The mantle transition zone — the region between 410 and 660 kilometers depth — contains mineral phases that can incorporate water into their crystal structures at concentrations far exceeding the upper and lower mantle. Wadsleyite and ringwoodite, the high-pressure polymorphs of olivine that define this zone, can hold up to 2-3 weight percent water as hydroxyl defects. If the transition zone were saturated, it would contain several times the mass of Earth's surface oceans. Seismic observations suggest it contains some water. The question is whether it acts as a permanent reservoir — a deep ocean that filled early in Earth's history and stayed full — or something else.
Gerya, Bardi, Karato, and Murakami (arXiv 2602.13820, February 2026) model the transition zone as a transient buffer. Water enters from above through subducting slabs and from below through upwelling mantle. Once incorporated into the transition zone minerals, the water reduces the density of the host rock. This buoyancy difference drives convective instabilities: hydrous regions become lighter than their surroundings and rise. When the rising material crosses the 410-kilometer phase boundary, the transition from wadsleyite to olivine releases the stored water (olivine holds far less), triggering partial melting. The water is expelled upward, and the now-dry residue sinks back.
The cycle operates on timescales of 80 to 430 million years, depending on the water concentration and the vigor of convection. The transition zone fills, becomes buoyant, overturns, and empties — then refills from the next pulse of subduction. The equilibrium water content is modest: less than 0.1 weight percent, far below the theoretical capacity. The zone can absorb more, but doesn't hold it long enough for the concentration to build up.
This makes the transition zone a regulator rather than a reservoir. It buffers the water cycle between the surface and the deep mantle, absorbing surges from subduction and releasing them through buoyancy-driven overturn. The surface ocean mass stays relatively stable not because the mantle doesn't exchange water with it, but because the exchange is self-correcting: more water in the transition zone means more buoyancy, means faster overturn, means faster return to the surface. The system has negative feedback built into its mineralogy.
The mechanism depends on a physical asymmetry: the minerals that can hold water are only stable in a narrow pressure range. Water that enters the transition zone from above is captured by wadsleyite. Water that would enter from below is released at the 660-kilometer phase boundary where ringwoodite transforms to bridgmanite and ferropericlase, neither of which stores water efficiently. The transition zone is a sponge bounded on both sides by phases that squeeze it out. The sponge absorbs, swells, rises, and wrings itself dry at the upper boundary.
The planetary implication extends to any rocky world with a similar mineralogical structure. The transition zone minerals are pressure-stabilized polymorphs of magnesium silicate — they should exist in any terrestrial planet above a threshold mass. If the buffering mechanism is generic, then the surface ocean mass of habitable-zone planets may be self-regulating: not set by the initial water inventory of the planet but maintained by the mantle's capacity to absorb and return water on hundred-million-year timescales.