Diferencia entre revisiones de «Ciclo del carbono profundo»
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|+ [[Reservoir|Reservorios]] de carbono en la corteza, el manto y la superficie.<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}}</ref>
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| En superficie || <math>(43-45) \times 10^3</math>
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| Manto inferior || <math><1.0 \times 10^9</math>
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Hay alrededor de 44.000 [[Gigatón|gigatones]] de carbono en la atmósfera y los océanos. Un gigatón es un [[millardo]] de [[toneladas]], equivalentes a la [[masa]] de [[agua]] en más de 400.000 [[Piscina olímpica|piscinas olímpicas]].<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 de diciembre de 2019 |work=Deep Carbon Observatory |date=1 de octubre de 2019 |language=en}}</ref>
En la [[Fotosfera|fotósfera]] del [[Sol]], el carbono es el [[Abundancia de los elementos químicos|cuarto elemento más abundante]]. Es probable que la Tierra haya comenzado con una proporción similar, pero fue perdiéndolo por [[evaporación]] de la atmósfera durante el [[acreción|proceso de acreción]]. Incluso teniendo en cuenta la evaporación, los [[Silicato|silicatos]] que forman la corteza y el manto terrestres presentan una [[concentración]] entre cinco y diez veces menor frente a la observada en [[Condrita|meteoritos condríticos]], un tipo de meteorito que se considera mantiene la composición media de la [[nebulosa solar]], anterior de la [[Disco protoplanetario|formación de los planetas]]. Parte de dicho carbono, puede haber quedado en el núcleo terrestre y, dependiendo del modelo utilizado, puede predecirse que representa entre 0,2 y 1 % de la masa del núcleo. A pesar de presentar menor concentración, ello representaría la mitad del carbono presente en la Tierra.
Estimaciones del contenido de carbono en el manto superior, obtenidos a partir de mediciones de la química de los [[Basalto|basaltos]] de la [[dorsal meso-oceánica]], conocidos como MORBs, por su [[acrónimo]] en [[Lengua Inglesa|inglés]]. Estos datos deben ser sometidos a corrección para dar cuenta del desgasamiento de carbono y otros elementos. Desde la formación del la Tierra, el manto superior ha perdido entre 40% y 90% del cabono original, ya sea debido a evaporación o por transporte hacia el núcleo, mediante la formación de compuestos ferrosos. Las estimaciones más rigurosas dan un contenido de carbono de 30 [[partes por millón]] (ppm). En cambio, en el manto inferior la concentración presentaría una disminución menor, con 350 ppm.<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}}</ref>
== Manto inferior ==
[[Archivo:Flux of crustal material in the mantle.jpg|thumb|304x304px|Movimiento de las placas tectónicas oceánicas, que transportan compuestos de carbono a través del manto.]]
Principalmente, el [[carbono]] entra al [[Manto terrestre|manto]] en forma de [[sedimento]]s ricos en [[carbonato]] a través de la [[tectónica de placas]] propia de la [[corteza oceánica]], que lleva el carbono hacia dentro del manto mediante [[subducción]]. Se conoce poco sobre su [[Convección del manto|circulación dentro del manto]] profundo de la Tierra, pero varios estudios han procurado mejorar la comprensión de su movimiento y sus compuestos en dicha región. Por ejemplo, un estudio de 2011 demostró que el [[ciclo del carbono]] se extiende bien hasta el [[manto inferior]]. El estudio se basó en el análisis de los raros [[diamantes]] superprofundos en un sitio de [[Mato Grosso|Juína, Mato Grosso]], [[Brasil]], que determinó que la composición bruta de algunas [[Inclusión (mineralogía)|inclusiones]] diamantíferas se correspondían con las esperadas para la [[Fusión (cambio de estado)|fusión]] y [[cristalización]] del [[basalto]] en condiciones de presión y temperatura como las reinantes en el manto inferior.<ref>{{Cite news |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>
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<!-- == Lower mantle ==
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. |last3=McCammon |first3=C. A. |last4=Mohseni |first4=H. |last5=Fei |first5=Y. |title=The oxygen fugacity at which graphite or diamond forms from carbonate-bearing melts in eclogitic rocks |journal=Contributions to Mineralogy and Petrology |date=5 February 2015 |volume=169 |issue=2 |pages=16 |doi=10.1007/s00410-015-1111-1 |bibcode=2015CoMP..169...16S }}</ref>
[[File:Carbon tetrahedral oxygen.png|thumb|Diagram of carbon tetrahedrally bonded to oxygen]]
Carbonates descending to the lower mantle form other compounds besides diamonds. In 2011, carbonates were subjected to an environment similar to that of 1800 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|last=Fiquet|first=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}}</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|last=Dorfman|first=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}}</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|last=Kelley|first=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}}</ref> The presence of reduced, elemental forms of carbon like graphite would indicate that carbon compounds are reduced as they descend into the mantle.
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|last=Mao|first=Wendy L.|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|doi=10.1038/ncomms7311 |pmid=25692448|bibcode=2015NatCo...6.6311B|arxiv=1503.03538}}</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|last=Carmody|first=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|last=Dasgupta|first=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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