The Earth Has A 30-Million-Year Carbon Cycle Driven From The Depths Of The Ocean

Much of the variation in the rate at which carbon is drawn into the mantle and then released by volcanoes is driven by cycles of how fast new crust is created. Adriana Dutkiewicz

Before humans sent atmospheric concentrations of carbon dioxide soaring in the blink of an eye, geologically speaking, these levels had fluctuated over much longer timespans. New University of Sydney research has found a pattern in these cycles and relates it to plate tectonics. The work helps explain why the Earth has remained habitable over such a long period of time.

Only geologists or astronomers could call a cycle lasting 26-32 million years short, but that is exactly how Professor Dietmar Müller and Dr Adriana Dutkiewicz have described the pattern they found in Science Advances.

“Studying the geological record to see how the Earth responds in the long-run to phases of hothouse or icehouse, we see that none of these become a runaway,” Müller told IFLScience. “The temperature always returns to moderation because the Earth has mechanisms that safeguard the climate. One of these is weathering of rocks, which increases when carbon dioxide makes rain more acidic, releasing silicates and carbonates into the oceans, but the less well-known one is carbonate precipitation in the deep ocean.”

Müller explained to IFLScience that carbon in seawater precipitates on the ocean floor. As this happens the oceans get out of balance with the atmosphere and absorb more CO2 from the air to compensate. The speed at which carbonates precipitate depends on the temperature of the water at the bottom of the oceans, and the rate at which new seafloor is created. When the new crustal material appears more rapidly, there is a faster drawdown of atmospheric carbon, eventually leading to a cooler planet. Over 26-32 million years, seafloor creation accelerates, slows, and speeds up again.

The good news, Müller said, is we are entering a period where seafloor production is accelerating. Eventually, this will dispose of the carbon dioxide we are pumping into the atmosphere. The bad news is this will take millions of years, by which time the damage will be well and truly done.

Exactly what causes this stop-start pattern in seafloor creation is unknown. Müller said he thinks it may relate to crustal material sinking through the relatively fluid upper mantle and the more rigid lower mantle, but acknowledged this is speculation.

Another uncertainty is whether this cycle has anything to do with other previously noticed patterns of similar length, including upticks in extinctions, volcanism, and deposition of salts. All have been unconvincingly attributed to astronomical effects, such as showers of comets or the Sun's movement through the galactic plane.

The cycle is short compared to a better-known carbon oscillation that runs over periods of hundreds of millions of years, and is believed to be caused by the formation and breakup of supercontinents like Pangaea


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