Scientists Have Discovered a “Sixth Ocean” on Earth, But It’s Not on the Planet’s Surface.
There are five oceans on the surface of the Earth: the Atlantic Ocean, Pacific Ocean, Indian Ocean, Arctic Ocean, and Southern Ocean. An international group of scientists has now found evidence of a significant amount of water located between the upper and lower mantle of the Earth.
According to an international study, scientists have discovered a water reservoir with a volume three times greater than all the oceans on the Earth’s surface combined. However, instead of existing on the planetary surface, this water was found in the transition zone between the Earth’s upper and lower mantle.
What is the “Sixth Ocean”?
Evidence suggests that water exists in the transition zone (TZ), the boundary separating the Earth’s upper mantle from the lower mantle. This boundary lies at depths of 410 to 660 km, where extreme pressures of up to 23,000 bars cause olive-green olivine minerals to alter their crystalline structure.
The study has confirmed a long-held theoretical notion that ocean water accompanies subducting plates and thus enters the transition zone. This means that the water cycle on our planet includes the interior of the Earth.
The transition zone between the upper (brown) and lower (orange) mantle of the Earth is believed to contain a significant amount of water bound within rocks. (Image: Worldatlas).
Professor Frank Brenker from the Institute of Geosciences at Goethe University in Frankfurt, Germany, explains: “These mineral transformations significantly hinder the movement of rocks in the mantle. For example, mantle plumes—an upwelling of anomalously hot rock within the mantle—sometimes stall just below the transition zone. The movement of mass in the opposite direction also becomes blocked. Subducting plates (an essential component of subduction zones) often struggle to break through the entire transition zone. Therefore, there is a graveyard of such subducting plates in this area beneath Europe“.
However, it is still unclear what the long-term effects of “pulling” material into the transition zone will have on its geochemical composition and whether a larger amount of water exists there.
Professor Brenker explains: “Subducting plates also carry deep-sea sediments into the Earth. These sediments may contain a significant amount of water and CO2. But it remains unclear how much of this enters the transition zone in more stable forms, such as hydrated minerals and carbonates—therefore, it is also unclear whether a large amount of water is truly stored there“.
The current conditions are certainly conducive to this. Dense minerals such as wadsleyite and ringwoodite can (unlike olivine at shallower depths) store significant amounts of water—in fact, the transition zone could theoretically absorb six times the amount of water contained in all of our oceans.
“So, we know that the transition zone (TZ) has a tremendous capacity to store water. However, we do not know if it actually does” – Professor Brenker stated.
Distinct Oceans Within the Earth
Evidence for the “Sixth Ocean” was found during the analysis of a rare diamond formed 660 km below the Earth’s surface.
After studying a rare diamond found in Botswana, Africa—this diamond formed at a depth of 660 km, right in the middle of the transition zone (TZ) and the lower mantle, where ringwoodite is the prevalent mineral—scientists found a substantial amount of ringwoodite impurities with high water content surrounding the diamond.
Diamonds from this region are extremely rare, even among those sourced from super-deep origins, making up only 1% of diamonds. Furthermore, the research team was able to determine the chemical composition of the rare diamond. It closely resembles every fragment of mantle rock found in basalt anywhere in the world. This indicates that the diamond certainly originates from the Earth’s mantle.
Professor Frank Brenker from the Institute of Geosciences at Goethe University in Frankfurt, Germany. (Image: MARCUS KAUFHOLD / FAZ).
“The impurities in the 1.5 cm diamond are large enough to allow precise determination of the chemical composition. In this study, we have demonstrated that the transition zone is not a dry sponge, but contains a substantial amount of water. This also brings us one step closer to the idea of French author Jules Verne [writer of the famous works “Journey to the Center of the Earth” (1864) and “Twenty Thousand Leagues Under the Sea” (1870)] about an ocean inside the Earth. The difference is that there is no liquid water ocean down there, but rather water-bearing rocks” – Professor Brenker shared.
The research team explains that the high water content of the transition zone has far-reaching implications for the dynamics within the Earth, and if disrupted, it could lead to large-scale mass movements in the crust.
The study by the German-Italian-American research team has been published in the journal Nature, stating that the internal structure and dynamics of the Earth are shaped by the 660 km boundary between the upper and lower mantle transition zones.
