Scientists have long speculated about a vast water reservoir deep within Earth’s interior, often called a “hidden ocean.” While not a liquid ocean, this deep-earth water is a significant scientific finding, reshaping our understanding of Earth’s composition and internal processes.
Understanding the Earth’s Deep Water Reservoir
This substantial internal water reservoir is not a liquid body of water like surface oceans. Instead, it consists of water molecules chemically bound within mineral crystal structures. The primary mineral identified as a significant host is ringwoodite, a high-pressure form of olivine common in the upper mantle.
This deep water is located primarily within the mantle transition zone, roughly 410 to 660 kilometers beneath the surface. Immense pressures and temperatures over 1,000 degrees Celsius prevent the water from existing in liquid form. Estimates suggest this transition zone could hold an amount of water equivalent to, or even exceeding, all the world’s surface oceans combined.
The Discovery and Key Research Teams
The discovery of this deep-earth water reservoir emerged from research in the early 2010s. One significant breakthrough came from Professor Graham Pearson of the University of Alberta. In 2014, Pearson and his team announced the first terrestrial discovery of ringwoodite within a tiny diamond found in Brazil. This diamond contained a microscopic inclusion of ringwoodite, containing approximately 1.5 percent water by weight. This finding provided direct evidence that ringwoodite could indeed hold substantial amounts of water.
Steven Jacobsen, a geophysicist, and Brandon Schmandt, a seismologist, provided broad-scale evidence through seismic studies. Their 2014 research identified deep pockets of magma beneath North America, indicating water within high-pressure minerals at those depths. Jacobsen’s lab experiments on synthetic ringwoodite further showed this mineral absorbs water under mantle conditions, and that water can lead to partial melting at the base of the transition zone.
Unveiling the Deep: Scientific Approaches
Scientists employed several advanced techniques to detect and confirm this deep water reservoir.
Seismic tomography, using earthquake waves, maps Earth’s interior. Changes in wave speed indicate variations in mineral composition or the presence of water, as waves slow down in hydrated rock.
Laboratory experiments simulate mantle conditions, recreating immense pressures and high temperatures. These experiments study how minerals like ringwoodite incorporate water and how water affects their properties, such as melting point.
Analysis of rare deep-earth minerals, like ringwoodite inclusions found in diamonds, provides direct mantle samples. These tiny samples, preserved within diamonds, allow scientists to measure water content and confirm the water-holding capacity of transition zone minerals.
Broader Significance for Earth
This discovery has profound implications for understanding Earth’s fundamental processes. It sheds new light on the global water cycle, suggesting a “whole-Earth water cycle” where water moves between the surface and deep interior over geological timescales. Subducting oceanic plates carry water-laden minerals deep into the mantle, and some water can eventually return to the surface through volcanic activity.
This deep water also influences mantle dynamics and plate tectonics. Water can significantly affect the viscosity and melting point of mantle rocks, impacting tectonic plate movement. The presence of water can weaken rocks, making them more pliable and facilitating the large-scale convection currents that drive plate motion.
Understanding this internal water reservoir also contributes to our knowledge of Earth’s habitability. The stability of Earth’s surface water, and its ability to support life, is likely linked to this deep-earth water cycle. The balance between water entering and exiting the mantle over billions of years has likely helped maintain relatively stable sea levels and conditions conducive to life on our planet.