Researchers affiliated with several institutions in Japan and Germany have found evidence that suggests the middle of Earth’s mantle holds as much water as the planet’s oceans. In the study published in the journal Science Advances, researchers write that the uppermost part of the mantle and lower part closest to the core are relatively “water free” because their dominant minerals, “olivine and bridgmanite,” have limited water storage capacity. However, the layer in between (the mantle transition zone [MTZ] at 410 to 660 kilometers below the surface) could harbor massive amounts of water because it is dominated by the minerals “wadsleyite and ringwoodite,” which are known to be able to hold a lot of water.
The viscosity of the middle zone of the mantle is lower than that of both the upper mantle and lower mantle. Using this information, the team replicated the mantle-like conditions in the lab. They created synthetic ringwoodite to represent the middle mantle and bridgmanite to represent material from the lower mantle. The researchers then measured dislocation mobility to infer viscosity and then added water to the ringwoodite to reduce its viscosity and match measurements taken of the real mantle. Adjusting the amount of water added to their synthetic mantle and calculating changes in viscosity, researchers were able to estimate how waterlogged the real-world minerals are. So by using that information to calculate how much water is in the entire mid-mantle, researchers came to conclusion that “it is very nearly equal to the amount of water in all of the world’s oceans.”