View source for Carbonate–silicate cycle

{{Short description|Geochemical transformation of silicate rocks}}
{{Not to be confused with|Carbon cycle}}
[[File:Carbonate-Silicate Cycle (Carbon Cycle focus).jpg|thumb|upright=2|This figure describes the geological aspects and processes of the carbonate silicate cycle, within the long-term carbon cycle.|alt=]]
The '''carbonate–silicate [[geochemical cycle]]''', also known as the '''inorganic carbon cycle''', describes the long-term transformation of [[silicate]] rocks to [[Carbonate rock|carbonate]] rocks by [[weathering]] and [[sedimentation]], and the transformation of carbonate rocks back into silicate rocks by [[metamorphism]] and [[volcanism]].<ref name="Urey_1952">[[Harold Urey|Urey, H. C.]] (1952). The planets: their origin and development. Mrs. Hepsa Ely Silliman Memorial Lectures.</ref><ref name=":0">{{Cite journal |last1=Berner|first1=Robert |last2=Lasaga|first2=Antonio |last3=Garrels|first3=Robert |date=1983 |title=The carbonate-silicate geochemical cycle and its effect on atmospheric carbon dioxide over the past 100 million years |journal=American Journal of Science|volume=283|issue=7|pages=641–683 |bibcode=1983AmJS..283..641B |doi=10.2475/ajs.283.7.641|doi-access=free}}</ref><ref name=Walker1981>{{Cite journal|last1=Walker|first1=James C. G.|last2=Hays|first2=P. B.|last3=Kasting|first3=J. F. |date=1981 |title=A negative feedback mechanism for the long-term stabilization of Earth's surface temperature|journal=Journal of Geophysical Research: Oceans|language=en|volume=86|issue=C10|pages=9776–9782 |bibcode=1981JGR....86.9776W |doi=10.1029/JC086iC10p09776 |issn=2156-2202}}</ref> [[Carbon dioxide]] is removed from the atmosphere during burial of weathered minerals and returned to the [[atmosphere]] through [[volcanism]]. On million-year time scales, the carbonate-silicate cycle is a key factor in controlling Earth's climate because it regulates [[carbon dioxide]] levels and therefore global temperature.<ref name=Walker1981 />

The rate of weathering is sensitive to factors that change how much land is exposed. These factors include [[sea level]], [[topography]], [[lithology]], and [[vegetation]] changes.<ref name="Walker1993">{{cite book |last1=Walker |first1=James C. G. |title=Biogeochemistry of Global Change: Radiatively Active Trace Gases Selected Papers from the Tenth International Symposium on Environmental Biogeochemistry |date=1993 |publisher=Springer |location=Boston, MA |isbn=978-1-4613-6215-9 |pages=3–28 |doi=10.1007/978-1-4615-2812-8_1 |chapter=Biogeochemical Cycles of Carbon on a Hierarchy of Timescales}}</ref> Furthermore, these geomorphic and chemical changes have worked in tandem with solar forcing, whether due to orbital changes or stellar evolution, to determine the [[global surface temperature]]. Additionally, the carbonate-silicate cycle has been considered a possible solution to the [[faint young Sun paradox]].<ref name=":0" /><ref name="Walker1981" />

== Overview of the cycle ==
[[File:Carbon-Slicate Cycle Feedbacks.jpg|thumb|upright=2|This schematic shows the relationship between the different physical and chemical processes that make up the carbonate-silicate cycle.]]The carbonate-silicate cycle is the primary control on [[carbon dioxide]] levels over long timescales.<ref name=Walker1981 /> It can be seen as a branch of the [[carbon cycle]], which also includes the [[Terrestrial biological carbon cycle|organic carbon cycle]], in which biological processes convert carbon dioxide and water into organic matter and oxygen via [[photosynthesis]].<ref name="SullivanBaross2007">{{cite book |last1=Sullivan |first1=Woodruff T. |last2=Baross |first2=John A. |title=Planets and Life |date=2007 |publisher=Cambridge University Press |location=Cambridge, UK |isbn=978-0-521-53102-3 |pages=91–116 |chapter=Planetary Atmospheres and Life}}</ref>

=== Physical and chemical processes ===
[[File:MicroscopicShell_IceAge.jpg|thumb|upright=1.2|Microscopic shells of [[Foraminifera]] found in sediment cores may be used to determine past climate conditions including ocean temperatures and aspects of atmospheric chemistry.|alt=]]
{{Carbon cycle|Biogeochemical}}

The inorganic cycle begins with the production of [[carbonic acid]] (H<sub>2</sub>CO<sub>3</sub>) from rainwater and gaseous carbon dioxide.<ref name="Bonan2013">{{cite book |last1=Bonan |first1=Gordon |title=Ecological Climatology: Concepts and Applications |date=2013 |publisher=Cambridge University Press |location=New York |isbn=978-0-521-69319-6 |pages=105–128 |edition=2nd}}</ref> Due to this process, normal rain has a pH of around 5.6.<ref>{{cite web |url=https://www.epa.gov/acidrain/what-acid-rain |title=What is Acid Rain? |author=Environmental Protection Agency|date=9 February 2016 }}</ref> Carbonic acid is a [[Acid strength#Weak acids|weak acid]], but over long timescales, it can dissolve silicate rocks (as well as carbonate rocks). Most of the Earth's crust (and mantle) is composed of silicates.<ref name="ACSClimateToolkit" /> These substances break down into dissolved ions as a result. For example, [[calcium silicate]] (CaSiO<sub>3</sub>), or [[wollastonite]], reacts with carbon dioxide and water to yield a calcium ion, Ca<sup>2+</sup>, a bicarbonate ion, HCO<sub>3</sub><sup>−</sup>, and dissolved silica. This reaction structure is representative of general silicate weathering of calcium silicate minerals.<ref name="CatlingKasting2017">{{cite book |last1=Catling |first1=David C. |last2=Kasting |first2=James F. |title=Atmospheric Evolution on Inhabited and Lifeless Worlds |date=2017 |publisher=Cambridge University Press |location=Cambridge, UK |isbn=978-0-521-84412-3 |pages=299–326}}</ref> The chemical pathway is as follows:

: <chem> 2CO2 + H2O + CaSiO3 -> Ca^2+ + 2HCO3- + SiO2 </chem>

River runoff carries these products to the ocean, where marine calcifying organisms use Ca<sup>2+</sup> and HCO<sub>3</sub><sup>−</sup> to build their shells and skeletons, a process called '''carbonate precipitation''':

: <chem> Ca^2+ + 2HCO3- -> CaCO3 + CO2 + H2O </chem>

Two molecules of CO<sub>2</sub> are required for silicate rock weathering; marine calcification releases one molecule back to the atmosphere. The calcium carbonate (CaCO<sub>3</sub>) contained in shells and skeletons sinks after the marine organism dies and is deposited on the ocean floor.

The final stage of the process involves the movement of the seafloor. At [[Subduction|subduction zones]], the carbonate sediments are buried and forced back into the [[Mantle (geology)|mantle]]. Some carbonate may be carried deep into the mantle where high pressure and temperature conditions allow it to combine metamorphically with SiO<sub>2</sub> to form CaSiO<sub>3</sub> and CO<sub>2</sub>, which is released from the interior into the atmosphere via volcanism, thermal vents in the ocean, or [[Spring (hydrology)|soda springs]], which are natural springs that contain carbon dioxide gas or soda water:

:<chem> CaCO3  +  SiO2 -> CaSiO3  +  CO2 </chem>

This final step returns the second CO<sub>2</sub> molecule to the atmosphere and closes the inorganic [[Emissions budget|carbon budget]]. 99.6% of all carbon on Earth (equating to roughly 10<sup>8</sup> billion tons of carbon) is sequestered in the longterm rock reservoir. And essentially all carbon has spent time in the form of carbonate. By contrast, only 0.002% of carbon exists in the biosphere.<ref name=ACSClimateToolkit />

=== Feedbacks ===
Changes to the surface of the planet, such as an absence of volcanoes or higher sea levels, which would reduce the amount of land surface exposed to weathering can change the rates at which different processes in this cycle take place.<ref name="ACSClimateToolkit">{{cite web |title=Geology and Climate: ACS Climate Science Toolkit |url=https://www.acs.org/content/acs/en/climatescience/oceansicerocks/geologyandclimate.html |website=American Chemical Society}}</ref> Over tens to hundreds of millions of years, carbon dioxide levels in the atmosphere may vary due to natural perturbations in the cycle<ref name="Berner1991">{{cite journal |last1=Berner |first1=Robert A. |title=A model for atmospheric CO<sub>2</sub> over Phanerozoic time |journal=American Journal of Science |date=April 1, 1991 |volume=291 |issue=4 |pages=339–376 |doi=10.2475/ajs.291.4.339|bibcode=1991AmJS..291..339B |doi-access=free }}</ref><ref name="Berner1998">{{cite journal |last1=Berner |first1=Robert A. |title=The carbon cycle and carbon dioxide over Phanerozoic time: the role of land plants |journal=  Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences|date=January 29, 1998 |volume=353 |issue=1365 |pages=75–82 |doi=10.1098/rstb.1998.0192|pmc=1692179 }}</ref><ref name="Berner2003">{{cite journal |last1=Berner |first1=Robert A. |last2=Beerling |first2=David J. |last3=Dudley |first3=Robert |last4=Robinson |first4=Jennifer M. |last5=Wildman, Jr. |first5=Richard A. |title=Phanerozoic Atmospheric Oxygen |journal=Annual Review of Earth and Planetary Sciences |date=2003 |volume=31 |issue=31 |pages=105–134|doi=10.1146/annurev.earth.31.100901.141329 |bibcode=2003AREPS..31..105B }}</ref> but even more generally, it serves as a critical [[negative feedback]] loop between carbon dioxide levels and climate changes.<ref name=Bonan2013 /><ref name=CatlingKasting2017 /> For example, if CO<sub>2</sub> builds up in the atmosphere, the greenhouse effect will serve to increase the surface temperature, which will in turn increase the rate of rainfall and silicate weathering, which will remove carbon from the atmosphere. In this way, over long timescales, the carbonate-silicate cycle has a stabilizing effect on the Earth's climate, which is why it has been called the Earth's thermostat.<ref name=SullivanBaross2007 /><ref name="DiVenere">{{cite web |last1=DiVenere |first1=Vic |title=The Carbon Cycle and Earth's Climate |url=http://www.columbia.edu/~vjd1/carbon.htm |publisher=Columbia University}}</ref>

== Changes through Earth history ==
{{Biogeochemical cycle sidebar|rock}}

Aspects of the carbonate-silicate cycle have changed through [[History of Earth|Earth history]] as a result of [[biological evolution]] and [[Tectonics|tectonic]] changes.<ref>{{Cite journal |last=Penman |first=Donald E. |last2=Caves Rugenstein |first2=Jeremy K. |last3=Ibarra |first3=Daniel E. |last4=Winnick |first4=Matthew J. |date=October 2020 |title=Silicate weathering as a feedback and forcing in Earth's climate and carbon cycle |url=https://www.sciencedirect.com/science/article/pii/S0012825220303445 |journal=[[Earth-Science Reviews]] |language=en |volume=209 |pages=103298 |doi=10.1016/j.earscirev.2020.103298 |access-date=30 October 2024 |via=Elsevier Science Direct}}</ref> Generally, the formation of carbonates has outpaced that of silicates, effectively removing carbon dioxide from the atmosphere. The advent of carbonate [[biomineralization]] near the [[Precambrian]]-[[Cambrian]] boundary would have allowed more efficient removal of weathering products from the ocean.<ref name=":1">{{Cite journal|last1=Ridgewell|first1=A|last2=Zeebe|first2=R|date=2005|title=The role of the global carbonate cycle in the regulation and evolution of the Earth system |journal=Earth and Planetary Science Letters|volume=234|issue=3–4|pages=299–315|doi=10.1016/j.epsl.2005.03.006|issn=0012-821X}}</ref> Biological processes in soils can significantly increase weathering rates.<ref name=":2">{{Cite journal |last1=Taylor |first1=Lyla L. |last2=Banwart|first2=Steve A.|last3=Valdes|first3=Paul J.|last4=Leake|first4=Jonathan R.|last5=Beerling|first5=David J.|date=2012|title=Evaluating the effects of terrestrial ecosystems, climate and carbon dioxide on weathering over geological time: a global-scale process-based approach|journal=Philosophical Transactions of the Royal Society B: Biological Sciences|volume=367|issue=1588|pages=565–582|doi=10.1098/rstb.2011.0251|issn=0962-8436 |pmc=3248708|pmid=22232768}}</ref> Plants produce [[organic acid]]s that increase [[weathering]]. These acids are secreted by root and [[mycorrhizal fungi]], as well as [[decomposition#Plant decomposition|microbial plant decay]]. [[soil respiration#root respiration|Root respiration]] and [[oxidation]] of [[soil organic matter|organic soil matter]] also produce [[carbon dioxide]], which is converted to [[carbonic acid]], which increases weathering.<ref>{{Cite journal|last=Berner|first=Robert A.|date=1992|title=Weathering, plants, and the long-term carbon cycle |journal=Geochimica et Cosmochimica Acta|volume=56|issue=8|pages=3225–3231 |doi=10.1016/0016-7037(92)90300-8|issn=0016-7037|bibcode=1992GeCoA..56.3225B|doi-access=free}}</ref>

Tectonics can induce changes in the carbonate-silicate cycle. For example, the uplift of major mountain ranges, such as [[Himalayas]] and the [[Andes]], is thought to have initiated the [[Late Cenozoic Ice Age]] due to increased rates of silicate weathering and draw down of [[carbon dioxide]].<ref>{{Cite journal|last1=Raymo|first1=Maureen E.|last2=Ruddiman|first2=William F.|last3=Froelich|first3=Philip N.|date=1988|title=Influence of late Cenozoic mountain building on ocean geochemical cycles|journal=Geology|language=en|volume=16|issue=7|pages=649|doi=10.1130/0091-7613(1988)016<0649:iolcmb>2.3.co;2|issn=0091-7613|bibcode=1988Geo....16..649R}}</ref> 
Seafloor weather is linked both to solar luminosity and carbon dioxide concentration.<ref name="Brady1997">{{cite journal |last1=Brady |first1=Patrick |last2=Gíslason |first2=Sigurdur R. |title=Seafloor weathering controls on atmospheric CO<sub>2</sub> and global climate Author links open overlay panel |journal=Geochimica et Cosmochimica Acta |date=March 1997 |volume=61 |issue=5 |pages=965–973 |doi=10.1016/S0016-7037(96)00385-7 }}</ref> However, it presented a challenge to modelers who have tried to relate the rate of outgassing and subduction to the related rates of seafloor change. Proper, uncomplicated proxy data is difficult to attain for such questions. For example, sediment cores, from which scientists can deduce past sea levels, are not ideal because sea levels change as a result of more than just seafloor adjustment.<ref name="Berner1989">{{cite journal |last1=Berner |first1=Robert A. |last2=Lasaga |first2=Antonio C. |title=Modeling the Geochemical Carbon Cycle |journal=Scientific American |volume=260 |issue=3 |date=March 1989 |pages=74–81 |doi=10.1038/scientificamerican0389-74|bibcode=1989SciAm.260c..74B }}</ref> Recent modeling studies have investigated the role of seafloor weathering on the early evolution of life, showing that relatively fast seafloor creation rates worked to draw down carbon dioxide levels to a moderate extent.<ref name="Krissansen2018">{{cite journal |last1=Krissansen-Totton |first1=Joshua |last2=Arney |first2=Giada N. |last3=Catling |first3=David C. |title=Constraining the climate and ocean pH of the early Earth with a geological carbon cycle model |journal=PNAS |date=April 17, 2018 |volume=115 |issue=16 |pages=4105–4110 |doi=10.1073/pnas.1721296115|pmid=29610313 |pmc=5910859 |arxiv=1804.00763 |bibcode=2018PNAS..115.4105K |doi-access=free }}</ref>
 
Observations of so-called [[deep time]] indicate that Earth has a relatively insensitive rock weathering feedback, allowing for large temperature swings. With about twice as much carbon dioxide in the atmosphere, paleoclimate records show that global temperatures reached up to 5 to 6&nbsp;°C higher than current temperatures.<ref name="Krissansen2017">{{cite journal |last1=Krissansen-Totton |first1=Joshua |last2=Catling |first2=David C. |title=Constraining climate sensitivity and continental versus seafloor weathering using an inverse geological carbon cycle model |journal=Nature Communications |date=22 May 2017 |volume=8 |page=15423 |doi=10.1038/ncomms15423|pmid=28530231 |pmc=5458154 |bibcode=2017NatCo...815423K }}</ref> However, other factors such as changes in [[Orbital forcing|orbital/solar forcing]] contribute to global temperature change in the paleo-record.

Human emissions of CO<sub>2</sub> have been steadily increasing, and the consequent concentration of CO<sub>2</sub> in the Earth system has reached unprecedented levels in a very short amount of time.<ref name="IPCC2014">{{cite book |editor1=Core Writing Team |editor2=R.K. Pachauri |editor3=L.A. Meyer |title=IPCC, 2014: Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change |date=2014 |publisher=IPCC |location=Geneva, Switzerland| url=https://www.ipcc.ch/report/ar5/syr/}}</ref> Excess carbon in the atmosphere that is dissolved in seawater can alter the rates of carbonate-silicate cycle. Dissolved CO<sub>2</sub> may react with water to form bicarbonate ions, HCO<sub>3</sub><sup>−</sup>, and hydrogen ions, H<sup>+</sup>. These hydrogen ions quickly react with carbonate, CO<sub>3</sub><sup>2-</sup> to produce more bicarbonate ions and reduce the available carbonate ions, which presents an obstacle to the carbon carbonate precipitation process.<ref name="WHOI">{{cite web |title=Ocean Acidification |url=https://www.whoi.edu/OCB-OA/page.do?pid=112136 |website=Ocean Carbon and Biogeochemistry |publisher=Woods Hole Oceanographic Institute (WHOI)}}</ref> Put differently, 30% of excess carbon emitted into the atmosphere is absorbed by the oceans. Higher concentrations of carbon dioxide in the oceans work to push the carbonate precipitation process in the opposite direction (to the left), producing less CaCO<sub>3</sub>. This process, which harms shell-building organisms, is called [[ocean acidification]].<ref name="NASA_EO">{{cite web |title=Carbon Cycle |url=https://earthobservatory.nasa.gov/features/CarbonCycle |website=Earth Observatory |publisher=NASA|date=2011-06-16 }}</ref>

== The cycle on other planets ==
One should not assume that a carbonate-silicate cycle would appear on all [[terrestrial planet]]s. To begin, the carbonate-silicate cycle requires the presence of a water cycle. It therefore breaks down at the inner edge of the Solar System's [[Circumstellar habitable zone|habitable zone]]. Even if a planet starts out with liquid water on the surface, if it becomes too warm, it will undergo a [[Runaway greenhouse effect|runaway greenhouse]], losing surface water. Without the requisite rainwater, no weathering will occur to produce carbonic acid from gaseous CO<sub>2</sub>. Furthermore, at the outer edge, CO<sub>2</sub> may condense, consequently reducing the [[greenhouse effect]] and reducing the surface temperature. As a result, the atmosphere would collapse into polar caps.<ref name=SullivanBaross2007 />

[[Mars]] is such a planet. Located at the edge of the solar system's habitable zone, its surface is too cold for liquid water to form without a greenhouse effect. With its thin atmosphere, Mars' mean surface temperature is {{convert|210|K|C|abbr=on}}. In attempting to explain topographical features resembling fluvial channels, despite seemingly insufficient incoming solar radiation, some have suggested that a cycle similar to Earth's carbonate-silicate cycle could have existed – similar to a retreat from [[Snowball Earth]] periods.<ref>{{cite journal|last1=Batalha|first1=Natasha E.|last2=Kopparapu|first2=Ravi Kumar|last3=Haqq-Misra|first3=Jacob|last4=Kasting|first4=James F. |date=2016|title=Climate cycling on early Mars caused by the carbonate-silicate cycle |journal=Earth and Planetary Science Letters |volume=455 |pages=7–13 |doi=10.1016/j.epsl.2016.08.044 |arxiv=1609.00602|bibcode=2016E&PSL.455....7B|s2cid=119257332}}</ref> It has been shown using modeling studies that gaseous CO<sub>2</sub> and H<sub>2</sub>O acting as greenhouse gases could not have kept Mars warm during its early history when the Sun was fainter because CO<sub>2</sub> would condense out into clouds.<ref>{{cite journal |last1=Kasting |first1=J. F. |title=CO<sub>2</sub> condensation and the climate of early Mars |journal=Icarus |date=1991 |volume=94 |issue=1 |pages=1–13 |doi=10.1016/0019-1035(91)90137-I|pmid=11538088 |bibcode=1991Icar...94....1K }}</ref> Even though CO<sub>2</sub> clouds do not reflect in the same way that water clouds do on Earth,<ref name="Forget1997">{{cite journal |last1=Forget |first1=François |last2=Pierrehumbert |first2=Raymond T. |title=Warming Early Mars with Carbon Dioxide Clouds that Scatter Infrared Radiation |journal=Science |date=1997 |volume=278|issue=5341 |pages=1273–1276 |bibcode=1997Sci...278.1273F |doi=10.1126/science.278.5341.1273 |pmid=9360920 }}</ref> it could not have had much of a carbonate-silicate cycle in the past.

By contrast, [[Venus]] is located at the inner edge of the habitable zone and has a mean surface temperature of {{convert|737|K|C|abbr=on}}. After losing its water by [[photodissociation]] and [[Atmospheric escape#Jeans escape|hydrogen escape]], Venus stopped removing carbon dioxide from its atmosphere, and began instead to build it up, and experience a [[runaway greenhouse effect]].

On tidally locked [[exoplanet]]s, the location of the [[substellar point]] will dictate the release of [[carbon dioxide]] from the [[lithosphere]].<ref>{{Cite journal|last1=Edson|first1=Adam R.|last2=Kasting|first2=James F. |last3=Pollard |first3=David |last4=Lee |first4=Sukyoung |author-link4=Sukyoung Lee|last5=Bannon |first5=Peter R. |date=2012 |title=The Carbonate-Silicate Cycle and CO<sub>2</sub>/Climate Feedbacks on Tidally Locked Terrestrial Planets |journal=Astrobiology |volume=12 |issue=6 |pages=562–571 |bibcode=2012AsBio..12..562E |doi=10.1089/ast.2011.0762 |issn=1531-1074 |pmid=22775488}}</ref>

== See also ==
* [[Biological pump]]
* [[Carbon cycle]]
* [[Carbonate compensation depth]]
* [[Reverse weathering]]
* [[Daisyworld]]
* [[Gaia hypothesis]]
* [[Runaway greenhouse effect#Positive feedback and runaway greenhouse effect|Runaway greenhouse effect]]
* [[Ice age#Major ice ages|Major ice ages]]
* [[Snowball earth]]
* [[Lake Nyos]]
* [[Ocean acidification]]
* [[Faint young Sun paradox]]

== References ==
{{Reflist}}

== External links ==
* [https://skepticalscience.com/weathering.html Understanding the long-term carbon-cycle: Weathering of rocks – a vitally important carbon-sink] by John Mason, Skeptical Science

{{Biogeochemical cycle}}

{{DEFAULTSORT:Carbonate-silicate cycle}}
[[Category:Geochemistry]]
[[Category:History of climate variability and change]]
[[Category:Climate forcing]]