The slowing of plate tectonics could have led to the Earth’s ice sheets Science

By Paul VoosenDecember 22, 2020, 11:05

In trenches on the seabed around the world, plates of old oceanic crust fall slowly into the mantle, while fresh plates are built on midocean ridges, where magma appears at the seams between the separate tectonic plates. The engine is tireless – but perhaps not so steady: since about 15 million years ago, at the end of the Miocene, the production of oceanic crusts fell by a third over 10 million years, at a slow pace, which continues almost today, says Colleen Dalton, a geophysicist at Brown University who presented the paper this month at a virtual meeting of the American Geophysical Union. “It’s a global phenomenon.”

Although previous recordings of the spread of the ocean have shown signs of a slowdown, nothing has suggested such a sharp decline, says Clint Conrad, a non-work-related mantle dynamist at the University of Oslo. The delay was also widespread: Dalton found that crust production slowed or remained constant at 15 of the Earth’s 16 oceanic ridges. And its effect on the climate could have been strong, says Conrad. “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 in Antarctica after a long hiatus.

The spread of the seabed is captured in the magnetic zones on the ocean floor. Every million years or so, the Earth’s magnetic field overturns and this reversal is frozen in forged rocks at midocean ridges. Naval observations of the alternating magnetic “stripes” that result from oceanic crustal plates unfolding from seabed scattering centers helped to give credence to the theory of plate tectonics in the 1960s.

The ridges of the Atlantic and Indian Oceans are spreading slowly, however, which means that ships have managed to map these stripes with a temporal resolution of only about 10 million years. But geophysicists Charles DeMets of the University of Wisconsin, Madison and Sergei Merkuryev of St. Petersburg State University used previously unused data from Russian naval vessels, which – like those of other nations – tow magnetometers to aid hunting. enemy submarines. New data have improved the resolution in these ocean basins to 1 million years. “And there seem to be surprising signs hiding in many places we didn’t know about,” says DeMets, who has identified some of the slowdown in his recordings.

Dalton and her colleagues added to the image by assembling a complementary high-resolution record for the Pacific Ocean, where the spread of the seabed is faster and more complex. With this global vision, the weakening became immediately apparent. The deceleration appears to have come in two waves, says DeMets: 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 the subducter plates have stopped firing as hard on the seabed as they move during this time, Dalton speculates, because they have become thinner or less dense. Or maybe the subduction zones, usually as long as the midocean ridges, have decreased in length, reducing their attraction. Another possibility is that the areas have changed their orientation, causing the subducer plates to encounter more resistance as they sink into the mantle, which has a kind of natural grain, such as wood. Or a plate could have broken completely, changing the heat flow inside the mantle and altering the sliding of the tectonic plates above the head, says Conrad. “Even if you change a board, it affects all boards.”

By administering volcanic CO₂ emissions related to today’s oceanic crust production and adjusting them for late Miocene speed, the team found a decrease in atmospheric CO₂ which could plausibly explain the global cooling of that time. But Dalton says other explanations are possible – for example, ancient volcanic rocks, raised from the ocean to form fresh mountain peaks in places like Indonesia, could have begun to soak up more CO₂. Both mechanisms probably explain part of the decline, says Nicholas Swanson-Hysell, a paleogeographer at the University of California, Berkeley. “But which is more important?”

Beyond decreasing CO₂, the slowing of the crust would have reshaped the surface of the Earth. With less volcanism on the seabed, the mid-ocean ridges would have been smaller, increasing the capacity of the oceans. Sea levels would have dropped by 22 meters, Dalton calculates, exposing new tracts of land. And as the volcanoes calmed down, the planet itself would have grown 5% less efficient to lose its internal heat, losing about 1.5 terabytes of production – about equal to the production of 1,500 nuclear power plants. This decrease in heat flux would not have made much of a difference in terms of atmospheric temperatures, but Dalton says it calls into question reconstructions of the Earth’s cooling history that involve constant heat loss over the centuries.

Although there is much to eliminate, it is clear that when viewed over a relatively short period of geological time, there is nothing constant about plate tectonics, says Karin Sigloch, a geophysicist at Oxford University. “Variation should always be expected.” The plates break, the monstrous panels of magma on the seabed suddenly burst – all with huge climatic repercussions for the thin biosphere that clings to life on the surface. However, they are just belching in a planetary engine that shatters into a deep and hidden underworld.