Earth’s Ancient Water Reservoir: A Revolutionary Discovery
In a groundbreaking study published in Science on December 11, researchers have dramatically revised our understanding of Earth’s water history. Scientists now believe that the young Earth had a far greater capacity to retain water than previously thought. According to this new research, rocks in Earth’s early mantle may have contained up to 100 times more water than previous estimates suggested – potentially an entire ocean’s worth of water stored in ancient mantle rocks.
This discovery came from the work of geochemist Wenhua Lu of the Chinese Academy of Sciences and colleagues, who investigated bridgmanite, one of Earth’s first and most abundant minerals. By recreating the extreme conditions of Earth’s deep mantle in laboratory settings, they found something remarkable: as heat increased, bridgmanite could incorporate more water into its crystal structure than scientists had believed possible.
“The findings add another vital piece to an intricate and multifaceted puzzle,” notes petrologist Michael Walter of Carnegie Science in his commentary on the research. This new understanding of how water was integrated into ancient minerals provides crucial insights into the earliest origins of Earth’s water cycle – the fundamental system that makes our planet habitable.
Long before Earth’s surface featured the oceans we know today, water existed in abundance, locked deep within the rocks of the planet’s lower mantle. During the Hadean Eon, approximately 4.4 billion years ago, Earth’s mantle began to form as the planet’s surface magma ocean gradually cooled and crystallized. Bridgmanite emerged as the first and most prevalent mineral during this process, and today constitutes about 60 percent of the mantle. This mineral forms only under conditions of extreme heat and pressure – in the deepest regions of the mantle, temperatures can exceed 4000° Celsius with pressures reaching up to 700,000 atmospheres.
As the primordial magma ocean cooled, water molecules dissolved in the molten rock became incorporated into the newly forming bridgmanite, becoming trapped within the mineral’s crystalline structure. This process continues even in modern times: water travels into the deep Earth with subducting tectonic plates, becomes temporarily locked in minerals like bridgmanite, and eventually returns to the surface through volcanic eruptions, completing a geological water cycle.
The key question that Lu and colleagues sought to answer was how much water existed in Earth’s depths during its earliest days. To determine this, they needed to understand bridgmanite’s water-holding capacity under extreme conditions. Using a laser-heated diamond anvil – a tool that generates intense pressure by compressing samples between diamonds while simultaneously heating them with focused lasers – they recreated the lower mantle’s environment. Their experiments yielded surprising results: heat significantly increased bridgmanite’s capacity to hold water, suggesting the lower mantle could store substantially more water than the cooler upper mantle. While previous estimates suggested bridgmanite was nearly dry, containing less than 220 parts per million of water by weight, this new research indicates there was a substantial deep water reservoir.
Over Earth’s long history, the researchers propose, the movement of tectonic plates and the upward flow of mantle plumes helped redistribute this water, bringing much of it to the surface. However, some of this primordial water may still remain deep within our planet, a hidden reservoir from Earth’s earliest days that continues to influence the complex geochemical cycles that make our planet unique in the solar system.
