Jr. Lister et Ba. Buffett, STRATIFICATION OF THE OUTER CORE AT THE CORE-MANTLE BOUNDARY, Physics of the earth and planetary interiors, 105(1-2), 1998, pp. 5-19
The thermal regime of the core is controlled by the heat flux taken by
the mantle across the core-mantle boundary (CMB) and its ratio Nu, th
e Nusselt number, to the conductive heat flux along the core adiabat.
If Nu > 1, then compositional convection from the inner-core boundary
(ICE) will be augmented by thermal convection from the CMB, and the ou
ter core will be well-mixed and unstratified. If Nu < 1, as some recen
t estimates suggest, there is a competition between a stabilising ther
mal buoyancy flux from the CMB and the convective compositional buoyan
cy flux from the ICE. An entirely analogous competition arises if ligh
t elements diffuse from the mantle into the core. A fundamental questi
on is whether such a stabilising buoyancy flux can stratify a layer at
the CMB and, if so, how thick the layer would be. A simple sphericall
y averaged fluid-mechanical and thermodynamic model is derived and use
d to examine the possibilities. Previous suggestions that compositiona
l convection is sufficiently vigorous to maintain a well-mixed state r
equire active entrainment downwards of buoyant fluid, which is doubtfu
l if inertial effects are negligible. It is suggested instead that a s
tratified layer accumulates below the CMB, which is not incorporated i
nto the underlying convection zone, though it may be mixed slowly by d
ouble-diffusive 'salt-fingering.' The thickness of the layer is calcul
ated theoretically as a function of the controlling fluxes of heat and
composition across the CMB. In the thermal case, the layer thickness
adjusts rapidly to reflect the current value of Nu and is largely inde
pendent of the cooling history. The layer thickness is of order 100 km
for Nu = 0.9, with a buoyancy frequency less than 10(-4) s(-1). The c
ompositional case typically gives a thinner but more strongly stratifi
ed layer and adjusts more slowly. Seismic, geomagnetic and length-of-d
ay evidence for such layers is assessed. (C) 1998 Elsevier Science B.V
.