Deep carbon cycle

Carbon in the Lower Mantle

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. 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. 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.

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. 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. 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. 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. 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. 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.

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. Carbon is then 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.

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. 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." 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. 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.

Numerous questions regarding the movement of carbon within the earth remain unanswered, especially those related to carbon-bearing phases and minerals' geochemical properties

-concentration, bonding properties, and mineralogy of carbon in Earth’s core

-genesis and nature of some deep hydrocarbons

-Fractionation of siderophile elements between metallic core and silicate mantle during core formation

This fractionation contributed to the bulk distribution of carbon and other terrestrial volatiles

-Recent experiments have indicated Dcarbon metal/silicate measurements of ~5500 - >150 at shallow magma ocean conditions. The experiment also displays that carbon has high metal affinity at high pressures and low affinity at high temperatures, silicate melt depolymerization, hydration ,and oxygen fugacity

Marty (2012) thinks the core does not store any carbon, whereas Dasgupta and Walker's calculations show that the core contains about 4.8 ± 2.9 * 10 ^24 grams of carbon, suggesting that the core contributes up to 803 ppm of carbon to the planet's carbon cycling

Earth's interior

nature and extent of the deep microbial biosphere (earth's interior could very well be the oldest ecosystem on the planet)

-look up regional and retrograde metamorphism

Carbon Inheritanceinto the Mantle and Core

Figure 1: Hadean (Young Earth) to Phanerozoic (Matured Earth)

Figure 2: Bulk Earth Carbon during core formation (ppm)

Figure 3: Schematic cross sections illustrating the range of deep Earth processes from magma ocean stage of the Hadean Eon to the plate tectonic framework of the modern world

-Dcarbon metal/silicate = (mass fraction of C in metal melt / mass fraction of carbon in silicate melt) — studies investigating it are scarce, so we do not know

-If it is less than 1, the outer silicate layer would contain most carbon

-If it much greater than 1, the core would possess large carbon deposits, limiting the impact on the long-term carbon cycle

Common assumption is that "equilibrium partitioning of carbon between metallic and silicate melt in a magma ocean" leads to the carbon concentration in mantle (30-110 ppm, we know this because of mantle-derived magma samples) , also that magma ocean conditions can only store about 6-7 wt% dissolved C

Retention of Mantle Carbon

Carbon Outgassing

Works Cited

http://www.minsocam.org/msa/rim/RiMG075/RiMG075_Ch01.pdf

Terrestrial Carbon through Geologic Time, from Carbon in Earth book

https://eps.harvard.edu/files/eps/files/dasgupta_2013.pdf

Potential Ideas

I'd definitely consider attempting to improve the article mentioned below, specifically focusing on writing more about the elements of deep earth carbon degassing that the project investigates. For instance, I could write details about the database of volcanic and hydrothermal gas compositions and fluxes that play a role in the investigation. Furthermore, I could add a short section about each of the volcanoes mentioned in the article and discuss how their carbon fluxes might be similar or different according to their structures and roles in deep carbon cycling. I could also see the new field and analytical instrumentation for carbon measurements that the project is attempting to develop and possibly include the progress of these innovations in a separate section in the article, being sure to describe how they impact geophysics overall and what specific insight they might provide. One last possible addition is adding more information to the description of tectonic degassing as a method of carbon movement from the deep earth to the surface, as well as how volatiles move in volcanoes and how the project measures such movement.

Article Evaluation: Deep Earth Carbon Degassing Project

• What kinds of conversations, if any, are going on behind the scenes about how to represent this topic?
  • There don't seem to be any conversations behind the scenes

• Check a few citations. Do the links work? Does the source support the claims in the article?
  • Yes, the links work, and the sources tend to apply to the information mentioned
• Is each fact referenced with an appropriate, reliable reference? Where does the information come from? Are these neutral sources? If biased, is that bias noted?
  • The information mainly comes from the project's website itself, which definitely presents a bias. It is noteworthy that this bias is not referenced; however, it is understandable given that the article mainly sites said website when it describes the project's goals and accomplishments, something that the project itself is evidently going to be a place to look. Most of the scientific material referenced comes from scholarly articles related to carbon fluxes, volcanoes, and subduction.

• Is the article neutral? Are there any claims that appear heavily biased toward a particular position?
  • Yes, the article seems quite neutral and doesn't attempt to manifest a particular opinion of the project and its aims
• Is everything in the article relevant to the article topic? Is there anything that distracted you?
  • Everything was quite relevant, and there was no distracting information
• Is any information out of date? Is anything missing that could be added?
  • The information in terms of scientific research conducted and achievements of the organisation is dated to 2016, so I wonder if any new breakthroughs or significant findings/projects have occurred since then
• What else could be improved?
  • They have a list of the volcano installations the group has, but I wish they'd added more about each site and why they chose to install equipment in those specific volcanoes. I also wish they went more into depth regarding the five main component's of the group's goal.Is the article neutral? Are there any claims that appear heavily biased toward a particular position