Where does the water in the oceans come from? Three key stages

A planetary mystery with multiple origins

An omnipresent molecule in the Universe

Water (H₂O) is one of the abundant molecules after (H₂) and (CO) in the Universe, and its existence on Earth has its origins in the protosolar cloud, the vast reservoir of gas and dust from which the Solar System formed.

Three ages to explain the hydrosphere

Water covers more than 70% of the Earth's surface, but its origins still raise many questions. Has the planet always been hydrated? Is it a heritage of the protosolar cloud, or the result of chance collisions? Today, geochemists and planetary scientists identify three major phases or "ages" in the history of Earth's water. These multiple origins make the hydrosphere an object of study in its own right.

First age: primordial water incorporated into the solar nebula

Hydrated minerals from accretion onward

During the formation of the Earth 4.56 billion years ago, a significant portion of the accreted matter already contained hydrated minerals, notably silicates from carbonaceous chondrites. These water-rich meteorites in the form of phyllosilicates contain up to 10% of their mass in H\(_2\)O.
The inner Earth was thus able to trap water in its mantle from accretion onward, with sufficient pressure to stabilize this water in bound form. It is estimated that the mantle could still today contain up to 2 to 10 times the mass of the current oceans.

N.B.:
Phyllosilicates are minerals that have a sheet-like structure in which water and hydroxyl ions \((OH^-)\) are integrated between atomic layers. Their presence in carbonaceous meteorites testifies to an ancient interaction with liquid water, which makes them key tracers for understanding the first phases of hydration of bodies in the Solar System.

Second age: the Late Heavy Bombardment

A massive input of icy bodies

Between 4.1 and 3.8 billion years ago, the Earth underwent a massive influx of celestial bodies resulting from planetary reorganization (Nice model). Ice-rich comets and asteroids then delivered water to the surface.
Isotopic analyses of deuterium (\(D/H\)) in the oceans show a signature close to that of certain classes of chondrites (notably CM), but different from that of Oort-type comets.
The Late Heavy Bombardment also allowed part of the water trapped in the depths to degas through thermal effects, initiating the primitive hydrosphere.

N.B.:
Chondrites (CM for "Mighei-type carbonaceous chondrites") are primitive meteorites particularly rich in hydrated minerals, containing up to 10% water in bound form. Their hydrogen isotopic composition (\(D/H\)) is very close to that of Earth's oceans, which makes them serious candidates as a major source of water for the primitive Earth.

Third age: recycling and tectonic stabilization

The establishment of plate tectonics

From 3.5 billion years ago, the Earth entered a phase of relative internal and external stability, marked by the progressive establishment of plate tectonics. This dynamic system, unique in the Solar System, allows a continuous recycling of water between the surface (oceans, crust) and the planet's interior (upper mantle). This water also influences plate tectonics by reducing friction along faults.

The fundamental role of subduction

During subduction, oceanic plates loaded with hydrated sediments and basaltic crust penetrate into the mantle. Under the effect of increasing pressure and temperature, these materials release their water above the descending plate, triggering partial melting of the mantle and feeding the volcanoes of island arcs. This process releases water vapor back into the atmosphere through volcanic degassing.

Hydrated minerals and thermal stability

This deep water cycle is controlled by the thermodynamic equilibria of water-bearing minerals (such as lawsonite, chlorite or serpentine), which dehydrate at precise pressures (3 to 10 GPa) and temperatures of the order of 500 to 800 °C. The current geochemical model suggests that about 2 to 3 km\(^3\) of water are subducted each year, while a comparable quantity is released by volcanic activity.

A regulatory cycle on a planetary scale

This mechanism ensures long-term regulation of ocean volume and global climate via the tectonic thermostat. Water also plays a lubricating role in the sliding of lithospheric plates, lowering their effective viscosity and facilitating mantle convective movements.

A huge reservoir of water in the mantle

This recycling, coupled with the deep storage of water in the mantle transition zones (410–660 km depth), allows a stable hydrosphere to be maintained for more than 3 billion years. Some models estimate that the mantle could contain up to three times the current mass of the oceans, trapped in minerals such as wadsleyite and ringwoodite. This mantle transition zone therefore constitutes a major reservoir.

N.B.:
Wadsleyite and ringwoodite are polymorphs of magnesium silicate capable of incorporating several percent of water into their crystal structure under high pressures. Their presence in the Earth's mantle transition zone suggests a huge underground water reservoir.

Water on Earth: a precarious balance

A stable but minority hydrosphere

The total mass of water on Earth is estimated at about \(1.4 \times 10^{21}\) kg. But only a available fraction (0.023%) is accessible at the surface in liquid form. The rest is trapped in ice, groundwater and hydrated minerals. The Earth remains a singular case due to its hydrosphere that has been stable for more than 3 billion years.

The table below summarizes the three ages of Earth's water and their isotopic signatures.

Earth's water: a triple origin

A hybrid model resulting from several processes

For a long time, researchers debated whether Earth's water came essentially from internal degassing (accretion), from an external contribution (cometary and asteroidal bombardment), or from geodynamic recycling. Modern isotopic, mineralogical and geophysical data now suggest a hybrid model, in which each age of planetary history contributed to shaping the current hydrosphere.

First age: the heritage of primitive accretion

The first age, marked by the accretion of hydrated primitive materials, provided an internal reservoir of water trapped in the mantle. This water is invisible at the surface but plays a fundamental role in the Earth's internal dynamics. The isotopic ratio \(D/H\) of certain mantle inclusions corroborates the idea that this water has been present since the very first phases of planetary formation.

Second age: exogenous input by chondrites

The second age, associated with the Late Heavy Bombardment, enriched the surface layers with exogenous water. CM-type carbonaceous chondrites, in particular, have a \(D/H\) ratio close to that of the oceans, suggesting compatibility with current surface water. This input probably allowed the appearance of the first liquid oceans during the Hadean.

Third age: stabilization by tectonics

Finally, the third age plays a stabilizing role: through plate tectonics, the Earth continuously recycles water between the surface and the interior. This mechanism explains why the quantity of liquid water accessible at the surface has remained globally stable over several billion years, despite atmospheric losses and climatic variations.

A synthesis of successive contributions

Thus, the Earth's hydrosphere is not the heritage of a single event but the result of a succession of processes, where each era consolidated the existence of water in different forms (liquid, mineral, gaseous) and at different depths. This multi-stage scenario accounts for the complexity and resilience of the Earth's water system.

N.B.:
This three-age model is now reinforced by the convergence between isotopic data (\(D/H\), \(^{18}O/^{16}O\)), thermal modeling of the mantle, and analyses of deep minerals containing water, such as ringwoodite inclusions in superdeep diamonds.