New Seafloor Sponges Up Carbon to Stabilize the Climate

Several worldwide phenomena related to land, sea, and sky occur in 26 to 30 million-year cycles. Such phenomena include ocean anoxic events where parts of the ocean run out of oxygen, the deposition of evaporates (chemical precipitates that accumulate as layers through evaporation of marine waters), fluctuations in atmospheric carbon dioxide levels, and even the growth of mountain ranges. Some scientists have suggested “extraterrestrial” causes for these cycles, such as cosmic showers periodically pelting the planet as it moves through the plane of the Milky Way, but no one could provide clear evidence of what causes these cycles.

In a new paper in the journal Science Advances [1], DCO members Dietmar Müller and Adriana Dutkiewicz (both at the University of Sydney, Australia) describe a process that ties together these multi-million-year cycles, the spreading of new seafloor and its ability to store and release carbon. After new ocean crust emerges from the diverging boundaries of oceanic plates, called mid-ocean ridge spreading centers, the crust takes in varying amounts of carbon dioxide depending on ocean water temperature. The researchers used their model of how tectonic plates have moved over the last 230 million years to understand how much carbon the ocean crust has soaked up over this timeframe, how carbon storage has fluctuated, and how the recycling of the seafloor relates to 26 to 30 million-year cycles in atmospheric carbon dioxide concentrations. Their findings indicate that this process is an important mechanism linking plate tectonics with atmospheric carbon dioxide, helping to maintain a stable climate.

Müller and Dutkiewicz did not start out looking for the cause of these mysterious 26 to 30 million-year cycles. “It came as a surprise,” said Müller. “Once you put all these different data sets together, there seems to be this giant cycling that indicates that there’s a connection between plate tectonic processes, surface environments, and the atmosphere.”

Initially, Müller and Dutkiewicz wanted to know how much carbon the ocean crust has stored over time. The carbon content of seafloor ranges between close to zero and 3%, but considering the size of the ocean, the ocean crust represents a huge reservoir with the potential to change significantly over Earth’s history. Recent research has shown that the bulk of oceanic crustal carbon dioxide uptake, called seafloor weathering, occurs when crust is “young” meaning it is less than about 20 million years old. Also, warmer seafloor temperatures, up to 25 degrees higher during past hothouse climates, cause the crust to take in more carbon.

Starting with GPlates, a software tool to reconstruct the movement of tectonic plates through Earth’s history developed by Müller’s EarthByte group at the University of Sydney including international collaborators, particularly Mike Gurnis (California Institute of Technology, USA), the researchers added information regarding the age of different areas of crust, and ocean temperatures over the last 230 million years. This enabled them to estimate how much carbon each new section of crust could absorb.

The researchers also took into account the amount of carbon dioxide emitted from plate boundaries (where new seafloor forms) and the total length of these seams on the seafloor, which gradually doubled after the break up of Pangaea.

Ultimately, each parcel of ocean crust ages, migrates, and becomes recycled through subduction, when it sinks into the mantle. This process releases some carbon dioxide back into the atmosphere through nearby volcanoes. Scientists have estimated that between ~35 to 65% of crustal carbon comes back up to the atmosphere through volcanoes and diffuse degassing along subduction zones. Müller and Dutkiewicz find that their model agrees better with reconstructed atmospheric carbon dioxide fluctuations when assuming that only 35% of the subducting carbon re-enters the atmosphere along volcanic “rings of fire,” said Müller, “but of course, the jury is still out.”

By combining all this information with the timing of the reconstructed speed of the lateral migration of deep sea trenches, they find that subduction zone migration creates ~26 million-year tectonic cycles, driving cyclic changes in seafloor spreading rates and thus the capacity of the seafloor to store carbon dioxide in young ocean floor, linking the deep carbon cycle to observed cycles in atmospheric carbon.

Much like continental weathering, the ocean crust offers another feedback mechanism for soaking up extra carbon dioxide from the atmosphere when temperatures get a little too hot. “As Earth heats up, the ocean’s bottom water also eventually becomes warmer, which will enhance the capacity of the ocean floor to store carbon dioxide. This is another mechanism to prevent runaway greenhouse climate development on Earth,” said Müller.

A better understanding of seafloor weathering also may help us to understand the extent to which the seafloor can mitigate excess carbon dioxide generated by humans, and about how many million years that process might take.

 

The ocean crust can be a source or sink of carbon depending on the age of the crust and the ocean water temperature. Credit: Adriana Dutkiewicz/Science Advances
 
Modeled oceanic upper crustal carbon dioxide content through time from 230 million years ago to the present. Credit: Dietmar Müller/EarthByte
 
Bottom water temperature at the time of oceanic crust formation from 230 million years ago to present time. Credit: Dietmar Müller/EarthByte

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