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A previously unknown chemical reaction with water likely creates two mysterious features in the Earth’s core. Consequently, the core, which is made mainly of iron, enriches itself with hydrogen and releases silicon into the Earth’s mantle above. This would explain, on the one hand, why the Earth’s outer core is around ten percent lighter than expected and, on the other hand, what is happening to a thin layer that, according to seismic data, is on the surface of the Earth. essential.
A team led by Sang-Heon Shim of Arizona State University and Yongjae Lee of Yonsei University in Seoul examined the behavior of core and shell components at high pressures and temperatures in a diamond anvil. As the group reports in the journal Nature Geoscience, water reacts with silicon in the iron alloy in the Earth’s core. This creates silicate, which binds to mantle rocks, while hydrogen remains dissolved in the iron.
Earth’s liquid outer core consists of an iron-nickel alloy, although its exact composition is controversial. Its density is much lower than would be expected for such an alloy. It is therefore assumed that it contains large quantities of lighter elements – but it is not clear which ones and, above all, how they get there. Part of the answer may lie in a gigantic conveyor belt that stretches from Earth’s surface to its iron core. In the deep ocean trenches that mark the boundaries between Earth’s plates, water-rich oceanic crustal rocks descend deep into the Earth’s mantle. They release much of this water into the Earth’s mantle. But some water-bearing minerals are so stable that they are transported with the Earth’s ancient crust to the edge of the Earth’s core, 2,900 kilometers away.
Shim and Lee’s team has now investigated what happens to water in high-pressure experiments. In a laser-heated diamond anvil, in which pressures and temperatures at the core-shell boundary can be reached, water-bearing minerals were allowed to react with an alloy of iron and nine percent silicon. Regardless of the exact source of water and the respective conditions, silicate was always formed on the one hand and hydrogen dissolved in iron on the other. In the experiment, the iron with the dissolved hydrogen also had a significantly lower density than the surrounding alloy, so the reaction could explain why Earth’s outer core is lighter than expected.
The results of this experiment also suggest that the significantly lighter, hydrogen-rich iron is poorly distributed throughout Earth’s outer core. It is simply too light to be distributed by currents in the metal and instead remains on the surface of the liquid core. This discovery matches surprisingly well with another intriguing observation from the 1990s. Consequently, earthquake waves that travel through the Earth and are diffracted in the various layers indicate the existence of a thin, stable layer of liquid around the Earth’s outer core.
Until now, there has been no adequate explanation for this layer, referred to as E’. However, the working group’s experiments now indicate that the liquid is the result of a gigantic process that took place over billions of years. Water, along with the subduction of rocks, travels from the surface to the edge of the Earth’s mantle. There it reacts with the metal in the Earth’s core, producing a hydrogen-rich molten iron that floats in the iron-nickel alloy of the Earth’s core – and is visible as a separate layer in the earthquake data.