Erosion of chemical stratification at the top of the Earth's core
Abstract
Seismic and geomagnetic observations suggest a layer with stable stratification at the top of Earth's core. Recent geodynamo simulations indicate that this strongly affects the morphology of the geomagnetic field. Yet the origin of this layer is debated. Previous models proposed that it built up over geological time by accumulation of light elements from the inner-core, barodiffusion or chemical interaction with the mantle. Alternatively, the layer could be primordial, resulting from incomplete mixing after a giant impact. However, we do not know whether a primordial stratification can be preserved for 4.5 Gy or whether it will be eroded by convection. Furthermore, it is unclear whether an early stratification is compatible with the initiation of the geodynamo prior to 3.4 Ga, as indicated by paleomagnetic observations.
To address these questions, we use numerical simulations on the erosion of a chemically stratified layer by thermal convection in a fully-molten core. In contrast with previous investigations, these include rotation, the spherical geometry of the core and the weak diffusivity of chemical elements. For large chemical density anomaly, we observe that thermal convection gradually erodes the layer from underneath. We obtain scaling laws for the erosion rate as a function of the relative importance of stratification to convection. Our scalings predict that a primordial layer thicker than 100 kilometers and with a density anomaly of about 1%, as suggested by seismic observations, can persist up to the present day. The stratified layer at the top of Earth's core could therefore be a vestige of core formation. In some simulations, we observe double-diffusive convection with strong radial flows within the stable stratification. This may help the initiation of the geodynamo from a chemically stratified core.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMGP31A..06L
- Keywords:
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- 1507 Core processes;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1510 Dynamo: theories and simulations;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1517 Magnetic anomalies: modeling and interpretation;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1527 Paleomagnetism applied to geologic processes;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1560 Time variations: secular and longer;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1595 Planetary magnetism: all frequencies and wavelengths;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 5440 Magnetic fields and magnetism;
- PLANETARY SCIENCES: SOLID SURFACE PLANETS