Water exists under the ocean floor, but it is not an open sea. This subsurface water is integrated into the crust and Earth’s mantle in various forms, making it one of the planet’s largest reservoirs. This buried water is chemically or physically trapped within seafloor sediments and rock, playing a role in regulating Earth’s geology and sustaining unique life forms. It is part of a slow, deep global cycle that influences the entire planet.
Forms of Water Stored in the Ocean Crust
Water is stored beneath the seafloor in three distinct states, reflecting increasing pressure and temperature with depth. The shallowest form is pore water, which is free liquid water trapped within the spaces of marine sediments and fractured crust. This water is essentially seawater that has seeped into the ground, and in thick sediment layers, it can be disconnected from the ocean for vast periods.
Deeper within the crust, particularly along mid-ocean ridges, water circulates as superheated hydrothermal fluids. Cold seawater seeps into cracks, is heated by the underlying magma chamber to temperatures between \(350^{\circ}\text{C}\) and \(400^{\circ}\text{C}\), and chemically reacts with the rock. These fluids become enriched in dissolved metals and reduced compounds.
The deepest and largest reservoir of subsurface water is chemically bound into the crystal structure of rocks, forming hydrated minerals. During serpentinization, mantle rock, rich in olivine, reacts with seawater to form serpentine, which can hold up to 13% of its weight in water. Other hydrous minerals, such as amphiboles and chlorite, also incorporate water into their molecular lattice. As the oceanic crust is pushed deeper, the water transitions to this chemically locked state, persisting deep into the Earth’s interior.
The Scale of the Subsurface Water Reservoir
The total volume of water stored beneath the ocean floor and carried into the deep Earth is immense. While surface oceans contain about \(1.35\) billion cubic kilometers of water, estimations suggest the deep mantle may hold one to three times that volume. This water is not liquid but is stored as hydroxyl ions within the crystal structures of high-pressure minerals.
This reservoir primarily exists in the mantle transition zone, located between \(410\) and \(660\) kilometers beneath the surface. Here, the mineral ringwoodite acts like a planetary sponge, absorbing water into its crystalline lattice under extreme pressure. Seismic evidence, showing waves slowing down in this region, supports the idea that the mantle rock is “wet.” This transition zone is believed to represent the largest water reservoir on Earth, making the planet’s water cycle a whole-Earth process.
Water’s Role in Plate Tectonics
The water carried by the oceanic crust is important for plate tectonics, especially at subduction zones. As one tectonic plate slides beneath another, the water-soaked oceanic crust and its sediments descend into the mantle. Increasing pressure and temperature cause the hydrous minerals within the subducting plate to become unstable and break down.
At depths of around \(100\) kilometers, this metamorphic dewatering process liberates the chemically bound water. The released water, now a supercritical fluid, rises and infiltrates the hot, overlying wedge of mantle rock. This introduction of water changes the physical properties of the mantle, acting as a fluxing agent.
The water lowers the melting temperature of the mantle rock by hundreds of degrees, a process known as flux melting. This melting generates buoyant magma that rises toward the surface. The resulting magmas feed the chains of volcanoes, known as volcanic arcs, such as the “Ring of Fire” that circles the Pacific Ocean. This process demonstrates a direct link between deep-earth water and the formation of surface geology.
Supporting the Deep Subsurface Biosphere
The subsurface water supports a deep biosphere, an ecosystem of microbial life that operates independent of sunlight. This life is sustained by the chemical energy within the circulating fluids and rocks rather than photosynthesis. The presence of hydrothermal fluids creates the necessary chemical gradients and transport mechanisms for this life to thrive kilometers beneath the seafloor.
Microorganisms in the deep biosphere, primarily archaea and bacteria, are chemosynthetic, using reduced chemical compounds as their energy source. Compounds like hydrogen, methane, and hydrogen sulfide, produced by water-rock reactions, serve as electron donors for these microbes. This continuous supply supports life in a dark, high-pressure environment that stretches deep into the crust. In some deep marine sediments, these microbes live for thousands of years with extremely slow metabolic rates, representing a significant portion of the Earth’s total biomass.