In the deep carbon cycle, mantle and lithospheric carbon continually escapes through degassing (volcanoes and mid-ocean ridges) and returns back to mantle via the process of rock weathering, carbonate formation, accumulation and subduction of oceanic plates. It plays the key role in regulating Earth’s climate over very long – million year – timescales. In the words, earth’s climate histories are determined by the balance between CO₂ emitted by volcanic degassing, CO₂ removed by weathering of rocks, and stabilizing feedback loops embedded in the deep carbon cycle.

Deep Carbon Emissions. Atmospheric release from the mantle is straight forward: it is CO₂ released in volcanoes, mid-ocean ridges, and oceanic hotspots. Most visible are the occasional volcanic eruptions, but a more steady seeps comes from at tectonic plates divergence boundaries: as oceanic plates drift apart, mid-ocean ridges form allowing magma to rise and solidify as new oceanic crust thereby releasing deep carbon.

Deep Carbon Capture. Atmospheric carbon’s capture back into mantle is a bit more complicated. This carbon capture starts with rain, where atmospheric CO₂ combines with water to form weak carbonic acid. Upon contact the acid in turn dissolves rock in a process known as chemical weathering, releasing magnesium, potassium, sodium and, in particular calcium ions.¹ Rivers carry these calcium ions into the ocean where they react with dissolved bicarbonate.  The product of that reaction, solid calcium carbonate (CaCO₃), slowly sinks onto the ocean floor, where it over time becomes limestone.

Eventually as a result of continental shift, the oceanic plates collide with continental ones,. The denser oceanic plates are forced beneath the continental plates in process called subduction, and the limestone embedded carbon moves back into the lithosphere and upper mantle.

These subducted carbonates can be further transformed under mantle’s high pressure and temperature via metamorphism and mineralization. Some carbon may even be  transported into the lower mantle via (upwellings from Earth’s core-mantle boundary).  Together this creates a VERY long term (hundred million to billion year) storage of carbon within Earth’s interior, whose total size dwarfs the surface carbon.

From the perspective of atmospheric carbon, deep carbon cycle has two main characteristics: highly stabilising feedback mechanism, but very slow speed.

Stabilizing feedback mechanism. The keys aspect of deep carbon cycle is that weathering acts as a natural feedback mechanism that regulates atmospheric CO₂levels.

Higher CO₂concentrations increase the rate of weathering, which in turn draws down CO₂, stabilizing the climate.

Higher CO₂ concentrations, warmer and wetter climates enhance weathering rates, leading to increased CO₂sequestration. Similarly lower CO₂, with colder and drier climates reduced the capturing process.

Thereby for example  increased tectonic activity and CO₂release initially lead to higher atmospheric carbon, but the newly exposed surfaces along with more active weathering process slowly turn the atmospheric carbon levels back to equilibrium. The deep carbon cycle prevents long-term runaway type climate change, and has kept Earth’s climate habitable for 4 billion years. (Isbell et al. 2012, Kump 2016, Park et al. 2020, Mitchell et al. 2021).

Slow speed. Whilst deep carbon cycle stabilises the climate, it only over very long, million year timeframes. It has been estimated that carbon takes between 100-200 million years to move through the slow carbon cycle. Indeed, the current estimated magnitude of chemical weathering on Earth is approximately 0.3 Gt of removed carbon per year (Bufe  2024). Similarly, total mantle degassing form volcanoes and volcanic regions is estimated at 0.28 to 0.36 Gt of Carbon per year. (Black and Gibson 2019). Thus the current system is roughly in balance, and in magnitude about one thirtieth of anthropogenic emissions.

Therefore, whilst deep carbon cycle is assuring from the perspective that it will correct any run-away climate change over the VERY long term, it cannot offset the current rapid increase in CO2 in any biological scale.

1. Carbonate Weathering: Involves the dissolution of carbonate rocks (e.g., limestone CaCO₃) to form bicarbonate ions: CaCO₃+CO₂+H₂O→Ca²⁺+2HCO₃
   Silicate Weathering: Involves the breakdown of silicate minerals (e.g., calcium silicate CaSiO₃) to form clay minerals and bicarbonate ions: CO₂ + H₂O + CaSiO₃ -> CaCO₃ + SiO₂ + H₂O
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