Gradients in the Earth's outermost core

Dec 08, 2010
Gradients in the Earth's outermost core

Evidence that the outermost portion of the Earth’s core is stratified is provided by earthquake data reported by scientists at the University of Bristol this week in Nature.

The Earth's core is composed mainly of iron, but it is also known to contain a small amount of lighter elements, such as oxygen and sulphur. As the ’s inner core solidified, it is thought to have expelled most of these light elements, which then migrated up through the liquid outer core, perhaps becoming concentrated in the outermost portion of the core, near the core/mantle boundary.

Professor George Helffrich and Satoshi Kaneshima observed seismic waves which travelled through the outermost core, from earthquakes in South America and the southwestern Pacific Ocean, recorded at arrays of seismometers in Japan and northern Europe.

The speeds at which seismic waves travelled through the outer core at different depths suggest that it is not a homogenous liquid, but that the uppermost 300 kilometres or so of the core is stratified, with the portion nearest the core/mantle boundary containing up to five per cent by weight light elements.

This stratification would affect temperature gradients in the outermost core and have implications for our understanding of the thermal evolution of the core and lowermost mantle.

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More information: Outer-core compositional stratification from observed core wave speed profiles by George Helffrich and Satoshi Kaneshima in Nature.

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Graeme
not rated yet Dec 12, 2010
Some of these layers could be a form of sedimentary deposit, where something (perhaps iron oxide) crystallizes out from the iron liquid, and floats to the top of the core and forms a slushy dross just below the mantle. Over time this should become compressed and more solid as the molten iron is squeezed out of the pores.

Another interesting possibility is whether a form of life could arise at the core boundary, using some different chemistry. However where would it get energy from?

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