Events / Year of Carbon

Defining Earth in five reactions

At the Geological Society’s Janet Watson Meeting in February 2019, Dr Simon Redfern from the Deep Carbon Observatory explained their recent work to define the five key reactions that control carbon in the Earth.

Since 2015 the Deep Carbon Observatory has been aiming to define and identify the five key chemical reactions that govern how carbon moves and changes in the Earth.

These form a framework under which we can develop our understanding of the carbon system, from the biosphere to the inner core of our planet.

Carbon is a fundamental part of Earth’s chemical system. It is essential for life, important for fuel, key to the climate, and ubiquitous. However, relatively little is known about the behaviour of carbon deeper inside the Earth.

Scientists from the Deep Carbon Observatory aim to share the forefront of deep carbon research by communicating their findings under the headings of five key chemical reactions that define how carbon is stored, released, transformed and moved around the Earth. These five key reactions are illustrated below in grey.


DCO E5R.png

Image: The Earth in five key reactions (and three runners up). Source: Deep Carbon Observatory, E5R.


  1. Hydrogenation / Dehydrogenation – CH4 + 2H2O ⇌ CO2 + 4H2

This reaction determines the formation of methane by microbes, such as in rocks containing organic matter and methanogens (methane producing microorganisms), or in the digestive systems of animals. This reaction is thought to be importantly linked to the emergence of life on Earth through the role of methanogens in the regulation of atmospheric greenhouse gases. Without balanced amounts of greenhouse gases, the atmosphere would not be habitable.

  1. Carboxylation / Decarboxylation – 6CO2 + 6H2O ⇌ C6H12O6 + 6O2

Central to photosynthesis, carboxylation fixes (or stores) carbon from carbon dioxide, producing water and sugar as by products. This reaction is key to the sustenance of all carbon-based life.

  1. Carbonisation / Decarbonisation – CO2 + CaSiO3 ⇌ CaCO3 + SiO2

This reaction removes or fixes CO2 from the atmosphere by weathering of silicate rocks into carbonate rocks. It is this reaction that governs the rate at which the Earth can naturally regulate CO2 in the atmosphere.

  1. CO2 Dissolution / Outgassing – CO2(aq) ⇌ CO2(g)

Similar to the release of CO2 from a bottled carbonated drink after the lid has been opened, outgassing releases CO2 from volcanic arcs and mantle plumes into the atmosphere.

  1. Hydration/ Dehydration – H2O + CO2 ⇌ H2CO3

An example of the hydration reaction is ocean acidification or the formation of acid rain. Carbon dioxides dissolves into sea or rain water causing it to turn more acidic. This degrades building stones and the shells of marine life which are susceptible to acidic erosion. Serpentinization is another important example of hydration whereby the crystal structure within a rock is changed by the addition of water into the structure.

Deeper carbon reactions such as (a) Redox Freezing, (b) Metal-Silicate Reactions or (c) Metal Freezing were close runners up to the top five carbon reactions, losing out mainly because of the much deeper settings at which they commonly occur. Perhaps their place in the overall standings will change as deep carbon research develops into the future, and their relevance becomes more widely communicated.

  • To find out more about the work of the Deep Carbon Observatory or the ‘Earth in 5 Reactions’ concept you can visit their webpage, follow them on twitter @deepcarb, or check out the Deep Carbon Observatory YouTube.
  • The Janet Watson Meeting took place at the Geological Society from 26th – 28th February 2019. More information about this event can be found here. Follow the event on twitter at #JWatson19.
  • Read more about this year’s Janet Watson meeting on the Deep Carbon Observatory website.

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