In seafloor trenches around the world, slabs of oceanic crust fall slowly into the mantle, while new slabs are formed at mid ocean ridges, where magma emerges at the seams separating tectonic plates. Beginning about 15 million years ago, ocean crust production declined by a third over 10 million years to a slow pace that continues to today.
The lag was widespread with crust production slowing down or staying steady at 15 of Earth’s 16 ocean ridges. Its effect on climate may have been substantial. Such a dramatic slowdown in plate tectonics in such a short period of time results in the release of a lot less carbon dioxide gas from volcanism. The slowdown corresponds to a 10 degree celsius drop in temperatures in the late Miocene period, when ice sheets began to form across Antarctica after a long break.
By looking at volcanic CO2 emissions tied to today’s ocean crust production and adjusting them for late Miocene speeds, the team found a drop in atmospheric CO2 that could potentially explain the global cooling at the time.
Apart from lowering CO2, the crustal slowdown would have reshaped Earth’s surface. With less seafloor volcanism, the mid ocean ridges would have been smaller which would increase the capacity of the oceans. Sea levels would have fallen by 22 meters, Dalton calculates, exposing vast areas of land.
Voosen, P. (2021). Slowdown in plate tectonics may have led to ice sheets. Science, 371(6524), 14. https://science.sciencemag.org/content/371/6524/14