Since the 1930s, the mainstream view of the 'Inner Core' of planet Earth is that it's solid, and is composed primarily of iron, along with small percentages of nickel and some other light elements.
Since the 1980s, it has been known that the core is able to transmit seismic shear waves (transverse seismic waves) - called S waves (see Wikipedia ).
However, the speed of transmission of the S waves has been found to be anomalous. Theoretical calculations, confirmed by high pressure lab. experiments, have shown that the S waves should be traveling at roughly twice the speed as has been observed in the Earth seismic studies.
This anomaly is currently unexplained, leading some researchers to question the core's solidity. But, given the extreme pressure at the depth of the core, there is currently no widely accepted theory which shows how the core could be anything but solid.
A 2014 research team put forward an alternative possible explanation - that the core could be formed from iron carbide ( Fe7C3 ) rather than free iron.
If so, the core would be by far the most significant carbon store on Earth.
An Fe7C3-dominant inner core would match seismic observations and imply a major carbon reservoir in Earth’s deepest interior.
See: Hidden carbon in Earth’s inner core revealed by shear softening in dense Fe7C3 Proc Natl Acad Sci U S A, 111(50): 17755–17758.
Also see :
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