HR: 0830h
AN: U51B-0020    [PDF]
TI: Origin and Evolution of a Thermally Stratified Layer at the Top of the Core
AU: * Buffett, B A
EM: buffett@geosci.uchicago.edu
AF: Department of Geophysical Sciences, The University of Chicago, 5734 S. Ellis Avenue, Chicago, IL 60637 United States
AU: Bunge, H
EM: bunge@lmu.de
AF: Department of Geo and Environmental Sciences, Ludwig Maximilians University, Theresienstrasse 41, Munich, D-80333 Germany
AB: A substantial amount of heat is transported through the Earth's core by conduction down the adiabatic temperature gradient. Typical estimates for the conductive heat flow near the core-mantle boundary (CMB) are 5 to 6 TW. These values are within a factor of two of the values commonly cited for the heat flow into the base of the mantle. If the heat flow into the base of the mantle exceeds the heat flow conducted doewn the adiabat, then cold, dense fluid at the CMB helps to drive vigorous convection in the core. However, if the heat flow into the mantle is less than the conductive heat flow (e.g. subadiabatic), then a warm stratified layer can develop at the top of the core. Under these circumstances convection in the core is sustained by compositional buoyancy from the inner-core boundary. Light elements gradually accumulate in the convecting region of the core, whereas warm fluid accumulates at the top of the core because heat is conducted toward the CMB faster than it can be removed by mantle convection. A competition between the density deficits in the convecting and stratified regions govern the growth or disappearance of the stratified layer. We investigate the evolution of the stratified layer using estimates of the CMB heat flow from a three-dimensional numerical model of mantle convection in a spherical shell. Geological estimates of plate motions over the past 120 Ma are imposed as boundary conditions in the numerical calculations to quantify the resulting changes in CMB heat flow. The calculations are extended to earlier times by cyclically repeating the plate motion record. Averaging the predictions over several cycles reduces the effect of initial conditions. The results indicate that minima in CMB heat flow occur 40 to 50 Ma after minima in the surface heat flow. An additional time delay is incurred by the time required to develop the stratified layer once the heat flow becomes subadiabatic. The net time delay is sufficient to cause the stratified layer to reach its maximum thickness during or shortly after the peak surface heat flow. The predicted growth of the layer in our simulation reaches a maximum thickness of approximately 50 km about 100 Ma ago. We expect the occurrence of a stratified layer to suppress variability in the magnetic field at the surface due to magnetic screening, and may even promote a stable polarity. The temporal correlation between the maximum layer thickness and the occurrence of the Cretaceous superchron suggests a caustive relationship.
DE: 8125 Evolution of the Earth
DE: 8130 Heat generation and transport
DE: 8147 Planetary interiors (5430, 5724)
SC: U
MN: 2003 Fall Meeting