Half of Carbon Transported to Mid-Ocean Ridges Remains Trapped Below

Much of the volcanic activity happening on Earth occurs where we can’t see it, in a network of mid-ocean ridges running along the bottom of the world’s oceans. As tectonic forces pull oceanic plates apart, they pull hot rocks up from the mantle. Some of these rocks melt in the process, producing magma that flows upward to fill the rift between the spreading plates with newly formed oceanic crust.

Researchers thought that this process efficiently moves deep carbon, carried by the magma, and releases it into the ocean. But a new simulation study finds that up to half of the carbon initially removed from mantle rocks remains trapped below the oceanic plates.

Tobias Keller (Stanford University, USA), Richard Katz (University of Oxford, UK), and Marc Hirschmann (University of Minnesota, USA), members of the DCO Reservoirs and Fluxes and Modeling and Visualization communities, simulated the global carbon output from mid-ocean ridges using a recently developed model of carbon transport in the mantle. While their simulations match existing estimates of global carbon release, they find that the carbon that doesn’t erupt crystallizes along with deep water that behaves similarly, in a layer at the base of the lithosphere, the plate-like part of Earth’s uppermost mantle. They publish their results in a recent paper in Earth and Planetary Science Letters [1].

Mid-ocean ridges make up an almost continuous underwater mountain range running about 65,000 kilometers worldwide. The ridges occur along the boundaries of spreading tectonic plates, which allow magma to rise up and form new seafloor as it cools. In the process, carbon dioxide degasses into the surrounding seawater.

Keller and Katz’s model is the first to use detailed physical processes to make predictions about carbon transport to the seafloor. The computer simulations take into account the melting of the mantle, the dissolution of deep carbon and water into the magma, and their transport toward the surface.

After running about 30 simulations at different conditions found around the globe, taking into account mantle temperature and composition, ridge spreading rate, and crust thickness, the researchers used their results to extrapolate the total output of carbon from mid-ocean ridges worldwide. The model’s estimates of carbon output from the deep align with previous estimates based on the chemistry of rocks at the seafloor.

“This process releases about the same amount of carbon dioxide into the surface environment as all land-based volcanoes taken together,” said Keller, however, “mid-ocean ridges still only release roughly 1,000 times less carbon than what human activity is currently doing to the planet.”

The researchers were surprised to find that, contrary to what previous models had assumed, not all of the carbon dissolved from the mantle by melting makes it to the surface. Up to half of that carbon and water gets stuck along the sides of the ridges where it freezes into the base of the oceanic plate above.

This finding may provide an explanation for a peculiar observation from seismic studies. When seismic waves travel through the Earth after an earthquake, seismologists observe unexplained vibrations bouncing back from the base of the lithosphere. Traditionally, scientists thought that boundary was too smooth to reflect waves.

“A sharp boundary that sends seismic reflections back to the surface is found at consistent depths throughout all the oceanic plates,” said Keller. “So far there have been suggestions of a hydrated or carbonated layer there to explain this observation, but no one has been able to show exactly how the volatiles would end up along this layer.”

The researchers are now working with seismologists who will combine their model with seismic calculations to see if their predictions of this layer match up with what they observe in the natural world.

“There is a lot of detail in the physics of these processes that have not been very well studied before,” said Keller. Models like this one can confirm and expand our existing understanding of the processes occurring deep in the subsurface.

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