Imaging ancient ocean seafloors sunk to the core-mantle boundary

Amazing stuff!

"Through global-scale seismic imaging of Earth’s interior, research ... revealed a layer between the core and the mantle that is likely a dense, yet thin, sunk ocean floor ...
Seen only in isolated patches previously, the latest data suggests this layer of ancient ocean floor may cover the core-mantle boundary. Subducted underground long ago as the Earth’s plates shifted, this ultra-low velocity zone, or ULVZ, is denser than the rest of the deep mantle, slowing seismic waves reverberating beneath the surface.
Seismic investigations, such as ours, provide the highest resolution imaging of the interior structure of our planet, and we are finding that this structure is vastly more complicated than once thought,” ...

Understanding the composition of the core-mantle boundary on a large scale is difficult, but a seismic network deployed ... to Antarctica collected data for three years. Similar to a medical scan of the body, the 15 stations in the network buried in Antarctica used seismic waves created by earthquakes from around the globe to create an image of the Earth below.

The project was able to probe in high-resolution a large portion of the southern hemisphere for the first time using a detailed method that examines sound wave echoes from the core-mantle boundary. Hansen and the international team identified unexpected energy in the seismic data that arrives within several seconds of the boundary-reflected wave. ...

These subtle signals were used to map a variable layer of material across the study region that is pencil thin, measuring in the tens of kilometers, compared to the thickness of the Earth’s dominant layers. The properties of the anomalous core-mantle boundary coating include strong wave speed reductions, leading to the name of ultra-low velocity zone.

ULVZs can be well explained by former oceanic seafloors that sunk to the core-mantle boundary. Oceanic material is carried into the interior of the planet where two tectonic plates meet and one dives beneath the other, known as subduction zones. Accumulations of subducted oceanic material collect along the core-mantle boundary and are pushed by the slowly flowing rock in the mantle over geologic time. The distribution and variability of such material explains the range of observed ULVZ properties. ..."

From the absract:

"Ultralow velocity zones (ULVZs) are the most anomalous structures within the Earth’s interior; however, given the wide range of associated characteristics (thickness and composition) reported by previous studies, the origins of ULVZs have been debated for decades. Using a recently developed seismic analysis approach, we find widespread, variable ULVZs along the core-mantle boundary (CMB) beneath a largely unsampled portion of the Southern Hemisphere. Our study region is not beneath current or recent subduction zones, but our mantle convection simulations demonstrate how heterogeneous accumulations of previously subducted materials could form on the CMB and explain our seismic observations. We further show that subducted materials can be globally distributed throughout the lowermost mantle with variable concentrations. These subducted materials, advected along the CMB, can provide an explanation for the distribution and range of reported ULVZ properties."

Beneath the Earth, Ancient Ocean Floor Likely Surrounds the Core – University of Alabama News | The University of Alabama

Globally distributed subducted materials along the Earth’s core-mantle boundary: Implications for ultralow velocity zones (open access)

Credits: An ancient ocean floor has landed next to the Earth's core, says study

In this representation of the underground imaging, seismic waves from earthquakes in the southern hemisphere sample the ULVZ structure along the Earth’s core-mantle boundary and are recorded by sensors in Antarctica