The extrusion of fresh ocean crust at mid-ocean ridges began to weaken 15 million years ago, perhaps cooling the planet.
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By Paul Voosen
In seafloor trenches around the world, plates of ancient ocean crust fall into the mantle in slow motion, while fresh plates are being built on mid-ocean ridges, where magma emerges in the seams between the separating tectonic plates. The engine is relentless – but perhaps not that stable: From about 15 million years ago, in the late Miocene, ocean crust production fell by a third over 10 million years to a slow rate that continues to this day, says Colleen Dalton, a geophysicist at Brown University who presented the work at a virtual meeting of the American Geophysical Union this month. “It’s a global phenomenon.”
While previous ocean dispersal data suggests a slowdown, nothing suggested such a sharp decline, says Clint Conrad, a mantle dynamist at the University of Oslo who is not affiliated with the work. The slowdown was also widespread: Dalton found that crust production slowed or remained stable on 15 of Earth’s 16 ocean ridges. And its effect on the climate may have been stark, Conrad says. “If you drastically slow down plate tectonics in such a short time, you can reduce the amount of carbon dioxide (CO2) gas from volcanism. “The delay corresponds to a 10 ° C drop in temperature in the late Miocene, when ice sheets began to grow over Antarctica after a long hiatus.
Seafloor spreading is captured in magnetic zones on the ocean floor. About every million years, Earth’s magnetic field changes, and this reversal is frozen in the rocks forged on the mid-ocean ridges. From the ship, observations of the alternating magnetic “streaks” that form as ocean floor plates unfold from the seafloor distribution centers have added credibility to the theory of plate tectonics in the 1960s.
However, the ridges in the Atlantic and Indian oceans are slowly spreading, which means that ships have been able to map these streaks at a temporal resolution of only about 10 million years. But geophysicists Charles DeMets of the University of Wisconsin, Madison, and Sergey Merkuryev of Saint Petersburg State University draw on previously unused data from Russian naval ships, which – like those from other countries – tow magnetometers to aid the hunt for enemy submarines. . The new data tightens resolution in these ocean basins to 1 million years. “And it turns out that there are surprising signs in many places that we were not aware of,” said DeMets, who identified some of the delay in his administration.
Dalton and her colleagues added to the picture by compiling a complementary high-resolution record for the Pacific Ocean, where seafloor spread is faster and more complex. With that global vision, the slackening became immediately apparent. It seems the slowdown came in two waves, DeMets says: first between 12 million and 13 million years ago in the Pacific and then 7 million years ago in the Atlantic and Indian Oceans.
Perhaps during this time the sinking plates stopped pulling the moving seafloor so hard, Dalton speculates, as they thinned or became less dense. Or perhaps the subduction zones, typically as long as the mid-ocean ridges, have shrunk in length, reducing their pull. Another possibility is that the zones changed orientation, causing the sinking plates to experience more resistance as they dove into the mantle, which has a kind of natural grain, like wood. Or a plate could have completely broken down, altering the heat flow in the mantle and altering the sliding motion of the tectonic plates overhead, Conrad says. “Even if you change one board, it will affect all boards.”
By taking volcanic CO2 emissions related to current ocean crust production and adjusting them to late Miocene rates, the team found a drop in atmospheric CO2 that could plausibly explain the then global cooling. But Dalton says other explanations are possible – for example, ancient volcanic rocks, lifted from the ocean to form fresh mountain peaks in places like Indonesia, could absorb more CO.2. Both mechanisms likely explain some of the decline, says Nicholas Swanson-Hysell, a paleogeographer at the University of California, Berkeley. “But what’s more important?”
Beyond lowering CO2the slowdown in the Earth’s crust would have reshaped the Earth’s surface. With less volcanism on the seafloor, the mid-ocean ridges would have been smaller, increasing the capacity of the oceans. The sea level would have dropped by 22 meters, Dalton calculates, exposing huge new areas of land. And as the volcanoes went still, the planet itself would have become 5% less efficient at releasing its internal heat, losing about 1.5 terawatts of output – roughly equivalent to the production of 1,500 nuclear power plants. That decrease in heat flow wouldn’t have made much of a difference to atmospheric temperatures, but Dalton says it questions reconstructions of Earth’s cooling history that assume constant heat loss over the centuries.
While there is much to be teased, it is clear that given relatively short geologic time frames, there is nothing constant about plate tectonics, says Karin Sigloch, a geophysicist at the University of Oxford. “Variation should always be expected.” Plates break, monster plumes of magma on the seafloor suddenly erupt – all with enormous climatic consequences for the thin biosphere that clings to the surface. Yet they are just farmers in a planetary engine rummaging away in a deep and hidden underworld.