Imagine the Earth as a giant onion, with many layers. The deepest layer, just above the molten core, is called the D" layer. Scientists from HPSTAR and Institute Academy for Precision Measurement Science and Technology of CAS, have discovered that there is more water in this layer than they previously thought. Their findings are recently published by Innovations.

Scientists use earthquakes to study the Earth's interior. When an earthquake happens, it sends out seismic waves that travel through the Earth. By measuring how fast these waves travel, scientists can learn about the interior’s structures and even the materials they pass through. A simple example is found at Earth’s core mantle boundary, where temperature sharply increases. Higher temperature usually slows down the waves and this core-mantle boundary are often featured by decreasing seismic velocity with depth.

However, in some places, like under Central America, the waves speed up at the core-mantle boundary. The secret lies in the presence of water. Water can affect how fast these seismic waves travel. The team measured the velocities travelling through a special hydrous mineral, which can exist in Earth’s deep lower mantle. This hydrous mineral, called hydrous Phase H, substantially slow down the waves. In water-enriched pockets like the bottom of Central America, the seismic velocity starts low. The abnormal increasing velocity suggests that water is being released from minerals in these areas.

This discovery is important because it tells us that water can travel much deeper into the Earth than we previously thought. It also helps us understand how the Earth's plates move and how the Earth's interior is changing over time.

Caption: Fine seismic structures at Earth’s core mantle boundary induced by dehydration.

北京高压科学研究中心胡清扬研究员联合中国科学院精密测量科学与技术创新研究院的侯明强教授等在距地表2850千米的地核地幔边界发现水循环的踪迹，深部水刻画了D”层的二级精细结构。研究成果以“识别核幔边界脱水引起的波速异常（Identifying dehydration-induced shear velocity anomaly in the Earth’s core-mantle boundary）”为题，于1月6日在线发表于国际学术期刊《The Innovations》杂志上，论文链接：https://doi.org/10.1016/j.xinn.2024.100740。