Antarctic seismic data has revealed a fascinating discovery about the Earth's core-mantle boundary. A thin layer, only a few to a few dozen kilometers thick, may be draped across this violent boundary, and it likely consists of ancient ocean floor pushed deep underground over geologic time. This layer, known as ultralow velocity zones (ULVZs), has been found to be widespread along the core-mantle boundary, challenging previous assumptions. The study, led by The University of Alabama, used seismic data from Antarctica to probe a vast stretch of the Southern Hemisphere nearly 2,000 miles below the surface. The team found evidence that ULVZs are not isolated patches but may be widespread, slowing seismic waves and appearing denser than the surrounding deep mantle. This discovery has significant implications for our understanding of the Earth's structure and processes.
The core-mantle boundary is one of the sharpest transitions inside the planet, with a greater change in physical properties than the shift between solid rock at Earth's surface and the air above it. ULVZs, thin regions about 5 to 50 kilometers thick, have been a subject of debate among scientists. The new study leans towards a broad role for subducted materials, especially former oceanic crust and sediments carried downward by plate tectonics. This conclusion is based on a rare southern view of Earth's deep interior, provided by data from the Transantarctic Mountains Northern Network (TAMNNET).
The researchers used a method called historical interstation pattern referencing (HIPR) to improve the signal and detect subtle signals arriving just before or after the main reflection. By comparing real waveforms with synthetic models, they identified 152 events with robust ULVZ evidence. This broad pattern stood out because the study area lies away from giant large low-velocity provinces (LLVPs) and present-day subduction zones. The data pointed to widespread thin anomalous layers across the region, suggesting 'mountains' along the core-mantle boundary with heights ranging from less than about 3 miles to more than 25 miles.
The study's findings favor the idea that old oceanic crust and sediments can sink with slabs, segregate due to density, and later migrate along the core-mantle boundary. This matches mantle convection simulations, which show subducted materials spreading widely along the boundary. The authors also considered alternatives, such as remnants of an ancient magma ocean and partial melting of pyrolite, but argue that subducted material, possibly partially molten near the core, better explains the widespread distribution. However, the study has limitations, as it cannot resolve very thin oceanic crust under 10 kilometers and may miss complexities tied to topography or layering.
The practical implications of this research are significant. If ancient seafloor forms a widespread coating above Earth's core, it could influence heat escape from the core, shaping Earth's magnetic field. It may also affect mantle plume formation and composition, helping to explain small-scale chemical and seismic heterogeneity deep inside the planet. This discovery highlights the ongoing evolution of our understanding of the Earth's interior and the interconnectedness of its processes.
