A Ridge’s Race to Capture CO2

Coggon, R.M., Carter, E.J., Grant, L.J.C. et al. A geological carbon cycle sink hosted by ocean crust talus breccias. Nat. Geosci. 18, 1279–1286 (2025). https://doi.org/10.1038/s41561-025-01839-5

Volcanic CO₂ exchange

They say slow and steady wins the race, and while that may not be true for sprinters, it could be true for mid-ocean ridges and the fight against climate change. As volcanoes erupt, carbon dioxide (CO₂) leaves the magma in a process called degassing. This is how large amounts of CO₂ stored inside the planet are put into the ocean and atmosphere. As temperatures rise and climate changes on Earth, it is increasingly important to understand how much CO₂ is released from inside the planet, as well as the processes that may keep it trapped and out of the warming atmosphere. This, however, is challenging because estimates of how much CO₂ is degassed from volcanoes and how much CO₂ gets trapped each year vary. Coggon and her team investigate the possibility that chemical reactions that produce calcium carbonate (CaCO₃) in oceanic crust may help to trap CO₂ in the ocean and balance what is pumped out through volcanism at mid-ocean ridges.

Mid-ocean ridge volcanism

Mid-ocean ridges form at the boundary where two tectonic plates move away from one another on the seafloor (Fig. 1A). As the two plates pull apart, there is a decrease in pressure on the material inside the Earth (a.k.a. the mantle) that is beneath the boundary. Mantle material wants to fill in the space that is left behind as the plates move away from one another. It rises, melts, and erupts as lava at the bottom of the ocean (Fig. 1B). The lava cools to form a new section of oceanic crust, or seafloor.

Mid-ocean ridges are in every ocean basin around the globe but can have different characteristics. For example, the Mid-Atlantic Ridge (labeled in Fig. 1) spreads more slowly than others and this can lead to a distinct ridge shape. This shape is a valley, or low point, surrounded by faults on both sides (Fig. 2). The movement and angle of the faults can make them prone to mass wasting events which are like rockslides. This can cause a buildup of talus (pieces of broken up lava) around these faults (Fig. 2).

South Atlantic cores

The scientists examined cores, or long tubes of sediment and rock, from the seafloor collected during International Ocean Discovery Program South Atlantic Transect Expedition 390 west of the Mid-Atlantic Ridge (Fig. 3A; red box on world map in top left). Geophysical imaging revealed faults in the oceanic crust (sawtooth pattern in Fig. 3C), which suggests there may be talus buildup.

Closer examination of core U1557 indeed revealed thick layers of talus breccia. Breccia is a type of rock that is formed when large, angular pieces of rock are cemented together by some other material. In this case, Coggon and her team analyzed the material cementing the talus together and found that it was CaCO₃ (Fig. 4). The formation of CaCO₃ traps CO₂ and is common when seawater and other fluids circulate through oceanic crust. All the space left between the talus pieces (up to 42 volume % of the cores) provided the perfect setting for this reaction and the resulting buildup of CaCO₃.

A new carbon cycle sink

Further analysis of the CaCO₃ cement showed extremely high CO₂ contents between 4.9 and 14.1 weight percent (wt%), which is up to 40 times that of what had previously been measured in oceanic crust (Fig. 5). Thus, the group produced a model to see if the CO₂ trapped in the CaCO₃ could balance the CO₂ degassing during volcanism. Their calculations suggest that the chemical reaction occurring in between talus pieces could, in fact, balance the CO₂ degassing at slow-spreading mid-ocean ridges if the thickness of the breccia layer was 2.5 to 50% that of the oceanic crust beneath it.

This study has implications for influencing atmospheric CO₂ concentrations over long timescales (millions of years). As the oceanic crust gets older, it sequesters more CO₂ dissolved in seawater, which decreases the amount of CO₂ that can be exchanged with the atmosphere. Ultimately, this may help to balance a warming climate, but only if mid-ocean ridges remain slow and steady.

Cover image modified from Wikimedia by author.   

Rebecca Lippitt

I am a Ph.D. Candidate in Geological Oceanography at the University of Rhode Island, Graduate School of Oceanography. I received my B.S. in Geology from Union College (NY). I study submarine volcanoes! I use the chemical composition of lava to figure out what is happening inside the Earth and how magma is formed. When I’m not working with rocks, I enjoy reading on the beach, cooking, and hiking.