth core, finding that carbon dissolved in iron and formed a stable phase.<ref>{{Cite journal|last=Hanfland|first=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=2015-03|title=High Poisson's ratio of Earth's inner core explained by carbon alloying|url=https://www.nature.com/articles/ngeo2370|journal=Nature Geoscience|language=en|volume=8|issue=3|pages=220–223|doi=10.1038/ngeo2370|issn=1752-0908}}</ref> This phase had a different structure than the one that Li and her team found; nevertheless, both iron carbide phases had the same Fe<sub>7</sub>C<sub>3</sub> composition. Hence, the movement of carbon within the Earth's core is not known, but recent research trends seem point to the presence of these iron carbides.
Although the presence of carbon in the earth's deep interior is not known, recent studies suggest the core possessing large inventories of the element. For instance, [[S-wave|shear (S) waves]] moving through the core travel at about fifty percent of the speed expected for most iron alloys. One theory postulates that such a phenomena is the result of various light elements, including carbon, in the core.<ref>{{Cite web|url=https://deepcarbon.net/feature/does-earths-core-host-deep-carbon-reservoir|title=Does Earth's Core Host a Deep Carbon Reservoir? {{!}} Deep Carbon Observatory|website=deepcarbon.net|access-date=2019-02-05}}</ref> In fact, the results from a study run by scientists at [[Deep Carbon Observatory]] 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. [[University of Michigan]] professor Jackie Li—who was on the investigation team—states: "Should it hold up to various tests, the model would imply that as much as two-thirds of the planet's carbon is hidden in its center sphere, making it the largest reservoir of carbon on Earth."<ref>{{Cite journal|last=Li|first=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 Fe7C3|url=https://www.pnas.org/content/111/50/17755|journal=Proceedings of the National Academy of Sciences|language=en|volume=111|issue=50|pages=17755–17758|doi=10.1073/pnas.1411154111|issn=0027-8424|pmid=25453077}}</ref> Furthermore, Clemens Prescher of the [[University of Bayreuth]] and his colleagues utilised a diamond anvil cell to replicate the conditions in the earth core, finding that carbon dissolved in iron and formed a stable phase.<ref>{{Cite journal|last=Hanfland|first=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=2015-03|title=High Poisson's ratio of Earth's inner core explained by carbon alloying|url=https://www.nature.com/articles/ngeo2370|journal=Nature Geoscience|language=en|volume=8|issue=3|pages=220–223|doi=10.1038/ngeo2370|issn=1752-0908}}</ref> This phase had a different structure than the one that Li and her team found; nevertheless, both iron carbide phases had the same Fe<sub>7</sub>C<sub>3</sub> composition. Hence, the movement of carbon within the Earth's core is not known, but recent research trends seem point to the presence of these iron carbides.

Revision as of 01:44, 13 February 2019

Deep Carbon Cycling

Although deep carbon cycling is not as well-researched as carbon movement through the atmosphere, terrestrial biosphere, and ocean, it is nonetheless an incredibly important process. In fact, studying the composition of basaltic magma and measuring carbon dioxide flux out of volcanoes reveals that the amount of carbon in the mantle is actually greater than that on the Earth's surface by a factor of one thousand.[1] Not much is conclusively known regarding the role of carbon in the deep earth, as drilling down and physically observing the processes there is evidently extremely difficult. Nonetheless, several pieces of evidence—some 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 said layer. Furthermore, scientists are beginning to augment their understanding of the presence of carbon in the core.

Carbon in the Lower Mantle

File:Tectonic-plates-subduction-zone-17280738.jpg
A diagram of an oceanic tectonic plate subducting beneath another, transporting carbonates to the mantle

Carbon principally enters the mantle in the form of carbonate-rich sediments on 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 by the University of Bristol's Michael Walter and his team demonstrated that carbon cycling extends all the way to the lower mantle. The study analysed rare, super-deep diamonds at a site in Juina, Brazil, determining that the bulk composition of some of the diamonds' inclusions matched the expected result of basalt melting and crytallisation under lower mantle temperatures and pressures.[2] Thus, the investigation's findings indicate that pieces of basaltic oceanic lithosphere act as the principle transport mechanism for carbon to Earth's deep interior. These subducted carbonates can interact with lower mantle silicates and metals, eventually forming super-deep diamonds like the one found.[3]

However, carbonates descending to the lower mantle encounter other fates in addition to forming diamonds. In 2011, a team of French scientists subjected carbonates to an environment similar to that of 1800km deep into the earth, well within the lower mantle. Doing so resulted in the formations of magnesite, siderite, and numerous varieties of graphite.[4] Other experiments—as well as 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.[5] According to Elizabeth Cottrell and Katherine Kelley, this is because carbonates undergo 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.[6] The presence of reduced, element forms like graphite in the 2011 study further displays that carbon compounds undergo reduction as they descends into the mantle. Nonetheless, it is noteworthy that polymorphism alters carbonate compounds' stability at different depths within the Earth. To illustrate, laboratory simulations and density functional theory calculations suggest that tetrahedrally-coordinated carbonates are most stable at depths approaching the Core-Mantle Boundary.[7][8] A 2015 study indicates that the lower mantle's high pressures causes carbon bonds to transition from sp2 to sp3 hybridised orbitals, resulting in carbon tetrahedrally bonding to oxygen.[9] CO3 trigonal groups cannot form polymerisable networks, while tetrahedral CO4 can, signifying an increase in carbon's coordination number, and therefore drastic changes in carbonate compounds' properties in the lower mantle. Preliminary theoretical studies suggest that high pressures causes carbonate melt viscosity to increase; the melts' lower mobility as a result of this property change is evidence for large deposits of carbon deep into the mantle.[10]

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. Rajdeep Dasgupta claims that "outgassing from the mantle is controlled by decompression melting of carbonated mantle", as well as carbonates rising due to mantle plumes.[11] Carbon is oxidised upon its ascent towards volcanic hotspots, where it is then released as CO2. This occurs so that the carbon atom matches the oxidation state of the basalts erupting in such areas.[12]

Analysis of shear wave velocities has played an integral role in the development of knowledge about carbon's existence in the core

Carbon in the Core

Although the presence of carbon in the earth's deep interior is not known, recent studies suggest the core possessing large inventories of the element. For instance, shear (S) waves moving through the core travel at about fifty percent of the speed expected for most iron alloys. One theory postulates that such a phenomena is the result of various light elements, including carbon, in the core.[13] In fact, the results from a study run by scientists at Deep Carbon Observatory indicate that iron carbide (Fe7C3) matches the inner core's sound and density velocities considering its temperature and pressure profile. University of Michigan professor Jackie Li—who was on the investigation team—states: "Should it hold up to various tests, the model would imply that as much as two-thirds of the planet's carbon is hidden in its center sphere, making it the largest reservoir of carbon on Earth."[14] Furthermore, Clemens Prescher of the University of Bayreuth and his colleagues utilised a diamond anvil cell to replicate the conditions in the earth core, finding that carbon dissolved in iron and formed a stable phase.[15] This phase had a different structure than the one that Li and her team found; nevertheless, both iron carbide phases had the same Fe7C3 composition. Hence, the movement of carbon within the Earth's core is not known, but recent research trends seem point to the presence of these iron carbides.

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Works Cited

  1. ^ physicstoday.scitation.org. doi:10.1063/1.1628990 https://physicstoday.scitation.org/action/captchaChallenge?redirectUrl=https%3A%2F%2Fphysicstoday.scitation.org%2Fdoi%2Ffull%2F10.1063%2F1.1628990&. Retrieved 2019-02-06. {{cite web}}: Missing or empty |title= (help)
  2. ^ "Carbon cycle reaches Earth's lower mantle: Evidence of carbon cycle found in 'superdeep' diamonds From Brazil". ScienceDaily. Retrieved 2019-02-06.
  3. ^ Stagno, V.; Frost, D. J.; McCammon, C. A.; Mohseni, H.; Fei, Y. (2015-02-05). "The oxygen fugacity at which graphite or diamond forms from carbonate-bearing melts in eclogitic rocks". Contributions to Mineralogy and Petrology. 169 (2): 16. doi:10.1007/s00410-015-1111-1. ISSN 1432-0967.
  4. ^ Fiquet, Guillaume; Guyot, François; Perrillat, Jean-Philippe; Auzende, Anne-Line; Antonangeli, Daniele; Corgne, Alexandre; Gloter, Alexandre; Boulard, Eglantine (2011-03-29). "New host for carbon in the deep Earth". Proceedings of the National Academy of Sciences. 108 (13): 5184–5187. doi:10.1073/pnas.1016934108. ISSN 0027-8424. PMID 21402927.
  5. ^ Dorfman, Susannah M.; Badro, James; Nabiei, Farhang; Prakapenka, Vitali B.; Cantoni, Marco; Gillet, Philippe (2018-05-01). "Carbonate stability in the reduced lower mantle". Earth and Planetary Science Letters. 489: 84–91. doi:10.1016/j.epsl.2018.02.035. ISSN 0012-821X.
  6. ^ Kelley, Katherine A.; Cottrell, Elizabeth (2013-06-14). "Redox Heterogeneity in Mid-Ocean Ridge Basalts as a Function of Mantle Source". Science. 340 (6138): 1314–1317. doi:10.1126/science.1233299. ISSN 0036-8075. PMID 23641060.
  7. ^ "ScienceDirect". www.sciencedirect.com. Retrieved 2019-02-07.
  8. ^ Fiquet, Guillaume; Guyot, François; Perrillat, Jean-Philippe; Auzende, Anne-Line; Antonangeli, Daniele; Corgne, Alexandre; Gloter, Alexandre; Boulard, Eglantine (2011-03-29). "New host for carbon in the deep Earth". Proceedings of the National Academy of Sciences. 108 (13): 5184–5187. doi:10.1073/pnas.1016934108. ISSN 0027-8424. PMID 21402927.
  9. ^ Mao, Wendy L.; Liu, Zhenxian; Galli, Giulia; Pan, Ding; Boulard, Eglantine (2015-02-18). "Tetrahedrally coordinated carbonates in Earth's lower mantle". Nature Communications. 6: 6311. doi:10.1038/ncomms7311. ISSN 2041-1723.
  10. ^ Carmody, Laura; Genge, Matthew; Jones, Adrian P. (2013-01-01). "Carbonate Melts and Carbonatites". Reviews in Mineralogy and Geochemistry. 75 (1): 289–322. doi:10.2138/rmg.2013.75.10. ISSN 1529-6466.
  11. ^ Dasgupta, Rajdeep; Hirschmann, Marc M. (2010-09-15). "The deep carbon cycle and melting in Earth's interior". Earth and Planetary Science Letters. 298 (1): 1–13. doi:10.1016/j.epsl.2010.06.039. ISSN 0012-821X.
  12. ^ "Shibboleth Authentication Request". login.stanford.idm.oclc.org. doi:10.1146/annurev.earth.36.031207.124322. Retrieved 2019-02-07.
  13. ^ "Does Earth's Core Host a Deep Carbon Reservoir? | Deep Carbon Observatory". deepcarbon.net. Retrieved 2019-02-05.
  14. ^ Li, Jie; Chow, Paul; Xiao, Yuming; Alp, E. Ercan; Bi, Wenli; Zhao, Jiyong; Hu, Michael Y.; Liu, Jiachao; Zhang, Dongzhou (2014-12-16). "Hidden carbon in Earth's inner core revealed by shear softening in dense Fe7C3". Proceedings of the National Academy of Sciences. 111 (50): 17755–17758. doi:10.1073/pnas.1411154111. ISSN 0027-8424. PMID 25453077.
  15. ^ Hanfland, M.; Chumakov, A.; Rüffer, R.; Prakapenka, V.; Dubrovinskaia, N.; Cerantola, V.; Sinmyo, R.; Miyajima, N.; Nakajima, Y. (2015-03). "High Poisson's ratio of Earth's inner core explained by carbon alloying". Nature Geoscience. 8 (3): 220–223. doi:10.1038/ngeo2370. ISSN 1752-0908. {{cite journal}}: Check date values in: |date= (help)
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