In seabed trenches around the world, plates of old oceanic crust slowly fall into the mantle, while fresh plates form on the reefs of the middle sea, where magma erupts at the seams between the separating tectonic plates. The engine is relentless – but perhaps not so stable: starting about 15 million years ago, in the late Miocene, ocean crust production declined by a third over 10 million years at a slow pace that continues to this day, says Colleen Dalton, a geophysicist at the University. Brown, who presented the paper this month at a virtual meeting of the American Geophysical Union. “It’s a global phenomenon.”
Although previous records of ocean expansion have predicted a slowdown, nothing has suggested such a sharp decline, says Clint Conrad, a dynamic mantle from the University of Oslo that is unrelated to work. The lag was also widespread: Dalton found that crust production slowed or remained stable on 15 of the Earth’s 16 oceanic reefs. And its effect on the climate could have been strong, Conrad says. “If you dramatically slow down plate tectonics in such a short time, you can emit much less carbon dioxide (CO2) gas from volcanism. “The slowdown corresponds to a 10 ° C drop in temperature in the late Miocene, when ice sheets began to rise across Antarctica after a long hiatus.
The expansion of the seabed is affected by magnetic zones on the ocean floor. Every million years or so, the Earth’s magnetic field reverses, and this reversal is frozen in rocks forged on the mid-ocean reefs. Ship observations of alternating magnetic “stripes” that occur when oceanic crust plates develop from seabed expansion centers helped give credibility to plate theory tectonics in the 1960s.
The reefs in the Atlantic and Indian Oceans are slowly expanding, however, which means that ships were able to map these lines with a time resolution of only about 10 million years. But geophysicists Charles DeMets of the University of Wisconsin at Madison and Sergey Merkuryev of St. Petersburg State University drew on previously unused data from Russian naval ships, which – like data from other countries – pull magnetometers to help hunt down enemy submarines. New data has sharpened the resolution in these ocean basins to a million years. “It turned out that in many places there were surprising signals lurking that we didn’t know about,” says DeMets, who identified part of the slowdown in his records.
Dalton and her colleagues added a picture by compiling a high-resolution complementary record for the Pacific Ocean, where seabed expansion is faster and more complex. With that global view, relaxation immediately became apparent. The slowdown appears to have come in two waves, DeMets says: first 12 to 13 million years ago in the Pacific, and then 7 million years ago in the Atlantic and Indian Oceans.
Perhaps the drainage plates at that time had stopped pulling such a strong seabed in motion, Dalton suggests, because they had become thinner or less dense. Or perhaps the subduction zones, typical as long as the medieval ridges, have decreased in length, reducing their stroke. Another possibility is that the zones have changed orientation, which is why underwater slabs have encountered greater resistance as they dive into a mantle that has some kind of natural grain, like wood. Or the plate could have broken completely, changing the heat flow inside the mantle and changing the slippage of the tectonic plates overhead, Conrad says. “Even if you change one tile, it affects all the tiles.”
By taking volcanic CO2 emissions related to today’s oceanic crust production and adapting them to Late Miocene velocities, the team discovered a decline in atmospheric CO2 which could probably explain the global cooling at the time. But Dalton says other explanations are possible – for example, ancient volcanic rocks, raised from the ocean to create fresh mountain peaks in places like Indonesia, could have started to absorb more CO2. Both mechanisms probably explain the decline, says Nicholas Swanson-Hysell, a paleogeographer at the University of California, Berkeley. “But what’s more important?”
In addition to lowering CO2, a slowing of the crust would reshape the Earth’s surface. With less volcanism in the seabed, the Mediterranean reefs would be smaller, increasing the capacity of the oceans. Sea levels would drop by 22 meters, Dalton calculates, exposing huge new areas. And in order for the volcanoes to subside, the planet itself would become 5% less efficient in discharging its internal heat, losing about 1.5 terawatts of power – roughly equal to the production of 1,500 nuclear power plants. That drop in heat flow would not have much difference from atmospheric temperatures, but Dalton says he questions reconstructions of the Earth’s cooling history that assume constant heat losses over the centuries.
While there is much to be surprised about, it is clear that, when viewed in a relatively short geological time span, there is nothing constant in plate tectonics, says Karin Sigloch, a geophysicist at Oxford University. “Variations should always be expected.” The plates are breaking, monstrous plumes of seabed magma suddenly erupting – all with huge climate consequences for the thin biosphere that clings to life on the surface. And yet, these are just burps in a planetary engine that bounces off in the deep and hidden underworld.